Determining the severity of burns, diagnosing rheumatism in good time, identifying the risk potential of plaque in the carotid artery – photoacoustics may facilitate all these procedures. The potential of this novel imaging method is currently being tested by medical engineers from Bochum and their research partners under the umbrella of the EU project “Fullphase”.
“Photoacoustics is said to combine the best of two worlds,” says Prof Dr Georg Schmitz from the Chair for Medical Engineering. It refers to the world of light and to the world of sound. The process is based on the photoacoustic effect that was discovered by Alexander Graham Bell in 1880. Light at a certain wavelength irradiates the body where tissue takes up a percentage of the radiation; this process is called absorption. Due to the thus absorbed energy, the temperature rises by the fraction of a degree, the tissue expands momentarily and an acoustic wave is created. An ultrasound scanner analyses the signals that are generated within the body. As the analysis is time-resolved, the scanner does not show a static image but a “film”, just like in ultrasonic imaging.
Photoacoustics provides different insights into the body than an ultrasonic scan. It assesses the capacity to absorb light to differentiate between different types of tissue. Different tissues absorb different amounts of light, resp. they absorb light at different wavelengths. An example: blood that is low in oxygen absorbs short-wavelength radiation better than blood that is rich in oxygen. Using this method to track the changes in oxygen levels in tumours, doctors could be able to judge which tumour stage they are dealing with. Whether this is going to be possible in future depends on how deeply the irradiated light penetrates the body. Radiation in the near-infrared spectrum, i.e. just below visible red light, is best suited. If too much is used, however, it may result in burns. The researchers involved in the “Fullphase” project work in accordance with operational health and safety standards. “Consequently, we comply with the threshold limit values that apply in a normal workplace, e.g. for scattered light in a laser-welding machine which is handled by the operator eight hours a day,” explains Georg Schmitz. “When deployed correctly, the laser radiation we use is entirely harmless to the patient.”
Schmitz plans to render photoacoustics fully functional with up to four different laser wavelengths simultaneously. The medical engineers explore the optimal method of economising the energy budget determined by threshold limit values. They analyse how much energy they should use to irradiate light at a certain wavelength in order to get the best possible image without exceeding the threshold limit value. For the purpose of those tests, the researchers produced objects from polymers like PVC in the lab, so-called phantoms (fig. 1). They used suitable chemicals to create scattering particles in order to simulate certain types of tissue. With the photoacoustic system that is currently available, the radiation penetrates at a depth of approx. one and a half centimetres. For many applications, e.g. examination of superficial blood vessels, this is sufficient, as Schmitz concludes.
The team from Bochum is also researching into image reconstruction. They eradicate artefacts and search for algorithms that are best suited for calculating the signal’s sources on the basis of the measured acoustic waves, i.e. the tissue from which the signal originated. The ultrasonic transducer receives signals from adjoining locations at many channels at the same time – 256 is a typical number. Schmitz and his colleagues use it to reconstruct which object the machine is currently “seeing”. To this end, they continue to optimise the existing analytical algorithms, falling back on research done in other fields as well, not just in medicine: “Researchers in other disciplines likewise study algorithms that describe wave propagation, including seismographers who look for structures beneath the surface of the earth,” says the engineer.
The way the project has been going to date is certainly encouraging. This is because the “Fullphase” consortium has made a huge progress, which Georg Schmitz would not have thought possible in the short space of time. The industrial partners have developed a laser with integrated ultrasonic transducer that is so small it fits into the palm of your hand. “In most studies that research into photoacoustic imaging, scientists utilise high-performance lasers that fill out an entire table,” says Schmitz. The “Fullphase” team’s trick: laser diodes. Originally, this technique generated a pulse power of a mere several hundred Watt during very short pulses. That’s nothing compared to the large lasers that operate in the Megawatt range. Today, the diodes manage several Kilowatts; but this means their performance is still below that of the large lasers by the factor one hundred. The “Fullphase” team compensates by irradiating laser pulses at much shorter intervals than it would be possible with a large laser.
The experiments conducted in the course of the project, which is running until October 2016, are about to enter the preclinical phase; that means the first tests with volunteers have been scheduled. A follow-up project has already been submitted, and the medical engineers are currently establishing a collaborative partnership with the Universitätsklinikum Bergmannsheil. Their objective: testing photoacoustic imaging for assessing severe burns.
So far, “Fullphase” has been proceeding according to plan. Georg Schmitz’s interim conclusion: “We are even slightly ahead of schedule. The Fullphase researchers are currently the only teams worldwide who have an integrated ultrasonic transducer with built-in laser directly at their disposal that can be used to experiment. I am thrilled by the options that are open to us.”
Further information: www.fullphase-fp7.eu