CAT and PET Scan

Explanation of PET and CAT scans

Despite PET and CAT scans both being scanning techniques, the way they gain insight into the body's internal state is different. Both expose you to some form of radiation. Before we identify the differences between the CAT scan and PET scan, let's quickly define tomography so we know what we're talking about.

Positron Emission Tomography (PET) Scan

PET scans identify functional or dysfunctional high-activity and low-activity areas of the brain and body when scanning their internal state using a radioactive tracer that behaves similarly to glucose. PET scans identify areas of function and abnormalities by monitoring the metabolic rate and activity of cells; how your brain and body tissues use oxygen and glucose.

PET scans can identify blood flow based on these parameters, and then infer function and identify potential abnormalities based on this information.

How Does a PET Scan Work?

Before a PET scan, a doctor injects a tracing radioactive dye (called a radiotracer) into your arm, which is then absorbed into the bloodstream. It can take up to an hour for the dye to circulate to the right cells, significantly increasing the time needed for the procedure.

The radiotracer has a very similar structure to glucose, so when the body comes into contact with it, it treats it the same way. Glucose, as you know, is a form of sugar your body uses as fuel. Once an hour has passed, patients will enter the PET scanner, a tube-like machine which takes close to another hour to conduct.

• When certain tissues are active more than others, they use more oxygen and glucose to fuel this activity. This is particularly prominent for the brain, so when we inject the dye and ask patients to think of certain things or perform specific actions, it should cause the brain (and the body) to use more of the dye to fuel the activity (increasing blood flow and oxygen use).

The radiotracer gives off energy in the form of gamma rays which the PET scanner can detect. When dye builds up in a particular area, we can make educated assumptions about what functions and potential abnormalities are happening in those areas.

These show up on scans/images as hot or cold areas:

• Red/yellow areas are places of high activity.
• Blue/green areas are places of low activity.

It can help us identify certain disorders or diseases within the brain. Have a look at this PET scan of a healthy patient compared to a patient with Alzheimer's disease.

Fig. 2: PET scans reveal information on a healthy versus unhealthy brain.

We can see many active areas of the brain showing up red in the normal brain patient, but a lot of low activity, blue areas in the Alzheimer's patient, showing how the disease has affected the brain and its functions.

Raine et al. (1997) used a PET scan to identify brain abnormalities in murderers and found that they indeed did have lower activity in areas associated with controlling aggressive behaviours (such as the prefrontal cortex).

Computerised Axial Tomography (CAT) Scan

CAT scans use X-rays to create detailed images of the internal state of the body and brain. They can be used in conjunction with PET scans. A CAT scan can identify the differences between the various tissues, showing bone, soft tissues, and other internal structures.

Usually, a CAT scan lasts for around 10 to 20 minutes, although it can be as long as an hour if necessary.

How Does a CAT Scan Work?

A CAT scan works by sending x-ray beams through your body to capture an image of what's going on inside of you. The X-ray takes images from many different angles and produces cross-sections of images, known as slices. The computer then combines these angles and sections to provide a detailed image of the area of the body you're scanning.

As we mentioned above, a contrast dye can aid this process, although it is not required. The images are usually given in black and white, so dyes increase the contrast and resolution of the images. Overall, a CAT scan provides more detailed images than a normal X-ray.

CAT scans are used to identify disorders of the muscle and bone and detect other conditions such as cancer, heart disease, and so on. They can be used to monitor the progress of these diseases, identifying if the treatment is helping.

The image below depicts a CAT scan (CT scan) of a human brain. As you can see, it shows the different layers of the brain going from the base of the skull to the top.

Fig. 3: CT scan images of the human brain reveal information on the organ.

As we can see, the CAT and PET scan differences are quite evident in the two example photos we have given in the respective sections. Let's explore the differences between the two scanning techniques further.

CAT and PET Scan Difference

CAT and PET scans produce different images and provide different functions, as a result. CAT scans differ from PET scans in a few ways, most notably in their use of X-rays to create images of the body and brain, although they may use a dye to gain more detail in their computerised images. Mainly, PET scans provide insight into brain and body function and possible sources of dysfunction, and CAT scans can identify areas of dysfunction by locating abnormalities.

CAT scans are usually faster than PET scans overall.

Fig. 4: Here we can see a composite of images resulting from brain scans using a.) CT, b.) PET, and c.) fMRI¹.

PET Scan Pros and Cons

PET scans provide helpful information, but the technique has its issues.

Pros

First, let's explore the pros of PET scans.

• They can detect and monitor conditions affecting heart and brain function and identify diseases and cancer.
• PET scans can detect cancer earlier than most other tests available; a significant strength when catching cancer early can mean the difference between life and death.
• They can detect areas that are dysfunctional or are not functioning normally. This could indicate many things, as mentioned above (for instance, it can show the location of a tumour).
• The scan itself is non-invasive and painless (the injection of the dye may cause slight discomfort, but the scanning process is painless, and the dye can also be ingested, so this can be avoided altogether).
• The information a PET scan provides can actually improve and alter the trajectory of the treatment plans. It allows doctors to monitor treatment, identify areas to target specifically and monitor the effectiveness of their current treatment plan.

Cons

Now, let's examine the cons of PET scans.

• The PET scan is a little more invasive than a CAT scan in its need to inject the patient with the dye, although sometimes the dye can be ingested.
• PET scans typically take a little longer than CAT scans, especially when we consider the time needed for the dye to circulate around the body.
• The radiation from a PET scan can remain in the body for some time after the scan. However, the radiation is almost negligible in most cases, so it is not something concerning.
• Being in a scanner can cause stress and feelings of claustrophobia in the patient, which is a con for both CAT and PET scans.
• The dyes used can sometimes cause side effects such as nausea, headaches, vomiting, itching, and other not-so-pleasant feelings. People with certain disorders such as diabetes who use the FDS dye, for instance, may have to take extra precautions, as the FDS dye is very similar to glucose and can present problems in terms of increasing the time needed to address the concerns.
• Although the PET scan is excellent at detecting cancer, if the tumours are slow-growing and not as active, they may not show up on the scans.
• If you're pregnant, you are advised not to undergo a PET scan or CAT scan as the small amounts of radiation pose a risk to the unborn baby.

CAT Scan Pros and Cons

CAT scans provide helpful information, but the technique also has its issues.

Pros

First, let's highlight some of the pros of CAT scans.

• CAT scans are typically faster than PET scans, and you do not have to wait for the same time for any dyes to take effect (unless one is administered to increase the contrast and resolution of the images).
• For the same reason, they are also less invasive than PET scans, as there's no need to inject a dye for the scan to produce an image.
• The radiation a person is exposed to during a scan lasts only during the scan; it does not stay in the body.
• By using a CAT scan, the need to explore a condition through surgery or biopsy is reduced.
• The CAT scan is not as rigid in its requirements, especially when we compare it to something like a magnetic resonance imaging (MRI) technique. MRIs require a patient to be still, whereas patients can move a little when having a CAT scan.
• The images created from a CAT scan are very detailed, which creates a better environment for accurate diagnosis.

Cons

Now, the cons of CAT scans.

• Although the ionising radiation only occurs for the time you're getting the scan, the radiation is greater than an average X-rays radiation output, so you're exposed to more radiation by getting a CAT scan. This can increase the risk of developing cancer.
• Because of this risk of ionising radiation, you are limited in how many CAT scans you can get. They also assess if a CAT scan is necessary, ensuring a clear benefit to getting a scan that outweighs the potential risks.
• Similar to a PET scan, pregnant people are advised not to get a CAT scan due to the risks posed by radiation exposure to the baby. Although, again, it is worth pointing out that the radiation dose is extremely low and poses little to no risk to humans.
• If you are asked to take a dye to increase the image's detail, you face the same side effects the dye may induce in a PET scan.

A PET scan and a CAT scan can be combined to produce an even more detailed image of what's occurring internally.