CT Scan & MRI - What is Good, What is Best ?

by Dr. Prakash Bhave

If X-Ray, Sonography etc routine examinations fail to reveal satisfactory diagnosis, then the patient is advised CT Scan or MRI.
Both these tests are expensive and therefore it would be illuminating to understand which modality is best for which ailment.
MRI shows a three-dimensional image of the body and this test has no radiation hazard. This is superior in imaging soft organs, glands and tissue. However, this test requires prolonged state of immobility in supine position inside or under the machine for the patient. The noise of the machine can also be irritating.
On the other hand, let us now see the benefits and drawbacks of CT Scan. It is less time consuming and less expensive than the MRI. However, there is certain amount of radiation hazard. But it is very effective in imaging bones and skeletal system.
PET test is also now gaining popularity along with CT Scan. In this test a radioactive material is first injected in the body in very minimal dose and then a CT Scan is done. This test is extremely useful in determining the degree and extent of a cancer.
It is necessary to get information about tendons and ligaments of the joints in various forms of arthritis. Here, MRI excels over CT scan, which is of no use in such cases. MRI is also more useful than CT Scan in imaging nerves.
CT Scan is very useful in the cases of accidental trauma and takes little time. It is also ideal for imaging lungs and chest organs. But the image cannot be viewed in real time or instantly.
In short, it is best that the doctor and the patient should jointly determine which is the best suited test.
While we are at it, it would be rational to understand the radiation hazards.
The general population is exposed to one mili-Sievert of radiation from the atmosphere and the sun annually. This dose is considered safe for humans. Let us now examine the level of radiation that is received by a patient in tests like X-Ray and CT Scan.

X-Ray of chest - 0.02 mili-Sievert

X-Ray of skull - 0.03 mili-Sievert

X-Ray of lumbar - 0.07 mili-Sievert
sacral spine

X-Ray of hands/feet - 0.005 mili-Sievert

Dental X-Ray (small) - 0.02 mili-Sievert

CT Scan of head - 2 mili-Sievert

CT Scan of chest - 8 mili-Sievert

Coronary Angiography - 4.6 to 15.8 mili-Sievert

Whole body CT/PET Scan - 14 mili-Sievert