Magnetic resonance imaging: brain

Introduction

• Must be carried out with the patient immobilized using deep sedation or general anesthetic.
• Anesthetic and monitoring equipment must be:
  Either Non-ferrous and MRI-compatible.
  Or Operated at a sufficient distance from the scanner such that the scanner's fringe magnetic field does not interfere with the equipment nor attract it towards the scanner.
• Even tiny pieces of non-ferrous metal will distort the homogeneity of the magnetic field resulting in grossly-distorted images.
  All ferrous equipment must be excluded from the same area and collars must be removed before the procedure
• Identity chips in the neck do not cause interference with the images and are not erased by the magnetic field.

• Small animals are usually scanned under general anesthesia and are intubated and ventilated with isoflurane (which is brain-sparing) vaporized in oxygen.
• Muscle relaxants are not used but the ventilation (provided by a medical non-ferrous system) is well tolerated and ensures an even plane of anesthesia.
  Essential if the patient is to keep still for long periods and not be disturbed by the knocking noise created by the gradient coils during scanning
• Monitoring is performed by means of capnography and blood pressure assessment.
• If raised intracranial pressure is suspected on clinical grounds, intravenous steroids or mannitol may be given prior to induction of anesthesia.
• The morbidity rate is remarkably low given the severe nature of brain disease subsequently diagnosed in some of the patients.
• The animal is anaesthetised in the adjacent preparation room and moved to the scanner's handling table on a non-ferrous trolley.

• For brain scans, the patient is positioned in sternal recumbency with its head in one of the cylindrical RF coils, the smallest coil possible being used for greatest image definition .
• After an initial procedure during which the coil is " tuned" to reflect its loading by the patient, three single slice " pilot" or " scout" scans are obtained in the transverse, sagittal and dorsal planes.
• Over these are series of lines are placed on the computer monitor, indicating the location, orientation, number and thickness of the slices in the intended scan.
• A standard protocol for brains scans might include:
  • Transverse T1W scan.
  • Transverse T2W scan.
  • Injection of intravenous contrast medium (gadodiamide - OMNISCAN; Nycomed UK Ltd.) at the recommended human dose rate of 1 ml/5 kg BW.
  • Repeat transverse T1W scan to look for abnormal areas of enhancement.
  • Sagittal and/or dorsal T1W scans - both if a lesion is visible.
• The T1W scans are always shorter than the T2W scans and with the current system these take 5 minutes and 13 minutes respectively.
• Total scanning time is usually about 45 minutes.
• A CSF tap is performed after the scan if the brain has appeared normal or if there is a suggestion of inflammatory disease.
• CSF removal is not advised in cases of presumed brain swelling or when large masses are present because of the risk of herniation, although lumbar puncture may be safer than cisternal puncture.
• Recovery from anesthesia is usually quick and patients are ready to be reunited with their owners within 30 minutes of the study.

Sources

Publications

Refereed papers

• Garosi L, McConnell J F, Platt S R, Barone G, Baron J C, de Lahunta A & Schatzberg S J (2006) Clinical and topographic magnetic resonance characteristics of suspected brain infarction in 40 dogs. J Vet Intern Med 20 , 311-321 PubMed.
• van der Merwe L L & Lane E (2001) Diagnosis of cerebellar cortical degeneration in a Scottish terrier using MRI. JSAP 42 , 403-408.
• Klopp L S, Hathcock J T & Sorjonen D C (2000) Magnetic resonance imaging features of brain stem abscessation in two cats. Vet Rad Ultra 41 (4), 300-307.
• Platt S R et al(1999) Canine intracranial epidermoid cyst. Vet Radiol Ultrasound 40 (5), 454-458.
• Le Couteur R A (1999) Current concepts in the diagnosis and treatment of brain tumors in dogs and cats. J Small Anim Pract 40 (9), 411-416.
• Forrest L J (1999) The head. Clin Tech Small Anim Pract 14 (3), 170-176.
• Thomas W B (1999) Non-neoplastic disorders of the brain. Clin Tech Small Anim Pract 14 (3), 125-147.
• Harkin K R et al(1999) Magnetic resonance imaging of the brain of a dog with hereditary polioencephalomyelopathy. JAVMA 214 (9), 1342-1344, 1334.
• Snellman M et al(1999) Low-field magnetic resonance imaging of beagle brain with a dedicated receiver coil. Vet Radiol Ultrasound 40 (1), 36-39.
• Targett M P et al(1999) Magnetic resonance imaging of a medullary dermoid cyst with secondary hydrocephalus in a dog. Vet Radiol Ultrasound 40 (1), 23-26.
• Kuwabara M et al(1998) Magnetic resonance imaging and histopathology of encephalitis in a Pug. J Vet Med Sci 60 (12), 1353-1355.
• Taga A et al(1998) The advantage of magnetic resonance imaging in diagnosis of cauda equina syndrome in dogs. J Vet Med Sci 60 (12), 1345-1348.
• Saunders J H et al(1998) Probable trigeminal nerve schwannoma in a dog. Vet Radiol Ultrasound 39 (6), 539-542.
• Lorenzo V et al(1998) Meningioangiomatosis in a dog - magnetic resonance imaging and neuropathological studies. J Small Anim Pract 39 (10), 486-489.
• Ramirez O III et al(1998) A review of imaging techniques for canine cauda equina syndrome. Vet Radiol Ultrasound 39 (4), 283-296.
• Kraft S L et al(1997) Retrospective review of 50 canine intracranial tumors evaluated by magnetic resonance imaging. J Vet Intern Med 11 (4), 218-225.
• Bayens-Simmond S J et al(1997) Use of magnetic resonance imaging in the diagnosis of central vestibular disease. Can Vet J 38 (1), 38.