C.L. de Korte

Noninvasive Carotid Strain Imaging Using Angular Compounding at Large Beam Steered Angles: Validation in Vessel Phantoms

Stroke and myocardial infarction are initiated by rupturing vulnerable atherosclerotic plaques. With noninvasive ultrasound elastography, these plaques might be detected in carotid arteries. However, since the ultrasound beam is generally not aligned with the radial direction in which the artery pulsates, radial and circumferential strains need to be derived from axial and lateral data. Conventional techniques to perform this conversion have the disadvantage that lateral strain is required. Since the lateral strain has relatively poor accuracy, the quality of the radial and circumferential strains is reduced. In this study, the radial and circumferential strain estimates are improved by combining axial strain data acquired at multiple insonification angles. Adaptive techniques to correct for grating lobe interference and other artifacts that occur when performing beam steering at large angles are introduced. Acquisitions at multiple angles are performed with a beam steered linear array. For each beam steered angle, there are two spatially restricted regions of the circular vessel cross section where the axial strain is closely aligned with the radial strain and two spatially restricted regions (different from the radial strain regions) where the axial strain is closely aligned with the circumferential strain. These segments with high quality strain estimates are compounded to form radial or circumferential strain images. Compound radial and circumferential strain images were constructed for a homogeneous vessel phantom with a concentric lumen subjected to different intraluminal pressures. Comparison of the elastographic signal-to-noise ratio (SNRe) and contrast-to-noise ratio ( CNRe) revealed that compounding increases the image quality considerably compared to images from 0deg information only. SNRe and CNRe increase up to 2.7 and 6.6 dB, respectively. The highest image quality was achieved by projecting axial data, completed with a small segment determined by either principal component analysis or by application of a rotation matrix.

Full 2D displacement vector and strain tensor estimation for superficial tissue using beam-steered ultrasound imaging.

Ultrasound strain imaging is used to measure local tissue deformations. Usually, only strains along the ultrasound beam are estimated, because those estimates are most precise, due to the availability of phase information. For estimating strain in other directions we propose to steer the ultrasound beam at an angle, which allows estimating different projections of the 2D strain tensor, while phase information remains available. This study investigates beam steering at maximally three different angles to determine the full 2D strain tensor. The method was tested on simulated and experimental data of an inclusion phantom and a vessel phantom. The combination of data from a non-steered acquisition and acquisitions at a large positive and an equally large but negative steering angle enabled the most precise estimation of the strain components. The method outperforms conventional methods that do not use beam steering.

Computer-aided B-mode ultrasound diagnosis of hepatic steatosis: a feasibility study

Fatty liver (steatosis) occurs in obese patients, among others, and is related to the development of diabetes type-2. Timely diagnosis of steatosis is therefore of great importance. Steatosis is also the most common liver disease of high-yielding dairy cattle during early lactation. This makes it a suitable animal model for studying liver steatosis. Furthermore, reference of derived ultrasound parameters against a gold standard is possible in cattle by taking a liver biopsy for the assessment of fat concentration. The authors undertook this pilot study to investigate the hypothesis that quantitative, computer-aided B- mode ultrasound enables the noninvasive detection of hepatic steatosis. Echographic images were obtained postpartum from dairy cows (n = 12) in transcutaneous and direct (intraoperative) applications using a convex array transducer at 4.2 MHz. During surgery, a biopsy was taken from the caudate lobe to assess the liver fat content (fat score). A custom-designed software package for computer-aided ultrasound diagnosis (CAUS) was developed. After linearizing the post-processing look-up-table (LUT), the image gray levels were transferred into echo levels in decibels relative to the mean echo level in a tissue-mimicking phantom. The quantitative comparison of transcutaneous and intraoperative images enabled the correction for the attenuation effect of skin and subcutaneous fat layer on the mean echo level in the liver, as well as for the effects of the beam formation and attenuation of liver tissue on the echo level vs. depth. The residual attenuation coefficient (dB/cm) in fatty liver vs. normal liver was estimated and compensated for. Finally, echo level was estimated relative to the phantom used for calibration, and echo texture was characterized by the mean axial and lateral speckle size within the regions of interest. In the no fat/low fat group (n = 5) skin plus fat layer attenuation was 3.4 dB/cm. A correlation of skin layer thickness vs. fat score of r = 0.48 was found. The mean transcutaneous liver tissue echo level correlated well with fat score: r = 0.80. A residual liver attenuation coefficient of 0.76 dB/cm and 1.19 dB/cm was found in medium and high fat liver, respectively. In transcutaneous images, correlation of residual attenuation coefficient with fat score was r = 0.69. Axial and lateral speckle sizes were on the order of 0.2 and 1.0 cm, respectively, and no correlation was found with liver fat content. Results for transcutaneous and intraoperative images were similar. The authors conclude that this pilot study shows the feasibility of calibrated, computer-aided ultrasound for noninvasively diagnosing, possibly even screening, steatosis of the liver.

Ultrafast vascular strain compounding using plane wave transmission

Deformations of the atherosclerotic vascular wall induced by the pulsating blood can be estimated using ultrasound strain imaging. Because these deformations indirectly provide information on mechanical plaque composition, strain imaging is a promising technique for differentiating between stable and vulnerable atherosclerotic plaques. This paper first explains 1-D radial strain estimation as applied intravascularly in coronary arteries. Next, recent methods for noninvasive vascular strain estimation in a transverse imaging plane are discussed. Finally, a compounding technique that our group recently developed is explained. This technique combines motion estimates of subsequently acquired focused ultrasound images obtained at various insonification angles. However, because the artery moves and deforms during the multi-angle acquisition, errors are introduced when compounding. Recent advances in computational power have enabled plane wave ultrasound acquisition, which allows 100 times faster image acquisition and thus might resolve the motion artifacts. In this paper the performance of strain imaging using plane wave compounding is investigated using simulations of an artery with a vulnerable plaque and experimental data of a two-layered vessel phantom. The results show that plane wave compounding outperforms 0° focused strain imaging. For the simulations, the root mean squared error reduced by 66% and 50% for radial and circumferential strain, respectively. For the experiments, the elastographic signal-to-noise and contrast-to-noise ratio (SNR(e) and CNR(e)) increased with 2.1 dB and 3.7 dB radially, and 5.6 dB and 16.2dB circumferentially. Because of the high frame rate, the plane wave compounding technique can even be further optimized and extended to 3D in future.

Noninvasive detection of hepatic lipidosis in dairy cows with calibrated ultrasonographic image analysis.

The aim was to test the accuracy of calibrated digital analysis of ultrasonographic hepatic images for diagnosing fatty liver in dairy cows. Digital analysis was performed by means of a novel method, computer-aided ultrasound diagnosis (CAUS), previously published by the authors. This method implies a set of pre- and postprocessing steps to normalize and correct the transcutaneous ultrasonographic images. Transcutaneous hepatic ultrasonography was performed before surgical correction on 151 German Holstein dairy cows (mean +/- standard error of the means; body weight: 571+/-7 kg; age: 4.9+/-0.2 yr; DIM: 35+/-5) with left-sided abomasal displacement. Concentration of triacylglycerol (TAG) was biochemically determined in liver samples collected via biopsy and values were considered the gold standard to which ultrasound estimates were compared. According to histopathologic examination of biopsies, none of the cows suffered from hepatic disorders other than hepatic lipidosis. Hepatic TAG concentrations ranged from 4.6 to 292.4 mg/g of liver fresh weight (FW). High correlations were found between the hepatic TAG and mean echo level (r=0.59) and residual attenuation (ResAtt; r=0.80) obtained in ultrasonographic imaging. High correlation existed between ResAtt and mean echo level (r=0.76). The 151 studied cows were split randomly into a training set of 76 cows and a test set of 75 cows. Based on the data from the training set, ResAtt was statistically selected by means of stepwise multiple regression analysis for hepatic TAG prediction (R(2)=0.69). Then, using the predicted TAG data of the test set, receiver operating characteristic analysis was performed to summarize the accuracy and predictive potential of the differentiation between various measured hepatic TAG values, based on TAG predicted from the regression formula. The area under the curve values of the receiver operating characteristic based on the regression equation were 0.94 (<50 vs. >or=50mg of TAG/g of FW), 0.83 (<100 vs. >or=100mg of TAG/g of FW), and 0.97 (<50 vs. >or=100mg of TAG/g of FW). The CAUS methodology and software for digitally analyzing liver ultrasonographic images is considered feasible for noninvasive screening of fatty liver in dairy herd health programs. Using the single parameter linear regression equation might be ideal for practical applications.

Predicting Target Displacements Using Ultrasound Elastography and Finite Element Modeling

Soft tissue displacements during minimally invasive surgical procedures may cause target motion and subsequent misplacement of the surgical tool. A technique is presented to predict target displacements using a combination of ultrasound elastography and finite element (FE) modeling. A cubic gelatin/agar phantom with stiff targets was manufactured to obtain pre- and post-loading ultrasound radio frequency (RF) data from a linear array transducer. The RF data were used to compute displacement and strain images, from which the distribution of elasticity was reconstructed using an inverse FE-based approach. The FE model was subsequently used to predict target displacements upon application of different boundary and loading conditions to the phantom. The influence of geometry was investigated by application of the technique to a breast-shaped phantom. The distribution of elasticity in the phantoms as determined from the strain distribution agreed well with results from mechanical testing. Upon application of different boundary and loading conditions to the cubic phantom, the FE model-predicted target motion were consistent with ultrasound measurements. The FE-based approach could also accurately predict the displacement of the target upon compression and indentation of the breast-shaped phantom. This study provides experimental evidence that organ geometry and boundary conditions surrounding the organ are important factors influencing target motion. In future work, the technique presented in this paper could be used for preoperative planning of minimally invasive surgical interventions.

2H-1 In Vivo 3D Cardiac and Skeletal Muscle Strain Estimation

In this study, BiPlane imaging was adapted for measuring strain in actively deforming tissue in three orthogonal directions. BiPlane imaging assures a sufficient frame rate (75-120 Hz) for accurate strain estimation. A coarse-to-fine iterative 2D strain algorithm using spatial correction and local stretching was implemented. Considering the huge amount of generated data, a fast interpolation scheme was implemented for measuring sub-sample and sub-line displacements. Assuming a 2D parabolic shape of the cross-correlation function, a straightforward and direct calculation of the displacements is possible. The strain estimation method was validated by means of a simulation study and phantom experiments. Rf-data were acquired with a 3D X4 matrix array transducer (Philips Sonos 7500) in BiPlane mode. In vivo verification in human skeletal muscle was performed. Furthermore, cardiac strain imaging was conducted using cardiac BiPlane data of dogs. In a pilot animal study, beagles with an induced valvular aortic stenosis were monitored. The Field II simulation was used for determining the accuracy and detectibility of the algorithm and revealed excellent correlation between applied and measured axial strain (SNR = 43 dB) for a window of 0.60 mm. Obviously, a lower SNR was found in lateral and elevational direction. The in vivo verification experiment in the skeletal muscles revealed similar cumulative axial strain curves (up to 8%) in both the azimuth and elevational direction. The shape of the strain curve matched perfectly with the curve of the measured force. The lateral strain values parallel to the direction of the muscle fibers matched the axial strain curves, whereas the shape of the lateral strain in the perpendicular plane differed due to anisotropy. Finally, strain images of the beagles were calculated. The beagle with the most excessive pressure gradient revealed a decrease of the radial strain. Furthermore, an elongated plateau in the radial strain indicated hypertrophy. In conclusion, 3D cardiac and strain estimation is feasible using a real-time 3D scanner. Additional validation studies of full 3D imaging modes are required to fully validate the technique

Increased prevalence of testicular adrenal rest tumours during adolescence in congenital adrenal hyperplasia.

Testicular adrenal rest tumours (TART) are one of the most important causes of infertility in adult male patients with congenital adrenal hyperplasia (CAH). These benign tumours are already detected in children, but screening of TART is not routinely performed. Objective: To define retrospectively the prevalence of TART in 41 paediatric male CAH patients aged 0-19 years regularly followed by high-frequency (Fcentral 12-MHz) ultrasound techniques. Results: Above the age of 10 years, there was a clear increase in the prevalence of TART: 10-12 years, 28% (2 of 7 patients), 13-14 years, 50% (4/8), and 15-16 years, 75% (3/4). Above the age of 16 years, TART were detected in 100% of the patients (7/7). The tumours were not detectable by palpation. Conclusion: TART is already present in childhood with an increasing prevalence after onset of puberty. We recommend regular ultrasound from the onset of puberty in all boys with classic CAH.

10B-4 4D Cardiac Strain Imaging: Methods and Initial Results

In this study, four-dimensional (3D+t) ultrasound imaging techniques were used for the development and in vivo verification of 3D strain imaging. Two different iterative coarse- to-fine 3D strain estimation methods were developed. One method was based on measuring displacements using 3D kernels and a 3D cross-correlation function. The second method used 2D kernels and cross-correlation, and estimated 3D displacements in an iterative process. A 3D or 2D parabolic interpolation was used for sub-sample displacement estimates. The strain estimation methods were experimentally validated using a gelatin phantom with a hard cylindrical inclusion (four times stiffer). The phantom was compressed with a plate in steps of 0.5 mm up to 3.0 mm (3% strain). Rf-data were acquired with a 3D matrix array transducer (X4, Philips Sonos 7500) in ECG-triggered 3D full volume mode. Preliminary in vivo validation was performed by acquiring 3D full volume data (frame rate = 19 Hz) of the left ventricle of a trained athlete. Both methods were able to produce high quality elastograms of the inclusion model up to an applied compression of 3% strain (resulting in 0.5% - 5% axial strain in the phantom). No significant difference in elastographic signal-to- noise ratio (SNRe) was found between the two methods. The iterative 2D algorithm is favored for the shorter computation time. The signal- and contrast-to-noise ratios (SNRe, CNRe) of the axial elastograms increased to 28 and 53 dB, respectively (compared to previously described BiPlane axial strain images). Lateral and elevational elastograms were also in accordance with finite element solutions of the phantom model. However, the SNRe and CNRe were considerably lower (16 and 33 dB), which is presumably caused by the lower in-plane spatial resolution of the 3D full volume data. Initial in vivo results revealed mean strain profiles in three orthogonal directions comparable with our previous studies, although, the maximum radial strain was lower than expected (20%). Hence, 3D cardiac strain imaging is feasible even at a relatively low frame rate.

Non-invasive staging of hepatic steatosis using computer-aided ultrasound diagnosis

In this study, a computer-aided ultrasound (CAUS) diagnostic method for the detection and staging of hepatic steatosis (non-alcoholic fatty liver disease, NAFLD) is investigated using a bovine model (n=151). In humans as well as in cows, hepatic steatosis increases the risk of co morbidity. Assessment of liver fat content is mostly done by taking liver biopsies. The authors goal was to estimate the liver fat content by using echographic parameters, i.e., non-invasively. Since skin and subcutaneous fat layer influence the characteristics of echographic B-mode liver-images, both transcutaneous and intraoperative images were acquired to study this effect. During image acquisition a fixed preset of the ultrasound equipment controls was used. One liver biopsy was taken from each animal to stage the NAFLD. Apart from the fat percentage, the triglyceride (TG) level was biochemically assessed and used in this study. The software package CAUS was developed to perform objective and unambiguous analysis on echographic B-mode images. Prior to image analysis, certain preprocessing steps were performed in order to achieve the relative echo strength in decibel (dB), rather than image gray level, as a quantitative parameter. Furthermore, corrections were made for ultrasound propagation through skin/fat layer and through an average ldquohealthy liverrdquo (automatic gain control, AGC). After these corrections, the estimated echographic parameters were correlated to the TG level by uni- and multivariate linear regression analysis. The regression formula was then used to predict the TG level in each animal. A retrospective classification was performed and ROC curves were obtained. High correlations with the liver TG score were found for several echographic and image parameters. ROC curve analysis show promising results for sensitivity (0.93), specificity (0.86) and area under the curve (0.93) in distinguishing fatty livers from healthy livers. This study showed the feasibility of computer-aided ultrasound for non-invasively diagnosing, or even screening, of liver steatosis.