Population rates of computed tomography (CT) and magnetic resonance imaging (MRI) continue to increase markedly.  However, little is known about the indications for and results of these imaging tests.

A cross-sectional chart-abstraction study was used to determine the indications for and results of outpatient CT and MRI scans performed on or after January 1, 2005 at randomly selected Ontario hospitals.

CT (11,824) scans and MRI (11,867) scans were studied.  Cancer-related indications accounted for over 50% of CT scans of the abdomen/pelvis and chest.  Headache was the most frequent indication for CT of the brain.  More than one-half of MRI scans of the extremity were for knee pain or suspected meniscal tear.  Back pain and radiculopathy were the most frequent indications for MRI of the spine.  There was considerable variation between institutions in ordering patterns, with as much as a 70-fold difference between hospitals in the frequency of scans ordered for a specific indication.  Less than 2% of CT scans of the brain for headache found abnormalities that could explain the headache, while over 90% of MRI scans of the spine for back pain were abnormal, although the clinical importance of the abnormalities was unclear.

These data are a starting point for a discussion about appropriateness.  Further information will be obtained by examining individual indications more closely, and linking these data to administrative databases to evaluate the impact of these imaging tests on clinical practice.