Remote temperature estimation in intravascular photoacoustic imaging.

Intravascular photoacoustic (IVPA) imaging is based on the detection of laser-induced acoustic waves generated within the arterial tissue under pulsed laser irradiation. In general, laser radiant energy levels are kept low (20 mJ/cm(2)) during photoacoustic imaging to conform to general standards for safe use of lasers on biological tissues. However, safety standards in intravascular photoacoustic imaging are not yet fully established. Consequently, monitoring spatio-temporal temperature changes associated with laser-tissue interaction is important to address thermal safety of IVPA imaging. In this study we utilize the IVUS-based strain measurements to estimate the laser-induced temperature increase. Temporal changes in temperature were estimated in a phantom modeling a vessel with an inclusion. A cross-correlation-based time delay estimator was used to assess temperature-induced strains produced by different laser radiant energies. The IVUS-based remote measurements revealed temperature increases of 0.7+/-0.3 degrees C, 2.9+/-0.2 degrees C and 5.0+/-0.2 degrees C, for the laser radiant energies of 30 mJ/cm(2), 60 mJ/cm(2) and 85 mJ/cm(2), respectively. The technique was then used in imaging of ex vivo samples of a normal rabbit aorta. For arterial tissues, a temperature elevation of 1.1 degrees C was observed for a laser fluence of 60 mJ/cm(2) and lesser than 1 degrees C for lower energy levels normally associated with IVPA imaging. Therefore, the developed ultrasound technique can be used to monitor temperature during IVPA imaging. Furthermore, the analysis based on the Arrhenius thermal damage model indicates no thermal injury in the arterial tissue, suggesting the safety of IVPA imaging.