Cochin Cardiac Club

Health Blog by Dr.Uday Nair


CT (computerized tomography) uses a sophisticated X ray machine combined with a computer to create a detailed picture of the body’s tissues and structure. Usually a special dye called a contrast material will be injected prior to the scan. This makes it easier to see abnormal tissue due to specific absorption rates. 

Nuclear magnetic resonance is produced by measuring the magnetism of spinning electrons and protons and their interactions with nearby atoms (usually protons) when they absorb energy. This provides information about the chemical structure of organic molecules. The use of the word “nuclear” has recently been avoided and Magnetic Resonance Imaging (MRI) is now preferred. MRI uses a magnetic field from super-cooled magnets and can often distinguish more accurately between healthy and diseased tissue. A contrast agent is usually used. MRI can provide pictures from various angles and construct a three dimensional image. Some patients who have received certain types of surgical clips, metallic fragments, cardiac monitors or pacemakers cannot receive this type of scan. 

One of the more common questions about MRI vs CT scan asked by patients is what's the difference between CT and MRI? Why should I have a CT instead of an MRI? Or why should I have an MRI instead of a CT?

CT Scan does not show you tendons and ligaments very well at all; at least not yet. MRI is the best choice for that. Tendons and ligaments around the shoulder and knee are best seen by the physics used in MRI. This is due to the density of the tissues that compose the tendons and ligaments.

Spinal cord is best seen by MRI for the same reason. The density of the spinal cord and the composition of it is such that MRI physics can show it to us much better than CT.

There are also reasons why CT is the exam of choice over MRI. It is the preferred modality for cancer, pneumonia, and abnormal chest x-rays.

Bleeding in the brain, especially from injury, is better seen on CT than MRI. But a tumor in the brain is better seen on MRI.

Many people suffer from ringing in the ears; CT displays the inner auditory canals well and there are natural remedies available to relieve it.

If you've been in an accident, organs can get torn or damaged. CT shows organ tear and organ injury quickly and efficiently. Broken bones and vertebral bodies of the spine are better seen on CT but injury to the spinal cord itself is displayed on MRI far better than CT.

CT is far superior at visualizing the lungs and organs in the chest cavity between the lungs. MRI is not a good tool for visualizing the chest or lungs at all.

MRI vs CT scan depends on what needs to be visualized and the reason you need the test. Radiologists are the doctors that specialize in reading images of the body and therefore know which test is best for showing anatomy according to the reason for the exam. Very experienced CT and MRI technologists will also know from working with Radiologists.

The difference in the way the images are produced in MRI vs CT is the physics involved. CT scan uses an x-ray beam that slices through you like a knife carving a spiral ham.
MRI ( magnetic resonance imaging) uses a magnetic field with radio frequencies introduced into it. When your body is placed inside the magnetic field, the molecules of water in your body (hydrogen molecules) will start to spin like a kid's top when he spins it. The top will begin to wobble as it slows down.
Your hydrogen molecules will start to wobble just like the top does at a certain rate of speed; the stronger the magnetic field, the faster they will wobble and the weaker the magnetic field, the slower they will wobble; it depends on the strength of the magnetic field.
Then a radio frequency is introduced into the magnetic field at the same rate of speed at which the "tops wobble" causing the wobbling tops and radio frequency to sing out together sharing the same signal frequency. (Thus the term resonance) That signal is used by the MRI computer to produce the image.

MRI and CT are very different and used for different needs and reasons; both are valuable and both have specific applications; they are not interchangeable and one is not a better test than the other for all things.

The decision whether to use one or the other is based according to the density of the body tissue that needs to be seen. Softer tissues that have more water molecules or hydrogen atoms in them are better seen by MRI because of the physics used.

Dangers involved with CT scanning?Are CT Scanning misused?

News reports  raises serious concerns about the use, and overuse, of CT scanning. While individual risks of developing cancer from a CT scan, which emits high doses of radiation, are relatively low, the researchers worry that their rapid growth as a highly accurate diagnostic tool is exposing too much of the population—and an increasing amount of vulnerable children—to radiation and might be setting the stage for higher incidence of cancer in years to come. Around 62 million scans are performed per year, compared with only 3 million in 1980. Moreover, the researchers estimate that a third of those CT scans are entirely unnecessary—many of them now performed by cautious doctors on worried people with no symptoms at all.

How is a CT scan different from a traditional X-ray?

With a traditional X-ray—a chest X-ray, for example—radiation goes through you from one side to the other, with 3-D information ultimately projected onto a two-dimensional picture. With a CT scan, an X-ray tube rotates around the patient and presents the results to you as a three-dimensional picture. The advantage is that it's much more sensitive, is high resolution, and offers much more anatomically specific information with great detail.
But a typical chest CT means around a 175-times-greater dose of radiation than a similar chest X-ray—that's like 20 sets of mammograms. That range can be between 20 and 200 times higher depending on which part of the body is being scanned.

When is a CT scan 

If you think something is desperately wrong with you and you require an immediate answer. For example, an incredible abdominal pain or a severe acute headache as opposed to a migraine you have had off and on for years. If the CT scan is medically needed, don't think about radiation. If you have excruciating head or abdominal pain, radiation exposure should not be high on the list of concerns.

What are the risks involved in CT scanning? 

Certainly a CT scan won't make your hair fall out or anything so drastic. But the risk of cancer is there, and it depends on a few things. There's the dose of the radiation, which depends substantially on the age of the patient. We know that kids are more sensitive and their risk of getting cancer is higher [when they are given] an adult dose. But these cancers tend not to occur for years or decades at least. For children, the risk of developing a fatal cancer is somewhere around 1 in 500 or 1 in 1,000—the older you get, the lower the risk becomes. So if someone is 90, for instance, there is virtually no cancer risk. For an adult, the risk is around 1 in 2,000.At this point, people believe there is [a] linear relationship between the dose and the risk of cancer. So if you cut the dose in half, you also cut the risk in half. The question should be, can you optimize the dose by using a lower amount of radiation or can the scan be done only once instead of three times? For kids especially, doctors should really optimize with the lowest dose possible if a CT scan is warranted.

What are some alternatives to CT scanning that patients should know about or ask their doctors?

Using ultrasound, for example, or doing an MRI scan is an alternative. Those don't use ionizing radiation, so there is virtually no risk. And if the scan is definitely going to be done, ask about the dose you will be getting. If a child is getting a scan, a concerned parent should make sure the radiology technician is using the correct pediatric doses.

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