Clinical use of nuclear magnetic resonance imaging in stroke.

Abstract

SUMMARY There are many positive aspects to the use of MRI in the evaluation of cerebrovascular disease. First, the MR imaging technique appears to be essentially without hazard. 26 It does not rely on ionizing radiation, and no intravenous injections of contrast agent are necessary. MRI exploits the tissue's inherent biophysical characteristics to provide superior contrast. Infarctions are well delineated by MRI, often better and earlier than CT.13 ' 15> M Because of the lack of MRI signal from bone and thus the lack of transverse artifact from bone often seen with CT, lesions in the posterior fossa are very well visualized. 21With MRI it is possible to obtain images in the transverse, coronal, and sagittal planes, which provides for good evaluation of lesion size and extent. 20 Arteriovenous malformations have been visualized by MRI, but it is still too early to know whether MRI has any detection capability over CT in this disorder. WHEN CERTAIN ATOMIC NUCLEI are placed in a magnetic field and stimulated by radiowaves of a particular frequency, they will re-emit some of the absorbed energy in the form of radio signals. This phenomenon is known as nuclear magnetic resonance (NMR) and was first described in the 1940's, almost simultaneously, by Bloch 1 and Purcell. 2 Then, in 1973 Lauterbur 3 devised a method of determining the location of stimulated nuclei by spatially encoding the emitted signal so that it could be used to create an image of the distribution of nuclei within the sample. This resulted in the implementation of NMR imaging (MRI) of biological systems in living animals and humans. The clinical usefulness of MRI is now widely apparent.1 " 5Because protons (1H) are among the most easily detected of NMR-sensitive nuclei and because there is such a high concentration of proton-rich water and lipids in biological systems, especially the brain, most current clinical MRI is generated using the 1H resonance. The resultant image intensity is a function of the distribution of proton density, modified by the re- laxation times, Tl (spin-lattice) and T2 (spin-spin). The extent to which the parameters of proton density and Tl and T2 relaxation times contribute to image intensity can be varied by the use of different imaging methods, in particular different radiofrequency pulse sequences. MRI images are determined by the physical and chemical characteristics of tissues, as opposed to their pathology or anatomy. An understanding of the nature of the physicochemical properties underlying image intensity and knowledge of the imaging technique used are needed to appreciate fully the implications of the resultant images. TechniquesThe principles of MRI 6 and details of pulse sequences 7 -8 have been outlined in detail elsewhere. Briefly, the most commonly used pulse sequences at present are inversion recovery (IR) and spin-echo (SE). With the inversion recovery technique the image intensity is related to T l . Water has a very long Tl; CSF, which behaves like water, has a relatively long Tl and appears dark on invers...

Lacunar Cerebral Infarct and Nuclear Magnetic Resonance

“…A significantly (p < 0.001) greater num ber of silent infarcts (asymptomatics or unreported strokes, mainly old LIs) was detected by MR (42%) than by CT scan (18.5 %) in our series. This result is consistent with the observations of other authors [1,23].…”

“…On these occasions, a greater number of associated cerebrovascular lesions and a superior def inition of the lesion, in comparison with the brain CT scan, has been observed [1,15,16]. The only series pub lished to date in which MR was used to diagnose an LI is that of Rothrock et al [ 17], who studied 31 patients and obtained positive results in 74%.…”

“…Nuclear magnetic resonance (MR) is a recently intro duced complementary examination used in the study of stroke [1][2][3]. Lacunar cerebral infarcts (Lis) comprise 11-19% of all cerebrovascular pathologies [4][5][6][7], Until now, the complementary examination of choice in the diagnosis of Lis has been computerized tomography (CT).…”

Clinical study of 227 patients with lacunar infarcts.

“…9 Publications including the application of magnetic resonance imaging (MRI) to the diagnosis of lacunes are scanty. 20 - 28 We describe our experience with lacunes through the clinical analysis of 227 patients, 45 of whom were studied using both CT and MRI. The aims of our study were to conduct a statistical analysis of a prospective clinical series and to compare, in approximately one fifth of these patients, the diagnostic usefulness of MRI and CT.…”

The Role of Magnetic Resonance Imaging for Acute Ischemic Stroke

“…Until the beginning of 1990s, MRI was reserved mainly for the subacute phase of ischemic stroke, because the most widely available conventional sequence -T2-weighted imaging -shows infarct lesions only after 6-8 hours as a hyperintense zone 9, 10 and a T1-weighted sequence even later and as a hypointense lesion 11 . Conventional MRI offered mainly higher resolution capability for detection of relatively smaller infarcts, especially in the brain stem and cerebellum compared to CT 12,13 . The use of urgent MRI in acute stroke was also limited for generally prevailing pessimism for low sensitivity in the detection of acute ICH or SAH compared to CT. Fortunately, this pessimism disappeared completely when more accurate MRI machines and new echoplanar sequences (T2*) capable of safely detecting brain hemorrhage became established in clinical practice [14][15][16][17] .…”