A model to assess SAR for surface coil magnetic resonance spectroscopy measurements.

Surface coils are widely used in magnetic resonance (MR) studies due to their superior signal to noise properties. Application of excessive power levels to transmit surface coils may result in local tissue damage. A homogeneous muscle tissue model for the conservative prediction of surface coil specific absorption rate (SAR) is introduced. Based on this model, sequence parameters can be limited to provide operational levels within safety guidelines. It is demonstrated that this model provides worst-case SAR estimates at MR frequencies of 25.75 MHz and 63.6 MHz. The dependence of SAR on model structure and geometry is analysed and conclusions on the relationship between SAR levels and local anatomy are drawn. By making a worst-case assumption for the tissue parameters the model provides safe operational levels for all tissue types. Power-demanding proton-decoupled 31P magnetic resonance spectroscopy experiments are possible based on the SAR estimates provided. To date SAR values are calculated for 1 g of tissue. Changes in regulations to calculate SAR values for 10 g tissue masses, and the according averaging of local SAR over a larger volume, have been proposed by the International Electrotechnical Commission. A comparative study shows that up to 100% more energy may be applied to surface coils if SAR values are determined for 10 g tissue masses rather than 1 g tissue masses.

Human rectal adenocarcinoma: demonstration of 1H-MR spectra in vivo at 1.5 T.

This study was designed to determine whether 1H-MR spectra of locally advanced human rectal adenocarcinoma could be acquired in vivo at 1.5 T. Despite the relatively large size of these neoplasms, only six out of 21 tumors accommodated a voxel size of 8 cm3. This was due to air pockets within the tumor mass, which limited voxel positioning. Localized proton spectra were acquired at short (20 ms) and long (135 ms) echo times (TEs) using a single-voxel technique. The most commonly detected metabolites were choline and lipid.

An algorithm for the optimum combination of data from arbitrary magnetic resonance phased array probes.

When summing the spectra acquired with phased array coils, signals with low signal-to-noise ratio or wrongly corrected phase may degrade the overall signal-to-noise ratio (SNR). Here we present a mathematical expression predicting the dependence of combined SNR on the signal-to-noise ratios and errors in phase correction of composite signals. Based on this equation, signals that do not lead to an overall increase in signal-to-noise ratio can be identified and excluded from the weighted sum of signals. This tool is particularly useful for the combination of large numbers of signals. Additionally, a simple and robust algorithm for calculating the complex weighting factors necessary for the signal-to-noise weighted combination of spectroscopic data is presented. Errors in the calculation and correction of relative phase differences between composite spectra are analysed. The errors have a negligible effect on the overall spectral SNR for typical clinical magnetic resonance spectroscopy (MRS). The signal combination routine developed here has been applied to the first in vivo MRS study of human rectal adenocarcinomas at 1.5 T (Dzik-Jurasz A S K, Murphy P S, George M, Prock T, Collins D J, Swift I and Leach M O 2001 Magn. Reson. Med. at press), showing improvements of combined spectral SNR of up to 34% over the maximum SNR from a single element.

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.

The quantitative 19F-imaging of albumin at 1.5 T: a potential in-vivo tool.

19F-MR-imaging has been used to quantitate albumin concentration in a phantom at 1.5 T. The experimentally derived relationship between albumin concentration and the T1 relaxation time of a fluorinated marker, tetrafluorosuccinic acid (TFSA) was used to calculate the albumin concentration from a quantitative 19F T1 map acquired using a gradient echo sequence. There was close correlation between calculated and actual BSA concentrations (r = 0.99, SE = 0.15). The potentially interfering effect of paramagnetic species on T1 relaxation times was also investigated. Relaxivity data show that albumin concentration measurements should be performed prior to any contrast agent administration.

Numerical evaluation of shaped surface coil sensitivity at 63 MHz.

Surface coils are widely used in magnetic resonance studies due to their superior signal to noise (SNR) properties. When shaping planar coils to cylindrical surfaces, the region with maximum sensitivity migrates from the coil plane towards the centre of the shaping radius. The influence of the coil current, the probe and tissue dimensions, the electrical tissue properties and the operating frequency on the B1 field strength of a coil has been studied using statistical methods. This analysis allows the dependence of the axial SNR distribution of circular and square surface coils on shaping radius and coil dimensions to be evaluated quantitatively using 3D finite element methods. An empirically derived equation describing the dependence of the SNR distributions on coil geometry and depth allows the optimum coil dimensions to be predicted for a given shaping radius and desired optimized depth of sensitivity. Simulations are validated experimentally using both B1 and SNR mapping techniques. A comparison between the axial SNR of circular and square coils demonstrated equal SNR distributions of coils with equivalent area at depth.

SAR and tissue heating with a clinical (31)P MRS protocol using surface coils, adiabatic pulses, and proton-decoupling.

In MRS studies using surface transmit coils, accurate assessment of local SAR and RF heating represents a difficult problem involving the coil geometry and electromagnetic and geometric tissue properties. Methodologies to determine the optimum operating parameters for dual-resonant surface coil measurements are presented, based on a standardized coil and protocol used in a multicenter (31)P MRS clinical trial, using adiabatic pulses and bilevel proton decoupling. Spatial distributions of absorbed radiation in human calf and in a tissue-equivalent gel phantom were modeled using finite-element simulations and realistic conductivity and permittivity values. Local SAR in worst-case 1 cm(3) volumes of interest (VOIs) in calf is predicted to be below international guidelines, and the temperature at the skin surface was found to increase due to the RF by less than 2 degrees C and remain below 37 degrees C. The heating rate and maximum temperature in the gel, at positions guided by the simulations, were within guideline values for both extremities and trunk and in reasonable agreement with that predicted.