State of the art medical imaging for quantitative blood flow assessment includes advanced diagnostic techniques such as Ultrasound Vector Doppler Imaging (VFI) and Phase Contrast Magnetic Resonance Imaging (PC MRI). 4D flow Phase Contrast MRI has great potential for estimating complex flow fields typical of healthy and pathological conditions of the cardiovascular system. However, Magnetic Resonance devices are very expensive and the 4D flow MRI technique is not clinically accepted (yet) because of long scanning and post-processing times. The use of Doppler ultrasound techniques over Magnetic Resonance Imaging offers numerous advantages and disadvantages. Doppler ultrasound is non-invasive, cost-effective, portable, universally available and compatible with all types of patient. By the time an MRI scan is performed (usually 1 to 2 hours), up to 5 ultrasound scans can be completed. Moreover, a hospital commonly has a larger number of ultrasound machines than Magnetic Resonance rooms. The use of Doppler ultrasound can significantly reduce waiting lists with comparable diagnostic results to MRI. This allows in quicker diagnosis and improved outcomes. Conventional Doppler ultrasound techniques provide real-time qualitative and quantitative information about intravascular blood flow patterns with high accuracy. However, these techniques are strictly angle dependent. Advanced angle independent Ultrasound Vector Flow Imaging (VFI) techniques have been developed to overcome this limitation. Ultrasound Vector Flow Imaging has attracted great interest in recent years. VFI accuracy and clinical efficacy have been widely
46 demonstrated. Several 2D VFI approaches have been validated and are already implemented on clinical and research ultrasound scanners. In 2013, VFI was FDA approved and recommended as the first choice for the assessment of cardiovascular flows and analysis of blood fluid dynamics. A major problem for medical physics departments involved in medical devices Quality Control is the lack of test objects technologies on the market able to challenge these innovative devices. Calibration of medical imaging devices is recommended as part of the Quality Management System stated by the ISO 9000-2015. Quality Control checks should be performed with periodic acquisition of images on specially designed test objects. Specific imaging quality parameters are quantified and if the values of interest differ from pre-defined threshold values, corrective actions must be undertaken. In order to challenge state of the art technologies, phantom designs must ideally develop alongside the medical imaging device market. The main advantages of using test objects over human (or animal) subjects and biological tissues are related to the reproducibility of results (it is possible to carry out comparative studies over time), high control (high precision in manufacturing) and absence of ethical issues (licencing, health and safety regulations, and ethics). For these reasons, phantoms play a fundamental role and find wide application for calibration, Quality Control, training, research and development of medical imaging devices, testing and validation of novel techniques at the research level. Flow phantoms are commonly used for the investigation of flow field velocities, the cross- validation of flow-field data obtained from independent measurement methods (i.e. medical imaging modalities and optical imaging systems) and for the investigation of a potential relationship between fluid dynamics and biological functions. A flow phantom can be designed as anthropomorphic, semi- anthropomorphic or non-anthropomorphic. It should invoke motion, mimic relevant physiological conditions and produce a known flow field within specified tolerances. From an analysis of the literature and of the market, there are few flow test objects available. Current flow phantom technologies are expensive, inappropriate, they fail to mimic relevant physiological conditions and often lack accuracy specifications. There are no cost-effective flow phantoms technologies able to reproduce complex flow patterns to challenge advanced medical imaging technologies clinically available, such as Ultrasound VFI and PC MRI. Consequently, calibration, testing and validation of medical imaging scanners for quantitative blood flow assessment and definition of standards for Quality Control checks is very challenging. Novel emerging Ultrasound and MRI methods would benefit from a routine standardisation and a calibration tool. A complex flow test object would allow the undertaking of measurements, collection of data, optimisation of scanning parameters for optimising procedures and training to the clinicians. A standard tool is essential to engage with scientific community, compare results across centres and widespread the use of these innovative medical imaging technologies. Appropriate flow phantom designs can create well-defined international standard guidelines (IPEM 2010; Browne 2014; Dudley and McKenna 2017; Keanan et al 2018) and validate new research techniques. This thesis consists to the design, development, testing and validation of a semi-anthropomorphic flow phantom that offers complex flow patterns comparable to relevant physiological conditions. The flow phantom must comply with all of the requirements (stability, controllability, predictability, reproducibility, reliability) recommended for test objects and manufactured from material compatible with multiple imaging technologies for flow measurements. However, the choice of materials compatible with both Ultrasound and Magnetic Resonance technologies is very challenging. The test object aims to deliver a flow benchmark for calibration of clinical scanners and for the validation of advanced velocity estimating algorithms at research level. The key element is the flow itself, rather attempting to mimic surrounding tissues or vessels. Therefore, the first objective is the identification and characterisation of a flow that can satisfy all the desirable requirements. The prototype is designed to operate in free-field (deionised water) and to be compatible with Doppler Ultrasound Vector Flow Imaging modalities, already clinically available since 2013. Optically transparent materials are preferred, in order to allow the undertaking
47 of comparative experiments with optical imaging techniques, such as Laser-PIV and video cameras. After the design characterisation and validation, a cost-effective solution for the adaptation to a Magnetic Resonance environment is proposed in Chapter 8.