A high-speed PAT data acquisition system was assembled using a commercial US scanner working with a P4-1 phased array (z.one, ZONARE Medical Systems, Inc.), as shown in
. A Nd:YAG laser (Powerlite, Continuum) pumped dye laser (ND6000, Continuum) provides laser pulses with a repetition rate of 10 Hz, wavelength of 710 nm, and a pulse length of 5 ns. The laser-generated photoacoustic signals are acquired by the US unit; synchronization between the US receive cycle and the laser firing is achieved using a frame-trigger signal tapped into the ultrasonic probe connector. To acquire a 2D PAT image in a prostate, the US unit operates in receive-only mode (with the transmitter turned off) at a frame rate that matches the 10 Hz repetition rate of the laser; while working on the US mode with the transmitter turned on, B-mode US images of the same imaging plane in the prostate can be taken. For command and control, a serial (RS-232) data link is set up between the US unit and an external PC. Through a Tera Term console that supports serial communications between the US unit and the PC, the user runs a command script that sends imaging control parameters to the US unit, including the start of data acquisition, transmitter on/off, transmit delays, acquisition frame rate, time-gain compensation, and demodulation frequency.
Schematic of the experimental system for PAT and US imaging of a canine prostate in vivo.
For each laser pulse, 64 channels of 16-bit in-phase quadrature (I/Q) data are acquired from a selectable 64-element sub-aperture of the array. Therefore, only 2 laser pulses are required for the acquisition of a complete 128-element data set for a 2D PAT image, at an effective full frame rate acquisition of 5 Hz. If needed, repeat acquisitions are made to support coherent data averaging for improved signal-to-noise ratios (SNR). Once the I/Q data including both reconstructed and channel data are captured in the cine buffer, they can be transferred as binary raw data files to an external USB storage device. On the PC, a MATLAB tool is used to extract the raw I/Q data and associated imaging parameters from binary data files. The RF data from the 64 or 128 elements are then processed using a standard ultrasound dynamic receive focusing algorithm to reconstruct a PAT image.
Before the experiment on canine prostates in vivo
, the lateral and axial resolution of this PAT system were evaluated by imaging a micro-flow vessel phantom made from a transparent soft tube (I.D. 0.5 mm, Cole-Parmer) filled with fresh canine blood. With the orientation of the vessel orthogonal to the transducer array, a point object was formed in the 2D B-scan plane when the vessel was illuminated by a narrow laser beam. The delay curve in
shows the I/Q data of received photoacoustic signals from the point object acquired by the 128 array elements. This signal was induced by a single laser pulse with an incident energy density within the ANSI safety limit [23
]. The fast Fourier transform (FFT) of the RF data converted from the I/Q data indicated that the imaging system working with the P4-1 array was sensitive in the spectrum of 1.5-4.2 MHz with a fairly broad −6 dB receiving bandwidth of 94%. Using the standard ultrasound dynamic receive focusing algorithm, a 2D PAT image of the point object was reconstructed, as shown in . By studying the image intensities through the center of the point object along the axial (depth) and the lateral (azimuthal) directions [as shown in and respectively], the −6 dB axial and lateral resolution achieved by this system were 0.65 mm and 0.52 mm, respectively, at a depth of 20 mm where the azimuthal f-number was approximately 0.7.
Fig. 2 (A) 128-channel I/Q data from a point object. (B) Reconstructed 2D PAT image of the point object. (C) System axial resolution. (D) System lateral resolution. (E) Photograph of a gel phantom with two artificial vessels embedded. (F) 2D gray scale US image (more ...)
In order to examine the capability of this system in mapping highly absorbing vasculature, a micro-flow vessel phantom with known vascular structure was imaged. As shown in
, two vessels made with the same transparent soft tubing (I.D. 0.5 mm) as above, filled with fresh canine blood, were embedded in the same 2D plane in a block of porcine gel. First, a 2D gray scale pulse echo US image of this phantom was taken, as shown in . The apparent poor axial resolution is caused by reverberations in the tube. Then without moving the sample or the probe, the system shifted to PAT mode with the US transmitter turned off. When illuminated by laser light at 850-nm wavelength with incident light fluence within the ANSI safety limit, a PAT image of the same 2D cross-section was taken, as shown in . The B-scan PAT image was reconstructed using the signals from the full 128 elements without apodization. Based on the strong optical absorption contrast between the canine blood and the background gel, the two vessels were imaged with satisfactory continuity, resulting in a good match with the sample photograph.