Novel Ultrasound Elastography Imaging System for Breast Cancer Assessment

Abstract Most conventional methods of breast cancer screening such as X-ray, Ultrasound (US) and MRI have some issues ranging from weaknesses associated with tumour detection or classification to high cost or excessive time of image acquisition and reconstruction. Elastography is a non- invasive technique to visualize suspicious areas in soft tissues such as the breast, prostate and myocardium using tissue stiffness as image contrast mechanism. In this study, a breast Elastography system based on US imaging is proposed. This technique is fast, expected to be cost effective and more sensitive and specific compared to conventional US imaging. Unlike current Elastography techniques that image relative elastic modulus, this technique is capable of imaging absolute Young\u27s modulus (YM). In this technique, tissue displacements and surface forces used to mechanically stimulate the tissue are acquired and used as input to reconstruct the tissue YM distribution. For displacements acquisition, two techniques were used in this research: 1) a modified optical flow technique, which estimates the displacement of each node from US pre- and post-compression images and 2) Radio Frequency (RF) signal cross-correlation technique. In the former, displacements are calculated in 2 dimensions whereas in the latter, displacements are calculated in the US axial direction only. For improving the quality of elastography images, surface force data was used to calculate the stress distribution throughout the organ of interest by using an analytical model and a statistical numerical model. For force data acquisition, a system was developed in which load cells are used to measure forces on the surface of the breast. These forces are input into the stress distribution models to estimate the tissue stress distribution. By combining the stress field with the strain field calculated from the acquired displacements using Hooke\u27s law, the YM can be reconstructed efficiently. To validate the proposed technique, numerical and tissue mimicking phantom studies were conducted. For the numerical phantom study, a 3D breast-shape phantom was created with synthetic US pre- and post-compression images where the results showed the feasibility of reconstructing the absolute value of YM of tumour and background. In the tissue mimicking study, a block shape gelatine- agar phantom was constructed with a cylindrical inclusion. Results obtained from this study also indicated reasonably accurate reconstruction of the YM. The quality of the obtained elasticity images shows that image quality is improved by incorporating the adapted stress calculation techniques. Furthermore, the proposed elastography system is reasonably fast and can be potentially used in real-time clinical applications

The Use of Susceptibility Effects for Tracking and Encoding in MR-guided Interventional Procedures

Magnetic Resonance Imaging (MRI) provides volumetric imaging with superior soft tissue contrast and is now standard-of-care for many diagnostic applications. A growing area is the use of MRI for minimally invasive procedures (a.k.a interventions) such as MRI-guided biopsy. In this thesis, the target procedure was placement of a drainage catheter in the upper urinary tract, the technical term for which is nephrostomy. Tracking and visualization of the interventional devices is crucial in achieving a successful outcome of the procedure. This thesis is a study of MRI methodologies to track interventional devices in MRI-guided procedures. A particular focus was the application of magnetic materials used to perturb the main magnetic field, an effect that was used to develop tools and techniques for tracking devices and making images at the device tip. The experiments in Chapter 2 led to the development of a new technique using spectrally selective excitation, which is capable of simultaneously measuring the location and orientation of an interventional device. In Chapter 3, a new mechanism for tracking is proposed based on the magneto-optical effect in nickel nanoparticles, where fiber optics were used to deliver photons from a near infrared (NIR) laser. This development is intended to enable the incorporation of small tracking devices attached to the interventional tools, which can be pulsed on and off using a laser waveform. In Chapter 4, magnetic materials have been used to generate spatial encoding fields at the tip of a device, to generate endoscopic MR images. These results have contributed to the development of tracking devices and to the advancement of MRI acquisition methods for passive tracking in MR-guided procedures, with an outlook towards clinical application in nephrostomy and beyond.Ph.D