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Purpose: To introduce an alternative idea for fat suppression that is suited both for low‐field applications where conventional fat‐suppression approaches become ineffective due to narrow spectral separation and for applications with strong B0 homogeneities. Methods: Separation of fat and water is achieved by sweeping the frequency of RF saturation pulses during continuous radial acquisition and calculating frequency‐resolved images using regularized iterative reconstruction. Voxel‐wise signal‐response curves are extracted that reflect tissue's response to RF saturation at different frequencies and allow the classification into fat or water. This information is then utilized to generate water‐only composite images. The principle is demonstrated in free‐breathing abdominal and neck examinations using stack‐of‐stars 3D balanced SSFP (bSSFP) and gradient‐recalled echo (GRE) sequences at 0.55 and 3T. Moreover, a potential extension toward quantitative fat/water separation is described. Results: Experiments with a proton density fat fraction (PDFF) phantom validated the reliability of fat/water separation using signal‐response curves. As demonstrated for abdominal imaging at 0.55T, the approach resulted in more uniform fat suppression without loss of water signal and in improved CSF‐to‐fat signal ratio. Moreover, the approach provided consistent fat suppression in 3T neck exams where conventional spectrally‐selective fat saturation failed due to strong local B0 inhomogeneities. The feasibility of simultaneous fat/water quantification has been demonstrated in a PDFF phantom. Conclusion: The proposed principle achieves reliable fat suppression in low‐field applications and adapts to high‐field applications with strong B0 inhomogeneity. Moreover, the principle potentially provides a basis for developing an alternative approach for PDFF quantification. [ABSTRACT FROM AUTHOR]

Ultrahigh field magnetic resonance imaging (MRI) (≥ 7 T) has the potential to provide superior spatial resolution and unique image contrast. Apart from radiofrequency transmit inhomogeneities in the body at this field strength, imaging of the upper abdomen faces additional challenges associated with motion‐induced ghosting artifacts. To address these challenges, the goal of this work was to develop a technique for high‐resolution free‐breathing upper abdominal MRI at 7 T with a large field of view. Free‐breathing 3D gradient‐recalled echo (GRE) water‐excited radial stack‐of‐stars data were acquired in seven healthy volunteers (five males/two females, body mass index: 19.6–24.8 kg/m2) at 7 T using an eight‐channel transceive array coil. Two volunteers were also examined at 3 T. In each volunteer, the liver and kidney regions were scanned in two separate acquisitions. To homogenize signal excitation, the time‐interleaved acquisition of modes (TIAMO) method was used with personalized pairs of B1 shims, based on a 23‐s Cartesian fast low angle shot (FLASH) acquisition. Utilizing free‐induction decay navigator signals, respiratory‐gated images were reconstructed at a spatial resolution of 0.8 × 0.8 × 1.0 mm3. Two experienced radiologists rated the image quality and the impact of B1 inhomogeneity and motion‐related artifacts on multipoint scales. The images of all volunteers showcased effective water excitation and were accurately corrected for respiratory motion. The impact of B1 inhomogeneity on image quality was minimal, underscoring the efficacy of the multitransmit TIAMO shim. The high spatial resolution allowed excellent depiction of small structures such as the adrenal glands, the proximal ureter, the diaphragm, and small blood vessels, although some streaking artifacts persisted in liver image data. In direct comparisons with 3 T performed for two volunteers, 7‐T acquisitions demonstrated increases in signal‐to‐noise ratio of 77% and 58%. Overall, this work demonstrates the feasibility of free‐breathing MRI in the upper abdomen at submillimeter spatial resolution at a magnetic field strength of 7 T. [ABSTRACT FROM AUTHOR]

Respiratory motion‐induced image blurring and artifacts can compromise image quality in dynamic contrast‐enhanced MRI (DCE‐MRI) of the liver. Despite remarkable advances in respiratory motion detection and compensation in past years, these techniques have not yet seen widespread clinical adoption. The accuracy of image‐based motion detection can be especially compromised in the presence of contrast enhancement and/or in situations involving deep and/or irregular breathing patterns. This work proposes a framework that combines GRASP‐Pro (Golden‐angle RAdial Sparse Parallel MRI with imProved performance) MRI with a new radial sampling scheme called navi‐stack‐of‐stars for free‐breathing DCE‐MRI of the liver without the need for explicit respiratory motion compensation. A prototype 3D golden‐angle radial sequence with a navi‐stack‐of‐stars sampling scheme that intermittently acquires a 2D navigator was implemented. Free‐breathing DCE‐MRI of the liver was conducted in 24 subjects at 3T including 17 volunteers and 7 patients. GRASP‐Pro reconstruction was performed with a temporal resolution of 0.34–0.45 s per volume, whereas standard GRASP reconstruction was performed with a temporal resolution of 15 s per volume. Motion compensation was not performed in all image reconstruction tasks. Liver images in different contrast phases from both GRASP and GRASP‐Pro reconstructions were visually scored by two experienced abdominal radiologists for comparison. The nonparametric paired two‐tailed Wilcoxon signed‐rank test was used to compare image quality scores, and the Cohen's kappa coefficient was calculated to evaluate the inter‐reader agreement. GRASP‐Pro MRI with sub‐second temporal resolution consistently received significantly higher image quality scores (P < 0.05) than standard GRASP MRI throughout all contrast enhancement phases and across all assessment categories. There was a substantial inter‐reader agreement for all assessment categories (ranging from 0.67 to 0.89). The proposed technique using GRASP‐Pro reconstruction with navi‐stack‐of‐stars sampling holds great promise for free‐breathing DCE‐MRI of the liver without respiratory motion compensation. [ABSTRACT FROM AUTHOR]

Purpose: To describe an inversion‐recovery T1‐weighted radial stack‐of‐stars 3D gradient echo (GRE) sequence with comparable image quality to conventional MP‐RAGE and to demonstrate how the radial acquisition scheme can be utilized for additional retrospective motion correction to improve robustness to head motion. Methods: The proposed sequence, named MP‐RAVE, has been derived from a previously described radial stack‐of‐stars 3D GRE sequence (RAVE) and includes a 180° inversion recovery pulse that is generated once for every stack of radial views. The sequence is combined with retrospective 3D motion correction to improve robustness. The effectiveness has been evaluated in phantoms and healthy volunteers and compared to conventional MP‐RAGE acquisition. Results: MP‐RAGE and MP‐RAVE anatomical images were rated "good" to "excellent" in overall image quality, with artifact level between "mild" and "no artifacts", and with no statistically significant difference between methods. During head motion, MP‐RAVE showed higher inherent robustness with artifacts confined to local brain regions. In combination with motion correction, MP‐RAVE provided noticeably improved image quality during different head motion and showed statistically significant improvement in image sharpness. Conclusion: MP‐RAVE provides comparable image quality and contrast to conventional MP‐RAGE with improved robustness to head motion. In combination with retrospective 3D motion correction, MP‐RAVE can be a useful alternative to MP‐RAGE, especially in non‐cooperative or pediatric patients. [ABSTRACT FROM AUTHOR]

Purpose: This study aimed to (i) develop Magnetization-Prepared Golden-angle RAdial Sparse Parallel (MP-GRASP) MRI using a stack-of-stars trajectory for rapid free-breathing T1 mapping and (ii) extend MP-GRASP to multi-echo acquisition (MP-Dixon-GRASP) for fat/water-separated (water-specific) T1 mapping. Methods: An adiabatic non-selective 180° inversion-recovery pulse was added to a gradient-echo-based golden-angle stack-of-stars sequence for magnetization-prepared 3D single-echo or 3D multi-echo acquisition. In combination with subspace-based GRASP-Pro reconstruction, the sequence allows for standard T1 mapping (MP-GRASP) or fat/water-separated T1 mapping (MP-Dixon-GRASP), respectively. The accuracy of T1 mapping using MP-GRASP was evaluated in a phantom and volunteers (brain and liver) against clinically accepted reference methods. The repeatability of T1 estimation was also assessed in the phantom and volunteers. The performance of MP-Dixon-GRASP for water-specific T1 mapping was evaluated in a fat/water phantom and volunteers (brain and liver). Results: ROI-based mean T1 values are correlated between the references and MPGRASP in the phantom (R² = 1.0), brain (R² = 0.96), and liver (R² = 0.73). MPGRASP achieved good repeatability of T1 estimation in the phantom (R² = 1.0), brain (R² = 0.99), and liver (R² = 0.82). Water-specific T1 is different from in-phase and out-of-phase composite T1 (composite T1 when fat and water signal are mixed in phase or out of phase) both in the phantom and volunteers. Conclusion: This work demonstrated the initial performance of MP-GRASP and MP-Dixon-GRASP MRI for rapid 3D T1 mapping and 3D fat/water-separated T1 mapping in the brain (without motion) and in the liver (during free breathing). With fat/water-separated T1 estimation, MP-Dixon-GRASP could be potentially useful for imaging patients with fatty-liver diseases. [ABSTRACT FROM AUTHOR]

Purpose: To describe an approach for detection of respiratory signals using a transmitted radiofrequency (RF) reference signal called Pilot‐Tone (PT) and to use the PT signal for creation of motion‐resolved images based on 3D stack‐of‐stars imaging under free‐breathing conditions. Methods: This work explores the use of a reference RF signal generated by a small RF transmitter, placed outside the MR bore. The reference signal is received in parallel to the MR signal during each readout. Because the received PT amplitude is modulated by the subject's breathing pattern, a respiratory signal can be obtained by detecting the strength of the received PT signal over time. The breathing‐induced PT signal modulation can then be used for reconstructing motion‐resolved images from free‐breathing scans. The PT approach was tested in volunteers using a radial stack‐of‐stars 3D gradient echo (GRE) sequence with golden‐angle acquisition. Results: Respiratory signals derived from the proposed PT method were compared to signals from a respiratory cushion sensor and k‐space‐center‐based self‐navigation under different breathing conditions. Moreover, the accuracy was assessed using a modified acquisition scheme replacing the golden‐angle scheme by a zero‐angle acquisition. Incorporating the PT signal into eXtra‐Dimensional (XD) motion‐resolved reconstruction led to improved image quality and clearer anatomical depiction of the lung and liver compared to k‐space‐center signal and motion‐averaged reconstruction, when binned into 6, 8, and 10 motion states. Conclusion: PT is a novel concept for tracking respiratory motion. Its small dimension (8 cm), high sampling rate, and minimal interaction with the imaging scan offers great potential for resolving respiratory motion. [ABSTRACT FROM AUTHOR]

Purpose: To develop a free‐breathing hepatic fat and R2∗ quantification method by extending a previously described stack‐of‐stars model‐based fat‐water separation technique with additional modeling of the transverse relaxation rate R2∗. Methods: The proposed technique combines motion‐robust radial sampling using a stack‐of‐stars bipolar multi‐echo 3D GRE acquisition with iterative model‐based fat‐water separation. Parallel‐Imaging and Compressed‐Sensing principles are incorporated through modeling of the coil‐sensitivity profiles and enforcement of total‐variation (TV) sparsity on estimated water, fat, and R2∗ parameter maps. Water and fat signals are used to estimate the confounder‐corrected proton‐density fat fraction (PDFF). Two strategies for handling respiratory motion are described: motion‐averaged and motion‐resolved reconstruction. Both techniques were evaluated in patients (n = 14) undergoing a hepatobiliary research protocol at 3T. PDFF and R2∗ parameter maps were compared to a breath‐holding Cartesian reference approach. Results: Linear regression analyses demonstrated strong (r > 0.96) and significant (P ≪.01) correlations between radial and Cartesian PDFF measurements for both the motion‐averaged reconstruction (slope: 0.90; intercept: 0.07%) and the motion‐resolved reconstruction (slope: 0.90; intercept: 0.11%). The motion‐averaged technique overestimated hepatic R2∗ values (slope: 0.35; intercept: 30.2 1/s) compared to the Cartesian reference. However, performing a respiratory‐resolved reconstruction led to better R2∗ value consistency (slope: 0.77; intercept: 7.5 1/s). Conclusions: The proposed techniques are promising alternatives to conventional Cartesian imaging for fat and R2∗ quantification in patients with limited breath‐holding capabilities. For accurate R2∗ estimation, respiratory‐resolved reconstruction should be used. [ABSTRACT FROM AUTHOR]

Purpose: To study the effects of magnetization transfer (MT, in which a semi‐solid spin pool interacts with the free pool), in the context of magnetic resonance fingerprinting (MRF). Methods: Simulations and phantom experiments were performed to study the impact of MT on the MRF signal and its potential influence on T1 and T2 estimation. Subsequently, an MRF sequence implementing off‐resonance MT pulses and a dictionary with an MT dimension, generated by incorporating a two‐pool model, were used to estimate the fractional pool size in addition to the B1+, T1, and T2 values. The proposed method was evaluated in the human brain. Results: Simulations and phantom experiments showed that an MRF signal obtained from a cross‐linked bovine serum sample is influenced by MT. Using a dictionary based on an MT model, a better match between simulations and acquired MR signals can be obtained (NRMSE 1.3% vs. 4.7%). Adding off‐resonance MT pulses can improve the differentiation of MT from T1 and T2. In vivo results showed that MT affects the MRF signals from white matter (fractional pool‐size ~16%) and gray matter (fractional pool‐size ~10%). Furthermore, longer T1 (~1060 ms vs. ~860 ms) and T2 values (~47 ms vs. ~35 ms) can be observed in white matter if MT is accounted for. Conclusion: Our experiments demonstrated a potential influence of MT on the quantification of T1 and T2 with MRF. A model that encompasses MT effects can improve the accuracy of estimated relaxation parameters and allows quantification of the fractional pool size. [ABSTRACT FROM AUTHOR]