Radiography has long been the primary method for imaging in veterinary medicine. Although CT and magnetic resonance imaging (MRI) have been available in large specialist and teaching hospitals for decades, they have recently become more readily available in smaller settings due to falling operating costs. In addition, improved technology with less need for radiation protection has given rise to a small number of mobile CT practices across the country. Although radiography will always have a place in veterinary medicine, improved access to advanced imaging modalities will enable better diagnosis and treatment of common diseases. In particular, many diseases of the head, the brain, the spine, the vascular system and the musculoskeletal system can be diagnosed more easily with these advanced modalities.
The compact anatomy and overlay make CT the ideal modality for imaging the head. One of the most common clinical complaints from our internal medicine service is chronic nasal discharge in geriatric patients. Skull x-rays are limited in their ability to provide critical information for a diagnosis, are technically difficult, and require heavy sedation or anesthesia for well-positioned images. Although x-rays can help the clinician identify increased or decreased opacity in the nasal cavity, as well as regional osteolysis, a subtle disorder is often overlooked. Important aspects of the disease such as cribriform plate lysis, nasopharyngeal polyps, or intranasal foreign bodies can be difficult to diagnose with radiography. CT is not only useful for assessing the nasal cavity in these patients, but also provides additional information about the disease of the regional lymph nodes (Figures 1 and 2). In these cases, CT is best used as a complementary method to rhinoscopy with biopsy specimens, both of which are performed during the same anesthetic event.
Similarly, in the case of head trauma, subtle and nondisplaced fractures due to the sum of several bones can be overlooked. In addition, suitable skull x-rays require strong sedation or anesthesia with numerous oblique views. Although CT also requires sedation, images are captured quickly in all planes, with the ability to reconstruct images in inclined planes as well as in three-dimensional volumes.
Both MRI and CT scans can be used in patients who have clinical symptoms related to the brain. Radiography is only used to a very limited extent in these cases, with findings such as calvarial hyperostosis reported in meningiomas.1 MRI has traditionally been more expensive and less readily available than CT, but because of its superior soft tissue contrast and detail, it is ideal modality for most neurological patients.
CT is particularly useful in trauma patients because the rapid image acquisition can minimize sedation. CT can be used to easily identify skull fractures, examine cerebellar hernias, and examine intracranial bleeding (Figure 3). If the cost of MRI is prohibitive, CT can also be used to assess brain neoplasms, particularly those that are typically contrast-enhancing, such as meningiomas.
MRI offers superiority in diagnosing brain neoplasms, inflammatory diseases, and ischemic infarcts. The main disadvantages are the cost and the lack of availability (Figure 4).
Similar to brain imaging, both MRI and CT are useful for imaging the spine (Figure 5). When a special focus on the bony structures is required, e.g. B. in trauma, the CT is superior. CT is also beneficial when there are concerns about a pathological fracture, osteolytic lesions such as discospondylitis, or osteoproliferative lesions such as bone-associated caudal cervical spondylomyelopathy. In addition to standard imaging, CT myelography can in many cases improve the conspicuity of compression lesions and increase diagnostic accuracy.
Although radiographic myelography remains a commonly used method for patients with disc degeneration, its strength is in quickly identifying the location of the compression of the spinal cord rather than diagnosing spinal cord disease. In clinics where it is available, MRI allows assessment of the spinal cord at the compression site. In addition, it can aid in the diagnosis of spinal cord neoplasia, ischemic myelopathy, acute extrusions of the hydrated nucleus pulposus, and congenital abnormalities.
Ultrasound has long been the most common method of assessing abdominal and extremity vessels. Although ultrasound is a non-invasive point-of-care tool for assessing thrombi in the great vasculature (Figure 6) like the abdominal aorta, their use in diagnosing abnormal vessels such as portosystemic shunts is limited by the operator’s experience. In addition, gastrointestinal gases and non-compliant patients can further hinder the identification of abnormal vessels.
CT angiography (CTA) has largely replaced sonography in academic institutions for diagnosing portosystemic shunts (Figure 7). In one study, the sensitivity of CTA in diagnosing portosystemic shunts was 96% compared to only 68% for ultrasound. In addition to better characterizing shunt morphology, CTA was 5.5 times more likely to determine whether a patient had a portosystemic shunt.2 In the past, the need for anesthesia may have deterred clinicians from choosing CTA as the first imaging step Speed of image acquisition with newer machines allows strong sedation to be used instead.
Patients with traumatic fractures, especially after a vehicle trauma, are often present at our rescue service. X-rays are a first-line imaging method in these cases, but patients often have complex fractures or dislocations, particularly of the pelvis. The summation can make it difficult to accurately diagnose these fractures, especially in small bones such as the carpal bones. CT not only makes fractures more noticeable, but three-dimensional models can also be created to facilitate surgical planning.
In addition to traumatic cases, common disease processes such as medial coronoid fragmentation, pathological fractures and deformities of the angled limbs can be diagnosed much more sensitively and accurately with CT compared to radiography (Figure 8).
MRI is not yet widely used in veterinary medicine for musculoskeletal imaging, but its ability to diagnose tendinopathies and desmopathies is unmatched in other modalities. Although MRI lacks the portability and cost-effectiveness of musculoskeletal ultrasound, some deeper structures can be imaged that otherwise cannot be assessed (Figure 9).
- Jeffery ND, Thakkar CH, Yarrow TG. Introduction to Computed Tomography of the Dog’s Brain. J Small animation practice. 1992; 33 (1): 2-10. doi: 10.1111 / j.1748-5827.1992.tb01004.x
- Kim SE, Giglio RF, Reese DJ, Reese SL, Bacon NJ, Ellison GW. Comparison of computed tomography angiography and ultrasound for the detection and characterization of portosystemic shunts in dogs. Vet Radiol Ultrasound. 2013; 54 (6): 569- 574. doi: 10.1111 / vru.12059
Eric Van Eerde, DVM, DACVR, is a 2012 graduate of the University of Florida College of Veterinary Medicine, Gainesville. He has worked as an associate radiologist and director of radiology at BluePearl Specialty and Emergency Pet Hospital in Tampa, Florida.