What’s the difference between an MRI and a CT (Cat) Scan? Is the CT Scan technology highly different than an MRI? How does each form an image? - Jinx
MRI uses magnetic resonance of water molecules. CAT scan is based on X-ray.
“Questions frequently arise regarding the utilization of CT versus MRI scans to obtain the best diagnostic outcome for the patient. In general, MRI is the procedure of choice for all neurologic and orthopedic diagnoses, while CT is the preferred modality for examining the chest, abdomen, pelvis, and skull base. Note that CT and MRI can also be used in cross-over problem solving from the alternative modality.”
A CT scan will X-ray you from many different angles and then use a computer to assemble all of the resulting x-ray image data into a 3D image or a cross-sectional slice. The x-ray beam can only “see” things that block x-rays such as bones, calcium deposits, etc. They can inject iodine or other dies to make soft tissue show up better.
An MRi puts your body into a very strong magnetic field where every point in your body is at a slightly different field strength. They then hit you with a strong radio signal tuned to the magnetic field strength of one point in your body. Only that one spot in your body will send back a tiny radio signal echo. The nature of the echo tells the MRI what kind of tissue is there. This process is repeated many times for different points in your body and a computer combines the data into a picture.
www.howstuffworks.com has much better descriptions of these things.
The real difference is that CT exposes your body to ionizing radiation and the MRI can totally screw up your watch. At the moment, my watch is really suffering.
Similar to what others have said above, MRI is better suited for visualizing soft tissue. CT works better on bony structure. There is some overlap, though.
Side note: We often do x-rays for people before they have an MRI procedure done to see if they’re harboring any small metal fragments, splinters, etc. You’d be surprised at how many people in the construction trades are walking around with mini-shrapnel in their bodies.
About two months ago I received a CT scan in the hospital while I was having severe abdominal pain. They injected me with a dye (probably iodine based - BTW this was the weirdest feeling I have ever had. You could actually taste it when it was injected!) and I was forced to drink about two liters of a related dye during the hour or so before the procedure.
Why? With abdominal pain there were only a couple of possibilities. Thoughts were:
[ul][li]appendicitis[/li][li]An infection[/li][li]kidney stones[/li][li]diverticulitus (this is what it was, and I’m only 33!)[/li][li]Or an alien was about to pop out of my chest[/ul][/li]All of these conditions involve soft tissue (except possibly the stones). Why didn’t I get an MRI?
On another note, I have spent the last several years working with 3D X-ray imaging, namely laminography. Laminography is very similar to a CT scan except mechanical motion is used to create a “slice” instead of using a computer to assemble the multitude of pictures. Why I am bringing this up is that I was shocked and disturbed by the amount of X-rays my body was exposed to (135kV, 80uA, 5 min IIRC). Have there been any studies of the somatic or genetic effects from CT scans?
I know the magnetic fields are very stong (amusingly strong), but strong enough to pull a splinter out?
What do you do if you find something?
The stuff you drink when you are given a CAT scan is definitely iodine. They ask you ahead of time if you are allergic to it. Actually, they asked me if I was allergic to shellfish.
The technician told me, “You’ll feel warm after this”. And did I ever.
I had a CAT scan on my head, I suppose they could have done an MRI, but I suppose they didn’t think they were going to find anything wrong anyway. Or else my HMO was cheap.
Regardless, I was fine.
Or else that brain tumor has been hanging around for 8 years.
Wikkit,
The magnetic field is not likely to yank a metal splinter out of your body but it can cause it to move around. If you have a splinter in your eye, the MRI can move it around which could rip up the delicate tissue. Aneurysm clamps placed in your brain is another thing you don’t want to have yanked around. A pacemaker is bad news too. If the hospital finds any of these ‘counter-indications’ through X-ray or just by asking you, then they won’t take you any where near an MRI. Things like bone screws are not a problem because they are securely hastened in bone and can’t move around.
How does a PET scan fit it to the picture?
I remember for my pscy days a long time ago that these machines seemed to be able to show and megga detail between certain types of tissue?
PET scans show function, not structure. They show which parts of the brain (or other organs) are most actively metabolizing glucose.
PET stands for “positron emission tomography”; the patient is given glucose (a basic metabolic fuel) which has been “tagged” with a radioisotope, and is therefore capable of being tracked as it travels through the body. PET scans can therefore be used to chart not just soft tissue but metabolism–to see if certain tissues (in the brain, for example) are or are not burning glucose. This is obviously useful for both medical reasons (to chart damage done by a stroke by seeing which parts of the brains are no longer “lighting up”) and for basic research (have volunteers do various tasks–remember lists, recite poetry, recognize patterns, etc.–and see which regions of the brain are activated).