Mammography


## Introduction

Computed tomography, more commonly known as "cat scans" or "CT scans", are cross sectional, two dimensional gray scale images. They are very commonly used in the practice of clinical medicine. This module will serve as an introduction to CT, and provide you with information that will help you to understand what you are looking at when you view an image.

## How Images Are Obtained

CT images use x- rays to obtain a thin axial image of the patient- a “slice”.  To obtain CT images, the beam of x-rays and the x-ray detector circle around the patient, continuously producing and detecting a thin beam of x-rays that pass through the patient. This is in contrast to radiographs, where a stationary x-ray beam source and detector are used.

### The CT Scanner

The patient lies on a table that slowly moves through a circular tube, called the gantry. The gantry houses the generator that produces the x-rays as well as a specialized x-ray detector. The two are located 180 degrees apart from one another. The generator and detector move in unison around the patient, continuously transmitting a thin beam of x-rays through the patient. Those that are not absorbed by the patient are sensed by the detector and transmitted to a computer. The data is analyzed and an image produced which represents one "slice" of the patient.

Traditionally, with each 360 degree rotation of the x-ray beam and detector, data is obtained from that axial section of the patient.  The table then moves, and another axial section of the patient is obtained.  With newer CT scanners, the table moves continuously through the gantry, and the generator and detector continuously circle the patient.  As a result, instead of a single “slice” of the patient being obtained with each rotation, a continuous helix of data is obtained.  A specialized computer interprets the information from the detector and transforms it into images, which represent sequential “slices” of the patient.

![CT-scanner](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/CT/images/CT-scanner.gif)

### Viewing the Image

Images are viewed as though the patient is lying down on his/her back and you are standing at the foot of the patient looking towards their head. The patient's right side is displayed on the left of the image, and the left side on the right. Images are generally displayed in the order they are obtained, from superior to inferior. In this way, structures can be followed from image to image. If you aren't sure which structure you are looking at, try looking at the slice above or the slice below to help follow the structure and determine its origin.

Below are two sequential CT images of the upper abdomen; the image on the left is 5mm superior to the image on the right. Each of these images represents a slice of the patient which is 5 mm thick.

The superior image is taken through the superior aspect of the pancreas ("P") and not much of the organ is visible on this slice. This makes it difficult to identify. Looking at the image just below will help, as more of the pancreas is now visible allowing appropriate identification of the structure. Note the right kidney is not visible on the superior image. The second, more inferior image shows the most superior part of the kidney, "K".

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## Factors Affecting the Appearance

CT images are produced with ionizing radiation, similar to radiographs. Therefore, some of the factors affecting the appearance of the CT images are similar to radiography (for example density).

On CT, the density of a structure, the image display, and the use of iv contrast enhancement are the main factors affecting the appearance of structures. Shape and thickness do not affect the final CT image as much as they do in radiographs, since CT images are summations of only thin slices of superimposed structures.

The data obtained can be displayed (or "windowed") to highlight certain structures. The two chest CT images shown are from the same set of data at the same level. The left image is displayed in "lung" windows and the right in "soft tissue" windows. Note that lung windows show the lung parenchyma (small blood vessels, bronchi, and airspaces), but one is unable to distinguish the soft tissue structures well. Soft tissue windows show the muscles, fat and blood vessels well, but not the lung parenchyma.

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### Density

Since x-rays are used to obtain CT images, the density of a structure affects the final image of a CT in a similar manner.

Dense structures, like bone, attenuate more of the x-ray beam and appear white.  Less dense structures, like air, attenuate less of the x-ray beam and appear black.  Soft tissue structures are of intermediate densities and appear as varying shades of gray.

The CT detectors are much more sensitive than conventional radiograph cassettes.  With the detector and beam rotation allowing for the accumulation of many data points in one slice, spatial resolution is excellent, and subtle differences in soft tissue densities are visible which are not noted on routine x-ray.

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On CT, soft tissue structures appear as varying shades of gray, depending on the density of the structure. Structures of similar density are easier to differentiate from each other on CT than on radiography.

Note that the fluid filled gallbladder can be differentiated from the adjacent liver, which is higher in density and appears a lighter shade of gray.  The spleen and left kidney can be seen distinctly from each other, partially due to the excellent spatial resolution of CT, but also due to differences in density.

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Similar to radiographs, when structures of differing densities are next to one another, they can be distinguished from each other.

Note that the low density fat between the right kidney and liver helps to differentiate them. In contrast, it is not possible to see the border between the duodenum and pancreas, since they are of similar densities and there is no intervening tissue of a different density present to help distinguish them from each other.

### Contrast Enhancement

In order to discretely identify tissues that are immediately adjacent to one another on CT, their densities must be different enough so that they appear as varying shades of gray on the CT image.

To aid in the differentiation of structures seen on CT, particularly if they are of similar density, intravenous contrast dye (x-ray dye) is often used. The dye is composed of iodine which is quite dense and therefore attenuates x-rays, thus appearing white on CT. The dye is administered to the patient by intravenous injection. When this is done, tissues in the body will take up the contrast and appear a lighter shade of gray than they would without the dye. The more dye that a tissue takes up, the lighter shade of gray it appears on CT; the more it is said to "enhance".

Look at the two images below.  The image on the left is "unenhanced" (no x-ray dye was given). Note that the kidneys, liver, pancreas, spleen and blood vessels all appear nearly the same shade of gray. The image on the right is "enhanced" (obtained after intravenous contrast was administered). Note that each organ is a lighter shade of gray than on the unenhanced image, and each now differs from each other in its subtle shade of gray. This results from differences in the amount of x-ray dye that each organ absorbs; the more absorbed, the lighter the shade of gray. The kidneys absorb the most dye and so appear the lightest. Also note that you can see blood vessels in the liver, since they contain a higher concentration of the dye than the adjacent liver parenchyma. Also note the splenic vein, which sits immediately posterior to the pancreas, is now seen discretely apart from it.

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## What Anatomical Structures Are Well Seen

CT can display differences between soft tissues that differ in physical density by as little as 1%. Most anatomic structures are visualized on CT scans, including bone, air, fluid, fat, and soft tissues.

All of these structures are better demonstrated on CT than on radiography. This results from a combination of factors, including the very sensitive CT detectors that differentiate smaller differences in densities, the excellent spatial resolution CT affords, and the fact that the images are single "slices" of the body (while radiographs represent a superimposition of a number of structures).

### Abdominal Structures

Compare these images of the abdomen. The image on the left is a radiograph and the right is a CT image of the mid-abdomen. The radiograph shows the bones well, and air within the bowel, however airless bowel, the individual organs, blood vessels, lymph nodes and other soft tissue structures cannot be seen. In contrast, the CT shows the individual organs, the bowel that contains air as well as the bowel which is airless (fluid filled), blood vessels and all of the fat intervening between and around these structures.

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### Thorax

Structures in the chest are also well seen with CT, including bones, blood vessels, lungs, trachea, esophagus, fat, as well as the muscles of the body wall.

Compare what soft tissues structures look like on the chest radiograph on the left and the chest CT image on the right. The trachea is seen on the radiograph, since it is air filled and more lucent than the adjacent structures of the mediastinum. Note that the esophagus, blood vessels and muscles cannot be differentiated on the radiograph, but they are easily seen on the CT.

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### Brain

CT is also an excellent way to view the brain and contents within the skull. None of these can be seen with radiographs.

The image on the left is of the inferior part of the brain, and the image on the right is slightly more superior. Note the low density fluid of the subarachnoid space and lateral ventricles is well differentiated from the varying shades of gray of the arteries supplying the brain, and the brain parenchyma itself. CT also demonstrates the subtle differences in the gray and white matter tracts of the brain parenchyma, which have different compositions and functions. The white matter appears darker than the gray matter.

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## Indications For Use

CT is widely used to help diagnose a variety of different problems and symptoms. It is excellent for evaluating lung parenchyma, as well as soft tissues of the neck, chest, abdomen and pelvis. It is also a quick and relatively inexpensive way to evaluate the brain.

CT can be used as a primary imaging tool to evaluate a patient's symptoms, or can provide additional information when other tests are inconclusive. CT is currently widely available and easily accessible.

It is beyond the intentions of this submodule to discuss all of the potential uses of CT, but this should serve as a broad guideline to help one understand the general uses of CT.

### Chest

CT is widely used to evaluate structures in the thorax. Tumors of the lung or chest wall, infections, emphysema and other chronic lung diseases are well demonstrated on CT.

More recently, CT has become a more powerful way to evaluate the blood vessels in the thorax and the heart. This includes evaluation for blood clots in the pulmonary arteries, atherosclerosis and narrowing in the coronary arteries and aorta, abnormal dilatations of blood vessels (aneurysms), and cardiac function.

Below is a chest CT image displayed in lung windows. Note how well the lung parenchyma is seen; the individual blood vessels and airspaces are visible. Also note that the soft tissues of the chest wall and mediastinum are not seen adequately. When viewing a CT, it is important that the display is set appropriately to allow visualization of the structures of interest. Do you see the mass in the lung?

![indic-lung-mass1](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/CT/images/indic-lung-mass1.jpg)

Below are identical images displayed in different windows. The image on the left is in soft tissue windows and that on the right in lung windows. Both images show the tumor mass in the medial right lung.

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Note that the tumor extension into the mediastinum, adjacent to the aorta and trachea is only appreciated on the soft tissue windows. This is not well seen on the lung windows and was not visible on the patient's chest radiograph shown below.

![indic-lung-mass](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/CT/images/indic-lung-mass.gif)

### Bone

As discussed earlier in this learning module, radiography is an excellent modality to evaluate the bones. There are instances in which CT is indicated for further evaluation or problem solving. This occurs most commonly in complex intra-articular fractures of the hip, ankle, and knee. CT is also used to evaluate tumor infiltration into bone.

Note in the radiograph of the knee on the left, the fracture of the proximal tibia is easily seen (arrows). The CT image on the right is a coronal reformat of a CT of the knee. Note that the complexity of the fracture is shown with greater detail on the CT image. It is easier to see that there are multiple fracture lines and that the fracture lines extend through the epiphysis into the knee joint space. This information impacts how this fracture is treated.

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### Abdomen

There are extensive uses for CT of the abdomen. These include a) abdominal pain, b) inflammatory conditions such as suspected appendicitis or diverticulitis, c) tumor assessment, d) enlargement of organs, e) kidneys stones, and f) obstruction of the bowel, kidneys or bile ducts.

The CTs images below are from two different patients who presented to the ER with abdominal pain.
The left image demonstrates a dilated right renal collecting system (the calyces and renal pelvis) due to obstruction of the right ureter. This is commonly caused by a renal stone lodging within the ureter and obstructing the drainage of the kidney. The right image demonstrates acute appendicitis. The appendix is abnormally dilated, with swelling in the wall and inflammation around it.

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## New Technology

Faster CT scanners that can acquire thinner sections, and advances in computer software have significantly impacted the field of CT. Reformations into any imaging plane are now commonly performed. This augments visualization of anatomy and aids in diagnosis.

### Coronal Reformats

Below are coronal reformats of an abdomen CT. The image on the left shows the relationship of the heart to the diaphragm and liver, and the course of the small and large bowel and mesenteric blood vessels.

The image on the right shows the relationship of the kidneys to the adrenal glands, and demonstrates that the kidneys are located quite posteriorly in the abdomen; as posterior as the spine, since they are both seen in this same coronal section.

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### 3D Volume Rendering

This is a way of reformatting CT image data to produce a 3 dimensional image of selected structures, excluding the structures which are not of interest. The images can be rotated in any orientation to see the anatomy from any perspective. 3D volume rendered images are most useful in preoperative planning for complex vascular procedures or complex fractures, such as those of the pelvis or proximal tibia. A 3D volume rendered image from an abdominal CT angiogram is shown. Note that the arteries and their branching pattern is displayed. The kidneys and spine are visible, but most other soft tissue structures are not demonstrated with this technique.

![new-tech-VR2](http://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/CT/images/new-tech-VR2.gif)

CT Angiography (CTA) is a CT technique targeted to display the arteries. Traditionally, in order to see the blood vessels with this level of detail, patients would have to undergo an angiogram, which is an invasive procedure in which catheters are placed directly into the arteries and contrast injected directly into the blood vessel.

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The image on the right is another example of a 3D volume rendered image from a CT angiogram of the brain. This shows the arteries supplying the brain. The tissue seen in the background is the base of the skull. Traditionally, in order to see the blood vessels to this level of detail, patients would have to undergo an angiogram, which is an invasive procedure in which catheters are placed directly into the carotid or vertebral arteries with direct intraarterial contrast injection.

*This is a 3 dimensional volume rendered image of the blood vessels forming the circle of willis at the base of the brain.*


Computed Tomography (CT)


## Introduction

Ultrasonography uses high frequency sound waves to create images. The sound waves are directed into the body and are reflected back to the ultrasound transducer. The reflected sound waves provide information about the depth, type of tissue, and tissue interfaces. This information is converted into grayscale images.

Unlike radiography, fluoroscopy and CT, ultrasound does not use ionizing radiation, so is low risk to the patient. It is non-invasive, relatively inexpensive, and allows one to perform "real-time" or "live" imaging and to observe motion.

## How Images Are Obtained

An ultrasound transducer (shown below) is placed onto the patients skin with the gray area in full contact with the skin. Gel is used between the skin and transducer to prevent air from intervening between the two. The transducer emits a narrow beam of sound waves of a specific high frequency. Depending on the frequency, the sound waves penetrate the patient's tissues to a variable depth and are refracted, scattered, absorbed or reflected. The reflected waves (called an echo) bounce from tissues back to the transducer, which also acts as a sensor, or detector, and are used to generate the image.

![US-transducer](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/US/images/US-transducer.jpg)

High frequency sound waves provide high resolution images, but don't penetrate deeply into tissues, so are best for superficial structures. Lower frequency sound waves are used to image deeper structures, but create lower resolution images.

Sound waves are reflected at the interfaces of tissues with different acoustic impedance (a product of the density of the tissue and the velocity of sound in the medium). The intensity of the reflected US beam depends upon the different characteristics of adjacent tissues and tissue interfaces. Larger differences in tissue types yields higher intensity of the reflected sound. High intensity echos appear white on the US image, "echogenic or hyperechoic". Low intensity echos appear dark, or "hypoechoic".

An US scanner is shown below. As the transducer is moved over the patient, the screen displays the image in real-time. Since the transducer can be held in any position, images in the axial, sagittal, coronal planes, as well as any obliquity can be generated.

![US-machine](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/US/images/USmachine.jpg)

## Factors Affecting the Appearance

### Composition of the Tissue

Solid organs, like the liver, have many internal tissue interfaces such as the bile ducts, portal vein branches, hepatic artery branches, hepatic veins, peri-portal fat, etc. This results in multiple reflected echos of variable intensity, and so the liver looks a medium shade of gray on US. Organs are slightly different from each other in their shade of gray on ultrasound based on differences in their tissue composition.

Fluid filled structures (gallbladder or urinary bladder) have no internal tissue interfaces and therefore do not reflect sound waves. As a result, the fluid filled portion of the structure will appear black and the walls gray to white. The US image of the gallbladder below illustrates this.

![liverGB](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/US/images/liverGB.jpg)

### Composition of Adjacent Tissues

The greater the differences between two adjacent structures, the greater the reflected sound waves and the more echogenic (white) the border between the tissues appears on the image. The difference in acoustic impedance between bone or air and adjacent tissues is large, and thus at such an interface most of the sound waves are reflected. Very few sound waves are left to penetrate and reach deeper structures. As a result ultrasound can't be used to image the area deep to bone or air, and since no echos are reflected from these locations, they look black on the US image.

The image below is a superficial ultrasound of the chest wall. The ribs, seen in cross section, are marked with stars. Note the anterior edges of the ribs are echogenic, and the black "shadow" posterior to the ribs, where no sound has been transmitted. The lung edge, running just posterior to the ribs, is also echogenic, but the lung parenchyma can't be imaged and appears black

![ribsUS](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/US/images/ribsUS.jpg)

## Anatomical Structures

The solid organs of the abdomen, including the liver, kidneys and spleen, can be evlauated well with US. Deeper structures can't be seen with the same level of resolution as superficial structures because of the number of scattered, absorbed and reflected sound waves occuring, leaving less sound available to penetrate deeper tissues and reflect back to the transducer. As an example of this, look at the difference in resolution of the upper vs the lower pole of the right kidney, shown below. The lower pole, on the right side of the image, is more superifical and is seen with better resolution than the upper pole.

![liverkid](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/US/images/liverkid.jpg)

Superficial structures (like the thyroid and testicles) can also be evaluated with ultrasound, with production of high resolution images. The image below is of a testicle. The black (anechoic) structure posterior to it (*) is an abnormal collection of fluid around the testicle, called a hydrocele.

![testicle](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/US/images/testicle.jpg)


US is widely used in obstetrics (OB), as the fluid in the amniotic sac provides a great "acoustic window" through which to image the fetus. The two images below are an US of a 19 week old fetus. The amniotic fluid is black (anechoic). The top part of the images shows the medium gray of the uterine wall and abdominal wall. Just deep to this is the black amniotic fluid, and then the fetus. The left image is a sagittal image of the fetus. The right image shows the fetus's hand in the center of the image.

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Fluid or blood filled structures, for example the gallbladder, urinary bladder, and the heart, can be evaluated well with US. Depending on their location and depending on the composition of adjacent tissues, some blood vessels are seen well. For example, the descending thoracic aorta cannot be seen with ultrasound, due to its location deep to bone and air.

## Indications for US

### Pregnancy

US is used in early pregnancy to confirm the presence of an embryo. In mid pregnancy, a "screening" ultrasound is performed to evaluate for fetal anomalies. The brain, heart, stomach, kidneys, umbilical cord and other structures are carefully evaluated to determine if they are morphologically normal.

The image on the right shows the fetal head in the axial plane. The outer skull is not yet ossified, which is why we are able to US "through" it. The stars depict the right and left cerebellar hemispheres. The + symbols are located on the thalamus. Identifying these normal structures and confirming their normal size helps to exclude major congenital anomalies.

![OBhead](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/US/images/OBhead.jpg)

### Gynecology

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Beautiful images of the uterus and ovaries are generated when US is performed with a transvaginal transducer (shown on bottom left). It is inserted into the vagina and positioned in the vaginal fornix. This places the transducer right next to the uterus and close to the ovaries, enabling high resolution images to be obtained.

In the image to the right, the vaginal wall is depicted by the green dots. The transducer sits against the wall and you can see its proximity to the uterus. The image on the bottom-right is a normal ovary, with the stars depicting follicles.

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### Abdominal Organs

The liver, kidneys, and spleen are routinely evaluated by ultrasound. Cysts, tumors, kidney stones and gallbladder calculi are some of the findings which can be seen with ultrasound. The below image on the left is a transverse US of the epigastrium. The body of the pancreas is clearly depicted (marked with the stars), which is unusual because gas within the stomach, lying anterior to the pancreas, often obscures it from being imaged.

It is easy to distinguish blood vessels from parenchyma. The image on the right is a transverse US of the upper liver. This shows the confluence of the right, middle, and left hepatic veins which are marked with stars.

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### Specialized Technology

#### **Color & Spectral Doppler Ultrasound**

Blood vessels can be identified with grayscale ultrasound, as shown above. Doppler ultrasound is used to gain additional information about the blood vessels and the blood flow through them. It is used to measure the velocity and direction of blood flow, to evaluate for stenosis (narrowing) of blood vessels, and for vascular resistance and patency.

Doppler US relies on the doppler effect; the change in frequency of sound waves reflected by a moving object. Blood moving towards the US transducer appears to have a higher frequency and shorter wavelength, while that moving away from the transducer appears to have a lower frequency and longer wavelength.

The image on the left is a color (power) doppler image showing blood flow in the common, internal and external carotid arteries.  The image on the right is a spectral doppler image of the internal carotid artery. Spectral doppler provides information on direction of blood flow and flow velocity (in cm/second).

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Ultrasonography


## Introduction

Fluoroscopy is a specialized x-ray technique which images moving structures and displays them in real time on a television monitor. Fluoroscopic images look just like x-ray images, as they are obtained with a similar technique.

The benefit of fluoroscopy is the ability to visualize dynamic processes as they are occurring; think of fluoroscopy as a live x-ray movie. The downside of fluoroscopy, which is important to keep in mind, is the relatively high radiation dose to the patient.

Angiography is the use of fluoroscopy to place catheters and other interventional devices into blood vessels and image blood flow in vessels.

## How Images Are Obtained

### Fluoroscopy Unit

The patient lies on a specialized table between the x-ray generator and the detector. The generator produces an x-ray beam which travels through the patient towards a specialized detector called the image intensifier.

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The image intensifier absorbs the transmitted x-rays and emits part of the absorbed energy as light photons. Through a sophisticated process, the number of light photons emitted is amplified and they are converted into a digital image. This is then displayed live on a tv monitor.

The physician observes the images on the monitor live, as the study is being performed.

### Angiography

Angiography is a general term describing the imaging of blood vessels with x-ray dye. Arteriograms refer to imaging of arteries, while venograms refer to imaging of veins.

Angiograms are obtained by injecting radioopaque contrast material (x-ray dye-just like that used in CT) into a blood vessel. The contrast mixes with the blood in the vessel, and is propelled forward by cardiac action (arteriograms) or towards the heart with venous return (venograms). The flow of the blood/contrast mixture can be observed fluoroscopically, or a sequence of static x-ray images can be obtained.

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Fluoroscopy is used to guide the placement and the positioning of the catheter into the blood vessel which is being studied.

The movie on the right is an arteriogram of the lower extremities. The catheter enters the right common femoral artery (catheter appears as a faint black line going across the right femur). The tip of the catheter is in the aorta, and this is where the contrast has been injected.

It is important to understand these concepts when looking at angiograms:

1. It is the vessel lumen that is being opacified - the vessel wall is not seen.
2. The images can be displayed with the opacified blood vessels appearing white or black.
3. You must look at the entire sequence of films, not just one film, in order to interpret angiograms optimally.
4. The degree of radioopacity produced by a contrast injection will depend on the relative proportions of blood and contrast. As more (unopacified) blood mixes in with the contrast bolus, the vessel lumen will become fainter.
5. If the heart is not pumping forcefully, and/or if the flow of blood is slow through a region (e.g. the main artery supplying the region is obstructed), the bolus of contrast will move comparatively slowly through the arterial tree.

## Factors Affecting the Appearance

As fluoroscopy images are live images using x-rays, the same factors which affect the appearance of radiographs will influence the fluoroscopic image, such as density, thickness and shape. Click here if you need to review these factors.
The main difference in the appearance of radiographs vs. fluoroscopic images is in the use of contrast. Contrast agents are almost always used to highlight areas of interest and improve the diagnostic yield of the study.

Contrast agents can be placed in many different cavities of the body to allow them to be seen, including blood vessels, GI tract, biliary system, CNS, joints, uterus, and the urinary tract. What contrast agent is used will affect the appearance as well.

### Display

Importantly, fluoroscopic images are digital images which can be displayed such that dense structures appear white, like radiographs, or the images can be inverted such that dense structures appear black.

### Intraluminal Contrast Agent: Barium

<br/>

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The first image shows barium which has been swallowed and is traveling down the esophagus into the stomach. Barium is very dense, absorbing much of the incident x-ray beam and so appears white on the film. Without the barium, the stomach and esophagus wouldn't be visible on an x-ray. Barium coats the mucosa and fills the lumen. Note the lungs appear dark and the ribs white, just like on an X-ray.

The second image is from a barium enema. Through a rectal catheter, barium is instilled into the rectum and flows retrograde into the colon. Its movement is observed fluoroscopically to ensure that the entire colon is coated and evaluated. Air is also instilled into the colon to distend it so that all of the walls are seen. This is called a "double contrast barium enema". In this image, the patient has been rolled so that they are lying on their right side. (Note the air-barium level, with the barium located dependently). The flow of barium is observed fluoroscopically as the patient is moved. Barium coats the mucosa of the colon to make it visible on x-rays.

## Intraarterial and Intravenous Contrast

Angiograms (arteriograms and venograms) are obtained by injecting radioopaque contrast directly into a blood vessel via a needle or catheter. The contrast is iodine, which mixes with the blood and attenuates the x-ray beam, making the lumen of the blood vessel high density. The iodine is subsequently filtered through the kidneys and excreted in the urine.

The forward movement of the contrast bolus that occurs concurrent with venous return (venogram) or arterial flow (arteriogram) is observed fluoroscopically. Without the injected x-ray dye, the blood vessels would not be visible.

Angiograms are displayed digitally, and can be manipulated to show the vessel lumen as white or black. The movie below, of an abdominal aortic angiogram, is "subtracted" which means that the bones and other radiodense structures have been subtracted from the image so that only the opacified blood vessels are visible.

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  caption="How Angiography is obtained."
/>

### Intraarticular Contrast Agent: Iodine

#### Arthrography

Under fluoroscopic guidance, a needle can be placed into a joint, and iodinated contrast can be injected while watching fluoroscopically to better define and outline a joint space or bursa. This is called arthrography. Without the contrast, the joint space or bursa is not visible.

After the joint has been injected, it is put through its range of motion to allow for complete filling of the joint space, as well as to assess the capsule, look for fragments or loose bodies in the joint space, and assess the cartilage and underlying bone.

Images from a left shoulder arthrogram are displayed. Dense structures, like bone, appear dark. The injected contrast is dark. Note the size of the joint space, and how far inferiorly it extends. The lower image shows the thickness of the articular cartilage. The white arrows indicate contrast in the joint space and the lucency between this contrast and the bone is the articular cartilage.

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## Indications for Use

Fluoroscopy is used to evaluate dynamic processes and structures in motion. It is used both in the diagnosis and in the treatment of disease. Diagnostic uses include evaluation of swallowing function, study of the gastrointestinal tract, motion of the diaphragm and motion of joints, evaluation of blood vessel patency and size. Therapeutically, it is used in everyday practice as guidance for a wide variety of interventional procedures, performed by radiologists and other specialists.

### Gastrointestinal Tract

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Fluoroscopy is invaluable in the evaluation of the gastrointestinal tract. Ingested barium is watched fluoroscopically as it moves throughout the gastrointestinal structures. Things that can be observed fluoroscopically include swallowing function, motility and peristalsis of the esophagus, stomach, small and large bowel, and the rapidity of transit throughout these gastrointestinal structures. The fold pattern, distensibility, and caliber of these structures is also observed.

Clinically, fluoroscopy of the GI tract allows diagnosis of obstructions, strictures, abnormal peristalsis, bowel wall edema, and impaired swallowing function such as aspiration into the trachea. The image on the left is an x-ray taken during ingestion of barium under fluoroscopic guidance. Note how well the fold pattern of the small bowel loops is seen, outlined with barium.

### Joint Motion

Joint motion can be observed fluoroscopically. This allows assessment of the range of motion of a joint, and will display any limitations of motion. Joint subluxations can sometimes be detected by having the patient perform certain maneuvers.

The movie on the right demonstrates the range of motion of the radiocarpal joint as seen fluoroscopically.

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  caption="Joint motion assessment via fluoroscopy."
/>

### Arteriography and Arterial Interventions

Arteriography is performed to look for areas of narrowing (stenosis), occlusions, aneurysms, traumatic injuries, blood clots, and anomalies of blood vessels.

In order to perform arteriography, a catheter has to be placed into the blood vessel of interest and iodinated contrast is injected through the catheter. Fluoroscopy is used to guide placement of the catheter in the blood vessel as well as to observe the flowing blood and contrast.

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Stenosis of blood vessels can also be treated using fluoroscopy to guide in the balloon dilatation of an area of narrowing, or in the placement of a stent, as shown in the images on the right from a lower extremity arteriogram. The arrow in the first image indicates a stenosis of the superficial femoral artery. This prevents adequate blood flow from getting to the more distal leg, causing symptoms of pain. Under fluoroscopic visualization, a wire was advanced down the artery, through the area of narrowing, and over the wire a metal stent was opened in the artery, stretching the area of narrowing. The white arrows in the second arteriogram image show the superior and inferior edges of the stent which was placed across the stenosis. Note the lumen appears widely patent after the stent was placed. The black arrow is pointing to a fine wire in the artery. Interventions have to be performed over a wire, which acts as a conduit over which to deliver the catheters, stents, or balloons.

### Venography

Venograms are used primarily to determine the patency of veins, to look for blood clots, and to look for areas of narrowing.

The movie on the right is a venogram of the left arm and chest. The contrast has been injected through a small catheter which had been placed into the left antecubital vein. The contrast, mixed with blood, is seen flowing through the left brachial, axillary, subclavian and brachiocephalic veins and into the superior vena cava. The left cephalic vein is also faintly seen.

<Video
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  caption="Venogram of the left arm and chest."
/>

## Guidance for Interventional Procedures

Fluoroscopy provides real time guidance when performing procedures. The target tissue is visualized, and a needle, catheter or wire can be advanced at that target under direct visualization. This increases the accuracy, and decreases the potential complications of procedures.

**Example: Percutaneous drainage of an obstructed renal collecting system**

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The images above demonstrate a fluoroscopically guided percutaneous nephroureteral stent placement. In the first image, a needle has been advanced into the renal collecting system and contrast injected. In the second image, a wire has been placed through the needle and coiled in the collecting system. In the third image, a tube has been placed over the wire into the lower collecting system and extends down the ureter. The tube carries urine from the renal collecting system to the bladder, bypassing a ureteral obstruction. All of these maneuvers are performed while watching with fluoroscopy.

**Example: Fluoroscopically guided fallopian tube recannalization**

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  alt="fallop-recan"
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  caption="Fallopian Tube Recannalization 1"
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  caption="Fallopian Tube Recannalization 2"
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Blocked fallopian tubes is one cause of infertility. The tubes can be opened up again or "recannalized" through a relatively minor intervention. Under fluoroscopic guidance, a catheter is placed through the cervix into the lower uterus and contrast is injected. The top image on the right shows that the injected contrast fills the uterine cavity and left fallopian tube. The right fallopian tube is not seen and is occluded.

While watching fluoroscopically, a tiny flexible wire and catheter are passed through the occlusion in the right fallopian tube, thus recannalizing it and increasing the patient's chances of conceiving. This is shown in the bottom image.

&nbsp;


Fluoroscopy & Angiography


## Introduction

Magnetic resonance imaging, more commonly known as "MRI", is a sophisticated method of imaging the body which, unlike CT and radiography, does not use ionizing radiation. MRI capitalizes on the molecular composition of tissues to produce an image.

Unlike with CT, images can be obtained directly in the axial, coronal, or sagittal planes, and 3 dimensional volumes of data can also be obtained. Advantages of MRI include the excellent soft tissue contrast and differentiation, reducing the need for intravenous contrast enhancement, the ability to evaluate blood flow, and the ability to subtract out background tissues that are not of interest.

The goal of this module is to serve as an introduction to MRI, and to provide you with information to aid in understanding what you are looking at when you view an image.

You can view individual sections of this module using the selections on the right.

## How Images Are Obtained

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  src="http://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/MRI/images/mri-scanner.gif"
  alt="mri-scanner"
  wrap="right"
  size="md"
/>

MRI is a technically complex imaging modality, and it is beyond the intentions of this module to discuss image acquisition in great detail. The following provides a brief summary of how MRI images are obtained and viewed.

The patient lies on a table which is moved into the bore of a large, powerful magnet (most of those in clinical use are 1.5 Tesla). Protons in the patient's tissues will naturally align themselves with this powerful external magnetic field.

Radio frequency pulses are sent into the patient, causing disruption of the alignment of the protons (the protons absorb energy and are displaced from their aligned position). Over time, the protons will naturally realign themselves with the external magnetic field. When this occurs, they emit the absorbed energy as radio frequency waves which are detected by the MRI scanner. These emitted radio frequency waves are the MRI signal.

The location and distribution of the emitted MRI signal is detected and analyzed by a computer, and used to produce an image. Thus, a radio frequency pulse is sent in, the patient emits a signal back which the machine reads and uses to create an image.

*What types of tissue emit the MRI signal?*

Only molecules with an odd number of protons have their own magnetic field and align themselves with the external magnetic field. It is these molecules that are responsible for emitting the MRI signal which is used to create the image.

Hydrogen atoms are the most abundant of these molecules in the human body and provide the basis for MRI imaging. The body is largely composed of water, and the hydrogen atoms in water are responsible for producing much of the MRI signal.

MRI images are displayed in shades of gray, like radiographs and CTs. The shade of gray that a given structure appears on MRI will vary with the MRI technique used to obtain the image. This will be discussed later in this module.

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  src="http://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/MRI/images/axialabd.gif"
  alt="axialabd"
  wrap="center"
  size="lg"
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Images can be obtained directly in the axial, coronal, or sagittal plane. Like CT, axial images are viewed as though looking at the patient from the foot towards the head, placing the right side of the patient on the left of the image. Like radiographs, coronal images are viewed as though looking at the front of the patient, placing the right side of the patient on the left of the image. Sagittal images are viewed such that anterior is on the left of the image and posterior on the right.

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## Factors Affecting the Appearance

A myriad of factors affect the appearance of the final MRI image, including tissue composition, machine parameters such as the timing with which the radio frequency (RF) pulse is sent in and when the machine "listens" for the returning MRI signal, the use of intravenous contrast, and whether the structure being imaged is static or dynamic (i.e. flowing blood).

A complete discussion of this is beyond the goals of this introductory module. It is not necessary to memorize the physics or the appearance of certain tissues on MRI, however you should be familiar with the concepts and realize that the same structures might look different on different types of MRI images.

### Tissue Composition

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  src="http://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/MRI/images/mri-factors1.gif"
  alt="mri-factors"
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As discussed in "how images are obtained", hydrogen atoms provide the basis for MRI imaging of the body. Tissues which are abundant in hydrogen atoms, such as water and fat, are responsible for producing most of the MR signal. Likewise, tissues which are devoid of hydrogen atoms, such as cortical bone, do not give off much signal.

Tissues which emit a strong MRI signal will appear white on MRI images and are termed "high signal intensity" and those which emit a weak MRI signal appear blacker and are termed "low signal intensity".

This is a sagittal MRI of the cervical spine. Note the cerebrospinal fluid which has a very high water content, has a high signal intensity on this image, appearing white. In contrast, the cortical bone appears black, "low signal intensity" due to its low water content.

### Contrast Enhancement

Similar to the use of intravenous contrast in CT, the conspicuity of and contrast between structures of similar composition can be augmented by the use of intravenous contrast in MRI.

The contrast used in MRI is made of gadolinium, a paramagnetic substance with a strong magnetic field. Gadolinium affects the appearance of tissues on T1 weighted images, causing tissues which take it up to appear brighter on T1 weighted images. Gadolinium is administered to the patient by intravenous injection. Tissues that appear brighter on images obtained after gadolinium are said to "enhance".

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The first image is a T1 weighted axial image of the abdomen. The next image was obtained after intravenous gadolinium was administered. Note that the kidneys, aorta and pancreas are brighter due to the gadolinium. In addition, the renal medulla and cortex can be differentiated on the enhanced images, as they have taken up different amounts of gadolinium. Tumors and other abnormal tissues take up gadolinium differently from normal tissues, and gadolinium is to augment detection of these.

Unlike CT, not all MRI images are obtained the same way, and as a result, the images can look quite different. MRI machine parameters, such as the strength of the RF pulse, the timing of repetitive RF pulses, the time the machine "listens" for the returning signal from the patient can all be selected and manipulated to create specific types of images and to optimize visualization of certain structures.

Two common "sequences" or types of MRI images which you should be familiar with are T1 and T2 weighted images. The images are "weighted" to bring out various characteristics of tissues. Some tissues look the same on both T1 and T2 images and some look different.

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These sagittal lumbar spine MRI images are both T1 weighted and T2 weighted. Note that subcutaneous fat is bright on both images. The cerebrospinal fluid is dark on T1 and bright on T2. Note the difference in the appearance of the intervertebral discs on the two images. The central portion of a disc contains fluid and so appears brighter on T2 than T1.

It is not necessary to understand the physics or memorize the appearances of tissues on T1 or T2 weighted images. The goal of this section is for you to become familiar with the concepts and realize why the same structures might look so different on different MRI images or "sequences".

The two images the below are the same axial section of the upper abdomen. The image on the top is T1 weighted and the image on the bottom is T2 weighted. Note the fluid filled gallbladder and the cerebrospinal fluid are different shades of grayscale on the two types of images. Also note that the soft tissues, such as the liver, appear slightly different shades of gray on the two sequences.

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Blood vessels, such as the aorta (labeled above) are even more complex. Not only do the machine parameters affect their appearance, but the speed and direction in which the blood flows also impacts the final image.

## Anatomical Structures

MRI provides excellent soft tissue contrast and can readily demonstrate and differentiate adjacent soft tissue structures, far superior to that possible with CT.

Everything is so beautifully seen on MRI that it is easier to discuss what isn't well seen. Air does not give off an MRI signal and produces a void on MRI, thus air containing structures such as the lung appear black. Likewise, MRI of the bowel is limited due to the presence of intestinal gas. Bowel peristalsis also causes motion artifact, limiting the utility of MRI in the evaluation of the bowel.

### Abdomen

The images below were both obtained without iv contrast enhancement. The image on the top is a CT and the image on the bottom is a T2 weighted MRI. Note that on the CT all of the blood vessels and solid organs (liver, pancreas, spleen and kidneys) appear the same shade of gray. Note that on the MRI all of these structures are subtly different shades of gray, allowing us to distinguish them. Also note that the portal vein can be discretely seen from the adjacent pancreas on MRI, and individual nerve roots of the spinal canal are visible.

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The image below is a T2 coronal MRI image of the abdomen. Note that the lungs appear black, since air does not give off an MRI signal.

![mri-structures-1](http://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/MRI/images/mri-structures-1.gif)

The next image is a T1 axial image of the abdomen. Note the fuzzy, ill defined margins of the bowel. This is caused by motion artifact, as the bowel is constantly moving (peristalsing). In the evaluation of bowel, CT is superior to MRI. The remaining soft tissues are well demonstrated, including the muscles of the back and abdominal wall.

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### Extremities

Ligaments, tendons and cartilage are easily displayed and evaluated on MRI, as demonstrated on the below coronal MRI of the knee. This "internal anatomy" of joints can only be seen with MRI, not with any other type of radiologic imaging.

Soft tissues of the extremities are also better evaluated with MRI than other imaging modalities, as demonstrated on the next image which is an axial MRI of the thigh. The medium gray muscles can be differentiated from the darker tendons, and the bright intervening fat helps to make individual muscles conspicuous. Note that the sciatic nerve is even discretely seen!

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Important to note is that dense cortical bone, such as of the femur, appears black. Cortical bone has a low water content and gives off very little signal. CT is superior for evaluating cortical bone. In contrast, the medullary bone, containing the bone marrow, does give off MRI signal and can be adequately seen on MRI.

### Pelvis

These are axial and sagittal T2 weighted MRI images of the female pelvis. The uterus and ovaries are beautifully displayed on MRI. The different soft tissue components of the uterus can be differentiated on the sagittal image; the endometrium is bright and the myometrium is darker. Note that the ovarian follicles are seen as white (fluid filled) structures. The final image shows CT image of the pelvis to compare with the MRI images. The rectum, pelvic sidewalls, prostate and seminal vesicles are also well seen on MRI.

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### Brain and Spinal Column

The following images show an axial T2 MRI of the spine, and an axial T2 MRI of the brain which includes the orbits.

The darker spinal cord is seen discretely, surrounded by the brighter subarachnoid fluid. The white dot in the center of the spinal cord is an abnormal fluid filled cavity, called a syrinx. The nucleus pulposis, the fluid component of the intervertebral disc, can be differentiated from the annulus fibrosis, the outer dark fibrous band of the intervertebral disc.

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These images demonstrate how exquisitely the internal anatomy of the brain and spinal cord is seen. No other imaging modality can compare with MRI for imaging the central nervous system.

## Indications and Specialized MRI Techniques

Since MRI has excellent soft tissue contrast, its greatest use is for evaluating soft tissue structures, as discussed previously in this module. MRI is indicated in the evaluation of joints and soft tissue tumors and masses of the extremities. In the neck, chest, abdomen and pelvis, MRI is indicated in the evaluation of congenital or structural abnormalities, tumors or other masses of organs and soft tissues. MRI is indicated to evaluate the brain, face, spinal cord and spine for a wide variety of symptoms and problems. Specialized techniques include specifically imaging blood vessels and fluid filled structures.

### Tumors

MRI is widely used to evaluate for tumors throughout the body, including the neck, chest, abdomen, pelvis, extremities, spine and brain. It is excellent in the evaluation of brain tumors. The two images demonstrate brain tumors in two different patients. The images were obtained after IV contrast was given, and the tumor tissue has taken up the contrast and appears brighter than the adjacent brain parenchyma on these T1 weighted images.

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### Magnetic Resonance Angiography (MRA)

This specialized technique takes advantage of the flow properties of blood vessels to obtain images. MRA is commonly used to evaluate arterial blood flow to the central nervous system. It is also used to evaluate arterial blood flow to the extremities, abdominal organs and the intestines.

MRA displays the caliber of a blood vessel, allowing evaluation for occlusions, stenosis or aneurysms, for congenital abnormalities, and for the presence of blood clots or atherosclerosis.

The image below is an MRA of the arch of the aorta, the subclavian arteries and arteries of the neck. Note the arteries appear bright, while the background tissues are dark and not well seen with this specialized angiographic technique.

![mri-indications-mra](http://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/MRI/images/mri-indications-mra.gif)

### Magnetic Resonance Venography (MRV)

This specialized technique takes advantage of the flow properties of veins to obtain images.

MRV is commonly used to evaluate venous drainage from the central nervous system. The image below is a sagittal view of an MRV of the brain. Flowing blood in the veins appears white while the background tissue is poorly seen. Blood flow in the arteries is not seen with this technique.

![mri-indications-mrv](http://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/MRI/images/mri-indications-mrv.gif)

MRV is also commonly used to evaluate the veins of the chest, abdomen, pelvis and extremities, to detect abnormal areas of narrowing or dilatation or for the presence of blood clots.

### MRCP

#### **Magnetic Resonance Cholangiopancreatography**

This specialized technique takes advantage of how bright fluid appears on T2 weighted images in order to specifically display fluid containing structures. The bile within the biliary ducts of the liver, the common bile duct and gallbladder, and the fluid in the pancreatic duct all appear very bright on these specialized images, while the background tissue is poorly seen.

![mri-indications-mrcp](http://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/MRI/images/mri-indications-mrcp.gif)

MRCP is commonly used to evaluate for obstruction of the bile ducts or pancreatic ducts, for stones within the ducts or gallbladder, and for congenital anomalies of these structures.

A normal coronal MRCP is shown above. Note how well the fluid containing structures are seen. The stomach and duodenum contain fluid in this patient, so they are visible as well.


Magnetic Resonance Imaging

Radiography


## Introduction

Nuclear medicine (or nuclear radiology) is unique within the field of medical imaging. While most images created in radiology are direct depictions of anatomy and structure, nuclear medicine studies are depictions of physiology and function. The studies are specifically designed to take advantage of changes in function that occur with pathologic processes. Functional changes often become apparent earlier than derangement of macroscopic anatomy, making nuclear imaging highly sensitive.

This introduction serves to explain the basis for, and techniques utilized to create nuclear medicine imaging studies and will guide you to an understanding of nuclear imaging. The role that nuclear medicine plays in the rapidly developing field of personalized medicine will also be touched upon.

## The tracer principle

The key to understanding nuclear medicine is the realization that when disease occurs, the physiologic processes at the site of disease change. This will be elucidated with the three most commonly performed examinations:

1. FDG PET for malignancy evaluation
2. Bone scans
3. Myocardial perfusion scans.

### FDG PET for malignancy evaluation

Nuclear imaging for malignancy exploits the differences in the way the tumor functions relative to normal tissue. The majority of malignancies have excessively high energy demands and meet these by metabolizing glucose. Thus, the glucose uptake by most tumors is markedly elevated compared to background (normal) tissue.

The glucose uptake in the body can be measured and imaged by administering a small dose of a glucose analog labeled with a radiation emitter (fluoro-deoxy glucose or FDG) intravenously and then placing the patient into a scanner (which is a radiation detector). Areas of malignancy will be manifest as sites of increased uptake of FDG. The FDG administered is a “tracer.” It will distribute in a manner similar to glucose but will have no impact on the body: it does not change organ function, is not recognized as a foreign substance and will cause no apparent side effects (other than the uncertain impact that people receive from low doses of radiation). In effect, only a “trace” dose is given.

### Bone scans

Bone scans are a depiction of bone turnover. Almost all processes that cause bone injury lead to a reparative process accomplished by increasing bone turnover. These scans are performed by administering a radio-labeled phosphate compound (methylene diphosphonate or MDP) intravenously. The phosphate is incorporated into the skeleton and areas of increased turnover (as occurs with skeletal metastases, fractures or osteomyelitis) are identified with the scanner.

### Myocardial perfusion scans

Finally, myocardial perfusion scans are performed principally for the evaluation of blood flow to the left ventricle. The tracers available (including sestamibi and tetrofosmin) are picked up by myocardial mitochondria relative to coronary flow. The state of flow to the heart is typically evaluated during phases of both rest and stress (usually with treadmill exercise or simulated with a vasodilator) for the detection of coronary artery disease.

## Isotopes and emissions

An element is defined by its atomic number (number of protons), but may have multiple different isotopes: versions of that element with different numbers of neutrons and, therefore, different atomic weights. For example, carbon (six protons) has multiple isotopes: carbon-11, carbon-12, carbon-14. Some isotopes may be stable and some are radioactive. Some elements have no stable isotopes.
Radioactive elements undergo decay events and emit energy. Energy can be emitted in multiple forms, either as energetic particles (e.g., alpha and beta particles) or as photons (having no mass; e.g., x-rays and gamma rays). Only photons are used to create images. Most studies are performed with gamma rays.

The four most commonly used isotopes in clinical nuclear medicine are:

1. Technetium-99m - used for a wide variety of study types
2. Indium-111 - for tagged white blood cell scans
3. Iodine-131/123 - thyroid imaging
4. Fluorine-18 - PET imaging

## Scanners and techniques

The scanners used in nuclear medicine are, much like Geiger counters, radiation detectors; they emit no radiation. Scanners record the photons being emitted from the patient, who becomes the source of radiation following the administration of the isotope. The emissions can be quantified, localized, and converted into images.

Gamma cameras v PET scanners

Scanners can be grouped into two large, but quite distinct, categories: gamma cameras and PET scanners. They are both radiation (photon) detectors that convert the emissions into images.

Gamma cameras count photons emitted, one at a time. The most commonly used isotope in gamma cameras is technetium-99 metastable or (Tc-99m).
PET scanners are used exclusively for studies performed with positron emitting isotopes. (A positron is a beta particle with a positive charge). After emission from a nucleus, it will travel a short distance and then undergo what is known as “annihilation” with an electron. In the annihilation reaction, the mass of the positron and of the electron are converted completely into energy in the form of two photons traveling in opposite directions. The scanner needs to detect both photons simultaneously for it to be counted. The most commonly used isotope in PET scanners is fluorine-18 (F-18).

## Planar vs. tomographic images

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  src="https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/NucMed/images/BoneScan.jpg"
  alt="bonescan-1"
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Simply, planar imaging is a two dimensional representation of a three dimensional object (think chest x-ray).

Some studies are typically done in a planar format (e.g. bones scans). The image on the right is a bone scan of a patient with prostate cancer, anterior view on the left and posterior view on the right. Areas of high radiotracer uptake are black. Multiple areas of intensely increased activity in the axial and appendicular skeleton represent sites of metastases.

A gamma camera can be stationary and acquire planar images, as shown, or it can rotate around the patient and acquire tomographic images (more details on tomographic imaging are on the next page).

Tomographic imaging is a two dimensional representation of a two dimensional object (think slices from a CT). In actuality, CT images do have some thickness, but not much. Tomographic images are created by rotating a gamma camera around the patient while taking pictures and then using a computer to reconstruct the image (hence, “computed tomography”-A CT scanner does this with x-rays).

A gamma camera does the same with the gamma rays emitted by the patient, creating tomographic images known as SPECT: single photon emission computed tomography. A gamma camera can take planar images or tomographic (SPECT) images. Some studies are done exclusively in tomographic format (cardiac perfusion scans).
The image displayed here is a dual headed gamma camera. There is a head positioned above the patient table (labeled “1”) and the second head is underneath and obscured by the table. The heads can stay stationary, to acquire planar images, or rotate around the patient to acquire tomographic images. The scanner shown is also a hybrid scanner which combines a CT scanner (the ring labelled "Symbia") and a gamma camera.

![scanner-1](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/NucMed/images/SPECT_CT.jpg)

## Hybrid imaging

Because nuclear studies are depictions of functional processes, the images have low spatial resolution and poor anatomic detail. It is often necessary to view them in conjunction with anatomic studies (like radiographs and CT scans) to provide useful additional localizing information. To improve this process, hybrid cameras were developed, such as the PET-CT. A PET-CT scanner performs both the CT scan and the PET scan, and the hardware for both studies is ‘housed’ in the same unit.
This allows the images to be overlain, or “registered.” Following the wide clinical acceptance of PET-CT, gamma cameras were coupled to CT scanners, called SPECT CT (see prior page) to improve anatomic localization of tomographic nuclear images. PET-MRI is now available as well.

The image shown is a PET-CT. This device combines a PET scanner (which acquires tomographic images) and a CT scanner under one housing. Be aware that with PET-CT, there are two sources of radiation exposure to the patient, one from the CT scan, and one from the injected radiotracer.

![scanner-2](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/NucMed/images/PETscanner.jpg)

## Terminology & image display

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  src="https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/NucMed/images/terminology.jpg"
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Nuclear images are most commonly depicted in grayscale, although studies can be depicted in color as well.

On grayscale images, by most conventions, areas of high radiotracer uptake are displayed in black, and areas of low uptake are white, with varying intermediate shades of gray. Areas of high uptake are commonly called “hot or radiotracer avid”. Areas of low uptake are called “cold or photopenic”.

The bone scan images shown are from two different time points. That on the left shows a hot spot in the medial left knee (red arrow) due to increased tracer uptake from bone turnover related to osteoarthritis. The patient then had a total knee replacement using a metal prosthesis. The bone scan on the right, done post-operatively, shows a cold spot in the left knee (yellow arrow) corresponding to the metal prosthesis, which cannot take up any tracer.

## Indications for nuclear imaging

The scans offered in nuclear medicine are numerous, diverse, and cover all organ systems. Presently, the most commonly performed studies are FDG PET-CT, myocardial perfusion scans and bone scans.

### FDG PET-CT

FDG is fluoro-deoxy glucose (a glucose analog labeled with a radioactive isotope of fluorine, F-18). Scanning with FDG gives a depiction of glucose metabolism throughout the body. A majority of malignancies are highly dependent on glucose for energy and avidly pick up this tracer. This includes such common tumors as lung, breast cancer, and lymphoma. FDG PET-CT has proven highly valuable for diagnosis, the evaluation of treatment response and the monitoring of recurrence.
The images shown are from a PET-CT in a smoker with an indeterminate lung nodule. The coronal PET image (right) shows intense activity in the right upper lobe corresponding to a nodule demonstrated on the accompanying CT (left). This indicates the presence of markedly increased glucose metabolism, consistent with (and subsequently biopsy proven to be) lung cancer. (Also note the intense tracer uptake by the myocardium, since cardiac muscle can utilize glucose for metabolism).

![indication-1](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/NucMed/images/PETCTLung.jpg)

### Myocardial perfusion scans

Myocardial perfusion imaging (MPI) is a method to map blood flow to the left ventricle. The most commonly used tracers are Tc-99m sestamibi, Tc-99m tetrofosmin and thallium-201 chloride. Scans are usually performed for the detection of ischemic heart disease, assessment of the significance of a coronary stenosis and the follow up of patients after therapy. Most often patients will be scanned in two stages: during rest and under stress (induced by either exercise or simulated with a vasodilator). Areas of the myocardium with poor radiotracer uptake are called ‘defects’. Those that occur during stress typically represent ischemia and those found at both stress and rest indicate myocardial infarction.

The myocardial perfusion scan shows left ventricular radiotracer uptake during stress (treadmill exercise, top row) and at rest (bottom row). Stress images show reduced tracer uptake in the lateral wall (arrow) indicating stenosis of the left circumflex coronary artery.

![myocardial-1](https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/NucMed/images/Myocardial.jpg)

### Bone scans

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  src="https://anatomy.host.dartmouth.edu/HAE/Radiology_Intro/NucMed/images/BoneScan.jpg"
  alt="bonescan-3"
  wrap="right"
  size="sm"
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Bone scans, or skeletal scintigraphy are performed with a radiolabeled phosphate analog (most commonly Tc-99m MDP). Bone turnover is a continual process; pathology that causes bone injury will lead to bone repair and the increased incorporation of phosphates and MDP. The processes typically evaluated with bone scans are skeletal metastatic disease, fractures and osteomyelitis. The most common indications are the staging and follow up of patients with prostate and breast cancer.

Shown on the right is the same bone scan shown earlier in this module, demonstrating multifocal "hot spots" due to prostate cancer metastasis to the spine, ribs and pelvis.
