Imaging blood flow in bones using photoacoustics

When we think of bones, we tend to think of them as being rigid, passive, and lifeless structures that are solely meant to provide a framework to support the human body. But biologically, bones are anything but static. They are living tissues that house a network of blood vessels, produce blood cells, and are capable of remodelling and repairing themselves. Blood flow within bones is vital for healing and overall bone health. However, despite wide consensus about the vitality of blood flow within bones, it remains largely unexamined in modern clinical practice. This has more to do with technological gaps than a lack of interest.

Dr Jami Shepherd, a senior research fellow in the Department of Physics at the University of Auckland, is among a group of imaging scientists seeking to close that gap. Her work in combining photoacoustic imaging with existing ultrasound technologies offers a window into the vascular life of bones, which can be crucial to understanding, detecting, and treating bone diseases.

Why care about blood flow in bones

Blood supply to bones is rarely assessed in clinical settings, but that’s not because it’s insignificant. Studies have shown that adequate bone perfusion (blood flow) is essential to ensuring adequate oxygenation and delivery of nutrients within bones. This is crucial to bone growth, healing, and overall health. “There have been links found between reduced blood supply and bone diseases like osteoporosis and even bone death,” Jami says. “Or in oncology, if we can say something about the blood flow in a tumour in the bone, you might be able to say how aggressive it is and how it’s responding to treatment.”

Even in the case of fractures, adequate blood flow is essential to the healing process, as Jami explains by recalling her own experience. “Before I did this project, I had a fracture in my foot. It’s called a Jones fracture and it’s in an area that’s known to have low blood flow, so it took forever to heal. It did heal on its own, but sometimes they have to go in and perform a surgery to increase blood supply to that area.” 

Where are the technological gaps?

While current imaging techniques each have their strengths, none of them can accurately, safely, and non-invasively measure real-time blood flow within bones.

Ultrasound is safe, portable, inexpensive, and can provide real-time imaging. However, it’s still not the whole package. “Ultrasound is great for imaging soft tissues and measuring blood flow in large, fast-flowing vessels. However, existing clinical ultrasound techniques cannot image the interior of bones or measure blood flow in small, slow-flowing vessels in bone,” says Jami.

Filling the gaps with Photoacoustics

This is the gap that photoacoustic imaging fills. “Photoacoustic imaging is essentially an add-on to ultrasound which gives us chemical and functional details,” she says. A laser fitted onto the ultrasound probe sends a pulse of light, which is absorbed by red blood cells. These red blood cells heat up and expand, generating ultrasound waves which are recorded by the ultrasound probe at the surface of the tissue.

As Jami explains, “Basically, you’re using light to generate sound, then using an ultrasound probe to detect that sound and reconstruct the image. Since red blood cells absorb the light, you can image where the blood is; and if you take images over time, you can quantify how it’s moving.”

The advantage of photoacoustics is that it provides functional information, such as oxygen saturation and direction and speed of blood flow, as a very important add-on to the structural information that ultrasound provides. Using this method, it’s possible to non-invasively measure real-time data on blood flow within bones without needing to expose patients to radiation.

However, imaging of bone perfusion presents a unique problem. “Most ultrasound and photoacoustic systems in use today work under the principle that the entire body is made up of water or a uniform medium,” Jami says. “While that’s accurate for most of our organs and tissues, the bone, of course, is solid.” As high school physics tells us, sound travels at different speeds in different mediums. Sound travels in bone at more than twice the speed than it does through other tissues. Therefore, the physical properties of the bone have to be accounted for in order to reconstruct an accurate image using photoacoustics and ultrasound.

Jami and her international colleagues at TU Delft in the Netherlands are developing new models and software that correct for these unique properties of bone. With the support of external funding through the Dodd-Walls Centre and Marsden Fund, Jami has been able to supervise a PhD student (Caitlin Smith) and make significant advancements on this project. “We’re testing our approach on ex-vivo samples, with bone models, and on healthy volunteers to prove that we can measure blood flow accurately,” she says.

Further to proving that blood flow in bone can be accurately measured, the next step in their research is to detect differences between normal and impaired bones. “We want to measure how blood flow is different for someone with osteoporosis. How a cancerous tumour affects perfusion; how a tumour responds to treatment; or how different blood flow is at various stages of fracture healing,” says Jami. “Ideally, since early detection is key, we want to discover changes in blood flow before structural degradation occurs.”

Clinical future

Photoacoustic imaging is relatively cheap because the technique can be integrated into existing ultrasound machines with the addition of a laser and new software. It also doesn’t have the logistical challenges that come with machines like MRIs and CTs, where you need an entire dedicated room. So it would seem that the barrier to clinical use is relatively low. Yet there are stages of more research, clinical trials and approvals that it would need to go through before being introduced clinically. Then come the subsequent challenges of convincing clinicians of this new technology, training sonographers in using it and integrating it into clinical workflows.

If successful, Jami and her team’s research could lead to earlier diagnoses, better monitoring of bone healing, and even help in tailoring treatments for diseases like osteoporosis and bone cancer. And it may shift our entire perception of bones – not as mere structural support, but as dynamic, living tissues deserving of closer scrutiny.