Saturday, October 27, 2012

What are CT, MRI and PET?

A patient has an MRI scan in this file photo. / Korea Times file

The development of radiology has enabled early diagnosis, and greatly enhanced the survival rate of people with serious diseases.

Most people have heard of imaging devices such as CT, MRI and PET, but few seem to understand when each of these should be used. There is also growing concern that few people are aware of the risks involved in these scanning methods such as exposure to radiation.


The most familiar form of medical imaging is the X-ray, which has been used for over 50 years. As X-ray passes through specific areas of the body, it shows lesions or injuries inside. X-rays are often used to check bones, chests and abdomens.

It is easy and cheap. One only has to hold a posture as directed and hold the breath while an X-ray is taken. However, as a person is exposed to radiation during this process, pregnant women had best avoid it. It is also used for diagnosis of breast cancer, but it is often useless for women with dense breast tissue. These women should take ultrasonography instead.


CT, or computed tomography, is basically another form of X-ray, but the computer reorganizes the numerous X-ray images to produce tomographic and three-dimensional images of specific areas of the body. Sometimes contrast agents are injected into blood vessels to produce clearer imaging. It is especially useful in diagnosing problems in the brain and digestive system and for identifying lung cancer.

CT is non-invasive, accurate, and gives a lot of information from only one scanning. As the scanning time is short, it is appropriate for getting images of moving organs such as lungs, heart and intestines. However, as CT is a combination of multiple X-rays taken from diverse directions, one is exposed to much bigger radiation than when taking X-rays. Pregnant women and those concerned over risks of cancer should consult a doctor in advance. One should be especially careful when using the device on children because exposure to radiation is much more harmful to children than adults.


MRI, or magnetic resonance imaging, is free from the risk of exposure to radiation.

As the powerful magnet shoots a high-frequency wave on the body, it produces signals similar to resonance. The scanner detects the signals and transforms them into digital information that then forms images, visualizing internal structures of the body in detail.

As it provides a good contrast between the different soft tissues of the body, it is useful in imaging the brain, muscles, joints and the heart. It is also used for an early diagnosis of cancer. Imaging technology companies are developing MRI to substitute biopsies. Traditionally, doctors used a long needle to collect liver tissue for diagnoses of cirrhosis, but now they can use MRIs instead.

However, as the person has to lie in the magnetic device for over 20 minutes, those with claustrophobia should consult a doctor in advance.


PET, or positron emission tomography, is especially useful in diagnoses of cancer. When positron-emitting radionuclide, or tracer, is injected into the body, it indirectly emits pairs of gamma rays. The PET detects the gamma rays and turns them into three-dimensional images. While CT and MRI checks structural irregularities, PET shows functional and biochemical problems.

PET is thus very useful for diagnosing cancer, but alone it isn’t enough to accurately show the location. Hence, PET is often combined with CT to show both functional and structural images.

Korea Times

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