The possible influence of third-order shim coils on gradient-magnet interactions: an inter-field and inter-site study.

OBJECTIVE: To assess the possible influence of third-order shim coils on the behavior of the gradient field and in gradient-magnet interactions at 7 T and above. MATERIALS AND METHODS: Gradient impulse response function measurements were performed at 5 sites spanning field strengths from 7 to 11.7 T, all of them sharing the same exact whole-body gradient coil design. Mechanical fixation and boundary conditions of the gradient coil were altered in several ways at one site to study the impact of mechanical coupling with the magnet on the field perturbations. Vibrations, power deposition in the He bath, and field dynamics were characterized at 11.7 T with the third-order shim coils connected and disconnected inside the Faraday cage. RESULTS: For the same whole-body gradient coil design, all measurements differed greatly based on the third-order shim coil configuration (connected or not). Vibrations and gradient transfer function peaks could be affected by a factor of 2 or more, depending on the resonances. Disconnecting the third-order shim coils at 11.7 T also suppressed almost completely power deposition peaks at some frequencies. DISCUSSION: Third-order shim coil configurations can have major impact in gradient-magnet interactions with consequences on potential hardware damage, magnet heating, and image quality going beyond EPI acquisitions.

From Black Holes Entropy to Consciousness: The Dimensions of the Brain Connectome.

It has been shown that the theory of relativity can be applied physically to the functioning brain, so that the brain connectome should be considered as a four-dimensional spacetime entity curved by brain activity, just as gravity curves the four-dimensional spacetime of the physical world. Following the most recent developments in modern theoretical physics (black hole entropy, holographic principle, AdS/CFT duality), we conjecture that consciousness can naturally emerge from this four-dimensional brain connectome when a fifth dimension is considered, in the same way that gravity emerges from a 'flat' four-dimensional quantum world, without gravitation, present at the boundaries of a five-dimensional spacetime. This vision makes it possible to envisage quantitative signatures of consciousness based on the entropy of the connectome and the curvature of spacetime estimated from data obtained by fMRI in the resting state (nodal activity and functional connectivity) and constrained by the anatomical connectivity derived from diffusion tensor imaging.

The diffusion MRI signature index is highly correlated with immunohistochemical status and molecular subtype of invasive breast carcinoma.

OBJECTIVES: To evaluate the relationship of the signature index (S-index), an advanced diffusion MRI marker, and the immunohistochemical (IHC) status, proliferation rate, and molecular subtype of invasive breast cancers. METHODS: A retrospective study of patients with invasive carcinoma was conducted between 2017 and 2021. All patients underwent dynamic contrast-enhanced MRI and DWI using a 3-T system. For DWI, three b values (0, 200, and 1500 s/mm2) were used to derive the S-index. Three-dimensional ROIs were manually placed over the whole tumor on DWI. Mean and 85th percentile S-index values were compared to the IHC status, proliferation rate, and molecular subtypes of lesions. RESULTS: The study included 153 patients (mean age, 60 ± 13 years) with 160 invasive carcinomas. S-index values were significantly higher in estrogen receptor-positive (mean, p = .005; 85th percentile, p < .001) and progesterone receptor-positive (mean, p = .003; 85th percentile, p < .001) tumors, and significantly lower in human epidermal growth factor receptor 2 (HER2) - positive tumors (mean, p = .023; 85th percentile, p < .001). Mean and 85th percentile S-index values were significantly different among breast cancer subtypes (mean, p = .015; 85th percentile, p = .002), and the AUC of these values for the prediction of IHC status were 0.64 and 0.66 for HER2, and 0.70 and 0.74 for hormone receptors, respectively. CONCLUSIONS: The DWI S-index showed significantly higher values in invasive carcinomas with immunohistochemical status associated with good prognosis, suggesting its usefulness as a noninvasive imaging biomarker to estimate IHC status and orient treatment. KEY POINTS: • The signature index, an advanced diffusion MRI marker, showed good discrimination of immunohistochemical status in invasive breast carcinomas. • The signature index has the potential to differentiate noninvasively invasive breast carcinoma subtypes and appears as an imaging biomarker of prognostic factors and molecular phenotypes.

A survey by the European Society of Breast Imaging on the implementation of breast diffusion-weighted imaging in clinical practice.

OBJECTIVES: To perform a survey among all European Society of Breast Imaging (EUSOBI) radiologist members to gather representative data regarding the clinical use of breast DWI. METHODS: An online questionnaire was developed by two board-certified radiologists, reviewed by the EUSOBI board and committees, and finally distributed among EUSOBI active and associated (not based in Europe) radiologist members. The questionnaire included 20 questions pertaining to technical preferences (acquisition time, magnet strength, breast coils, number of b values), clinical indications, imaging evaluation, and reporting. Data were analyzed using descriptive statistics, the Chi-square test of independence, and Fisher's exact test. RESULTS: Of 1411 EUSOBI radiologist members, 275/1411 (19.5%) responded. Most (222/275, 81%) reported using DWI as part of their routine protocol. Common indications for DWI include lesion characterization (using an ADC threshold of 1.2-1.3 × 10-3 mm2/s) and prediction of response to chemotherapy. Members most commonly acquire two separate b values (114/217, 53%), with b value = 800 s/mm2 being the preferred value for appraisal among those acquiring more than two b values (71/171, 42%). Most did not use synthetic b values (169/217, 78%). While most mention hindered diffusion in the MRI report (161/213, 76%), only 142/217 (57%) report ADC values. CONCLUSION: The utilization of DWI in clinical practice among EUSOBI radiologists who responded to the survey is generally in line with international recommendations, with the main application being the differentiation of benign and malignant enhancing lesions, treatment response assessment, and prediction of response to chemotherapy. Report integration of qualitative and quantitative DWI data is not uniform. KEY POINTS: • Clinical performance of breast DWI is in good agreement with the current recommendations of the EUSOBI International Breast DWI working group. • Breast DWI applications in clinical practice include the differentiation of benign and malignant enhancing, treatment response assessment, and prediction of response to chemotherapy. • Report integration of DWI results is not uniform.

Diffusion-weighted MRI-based Virtual Elastography for the Assessment of Liver Fibrosis.

Background Diffusion-weighted (DW) MRI-based elastography has recently been proposed for noninvasive liver fibrosis staging but requires evaluation in a larger number of patients. Purpose To compare DW MRI and MR elastography for the assessment of liver fibrosis. Materials and Methods In this retrospective study, patients underwent MR elastography and DW MRI between November 2017 and April 2018. Shear modulus measured by MR elastography (µMRE) was obtained in each patient from regions of interest placed on liver stiffness maps by two independent readers. Shifted apparent diffusion coefficient (ADC) was calculated from DW MRI (b = 200 and 1500 sec/mm2) and converted to DW MRI-based virtual shear modulus (µDiff). MRI-based liver fibrosis stages were estimated from µMRE and µDiff values (F0-F4) and serum fibrosis markers were assessed. Statistical analyses included Bland-Altman plots, Bayesian prediction, and receiver operating characteristic analyses. Results Seventy-four patients (mean age, 68 years ± 9 [standard deviation]; 45 men) were evaluated. Interreader coefficient of reproducibility was 0.86 kPa for DW MRI and 1.2 kPa for MR elastography. Strong correlation between shifted ADC and µMRE was observed (r2 = 0.81; P < .001), showing high agreement between µMRE and µDiff values (mean difference, -0.02 kPa ± 0.88; P < .001). DW MRI-based fibrosis staging agreed with MR elastography-based staging in 55% of patients (41 of 74) and within one stage difference in 35% of patients (26 of 74). Binarization into insignificant (F0-F1) and significant fibrosis (F2-F4) showed agreement in 85% of patients (63 of 74; κ = 0.85). Compared with serum markers (area under the receiver operating characteristic curve [AUC], 0.50-0.69), µDiff showed better performance in discriminating fibrosis stages F0-F2 from F3-F4 (AUC, 0.79; 95% confidence interval: 0.69, 0.90), whereas serum markers showed slightly better results for F0-F1 versus F2-F4 differentiation (fibrosis stages were estimated by using MR elastography). Combining DW MRI with serum markers provided a trend toward highest discriminative performance (AUC, µDiff + aspartate aminotransferase-to-platelet radio index: F0-F1 vs F2-F4, 0.81 [95% confidence interval: 0.69, 0.93], P = .17; F0-F2 vs F3-F4, 0.83 [95% confidence interval: 0.74, 0.92], P = .07; and AUC µDiff + Fibrosis 4 score: F0-F1 vs F2-F4, 0.78 [95% confidence interval: 0.64, 0.92], P < .30; F0-F2 vs F3-F4, 0.81 [95% confidence interval: 0.71, 0.91], P = .08). Conclusion MR elastography and diffusion-weighted (DW) MRI-based estimation of liver fibrosis stage showed high agreement. DW MRI shows potential as an alternative to MR elastography for noninvasive fibrosis staging without the need for mechanical vibration setup. © RSNA, 2020 Online supplemental material is available for this article.

Simultaneous proton density, T1 , T2 , and flip-angle mapping of the brain at 7 T using multiparametric 3D SSFP imaging and parallel-transmission universal pulses.

PURPOSE: Performing simultaneous quantitative MRI at ultrahigh field is challenging, as B0 and B1+ heterogeneities as well as specific absorption rate increase. Too large deviations of flip angle from the target can induce biases and impair SNR in the quantification process. In this work, we use calibration-free parallel transmission, a dedicated pulse-sequence parameter optimization and signal fitting to recover 3D proton density, flip angle, T1 , and T2 maps over the whole brain, in a clinically suitable time. METHODS: Eleven optimized contrasts were acquired with an unbalanced SSFP sequence by varying flip-angle amplitude and RF phase-cycling increment, at a 1.0 × 1.0 × 3.0 mm3 resolution. Acquisition time was 16 minutes 36 seconds for the whole brain. Parallel transmission and universal pulses were used to mitigate B1+ heterogeneity, to improve the results' reliability over 6 healthy volunteers (3 females/3 males, age 22.6 ± 2.7 years old). Quantification of the physical parameters was performed by fitting the acquired contrasts to the simulated ones using the Bloch-Torrey equations with a realistic diffusion coefficient. RESULTS: Whole-brain 3D maps of effective flip angle, proton density, and relaxation times were estimated. Parallel transmission improved the robustness of the results at 7 T. Results were in accordance with literature and with measurements from standard methods. CONCLUSION: These preliminary results show robust proton density, flip angle, T1 , and T2 map retrieval. Other parameters, such as ADC, could be assessed. With further optimization in the acquisition, scan time could be reduced and spatial resolution increased to bring this multiparametric quantification method to clinical research routine at 7 T.

Water diffusion closely reveals neural activity status in rat brain loci affected by anesthesia.

Diffusion functional MRI (DfMRI) reveals neuronal activation even when neurovascular coupling is abolished, contrary to blood oxygenation level-dependent (BOLD) functional MRI (fMRI). Here, we show that the water apparent diffusion coefficient (ADC) derived from DfMRI increased in specific rat brain regions under anesthetic conditions, reflecting the decreased neuronal activity observed with local field potentials (LFPs), especially in regions involved in wakefulness. In contrast, BOLD signals showed nonspecific changes, reflecting systemic effects of the anesthesia on overall brain hemodynamics status. Electrical stimulation of the central medial thalamus nucleus (CM) exhibiting this anesthesia-induced ADC increase led the animals to transiently wake up. Infusion in the CM of furosemide, a specific neuronal swelling blocker, led the ADC to increase further locally, although LFP activity remained unchanged, and increased the current threshold awakening the animals under CM electrical stimulation. Oppositely, induction of cell swelling in the CM through infusion of a hypotonic solution (-80 milliosmole [mOsm] artificial cerebrospinal fluid [aCSF]) led to a local ADC decrease and a lower current threshold to wake up the animals. Strikingly, the local ADC changes produced by blocking or enhancing cell swelling in the CM were also mirrored remotely in areas functionally connected to the CM, such as the cingulate and somatosensory cortex. Together, those results strongly suggest that neuronal swelling is a significant mechanism underlying DfMRI.

Six DWI questions you always wanted to know but were afraid to ask: clinical relevance for breast diffusion MRI.

Diffusion MRI (often called diffusion-weighted imaging or DWI) has enjoyed a tremendous growth since its introduction in the mid-1980s, especially to investigate neurological disorders and in oncology. At a time when standardization and quality control appear as critical as ever to support widespread utilization, our aim was to address common fundamental questions that arise regarding results obtained with DWI. We focus on six questions taking breast DWI as an example, as breast DWI is increasingly used in clinical practice, but most of our conclusions would apply to DWI in general. We show especially that noise can act in a pernicious way specific to DWI. Ignoring such noise effects could lead to incorrect data interpretations or conclusions, of which authors and readers may be genuinely unaware. While addressing these six questions, we give practical examples of how noise effects can be understood, corrected when possible, or taken to our advantage. Key Points • Ignoring noise effects in DWI could lead to incorrect data interpretations or conclusions, of which authors and readers may be genuinely unaware. • In vivo apparent diffusion coefficient (ADC) decreases with b value, which must therefore be reported along with ADC. • Synthesized DWI boosts contrast at the expense of accurate diffusion/microstructure characterization.

Diffusion-weighted imaging of the breast-a consensus and mission statement from the EUSOBI International Breast Diffusion-Weighted Imaging working group.

The European Society of Breast Radiology (EUSOBI) established an International Breast DWI working group. The working group consists of clinical breast MRI experts, MRI physicists, and representatives from large vendors of MRI equipment, invited based upon proven expertise in breast MRI and/or in particular breast DWI, representing 25 sites from 16 countries. The aims of the working group are (a) to promote the use of breast DWI into clinical practice by issuing consensus statements and initiate collaborative research where appropriate; (b) to define necessary standards and provide practical guidance for clinical application of breast DWI; (c) to develop a standardized and translatable multisite multivendor quality assurance protocol, especially for multisite research studies; (d) to find consensus on optimal methods for image processing/analysis, visualization, and interpretation; and (e) to work collaboratively with system vendors to improve breast DWI sequences. First consensus recommendations, presented in this paper, include acquisition parameters for standard breast DWI sequences including specifications of b values, fat saturation, spatial resolution, and repetition and echo times. To describe lesions in an objective way, levels of diffusion restriction/hindrance in the breast have been defined based on the published literature on breast DWI. The use of a small ROI placed on the darkest part of the lesion on the ADC map, avoiding necrotic, noisy or non-enhancing lesion voxels is currently recommended. The working group emphasizes the need for standardization and quality assurance before ADC thresholds are applied. The working group encourages further research in advanced diffusion techniques and tailored DWI strategies for specific indications. Key Points • The working group considers breast DWI an essential part of a multiparametric breast MRI protocol and encourages its use. • Basic requirements for routine clinical application of breast DWI are provided, including recommendations on b values, fat saturation, spatial resolution, and other sequence parameters. • Diffusion levels in breast lesions are defined based on meta-analysis data and methods to obtain a reliable ADC value are detailed.

What can we see with IVIM MRI?

Intravoxel Incoherent Motion (IVIM) refers to translational movements which within a given voxel and during the measurement time present a distribution of speeds in orientation and/or amplitude. The IVIM concept has been used to estimate perfusion in tissues as blood flow in randomly oriented capillaries mimics a pseudo-diffusion process. IVIM-based perfusion MRI, which does not require contrast agents, has gained momentum recently, especially in the field oncology. In this introductory review the basic concepts, models, technical requirements and limitations inherent to IVIM-based perfusion MRI are outlined, as well as new, non-perfusion applications of IVIM MRI, such as virtual MR Elastography.

The functional database of the ARCHI project: Potential and perspectives.

More than two decades of functional magnetic resonance imaging (fMRI) of the human brain have succeeded to identify, with a growing level of precision, the neural basis of multiple cognitive skills within various domains (perception, sensorimotor processes, language, emotion and social cognition …). Progress has been made in the comprehension of the functional organization of localized brain areas. However, the long time required for fMRI acquisition limits the number of experimental conditions performed in a single individual. As a consequence, distinct brain localizations have mostly been studied in separate groups of participants, and their functional relationships at the individual level remain poorly understood. To address this issue, we report here preliminary results on a database of fMRI data acquired on 78 individuals who each performed a total of 29 experimental conditions, grouped in 4 cross-domains functional localizers. This protocol has been designed to efficiently isolate, in a single session, the brain activity associated with language, numerical representation, social perception and reasoning, premotor and visuomotor representations. Analyses are reported at the group and at the individual level, to establish the ability of our protocol to selectively capture distinct regions of interest in a very short time. Test-retest reliability was assessed in a subset of participants. The activity evoked by the different contrasts of the protocol is located in distinct brain networks that, individually, largely replicate previous findings and, taken together, cover a large proportion of the cortical surface. We provide detailed analyses of a subset of regions of relevance: the left frontal, left temporal and middle frontal cortices. These preliminary analyses highlight how combining such a large set of functional contrasts may contribute to establish a finer-grained brain atlas of cognitive functions, especially in regions of high functional overlap. Detailed structural images (structural connectivity, micro-structures, axonal diameter) acquired in the same individuals in the context of the ARCHI database provide a promising situation to explore functional/structural interdependence. Additionally, this protocol might also be used as a way to establish individual neurofunctional signatures in large cohorts.

Adding a Model-free Diffusion MRI Marker to BI-RADS Assessment Improves Specificity for Diagnosing Breast Lesions.

Background The apparent diffusion coefficient (ADC) is a commonly used quantitative diffusion-weighted (DW) imaging marker in breast lesion assessment; however, reported ADC values to distinguish malignant and benign lesions show wide variability. Purpose To investigate the diagnostic performance of a tissue signature index (S-index) as a model-free diffusion marker to differentiate malignant and benign breast lesions. Materials and Methods This was a single-institution retrospective study of patients who underwent breast MRI from April 2017 to September 2018. Dynamic contrast-enhanced (DCE) MRI and DW imaging were performed with a 3-T MRI system. For DW imaging, three b values (0, 200, and 1500 sec/mm2) were used for Breast Imaging Reporting and Data Systems (BI-RADS) scoring and to calculate the S-index and a shifted ADC. The diagnostic performances of S-index, shifted ADC, and BI-RADS scoring were evaluated by using receiver operating coefficient analysis. Results The study involved 99 women (mean age, 54 years ± 14 [standard deviation]) with 69 malignant and 38 benign lesions. The S-index was higher for malignant lesions (mean, 75.9 ± 17.4) than for benign lesions (mean, 31.6 ± 21.0; P < .001). Overall diagnostic performance was identical for S-index and shifted ADC (area under the receiver operating characteristic curve [AUC], 0.95; 95% confidence interval [CI]: 0.91, 0.99) and slightly higher than for BI-RADS (AUC, 0.91; 95% CI: 0.87, 0.96; P = .22). The AUC of S-index combined with BI-RADS reached 0.98 (95% CI: 0.96, 1.00), higher than for BI-RADS alone (P < .001), yielding high sensitivity (65 of 69 [94%]; 95% CI: 85%, 98%) and specificity (36 of 38 [95%]; 95% CI: 81%, 99%). Significant differences were identified with the S-index for progesterone receptor and human epidermal growth factor receptor type 2 status (P = .003 and P < .001, respectively). Conclusion The signature index has the potential to enable classification of breast lesion types with high accuracy, especially in combination with dynamic contrast-enhanced MRI and correlates with histologic prognostic factors in invasive breast cancer. © RSNA, 2019 Online supplemental material is available for this article.

Robust nonadiabatic T2 preparation using universal parallel-transmit kT -point pulses for 3D FLAIR imaging at 7 T.

PURPOSE: The fluid attenuated inversion recovery sequence is a pillar technique to detect brain lesions in MRI. At ultrahigh field, the lengthening of T1 often advocates a T2 -weighting preparation module to regain signal and contrast between tissues, which can be affected by transmit RF field inhomogeneity. In this note, we report an extension of a previous fluid attenuated inversion recovery study that now incorporates the T2 preparation with parallel transmission calibration-free universal pulses to mitigate the problem. METHODS: The preparation consisted of a 90°-τ-180° -τ-90° module to implement an effective inversion in the CSF and a saturation in the brain tissues. Care was taken for the pulses to have the desired phase relationship in every voxel by appropriate pulse design. The RF pulse design made use of the kT -point parametrization and was based on a database of 20 B1+ and ΔB0 maps previously acquired on different subjects at 7 T. Simulations and experiments on 5 volunteers, not contained in the database, were performed for validation. RESULTS: Simulations reported very good inversion efficiency for the preparation module with 8% variation, with respectively 4 and 6 times less power and specific absorption rate than for the adiabatic version. Experiments revealed fluid attenuated inversion recovery images free of B1+ artifacts. CONCLUSION: This work contributes further to the panel of 3D sequences validated and now available with universal pulses at 7 T. The drop in power and specific absorption rate demand compared with adiabatic pulses in the T2 preparation leads to more freedom for the design of the readout train.

Design of universal parallel-transmit refocusing kT -point pulses and application to 3D T2 -weighted imaging at 7T.

PURPOSE: T2 -weighted sequences are particularly sensitive to the radiofrequency (RF) field inhomogeneity problem at ultra-high-field because of the errors accumulated by the imperfections of the train of refocusing pulses. As parallel transmission (pTx) has proved particularly useful to counteract RF heterogeneities, universal pulses were recently demonstrated to save precious time and computational efforts by skipping B1 calibration and online RF pulse tailoring. Here, we report a universal RF pulse design for non-selective refocusing pulses to mitigate the RF inhomogeneity problem at 7T in turbo spin-echo sequences with variable flip angles. METHOD: Average Hamiltonian theory was used to synthetize a single non-selective refocusing pulse with pTx while optimizing its scaling properties in the presence of static field offsets. The design was performed under explicit power and specific absorption rate constraints on a database of 10 subjects using a 8Tx-32Rx commercial coil at 7T. To validate the proposed design, the RF pulses were tested in simulation and applied in vivo on 5 additional test subjects. RESULTS: The root-mean-square rotation angle error (RA-NRMSE) evaluation and experimental data demonstrated great improvement with the proposed universal pulses (RA-NRMSE ∼8%) compared to the standard circularly polarized mode of excitation (RA-NRMSE ∼26%). CONCLUSION: This work further completes the spectrum of 3D universal pulses to mitigate RF field inhomogeneity throughout all 3D MRI sequences without any pTx calibration. The approach returns a single pulse that can be scaled to match the desired flip angle train, thereby increasing the modularity of the proposed plug and play approach. Magn Reson Med 80:53-65, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Correction: Diffusion Magnetic Resonance Imaging: What Water Tells Us about Biological Tissues.

[This corrects the article DOI: 10.1371/journal.pbio.1002203.].

Diffusion Magnetic Resonance Imaging: What Water Tells Us about Biological Tissues.

Since its introduction in the mid-1980s, diffusion magnetic resonance imaging (MRI), which measures the random motion of water molecules in tissues, revealing their microarchitecture, has become a pillar of modern neuroimaging. Its main clinical domain has been the diagnosis of acute brain stroke and neurogical disorders, but it is also used in the body for the detection and management of cancer lesions. It can also produce stunning maps of white matter tracks in the brain, with the potential to aid in the understanding of some psychiatric disorders. However, in order to exploit fully the potential of this method, a deeper understanding of the mechanisms that govern the diffusion of water in tissues is needed.

The Brainomics/Localizer database.

The Brainomics/Localizer database exposes part of the data collected by the in-house Localizer project, which planned to acquire four types of data from volunteer research subjects: anatomical MRI scans, functional MRI data, behavioral and demographic data, and DNA sampling. Over the years, this local project has been collecting such data from hundreds of subjects. We had selected 94 of these subjects for their complete datasets, including all four types of data, as the basis for a prior publication; the Brainomics/Localizer database publishes the data associated with these 94 subjects. Since regulatory rules prevent us from making genetic data available for download, the database serves only anatomical MRI scans, functional MRI data, behavioral and demographic data. To publish this set of heterogeneous data, we use dedicated software based on the open-source CubicWeb semantic web framework. Through genericity in the data model and flexibility in the display of data (web pages, CSV, JSON, XML), CubicWeb helps us expose these complex datasets in original and efficient ways.

Diffusion and Intravoxel Incoherent Motion MR Imaging-based Virtual Elastography: A Hypothesis-generating Study in the Liver.

Purpose To investigate the potential of diffusion magnetic resonance (MR) imaging to provide quantitative estimates of tissue stiffness without using mechanical vibrations in patients with chronic liver diseases and to generate a new elasticity-driven intravoxel incoherent motion (IVIM) contrast. Materials and Methods This retrospective study, conducted from January to April 2016, was approved by an institutional review board that waived the requirement for informed consent. Fifteen subjects were included (13 men and two women; mean age ± standard deviation, 73 years ± 8). MR elastography and diffusion MR imaging were performed at 3 T. A search for an empirical relationship between MR elastographic shear modulus, µMRE, and a shifted apparent diffusion coefficient (sADC) was performed. The sADC was then inverted to estimate patient liver shear modulus directly from diffusion MR imaging signals. Results A significant correlation (r2 = 0.90, P = 1 · 10-7) was observed between µMRE and sADC calculated by using diffusion MR imaging signals acquired with b values of 200 (S200) and 1500 (S1500 ) sec/mm2 (sACD200-1500). On the basis of the relationship between the µMRE and sADC200-1500, a diffusion-based shear modulus, µdiff, could be estimated with the following equation: µdiff = (-9.8 ± 0.8) ln(S200/S1500) + (14.0 ± 0.9). IVIM virtual elastograms also could be generated to reveal new contrast features in lesions, depending on pseudovibration frequency and amplitude. Conclusion Diffusion MR imaging, through a calibration of sADC200-1500 with standard MR elastography, can be converted quantitatively into shear modulus without using mechanical vibrations to provide information on the degree of liver fibrosis; these virtual elastograms require only two b values to be acquired and processed. Propagating shear wave can also be emulated, leading to a new elasticity-driven IVIM contrast with ranges of virtual vibration frequencies and amplitudes not limited by MR elastography or MR imaging hardware capacities. © RSNA, 2017 Online supplemental material is available for this article.

Universal pulses: A new concept for calibration-free parallel transmission.

PURPOSE: A calibration-free parallel transmission method is investigated to mitigate the radiofrequency (RF) field inhomogeneity problem in brain imaging at 7 Tesla (T). THEORY AND METHODS: Six volunteers were scanned to build a representative database of RF and static field maps at 7T. Small-tip-angle and inversion pulses were designed with joint kT -points trajectory optimization to work robustly on all six subjects. The returned "universal" pulses were then inserted in an MPRAGE sequence implemented on six additional volunteers without further field measurements and pulse optimizations. Similar acquisitions were performed in the circularly polarized mode and with subject-based optimizations for comparison. Performance of the different approaches was evaluated by means of image analysis and computation of the flip angle normalized root mean square errors (NRMSE). RESULTS: For both the excitation and inversion, the universal pulses (NRMSE∼11%) outperformed the circularly polarized (NRMSE∼28%) and RF shim modes (NRMSE∼20%) across all volunteers and returned slightly worse results than for subject-based optimized pulses (NRMSE∼7%). CONCLUSION: RF pulses can be designed to robustly mitigate the RF field inhomogeneity problem over a population class. This appears as a first step toward another plug and play parallel transmission solution where the pulse design can be done offline and without measuring subject-specific field maps. Magn Reson Med 77:635-643, 2017. © 2016 International Society for Magnetic Resonance in Medicine.