Magnetic resonance imaging: spine

Introduction

MRI Magnetic resonance imaging: basic principles is the best imaging modality for investigation of many spinal diseases, due to its multiplanar imaging capability (cross-sectional images can be obtained in any plane) and the high resolution of the resulting images.
CT Computed tomography is also helpful, but soft tissue contrast is poorer, and images in the sagittal and dorsal planes may require reformatting from the transverse plane.

Advantages of MRI over myelography and CT

It is non-invasive, as no subarachnoid injection of contrast medium is required.
It can differentiate types of spinal cord swelling, eg solid and cystic.
It will show lesions which do not produce a mass effect and which therefore are not seen on myelography Radiology: myelography Radiography: myelography.
It detects early disk degeneration and other soft changes in the disk space, such as diskospondylitis Diskospondylitis.
Acquisition of images in the transverse plane provides a dimension that cannot be assessed radiographically.
Spinal nerves can be seen.
The investigation is often quicker than myelography, dependant on scanner type, and is physically easier as no repositioning of the patient is needed.
If the clinical signs are multifocal, the brain can easily be scanned at the same time.
Paraspinal soft tissues can be assessed for involvement in pathology.
MRI provides superior contrast resolution than CT and is better suited for imaging soft tissues, such as the spinal cord, nerve roots, and intervertebral disks.

Disadvantages of MRI compared with myelography and CT

It is more expensive, and is often not immediately available for acute cases requiring urgent surgery.
Localization of lesions with respect to the meninges may be harder, eg differentiation of extradural and intradural lesions.
Stressed and traction views are more difficult; in particular, stressed views which may compress the cord further cannot be justified because of the length of time for which the position must be maintained.
There is reduced fine bone detail compared with radiography, and periosteal new bone is hard to see (although medullary and subchondral bone changes are better demonstrated).
Whole spine screens can take a long time, especially in large dogs.
CT provides superior spatial resolution and is better suited for imaging bone.
Thinner slice acquisition (submillimeter) is possible with CT.
CT is cheaper to buy, requires fewer maintenance requirement and associated expenses as well as rapidity of imaging.
CT can be used to successfully guide needle aspirations or biopsies once a lesion is localized, but this is not generally done with MRI guidance because of prolonged imaging times and inability to use metal implements for tissue sampling.
Interpretation requires considerable experience, and false positive diagnoses are easy to make.
Resolution may be inadequate for a limited number of small cats and dogs.
The common presence of clinically-silent disk changes, especially in older dogs, means that an accurate neurological examination is necessary to identify the part of the spine affected, in particular to differentiate T3-L3 lesions from lumbosacral lesions, since incidental LS pathology is extremely common.