Quantitative single‐voxel spectroscopy: The reciprocity principle for receive‐only head coils

Abstract

Purpose: To correct MR spectra for local changes in the coil sensitivity for a widely used coil setup, consisting of a transmitting body coil and a receive-only head coil. Materials and Methods:The method relies on the reciprocity principle for the body coil and a correction factor for signal amplitudes between body coil and head coil. The correction is based either on the local flip angle dependence of the stimulated echo acquisition mode signal (TFC) or on the automatic RF calibration (RFC). Water phantoms of different volumes were used to simulate variable coil loads, and B1 field inhomogeneities were assessed by varying the voxel position. Furthermore, the correction was tested by longitudinal measurements in one volunteer. Results:The correction in vitro yields a reduction of the variation coefficient of the water signal by about 77% (TFC) and 66% (RFC) for different coil loads, as well as 55% (TFC) for variable voxel positions. Slightly lower reductions were assessed for the variation coefficients of the metabolite signals in vivo. Conclusion:This approach adequately compensates for local changes in coil sensitivity, when acquiring MR spectra with a receive-only head coil.

Quantitative SENSE-MRSI of the human brain

“…As has been shown previously [16,17], for transmit–receive coils the concentration [M] in voxel ( x,y ) can be determined from $$\left[\mathrm{normalM}\right]=\left[\mathrm{normalP}\right]\frac{S_{\mathrm{ti}}(x,y)k_{\mathrm{tp}}{F}_{\mathrm{ti}}}{S_{\mathrm{tp}}(x,y)k_{\mathrm{ti}}{F}_{\mathrm{tp}}}$$ where the factors k tp and k ti refer to the k t factors in the phantom and in vivo experiments, respectively, and it is assumed that the receiver gain is not changed between experiments. It is desirable to keep k tp and k ti as similar as possible by using identical conditions for both experiments; however, usually, correction factors will have to be applied, for example, to account for differences in relaxation times between the phantom and the in vivo metabolite.…”

“…Fortunately, both the B 1 inhomogeneity and the variable receiver coil loading can be measured (as described below) relative to the body coil, for which a load factor can be determined, since it is a transmit–receive coil. This procedure has previously been applied for single-voxel spectroscopy of the brain with phased-array coils [16,17], but not previously for quantitative MRSI as far as we are aware.…”

Quantitative Proton Magnetic Resonance Spectroscopy and Spectroscopic Imaging of the Brain

“…G RC depends on 1) subject specific parameters 2) Static magnetic field strength (B0), 3) the location of the volume of interest in the brain and 4) the design of the coil system. B1 field imaging allows G RC to be estimated (5, 6), but such measurements are usually are not made because they are subject specific and consume valuable scanner and patient time.…”

Proton magnetic resonance spectroscopy of skeletal muscle: A comparison of two quantitation techniques

“…Nevertheless, despite its advantages, the method remains sensitive to B 1 inhomogeneity and variable radiofrequency coil loading [9]. For transmit-receive coils, the coil loading can be estimated, and corrected for, using the reciprocity theorem [8], and this may be extended for use with receive-only phased array coils by comparing the relative sensitivities of each element of the receive array to that of the transmit coil [9, 16, 17]. …”