Magnetic resonance imaging (MRI) is a non-invasive technique
that relies on magnetically-induced changes in proton energy to
generate highly detailed views of the body. In most cases, MRI is
the best method of imaging the central nervous system, including
the brain and spinal cord. Unfortunately many veterinary specialty
clinics do not have access to this technology, have low-field
magnets which preclude certain regions from being imaged, or may
not have extensive experience interpreting or performing veterinary
MRI. Since 2007, we have performed over 400 MR examinations on dogs
and cats with nervous system disease.
MRI is universally regarded as the best means of imaging the
brain. It is able to detect many lesions that are challenging or
impossible to recognize using computed tomography (CT). These
include certain forms of stroke,
neurodegenerative diseases, Chiari
malformation, and various types of nervous system inflammation. Even tumors can sometimes be
challenging to detect with CT, especially if they are located in
the area of the brainstem or are relatively small. The following
images were obtained 1 month apart and illustrate this point
well:
Glioma
A middle aged dog presented for evaluation of a seizure
disorder. A CT scan had been obtained 1 month prior to admission
which was judged to be normal (left). Given the progressive nature
of the neurological signs an MRI was performed at Texas A&M. A
T2W transverse MR image of the brain showed an expansive,
hyperintense lesion consistent with a glial tumor
Increasing attention has been paid to the use of MRI for
vertebral column diseases in dogs and cats. Compared to myelogram,
MRI is non-invasive and unlike both CT and myelogram, it allows for
direct visualization of the spinal cord. Many diseases that are
common causes of spinal cord dysfunction in dogs - such as
fibrocartilagenous embolic myelopathy - cannot be detected without
MRI (De Risio et al J Vet Intern Med 2007). Additionally, MRI
provides prognostic information not available through other imaging
modalities concerning outcome after disk
herniation in dogs (Ito et al J Am Vet Med Assoc 2005). And
even in animals with disk herniation, MRI may be superior to
detecting and locating areas of spinal cord compression compared to
CT or myelogram. The following images were obtained at the same
time and illustrate this point:
Vertebral Trauma:
These images are from a middle aged dog that was hit by a car
and had pelvic limb neurologic deficits. The CT image (left) did
not show any definitive lesions. The T2W sagittal MRI showed an
area of hyperintensity (brightness) within the spinal cord, the
extent of which is marked by white arrows. The hyperintensity was
felt to represent spinal cord swelling secondary to trauma
Fibrocartilagenous Embolism
A young, large breed dog was presented to Texas A&M
University with acute onset signs of cervical spinal cord
dysfunction. A myelogram (left) had been performed which showed no
abnormalities. The T2W sagittal MRI on the right shows an extensive
area of hyperintensity (brightness) within the spinal cord, the
limits of which are marked by white arrows. The hyperintensity was
felt to represent swelling secondary to fibrocartilage within the
spinal cord.
Sagittal and transverse T2 weighted MRI images. Note the large
hyperintense lesion within the cervical spinal cord (white arrows),
consistent with fibrocartilagenous embolic myelopathy.
Nerve Sheath Tumor
An older, large breed dog was evaluated at Texas A&M
University for progressive right thoracic limb weakness and pain. A
CT (left) had been performed which was judged to only show muscle
atrophy on the right thoracic limb, the cause of which was not
apparent. On the right is a transverse STIR MRI image showing a
nerve sheath tumor, outlined by the white arrows.
Disk herniation
MRI images from 3 different animals with disk extrusion. Lesions
identified with white arrows.
A middle aged Dachshund was evaluated for back pain and pelvic
limb weakness. A myelogram had been performed (left) which was
inconclusive. A sagittal T2W and transverse T1W MR image are
presented on the right. The T2W image shows hyperintensity within
the spinal cord (white arrows) indicating potential swelling or
inflammation. The T1W image shows fragments of disk (white arrows)
and other material compressing the spinal cord.
Stroke
T2 weighted transverse image showing an ischemic territorial
infarction within the thalamus
Chiari Malformation
T2 weighted sagittal image showing compression of the caudal
cerebellum, kinking of the brainstem, hydrocephalus, and
hyperintensity within the cervical spinal cord consistent with
syringohydromyelia. These are the MRI features of Chiari-like
malformation (caudal occipital malformation)
Inflammation
T2 weighted sagittal MRI from a Pug with necrotizing
meningoencephalitis (Pug Dog Encephalitis). Not the hyperintense
regions highlighted with arrows within the cerebrum.