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The Difference Between MRI & CT (CAT Scans)



CAT Scans -

Cat scans are a specialized type of x-ray.  The patient lies down on a couch  which slides into a large circular opening.  The x-ray tube rotates around the patient and a computer collects the results.  These results are translated into images that look like a "slice" of the person. 

Sometimes a radiologist will decide that contrast agents should be used.  Contrast agents are iodine based and are absorbed by abnormal tissues.  They make it easier for the doctor to see tumors within the brain tissue.  There are some (rare) risks associated with contrast agents and you should make sure that you discuss this with the doctor before arriving for the examination. 

CT is very good for imaging bone structures.  In fact, it's usually the imaging mode of choice when looking at the inner ears.  It can easily detect tumors within the auditory canals and  can demonstrate the entire cochlea on most patients.



MRI is a completely different animal!  Unlike CT it uses magnets and radio waves to create the images.  No x-rays are used in an MRI scanner. 

The patient lies on a couch that looks very similar the ones used for CT.  They are then placed in a very long cylinder and asked to remain perfectly still.  The machine will produce a lot of noise and examinations typically run about 30 minutes. 

The cylinder that you are lying in is actually a very large magnet.  The computer will send radio waves through your body and collect the signal that is emitted from the hydrogen atoms in your cells.  This information is collected by an antenna and fed into a sophisticated computer that produces the images.  These images look similar to a CAT scan but they have much higher detail in the soft tissues.  Unfortunately, MRI does not do a very good job with bones. 

One of the great advantages of MRI is the ability to change the contrast of the images.  Small changes in the radio waves and the magnetic fields can completely change the contrast of the image.  Different contrast settings will highlight different types of tissue. 

Another advantage of MRI is the ability to change the imaging plane without moving the patient.  If you look at the images to the left you should notice that they look very different.  The top two images are what we call axial images.  This is what you would see if you cut the patient in half and looked at them from the top.  The image on the bottom is a coronal image.  This slices the patient from front to back.  Most MRI machines can produce images in any plane.  CT can not do this. 

Contrast agents are also used in MRI but they are not made of iodine.  There are fewer documented cases of reactions to MRI contrast and it is considered to be safer than x-ray dye.  Once again, you should discuss contrast agents with your physician before you arrive for the examination.

Frequently asked questions

Question:  My child is hearing impaired.  Which study is right for him?

Answer:  You need to discuss this with the neurologist that is seeing your child.  CT scans are great for looking at the bones and can be very useful in diagnosing inner ear malformations that could be contributing to the deafness.  MRI scans are used for looking at soft tissue.  This might be appropriate if your doctor suspects a brain lesion.  Either way, make sure that the prescribing doctor answers all of your questions before you leave the office.

QuestionIs MRI dangerous?

AnswerNot usually.....There are some people that should not have an MRI.  Top of the list are folks that have cochlear implants or pacemakers.  People with metal fragments in their eyes or aneurysm clips implanted in their brain should also avoid MRI examinations.  The strong magnetic fields can dislodge metal objects that are in the body.  Even worse they can overload implanted electronic devices and render them useless.  Always tell your physician and the technologist if you have implanted medical devices before you enter the magnet suite.

Question:  Are the pictures on your web page normal?

Answer:  No they are not.  All of these patients were very ill at the time the images were obtained.

Question:  Why did the technologist make me remove my eye make up before the (MRI) scan?

Answer:  Eye shadows often contain small metal particles than can interfere with the study.  Sometimes they can heat up and cause eye irritation.

Question: If I'm unconscious in an emergency room and they order an MRI, will they know I have an implant?

Answer:  Emergency departments and MRI facilities often rely on relatives to provide this information when you are unable to communicate.  The problem arises when a patient presents with a loss of consciousness and family is not present.  If you are in a hospital that has not seen you before and the external portion of your implant has been removed prior to arrival in MRI there may be a problem.  As you know, a CI is almost impossible to see once the processor is disconnected.

MRI is very common for patients that present with seizure activity, spinal trauma, and unexplained loss of consciousness that does not appear to be stroke induced.  Sometimes they get a CT scan first which shows the implant but sometimes they do not.  If you are concerned about this possibility, you should wear a medic alert tag or bracelet.

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