John S. Mattoon, DVM, DACVR
Associate Professor, Radiology, College of Veterinary Medicine, Washington State University
Pullman, WA, USA
CT provides exquisite bony detail and as such is an excellent modality for assessment of skeletal disorders. Bony lesions can be identified on CT examination that is not visible on radiographs, even in retrospect radiographic evaluation. For example, in spinal imaging CT is capable of diagnosing early neoplasia or infection, well before radiographic signs are present. The extent of bony neoplasms is better assessed with CT than conventional radiographs and can play a role in surgical planning. The canine elbow has been studied extensively using CT and has been shown to be the most sensitive image test for in evaluation of canine elbow dysplasia. The canine shoulder, carpus, stifle, and tarsus have all been studied using CT. Clinical applications include suspected osteochondral (OC) lesions, erosive arthritis, soft tissue and bony neoplasms, and fractures to name a few. And of course, CT has been used to assess the spine and various neurological diseases.(1)
It must be noted that while CT is the best diagnostic imaging test to evaluate bone structure, MRI is superior in detecting bone physiology and soft tissue pathology including joints, menisci, cruciate and collateral ligaments, and articular cartilage. Still, CT is useful in evaluation of soft tissue lesions of the musculoskeletal system. Basic tenants of assessment include delineation of specific muscles or muscle groups and the presence or absence of underlying or adjacent bony reaction. The extent of the mass can be assessed for surgical planning and in the case of lipomas, CT is diagnostic because of HU values characteristic of fat (less than the surrounding muscles); liposarcomas have a wispy appearance and HU values intermediate between lipomas and soft tissues masses.(2)
The elbow has been extensively studied using CT and is especially valuable in defining the various pathologies associated with elbow dysplasia and surgical planning.(3-8) Plain radiographic examination, while capable of assessing the majority of changes associated with elbow dysplasia, cannot reliably determine the presence of a fragmented medial coronoid process or the degree of joint incongruity, areas easily reviewed with CT.
Under general anesthesia, the patient is placed in sternal recumbency and the fore limbs extended cranially as far as possible. Both elbows can be imaged in the same scan plane, and if symmetrically positioned, can be easily compared to each other during image review. Axial images are obtained from the distal humerus through the proximal radius and ulna. Three millimeter slices can be made through the entire elbow, with thinner slices obtained through the medial coronoid process (MCP). Overlap of the axial slices allows better reconstruction into dorsal and sagittal images. Sagittal image reconstruction allows the best assessment of joint incongruity.
The appearance MCP is of particular interest and should be assessed for fragmentation or fissures, size and shape, osteophyte formation, and sclerosis.
Other important areas of interest include the humeroulnar, humeroradial and radioulnar relationships (joint congruity), evaluation of the humeral condyle for subchondral bony lysis and sclerosis, indicative of osteochondrosis, and the medial and lateral collateral bony and soft tissues. Keeping in mind the anatomic areas involved in elbow dysplasia can help in the evaluation process. These include: medial and lateral epicondylar bony response, anconeal process bony reaction proximally, ununited anconeal process, joint incongruity, MCP remodeling, radial head osteophytosis, sclerosis of the ulnar trochlear notch.
CT evaluation of the canine tarsus has valuable in assessing fractures, particularly those sustained in racing injuries, and in osteochondrosis.(9,10) While radiographs maintain a place in initial evaluation, CT can provide otherwise unattainable information regarding the severity of pathology necessary for surgical planning and intervention.
As with the elbow, sagittal and dorsal reconstruction planes made from axial images yield significant information. Three-millimeter contiguous collimation of the distal tibia through the proximal metatarsal bones provides baseline information and can be followed with 1 mm slices with or without slice overlap for better detail and multiplanar reconstruction. 3-D reconstruction is also valuable.
The canine stifle has been evaluated with CT and more recently has been used in conjunction with stifle arthrography, in which diluted iodinated contrast medium is injected into the stifle prior to imaging. The use of intraarticular contrast allows exquisite anatomic assessment of the cruciate ligaments and the menisci. Without stifle arthrography, CT is useful for determining the extent of degenerative joint disease, and the depth of the femoral trochlea and relationship to the patellar articulation in luxation cases, but lacks the soft tissue differentiation of MRI, probably the reference standard in canine stifle imaging (though still a young and developing science). (11,12)
The shoulder joint is amenable to CT assessment. CT's strong suit is in the evaluation of tendon size and calcification, usually of the biceps and supraspinatous muscles. MRI is the preferred imaging test, however, as it cannot only assess size and mineralization but can detect abnormal intratendinous signals. It should be noted that ultrasound is useful to evaluate the tendinous structures of the shoulder and because it can be performed without anesthesia, can be thought of as a first-line test for shoulder musculoskeletal disorders. Commonly diagnosed diseases of the canine shoulder include bicipital tendonitis (enlargement, mineralization, synovitis), bicipital origin injury, and mineralization of the supraspinatous insertion. Evaluation of the opposite shoulder is often informative.
Nerve Sheath Tumors
Nerve sheath tumors are often responsible for lameness and can pose a diagnostic challenge. They can occur anywhere along the peripheral nerves of the fore and hind limbs, from the spinal cord to a distal extremity. The more central lesions are easiest to detect while more distally located tumors are more problematic. When suspected, axial images of the patient to include the brachial plexus or lumbar plexus must be made. Observation of muscle atrophy helps to localize tumor location, especially if specific muscle groups can be identified (i.e., what nerve is involved?). On CT examinations, these tumors are often large enough to identify as a nodule or mass lesion. (13)
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