Both CT and MRI modalities present risks, causing researchers to evaluate the safety of each.
Issues such as radiation versus energy deposition, allergic reactions, renal issues and nephrogenic systemic fibrosis (NSF), pregnancy or possible pregnancy, and issues involving the pediatric patient, must be considered when determining the safest modality for the individual patient.
MRI has always been thought to be a very safe modality, but putting certain devices such as a pacemaker, clip, or aneurysm clip into a magnetic field can certainly cause problems.
In addition, the FDA recently came out with an alert concerning the use of CT radiation or radiation in general and medical devices, such as pacemakers and neurostimulators.
The amount of radiation that the patient is exposed to poses extremely low to almost negligible risk. Obviously, the issue of cancer induction with radiation has been discussed, and it does make sense to limit the number of CT scans a patient gets, especially in younger patients where MR can be used to obtain similar information and have the exposure to the radiation risk limited.
The pregnant patient is a different matter entirely. While we avoid using gadolinium with the pregnant patient, iodinated contrast probably doesn’t have to be avoided in the amount that we give, as the amount that crosses the placenta is not going to cause a fetal goiter.
Therefore, if you really need to give contrast in the pregnant patient for whatever reason, it is probably safer to use CT rather than MRI.
Renal impairment and issues related to NSF with MRI is something to consider. The gadolinium-based contrast agent called ProHance appears to be very safe even in patients who have renal impairment.Related CME:
For pancreatic lesions, there is no question: CT. It is quick, and if you are trying to diagnose pancreatic cancer, you can almost always tell at CT. For the most part, CT is going to be able to show you the ductal dilatation and show you if the pancreatic cancer is there.
MRI does have a little advantage in terms of cyst characterization as you can see the septations and ductal communication better. No contrast is needed when looking for debris inside of the cyst or measuring growth, and these can be done very quickly with just few sequences.
The other area in which I find MRI to be helpful other than in characterizing lesions that I am not sure about at CT is in the evaluation of the bile duct. For gallstone disease, I think MR certainly has an advantage in evaluation of the biliary tree.
About 15% of our study are hematuria studies. You just cannot see kidney stones with MRI imaging but a CT scan can look at the urothelium, the parenchyma of the kidney for tumors, the prostate, the urinary bladder, and you can see stones and other abnormalities with the CT scan.
Overall, in staging cancer, I think in general your first line always is CT and for the most part, we use MR imaging for problem solving.
Author: Michal Macari, MD
CME: Body Imaging: Abdominal, Thoracic and Vascular
University of California, San Francisco, Department of Radiology
This CME program carries 18.5 AMA PRA Category 1 Credits and is designed for the radiologist in clinical practice.
Hot topics include cardiac imaging, multidetector CT, CT/MR angiography, virtual colonoscopy, and tumor ablation.
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