Marco M. Voormolen

Microbubble spectroscopy of ultrasound contrast agents

A new optical characterization of the behavior of single ultrasound contrast bubbles is presented. The method consists of insonifying individual bubbles several times successively sweeping the applied frequency, and to record movies of the bubble response up to 25 million frames/s with an ultrahigh speed camera operated in a segmented mode. The method, termed microbubble spectroscopy, enables to reconstruct a resonance curve in a single run. The data is analyzed through a linearized model for coated bubbles. The results confirm the significant influence of the shell on the bubble dynamics: shell elasticity increases the resonance frequency by about 50%, and shell viscosity is responsible for about 70% of the total damping. The obtained value for shell elasticity is in quantative agreement with previously reported values. The shell viscosity increases significantly with the radius, revealing a new nonlinear behavior of the phospholipid coating.

Guiding and optimization of resynchronization therapy with dynamic three-dimensional echocardiography and segmental volume–time curves: a feasibility study

To assess a new approach for guiding and hemodynamic optimization of resynchronization therapy, using three‐dimensional (3D) transthoracic echocardiography.

Assessment of left ventricular function by three-dimensional echocardiography.

Accurate determination of LV volume, ejection fraction and segmental wall motion abnormalities is important for clinical decision-making and follow-up assessment. Currently, echocardiography is the most common used method to obtain this information. Three-dimensional echocardiography has shown to be an accurate and reproducible method for LV quantitation, mainly by avoiding the use of geometric assumptions. In this review, we describe various methods to acquire a 3D-dataset for LV volume and wall motion analysis, including their advantages and limitations. We provide an overview of studies comparing LV volume and function measurement by various gated and real-time methods of acquisition compared to magnetic resonance imaging. New technical improvements, such as automated endocardial border detection and contrast enhancement, will make accurate on-line assessment with little operator interaction possible in the near future.

Sparse Registration for Three-Dimensional Stress Echocardiography

Three-dimensional (3-D) stress echocardiography is a novel technique for diagnosing cardiac dysfunction. It involves evaluating wall motion of the left ventricle, by visually analyzing ultrasound images obtained in rest and in different stages of stress. Since the acquisitions are performed minutes apart, variabilities may exist in the visualized cross-sections. To improve anatomical correspondence between rest and stress, aligning the images is essential. We developed a new intensity-based, sparse registration method to retrieve standard anatomical views from 3-D stress images that were equivalent to the manually selected views in the rest images. Using sparse image planes, the influence of common image artifacts could be reduced. We investigated different similarity measures and different levels of sparsity. The registration was tested using data of 20 patients and quantitatively evaluated based on manually defined anatomical landmarks. Alignment was best using sparse registration with two long-axis and two short-axis views; registration errors were reduced significantly, to the range of interobserver variabilities. In 91% of the cases, the registration result was qualitatively assessed as better than or equal to the manual alignment. In conclusion, sparse registration improves the alignment of rest and stress images, with a performance similar to manual alignment. This is an important step towards objective quantification in 3-D stress echocardiography.

Three-Dimensional Echocardiographic Analysis of Left Ventricular Function during Hemodialysis

Background: The effects of hemodialysis (HD) on left ventricular (LV) function have been studied by various echocardiographic techniques (M-mode, 2D echocardiography). These studies are hampered by a low accuracy of measurements because of geometric assumptions regarding LV shape. Three-dimensional echocardiography (3DE) overcomes this limitation. Methods: We tested the feasibility of 3DE assessment of LV function during HD. Conventional biplane Simpson rule (BSR) and single plane area length method (SPM) for LV function analysis were used as a reference. Results: 12 HD patients were studied and in 10 (83%) a total of 80 3D datasets were acquired. In 3 patients, one dataset (4%) was of insufficient quality and excluded from analysis. Correlation between SPM, BSR and 3DE for calculation of end-diastolic (EDV, r = 0.89 and r = 0.92, respectively), end-systolic volume (ESV, r = 0.92 and r = 0.93, respectively) and for ejection fraction (EF, r = 0.90 and r = 0.88, respectively) was moderate. Limits-of-agreement results for EDV and ESV were poor with confidence intervals larger than 30 ml. Both 2DE methods underestimated end-diastolic and end-systolic volume, while overestimating ejection fraction. Conclusion: 3DE is feasible for image acquisition during HD, which opens the possibility for accurate and reproducible measurement of LV function during HD. This may improve the assessment of the acute effect of HD on LV performance, and guide therapeutic strategies aimed at preventing intradialytic hypotension.

Contrast-enhanced three-dimensional dobutamine stress echocardiography: between Scylla and Charybdis?

AIMS Real-time three-dimensional echocardiography (RT3DE) allows quick volumetric scanning of the left ventricle (LV). We evaluated the diagnostic accuracy of contrast-enhanced stress RT3DE for the detection of coronary artery disease (CAD) in comparison with coronary arteriography as the reference technique. METHODS AND RESULTS Forty-five consecutive patients (age 59 +/- 10, 31 males) referred for coronary angiography were examined by contrast-enhanced RT3DE. Wall motion analysis was performed off-line by dedicated software. New or worsening wall motion abnormalities were detected in 17 of 28 patients with significant CAD (sensitivity 61%), and in two of 17 patients without significant CAD (specificity 88%). The sensitivity for detection of single-vessel CAD was 8/15 patients (53%), for two-vessel CAD 4/6 (67%), and for three-vessel CAD 5/7 (71%). In 35 patients, comparison with conventional RT3DE was available. The image quality index at rest improved from 2.5 +/- 1.2 to 3.2 +/- 1.0 (P < 0.001) with contrast and at peak stress from 2.3 +/- 1.2 to 3.1 +/- 1.0 (P < 0.001). Interobserver agreement on the diagnosis of myocardial ischaemia improved from 26 of 35 studies (74%, kappa = 0.44) with conventional stress RT3DE to 30 of 35 studies (86%, kappa = 0.69) with contrast-enhanced stress RT3DE. Sensitivity increased from 50 to 55% and specificity from 69 to 85% with contrast-enhanced stress RT3DE in this subset of patients. CONCLUSION Despite some important practical and theoretical benefits, contrast-enhanced stress RT3DE currently has only moderate diagnostic sensitivity due to several technical limitations as temporal and spatial resolution.

Rapid and Accurate Measurement of Left Ventricular Function with a New Second‐Harmonic Fast‐Rotating Transducer and Semi‐Automated Border Detection

Measurement of left ventricular (LV) volume and function are the most common clinical referral questions to the echocardiography laboratory. A fast, practical, and accurate method would offer important advantages to obtain this important information. To validate a new practical method for rapid measurement of LV volume and function. We developed a continuous fast‐rotating transducer, with second‐harmonic capabilities, for three‐dimensional echocardiography (3DE). Fifteen cardiac patients underwent both 3DE and magnetic resonance imaging (reference method) on the same day. 3DE image acquisition was performed during a 10‐second breath‐hold with a frame rate of 100 frames/sec and a rotational speed of 6 rotations/sec. The individual images were postprocessed with Matlab software using multibeat data fusion. Subsequently, with these images, 12 datasets per cardiac cycle were reconstructed, each comprising seven equidistant cross‐sectional images for analysis in the new TomTec 4DLV analysis software, which uses a semi‐automated border detection (ABD) algorithm. The ABD requires an average analysis time of 15 minutes per patient. A strong correlation was found between LV end‐diastolic volume (r = 0.99; y = 0.95x – 1.14 ml; SEE = 6.5 ml), LV end‐systolic volume (r = 0.96; y = 0.89x + 7.91 ml; SEE = 7.0 ml), and LV ejection fraction (r = 0.93; y = 0.69x + 13.36; SEE = 2.4%). Inter‐ and intraobserver agreement for all measurements was good. The fast‐rotating transducer with new ABD software is a dedicated tool for rapid and accurate analysis of LV volume and function.

Novel spatiotemporal voxel interpolation with multibeat fusion for 3D echocardiography with irregular data distribution

We developed a novel multi-beat image fusion technique using a special spatiotemporal interpolation for sparse, irregularly sampled data (ISI). It is applied to irregularly distributed 3D cardiac ultrasound data acquired with a fast rotating ultrasound (FRU) transducer. ISI is based on Normalized Convolution with Gaussian kernels tuned to irregular beam data spacing over cardiac phase (τ), and beam rotation (θ) and elevation angles (φ). Methods: images are acquired with the FRU transducer developed in our laboratory, a linear phased array rotating mechanically continuously at very high speed (240-480rpm). High-quality 2D images are acquired at ~100 frames/s over 5-10 seconds. ECG is recorded simultaneously. Images are irregularly distributed over τ and θ, because rotation is not synchronized to heartrate. ISI was compared quantitatively to spatiotemporal nearest neighbor interpolation (STNI) on synthetic (distance function) data of a pulsating ellipsoid for 32 angles (θ) and 37 phases (τ). ISI was also tested qualitatively on 20 in-vivo cardiac image sets and compared to classical temporal binning with trilinear voxel interpolation, at resolutions of 256*256*400 for 16 phases. Results: From the synthetic data simulations, ISI showed absolute distance errors (mean±SD) of 1.23 ± 1.52mm; considerably lower than for STNI (3.45 ± 3.03mm). For in-vivo images, ISI voxel sets showed reduced motion artifacts, suppression of noise and interpolation artifacts and better delineation of endocardium. Conclusions: ISI improves the quality of 3D+T images acquired with a fast rotating transducer in simulated and in-vivo data. It may also be useful for similar spatiotemporal irregularly distributed data, e.g. freehand 3D echocardiography.

Semi-automatic border detection method for left ventricular volume estimation in 4D ultrasound data

We propose a semi-automatic endocardial border detection method for LV volume estimation in 3D time series of cardiac ultrasound data. It is based on pattern matching and dynamic programming techniques and operates on 2D slices of the 4D data requiring minimal user-interaction. We evaluated on data acquired with the Fast Rotating Ultrasound (FRU) transducer: a linear phased array transducer rotated at high speed around its image axis, generating high quality 2D images of the heart. We automatically select a subset of 2D images at typically 10 rotation angles and 16 cardiac phases. From four manually drawn contours a 4D shape model and a 4D edge pattern model is derived. For the selected images, contour shape and edge patterns are estimated using the models. Pattern matching and dynamic programming is applied to detect the contours automatically. The method allows easy corrections in the detected 2D contours, to iteratively achieve more accurate models and improved detections. An evaluation of this method on FRU data against MRI was done for full cycle LV volumes on 10 patients. Good correlations were found against MRI volumes [r=0.94, y=0.72x + 30.3, difference of 9.6 +/- 17.4 ml (Av +/- SD)] and a low interobserver variability for US (r=0.94, y=1.11x - 16.8, difference of 1.4 +/- 14.2 ml). On average only 2.8 corrections per patient were needed (in a total of 160 images). Although the method shows good correlations with MRI without corrections, applying these corrections can make significant improvements.

Sparse appearance model based registration of 3D ultrasound images

In this paper, we propose a sparse appearance model based registration algorithm for segmenting 3D echocardiograms. The end-diastolic model is built in 3D sparsely on 2D planes, representing the anatomical 4-chamber, 2-chamber, and short-axis views. Ultrasound specific intensity normalization and shape-based intensity modeling are employed. The model is matched in an intensity-based registration approach, by perturbing appearance and pose parameters simultaneously. Leave-one-out experiments on 10 patients reveal significant improvement in the segmentation using the normalized cross-correlation metric. The registration method will allow fully automatic extraction of the standard views as used in echocardiography. This will aid in the selection of images for inter- and intra-patient comparison and may provide an alternative for a complete 3D AAM.