4-D photoacoustic tomography

Abstract

Photoacoustic tomography (PAT) offers three-dimensional (3D) structural and functional imaging of living biological tissue with label-free, optical absorption contrast. These attributes lend PAT imaging to a wide variety of applications in clinical medicine and preclinical research. Despite advances in live animal imaging with PAT, there is still a need for 3D imaging at centimeter depths in real-time. We report the development of four dimensional (4D) PAT, which integrates time resolutions with 3D spatial resolution, obtained using spherical arrays of ultrasonic detectors. The 4D PAT technique generates motion pictures of imaged tissue, enabling real time tracking of dynamic physiological and pathological processes at hundred micrometer-millisecond resolutions. The 4D PAT technique is used here to image needle-based drug delivery and pharmacokinetics. We also use this technique to monitor 1) fast hemodynamic changes during inter-ictal epileptic seizures and 2) temperature variations during tumor thermal therapy.

Figure 1. 4D PAT system.

(a) Schematic of the 4D PAT system. Pulsed laser light from a Ti: Sapphire laser tunable (690 to 950 nm) is delivered to tissue surfaces through an optical fiber. A 192 element 3D sphere transducer array is used to capture the PA signals generated by the laser light. A 192 channel data acquisition system consists of preamplifiers, secondary stage amplifiers (for optimizing the signal-to-noise ratio), and a 3:1 electronic multiplexer coupled with a 64-channel analog-to-digital converter. The average speed of PAT data acquisition system is 0.33 s/frame and was limited by the 10 Hz laser repetition. The spatial resolution of this system is better (smaller) than 0.19 mm in the x-y direction, and better (smaller) than 0.27 mm in z direction (Supplemental data Figure 1, online).

Figure 2. Schematic representation of time-resolved 4D photoacoustic tomography.

3-D spatial phase-controlled algorithm was used to reconstruct the imaging. A series of 2D images at various projection angles, θ and time steps t can be obtained to construct the 4D movies of objects in motion. r_mk is the distance between photoacoustic source at position m and No. k detector.

Figure 3. Image-guiding curved needle insertion and drug delivery.

3D PA guidance of a curved needle, a snapshot of needle insertion is tracked in real-time (Video 1). From left to right, the needle positions were monitored at different time serials from 2.331 s to 9.990 s. scale bars throughout figure, 2 mm.

Figure 4. 4D visualization of ICG-based pharmacokinetics of drugs.

(a) Representative time frames of 3D volume images taken at t = 19.647 s and (b) t = 30.303 s for brain vascular structures. ICG pharmacokinetics was also monitored within a rat brain in real time with 810 nm (Video 2). The PA image of the ICG-dyed brain acquired at 30.303 s post-injection shows ICG accumulation enhanced the photoacoustic signal (area B) by 8.5±2.6 (standard deviation). Video 3 shows a three dimensional rendering of the ICG accumulation at a given time point. (c) The 2D noninvasive PAT image extracted from the 3D tomogram of the rat cortical vasculature (d) Open-skull photograph of the rat brain surface acquired after the PAT experiment. Numbers 1–3 indicate the corresponding blood vessels in the PAT image and rat brain photograph. Scale bars in figure 4(c), 2 mm.

Figure 5. Interictal events monitoring.

(a) Schematic showing the location of the imaged region. (b) Simultaneous photoacoustic signal (below) and EEG recording (upper) show that each interictal spike had discrete photoacoustic correlates. (c) Change in reflectance (−ΔR/R) as a function of time for the 0.5 s before and 1 s after each spike. Error bars (± s.d.) were calculated from 5 consecutive spikes. Notice the rapid onset of the signal.*, and the time of occurrence of the spike (shown on the left). Numbers correspond to images in next panel. (d) Spike-triggered (ST) epilepsy PA maps were obtained by dividing each frame after the spike by the frame prior to the spike (denominator frame). A significant increase of optical absorption is seen during the interictal onset, scale bars throughout figure, 2 mm.

Figure 6. Tumor margin detection and thermal therapy monitoring.

(a)Three-dimensional stack of consecutive photoacoustic image sections from a breast tumor on mouse, the 0.2-mm slice corresponds to sections 1–5. Maximum intensity projection of the entire z stack is shown in 6; 7 is the 3D reconstruction of the entire z-stack. Yellow dash circle represents the tumor margin in different slices as in 1–6 are correlated well with tumor histology in 8. (b) 3D photoacoustic-based thermal images at different time point of 90 s, 120 s, 150 s and 180 s after photothermal therapy, respectively. (c) Photoacoustic measurement of temperature rise during photothermal therapy, results in maximum temperature rise of ~6.6°C within the tumor, Error bars (± s.d.) were calculated from 5 consecutive measurements.