The modification of the RF field distribution of surface coils by weakly conducting saline samples

Reduction of field of view in MRI using a surface-spoiling local gradient insert

Herein is presented a method for suppressing the magnetic resonance signal to a controlled depth by applying a spatially heterogeneous spoiler field between the slice-select and readout pulses. Eliminating the signal from near-surface regions allows one to shrink the field of view without introducing aliasing artifacts, thereby decreasing imaging time over a smaller defined volume. A unique planar magnetic gradient coil was constructed to generate the spoiler field. Phantom and human subject studies showed that the signal can be suppressed to controlled distances of up to 90 mm from the coil, with modest requirements on power supplies, pulse sequences, and materials, and with no increase in imaging time.

The intrinsic signal-to-noise ratio in human cardiac imaging at 1.5, 3, and 4 T

Cardiac imaging is inherently demanding on the signal-to-noise performance of the MR scanner and may benefit from high field strengths. However, the complex behavior of the radiofrequency field in the human body at high frequencies makes model-based analyses difficult. This study aims to obtain reliable comparisons of the signal-to-noise profile in the human chest in vivo at 1.5, 3, and 4 T. By using an RF-field-mapping method, it is shown that the intrinsic signal-to-noise increases with the field strength up to 4 T with a less than linear relation. The RF field profile is markedly distorted at 4 T, and the onset of this distortion is dependent on the body size. The high power deposition and the consequences of the RF field distortion are discussed.

31P magnetic resonance spectroscopy detects a functional abnormality in liver metabolism after acetaminophen poisoning

Eighteen patients with acetaminophen poisoning were studied with 31P magnetic resonance spectroscopy to measure phosphorus-containing metabolites in their livers. The concentrations of all magnetic resonance-detectable metabolites fell in parallel with a decrease in the synthetic ability of the liver, indicated by the prothrombin time ratio (international normalized ratio). In particular, ATP fell to about 20% of its normal concentration in severely affected patients, as did the phosphodiester signal, which is thought to arise mainly from the endoplasmic reticulum in the liver. The correlation between ATP levels and international normalized ratio suggests that the international normalized ratio is a more accurate measure of the number of viable hepatocytes than are other biochemical tests.

Development of an in vivo 19F magnetic resonance method for measuring oxygen deficiency in tumors

A 19F magnetic resonance spectroscopy (MRS) approach to measuring hypoxia in experimental tumors in rats at a field strength of 2.35 T has been investigated in a combined study of in situ and excised tumors. The detection of tumor hypoxia is based on the hypoxia marker approach which depends on the selective, covalent binding of a fluorinated 2-nitroimidazole to hypoxic cells. The 19F MRS measurement of in vivo hypoxia marker binding was made at a fixed postinjection time when unbound, circulating marker molecule concentrations had dropped below detectable levels. A correlation between 19F MRS and scintillation counting measures of tumor-bound, tritium-labeled hypoxia marker was observed. There was no correlation between integrated 19F MRS hypoxia marker signals and the in vivo 31P MRS parameters of hypoxia which have been developed to measure normal tissue ischemia. Radiolabeling studies and previous immunohistochemical studies with the fluorinated hypoxia marker support the conclusion that the 19F MRS approach has promise as a physically noninvasive guide to the use of hypoxia-dependent therapies at clinically usable MRS field strengths.

Quantitative spectroscopy using multiple surface coil probes

The performance of the rotating frame localization method is discussed, with emphasis on performing the experiment and processing the data in the optimum manner. Studies on phantom samples are presented to illustrate various imperfections which affect the confidence that can be placed on acquired data.