Real-time diagnosis of hifu lesion formation by ultrasound elastography

Abstract

Cancer is a serious disease for human beings for centuries. Despite great progress in oncology in the past a few decades there are still significant challenges in this field. Ultrasound elastography is a new imaging modality to detect the relative stiffness of tissue in the diagnosis of cancer in the early stage or calcified tissue or the lesions which have stiffness 3-5 times as the surrounding healthy tissue but much less difference in acoustic impedance (<10%) in forming the conventional B-mode sonography. Ultrasound elastography tracks the movement of acoustic scatterers within the tissue in the sonography with response to quasi-static compression applied to the tissue, and then derives the stiffness distribution. In the last twenty years, conventional axial strain imaging method, shear wave imaging, and other stiffness-detection methods have been well-established. High-intensity focused ultrasound (HIFU) is emerging as a noninvasive and effective ablation approach in the treatment of solid tumors and cancers, such as breast, liver, prostate, kidney, bone cancers and uterine fibroids. The major mechanism of HIFU ablation is to elevate the temperature of tissue in the focal region of HIFU field over 65C on the order of seconds or ten seconds to form irreversible necrosis. Quantitatively evaluating the shape of lesions and gaps between them, which are much harder than the surrounding tissue, is critical in monitoring the progress and success of HIFU treatment. Although elastography has been tried to detect HIFU- or chemotherapy-induced lesions after the treatment, real-time monitoring and feedback are more important. Especially the thin gap between two vertical aligned lesions must be avoided. Otherwise, cancer cells in the untreated region have the much high possibility of relapsing. However, there are currently no solutions for detecting the lesion gap in real time. In this study, a technique was developed and evaluated for this specific problem. We utilized the acoustic radiation force on the lesion area produced by the therapeutic HIFU pulse during ablation to generate high-contrast axial shear strain around the gap region and construct the axial shear strain elastograms (ASSEs). In order to establish the utility of this novel approach, all related fundamental knowledge, such as strain distribution in the tissue induced by the quasi-static compression, displacement of acoustic scatterers using raw radio-frequency (RF) data, reconstruction of elastogram, collection of RF data from an ultrasound research platform, and plane wave imaging for ultrafast sonography, was introduced at first in Chapter 2. Subsequently, numerical simulation was carried out to show the feasibility of using acoustic radiation force from the therapeutic HIFU exposure to illustrate the gap between two vertically aligned lesions in Chapter 3. The effects of lesion size, lesion gap, and acoustic radiation force from HIFU burst on the axial shear strain elastogram were investigated to estimate the feasibility and sensitivity of the proposed method. Overall, the preliminary numerical simulation demonstrated that it is highly feasible to detect the gap between two vertically aligned lesions by axial shear strain (ASS) using acoustic radiation force in real time during HIFU ablation. From the numerical simulation of various conditions, a criterion of ASS ratio was established for detecting the lesion gap. It is found that the threshold is about 0.75. Finally, gel phantom and ex vivo tissue (porcine kidney and liver) sample experimental studies were carried out to further evaluate the performance of our proposed technology. An array of markers were produced in bovine serum albumin (BSA) included polyacrylamide to track the displacement by high-speed photography. Axial strain elastograms (ASEs) under static compression can show the contour of lesions in the gel phantom, but cannot discern whether gap zone exists between two HIFU-induced lesions. However, remote palpation from acoustic radiation force using HIFU exposure can produce axial shear strain elastogram and generate large axial shear strain in the gap zone, which is quite sensitive and reliable in the HIFU monitoring. Because of fast relaxation of gel phantom and ex vivo tissue samples, sonography at the high frame rate (about 1000 Hz) using planar wave imaging (PWI) is required. However, the memory size of ultrasound research platform used to collect RF data limits only a few scanlines around the lesions. Experiments in the porcine kidney and liver samples are also very promising. Lesion gap correlates quite well with the presence of large axial shear strain or high ratio of ASS between two lesions to that at the left side of being treated lesion, which provides good compromise for the future animal experiments and clinical trials. Overall, in this study, we proposed a practical and cost-effective approach of detecting lesion gap during HIFU treatment. Our technology does not need to interrupt the ablation procedure and apply quasi-static compression or ARF to the target using the ultrasound imaging probe. Detection seems to be sensitive and reliable without much intervention and physical background of the operator. It has a high possibility of clinical trials for most of sonography-guided HIFU treatment system with little modification of configurations and HIFU treatment protocols.