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Review Article: Magnetic Resonance Imaging – An Overview of the Theory and Applications in Small Animal Practice.

The concept of the Magnetic Resonance Image (MRI) is a chemical analysis of constituent body tissues, displayed as a grey-scale in an anatomical context. It uses a combination of magnetic fields and radiofrequency signals to map hydrogen nuclei within the tissues.  The MRI scanner contains a powerful magnet, which causes the protons to align and spin around the long axis of the magnetic field. Radiofrequency pulses are then produced to ‘knock’ the aligned protons off this axis, which subsequently emit a measurable signal as they ‘relax’ back into position.  The characteristics of the signal emitted relate to the location of the proton and its immediate chemical environment (which influences the rate of  said relaxation) within the tissue.  The image takes several minutes to acquire, and as any movement blur can render the image non-diagnostic, general anaesthesia is normally required.

Different radiofrequency pulse sequences can be used to highlight tissues in different ways. For example a ‘T1-weighted’ (T1W) scan shows free fluid as dark grey, whereas ‘T2-weighted’ (T2W) scans shows fluid as white. Fat appears bright with both weightings. Most acute pathological processes will lower the fat content and raise the water content, so the difference between the T1 and T2-weighted images – known as an ‘irritation pattern’- will often highlight areas of interest.

Additional processing of T2-weighted scans can be done to suppress the fat-signal, to produce a ‘STIR’ image (Short Tau Inversion Recovery – especially useful in identifying nerve-root and bone pathology).  Alternatively, 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.

fig.1 - post-gadolinium T1W sagittal MRI scan of a hypophyseal tumour in a cat - bright area indicates uptake of contrast.

Another useful technique is to use contrast enhancement.  Agents containing gadolinium salts are normally used as these are inherently paramagnetic.  They are administered as an intravenous injection, when the salts rapidly accumulate in tissues with abnormal capillary flow.  More specifically, they are used to identify breakdown of the blood/ brain barrier and are therefore primarily indicated in cases with suspect neoplasia or localised infectious/ inflammatory processes within the CNS (see fig.1).

How is an MRI scan performed?

fig.2 - scanner located within radio-frequency shielded room.

In the vast majority of cases, general anaesthesia is required to correctly position the patient and prevent movement blur. The scanner is located within a radiofrequency-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 (fig.2).  A low resolution, real-time scan is initially performed to aid with positioning of the animal. A ‘scout’-scan is then performed in three 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 for a typical series, may take 30mins to an hour.  With low-field units (<0.2T) such at the one currently in use here, the effective field-of-view is smaller than that of scanners with higher field-strength magnets.  This may necessitate repositioning the animal between scans to survey the entire area of interest (e.g.the spine).  There are also physical size limitations with low-field units; the region of interest must fit between the poles of the magnet, and this effectively means giant breeds may need to be scanned in a mid- (0.2-0.5T) or high-field scanner if the relevant body part of interest does not fit.  However, the low-field unit can adequately image approximately 95% of our current caseload here at Downs.  The images are produced in digital format, which can be burnt to CD, e-mailed, or printed on X-ray viewer compatable film.

Clinical Indications

Essential:

  • Suspected intra-cranial space-occupying lesion
  • Suspected raised intra-cranial pressure (especially prior to CSF tap)
  • Head trauma

Important:

  • Epilepsy (particularly if unusual signalment/ poor response to therapy)
  • Inflammatory and congenital brain disease
  • Spinal disease (is the only reliable method of assessing lumbosacral syndrome/ other foraminal disease)
  • Chronic nasal discharge
  • Surgical planning/ feasibilty for oncological resections
  • Middle/inner ear disease
  • Stick injuries/ suspected foreign body localisation
  • Orbital disease

Useful:

  • Various orthopaedic conditions (e.g. myositis, tendinopathy, osteomyelitis/ bone-sequestra, incomplete ossification of the humeral condyle, tarsal osteochondritis dissecans, suspected cruciate disease/ meniscal tear)
  • Renal disease
  • Hepatic disease
  • Vascular disease (e.g. portosystemic shunts, embolic disease)
  • Various soft tissue conditions (e.g. prostatic cyst/ abscess/ neoplasia/ congenital dysplasia such as branchial cleft retention cyst)

In summary, the most common indications at Downs Veterinary Referrals are the diagnosis of intracranial disease, spinal disease, soft-tissue oncology 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 (see again fig.1).

While relatively uncommon in small animals, the incidence of CNS neoplasia has probably been under-recognised historically, due to lack of appropriate imaging facilities. In some cases, cure or long term remission can be achieved by surgical removal or ‘debulking’.  Many intraxial masses will also respond to radiotherapy, at least in the short-to-medium term.  In addition, we have found that many clients appreciate a definitive diagnosis and 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.

Clinical signs associated with intracranial space occupying lesions.

Behavioural changes

Temperament changes

Seizures

Altered mental status

Circling

Gait abnormalities

Proprioceptive deficits

Locomotor disturbances

Blindness

Cranial nerve deficits (e.g. strabismus, anisocoria, loss of facial sensation or muscle tone).

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.

Nasal disease

fig.3 - transverse T2W slice of muzzle showing left-sided nasal tumour (fibrosarcoma) with metastatic disease adjacent to left mandible

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.  Endoscopy may 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

fig.4 - transverse T2W scan of orbit. There is an irritation pattern dorsal to the right globe with a signal void (stick foreign body) at the centre

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 (fig.4).  .

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Spinal disease

fig.5 - hansen type I Ce4-5 disc extrusion to compress overlying spinal cord

fig.6 - sagittal T1W scan of lumbar spine showing compressive mass (vertebral body neoplasia) encircling spinal cord

Imaging of spinal disease has traditionally relied upon radiography and in particular, 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 (1:1000). 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 parenchymal changes.  MRI in comparison, allows rapid localisation and characterisation of spinal lesions, with negligible risks.

ENT/Soft Tissues

fig.7 - transverse T2W scan of head showing large, fluid-filled branchial cleft retention cyst to left of pharynx

fig.8 - Masticatory myositis: T2W transverse slice of head showing increased signal in temporalis muscles bilaterally.

MRI may be useful in characterising hepatic, splenic, renal and pelvic lesions with accuracy. The difficulty with this is the time required to achieve a scan, which can mean that abdominal movement lead 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.

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Orthopaedics

CT is commonly cited as a preferable imaging modality, due to its inherently superior resolution of fine bony detail.  However MRI may be useful for specific conditions, especially those involving soft-tissue structure or abnormal bone metabolism (neoplasia) – see figs. 9 & 10

fig.10 - plain radiograph of distal femur

fig.9 - STIR scan of distal femour showing abnormal high signal within marrow (occult neoplasia)