Regularized SUPER-CAIPIRINHA: Accelerating 3D variable flip-angle T1 mapping with accurate andefficient reconstruction.

PurposeThe variable flip angle (VFA) approach to T1 mapping assumes perfectly spoiled transverse magnetisation at the end of each repetition time (TR). Despite radiofrequency (RF) and gradient spoiling, this condition is rarely met, leading to erroneous T1 estimates (T1app). Theoretical corrections can be applied but make assumptions about tissue properties, for example, a global T2 time. Here, we investigate the effect of imperfect spoiling at 7T and the interaction between the RF and gradient spoiling conditions, additionally accounting for diffusion. We provide guidance on the optimal approach to maximise the accuracy of the T1 estimate in the context of 3D multi‐echo acquisitions.MethodsThe impact of the spoiling regime was investigated through numerical simulations, phantom and invivo experiments.ResultsThe predicted dependence of T1app on tissue properties, system settings, and spoiling conditions was observed in both phantom and in vivo experiments. Diffusion effects modulated the dependence of T1app on both B1+ efficiency and T2 times.ConclusionError in T1app can be minimized by using an RF spoiling increment and gradient spoiler moment combination that minimizes T2‐dependence and safeguards image quality. Although the diffusion effect was comparatively small at 7T, correction factors accounting for this effect are recommended.

Comparison of DCE-MRI parametric mapping using MP2RAGE and variable flip angle T1 mapping

Motivation: A rapid 3D whole-brain high-resolution T1 quantification using IR is desired but time-consuming. Goal(s): Optimize IR protocols to make the acquisition faster with sufficient accuracy. Approach: Conventional IR was accelerated using a long 3D stack-of-spirals FLASH readout. Its accuracy was evaluated by comparing protocols using varying saturation times with a reference 2D-EPI readout and with different undersampling factors. Results: This study presented a method for 3D whole-brain 1mm-resolution T1 mapping using IR with stack-of-spirals FLASH acquisition under 3.6min, employing a short presaturation time, a long echo train, and a large undersampling factor. Impact: This study demonstrates a 3D whole-brain 1mm-resolution IR-based T1 mapping method under 3.6min. The optimized IR technique was accelerated using a 3D SoS-TFL readout with a short presaturation time, a long echo train, as well as a large undersampling factor.

Objective: Non-invasive methods have been investigated as an alternative to biopsy in assessing liver fibrosis. This study aimed to evaluate the relationship between liver T1 relaxation time and liver fibrosis as a non-invasive alternative method. Patients and Methods: This study analyzed 1.5T magnetic resonance (MR) images of 86 patients retrospectively. The participants were divided into two groups: patients with chronic hepatitis and the control group. Native variable flip angle (VFA) T1 mapping technique was used to estimate liver T1 relaxation time. T1 mapping sequence, T2* mapping sequence, and image analysis were performed. The liver size, the spleen size, the liver T1 relaxation time, and the liver T2* relaxation time were recorded. Results: The T1 relaxation time was 758.4 ± 121.1 ms in the chronic hepatitis group and 600.2 ± 67 ms in the control group. The T1 relaxation time of the patient group was significantly higher than that of the control group (p

Background Clinical guidelines recommend the use of established T2 mapping sequences to detect and quantify myocarditis and edema, but T2 mapping is performed in two dimensions with limited coverage and repetitive breath holds. Purpose To assess the reproducibility of an accelerated free-breathing three-dimensional (3D) whole-heart T2 MRI mapping sequence in phantoms and participants without a history of cardiac disease and to investigate its clinical performance in participants with suspected myocarditis. Materials and Methods Eight participants (three women, mean age, 31 years ± 4 [standard deviation]; cohort 1) without a history of cardiac disease and 25 participants (nine women, mean age, 45 years ± 17; cohort 2) with clinically suspected myocarditis underwent accelerated free-breathing 3D whole-heart T2 mapping with 100% respiratory scanning efficiency at 1.5 T. The participants were enrolled from November 2018 to August 2020. Three repeated scans were performed on 2 separate days in cohort 1. Segmental variations in T2 relaxation times of the left ventricular myocardium were assessed, and intrasession and intersession reproducibility were measured. In cohort 2, segmental myocardial T2 values, detection of focal inflammation, and map quality were compared with those obtained from clinical breath-hold two-dimensional (2D) T2 mapping. Statistical differences were assessed using the nonparametric Mann-Whitney and Kruskal-Wallis tests, whereas the paired Wilcoxon signed-rank test was used to assess subjective scores. Results Whole-heart T2 maps were acquired in a mean time of 6 minutes 53 seconds ± 1 minute 5 seconds at 1.5 mm3 resolution. Breath-hold 2D and free-breathing 3D T2 mapping had similar intrasession (mean T2 change of 3.2% and 2.3% for 2D and 3D, respectively) and intersession (4.8% and 4.9%, respectively) reproducibility. The two T2 mapping sequences showed similar map quality (P = .23, cohort 2). Abnormal myocardial segments were identified with confidence (score 3) in 14 of 25 participants (56%) with 3D T2 mapping and only in 10 of 25 participants (40%) with 2D T2 mapping. Conclusion High-spatial-resolution three-dimensional (3D) whole-heart T2 mapping shows high intrasession and intersession reproducibility and helps provide T2 myocardial characterization in agreement with clinical two-dimensional reference, while enabling 3D assessment of focal disease with higher confidence. © RSNA, 2021 Online supplemental material is available for this article. See also the editorial by Friedrich in this issue.

Cardiac amyloidosis (CA) is a significant contributor to heart failure with preserved ejection fraction and is appreciating expanding therapeutic options. Non-invasive tools aimed at accurate identification and surveillance of therapeutic response are of immediate and expanding need. While native and post-contrast T1 mapping quantify expansion of the extra-cellular compartment from amyloid protein deposition, 3D strain analysis of non-contrast cine images offers unique advantages relevant to high prevalence of renal insufficiency in this population and reduced dependency on field strength, pulse sequence, and vendor implementation. We aimed to evaluate global and segmental associations between 3D strain and T1 mapping in patients with cardiac amyloidosis. Twenty consecutive patients with confirmed CA were recruited and underwent a standardized cardiovascular magnetic resonance imaging protocol at 3T including using multi-planar cine imaging and T1 mapping using a shortened modified look-locker inversion recovery sequence. T1 mapping was performed pre- and (when permitted by renal function) post-contrast and measured for segmental T1 values. Spatially-matched 3D strain-based measures were similarly calculated. Mean left ventricular ejection fraction was 61 ± 21% (range 30-73%). Mean global native T1 was 1308 ± 96ms. Post-contrast T1 and partition coefficient were 558 ± 104ms and 0.85 ± 0.31, respectively. Global myocardial strain values were 8.1 ± 2.9% in the longitudinal direction, - 9.2 ± 3.4% in the circumferential direction, and 41.7 ± 22.8% in the maximum principal direction. Segmental analyses confirmed relative worsening in T1 values and reductions in strain values in the basal myocardial segments with relative sparing of the apical segments. Significant associations between T1 and strain-based measures were observed globally and segmentally, with the strongest associations found both globally and segmentally in the circumferential and minimum principal directions of deformation. This study identifies strong associations between 3D myocardial strain and T1-mapping based markers of regional amyloid protein deposition. These findings support expanded investigation of myocardial strain as a surrogate marker of response to novel therapeutic strategies in patients with cardiac amyloidosis.

Variable flip angle (VFA) imaging is widely used for the estimation of T1 relaxation in the prostate, but may have limited repeatability and reproducibility due to its sensitivity to B1 + inhomogeneity. To assess the repeatability and reproducibility of prostate T1 estimation with and without compensating for B1 + variation. Prospective. Twenty-one volunteers were prospectively recruited and scanned twice on two 3 T MRI scanners, resulting in 84 VFA T1 exams. 3 T/2D saturated turbo fast low angle shot (FLASH) and 3D dual-echo FLASH. Two B1 + mapping techniques, including reference region VFA (RR-VFA) and saturated turbo FLASH (satTFL), were used for B1 + correction, and T1 maps with and without B1 + correction were tested for intrascanner repeatability and interscanner reproducibility. Volumetric regions of interest (ROIs) were drawn on the transition zone, peripheral zone of the prostate, and the obturator internus left and right muscles in the corresponding slices. The average T1 within each ROI for each scan was compared for both intra- and interscanner variability using concordance correlation coefficient and a Bland-Altman plot. Both RR-VFA-corrected T1 and satTFL-corrected T1 showed higher intra- and interscanner correlation (0.89/0.87 and 0.87/0.84, respectively) than VFA T1 (0.84 and 0.74). Bland-Altman plots showed that VFA T1 had wider 95% limits of agreement and a larger range of T1 for each tissue compared with T1 with B1 + correction. The application of B1 + correction (both RR-VFA and satTFL) to VFA T1 results in more repeatable and reproducible T1 estimation than VFA T1 . This can potentially provide improved quantification of the prostate dynamic contrast-enhanced MRI parameters. Level of Evidence 1. Technical Efficacy Stage 1. J. Magn. Reson. Imaging 2018.

To develop a three-dimensional (3D) free-breathing myocardial T1 mapping sequence for assessment of left ventricle diffuse fibrosis after contrast administration. In the proposed sequence, multiple 3D inversion recovery images are acquired in an interleaved manner. A mixed prospective/retrospective navigator scheme is used to obtain the 3D Cartesian k-space data with fully sampled center and randomly undersampled outer k-space. The resulting undersampled 3D k-space data are then reconstructed using compressed sensing. Subsequently, T1 maps are generated by voxel-wise curve fitting of the individual interleaved images. In a phantom study, the accuracy of the 3D sequence was evaluated against two-dimensional (2D) modified Look-Locker inversion recovery (MOLLI) and spin-echo sequences. In vivo T1 times of the proposed method were compared with 2D multislice MOLLI T1 mapping. Subsequently, the feasibility of high-resolution 3D T1 mapping with spatial resolution of 1.7 × 1.7 × 4 mm(3) was demonstrated. The proposed method shows good agreement with 2D MOLLI and the spin-echo reference in phantom. No significant difference was found in the in vivo T1 times estimated using the proposed sequence and the 2D MOLLI technique (myocardium, 330 ± 66 ms versus 319 ± 93 ms; blood pools, 211 ± 68 ms versus 210 ± 98 ms). However, improved homogeneity, as measured using standard deviation of the T1 signal, was observed in the 3D T1 maps. The proposed sequence enables high-resolution 3D T1 mapping after contrast injection during free-breathing with volumetric left ventricle coverage.

Variable flip angle (VFA) imaging is widely used for whole-brain T1 quantification. Because of the requirement to acquire at least two sets of MR images at different flip angles, VFA relaxometry is relatively slow. Here, whole-brain VFA T1 mapping at 1.5 T is accelerated by using efficient spiral non-Cartesian imaging METHODS: To quantify T1 in the human brain, radiofrequency spoiled gradient-echo imaging is performed at two optimized flip angles using an interleaved 2D multislice sequence with high spoiling efficiency. The acquisitions are accelerated by using a spiral trajectory with 10 interleaves combined with a dedicated magnetization preparation to ensure steady-state conditions in minimal time. The investigated MR scan protocol allowed the acquisition of whole-brain T1 maps at a clinically relevant resolution in only 40 s (0.7 s per slice) with high reproducibility. White and gray matter T1 peaks clearly could be delineated by calculation of whole-brain T1 histograms, and the delivered T1 values showed good agreement with the reference method in selected regions of interest. Due to the use of a fast spiral k-space trajectory, whole-brain VFA T1 mapping could be accelerated by an order of magnitude compared to conventional Cartesian sampling. Magn Reson Med 79:856-866, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

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. 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). ROI-based mean T1 values are correlated between the references and MP-GRASP in the phantom (R2 = 1.0), brain (R2 = 0.96), and liver (R2 = 0.73). MP-GRASP 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. 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.

Quantitative magnetic resonance imaging (qMRI) offers reliable biomarkers in clinic. Nevertheless, most qMRI methods are time-consuming and sensitive to motion. Single-shot multiple overlapping-echo detachment (MOLED) magnetic resonance imaging can deliver robust T2 mapping in about 100ms with high motion tolerance. However, its spatial resolution is relatively low due to the limitations of signal-to-noise ratio (SNR) and echo-train length. At the mean time, the number of echoes with different evolution times collected is usually limited, which is not conducive to T2 mapping in high accuracy. To propose a novel method to improve the spatial resolution and quantification accuracy of single-shot MOLED T2 mapping. A new method called switching modulation patterns multiple overlapping-echo detachment imaging (SWP-MOLED) was designed for multi-slice information sharing via switching the k-space modulation pattern of MOLED imaging. In the SWP-MOLED pulse sequence, three different k-space modulation patterns were devised, making the 12 main echoes of any three adjacent slices symmetrically and uniformly distributed around their k-space centers to obtain diverse contrast weighting information. A multi-slice fusion three-dimensional spatial attention context-guided U-Net was trained with 3000/7000 synthetic data with geometric/brain patterns to efficiently learn the mapping relationship between SWP-MOLED signals and T2 maps. Experiments on numerical human brains, a phantom containing MnCl2 solutions with different concentrations, three healthy volunteers, and three patients diagnosed with meningioma or glioblastoma were performed. The effectiveness of the new method was quantitatively assessed using the structure similarity index measure (SSIM) and root mean square error (RMSE). Multiple statistical analyses were utilized to evaluate the accuracy and significance of the method, including linear regression, Bland-Altman analysis, Mann-Whitney test, Wilcoxon signed rank test, and Friedman test with Bonferroni correction, with the p-value significance level of 0.05. The results from numerical human brain (The average SSIM of the reconstructed T2 maps was 0.9742/0.9782/0.9826 for MOLED/MS-MOLED/SWP-MOLED) and phantom (The slope of linear fitting of the predicted T2 values vs. reference values was 0.9934/9942/0.9972 for MOLED/MS-MOLED/SWP-MOLED) demonstrated that more accurate T2 maps were delivered by the proposed method, closely resembling the reference maps. From the Friedman test performed on the results of the test data set after the multi-comparison correction, we found that the pairwise performance differences among different reconstruction networks were all statistically significant (p<0.001). In healthy human brain experiments, the comparison of SWP-MOLED reconstruction with reference measurements indicated no significant difference (p=0.4504). SWP-MOLED was quite repeatable (average coefficient of variation [CV]=4.17%) and was not corrupted by motion (average CV=7.49%). Moreover, the proposed method exhibited clearer lesion contours in clinical cases, demonstrating the potential of the proposed method for clinical applications. SWP-MOLED can efficiently exploit the structural similarity and parameter-weighted information diversity of adjacent slices to improve the spatial resolution and quantification accuracy of MOLED T2 mapping. It also exhibits excellent motion robustness. This technique would extend the application of MOLED imaging.

Detecting variations in myocardial water content with T2 mapping is superior to conventional T2 -weighted MRI since quantification enables direct observation of complicated pathology. Most commonly used T2 mapping techniques are limited in achievable spatial and/or temporal resolution, both of which reduce accuracy due to partial-volume averaging and misregistration between images. The goal of this study was to validate a novel free breathing T2 mapping sequence that overcomes these limitations. The proposed technique was made insensitive to heart rate variability through the use of a saturation prepulse to reset magnetization every heartbeat. Respiratory navigator-gated, differentially T2 -weighted volumes were interleaved per heartbeat, guaranteeing registered images and robust voxel-by-voxel T2 maps. Free breathing acquisitions removed limits on spatial resolution and allowed short diastolic windows. Accuracy was quantified with simulations and phantoms. Homogeneous three-dimensional (3D) T2 maps were obtained from normal human subjects and swine. Normal human and swine left ventricular T2 values were 42.3 ± 4.0 and 43.5 ± 4.3 ms, respectively. The T2 value for edematous myocardium obtained from a swine model of acute myocardial infarction was 59.1 ± 7.1 ms. Free-breathing accurate 3D T2 mapping is feasible and may be applicable in myocardial assessment in lieu of current clinical black blood, T2 -weighted techniques.

Mapping the longitudinal relaxation time constant (T1) of the myocardium using Magnetic Resonance Imaging (MRI) is an emerging technique for quantitative assessment of the morphology and viability of the myocardium. However, three-dimensional (3D) T1 mapping of the heart is challenging due to the high dimensionality of the signal and the presence of cardiac and respiratory motions. We propose a subspace-based method for free-breathing 3D T1 mapping of the heart without respiratory gating. The image function is represented as a high-order partially separable (PS) function to explore the inherent spatiotemporal correlations of the underlying signal. A special data acquisition scheme enabled by the high-order PS model is used for sparse sampling of the (k,t)-space, where complementary sparse datasets are acquired, each covering only a small portion of the (k,t)-space to characterize a single subspace (spatial or temporal). High-resolution dynamic MR images are reconstructed from the highly undersampled (k,t)-space using low-rank tensor and sparsity constraints. We demonstrate the feasibility of our proposed method using in vivo data obtained from healthy subjects on a 3T MR scanner. The proposed method can enable new clinical applications of T1 mapping in cardiac MR.

BackgroundThe aim of the study was to quantify atherosclerotic plaque burden by volumetric assessment and T1 relaxivity measurement at 7T MRI using Gadospin F (GDF) in comparison to en face based measurements.Methods and results9-weeks old ApoE-/- (n = 5 for each group) and wildtype mice (n = 5) were set on high fat diet (HFD). Progression group received MRI at 9, 13, 17 and 21 weeks after HFD initiation. Regression group was reswitched to chow diet (CD) after 13 weeks HFD and monitored with MRI for 12 weeks. MRI was performed before and two hours after iv injection of GDF (100 μmol/kg) at 7T (Clinscan, Bruker) acquiring a 3D inversion recovery gradient echo sequence and T1 Mapping using Saturation Recovery sequences. Subsequently, aortas were prepared for en face analysis using confocal microscopy. Total plaque volume (TPV) and T1 relaxivity were estimated using ImageJ (V. 1.44p, NIH, USA).2D and 3D en face analysis showed a strong and exponential increase of plaque burden over time, while plaque burden in regression group was less pronounced. Correspondent in vivo MRI measurements revealed a more linear increase of TPV and T1 relaxivity for regression group. A significant correlation was observed between 2D and 3D en face analysis (r = 0.79; p<0.001) as well as between 2D / 3D en face analysis and MRI (r = 0.79; p<0.001; r = 0.85; p<0.001) and delta R1 (r = 0.79; p<0.001; r = 0.69; p<0.01).ConclusionGDF-enhanced in vivo MRI is a powerful non-invasive imaging technique in mice allowing for reliable estimation of atherosclerotic plaque burden, monitoring of disease progression and regression in preclinical studies.

ABSTRACTEstimation of the longitudinal relaxation time T1 from the MP2RAGE pulse sequence is based on a monoexponential signal evolution model. However, magnetization transfer (MT) caused by the inversion pulse induces a fast relaxation component, which appears as a reduction in the efficiency of the inversion. This may explain the underestimation of T1 derived from MP2RAGE. To address this systematic bias, an “apparent” inversion efficiency (finv) was introduced, which comprises all mechanisms that affect the inversion in the monoexponential MP2RAGE signal model. The model was then extended by calibrating an empirical linear dependence of finv on R1 = 1/T1, resulting in increased accuracy of the estimated T1. The apparent inversion efficiency finv and the apparent T1* (yielding T1 by auxiliary B1+ mapping) were mapped at 7T in healthy adults using phase‐sensitive inversion recovery (IR) with four consecutive RAGE trains (PS‐MP4RAGE) in conjunction with adiabatic inversion using time‐resampled (TR)–FOCI and hyperbolic secant pulses. Upon validation by conventional IR‐EPI, PS‐MP4RAGE was used to calibrate the linear finv model for the human brain. These 3D T1 maps also served as a reference to assess the improvement of the MP2RAGE‐based T1 estimates. The apparent inversion efficiency finv was consistently smaller in white matter (WM) than in gray matter (~0.73 vs. ~0.84). The difference in WM T1 between MP2RAGE and the reference PS‐MP4RAGE technique was reduced by more than 200 ms when using the suggested finv model. MT effects after spin inversion in MP2RAGE can be accounted for by calibrating the apparent inversion efficiency finv without introducing additional parameters. The proposed empirical model retains the B1+ compensation inherent to MP2RAGE and facilitates accurate T1 quantification in brain tissue.