Small-animal whole-body photoacoustic tomography: a review

Abstract

With the wide use of small animals for biomedical studies, in vivo small-animal whole-body imaging plays an increasingly important role. Photoacoustic tomography (PAT) is an emerging whole-body imaging modality that shows great potential for preclinical research. As a hybrid technique, PAT is based on the acoustic detection of optical absorption from either endogenous tissue chromophores, such as oxyhemoglobin and deoxyhemoglobin, or exogenous contrast agents. Because ultrasound scatters much less than light in tissue, PAT generates high-resolution images in both the optical ballistic and diffusive regimes. Using near-infrared light, which has relatively low blood absorption, PAT can image through the whole body of small animals with acoustically defined spatial resolution. Anatomical and vascular structures are imaged with endogenous hemoglobin contrast, while functional and molecular images are enabled by the wide choice of exogenous optical contrasts. This paper reviews the rapidly growing field of small-animal whole-body PAT and highlights studies done in the past decade.

Figure 1

a. Schematic of the deep reflection-mode photoacoustic imaging system. b. Sagittal photoacoustic MAP image of the blood vessels around the ribs and the liver. c. Sagittal photoacoustic MAP image of the kidney, the vena cava inferior, and the spinal cord. d. Anatomical photograph with the ribs (RB) intact, but the skin removed. e. Anatomical photograph with the skin and the ribs removed. The photographs and photoacoustic images were obtained from the right side of the rat. AA, arcuate blood vessels; IB, interlobar blood vessels; IT, intestines; KN, kidney; LF, lumbodorsal fascia; LV, liver; RV, blood vessels around ribs; SC, blood vessels in the spinal cord; and VC, vena cava inferior. Reproduced with permission from ref. [23].

Figure 2

(a) Schematic illustrating the operation of the photoacoustic imaging system. Photoacoustic waves are generated by the absorption of nanosecond optical pulses provided by a wavelength-tunable OPO laser and detected by a transparent Fabry-Perot polymer film ultrasound sensor. The sensor comprises a pair of dichroic mirrors separated by a 40 μm thick polymer spacer thus forming a Fabry-Perot interferometer (FPI). The waves are mapped in 2D by raster-scanning a CW focused interrogation laser beam across the sensor and recording the acoustically induced modulation of the reflectivity of the FPI at each scan point. (b)–(e) In situ images of the abdomen of a pregnant female mouse containing two embryos for different depth ranges. The excitation wavelength was 640 nm and the step size was 115 μm. (b) x – y MAP of complete 3-D image data set (depth 0 to 6 mm) showing the two embryos (shaded red), (c) x – y MAP for depths of 0 to 1 mm showing the blood vessel network in the abdominal skin and the uterus. (d) x – y MAP for depths of 1 to 3 mm. This shows the anatomy of the two embryos. The right atrium, the pulmonary vein, and the vessels in the embryo head are evident as are organs such as the liver and parts of the developing skeleton, such as the ribs. (e) x – y MAP for depths of 3 to 6 mm showing vascular features such as the abdominal aorta, the Azygos vein, and the inferior vena cava. The spinal region, which contains the intercostal arteries, is also visible. Reproduced with permission from ref. [42].

Figure 3

(a). Schematic drawing of the multispectral optoacoustic tomography system (MOTS). A curved array of wide-band and cylindrically focused ultrasound transducers enables parallel data acquisition. Optical fibers are used to homogeneously illuminate the object. (b)–(d). MOTS images of mouse anatomy taken at 750 nm. 1, kidneys; 2, spine; 3, spleen; 4, vena cava; 5, liver; and 6, brain. Reproduced with permission from ref. [50].

Figure 4

Schematic of the ring-shaped confocal photoacoustic computed tomography (RC-PACT) system. The dashed box shows a cross-sectional view of the confocal design. (b)–(e) are in vivo RC-PACT images of athymic mice acquired noninvasively at various anatomical locations: (b) liver, (c) kidneys, (d) bladder, and (e) brain. BL, bladder; BM, backbone muscle; CV, cortical vessels; EY, eyes; GI, GI tract; KN, kidney; LV, liver; PV, portal vein; SC, spinal cord; SP, spleen; and VC, vena cava. Reproduced with permission from ref. [51].

Figure 5

(a) Schematic of the photoacoustic scanner, showing the laser beam entering the bottom of the detector bowl and illuminating a mouse partially immersed in water and suspended above the bowl by a transparent tray. A beam splitter allows a portion of the laser beam to be monitored by a spectrometer for detection of laser power and wavelength. (b) The 128 radial projections captured by the detector geometry when data is collected for a single bowl angular position. (c) The increased density of projections available as the bowl is rotated through 360 degrees. (d) MAP image of mouse abdominal section. Reproduced with permission from ref. [19].

Figure 6

(a) Schematic drawing of the tri-modal imaging system and a photograph of the animal holder. (b) Drawing of the 3D detector for photoacoustic and thermoacoustic imaging. (c) The angular coverage as the target is rotated 360 degrees. (d) Three co-registered modalities are combined in a MAP image. Each modality is represented by a different color: green is ultrasound, red is photoacoustic tomography, and blue is thermoacoustic tomography. PCT, photoacoustic computed tomography; TCT, thermoacoustic computed tomography; and US, ultrasound B-mode imaging. Reproduced with permission from ref. [60].

Figure 7

(a) Picture of a mouse during a scan, showing the placement of the mouse and illumination with respect to the array of wideband ultrasonic transducers. (b) Three-dimensional photoacoustic volume of a female nude mouse. (c) Three-dimensional photoacoustic volume of a male nude mouse. Reproduced with permission from ref. [18].

Figure 8

In vivo RC-PACT images of orthotopically implanted 786-O kidney tumors. Top row: mouse 1. Bottom row: mouse 2. (a) Anatomical images acquired at 776 nm. (b) HbT images overlaid on the anatomical images. (c) IRDye800-2DG images overlaid on the anatomical images. CK, cancerous kidney; HK, healthy kidney; HM, hypermetabolic. Reproduced with permission from ref. [14].

Figure 9

In vivo RC-PACT and Deep-PAMac imaging of mouse mammary gland tumor. (a). Photograph of the mouse with the three-week-old tumor xenograft growing in the mammary pad (yellow arrow). (b). RC-PACT images of the tumor shown in the mouse cross section. The three images on the left were taken at different wavelengths (indicated); the right-most image is a spectrally separated one, with the tumor in blue and the blood in red. The spectrally resolved iRFP signal is normalized to the spectrally resolved signal for blood. The black dashed line shows the mouse body borders. Deep-PAMac MAP images of the tumor and surrounding major blood vessels, taken without (c) and with (d) 4 mm thick chicken breast tissue overlay. The three left images on each panel were taken at different wavelengths (indicated), and the images on each of the right-most panels are spectrally separated MAP images with orthogonal orientations. Again, the tumor is labeled blue, the blood is labeled red, and the spectrally resolved iRFP signal is normalized to the spectrally resolved blood signal. The tumor position is indicated by the green arrow. Reproduced with permission from ref. [17].

Figure 10

Three-dimensional photoacoustic images reconstructed from a mouse before and after intravenous injection of gold nanorods. (a) and (b) are dorsoventral images acquired at 765 nm and 1064 nm, respectively. (c) and (d) show changes of the averaged brightness inside the segmented parts of the photoacoustic images acquired at 765 nm and 1064 nm, respectively. Reproduced with permission from ref. [64].

Figure 11

(a) Cross-sectional photoacoustic images at different time points of the kidneys of a female CD1 mouse illuminated at 800 nm after injection of 300 nmol of ICG. (b) Superposition of the absorption difference and the single wavelength image acquired before injection. Reproduced with permission from ref. [21].