Photoacoustic imaging is a hybrid imaging technique that has received considerable attention in biomedical studies. In contrast to pure optical imaging techniques, photoacoustic imaging enables the visualization of optical absorption properties at deeper imaging depths. In preclinical small animal studies, photoacoustic imaging is widely used to visualize biodistribution at the molecular level. PAI systems used single-element US transducers to acquire data. Single-element transducers scanned around the animals to produce tomographic images.
1. Introduction
Photoacoustic imaging (PAI) is a non-invasive biomedical imaging technique based on the photoacoustic (PA) effect that involves energy transduction from light to sound
[1]. In recent decades, PAI has gained considerable attention in biomedical research owing to its unique characteristics
[2][3][4]. PAI is cost efficient and easy to implement compared to other medical imaging techniques, such as X-ray imaging, X-ray computed tomography, magnetic resonance imaging, and positron emission tomography. In addition, PAI is free from ionizing radiation, which may cause side effects in biological tissues. Similar to optical imaging techniques, PAI can provide molecular functional information using multispectral data acquisition
[5][6][7][8][9], which is not available in ultrasound imaging (USI). By contrast, PAI can deeply penetrate biological tissue, similar to USI, whereas the typical imaging depth of pure optical imaging is ~1 mm (i.e., optical mean free path)
[10][11].
The PA effect occurs when a short (~10 ns) pulsed laser is absorbed by chromophores in biological tissue. The absorbed light energy is released as thermal energy, and thermoelastic expansion causes a volumetric change in the surrounding tissues. Because thermal energy is rapidly dissipated owing to the short pulse width of the excitation laser, the expanded tissues shrink to their original size. The repeated volume changes generate vibrations that propagate in the form of acoustic waves called PA waves. By detecting these acoustic waves using US transducers, PA images can be obtained through an image generation procedure, which is similar to that of US image generation.
PAI has another unique characteristic: scalable resolution and imaging depth according to the target
[12]. Based on the principles of PAI, the resolution and imaging depth can be controlled by adjusting the light illumination method and transducer geometry. When light is tightly focused, high-resolution (~5–50 μm) photoacoustic microscopy (PAM) can be implemented
[13][14][15]. However, PAM is limited in imaging depth (~1 mm); thus, it is primarily applied to imaging superficial areas, including the ear, eye, brain, and skin in small animals
[16][17][18][19][20][21]. The imaging depth of PAI can be enhanced (up to ~10–20 mm) by sacrificing its spatial resolution (~100–500 μm)
[22]. In such configurations, the light is moderately focused or even diffused in biological tissue, and the resolution of images is determined by the acoustic focal zone of the US transducers.
2. Photoacoustic Imaging with Single-Element Transducer
Wang et al. demonstrated a tomography device with cross-sectional brain images of rats using a Nd:YAG pulsed dye laser
[23]. In the system, an unfocused US transducer with a center frequency of 10.4 MHz was submerged in a water tank to detect PA waves. A circular rotational scan using a US transducer with a step angle of 1.5° and scanning radius of 3 cm was performed around a rat’s head to acquire cross-sectional PA images of the brain. To increase contrast, light-absorbing metal nanoshells were injected into the rat intravenously. The resulting images visualized the blood vessels in the brain with a spatial resolution of ~60 μm and an imaging depth of ~1 cm. Differential images before and after the injection of the contrast agent delineated the blood vessels in the brain. However, the imaging time required for a single cross-sectional image was slow (~24 min) owing to the mechanical scanning of the transducer using a stepping motor.
Ma et al. developed a multispectral PAI system to improve the imaging speed using a Nd:YAG pulsed optical parametric oscillator (OPO) laser
[24]. The system was equipped with a single-element focused US transducer with a center frequency of 3.5 MHz. Interestingly, a rotational stage was used to move the imaging targets instead of scanning the US transducer. The thoracic and neck regions of mice were successfully visualized with a spatial resolution of 32 μm and depth of ~7 mm. The imaging speed required for a single cross-sectional image was reduced to 9 s owing to the faster rotational scan with an angular speed of 40°/s, which corresponded to 6.7 rotations per minute.
Deng et al. also attempted to improve the imaging speed using a slip-ring-based PAI system with a Ti:Sapphire laser
[25]. They used two single-element focused US transducers with a center frequency of 5 MHz. The two transducers were symmetrically positioned in the slip-ring-based scanning stage; thus, full data for the cross-sectional image could be obtained with only half a rotation. Tomographic images of the abdominal regions of mice were acquired. The resulting images had a spatial resolution of ~129 μm with a total imaging time of 9 min.
For a simple configuration, Jeon et al. used raster scanning with a single-element focused transducer to achieve whole-body images of mice in vivo
[26]. The system could switch the US transducers between two different center frequencies of 5 and 40 MHz. Using raster scanning, the system obtained landscape images by projecting the most significant signal in the volumetric data to the transverse plane. The two different center frequencies of the transducers produced different imaging performances: high spatial resolution (~85 μm) in the shallow region (~3.1 mm) and a relatively low spatial resolution (~590 μm) in the deeper region (~10.3 mm). However, the imaging speed of this system was limited (~20 min for a scanning region of 60 × 32 mm
2) owing to the slow mechanical scanning of the transducer. Although volumetric data were obtained in this system, three-dimensional (3D) visualization was limited owing to breath-related distortion during the long scanning time. Recently, Lee et al. improved the system to achieve both US and PA images simultaneously and produced breath-compensated 3D PA whole-body images of mice
[27][28]. Breath-related distortion was corrected by segmenting the skin profile in US images, realigning the signals in the axial direction, and applying the realignment parameters to the corresponding PA data. Breath compensation was also applied to the multispectral data to generate 3D hemoglobin oxygen saturation whole-body maps of mice.
3. Conclusion
PAI is a promising biomedical imaging technique that can be used to assess biodistribution in small animals in vivo. By detecting the optical absorption characteristics of biological tissue with US resolution, PAI can visualize molecular functional information in deep tissue better than pure optical imaging techniques. Whole-body visualization of small animals is widely applied in preclinical biomedical studies, including drug-delivery monitoring
[29], treatment assessment
[30], and contrast-enhanced imaging
[31]. For these purposes, various configurations of PAI systems have been demonstrated using various combinations of US transducers and scanning mechanisms.