Accurate actual flip angle imaging (AFI) in the presence of fat.

PURPOSE: To investigate the effect of incomplete fat spoiling on the accuracy of B1 mapping with actual flip angle imaging (AFI) and to propose a method to minimize the errors using the chemical shift properties of fat. THEORY AND METHODS: Diffusion-based dephasing is the main spoiling mechanism exploited in AFI. However, a very low diffusion in fat may make the spoiling insufficient, leading to ghosts in the B1 maps. As the errors retain the chemical-shift signature of fat, their impact can be minimized using chemical-shift-based fat signal removal from AFI acquisition modified to include multi-echo readout. The source of the errors and the proposed correction were studied in simulations and phantom and in-vivo imaging experiments. RESULTS: Our results support that AFI artifacts are caused by the incomplete fat spoiling present in clinically attractive short TR acquisition regimes. The correction eliminated the ghosting and significantly improved the B1 mapping accuracy as well as the accuracy of R1 mapping performed with AFI-derived B1 maps. CONCLUSIONS: The incomplete fat signal spoiling may be a source of AFI B1 mapping errors, especially in subjects with high fat content. Achieving complete fat spoiling requires longer TR, which is undesirable in clinical applications. The proposed approach based on fat signal removal can reduce errors without significant prolongation of the AFI pulse sequence. We propose that, when attaining complete fat spoiling is not feasible, AFI mapping should be performed in a multi-echo regime with appropriate fat separation or suppression to minimize these errors.

Head-and-neck multichannel B1+ mapping and RF shimming of the carotid arteries using a 7T parallel-transmit head coil.

PURPOSE: Neurovascular MRI suffers from a rapid drop in B1 + into the neck when using transmit head coils at 7 T. One solution to improving B1 + magnitude in the major feeding arteries in the neck is to use custom RF shims on parallel-transmit head coils. However, calculating such shims requires robust multichannel B1 + maps in both the head and the neck, which is challenging due to low RF penetration into the neck, limited dynamic range of multichannel B1 + mapping techniques, and B0 sensitivity. We therefore sought a robust, large-dynamic-range, parallel-transmit field mapping protocol and tested whether RF shimming can improve carotid artery B1 + magnitude in practice. METHODS: A pipeline is presented that combines B1 + mapping data acquired using circularly polarized (CP) and CP2-mode RF shims at multiple voltages. The pipeline was evaluated by comparing the predicted and measured B1 + for multiple random transmit shims, and by assessing the ability of RF shimming to increase B1 + in the carotid arteries. RESULTS: The proposed method achieved good agreement between predicted and measured B1 + in both the head and the neck. The B1 + magnitude in the carotid arteries can be increased by 43% using tailored RF shims or by 37% using universal RF shims, while also improving the RF homogeneity compared with CP mode. CONCLUSION: B1 + in the neck can be increased using RF shims calculated from multichannel B1 + maps in both the head and the neck. This can be achieved using universal phase-only RF shims, facilitating easy implementation in existing sequences.

Fast 3D 31P B 1 + mapping with a weighted stack of spiral trajectory at 7 T.

PURPOSE: Phosphorus MRS (31P MRS) enables noninvasive assessment of energy metabolism, yet its application is hindered by sensitivity limitations. To overcome this, often high magnetic fields are used, leading to challenges such as spatial B 1 + $$ {\mathrm{B}}_1^{+} $$ inhomogeneity and therefore the need for accurate flip-angle determination in accelerated acquisitions with short TRs. In response to these challenges, we propose a novel short TR and look-up table-based double-angle method for fast 3D 31P B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping (fDAM). METHODS: Our method incorporates 3D weighted stack-of-spiral gradient-echo acquisitions and a frequency-selective pulse to enable efficient B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping based on the phosphocreatine signal at 7 T. Protocols were optimized using simulations and validated through phantom experiments. The method was validated in the human brain using a 31P 1Ch-trasmit/32Ch-receive coil and skeletal muscle using a birdcage 1H/31P volume coil. RESULTS: The results of fDAM were compared with the classical DAM. A good correlation (r = 0.95) was obtained between the two B 1 + $$ {\mathrm{B}}_1^{+} $$ maps. A 3D 31P B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping in the human calf muscle was achieved in about 10:50 min using a birdcage volume coil, with a 20% extended coverage (number of voxels with SNR > 3) relative to that of the classical DAM (24 min). fDAM also enabled the first full-brain coverage 31P 3D B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping in approximately 10:15 min using a 1Ch-transmit/32Ch-receive coil. CONCLUSION: fDAM is an efficient method for 31P 3D B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping, showing promise for future applications in rapid 31P MRSI.

Deep learning-based whole-brain B1 +-mapping at 7T.

PURPOSE: This study investigates the feasibility of using complex-valued neural networks (NNs) to estimate quantitative transmit magnetic RF field (B1 +) maps from multi-slice localizer scans with different slice orientations in the human head at 7T, aiming to accelerate subject-specific B1 +-calibration using parallel transmission (pTx). METHODS: Datasets containing channel-wise B1 +-maps and corresponding multi-slice localizers were acquired in axial, sagittal, and coronal orientation in 15 healthy subjects utilizing an eight-channel pTx transceiver head coil. Training included five-fold cross-validation for four network configurations: NN cx tra $$ {\mathrm{NN}}_{\mathrm{cx}}^{\mathrm{tra}} $$ used transversal, NN cx sag $$ {\mathrm{NN}}_{\mathrm{cx}}^{\mathrm{sag}} $$ sagittal, NN cx cor $$ {\mathrm{NN}}_{\mathrm{cx}}^{\mathrm{cor}} $$ coronal data, and NN cx all $$ {\mathrm{NN}}_{\mathrm{cx}}^{\mathrm{all}} $$ was trained on all slice orientations. The resulting maps were compared to B1 +-reference scans using different quality metrics. The proposed network was applied in-vivo at 7T in two unseen test subjects using dynamic kt-point pulses. RESULTS: Predicted B1 +-maps demonstrated a high similarity with measured B1 +-maps across multiple orientations. The estimation matched the reference with a mean relative error in the magnitude of (2.70 ± 2.86)% and mean absolute phase difference of (6.70 ± 1.99)° for transversal, (1.82 ± 0.69)% and (4.25 ± 1.62)° for sagittal ( NN cx sag $$ {\mathrm{NN}}_{\mathrm{cx}}^{\mathrm{sag}} $$ ), as well as (1.33 ± 0.27)% and (2.66 ± 0.60)° for coronal slices ( NN cx cor $$ {\mathrm{NN}}_{\mathrm{cx}}^{\mathrm{cor}} $$ ) considering brain tissue. NN cx all $$ {\mathrm{NN}}_{\mathrm{cx}}^{\mathrm{all}} $$ trained on all orientations enables a robust prediction of B1 +-maps across different orientations. Achieving a homogenous excitation over the whole brain for an in-vivo application displayed the approach's feasibility. CONCLUSION: This study demonstrates the feasibility of utilizing complex-valued NNs to estimate multi-slice B1 +-maps in different slice orientations from localizer scans in the human brain at 7T.

Fast bias-corrected conductivity mapping using stimulated echoes.

OBJECTIVE: To demonstrate the potential of a double angle stimulated echo (DA-STE) method for fast and accurate "full" homogeneous Helmholtz-based electrical properties tomography using a simultaneous B 1 + magnitude and transceive phase measurement. METHODS: The combination of a spin and stimulated echo can be used to yield an estimate of both B 1 + magnitude and transceive phase and thus provides the means for "full" EPT reconstruction. An interleaved 2D acquisition scheme is used for rapid acquisition. The method was validated in a saline phantom and compared to a double angle method based on two single gradient echo acquisitions (GRE-DAM). The method was evaluated in the brain of a healthy volunteer. RESULTS: The B 1 + magnitude obtained with DA-STE showed excellent agreement with the GRE-DAM method. Conductivity values based on the "full" EPT reconstruction also agreed well with the expectations in the saline phantom. In the brain, the method delivered conductivity values close to literature values. DISCUSSION: The method allows the use of the "full" Helmholtz-based EPT reconstruction without the need for additional measurements. As a result, quantitative conductivity values are improved compared to phase-based EPT reconstructions. DA-STE is a fast complex- B 1 + mapping technique that could render EPT clinically relevant at 3 T.

Effects of radiofrequency channel numbers on B1+ mapping using the Bloch-Siegert shift method.

PURPOSE: This paper aims to investigate the impact of the channel numbers on the performance of B1+ mapping, by using the Bloch-Siegert shift (BSS) method. B1+ mapping plays a crucial role in various brain imaging protocols. THEORY AND METHODS: We simulated the radiofrequency field of the human head model in six groups of multi-channel receive coil with a range of different channel numbers. MR signals were synthesized according to the standard BSS sequence, with quantified Gaussian added. Next, we combined the signals of each channel to reconstruct the B1+ map by weighted averaging and maximum likelihood estimation strategies and evaluate the bias by relative standard deviation of each coil. RESULTS: The simulation results revealed that the accuracy of B1+ maps improved with the increasing of channel numbers, meanwhile the per channel efficiency of B1+maps accuracy gradually decrease. Both trends slowed down when the channel numbers reached 12 or above. CONCLUSION: Our finding suggests that increasing the channel numbers can improve the accuracy of B1+map. However, a diminishing efficiency of per channel accuracy improvement was overserved, indicating that the relationship between quality of B1+ map and the channel numbers is nonlinear. Based on these findings, our study provides a reference for determining channel numbers to achieve a balance of coil selection and manufacturing cost. It also provides a theoretical basis for evaluating other B1+ mapping techniques.

B1 mapping using an EPI-based double angle approach: A practical guide for correcting slice profile and B0 distortion effects.

PURPOSE: Aim of this study was to develop a reliable B1 mapping method for brain imaging based on vendor MR sequences available on clinical scanners. Correction procedures for B0 distortions and slice profile imperfections are proposed, together with a phantom experiment for deriving the approximate time-bandwidth-product (TBP) of the excitation pulse, which is usually not known for vendor sequences. METHODS: The double angle method was used, acquiring two gradient echo echo-planar imaging data sets with different excitation angles. A correction factor C (B1 , TBP, B0 ) was derived from simulations for converting double angle method signal quotients into bias-free B1 maps. In vitro and in vivo tests compare results with reference B1 maps based on an established in-house sequence. RESULTS: The simulation shows that C has a negligible B1 dependence, allowing for a polynomial approximation of C (TBP, B0 ). Signal quotients measured in a phantom experiment with known TBP reconfirm the simulation results. In vitro and in vivo B1 maps based on the proposed method, assuming TBP = 5.8 as derived from a phantom experiment, match closely the reference B1 maps. Analysis without B0 correction shows marked deviations in areas of distorted B0 , highlighting the importance of this correction. CONCLUSION: Double angle method-based B1 mapping was set up for vendor gradient echo-echo-planar imaging sequences, using a correction procedure for slice profile imperfections and B0 distortions. This will help to set up quantitative MRI studies on clinical scanners with release sequences, as the method does not require knowledge of the exact RF-pulse profiles or the use of in-house sequences.

Optimized interferometric encoding of presaturated TurboFLASH B1 mapping for parallel transmission MRI at 7 T: Preliminary application for quantitative T1 mapping in the spinal cord.

PURPOSE: The acquisition of accurate B1 maps is critical for parallel transmit techniques (pTx). The presaturated turboFLASH (satTFL) method has been widely used in combination with interferometric encoding to provide robust and fast B1 maps. However, typical encodings, mostly evaluated on brain, do not necessarily fit all coils and organs. In this work, we evaluated and improved the accuracy of the satTFL for cervical spine at 7 T, proposing a novel interferometric encoding optimization. The benefits of such improvements were investigated in an exploratory study of quantitative T1 mapping with pTx-MP2RAGE. METHODS: Global optimization of interferometric encoding was implemented by simulating the ability of the satTFL to reconstruct B1 maps, with varying encoding and inclusion of complex noise, inside a region of interest covering the cervical spine. The performance of satTFL before and after optimization was compared to actual flip angle imaging. Optimized and non-optimized B1 maps were then used to calculate pTx pulses for MP2RAGE T1 mapping. RESULTS: Interferometric encoding optimization resulted in satTFL closer to actual flip angle imaging, with substantial gain of signal in regions where non-optimized satTFL could fail. T1 maps measured with non-adiabatic pTx pulses were closer to standard non-pTx results (which used adiabatic pulses) when using optimized-satTFL, with substantially lower specific absorption rate. CONCLUSION: Optimization of the satTFL interferometric encoding improves B1 maps in the spinal cord, in particular in low SNR regions. A linear correction of the satTFL was additionally shown to be required. The method was successfully used for quantitative phantom and in vivo T1 mapping, showing improved results compared to non-optimized satTFL thanks to improved pTx-pulse generation.

Optimized ultrahigh field parallel transmission workflow using rapid presaturated TurboFLASH transmit field mapping with a three-dimensional centric single-shot readout.

PURPOSE: To evaluate the usage of three-dimensional (3D) presaturated TurboFLASH (satTFL) for B 1 + $$ {\mathrm{B}}_1^{+} $$ and B 0 $$ {\mathrm{B}}_0 $$ mapping on single channel and parallel transmission (pTx) systems. METHODS: B 1 + $$ {\mathrm{B}}_1^{+} $$ maps recorded with 3D satTFL were compared to maps from three other 3D B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping sequences in an agar phantom. Furthermore, individual-channel B 1 + $$ {\mathrm{B}}_1^{+} $$ maps of 18 human subjects were recorded with 3D satTFL using B 1 + $$ {\mathrm{B}}_1^{+} $$ interferometry. A neural network was trained for masking of the maps. RESULTS: Out of the sequences compared satTFL was the only one with a mapping range exceeding well over 90°. In regions with lower flip angles there was high correspondence between satTFL and AFI. DREAM and double angle method also showed high qualitative similarity, however the magnitude differed from the other two measurements. The individual-channel B 1 + $$ {\mathrm{B}}_1^{+} $$ maps were successfully used for pTx pulse calculation in a separate study. CONCLUSION: 3D satTFL can record high-quality B 1 + $$ {\mathrm{B}}_1^{+} $$ maps with a high dynamic range in a short time. Correspondence with AFI maps is high, while measurement duration is reduced drastically.

Rapid 3D absolute B1 + mapping using a sandwiched train presaturated TurboFLASH sequence at 7 T for the brain and heart.

PURPOSE: To shorten the acquisition time of magnetization-prepared absolute transmit field (B1 + ) mapping known as presaturation TurboFLASH, or satTFL, to enable single breath-hold whole-heart 3D B1 + mapping. METHODS: SatTFL is modified to remove the delay between the reference and prepared images (typically 5 T1 ), with matching transmit configurations for excitation and preparation RF pulses. The new method, called Sandwich, is evaluated as a 3D sequence, measuring whole-brain and gated whole-heart B1 + maps in a single breath-hold. We evaluate the sensitivity to B1 + and T1 using numerical Bloch, extended phase graph, and Monte Carlo simulations. Phantom and in vivo images were acquired in both the brain and heart using an 8-channel transmit 7 Tesla MRI system to support the simulations. A segmented satTFL with a short readout train was used as a reference. RESULTS: The method significantly reduces acquisition times of 3D measurements from 360 s to 20 s, in the brain, while simultaneously reducing bias in the measured B1 + due to T1 and magnetization history. The mean coefficient of variation was reduced by 81% for T1 s of 0.5-3 s compared to conventional satTFL. In vivo, the reproducibility coefficient for flip angles in the range 0-130° was 4.5° for satTFL and 4.7° for our scheme, significantly smaller than for a short TR satTFL sequence, which was 12°. The 3D sequence measured B1 + maps of the whole thorax in 26 heartbeats. CONCLUSION: Our adaptations enable faster B1 + mapping, with minimal T1 sensitivity and lower sensitivity to magnetization history, enabling single breath-hold whole-heart absolute B1 + mapping.

Ultrafast B1 mapping with RF-prepared 3D FLASH acquisition: Correcting the bias due to T1 -induced k-space filtering effect.

PURPOSE: The traditional radiofrequency (RF)-prepared B1 mapping technique consists of one scan with an RF preparation module for flip angle-encoding and a second scan without this module for normalizing. To reduce the T1 -induced k-space filtering effect, this method is limited to 2D FLASH acquisition with a two-parameter method. A novel 3D RF-prepared three-parameter method for ultrafast B1 -mapping is proposed to correct the T1 -induced quantification bias. THEORY: The point spread function analysis of FLASH shows that the prepared longitudinal magnetization before the FLASH acquisition and the image signal obeys a linear (not proportional) relationship. The intercept of the linear function causes the quantification bias and can be captured by a third saturated scan. METHODS: Using the 2D double-angle method (DAM) as the reference, a 3D RF-prepared three-parameter protocol with 9 s duration was compared with the two-parameter method, as well as the saturated DAM (SDAM) method, the dual refocusing echo acquisition mode (DREAM) method, and the actual flip-angle imaging (AFI) method, for B1 mapping of brain, breast, and abdomen with different orientations and shim settings at 3T. RESULTS: The 3D RF-prepared three-parameter method with complex-subtraction delivered consistently lower RMS error, error mean, error standard deviation, and higher concordance correlation coefficients values than the two-parameter method, the three-parameter method with magnitude-subtraction, the multi-slice DREAM and the 3D AFI, and were close to the results of 2D or multi-slice SDAM. CONCLUSION: The proposed ultrafast 3D RF-prepared three-parameter method with complex-subtraction was demonstrated with high accuracy for B1 mapping of brain, breast, and abdomen.

Complex B1+ mapping with Carr-Purcell spin echoes and its application to electrical properties tomography.

PURPOSE: To present a new complex-valued B1+ mapping method for electrical properties tomography using Carr-Purcell spin echoes. METHODS: A Carr-Purcell (CP) echo train generates pronounced flip-angle dependent oscillations that can be used to estimate the magnitude of B1+ . To this end, a dictionary is used that takes into account the slice profile as well as T2 relaxation along the echo train. For validation, the retrieved B1+ map is compared with the actual flip angle imaging (AFI) method in a phantom (79 ε0 , 0.34 S/m). Moreover, the phase of the first echo reflects the transceive phase. Overall, the CP echo train yields an estimate of the complex-valued B1+ , allowing electrical properties tomography with both permittivity and conductivity. The presented method is evaluated in phantom scans as well as for in vivo brain at 3 T. RESULTS: In the phantom, the obtained magnitude B1+ maps retrieved from the CP echo train and the AFI method show excellent agreement, and both the reconstructed estimated permittivity (79 ± 3) ε0 and conductivity (0.35 ± 0.04) S/m values are in accordance with expectations. In the brain, the obtained electrical properties are also close to expectations. In addition to the retrieved complex B1+ information, the decay of the CP echo trains also yields an estimate for T2 . CONCLUSION: The CP sequence can be used to simultaneously provide both B1+ magnitude and phase estimations, and therefore allows for full reconstruction of the electrical properties.

Free-breathing self-gated continuous-IR spiral T1 mapping: Comparison of dual flip-angle and Bloch-Siegert B1-corrected techniques.

PURPOSE: To develop a B1-corrrected single flip-angle continuous acquisition strategy with free-breathing and cardiac self-gating for spiral T1 mapping, and compare it to a previous dual flip-angle technique. METHODS: Data were continuously acquired using a spiral-out trajectory, rotated by the golden angle in time. During the first 2 s, off-resonance Fermi RF pulses were applied to generate a Bloch-Siegert shift B1 map, and the subsequent data were acquired with an inversion RF pulse applied every 4 s to create a T1* map. The final T1 map was generated from the B1 and the T1* maps by using a look-up table that accounted for slice profile effects, yielding more accurate T1 values. T1 values were compared to those from inversion recovery (IR) spin echo (phantom only), MOLLI, SAturation-recovery single-SHot Acquisition (SASHA), and previously proposed dual flip-angle results. This strategy was evaluated in a phantom and 25 human subjects. RESULTS: The proposed technique showed good agreement with IR spin-echo results in the phantom experiment. For in-vivo studies, the proposed technique and the previously proposed dual flip-angle method were more similar to SASHA results than to MOLLI results. CONCLUSIONS: B1-corrected single flip-angle T1 mapping successfully acquired B1 and T1 maps in a free-breathing, continuous-IR spiral acquisition, providing a method with improved accuracy to measure T1 using a continuous Look-Locker acquisition, as compared to the previously proposed dual excitation flip-angle technique.

One-minute whole-brain magnetization transfer ratio imaging with intrinsic B1 -correction.

PURPOSE: Magnetization transfer ratio (MTR) histograms are used widely for the assessment of diffuse pathological changes in the brain. For broad clinical application, MTR scans should not only be fast, but confounding factors should also be minimized for high reproducibility. To this end, a 1-minute whole-brain spiral MTR method with intrinsic B1 -field correction is introduced. METHODS: A spiral multislice spoiled gradient-echo sequence with adaptable magnetization-transfer saturation pulses (angle ß) is proposed. After a low-resolution single-shot spiral readout and a dummy preparation period, high-resolution images are acquired using an interleaved spiral readout. For whole-brain MTR imaging, 50 interleaved slices with three different magnetization-transfer contrasts (ß = 0°, 350°, and 550°) together with an intrinsic B1 -field map are recorded in 58.5 seconds on a clinical 3T system. From the three contrasts, two sets of MTR images are derived and used for subsequent B1 correction, assuming a linear dependency on ß. For validation, a binary spin bath model is used. RESULTS: For the proposed B1 -correction scheme, numerical simulations indicate for brain tissue a decrease of about a factor of 10 for the B1 -related bias on MTR. As a result, following B1 correction, MTR differences in gray and white matter become markedly accentuated, and the reproducibility of MTR histograms from scan-rescan experiments is improved. Furthermore, B1 -corrected MTR histograms show a lower variability for age-matched normal-appearing brain tissue. CONCLUSION: From its speed and offering intrinsic B1 correction, the proposed method shows excellent prospects for clinical studies that explore magnetization-transfer effects based on MTR histogram analysis.

Demonstration and suppression of respiration-related artifacts in Bloch-Siegert shift-based B1+ maps of the human brain.

Respiration-induced movement of the chest wall and internal organs causes temporal B0 variations extending throughout the brain. This study demonstrates that these variations can cause significant artifacts in B1+ maps obtained at 7 T with the Bloch-Siegert shift (BSS) B1+ mapping technique. To suppress these artifacts, a navigator correction scheme was proposed. Two sets of experiments were performed. In the first set of experiments, phase shifts induced by respiration-related B0 variations were assessed for five subjects at 7 T by using a gradient echo (GRE) sequence without phase-encoding. In the second set of experiments, B1+ maps were acquired using a GRE-based BSS pulse sequence with navigator echoes. For this set, the measurements were consecutively repeated 16 times for the same imaging slice. These measurements were averaged to obtain the reference B1+ map. Due to the periodicity of respiration-related phase shifts, their effect on the reference B1+ map was assumed to be negligible through averaging. The individual B1+ maps of the 16 repetitions were calculated with and without using the proposed navigator scheme. These maps were compared with the B1+ reference map. The peak-to-peak value of respiration-related phase shifts varied between subjects. Without navigator correction, the interquartile range of percentage error in B1+ varied between 4.0% and 8.3% among subjects. When the proposed navigator scheme was used, these numbers were reduced to 2.5% and 2.9%, indicating an improvement in the precision of GRE-based BSS B1+ mapping at high magnetic fields.

Scan-Rescan Repeatability and Impact of B0 and B1 Field Nonuniformity Corrections in Single-Point Whole-Brain Macromolecular Proton Fraction Mapping.

BACKGROUND: Single-point macromolecular proton fraction (MPF) mapping is a recent quantitative MRI method for fast assessment of brain myelination. Information about reproducibility and sensitivity of MPF mapping to magnetic field nonuniformity is important for clinical applications. PURPOSE: To assess scan-rescan repeatability and a value of B0 and B1 field inhomogeneity corrections in single-point synthetic-reference MPF mapping. STUDY TYPE: Prospective. POPULATION: Eight healthy adult volunteers underwent two scans with 11.5 ± 2.3 months interval. FIELD STRENGTH/SEQUENCE: 3T; whole-brain 3D MPF mapping protocol included three spoiled gradient-echo sequences providing T1 , proton density, and magnetization transfer contrasts with 1.25 × 1.25 × 1.25 mm3 resolution and B0 and B1 mapping sequences. ASSESSMENT: MPF maps were reconstructed with B0 and B1 field nonuniformity correction, B0 - and B1 -only corrections, and without corrections. Mean MPF values were measured in automatically segmented white matter (WM) and gray matter (GM). STATISTICAL TESTS: Within-subject coefficient of variation (CV), intraclass correlation coefficient (ICC), Bland-Altman plots, and paired t-tests to assess scan-rescan repeatability. Repeated-measures analysis of variance (ANOVA) to compare field corrections. RESULTS: Maximal relative local MPF errors without correction in the areas of largest field nonuniformities were about 5% and 27% for B0 and B1 , respectively. The effect of B0 correction was insignificant for whole-brain WM (P > 0.25) and GM (P > 0.98) MPF. The absence of B1 correction caused a positive relative bias of 4-5% (P < 0.001) in both tissues. Scan-rescan agreement was similar for all field correction options with ICCs 0.80-0.81 for WM and 0.89-0.92 for GM. CVs were 1.6-1.7% for WM and 0.7-1.0% for GM. DATA CONCLUSION: The single-point method enables high repeatability of MPF maps obtained with the same equipment. Correction of B0 inhomogeneity may be disregarded to shorten the examination time. B1 nonuniformity correction improves accuracy of MPF measurements at 3T. Reliability of whole-brain MPF measurements in WM and GM is not affected by B0 and B1 field corrections. LEVEL OF EVIDENCE: 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2020;51:1789-1798.

Accuracy and precision of electrical permittivity mapping at 3T: the impact of three B1+ mapping techniques.

PURPOSE: To investigate the sequence-specific impact of B1+ amplitude mapping on the accuracy and precision of permittivity reconstruction at 3T in the pelvic region. METHODS: B1+ maps obtained with actual flip angle imaging (AFI), Bloch-Siegert (BS), and dual refocusing echo acquisition mode (DREAM) sequences, set to a clinically feasible scan time of 5 minutes, were compared in terms of accuracy and precision with electromagnetic and Bloch simulations and MR measurements. Permittivity maps were reconstructed based on these B1+ maps with Helmholtz-based electrical properties tomography. Accuracy and precision in permittivity were assessed. A 2-compartment phantom with properties and size similar to the human pelvis was used for both simulations and measurements. Measurements were also performed on a female volunteer's pelvis. RESULTS: Accuracy was evaluated with noiseless simulations on the phantom. The maximum B1+ bias relative to the true B1+ distribution was 1% for AFI and BS and 6% to 15% for DREAM. This caused an average permittivity bias relative to the true permittivity of 7% to 20% for AFI and BS and 12% to 35% for DREAM. Precision was assessed in MR experiments. The lowest standard deviation in permittivity, found in the phantom for BS, measured 22.4 relative units and corresponded to a standard deviation in B1+ of 0.2% of the B1+ average value. As regards B1+ precision, in vivo and phantom measurements were comparable. CONCLUSIONS: Our simulation framework quantitatively predicts the different impact of B1+ mapping techniques on permittivity reconstruction and shows high sensitivity of permittivity reconstructions to sequence-specific bias and noise perturbation in the B1+ map. These findings are supported by the experimental results.

Steady-state double-angle method for rapid B1 mapping.

PURPOSE: To introduce an accurate, rapid, and practical method for active B1 field mapping based on the double-angle method (DAM) in the steady-state (SS) signal regime. METHODS: We introduced and evaluated the performance of the SS-DAM approach to map the B1 field and compared the results to those calculated from the conventional DAM approach. Similar to DAM, SS-DAM uses the signal intensity ratio of 2 magnitude images acquired with different flip angles using the spoiled gradient recalled echo sequence. However, unlike DAM, in SS-DAM, these 2 spoiled gradient recalled echo images are acquired with very short TR, which allows substantially reduced acquisition time. Numerical, phantom, and in vivo brain imaging analyses, representing a wide range of T1 s and large B1 variation, were conducted. Methods for further accelerating acquisition were also investigated. RESULTS: Our results demonstrate the potential of the SS-DAM approach to be applied widely in the clinical setting. B1 maps derived from SS-DAM were demonstrated to be quantitatively comparable to those derived from DAM but were derived much more rapidly. Large-volume B1 maps were obtained at a field strength of 3 tesla within clinically acceptable acquisition times. CONCLUSION: SS-DAM permits accurate B1 mapping in the clinical setting, with whole-brain coverage in less than 1 min.

Mapping electrical properties heterogeneity of tumor using boundary informed electrical properties tomography (BIEPT) at 7T.

PURPOSES: To develop and evaluate a boundary informed electrical properties tomography (BIEPT) technique for high-resolution imaging of tumor electrical properties (EPs) heterogeneity on a rodent tumor xenograft model. METHODS: Tumor EP distributions were inferred from a reference area external to the tumor, as well as internal EP spatial variations derived from a plurality of relative transmit B1 measurements at 7T. Edge sparsity constraint was enforced to enhance numerical stability. Phantom experiments were performed to determine the imaging accuracy and sensitivity for structures of various EP values, as well as geometrical sizes down to 1.5 mm. Numerical simulation of a realistic rodent model was used to quantify the algorithm performance in the presence of noise. Eleven athymic rats with human breast cancer xenograft were imaged in vivo, and representative pathological samples were acquired for comparison. RESULTS: Reconstructed EPs of the phantoms correspond well to the ground truth acquired from dielectric probe measurements, with the smallest structure reliably detectable being 3 mm. EPs heterogeneity inside a tumor is successfully retrieved in both simulated and experimental cases. In vivo tumor imaging results demonstrate similar local features and spatial patterns to anatomical MRI and pathological slides. The imaged conductivity of necrotic tissue is higher than that of viable tissues, which agrees with our expectation. CONCLUSION: BIEPT enables robust detection of tumor EPs heterogeneity with high accuracy and sensitivity to small structures. The retrieved quantitative EPs reflect tumor pathological features (e.g., necrosis). These results provide strong rationale to further expand BIEPT studies toward pathological conditions where EPs may yield valuable, non-invasive biomarkers.

A regional bolus tracking and real-time B1 calibration method for hyperpolarized 13 C MRI.

PURPOSE: Acquisition timing and B1 calibration are two key factors that affect the quality and accuracy of hyperpolarized 13 C MRI. The goal of this project was to develop a new approach using regional bolus tracking to trigger Bloch-Siegert B1 mapping and real-time B1 calibration based on regional B1 measurements, followed by dynamic imaging of hyperpolarized 13 C metabolites in vivo. METHODS: The proposed approach was implemented on a system which allows real-time data processing and real-time control on the sequence. Real-time center frequency calibration upon the bolus arrival was also added. The feasibility of applying the proposed framework for in vivo hyperpolarized 13 C imaging was tested on healthy rats, tumor-bearing mice and a healthy volunteer on a clinical 3T scanner following hyperpolarized [1-13 C]pyruvate injection. Multichannel receive coils were used in the human study. RESULTS: Automatic acquisition timing based on either regional bolus peak or bolus arrival was achieved with the proposed framework. Reduced blurring artifacts in real-time reconstructed images were observed with real-time center frequency calibration. Real-time computed B1 scaling factors agreed with real-time acquired B1 maps. Flip angle correction using B1 maps results in a more consistent quantification of metabolic activity (i.e, pyruvate-to-lactate conversion, kPL ). Experiment recordings are provided to demonstrate the real-time actions during the experiment. CONCLUSIONS: The proposed method was successfully demonstrated on animals and a human volunteer, and is anticipated to improve the efficient use of the hyperpolarized signal as well as the accuracy and robustness of hyperpolarized 13 C imaging.