Wavelet MRE: Imaging propagating broadband acoustic waves with wavelet-based motion-encoding gradients.

PURPOSE: To demonstrate a novel MR elastography (MRE) technique, termed here wavelet MRE. With this technique, broadband motion sensitivity is achievable. Moreover, the true tissue displacement can be reconstructed with a simple inverse transform. METHODS: A wavelet MRE sequence was developed with motion-encoding gradients based on Haar wavelets. From the phase images' displacement was estimated using an inverse transform. Simulations were performed using a frequency sweep and a transient as ground-truth motions. A PVC phantom was scanned using wavelet MRE and standard MRE with both transient (one and 10 cycles of 90-Hz motion) and steady-state dual-frequency motion (30 and 60 Hz) for comparison. The technique was tested in a human brain, and motion trajectories were estimated for each voxel. RESULTS: In simulation, the displacement information estimated from wavelet MRE closely matched the true motion. In the phantom test, the MRE phase data generated from the displacement information derived from wavelet MRE agreed well with standard MRE data. Testing of wavelet MRE to assess transient motion waveforms in the brain was successful, and the tissue motion observed was consistent with a previous study. CONCLUSION: The uniform and broadband frequency response of wavelet MRE makes it a promising method for imaging transient, multifrequency motion, or motion with unknown frequency content. One potential application is measuring the response of brain tissue undergoing low-amplitude, transient vibrations as a model for the study of traumatic brain injury.

Self-Gated Radial Free-Breathing Liver MR Elastography: Assessment of Technical Performance in Children at 3 T.

BACKGROUND: Conventional liver magnetic resonance elastography (MRE) requires breath-holding (BH) to avoid motion artifacts, which is challenging for children. While radial free-breathing (FB)-MRE is an alternative for quantifying liver stiffness (LS), previous methods had limitations of long scan times, acquiring two slices in 5 minutes, and not resolving motion during reconstruction. PURPOSE: To reduce FB-MRE scan time to 4 minutes for four slices and to investigate the impact of self-gated (SG) motion compensation on FB-MRE LS quantification in terms of agreement, intrasession repeatability, and technical quality compared to conventional BH-MRE. STUDY TYPE: Prospective. POPULATION: Twenty-six children without fibrosis (median age: 12.9 years, 15 females). FIELD STRENGTH/SEQUENCE: 3 T; Cartesian gradient-echo (GRE) BH-MRE, research application radial GRE FB-MRE. ASSESSMENT: Participants were scanned twice to measure repeatability, without moving the table or changing the participants' position. LS was measured in areas of the liver with numerical confidence ≥90%. Technical quality was examined using measurable liver area (%). STATISTICAL TESTS: Agreement of LS between BH-MRE and FB-MRE was evaluated using Bland-Altman analysis for SG acceptance rates of 40%, 60%, 80%, and 100%. LS repeatability was assessed using within-subject coefficient of variation (wCV). The differences in LS and measurable liver area were examined using Kruskal-Wallis and Wilcoxon signed-rank tests. P < 0.05 was considered significant. RESULTS: FB-MRE with 60% SG achieved the closest agreement with BH-MRE (mean difference 0.00 kPa). The LS ranged from 1.70 to 1.83 kPa with no significant differences between BH-MRE and FB-MRE with varying SG rates (P = 0.52). All tested methods produced repeatable LS with wCV from 4.4% to 6.5%. The median measurable liver area was smaller for FB-MRE (32%-45%) than that for BH-MRE (91%-93%) (P < 0.05). DATA CONCLUSION: FB-MRE with 60% SG can quantify LS with close agreement and comparable repeatability with respect to BH-MRE in children. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 1.

MR elastography outperforms shear wave elastography for the diagnosis of clinically significant portal hypertension.

OBJECTIVES: Portal hypertension (PH) is associated with complications such as ascites and esophageal varices and is typically diagnosed through invasive hepatic venous pressure gradient (HVPG) measurement, which is not widely available. In this study, we aim to assess the diagnostic performance of 2D/3D MR elastography (MRE) and shear wave elastography (SWE) measures of liver and spleen stiffness (LS and SS) and spleen volume, to noninvasively diagnose clinically significant portal hypertension (CSPH) using HVPG measurement as the reference. METHODS: In this prospective study, patients with liver disease underwent 2D/3D MRE and SWE of the liver and spleen, as well as HVPG measurement. The correlation between MRE/SWE measures of LS/SS and spleen volume with HVPG was assessed. ROC analysis was used to determine the utility of MRE, SWE, and spleen volume for diagnosing CSPH. RESULTS: Thirty-six patients (M/F 22/14, mean age 55 ± 14 years) were included. Of the evaluated parameters, 3D MRE SS had the strongest correlation with HVPG (r = 0.686, p < 0.001), followed by 2D MRE SS (r = 0.476, p = 0.004). 3D MRE SS displayed the best performance for diagnosis of CSPH (AUC = 0.911) followed by 2D MRE SS (AUC = 0.845) and 3D MRE LS (AUC = 0.804). SWE SS showed poor performance for diagnosis of CSPH (AUC = 0.583) while spleen volume was a fair predictor (AUC = 0.738). 3D MRE SS was significantly superior to SWE LS/SS (p ≤ 0.021) for the diagnosis of CSPH. CONCLUSION: SS measured with 3D MRE outperforms SWE for the diagnosis of CSPH. SS appears to be a useful biomarker for assessing PH severity. These results need further validation. KEY POINTS: • Spleen stiffness measured with 2D and 3D MR elastography correlates significantly with hepatic venous pressure gradient measurement. • Spleen stiffness measured with 3D MR elastography demonstrates excellent performance for the diagnosis of clinically significant portal hypertension (AUC 0.911). • Spleen stiffness measured with 3D MR elastography outperforms liver and spleen stiffness measured with shear wave elastography for diagnosis of clinically significant portal hypertension.

Free-breathing radial magnetic resonance elastography of the liver in children at 3 T: a pilot study.

BACKGROUND: Magnetic resonance (MR) elastography of the liver measures hepatic stiffness, which correlates with the histopathological staging of liver fibrosis. Conventional Cartesian gradient-echo (GRE) MR elastography requires breath-holding, which is challenging for children. Non-Cartesian radial free-breathing MR elastography is a potential solution to this problem. OBJECTIVE: To investigate radial free-breathing MR elastography for measuring hepatic stiffness in children. MATERIALS AND METHODS: In this prospective pilot study, 14 healthy children and 9 children with liver disease were scanned at 3 T using 2-D Cartesian GRE breath-hold MR elastography (22 s/slice) and 2-D radial GRE free-breathing MR elastography (163 s/slice). Each sequence was acquired twice. Agreement in the stiffness measurements was evaluated using Lin's concordance correlation coefficient (CCC) and within-subject mean difference. The repeatability was assessed using the within-subject coefficient of variation and intraclass correlation coefficient (ICC). RESULTS: Fourteen healthy children and seven children with liver disease completed the study. Median (±interquartile range) normalized measurable liver areas were 62.6% (±26.4%) and 44.1% (±39.6%) for scan 1, and 60.3% (±21.8%) and 43.9% (±44.2%) for scan 2, for Cartesian and radial techniques, respectively. Hepatic stiffness from the Cartesian and radial techniques had close agreement with CCC of 0.89 and 0.94, and mean difference of 0.03 kPa and -0.01 kPa, for scans 1 and 2. Cartesian and radial techniques achieved similar repeatability with within-subject coefficient of variation=1.9% and 3.4%, and ICC=0.93 and 0.92, respectively. CONCLUSION: In this pilot study, radial free-breathing MR elastography was repeatable and in agreement with Cartesian breath-hold MR elastography in children.

Efficient shear wave elastography using transient acoustic radiation force excitations and MR displacement encoding.

PURPOSE: To present a novel MR shear wave elastography (MR-SWE) method that efficiently measures the speed of propagating wave packets generated using acoustic radiation force (ARF) impulses. METHODS: ARF impulses from a focused ultrasound (FUS) transducer were applied sequentially to a preselected set of positions and motion encoded MRI was used to acquire volumetric images of the propagating shear wavefront emanating from each point. The wavefront position at multiple propagation times was encoded in the MR phase image using a train of motion encoding gradient lobes. Generating a transient propagating wavefront at multiple spatial positions and sampling each at multiple time-points allowed for shear wave speed maps to be efficiently created. MR-SWE was evaluated in tissue mimicking phantoms and ex vivo bovine liver tissue before and after ablation. RESULTS: MR-SWE maps, covering an in-plane area of ~5 × 5 cm, were acquired in 12 s for a single slice and 144 s for a volumetric scan. MR-SWE detected inclusions of differing stiffness in a phantom experiment. In bovine liver, mean shear wave speed significantly increased from 1.65 ± 0.18 m/s in normal to 2.52 ± 0.18 m/s in ablated region (n = 581 pixels; P-value < 0.001). CONCLUSION: MR-SWE is an elastography technique that enables precise targeting and excitation of the desired tissue of interest. MR-SWE may be particularly well suited for treatment planning and endpoint assessment of MR-guided FUS procedures because the same device used for therapy can be used as an excitation source for tissue stiffness quantification.

Magnetic resonance imaging-based measurement of internal deformation of vibrating vocal fold models.

A method is presented for tracking the internal deformation of self-oscillating vocal fold models using magnetic resonance imaging (MRI). Silicone models scaled to four times life-size to lower the flow-induced vibration frequency were embedded with fiducial markers in a coronal plane. Candidate marker materials were tested using static specimens, and two materials, cupric sulfate and glass, were chosen for testing in the vibrating vocal fold models. The vibrating models were imaged using a gated MRI protocol wherein MRI acquisition was triggered using the subglottal pressure signal. Two-dimensional image slices at different phases during self-oscillation were captured, and in each phase the fiducial markers were clearly visible. The process was also demonstrated using a three-dimensional scan at two phases. The benefit of averaging to increase signal-to-noise ratio was explored. The results demonstrate the ability to use MRI to acquire quantitative deformation data that could be used, for example, to validate computational models of flow-induced vocal fold vibration and quantify deformation fields encountered by cells in bioreactor studies.

Multiecho pseudo-golden angle stack of stars thermometry with high spatial and temporal resolution using k-space weighted image contrast.

PURPOSE: Implement and evaluate a 3D MRI method to measure temperature changes with high spatial and temporal resolution and large field of view. METHODS: A multiecho pseudo-golden angle stack-of-stars (SOS) sequence with k-space weighted image contrast (KWIC) reconstruction was implemented to simultaneously measure multiple quantities, including temperature, initial signal magnitude M(0), transverse relaxation time ( T2*), and water/fat images. Respiration artifacts were corrected using self-navigation. KWIC artifacts were removed using a multi-baseline library. The phases of the multiple echo images were combined to improve proton resonance frequency precision. Temperature precision was tested through in vivo breast imaging (N = 5 healthy volunteers) using both coronal and sagittal orientations and with focused ultrasound (FUS) heating in a pork phantom using a breast specific MR-guided FUS system. RESULTS: Temperature measurement precision was significantly improved after echo combination when compared with the no echo combination case (spatial average of the standard deviation through time of 0.3-1.0 and 0.7-1.9°C, respectively). Temperature measurement accuracy during heating was comparable to a 3D seg-EPI sequence. M(0) and T2* values showed temperature dependence during heating in pork adipose tissue. CONCLUSION: A self-navigated 3D multiecho SOS sequence with dynamic KWIC reconstruction is a promising thermometry method that provides multiple temperature sensitive quantitative values. Magn Reson Med 79:1407-1419, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Liver MR Elastography at 3 T: Agreement Across Pulse Sequences and Effect of Liver R2* on Image Quality.

OBJECTIVE: The objectives of our study were to compare MR elastography (MRE) based on gradient-recalled echo (GRE) imaging with spin-echo echo-planar imaging (SEEPI) and rapid fractional (RF)-GRE MRE sequences at 3 T in terms of liver stiffness (LS) and image quality and to evaluate the effect of liver R2* on image quality. MATERIALS AND METHODS: Eighty-one patients underwent 3-T liver MRE with GRE, SE-EPI, and RF-GRE sequences performed in variable order in this study. LS and ROI areas on the LS 95% confidence maps were compared among the three sequences. The relationship between liver R2* and ROI area was investigated. RESULTS: There was no significant difference in mean LS among the three sequences (p = 0.49). Mean ROI area was significantly larger for RF-GRE (18,213 ± 9292 [SD] mm2) than for GRE (13,196 ± 8149 mm2) and SE-EPI (12,896 ± 8656 mm2) (p < 0.0001). Liver R2* was significantly higher among patients with one or more failed sequences (mean ± SD, 116 ± 76 s-1) than for patients with no failed sequences (59 ± 26 s-1) (p = 0.001). Technical failure rates were 10% (8/81), 4% (3/81), and 2% (2/81) for GRE, SE-EPI, and RF-GRE, respectively. Among patients with iron overload (R2* ≥ 100 s-1), there was a trend toward larger ROI area for SE-EPI (p = 0.09). CONCLUSION: SE-EPI-and RF-GRE-based MRE sequences provide equivalent measures of LS compared with GRE-based MRE, and both have lower technical failure rates. The RF-GRE sequence yielded the largest measurable area of LS. Among patients with iron overload, there was a trend toward larger measurable area of LS for the SE-EPI sequence.

Focal point determination in magnetic resonance-guided focused ultrasound using tracking coils.

PURPOSE: To develop a method for rapid prediction of the geometric focus location in MR coordinates of a focused ultrasound (US) transducer with arbitrary position and orientation without sonicating. METHODS: Three small tracker coil circuits were designed, constructed, attached to the transducer housing of a breast-specific MR-guided focused US (MRgFUS) system with 5 degrees of freedom, and connected to receiver channel inputs of an MRI scanner. A one-dimensional sequence applied in three orthogonal directions determined the position of each tracker, which was then corrected for gradient nonlinearity. In a calibration step, low-level heating located the US focus in one transducer position orientation where the tracker positions were also known. Subsequent US focus locations were determined from the isometric transformation of the trackers. The accuracy of this method was verified by comparing the tracking coil predictions to thermal center of mass calculated using MR thermometry data acquired at 16 different transducer positions for MRgFUS sonications in a homogeneous gelatin phantom. RESULTS: The tracker coil predicted focus was an average distance of 2.1 ± 1.1 mm from the thermal center of mass. The one-dimensional locator sequence and prediction calculations took less than 1 s to perform. CONCLUSION: This technique accurately predicts the geometric focus for a transducer with arbitrary position and orientation without sonicating. Magn Reson Med 77:2424-2430, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Interchangeable neck shape-specific coils for a clinically realizable anterior neck phased array system.

PURPOSE: To demonstrate the interchangeable neck shape-specific (NSS) coil concept that supplements standard commercial spine and head/neck coils to provide simultaneous high-resolution (hi-res) head/neck imaging with high signal-to-noise ratio (SNR). METHODS: Two NSS coils were constructed on formers designed to fit two different neck shapes. A 7-channel (7ch) ladder array was constructed on a medium neck former, and a 9-channel (9ch) ladder array was constructed on large neck former. Both coils were interchangeable with the same preamp housing. RESULTS: The 7ch and 9ch coils demonstrate SNR gains of approximately 4 times and 3 times over the Siemens 20-channel head/neck coil in the carotid arteries of our volunteers, respectively. Coupling between the Siemens 32-channel spine coil, Siemens 20-channel head/neck coil, and the NSS coils was negligible, allowing for simultaneous hi-res head/neck imaging with high SNR. CONCLUSIONS: This study demonstrates that supplementing existing commercial spine and head/neck coils with an NSS coil allows uniform simultaneous hi-res imaging with high SNR in the anterior neck, while maintaining SNR of the commercial coil in the head and posterior neck. Magn Reson Med 78:2460-2468, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Motion-insensitive carotid intraplaque hemorrhage imaging using 3D inversion recovery preparation stack of stars (IR-prep SOS) technique.

PURPOSE: Carotid artery imaging is important in the clinical management of patients at risk for stroke. Carotid intraplaque hemorrhage (IPH) presents an important diagnostic challenge. 3D magnetization prepared rapid acquisition gradient echo (MPRAGE) has been shown to accurately image carotid IPH; however, this sequence can be limited due to motion- and flow-related artifact. The purpose of this work was to develop and evaluate an improved 3D carotid MPRAGE sequence for IPH detection. We hypothesized that a radial-based k-space trajectory sequence such as "Stack of Stars" (SOS) incorporated with inversion recovery preparation would offer reduced motion sensitivity and more robust flow suppression by oversampling of central k-space. MATERIALS AND METHODS: A total of 31 patients with carotid disease (62 carotid arteries) were imaged at 3T magnetic resonance imaging (MRI) with 3D IR-prep Cartesian and SOS sequences. Image quality was determined between SOS and Cartesian MPRAGE in 62 carotid arteries using t-tests and multivariable linear regression. Kappa analysis was used to determine interrater reliability. RESULTS: In all, 25 among 62 carotid plaques had carotid IPH by consensus from the reviewers on SOS compared to 24 on Cartesian sequence. Image quality was significantly higher with SOS compared to Cartesian (mean 3.74 vs. 3.11, P < 0.001). SOS acquisition yielded sharper image features with less motion (19.4% vs. 45.2%, P < 0.002) and flow artifact (27.4% vs. 41.9%, P < 0.089). There was also excellent interrater reliability with SOS (kappa = 0.89), higher than that of Cartesian (kappa = 0.84). CONCLUSION: By minimizing flow and motion artifacts and retaining high interrater reliability, the SOS MPRAGE has important advantages over Cartesian MPRAGE in carotid IPH detection. LEVEL OF EVIDENCE: 1 J. Magn. Reson. Imaging 2017;45:410-417.

Magnetic resonance elastography (MRE) for measurement of muscle stiffness of the shoulder: feasibility with a 3 T MRI system.

BACKGROUND: Magnetic resonance elastography (MRE) at 3 T MR has the potential to improve the objective detection of skeletal muscle stiffness. PURPOSE: To determine the feasibility of MRE using 3 T MR for measurement of the stiffness of shoulder muscles in subjects. MATERIAL AND METHODS: This study prospectively evaluated 16 healthy subjects (mean age, 29.8 years; range, 25-51 years). MRE was acquired with 3 T MR through the use of a 2D-gradient-echo-based MRE sequence at two different excitation frequencies (90 and 120 Hz). The mean stiffness values (MSV) of the trapezius and infraspinatus muscles were measured by two radiologists. Differences between the MSV in the x, y, and z motion-sensitization directions were assessed. Inter-observer agreement was also measured. RESULTS: The MSV of the trapezius muscle were 2.72 kPa ± 0.6 (SD) at 90 Hz and 4.66 kPa ± 1.2 at 120 Hz, while the MSV for the infraspinatus muscle were 3.2 kPa ± 0.52 at 90 Hz and 4.38 kPa ± 0.92 at 120 Hz. The MSV for both muscles were significantly higher at 120 Hz than at 90 Hz (P < 0.05). The MSV in the three different directions were significantly different from each other in the infraspinatus muscle (P < 0.05). Levels of inter-observer agreement regarding MSV were good to excellent for both the trapezius (intraclass correlation coefficient [ICC] = 0.979-0.996) and infraspinatus muscles (ICC = 0.614-0.943). CONCLUSION: MRE at 3 T is a feasible technique for the evaluation of shoulder muscle stiffness. Extended application of skeletal muscle MRE at 3 T will contribute to the evaluation and treatment of skeletal muscle disorders.

Characterization of multiple sclerosis plaques using susceptibility-weighted imaging at 1.5 T: can perivenular localization improve specificity of imaging criteria?

BACKGROUND AND PURPOSE: The purpose of this study was to determine if magnetic resonance (MR) susceptibility-weighted imaging (SWI) can increase the conspicuity of corticomedullary veins within the white matter lesions of multiple sclerosis (MS) and, thus, aid in distinguishing plaques from leukoaraiosis. METHODS: We retrospectively reviewed MR examinations in 21 patients with a clinical diagnosis of MS and 18 patients with a clinical diagnosis of dementia. Examinations included fluid-attenuated inversion recovery (FLAIR) and SWI sequences obtained in the axial plane. Lesions greater than 5 mm in diameter on the axial FLAIR sequence were identified as periventricular or subcortical. Three neuroradiologists evaluated SWI images, compared with FLAIR, for a centrally located signal void in each lesion that was scored as present, absent, or indeterminate. RESULTS: In patients with MS, central veins were present in both periventricular lesions (75%, P < 0.001) and subcortical lesions (52%, P < 0.005). In patients with dementia, central veins were seen much less frequently in subcortical lesions (14%, P < 0.001); their association with periventricular lesions was not significant. CONCLUSIONS: Central veins were detected in MS lesions with a significantly greater frequency than that in patients with dementia. Susceptibility-weighted imaging increases the conspicuity of corticomedullary veins and may improve the specificity of MR findings in MS.

Three-dimensional dynamic contrast enhanced imaging of the carotid artery with direct arterial input function measurement.

PURPOSE: Kinetic analysis using dynamic contrast enhanced MRI to assess neovascularization of carotid plaque requires images with high spatial and temporal resolution. This work demonstrates a new three-dimensional (3D) dynamic contrast enhanced imaging sequence, which directly measures the arterial input function with high temporal resolution yet maintains the high spatial resolution required to identify areas of increased adventitial neovascularity. THEORY AND METHODS: The sequence consists of multiple rapid acquisitions of a saturation prepared dynamic 3D gradient recalled echo (GRE) sequence temporally interleaved with multiple acquisitions of a 2D slice. The saturation recovery time was adjusted to maintain signal linearity with the very different contrast agent concentrations in the 2D slice and 3D volume. The K(trans) maps were obtained from the 3D dynamic contrast measurements while the 2D slice was used to obtain the arterial input function. Calibration and dynamic studies are presented. RESULTS: For contrast agent concentrations up to 5 mM, a saturation recovery time for the 2D slice of 20 ms resulted in less than a 10% deviation from the desired linear response of signal intensity with contrast agent concentration. The corresponding saturation recovery time of 83 ms for the 3D volume maintained less than a 10% deviation from the linear response up to contrast agent concentrations of 2 mM while a contrast agent concentration of 5 mM had almost a 30% deviation. There was a significant improvement in signal attenuation (9 ± 3% versus 23 ± 5% at 40 cm/s) when flow compensation was added to the slice select gradients. For patient studies, volume transfer and plasma fraction maps were calculated with data from the proposed sequence. CONCLUSION: This work demonstrated a novel sequence for 3D dynamic contrast enhanced imaging with a simultaneously acquired 2D slice that directly measures the arterial input function with high temporal resolution. Acquisition parameters can be adjusted to accommodate the full range of contrast agent concentration values to be encountered and the kinetic parameters obtained were consistent with expected values.

Magnetic resonance 4D flow reveals unusual hemodynamics associated with aneurysm formation and a possible cause of cryptogenic stroke in a patient with aortic dissection.

Four-dimensional flow is a magnetic resonance technology that has undergone significant technical improvements in recent years. With increasingly rapid acquisition times and new postprocessing tools, it can provide a tool for demonstrating and visualizing cardiovascular flow phenomena, which may offer new insights into disease. We present an interesting clinical case in which 4-dimensional flow demonstrates potential etiologies for 2 interesting phenomena in the same patient: (1) development of an unusual aneurysm and (2) cryptogenic stroke.

Accuracy of MR elastography and anatomic MR imaging features in the diagnosis of severe hepatic fibrosis and cirrhosis.

PURPOSE: To compare the diagnostic accuracy of magnetic resonance imaging elastography (MRE) and anatomic MRI features in the diagnosis of severe hepatic fibrosis and cirrhosis. MATERIALS AND METHODS: Three readers independently assessed presence of morphological changes associated with hepatic fibrosis in 72 patients with liver biopsy including: caudate to right lobe ratios, nodularity, portal venous hypertension (PVH) stigmata, posterior hepatic notch, expanded gallbladder fossa, and right hepatic vein caliber. Three readers measured shear stiffness values using quantitative shear stiffness maps (elastograms). Sensitivity, specificity, and diagnostic accuracy of stiffness values and each morphological feature were calculated. Interreader agreement was summarized using weighted kappa statistics. Intraclass correlation coefficient was used to assess interreader reproducibility of stiffness measurements. Binary logistic regression was used to assess interreader variability for dichotomized stiffness values and each morphological feature. RESULTS: Using 5.9 kPa as a cutoff for differentiating F3-F4 from F0-2 stages, overall sensitivity, specificity, and diagnostic accuracy for MRE were 85.4%, 88.4%, and 87%, respectively. Overall interreader agreement for stiffness values was substantial, with an insignificant difference (P = 0.74) in the frequency of differentiating F3-4 from F0-2 fibrosis. Only hepatic nodularity and PVH stigmata showed moderately high overall accuracy of 69.4% and 72.2%. Interreader agreement was substantial only for PVH stigmata, moderate for C/R m, deep notch, and expanded gallbladder fossa. Only posterior hepatic notch (P = 0.82) showed no significant difference in reader rating. CONCLUSION: MRE is a noninvasive, accurate, and reproducible technique compared with conventional features of detecting severe hepatic fibrosis.

Intra-patient comparison of 3D and 2D magnetic resonance elastography techniques for assessment of liver stiffness.

PURPOSE: To evaluate performance of 3D magnetic resonance elastography (MRE) using spin-echo echo-planar imaging (seEPI) for assessment of hepatic stiffness compared with 2D gradient-recalled echo (GRE) and 2D seEPI sequences. METHODS: Fifty-seven liver MRE examinations including 2D GRE, 2D seEPI, and 3D seEPI sequences were retrospectively evaluated. Elastograms were analyzed by 2 radiologists and polygonal regions of interests (ROIs) were drawn in 2 different fashions: "curated" ROI (avoiding liver edge, major vessels, and areas of wave interferences) and "non-curated" ROI (including largest cross section of liver, to assess the contribution of artifacts). Liver stiffness measurement (LSM) was calculated as the arithmetic mean of individual stiffness values for each technique. For 3D MRE, LSMs were also calculated based on 4 slices ("abbreviated LSM"). Intra-patient variations in LSMs and different methods of ROI placement were assessed by univariate tests. A p-value of < 0.05 was set as a statistically significant difference. RESULTS: Mean surface areas of the ROIs were 50,723 mm2, 12,669 mm2, 5814 mm2, and 10,642 mm2 for 3D MRE, abbreviated 3D MRE, 2D GRE, and 2D seEPI, respectively. 3D LSMs based on curated and non-curated ROIs showed no clinically significant difference, with a mean difference less than 0.1 kPa. Abbreviated 3D LSMs had excellent correlation with 3D LSMs based on all slices (r = 0.9; p < 0.001) and were not significantly different (p = 0.927). CONCLUSION: 3D MRE allows more reproducible measurements due to its lower susceptibility to artifacts and provides larger areas of parenchyma, enabling a more comprehensive evaluation of the liver.

Assessment of chronic hepatitis and fibrosis: comparison of MR elastography and diffusion-weighted imaging.

OBJECTIVE: The purpose of our study was to compare the utility of MR elastography (MRE) and diffusion-weighted imaging (DWI) in characterizing fibrosis and chronic hepatitis in patients with chronic liver diseases. SUBJECTS AND METHODS: Seventy-six patients with chronic liver disease underwent abdominal MRI, MRE, and DWI. Severities of liver fibrosis and chronic hepatitis were graded by histopathologic analysis according to standard disease-specific classifications. The overall predictive ability of MRE and DWI in assessment of fibrosis was compared by constructing a receiver operating characteristic (ROC) curve and calculating the area under the curve (AUC) on the basis of histopathologic analysis. RESULTS: Using ROC analysis, MRE showed greater capability than DWI in discriminating stage 2 or greater (≥ F2), stage 3 or greater (≥ F3), and cirrhosis (≥ F4), shown as significant differences in AUC (p = 0.003, p = 0.001, and p = 0.001, respectively). Higher sensitivity and specificity were shown by MRE in predicting fibrosis scores ≥ F2 (91% and 97%), scores ≥ F3 (92% and 95%), and scores F4 (95% and 87%) compared with DWI (84% and 82%, 88% and 76%, and 85% and 68%, respectively). Although MRE had higher ability in identification of liver with fibrosis scores ≥ F1 than DWI, a significant difference was not seen (p = 0.398). Stiffness values on MRE increased in relation to increasing severity of fibrosis confirmed by histopathology scores; however, a consistent relationship between apparent diffusion coefficient (ADC) values and stage of fibrosis was not shown. In addition, liver tissue with chronic hepatitis preceding fibrosis may account for mild elevation of liver stiffness. CONCLUSION: MRE had greater predictive ability in distinguishing the stages of liver fibrosis than DWI.

Magnetic resonance shear wave elastography using transient acoustic radiation force excitations and sinusoidal displacement encoding.

A magnetic resonance (MR) shear wave elastography technique that uses transient acoustic radiation force impulses from a focused ultrasound (FUS) transducer and a sinusoidal-shaped MR displacement encoding strategy is presented. Using this encoding strategy, an analytic expression for calculating the shear wave speed in a heterogeneous medium was derived. Green's function-based simulations were used to evaluate the feasibility of calculating shear wave speed maps using the analytic expression. Accuracy of simulation technique was confirmed experimentally in a homogeneous gelatin phantom. The elastography measurement was compared to harmonic MR elastography in a homogeneous phantom experiment and the measured shear wave speed values differed by less than 14%. This new transient elastography approach was able to map the position and shape of inclusions sized from 8.5 to 14 mm in an inclusion phantom experiment. These preliminary results demonstrate the feasibility of using a straightforward analytic expression to generate shear wave speed maps from MR images where sinusoidal-shaped motion encoding gradients are used to encode the displacement-time history of a transiently propagating wave-packet. This new measurement technique may be particularly well suited for performing elastography before, during, and after MR-guided FUS therapies since the same device used for therapy is also used as an excitation source for elastography.

Magnetic resonance elastography SE-EPI vs GRE sequences at 3T in a pediatric population with liver disease.

PURPOSE: The goal of our study is to compare hepatic stiffness measures using gradient-recalled echo (GRE) versus spin-echo echo planar imaging (SE-EPI)-based MR Elastography (MRE) at 3T used to measure hepatic stiffness in a patients with suspected liver diseases. MATERIALS AND METHODS: This retrospective study included 52 patients with liver disease who underwent a 3T MRE exam including both an investigational SE-EPI-based technique and a product GRE-based technique. Regions of interest (ROI) were placed on the elastograms to measure elastography-derived liver stiffness as well as the area included within the ROIs. The mean liver stiffness values and area of ROIs were compared. RESULTS: The mean liver stiffness was 3.72 kilopascal (kPa) ± 1.29 using GRE MRE and 3.78 kPa ± 1.13 using SE-EPI MRE. Measurement of liver stiffness showed excellent agreement between the two pulse sequences with a mean bias of - 0.1 kPa (range - 1.8 to 1.7 kPa) between sequences. The mean measurable ROI area was higher with SE-EPI (313.8 cm2 ± 213.8) than with the GRE technique (208.6 cm2 ± 114.8), and the difference was statistically significant (P < 0.05). CONCLUSIONS: Our data shows excellent agreement of measured liver stiffness between GRE and SE-EPI-based sequences at 3T. Our results show the advantage of a SE-EPI MRE sequence in terms of image quality, ROI size and acquisition time with equivalent liver stiffness measurements as compared to GRE-MRE sequence.