When using heat and other forms of radiation to ablate tumors, it is usually difficult to know just how hot the tissues around your target are getting, particularly when working deep within the body. MRI and ultrasound can be useful in many cases, but they have limitations and can produce misleading readings. To have a better option, researchers at Duke University have been working on using photoacoustic imaging as a tool to remotely measure the temperature of deep-seated tissues.
Photoacoustic imaging involves shining light onto an object, heating it ever-so-slightly, and then using ultrasound to detect the waves that result from the thermal expansion of the tissue. What’s interesting for this research is that the conversion of light via heat into sound waves is dependent on the temperature of the tissue being studied.
The researchers used this fact to create a highly sensitive phototoacoustic system that can detect the differences in signals that the ultrasound measures when the tissue temperature changes.
An announcement about the study from Duke University provides a good explanation on the challenges involved:
“The conversion efficiency between light and sound is temperature-dependent, so we know it’s possible to measure temperature by listening to soundwaves generated by light,” [Junjie Yao, assistant professor of biomedical engineering at Duke] said. “However, we haven’t previously been able to measure absolute temperature because the process itself needs to know how many photons are reaching the tissue, which is technically challenging.”
To get around this missing information, Yao is working with Pei Zhong, a professor in the department of mechanical engineering and materials science who has generated deep tissue heating using high intensity focused ultrasound (HIFU). Their team devised a new approach named thermal-energy-memory-based photoacoustic thermometry, or TEMPT, which uses photoacoustic imaging to measure the tissue’s “thermal memory.”
With TEMPT, researchers take a baseline temperature reading before bombarding the tissue with a burst of nanosecond-long laser pulses. The pulses temporarily increase the tissue’s temperature, which is then measured using another photoacoustic pulse.
The research team was able to use these measurements and a mathematical model to estimate the absolute temperature without knowing how many photons were delivered.
Study in journal Optica: Thermal memory based photoacoustic imaging of temperature…