2.1.

PADL Design

Building on our previous results, the PADLs were designed to provide good mechanical stability and acoustic performance. Due to their broad availability, low cost, and low acoustic attenuation at ultrasonic frequencies (i.e., MHz), fused-silica optical fibers (CeramOptec Inc., East Longmeadow, Massachusetts) with a total diameter of $\sim 220\mu \mathrm{m}$ were used in the PADLs. In a cylindrical wire-type ultrasound delay line, the acoustic delays can vary with different frequencies. To transmit desirable ultrasound signals in the wire-type delay lines, the second and higher-orders of longitudinal mode and mode dispersion should be suppressed by satisfying the following condition^10,11

The acoustic time delays were controlled by varying the length of the delay lines which was determined using the following two criteria.⁹ First, the time delay between each delay line had to be long enough to prevent signal interference from neighboring delay lines. In our PADLs, the length differences between two adjacent delay lines were determined to be 5.00 cm long, which corresponded to an incremental time delay of $10\mu \mathrm{s}$. Since the time duration of the PA signal from each delay line is less than $6\mu \mathrm{s}$, a $10\text{-}\mu \mathrm{s}$ delay is adequate. Second, the PA signal in the longest (eighth) delay line had to reach the receiving transducers before the first reflected signal (first echo, a signal reflected twice by the two ends of the optical fiber) in the shortest delay line, to avoid possible interference between them. Representing the time delay of the first delay line as $T_1$ (in microseconds), the eighth signal will reach the receiving transducer after $T_1+70\mu \mathrm{s}$, and the first echo will be detected at $3T_1$. To avoid interference, an inequality of $3T_1>T_1+70$ is required, and, therefore, $T_1$ should be greater than $35\mu \mathrm{s}$. Based on this calculation, the time delays of the shortest (first) delay line and longest (eighth) delay line were designed for 40 and $110\mu \mathrm{s}$, respectively. The time delays and delay lengths for the 8-channel PADL array are summarized in Table 1. In common ultrasonic array systems, the transducer elements are arranged with $p<\lambda /2$, where $p$ is the pitch and $\lambda$ is the ultrasound wavelength. Based on the ultrasound frequency (2.25 MHz) of our ultrasonic transducers, the pitches between two adjacent fibers were determined to be $500\mu \mathrm{m}$. Since the optical fibers have an overall diameter of $220\mu \mathrm{m}$, a fiber-to-fiber spacing of $300\mu \mathrm{m}$ was chosen for arranging the PADLs at the input port.

Table 1

Time delays and fiber lengths for the 8-channel parallel acoustic delay line array.

2.2.

Fiber Spacer Design and Fabrication

To securely position the optical-fiber PADLs, four pairs of spacer structures were designed and fabricated [Fig. 3(a)], each having horizontal and a vertical members assembled into a cross. On each spacer structure, there is a 2-D matrix of elliptical holes ($800\text{-}\mu \mathrm{m}$ long and $300\text{-}\mu \mathrm{m}$ wide) for assembling the optical fibers. Unlike circular holes, the elliptic holes can provide relief clearance in the direction of optical fiber bending, which helps to reduce the chance of breaking the optical fibers during threading. To reduce possible acoustic coupling between two adjacent optical fibers passing through the spacer structure, an “L” shaped isolation trench was cut around each elliptic hole, as seen in Fig. 3(b). The length and width of the isolation trenches were 2.15 mm and $250\mu \mathrm{m}$, respectively. To accommodate even the longest optical fiber ($\sim 56\mathrm{cm}$), the horizontal spacer structure has an array of 4-by-30 elliptic holes and the vertical spacer structure has an array of 4-by-18 elliptic holes. Overall, the spacer structures measure 6.2 mm by 60 mm horizontally ($x\text{-}z$) and 6.2 mm by 35.5 mm vertically ($x\text{-}y$). These dimensions allow the longest fiber to be snugly wound with a smallest radius of curvature of around 5 mm without breaking.

Fig. 3

(a) Horizontal and vertical spacers fabricated using laser micromachining; and (b) zoom-in view of the horizontal spacer showing the elliptical threading holes and “L” shaped isolation trenches.

When a sound wave is incident at an oblique angle onto an interface of two materials with different acoustic impedances, one form of vibrational motion (e.g., longitudinal) can be transformed into another one (e.g., transverse). This phenomenon is called (acoustic) mode conversion. The mode conversion could occur at the bent section of an optical-fiber PADL, depending on the both the wavelength of the PA signals and the bending radius. Generally, a larger bending radius creates less mode conversion. In PA imaging, the PA signals are launched and received in the longitudinal mode. Therefore, any conversion to other modes during their transmission in the optical-fiber PADLs should be avoided. Based on our previous studies,⁹ a smallest bending radius of 5 mm was maintained in winding the optical fiber PADLs to minimize the mode conversion.

2.3.

Probe Construction

The first step to construct the handheld PADL probe was preparing the optical-fiber PADLs. 16 optical fibers were cut to their requisite lengths, and both ends of each optical fiber were polished. Then, a laser micromachining system (Universal Laser Systems, PLS6.75, Scottdale, Arizona) was used to cut the input/output fiber holders, the top, bottom, front, and back plates of the PADL housing unit, and four pairs of optical fiber spacers from a 1.5-mm-thick acrylic sheet. To assemble the PADL probe, the input and the output holders for fiber arrangement were first fixed on the bottom plate. Second, the front and back panels were fixed on the bottom plate. Third, the horizontal and the vertical spacers were assembled crosswise. The main housing part, including the front/back panels and crossed horizontal/vertical spacers, was fixed on the bottom plate. The last step was carefully threading the optical probe fibers through the input port, front panel, vertical/horizontal spacers, back panel, and output port. Since the coiled optical fibers are flexible springs under compression, the lengths of the input and output port were carefully adjusted to provide the best contact condition. The fully assembled PADL probe is shown in Fig. 4.

Fig. 4

Fully assembled optical fiber PADL probe.

3.1.

Ultrasonic Testing Setup

We set up a simple experimental system to characterize the ultrasonic transmission properties of the 16-channel PADLs. The pulser/receiver (Olympus NDT, 5072PR, Houston, Texas) was set to transmission mode. Three ultrasonic transducers (Olympus NDT, V106-RB, 2.25 MHz) were used in the testing, with one serving as the transmitter and the other two serving as the receivers. The transmitting transducer was interfaced with the input port of the 16-channel PADLs. The two receiving transducers were interfaced with the two output ports of the PADLs, respectively. To enhance coupling efficiency and minimize unwanted reverberation, mineral oil was applied between the surfaces of the transducers and the ends of the PADLs. The ultrasound signals (generated by the transmitting transducer) traveled from the input port to the two output ports through the 16-channel PADLs with different time delays. Since the pulser/receiver had only one receiving port, the two receiving transducers were connected to the pulser/receiver one at a time to receive all ultrasound signals from the 16 optical fibers, which were averaged 16 times to improve the signal-to-noise ratio, displayed, and recorded on the digital oscilloscope (Tektroniks, TDS2014B, Beaverton, Oregon).

3.2.

PAT Imaging Setup

The experimental setup of the PA imaging experiment is shown in Fig. 5. The light source was a frequency-doubled Q-switched Nd:YAG laser (Continuum, SL2-10, San Jose, California) with a pulse duration of 5 ns, a pulse repetition rate of 10 Hz, and a wavelength of 532 nm. After it passed through several prisms, the light beam was focused on a phantom via a cylindrical lens, parallel to the alignment of the delay lines. The laser pulse energy was less than $10\mathrm{mJ}/{\mathrm{cm}}^2$, much lower than the ANSI safety limit ($20\mathrm{mJ}/{\mathrm{cm}}^2$ at 532 nm).

Fig. 5

Photoacoustic tomography imaging setup using the 16-channel PADL probe.

The induced PA waves propagated from the phantom and traveled along the PADLs into two DAQ channels. In each DAQ channel, eight time-delayed PA signals were sequentially detected by a single-element unfocused ultrasonic transducer (Olympus NDT, V303). The delivered PA signals were amplified by an ultrasound pulser/receiver (Olympus NDT, 5072PR). An oscilloscope (Tektronix, TDS5054) recorded these signals at a sampling rate of 5 MHz.

3.3.

DAQ and Image Reconstruction

The PA signals were averaged 10 times to generate one PA image. Thus, the imaging speed was 1 Hz. The end of each delay line on the sample surface was considered as a single-ultrasound element in an ultrasound array system. Each PA signal sequentially detected by one DAQ channel was reshaped into eight separate PA signals, based on the predefined time positions and measured by the ultrasound transmission experiment. The differences in acoustic attenuation in each fiber were compensated for ($0.2\mathrm{dB}/\mathrm{cm}$). Then, a commonly used delay-and-sum image reconstruction method was applied to reconstruct the PA images.¹² Briefly, the PA images were reconstructed by summing the data from all channels after compensating for time delays at each fiber based on its length and matching phase. In a 2-D beam field along the $x$ and $z$ directions, a time delay ${\tau }_n$ for the $n$’th PADL located on $(x_n,z_n)$ can be calculated as follows:

4.1.

Ultrasonic Transmission through the PADLs

Figure 6(a) shows the eight channels of time-delayed ultrasound signals followed by the first channel’s echo signal received by ultrasonic transducer UT1, and Fig. 6(b) shows a zoom-in view of the first three ultrasound signals. Similarly, Fig. 6(c) shows the eight channels of time-delayed ultrasound signals followed by the first channel’s echo signal received by ultrasonic transducer UT2, and Fig. 6(d) shows a zoom-in view of the first three ultrasound signals. The ultrasound signals naturally attenuated along the length of the PADLs. Theoretically, acoustic attenuation should be exponential to the delay length according to Beer’s law. However, the observed nonlinear attenuation of the PADLs could be caused by nonuniform contacts between the PADLs and the transducers. The average attenuation was calculated to be $0.19\mathrm{dB}/\mathrm{cm}$ at 2.25 MHz by measuring the signal amplitude difference between the shortest fiber and the longest one. This value is similar to that observed before ($0.2\mathrm{dB}/\mathrm{cm}$ at 1 MHz).⁹ Figures 6(b) and 6(d) show incremental time delays of $\sim 9.5\mu \mathrm{s}$.

Fig. 6

Plots of acoustic waveforms propagating through the optical-fiber PADLs with various lengths ranging from 21 to 56 cm: (a) Raw A-line signals obtained by ultrasonic transducer UT1; (b) zoom-in view of the first three signals in (a); (c) raw A-line image obtained by ultrasonic transducer UT2; and (d) zoom-in view of the first three signals in (c).

4.2.

PAT Imaging

To demonstrate the PA imaging capability of the handheld PADL-PAT probe, we imaged an optically absorptive target ($2.5\times 1\times 2.5{\mathrm{mm}}^3$ along the $X$, $Y$, and $Z$ axes, respectively) embedded in an optically scattering tissue phantom ($100\times 100\times 50{\mathrm{mm}}^3$ along the $X$, $Y$, and $Z$ axes, respectively), shown in Fig. 7(a). The phantom consisted of 10% gelatin by weight and 1% intralipid by volume, and its reduced scattering coefficient was $\sim 9{\mathrm{cm}}^{-1}$. The object was located 2.5 mm below the phantom’s surface. The optical absorption coefficient of the target was $\sim 100{\mathrm{cm}}^{-1}$. The raw A-line data with Hilbert transformation acquired by both channels are shown in Figs. 7(b) and 7(c).

Fig. 7

(a) Photograph of an optically absorbing target embedded in an optically scattering medium; (b) raw A-line PA signals with Hilbert transformation obtained by the ultrasonic transducer (UT 1); and (c) raw A-line PA signals with Hilbert transformation obtained by the ultrasonic transducer (UT 2).

The A-line PA data from both channels were rearranged into one raw Hilbert-transformed PA B-scan image, shown in Fig. 8(a). The reconstructed PA images are presented with compensation for the ultrasound attenuation ($0.2\mathrm{dB}/\mathrm{cm}$) in each fiber, as shown in Fig. 8(b). The reconstructed PA images match well with the photograph in Fig. 7(a). The image contrast, defined as $({\mathrm{PA}}_{\mathrm{target}}-{\mathrm{PA}}_{\mathrm{background}})/{\mathrm{PA}}_{\mathrm{background}}$, is calculated to be $\sim 2.4$. The spatial resolution, defined as the one-way distance between 10% and 90% of the maximum over the minimum, is $\sim 1.1\mathrm{mm}$. Compared to our previous results,⁹ we found that the PA signals were broadened and more noisy due to the increased contact junctions and bends of the optical fibers in the small housing unit. Thus, the image contrast and spatial resolution were slightly diminished, by 20% and 37%, respectively. Further, the enhanced PA image can be presented after the signal threshold of 30% [Fig. 8(c)].

Fig. 8

(a) Rearranged Hilbert-transformed raw data acquired by 16 optical fibers; (b) reconstructed PA images with compensation for the acoustic attenuation in each fiber; and (c) thresholded (30%) PA image of (b).