High-spatial-resolution, instantaneous passive cavitation imaging with temporal resolution in histotripsy: a simulation study

Purpose@#In histotripsy, a shock wave is transmitted, and the resulting inertial bubble cavitation that disrupts tissue is used for treatment. Therefore, it is necessary to detect when cavitation occurs and track the position of cavitation occurrence using a new passive cavitation (PC) imaging method. @*Methods@#An integrated PC image, which is constructed by collecting the focused signals at all times, does not provide information on when cavitation occurs and has poor spatial resolution. To solve this problem, we constructed instantaneous PC images by applying delay and sum beamforming at instantaneous time instants. By calculating instantaneous PC images at all data acquisition times, the proposed method can detect cavitation when it occurs by using the property that when signals from the cavitation are focused, their amplitude becomes large, and it can obtain a high-resolution PC image by masking out side lobes in the vicinity of cavitation. @*Results@#Ultrasound image simulation confirmed that the proposed method has higher resolution than conventional integrated PC imaging and showed that it can determine the position and time of cavitation occurrence as well as the signal strength. @*Conclusion@#Since the proposed novel PC imaging method can detect each cavitation separately when the incidence of cavitations is low, it can be used to monitor the treatment process of shock wave therapy and histotripsy, in which cavitation is an important mechanism of treatment.

A new method for assessing the performance of signal processing filters in suppressing the side lobe level

Purpose@#This study aimed to propose a new ground truth ultrasound imaging method and to confirm its efficacy when applied to side lobe suppression filtering. @*Methods@#Using a computer simulation, we synthesized a side lobe-free image (i.e., with no side lobe whatsoever) by separating the main and side lobe signals in the construction of point target, speckled cyst, and pseudo-kidney images. During signal processing, we assessed the quality of the filtered image by comparing it with a ground truth image (i.e., the main lobe image). @*Results@#We examined the effect of reducing the side lobe by applying aperture apodization, side lobe estimation and reduction filtering, and minimum variance beamforming, which are widely used as side lobe suppression techniques. Despite the drawback of decreased resolution, the commonly used apodization method increases the contrast, which improves ultrasound image quality and enables a better diagnosis. Although side lobe estimation and reduction filtering and minimum variance beamforming are demanding in terms of computational resources, they can considerably improve ultrasound images. Compounding of ultrasound images processed by various signal processing methods increases the resolution and contrast, while reducing the speckle noise. @*Conclusion@#Although it appears that the proposed method can only be used for computer-generated radiofrequency data, this method can improve ultrasound image quality by identifying the characteristics of signal processing filters for side lobe suppression and applying appropriately adjusted filters to in vivo human imaging data.

Side lobe free medical ultrasonic imaging with application to assessing side lobe suppression filter

When focusing using an ultrasonic transducer array, a main lobe is formed in the focal region of an ultrasound field, but side lobes also arise around the focal region due to the leakage. Since the side lobes cannot be completely eliminated in the focusing process, they are responsible for subsequent ultrasound image quality degradation. To improve ultrasound image quality, a signal processing strategy to reduce side lobes is definitely in demand. To this end, quantitative determination of main and side lobes is necessary. We propose a theoretically and actually error-free method of exactly discriminating and separately computing the main lobe and side lobe parts in ultrasound image by computer simulation. We refer to images constructed using the main and side lobe signals as the main and side lobe images, respectively. Since the main and side lobe images exactly represent their main and side lobe components, respectively, they can be used to evaluate ultrasound image quality. Defining the average brightness of the main and side lobe images, the conventional to side lobe image ratio, and the main to side lobe image ratio as image quality metrics, we can evaluate image characteristics in speckle images. The proposed method is also applied in assessing the performance of side lobe suppression filtering. We show that the proposed method may greatly aid in the evaluation of medical ultrasonic images using computer simulations, albeit lacking the use of actual experimental data.

Estimation and suppression of side lobes in medical ultrasound imaging systems

This paper estimates the side lobe levels from the received echo data, and proposes and compares three types of filters that can be used to suppress them in an ultrasound image. Ultrasound echo signals from the offaxis scatterers can be modeled as a sinusoidal wave whose spatial frequency in the lateral direction of a transducer array varies as a function of the incident angle. The received channel data waveform due to side lobes have a spatial frequency of an integer plus a half. Doubling the length of the channel data by appending zeros and taking the discrete Fourier transform of the elongated data makes the spatial frequency of the channel data due to side lobes become an integer. Thus, it is possible to estimate the complex amplitude of the side lobes. Adding together all the channel data of the estimated side lobes, we can obtain the side lobe levels present in ultrasound field characteristics. We define the summed value as a quality factor that is used as a parameter of side lobe suppression filters. Computer simulations as well as experiments on wires in a water tank and a cyst phantom show that the proposed filters are very effective in reducing side lobe levels and that the amount of computation is smaller than that of the minimum variance beamforming method while showing comparable performance. A method of estimating and suppressing side lobes in an ultrasound image is presented, and the performance of the proposed filters is found to be viable against the conventional B-mode imaging and minimum variance beamforming methods.

Advances in ultrasound elasticity imaging

The most troublesome of ultrasonic B-mode imaging is the difficulty of accurately diagnosing cancers, benign tumors, and cysts because they appear similar to each other in B-mode images. The human soft tissue has different physical characteristics of ultrasound depending on whether it is normal or not. In particular, cancers in soft tissue tend to be harder than the surrounding tissue. Thus, ultrasound elasticity imaging can be advantageously used to detect cancers. To measure elasticity, a mechanical force is applied to a region of interest, and the degree of deformation measured is rendered as an image. Depending on the method of applying stress and measuring strain, different elasticity imaging modalities have been reported, including strain imaging, sonoelastography, vibro-acoustography, transient elastography, acoustic radiation force impulse imaging, supersonic imaging, and strain-rate imaging. In this paper, we introduce various elasticity imaging methods and explore their technical principles and characteristics.