MRI, CT & PET Scans...

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What Are The Differences

What’s the Difference between CT, MRI, and PET Scans?

It always seems like someone you know has to have a CT scan, or maybe just had an MRI. Occasionally you may hear about the results of a friend’s PET scan. Then there are the people who just get plain old x-rays. What these tests have in common is that they are all used as diagnostic tools. So how do doctors know which one to order? Although it may seem as though it depends on which machine is conveniently available, it isn’t. It depends on what the doctors are looking for, or want to rule out.

CT or CAT Scan
Your Little Leaguer takes a mighty line drive to the shin, and it’s fairly apparent he has a broken bone or two. The emergency room doctor wants detailed information about the break and any possible internal bleeding or nerve damage, so he orders a CT scan.

CT is short for Computed Tomography (formerly referred to as Computed Axial Tomography, or CAT). A CT scan is an especially sophisticated x-ray machine. An
x-ray is a type of radiation, focused as a beam, that makes a kind of “photograph” of the inside of the body where the rays are directed, such as the foot or arm, so it can reveal, for example, broken bones. A CT uses specialized x-ray and computer equipment that convert very detailed x-rays to digital images, including cross-sectional views, that will provide considerably more information about your son’s injuries than standard x-rays. So your pint-sized Mickey Mantle will lie comfortably on a table while the CT scanner, which looks like an over-sized doughnut, virtually “slices up” the structures it sees and, while rotating around the area, takes a picture of a succession of “slices” every second. Each 3D image is saved on the computer so that it can be studied by the radiologist, a doctor who specializes in x-ray technology.

CTs are also useful for diagnosing bone disorders such as tumors and fractures, cancer or heart disease, and internal injuries.

PET Scan
Your mother, who has already had one heart attack, is rushed to the emergency room for another suspected heart attack. Because of her history, the ER doctor orders a PET scan to determine the extent of damage to the heart muscle.

PET is shorthand for Positron Emission Tomography, a test that uses a “radiotracer,” or simply “tracer,” a radioactive chemical that enters your body through an IV and is followed by a high-technology camera as it courses through your bloodstream. The tracer collects in the area being examined, where it emits positively charged atoms that are recorded by the camera and converted to digital images on a computer screen. A PET does not reveal clear structural images but rather any “hot spots” of increased metabolic activity as revealed by the tracer, which may suggest decreased blood flow, non-functioning heart muscle, or an area of disease. Combining the images of a CT scan with PET scan images provides more detailed information than either does alone, thereby assisting the doctors in their search for a definitive diagnosis.

PET scans are used to detect the presence of cancer, which would appear as a white hot spot, to determine if a previously diagnosed cancer has spread, and to assess the effectiveness of cancer treatment. It is also useful to evaluate brain abnormalities and central nervous system disorders. Because it is quite costly, PET scans are usually used only after other inconclusive testing has taken place.

MRI
Your wife had a diagnostic mammogram that shows an unidentified mass. A follow-up ultrasound failed to detect the same mass. Because of her diagnosis of breast cancer 10 years ago, her doctor orders an MRI.

MRI or Magnetic Resonance Imaging is a diagnostic tool that relies on a magnetic field and radio wave energy to produce pictures of internal organs and structures. MRIs give different information about structures in the body than x-rays, ultrasound, or CTs. MRIs may also reveal problems that other imaging methods cannot. Pictures from an MRI scan are digital images that can be saved and stored on a computer for detailed analysis.

After a pre-screening interview to determine the presence of any forms of metal in her body, including tattoos, implants, shrapnel, and body piercings, your wife lies comfortably on a table in a position that allows the area of the body being studied to be exposed to a strong magnet, and the table slides into position inside a cylindrical piece of equipment. The resulting images are reviewed by a radiologist, who sends a report to your wife’s doctor. In some cases a contrast material is used during the MRI to provide additional information.

MRIs are also used to detect internal bleeding, infections, damage caused by a stroke, abnormal blood flow, kidney disease, aneurysms, and several other internal problems.

MRI images show a clearer contrast among the different kinds of soft tissue than do x-rays, which do a better job of showing the contrast between soft tissue and bone.

Although it sounds like alphabet soup, each test provides different information. The more information the doctors need, the more testing that will be done. Combining the results of all tests given can either confirm what may have been suspected, or rule it out. Definitive negative results will bring relief, and definitive positive results will enable the patient and his or her medical team to plan the most effective treatment regimen available.

Although there are numerous tools available to doctors for diagnosing illnesses, three of the most common are known by their nicknames: CT, PET, and MRI. Each method of testing uses technology differently, and when one isn't enough to draw a conclusion, they can be used in tandem to provide more detailed information that can aid in diagnoses.


by GIGI

[Rita Rimmer]

Thanks for elucidating on the various tests that are available to evaluate things that may abnormal in our bodies. I'd like to add some caution to the discussion of these tests. There should be concerns about having too many ionizing radiation tests and unfortunately those who order these tests don't ask how many you've previously had. Radiation tests, such as CT scans and PET scans have a very great amounts of radiation. They should be used very cautiously and should be last resort. There is a growing concern as to the rise in cancers related to medical radiology testing and their indiscriminate use. We all want to find out what we need to know but need to keep track of the radiation that over time can cause the very thing we're all afraid we'll find. Children are at especially high risk from CT scans since their cells are new and dividing very fast.