VETERINARY MRI

Our dedicated veterinary MRI scanner is now fully open.

As a fully integrated and permanently staffed facility, the scanner forms a central part of our mission: to bring a comprehensive referral service to the Southwest region, which can take cases quickly and efficiently, thus minimising delay for your clients’ animals.   All staff here have undergone extensive training, both on-site with Easote imaging experts, and alongside human radiologists at the Oxford MRI Centre, John Radcliffe Hospital, Oxford.  This enables us to offer an advanced imaging service which can safely and reliably investigate and treat cases with the minimum of delay.

  We also have the backing of eminent neuroradiologists in both the human and veterinary fields who are available for interpretation of non-routine cases - especially important in a field where the knowledge-base is rapidly expanding all the time.

  As with all our other services, the extent of your input is tailored to your requirements, with the option of arranging the imaging alone, or having us manage the case from the initial consultation to a full investigation and appropriate medical or surgical therapy.

Magnetic Resonance Imaging -

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 anaesthesia, or at least sedation is required for the majority of cases.

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 or foreign body localisation), 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.

How is an MR-scan performed?

The Scanner is located within a radio-shielded, internal room with an integral viewing window.  The anaesthetic gas-tubing and monitoring equipment run through wave-guides in the walls of this room to the anaesthetic machine.

A low resolution, real-time scan is initially performed to aid with positioning of the animal. A ‘scout’-scan is then performed in sagittal, dorsal and transverse planes to aid orientation and confirm that the area of interest is centralised within the magnetic field. A number of high resolution scans are then performed, which typically take from 30mins to an hour.  With low-field units such as ours, the effective field-of-view is smaller than that of a high-field system.  This may necessitate repositioning the animal between scans to survey the entire spine. However, in the majority of cases the clinical assessment, including neurological examination, should localise the lesion to allow for rapid reporting.  There are also physical size limitations with a low field magnet; the region of interest must fit between the poles of the magnet, and this effectively means any animal over 100kgs may need to be scanned in a high-field scanner, depending on the body part to be imaged.

The images are produced in digital format, which can be burnt to CD, e-mailed, or printed on x-ray type film to be viewed on a conventional lightbox.

What are the indications for use?

Essential:

·                 Suspected intra-cranial space occupying lesion

·                 Suspected raised intra-cranial pressure                            (especially prior to CSF tap)

·                 Head trauma

Important:

·                 Epilepsy

·                 Inflammatory and congenital brain disease

·                 Spinal disease

·                 Chronic nasal discharge

·                 Pre-operative assessment of tumour margins

·                 Middle/inner ear disease

·                 Stick injuries/ suspected foreign body localisation

·                 Orbital disease

Useful:

·                 Various orthopaedic conditions

            e.g. suspected cruciate disease/ meniscal tear

·                 Renal disease

·                 Hepatic disease

·                 Various soft tissue conditions

  e.g. prostatic cyst/ abscess/ neoplasia

The most commonly used applications are for diagnosis of intracranial disease, spinal disease and nasal disease.

Intracranial disease

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 tumours.

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.

Epilepsy

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, fitting or seizures, irrespective of age would receive a brain scan to rule out underlying disease as part of a thorough work up.

Nasal disease

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 endoscopy can be hampered by lack of access to the entire nasal cavity and a field of view impeded by debris, discharge and haemorrhage.               

Historically, many cases of intra-nasal 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 tumours, foreign bodies and aspergillosis.

Orbital disease

In one study of 25 animals with orbital disease, MRI alone produced an accurate diagnosis in 22 cases. This was superior to radiography, which was only helpful in cases in which neoplastic disease extends markedly beyond the orbit, and ultrasonography, which gave 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.

 Spinal disease

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.  It is associated with significant risks, including iatrogenic cord/ nerve-root trauma, seizures and occasional fatalities. Interpretation may be compromised if cord swelling is present (as is often the case with acute disc disease). It may not be able to accuarately differentiate masses within the neural canal, and doesn’t provide any information about the cord parenchyma.  MRI in comparison, allows rapid localisation and characterisation of spinal lesions, with negligible risks.

Other Soft Tissues  

MRI shows some promise in characterising hepatic, splenic, renal and pelvic lesions with some accuracy. The difficulty with this is the time required to achieve a scan, which can mean that abdominal movement leads to respiratory blur. This is less of a problem for kidneys/ pelvic viscera, and more of problem for the spleen. Hyperventilating the animal can lead to breath holding sufficient to allow time for a scan.

Send mail to downsvets@btconnect.com with questions or comments about this web site.
Copyright © 2003 The Downs Veterinary Practice
Last modified: March 21, 2005