Radio-frequency probe for 1H decoupled 31P MRS of the head and neck region.

For optimal performance of 31P MRS at 1.5 Tesla, the use of a double resonant probe is essential to enable the application of 1H decoupling and Nuclear Overhauser Enhancement. This note describes the design, evaluation and safety validation of a versatile and compact probe optimized for 1H decoupled 31P MRS studies of tumors close to the surface of the body, in particular the head and neck region.

Skin temperature increase during local exposure to high-power RF levels in humans.

The local temperature response of the skin on heating due to prolonged exposure to RF radiation by a surface coil was investigated in five healthy volunteers. Temperature changes induced by RF radiation were measured at the skin of the calf muscle by a fluoroptic probe. Exposure to superficial specific absorption rate (SAR) levels of 6.5, 12 and 22 W/kg resulted in skin temperature increases, the highest temperature recorded was 38.3 degrees C. Although the maximum values of each temperature curve correlated with the applied superficial SAR levels, these values did not exceed the recommended temperature limit for the extremities such as given by the Food and Drug Administration (FDA).

Assessment of human muscle glycogen synthesis and total glucose content by in vivo 13C MRS.

BACKGROUND: Obesity is often accompanied by a decreased ability of insulin to stimulate glucose uptake and glycogenesis in skeletal muscle. The aim of this study was to investigate the rate of glycogen formation and of muscular glucose content in relation to insulin sensitivity under euglycemic conditions. MATERIALS AND METHODS: We applied a hyperinsulinemic (430 pmol m-2 min-1) euglycemic clamp with infusion of 20% glucose (30% enriched with 13C-1-glucose) to 8 subjects with a wide range of insulin sensitivities. Glycogen and glucose levels were monitored simultaneously by in vivo 13C MRS of the calf muscle on a clinical MR system at 1.5T field strength. RESULTS AND CONCLUSIONS: Glycogen synthesis rate showed a strong correlation with whole body glucose uptake during the clamp (r = 0.93, P < 0.01). With the use of 13C MRS, total muscular glucose content could be determined in vivo, and showed a positive, linear correlation with glycogen synthesis rate (r = 0.85, P < 0.01). 13C MRS provides important information regarding in vivo insulin action. Preliminary results indicate that the glycogen synthesis rate improves after treatment with troglitazone.

Human prostate: multisection proton MR spectroscopic imaging with a single spin-echo sequence--preliminary experience.

The authors investigated the feasibility of a multisection proton magnetic resonance (MR) spectroscopic imaging technique for the acquisition of metabolic information in the human prostate. Multisection MR spectroscopic imaging was performed of a citrate phantom and of the prostates of eight adult volunteers. High-quality proton MR spectra and citrate metabolite maps of the prostate were obtained with this method.

Heteronuclear cross polarization for enhanced sensitivity of in vivo 13C MR spectroscopy on a clinical 1.5 T MR system.

The potential of heteronuclear ¿1H-13C¿ cross polarization was studied for optimization of the signal-to-noise ratio in in vivo 13C MR spectroscopy at the clinical field strength of 1.5 T. Experiments on the human calf showed a significant chemical-shift selective signal enhancement on triglyceride signals of 3.9 by heteronuclear cross polarization, compared to a standard pulse-acquire sequence. Studies on a neonatal piglet brain showed an enhancement by cross polarization of 2.2 for the detection of 13C-1-glucose. This enhancement allowed a fourfold improvement in time resolution in dynamic 13C MR of 13C-1-glucose inflow in piglet brain. Phantom experiments demonstrated the efficiency of this technique for interleaved detection of two spectral regions. Tests with a volume coil showed the feasibility of signal enhancement by cross polarization over a large volume of interest.

Calibration of the 1H decoupling field strength and experimental evaluation of the specific RF absorption rate in 1H-decoupled human 13C-MRS.

For patient safety in human 1H decoupled 13C-MRS, it is absolutely necessary to evaluate the specific RF absorption rate (SAR) of the tissue exposed to 1H frequency irradiation. With the use of surface coils, the local SAR at the body surface is of most concern due to the inherent RF field inhomogeneity. An empirical procedure to spatially calibrate the decoupler power level and to evaluate the local SAR at the body surface is described. For head, liver, muscle gastrocnemius, and muscle vastus lateralis, the SAR at the body surface was estimated for an 1H/13C double surface coil setup. Optimized duty cycle values obtained with this procedure show that broad-band 1H-decoupled 13C-MR spectroscopy is clinically feasible at 1.5 T for such a coil configuration within safety guidelines.

Muscle glycogen recovery after exercise during glucose and fructose intake monitored by 13C-NMR.

The purpose of this study was to examine muscle glycogen recovery with glucose feeding (GF) compared with fructose feeding (FF) during the first 8 h after partial glycogen depletion using 13C-nuclear magnetic resonance (NMR) on a clinical 1.5-TNMR system. After measurement of the glycogen concentration of the vastus lateralis (VL) muscle in seven male subjects, glycogen stores of the VL were depleted by bicycle exercise. During 8 h after completion of exercise, subjects were orally given either GF or FF while the glycogen content of the VL was monitored by 13C-NMR spectroscopy every second hour. The muscular glycogen concentration was expressed as percentage of the glycogen concentration measured before exercise. The glycogen recovery rate during GF (4.2 +/- 0.2%/h) was significantly higher (P < 0.05) compared with values during FF (2.2 +/- 0.3%/h). This study shows that 1) muscle glycogen levels are perceptible by 13 C-NMR spectroscopy at 1.5 T and 2) the glycogen restoration rate is higher after GF compared with after FF.

Non-invasive in vivo localized 1H spectroscopy of human astrocytoma implanted in rat brain: regional differences followed in time.

Human astrocytoma cells were cultured and inoculated into the rat brain. From the pre-clinical to the terminal state, tumour growth was monitored by in vivo MR imaging and by localized water-suppressed 1H spectroscopy (0.12-0.15 cm3 volumes) and spectroscopic imaging (0.01 cm3 voxels) employing the ACE localization technique. The MR experiments were conducted completely non-invasively, leaving the scalp intact. Brain spectra were obtained, showing distinct resonances for more than five different brain metabolites; they were not contaminated with lipid signals because of the adequate localization. Tumour progression, monitored in a selected volume of interest, was reflected in the corresponding spectra by decreasing intensities for resonances of N-acetyl aspartate and (phospho)creatine and increasing intensities for resonances of choline compounds and lactate. From spectroscopic imaging experiments metabolic heterogeneity could be deduced within the tumorous region. At particular times during tumour development spectra were obtained greatly resembling localized 1H MR spectra obtained from patients with astrocytomas by the use of similar localization methods. This emphasizes the relevance of animal model study for the evaluation of MR spectroscopic investigations in human brain tumour diagnosis and therapy evaluation.

ACE: a single-shot method for water-suppressed localization and editing of spectra, images, and spectroscopic images.

A versatile method for localized (1H) NMR spectroscopy is presented. The method intrinsically combines B0-based spatial localization with the possibility of water suppression and spectral editing. With this sequence it is feasible to localize not only single spectra but also phase-encoded images and spectroscopic images. The technique essentially integrates the "Hahn spin-echo" with the "stimulated echo" sequence and is therefore called ACE (acquiring combined echoes). It realizes water-suppressed three-dimensional localization in a single shot and can be used for localized shimming. Studies in which the new method is applied to phantoms with metabolites diluted at low concentrations are presented. Discrimination between lactate and alanine, employing an adapted spectral editing method with complete inversion, combined with simultaneous water suppression and localization of a 0.06-cc volume is shown. The suppression of signals from outside the selected volume is greater than or equal to 24,000. Also, the method is demonstrated by in vivo experiments at 6.3 T. Localized water-suppressed 1H spectra are obtained completely noninvasively, leaving scalp and fur intact, from well-defined volumes of 0.15 cc in the brain of a living rat. Water-suppressed spectroscopic imaging over a localized volume with "body" coil excitation and noninvasive surface coil detection yielded spectra from voxels as small as 25 microliters in the in vivo rat brain.