Analysis and improvement of motion encoding in magnetic resonance elastography

Magnetic resonance elastography (MRE) utilizes phase contrast magnetic resonance imaging (MRI), which is phase locked to externally generated mechanical vibrations, to measure the three‐dimensional wave displacement field. At least four measurements with linear‐independent encoding directions are necessary to correct for spurious phase contributions if effects from imaging gradients are non‐negligible. In MRE, three encoding schemes have been used: unbalanced four‐ and six‐point and balanced four‐point (‘tetrahedral’) encoding. The first two sensitize to motion with orthogonal gradients, with the four‐point method acquiring a single reference scan without motion sensitization, whereas three additional scans with inverted gradients are used with six‐point encoding, leading to two‐fold higher displacement‐to‐noise ratio (DNR) and 50% longer scan duration. Balanced four‐point (tetrahedral) encoding encodes along the four diagonals of a cube, with one direction serving as a reference for the other three encoding directions, similar to four‐point encoding. The objective of this work is to introduce a theoretical framework to compare different motion sensitization strategies with respect to their motion encoding efficiency in two fundamental encoding limits, the gradient strength limit and the dynamic range limit, which are both placed in relation to conventional gradient recalled echo (GRE)‐ and spin echo (SE)‐based MRE sequences. We apply the framework to the three aforementioned schemes and show that the motion encoding efficiency of unbalanced four‐ and six‐point encoding schemes in the gradient‐limited regime can be increased by a factor of 1.5 when using all physical gradient channels concurrently. Furthermore, it is demonstrated that reversing the direction of the reference in balanced four‐point (tetrahedral) encoding results in the Hadamard encoding scheme, which leads to increased DNR by compared with balanced four‐point encoding and 2.8 compared with unbalanced four‐point encoding. As an example, we show that optimal encoding can be utilized to reduce the acquisition time of standard liver MRE in vivo from four to two breath holds.     

Rapid acquisition of multifrequency,multislice and multidirectional MR elastography data with a fractionally encoded gradient echo sequence

In MR elastography (MRE), periodic tissue motion is phase encoded using motion‐encoding gradients synchronized to an externally applied periodic mechanical excitation. Conventional methods result in extended scan time for quality phase images, thus limiting the broad application of MRE in the clinic. For practical scan times, researchers have been relying on one‐dimensional or two‐dimensional motion‐encoding, low‐phase sampling and a limited number of slices, and artifact‐prone, single‐shot, echo planar imaging (EPI) readout. Here, we introduce a rapid multislice pulse sequence capable of three‐dimensional motion encoding that is also suitable for simultaneously encoding motion with multiple frequency components. This sequence is based on a gradient‐recalled echo (GRE) sequence and exploits the principles of fractional encoding. This GRE MRE pulse sequence was validated as capable of acquiring full three‐dimensional motion encoding of isotropic voxels in a large volume within less than a minute. This sequence is suitable for monofrequency and multifrequency MRE experiments. In homogeneous paraffin phantoms, the eXpresso sequence yielded similar storage modulus values as those obtained with conventional methods, although with markedly reduced variances (7.11 ± 0.26 kPa for GRE MRE versus 7.16 ± 1.33 kPa for the conventional spin‐echo EPI sequence). The GRE MRE sequence obtained better phase‐to‐noise ratios than the equivalent spin‐echo EPI sequence (matched for identical acquisition time) in both paraffin phantoms and in vivo data in the liver (59.62 ± 11.89 versus 27.86 ± 3.81, 61.49 ± 14.16 versus 24.78 ± 2.48 and 58.23 ± 10.39 versus 23.48 ± 2.91 in the X, Y and Z components, respectively, in the case of liver experiments). Phase‐to‐noise ratios were similar between GRE MRE used in monofrequency or multifrequency experiments (75.39 ± 14.93 versus 86.13 ± 18.25 at 28 Hz, 71.52 ± 24.74 versus 86.96 ± 30.53 at 56 Hz and 95.60 ± 36.96 versus 61.35 ± 26.25 at 84Hz, respectively). Copyright © 2013 John Wiley & Sons, Ltd.     

梯度回波成象技术分析

当今磁共振快速成象技术大多是建立在梯度回波脉冲序更的基础上，这种序列采用小的射频（ＲＦ）偏转角、短重复时间（ＴＲ）。当ＴＲ小到自与自旋－自旋驰豫时间Ｔ２同数量组或更短时，序列上的细微差异将引起图象对比上的较大变化，从而产生了多种多样的梯度回波序列，它们在结构上大致相似，但通过一些细微的差异保留各自的特色。梯度回波的物理基础是磁共振波谱学中早期发展的稳态自由进动即ＳＳＦＰ技术，本文从这一基本理论出发     

Comparison of spoiled gradient echo and steady‐state free‐precession imaging for native myocardial T1 mapping using the slice‐interleaved T1 mapping (STONE) sequence

Cardiac T₁ mapping allows non‐invasive imaging of interstitial diffuse fibrosis. Myocardial T₁ is commonly calculated by voxel‐wise fitting of the images acquired using balanced steady‐state free precession (SSFP) after an inversion pulse. However, SSFP imaging is sensitive to B₁ and B₀ imperfection, which may result in additional artifacts. A gradient echo (GRE) imaging sequence has been used for myocardial T₁ mapping; however, its use has been limited to higher magnetic field to compensate for the lower signal‐to‐noise ratio (SNR) of GRE versus SSFP imaging. A slice‐interleaved T₁ mapping (STONE) sequence with SSFP readout (STONE–SSFP) has been recently proposed for native myocardial T₁ mapping, which allows longer recovery of magnetization (>8 R–R) after each inversion pulse. In this study, we hypothesize that a longer recovery allows higher SNR and enables native myocardial T₁ mapping using STONE with GRE imaging readout (STONE–GRE) at 1.5T. Numerical simulations and phantom and in vivo imaging were performed to compare the performance of STONE–GRE and STONE–SSFP for native myocardial T₁ mapping at 1.5T. In numerical simulations, STONE–SSFP shows sensitivity to both T₂ and off resonance. Despite the insensitivity of GRE imaging to T₂, STONE–GRE remains sensitive to T₂ due to the dependence of the inversion pulse performance on T₂. In the phantom study, STONE–GRE had inferior accuracy and precision and similar repeatability as compared with STONE–SSFP. In in vivo studies, STONE–GRE and STONE–SSFP had similar myocardial native T₁ times, precisions, repeatabilities and subjective T₁ map qualities. Despite the lower SNR of the GRE imaging readout compared with SSFP, STONE–GRE provides similar native myocardial T₁ measurements, precision, repeatability, and subjective image quality when compared with STONE–SSFP at 1.5T.     

Gradient moment nulling for steady-state free precession MR imaging of cerebrospinal fluid

Steady-state free precession (SSFP) pulse sequences can produce magnetic resonance (MR) images rapidly, in which cerebrospinal fluid (CSF) is several times more intense than the other tissues. However, motion in the presence of magnetic field gradients reduces the intensity of CSF drastically, unless the time integral of the gradient waveform between each radio-frequency (rf) pulse vanishes. The consequences of motion on SSFP are explored here in detail theoretically and experimentally. The principle of gradient moment nulling is applied with the objective of giving CSF in SSFP images uniformly high intensity everywhere, in spite of motion. Theoretical analysis of the phase of the transverse magnetization from a group of isochromats, with a trajectory described by a Taylor series, reveals how motion along each direction disrupts SSFP and also causes ghost artifacts. Images of CSF in the cervical spine are found to have less extensive flow voids and weaker ghosts from pulsation if the first moment calculated from the rf pulse to the center of the gradient echo vanishes for both the frequency encoding and slice selection gradient waveforms. However, first-order moment nulling of the phase encoding gradient waveform is unnecessary for SSFP imaging of CSF.     

Magnetic Resonance Elastography of kidneys: SE‐EPI MRE reproducibility and its comparison to GRE MRE

The purpose of this study is 1) to demonstrate reproducibility of spin echo‐echo planar imaging (SE‐EPI) magnetic resonance elastography (MRE) to estimate kidney stiffness; and 2) to compare SE‐EPI MRE and gradient recalled echo (GRE) MRE‐derived stiffness estimations in various anatomical regions of the kidney. Kidney MRE was performed on 33 healthy subjects (8 for SE‐EPI MRE reproducibility and 25 for comparison with GRE MRE; age range: 22–66 years) in a 3 T MRI scanner. To demonstrate SE‐EPI MRE reproducibility, subjects were scanned for the first scan and then asked to leave the scan room and repositioned again for the second (repeat) scan. Similar set‐up was used for GRE MRE as well. The displacement data was then processed to obtain overall stiffness estimates of the kidney. Concordance correlation analyses were performed to determine SE‐EPI MRE reproducibility and agreement between GRE MRE and SE‐EPI MRE derived stiffness. A high concordance correlation (ρ_c = 0.95; p‐value<0.0001) was obtained for SE‐EPI MRE reproducibility. Good concordance correlation was observed (ρ_c = 0.84; p < 0.0001 for both kidneys, ρ_c = 0.91; p < 0.0001 for right kidney and ρ_c = 0.78; p < 0.0001 for left kidney) between GRE MRE and SE‐EPI MRE derived stiffness measurements. Paired t‐test results showed that stiffness value of medulla was significantly (p < 0.0001) greater than cortex using SE‐EPI MRE as well as GRE MRE. SE‐EPI MRE was reproducible and good agreement was observed in MRE‐derived stiffness measurements obtained using SE‐EPI and GRE sequences. Therefore, SE‐EPI can be used for kidney MRE applications.     

Ristretto MRE: A generalized multi‐shot GRE‐MRE sequence

In order to acquire consistent k‐space data in MR elastography, a fixed temporal relationship between the MRI sequence and the underlying period of the wave needs to be ensured. To this end, conventional GRE‐MRE enforces synchronization through repeated triggering of the transducer and forcing the sequence repetition time to be equal to an integer multiple of the wave period. For wave frequencies below 100 Hz, however, this leads to prolonged acquisition times, as the repetition time scales inversely with frequency. A previously developed multi‐shot approach (eXpresso MRE) to multi‐slice GRE‐MRE tackles this issue by acquiring an integer number of slices per wave period, which allows acquisition to be accelerated in typical scenarios by a factor of two or three. In this work, it is demonstrated that the constraints imposed by the eXpresso scheme are overly restrictive. We propose a generalization of the sequence in three steps by incorporating sequence delays into imaging shots and allowing for interleaved wave‐phase acquisition. The Ristretto scheme is compared in terms of imaging shot and total scan duration relative to eXpresso and conventional GRE‐MRE and is validated in three different phantom studies. First, the agreement of measured displacement fields in different stages of the sequence generalization is shown. Second, performance is compared for 25, 36, 40, and 60 Hz actuation frequencies. Third, the performance is assessed for the acquisition of different numbers of slices (13 to 17). In vivo feasibility is demonstrated in the liver and the breast. Here, Ristretto is compared with an optimized eXpresso sequence, leading to scan accelerations of 15% and 5%, respectively, without compromising displacement field and stiffness estimates in general. The Ristretto concept allows us to choose imaging shot durations on a fine grid independent of the number of slices and the wave frequency, permitting 2‐ to 4.5‐fold acceleration of conventional GRE‐MRE acquisitions.     

Calculation of shear stiffness in noise dominated magnetic resonance elastography data based on principal frequency estimation

Magnetic resonance elastography (MRE) is a non-invasive phase-contrast-based method for quantifying the shear stiffness of biological tissues. Synchronous application of a shear wave source and motion encoding gradient waveforms within the MRE pulse sequence enable visualization of the propagating shear wave throughout the medium under investigation. Encoded shear wave-induced displacements are then processed to calculate the local shear stiffness of each voxel. An important consideration in local shear stiffness estimates is that the algorithms employed typically calculate shear stiffness using relatively high signal-to-noise ratio (SNR) MRE images and have difficulties at an extremely low SNR. A new method of estimating shear stiffness based on the principal spatial frequency of the shear wave displacement map is presented. Finite element simulations were performed to assess the relative insensitivity of this approach to decreases in SNR. Additionally, ex vivo experiments were conducted on normal rat lungs to assess the robustness of this approach in low SNR biological tissue. Simulation and experimental results indicate that calculation of shear stiffness by the principal frequency method is less sensitive to extremely low SNR than previously reported MRE inversion methods but at the expense of loss of spatial information within the region of interest from which the principal frequency estimate is derived.     

A low‐cost,mechanically simple apparatus for measuring eddy current‐induced magnetic fields in MRI

The fidelity of gradient waveforms in MRI pulse sequences is essential to the acquisition of images and spectra with minimal distortion artefacts. Gradient waveforms can become nonideal when eddy currents are created in nearby conducting structures; however, the resultant magnetic fields can be characterised and compensated for by measuring the spatial and temporal field response following a gradient impulse. This can be accomplished using a grid of radiofrequency (RF) coils. The RF coils must adhere to strict performance requirements: they must achieve a high sensitivity and signal‐to‐noise ratio (SNR), have minimal susceptibility field gradients between the sample and surrounding material interfaces and be highly decoupled from each other. In this study, an apparatus is presented that accomplishes these tasks with a low‐cost, mechanically simple solution. The coil system consists of six transmit/receive RF coils immersed in a high‐molarity saline solution. The sensitivity and SNR following an excitation pulse are sufficiently high to allow accurate phase measurements during free‐induction decays; the intrinsic susceptibility matching of the materials, because of the unique design of the coil system, results in sufficiently narrow spectral line widths (mean of 19 Hz), and adjacent RF coils are highly decoupled (mean S₁₂ of ?47 dB). The temporal and spatial distributions of eddy currents following a gradient pulse are measured to validate the efficacy of the design, and the resultant amplitudes and time constants required for zeroth‐ and first‐order compensation are provided. Copyright © 2013 John Wiley & Sons, Ltd.     

Motion‐compensated b‐tensor encoding for in vivo cardiac diffusion‐weighted imaging

Motion is a major confound in diffusion‐weighted imaging (DWI) in the body, and it is a common cause of image artefacts. The effects are particularly severe in cardiac applications, due to the nonrigid cyclical deformation of the myocardium. Spin echo‐based DWI commonly employs gradient moment‐nulling techniques to desensitise the acquisition to velocity and acceleration, ie, nulling gradient moments up to the 2nd order (M2‐nulled). However, current M2‐nulled DWI scans are limited to encode diffusion along a single direction at a time. We propose a method for designing b‐tensors of arbitrary shapes, including planar, spherical, prolate and oblate tensors, while nulling gradient moments up to the 2nd order and beyond. The design strategy comprises initialising the diffusion encoding gradients in two encoding blocks about the refocusing pulse, followed by appropriate scaling and rotation, which further enables nulling undesired effects of concomitant gradients. Proof‐of‐concept assessment of in vivo mean diffusivity (MD) was performed using linear and spherical tensor encoding (LTE and STE, respectively) in the hearts of five healthy volunteers. The results of the M2‐nulled STE showed that (a) the sequence was robust to cardiac motion, and (b) MD was higher than that acquired using standard M2‐nulled LTE, where diffusion‐weighting was applied in three orthogonal directions, which may be attributed to the presence of restricted diffusion and microscopic diffusion anisotropy. Provided adequate signal‐to‐noise ratio, STE could significantly shorten estimation of MD compared with the conventional LTE approach. Importantly, our theoretical analysis and the proposed gradient waveform design may be useful in microstructure imaging beyond diffusion tensor imaging where the effects of motion must be suppressed.     

Continuous prospectively navigated multi‐echo GRE for improved BOLD imaging of the kidneys

The objective of this study is to develop improved methods for renal blood oxygenation level dependent (BOLD) imaging. T2* mapping of the kidneys, or renal BOLD imaging, may depict renal oxygen levels and may be valuable as a noninvasive means of following the progression of renal disease. Current renal BOLD data is limited by imaging in a single breath hold, which results in low resolution and low signal‐to‐noise ratio (SNR). We compare a new free‐breathing renal BOLD method with conventional breath‐hold BOLD (BH‐BOLD). A multi‐echo GRE sequence with continuous prospective respiratory navigation and real‐time feedback was developed that allows high resolution and high SNR renal BOLD imaging with constant sequence repetition time (TR) during free‐breathing BOLD (FB‐BOLD). The sequence was evaluated in 10 normal volunteers and compared with conventional BH‐BOLD. Scan time for the FB‐BOLD sequence was approximately three minutes, compared with 15 seconds for the BH‐BOLD sequence. SNR of source images and residual error of T2* fitting were compared between the two methods. The FB‐BOLD sequence produced motion‐free T2* maps of the kidneys with SNR 1.9 times higher than BH‐BOLD images. Residual error of T2* fitting was consistently lower in the right kidney with FB‐BOLD (30% less than BH‐BOLD) but higher in the left kidney (80% more than BH‐BOLD), likely related to placement of the navigator on the right hemidiaphragm. A free‐breathing prospectively navigated renal BOLD sequence allows flexible tradeoff between scan time, resolution, and SNR.     

Snapshot‐CEST: Optimizing spiral‐centric‐reordered gradient echo acquisition for fast and robust 3D CEST MRI at 9.4 T

Gradient echo (GRE)‐based acquisition provides a robust readout method for chemical exchange saturation transfer (CEST) at ultrahigh field (UHF). To develop a snapshot‐CEST approach, the transient GRE signal and point spread function were investigated in detail, leading to optimized measurement parameters and reordering schemes for fast and robust volumetric CEST imaging. Simulation of the transient GRE signal was used to determine the optimal sequence parameters and the maximum feasible number of k‐space lines. Point spread function analysis provided an insight into the induced k‐space filtering and the performance of different rectangular reordering schemes in terms of blurring, signal‐to‐noise ratio (SNR) and relaxation dependence. Simulation results were confirmed in magnetic resonance imaging (MRI) measurements of healthy subjects. Minimal repetition time (TR) is beneficial for snapshot‐GRE readout. At 9.4 T, for TR = 4 ms and optimal flip angle close to the Ernst angle, a maximum of 562 k‐space lines can be acquired after a single presaturation, providing decent SNR with high image quality. For spiral‐centric reordered k‐space acquisition, the image quality can be further improved using a rectangular spiral reordering scheme adjusted to the field of view. Application of the derived snapshot‐CEST sequence for fast imaging acquisition in the human brain at 9.4 T shows excellent image quality in amide and nuclear Overhauser enhancement (NOE), and enables guanidyl CEST detection. The proposed snapshot‐CEST establishes a fast and robust volumetric CEST approach ready for the imaging of known and novel exchange‐weighted contrasts at UHF.     

An octahedral shear strain-based measure of SNR for 3D MR elastography

A signal-to-noise ratio (SNR) measure based on the octahedral shear strain (the maximum shear strain in any plane for a 3D state of strain) is presented for magnetic resonance elastography (MRE), where motion-based SNR measures are commonly used. The shear strain, γ, is directly related to the shear modulus, μ, through the definition of shear stress, τ = μγ. Therefore, noise in the strain is the important factor in determining the quality of motion data, rather than the noise in the motion. Motion and strain SNR measures were found to be correlated for MRE of gelatin phantoms and the human breast. Analysis of the stiffness distributions of phantoms reconstructed from the measured motion data revealed a threshold for both strain and motion SNR where MRE stiffness estimates match independent mechanical testing. MRE of the feline brain showed significantly less correlation between the two SNR measures. The strain SNR measure had a threshold above which the reconstructed stiffness values were consistent between cases, whereas the motion SNR measure did not provide a useful threshold, primarily due to rigid body motion effects.     

High‐resolution MRI encoding using radiofrequency phase gradients

Although MRI offers highly diagnostic medical imagery, patient access to this modality worldwide is very limited when compared with X‐ray or ultrasound. One reason for this is the expense and complexity of the equipment used to generate the switched magnetic fields necessary for MRI encoding. These field gradients are also responsible for intense acoustic noise and have the potential to induce nerve stimulation. We present results with a new MRI encoding principle which operates entirely without the use of conventional B₀ field gradients. This new approach – ‘Transmit Array Spatial Encoding’ (TRASE) – uses only the resonant radiofrequency (RF) field to produce Fourier spatial encoding equivalent to conventional MRI. k‐space traversal (image encoding) is achieved by spin refocusing with phase gradient transmit fields in spin echo trains. A transmit coil array, driven by just a single transmitter channel, was constructed to produce four phase gradient fields, which allows the encoding of two orthogonal spatial axes. High‐resolution two‐dimensional‐encoded in vivo MR images of hand and wrist were obtained at 0.2 T. TRASE exploits RF field phase gradients, and offers the possibility of very low‐cost diagnostics and novel experiments exploiting unique capabilities, such as imaging without disturbance of the main B₀ magnetic field. Lower field imaging (<1 T) and micro‐imaging are favorable application domains as, in both cases, it is technically easier to achieve the short RF pulses desirable for long echo trains, and also to limit RF power deposition. As TRASE is simply an alternative mechanism (and technology) of moving through k space, there are many close analogies between it and conventional B₀‐encoded techniques. TRASE is compatible with both B₀ gradient encoding and parallel imaging, and so hybrid sequences containing all three spatial encoding approaches are possible. Copyright © 2013 John Wiley & Sons, Ltd.     

Resolution limit of cylinder diameter estimation by diffusion MRI: The impact of gradient waveform and orientation dispersion

Diffusion MRI has been proposed as a non‐invasive technique for axonal diameter mapping. However, accurate estimation of small diameters requires strong gradients, which is a challenge for the transition of the technique from preclinical to clinical MRI scanners, since these have weaker gradients. In this work, we develop a framework to estimate the lower bound for accurate diameter estimation, which we refer to as the resolution limit. We analyse only the contribution from the intra‐axonal space and assume that axons can be represented by impermeable cylinders. To address the growing interest in using techniques for diffusion encoding that go beyond the conventional single diffusion encoding (SDE) sequence, we present a generalised analysis capable of predicting the resolution limit regardless of the gradient waveform. Using this framework, waveforms were optimised to minimise the resolution limit. The results show that, for parallel cylinders, the SDE experiment is optimal in terms of yielding the lowest possible resolution limit. In the presence of orientation dispersion, diffusion encoding sequences with square‐wave oscillating gradients were optimal. The resolution limit for standard clinical MRI scanners (maximum gradient strength 60–80 mT/m) was found to be between 4 and 8 μm, depending on the noise levels and the level of orientation dispersion. For scanners with a maximum gradient strength of 300 mT/m, the limit was reduced to between 2 and 5 μm.     

Measurement of distinctive features of cortical spreading depolarizations with different MRI contrasts

Growing clinical evidence suggests critical involvement of spreading depolarizations (SDs) in the pathophysiology of neurological disorders such as migraine and stroke. MRI provides powerful tools to detect and assess co‐occurring cerebral hemodynamic and cellular changes during SDs. This study reports the feasibility and advantages of two MRI scans, based on balanced steady‐state free precession (b‐SSFP) and diffusion‐weighted multi‐spin‐echo (DT2), heretofore unexplored for monitoring SDs. These were compared with gradient‐echo MRI. SDs were induced by KCl application in rat brain. Known for high SNR, the T₂‐ and T₁‐based b‐SSFP contrast was hypothesized to provide higher spatiotemporal specificity than ‐based gradient‐echo scanning. DT2 scanning was designed to provide simultaneous T₂ and apparent diffusion coefficient (ADC) measurements, thus enabling combined quantitative assessment of hemodynamic and cellular changes during SDs. Procedures were developed to automate identification of SD‐induced responses in all the scans. These responses were analyzed to determine detection sensitivity and temporal characteristics of signals from each scanning method. Cluster analysis was performed to elucidate unique temporal patterns for each contrast. All scans allowed detection of SD‐induced responses. b‐SSFP scans showed significantly larger relative intensity changes, narrower peak widths and greater spatial specificity compared with gradient‐echo MRI. SD‐induced effects on ADC, calculated from DT2 scans, showed the most pronounced signal changes, displaying about 20% decrease, as against 10–15% signal increases observed with b‐SSFP and gradient‐echo scanning. Cluster analysis revealed additional temporal sub‐patterns, such as an initial dip on gradient‐echo scans and temporally shifted T₂ and proton density changes in DT2 data. To summarize, b‐SSFP and DT2 scanning provide distinct information on SDs compared with gradient‐echo MRI. DT2 scanning, with its potential to simultaneously provide cellular and hemodynamic information, can offer unique information on the inter‐relationship between these processes in pathologic brain, which may improve monitoring of spreading depolarizations in (pre)clinical settings. Copyright © 2015 John Wiley & Sons, Ltd.     

Optimized motion estimation for MRE data with reduced motion encodes

Motion estimation is an essential step common to all magnetic resonance elastography (MRE) methods. For dynamic techniques, the motion is obtained from a sinusoidal fit of the image phase at multiple, uniformly spaced relative phase offsets, phi, between the motion and the motion encoding gradients (MEGs). Generally, eight values of phi sampled at the Nyquist interval pi/4 over [0, 2pi). We introduce a method, termed reduced motion encoding (RME), that reduces the number of phi required, thereby reducing the imaging time for an MRE acquisition. A frequency-domain algorithm was implemented using the discrete Fourier transform (DFT) to derive the general least-squares solution for the motion amplitude and phase given an arbitrary number of phi. A closed form representation of the condition number of the transformation matrix which is used for estimating motion was introduced to determine the sensitivity to noise for different sampling patterns of phi. Simulation results confirmed the minimum error sampling patterns suggested from the condition number maps. The minimum noise in the motion estimate is obtained when the sampled phi are essentially evenly distributed over the range [0, pi) with an interval pi/n, where n is the number of phi sampled, or alternatively with an interval 2pi/n over the range [0, 2pi) which represents the Nyquist interval. Simulations also show that the noise level decreases as n increases as expected. The decrease in noise is the largest when n is small and it becomes less significant as n increases. The algorithm also makes it possible to estimate the motion from only two values of phi, which cannot be accomplished with traditional methods because sampling at the Nyquist interval is indeterminate. Finally, noise levels in motion estimated from phantom studies and in vivo results taken with different n agreed with that predicted by simulation and condition number calculations.     

Magnetic resonance elastography using 3D gradient echo measurements of steady-state motion

Magnetic resonance elastography (MRE) is an important new method used to measure the elasticity or stiffness of tissues in vivo. While there are many possible applications of MRE, breast cancer detection and classification is currently the most common. Several groups have been developing methods based on MR and ultrasound (US). MR or US is used to estimate the displacements produced by either quasi-static compression or dynamic vibration of the tissue. An important advantage of MRE is the possibility of measuring displacements accurately in all three directions. The central problem in most versions of MRE is recovering elasticity information from the measured displacements. In previous work, we have presented simulation results in two and three dimensions that were promising. In this article, accurate reconstructions of elasticity images from 3D, steady-state experimental data are reported. These results are significant because they demonstrate that the process is truly three-dimensional even for relatively simple geometries and phantoms. Further, they show that the integration of displacement data acquisition and elastic property reconstruction has been successfully achieved in the experimental setting. This process involves acquiring volumetric MR phase images with prescribed phase offsets between the induced mechanical motion and the motion-encoding gradients, converting this information into a corresponding 3D displacement field and estimating the concomitant 3D elastic property distribution through model-based image reconstruction. Fully 3D displacement fields and resulting elasticity images are presented for single and multiple inclusion gel phantoms.     

Overhauser‐enhanced magnetic resonance elastography

Magnetic resonance elastography (MRE) is a powerful technique to assess the mechanical properties of living tissue. However, it suffers from reduced sensitivity in regions with short T₂ and T₂* such as in tissue with high concentrations of paramagnetic iron, or in regions surrounding implanted devices. In this work, we exploit the longer T₂* attainable at ultra‐low magnetic fields in combination with Overhauser dynamic nuclear polarization (DNP) to enable rapid MRE at 0.0065 T. A 3D balanced steady‐state free precession based MRE sequence with undersampling and fractional encoding was implemented on a 0.0065 T MRI scanner. A custom‐built RF coil for DNP and a programmable vibration system for elastography were developed. Displacement fields and stiffness maps were reconstructed from data recorded in a polyvinyl alcohol gel phantom loaded with stable nitroxide radicals. A DNP enhancement of 25 was achieved during the MRE sequence, allowing the acquisition of 3D Overhauser‐enhanced MRE (OMRE) images with (1.5 × 2.7 × 9) mm³ resolution over eight temporal steps and 11 slices in 6 minutes. In conclusion, OMRE at ultra‐low magnetic field can be used to detect mechanical waves over short acquisition times. This new modality shows promise to broaden the scope of conventional MRE applications, and may extend the utility of low‐cost, portable MRI systems to detect elasticity changes in patients with implanted devices or iron overload. Copyright © 2016 John Wiley & Sons, Ltd.     

Intrinsic diffusion sensitivity of the balanced steady‐state free precession (bSSFP) imaging sequence

The purpose of this work was to analyze the intrinsic diffusion sensitivity of the balanced steady‐state free precession (bSSFP) imaging sequence, meaning the observation of diffusion‐induced attenuation of the bSSFP steady‐state signal due to the imaging gradients. Although these diffusion effects are usually neglected for most clinical gradient systems, such strong gradient systems are employed for high resolution imaging of small animals or MR Microscopy. The impact on the bSSFP signal of the imaging gradients characterized by their b‐values was analyzed with simulations and experiments at a 7T animal scanner using a gradient system with maximum gradient amplitude of approx. 700 mT/m. It was found that the readout gradients have a stronger impact on the attenuation than the phase encoding gradients. Also, as the PE gradients are varying with each repetition interval, the diffusion effects induce strong modulations of the bSSFP signal over the sequence repetition cycles depending on the phase encoding gradient table. It is shown that a signal gain can be obtained through a change of flip angle as a new optimal flip angle maximizing the signal can be defined. The dependency of the diffusion effects on relaxation times and b‐values were explored with simulations. The attenuation increases with T₂. In conclusion, diffusion attenuation of the bSSFP signal becomes significant for high resolution imaging voxel size (roughly < 100 μm) of long T₂ substances. Copyright © 2015 John Wiley & Sons, Ltd.