An NMR Phased Array for Human Cardiac ³¹P Spectroscopy

Abstract

A four-coil phased-array 31P NMR receiver was designed and tested for human cardiac applications, to determine whether the combination of relatively high signal-to-noise ratio (SNR) and large field of view produced in 1H imaging is also realized for in vivo 31P spectroscopy. Spectra were acquired in parallel from an array of four overlapping 6.5-cm surface coils using one- and two-dimensional phase-encoding pulse sequences and were optimally combined to yield composite spectroscopic images. The phased array was found to generate useful 31P spectra from a 2.5-fold wider lateral region around the anterior myocardium than a single receiver of the same size as the array elements, with no increase in imaging time. In addition, the sensitive depth was increased by up to 2 cm over that of a single coil. Spectra could be acquired in roughly 15 min from a region extending to the middle of the heart, with voxel sizes of 2 x 2 x 4 cm3. For the average heart voxel, the SNR of the combined spectrum was higher than that of the best spectrum from any one coil in the array by 30%, with some voxels showing an increase as high as 60%.

4T Actively detuneable double-tuned 1H/31P head volume coil and four-channel 31P phased array for human brain spectroscopy

“…"Double tuning" of the balun is obtained when the impedance of this resonance circuit equals to j/ωC 1 at 170MHz and -jωL 1 at 69 MHz. Thus, L 0 and C 0 are given by [1] [ 2] where ω 1 = 69 MHz and ω 2 = 170 MHz. Values of C 1 and L 1 were measured at 170 MHz and 69 MHz, respectively, using a network analyzer in the impedance meter mode.…”

“…Values of C 1 and L 1 were measured at 170 MHz and 69 MHz, respectively, using a network analyzer in the impedance meter mode. L 1 and C 1 were measured individually for each of the four tri-axial baluns and then L 0 and C 0 were calculated using equations [1] and [2]. Table 1 presents an example of calculated values for one of the four tri-axial baluns where presents the resonance frequency of the series resonance circuit.…”

“…Although 31 P in vivo magnetic resonance spectroscopic imaging (MRSI) is often limited by the available SNR, there are few reports of the use of 31 P phased arrays to increase sensitivity for human brain studies (2,3). Although receive-only 31 P phased arrays in combination with larger transmit-only surface coils have been reported (2,3); to our knowledge their use with homogeneous transmit volume coils has not been yet reported. To provide highresolution anatomical imaging and shimming with a homogeneous volume coil without removal of the coil or patient from the magnet, double-tuning of the transmit volume coil is required.…”

In vivo method for correcting transmit/receive nonuniformities with phased array coils

“…A potential drawback to using such coils, however, is the greater intensity nonuniformity, relative to a volume coil. Images acquired using phased array coils include inhomogeneous signal intensities (SI) resulting from the intrinsic configuration of the coils (2,8,9). Additionally, with the increasing field strength, other sources of intensity variations become bigger, including gradient-driven eddy currents, wave behavior, radio-frequency (RF) penetration effects, and the interaction of the RF with the electromagnetic properties of the loaded tissues.…”

Receive array magnetic resonance spectroscopy: Whitened singular value decomposition (WSVD) gives optimal Bayesian solution

“…Their algorithms require calibration experiments or detailed calculations (3,4) to determine the noise statistics and to determine the sensitivity and phase shift for each voxel and array element. In practice, this approach has not been widely used because the coil sensitivity and phase maps vary significantly with positioning and composition of the subject (3), especially with flexible array coils (4).…”