Ultrasound image guidance and therapy with random phased arrays
Creators: name: Zubair, Muhammad
Publisher: name: Imperial College London
Year: 2019

description: The dream of replacing surgeons' knife with a non-invasive procedure to treat cancer and other major ailments is no more a fantasy. Ultrasound is a leading technology behind non-invasive therapy. High intensity focused ultrasound (HIFU) has been proposed for treatment of various ailments. HIFU transducers produce high intensity levels needed for ablative therapy or mechanical destruction of tissue. With the use of advanced imaging modalities such as MRI, CT scan and diagnostic ultrasound, it has got widespread recognition. Each one of them has its own advantages and shortcomings. These modalities, when integrated with HIFU, not only increase the cost and complexity of the system but also suffer from low frame rate, difficulty in alignment, image registration problems and low spatial resolution and thus cause hindrance in the clinical use of HIFU. Imaging with the therapeutic transducer is a natural solution due to the intrinsic ability of the imaging transducer to have same coordinates as the therapy transducer and thus can benefit from the inherent registration between the imaging and therapeutic frames of reference. Furthermore, the operating bandwidth of these transducers is wide enough to offer image quality suitable for planning and monitoring the treatment procedure. However, therapeutic arrays are optimized for therapy only with large, directional elements operating at low MHz frequency, conventional beamforming techniques are thus not useful to produce adequate images with such an array. The objective of this thesis was to investigate the feasibility of using synthetic aperture imaging techniques with a therapeutic random phased array. Various synthetic aperture techniques were implemented, in simulation and experiments, with the random phased array and its potential to accurately image the target was demonstrated for the first time. One of the difficulties that currently hinders the clinical application of HIFU to treat liver tumours is the transmission of sufficient energy to cause tissue necrosis while avoiding the ribs from being heated. The rib bones not only absorb and reflect ultrasonic energy causing overheating of the surrounding tissue but also distort the focus and create secondary maxima. The occurrence of secondary maxima leads to tissue heating in undesired areas. Thus it is important to devise techniques to prevent heating on the ribs while performing tissue ablation intercostally. Various techniques have been proposed to avoid thermal damage to the ribs but they either rely on MR/CT images, the physical existence of a point target at the focus or operate on pulses. Imaging with the therapeutic transducer not only eliminates the cost of an external imaging modality such as MRI but also offers the opportunity to detect strong scattering objects such as ribs during or before the therapy procedure. In this work, a binarized apodization model based on geometric ray tracing was used to minimize heating of the ribs while maintaining sufficient intensity at the focus. This model rely on simple shadowing technique overcoming the computational challenges for large domain dimensions at 1 MHz frequency. Focusing through the ribs was investigated in detail and intensity distributions were calculated both in the presence of idealized ribs and anatomically correct rib structures. Various practical situations that can arise during HIFU therapy were taken into consideration and focus quality was analyzed in terms of various practically relevant parameters. A model capable of predicting 3D intensity distribution in the focal plane and in the plane of the ribs was thus developed which is helpful in devising treatment planning strategy for HIFU therapy of liver tumour behind the ribs. The final achievement of this work was to design and model an improved random phased array by modifying the existing 256-element array. This overcame the limitation of reduced intensity at the focus due to a central aperture hole for imaging and the turning off of some elements in the existing transducer to spare the ribs. Since very high peak output power of the array is required for transcostal therapy, the improved design had an increased gain resulting in higher intensity at the focus as well as an extended steering range with out any increase in the grating lobes away from the focus. Moreover, the sidelobes were significantly reduced which helped in improving the imaging resolution of the system.