Application of CT versus MRI: A Basic Approach

Friday, July 29, 2005 | 0

The two primary cross-sectional anatomic imaging techniques used in radiology to evaluate injuries are CT and MRI. The mechanisms to show contrast are different for these two modalities, and therefore, they can demonstrate different and complimentary data. As such, each can be ideally suited for evaluating specific body parts with specific history.

Volumes of books have been written on the physics of MRI and CT (in fact the Nobel Prize was recently awarded to the invention of MRI), so obviously detailed discussion of the physics of these 2 modalities is beyond the scope of this article. But in short, CT uses x-ray beams to determine the density of the different tissues in the body, while MRI uses a magnetic field to differentiate the different tissue character of the body.

As a result of this difference in technology, there are advantages to each modality. CT (using the new multi-detector CT scanners) can scan the entire body in a few seconds, where as an MRI exam typically takes between 15-45 minutes, dependent on the scanner used. Therefore, MRI is much more sensitive to motion artifacts from the patient, and claustrophobic patients have a much more difficult time with a typical MRI examination and may need sedation. Secondary, the resolution of CT for small pathology is much better than MRI. Prior to the advent of multi-detector CT scanners, MRI had the advantage that images could be obtained in all planes, but with the newer generation of CT scanners, images can be post processed after the examination in all imaging planes including 3-dimentional imaging with no "step" artifact. The major advantage that MRI has over CT scan is its better tissue characterization, which makes it usually the ideal choice for musculoskeletal or neurological applications.

SPINE

The first test of choice for most spine applications is MRI. Because of its better tissue characterization, MRI demonstrates better than CT - abnormalities of the disc, including loss of its normal hydration, tears of annulus fibrosis, abnormalities of the vertebral bodies such as edema, and abnormalities of the spinal cord.

CT can be the imaging test of choice for the spine in a few situations. 1) If there is a known compression fracture as CT is better at accurately characterizing the bony details. 2) If the most accurate measurement of the neural foramen is needed, as CT has a better resolution for small anatomy. 3) If there is a contraindication for MRI. Finally, it should be noted that CT myelogram is probably the best way to characterize the spinal canal using CT examination, but requires a lumbar puncture, an invasive test.

MUSCULOSKELETAL

Because of its better tissue characterization, MRI is the study of choice for virtually all muscloskeletal applications. MRI can better characterize edema in bone, and soft tissue abnormalities such as: ligamentous, tendon, labral, and meniscal pathology.

Again CT, can be the study of choice in a few situations: 1) If accurate body anatomy is needed, or if looking for loose body prior to surgery or in contemplating surgery in a complex fracture. 3-Dimentional and multi-planar reformations can be helpful in this situation. 2) In evaluation of tumors or tumor-like conditions, CT can play a complimentary role as it better displays calcification which can characterizes the tumor itself. 3) Finally, if there is a contraindication for MRI. In this instance, if soft tissue abnormality is considered (such as a rotator cuff tear in the shoulder or a meniscal tear in the knee), use of intra-articular contrast (i.e. CT arthrogram) can be very useful, but requires an interventional procedure.

BRAIN

In general, and in the setting of injury, initially screening of the brain CT is similar to MRI in diagnosing acute traumatic injuries of the brain such as an intracranial bleed. If the CT is normal or equivacol, MRI can be a complimentary study, and can be useful to confirm the abnormalities in CT or demonstrate more subtle abnormalities.

In summary, MRI and CT are invaluable tools in evaluating pathology. As they use different mechanisms, they can be the primary or secondary imaging tools to evaluate a clinical complaint. This article is only a general overview and specific questions should be addressed to your radiology professional. We, the radiologists at Carlsbad Imaging Center, are always available to discuss and guide our referring physicians to the appropriate study for the clinical circumstance.

Please direct your questions to Dr Afsaneh Maghsoudy or Dr. Brian Bigoni
Carlsbad Imaging Center
3144 El Camino Real #100
Carlsbad, CA 92008
(760) 730-3536

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