Noise correlations and SNR in phased‐array MRS

The acquisition of magnetic resonance spectroscopy (MRS) signals by multiple receiver coils can improve the signal‐to‐noise ratio (SNR) or alternatively can reduce the scan time maintaining a reliable SNR. However, using phased array coils in MRS studies requires efficient data processing and data combination techniques in order to exploit the sensitivity improvement of the phased array coil acquisition method. This paper describes a novel method for the combination of MRS signals acquired by phased array coils, even in presence of correlated noise between the acquisition channels. In fact, although it has been shown that electric and magnetic coupling mechanisms produce correlated noise in the coils, previous algorithms developed for MRS data combination have ignored this effect. The proposed approach takes advantage of a noise decorrelation stage to maximize the SNR of the combined spectra. In particular Principal Component Analysis (PCA) was exploited to project the acquired spectra in a subspace where the noise vectors are orthogonal. In this subspace the SNR weighting method will provide the optimal overall SNR. Performance evaluation of the proposed method is carried out on simulated ¹H‐MRS signals and experimental results are obtained on phantom ¹H‐MR spectra using a commercially available 8‐element phased array coil. Noise correlations between elements were generally low due to the optimal coil design, leading to a fair SNR gain (about 0.5%) in the center of the field of view (FOV). A greater SNR improvement was found in the peripheral FOV regions. Copyright © 2009 John Wiley & Sons, Ltd.     

Size‐adaptable 13‐channel receive array for brain MRI in human neonates at 3 T

Neonatal brain injury suffered by preterm infants and newborns with some medical conditions can cause significant neurodevelopmental disabilities. MRI is a preferred method to detect these accidents and perform in vivo evaluation of the brain. However, the commercial availability and optimality of receive coils for the neonatal brain is limited, which in many cases leads to images lacking in quality. As extensively demonstrated, receive arrays closely positioned around the scanned part provide images with high signal‐to‐noise ratios (SNRs). The present work proposes a pneumatic‐based MRI receive array that can physically adapt to infant head dimensions from 27‐week premature to 1.5 months old. Average SNR increases of up to 68% in the head region and 122% in the cortex region, compared with a 32‐channel commercial head coil, were achieved at 3 T. The consistent SNR distribution obtained through the complete coil size range, specifically in the cortex, allows the acquisition of images with similar quality across a range of head dimensions, which is not possible with fixed‐size coils due to the variable coil‐to‐head distance. The risks associated with mechanical pressure on the neonatal head are minimal and the head motion is restricted. The method could be used in coil designs for other age groups, body parts and subjects.     

Development of a 7 T RF coil system for breast imaging

In ultrahigh‐field MRI, such as 7 T, the signal‐to‐noise ratio (SNR) increases while transmit (Tx) field (B₁⁺) can be degraded due to inhomogeneity and elevated specific absorption rate (SAR). By applying new array coil concepts to both Tx and receive (Rx) coils, the B₁⁺ homogeneity and SNR can be improved. In this study, we developed and tested in vivo a new RF coil system for 7 T breast MRI. An RF coil system composed of an eight‐channel Tx‐only array based on a tic‐tac‐toe design (can be combined to operate in single‐Tx mode) in conjunction with an eight‐channel Rx‐only insert was developed. Characterizations of the B₁⁺ field and associated SAR generated by the developed RF coil system were numerically calculated and empirically measured using an anatomically detailed breast model, phantom and human breasts. In vivo comparisons between 3 T (using standard commercial solutions) and 7 T (using the newly developed coil system) breast imaging were made. At 7 T, about 20% B₁⁺ inhomogeneity (standard deviation over the mean) was measured within the breast tissue for both the RF simulations and 7 T experiments. The addition of the Rx‐only array enhances the SNR by a factor of about three. High‐quality MR images of human breast were acquired in vivo at 7 T. For the in vivo comparisons between 3 T and 7 T, an approximately fourfold increase of SNR was measured with 7 T imaging. The B₁⁺ field distributions in the breast model, phantom and in vivo were in reasonable agreement. High‐quality 7 T in vivo breast MRI was successfully acquired at 0.6 mm isotropic resolution using the newly developed RF coil system.     

Coil combination of multichannel MRSI data at 7 T: MUSICAL

The goal of this study was to evaluate a new method of combining multi‐channel ¹H MRSI data by direct use of a matching imaging scan as a reference, rather than computing sensitivity maps. Seven healthy volunteers were measured on a 7‐T MR scanner using a head coil with a 32‐channel array coil for receive‐only and a volume coil for receive/transmit. The accuracy of prediction of the phase of the ¹H MRSI data with a fast imaging pre‐scan was investigated with the volume coil. The array coil ¹H MRSI data were combined using matching imaging data as coil combination weights. The signal‐to‐noise ratio (SNR), spectral quality, metabolic map quality and Cramér–Rao lower bounds were then compared with the data obtained by two standard methods, i.e. using sensitivity maps and the first free induction decay (FID) data point. Additional noise decorrelation was performed to further optimize the SNR gain. The new combination method improved significantly the SNR (+29%), overall spectral quality and visual appearance of metabolic maps, and lowered the Cramér–Rao lower bounds (?34%), compared with the combination method based on the first FID data point. The results were similar to those obtained by the combination method using sensitivity maps, but the new method increased the SNR slightly (+1.7%), decreased the algorithm complexity, required no reference coil and pre‐phased all spectra correctly prior to spectral processing. Noise decorrelation further increased the SNR by 13%. The proposed method is a fast, robust and simple way to improve the coil combination in ¹H MRSI of the human brain at 7 T, and could be extended to other ¹H MRSI techniques. © 2013 The Authors. NMR in Biomedicine published by John Wiley & Sons, Ltd.     

Optimized 31P MRS in the human brain at 7 T with a dedicated RF coil setup

The design and construction of a dedicated RF coil setup for human brain imaging (¹H) and spectroscopy (³¹P) at ultra‐high magnetic field strength (7 T) is presented. The setup is optimized for signal handling at the resonance frequencies for ¹H (297.2 MHz) and ³¹P (120.3 MHz). It consists of an eight‐channel ¹H transmit–receive head coil with multi‐transmit capabilities, and an insertable, actively detunable ³¹P birdcage (transmit–receive and transmit only), which can be combined with a seven‐channel receive‐only ³¹P array. The setup enables anatomical imaging and ³¹P studies without removal of the coil or the patient. By separating transmit and receive channels and by optimized addition of array signals with whitened singular value decomposition we can obtain a sevenfold increase in SNR of ³¹P signals in the occipital lobe of the human brain compared with the birdcage alone. These signals can be further enhanced by 30 ± 9% using the nuclear Overhauser effect by B₁‐shimmed low‐power irradiation of water protons. Together, these features enable acquisition of ³¹P MRSI at high spatial resolutions (3.0 cm³ voxel) in the occipital lobe of the human brain in clinically acceptable scan times (~15 min). © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.     

An embedded four‐channel receive‐only RF coil array for fMRI experiments of the somatosensory pathway in conscious awake marmosets

fMRI has established itself as the main research tool in neuroscience and brain cognitive research. The common marmoset (Callithrix jacchus) is a non‐human primate model of increasing interest in biomedical research. However, commercial MRI coils for marmosets are not generally available. The present work describes the design and construction of a four‐channel receive‐only surface RF coil array with excellent signal‐to‐noise ratio (SNR) specifically optimized for fMRI experiments in awake marmosets in response to somatosensory stimulation. The array was designed as part of a helmet‐based head restraint system used to prevent motion during the scans. High SNR was obtained by building the coil array using a thin and flexible substrate glued to the inner surface of the restraint helmet, so as to minimize the distance between the array elements and the somatosensory cortex. Decoupling between coil elements was achieved by partial geometrical overlapping and by connecting them to home‐built low‐input‐impedance preamplifiers. In vivo images show excellent coverage of the brain cortical surface with high sensitivity near the somatosensory cortex. Embedding the coil elements within the restraint helmet allowed fMRI data in response to somatosensory stimulation to be collected with high sensitivity and reproducibility in conscious, awake marmosets. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

Brain γ‐aminobutyric acid (GABA) detection in vivo with the J‐editing 1H MRS technique: a comprehensive methodological evaluation of sensitivity enhancement,macromolecule contamination and test–retest reliability

Abnormalities in brain γ‐aminobutyric acid (GABA) have been implicated in various neuropsychiatric and neurological disorders. However, in vivo GABA detection by ¹H MRS presents significant challenges arising from the low brain concentration, overlap by much stronger resonances and contamination by mobile macromolecule (MM) signals. This study addresses these impediments to reliable brain GABA detection with the J‐editing difference technique on a 3‐T MR system in healthy human subjects by: (i) assessing the sensitivity gains attainable with an eight‐channel phased‐array head coil; (ii) determining the magnitude and anatomic variation of the contamination of GABA by MM; and (iii) estimating the test–retest reliability of the measurement of GABA with this method. Sensitivity gains and test–retest reliability were examined in the dorsolateral prefrontal cortex (DLPFC), whereas MM levels were compared across three cortical regions: DLPFC, the medial prefrontal cortex (MPFC) and the occipital cortex (OCC). A three‐fold higher GABA detection sensitivity was attained with the eight‐channel head coil compared with the standard single‐channel head coil in DLPFC. Despite significant anatomical variation in GABA + MM and MM across the three brain regions (p < 0.05), the contribution of MM to GABA + MM was relatively stable across the three voxels, ranging from 41% to 49%, a non‐significant regional variation (p = 0.58). The test–retest reliability of GABA measurement, expressed as either the ratio to voxel tissue water (W) or to total creatine, was found to be very high for both the single‐channel coil and the eight‐channel phased‐array coil. For the eight‐channel coil, for example, Pearson's correlation coefficient of test vs. retest for GABA/W was 0.98 (R² = 0.96, p = 0.0007), the percentage coefficient of variation (CV) was 1.25% and the intraclass correlation coefficient (ICC) was 0.98. Similar reliability was also found for the co‐edited resonance of combined glutamate and glutamine (Glx) for both coils. Copyright © 2016 John Wiley & Sons, Ltd.     

Low SAR 31P (multi‐echo) spectroscopic imaging using an integrated whole‐body transmit coil at 7T

Phosphorus (³¹P) MRSI provides opportunities to monitor potential biomarkers. However, current applications of ³¹P MRS are generally restricted to relatively small volumes as small coils are used. Conventional surface coils require high energy adiabatic RF pulses to achieve flip angle homogeneity, leading to high specific absorption rates (SARs), and occupy space within the MRI bore. A birdcage coil behind the bore cover can potentially reduce the SAR constraints massively by use of conventional amplitude modulated pulses without sacrificing patient space. Here, we demonstrate that the integrated ³¹P birdcage coil setup with a high power RF amplifier at 7 T allows for low flip angle excitations with short repetition time (T_R) for fast 3D chemical shift imaging (CSI) and 3D T₁‐weighted CSI as well as high flip angle multi‐refocusing pulses, enabling multi‐echo CSI that can measure metabolite T₂, over a large field of view in the body. B₁⁺ calibration showed a variation of only 30% in maximum B₁ in four volunteers. High signal‐to‐noise ratio (SNR) MRSI was obtained in the gluteal muscle using two fast in vivo 3D spectroscopic imaging protocols, with low and high flip angles, and with multi‐echo MRSI without exceeding SAR levels. In addition, full liver MRSI was achieved within SAR constraints. The integrated ³¹P body coil allowed for fast spectroscopic imaging and successful implementation of the multi‐echo method in the body at 7 T. Moreover, no additional enclosing hardware was needed for ³¹P excitation, paving the way to include larger subjects and more space for receiver arrays. The increase in possible number of RF excitations per scan time, due to the improved B₁⁺ homogeneity and low SAR, allows SNR to be exchanged for spatial resolution in CSI and/or T₁ weighting by simply manipulating T_R and/or flip angle to detect and quantify ratios from different molecular species.     

Maximizing sensitivity for fast GABA edited spectroscopy in the visual cortex at 7 T

The combination of functional MRI (fMRI) and MRS is a promising approach to relate BOLD imaging to neuronal metabolism, especially at high field strength. However, typical scan times for GABA edited spectroscopy are of the order of 6‐30 min, which is long compared with functional changes observed with fMRI. The aim of this study is to reduce scan time and increase GABA sensitivity for edited spectroscopy in the human visual cortex, by enlarging the volume of activated tissue in the primary visual cortex. A dedicated setup at 7 T for combined fMRI and GABA MRS is developed. This setup consists of a half volume multi‐transmit coil with a large screen for visual cortex activation, two high density receive arrays and an optimized single‐voxel MEGA‐sLASER sequence with macromolecular suppression for signal acquisition. The coil setup performance as well as the GABA measurement speed, SNR, and stability were evaluated. A 2.2‐fold gain of the average SNR for GABA detection was obtained, as compared with a conventional 7 T setup. This was achieved by increasing the viewing angle of the participant with respect to the visual stimulus, thereby activating almost the entire primary visual cortex, allowing larger spectroscopy measurement volumes and resulting in an improved GABA SNR. Fewer than 16 signal averages, lasting 1 min 23 s in total, were needed for the GABA fit method to become stable, as demonstrated in three participants. The stability of the measurement setup was sufficient to detect GABA with an accuracy of 5%, as determined with a GABA phantom. In vivo, larger variations in GABA concentration are found: 14‐25%. Overall, the results bring functional GABA detections at a temporal resolution closer to the physiological time scale of BOLD cortex activation.     

Non‐water‐suppressed short‐echo‐time magnetic resonance spectroscopic imaging using a concentric ring k‐space trajectory

Water‐suppressed MRS acquisition techniques have been the standard MRS approach used in research and for clinical scanning to date. The acquisition of a non‐water‐suppressed MRS spectrum is used for artefact correction, reconstruction of phased‐array coil data and metabolite quantification. Here, a two‐scan metabolite‐cycling magnetic resonance spectroscopic imaging (MRSI) scheme that does not use water suppression is demonstrated and evaluated. Specifically, the feasibility of acquiring and quantifying short‐echo (T_E = 14 ms), two‐dimensional stimulated echo acquisition mode (STEAM) MRSI spectra in the motor cortex is demonstrated on a 3 T MRI system. The increase in measurement time from the metabolite‐cycling is counterbalanced by a time‐efficient concentric ring k‐space trajectory. To validate the technique, water‐suppressed MRSI acquisitions were also performed for comparison. The proposed non‐water‐suppressed metabolite‐cycling MRSI technique was tested for detection and correction of resonance frequency drifts due to subject motion and/or hardware instability, and the feasibility of high‐resolution metabolic mapping over a whole brain slice was assessed. Our results show that the metabolite spectra and estimated concentrations are in agreement between non‐water‐suppressed and water‐suppressed techniques. The achieved spectral quality, signal‐to‐noise ratio (SNR) > 20 and linewidth <7 Hz allowed reliable metabolic mapping of five major brain metabolites in the motor cortex with an in‐plane resolution of 10 × 10 mm² in 8 min and with a Cramér‐Rao lower bound of less than 20% using LCModel analysis. In addition, the high SNR of the water peak of the non‐water‐suppressed technique enabled voxel‐wise single‐scan frequency, phase and eddy current correction. These findings demonstrate that our non‐water‐suppressed metabolite‐cycling MRSI technique can perform robustly on 3 T MRI systems and within a clinically feasible acquisition time.     

A 22‐channel receive array with Helmholtz transmit coil for anesthetized macaque MRI at 3 T

The macaque monkey is an important model for cognitive and sensory neuroscience that has been used extensively in behavioral, electrophysiological, molecular and, more recently, neuroimaging studies. However, macaque MRI has unique technical differences relative to human MRI, such as the geometry of highly parallel receive arrays, which must be addressed to optimize imaging performance. A 22‐channel receive coil array was constructed specifically for rapid high‐resolution anesthetized macaque monkey MRI at 3 T. A local Helmholtz transmit coil was used for excitation. Signal‐to‐noise ratios (SNRs) and noise amplification for parallel imaging were compared with those of single‐ and four‐channel receive coils routinely used for macaque MRI. The 22‐channel coil yielded significant improvements in SNR throughout the brain. Using this coil, the SNR in peripheral brain was 2.4 and 1.7 times greater than that obtained with single‐ or four‐channel coils, respectively. In the central brain, the SNR gain was 1.5 times that of both the single‐ and four‐channel coils. Finally, the performance of the array for functional, anatomical and diffusion‐weighted imaging was evaluated. For all three modalities, the use of the 22‐channel array allowed for high‐resolution and accelerated image acquisition. Copyright © 2013 John Wiley & Sons, Ltd.     

Boosting the SNR by adding a receive‐only endorectal monopole to an external antenna array for high‐resolution,T2‐weighted imaging of early‐stage cervical cancer with 7‐T MRI

The aim of this study was to investigate the signal‐to‐noise ratio (SNR) gain in early‐stage cervical cancer at ultrahigh‐field MRI (e.g. 7 T) using a combination of multiple external antennas and a single endorectal antenna. In particular, we used an endorectal monopole antenna to increase the SNR in cervical magnetic resonance imaging (MRI). This should allow high‐resolution, T₂‐weighted imaging and magnetic resonance spectroscopy (MRS) for metabolic staging, which could facilitate the local tumor status assessment. In a prospective feasibility study, five healthy female volunteers and six patients with histologically proven stage IB1–IIB cervical cancer were scanned at 7 T. We used seven external fractionated dipole antennas for transmit–receive (transceive) and an endorectally placed monopole antenna for reception only. A region of interest, containing both normal cervix and tumor tissue, was selected for the SNR measurement. Separated signal and noise measurements were obtained in the region of the cervix for each element and in the near field of the monopole antenna (radius < 30 mm) to calculate the SNR gain of the endorectal antenna in each patient. We obtained high‐resolution, T₂‐weighted images with a voxel size of 0.7 × 0.8 × 3.0 mm³. In four cases with optimal placement of the endorectal antenna (verified on the T₂‐weighted images), a mean gain of 2.2 in SNR was obtained at the overall cervix and tumor tissue area. Within a radius of 30 mm from the monopole antenna, a mean SNR gain of 3.7 was achieved in the four optimal cases. Overlap between the two different regions of the SNR calculations was around 24%. We have demonstrated that the use of an endorectal monopole antenna substantially increases the SNR of 7‐T MRI at the cervical anatomy. Combined with the intrinsically high SNR of ultrahigh‐field MRI, this gain may be employed to obtain metabolic information using MRS and to enhance spatial resolutions to assess tumor invasion.     

Optimizing diffusion MRI acquisition efficiency of rodent brain using simultaneous multislice EPI

Diffusion tensor imaging (DTI) of the brain provides essential information on the white matter integrity and structural connectivity. However, it suffers from a low signal‐to‐noise ratio (SNR) and requires a long scan time to achieve high spatial and/or diffusion resolution and wide brain coverage. With recent advances in parallel and simultaneous multislice (multiband) imaging, the SNR efficiency has been improved by reducing the repetition time (T_R). However, due to the limited number of RF coil channels available on preclinical MRI scanners, simultaneous multislice acquisition has not been practical. In this study, we demonstrate the ability of multiband DTI to acquire high‐resolution data of the mouse brain with 84 slices covering the whole brain in 0.2 mm isotropic resolution without a coil array at 9.4 T. Hadamard‐encoding four‐band pulses were used to acquire four slices simultaneously, with the reduction in the T_R maximizing the SNR efficiency. To overcome shot‐to‐shot phase variations, Hadamard decoding with a self‐calibrated phase was developed. Compared with single‐band DTI acquired with the same scan time, the multiband DTI leads to significantly increased SNR by 40% in the white matter. This SNR gain resulted in reduced variations in fractional anisotropy, mean diffusivity, and eigenvector orientation. Furthermore, the cerebrospinal fluid signal was attenuated, leading to reduced free‐water contamination. Without the need for a high‐density coil array or parallel imaging, this technique enables highly efficient preclinical DTI that will facilitate connectome studies.     

Cardiac proton spectroscopy using large coil arrays

Large coil arrays are widely used in clinical routine for cardiovascular imaging providing extended spatial coverage and enabling accelerated acquisition using parallel imaging approaches. This work investigates the use of large coil arrays in single‐voxel cardiac spectroscopy for the detection of myocardial creatine and triglyceride content. For this purpose, a navigator‐gated and cardiac‐triggered point‐resolved spectroscopy sequence was implemented, and data obtained in 11 healthy volunteers using 32‐ and 5‐element coil arrays were compared. For combination of the individual coil element signals, four strategies were evaluated differing in the manner of estimation of the complex coil weights and the amount of additional information required for coil combination. In all volunteers, and with both the 32‐ and 5‐channel coil arrays, triglyceride‐to‐water (0.44 ± 0.19% and 0.45 ± 0.17%) and total creatine‐to‐water (0.05 ± 0.02% and 0.05 ± 0.01%) contents were computed. The values were found to agree well, showing an intraclass correlation coefficient of 0.76 (p < 0.003). The results revealed a gain in signal‐to‐noise ratio of approximately 24% with the 32‐channel coil relative to the 5‐channel array. The findings may foster the integration of cardiac spectroscopy into clinical practice using large coil arrays, provided that appropriate reconstruction algorithms are implemented. Copyright © 2012 John Wiley & Sons, Ltd.     

Spatial Hadamard encoding of J‐edited spectroscopy using slice‐selective editing pulses

A new approach for simultaneous dual‐voxel J‐difference spectral editing is described, which uses spatially selective spectral‐editing pulses and Hadamard encoding. A theoretical framework for spatial Hadamard editing and reconstruction for parallel acquisition (SHERPA) was developed, applying gradient pulses during the frequency‐selective editing pulses. Spectral simulations were performed for either one (gamma‐aminobutyric acid, GABA) or two molecules (glutathione and lactate) simultaneously detected in two voxels. The method was tested in a two‐compartment GABA phantom, and finally applied to the left and right hemispheres of 10 normal control subjects, scanned at 3 T. SHERPA was successfully implemented at 3 T and gave results in close agreement with conventional MEGA‐PRESS scans in both the phantom and in vivo experiments. Simulations for GABA editing for (3 cm)³ voxels in the left and right hemispheres suggest that both editing efficiency losses and contamination between voxels are about 2%. Compared with conventional single‐voxel single‐metabolite J‐difference editing, two‐ or fourfold acceleration is possible without significant loss of SNR using the SHERPA method. Unlike some other dual‐voxel methods, the method can be used with single‐channel receiver coils, and there is no SNR loss due to unfavorable receive‐coil geometry factors.     

Combining parallel detection of proton echo planar spectroscopic imaging (PEPSI) measurements with a data‐consistency constraint improves SNR

One major challenge of MRSI is the poor signal‐to‐noise ratio (SNR), which can be improved by using a surface coil array. Here we propose to exploit the spatial sensitivity of different channels of a coil array to enforce the k‐space data consistency (DC) in order to suppress noise and consequently to improve MRSI SNR. MRSI data were collected using a proton echo planar spectroscopic imaging (PEPSI) sequence at 3 T using a 32‐channel coil array and were averaged with one, two and eight measurements (avg‐1, avg‐2 and avg‐8). The DC constraint was applied using a regularization parameter λ of 1, 2, 3, 5 or 10. Metabolite concentrations were quantified using LCModel. Our results show that the suppression of noise by applying the DC constraint to PEPSI reconstruction yields up to 32% and 27% SNR gain for avg‐1 and avg‐2 data with λ = 5, respectively. According to the reported Cramer–Rao lower bounds, the improvement in metabolic fitting was significant (p < 0.01) when the DC constraint was applied with λ ≥ 2. Using the DC constraint with λ = 3 or 5 can minimize both root‐mean‐square errors and spatial variation for all subjects using the avg‐8 data set as reference values. Our results suggest that MRSI reconstructed with a DC constraint can save around 70% of scanning time to obtain images and spectra with similar SNRs using λ = 5. Copyright © 2015 John Wiley & Sons, Ltd.     

A dedicated eight‐channel receive RF coil array for monkey brain MRI at 9.4 T

The neuroimaging of nonhuman primates (NHPs) realised with magnetic resonance imaging (MRI) plays an important role in understanding brain structures and functions, as well as neurodegenerative diseases and pathological disorders. Theoretically, an ultrahigh field MRI (≥7 T) is capable of providing a higher signal‐to‐noise ratio (SNR) for better resolution; however, the lack of appropriate radiofrequency (RF) coils for 9.4 T monkey MRI undermines the benefits provided by a higher field strength. In particular, the standard volume birdcage coil at 9.4 T generates typical destructive interferences in the periphery of the brain, which reduces the SNR in the neuroscience‐focused cortex region. Also, the standard birdcage coil is not capable of performing parallel imaging. Consequently, extended scan durations may cause unnecessary damage due to overlong anaesthesia. In this work, assisted by numerical simulations, an eight‐channel receive RF coil array was specially designed and manufactured for imaging NHPs at 9.4 T. The structure and geometry of the proposed receive array was optimised with numerical simulations, so that the SNR enhancement region was particularly focused on monkey brain. Validated with rhesus monkey and cynomolgus monkey brain images acquired from a 9.4 T MRI scanner, the proposed receive array outperformed standard birdcage coil with higher SNR, mean diffusivity and fractional anisotropy values, as well as providing better capability for parallel imaging.     

Simultaneous multi‐slice spin‐ and gradient‐echo dynamic susceptibility‐contrast perfusion‐weighted MRI of gliomas

Although combined spin‐ and gradient‐echo (SAGE) dynamic susceptibility‐contrast (DSC) MRI can provide perfusion quantification that is sensitive to both macrovessels and microvessels while correcting for T₁‐shortening effects, spatial coverage is often limited in order to maintain a high temporal resolution for DSC quantification. In this work, we combined a SAGE echo‐planar imaging (EPI) sequence with simultaneous multi‐slice (SMS) excitation and blipped controlled aliasing in parallel imaging (blipped CAIPI) at 3 T to achieve both high temporal resolution and whole brain coverage. Two protocols using this sequence with multi‐band (MB) acceleration factors of 2 and 3 were evaluated in 20 patients with treated gliomas to determine the optimal scan parameters for clinical use. ΔR₂*(t) and ΔR₂(t) curves were derived to calculate dynamic signal‐to‐noise ratio (dSNR), ΔR₂*‐ and ΔR₂‐based relative cerebral blood volume (rCBV), and mean vessel diameter (mVD) for each voxel. The resulting SAGE DSC images acquired using MB acceleration of 3 versus 2 appeared visually similar in terms of image distortion and contrast. The difference in the mean dSNR from normal‐appearing white matter (NAWM) and that in the mean dSNR between NAWM and normal‐appearing gray matter were not statistically significant between the two protocols. ΔR₂*‐ and ΔR₂‐rCBV maps and mVD maps provided unique contrast and spatial heterogeneity within tumors.     

Accelerated spectroscopic imaging of hyperpolarized C‐13 pyruvate using SENSE parallel imaging

The ability to accelerate the spatial encoding process during a chemical shift imaging (CSI) scan of hyperpolarized compounds is demonstrated through parallel imaging. A hardware setup designed to simultaneously acquire ¹³C data from multiple receivers is presented here. A system consisting of four preamplifiers, four gain stages, a transmit coil, and a four receive channel rat coil was built for single channel excitation and simultaneous multi‐channel detection of ¹³C signals. The hardware setup was integrated with commercial scanner electronics, allowing the system to function similar to a conventional proton multi‐channel setup, except at a different frequency. The ability to perform parallel imaging is demonstrated in vivo. CSI data from the accelerated scans are reconstructed using a self‐calibrated multi‐spectral parallel imaging algorithm, by using lower resolution coil sensitivity maps obtained from the central region of k‐space. The advantages and disadvantages of parallel imaging in the context of imaging hyperpolarized compounds are discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Non‐invasive detection of glycine as a biomarker of malignancy in childhood brain tumours using in‐vivo 1H MRS at 1.5 Tesla confirmed by ex‐vivo high‐resolution magic‐angle spinning NMR

Management of brain tumours in children would benefit from improved non‐invasive diagnosis, characterisation and prognostic biomarkers. Metabolite profiles derived from in‐vivo MRS have been shown to provide such information. Studies indicate that using optimum a priori information on metabolite contents in the construction of linear combination (LC) models of MR spectra leads to improved metabolite profile estimation. Glycine (Gly) is usually neglected in such models due to strong overlap with myo‐inositol (mI) and a low concentration in normal brain. However, biological studies indicate that Gly is abundant in high‐grade brain tumours. This study aimed to investigate the quantitation of Gly in paediatric brain tumours using MRS analysed by LCModel?, and its potential as a non‐invasive biomarker of malignancy. Single‐voxel MRS was performed using PRESS (TR 1500 ms, TE 30 ms/135 ms) on a 1.5 T scanner. Forty‐seven cases (18 high grade (HG), 17 low grade (LG), 12 ungraded) were retrospectively selected if both short‐TE and long‐TE MRS (n = 33) or short‐TE MRS and high‐resolution magic‐angle spinning (HRMAS) of matched surgical samples (n = 15) were available. The inclusion of Gly in LCModel? analyses led to significantly reduced fit residues for both short‐TE and long‐TE MRS (p < 0.05). The Gly concentrations estimated from short‐TE MRS were significantly correlated with the long‐TE values (R = 0.91, p < 0.001). The Gly concentration estimated by LCModel? was significantly higher in HG versus LG tumours for both short‐TE (p < 1e‐6) and long‐TE (p = 0.003) MRS. This was consistent with the HRMAS results, which showed a significantly higher normalised Gly concentration in HG tumours (p < 0.05) and a significant correlation with the normalised Gly concentration measured from short‐TE in‐vivo MRS (p < 0.05). This study suggests that glycine can be reliably detected in paediatric brain tumours using in‐vivo MRS on standard clinical scanners and that it is a promising biomarker of tumour aggressiveness. Copyright © 2009 John Wiley & Sons, Ltd.