E. Auriemma1, DVM, DECVDI
Magnetic resonance imaging (MRI) is an advanced imaging technique more and more available in the veterinary market. MRI system doesn't use ionizing radiation but needs a magnetic field, radiofrequency pulses, and a computer in order to produce an image. The patient is placed in a magnetic field and protons in tissues are used to create a signal, which is detected by receiving coils and then encoded and processed to produce an image. Magnets producing the magnetic fields can be classified by their design (permanent, resistive and superconducting) or by their field strength (low field, high field). The clinical applications of this modality strongly depend on the type of machine and settings available. Low-field MRI units can provide adequate results in small animal practice but have some limitations such as limited field of view, long time of acquisition, and lower image quality, as compared to high-field units. On the other hand, a low-field MRI machine is certainly cheaper and has lower maintenance cost as compared to a high-field MRI system. A high-field MRI unit can provide exquisite contrast resolution and is capable of acquiring images with a large field of view in a limited amount of time.
Clinical Applications of MRI
In veterinary medicine the widest application of MRI is certainly linked to the nervous system and in minor extent to orthopaedic diseases. Research studies are available on cardiac and functional MRI but are still sporadic.
The exquisite contrast resolution and the absence of superimposition of structures allow studying neurological pathology with high diagnostic accuracy.
Congenital brain diseases (ex. hydrocephalus) can be initially evaluated with other imaging techniques (ultrasonography, computed tomography), but only with MRI is possible to clearly classify these pathologies. In cases of hydrocephalus for example, is necessary to understand if the disease process is obstructive (secondary to space-occupying lesions, stenosis) or compensatory (increased volume of the liquoral space secondary to brain atrophy).
Acquired inflammatory or neoplastic brain diseases need to be characterized with MRI; the number of lesions and their MRI features (signal intensity, margins, location, mass effect, etc.) are helpful in differentiating a neoplastic from a non-neoplastic pathology. Cerebrospinal fluid tap is often required to complete the MRI study and to provide answers to the questions of the clinician.
MRI studies of the spine, particularly with high-field units, provide a better contrast resolution as compared to CT and allow not only to characterize the site of potential spinal cord compression (intradural, extradural, intramedullary) but also its signal intensity. Intramedullary lesions, such as ischemic myelopathy, myelitis, and sometimes spinal cord neoplasia, can be undetectable with other imaging modalities (i.e., CT). MRI represents the gold standard imaging technique for these patients and is useful not only for pre-surgical planning, but also for providing important prognostic factors.