An Overview of the Theory for Small Animal Veterinary Practice
What is it?
The concept of the MR image is a chemical analysis of constituent body tissues, displayed as a grey-scale in anatomical context. It uses a combination of magnetic fields and radiofrequency signals to map hydrogen nuclei. The MR-scanner contains a powerful magnet, which aligns the protons and causes them to spin around the long axis of the magnetic field. Radiofrequency bombardment causes the nuclei to emit a signal which is related to position and chemical environment within the tissues. The image takes several minutes to acquire, and any movement blur can render the image non-diagnostic, so general anesthesia is required.
Different radiofrequency pulse sequences can be used to highlight tissues in different ways. For example, a “T1-weighted” scan shows fluid as dark, and a “T2-weighted” scan shows fluid as bright. Fat will appear bright in both scans. Most pathological processes will lower the fat content and raise the water content, so comparing the T1 and T2 weighted scans (known as an irritation pattern) will highlight this.
Additional processing of T2-weighted scans can be done to suppress the fat-signal, to produce a “STIR” image (Short Tau Inversion Recovery – useful in identifying vertebral body pathology), and suppression of the water-signal produces a “FLAIR” image (Fluid Attenuated Inversion Recovery – useful for identifying pathology in or near the ventricles in CNS studies). Scans can also be run to provide 3-D/high resolution protocols for examination of small areas of tissue in greater detail.
Another useful technique is to use contrast enhancement. Gadolinium injection identifies a breakdown in the blood/brain barrier and abnormal capillary flow and so helps identify neoplasia or acute infectious processes.