Developing a portable, high-speed, low-cost desktop photoacoustic tomography imaging system
Kalva, Sandeep Kumar
Date of Issue2019
School of Chemical and Biomedical Engineering
Photoacoustic tomography (PAT) is a non-invasive, hybrid biomedical imaging modal- ity. Conventional PAT system uses Nd:YAG-OPO laser as excitation source and a single-element ultrasound transducer (SUT) for detecting the generated photoacoustic (PA) waves from the imaging object. Usually Nd:YAG-OPO laser is bulky, heavy and needs an optical table to deliver non-fluctuating laser beam and it needs external op- tics (reflectors/prisms) to direct the laser beam onto the sample (there are portable OPO lasers from OpoTek, EKSPLA, and various other companies that do not require optical table but they are more expensive than conventional Nd:YAG-OPO lasers). Also, the detecting element of the SUT is placed at orthogonal direction to the laser illumination for deep tissue imaging. Due to this configuration, the length of the transducer body and the connecting cable requires a lot of space inside the water (coupling medium) tank, thereby, increasing the size of the PAT scanner area. All these (laser, additional optics, SUT alignment) make the whole PAT imaging system very large in size and non- portable. Therefore, it is limited to bench-side laboratory imaging purposes. Moreover, this Nd:YAG laser is very expensive and has low pulse repetition rate of (10-100) Hz. So, one SUT takes several minutes to acquire a single cross-sectional PA data. This in turn, inhibits real-time imaging. The acquired PA data using this SUT is reconstructed conventionally using a simple delay-and-sum algorithm to obtain cross-sectional PA images. When this technique is employed for reconstruction, there will be blurring in tangential direction due to the effect of SUT aperture size, thereby, resulting in the degradation in quality (poor tangential resolution) of the PA images, especially when the target is far from the scanning center. These are some of the limitations in using a conventional PAT system in terms of size, cost, speed and image quality that inhibit the translation of the PAT system into clinics for real-time high-speed imaging applications. Thus, there is a need for a low- cost, portable and high-speed PAT imaging system that can render better quality images. To minimize the size of the overall PAT system, initially we have reduced the space occupied by the SUT in the PAT scanner. To achieve this, we have placed the transducer vertically instead of conventional horizontal direction and augmented it with an external acoustic reflector to the transducer body. This will reflect/direct the incoming PA waves from the imaging object to the detector element of the vertically placed SUT. We have validated the effectiveness of using the acoustic reflector augmented SUT (SUTR) for PA imaging using phantom and in vivo studies. Further to overcome the space occupied by the bulky Nd:YAG-OPO laser, we have used pulsed laser diode (PLD) as an alternate excitation source. With advancement in laser technology, pulsed laser diodes have come into existence and been used effectively for PA imaging over the past few years. These PLD lasers are compact in size, light in weight and less expensive compared to the conventional Nd:YAG lasers. Taking these advantages into consideration, we have used a PLD laser directly inside the PAT scanner by mounting it on top of the imaging area (PLD laser is placed directly above the imaging object at the scanning centre). Due to this we saved a humongous amount of space occupied by the previously used Nd:YAG-OPO laser (placed outside the scanning area). In addition, there was no requirement of using any additional optics in this configuration as the laser source directly illuminates the sample. Hence, by employing the PLD excitation source, we have made the PAT system more compact, portable and easy to use for bed-side imaging. Moreover, the pulse repetition rate of this PLD laser is very high (2 kHz) compared to Nd:YAG laser (10 Hz). This is more advantageous for high speed imaging. Therefore, to improve the imaging speed, we have used PLD along with 8 SUTRs rotating in 45 degree (instead of 1 SUTR rotating in 360 degree) and obtained a high scan speed of 0.5 s in vivo. Experimentally, all these transducers cannot be placed exactly at the same distance from the scanning center manually. Hence the acquired PA data from each transducer needs to be reconstructed with the corresponding radii while using delay-and-sum al- gorithm. This requires the exact location of each transducer from the scanning center. Hence, we proposed a calibration method to find the scanning radius of each transducer. We validated the efficacy of this method using numerical and experimental data. To improve the image quality in terms of tangential resolution, we have used a mod- ified delay-and-sum algorithm, an improved version of conventional simple delay-and- sum algorithm for reconstructing the PA images and observed three-fold improvement in tangential resolution. In addition to it, this algorithm also preserved the shape of the target object which is very useful for diagnosis and treatment purposes. Overall, by combining all the innovations described earlier, we have developed an affordable, portable, high-speed desktop PLD based PAT imaging system. Using this system we achieved a high spatial resolution of 165 μm and an in vitro imaging depth of 3 cm. We also demonstrated its dynamic imaging ability at a high scan speed of 2 frames/s by monitoring the fast uptake and clearance process of indocyanine green (ICG) dye in rat cortical vasculature.