Researchers at the Keck School of Medicine of USC and the California Institute of Technology (Caltech) have developed an innovative imaging technique that can overcome significant limitations of existing medical imaging. In a proof-of-concept study, they demonstrate that a new, non-invasive method can be used to quickly create three-dimensional images of the human body, from head to toe.
The technique combines ultrasound with photoacoustic imaging, in which sound waves are generated by light. By combining these methods, the system can simultaneously produce images of both body tissue and blood vessels. The results of the research have been published in Nature Biomedical Engineering and demonstrate the broad potential of this new imaging approach.
Limitations of existing imaging
Medical imaging is indispensable in modern healthcare and plays a crucial role in the diagnosis and treatment of trauma, infections, cancer and chronic conditions, among other things. However, current “gold standard” techniques, such as ultrasound, X-ray, CT and MRI, have clear limitations. These range from high costs and long scan times to limitations in image range, depth and level of detail.
‘The importance of medical imaging for clinical practice can hardly be overestimated,’ says Charles Liu, professor at the Keck School of Medicine and co-senior author of the study. ‘Our team has identified the main limitations of existing techniques and developed a new approach to address them.’
Two techniques in one platform
For the first time in humans, the research team combined two imaging techniques: rotational ultrasound tomography (RUST) and photoacoustic tomography (PAT). Together, they form the new RUS-PAT platform.
RUST works similarly to conventional ultrasound, but does not use a single detector to create a two-dimensional image. Instead, the system uses an arc of detectors to build up a three-dimensional volumetric image of the tissue. PAT focuses on blood vessels: laser light is absorbed by haemoglobin in the blood, causing vibrations that emit ultrasonic signals. These signals are measured by the same detectors and translated into 3D images of the vascular system.
According to co-senior author Lihong Wang, the combination of these techniques is essential. ‘We have developed a new way in which ultrasound and photoacoustic imaging work together. This allows us to create much more detailed images at clinically relevant depths, without ionising radiation or strong magnets.’
Fast images of multiple body parts
To demonstrate its broad applicability, the researchers used RUS-PAT to image various body parts, including the brain, breast, hand and foot. Brain imaging involved patients with traumatic brain injury who underwent surgery in which part of the skull was temporarily removed. In this setting, the system was able to capture both tissue structures and blood vessels over an area of approximately ten centimetres in about ten seconds.
RUS-PAT builds on previous work by the USC-Caltech team, which demonstrated that PAT can also be used to image brain activity.
Advantages over MRI and CT
According to the researchers, the new platform offers several advantages. The system is cheaper to build than an MRI scanner, avoids the radiation required for X-ray and CT examinations, and provides richer images than conventional ultrasound. ‘When you look at cost, image range, spatial resolution and scan time, this platform addresses many of the key bottlenecks in current medical imaging,’ said Liu.
The possibilities of RUS-PAT extend beyond a single specialism. Brain imaging is crucial in conditions such as stroke, traumatic brain injury and neurological diseases. Breast imaging plays a key role in the diagnosis and treatment of one of the most common forms of cancer worldwide.
‘Photoacoustics opens up a new field of research in humans,’ says Jonathan Russin, co-first author of the study. ‘We believe this technology could be of great importance for the development of new diagnostics and patient-specific therapies.’ Fast, accessible imaging of the foot can also be valuable for people with diabetes and vascular disease. According to Tze-Woei Tan, the technique can help to identify high-risk situations earlier and support targeted interventions.
Further development needed
Although the results are promising, further development is needed before RUS-PAT can be used clinically. A major challenge in brain imaging is that the human skull distorts signals. The Caltech team is therefore investigating new solutions, such as adjustments in ultrasonic frequencies. Work is also being done to further improve and standardise image quality.
‘This is an early but important step,’ concludes Liu. ‘We have shown that RUS-PAT can produce medically relevant images of multiple body parts. We are now further refining the system with a view to future clinical use.’
MRI innovations
Last year, researchers at Children's Hospital Los Angeles developed a new method to correct errors in MRI images of cerebral blood flow. Using advanced mathematical models, missing or inaccurate measurement data can be reconstructed without the need for an additional scan. This is important because motion artefacts and complex vascular structures often make it impossible to correctly image all brain vessels.
These models have been tested on hundreds of phase-contrast MRIs and have proven to be reliable, even in patients with abnormal blood flow. A major advantage is that the method can be applied to existing 3T MRI systems. In the future, the researchers want to enable real-time error correction during the scan in order to diagnose neurological disorders earlier and more accurately.
