Current Concepts

Veterinary magnetic resonance imaging (MRI) is a well-established and highly valued tool for the evaluation of neurologic disease in dogs and cats due to its unrivaled ability to distinguish normal and abnormal soft tissues (known as tissue contrast). However, this same exquisite sensitivity of MRI for detecting lesions in the brain and spinal cord can be of great benefit when evaluating other organ systems and diseases.

MRI is widely used in human medicine across a variety of conditions; in fact, the use of non-neurologic MRI of small animals is expanding. The major areas of developing veterinary use include oncology staging and pre-surgical planning, abdominal imaging and evaluating musculoskeletal diseases.

Figure 1. This pre-operative T1-weighted post-contrast image of a low grade fibrosarcoma demonstrates a poorly marginated enhancing mass (black arrowheads) associated with the right scapula, axillary region and thoracic wall. On the basis of this MRI it was determined a forelimb amputation would be insufficient and that resection of a portion of the thoracic wall would be needed to ensure complete removal of the tumor.

MRI is a standard part of the pre-surgical workup of soft tissue sarcomas in people. Its ability to distinguish normal from abnormal tissue allows for the best appreciation of tumor margins and extension, which greatly facilitates surgical planning. At the Foster Hospital for Small Animals, MRI is commonly used in patients with large masses on the trunk or proximal limbs where tumor margins may be difficult to determine via palpation and conventional radiography. Having complete information in the diagnostic workup allows our clinicians to determine the feasibility of complete resection and to offer the best treatment plan to owners, whether that be surgical or non-surgical.  (Figure 1.)

Figure 2. Sagittal T1-weighted post-contrast sagittal image of the caudal abdomen and pelvis. A large vaginal mass was detected on physical examination and an MRI examination was performed to determine if the mass was resectable. The vaginal mass (black arrowheads) was determined to be originating from the dorsal vaginal wall and was not involving the urethra (white arrow), urinary bladder (#) or colon / rectum (*).

The same benefits of tissue contrast resolution apply to the investigation of abdominal disease with MRI.  At our hospital we have found it useful for the investigation of large abdominal or intrapelvic masses  (figure 2.) where the origin and margins cannot be clearly defined and for the evaluation of small tumors (such as insulinomas) that are difficult to detect with ultrasound in dogs.

While our equine community has been quick to embrace the benefits of musculoskeletal MRI, its use for that purpose in small animals is less common. Although MRI has a reputation for being less useful than CT for investigating cortical bone, its superior ability to evaluate muscle, tendon, cartilage and the medullary cavities of bones makes it a very good choice for imaging musculoskeletal disease. Current musculoskeletal MR applications at the Cummings School include imaging of patients with inability to open or close the mouth, suspected iliopsoas muscle injury and recurrent soft tissue swellings of the head and neck.

In conclusion, MRI is a useful diagnostic imaging modality in a variety of challenging clinical situations. However, owners need to be aware that MRI is relatively expensive compared to other imaging modalities and requires general anesthesia during the 1-2 hour scan time. For these reasons, use of MRI is limited to those cases where other less costly imaging modalities such as ultrasound or CT are not expected to provide a diagnosis.