MRI Theory


An Overview of the Theory for Small Animal Veterinary Practice

The concept of the MRI is a chemical analysis of constituent body tissues displayed as a grey-scale in anatomical context. It uses a powerful magnetic field and radiofrequency signals to map hydrogen nuclei. The MRI machine is 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 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, T1-weighted images (T1WI) show fluid as dark, and T2-weighted images (T2WI) show fluid as bright. Fat will appear bright in both sequences. Most pathological processes will lower the fat content and raise the water content, so comparing the T1 and T2 weighted images (known as an irritation pattern) is helpful in highlighting various pathologies.

Additional sequences can be obtained by modifying T2WI to suppress the fat-signal which produces a STIR sequence (Short Tau Inversion Recovery). STIR images are useful in identifying vertebral body pathology. T2WI can also be altered to suppress water-signal producing a FLAIR sequence (Fluid Attenuated Inversion Recovery). FLAIR sequences are useful for identifying pathology in or near the ventricles in CNS studies. These sequences can also be run to provide 3-D/high resolution protocols for examination of small areas of tissue in greater details.

Another useful technique is to use contrast enhancement. Gadolinium injection identifies a breakdown in the blood/brain barrier and abnormal capillary flow which helps identifying neoplasia or acute infectious processes.

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