Any-nucleus distributed active programmable transmit coil.

PURPOSE: There are 118 known elements. Nearly all of them have NMR active isotopes and at least 39 different nuclei have biological relevance. Despite this, most of today's MRI is based on only one nucleus-1H. To facilitate imaging all potential nuclei, we present a single transmit coil able to excite arbitrary nuclei in human-scale MRI. THEORY AND METHODS: We present a completely new type of RF coil, the Any-nucleus Distributed Active Programmable Transmit Coil (ADAPT Coil), with fast switches integrated into the structure of the coil to allow it to operate at any relevant frequency. This coil eliminates the need for the expensive traditional RF amplifier by directly converting direct current (DC) power into RF magnetic fields with frequencies chosen by digital control signals sent to the switches. Semiconductor switch imperfections are overcome by segmenting the coil. RESULTS: Circuit simulations demonstrated the effectiveness of the ADAPT Coil approach, and a 9 cm diameter surface ADAPT Coil was implemented. Using the ADAPT Coil, 1H, 23Na, 2H, and 13C phantom images were acquired, and 1H and 23Na ex vivo images were acquired. To excite different nuclei, only digital control signals were changed, which can be programmed in real time. CONCLUSION: The ADAPT Coil presents a low-cost, scalable, and efficient method for exciting arbitrary nuclei in human-scale MRI. This coil concept provides further opportunities for scaling, programmability, lowering coil costs, lowering dead-time, streamlining multinuclear MRI workflows, and enabling the study of dozens of biologically relevant nuclei.

Wideband receive-coil array design using high-impedance amplifiers for broadband decoupling.

PURPOSE: Multinuclear MRI/S is of increasing interest. Currently, most multinuclear receive array coils are constructed by nesting multiple single-tuned array coils or using switching elements to control the operating frequency, in which case more than one set of conventional isolation preamplifiers and associated decoupling circuits is required. These conventional configurations rapidly become complicated when greater numbers of channels or nuclei are needed. In this work, a novel coil decoupling mechanism is proposed to enable broadband decoupling for array coils with one set of preamplifiers. METHODS: Instead of using conventional isolation preamplifiers, a high-input impedance preamplifier is proposed to create broadband decoupling of the array elements. A matching network consisting of a single inductor-capacitor-capacitor multi-tuned network and a wire-wound transformer was used to interface the surface coil to the high-impedance preamplifier. To validate the concept, the proposed configuration was compared to the conventional preamplifier decoupling configuration on both bench and scanner. RESULTS: 2 The approach can provide more than 15dB decoupling over a range of 25MHz, covering the Larmor frequencies of 23 Na and 2 H at 4.7T. This multi-tuned prototype obtained 61% and 76% of the imaging SNR at 2 H and 23 Na respectively, 76 and 89% in a higher loading test phantom, when compared to the conventional single-tuned preamplifier decoupling configuration. CONCLUSION: With the multinuclear array operation and decoupling achieved using only one layer of array coil and preamplifiers, this work provides a simple approach of building high element-count arrays to enable accelerated imaging or SNR improvement from multiple nuclei.

Influence of image contrasts and reconstruction methods on the classification of multiple sclerosis-like lesions in simulated sodium magnetic resonance imaging.

PURPOSE: To evaluate the classifiability of small multiple sclerosis (MS)-like lesions in simulated sodium (23 Na) MRI for different 23 Na MRI contrasts and reconstruction methods. METHODS: 23 Na MRI and 23 Na inversion recovery (IR) MRI of a phantom and simulated brain with and without lesions of different volumes (V = 1.3-38.2 nominal voxels) were simulated 100 times by adding Gaussian noise matching the SNR of real 3T measurements. Each simulation was reconstructed with four different reconstruction methods (Gridding without and with Hamming filter, Compressed sensing (CS) reconstruction without and with anatomical 1 H prior information). Based on the mean signals within the lesion volumes of simulations with and without lesions, receiver operating characteristics (ROC) were determined and the area under the curve (AUC) was calculated to assess the classifiability for each lesion volume. RESULTS: Lesions show higher classifiability in 23 Na MRI than in 23 Na IR MRI. For typical parameters and SNR of a 3T scan, the voxel normed minimal classifiable lesion volume (AUC > 0.9) is 2.8 voxels for 23 Na MRI and 19 voxels for 23 Na IR MRI, respectively. In terms of classifiability, Gridding with Hamming filter and CS without anatomical 1 H prior outperform CS reconstruction with anatomical 1 H prior. CONCLUSION: Reliability of lesion classifiability strongly depends on the lesion volume and the 23 Na MRI contrast. Additional incorporation of 1 H prior information in the CS reconstruction was not beneficial for the classification of small MS-like lesions in 23 Na MRI.

RF coil design for accurate parallel imaging on 13 C MRSI using 23 Na sensitivity profiles.

PURPOSE: To develop a coil-based method to obtain accurate sensitivity profiles in 13 C MRI at 3T from the endogenous 23 Na. An eight-channel array is designed for 13 C MR acquisitions. As application examples, the array is used for two-fold accelerated acquisitions of both hyperpolarized 13 C metabolic imaging of pig kidneys and the human brain. METHODS: A flexible coil array was tuned optimally for 13 C at 3T (32.1 MHz), with the coil coupling coefficients matched to be nearly identical at the resonance frequency of 23 Na (33.8 MHz). This is done by enforcing a high decoupling (obtained through highly mismatched preamplifiers) and adjusting the coupling frequency response. The SNR performance is compared to reference coils. RESULTS: The measured sensitivity profiles on a phantom showed high spatial similarity for 13 C and 23 Na resonances, with average noise correlation of 9 and 11%, respectively. For acceleration factors 2, 3, and 4, the obtained maximum g-factors were 1.0, 1.1, and 2.6, respectively. The 23 Na profiles obtained in vivo could be used successfully to perform two-fold acceleration of hyperpolarized 13 C 3D acquisitions of both pig kidneys and a healthy human brain. CONCLUSION: A receive array has been developed in such a way that the 13 C sensitivity profiles could be accurately obtained from measurements at the 23 Na frequency. This technique facilitates accelerated acquisitions for hyperpolarized 13 C imaging. The SNR performance obtained at the 13 C frequency, compares well to other state-of-the-art coils for the same purpose, showing slightly better superficial and central SNR.

Quantitative 23 Na-MRI of the intervertebral disk at 3 T.

Monitoring the tissue sodium content (TSC) in the intervertebral disk geometry noninvasively by MRI is a sensitive measure to estimate changes in the proteoglycan content of the intervertebral disk, which is a biomarker of degenerative disk disease (DDD) and of lumbar back pain (LBP). However, application of quantitative sodium concentration measurements in 23 Na-MRI is highly challenging due to the lower in vivo concentrations and smaller gyromagnetic ratio, ultimately yielding much smaller signal relative to 1 H-MRI. Moreover, imaging the intervertebral disk geometry imposes higher demands, mainly because the necessary RF volume coils produce highly inhomogeneous transmit field patterns. For an accurate absolute quantification of TSC in the intervertebral disks, the B1 field variations have to be mitigated. In this study, we report for the first time quantitative sodium concentration in the intervertebral disks at clinical field strengths (3 T) by deploying 23 Na-MRI in healthy human subjects. The sodium B1 maps were calculated by using the double-angle method and a double-tuned (1 H/23 Na) transceive chest coil, and the individual effects of the variation in the B1 field patterns in tissue sodium quantification were calculated. Phantom measurements were conducted to evaluate the quality of the Na-weighted images and B1 mapping. Depending on the disk position, the sodium concentration was calculated as 161.6 mmol/L-347 mmol/L, and the mean sodium concentration of the intervertebral disks varies between 254.6 ± 54 mmol/L and 290.1 ± 39 mmol/L. A smoothing effect of the B1 correction on the sodium concentration maps was observed, such that the standard deviation of the mean sodium concentration was significantly reduced with B1 mitigation. The results of this work provide an improved integration of quantitative 23 Na-MRI into clinical studies in intervertebral disks such as degenerative disk disease and establish alternative scoring schemes to existing morphological scoring such as the Pfirrmann score.

3D sodium (23 Na) magnetic resonance fingerprinting for time-efficient relaxometric mapping.

PURPOSE: To develop a framework for 3D sodium (23 Na) MR fingerprinting (MRF), based on irreducible spherical tensor operators with tailored flip angle (FA) pattern and time-efficient data acquisition for simultaneous quantification of T1 , T2l∗ , T2s∗ , and T2∗ in addition to ΔB0 . METHODS: 23 Na-MRF was implemented in a 3D sequence and irreducible spherical tensor operators were exploited in the simulations. Furthermore, the Cramér Rao lower bound was used to optimize the flip angle pattern. A combination of single and double echo readouts was implemented to increase the readout efficiency. A study was conducted to compare results in a multicompartment phantom acquired with MRF and reference methods. Finally, the relaxation times in the human brain were measured in four healthy volunteers. RESULTS: Phantom experiments revealed a mean difference of 1.0% between relaxation times acquired with MRF and results determined with the reference methods. Simultaneous quantification of the longitudinal and transverse relaxation times in the human brain was possible within 32 min using 3D 23 Na-MRF with a nominal resolution of (5 mm)3 . In vivo measurements in four volunteers yielded average relaxation times of: T1,brain = (35.0 ± 3.2) ms, T2l,brain∗ = (29.3 ± 3.8) ms and T2s,brain∗ = (5.5 ± 1.3) ms in brain tissue, whereas T1,CSF = (61.9 ± 2.8) ms and T2,CSF∗ = (46.3 ± 4.5) ms was found in cerebrospinal fluid. CONCLUSION: The feasibility of in vivo 3D relaxometric sodium mapping within roughly ½ h is demonstrated using MRF in the human brain, moving sodium relaxometric mapping toward clinically relevant measurement times.

Characterization and correction of center-frequency effects in X-nuclear eddy current compensations on a clinical MR system.

PURPOSE: The aim of the study was to investigate whether incorrectly compensated eddy currents are the source of persistent X-nuclear spectroscopy and imaging artifacts, as well as methods to correct this. METHODS: Pulse-acquire spectra were collected for 1 H and X-nuclei (23 Na or 31 P) using the minimum TR permitted on a 3T clinical MRI system. Data were collected in 3 orientations (axial, sagittal, and coronal) with the spoiler gradient at the end of the TR applied along the slice direction for each. Modifications to system calibration files to tailor eddy current compensation for each X-nucleus were developed and applied, and data were compared with and without these corrections for: slice-selective MRS (for 23 Na and 31 P), 2D spiral trajectories (for 13 C), and 3D cones trajectories (for 23 Na). RESULTS: Line-shape distortions characteristic of eddy currents were demonstrated for X-nuclei, which were not seen for 1 H. The severity of these correlated with the amplitude of the eddy current frequency compensation term applied by the system along the axis of the applied spoiler gradient. A proposed correction to eddy current compensation, taking account of the gyromagnetic ratio, was shown to dramatically reduce these distortions. The same correction was also shown to improve data quality of non-Cartesian imaging (2D spiral and 3D cones trajectories). CONCLUSION: A simple adaptation of the default compensation for eddy currents was shown to eliminate a range of artifacts detected on X-nuclear spectroscopy and imaging.

Feasibility of 31 P spectroscopic imaging at 7 T in lung carcinoma patients.

Currently, it is difficult to predict effective therapy response to molecular therapies for the treatment of lung cancer based solely on anatomical images. 31 P MR spectroscopic imaging could provide as a non-invasive method to monitor potential biomarkers for early therapy evaluation, a necessity to improve personalized care and reduce cost. However, surface coils limit the imaging volume in conventional 31 P MRSI. High-energetic adiabatic RF pulses are required to achieve flip angle homogeneity but lead to high SAR. Birdcage coils permit use of conventional amplitude modulated pulses, even over large FOV, potentially decreasing overall SAR massively. Here, we investigate the feasibility of 3D 31 P MRSI at 7 T in lung carcinoma patients using an integrated 31 P birdcage body coil in combination with either a dual-coil or a 16-channel receiver. Simulations showed a maximum decrease in SNR per unit of time of 8% for flip angle deviations in short TR low flip-angle excitation 3D CSI. The minimal SNR loss allowed for fast 3D CSI without time-consuming calibration steps (>10:00 min.). 31 P spectra from four lung carcinoma patients were acquired within 29:00 minutes and with high SNR using both receivers. The latter allowed discrimination of individual phosphodiesters, inorganic phosphate, phosphocreatine and ATP. The receiver array allowed for an increased FOV compared to the dual-coil receiver. 3D 31 P-CSI were acquired successfully in four lung carcinoma patients using the integrated 31 P body coil at ultra-high field. The increased spectral resolution at 7 T allowed differentiation of multiple 31 P metabolites related to phospholipid and energy metabolism. Simulations provide motivation to exclude 31 P B1 calibrations, potentially decreasing total scan duration. Employing large receiver arrays improves the field of view allowing for full organ coverage. 31 P MRSI is feasible in lung carcinoma patients and has potential as a non-invasive method for monitoring personalized therapy response in lung tumors.

Sodium relaxometry using 23 Na MR fingerprinting: A proof of concept.

PURPOSE: To evaluate the feasibility of 23 Na MR fingerprinting (MRF) for simultaneous quantification of T1 , T2l∗ , T2s∗ , T2∗ in addition to ΔB0 . METHODS: A framework for sodium relaxometry using MRF at 7T was developed, allowing simultaneous measurement of relaxation times and inhomogeneities in the static field. The technique distinguishes between bi- and monoexponential transverse relaxation and was validated in simulations with respect to the ground truth. In phantom measurements, a resolution of 2 × 2 × 12 mm3 was achieved within 1 h acquisition time, and the resulting parameter maps were compared to results from reference methods. Relaxation times in five healthy volunteers were measured with a resolution of 4 × 4 × 12 mm3 . RESULTS: Phantom experiments revealed an agreement between the relaxation times obtained via 23 Na-MRF and the reference methods. In white matter, a longitudinal relaxation constant of T1 = 38.9 ± 4.8 ms was found, while values of T2l∗ = 29.2 ± 4.9 ms and T2s∗ = 4.7 ± 1.2 ms were found for the long and short component of the transverse relaxation. In cerebrospinal fluid, T1 was 67.7 ± 6.3 ms and T2∗ = 41.5 ± 3.4 ms. CONCLUSION: This work demonstrates the feasibility of 23 Na-MRF for relaxometry in sodium MRI in both phantom and in vivo studies. Simultaneous quantification of T1 , T2l∗ , T2s∗ , T2∗ and ΔB0 was possible within a 1 h measurement time.

Compressed sensing effects on quantitative analysis of undersampled human brain sodium MRI.

PURPOSE: The clinical application of sodium MRI is hampered due to relatively low image quality and associated long acquisition times. Compressed sensing (CS) aims at a reduction of measurement time, but has been found to encompass quantitative estimation bias when used in low SNR x-Nuclei imaging. This work analyses CS in quantitative human brain sodium MRI from undersampled acquisitions and provides recommendations for tissue sodium concentration (TSC) estimation. METHODS: CS reconstructions from 3D radial acquisitions of 5 healthy volunteers were investigated over varying undersampling factors (USFs) and CS penalty weights on different sparsity domains, Wavelet, Discrete Cosine Transform (DCT), and Identity. Resulting images were compared with highly sampled and undersampled NUFFT-based images and evaluated for image quality (i.e. structural similarity), image intensity bias, and its effect on TSC estimates in gray and white matter. RESULTS: Wavelet-based CS reconstructions show highest image quality with stable TSC estimates for most USFs. Up to an USF of 4, images showed good structural detail. DCT and Identity-based CS enable good image quality, however show a bias in TSC with a reduction in estimates across USFs. CONCLUSIONS: The image intensity bias is lowest in Wavelet-based reconstructions and enables an up to fourfold acquisition speed up while maintaining good structural detail. The associated acquisition time reduction can facilitate a translation of sodium MRI into clinical routine.

X-nuclei imaging: Current state, technical challenges, and future directions.

1 H imaging is concerned with contrast generation among anatomically distinct soft tissues. X-nuclei imaging, on the other hand, aims to reveal the underlying changes in the physiological processes on a cellular level. Advanced clinical MR hardware systems improved 1 H image quality and simultaneously enabled X-nuclei imaging. Adaptation of 1 H methods and optimization of both sequence design and postprocessing protocols launched X-nuclei imaging past feasibility studies and into clinical studies. This review outlines the current state of X-nuclei MRI, with the focus on 23 Na, 35 Cl, 39 K, and 17 O. Currently, various aspects of technical challenges limit the possibilities of clinical X-nuclei MRI applications. To address these challenges, quintessential physical and technical concepts behind different applications are presented, and the advantages and drawbacks are delineated. The working process for methods such as quantification and multiquantum imaging is shown step-by-step. Clinical examples are provided to underline the potential value of X-nuclei imaging in multifaceted areas of application. In conclusion, the scope of the latest technical advance is outlined, and suggestions to overcome the most fundamental hurdles on the way into clinical routine by leveraging the full potential of X-nuclei imaging are presented. Level of Evidence: 1 Technical Efficacy Stage: 3 J. Magn. Reson. Imaging 2020;51:355-376.

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

Phosphorus (31 P) MRSI provides opportunities to monitor potential biomarkers. However, current applications of 31 P 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 31 P birdcage coil setup with a high power RF amplifier at 7 T allows for low flip angle excitations with short repetition time (TR ) 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 31 P 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 31 P 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 TR and/or flip angle to detect and quantify ratios from different molecular species.

Dynamic phosphocreatine imaging with unlocalized pH assessment of the human lower leg muscle following exercise at 3T.

PURPOSE: To develop a high temporal resolution imaging method that measures muscle-specific phosphocreatine (PCr) resynthesis time constant (τPCr ) and pH changes in muscles of the lower leg following exercise on a clinical 3T MRI scanner. METHODS: We developed a frequency-selective 3D non-Cartesian FLORET sequence to measure PCr with 17-mm nominal isotropic resolution (28 mm actual resolution) and 6-s temporal resolution to capture dynamic metabolic muscle activity. The sequence was designed to additionally collect inorganic phosphate spectra for pH quantification, which were localized using sensitivity profiles of individual coil elements. Nineteen healthy volunteers were scanned while performing a plantar flexion exercise on an in-house developed ergometer. Data were acquired with a dual-tuned multichannel coil array that enabled phosphorus imaging and proton localization for muscle segmentation. RESULTS: After a 90-s plantar flexion exercise at 0.66 Hz with resistance set to 40% of the maximum voluntary contraction, τPCr was estimated at 22.9 ± 8.8 s (mean ± standard deviation) with statistical coefficient of determination r2 = 0.89 ± 0.05. The corresponding pH values after exercise were in the range of 6.9-7.1 in the gastrocnemius muscle. CONCLUSION: The developed technique allows measurement of muscle-specific PCr resynthesis kinetics and pH changes following exercise, with a temporal resolution and accuracy comparable to that of single voxel 31 P-MRS sequences. Magn Reson Med 79:974-980, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Dynamic PCr and pH imaging of human calf muscles during exercise and recovery using (31) P gradient-Echo MRI at 7 Tesla.

PURPOSE: Simultaneous acquisition of spatially resolved (31) P-MRI data for evaluation of muscle specific energy metabolism, i.e., PCr and pH kinetics. METHODS: A three-dimensional (3D) gradient-echo sequence for multiple frequency-selective excitations of the PCr and Pi signals in an interleaved sampling scheme was developed and tested at 7 Tesla (T). The pH values were derived from the chemical shift-induced phase difference between the resonances. The achieved spatial resolution was ∼2 mL with image acquisition time below 6 s. Ten healthy volunteers were studied performing plantar flexions during the delay between (31) P-MRI acquisitions, yielding a temporal resolution of 9-10 s. RESULTS: Signal from anatomically matched regions of interest had sufficient signal-to-noise ratio to allow single-acquisition PCr and pH quantification. The Pi signal was clearly detected in voxels of actively exercising muscles. The PCr depletions were in gastrocnemius 42 ± 14% (medialis), 48 ± 17% (lateralis) and in soleus 20 ± 11%. The end exercise pH values were 6.74 ± 0.18 and 6.65 ± 0.27 for gastrocnemius medialis and lateralis, respectively, and 6.96 ± 0.12 for soleus muscle. CONCLUSION: Simultaneous acquisition of PCr and Pi images with high temporal resolution, suitable for measuring PCr and pH kinetics in exercise-recovery experiments, was demonstrated at 7T. This study presents a fast alternative to MRS for quantifying energy metabolism of posterior muscle groups of the lower leg. Magn Reson Med 75:2324-2331, 2016. © 2015 Wiley Periodicals, Inc.

(17)O relaxation times in the rat brain at 16.4 tesla.

PURPOSE: Measurement of the cerebral metabolic rate of oxygen (CMRO2 ) by means of direct imaging of the (17) O signal can be a valuable tool in neuroscientific research. However, knowledge of the longitudinal and transverse relaxation times of different brain tissue types is required, which is difficult to obtain because of the low sensitivity of natural abundance H2 (17) O measurements. METHODS: Using the improved sensitivity at a field strength of 16.4 Tesla, relaxation time measurements in the rat brain were performed in vivo and postmortem with relatively high spatial resolutions, using a chemical shift imaging sequence. RESULTS: In vivo relaxation times of rat brain were found to be T1 = 6.84 ± 0.67 ms and T2 * = 1.77 ± 0.04 ms. Postmortem H2 (17) O relaxometry at enriched concentrations after inhalation of (17) O2 showed similar T2 * values for gray matter (1.87 ± 0.04 ms) and white matter, significantly longer than muscle (1.27 ± 0.05 ms) and shorter than cerebrospinal fluid (2.30 ± 0.16 ms). CONCLUSION: Relaxation times of brain H2 (17) O were measured for the first time in vivo in different types of tissues with high spatial resolution. Because the relaxation times of H2 (17) O are expected to be independent of field strength, our results should help in optimizing the acquisition parameters for experiments also at other MRI field strengths.

A form-fitted three channel (31) P, two channel (1) H transceiver coil array for calf muscle studies at 7 T.

PURPOSE: To enhance sensitivity and coverage for calf muscle studies, a novel, form-fitted, three-channel phosphorus-31 ((31) P), two-channel proton ((1) H) transceiver coil array for 7 T MR imaging and spectroscopy is presented. METHODS: Electromagnetic simulations employing individually generated voxel models were performed to design a coil array for studying nonpathological muscle metabolism. Static phase combinations of the coil elements' transmit fields were optimized based on homogeneity and efficiency for several voxel models. The best-performing design was built and tested both on phantoms and in vivo. RESULTS: Simulations revealed that a shared conductor array for (31) P provides more robust interelement decoupling and better homogeneity than an overlap array in this configuration. A static B1 (+) shim setting that suited various calf anatomies was identified and implemented. Simulations showed that the (31) P array provides signal-to-noise ratio (SNR) benefits over a single loop and a birdcage coil of equal radius by factors of 3.2 and 2.6 in the gastrocnemius and by 2.5 and 2.0 in the soleus muscle. CONCLUSION: The performance of the coil in terms of B1 (+) and achievable SNR allows for spatially localized dynamic (31) P spectroscopy studies in the human calf. The associated higher specificity with respect to nonlocalized measurements permits distinguishing the functional responses of different muscles.

(19)F MRSI of capecitabine in the liver at 7 T using broadband transmit-receive antennas and dual-band RF pulses.

Capecitabine (Cap) is an often prescribed chemotherapeutic agent, successfully used to cure some patients from cancer or reduce tumor burden for palliative care. However, the efficacy of the drug is limited, it is not known in advance who will respond to the drug and it can come with severe toxicity. (19)F Magnetic Resonance Spectroscopy (MRS) and Magnetic Resonance Spectroscopic Imaging (MRSI) have been used to non-invasively study Cap metabolism in vivo to find a marker for personalized treatment. In vivo detection, however, is hampered by low concentrations and the use of radiofrequency (RF) surface coils limiting spatial coverage. In this work, the use of a 7T MR system with radiative multi-channel transmit-receive antennas was investigated with the aim of maximizing the sensitivity and spatial coverage of (19)F detection protocols. The antennas were broadband optimized to facilitate both the (1)H (298 MHz) and (19)F (280 MHz) frequencies for accurate shimming, imaging and signal combination. B1(+) simulations, phantom and noise measurements showed that more than 90% of the theoretical maximum sensitivity could be obtained when using B1(+) and B1(-) information provided at the (1)H frequency for the optimization of B1(+) and B1(-) at the (19)F frequency. Furthermore, to overcome the limits in maximum available RF power, whilst ensuring simultaneous excitation of all detectable conversion products of Cap, a dual-band RF pulse was designed and evaluated. Finally, (19)F MRS(I) measurements were performed to detect (19)F metabolites in vitro and in vivo. In two patients, at 10 h (patient 1) and 1 h (patient 2) after Cap intake, (19)F metabolites were detected in the liver and the surrounding organs, illustrating the potential of the set-up for in vivo detection of metabolic rates and drug distribution in the body.

MRI at 7 Tesla and above: demonstrated and potential capabilities.

With more than 40 installed MR systems worldwide operating at 7 Tesla or higher, ultra-high-field (UHF) imaging has been established as a platform for clinically oriented research in recent years. Along with technical developments that, in part, have also been successfully transferred to lower field strengths, MR imaging and spectroscopy at UHF have demonstrated capabilities and potentials for clinical diagnostics in a variety of studies. In terms of applications, this overview article focuses on already achieved advantages for in vivo imaging, i.e., in imaging the brain and joints of the musculoskeletal system, but also considers developments in body imaging, which is particularly challenging. Furthermore, new applications for clinical diagnostics such as X-nuclei imaging and spectroscopy, which only really become feasible at ultra-high magnetic fields, will be presented.

Rapid 2D 23Na MRI of the calf using a denoising convolutional neural network.

PURPOSE: 23Na MRI can be used to quantify in-vivo tissue sodium concentration (TSC), but the inherently low 23Na signal leads to long scan times and/or noisy or low-resolution images. Reconstruction algorithms such as compressed sensing (CS) have been proposed to mitigate low signal-to-noise ratio (SNR); although, these can result in unnatural images, suboptimal denoising and long processing times. Recently, machine learning has been increasingly used to denoise 1H MRI acquisitions; however, this approach typically requires large volumes of high-quality training data, which is not readily available for 23Na MRI. Here, we propose using 1H data to train a denoising convolutional neural network (CNN), which we subsequently demonstrate on prospective 23Na images of the calf. METHODS: 1893 1H fat-saturated transverse slices of the knee from the open-source fastMRI dataset were used to train denoising CNNs for different levels of noise. Synthetic low SNR images were generated by adding gaussian noise to the high-quality 1H k-space data before reconstruction to create paired training data. For prospective testing, 23Na images of the calf were acquired in 10 healthy volunteers with a total of 150 averages over ten minutes, which were used as a reference throughout the study. From this data, images with fewer averages were retrospectively reconstructed using a non-uniform fast Fourier transform (NUFFT) as well as CS, with the NUFFT images subsequently denoised using the trained CNN. RESULTS: CNNs were successfully applied to 23Na images reconstructed with 50, 40 and 30 averages. Muscle and skin apparent TSC quantification from CNN-denoised images were equivalent to those from CS images, with <0.9 mM bias compared to reference values. Estimated SNR was significantly higher in CNN-denoised images compared to NUFFT, CS and reference images. Quantitative edge sharpness was equivalent for all images. For subjective image quality ranking, CNN-denoised images ranked equally best with reference images and significantly better than NUFFT and CS images. CONCLUSION: Denoising CNNs trained on 1H data can be successfully applied to 23Na images of the calf; thus, allowing scan time to be reduced from ten minutes to two minutes with little impact on image quality or apparent TSC quantification accuracy.

A Framework for Predicting X-Nuclei Transmitter Gain Using 1H Signal.

Commercial human MR scanners are optimised for proton imaging, containing sophisticated prescan algorithms with setting parameters such as RF transmit gain and power. These are not optimal for X-nuclear application and are challenging to apply to hyperpolarised experiments, where the non-renewable magnetisation signal changes during the experiment. We hypothesised that, despite the complex and inherently nonlinear electrodynamic physics underlying coil loading and spatial variation, simple linear regression would be sufficient to accurately predict X-nuclear transmit gain based on concomitantly acquired data from the proton body coil. We collected data across 156 scan visits at two sites as part of ongoing studies investigating sodium, hyperpolarised carbon, and hyperpolarised xenon. We demonstrate that simple linear regression is able to accurately predict sodium, carbon, or xenon transmit gain as a function of position and proton gain, with variation that is less than the intrasubject variability. In conclusion, sites running multinuclear studies may be able to remove the time-consuming need to separately acquire X-nuclear reference power calibration, inferring it from the proton instead.