Photoacoustic tomography, or PAT, is an imaging hybrid biomedical imaging modality, combining both optical and ultrasound imaging for monitoring dynamic changes in small animal brains. This is a cost-effective, portable high speed imaging system that uses imaging speed as short as half a second. Demonstrating the procedure with Sandeep will be Praveen, a PhD student, Dr.Paul, a research fellow, and Rhonnie, a research assistant.
Begin by mounting all single element ultrasound transducers onto each transducer holder one at a time, such that the surface of each acoustic reflector faces toward the center of the scanning area. Use connecting cables to connect each transducer cable to the low noise signal amplifier. Switch on the power supply of the chiller and turn on the chiller switch to set the temperature between 20 to 25 degrees Celsius.
Switch on the low voltage power supply and slowly turn the current control to 0.3 amps. Set the voltage to 12 volts and verify that the current does not exceed 0.1 amp. Switch on the high voltage power supply and press the preset button to set the current to one amp and the voltage to zero volts.
Enable the output button and switch on the power supply of the function generator. Press the recall button and select a two kilohertz configuration to generate the laser pulses at this repetition rate. Place the acrylic tank inside the scanner and fill the tank with water until the detecting surfaces of the transducers are completely immersed.
Then switch on the power supply of the low noise signal amplifier. Place the anesthetized rat in the prone position on the workbench. After confirming a lack of response to toe pinch, use a hair trimmer to remove the fur on the scalp.
Gently apply depilatory cream to remove the fur completely from the exposed skin using a cotton swab to remove the cream after four to five minutes. Apply ointment to animal's eyes and mount a custom made animal holder equipped with a breathing mask on a lab jack. Place the rat in the prone position on the holder with the head resting on the horizontal platform of the holder.
Use surgical tape to secure the animal to the holder. Clamp the pulse oximeter to one hind limb to monitor its physiological condition. Use a cotton tipped applicator to apply a layer of colorless ultrasound gel to the scalp.
Then adjust the lab jack position to the center of the scanner. Adjust the height of the jack manually so that the imaging plane is at the center of the acoustic reflector. After setting the parameters in the data acquisition software for a 360 degree acquisition scan, enable the output of the function generator to turn on the pulsed laser diode laser emission.
Slowly increase the voltage of the variable high voltage power supply to 120 volts for maximum per pulse energy. Run the data acquisition software program to rotate all eight of the transducers 360 degrees over a four second scan time. Disable the output of the function generator to turn off the laser emission.
Using the reconstruction algorithm in the data processing software, determine the scanning radius of all eight of the transducers by trial and error, using the back-projection algorithm. Set the parameters in the data acquisition software for a 45 degree acquisition over a 0.5 second scan time. Enable the output of the function generator to turn on the laser emission.
Run the data acquisition software program to rotate all eight of the transducers 45 degrees to obtain the initial control data. Disable the output of the function generator to turn off the laser emission. Next, inject 0.3 milliliters of indocyanine green into the tail vein of the rat.
Enable the output of the function generator to turn on the laser emission. Then run the data acquisition software to acquire A-lines over a 0.5 second scan time in a 45 degree rotation. At the end of the acquisition, use the back-projection algorithm to reconstruct the cross sectional brain image from the saved A-lines.
Here, brain images from a 98 gram female rat, collected at four and 0.5 second scan speeds are shown. The sagittal sinus and the transfer sinus are clearly visible in both images. The photographs show the same region of the rat brain before and after removing the scalp.
Photoacoustic tomography imaging can achieve the same visualization noninvasively with the skin and skull intact. In this plot, increases in the average photoacoustic signal in the sagittal sinus due to increases and subsequently decreases in the optical absorption by indocyanine green at 860 nanometer wavelengths over time can be observed. Always take extra care when injecting the ICG dye into the tail vein of the animal.
This low cost desktop photoacoustic tomography system can be used for the dynamic imaging of small animal brain vasculatures with high special resolution at 0.5 seconds scan speed. Other applications of this imaging system include brain tumor imaging, the imaging of different organs in small animals and other therapeutic applications.
