Anatomic, differential diagnosis key to MRI success
Q. What are the pros and cons for magnetic resonance imaging (MRI) in dogs?
A. Dr. Laurent S. Garosi at the 2005 American College of Veterinary Internal Medicine Forum in Baltimore, Md., gave a lecture on use, misuse and abuse of MRI. Some relevant points in this lecture are provided below.
There has seen a dramatic increase in the availability of sophisticated neuro-diagnostic tests (computed tomography and MRI). Despite their relatively high sensitivity, these diagnostic tests often lack specificity in determining the exact nature of the disease process. It still stands true — a precise localization of the problem within the nervous system (anatomic diagnosis) and understanding of the suspected disease process (differential diagnosis) are the keys to successful management.With unlimited media availability provided by the Internet, the general public has become more informed and sometimes more critical of what can or cannot be done for their beloved companion dog or cat. Consequently, the demand from owners for diagnostic modalities such as MRI has increased. MRI is now more readily available to the veterinary community and considered as an indispensable tool in many referral practices. Unfortunately, advances in therapy seem to lag behind the progress made in the availability of this diagnostic imaging technique. MRI is the preferred imaging method for humans with central nervous system disease. Its main advantages rest on its highly accurate soft-tissue resolution, multiplanar capability and absence of ionizing radiation risk when compared to other imaging techniques.
Numerous MRI radiofrequency pulse sequences have been designed in order to improve soft-tissue contrast resolution. The possibilities are nearly endless. The most common pulse sequences used to image the brain and spinal cord are those based upon the spin-echo. T1-weighted images are useful for visualizing anatomy. Gadolinium-enhanced T1-weighted images allow for identification of regions within the CNS where the blood-brain barrier is not intact. T2-weighted images are used for identifying regions of increased free water (regions of edema, cellular infiltration or inflammation). Other pulse sequences used for small animal MRI include fluid-attenuated inversion recovery (FLAIR) (used to suppress CSF signals in order to examine lesions of the brain parenchyma which are near to the ventricle or subarachnoid space), gradient echo (increased sensitivity of blood products and calcification), diffusion- and perfusion-weighted (use for early detection of infarction), and fat-saturation techniques.
Common MRI artifacts are important to recognize and include motion, susceptibility, signal void, partial volume and signal drop-off artifacts. MRI does require that the animal be anesthetized. Other than the risks associated with this general anesthesia (especially in animals with severely compromised brain function), there is no conclusive evidence for irreversible or hazardous bio-effects related to short-term exposures of humans to static magnetic fields up to 2.0 Tesla. When compared with other imaging techniques, MRI also has the benefit of the absence of ionizing radiation risk.
MRI is likely to add information to that obtained by conventional imaging techniques when:
1. The area of the suspected lesion cannot be evaluated using other means,
2. The information produced by conventional imaging techniques is limited, and
3. A lesion is evident on other imaging techniques but more 3-D information is required for example for treatment planning.
Looking into brain diseases
Because of its excellent soft-tissue resolution, MRI of the brain is indicated in the diagnostic work-up of animals with neurological signs of brain disease. Diseases that affect the brain are divided into a. extra-cranial disease (toxic or metabolic), b. intra-cranial structural brain disease (cerebrovascular, inflammatory or infectious, neoplastic, degenerative, anomalous, or trauma) and c. intra-cranial functional brain disease (mainly diseases caused by abnormal neurotransmission or ion channel disorders, such as primary epilepsy, narcolepsy or diseases classified as movement disorders). Elimination of extra-cranial causes of brain disorder is a prerequisite to MRI evaluation of brain disease. Signal intensity on MRI scans is a reflection of subtle biochemical and biophysical tissue properties. As a result, MRI has excellent soft-tissue contrast resolution and high sensitivity to many disease processes affecting the brain.