Diego R. T. Sampaio

Ultrasound-based transient elastography using a magnetic excitation

Magneto-motive ultrasound is an imaging technique where magnetic particles, in tissues, are displaced by an external magnetic field and the resulting displacements are evaluated using ultrasonic echo signals. The magneto-motive ultrasound method has been, for example, used to detect movements of superparamagnetic nanoparticles. However, the viscoelastic properties of tissues greatly affect the ability of magneto-motive force to displace the particles and, consequently, its surrounding medium. Therefore, having knowledge about the elastic properties of the region where the magnetic particles are located is a useful parameter to improve the magneto-motive ultrasound images. Several quantitative methods to estimate the elastic properties of tissues using ultrasound have been developed. In the present study, we propose the magneto-motive ultrasound as a method to characterize the viscoelastic properties of tissues. We used a coil to induce movements to a viscoelastic phantom containing magnetic particles, and the transient response was evaluated cross-correlating ultrasonic echoes acquired during the magnetic excitation. The resulting seismic waves (compressional, shear and surface waves) were detected, and used to estimate the elastic properties of the phantom.

A hybrid transducer to evaluate stomach emptying by ultrasound and susceptometric measurements: an in vivo feasibility study

Gastric emptying reflects a diversity of important physiological functions. Alternating current biosusceptometry (ACB) is an inexpensive, radiation-free, and minimally invasive method to evaluate gastric emptying, but its response depends on the spatial distribution of the magnetized material and does not provide precise anatomical information. The hybrid transducer, which combines ACB and an ultrasound probe, is an alternative to improve susceptometry measurements, namely the spatial localization of the magnetized source. In this study, initial stomach emptying, in rats, was monitored with the aid of the hybrid transducer. Yogurt mixed with ferrite particles was injected into the rats stomach. The hybrid transducer was placed on the rats abdomen during experiments, and the susceptometry signal and magnetomotive ultrasound (MMUS) images were saved and postprocessed. MMUS highlighted the movement of magnetic particles due to magnetic force from ACB excitation coils, and showed the rats stomach location. In this feasibility study, we monitored the stomach emptying of 4 rats for 20 min. The mean relative ACB signal decayed by 4.6 ± 0.1%, and the mean relative area of MMUS images decreased by 4.5 ± 0.2%, after 20 min postingestion of the magnetic meal due to stomach emptying. In a second experiment, 3-D MMUS images from axial sequences were obtained by spatially translating the hybrid transducer, providing details of the stomach wall, which may enable minimally invasive detection of abnormalities. In conclusion, the MMUS image increased ACB spatial resolution and furnished additional anatomical information.

A magneto-motive ultrasound platform designed for pre-clinical and clinical applications

Introduction Magneto-motive ultrasound (MMUS) combines magnetism and ultrasound (US) to detect magnetic nanoparticles in soft tissues. One type of MMUS called shear-wave dispersion magneto-motive ultrasound (SDMMUS) analyzes magnetically induced shear waves (SW) to quantify the elasticity and viscosity of the medium. The lack of an established presets or protocols for pre-clinical and clinical studies currently limits the use of MMUS techniques in the clinical setting. Methods This paper proposes a platform to acquire, process, and analyze MMUS and SDMMUS data integrated with a clinical ultrasound equipment. For this purpose, we developed an easy-to-use graphical user interface, written in C++/Qt4, to create an MMUS pulse sequence and collect the ultrasonic data. We designed a graphic interface written in MATLAB to process, display, and analyze the MMUS images. To exemplify how useful the platform is, we conducted two experiments, namely (i) MMUS imaging to detect magnetic particles in the stomach of a rat, and (ii) SDMMUS to estimate the viscoelasticity of a tissue-mimicking phantom containing a spherical target of ferrite. Results The developed software proved to be an easy-to-use platform to automate the acquisition of MMUS/SDMMUS data and image processing. In an in vivo experiment, the MMUS technique detected an area of 6.32 ± 1.32 mm2 where magnetic particles were heterogeneously distributed in the stomach of the rat. The SDMMUS method gave elasticity and viscosity values of 5.05 ± 0.18 kPa and 2.01 ± 0.09 Pa.s, respectively, for a tissue-mimicking phantom. Conclusion Implementation of an MMUS platform with addressed presets and protocols provides a step toward the clinical implementation of MMUS imaging equipment. This platform may help to localize magnetic particles and quantify the elasticity and viscosity of soft tissues, paving a way for its use in pre-clinical and clinical studies.

Copolymer-in-oil phantoms for photoacoustic imaging

Photoacoustic imaging is a hybrid imaging modality, where ultrasound waves are generated from the interaction between light and biological tissue. Therefore, the image contrast is based on the optical absorption properties of tissues. A useful approach for studying photoacoustic imaging methods is through tissue-mimicking phantom experiments. Water-based materials are commonly used in phantoms for photoacoustic imaging. However, these materials have disadvantages such as easy degradation and low temporal stability. In this paper, copolymer-in-oil phantoms are presented for ultrasound and photoacoustic techniques, with the advantage of presenting low temporal degradation. Oil-based materials made of mixtures of styrene-ethylene/butylene-styrene (SEBS) and low-density polyethylene (LDPE) using paraffinic mineral oil as solvent was prepared, and annatto dye was used as optical absorber. Acoustic and optical characterization of the material was performed, where speed of sound, attenuation coefficient, optical absorption and scattering coefficients were evaluated. The phantom material showed tissue-mimicking optical and ultrasonic properties. A cubic phantom with a spherical absorber inclusion for photoacoustic imaging was manufactured. The photoacoustic signal from the inclusion showed the highest intensities at 532 nm with consistent variation for other wavelengths according to the measured absorption spectrum. In this case, the absorption spectrum acquired for the material containing annatto showed band between 350-550nm. Therefore, the results suggest the phantoms of SEBS and LDPE are promising for photoacoustic imaging.

X-ray acoustic imaging for external beam radiation therapy dosimetry using a commercial ultrasound scanner

Photoacoustic (PA) imaging has been used for numerous applications in clinical medicine and preclinical studies. Usually, nanosecond laser pulses are used to generate PA signals. In laser-based PA imaging, the contrast is based on optical absorption. The generated PA signals can be detected at greater depths, given that the limits imposed by photon scattering of purely optical based techniques are overcome. However, less effort has been directed towards the use of X-ray photons, which have more penetration depth than photons in the visible NIR region. Modern linear accelerators can provide polychromatic X-ray with sufficient power density to produce microseconds X-ray pulses capable of inducing ultrasonic waves in the material. Based on this concept, the X-ray acoustic computer tomography (XACT) was proposed to generate images by combining X-ray excitation and ultrasonic detection. In XACT an ultrasonic single element transducer with central frequency of 500 kHz was moved 360° around the sample. The present study proposes the use of X-ray photons to generate x-ray acoustic (XA) imaging using a commercial ultrasound system, where a linear ultrasound probe was used to acquire XA signals during external beam radiation therapy (RT). To validate our system, lead samples were positioned inside a water tank and then were irradiated to generate XA signals for 6 MV and 15 MV energies. The XA signals were captured by the ultrasound transducer operating in a frequency range between 5 MHz and 14 MHz, using the delay and sum beamforming to generate the images. We obtained XA images of lead samples consistent to XACT and the signals analysis showed XA signals with amplitude increased for higher dose-rates. These results demonstrate the feasibility of generating XA images, which provided dosimetric information during RT using a linear accelerator and a commercial ultrasound system.

Photoacoustic-based thermal image formation and optimization using an evolutionary genetic algorithm

Introduction: For improved efficiency and security in heat application during hyperthermia, it is important to monitor tissue temperature during treatments. Photoacoustic (PA) pressure wave amplitude has a temperature dependence given by the Gruenesein parameter. Consequently, changes in PA signal amplitude carry information about temperature variation in tissue. Therefore, PA has been proposed as an imaging technique to monitor temperature during hyperthermia. However, no studies have compared the performance of different algorithms to generate PA-based thermal images. Methods: Here, four methods to estimate variations in PA signal amplitude for thermal image formation were investigated: peak-to-peak, integral of the modulus, autocorrelation of the maximum value, and energy of the signal. Changes in PA signal amplitude were evaluated using a 1-D window moving across the entire image. PA images were acquired for temperatures ranging from 36oC to 41oC using a phantom immersed in a temperature controlled thermal bath. Results: The results demonstrated that imaging processing parameters and methods involved in tracking variations in PA signal amplitude drastically affected the sensitivity and accuracy of thermal images formation. The sensitivity fluctuated more than 20% across the different methods and parameters used. After optimizing the parameters to generate the thermal images using an evolutionary genetic algorithm (GA), the percentage of pixels within the acceptable error was improved, in average, by 7.5%. Conclusion: Optimization of processing parameters using GA could increase the accuracy of measurement for this experimental setup and improve quality of PA-based thermal images.

Imaging biomarker development based on microbubble perfusion and oxygen saturation in a rat model of liver cancer

Treatment of hepatocellular carcinoma (HCC) with sorafenib, a multikinase inhibitor, results in decreased microvessel density associated with increased levels of tumor hypoxia. However, the response rate is relatively poor, and recently it has been shown that tumor hypoxia and perfusion have predictive correlations with HCC response to sorafenib. In this study, we have investigated the correlation of oxygen saturation (SO2) and perfusion, estimated using photoacoustic-ultrasonic (PAUS) imaging, to the sorafenib treatment response in an orthotopic rat model of HCC. Following spectroscopic photoacoustic (sPA) imaging, microbubble contrast was introduced and harmonic imaging data were acquired for perfusion measurements. An FEM-based fluence correction model based on the diffusion approximation with empirically estimated tissue surface fluence and an SNR-based thresholding approach have been developed and validated on ex vivo and in vivo rat data to estimate SO2 using sPA imaging. The SO2 estimate has been obtained by solving an iterative minimization problem and then thresholded based on a pixel-wise empirically estimated SNR mask. For the treated cohort, the results show that the change in SO2 during an oxygen challenge is positively correlated with disease progression, while it is negatively correlated for the untreated cohort. Additionally, perfusion was significantly decreased in the treated group compared to baseline pretreatment and untreated cohort measurements. The reduced treatment-mediated perfusion leads to lack of oxygen supply and thus reduced oxygen levels. This study shows the potential of PAUS estimation of SO2 and perfusion to monitor and predict HCC sorafenib treatment response, ultimately leading to improved future treatment.

Shear wave Vibro Magneto Acoustography for measuring tissue mimicking phantom elasticity and viscosity

Estimating the rheological properties of soft tissues is a useful procedure to identify and evaluate pathologies in medical diagnosis. Quantification of soft tissue viscoelasticity can be achieved by generating and tracking shear wave propagation. Many ultrasound-based techniques have been used to evaluate the induced shear wave. The viscosity and elasticity can be measured by shear wave dispersion. The Magneto Motive Ultrasound induces motion within magnetically labeled tissue. This paper describes the Shear wave Dispersion Vibro Magneto Acoustography (SDVMA) technique for analysis and quantifying the viscoelasticity through shear wave propagation in gelatin phantoms labeled with ferromagnetic nanoparticles. Phantoms were excited applying an external magnetic field gradient and the movements induced in the internal phantom structure due to the interaction of the field with the particles were obtained through pulse-echo equipment ultrasound. The movement information were processed to get values of elasticity and viscosity of the medium.