Post #18:  Abdominal CT or Abdominal MRI?  Which imaging study to request? 

In a previous post (#12) on abdominal imaging I discussed using ultrasound instead of CT scan as the initial imaging study in many clinical scenarios. Ultrasound may provide the diagnostic answer in some cases, while in other situations the findings on ultrasound may redirect physicians to a different imaging study.  For example, when an ultrasound demonstrates a mass in the liver, pancreas or spleen, guidelines support performing a CT abdomen without and with contrast (a dual study) instead of the usual CT abdomen with contrast. The same guidelines would also support MRI abdomen as an alternative imaging study.  Once in a while, an ultrasound will show a dilated common bile duct, for which MRCP is the preferred follow-up imaging.  This posting will help sort out when an abdominal CT is the next imaging study and when an alternative is preferable.  Since prior authorization may be denied if you request a CT with contrast when guidelines specify CT without contrast is indicated, I will include in this discussion requesting tips when considering contrast. 

CT abdomen: pro and con

First, a few pros and cons for CT vs. MRI.  Generally, CT is a better imaging study for evaluating traumatic injury.  Bleeding and organ tears are better seen with CT scan, which also happens to be a faster study to perform than MRI and thus the preferred test in the emergency department.   Post injury CT for abdominal pain may also show extra-abdominal findings such as a vertebral fracture better. MRI, alternatively, is a better imaging study when a prior CT or Ultrasound has shown a mass in an internal organ, as MRI differentiates organs and soft tissue better than CT.  CT testing is associated with radiation exposure, while MRI has no such risk.  This may be important in pregnant women and women who may be pregnant, or in children, for whom radiation exposure should be avoided when possible. Radiation exposure from CT scan is also a consideration in patients with inflammatory bowel disease who have had countless CT scans whenever each flare-up occurs.  To avoid additional exposure to radiation in IBD, an MRI or MR enterogram can be requested and will be approved, as long as the provider tells the Prior authorization nurses that radiation avoidance is the reason for the MRI.  Magnetic resonance imaging is also preferred when patients are unable to have IV contrast, either because of past allergic reactions to CT dye or because of renal insufficiency.    

MRI abdomen: pro and con

MRI of the abdomen has its pros and cons too.  Patients with metal objects or devices in the body may not be candidates for an MRI.  This may include not only older (before 2007) metallic surgical implants such as pacemakers, orthopedic screws, rods and plates, iv stents and filters, but also foreign bodies such as hair pins, safety pins, bullets, shrapnel, BBs, and braces.  MRI facilities usually keep up with specific devices and metallic objects that are allowed for MRI scans.   Some facilities specify that a 6 -week period of time have elapsed after a heart valve insertion before performing a non-emergent MRI.   

Most MRI scans of the abdomen are performed without gadolinium (Gd) contrast and then repeated with IV contrast.  Gd does not have iodine and may be used in iodine-allergic patients, but is contraindicated in patients with moderate renal insufficiency and recently there have been concerns about the contrast retained in the brain.  Gadolinium should not be used in pregnant women so imaging performed in these patients should be non-contrast MRI.  Time to perform the study may be an important consideration: CT takes only a few minutes while MRI may take 30-45 minutes per study.  Almost all abdominal MRIs are performed without and with Gd except in pregnancy. 

Besides these obvious contraindications, there are also technical reasons to choose CT over MRI scan or vice versa.  MRI is more costly than CT scan but the study time frame is also a consideration. Patients who are in pain often cannot lie still for extended periods of the time that MRI requires.  Children, especially, may not be able to lie still, and may require sedation or anesthesia for the study. This may further increase costs, often doubling the total cost of the exam!  RBM guidelines do support using MRI as a preferred study in children, but it was my experience that many pediatricians still prefer CT (which takes only a few seconds) despite the radiation, to avoid the greater anesthesia costs. As stated in a prior post, the initial imaging study for children and many adults is often an ultrasound. 

CT scan is a more general test while MRI is often reserved for more targeted imaging.  If ultrasound or CT abdomen with contrast is equivocal, MRI may better delineate a lesion in the liver, pancreas, spleen or kidney.  If a suspected hemangioma in the liver is suspected by ultrasound or CT scan, options for follow-up testing include either an MRI without and with contrast OR CT abdomen triple phase (without and with contrast plus extra liver views using delayed shots).   If an ultrasound or CT suggests either liver adenoma or focal nodular hyperplasia, MRI with eovist may be a preferred contrast agent for differentiating these two conditions. As detailed on blog post #12, ultrasound is still an excellent screening test for abdominal symptoms before advanced imaging, and is a superb test for following a lesion over time for size change. 

CT vs. MRI: a tossup

In some diagnostic cases guidelines support either abdominal MRI or CT scan and either request should be approved. For example an adrenal lesion that is not considered definitively benign on a non-contrast CT can either have an abdominal CT with contrast plus delayed views as a follow up study, or an MRI.   

Requests for CT imaging are sometimes denied because the physician asks for a contrast modality that is not supported by the guidelines.  For example, a dual study (CT abdomen and pelvis without and with contrast) may be denied when abdomen guidelines support only a CT with contrast.  The next post will discuss CT contrast in more detail so as to avoid future denials, appeals, and peer-to-peer calls due to these types of imaging errors. 

An excellent summary of MRI indications vs. CT indications can be found at the following website: 
https://www.radiax.com/Portals/1/Documents/Downloadable%20Materials/General%20Radia/RadiaImCtrs_OrderingGuideF_Jun12.pdf

Lesson posting #18:  Imaging of the abdomen often is best initiated with ultrasound, which may direct clinicians to a different follow-up test than originally planned.  MRI and CT have advantages and disadvantages, strengths and weaknesses.  For a more detailed description of indications, see the Abdomen guidelines (post #3 will show you how to capture guidelines and paste them on your computer’s desktop).  Next post will discuss when to request CT with contrast and when to ask for a non-contrast imaging.