Mattoon JS, Wisner ER.
Historically, the most common feline intracranial neoplasm, meningioma, was diagnosed by the detection of adjacent calvarial hyperostosis on plain film radiographs. Later refinements included intracranial angiography for the diagnosis of abnormal vascular anatomy and arcane inva- sive studies such as ventriculography and optic thecography to document intracranial mass effect. With the introduction of contrast-enhanced x-ray computed tomography (CT) in veterinary medicine in the early 1980s, intracranial mass lesions were able to be detected noninvasively with a high degree of accuracy on cross-sectional images with excellent anatomic resolution.
Paralleling the technologic advances and medical appli- cations of CT has been the development of magnetic res- onance imaging (MRI). The medical application of MRI was conceived in 1971 by Raymond Damadian, who, by means of the well-known laboratory spectroscopic tech- nique of nuclear magnetic resonance (NMR), discovered that certain mouse tumors had prolonged relaxation times compared with normal tissues. Brilliant advances were made in the early to mid 1970s that allowed the conversion of NMR signals into anatomic images. Paul Lauterbur pro- posed that the precise location of NMR signals could be distinguished by the use of magnetic field gradients, and the idea of tomographic image acquisition was conceived by Sir Peter Mansfield in 1974. In fact, Lauterbur and Mansfield were awarded the 2003 Nobel Prize in Medicine last year for their pioneering work in this field.1 The 1st image of the human head was published in 1978 by the EMI Central Research Laboratories by means of a 0.1-T magnet. Picker International and Philips Medical Systems soon followed with commercial low-field-strength systems, and General Electric marketed a high-field-strength magnet in 1984.
Rapid technologic and medical advances have followed, with MRI becoming, with only a few exceptions, the mo- dality of choice for intracranial neuroimaging.2 Whereas x- ray film–based imaging modalities rely on tissue density differences to define tissue contrast, MRI relies on chemical differences, typically the differences in water hydrogen pro- ton microenvironments, to generate tissue contrast. A mul- titude of acquisition parameters or pulse sequences result in virtually unlimited ways to visualize the tissues of inter- est. In addition to delineating anatomic margins with high spatial resolution, we can now distinguish between gray and white matter, normal cerebrospinal fluid in the ventricular system, abnormal fluid from vasogenic and cytotoxic ede- ma, and hemorrhage. The combination of excellent tissue contrast and anatomic detail therefore provides an accurate depiction of the patient’s brain and associated pathology. The introduction of gadolinium chelate contrast agents fur- ther improved our ability to identify lesions with abnormal perfusion or vascular permeability. Returning to its roots, one of the cutting-edge topics in current MRI research is the use of spectroscopy to differentiate various types of brain pathology by chemical composition and metabolite concentration.