Signal-to-noise ratio versus field strength for small surface coils.

The increasing signal-to-noise ratio (SNR) is the main reason to use ultrahigh field MRI. Here, we investigate the dependence of the SNR on the magnetic field strength, especially for small animal applications, where small surface coils are used and coil noise cannot be ignored. Measurements were performed at five field strengths from 3 to 14.1 T, using 2.2-cm surface coils with an identical coil design for transmit and receive on two water samples with and without salt. SNR was measured in a series of spoiled gradient echo images with varying flip angle and corrected for saturation based on a series of flip angle and T1 measurements. Furthermore, the noise figure of the receive chain was determined and eliminated to remove instrument dependence. Finally, the coil sensitivity was determined based on the principle of reciprocity to obtain a measure for ultimate SNR. Before coil sensitivity correction, the SNR increase in nonconductive samples is highly supralinear with B0 1.6-2.7, depending on distance to the coil, while in the conductive sample, the growth is smaller, being around linear close to the surface coil and increasing up to a B0 2.0 dependence when moving away from the coil. After sensitivity correction, the SNR increase is independent of loading with B0 2.1. This study confirms the supralinear increase of SNR with increasing field strengths. Compared with most human measurements with larger coil sizes, smaller surface coils, as mainly used in animal studies, have a higher contribution of coil noise and thus a different behavior of SNR at high fields.

Dynamic parallel imaging at 9.4 T using reconfigurable receive coaxial dipoles.

Parallel imaging is one of the key MRI technologies that allow reduction of image acquisition time. However, the parallel imaging reconstruction commonly leads to a signal-to-noise ratio (SNR) drop evaluated using a so-called geometrical factor (g-factor). The g-factor is minimized by increasing the number of array elements and their spatial diversity. At the same time, increasing the element count requires a decrease in their size. This may lead to insufficient coil loading, an increase in the relative noise contribution from the RF coil itself, and hence SNR reduction. Previously, instead of increasing the channel number, we introduced the concept of electronically switchable time-varying sensitivities, which was shown to improve parallel imaging performance. In this approach, each reconfigurable receive element supports two spatially distinct sensitivity profiles. In this work, we developed and evaluated a novel eight-element human head receive-only reconfigurable coaxial dipole array for human head imaging at 9.4 T. In contrast to the previously reported reconfigurable dipole array, the new design does not include direct current (DC) control wires connected directly to the dipoles. The coaxial cable itself is used to deliver DC voltage to the PIN diodes located at the ends of the antennas. Thus, the novel reconfigurable coaxial dipole design opens a way to scale the dynamic parallel imaging up to a realistic number of channels, that is, 32 and above. The novel array was optimized and tested experimentally, including in vivo studies. It was found that dynamic sensitivity switching provided an 8% lower mean and 33% lower maximum g-factor (for Ry × Rz = 2 × 2 acceleration) compared with conventional static sensitivities.

Alpha-180 spin-echo based line-scanning method for high resolution laminar-specific fMRI.

Laminar-specific functional magnetic resonance imaging (fMRI) has been widely used to study circuit-specific neuronal activity by mapping spatiotemporal fMRI response patterns across cortical layers. Hemodynamic responses reflect indirect neuronal activity given limit of spatial and temporal resolution. Previous gradient-echo based line-scanning fMRI (GELINE) method was proposed with high temporal (50 ms) and spatial (50 µm) resolution to better characterize the fMRI onset time across cortical layers by employing 2 saturation RF pulses. However, the imperfect RF saturation performance led to poor boundary definition of the reduced region of interest (ROI) and aliasing problems outside of the ROI. Here, we propose α (alpha)-180 spin-echo-based line-scanning fMRI (SELINE) method to resolve this issue by employing a refocusing 180° RF pulse perpendicular to the excitation slice. In contrast to GELINE signals peaked at the superficial layer, we detected varied peaks of laminar-specific BOLD signals across deeper cortical layers with the SELINE method, indicating the well-defined exclusion of the large drain-vein effect with the spin-echo sequence. Furthermore, we applied the SELINE method with 200 ms TR to sample the fast hemodynamic changes across cortical layers with a less draining vein effect. In summary, this SELINE method provides a novel acquisition scheme to identify microvascular-sensitive laminar-specific BOLD responses across cortical depth.

Reconfigurable dipole receive array for dynamic parallel imaging at ultra-high magnetic field.

PURPOSE: To extend the concept of 3D dynamic parallel imaging, we developed a prototype of an electronically reconfigurable dipole array that provides sensitivity alteration along the dipole length. METHODS: We developed a radiofrequency array coil consisting of eight reconfigurable elevated-end dipole antennas. The receive sensitivity profile of each dipole can be electronically shifted toward one or the other end by electrical shortening or lengthening the dipole arms using positive-intrinsic-negative-diode lump-element switching units. Based on the results of electromagnetic simulations, we built the prototype and tested it at 9.4 T on phantom and healthy volunteer. A modified 3D SENSE reconstruction was used, and geometry factor (g-factor) calculations were performed to assess the new array coil. RESULTS: Electromagnetic simulations showed that the new array coil was capable of alteration of its receive sensitivity profile along the dipole length. Electromagnetic and g-factor simulations showed closely agreeing predictions when compared to the measurements. The new dynamically reconfigurable dipole array provided significant improvement in geometry factor compared to static dipoles. We obtained up to 220% improvement for 3 × 2 (Ry × Rz ) acceleration compared to the static configuration case in terms of maximum g-factor and up to 54% in terms of mean g-factor for the same acceleration. CONCLUSION: We presented an 8-element prototype of a novel electronically reconfigurable dipole receive array that permits rapid sensitivity modulations along the dipole axes. Applying dynamic sensitivity modulation during image acquisition emulates two virtual rows of receive elements along the z-direction, and therefore improves parallel imaging performance for 3D acquisitions.

Double-row 16-element folded-end dipole transceiver array for 3D RF shimming of the whole human brain at 9.4 T.

Homogeneity and longitudinal coverage of transmit (Tx) human head RF coils at ultrahigh field (UHF, ≥7 T) can be improved by 3D RF shimming, which requires using multi-row Tx arrays. Examples of 3D RF shimming using double-row UHF loop transceiver (TxRx) and Tx arrays have been described previously. Dipole antennas provide unique simplicity and robustness while offering comparable Tx efficiency and signal-to-noise ratio to conventional loop designs. Single-row Tx and TxRx human head UHF dipole arrays have been previously described by multiple groups. Recently, we developed a novel type of dipole antenna, a folded-end dipole, and presented single-row eight-element array prototypes for human head imaging at 7 and 9.4 T. These studies have shown that the novel antenna design can improve the longitudinal coverage and minimize peak local specific absorption rate (SAR) as compared with common unfolded dipoles. In this work, we developed, constructed, and evaluated a 16-element double-row TxRx folded-end dipole array for human head imaging at 9.4 T. To minimize cross-talk between neighboring dipoles located in different rows, we used transformer decoupling, which decreased coupling to a level below -20 dB. The developed array design was demonstrated to be capable of 3D static RF shimming and can be potentially used for dynamic shimming using parallel transmission. For optimal phase shifts between the rows, the array provides 11% higher SAR efficiency and 18% higher homogeneity than a folded-end dipole single-row array of the same length. The design also offers a substantially simpler and more robust alternative to the common double-row loop array with about 10% higher SAR efficiency and better longitudinal coverage.

Deuterium metabolic imaging of the human brain in vivo at 7 T.

PURPOSE: To explore the potential of deuterium metabolic imaging (DMI) in the human brain in vivo at 7 T, using a multi-element deuterium (2 H) RF coil for 3D volume coverage. METHODS: 1 H-MR images and localized 2 H MR spectra were acquired in vivo in the human brain of 3 healthy subjects to generate DMI maps of 2 H-labeled water, glucose, and glutamate/glutamine (Glx). In addition, non-localized 2 H-MR spectra were acquired both in vivo and in vitro to determine T1 and T2 relaxation times of deuterated metabolites at 7 T. The performance of the 2 H coil was assessed through numeric simulations and experimentally acquired B1 + maps. RESULTS: 3D DMI maps covering the entire human brain in vivo were obtained from well-resolved deuterated (2 H) metabolite resonances of water, glucose, and Glx. The T1 and T2 relaxation times were consistent with those reported at adjacent field strengths. Experimental B1 + maps were in good agreement with simulations, indicating efficient and homogeneous B1 + transmission and low RF power deposition for 2 H, consistent with a similar array coil design reported at 9.4 T. CONCLUSION: Here, we have demonstrated the successful implementation of 3D DMI in the human brain in vivo at 7 T. The spatial and temporal nominal resolutions achieved at 7 T (i.e., 2.7 mL in 28 min, respectively) were close to those achieved at 9.4 T and greatly outperformed DMI at lower magnetic fields. DMI at 7 T and beyond has clear potential in applications dealing with small brain lesions.

A 32-element loop/dipole hybrid array for human head imaging at 7 T.

PURPOSE: To improve whole-brain SNR at 7 Tesla, a novel 32-element hybrid human head array coil was developed, constructed, and tested. METHODS: Our general design strategy is based on 2 major ideas: Firstly, following suggestions of previous works based on the ultimate intrinsic SNR theory, we combined loops and dipoles for improvement of SNR near the head center. Secondly, we minimized the total number of array elements by using a hybrid combination of transceive (TxRx) and receive (Rx) elements. The new hybrid array consisted of 8 folded-end TxRx-dipole antennas and 3 rows of 24 Rx-loops all placed in a single layer on the surface of a tight-fit helmet. RESULTS: The developed array significantly improved SNR in vivo both near the center (∼20%) and at the periphery (∼20% to 80%) in comparison to a common commercial array coil with 8 transmit (Tx) and 32 Rx-elements. Whereas 24 loops alone delivered central SNR very similar to that of the commercial coil, the addition of complementary dipole structures provided further improvement. The new array also provided ∼15% higher Tx efficiency and better longitudinal coverage than that of the commercial array. CONCLUSION: The developed array coil demonstrated advantages in combining complementary TxRx and Rx resonant structures, that is, TxRx-dipoles and Rx-loops all placed in a single layer at the same distance to the head. This strategy improved both SNR and Tx-performance, as well as simplified the total head coil design, making it more robust.

Double-row dipole/loop combined array for human whole brain imaging at 7 T.

Important issues in designing radiofrequency (RF) coils for human head imaging at ultra-high field (UHF; ≥7 T) are the inhomogeneity and longitudinal coverage (along the magnet axis) of the transmit (Tx) RF field. Both the homogeneity and coverage produced by Tx volume coils can be improved by means of three-dimensional (3D) RF shimming, which requires the use of multirow Tx-arrays. In addition, according to recent findings of the ultimate intrinsic signal-to-noise ratio (UISNR) theory, the loop-only receive (Rx) arrays do not provide optimal SNR near the brain center at UHF. The latter can be obtained by combining complementary conductive structures carrying different current patterns (e.g., loops and dipole antennas). In this work, we developed, constructed, and evaluated a novel 32-element hybrid array design for human head imaging at 7 T. The array consists of 16 transceiver loops placed in two rows circumscribing the head and 16 folded-end Rx-only dipoles positioned in the centers of loops. By placing all elements in a single layer, we increased RF power deposition into the tissue and, thus, preserved the Tx-efficiency. Using this hybrid design also simplifies the coil structure by minimizing the total number of array elements. The array demonstrated whole brain coverage, 3D RF shimming capability, and high SNR. It provided ~15% higher SNR near the brain center and, depending on the RF shim mode, from 20% to 40% higher Tx-efficiency than a common commercial head array coil.

Accelerated MRI at 9.4 T with electronically modulated time-varying receive sensitivities.

PURPOSE: To investigate how electronically modulated time-varying receive sensitivities can improve parallel imaging reconstruction at ultra-high field. METHODS: Receive sensitivity modulation was achieved by introducing PIN diodes in the receive loops, which allow rapid switching of capacitances in both arms of each loop coil and by that alter B1- profiles, resulting in two distinct receive sensitivity configurations. A prototype 8-channel reconfigurable receive coil for human head imaging at 9.4T was built, and MR measurements were performed in both phantom and human subject. A modified SENSE reconstruction for time-varying sensitivities was formulated, and g-factor calculations were performed to investigate how modulation of receive sensitivity profiles during image encoding can improve parallel imaging reconstruction. The optimized modulation pattern was realized experimentally, and reconstructions with the time-varying sensitivities were compared with conventional static SENSE reconstructions. RESULTS: The g-factor calculations showed that fast modulation of receive sensitivities in the order of the ADC dwell time during k-space acquisition can improve parallel imaging performance, as this effectively makes spatial information of both configurations simultaneously available for image encoding. This was confirmed by in vivo measurements, for which lower reconstruction errors (SSIM = 0.81 for acceleration R = 4) and g-factors (max g = 2.4; R = 4) were observed for the case of rapidly switched sensitivities compared to conventional reconstruction with static sensitivities (SSIM = 0.74 and max g = 3.2; R = 4). As the method relies on the short RF wavelength at ultra-high field, it does not yield significant benefits at 3T and below. CONCLUSIONS: Time-varying receive sensitivities can be achieved by inserting PIN diodes in the receive loop coils, which allow modulation of B1- patterns. This offers an additional degree of freedom for image encoding, with the potential for improved parallel imaging performance at ultra-high field.

Simultaneous detection of metabolite concentration changes, water BOLD signal and pH changes during visual stimulation in the human brain at 9.4T.

This study presents a method to directly link metabolite concentration changes and BOLD response in the human brain during visual stimulation by measuring the water and metabolite signals simultaneously. Therefore, the metabolite-cycling (MC) non-water suppressed semiLASER localization technique was optimized for functional 1H MRS in the human brain at 9.4 T. Data of 13 volunteers were acquired during a 26:40 min visual stimulation block-design paradigm. Activation-induced BOLD signal was observed in the MC water signal as well as in the NAA-CH3 and tCr-CH3 singlets. During stimulation, glutamate concentration increased 2.3 ± 2.0% to a new steady-state, while a continuous increase over the whole stimulation period could be observed in lactate with a mean increase of 35.6 ± 23.1%. These increases of Lac and Glu during brain activation confirm previous findings reported in literature. A positive correlation of the MC water BOLD signal with glutamate and lactate concentration changes was found. In addition, a pH decrease calculated from a change in the ratio of PCr to Cr was observed during brain activation, particularly at the onset of the stimulation.

Relaxation-corrected macromolecular model enables determination of 1 H longitudinal T1 -relaxation times and concentrations of human brain metabolites at 9.4T.

PURPOSE: Ultrahigh field MRS has improved characterization of the neurochemical profile. To compare results obtained at 9.4T to those from lower field strengths, it is of interest to quantify the concentrations of metabolites measured. Thus, measuring T1 -relaxation times is necessary to correct for T1 -weighting that occurs in acquisitions for single-voxel spectroscopy and spectroscopic imaging. A macromolecule (MM) simulation model was developed to fit MM contributions to the short TE inversion series used to measure T1 -relaxation times. METHODS: An inversion series with seven time points was acquired with metabolite-cycled STEAM to estimate T1 -relaxation times of metabolites. A short TE was employed in this study to retain signals from metabolites with short T2 -relaxation times and J-couplings. The underlying macromolecule spectrum was corrected by developing a sequence-specific, relaxation-corrected simulated MM model. Quantification of metabolite peaks was performed using internal water referencing and relaxation corrections. RESULTS: T1 -relaxation times for metabolites range from approximately 750 to approximately 2000 ms and approximately 1000 to approximately 2400 ms in gray matter (GM)- and white matter (WM)- rich voxels, respectively. Quantification of metabolites was compared between GM and WM voxels, as well as between results that used a simulated MM spectrum against those that used an experimentally acquired MM spectrum. Metabolite concentrations are reported in mmol/kg quantities. CONCLUSION: T1 -relaxation times are reported for nonsinglet resonances for the first time at 9.4T by use of a MM simulation model to account for contributions from the MM spectrum. In addition to T1 -relaxation times, quantification results of metabolites from GM- and WM-rich voxels are reported.

Deuterium metabolic imaging in the human brain at 9.4 Tesla with high spatial and temporal resolution.

PURPOSE: To present first highly spatially resolved deuterium metabolic imaging (DMI) measurements of the human brain acquired with a dedicated coil design and a fast chemical shift imaging (CSI) sequence at an ultrahigh field strength of B0 = 9.4 T. 2H metabolic measurements with a temporal resolution of 10 min enabled the investigation of the glucose metabolism in healthy human subjects. METHODS: The study was performed with a double-tuned coil with 10 TxRx channels for 1H and 8TxRx/2Rx channels for 2H and an Ernst angle 3D CSI sequence with a nominal spatial resolution of 2.97 ml and a temporal resolution of 10 min. RESULTS: The metabolism of [6,6'-2H2]-labeled glucose due to the TCA cycle could be made visible in high resolution metabolite images of deuterated water, glucose and Glx over the entire human brain. CONCLUSION: X-nuclei MRSI as DMI can highly benefit from ultrahigh field strength enabling higher temporal and spatial resolutions.

9.4 T double-tuned 13 C/1 H human head array using a combination of surface loops and dipole antennas.

MRI at ultra-high field (UHF, ≥7 T) provides a natural strategy for improving the quality of X-nucleus magnetic resonance spectroscopy and imaging due to the intrinsic benefit of increased signal-to-noise ratio. Considering that RF coils require both local transmission and reception at UHF, the designs of double-tuned coils, which often consist of several layers of transmit and receive resonant elements, become quite complex. A few years ago, a new type of RF coil, ie a dipole antenna, was developed and used for human body and head imaging at UHF. Due to the mechanical and electrical simplicity of dipole antennas, combining an X-nucleus surface loop array with 1 H dipoles can substantially simplify the design of a double-tuned UHF human head array coil. Recently, we developed a novel bent folded-end dipole transceiver array for human head imaging at 9.4 T. The new eight-element dipole array demonstrated full brain coverage, and transmit efficiency comparable to that of the substantially more complex 16-element surface loop array. In this work, we developed, constructed and evaluated a double-tuned 13 C/1 H human head 9.4 T array consisting of eight 13 C transceiver surface loops and eight 1 H transceiver bent folded-end dipole antennas all placed in a single layer. We showed that interaction between loops and dipoles can be minimized by placing four 1 H traps into each 13 C loop. The presented double-tuned RF array coil substantially simplifies the design as compared with the common double-tuned surface loop arrays. At the same time, the coil demonstrated an improved 1 H longitudinal coverage and good transmit efficiency.

Open birdcage coil for head imaging at 7T.

PURPOSE: To theoretically describe, design, and test the new geometry of the birdcage coil for 7 Tesla anatomical brain imaging, which includes a large window on top, without deliberately jeopardizing its homogeneity and efficiency. This opencage will not only improve patient comfort but also enable the volunteer to follow functional MRI stimuli. This design could also facilitate the tracking of patient compliance and enable better correction of the movement. METHODS: Via the transfer matrix approach, a birdcage-like coil with a nonperiodic distribution of rungs is constructed with optimized currents in the coil rungs. Subsequently, the coil is adjusted in full-wave simulations. Then, the coil is assembled, fine-tuned, and matched on the bench. Finally, these results are confirmed experimentally on a phantom and in vivo. RESULTS: Indeed, the computed isolation of -14.9 dB between the feeding ports of the coil and the symmetry of the circular polarized mode pattern transmit RF magnetic field ( B1+ ) showed that the coil was properly optimized. An experimental assessment of the developed coil showed competitive transmit efficiency and coverage compared with the conventional birdcage coil of similar size. CONCLUSION: The proposed opencage coil can be designed and work without a dramatic drop of performance in terms of the B1+ field homogeneity, transmit efficiency ( B1+ / Pref ), peak local specific absorption rate ( SAR10g ) and SAR efficiency ( B1+ / SAR10g ).

Folded-end dipole transceiver array for human whole-brain imaging at 7 T.

The advancement of clinical applications of ultrahigh field (UHF) MRI depends heavily on advances in technology, including the development of new radiofrequency (RF) coil designs. Currently, the number of commercially available 7 T head RF coils is rather limited, implying a need to develop novel RF head coil designs that offer superior transmit and receive performance. RF coils to be used for clinical applications must be robust and reliable. In particular, for transmit arrays, if a transmit channel fails the local specific absorption rate may increase, significantly increasing local tissue heating. Recently, dipole antennas have been proposed and used to design UHF head transmit and receive arrays. The dipole provides a unique simplicity while offering comparable transmit efficiency and signal-to-noise ratio with the conventional loop design. Recently, we developed a novel array design in our laboratory using a folded-end dipole antenna. In this work, we developed, constructed and evaluated an eight-element transceiver bent folded-end dipole array for human head imaging at 7 T. Driven in the quadrature circularly polarized mode, the array demonstrated more than 20% higher transmit efficiency and significantly better whole-brain coverage than that provided by a widely used commercial array. In addition, we evaluated passive dipole antennas for decoupling the proposed array. We demonstrated that in contrast to the common unfolded dipole array, the passive dipoles moved away from the sample not only minimize coupling between the adjacent folded-end active dipoles but also produce practically no destructive interference with the quadrature mode of the array.

Electromagnetic simulation of a 16-channel head transceiver at 7 T using circuit-spatial optimization.

PURPOSE: With increased interest in parallel transmission in ultrahigh-field MRI, methods are needed to correctly calculate the S-parameters and complex field maps of the parallel transmission coil. We present S-parameters paired with spatial field optimization to fully simulate a double-row 16-element transceiver array for brain MRI at 7 T. METHODS: We implemented a closed-form equation of the coil S-parameters and overall spatial B1+ field. We minimized a cost function, consisting of coil S-parameters and the B1+ homogeneity in brain tissue, by optimizing transceiver components, including matching, decoupling circuits, and lumped capacitors. With this, we are able to compare the in silico results determined with and without B1+ homogeneity weighting. Using the known voltage range from the host console, we reconstructed the B1+ maps of the array and performed RF shimming with four realistic head models. RESULTS: As performed with B1+ homogeneity weighting, the optimized coil circuit components were highly consistent over the four heads, producing well-tuned, matched, and decoupled coils. The mean peak forward powers and B1+ statistics for the head models are consistent with in vivo human results (N = 8). There are systematic differences in the transceiver components as optimized with or without B1+ homogeneity weighting, resulting in an improvement of 28.4 ± 7.5% in B1+ homogeneity with a small 1.9 ± 1.5% decline in power efficiency. CONCLUSION: This co-simulation methodology accurately simulates the transceiver, predicting consistent S-parameters, component values, and B1+ field. The RF shimming of the calculated field maps match the in vivo performance.

Unshielded bent folded-end dipole 9.4 T human head transceiver array decoupled using modified passive dipoles.

PURPOSE: To develop an unshielded dipole transceiver array for human head imaging at 9.4 Tesla and to improve decoupling of adjacent dipole elements, a novel array design with modified passive dipole antennas was developed, evaluated, and tested. METHODS: The new array consisted of 8 bent folded-end dipole elements placed in a single row and surrounding the head. Adjacent elements of RF transceiver arrays are usually decoupled by introducing circuits electrically connected to elements. These methods are difficult to use for dipole arrays because of the distant location of the adjacent antennas. A recently developed decoupling technique using passive dipoles is simple and does not require any electrical connection. However, common parallel passive dipoles can produce destructive interference with the RF field of the array itself. To minimize this interference, we placed the passive dipoles perpendicularly to the active dipoles and positioned them at the ends of the array. We also evaluated the effect of different passive dipoles on the array transmit performance. Finally, we optimized the array transmit performance by varying the length of the dipole folded portion. RESULTS: By rotating the passive dipoles 90º and moving them toward the ends of the array, we minimized the destructive interference to an acceptable level without compromising decoupling and the transmit efficiency. CONCLUSION: While keeping the benefits of the passive dipole decoupling method, the new modified dipoles produce substantially less destructive interference with the RF field of the array than the common design. The constructed transceiver array demonstrated good decoupling and whole-brain coverage.

Comparison of four 31P single-voxel MRS sequences in the human brain at 9.4 T.

PURPOSE: In this study, different single-voxel localization sequences were implemented and systematically compared for the first time for phosphorous MRS (31 P-MRS) in the human brain at 9.4 T. METHODS: Two multishot sequences, image-selected in vivo spectroscopy (ISIS) and a conventional slice-selective excitation combined with localization by adiabatic selective refocusing (semiLASER) variant of the spin-echo full intensity-acquired localized spectroscopy (SPECIAL-semiLASER), and two single-shot sequences, semiLASER and stimulated echo acquisition mode (STEAM), were implemented and optimized for 31 P-MRS in the human brain at 9.4 T. Pulses and coil setup were optimized, localization accuracy was tested in phantom experiments, and absolute SNR of the sequences was compared in vivo. The SNR per unit time (SNR/t) was derived and compared for all four sequences and verified experimentally for ISIS in two different voxel sizes (3 × 3 × 3 cm3 , 5 × 5 × 5 cm3 , 10-minute measurement time). Metabolite signals obtained with ISIS were quantified. The possible spectral quality in vivo acquired in clinically feasible time (3:30 minutes, 3 × 3 × 3 cm3 ) was explored for two different coil setups. RESULTS: All evaluated sequences performed with good localization accuracy in phantom experiments and provided well-resolved spectra in vivo. However, ISIS has the lowest chemical shift displacement error, the best localization accuracy, the highest SNR/t for most metabolites, provides metabolite concentrations comparable to literature values, and is the only one of the sequences that allows for the detection of the whole 31 P spectrum, including ß-adenosine triphosphate, with the used setup. The SNR/t of STEAM is comparable to the SNR/t of ISIS. The semiLASER and SPECIAL-semiLASER sequences provide good results for metabolites with long T2 . CONCLUSION: At 9.4 T, high-quality single-voxel localized 31 P-MRS can be performed in the human brain with different localization methods, each with inherent characteristics suitable for different research issues.

A novel method to measure T1 -relaxation times of macromolecules and quantification of the macromolecular resonances.

PURPOSE: Macromolecular peaks underlying metabolite spectra influence the quantification of metabolites. Therefore, it is important to understand the extent of contribution from macromolecules (MMs) in metabolite quantification. However, to model MMs more accurately in spectral fitting, differences in T1 relaxation times among individual MM peaks must be considered. Characterization of T1 -relaxation times for all individual MM peaks using a single inversion recovery technique is difficult due to eventual contributions from metabolites. On the contrary, a double inversion recovery (DIR) technique provided flexibility to acquire MM spectra spanning a range of longitudinal magnetizations with minimal metabolite influence. Thus, a novel method to determine T1 -relaxation times of individual MM peaks is reported in this work. METHODS: Extensive Bloch simulations were performed to determine inversion time combinations for a DIR technique that yielded adequate MM signal with varying longitudinal magnetizations while minimizing metabolite contributions. MM spectra were acquired using DIR-metabolite-cycled semi-LASER sequence. LCModel concentrations were fitted to the DIR signal equation to calculate T1 -relaxation times. RESULTS: T1 -relaxation times of MMs range from 204 to 510 ms and 253 to 564 ms in gray- and white-matter rich voxels respectively at 9.4T. Additionally, concentrations of 13 MM peaks are reported. CONCLUSION: A novel DIR method is reported in this work to calculate T1 -relaxation times of MMs in the human brain. T1 -relaxation times and relaxation time corrected concentrations of individual MMs are reported in gray- and white-matter rich voxels for the first time at 9.4T.

Ultrahigh-resolution quantitative spinal cord MRI at 9.4T.

PURPOSE: To present the results of the first human spinal cord in vivo MRI scans at 9.4T. METHODS: A human brain coil was used to image the human spinal cord at 9.4T. All anatomical images were acquired with a T2 *-weighted gradient-echo sequence. A comparison of the influence of four different B0 shimming routines on the image quality was performed. Intrinsic signal-to-noise-ratio maps were determined using a pseudo-multiple replica approach. Measurements with different echo times were compared and processed to one multiecho data image combination image. Based on the multiecho acquisitions, T2 *-relaxation time maps were calculated. Algorithmic spinal cord detection and gray matter/white matter segmentation were tested. RESULTS: An echo time between 9 and 13.8 ms compromised best between gray matter/white matter contrast and image quality. A maximum in-plane resolution of 0.15 × 0.15 mm2 was achieved for anatomical images. These images offered excellent image quality and made small structures of the spinal cord visible. The scanner vendor implemented B0 shimming routine performed best during this work. Intrinsic signal-to-noise-ratio values of between 6600 and 8060 at the upper cervical spinal cord were achieved. Detection and segmentation worked reliably. An average T2 *-time of 24.88 ms ± 6.68 ms for gray matter and 19.37 ms ± 8.66 ms for white matter was calculated. CONCLUSION: The proposed human brain coil can be used to image the spinal cord. The maximum in-plane resolution in this work was higher compared with the 7T results from the literature. The 9.4T acquisitions made the small structures of the spinal cord clearly visible.