Ultrasound modulated tomography for high sensitivity, high spatial resolution 3D imaging
Light offers an alternative to techniques such as X-rays, CT, PET or MRI for imaging body tissues. Optical techniques are useful because they provide rich functional information and are non-hazardous within the average and peak intensity limits. One of the main drawbacks of using light is that it is heavily scattered by tissue and so images often have poor spatial resolution. The outlined research aims to develop a new imaging technique that combines bioluminescence optical imaging (BLI) with ultrasound (US) that will offer significant improvement in spatial resolution over conventional BLI. This is important in pre-clinical imaging of small animals where more accurate imaging techniques will help to reduce the number of animals used in such experiments. The improvements in spatial resolution will be achieved in two ways; firstly, by modulating the bioluminescent light emitted within the tissue using focused US beam to produce a modulated light ‘beacon’ in the region of the US focus that reduces the effects of light scattering and improves the spatial resolution; and secondly by using the US image to inform a reconstruction algorithm. Based on our concept we anticipate that 3D image spatial resolution will be improved by at least a factor of 5 (500 μm compared with 2.5 mm) enabling more accurate and consistent clinical data to be obtained from a smaller set of animals. The potential of system to outperform current BLI system will be demonstrated through a number of exemplar pre-clinical studies, including tracking of mesenchymal stem cells in nude mice.
We have developed a novel small animal US mediated BLI platform (Fig. 1a) in which US (Verasonics scanner) is applied from the underside of the animal and modulated bioluminescence signals can be detected using a photomultiplier tube (PMT). In addition a CCD camera is also used to record the conventional optical image for the image registration. Scanning the US and recording the modulated bioluminescence signal at each point allows a 3D structural and bioluminescence image to be obtained.
In initial experiments, tissue like ‘phantoms’ of known optical and acoustic properties embedded with low illumination sources are used to mimic small animal experiments in order to optimize the system with respect to spatial resolution and signal to noise ratio (SNR). Higher spatial resolution can be achieved by applying pulsed US with time gating and increasing the US frequency. This will inevitably reduce the magnitude of the US modulated signal due to reduction in the US focal volume. Fig 1(b) shows the improvement in spatial resolution using US modulation (AC) compared to the conventional unmodulated light (DC), whereas fig. 1(c) shows a plot between SNR and modulation depth at different surface radiances.
This research project will contribute to a significant impact on the 3Rs. Replacement: better imaging will inform more accurate computational models; Reduction: imaging enables longitudinal studies on the same cohort, more accurate quantitative imaging allows fewer animals to be used in a study; Refinement: through the improved quality of research findings. This research has been funded by the National Centre for the Replacement, Refinement and Reduction of animals in research (NC3Rs).