A computed tomography scanner uses radiation and high-level computer programming to produce accurate, detailed axial images that can be reformatted to provide similarly detailed coronal, sagittal, oblique or even 3-D views of the anatomy imaged. The scans can be done with or without IV and oral contrast, depending on the body part imaged and the reasons behind it. With the newer spiral multi-slice scanners, most exams are complete in less than 5 minutes.

A magnetic resonance imaging scanner uses a powerful electromagnetic field to produce its images, and does so without generating any ionizing radiation, a great advantage. The scanner gantry, however, is much longer and closer to the patient than with CT, leading to claustrophobia in many patients. Numerous imaging sequences are typically performed, often requiring re-adjusting of the local body coils needed. Scan times are much longer, often up to 45 minutes and patients must lie extremely still for extended periods or the images are distorted. There’s a loud repetitive banging sound associated with each scan, but earplugs are routinely distributed. Wait list times for MR scans are typically longer than CT in Canada, due to fewer machines, increasing demand and restricted access. Although IV contrast agents are used in certain scenarios, MRI can also provide detailed MR angiographic 3D mapping of vessels (i.e. circle of Willis) without any contrast at all. Some patients are contraindicated to use MRI, due to the powerful magnet involved. Examples include cardiac pacemakers and particular types of heart valves, ferromagnetic aneurysm clips and patients with metallic foreign bodies in the orbital regions (metal workers, carpenters).

MRI is superior for soft tissue structures (joints, muscles, brain and spinal cord elements) whereas CT is the modality of choice for imaging bone and lungs, and for acute trauma (intracranial hemorrhage, organ damage, etc.)

Feel free to contact your local radiologist to inquire which test is the best for your patient.