Research Interest

Hybrid ultrasound and Magnetic Resonance (MR) imaging

GR ter Haar, V Bull, J Civale, I Rivens;
Source of funding: ICR, NHS New and Emerging Applications of Technology (NEAT)

Image guidance of therapeutic ultrasound treatments is vital for ensuring positional accuracy, confluent tissue damage and sparing of healthy tissues. Current clinical systems use either ultrasound or MRI to monitor energy deposition within the body. However, each modality has its drawbacks and limitations. This project aims to combine both diagnostic ultrasound (US) and MR imaging with HIFU in order to capitalise on the advantages of both modalities - namely the temporal resolution and cost benefit of ultrasound, along with the accuracy of temperature imaging and the greater spatial resolution of MRI. Potential benefits also include the ability to compare quantitative measurement techniques (thermometry, blood flow, elastography) between the modalities. It is hoped that the scope of a hybrid system can be widened to a wealth of diagnostic studies in the future.

Proof of concept studies have been performed using a Siemens Avanto 1.5 T clinical MRI scanner and a Siemens Antares clinical ultrasound scanner. A custom built ultrasound probe with an 8 m cable allows real time imaging in the scanner whilst the ultrasound unit is situated outside the scanner room. A variety of MR sequences and US modes have been used simultaneously to image agar gel samples, in order to study the interplay between the two technologies.
A fibre-optic hydrophone system (Precision Acoustics) has been used within the MR environment for both thermometry and cavitation detection. Its limitations as a MR compatible cavitation detector compared with a standard laboratory detector have been explored, and detection and data processing techniques developed and optimised for exposures in agar gel and ex vivo liver tissue.

Figure 1: Hybrid imaging setup showing position of ultrasound probe and typical MR image slice (dashed lines show transverse cross section through water container and gel sample).

Figure 2: T2 weighted Turbo Spin Echo (TSE) MR image (left) and B-Mode ultrasound image (right) of an agar gel sample in cross section, acquired simultaneously. Red dashed lines indicate the field of view of the US image superimposed onto the MR slice.

Figure 3: Fibre-optic hydrophone system from Precision Acoustics, with a microscope image of the 120 µm diameter fibre tip, with 10 µm active element.

Figure 4: A typical plot of broadband signal as a function of  time, detected by the fibre-optic hydrophone, during a 3 second HIFU exposure in ex vivo liver tissue. Spikes indicate inertial cavitation activity within the sample.