Lidai Wang and his colleagues at Washington University in St. Louis (USA) describe a novel technique to measure blood-flow speed in deep tissue in “Ultrasound-heated photoacoustic flowmetry,” recently published in the Journal of Biomedical Optics.
The article describes the development of photoacoustic flowmetry assisted by high-intensity focused ultrasound (HIFU). This method employs HIFU to generate a heating impulse in the flow medium, followed by photoacoustic monitoring of the thermal-decay process.
Blood-flow speed is important in assessing tissue functionality and diagnosing many diseases. The unique attributes of photoacoustic (PA) imaging make it well suited for blood-flow measurement.
Photoacoustic flowmetry is highly sensitive to optical absorption, providing high contrast for blood sensing. Photoacoustic flowmetry can also be readily integrated with other photoacoustic-imaging modes, providing naturally co-registered imaging of vasculature, hemoglobin concentration, oxygen saturation, and blood flow as well as sensing of oxygen metabolic rate.
Depth of biophotonics imaging increased
Current PA flow-imaging techniques are limited to shallow depth (within ~1 mm) in biological tissue. Based on focused ultrasonic heating and PA thermal clearance detection, the new technique extends photoacoustic flowmetry to deep tissue.
Fig. 1 Photoacoustic signal decays under varied flow speeds (0-41 mm∙s–1).
First, a focused ultrasonic transducer generates a small, locally heated region in flowing blood. Blood flow accelerates thermal clearance, and PA temperature sensing is employed to monitor the thermal-clearance process.
The heat transfer and detection are mathematically modeled to calculate the flow speed from the PA signal. Both the ultrasonic heating and photoacoustic detection can focus beyond the optical diffusion limit (~1 mm deep in tissue), providing high spatial resolution at depths.
Blood flow speeds up to 41 mm·s–1 have been experimentally measured in a tube covered by 1.5-mm thick tissue, which can potentially be further improved.
Fig. 2 Measured blood-flow speed (vmeas) versus set-flow speed (vset).
The authors cite several advantages of ultrasound-heated photoacoustic flowmetry in biophotonics:
- The PA signal is generated from optical absorption and thus has a clean background in blood-flow measurement.
- The PA effect is inherently sensitive to temperature.
- The method does not rely on discrete scatterers or absorbers and can be potentially applied to homogenous media, e.g., flowing lymph.
- Ultrasonic heating and PA detection potentially can be performed by the same ultrasonic transducer, producing naturally confocal acoustic heating and detection.
Along with Lidai Wang, co-authors of “Ultrasound-heated photoacoustic flowmetry” include Junjie Yao; Konstantin I. Maslov; Wenxin Xing, and SPIE Fellow Lihong V. Wang, editor of the Journal of Biomedical Optics.
Photoacoustic tomography a ‘Hot Topic’
At SPIE Photonics West in February, SPIE Fellow Lihong Wang of Washington University in St. Louis (USA) discussed, “Photoacoustic Tomography: Ultrasonically Beating Optical Diffusion and Diffraction” as part of the BiOS Hot Topics session.
Wang, editor-in-chief of the SPIE Journal of Biomedical Optics, noted that a decade of research has pushed photoacoustic CT to the forefront of molecular-level imaging.
While modern optical microscopy has resolution and diffraction limitations, noninvasive functional photoacoustic CT has overcome this limit, Wang noted.
The imaging technique offers deep penetration with optical contrast and ultrasonic resolution of 1 cm depth or more — up to 7 cm of penetration in some cases.
This opens up applications in whole-body imaging, brain function, oxygen saturation, label-free cell analysis, and noninvasive cancer biopsies.
The ability to image the brain in vivo is “simply exhilarating,” the biophotonics expert said.