In human medicine examination of musculoskeletal abnormalities of the appendicular skeleton is the most common non-neurologic application of magnetic resonance imaging (MRI). The knee (menisci, ligaments), shoulder (rotator cuff), and hip joints (avascular necrosis) are the joints most often examined, but other joints and soft tissues also are assessed with MRI. In veterinary medicine there is an increasing number of reports describing normal MRI anatomy and pathology of the distal limbs in clinically normal and lame horses. In small animals reports using MRI as a diagnostic tool in orthopedic diseases are still sparse. They are concentrating on technical aspects including contrast procedures, specific regions such as the knee joint, assessment of specific diseases such as bone tumors and research on pathophysiology and treatment of osteoarthritis using the dog as model (e.g., Pond Nuki model for the development of osteoarthritis).

The following paragraphs will give a short review on applications of MRI in orthopedic diseases in small animals and discuss possible indications, benefits and limitations. Radiography is still the single most important diagnostic tool for the detection and diagnosis of musculoskeletal diseases, and Ultrasonography is an excellent method for soft tissue pathologies. Computed tomography (CT) and MRI are used as complementary techniques in selected cases. It is widely accepted that computed tomography has advantages over MRI in imaging the morphology of normal bone and in traumatology, whereas MRI is the method of choice in imaging soft tissues such as the muscles, tendons, ligaments, joint capsule and menisci, but also diseases of the bones leading to alteration in the bone marrow composition. The examination time of MRI is longer than for CT especially if helical CT scanners are used. This has consequences for the anesthesia and increases the costs.

Properties of MRI:

 Very high soft tissue (contrast) resolution

 Individual sequences and contrast media allow differentiation of different types of soft tissues and pathologies

 Imaging in any desired plane facilitates three-dimensional understanding of morphology

Sequences:

 STIR or other fat saturation sequences: high contrast, low spatial resolution--screening for lesions in general, bone marrow, menisci, muscles

 FE T2*: good contrast and moderate spatial resolution--ligaments in general, menisci

 FE 3D MPR or SPGR (T1)--native/+C : high spatial resolution--contrast sensitive, cartilage

 Completely balanced sequences (BASG; True FISP, B-FFE3D; other)--ligaments/cartilage

 FSE T2: high contrast moderate spatial resolution--tendons; trouble shooting for high SI in ligaments (cranial cruciate ligament)

 SE T1 in combination with post contrast studies

Review

MRI is an excellent tool for screening for bone metastases with a compared to scintigraphy higher sensitivity for specific soft issue tumors. However, assessment of bone marrow pathology requires knowledge of the MRI features of the normal bone marrow in different age groups. The different patterns are associated with the different composition of the bone marrow in growing, adult and aging bones. Fat suppression sequences such as the STIR (Short TI Inversion Recovery) will exhibit a bright signal in the diaphysis of long bones in 3 to 6 month old dogs because of the "red" bone marrow. In adult dogs, this will be replaced by fatty marrow and will be dark in the same sequence. However, reconversion to hematopoietic marrow is also described in older individuals.

In a series of 10 dogs with osteosarcoma of long bones, radiographic examination underestimated tumor length substantially in 1 limb and slightly in another limb. CT and MRI were least accurate but did not underestimate tumor length in any of the limbs. Obviously, MRI is overestimating the extent of osteosarcoma because it is very sensitive for any change in the adjacent bone marrow such as inflammatory reaction or hyperemia.

Most joints including the shoulder joints are rare indications to perform an MRI in dogs. Unlike as in humans where rotator cuff tear is the most typical indication for MRI of the shoulder, bicipital injuries are far more common in the dog. However, the supra- and infraspinatus muscle and tendon insertion, the bicipital tendon and tendon sheath, and the shoulder are readily assessed using ultrasonography and are rare indications for MRI.

As in humans, the knee joint is also the joint with the highest number of MRI reports in the dog. The primary indication for MRI of the knee is evaluation of internal derangement. MRI is helpful when physical examination findings and the results of other imaging modalities are equivocal or for assessing the entire joint with all its components. MRI primarily assesses the menisci and ligaments, but the joint capsule, the integrity of the osseous structures and muscles also can be evaluated. MRI is a sensitive method for assessing the menisci non-invasively. MRI of meniscal injuries of the knee in the dog is not fully understood yet and there are differences in the appearance depending on the field strength of the magnet. However, a meniscal tear usually appears as increased signal that reaches the articular surface within the normally dark menisci. Ligamentous injuries of the knee typically present as abnormally high signal and lack of a normal course for the ligament in question. MRI therefore is an excellent method for evaluation of cranial cruciate ligament tears and associated pathologies of the menisci and underlying bone of the tibial plateau and femoral condyles including for the presence of osteophytes. Bone marrow edema like lesions ("bone bruise") will appear dark on a STIR sequence, is usually masked by the fat in T2 sequences und will appear dark on T1 sequences. They may or may not take up contrast.

Due to its small thickness being in the range of the achievable resolution of the applied MR-sequences, cartilage imaging is still a challenge in veterinary MRI. Whereas in human medicine very high resolution sequences with fat suppression pulses are of great value, the best sequence for cartilage evaluation in veterinary MRI still has to be determined. However, because of its high sensitivity for bone marrow changes, a cartilage lesion can be suspected due to changes of the subchondral bone.

Muscular lesions are readily identified using fat suppression techniques (e.g., STIR). They are preferred to normal SE or FSE T2 sequences because hyperintense lesions may be masked by the hyperintense surrounding fat. In a STIR sequence, the normal muscle will appear dark, a lesion usually will present with high SI. They may or may not take up contrast material; if contrast medium is used, fat-water separation techniques will highlight contrast enhanced lesions. Fat suppression techniques are also helpful in distinguishing fatty degeneration of muscles from other lesion. Fatty degeneration (atrophy) will present with volume loss (usually) and relatively high SI in T1 and T2 sequences. These areas will present with low SI using fat suppression techniques.

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