Miklós Gyöngy

Variation of ultrasound image lateral spectrum with assumed speed of sound and true scatterer density

One method of estimating sound speed in diagnostic ultrasound imaging consists of choosing the speed of sound that generates the sharpest image, as evaluated by the lateral frequency spectrum of the squared B-mode image. In the current work, simulated and experimental data on a typical (47 mm aperture, 3.3-10.0 MHz response) linear array transducer are used to investigate the accuracy of this method. A range of candidate speeds of sound (1240-1740 m/s) was used, with a true speed of sound of 1490 m/s in simulations and 1488 m/s in experiments. Simulations of single point scatterers and two interfering point scatterers at various locations with respect to each other gave estimate errors of 0.0-2.0%. Simulations and experiments of scatterer distributions with a mean scatterer spacing of at least 0.5 mm gave estimate errors of 0.1-4.0%. In the case of lower scatterer spacing, the speed of sound estimates become unreliable due to a decrease in contrast of the sharpness measure between different candidate speeds of sound. This suggests that in estimating speed of sound in tissue, the region of interest should be dominated by a few, sparsely spaced scatterers. Conversely, the decreasing sensitivity of the sharpness measure to speed of sound errors for higher scatterer concentrations suggests a potential method for estimating mean scatterer spacing.

Cavitation-enhanced delivery of insulin in agar and porcine models of human skin.

Ultrasound-assisted transdermal insulin delivery offers a less painful and less invasive alternative to subcutaneous insulin injections. However, ultrasound-based drug delivery, otherwise known as sonophoresis, is a highly variable phenomenon, in part dependent on cavitation. The aim of the current work is to investigate the role of cavitation in transdermal insulin delivery. Fluorescently stained, soluble Actrapid insulin was placed on the surface of human skin-mimicking materials subjected to 265 kHz, 10% duty cycle focused ultrasound. A confocally and coaxially aligned 5 MHz broadband ultrasound transducer was used to detect cavitation. Two different skin models were used. The first model, 3% agar hydrogel, was insonated with a range of pressures (0.25-1.40 MPa peak rarefactional focal pressure-PRFP), with and without cavitation nuclei embedded within the agar at a concentration of 0.05% w/v. The second, porcine skin was insonated at 1.00 and 1.40 MPa PRFP. In both models, fluorescence measurements were used to determine penetration depth and concentration of delivered insulin. Results show that in agar gel, both insulin penetration depth and concentration only increased significantly in the presence of inertial cavitation, with up to a 40% enhancement. In porcine skin the amount of fluorescent insulin was higher in the epidermis of those samples that were exposed to ultrasound compared to the control samples, but there was no significant increase in penetration distance. The results underline the importance of instigating and monitoring inertial cavitation during transdermal insulin delivery.

Experimental validation of a convolution- based ultrasound image formation model using a planar arrangement of micrometer-scale scatterers.

The shift-invariant convolution model of ultrasound is widely used in the literature, for instance to generate fast simulations of ultrasound images. However, comparison of the resulting simulations with experiments is either qualitative or based on aggregate descriptors such as envelope statistics or spectral components. In the current work, a planar arrangement of 49-μm polystyrene microspheres was imaged using macrophotography and a 4.7-MHz ultrasound linear array. The macrophotograph allowed estimation of the scattering function (SF) necessary for simulations. Using the coefficient of determination R2 between real and simulated ultrasound images, different estimates of the SF and point spread function (PSF) were tested. All estimates of the SF performed similarly, whereas the best estimate of the PSF was obtained by Hanningwindowing the deconvolution of the real ultrasound image with the SF: this yielded R2 = 0.43 for the raw simulated image and R2 = 0.65 for the envelope-detected ultrasound image. R2 was highly dependent on microsphere concentration, with values of up to 0.99 for regions with scatterers. The results validate the use of the shift-invariant convolution model for the realistic simulation of ultrasound images. However, care needs to be taken in experiments to reduce the relative effects of other sources of scattering such as from multiple reflections, either by increasing the concentration of imaged scatterers or by more careful experimental design.

Histology-Based Simulations of Ultrasound Imaging: Methodology

Simulations of ultrasound (US) images based on histology may shed light on the process by which microscopic tissue features translate to a US image and may enable predictions of feature detectability as a function of US system parameters. This technical note describes how whole-slide hematoxylin and eosin-stained histology images can be used to generate maps of fractional change in bulk modulus, whose convolution with the impulse response of the US system yields simulated US images. The method is illustrated by two canine mastocytoma histology images, one with and the other without signs of intra-operative hemorrhaging. Quantitative comparisons of the envelope statistics with corresponding clinical US images provide preliminary validation of the method.

Comparison of Two Inexpensive Rapid Prototyping Methods for Manufacturing Filament Target Ultrasound Phantoms

Current use of 3-D printers to manufacture ultrasound phantoms is limited to relatively expensive photopolymer jetting printers. The present work investigates the feasibility of using two common and inexpensive 3-D printer technologies, fused deposition modeling (FDM) and digital light processing (DLP), to print custom filament target phantoms. Acoustic characteristics obtained from printed solid blocks indicated that the printing materials-acrylonitrile butadiene styrene and polylactic acid for FDM and a photopolymer for DLP printing-were appropriate for use as scatterers. A regular grid of filaments was printed to study printing accuracy. As a proof of concept of the phantom manufacturing process, a complex pattern of filament targets was placed in de-ionized water to create a phantom, which was then imaged using an ultrasound imager. The pattern was clearly identifiable, although multiple reflections were observed, which underscores the importance of future work to enhance printing resolution. This goal is deemed possible using improvement of the DLP printing setup.

Automated classification of common skin lesions using bioinspired features

Ultrasound (US) imaging of skin lesions provides information supplementary to dermoscopy and helps in improving diagnostic accuracy. The aim of the current work is to explore the feasibility of using ultrasound image features derived from radiological experience to distinguish between common skin lesions. 5-18 MHz B-mode ultrasound images were acquired of incoming patients. Images containing lesions 1-2 mm thick were selected (N=248), with histology used to diagnose suspicious lesions. 73 melanomas, 130 BCC, and 45 nevi were studied. Following semi-automatic segmentation, a number of relevant features expressing the geometry of the lesion boundary and boundary layer, as well as the image characteristics of the lesion, lesion boundary layer, and post-lesion region were considered. With the exception of lesion echogenicity, all features had an area under the curve (AUC) value of above 0.70. The AdaBoost and Support Vector Machine (SVM) classifiers were then trained and tested using cross-validation of 50 random equal populations of melanomas, BCC, and nevi; each population was then 2-fold (holdout) cross-validated 50 times. When detecting one group against two other groups, the detection of cancerous lesions fared best, with an AUC of at least 0.84 and a specificity of at least 19% at 100% sensitivity for both classifiers. The results demonstrate the potential of clinically useful ultrasound-based automatic differential diagnosis of skin lesions, which could perhaps be attained by better segmentation, having more training data, using several images of the same lesion when performing classification, as well as refinements in the definition of image features.

Temperature increase induced by modulated electrohyperthermia (oncothermia®) in the anesthetized pig liver

AIM OF STUDY Is to show the intrahepatic temperature development in anesthetized pig. MATERIALS AND METHODS Temperature development in the liver of anesthetized pig is measured to study the thermal effects of capacitive coupled energy transfer. The treatment was made by modulated electrohyperthermia (mEHT, trade name: oncothermia ®), controlled by a fluoroptical temperature sensing positioned by the ultrasound-guided process. Various fits of coupling were studied. RESULTS The intrahepatic temperature at the end of the treatment ranged 40.5-44.8°C, while the skin temperature ranged 36.8-41.8°C depending on the coupling arrangement. CONCLUSION mEHT is a feasible method to deliver deep heat to the liver of an anesthetized pig.

Temperature dependence of speed of sound and attenuation of porcine left ventricular myocardium

HIGHLIGHTSTemperature‐dependency of porcine myocardium acoustic parameters was investigated.Slope of phase velocity with temperature was measured as 1.10 ± 0.04 m/s/°C.Slope of attenuation was estimated as −0.11 ± 0.04 dB/cm/°C at 10 MHz.Attenuation power law coefficient was estimated as n = 1.434 ± 0.025.Speed of sound dispersion accurately predicted from Kramers‐Kronig relations. ABSTRACT The temperature dependence of soft tissue acoustic properties is relevant for monitoring tissue hyperthermia. In the current work, the propagation speed and attenuation of healthy porcine left ventricular myocardium (N = 5) was investigated in a frequency range relevant for clinical diagnostic imaging, i.e. 2.5–13.0 MHz. Each tissue sample was held in a water bath at a temperature T = 25 °C, heated to 45 °C, and allowed to cool back down to 25 °C. Due to initial tissue swelling, the data for decreasing temperatures was considered more reliable. In this case, the slope of the phase velocity versus temperature relation was measured to be 1.10 ± 0.04 m/s/°C, and the slope of the attenuation was −0.11 ± 0.04 dB/cm/°C at 10 MHz, or −0.0041 ± 0.0015 dB/cm/MHz1.4336/°C as a function of frequency.

Validation of image restoration methods on 3D-printed ultrasound phantoms

The resolution of ultrasound images is limited by the bandwidth of the imaging system and the features of the propagating medium. Using certain assumptions, image restoration can recover out-of-bandwidth data and improve resolution. Several resolution improvement methods have been reported in the literature. However, due to the lack of ground truth, their evaluation on experimental data remains an open issue. Indeed, to evaluate the performance of such methods, knowledge of the scattering function is necessary. Usually this is achieved with numerical simulations, since in traditional phantoms the exact distribution of scatterers is unknown. In the current work, based on a 3D-printed phantom, the feasibility of the evaluation of deconvolution is investigated. The deconvolution method used lp-norm-regularization terms with p=0.5 and p=2. Knowledge of the scattering function allows comparison of the deconvolved images with the ground truth. Thus, using the scattering function and the originally acquired B-mode image, performance of image restoration methods could be evaluated quantitatively through comparison of root mean square error and full width half maximum values. Preliminary results demonstrate the benefits of knowing the scattering function during experimental testing of image restoration methods. In summary, the current work shows the potential of an experimental method for evaluating the extent to which an image restoration method provides a faithful rendering of the underlying scattering structure.