Dr. Miller: Benefits of having a CT scan versus having an MRI. We're going to talk about that next on Scope Radio.
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Dr. Miller: I'm here with Dr. Ulrich Rassner and he is a radiologist here at the University of Utah. And we're going to talk a little bit about the difference between a CT scan and an MRI scan. Ulrich, can you tell us a little bit about the difference and kind of in a nutshell what would be the concern for patients? I know that some patients are concerned about the potential for radiation exposure with CT scans and MRIs use magnetic fields and there's no ionizing radiation in there. So should everybody have an MRI?
Dr. Rassner: No, not everybody should have an MRI. It depends very much on the question that we're trying to answer.
What Is a CT Scan?
In general terms, a CT is a fancy X-ray. And fancy in the way that we acquire a lot of individual X-rays and then the computer can assemble them and allow us to have cross-sections like we're cutting through the patient. And that is tremendously helpful with complex anatomy. So for example, the facial bone fractures, where if you tried to figure out what is going on a plain film on an X-ray, it is very difficult because there are so many overlapping structures. While on a CT scan, you can see them to much greater detail and see what is injured and needs treatment.
What Is an MRI?
Dr. Miller: Now, how about the MRI? Does it not do the same thing?
Dr. Rassner: The MRI does do the same thing. However, MRI uses the magnetic properties of tissues. So when we put the patient in the scanner, and the scanner's a very powerful magnet, the patient actually will become magnetic while they are in the scanner. And then we can manipulate that magnetization in the patient with radio waves and they are just the same wavelength as a TV channel or so you might tune in.
Dr. Miller: But that's not dangerous.
Dr. Rassner: That is not dangerous and there are very specific guidelines from the FDA in how much energy we can put in a patient. But there is no ionizing radiation so there is no cancer risk associated with that.
Dr. Miller: Now, a CT scan's ionizing radiation exposure is multiple times that of a plain X-ray so get that better imaging, it comes at a cost of greater ionizing radiation.
Dr. Rassner: With CT, yeah. CT has higher radiation doses than a radiograph, but in a lot of cases, the added information that you get is beneficial. And also, in some cases, for example, if you want to look at the skull with a radiograph, you need a lot of different views so you can't just look from the front. You may have to look from the side and then obliquely, and so all those individual radiographs can add up to a similar dose as a CT and yet you don't get as much information out of them. So sometimes, doing the correct study or the better study right away can actually save radiation, rather than doing the first one and then decide, "Well, we can't actually see what we need to see."
MRI, again, just to come back to that point, looks at a magnetization, and then we can look at tissue property. In a sense, when we manipulate it with the radio frequency waves, how long do they stay magnetized and how long does it take to regain magnetization? And the differences between soft tissues is quite striking with that and so we can resolve soft tissue detail that is invisible in CT. And so we can pick up lesions that cannot be seen with other imaging studies. And there are certain things we can see and certain things we cannot see so we are basically looking at hydrogen in the body, which is mostly is in water and which is in fat. And so those tissues that contain that, we can see very well. Other tissues, like bone, we do not see directly. We see the bone marrow, but not the bone.
Dr. Miller: So tell me how a patient would know or how they might sense they would be better off with an MRI versus the CT. I mean, there are a lot of articles out now in the press about the dangers of ionizing radiation. And some patients may think that they should have an MRI just simply because it doesn't have any exposure to ionizing radiation. We talked a little about the need for CT scan for certain types of tissues in the body and MRIs look at another set of tissues in the body, maybe better than the CT scan. So what would a patient to sort of distinguish whether they would benefit most from an MRI or a CT scan?
Dr. Rassner: It depends very much on the question that's trying to be answered. And again, if there are certain soft tissue tumors, it might easier to see them with MRI than with CT, but there is no general rule. What also enters into the equation is that an MRI scan is a much longer study.
Dr. Miller: More expensive, perhaps?
How Long Does an MRI Take Vs. a CT Scan?
Dr. Rassner: More expensive, as well and in part because it is longer. So while a CT scan, the actual scanning part, may take 10 seconds, an MRI examination may take 50 minutes to an hour or even longer. And so a lot of images, to just acquire them, may take on the order of five minutes. And so, for example, if you're imaging the chest, you can hold your breath for 10 seconds, but you can't hold your breath for five minutes. So there may be much more issues with breathing motion so some lesions that are easily seen on CT, we might see as well on MR because there's a lot of movement during the examination.
Dr. Miller: I suppose one appropriate question from a patient might be, "Have you discussed with radiologists which test might be best for me?"
Dr. Rassner: Yeah.
Dr. Miller: I don't think that happens very often, but that could be a very good question for a practitioner to hear. "Have you discussed that with the radiologist," and that might give them some additional comfort in which test was being ordered.
Dr. Rassner: Yeah. We are always happy to discuss it with our clinicians, and in fact, it often helps us, especially in complex cases, when we do know more of the history of the patient and what is specifically of concern, especially with MRI. The MRI is a much more targeted exam and there are many different ways we can acquire images. And we target those for a specific question. And so, in difficult cases, having that specific information helps us do the best study for that question.
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updated: July 31, 2018
originally published: January 28, 2016
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