4-D Photoacoustic Tomography Liangzhong Xiang, Bo Wang, Lijun

4-D Photoacoustic Tomography
Liangzhong Xiang, Bo Wang, Lijun Ji & Huabei Jiang*
J. Crayton Pruitt Family Department of Biomedical Engineering
University of Florida
Gainesville, FL 32611, U.S.A.
[email protected]
Supplementary Information
Ultrasound transducer array configuration and system characterization
A 192 element 3D sphere transducer array is used to capture the PA signals generated by the laser
light. The 192 transducers placed along a custom fabricated white ABS spherical interface containing
610 through holes with counter bores, formed 11 evenly spaced layers along the vertical direction of
the ball as shown in Fig. 1(a) and Fig. 1(b). The transducer positions on the spherical interface are
selectable which depends on geometry of the imaged target. The transducers were glued onto the
interface with epoxy which can be removed to allow the position change of the transducers. The
interface has an outer diameter of 160 mm and an inner diameter of 140 mm. The physical width of
the light beam measured ~20 mm at the center-of-curvature of the spherical-array. The mean
illumination fluence within this region was estimated to be ~1 mJ/cm 2. In this way, an imaging
region of about π × 10× 10 × 30 mm3 can be covered using a single laser pulse.
The point object used for system characterization was a small spherical graphite particle (0.1 mm in
diameter) located at the center of the spherical array and ensured an isotropic acoustic emission
profile for all directions. Fig. 1(c) and Fig. 1(d) present the reconstructed x-y and z-x cross-section
images of the point object located at the array center. The quality of these images is determined by
both the distribution and the characteristics of the transducers. The profiles of the two reconstructed
images were also extracted in x and z directions, as shown in Fig. 1(e) and Fig. 1(f), respectively.
The full width at half maximum (FWHM) of the profiles was measured to be 0.19 mm (x direction)
for Fig. 1(c), and 0.27 mm (z direction) for Fig. 1(d), compared to the theoretical value of 0.16 mm
for the 5 MHz central frequency transducer with an estimated cut off frequency of 7 MHz. For
targets located away from the center of the array, the radial resolution will stay nearly the same as
that for a centrally located target, while the lateral resolution will be linearly reduced with increased
distance away from the array center.
Figure 1 | Ultrasound transducer array configuration and system characterization. (a.) 3D schematic of the
transducer distribution on a white ABS interface. (b.) Photograph of the transducer array. (c.) Reconstructed
images of the point source object by the 4 D PAT system in x-y plane and (d. ) in x-z plane. (e.), (f.) are the
profiles extracted in x and z directions from (c.) and (d.), respectively. Units are in mm.
Photothermal therapy and temperature monitoring
An integrated imaging and therapy system was assembled to acquire photoacoustic imaging during
photothermal therapy in Fig.2. A 192 element spherical array transducer, with 64 parallel data
acquisition system was used to capture photoacoustic transients. A Ti: Sapphire pulsed laser system
operating at 810-nm wavelength, with a 10-Hz repetition rate was interfaced for photoacoustic
imaging of tissue samples. A continuous wave (CW) diode laser, operating at 755 nm with a
maximum power of 80mW, was used as a light source for photothermal therapy. The direction of the
laser beam was orthogonal to the imaging plane. During the five-minute exposure, photoacoustic
frames were recorded every 0.333 s. The captured data were stored offline for temperature
processing. The experiments were performed at 25°C.
Figure 2 | Diagram of the experimental setup of photothermal therapy and temperature monitoring.