Validation of Semiautomated Quantification of Mitral Valve Regurgitation by Three-Dimensional Color Doppler Transesophageal Echocardiography.

BACKGROUND: The aim of this study was to evaluate the accuracy of mitral regurgitation (MR) volume quantified on three-dimensional (3D) color Doppler transesophageal echocardiography (TEE) using new semiautomated software compared with conventional two-dimensional (2D) proximal isovelocity surface area (PISA) transthoracic echocardiography (TTE) and TEE and cardiac magnetic resonance imaging (CMR). METHODS: Fifty-one patients (mean age, 63 ± 16 years; 35 men) prospectively underwent TTE, TEE, and CMR for MR evaluation. Regurgitant volume (RVol) by 3D MR flow quantification was compared with 2D TTE, TEE, and CMR, and the accuracy of evaluation of severe MR by 3D MR flow quantification was compared against guideline criteria by TEE. RESULTS: Twenty-nine patients had severe MR, 16 had moderate MR, and six had mild MR. Three-dimensional MR flow quantification was feasible in all patients, including prolapse (n = 37), restriction (n = 9), functional MR (n = 5), and eccentric or multiple jects (n = 41). RVol on 3D MR flow quantification correlated well with RVol on 2D PISA TTE (interclass correlation coefficient [ICC] = 0.75, P < .001), quantitatively estimated RVol (ICC = 0.74, P < .001), and 2D PISA TEE (ICC = 0.79, P < .001). Three-dimensional MR flow quantification agreed better with CMR (ICC = 0.86, P < .001) than did RVol on 2D PISA TTE (ICC = 0.66, P < .001) and 2D PISA TEE (ICC = 0.69, P < .001), with narrower limits of agreement on Bland-Altman analysis. Three-dimensional MR flow quantification had high accuracy for diagnosing severe MR using TEE (area under the curve = 0.85, 95% CI 0.74-0.96, P < .001) or CMR (area under the curve = 0.95; 95% CI, 0.89-1.00; P < .001) as the criterion. CONCLUSIONS: The new software enabled semiautomated 3D MR flow quantification in complex MR with multiple and eccentric jets and showed better agreement with CMR than 2D PISA TTE or TEE, suggesting that this method is more accurate than conventional 2D PISA TTE and TEE.

Perfusion estimation using contrast-enhanced 3-dimensional subharmonic ultrasound imaging: an in vivo study.

OBJECTIVES: The ability to estimate tissue perfusion (in milliliter per minute per gram) in vivo using contrast-enhanced 3-dimensional (3D) harmonic and subharmonic ultrasound imaging was investigated. MATERIALS AND METHODS: A LOGIQ™ 9 scanner (GE Healthcare, Milwaukee, WI) equipped with a 4D10L probe was modified to perform 3D harmonic imaging (HI; f(transmit), 5 MHz and f(receive), 10 MHz) and subharmonic imaging (SHI; f(transmit), 5.8 MHz and f(receive), 2.9 MHz). In vivo imaging was performed in the lower pole of both kidneys in 5 open-abdomen canines after injection of the ultrasound contrast agent (UCA) Definity (Lantheus Medical Imaging, N Billerica, MA). The canines received a 5-µL/kg bolus injection of Definity for HI and a 20-µL/kg bolus for SHI in triplicate for each kidney. Ultrasound data acquisition was started just before the injection of UCA (to capture the wash-in) and continued until washout. A microvascular staining technique based on stable (nonradioactive) isotope-labeled microspheres (Biophysics Assay Laboratory, Inc, Worcester, MA) was used to quantify the degree of perfusion in each kidney (the reference standard). Ligating a surgically exposed branch of the renal arteries induced lower perfusion rates. This was followed by additional contrast-enhanced imaging and microsphere injections to measure post-ligation perfusion. Slice data were extracted from the 3D ultrasound volumes and used to generate time-intensity curves offline in the regions corresponding to the tissue samples used for microvascular staining. The midline plane was also selected from the 3D volume (as a quasi-2-dimensional [2D] image) and compared with the 3D imaging modes. Perfusion was estimated from the initial slope of the fractional blood volume uptake (for both HI and SHI) and compared with the reference standard using linear regression analysis. RESULTS: Both 3D HI and SHI were able to provide visualization of flow and, thus, perfusion in the kidneys. However, SHI provided near-complete tissue suppression and improved visualization of the UCA flow. Microsphere perfusion data were available for 4 canines (1 was excluded because of an error with the reference blood sample) and showed a mean (SD) perfusion of 9.30 (6.60) and 5.15 (3.42) mL/min per gram before and after the ligation, respectively. The reference standard showed significant correlation with the overall 3D HI perfusion estimates (r = 0.38; P = 0.007), but it correlated more strongly with 3D SHI (r = 0.62; P < 0.001). In addition, these results showed an improvement over the quasi-2D HI and SHI perfusion estimates (r = -0.05 and r = 0.14) and 2D SHI perfusion estimates previously reported by our group (r = 0.57). CONCLUSIONS: In this preliminary study, 3D contrast-enhanced nonlinear ultrasound was able to quantify perfusion in vivo. Three-dimensional SHI resulted in better overall agreement with the reference standard than 3D HI did and was superior to previously reported 2D SHI results. Three-dimensional SHI outperforms the other methods for estimating blood perfusion because of the improved visualization of the complete perfused vascular networks.

Chronic liver disease: noninvasive subharmonic aided pressure estimation of hepatic venous pressure gradient.

PURPOSE: To compare subharmonic aided pressure estimation (SHAPE) with pressure catheter-based measurements in human patients with chronic liver disease undergoing transjugular liver biopsy. MATERIALS AND METHODS: This HIPAA-compliant study had U.S. Food and Drug Administration and institutional review board approval, and written informed consent was obtained from all participants. Forty-five patients completed this study between December 2010 and December 2011. A clinical ultrasonography (US) scanner was modified to obtain SHAPE data. After transjugular liver biopsy with pressure measurements as part of the standard of care, 45 patients received an infusion of a microbubble US contrast agent and saline. During infusion, SHAPE data were collected from a portal and hepatic vein and were compared with invasive measurements. Correlations between data sets were determined by using the Pearson correlation coefficient, and statistical significance between groups was determined by using the Student t test. RESULTS: The 45 study patients included 27 men and 18 women (age range, 19-71 years; average age, 55.8 years). The SHAPE gradient between the portal and hepatic veins was in good overall agreement with the hepatic venous pressure gradient (HVPG) (R = 0.82). Patients at increased risk for variceal hemorrhage (HVPG ≥ 12 mm Hg) had a significantly higher mean subharmonic gradient than patients with lower HVPGs (1.93 dB ± 0.61 [standard deviation] vs -1.47 dB ± 0.29, P < .001), with a sensitivity of 100% and a specificity of 81%, indicating that SHAPE may be a useful tool for the diagnosis of clinically important portal hypertension. CONCLUSION: Preliminary results show SHAPE to be an accurate noninvasive technique for estimating portal hypertension.

Investigating the efficacy of subharmonic aided pressure estimation for portal vein pressures and portal hypertension monitoring.

The efficacy of using subharmonic emissions from Sonazoid microbubbles (GE Healthcare, Oslo, Norway) to track portal vein pressures and pressure changes was investigated in 14 canines using either slow- or high-flow models of portal hypertension (PH). A modified Logiq 9 scanner (GE Healthcare, Milwaukee, WI, USA) operating in subharmonic mode (f(transmit): 2.5 MHz, f(receive): 1.25 MHz) was used to collect radiofrequency data at 10-40% incident acoustic power levels with 2-4 transmit cycles (in triplicate) before and after inducing PH. A pressure catheter (Millar Instruments, Inc., Houston, TX, USA) provided reference portal vein pressures. At optimum insonification, subharmonic signal amplitude changes correlated with portal vein pressure changes; r ranged from -0.82 to -0.94 and from -0.70 to -0.73 for PH models considered separately or together, respectively. The subharmonic signal amplitudes correlated with absolute portal vein pressures (r: -0.71 to -0.79). Statistically significant differences between subharmonic amplitudes, before and after inducing PH, were noted (p ≤ 0.01). Portal vein pressures estimated using subharmonic aided pressure estimation did not reveal significant differences (p > 0.05) with respect to the pressures obtained using the Millar pressure catheter. Subharmonic-aided pressure estimation may be useful clinically for portal vein pressure monitoring.

Three-dimensional subharmonic ultrasound imaging in vitro and in vivo.

RATIONALE AND OBJECTIVES: Although contrast-enhanced ultrasound imaging techniques such as harmonic imaging (HI) have evolved to reduce tissue signals using the nonlinear properties of the contrast agent, levels of background suppression have been mixed. Subharmonic imaging (SHI) offers near complete tissue suppression by centering the receive bandwidth at half the transmitting frequency. The aims of this study were to demonstrate the feasibility of three-dimensional (3D) SHI and to compare it to 3D HI. MATERIALS AND METHODS: Three-dimensional HI and SHI were implemented on a Logiq 9 ultrasound scanner with a 4D10L probe. Four-cycle SHI was implemented to transmit at 5.8 MHz and receive at 2.9 MHz, while two-cycle HI was implemented to transmit at 5 MHz and receive at 10 MHz. The ultrasound contrast agent Definity was imaged within a flow phantom and the lower pole of two canine kidneys in both HI and SHI modes. Contrast-to-tissue ratios and rendered images were compared offline. RESULTS: SHI resulted in significant improvement in contrast-to-tissue ratios relative to HI both in vitro (12.11 ± 0.52 vs 2.67 ± 0.77, P< .001) and in vivo (5.74 ± 1.92 vs 2.40 ± 0.48, P = .04). Rendered 3D subharmonic images provided better tissue suppression and a greater overall view of vessels in a flow phantom and canine renal vasculature. CONCLUSIONS: The successful implementation of SHI in 3D allows imaging of vascular networks over a heterogeneous sample volume and should improve future diagnostic accuracy. Additionally, 3D SHI provides improved contrast-to-tissue ratios relative to 3D HI.

Harmonic source wavefront aberration correction for ultrasound imaging.

A method is proposed which uses a lower-frequency transmit to create a known harmonic acoustical source in tissue suitable for wavefront correction without a priori assumptions of the target or requiring a transponder. The measurement and imaging steps of this method were implemented on the Duke phased array system with a two-dimensional (2-D) array. The method was tested with multiple electronic aberrators [0.39π to 1.16π radians root-mean-square (rms) at 4.17 MHz] and with a physical aberrator 0.17π radians rms at 4.17 MHz) in a variety of imaging situations. Corrections were quantified in terms of peak beam amplitude compared to the unaberrated case, with restoration between 0.6 and 36.6 dB of peak amplitude with a single correction. Standard phantom images before and after correction were obtained and showed both visible improvement and 14 dB contrast improvement after correction. This method, when combined with previous phase correction methods, may be an important step that leads to improved clinical images.

A simple microindentation technique for mapping the microscale compliance of soft hydrated materials and tissues.

Several recent studies have shown that cells respond to the elastic modulus and elasticity gradients on soft substrates. However, traditional macroscale methods for measuring elastic modulus cannot resolve elastic gradients or differences between the macroscale and microscale elastic modulus of layered tissues. Here, we present a technique for measurement of the microscale elastic modulus of soft, hydrated gels and tissues. This technique requires less equipment than equivalent atomic force microscopy (AFM) and can easily measure larger samples with high adhesiveness. We validate this technique by measuring the microscale modulus of a hydrogel with elasticity that does not depend on measurement scale. We show that the elastic modulus measured using microindentation correlates with measurements using AFM and the macroscale tensile modulus. We verified the ability of this technique to characterize a hydrogel with an elastic gradient of 2.2 kPa/mm across 19 mm and to measure the microscale elastic modulus of the endothelial side of human greater saphenous vein, which is an order of magnitude less than the whole vein macroscale modulus. This simple, inexpensive system allows the measurement of the spatial organization of microscale elastic properties of fully hydrated, soft gels and tissues as a routine laboratory technique.