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Magnetic resonance imaging: brain

ISSN 2398-2950

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Synonym(s): MRI

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, pulse oximetry and blood pressure assessment.
  • If raised intracranial pressure Intracranial pressure measurement 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 anesthetized 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 a 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.
    • Transverse T2 FLAIR scan.
    • Injection of intravenous contrast medium, eg gadodiamide (OMNISCAN; Nycomed UK Ltd.) at the recommended human dose rate of 1 ml/5kg 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

  • With T1-weighting, brain parenchyma appears almost homogeneous and of mid-gray signal intensity, slightly paler than the overlying muscle  Brain: normal 01 - MRI scan (rostral transverse T1W) .
  • CSF is dark (hypointense) and bone marrow, subcutaneous and orbital fat is very bright (hyperintense).
  • Cortical bone and air in the frontal sinuses appear as a signal void (black).
  • On T2-weighted scans the brain is seen with a greater degree of contrast, with white matter appearing darker than gray matter, and CSF is hyperintense Brain: normal 03 - MRI scan (rostral transverse T2W) .
  • Surrounding tissues are usually darker and appear grainier than on T1-weighted images.
  • Generally, T1WI show better anatomic detail Brain: normal 02 - MRI scan (caudal transverse T1W) and are used to visualize enhancement of neoplastic or inflamed areas with contrast medium.
  • T2WI  Brain: normal 04 - MRI scan (caudal transverse T2W) is sometimes a more sensitive detector of minor pathology since increased water content of tissues, which occurs with most disease processes, creates dramatic hyperintense signal.

For this reason, most studies would include a combination of T1- and T2-weighted scans.

Pathology

Brain tumors

  • Brain tumors are surprisingly common in dogs and cats Brain: neoplasia.
  • Affected patients are usually middle-aged, but may occasionally be young adults.
  • In the cat meningiomas are by far the most common type of intracranial tumor.
  • Gliomas may also occur.
  • The two tumor types can be differentiated with reasonable confidence by their imaging characteristics.
  • Meningiomas:
    • Extra-axial (arise from tissues other than the actual brain parenchyma), compress rather than invade the brain parenchyma outside the brain and cause inward deviation of brain tissue  Brain: meningioma - MR transverse .
    • Most commonly rostrotentorial (especially olfactory or frontal lobe) but association with the caudal fossa or the ventricular system is possible.
    • Usually single lesion, but presence of multiple tumors is possible.
    • May be separated from the brain by a fine cleavage line.
    • Usually isointense with brain tissue on plain T1W scans.
    • Intravenous injection of contrast medium causes dense and usually uniform enhancement due to their high vascularity and location outside the blood brain barrier.
    • Peripheral, tapering projections of tissue are often seen between brain and bone as the tumors extends along the meninges (dural tail sign ).
    • Occasionally contain small dark specks due to calcification.
    • May be partly cavitated.
    • Overlying bone is often sclerotic due to hyperostosis, and this may be sufficiently severe to be visible radiographically Brain: meningioma - MR transverse .
  • Meningiomas may be surgically removed if they are located at an accessible site over the brain.
  • Compared to dogs, they tend in cats to be well-defined and non-infiltrative.
  • Also sensitive to X-radiation and can be treated with radiotherapy.
  • Gliomas:
    • Intra-axial (originate from brain parenchyma) and usually cause a marked mass effect with midline shift and ventricle compression depending on their size and location.
    • Usually ill-defined and hypointense in T1W scans (the hypointense signal resulting from both the mass itself and surrounding edema).
    • Contrast medium produces irregular enhancement which is often in ring-formation around a central, poorly-vascularized center.
    • T2W scans often show marked surrounding edema as a hyperintense signal, the lesion itself being of variable or mixed signal.
  • Gliomas may also be treated with radiotherapy alone or following surgical debulking.
  • Pituitary tumors Pituitary gland: neoplasia (also regularly diagnosed although less commonly than in the dog):
    • Histological type most commonly adenoma or carcinoma.
    • Well-defined masses extending dorsally out of the pituitary fossa.
    • Show marked contrast enhancement .
    • Hemorrhage is sometimes a feature of pituitary tumors and creates variable signal on both T1- and T2W images depending on its age and the stage of breakdown of hemoglobin.
  • Occasionally other histological types of tumor arise in the brain, but would not be recognized as such based on imaging characteristics.
  • Unlike man, brain metastases seem to be very rare in dogs and cats.

Inflammatory brain disease

  • Inflammatory brain disease typically produces multiple, ie multifocal, lesions of variable size and definition.
  • Modest, ill-defined enhancement may occur on T1W scans, but the lesions are usually best seen as ill-defined hyperintense zones on T2W scans.
  • Feline infectious peritonitis (FIP) Feline infectious peritonitis (dry form) is the most common cause of inflammatory CNS disease in cats. Usually seen in cats younger than 3 years of age.
  • The diagnosis can often be confirmed by CSF analysis Cerebrospinal fluid: cytology.
  • Meningitis Meningomyelitis: bacterial and fungal is unusual aside for FIP, and is diagnosed by thickening and abnormal enhancement of meninges.
  • In FIP, MRI may show T2 hyperintensity and contrast enhancement of ventricular lining, choroid plexus and meninges compatible with ependymitis, choroiditis and meningitis. Concurrent hydrocephalus is common, and herniation of the cerebellum secondary to increased intracranial pressure is possible.
  • Brain abscesses are rare in small animals, but would be seen as areas of fluid-nature signal with a surrounding, enhancing capsule. Common causes of brain abscess (or empyema) in cats include extension to the brain of otogenic infection or retrobulbar abscess as well as bite wound through the skull.

Congenital brain disease

  • The most common congenital condition diagnosed is hydrocephalus Hydrocephalus.
  • May affect the entire ventricular system or only the lateral ventricles .
  • Cerebellar hypoplasia Brain: cerebellar disease is an unusual condition the diagnosis of which is based on a subjective comparison of cerebellar size in the patient compared with other animal of similar conformation. It is most commonly caused by panleukopenia virus in cats Cerebellum: hypoplasia (feline panleukopenia related).

Traumatic conditions

  • Useful modality for assessment of brain trauma provided the patient can withstand the necessary anesthesia.
  • Cranial fractures , intracranial hematomata, parenchymal hemorrhage and parenchymal edema can be recognized.
  • MRI can also be used to exclude the possibility of previous trauma as a cause of epilepsy Epilepsy: traumatic.
  • In addition to conventional T1W and T2W images, the following sequences may add valuable information in the patient with head trauma:
    • T2-FLAIR (improves detection of brain contusions ans subarachnoid hemorrhages), gradient echo T2* (highly sensitive to detect blood and in evaluation of bony strucutes) and STIR (most helpful in identifying extracranial soft tissue trauma).
  • Grading system has been proposed:
    • Grade 1: normal parenchyma.
    • Grade 2: lesions only affecting the cerebral hemisphere, cerebellar parenchyma without midline shift or both.
    • Grade 3: lesions only affecting the cerebral hemisphere, cerebellar parenchyma, or both and causing midline shift.
    • Grade 4: lesions affecting corpus callosum, thalamus, or basal nuclei with or without any of the foregoing lesions of lesser grades.
    • Grade 5: unilateral lesions in the brainstem with or without any of the foregoing lesions of lesser grades.
    • Grade 6: bilateral lesions affecting the brainstem with or without any of the foregoing lesions of lesser grades.
  • Significant association exists between MRI grade and prognosis.
  • This grading system helps identify animals at higher risk of poorer outcome.
  • MRI prognostic indicators in head trauma include:
    • Degree of midline shift.
    • Extent of intraparenchymal lesions.
    • Skull fractures.
    • Injuries affecting the caudal fossa or both the rostral and caudal fossa.

Miscellaneous brain conditions

  • Brain infarcts Cerebrovascular disease:
    • Rarely occur in small animals.
    • Usually seen as wedge-shaped areas of hyperintensity on T2W scans with absent or feeble contrast enhancement on T1W scans.
    • If hemorrhage is present, mixed signal may be seen on both T1- and T2W scans depending on the age of the lesion.
    • Old infarcts may result in brain shrinkage and deviation of the midline towards the lesion.
  • Acute central vestibular disease:
    • Sometimes referred to as a 'stroke'.
    • Does not produce recognisable lesions on MRI.
    • MRI is very helpful in differentiating central from peripheral vestibular disease.
  • Other conditions:
    • Occasionally, neurological signs may result from expanding masses of surrounding tissue causing compression of underlying brain.
    • Caudally-extending nasal tumors, and tumors of the bases cranii and calvarium, can easily be recognized even when they are not visible clinically or with radiography.

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • 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.
  • Costanzo C, Garosi L S, Glass E N et al (2011) Brain abscess in seven cats due to bite wound: MRI findings, surgical management and outcome. J Feline Med Surg 13 (9), 672-680 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, 39-63 PubMed.
  • Negrin A, Lamb C R, Cappello R et al (2007) Results of magnetic resonance imaging in 14 cats with meningoencephalitis. J Feline Med Surg (2), 109-116 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.