Photoacoustic Tomography (PAT) / Photoacoustic Computed Tomography (PACT)
Photoacoustic tomography (PAT)
In photoacoustic tomography (PAT) the excitation laser source illuminates the sample/target homogenously. A nonfocus ultrasound detection records the photoacoustic waves at the boundary of the sample/target. The signals are recorded at multiple positions of the transducer. Typically a circular scanning geometry is used. Other scanning geometry such as linear scanning is also reported. In PAT image reconstruction method is used to form the cross-sectional image of the sample/target. Several reconstruction methods are also used. A simple and popular delay-and-sum reconstruction is typically used for circular geometry. Typical spatial resolution for PAT is around 0.2-0.7 mm (2-5 MHz detectors), depending on the center frequency and the bandwidth of the ultrasonic detectors used. The imaging depth in case of PAT is quite high. Upto several centimetre (5-7 cm) imaging depth have been reported in the literature. We are working on building low-cost, portable, fast PAT imaging system using pulsed diode lasers. In conventional PAT system typically Nd:YAG lasers are used a excitation sources. These are bulky, slow (10 Hz pulse repetition rate) and expensive, making such PAT systems difficult to translate into clinical use. On the contrary pulsed diode lasers are cheap, light-weight, portable, high repletion rate (several kHz). However, there is a compromise on the laser energy, which will impact the image quality (poor SNR). We believe pulsed diode laser based PAT systems will be easier to translate into clinical and pre-clinical applications.
Real-time temperature monitoring using photoacoustics:
Photoacoustic temperature sensing is based on the fact that the photoacoustic signal amplitude is dependent on the local temperature of the absorber. Thus by monitoring the PA signal amplitude one can monitor the local temperature. We demonstrate a pulsed laser diode (PLD) based photoacoustic temperature sensing (PATS) system for monitoring tissue temperature in real-time. The system is capable of providing local temperature information at high temporal resolution 1 millisecond and high sensitivity 0.31 oC. The temperature data measured with PLD-PATS system are compared with the data provided by the commercial fiber Bragg grating (FBG) sensor. The proposed system will find applications in radiofrequency ablation, photothermal therapy, and focused ultrasound, etc. used for cancer treatments. More details can be found in Upputuri et al., Optics Letters (2020).
Live chicken embryo imaging using photoacoustic tomography:
Chicken embryos have been proven to be an attractive vertebrate model for biomedical research. They have helped in making significant contributions for advancements in various fields like developmental biology, cancer research, and cardiovascular studies. However, a non-invasive, label-free method of imaging live chicken embryo at high resolution still needs to be developed and optimized. We have shown the potential of photoacoustic tomography (PAT) for imaging live chicken embryos cultured in bioengineered eggshells. Cross sections along different depths were imaged to gain knowledge of relative depth of different vessels and organs. Due to high optical absorption of vasculature and embryonic eye, images with good optical contrast could be acquired using this method. More details can be found in Sharma et al., Journal of Biophotonics (2020).
Second generation desktop Photoacoustic tomography:
Over the last few years we have made several technical improvement to make PAT faster, compact, low-cost. Merging all of them into a second generation low-cost, compact, fast, desktop pulsed laser diode based photoacoustic tomography (PLD-PAT-G2) we have built the fastest cross-sectional imaging speed of 0.5 seconds (2 Hz frame rate). Use of multiple single element ultrasound transducer, continuous scan instead of stop-n-go scan, and acoustic reflector to make the scanning system compact resulted 0.5 second scan speed without using any expensive array ultrasound transducer. In spite of low pulse energy we obtained high quality PAT cross-sectional imaging with 0.5 sec imaging speed and up to 3 cm imaging depth in biological tissue. Fast uptake and clearance of dye can be monitored with such system. More details can be found in Kalva et al., Optics Letters 44 (2019), Kalva et al., Journal of Visualized Experiments 147 (2019).
Pulsed laser diode Photoacoustic tomography (PLD-PAT):
Traditional PAT system uses Q-switched Nd:YAG laser as a pump source. This bulky, expensive pump laser is one of the reason getting PAT into clinical use is quite challenging. Moreover, the low repetition rate of this types of pump laser makes PAT data acquisition very slow. To improve the speed of imaging array of ultrasound transducers are used. However, transducer arrays are not easily available and the complicated electronics associated with the array transducer make such system very expensive. Another approach to overcome some of the challenges we have built a PAT system using low-cost, portable pulsed laser diode based photoacoustic tomography system (PLD-PAT). With one fourth the cost of traditional Q-switched pump lasers, ~200 gms weight of the laser head and a very high repetition rate (7 kHz), PLD-PAT holds promise for various clinical applications. The PLD is integrated directly into the PAT scanning system, making it very portable. With single element transducer, we were able to achieve an imaging speed of 3 seconds, at least 10 time faster than existing PAT systems. Still there are many challenges. The laser energy is quite small yet compared to traditional lasers. we were able to get ~1.4 mJ per pulse at 800 nm wavelength. In spite of low pulse energy we obtained high quality PAT cross-sectional imaging with 3 sec imaging speed and up to 2 cm imaging depth in biological tissue. PA signal can be observed from as deep as 3 cm within biological tissues. Good quality in vivo brain imaging can also be done with 5 sec imaging speed as well. Fast uptake and clearance of dyes/nanoparticles from the brain can be monitored with such PLD-PAT system. More details can be found in Upputuri et al., Biomedical Optics Express 6 (2015), Upputuri et al., Biomedical Physics & Engineering Express 1 (2015), Upputuri et al., Journal of Visualized Experiments 124 (2017), Upputuri et al., Journal of Biomedical Optics 22 (2017).
Compact PAT system with acoustic reflector:
A typical photoacoustic tomography (PAT) system uses Q-switched Nd:YAG laser for irradiating the sample and a single element ultrasound transducer (UST) for acquiring the photoacoustic data. Conventionally in PAT system, the UST is held in horizontal position and moved in a circular motion around the sample in full 2π radians. Horizontal positioning of the UST requires a large water tank to house and load on the motor is also high. To overcome this limitation, we used the UST in vertical plane instead of horizontal plane. The photoacoustic (PA) waves generated from the sample are directed to the detector surface using an acoustic reflector placed at 450 to the transducer body. Hence we can reduce the scanning radius which in turn will reduce the size of the water tank and load on the motor. Hence, the overall conventional PAT system size can be minimized. The use of acoustic reflector (made of stainless steel) does not affect the photoacoustic image quality, spatial resolution of the imaging system, or the bandwidth of the ultrasound detector. More details can be found in Kalva et al., Journal of Biomedical Optics 22 (2017).
Continuous versus stop-n-go scanning:
In a typical PAT system, photoacoustic (PA) waves are recorded using an ultrasound transducer rotating around the sample. Being economical and easily available, single element transducer (SET) is commonly employed. For each laser pulse the SET collects one time-resolved PA signal, known as an A-line. Acquisition of A-lines in a circular scanning PAT system by an SET can be done in two ways - (1) Stop-and-go scan, and (2) Continuous scan. We compared the two types of scanning methods in terms of image quality, signal-to-noise ratio (SNR), spatial accuracy, resolution, and scan-time for phantoms and in vivo imaging. We found that the image quality, spatial accuracy, and SNR did not change in continuous scans, as compared to stop-and-go scans. However, there was a significant decrease in scan time in continuous scans. This improvement in scan time was 2-4 folds for lasers with low pulse repetition rate (10 Hz), and up to 7-12 folds for lasers with higher pulse repetition rate (7 kHz). More details can be found in Sharma et al., IEEE Journal of Selected Topics in Quantum Electronics 25 (2019).
Handheld real-time clinical photoacoustic imaging system:
Translating photoacoustic imaging into clinical setup is a challenge. Handheld clinical real-time photoacoustic imaging systems are not common. We are working on integrated photoacoustic and clinical ultrasound imaging system by combining light delivery with the ultrasound probe for in vivo animal imaging and also for needle guidance. The open access clinical ultrasound platform allows seamless integration of photoacoustic imaging resulting in the development of handheld real-time photoacoustic imaging probe. Below is a movie showing the real-time imaging of sentinel lymph node in small animal as well as need guidance using photoacoustic imaging. The system can also be used for bladder imaging, non-invasive monitoring of nanoparticles clearance etc. More details can be found in Sivasubramanian et al., Journal of Biophotonics 11 (2018), Sivasubramanian et al., Journal of Visualized Experiments 128 (2017), and Sivasubramanian et al., Journal of Biophotonics 11 (2018).
7000 frames per second photoacoustic B-scan imaging:
Q-switched Nd:YAG laser with high pulse energy comes with low pulse repetition rate of few tens of hertz. This prevents them from being used in high frame rate photoacoustic imaging. Using a Pulsed laser diode up to 7000 Hz photoacoustic imaging (B-mode) was demonstrated and measured the flow rate of a fast moving object. A clinical ultrasound imaging system (Alpinion) with a liner array ultrasound transducer was used to capture and display the photoacoustic images. The excitation laser is ~803 nm in wavelength with ~1.4 mJ energy per pulse. This is the first report on 2-dimensional photoacoustic B-scan imaging with 7000 frames per second. We have demonstrated various flowrates ranging from 3-14 cm/s. This high framerate photoacoustic imaging can be useful for various clinical applications including cardiac related problems, where the blood flow rate is quite high, or other dynamic studies. More details can be found in Sivasubramanian et al., Biomedical Optics Express 7 (2016).
Label-free high framerate imaging of circulating blood clots:
Deep vein thrombosis (DVT) is a disorder when a blood clot (thrombus) is formed in one of the deep veins. These clots detach from the original sites and circulate in the blood stream at high velocities. Diagnosing these blood clots at an early stage is necessary to decide the treatment strategy. For label-free, in vivo and real-time detection, high framerate photoacoustic imaging can be used. In this work, a dual modal clinical ultrasound (US) and photoacoustic (PA) system is used for the high framerate PA imaging of circulating blood clots in blood at linear velocities up to 107 cm/sec. Blood clot had 1.4 times higher signal-to-noise ratio (SNR) in the static mode and 1.3 times higher SNR compared to blood PA signal in the flow experiments. This work demonstrates that fast-moving circulating blood clots are easy to recognize against the background PA signal and may aid in early diagnosis. More details can be found in Dhiman et al., Journal of Biophotonics 14 (2021) .