What is the difference between CT scan And MRI

Nurses General Nursing

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Someone asked me recently what the difference is between a CT scan and an MRI and I couldn't answer them. I am a relatively new grad working in LTC. I haven't had any exposure to these tests except what we went over in school. I've been studying on it and this is what I've come up with. A CT scan is cheaper, faster, and it uses radiation . It will show bones where an MRI will not. An MRI uses magnetism and radio waves.It shows the same thing as a CT scan except it will not show bones. Is there anything else I should know when explaining the difference to someone? If some one is having neurological problems why would a CT scan be ordered before an MRI? The person who asked me the difference thought that the 2 tests showed the same thing but that an MRI was better so they wanted to know why you wouldn't start with an MRI if the test wasn't needed to show bones.

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Specializes in LTC.

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.

source: google.

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Specializes in being a Credible Source.

A CT is basically just a whole mess of regular x-rays which are compiled by a computer to create the "slices" through the region of interest or even 3D images. Like any x-ray, a CT simply reveals at differences in density. Hence, they do pretty well showing things like bones, blood clots, etc. They can reveal tissue masses but (a) not reliably (again, depending on density), and (b) not with very good resolution.

On the positive side: They're relatively cheap and available almost anywhere.

On the negative side: They don't work so well in soft tissue (for example, looking for torn ligaments or for small masses in neurological tissue) and they utilize ionizing radiation which is generally the equivalent of hundreds of regular x-rays.

MRIs utilizing magnetic fields to alter the spin-states of hydrogen atoms (if you took upper division organic chemistry, think of your NMR spectroscopy) and utilizing the absorption characteristics to generate 2D and 3D images.

They are ideal for any soft tissue which contains hydrogen (which they all do, in abundance) and can reveal very subtle distinctions (for example, a brain lesion of only 1 millimeter).

The primary advantages are their resolution, ability to "see" differences in soft tissue, and their lack of radiation.

The primary disadvantage is COST (they're typically 3-5x a CT), the requirement to be perfectly stationary in a closed tube (though open MRIs are becoming more available) for 15-45 minutes, and that their strong magnetic fields preclude their use on patients with certain kinds of implants, pacemakers, and surgical clips.

The CT is generally ordered before the MRI primarily due to cost and because the CT will often reveal what needs to be revealed and can then be used to target the MRI to the zone of interest.

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Specializes in neurology, cardiology, ED.

The major difference, other than price that I can see as a nurse is the image quality. Most of my experience looking at scans is from a neurological stand point, so this is the example I will use:

For a patient who has had a stroke, a CT scan will show a bleed immediately, but not show a new infarct, only an old (weeks old at least) one. Technically, you are looking at 3-5mm "slices" of the patient's brain and skull.

The MRI, on the other hand will show you a 3D image of the brain in it's entirety. You can look at the brain as a whole, or you can choose to only look at the blood vessels. A MRI will show a non-bleeding stroke immediately, within hours, and you can see the actual blood vessels that are not receiving blood flow, to pinpoint the location of the infarct.

So to sum it up:

CT scan = less $, more radiation, good image quality

MRI = more $, no radiation, excellent 3D image

Oh, PS:

MRI can show bones

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Specializes in NICU, PICU, PACU.

Ct uses radiation, MRI uses a magnet to stand your hydrogen molecules on end to provide the image. When the magnet is in use it spins the molecules on their axis and makes them "stand up" so that you can see soft tissues more easily. It does show bone, but it more useful for soft tissues.

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I just do not know why a nurse would have to come here to ask this question?!:confused:

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My first thought was "wear metal and you will find out quickly!" ;)

My brother "forgot" he had shrapnel in his calf. Had an MRI of his foot after a MVA. OUCH!

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I just do not know why a nurse would have to come here to ask this question?!:confused:

Maybe because she wasn't clear on the subject?

I'm sure a negative, non-constructive critical reply didn't contribute anything.

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Surely, surely this is quite easy information to find for the public as well...

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Thanks to everyone who has replied positively. Didn't know we were suppose to only ask hard questions. I came here to ask the question first- because I thought this was allnurses.com where a nurse could come to ask questions from other people who might be more knowledgeable. Secondly- because the person who asked me this question is going through a rough time, they are scared for their loved one, have alot of hard decisions to make right now. I am a relatively new nurse who wanted to answer their questions correctly. I read about the scans in my medical books and did research but sometimes its better if it comes from an actual person. Yes there is information out there for the public but this is a family member I am dealing with, they didn't want to have to go look up info on top of dealing with a crisis. They look to me now "as the nurse" in the family and again being a new nurse I don't feel quite comfortable in this role yet so its nice to come here when I need clarification.Their doctor ordered a CT scan but someone who is not a medical professional told them that an MRI is the same thing as a CT scan only better so why waste time and money on a CT scan. To see if they could skip it and do an MRI since they felt in the end an MRI would be ordered. While doing my research I read everything you all said except I did read somewhere that an MRI did not show bones because an MRI uses water in you body to make images and it said your bones do not contain enough water that they would show up as a black spot on the MRI. Maybe this was not an accredited site? Anyways thank you for all the positive answers.`

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Specializes in being a Credible Source.
My first thought was "wear metal and you will find out quickly!" ;)

My brother "forgot" he had shrapnel in his calf. Had an MRI of his foot after a MVA. OUCH!

Oooh, that's a mistake you only make once, I'd bet.

Good thing it wasn't near a vessel :eek:

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source: google.

LOL! Really?

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