In recent years, there have been significant strides in the development of and access to advanced imaging in veterinary medicine. Techniques such as magnetic resonance imaging (MRI), computed tomography (CT) and fluoroscopy were once the preserve of human medical establishments or research facilities. However, several factors, namely:
- a rising awareness amongst veterinarians and clients as to the results these modalities can deliver,
- a wider uptake of pet insurance to ultimately fund private investment in these technologies, and
- on-going development of both hardware and software, resulting in falling equipment and maintenance cost, have all contributed to increased availability, where the majority of referral practices and many large veterinary groups have on-site equipment, or at least regular access to one or more of these techniques.
Given the inherent limitations of each technique, it is useful to have a broad understanding of the indications and disadvantages of each, so as to offer timely advice to your clients as to how further assessments might proceed, whilst making best use of available funds in each case; in many instances, radiography and ultrasonography will suffice, but these techniques may fail to yield useful information in certain circumstances and might even delay timely referral or possibly exacerbate an unstable patient – it is not unheard of for animals to present with grade V spinal lesions following radiology under general anaesthesia as part of initial assessment for spinal pain or weakness.
MRI has established itself as the gold-standard in human medicine for the assessment of the neurological patient. The only exception to this rule is peracute presentation of the head trauma patient or vascular accidents where CT is used for urgent assessment prior to surgery. The principle reason for the primacy of MRI is its ability to chemically analyse target tissues to create visible contrast between physiologically distinct areas of parenchyma which might otherwise lack contrast when relying on density alone (CT). In addition, the capacity of MRI to look inside the bony box of the skull, neural canal and pelvis lends itself to CNS imaging readily. More sophisticated MRI set-ups are also gated to negate motion artefacts produced by the heart and respiratory muscles, thus expanding the repertoire of body-systems it can assess.
The main indications for MRI in veterinary medicine are:
Seizuring/ Other Intracranial Signs
Spinal pain or Proprioceptive Deficits
Figures left sagittal, and right transverse, are T2W slices from an 8yo Rhodesian Ridgeback which had become per-acutely tetraplegic after colliding with a tree; the scans reveal cord contusion over the Ce4-5 disc, but no on-going compression; this patient was managed successfully with conservative management.
The figure left, is a post-contrast sagittal slice of a lumbar spine in a 12 year old springer spaniel with 5 month history of progressive pelvic limb weakness and caudal lumbar pain. Radiology had revealed lysis of the end plates in the lumbosacral junction consistent with disco spondyltitis -confirmed with MR-scans. However, there is also an unrelated, enhancing mass dorsal to L4. Transverse slices through this level revealed a malignant process involving the neural arch and extending into the neural canal – see figure, right .
MRI is also indispensible for assessing the extent of cord damage secondary to surgical lesions as well as other spinal cord parenchymal pathology, such as syringomyelia, myelitis and subarachnoid cysts.
These include lameness due to nerve-root pathology. The figure, right is a transverse slice of a STIR-weighted MR-scan of an 8 year old Lhasa Apso with chronic left thoracic limb lameness. Radiology was unremarkable. There is irregular enlargement and inflammation of the left, Ce6 nerve root consistent with malignant peripheral nerve sheath tumour..
.Developmental abnormalities such as incomplete ossification of the humeral condyle (IOHC) can often be seen more clearly with MRI; the case left shows a craniocaudal radiograph of the left elbow in a chronically lame springer spaniel. There is a suspicion of reduced bone density across the condyle. STIR-weighted MR scans of same joint confirms the diagnosis of IOHC. The contralateral elbow is also shown right, for comparison.
Muscles, tendons and ligaments/ joint capsule may also be assessed in more detail should conventional imaging modalities prove inconclusive. High resolution MRI may also be used to assess articular cartilage integrity without recourse to arthroscopy.
MRI has been shown to be a very sensitive tool for detecting primary bone pathology, including bruising or remodeling (see the lateral epicondyle in IOHC case above) in response to mechanical stress. It is also very sensitive for screening for primary bone neoplasia and metastatic disease within the musculoskeletal system; the figure left, is a survey radiograph of the humerous of a six-year-old Flat-coat Retriever with left axillary swelling. STIR MR-scans right, show bony and involvement of the surrounding soft tissues. Histological diagnosis was histiocytic sarcoma.
MRI has revolutionized pre-operative staging of many forms of neoplasia by clearly delineating the primary mass and the extent of local spread; inoperable tumors may be identified early on, thus sparing patients from unnecessary surgery and their owners the associated expense. This modality is especially useful with prostatic and other intrapelvic masses which can be challenging to image otherwise. The figure right, is a sagittal T2W slice of the pelvis of 10-year-old female, neutered Labrador Retriever presenting with a vulval discharge of one year’s duration. Scan shows a large, cavitating mass arising from the wall of the vagina to compress the large bowel. The mass was removed via episiotomy and found to be a leiomyoma. The patient made an unremarkable recovery.
Foreign bodies (e.g. grass awns and fragments from pharyngeal stick injury) may be more easily identified and located with MRI, thus minimizing surgical risk and morbidity. The figures left, parasagittal and right, transverse are T2W slices of the left pharyngeal region of a six-year-old female, neutered Cocker Spaniel presenting with left cervical swelling six months after pharyngeal stick injury. Scans reveal signal voids corresponding to wooden fragments within an irregular, fluid-filled space at the confluence of the linguofacial and maxillary veins. Two wooden fragments were retrieved at surgery.
Nasal and middle ear pathology can also be reliably assessed with this imaging modality prior to definitive diagnosis with targeted biopsies. The three figures shown here are all dogs presenting with nasal discharge. Images are transverse, T1W slices, mid nasal cavity.
The first patient left, reveals irregular soft tissue masses throughout both sides of the nasal cavity. Note that there is no septal deviation and no evidence of turbinate destruction. Histological diagnosis was confirmed as nasopharyngeal polyps.
The next patient right, shows a different pattern of pathology; there is significant, uniform destruction of the turbinates creating large areas of dead space. Diagnosis was confirmed on biopsy as Aspergillus infection and treated successfully with clotrimazole instillation.
The last patient left, shows a third distinctive pattern of pathology; there is an expansile mass within the right nasal cavity which has resulted in deviation and erosion of the septum with local spread to the left side. These changes are consistent with a malignant process and the diagnosis was confirmed histologically as nasal adenocarcinoma.
We have also used MRI for non-invasive characterization of porto-systemic shunting, particularly useful where the results of ultrasound are inconclusive. The figure left, is a dorsal, T1W slice at level of the caudal vena cava; there is a large vessel emptying into the vena cava cranial to the right renal vein and caudal to the liver; diagnosis – single, extra-hepatic portosystemic shunt. This case was successfully managed by placement of an ameroid constrictor around the vessel.
Computed tomography has the obvious advantage over MRI of lower start-up costs. It provides exquisite detail of bony structures (see figures, sagittal, left and transverse, right CT-slices of skull of a dog presenting with bony swelling – diagnosis: Multilobular osteosarcoma).
CT is often used with 3D-rendering software in human medicine for analysis of complex fractures prior to anatomical reconstruction.
In veterinary medicine, CT is particularly useful in screening for pulmonary, mediastinal or abdominal metastatic disease.
Given acquisition times are relatively short, CT has also been advocated for use in assessing conditions affecting the brain, spinal cord, and associated nerve roots. However, images lack the inherent physiological contrast of the tissues which differing MRI-sequences provide and so enable maximum information to be gained non-invasively for optimal treatment planning.
Much use is made of fluoroscopic-guided placement of orthopaedic implants in human medicine. This modality is also used routinely in people to assess cardiovascular abnormalities. These techniques are also gaining in popularity in the veterinary sphere also. However, the commonest indication for fluoroscopy in our patients remains the real-time assessment of the airways and alimentary canal. Therapeutically, fluoroscopy is also employed with minimally invasive implantation of pacemakers, embolisation coils (for PDAs, porto-systemic shunts, etc) and stents (e.g. urethral/ ureteric, collapsing trachea). Concerns remain over ionising radiation for both the patient and operator/ surgeon, but is thought less relevant in our context, given the relatively short lifespan of those under our care.