Introduction

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.

Anesthesia

• Small animals are usually scanned under general anesthesia General anesthesia: overview and are intubated and ventilated with isoflurane or sevoflurane (which are brain-sparing) vaporized in oxygen.
• Muscle relaxants Muscle relaxant: overview 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 Anesthetic monitoring: respiratory system (capnograph), pulse oximetry Anesthetic monitoring: pulse oximetry and blood pressure Blood pressure: Doppler ultrasound assessment.
• If raised intracranial pressure is suspected on clinical grounds, intravenous steroids or mannitol 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.

Scanning procedure

• 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 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.
  • Transverse T2 FLAIR 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.
• In addition to the above standard sequences, many new techniques have been developed to better evaluate parameters including:
  • CSF flow using cine MRI.
  • Enhanced diagnosis and anatomical study of tracts using diffusion-weighted images (DWI) and perfusion-weighted (PWI) techniques.
  • Functional characteristics of neuronal activity (functional MRI).
  • Metabolic characteristics of the CNS using MR spectroscopy.
• A CSF tap Cerebrospinal fluid: sampling 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, brain herniation through the foramen magnum or when large masses are present because of the risk of herniation. Lumbar puncture does not reduce the risks of brain herniation in case of suspected raised intracranial pressure.
• Recovery from anesthesia is usually quick and patients are ready to be reunited with their owners within 30 to 45 minutes of the study.

Normal appearance of the brain

Pathology

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.
• Noh D, Choi S, Choi H et al (2019) Evaluating traumatic brain injury using conventional magnetic resonance imaging and susceptibility-weighted imaging in dogs. J Vet Sci 20 (2), e10 PubMed.
• Beltran E, Platt S R, McConnell J F et al (2014) Prognostic value of early magnetic resonance imaging in dogs after traumatic brain injury: 50 cases. J Vet Intern Med 28 (4), 1256-1262 PubMed.
• Hecht S, Adams W H (2010) MRI of brain disease in veterinary patients part 1: Basic principles and congenital brain disorders. Vet Clin Small Anim North Am 40 (1), 21-38 PubMed.
• Hecht S, Adams W H (2010) MRI of brain disease in veterinary patients part 2: Acquired brain disorders. Vet Clin Small Anim North Am 40 (1), 39-63 PubMed.
• Garosi L, McConnell J F, Platt S R et al (2006) Clinical and topographic magnetic resonance characteristics of suspected brain infarction in 40 dogs. J Vet Intern Med 20 (2), 311-321 PubMed.
• van der Merwe L L & Lane E (2001) Diagnosis of cerebellar cortical degeneration in a Scottish terrier using magnetic resonance imaging. JSAP 42 (8), 409-12 PubMed.
• 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 PubMed.
• Forrest L J (1999) The head: excluding the brain and orbit.​ Clin Tech Small Anim Pract 14 (3), 170-176 PubMed.
• Harkin K R, Goggin J M, DeBey B M et al (1999) Magnetic resonance imaging of the brain of a dog with hereditary polioencephalomyelopathy. JAVMA 214 (9), 1342-1344, 1334 PubMed.
• 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 PubMed.
• Platt S R, Graham J, Chrisman C L et al (1999) Canine intracranial epidermoid cyst. Vet Radiol Ultrasound 40 (5), 454-458 PubMed.
• Targett M P, McInnes E, Dennis R (1999) Magnetic resonance imaging of a medullary dermoid cyst with secondary hydrocephalus in a dog. Vet Radiol Ultrasound 40 (1), 23-26 PubMed.
• Thomas W B (1999) Non-neoplastic disorders of the brain. Clin Tech Small Anim Pract 14 (3), 125-147 PubMed.
• Snellman M, Benczik J, Joensuu R et al (1999) Low-field magnetic resonance imaging of beagle brain with a dedicated receiver coil. Vet Radiol Ultrasound 40 (1), 36-39 PubMed.
• Kuwabara M, Tanaka S, Fujiwara K (1998) Magnetic resonance imaging and histopathology of encephalitis in a Pug. J Vet Med Sci 60 (12), 1353-1355 PubMed.
• Lorenzo V, Pumarola M, Muñoz A (1998) Meningioangiomatosis in a dog: magnetic resonance imaging and neuropathological studies. J Small Anim Pract 39 (10), 486-489 PubMed.
• Ramirez O 3rd, Thrall D E (1998) A review of imaging techniques for canine cauda equina syndrome. Vet Radiol Ultrasound 39 (4), 283-296 PubMed.
• Saunders J H, Poncelet L, Clercx C et al (1998) Probable trigeminal nerve schwannoma in a dog. Vet Radiol Ultrasound 39 (6), 539-542 PubMed.
• Taga A, Taura Y, Nishimoto T 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 PubMed.
• Bayens-Simmond S J, Purcell T P, Nation N P (1997) Use of magnetic resonance imaging in the diagnosis of central vestibular disease. Can Vet J 38 (1), 38 PubMed.
• Kraft S L, Gavin P R, DeHaan C et al (1997) Retrospective review of 50 canine intracranial tumors evaluated by magnetic resonance imaging. J Vet Intern Med 11 (4), 218-225 PubMed.