High resolution biomedical imaging using ultrasonic metamaterials

Ultrasound biomedical imaging is used routinely for the diagnosis of many diseases. It has the ability show detailed structures of soft tissues, and has the advantage over other imaging methods, such as CT or MRI imaging, of being relatively cheap and portable. It is thus a good method for many diagnostic clinical settings. The current resolution of ultrasonic imaging in the body is determined by various factors, including transducer design, frequency of operation, and depth of penetration required. However, there is a fundamental limit to the imaging performance of such systems - namely the diffraction limit. This sets the minimum spot size that a focused beam can achieve by conventional means, even in a perfect propagation medium. The present proposal aims to improve this by the use of metamaterials, which will be incorporated within an ultrasonic transducer system. These exotic materials are, in fact, made up of a complicated geometry, where the internal structure contains many sub-wavelength features. These can act together to make the material behave in a way that is totally different from normal structures. The result is that they can, for example, have a negative refractive index, noting that for conventional materials the value is always positive. Thus, a plate with flat parallel sides can focus ultrasound, provided it is designed correctly.

The research will identify the best designs that can be used at biomedical ultrasound frequencies, which in the present case will be 1-5 MHz. To date, acoustic metamaterials have been designed typically for much lower frequency, and for use in air. In this project, novel new designs are proposed, which will first be modelled theoretically, and then constructed using high-resolution additive manufacturing (3D printing) techniques. Once built, the new structures will be tested with biomedical ultrasound transducers, and their performance in imaging systems determined. In this way, it is hoped to produce a new approach to diagnostic ultrasound, with resolution enhancement that could be useful for cardiovascular disease, prostate and skin cancer diagnosis.

Biomedical ultrasound is one of the main imaging methods used worldwide for the diagnosis of disease. It ranks alongside Magnetic Resonance Imaging (MRI) and X-ray Computed Tomography (CT) methods as a minstram imaging tool by the NHS and other healthauthorities. It has the advantage that it is relatively cheap to acquire and use, and does not involve radiation exposure. It is considered safe at the exposure levels used in diagnostic ultrasound. Any improvement in performance is thus likely to have a significant impact in early disease detection and subsequent follow-on treatment, in turn reducing costs. Impact in the healthcare system would thus occur.

The research aims to improve imaging resolution by incorporating a new class of substance, known as a metamaterial, into the equipment used to create and detect ultrasound. This is likely to create impact in certain diseases where imaging over a limited range of distances is required. There are many examples, but one of the most important of these is intravascular ultrasonic imaging, which is becoming increasingly important. This is because it can detect changes to arteries, caused by coronary disease. In addition, certain cancers would be better diagnosed with the use of higher imaging resolution. For example, Trans-rectal Ultrasound (TRUS) can be used to image the prostate when a high PSA level is recorded, and can also guide biopsy needles into the prostate. Estimation of size of the prostate can also give information regarding the best treatment for a successful outcome. An example of a cancer close to the surface of the body is malignant melanoma (a form of skin cancer). Ultrasound can be used to estimate the size and depth of a lesion, and inform subsequent surgical treatment. In all these cases, impact in terms of better diagnosis, earlier treatment and increased information is likely to lead to significant impact to the NHS.

A related area of impact is the use of additive manufacturing, which we intend to use to create some new and novel structures. These give us the intended improvement in operation of ultasound system, but there is also general interest in metamaterials in many different fields - from noise control (more effective industrial noise reduction) to the defence sector (cloaking of submarines). Imaging is also important in nondestructive testing (NDT), where the detection of defects in industrial stuctures is performed. Ultrasonic inspection is a mainstream technique in NDT, and improvements in imaging resolution could also have significant imapct in this field.