A High-performance Compact Photoacoustic Tomography System for In Vivo Small-animal Brain Imaging

The overall goal of this procedure is to demonstrate the performance of a low-cost, compact photoacoustic system for in vivo brain imaging of small animals. Photoacoustic tomography or PAT is a hybrid imaging modality combining light and sound. PAT offers in vivo deep tissue imaging of organ to organelles with very high optical contrast.

This work presents a compact, high speed pulsar laser diode based photoacoustic tomography system for in vivo cross-sectional brain imaging in rats. Demonstrating the procedure will be Dr.Paul Kumar Upputuri, a research fellow, Sandeep Kumar Kalva, a PhD student, and Vijitha Periyasamy, a project officer from our lab. To begin the system preparation, turn on the pulsed-laser diode mounted in the circular scanner assembly.

Use the laser driver unit to set the repetition rate to 7, 000 Hertz. Set the laser power supply to 3.1 volts to achieve the pulse energy of 1.42 millijoules. Then connect a focused ultrasonic emersion transducer to a pulser-receiver unit.

Mount the transducer in its holder under the circular scanning plate, with the transducer oriented towards the center of the plate. Place the scanning assembly in an acrylic tank, with a polyethylene imaging window in the bottom of the tank. Add water to the tank until the transducer is fully immersed.

Verify that the photoacoustic signal is being detected by the pulser-receiver unit before proceeding to imaging. To begin the imaging procedure, obtain a healthy female rat. Following anesthesia, trim the fur on the scalp with a hair clipper.

Gently apply depilatory cream to the trimmed area. After four to five minutes, remove the cream with a cotton swab. Apply sterile ocular ointment to the rat's eyes to prevent dryness and to block scattered laser beams.

Fix a custom made imaging bed equipped with a breathing mask on a lab jack with surgical tape. Place the animal in a prone position on the imaging bed, with its nose and mouth in the mask. Secure the animal in the imaging bed with surgical tape.

Position the imaging bed assembly on the optical table. Connect the breathing mask to an anesthesia machine, and begin delivering the inhaled anesthetic. Affix a pulse oximeter to the animal's tail or hind paw to monitor its physiological condition throughout the procedure.

Apply a colorless ultrasound gel to the scalp of the animal. Ensure that the imaging bed assembly is securely positioned under the scanner, with the animal's scalp centered under the imaging window. Carefully raise the imaging bed until the ultrasound gel contacts the imaging window, taking care to minimize the pressure against the animal's head.

Acquire test images and adjust the vertical position of the bed until the imaging cross-sectional plane of the brain is at the focus point of the ultrasonic transducer. Then, set the parameters for the experiment and start the scan. Monitor the animal during the imaging period.

Use reconstruction software to obtain the cross-sectional brain image. Once imaging is complete, discontinue anesthesia and allow the animal to recover fully under observation. cross-sectional images of a healthy female rat brain were obtained with non-invasive PLD PAT imaging.

Scan times ranged from five to 20 seconds. The transverse sinus, superior sagittal sinus, and cerebral veins were visible at all tested scan speeds. The same performance was observed when the procedure was repeated with a second healthy female rat.

This compact photoacoustic system is used to acquire photoacousitc images of brain vasculature. The results promise that the system can provide high quality individual brain images in scanning time as low as five seconds. Future applications of the developed PAT system include brain tumor imaging, imaging organs in small animal, contrasts flowing in and out of the body, and other therapeutic applications.