Catherine M. Mills

The efficacy and clinical impact of brain imaging in neurologically symptomatic AIDS patients: a prospective CT/MRI study.

The relative efficacies of cranial magnetic resonance imaging (MRI) and computed tomography (CT) brain scans for the detection of intracranial pathology in patients with the acquired immune deficiency syndrome (AIDS) were evaluated prospectively. Fifty homosexual or bisexual men with AIDS and neurologic symptoms were evaluated using both modalities. In 24 patients, MR images and CT scans provided the same diagnostic information (within normal limits in 16, cerebral atrophy only in 6, and similar lesions in 2 patients). In only one instance did CT show the presence of a lesion not seen on MRI. In the 25 remaining patients, MRI was the more sensitive modality. MRI also reflected more consistently the histopathologically documented extent and distribution of central nervous system disease. The greater sensitivity of MRI suggested significant alterations in the diagnostic evaluation and therapeutic management of 20 patients. Thus, MRI was as good or better than CT for the detection of intracranial pathology in 49 of 50 neurologically symptomatic AIDS patients and significantly affected the diagnosis and treatment of 40% of these patients. Although MRI does not appear to be more specific than other modalities in the differentiation of human immunodeficiency virus (HIV)-related neurologic diseases, its greater sensitivity suggests that MRI may be the best neuroimaging procedure for the initial radiologic evaluation of AIDS patients with neurologic illness.

Magnetic Resonance Imaging of the Central Nervous System

Catherine M. Mills, M.D. Due to the geometry of conventional CT scanners, imaging Jonathan P. Posin, M.D. is typically performed in the transaxial plane, and, if views in Diagnostics Networks, Inc., and the University of California, other orthogonal planes are desired, complex computer San Francisco, CA reformations are necessary. These reformations are time consuming and often suffer from poor spatial resolution. As INTRODUCTION the MR information is encoded by alterations in radiofreTHERE have been few developments in the field of diagnosquency signal and orthogonal magnetic field gradients, imTtic imaging that have generated as much excitement, as ages can be obtained directly in the sagittal and coronal the recent emergence of magnetic resonance imaging (MRI). planes as well as the more usual transaxial plane. This Though the history and engineering fundamentals of this technique has proved to be of great value in central nervous technology (reviewed by Dr. Crooks in this issue) escape system imaging, especially involving the brainstem and spinal many of its clinical users, enthusiasm grows among radiolocord. gists and other diagnostic imaging specialists for this new Though CT provides enhanced soft tissue contrast when tool, which can offer increased sensitivity to pathologic compared to conventional radiographic techniques, it is change in an environment of relative safety (see the compannonetheless common to administer contrast agents, both ion article by Dr. Budinger) and, at present, without the intravascularly, and intrathecally into the cerebrospinal fluid routine use of contrast agents (see the companion article by which surrounds the brain and spinal cord. These agents, Dr. Gore). though generally safe, are still toxic substances and side One of the earliest applications (and now the leading one) effects along with major complications associated with their of magnetic resonance imaging is the diagnosis of diseases of use can and do occur. MRI, with its ability to evaluate the central nervous system. While MRIs nearest predeceschanges in many tissue parameters and not just a single sor, x-ray computed tomography (CT), was and still remains a diagnostic tool of great strength when compared to its predecessors, it suffers from several limitations not present with MRI. Cortical bone, metal, and other substances with high atomic number cause streaky image degradation known as beam hardening artifact, which limits the diagnostic utility of CT in the posterior fossa, and in areas coplanar with metallic objects including dental prostheses and surgical clips. Since cortical bone has a low mobile proton density, it appears of low intensity on MR images, and it creates no artifact to impede evaluation of soft tissue structures within a bony vault, such as the cerebellum or brainstem. Metallic objects can, however, be a problem for MRI with respect to biohazards. Although they do cause local signal dropout in MR images, typically, the resultant artifact is far less severe than that seen with CT.