Gl Wismer

Contrast in rapid MR imaging: T1- and T2-weighted imaging.

Partial saturation (PS) is an imaging technique that is useful in applications that require rapid image acquisitions (imaging time less than 1 min). Image contrast in PS imaging, as in other magnetic resonance methods, depends on the often conflicting effects of differences in proton density, T1, and T2. Previous analyses of pulse sequence optimization to maximize image contrast have assumed 90 degrees pulses and examined the effects of varying repetition times (TR) and echo times (TE). In this paper we present theoretical calculations and images made with a 0.6 T imager to show that the radiofrequency pulse tip angle alpha, and not the pulse sequence timing parameters, is the most important parameter for producing image contrast. For large tip angles (alpha greater than or equal to 60 degrees), contrast is primarily determined by differences in T1, but for small tip angles (alpha approximately equal to 25 degrees), contrast is primarily due to differences in T2. The T2-weighted images can be produced as quickly as T1-weighted images by using a small pulse angle and a long TE; it is not necessary to use a long TR to reduce the effects of T1 differences. Optimum pulse angles are calculated, and the potential advantages and disadvantages of T2-weighted and T1-weighted PS imaging are discussed.

Susceptibility induced MR line broadening: applications to brain iron mapping.

Magnetic susceptibility variations due to the presence of iron in neural tissue can result in a shift of local resonance frequency, decreased T2 resulting from water diffusion through local field gradients, and line broadening due to field inhomogeneity within a voxel. In this study, modified spin echo phase contrast pulse sequences were used to map proton resonance line widths in phantoms and in vivo. In agar gels containing varying concentrations of Fe3O4 (magnetite), line broadening mechanisms permitted accurate spatial localization of iron deposits from measurement of local resonance line widths. Furthermore, line widths estimated from the data were strongly correlated with iron concentrations, indicating the potential quantitative applications of the method. The technique was applied clinically to map iron in subacute brain hemorrhages. These data suggest that resonance line widths may be a useful measure of brain iron content.

Magnetic resonance imaging during acute myocardial infarction

Experimental canine studies have demonstrated the potential of magnetic resonance imaging (MRI) for detecting and characterizing acute myocardial infarction (AMI) in humans. Accordingly, electrocardiographic-gated spin-echo MR images of the left ventricular short axis were obtained in 34 patients a mean of 11 +/- 6 days (range 3 to 30) after AMI. This imaging technique allowed division of the left ventricle into segments corresponding to the left ventricular segments on angiography. Patients were separated into 2 groups; the first 16 patients (group I) were examined using a variety of imaging techniques. Information derived from this experience resulted in a standard imaging protocol and development of criteria for the presence of AMI. The imaging protocol and interpretation criteria were used in the assessment of a subsequent group of 18 patients (group II). Of the 14 patients in group II with satisfactory image quality, all showed an increase in myocardial signal intensity consistent with an AMI. In addition, the anterior or inferior location of the abnormal MR segments corresponded to the electrocardiographic infarct location. MR segments showing increased signal intensity corresponded with severely hypokinetic or akinetic segments on the left ventriculogram in 8 patients having both procedures. In a group of volunteers who underwent imaging and whose images were interpreted in the same manner as those of the patients with AMI, 1 of 9 subjects had regional variation in myocardial signal intensity compatible with an AMI. In summary, AMI is readily detected, located and characterized by electrocardiographic-gated MRI. These findings suggest that MRI techniques may have a role in the evaluation of AMI in humans.

Magnetic Resonance Imaging: Present and Future Applications

Magnetic resonance (MR) imaging has created considerable excitement in the medical community, largely because of its great potential to diagnose and characterize many different disease processes. However, it is becoming increasingly evident that, because MR imaging is similar to computed tomography (CT) scanning in identifying structural disorders and because it is more costly and difficult to use, this highly useful technique must be judged against CT before it can become an accepted investigative tool. At present MR imaging has demonstrated diagnostic superiority over CT in a limited number of important, mostly neurologic, disorders and is complementary to CT in the diagnosis of certain other disorders. For most of the remaining organ systems its usefulness is not clear, but the lack of ionizing radiation and MRs ability to produce images in any tomographic plane may eventually prove to be advantageous. The potential of MR imaging to display in-vivo spectra, multinuclear images and blood-flow data makes it an exciting investigative technique. At present, however, MR imaging units should be installed only in medical centres equipped with the clinical and basic research facilities that are essential to evaluate the ultimate role of this technique in the care of patients.