Precision and Test-Retest Repeatability of Stiffness Measurement with MR Elastography: A Multicenter Phantom Study.

Background MR elastography (MRE) has been shown to have excellent performance for noninvasive liver fibrosis staging. However, there is limited knowledge regarding the precision and test-retest repeatability of stiffness measurement with MRE in the multicenter setting. Purpose To determine the precision and test-retest repeatability of stiffness measurement with MRE across multiple centers using the same phantoms. Materials and Methods In this study, three cylindrical phantoms made of polyvinyl chloride gel mimicking different degrees of liver stiffness in humans (phantoms 1-3: soft, medium, and hard stiffness, respectively) were evaluated. Between January 2021 and January 2022, phantoms were circulated between five different centers and scanned with 10 MRE-equipped clinical 1.5-T and 3-T systems from three major vendors, using two-dimensional (2D) gradient-recalled echo (GRE) imaging and/or 2D spin-echo (SE) echo-planar imaging (EPI). Similar MRE acquisition parameters, hardware, and reconstruction algorithms were used at each center. Mean stiffness was measured by a single observer for each phantom and acquisition on a single section. Stiffness measurement precision and same-session test-retest repeatability were assessed using the coefficient of variation (CV) and the repeatability coefficient (RC), respectively. Results The mean precision represented by the CV was 5.8% (95% CI: 3.8, 7.7) for all phantoms and both sequences combined. For all phantoms, 2D GRE achieved a CV of 4.5% (95% CI: 3.3, 5.7) whereas 2D SE EPI achieved a CV of 7.8% (95% CI: 3.1, 12.6). The mean RC of stiffness measurement was 5.8% (95% CI: 3.7, 7.8) for all phantoms and both sequences combined, 4.9% (95% CI: 2.7, 7.0) for 2D GRE, and 7.0% (95% CI: 2.9, 11.2) for 2D SE EPI (all phantoms). Conclusion MRE had excellent in vitro precision and same-session test-retest repeatability in the multicenter setting when similar imaging protocols, hardware, and reconstruction algorithms were used. © RSNA, 2024 Supplemental material is available for this article. See also the editorial by Tang in this issue.

Performance evaluation of commercial and non-commercial shear wave elastography implementations for vascular applications.

BACKGROUND: Shear wave elastography (SWE) is mainly used for stiffness estimation of large, homogeneous tissues, such as the liver and breasts. However, little is known about its accuracy and applicability in thin (∼0.5-2 mm) vessel walls. To identify possible performance differences among vendors, we quantified differences in measured wave velocities obtained by commercial SWE implementations of various vendors over different imaging depths in a vessel-mimicking phantom. For reference, we measured SWE values in the cylindrical inclusions and homogeneous background of a commercial SWE phantom. Additionally, we compared the accuracy between a research implementation and the commercially available clinical SWE on an Aixplorer ultrasound system in phantoms and in vivo in patients. METHODS: SWE measurements were performed over varying depths (0-35 mm) using three ultrasound machines with four ultrasound probes in the homogeneous 20 kPa background and cylindrical targets of 10, 40, and 60 kPa of a multi-purpose phantom (CIRS-040GSE) and in the anterior and posterior wall of a homogeneous polyvinyl alcohol vessel-mimicking phantom. These phantom data, along with in vivo SWE data of carotid arteries in 23 patients with a (prior) head and neck neoplasm, were also acquired in the research and clinical mode of the Aixplorer ultrasound machine. Machine-specific estimated phantom stiffness values (CIRS phantom) or wave velocities (vessel phantom) over all depths were visualized, and the relative error to the reference values and inter-frame variability (interquartile range/median) were calculated. Correlations between SWE values and target/vessel wall depth were explored in phantoms and in vivo using Spearman's correlations. Differences in wave velocities between the anterior and posterior arterial wall were assessed with Wilcoxon signed-rank tests. Intra-class correlation coefficients were calculated for a sample of ten patients as a measure of intra- and interobserver reproducibility of SWE analyses in research and clinical mode. RESULTS: There was a high variability in obtained SWE values among ultrasound machines, probes, and, in some cases, with depth. Compared to the homogeneous CIRS-background, this variation was more pronounced for the inclusions and the vessel-mimicking phantom. Furthermore, higher stiffnesses were generally underestimated. In the vessel-mimicking phantom, anterior wave velocities were (incorrectly) higher than posterior wave velocities (3.4-5.6 m/s versus 2.9-5.9 m/s, p ≤ 0.005 for 3/4 probes) and remarkably correlated with measurement depth for most machines (Spearman's ρ = -0.873-0.969, p < 0.001 for 3/4 probes). In the Aixplorer's research mode, this difference was smaller (3.3-3.9 m/s versus 3.2-3.6 m/s, p = 0.005) and values did not correlate with measurement depth (Spearman's ρ = 0.039-0.659, p ≥ 0.002). In vivo, wave velocities were higher in the posterior than the anterior vessel wall in research (left p = 0.001, right p < 0.001) but not in clinical mode (left: p = 0.114, right: p = 0.483). Yet, wave velocities correlated with vessel wall depth in clinical (Spearman's ρ = 0.574-0.698, p < 0.001) but not in research mode (Spearman's ρ = -0.080-0.466, p ≥ 0.003). CONCLUSIONS: We observed more variation in SWE values among ultrasound machines and probes in tissue with high stiffness and thin-walled geometry than in low stiffness, homogeneous tissue. Together with a depth-correlation in some machines, where carotid arteries have a fixed location, this calls for caution in interpreting SWE results in clinical practice for vascular applications.

Cardiac 2-D Shear Wave Imaging Using a New Dedicated Clinical Ultrasound System: A Phantom Study.

OBJECTIVE: The purpose of this study was to assess cardiac shear wave imaging implemented in a new MACH 30 ultrasound machine (SuperSonic Imaging, Aix-en-Provence, France) and interfaced with a linear probe and a phased array probe, in comparison with a previously validated Aixplorer system connected to a linear probe (SuperSonic Imaging) using Elasticity QA phantoms (Models 039 and 049, CIRS Inc., Norfolk, VA, USA). METHODS: Quantile-quantile plots were used for distribution agreement. The accuracy of stiffness measurement was assessed by the percentage error and the mean percentage error (MPE), and its homogeneity, by the standard deviation of the MPE. A p value <0.01 was considered to indicate statistical significance. RESULTS: The accuracy of dedicated cardiac sequences for linear probes was similar for the two systems with an MPE of 8 ± 14% versus 20 ± 21% (p = not significant) with the SuperSonic MACH 30 and Aixplorer, respectively, and was influenced by target stiffness and location of the measurement in the field of view, but without drift over time. The optimal transthoracic cardiac probe workspace was located between 4 and 10 cm, with an MPE of 29.5 ± 25% compared with 93.3 ± 130% outside this area (p < 0.0001). In this area, stiffness below 20 kPa was significantly different from the reference (p < 0.0001). The sectorial probe revealed no MPE difference in any of the measurement areas, with no significant lateral or axial gradient. CONCLUSION: The new Supersonic MACH 30 system upgraded with a sectorial probe and specific cardiac settings provided homogenous stiffness measurements, especially when operating at depths between 4 and 10 cm. These phantom results may be useful in designing future in vivo studies.

A force-measuring device combined with ultrasound-based elastography for assessment of the uterine cervix.

INTRODUCTION: In this feasibility study, we hypothesize that the evaluation of cervical biomechanical strength can be improved if cervical length measurement is supplemented with quantitative elastography, which is a technique based on conventional ultrasound elastography combined with a force-measuring device. Our aims were to: (a) develop a force-measuring device; (b) introduce a cervical elastography index (CEI) and a cervical strength index (CSI; defined as cervical length × CEI); (c) evaluate how these indexes assess the cervical softening that takes place during normal pregnancy; and (d) how these indexes predict the cervical dilatation time from 4 to 10 cm. MATERIAL AND METHODS: An electronic force-measuring device was mounted on the handle of the transvaginal probe, allowing for force measurement when conducting elastography. The study group concerned with normal cervical softening included 44 unselected pregnant women. Outcomes were CEI and CSI at different gestational ages. The study group for labor induction included 26 singleton term pregnant women admitted for labor induction. Outcome was defined as cervical dilatation time from 4 to 10 cm. Elastography measured the changes in mean gray value (intensity) during manual compressions. Region of interest was set within the anterior cervical lip. RESULTS: We found that the mean of all variables regarding cervical softening decreased from early to late pregnancy: ie cervical length from 34 to 29 mm, CEI from 0.17 to 0.11 N, and CSI from 5.9 to 3.1 N mm. Moreover, the cervical dilatation time during labor induction was associated with CEI, although not statistically significantly (area under the ROC curve of 0.67), but not with the Bishop score, the cervical length, or the CSI. CONCLUSIONS: We propose that quantitative elastography based on changes in the intensity of the B-mode ultrasound recording, in combination with a force-measuring device on the handle of the vaginal probe, deserves further investigation as an approach for evaluation of cervical biomechanical strength.

M probe comes first: Impact of initial probe choice on diagnostic performance of vibration controlled transient elastography.

BACKGROUND & AIMS: Probe choice (M or XL) in transient elastography can be made by the user's own measure of skin-to-liver-capsule distance (SCD) or with an automated tool (AUTO). We studied how AUTO depends on initial probe choice. METHODS: Three fictive clinics were considered: The "M-first clinic" uses AUTO from the M probe, the "XL-first clinic" uses AUTO from the XL probe and a "reference clinic" measures SCD independently. Agreement and discrepancies to the reference clinic were measured. RESULTS: 200 patients with chronic liver disease were prospectively included (58% female, 56 years, BMI 28.1 kg/m²). Fleiss' kappa for agreement in probe selection was 0.11 (95% CI -0.09 to 0.31), but accuracy was above 0.8 for both. Probe failure occurred for 16 (M-first clinic), 4 (XL-first clinic) and 3 patients (reference clinic). Use of XL probe given M probe failure improved performance of the M-first approach. The odds ratio for discrepancy in the XL-first vs M-first clinic is 2.4 (95% CI 1.2 to 5.2, p = 0.012) for liver fibrosis and 4.8 (95% CI 1.8 to 16.1, p < 0.001) for steatosis. CONCLUSIONS: Agreement in AUTO between M and XL probes is poor although each has acceptable accuracy. The M-first approach leads to fewer discrepancies and should be adopted as a standard.

Impact of axisymmetric deformation on MR elastography of a nonlinear tissue-mimicking material and implications in peri-tumour stiffness quantification.

Solid tumour growth is often associated with the accumulation of mechanical stresses acting on the surrounding host tissue. Due to tissue nonlinearity, the shear modulus of the peri-tumoural region inherits a signature from the tumour expansion which depends on multiple factors, including the soft tissue constitutive behaviour and its stress/strain state. Shear waves used in MR-elastography (MRE) sense the apparent change in shear modulus along their propagation direction, thereby probing the anisotropic stiffness field around the tumour. We developed an analytical framework for a heterogeneous shear modulus distribution using a thick-shelled sphere approximation of the tumour and soft tissue ensemble. A hyperelastic material (plastisol) was identified to validate the proposed theory in a phantom setting. A balloon-catheter connected to a pressure sensor was used to replicate the stress generated from tumour pressure and growth while MRE data were acquired. The shear modulus anisotropy retrieved from the reconstructed elastography data confirmed the analytically predicted patterns at various levels of inflation. An alternative measure, combining the generated deformation and the local wave direction and independent of the reconstruction strategy, was also proposed to correlate the analytical findings with the stretch probed by the waves. Overall, this work demonstrates that MRE in combination with non-linear mechanics, is able to identify the apparent shear modulus variation arising from the strain generated by a growth within tissue, such as an idealised model of tumour. Investigation in real tissue represents the next step to further investigate the implications of endogenous forces in tissue characterisation through MRE.

An electromagnetic actuator for brain magnetic resonance elastography with high frequency accuracy.

Our goal is to design, test and verify an electromagnetic actuator for brain magnetic resonance elastography (MRE). We proposed a grappler-shaped design that can transmit stable vibrations into the brain. To validate its performance, simulations were carried out to ensure the electromagnetic field generated by the actuator did not interfere with the B0 field. The actuation vibration spectrum was analyzed to verify the actuation accuracy. Phantom and volunteer experiments were carried out to evaluate the performance of the actuator. Simulation of the magnetic field showed that the proposed actuator has a fringe field of less than 3 G in the imaging region. The phantom experiments showed that the proposed actuator did not interfere with the routine imaging sequences. The measured vibration spectra demonstrated that the frequency offset was about one third that of a pneumatic device and the transmission efficiency was three times higher. The shear moduli estimated from brain MRE were consistent with those from the literature. The actuation frequency of the proposed actuator has less frequency offset and off-center frequency components compared with the pneumatic counterpart. The whole actuator weighted only 980 g. The actuator can carry out multifrequency MRE on the brain with high accuracy. It is easy to use, comfortable for the patient and portable.

Liver stiffness measured by magnetic resonance elastography in early recurrence of hepatocellular carcinoma after treatment: A protocol for systematic review and meta analysis.

BACKGROUND: With high diagnostic accuracy, magnetic resonance elastography (MRE) is a noninvasive tool and can be adopted to measure liver stiffness (LS). In this study, meta-analysis was carried out to further evaluate whether LS measured by MRE can predict early recurrence in patients with hepatocellular carcinoma (HCC). METHODS: PUBMED, EMBASE, Web of Science, China National Knowledge Infrastructure, and Cochrane Library database were searched for studies related to LS measured by MRE in the prediction of recurrence in patients with HCC. Survival outcome was estimated by hazard ratios and 95% confidence intervals. Meta-analysis was conducted with the Stata 16.0. RESULTS: The results of this meta-analysis will be submitted to a peer-reviewed journal for publication. CONCLUSION: This study will provide evidence support for LS measured by MRE in predicting the recurrence of HCC. ETHICS AND DISSEMINATION: The private information from individuals will not be published. This systematic review also should not damage participants' rights. Ethical approval is not available. The results may be published in a peer-reviewed journal or disseminated in relevant conferences. OSF REGISTRATION NUMBER: DOI 10.17605/ OSF.IO / SURH3.

Longitudinal stability of a multimodal visco-elastic polyacrylamide gel phantom for magnetic resonance and ultrasound shear-wave elastography.

We evaluated the long-term stability of a newly developed viscoelastic phantom made of polyacrylamide (PAAm) gel for magnetic resonance elastography (MRE) and ultrasound-based shear-wave elastography (US SWE). The stiffness of the cylindrical phantom was measured at 0, 13 and 18 months. Storage and loss moduli were measured with MRE, and shear-wave speed (SWS) was measured with US SWE. Long-term stability was evaluated in accordance with the Quantitative Imaging Biomarker Alliance (QIBA) profiles for each modality. The initial storage and loss moduli of the phantom were 5.01±0.22 and 1.11±0.15 respectively, and SWS was 2.57±0.04 m/s. The weight of the phantom decreased by 0.6% over the 18 months. When measured with MRE, the stiffness of the phantom decreased and changes to the storage and loss moduli were -3.0% and -4.6% between 0 and 13 months, and -4.3% and 0.0% between 0 and 18 months. The US measurements found that SWS decreased by 2.4% over the first 13 months and 3.6% at 18 months. These changes were smaller than the tolerances specified in the QIBA profiles, so the viscoelastic PAAm gel phantom fulfilled the condition for long-term stability. This new phantom has the potential to be used as a quality assurance and quality control phantom for MRE and US SWE.

Attenuation parameter and liver stiffness measurement using FibroTouch vs Fibroscan in patients with chronic liver disease.

BACKGROUND & AIM: We studied FibroTouch (FT) and Fibroscan (FS) examination results and their repeatability when performed by healthcare personnel of different background. METHODS: FT and FS examinations were performed on patients with chronic liver disease by two operators, a doctor and a nurse, twice on each patient, at two different time points, independent of each other. RESULTS: The data for 163 patients with 1304 examinations was analyzed. There was strong correlation between FT and FS for attenuation parameter (Spearman's rho 0.76, p<0.001) and liver stiffness measurement (LSM) (Spearman's rho 0.70, p<0.001). However, FT produced higher value at lower attenuation parameter and LSM, and lower value at higher attenuation parameter and LSM. There was substantial agreement when using 15kPa LSM cut-off, but only moderate agreement when using 10kPa and 20kPa LSM cut-offs and 248dB/m, 268dB/m and 280dB/m attenuation parameter cut-offs. The IQR for attenuation parameter and IQR/median for LSM were significantly lower for FT compared with FS (4dB/m vs 27dB/m, p<0.001, and 10 vs 12, p<0.001, respectively). The intra- and inter-observer reliability of attenuation parameter and LSM using FT and FS were good to excellent with intraclass correlation coefficients 0.89-0.99. FT had shorter examination time (33s vs 47s, p<0.001) and less invalid measurements (0 vs 2, p<0.001). CONCLUSION: Measurements obtained with FT and FS strongly correlated, but significant differences in their absolute values, consistency, examination time and number of invalid measurements were observed. Either device can be used by healthcare personnel of different backgrounds when sufficiently trained.

Differential Imaging of Liver Tumors before and after Microwave Ablation with Electrode Displacement Elastography.

Liver cancer is a leading cause of cancer-related deaths; however, primary treatment options such as surgical resection and liver transplant may not be viable for many patients. Minimally invasive image-guided microwave ablation (MWA) provides a locally effective treatment option for these patients with an impact comparable to that of surgery for both cancer-specific and overall survival. MWA efficacy is correlated with accurate image guidance; however, conventional modalities such as B-mode ultrasound and computed tomography have limitations. Alternatively, ultrasound elastography has been used to demarcate post-ablation zones, yet has limitations for pre-ablation visualization because of variability in strain contrast between cancer types. This study attempted to characterize both pre-ablation tumors and post-ablation zones using electrode displacement elastography (EDE) for 13 patients with hepatocellular carcinoma or liver metastasis. Typically, MWA ablation margins of 0.5-1.0 cm are desired, which are strongly correlated with treatment efficacy. Our results revealed an average estimated ablation margin inner quartile range of 0.54-1.21 cm with a median value of 0.84 cm. These treatment margins lie within or above the targeted ablative margin, indicating the potential to use EDE for differentiating index tumors and ablated zones during clinical ablations. We also obtained a high correlation between corresponding segmented cross-sectional areas from contrast-enhanced computed tomography, the current clinical gold standard, when compared with EDE strain images, with r2 values of 0.97 and 0.98 for pre- and post-ablation regions.

Point Shear Wave Elastography for Assessing Liver Stiffness in Chronic Liver Diseases of Different Etiologies Compared to Biopsy.

BACKGROUND: Point shear-wave elastography (pSWE) is a new method to assess the degree of liver fibrosis. It has been shown to be effective in detecting stiffness in viral hepatitis. OBJECTIVES: To determine the feasibility of pSWE for assessing liver stiffness and fibrosis in liver diseases of different etiologies. METHODS: This prospective single-center study included a population of adult patients with chronic liver diseases from different etiologies, who were scheduled for liver biopsy, and a control group of healthy adults who prospectively underwent pSWE. Ten consecutive pSWE measurements of the liver were performed using a Philips iU22 ultrasound system. Stiffness degree was compared to liver biopsy results. Fibrosis degree was staged according to METAVIR scoring system. RESULTS: The study group was comprised of 202 patients who underwent liver biopsy and pSWE test and a control group consisting of 14 healthy adults who underwent pSWE for validation. In the study group, the median stiffness was 5.35 ± 3.37 kilopascal (kPa). The median stiffness for F0-1, F2, F3, and F4 as determined by liver biopsy results were 4.9 kPa, 5.4 kPa, 5.7 kPa, and 8 kPa, respectively. The median stiffness in the control group was 3.7 ± 0.6 kPa. Subgroup analyses were conducted for viral hepatitis vs. non-viral hepatitis and steatohepatitis vs. non-steatohepatitis groups. CONCLUSIONS: pSWE is a reproducible method for assessing liver stiffness and is in a linear relationship with fibrosis degree as seen in pathology. Compared with patients with non-significant fibrosis, healthy controls showed significantly lower values.

The prevalence of metabolic associated fatty liver detected by FibroScan® in women with gestational diabetes in a multiethnic population.

AIMS: Metabolic associated fatty liver disease (MAFLD) is a leading cause of chronic liver disease and has been increasingly associated with gestational diabetes (GDM). This study aimed to assess the prevalence of MAFLD in women with GDM in the antenatal period. METHODS: 108 pregnant women with GDM diagnosed on a 75-gram oral glucose tolerance test were enrolled from a multiethnic cohort attending a large obstetrics clinic in Sydney, Australia and had a single FibroScan® assessment after 24 weeks gestation to assess for hepatic steatosis and fibrosis. A control attenuated parameter (CAP) cut-off score of ≥ 233.5 dB/m was chosen to signify presence of hepatic steatosis which indicates MAFLD. Obstetric, anthropometric and metabolic measures were analysed. RESULTS: 29 (26.9%) women had evidence of FibroScan®-detected MAFLD, whilst none had evidence of hepatic fibrosis. Increased maternal BMI (aOR 1.12, 95% CI: 1.04-1.20) was associated with the finding of MAFLD in this cohort. CONCLUSIONS: We found a significant antenatal prevalence of FibroScan®-detected MAFLD in this cohort of multiethnic women with GDM. FibroScan® is a safe and rapid assessment tool which may have a role in screening for MAFLD in pregnancy in appropriate at-risk women.

Development of a method for measuring spleen stiffness by transient elastography using a new device and ultrasound-fusion method.

BACKGROUND: Hepatic venous pressure gradient (HVPG) is the gold standard index for evaluating portal hypertension; however, measuring HVPG is invasive. Although transient elastography (TE) is the most common procedure for evaluating organ stiffness, accurate measurement of spleen stiffness (SS) is difficult. We developed a device to demonstrate the diagnostic precision of TE and suggest this technique as a valuable new method to measure SS. METHODS: Of 292 consecutive patients enrolled in this single-centre, translational, cross-sectional study from June through September in 2019, 200 underwent SS measurement (SSM) using an M probe (training set, n = 130; inspection set, n = 70). We performed TE with B-mode imaging using an ultrasound-fusion method, printed new devices with a three-dimensional printer, and attached the magnetic position sensor to the convex and M probes. We evaluated the diagnostic precision of TE to evaluate the risk of esophagogastric varices (EGVs). RESULTS: The median spleen volume was 245 mL (range, 64-1,720 mL), and it took 2 minutes to acquire a B-mode image using the ultrasound-fusion method. The median success rates of TE were 83.3% and 57.6% in patients with and without the new device, respectively (p<0.001); it was 76.9% and 35.0% in patients with and without splenomegaly (<100 mL), respectively (p<0.001). In the prediction of EGVs, the areas under the receiver operating characteristic curve were 0.921 and 0.858 in patients with and without the new device, respectively (p = 0.043). When the new device was attached, the positive and negative likelihood ratios were 3.44 and 0.11, respectively. The cut-off value of SSM was 46.0 kPa. Data that were similar between the validation and training sets were obtained. CONCLUSIONS: The SS can be precisely measured using this new device with TE and ultrasound-fusion method. Similarly, we can estimate the bleeding risk due to EGV using this method.

Reliability of shoulder muscle stiffness measurement using strain ultrasound elastography and an acoustic coupler.

PURPOSE: Abnormal increases in muscle tone can be caused by various musculoskeletal disorders. The objective of this study was to evaluate intratester and intertester reliabilities in measuring the stiffness of the shoulder muscles using strain ultrasound elastography (USE) and an acoustic coupler. METHOD: Tissue stiffness was measured in the trapezius muscle and in the supraspinatus muscle of healthy young volunteers. RESULTS: The mean strain ratios measured by two experienced shoulder surgeons were significantly higher in the trapezius muscle than in the supraspinatus muscle (P < 0.001). Intratester reliability was rated as moderate to substantial for the trapezius muscle and substantial for the supraspinatus muscle. Intertester reliability was substantial for both muscles, with an intraclass correlation coefficient (2,1) of 0.62 [95% confidence interval (CI) 0.28-0.82] for the trapezius muscle and 0.69 (95% CI 0.40-0.86) for the supraspinatus muscle. CONCLUSIONS: We found substantial intratester and intertester reliabilities for the trapezius and supraspinatus muscles, suggesting that USE represents a promising modality for measuring the stiffness of shoulder muscles. However, the clinical application of this method will require the development of a device that can standardize the scanning technique to further increase the reliability.

Magnetic resonance elastography for estimating in vivo stiffness of the abdominal aorta using cardiac-gated spin-echo echo-planar imaging: a feasibility study.

INTRODUCTION: Magnetic resonance elastography (MRE)-derived aortic stiffness is a potential biomarker for multiple cardiovascular diseases. Currently, gradient-recalled echo (GRE) MRE is a widely accepted technique to estimate aortic stiffness. However, multi-slice GRE MRE requires multiple breath-holds (BHs), which can be challenging for patients who cannot consistently hold their breath. The aim of this study was to investigate the feasibility of a multi-slice spin-echo echo-planar imaging (SE-EPI) MRE sequence for quantifying in vivo aortic stiffness using a free-breathing (FB) protocol and a single-BH protocol. METHOD: On Scanner 1, 25 healthy subjects participated in the validation of FB SE-EPI against FB GRE. On Scanner 2, another 15 healthy subjects were recruited to compare FB SE-EPI with single-BH SE-EPI. Among all volunteers, five participants were studied on both scanners to investigate the inter-scanner reproducibility of FB SE-EPI aortic MRE. Bland-Altman analysis, Lin's concordance correlation coefficient (LCCC) and coefficient of variation (COV) were evaluated. The phase-difference signal-to-noise ratios (PD SNR) were compared. RESULTS: Aortic MRE using FB SE-EPI and FB GRE yielded similar stiffnesses (paired t-test, P = 0.19), with LCCC = 0.97. The FB SE-EPI measurements were reproducible (intra-scanner LCCC = 0.96) and highly repeatable (LCCC = 0.99). The FB SE-EPI MRE was also reproducible across different scanners (inter-scanner LCCC = 0.96). Single-BH SE-EPI scans yielded similar stiffness to FB SE-EPI scans (LCCC = 0.99) and demonstrated a low COV of 2.67% across five repeated measurements. CONCLUSION: Multi-slice SE-EPI aortic MRE using an FB protocol or a single-BH protocol is reproducible and repeatable with advantage over multi-slice FB GRE in reducing acquisition time. Additionally, FB SE-EPI MRE provides a potential alternative to BH scans for patients who have challenges in holding their breath.

Diffuse shear wave spectroscopy for soft tissue viscoelastic characterization.

In order to limit and slow the development of diseases, they have to be diagnosed early as possible to treat patients in a better and more rapid manner. In this paper, we focus on a noninvasive and quick method based on diffuse fields in elastography to detect diseases that affect the stiffness of organs. To validate our method, a phantom experiment numerically pre-validated is designed. It consists of seven vibrators that generate white noises in a bandwidth of [80-300] Hz and then a complex acoustic field in a phantom. Waves are tracked by a linear ultrasound probe L11-4v linked to a Verasonics Vantage System and are converted into a particle velocity 2D map as a function of time. The phase velocity of the shear waves is calculated using a temporal and 2D spatial Fourier transform and an adapted signal-processing method. Shear wave velocity dispersion measurement in the frequency bandwidth of the vibrators enables one to characterize the dynamic hardness of the material through the viscoelastic parameters µ and η in an acquisition time shorter than a second (Tacq = 300 ms). With the aim of estimating the consistency of the method, the experiment is performed N = 10 times. The measured elastic modulus and viscous parameter that quantify the dynamic properties of the medium correspond to the expected values: µ = 1.23 ± 0.05 kPa and η = 0.51 ± 0.09 Pa∙s.

Ultrasound elastography using a regularized modified error in constitutive equations (MECE) approach: a comprehensive phantom study.

Many of the current techniques in transient elastography, such as shear wave elastography (SWE) assume a dominant planar shear wave propagating in an infinite medium. This underlying assumption, however, can be easily violated in real scenarios in vivo, leading to image artifacts and reconstruction errors. Other approaches that are not bound to planar shear wave assumption, such solutions based on the partial differential equation, can potentially overcome the shortcomings of the conventional SWE. The main objective of this paper is to demonstrate the advantages of the modified error in constitutive equations (MECE) formulation with total variation regularization (MECE + TV) over SWE in reconstructing the elastic moduli of different tissue-mimicking phantoms. Experiments were conducted on phantoms with inclusions of well-defined shapes to study the reconstruction of specific features relevant to practical applications. We compared the performances of MECE + TV and SWE in terms of quantitative metrics to estimate reconstruction accuracy, inclusion shape recovery, edge preservation and edge sharpness, inclusion size representation, and shear elasticity and contrast accuracies. The results indicate that the MECE + TV approach outperforms SWE based on several of these metrics. It is concluded that, with further development, the proposed method may offer elastography reconstructions that are superior to SWE in clinical applications.

Toward clinical elastography of dermal tissues: A medical device to probe skin's elasticity through suction, with subsurface imaging via optical coherence tomography.

The mechanical behavior of dermal tissues is unarguably recognized for its diagnostic ability and in the last decades received a steadily increasing interest in dermatology practices. Among the various methods to investigate the mechanics of skin in clinical environments, suction-based ones are especially noteworthy, thanks to their qualities of minimal invasiveness and relative simplicity of setups and data analysis. In such experiments, structural visualization of the sample is highly desirable, both in its own right and because it enables elastography. The latter is a technique that combines the knowledge of an applied mechanical stimulus and the visualization of the induced deformation to result in a spatially resolved map of the mechanical properties, which is particularly important for an inhomogeneous and layered material such as skin. We present a device, designed for clinical trials in dermatology practices, that uses a handheld probe to (1) deliver a suction-based, controlled mechanical stimulus and (2) visualize the subsurface structure via optical coherence tomography. We also present a device-agnostic data-analysis framework, consisting of a Python library, released in the public domain. We show the working principle of the setup on a polymeric model and on a volunteer's skin.