Significant T2-weighted hyperintensity is present within the spinal cord overlying the C3-C4 vertebrae (arrowheads in A). This lesion was consistent with fibrocartilaginous embolic myelopathy (FCEM) based on other MRI features and was confirmed as FCEM on necropsy.
Cervical Spondylomyelopathy
Data comparing MRI and myelography are available from one study in dogs with cervical spondylomyelopathy.7 Both modalities identified the location of the main lesion with reasonable inter-rater agreement. Signal changes within the spinal cord were identified by MRI in 10 of 18 dogs,7 suggesting that this modality could assist in locating compressions associated with significant injury.
Other Diseases
Intravenous contrast can be administered to animals undergoing veterbral column MRI. In certain disease states in which the blood-spinal cord barrier has been disrupted or neoangiogenesis has occurred, contrast material can extravasate into surrounding abnormal tissue.
Contrast is often provided when neoplastic or inflammatory lesions are suspected. It is well known that certain tumor types commonly enhance strongly (eg, meningioma), whereas enhancement is uncommon in other lesions (eg, ischemic myelopathies).
Nonetheless, the ability of postcontrast studies to differentiate among diseases has not been carefully studied in veterinary medicine. Further, contrast enhancement is frequently identified in nonneoplastic diseases such as disk herniation.8
Disadvantages
The major disadvantages associated with MRI are the lack of availability, issues related to patient size, patient and operator safety, the need for general anesthesia, limited field of view, and the time required to obtain a scan.
Lack of Facilities
Although the number of veterinary MRI centers is expanding, many specialty clinics evaluating animals with vertebral column disorders do not have ready access to MRI facilities.
Patient Size Constraints
The physical size of patients can limit the use of MRI in veterinary medicine. Available MRI bore diameter and receiver coils may not accommodate the wide variety of patient sizes in veterinary medicine.
Safety Concerns
Safety is also a concern. Veterinary patients with loose metal debris close to vital structures may be at risk for metal heating or fragment migration, and the susceptibility artifact generated by the presence of metal adjacent to structures of interest may hinder image interpretation. The degree to which this is an issue depends on the ferromagnetic and paramagnetic properties of the metal and its volume. Microchips rarely impact the diagnostic value of vertebral column MRI, for example, but patients with cardiac pacing devices cannot be scanned due to magnetic field interference.
Technical MRI staff must be well versed in appropriate safety protocols. Metal instruments cannot come near an MRI machine as they can be strongly drawn toward the bore, which may lead to injury.
Anesthesia and Monitoring
General anesthesia is almost uniformly required to perform MRI on domestic species, and technicians must be proficient in anesthetizing these patients. Specialized anesthesia machines and monitoring equipment are required, and patient monitoring equipment in particular has certain limitations associated with interference.
MRI artifacts may be produced by monitoring equipment, and radiofrequency pulses associated with MRI acquisition may hinder readings obtained on monitoring equipment. In my experience, these limitations can be easily overcome, but they are equipment-dependent.
Limited Field of View
A field of view (the region that can be imaged) must be selected with vertebral column MRI. The size of the field of view varies between different units and scan protocols. Low-field (<1.0 Tesla [T]) and older MRI units may be particularly susceptible to this limitation. Newer, high-field MRI units can subvert field of view limitations using various techniques.
Scan Time
Finally, the time required to perform an MRI study is generally longer than that required for vertebral column CT. It must be remembered that MRI field strength, software, and the number/type of pulse sequences selected highly influence scan times. The time required to scan a particular region of interest varies from 10 to 20 minutes (eg, with a modern 3-T magnet) to 1 to 2 hours (with a low-field or older MRI unit).
Reliability of Results
As outlined here, not all MRI units are equal. Field strength, software, receiver coils, protocols, personnel operating the machine, and other factors heavily influence image quality—and thus the reliability of the results.
Economic Impact
The cost associated with MRI is greater than that for traditional imaging modalities. At our center, the cost for regional vertebral column MRI is about twice as much as that for myelography.
When determining which diagnostic study to undertake, cost must be balanced with the accuracy of the study as well as study-related adverse events and prognostic data. As previously stated, CT and myelography cannot reliably identify certain vertebral column diseases. Likewise, myelography may be less specific than MRI for identifying sites of extradural compression (eg, cervical spondylomyelopathy).7
Adverse events associated with myelography in one study included seizures (28%) and worsening of neurologic signs (33%).7 Although many of these adverse events associated with myelography are transient, there is a case to be made for avoiding even transient patient morbidity. Finally, MRI can provide a greater amount of prognostic data than myelography or CT, which is certainly valuable.
How MRI Works
The Basics
MRI is a technique for rendering high-contrast images of structures by manipulating the energy state of protons, which make up approximately 70% of most tissues. The MRI machine uses an extremely powerful magnet to orient all protons in 1 direction. A radiofrequency pulse is then delivered that flips protons into a high-energy orientation. Protons are then allowed to return to their original energy state, which is termed relaxation. Relaxation occurs in multiple planes and leads to the release of energy (known as signal), which is detected via a receiver coil. A computer is used to display signal data as an image.
Image Contrast
The contrast characteristics of images (referred to as image weighting) can be altered by changing when and how protons are flipped and when the machine “listens” for energy release. Common weightings include T1, T2, T2*, fluid attenuated inversion recovery (FLAIR), and short tau inversion recovery (STIR). T2-weighted images are often used to detect pathology such as edema, necrosis, or hemorrhage, which appear bright, or hyperintense, on the resulting image.
Contrast enhancement: Extravasation of intravenous contrast material into parenchyma, which is visible on MRI as hyperintensity. Enhancement may indicate neoangiogenesis or blood-spinal cord barrier breakdown.
CT–myelography: An imaging study in which iodinated contrast is injected into the subarachnoid space, after which a CT image is generated.
CT reconstruction: A technique whereby CT images obtained in a certain plane (ie, transverse, dorsal, sagittal) can be reformatted into a different plane. This technique may enhance lesion visualization.
Field of view: The anatomic area visualized in MRI.
Field strength: The strength of the magnetic field an MRI generates, which is usually measured in Tesla. Higher-field magnets produce a stronger signal, which usually translates into better image quality and/or more rapid image acquisition.
Gradient: Small differences in magnetic field produced by MRI that are used to help locate where signal originates within a patient.
Hyperintensity: A state in which the MRI signal of tissue is higher than normal. Hyperintense lesions appear bright on resulting images.
Hypointensity: A state in which the MRI signal of tissue is lower than normal. Hypointense lesions appear dark on resulting images.
Image weighting: The contrast characteristics of an image. Using multiple image weightings enhances lesion visualization and determination of underlying pathology.
Magnet bore: The cylindric opening within an MRI unit into which the patient is placed.
Pulse sequence: The timing of radiofrequent pulses and receiver coil detection of signal that determines image weighting.
Radiofrequent pulse: A short-duration radiofrequent wave that changes the orientation of protons within the patient and ultimately allows signal to be released as protons relax to a lower energy state.
Receiver coil: A coil placed around an anatomic area of interest to allow the detection of signal.
Signal: The brightness of an image, which relates to the amount of nuclear magnetic resonance detected by a receiver coil.