An Overview of the Theory and Applications for Small Animal Veterinary Practice
What is it?
The concept of the MR image is a chemical analysis of constituent body tissues, displayed as a grey-scale in anatomical context. It uses a combination of magnetic fields and radiofrequency signals to map hydrogen nuclei. The MR-scanner contains a powerful magnet, which aligns the protons and causes them to spin around the long axis of the magnetic field. Radiofrequency bombardment causes the nuclei to emit a signal which is related to position and chemical environment within the tissues. The image takes several minutes to acquire, and any movement blur can render the image non-diagnostic, so general anesthesia is required.
Different radiofrequency pulse sequences can be used to highlight tissues in different ways. For example, a “T1-weighted” scan shows fluid as dark, and a “T2-weighted” scan shows fluid as bright. Fat will appear bright in both scans. Most pathological processes will lower the fat content and raise the water content, so comparing the T1 and T2 weighted scans (known as an irritation pattern) will highlight this.
Additional processing of T2-weighted scans can be done to suppress the fat-signal, to produce a “STIR” image (Short Tau Inversion Recovery – useful in identifying vertebral body pathology), and suppression of the water-signal produces a “FLAIR” image (Fluid Attenuated Inversion Recovery – useful for identifying pathology in or near the ventricles in CNS studies). Scans can also be run to provide 3-D/high resolution protocols for examination of small areas of tissue in greater detail.
Another useful technique is to use contrast enhancement. Gadolinium injection identifies a breakdown in the blood/brain barrier and abnormal capillary flow and so helps identify neoplasia or acute infectious processes.
What are the indications for use?
Imaging of spinal disease has traditionally relied upon plain radiography and almost invariably, myelography. The latter can be a time consuming and technically demanding procedure for some specialists with some risk of post study complications (e.g. seizures) or non-diagnostic studies (e.g. due to diffuse cord swelling). MRI allows for rapid localization and characterization of spinal lesions, with negligible risks.
MRI is currently the best way of imaging intracranial disease, as it is not impeded by the bone of the cranial vault and it provides superior detail of soft tissue structures compared with CT-scans. This makes it invaluable for the diagnosis of brain tumors.
While relatively uncommon in small animals, the incidence of CNS neoplasia has probably been under-diagnosed historically, due to lack of appropriate imaging facilities. In some cases, long term remission can be achieved by surgical removal/debulking or radiotherapy. In addition, we have found that many clients appreciate a definitive diagnosis, and thus accurate prognosis in such cases, even if they elect not to proceed with further treatment.
Furthermore, ruling out intracranial space occupying lesions early can help direct other diagnostic and therapeutic efforts.
Primary epilepsy usually starts early in life, but can occur at any age. Given that late-onset epilepsy is also treatable with anti-convulsants, it is important to rule out neoplastic/inflammatory disease in the older patient. Ideally, all cases presenting with altered mentation or seizures, irrespective of age, would receive a brain scan to rule out underlying disease as part of a thorough work-up.
Radiography and endoscopy with cytology or biopsy are currently the mainstay of diagnosis in chronic nasal discharge, epistaxis, and sneezing. However, cytology has a low sensitivity for intra-nasal neoplasia; biopsies may be non-diagnostic (often due to non-representative sampling of associated inflammatory tissue); radiography is non-specific and does not delineate soft tissue masses clearly; and rhinoscopy can be hampered by lack of access to the entire nasal cavity and a field of view impeded by debris, discharge, and hemorrhage.
Historically, many cases of intranasal disease have only been diagnosed by exploratory rhinotomy – inherently an invasive procedure. MRI will evaluate the nature of abnormal soft tissue and determine the extent of the lesion, including any involvement of adjacent structures. It is therefore useful in the diagnosis of tumors, foreign bodies, and fungal disease.
MRI is superior to radiography, which is only helpful in cases where neoplastic disease extends markedly beyond the orbit, and ultrasonography, which can give both false positive and false negative diagnoses for neoplastic masses. MRI is recommended for patients in which radiography and ultrasonography fail to produce a confident diagnosis, or for which surgery is proposed.
Other Soft Tissues
MRI shows some promise in characterizing hepatic, splenic, renal, and pelvic lesions. The difficulty with this is the time required to achieve a scan, which can mean that abdominal movement leads to respiratory blur. As new protocols are devised, such complications may be avoided.