John S. Mattoon, DVM, DACVR
Professor and Chief of Radiology, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, Pullman, WA, USA
In human medicine, MRI has become a mainstay of orthopedic imaging and there have been rapid advances in abdominal MRI.1During the past 20 years, magnetic resonance imaging (MRI) has firmly established itself as the imaging modality of choice for assessment of veterinary neurological disease in small animals. During the same time, ultrasound has become the preferred imaging modality for abdominal disease for many practices due to its ease of use, availability and safety. With increasing availability in small animal practice, MRI is slowly but most assuredly becoming an important tool for the study non-neurological disease in our small animal practices.
This discussion will share our experience of MR imaging in non-neurological diseases.
Shoulder MRI is usually performed after radiography. Providing baseline and familiar information, radiography provides excellent detail of the bony surface of the greater tubercle, the intertubercular (bicipital) groove, the humeral head and proximal humerus and the periarticular margins of the glenoid. Positive radiographic findings do not negate an MR examination in most instances.
Normal MRI anatomy of the dog shoulder has been reported.2,3 A variety of routine sequences were used (T1, T2, PD, STIR) with sagittal, transverse, and dorsal scan planes. The biceps brachii and tendon sheath, supraspinatus, teres minor are seen best in sagittal and transverse planes; the infraspinatus in all planes; the subscapularis in transverse and dorsal planes; the capsuloligamentous complexes best seen in transverse and dorsal planes. Extending the limb increases conspicuity of the medial capsuloligamentous structures. MRI arthrography with the joint in extension has shown particular value in increasing visibility of the biceps tendon and sheath and essentially all of the structures of the shoulder.3,4
Disease processes that can be diagnosed include injuries to the biceps, supraspinatus, infraspinatus, and subscapularis tendons, the medial and lateral transverse humeral ligaments, and biceps tendon impingement.5 The changes are characterized by enlargement of these structures, abnormally high signal in them, and excessive bicipital tendon sheath and scapulohumeral joint fluid. Of course, MR can be used to evaluate the extent of bone or soft tissue pathology as with neoplasia or inflammatory disease.
A very recent report has described the normal MRI anatomic features of the dog elbow.6 All musculoskeletal structures could be identified. Flexor tendons were best seen in sagittal and dorsal planes, the flexor carpi ulnaris tendon best on transverse images. Extensor tendons and the lateral collateral ligament were seen on transverse and dorsal planes while the biceps brachii and triceps were best visualized on sagittal images. T1-weighted gave the best anatomic detail while T2 images were best for synovial cavities. Sagittal, transverse, and dorsal image planes were investigated.
MR examination of the stifle is probably the best diagnostic test available for assessment of intraarticular disease and there is relatively abundant literature on its use in dogs.7-10 CT arthrography has been successfully investigated and used.11 However, CT arthrography is more invasive due to the arthrography component. The biggest limitation to MRI is the relatively small size of dog (and cat) stifle and intraarticular ligaments and menisci. It is difficult, sometime the luck of the draw, to obtain adequate scan planes through the cruciate ligaments. In some cases, only one diagnostic slice is obtained, sometimes none. The other limitation is that we are limited in our ability to directly assess articular cartilage, often relying on indirect subchondral bony signal abnormalities to predict damage.
Sagittal, dorsal and transverse planes are obtained with T1 and T2 sequences include T1 and SPIR or STIR sequences. Intravenous contrast is usually not administered but intraarticular contrast has been used.
Stifle joint pathology is recognized by disruption of the cruciate ligaments, abnormal and excessive fluid accumulation, collateral ligaments and periligamentous thickening, meniscal tears, subchondral sclerosis and/or abnormally high signal. Of course, periarticular osteophytosis is seen, but this is perhaps better evaluated on pre-MRI radiographic examinations.
Carpus and Tarsus
There are two excellent papers describing the normal dog carpus.12,13 Carpal ligaments, the radioulnar articular disc, palmar fibrocartilage, and the accessorio-quartile ligament were identified. T2* weighted gradient echo imaging technique was found to provide the best images because thinner slices were possible (1 mm) vs. T1-weighted 3 mm slices.12 As mentioned, in our institution, CT is preferred over MRI for the dog and cat carpus and tarsus.
Neck and Trunk Soft Tissues
MRI is valuable in assessment of cervical tumors. It allows excellent visualization of tumor extent, surrounding vascular involvement, and local and regional lymph nodes. Paravertebral abscesses have been successfully imaged using MRI, with identification of the course of the fistulous tract(s).
MR has been a very effective imaging modality for accurate assessment of adrenal gland size and shape and identification of pathology14,15, often incidentally16. It is also valuable for evaluation of phrenicoabdominal and caudal vena cava invasion in cases of adrenal gland neoplasia, information essential for proper surgical planning. A recent study has shown MRI to be more accurate than ultrasound in determining adrenal gland size and shape.17
MRI has been used in the diagnosis and characterization of portosystemic shunts (PSS).
Time-of-flight MR angiography (TOF-MRA) and contrast-enhanced MRA (CE-MRA) protocols have been used with good success.18-20 A recent report describes the use of MRI to assess liver size before and after attenuation of extrahepatic portosystemic shunts.21 In our recent experience using multidetector CT, CT angiography has become our imaging modality of choice for assessment of portosystemic shunts.
Liver and Spleen
Many times during spinal MR imaging, liver and splenic nodules are encountered unexpectedly and ultrasound is used to obtain fine needle aspirates or biopsies. We have not used MR for routine investigation of hepatic or splenic disease. One report describes MRI of focal splenic and hepatic lesions in dogs22 where a 94% overall accuracy in differentiating malignant from benign lesions was found.
A few publications describe thoracic and cardiac MRI anatomy of the dog.23-25 We have little experience with dedicated thoracic MRI.
MRI for Radiation Therapy Planning
Cancer is routinely scanned to assess tumor margin and regional nodal involvement for radiation therapy planning.
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3. Agnello KA, Puchalski SM, Wisner ER et al. Effect of positioning, scan plane, and arthrography on visibility of periarticular canine shoulder soft tissue structures on magnetic resonance images. Vet Radiol Ultrasound 2008;49(6):529–539.
4. Schaefer SL, Baumel CA, Gerbig JR, Forrest LJ. Direct magnetic resonance arthrography of the canine shoulder Vet Radiol Ultrasound 2010;51(4):391–396.
5. Murphy Se, Ballegeer EA, Forrest LJ, Schaefer SL. Magnetic resonance imaging findings in dogs with confirmed shoulder pathology. Vet Surg 2008;37(7):631–638.
6. Baeumlin Y, De Rycke L, Van Caelenberg A, et al. Magnetic resonance imaging of the canine elbow: an anatomic study. Vet Surg 2010;39(5):566–573.
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9. Pujol E, Van Bree H, Cauzinille L, et al. Anatomic study of the canine stifle using low-field magnetic resonance imaging (MRI) and MRI arthrography. Vet Surg 2011;40(4): 395–401.
10. Barrett E, Barr F, Owen M, Bradley K. A retrospective study of the MRI findings in 18 dogs with stifle injuries. J Small Anim Pract 2009;50(9):448–455.
11. Samii VF, Dyce J, Pozzi A, et al. Computed tomography arthrography of the stifle for the detection of cranial and caudal cruciate ligament and meniscal tears in dogs. Vet Radiol Ultrasound 2009;50(2):144–150.
12. Nordberg CC, Johnson KA. Magnetic resonance imaging of normal canine carpal ligaments. Vet Radiol Ultrasound 1999;40(2):128–136.
13. Ober CP, Freeman LE. Computed tomographic, magnetic resonance imaging, and cross-sectional anatomic features of the manus in cadavers of dogs without forelimb disease. Am J Vet Res 2009;70(12):1450–1458.
14. Llabres-Diaz FJ, Dennis R. Magnetic resonance imaging of the presumed normal canine adrenal glands. Vet Radiol Ultrasound 2003;44(1):5–19.
15. Apall B, Chen AV, Tucker, et al. Imaging diagnosis-metastatic adrenal pheochromocytoma in a dog. Vet Radiol Ultrasound (Epub ahead of print). 2011.
16. Myers NC. Adrenal incidentalomas. Diagnostic workup of the incidentally discovered adrenal mass. Vet Clin North Am Small Anim Pract 1997;27(2):381–399.
17. Hayles JLF, Tucker RL, Gay JM. Comparison of adrenal gland dimensions and volume with MRI and ultrasound in 7 cadaver dogs. Proceedings American College of Veterinary Radiology Annual Scientific Meeting, 2010:68.
18. Seguin B, Tobias KM, Gavin PR, Tucker RL. Use of magnetic resonance angiography for diagnosis of portosystemic shunts in dogs. Vet Radiol Ultrasound 1999;40(3):251–258.
19. Breuhschwein A, Foltin I, Flatz K, et al. Contrast-enhanced magnetic resonance angiography for diagnosis of portosystemic shunts in 10 dogs. Vet Radiol Ultrasound 2010;51(2):116–121.
20. Schwarz T, Rossi F, Wray JD, et al. Computed tomographic and magnetic resonance imaging features of canine segmental caudal vena aplasia. J Small Anim Pract 2009;50(7):341–349.
21. Kummeling A, Vrakking DJ, Rothuizen J, et al. Hepatic volume measurements in dogs with extrahepatic congenital portosystemic shunts before and after surgical attenuation. J Vet Intern Med 2010;24(1):114–119.
22. Clifford CA, Pretorius ES, Weisse C, et al. Magnetic resonance imaging of focal splenic and hepatic lesions in the dog. J Vet Intern Med 2004;18(3):330–338.
23. Vilar JM, Arencibia A, Ramirez JA, et al. Magnetic resonance imaging of the thorax in three dogs. Vet Rec 2003;153(18):566–568.
24. Contreras S, Vazquez JM, Miguel AD, et al. Magnetic resonance angiography of the normal canine heart and associated blood vessels. Vet J 2008;178(1):130–132.
25. Contreras S, Arencibia A, Gil F, et al. Black and bright-blood sequences magnetic resonance angiography and gross section of the canine thorax: an anatomical study. Vet J 2010;185(2):231–234.