A high frame rate photoacoustic imaging system and its applications to perfusion measurements

A high frame rate photoacoustic imaging system is described. Applications of this system to perfusion measurements are also presented as a demonstration of its potential usage. The system consists of an ultrasound front-end sub-system for acquisition of acoustic array data. The ultrasound front-end sub-system is also known as the DiPhAS (digital phased array system) which is capable of simultaneously acquiring radio frequency data from 64 transducer channels at a rate up to 40 MSamples/sec per channel. In this study, an ultrasonic linear array with a 5 MHz center frequency was employed as part of the integrated photoacoustic probe. The photoacoustic probe also had two linear light guides mounted on the sides of the ultrasonic array for broad laser irradiation from a Q-switched Nd:YAG pulsed laser. After the acquired ultrasound array data were transferred to a personal computer via a high speed digital I/Q card, dynamic focusing and image reconstruction were done off-line. The 64-channel array data can be acquired and transferred every 4 milliseconds, thus making the frame rate of the system up to 250 Hz. The actual frame rate of the current system is limited by the pulse repetition frequency of the laser at 15 Hz. To demonstrate capabilities of the system, photoacoustic perfusion measurements with gold nanorods were performed. A previously proposed time-intensity based flow estimation technique utilizing the shape transitions of gold nanorods under laser irradiation was employed. Good estimation results were achieved and potential of this high frame rate photoacoustic imaging system is clearly demonstrated.