Supercomputer Technology for Ultrasound Tomographic Image Reconstruction: Mathematical Methods and Experimental Results

With the advantages of non-radiation and low-cost, ultrasound imaging has been widely used in clinical diagnosis. However, due to the compromise between temporal and spatial resolution, the traditional ultrasound imaging method which collects images line by line fails to obtain the images at a high frame rate. Ultrafast ultrasound imaging method based on the plane-wave emission can achieve a high frame rate with the capability for instantaneous hemodynamic imaging of neurovascular response. Besides, by the coherent compounding of the echo signals received from emitting a set of tilting plane-waves, the image quality can be improved in terms of resolution, contrast and signal-to-noise ratio (SNR). Considering the fact that received signals are the mixture of echo signals from the low-speed soft tissue and high-speed blood flow, a clutter filtering method is used to remove the signals of soft tissue. In this study, the ultrafast ultrasound sequence of emission and reception is designed to image the spinal cord by using the groups of multiple steering-angle plane-waves. The so-called f-k migration algorithm based on an exploding reflector model (ERM) is used for coherent plane-wave compounding. Eigenvalue decomposition (EVD) is then applied to tissue and blood signal extraction. The static tissue signals correspond to eigenvectors with low Doppler frequency shift and large magnitude eigenvalue while the high-speed blood flow signals correspond to eigenvectors with high Doppler frequency shift and small eigenvalue. Therefore, frequency and amplitude thresholds can be applied to the accurate separation of the blood component and the tissue component. After the signal extraction, the hemodynamic imaging of blood vessels can thus be obtained from the power Doppler results. The experiments are carried out by using a programmable ultrasonic array system and a high-frequency linear array transducer L22-14vX with a central frequency of 15.625 MHz. The sample acquisition frequency is set to be 62.5 MHz. In rat experiments <i>in vivo</i>, 14040 angle-stilting images per second are compounded into 520 images, each of which is compounded from 27 tilting images (the tilting angles ranging from –10° to 10°). The experiments are conducted on the anesthetized rats with part of the vertebral plate removed. The ultrafast B-mode images are acquired from a 14.0 mm ×12.7 mm region-of-interest. The 520 frames acquired in one second are processed by the clutter filter based on eigenvalue decomposition and Doppler shift analysis. The eigenvectors and eigenvalues corresponding to the soft tissue are discarded. The power Doppler images of the spinal cord micro-vessels are obtained. The experimental results indicate that the ultrafast ultrasound Doppler imaging method is effective for monitoring the hemodynamic variation in spinal cord. The micro-hemorrhage can be identified from the power Doppler images. The quantitative results indicate that the SNR increases with the steering angles increasing. Compared with the results obtained by using 9 steering angles, 5 dB SNR enhancement can be obtained by using the 27 steering angles. In conclusion, the ultrafast ultrasound Doppler technology has the potential applications in spinal cord microvascular imaging and hemodynamic evaluation of neurovascular function of spinal cord.

Assessment of Curve Flexibility on Scoliotic Surgical Candidates Using Ultrasound Imaging Method

An experimental method is described for estimating the mechanical properties of human soft tissues by calculating Young's modulus which was defined according to the slope of the hysteresis curve of the test subject, reflecting the dependence of its deformation on the force applied. The main causes of numerous difficulties in obtaining such data and the ways for their overcoming are considered. Experiments were made on the biceps in the isometric mode under different degrees of internal tension. It has been shown that the higher the physical development of the test subject the less the magnitude of Young's modulus. The dependence of its magnitude on the degree of muscular tension is sublinear in character in the group with good physical development and superlinear in that with poor physical development.

A key challenge of collaborative surgical simulation is to maintain a high level of state consistency among the distributed users under the limitation of network transmission capacity. In this paper, a framework integrating a scalable deformable model and an extensible communication protocol is proposed to meet this challenge. The parameters of the deformable model are obtained by making reference to the biomechanical properties of human soft tissues. Efficient collaboration is achieved by developing the communication protocol and implementing a series of network management approaches, including service management, computation policies, coupling control, token control and availability mechanism. A prototype has been developed by using the client-server network architecture. Experimental results demonstrate that this framework can support collaborative surgical simulation with acceptable network latencies.

This paper concentrated on how to apply meshless method to do physical based soft tissue deformation. First we approximated the deformation field using a Moving Least Squares (MLS) approximation method. Then, a compact force computation was applied to accelerate the physical computation speed. Finally we implemented a surface animation method to simulate the deformation of soft tissue under external forces. We made experiments with real properties of human soft tissue and the results showed that our method successfully made physical based deformation of human soft tissue.

Emerging biomedical applications require realistic phantoms for validation and testing of prototype systems. These phantoms require stable and flexible tissue-mimicking materials with realistic dielectric properties in order to properly model human tissues. To create a tissue-mimicking material to fulfill these needs, carbon powder and urethane rubber mixtures were created, and the dielectric properties were measured using a dielectric probe. Both graphite and carbon black were tested. Mixtures of graphite and urethane (0% to 50% by weight) provided relatively low permittivity and conductivity, suitable for mimicking fatty tissues. Mixtures of carbon black and urethane (0% to 15% by weight) provided a broad range of suitable properties. Samples with 15% carbon black had permittivity and conductivity similar to higher-water-content tissues, however the cured samples were not mechanically suitable for moulding into complex shapes. Finally, mixtures of graphite, carbon black, and urethane were created. These exhibited a range of dielectric properties and can be used to mimic a variety of soft tissues. The mechanical properties of these samples were tested and presented properties that exceed typical phantom requirements. This tissue-mimicking material will be useful when creating thin, flexible, and robust structures such as skin layers.

Acoustic properties are the main criteria to evaluate the compatibility of tested materials as a medical phantom. We determined the acoustic properties of konjac glucomannan (KGM) gel to verify its compatibility as a medical phantom. The acoustic properties measurement employed ultrasonic insertion technique and utilized two identical transducers of 5 MHz center frequencies. One KGM gel block with 5.62 mm thickness was used as sample to determine its longitudinal speed, acoustic impedance and attenuation coefficient at three different water temperatures; (21.5 ± 0.1), (22.5 ± 0.1) and (23.5 ± 0.1) °C. Findings indicated that its longitudinal speed and acoustic impedance increased from (1495 ± 1) to (1499 ± 1) m s-1 and (1.555 ± 0.001) to (1.559 ± 0.001) × 106 kg m-2 s-1 as the water temperature increased from (21.5 ± 0.1) to (23.5 ± 0.1) °C. It also indicated that its attenuation coefficient varied around (0.1303 ± 0.0107) to (0.1373 ± 0.0103) dB cm-1 with increasing water temperature. KGM gel is compatible to be a medical phantom since its acoustic properties are comparable to the acoustic properties of human soft tissue.

We develop efficient iterative methods for solving inverse problems of wave tomography in models incorporating both diffraction effects and attenuation. In the inverse problem the aim is to reconstruct the velocity structure and the function that characterizes the distribution of attenuation properties in the object studied. We prove mathematically and rigorously the differentiability of the residual functional in normed spaces, and derive the corresponding formula for the Fréchet derivative. The computation of the Fréchet derivative includes solving both the direct problem with the Neumann boundary condition and the reversed-time conjugate problem. We develop efficient methods for numerical computations where the approximate solution is found using the detector measurements of the wave field and its normal derivative. The wave field derivative values at detector locations are found by solving the exterior boundary value problem with the Dirichlet boundary conditions. We illustrate the efficiency of this approach by applying it to model problems. The algorithms developed are highly parallelizable and designed to be run on supercomputers. Among the most promising medical applications of our results is the development of ultrasonic tomographs for differential diagnosis of breast cancer.

Ultrasound transmission tomography promises a high potential and novel imaging method for early breast cancer diagnosis; it can quantitatively characterize tissues or materials by the attenuation and speed of sound (SoS). Reconstruction of ultrasound transmission tomography is an inverse problem that can be solved iteratively based on a paraxial approximation of the Helmholtz equation as forward model, which is highly non-linear and time-consuming. In order to address these problems and reconstruct desired images, we design a dual domain network architecture for ultrasound transmission tomography reconstruction. It can enhance the information of measurement domain and directly reconstruct from pressure field measurements without using any initialization of reconstruction and fully connected layer. We train the network on simulated ImageNet data and transfer it for ultrasound transmission tomography images to avoid overfitting when the amount of ultrasound transmission tomography images is limited. Our experimental results demonstrate that a dual domain network produces significant improvements over state-of-the-art methods. It improves the measured structural similarity measure (SSIM) from 0.54 to 0.90 and normalized root mean squared error (nRMSE) from 0.49 to 0.01 on average concerning the SoS reconstruction, and from 0.46 to 0.98 for SSIM, from 353 to 0.03 for nRMSE on average concerning the attenuation reconstruction.

Study of ultrasound stiffness imaging methods using tissue mimicking phantoms

H-scan ultrasound (US) is an innovative real-time imaging technology that depicts the relative size of acoustic scattering objects and structures. The purpose of this research was to develop a novel 3-dimensional (3D) H-scan US imaging approach for tissue classification in volume space. Using a programmable research scanner (Vantage 256, Verasonics Inc, Kirkland, WA) equipped with a custom-built volumetric imaging transducer (4DL7, Vermon, Tours, France), radio frequency (RF) data was collected for offline processing. H-scan US images were constructed after applying a set of convolutional filters based on Gaussian-weighted Hermite polynomials. These functions are related to different sized scattering objects. Preliminary studies were conducted using homogeneous gelatin-based tissue-mimicking phantom materials embedded with acoustic scatterers of varying size (15, 30 or 250 μm) and concentrations (0.1, 0.3, 0.5 or 1.0 %). In vitro results indicate that 3D H-scan US imaging can detect acoustic scatterers of varying size (p 0.05). Overall, our preliminary in vitro findings reveal that 3D H-scan US imaging allows the visualization of different tissue scatterer patterns.

Introduction: Thrombotic microvascular obstruction (MVO) poses a diagnostic challenge due to the lack of specific imaging techniques in clinical practice. This study introduces a novel ultrasound imaging method that utilizes gold nanoparticles (AuNPs) modified with the GPIIb/IIIa-targeting peptide Arg-Gly-Asp (RGD) to enhance the contrast of ultrasound imaging for microembolization. Methods: Targeted gold nanoparticles (RGD-AuNPs) and untargeted gold nanoparticles (GSH-AuNPs) were synthesized using the glutathione reduction method from chloroauric acid. A rat model of MVO was established by infusing microthrombi into the left hindlimb via the right femoral artery. The rats were randomly assigned to receive either RGD-AuNPs or GSH-AuNPs injected via an angiocatheter. Ultrasound imaging was performed on the left hindlimb for three hours at five-minute intervals, and the average video intensity of the images was measured. Histological staining and immunohistochemistry were performed to validate the co-localization of microthrombi and RGD-AuNPs. Results: In the RGD-AuNPs+MVO group, enhanced ultrasound signals were visible at 15 minutes and progressively increased over three hours. The average video intensity in this group was approximately three times higher than that in the GSH-AuNPs+MVO group (11.88±3.68 vs. 3.39±2.67, p<0.001). No significant difference in average video intensity was observed between the GSH-AuNPs+MVO group and the GSH-AuNPs control group (3.39±2.67 vs. 3.50±2.09, p>0.05), or between the GSH-AuNPs control group and the RGD-AuNPs control group (3.50±2.09 vs. 2.95±2.18, p>0.05). Histological staining and immunohistochemistry confirmed the co-localization of microthrombi and RGD-AuNPs. Conclusions: Gold nanoparticles modified with the RGD peptide effectively enhance the signal of ultrasound imaging, enabling targeted imaging of thrombotic microvascular obstruction. This approach holds promise for improving the diagnosis and monitoring of MVO in clinical settings.

Scoliosis is a three-dimensional deformity of spine with vertebral rotation. Brace treatment is the most effective non-surgical treatment for scoliosis. Spinal flexibility affects the amount of correction attainable by bracing. More flexible spines should be more correctable in a brace. However, lateral-bending radiographs are generally not required for brace patients in order to minimize exposure to undesirable ionizing radiation. Without knowing the spinal flexibility, orthotist usually designs the brace based solely on empirical knowledge derived from personal experiences. Ultrasound imaging method has been used to measure the spinal curvature reliably. The objective of this study was to determine the accuracy of the curve flexibility measurements on the ultrasound images. Twelve children who have adolescent idiopathic scoliosis and required brace treatment were recruited. Standing radiographs and prone side-bending ultrasound images were acquired. A total of 20 curvatures were measured and compared between 2 raters. The intra- and inter-rater reliablities of the flexibility measurements were (R1: 0.96; R2: 0.95) and 0.91, respectively. There was no statistically significant difference between the measurements from the 2 raters. However, since the number of participants is still small, more trials are required before making a definite conclusion.

Ultrasound (US) tomography enables quantitative measurement of acoustic properties. Robot assisted ultrasound tomography system enables alignment of two US probes. The alignment is done automatically by the robotic arm so that tomographic reconstruction of more anatomies becomes possible. In this study, we propose a new system setup for robot assistance in US tomographic imaging. This setup includes two robotic arms holding two US probes. One of the robotic arms is operated by the sonographer to determine the desired location for the tomographic imaging; this probe can also provide the B-mode US image during the search. The other robotic arm can then move automatically to align the two probes. One of the probes will act as transmitter and the other one as receiver to enable tomographic imaging. We provide an overview of the system setup and components together with the calibration procedures. In an attempt to provide a complete framework for the tomography system, we also provide a sample tomographic reconstruction method that can reconstruct speed of sound image using two aligned linear US probes. The reconstruction algorithm is however very prone to alignment inaccuracies. We provide an error propagation analysis to provide an estimation of the overall alignment error and then show the effect of the in-plane translational error in the tomographic reconstruction.

Due to the significant acoustic impedance contrast at cortical boundaries, highly inside attenuation, and the unknown sound velocity distribution, accurate ultrasound cortical bone imaging remains a challenge, especially for the traditional pulse-echo modalities using unique sound velocity. Moreover, the large amounts of data recorded by multielement probe results in a relatively time-consuming reconstruction process. To overcome these limitations, this article proposed an index-rotated fast ultrasound imaging method based on predicted velocity model (IR-FUI-VP) for cortical cross section ultrasound tomography (UST) imaging, utilizing ray-tracing synthetic aperture (RTSA). In virtue of ring probe, the sound velocity model was predicted in advance using bent-ray inversion (BRI). With the predicted velocity model, index-rotated fast ultrasound imaging (IR-FUI) was further applied to image the cortical cross sections in the sectors corresponding to the dynamic apertures (DAs) and ring center. The final result was merged by all sector images. One cortical bone phantom and two ex vivo bovine femurs were utilized to demonstrate the performance of the proposed method. Compared to the conventional synthetic aperture (SA) imaging, the method can not only accurately image the outer cortical boundary but also precisely reconstruct the inner cortical surface. The mean relative errors of the predicted sound velocity in the region of interest (ROI) were all smaller than 7%, and the mean errors of cortical thickness are all less than 0.31 mm. The reconstructed images of bovine femurs were in good agreement with the reference images scanned by micro-computed tomography ( μ CT) with respect to the morphology and thickness. The speed of IR-FUI is about 3.73 times faster than the traditional SA. It is proved that the proposed IR-FUI-VP-based UST is an effective way for fast and accurate cortical bone imaging.