Diya Wang

Ultrasound contrast plane wave imaging with higher CTR based on pulse inversion bubble wavelet transform

Although ultrasound contrast plane wave imaging can avoid the repeated disruption and capture the transient spatial distribution of microbubbles, it is still limited by lower contrast-to-tissue ratio (CTR) due to low negative peak pressure and lacks of transmit focus. The purpose of this paper was to develop an ultrasound contrast plane wave imaging method combined with pulse inversion bubble wavelet transform imaging (PIWI) technique to improve the CTR of plane wave images. First, a pair of “bubble wavelets” was constructed by microbubbles scattering echoes predicted by modified Herring equation driven by two inverted plane waves. Next, the original echoes from such plane waves were performed by bubble wavelet correlation analysis. Then, such echoes replaced by the maximal wavelet correlation coefficients were summed to distinguish echoes of microbubbles and tissues. In vivo rabbit kidney experiments, the CTR of plane wave imaging was improved to 15.19 dB by PIWI technique without the sacrifice of image frame, which was larger 4.48±0.96 dB than that of raw images. In summary, this method could contribute to plane wave imaging by allowing the continuous transient monitoring of the accumulation of microbubbles with higher CTR.

Parametric perfusion imaging with single-pixel resolution and high signal to clutter ratio

Parametric perfusion imaging (PPI) based on time-intensity-curves (TICs) can quantify and depict the spatial distribution of tumor perfusion information in liver cancer research. However, accurate diagnosis of PPI is seriously disturbed by the fluctuations of TICs and decreases of coded threshold induced by no-microbubble (MB) regions. Such disturbances, particularly the decrease of signal-to-clutter ratio (SCR) of TICs, are further exacerbated during selecting single-pixel region-of-interest (ROI) to obtain PPI with highest resolution. The objective of this study was to accurately obtain PPI with single-pixel resolution at the smallest ROI by the valid TICs filtration and the SCR enhancement of TICs. First, single-pixel TICs were obtained from the dynamic contrast images of a patient with gallbladder carcinoma liver metastasis. Next, these TICs in no-MB regions were then eliminated due to their low correlation with MB regions. Whereas the valid reserved TICs were filtered by the detrended fluctuation analysis to improve their SCR. Color-coded images were finally obtained based on the perfusion parameters extracted from the reserved TICs. After TICs filtration and denoising, the disturbances from no-MB regions were effectively removed; SCR of TICs was enhanced by 5.49 ± 0.34 dB; and operation time of PPI decreased 50.4 ± 0.1% because lots of invalid data were eliminated. Hot spots distribution and perfusion characteristic of neovascularization in the liver metastases were accurately distinguished and depicted by combining PPI with single-pixel resolution, especially the wash in time and wash out time. Besides, edge features of the invasion area and liver were clearly described without extra segmentation algorithm. It can contribute to accurately make a clinical decision in the liver cancer diagnoses by the single-pixel resolution PPI with comprehensive functional perfusion information.

Contrast-enhanced ultrasound imaging with high CTR and improved resolution by bubble-echo based deconvolution

In microvascular perfusion imaging by contrastenhanced ultrasound (CEUS), inhibiting the strong tissue backscattering to enhance contrast-to-tissue ratio (CTR) and improving the imaging resolution to distinguish small vessels are two critical aspects. CTR has been greatly enhanced based on the strong nonlinear responses of contrast microbubbles under ultrasound insonation. However, the resolution of CEUS image is still limited by the finite bandwidth of the imaging system. In this paper, a peculiar bubble-echo based deconvolution (BED) was proposed based on a modified convolution model for CEUS to improve both CTR and resolution. A novel bubble-echo based PSF was constructed using the modified Herring equation, where bubble-echo referred to the theoretical backscattered echo from a single bubble under the insonification. Deconvolution was implemented axially using regularized inverse Wiener filtering. The efficiency of the proposed BED was verified by combining with fundamental imaging, second harmonic imaging and pulse inversion technique. Meanwhile, as comparisons, the approved cepstrum based deconvolution (CEPD) and pulse inversion bubble-wavelet imaging (PIWI) were also performed. In vivo rabbit kidney perfusion experiments were carried out to evaluate the above methods from CTR, time-intensity-curve and resolution. BED performed better than CEPD and PIWI in enhancing CTR, and had a higher resolution than PIWI. All results indicate that BED provides new CEUS methods with higher CTR and good resolution, which has important significance to microvascular perfusion evaluation in deep tissue.

Influences of frequency-dispersive guided waves on contrast-enhanced ultrasound imaging

When emission waves (EWs) of imaging scanner hit bone, especially at an oblique incidence, guided waves (GWs) are generated from the bone cortex. GWs are multimodal and frequency-dispersive due to their non-linear propagation. Meanwhile, GWs leak to surrounding tissues and disturb SNR of acoustic signals. When microbubbles (MBs) flow through microvessels near a bone cortex, GWs can interact with MBs. The vibration and nonlinear scattering of MBs are unavoidably affected by GWs, which may disturb the image quality of ultrasound contrast imaging (UCI) near the bone surface. However, such disturbances on UCI with sub-harmonic (SH), fundamental (F), and ultra-harmonic (UH) have not been adequately addressed. This study aimed to elucidate the influences of frequency-dispersive GWs on UCI, especially with pulse inversion (PI) mode. The effects of GWs on UCI with B and PI modes were first elucidated through simulated comparison based on the modified Herring equation derived by GWs coupled EWs together and EWs alone, respectively. The GWs were reconstructed by the single-mode GWs, which were detected by using a transmitted-air gap-received (TAR) method and identified by using a smoothed pseudo-Wigner-Ville (SPWV) energy distribution superimposed with theoretical dispersion curves. Such effects were further verified by UCI in a vessel-plate flow phantom at incidence angles 0° and 22°. The influences of GWs on contrast and resolution of UCI were finally quantified by the differences in peak value (DPV) and half peak width (DHPW) of MB echoes, respectively. Based on TAR combined with SPWV methods, the coupling disturbances of EWs were removed; and the GWs with four single-modes (A0, A1, S0, and S2) and double-frequencies (3.35 and 1.62 MHz) were detected and identified. Due to the changes in MB echoes caused by GWs, the echo aliasing and gray changing occurred in all UCI methods. And DPV of MB echoes of PI SH was 4.0 times greater than that of SH, and DHPW of SH was 3.9 and 11.4 times higher than these of F and UH. Contrast of UCI was enhanced by GWs, but corresponding resolution was decreased, especially for SH and PI mode. So as a double-edged sword, the effects of GWs should be further explored to benefit UCI near the cortex.

Ultrasonic concentration imaging of cavitation bubbles using Nakagami statistical model

Cavitation is regarded as the primary mechanism in pulsed high-intensity focused ultrasound (pHIFU) therapy. Residual cavitation bubbles can be served as nuclei for the subsequent pHIFU cavitation. Therefore, information on spatial distribution especially concentration distribution of residual cavitation bubbles is important to better control pHIFU therapy. Acoustic spatial imaging of cavitation bubbles has been widely studied, while no research on quantitative bubble concentration imaging has been found. In this work, ultrasonic concentration imaging of cavitation bubbles was proposed based on Nakagami statistical model. The feasibility of Nakagami parameter m on concentration imaging of microbubbles was investigated firstly by simulation. Afterward, concentration imaging of cavitation bubbles by parameter m was performed in pHIFU experiments. Parameter m of cavitation bubbles increased with increasing concentrations and got saturated when concentrations above 16 bubbles/mm2, which indicated that parameter m could reflect bubble concentration at certain levels. Meanwhile, for the same concentration, parameter m kept almost constant under different imaging parameters, while the corresponding B-mode echoes varied greatly. Parameter m of cavitation bubbles in pHIFU decreased as bubbles dissipated gradually. All results showed that Nakagami parameter m can reflect the concentration distribution of cavitation bubbles, which provides a new method for better monitoring and optimization of pHIFU therapy.

Contrast-based transient flow vector distribution in arterial stenosis based on plane wave bubble wavelet imaging and modified optical flow method

The objective of this study was to accurately obtain the transient flow vector distribution (FVD) of microbubbles in arterial stenosis by modified optical flow method based on ultrasound contrast plane wave imaging (UCPI). First, a bolus injection experiment was performed in an arterial stenosis flow phantom using inverted plane waves at 400Hz frame rate controlled by Sonix-Touch. The contrast-to-tissue ratio (CTR) of UCPI images and detection sensitivity of microbubbles was then improved by pulse inversion bubble wavelet imaging (PIWI) technique based on a pair of inverted bubble wavelet constructed according to the modified Herring equation. Next, transient displacement and FVD of microbubbles among a set of UCPI images with high CTR and sensitivity were obtained by a modified optical flow method, which added a discontinuity preserving spatio-temporal smoothness constraint, to accurately mark and evaluate the hemodynamic feature in the stenosis. Finally, FVD of microbubbles in the arterial stenosis was depicted clearly by PIWI-based UCPI. Compared to those obtained using pulse inversion harmonic imaging, CTR of UCPI in the stenosis and detection sensitivity of microbubbles near arterial wall were improved by up to 10.09 ± 2.96 dB and 12.52 ± 2.10 dB, respectively. Axial microbubble velocity in the stenosis was 28.45 ± 3.05 mm/s; it was in accordance with the set value. Moreover, compared with results obtained by the conventional optical flow method, an anticlockwise vortex of microbubbles in the distal was described more accurately by the transient FVD obtained using the modified optical flow method. It indicated that contrast-based transient FVD in the arterial stenosis can be acquired accurately by PIWI-based UCPI combined with modified optical flow method.