| CATs 'n' PETs | |
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by Laura Dolson
Some people looking at the title of this article would think of small furry animals, but anyone who has any experience with cancer knows better. CAT (CT or C-T) scans, MRI and PET scans are all used to provide information about possible tumors and metastasis (spread) of cancer. Since proper staging of cancer is important in planning the best treatment, imaging methods such as these are important in getting the best cancer care. But the terminology can be confusing, and often it's difficult to tell the difference between them, or the reasons for using each. Here's a guide to help sort it all out.
C-T (CAT) Scans - CT stands for computed or computerized tomography. This is essentially a series of X-rays in "slices" through the body, which are then analyzed by a computer, and an image constructed from the data. It can show the precise location of a tumor, its shape, and whether it is solid or hollow. Although it can give clues as to whether or not a tumor is cancerous, only a biopsy can tell for sure.
Procedure - Usually before the test, a contrast medium is given, either as a drink or through a vein or port. When it's time, the person being tested lies down on a special table while the scanner is rotated around them. This usually takes around 15-20 minutes, although it can be shorter or longer depending upon how much of the body is being scanned.
Drawbacks - A CT scan is not reliable in helping to find tumors that are less than 2 cm. in size. Additionally, some find the contrast drink unpalatable. (Tip: There are several different contrast substances that can be used, and sometimes this problem can be helped by switching.)
MRI (Magnetic Resonance Imaging) Scans - MRI scans use magnets rather than X-rays to produce the image. The strength of the magnetic field causes the atoms of the body to respond, and the emissions are detected by the scanner, which are analyzed, and an image is produced. In many tissues, the image and detail are clearer with an MRI than a CT scan.
Procedure - The person to be tested lies on a table, which slides into the opening of a narrow cylinder. Loud noises are heard during the scan, which lasts approximately 30 minutes, although this, again, depends on the size of the area being scanned.
Drawbacks - For some tissues, a MRI image is not as clear as that of a CT. It is more difficult to differentiate inflammation and scar tissue from tumors on a MRI image. Also, it generally cannot be used on anyone who has metal implants in their body. People who have a difficult time in close places sometimes have a hard time coping with the MRI. The MRI is also more expensive than the CT.
PET (Positron Emission Tomography) Scans - This technology uses radioactive positrons (positively charged particles) to detect differences in metabolic and chemical activity in the body. An area with increased activity will show on a colored image. Notice that this is a distinct difference from other kinds of imaging - whereas CT and MRI scans look at structures in the body, a PET scan looks at function. Since cancer cells tend to divide more rapidly than other cells, they will generally show as having more metabolic activity. PETs can pick up very small areas of activity - much smaller than either of the above methods. They are also very good at differentiating scar tissue, which has very little metabolic activity.
Procedure - The person to be scanned lies on a table, which moves through a ring-shaped scanner. After a few minutes, a radioisotope called a tracer is inhaled or injected into a vein. (This substance will clear from the body rapidly as it has a short "half-life".) The scan then continues with the tracer active. The entire procedure can take anywhere from 30 minutes to two hours, depending upon the size of the body part being scanned.
Drawbacks - Frankly, the main drawbacks to PET scans are accessibility and expense. Until recently, PET was mainly used as a research tool. It is very expensive (often above $3000 per scan) and many areas do not yet have access to them. Even when PETs are available, insurance does not always cover them. Among other problems with PET: 1) that the image is not as clear as with CT scans and MRI, so sometimes localizing the exact place of the increased activity can be a problem 2) it takes more training to learn to read them well 3) since inflammation shows as "hot" on a PET scan, if there is a lot of inflammation present it can obscure other activity on the scan and 4) it tends to work better for higher grade tumors and metastasis, as well-differentiated tumors have less metabolic activity (they are usually growing more slowly).
Still, the prospects for PET scans being able to find cancer that goes undetected by other technologies is exciting, and the use of PETs will probably grow quite rapidly. At this point, many people are having success getting approval for PETs if their doctors submit a request as a necessary procedure.
NEW! CAT/PET Combo Scans! - The current "wave of the future" is a new type of scan that combines the technologies of CT and PET. Superimposing the images of function on structure seems to give the best of both worlds - the anatomical detail of the CT and the ability to find small clumps of cancer cells of the PET. (See sidebar for article) This type of scan is not yet widely available, but many hospitals have plans to add them in the coming months.
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Copyright © 2001 by Laura Dolson. All rights reserved. Please submit reprint requests to gyncancer@baymoon.com
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