2.1.

Development and Validation of Ring-Illumination Optical System with Linear Array Acquisition

The performance of the ring illumination mode was evaluated with regard to the PA imaging depth using two tissue-mimicking phantoms made of 10% porcine gelatin (G2500, SIGMA-ALDRICH, Missouri) and 0.2% cellulose (S5504, SIGMA-ALDRICH, Missouri). For the first experiment, a $700\text{-}\mu \mathrm{m}$ diameter graphite absorber was inserted obliquely into a gelatin phantom, and the PA signal was measured with respect to the horizontal depth [Fig. 1(a)]. The illumination source for the experiment was a pulsed 532 nm, Nd:YAG, 8-ns laser (Quanta-Ray Pro 270, Spectra-physics, California) with an output of 10 mJ per pulse. A 100-mm diameter parabolic mirror (45-944, Edmund Optics, New Jersey) and cone-shaped reflector (68-791, Edmund Optics, New Jersey) were used to create the ring beam by adjusting the distance between the cone mirror and the parabolic ring reflector [Fig. 1(b)]. For the second experiment, a 70-mm diameter gelatin phantom containing two graphite absorbers with $500\text{-}\mu \mathrm{m}$ diameters were placed in two planes [Fig. 1(c)] and imaged to determine the maximum imaging depth and the plane selectivity of the full-ring illumination method. Large scale parabolic reflector (P19-0300, Optiforms Inc., California) with 243-mm diameter was utilized with a cone-shaped reflector to create a large ring-shaped beam with 4-mm thickness. A 532-nm laser source (PhocusCore, Optotek, California) was used, resulting in $4.77\mathrm{mJ}/{\mathrm{cm}}^2$ fluence in the targeted cross-sectional area. In this experiment, it was not possible to adjust the position of the ring beam, so the beam was 17 mm below the targeted cross-sectional area [Fig. 1(c)]. In both experiments, a programmable US scanner (Vantage 128, Verasonics Inc., Washington, USA) was utilized with an L11-4 linear array transducer, operating at 8.4 MHz center frequency, for US and PA signal acquisition.

Fig. 1

(a) Experimental setup of the breast phantom embedded with a diagonal graphite absorber and (b) the photograph of the same experimental setup. (c) Diagram showing the dimensions of the tissue-mimicking phantom used for the second experiment. The US scanning area is enclosed in the red dashed lines, and the ring beam has fallen 17 mm below the targeted cross section.

2.2.

Photoacoustic Tomography Using Full-Ring Illumination and Ring-Shaped Transducer

The full-ring US transducer and illumination system, shown in Fig. 2(a), also used a gelatin phantom for system characterization. In this experiment, an $89\mathrm{mJ}/\text{pulse}$ laser source (PhocusCore, Optotek, California, USA) was expanded to a ring shape, resulting in $4.7\mathrm{mJ}/{\mathrm{cm}}^2$ of fluence at 532 nm. The ring US transducer has a 200-mm inner diameter and comprised 256 elements, with an element pitch of 2.45 mm and a height of 9 mm. In PAT mode, the scattered signals were recorded by all 256 elements at a sampling rate of 8.33 MHz. The 243-mm diameter parabolic reflector and the cone-shaped reflector were used to create the ring-shaped beam with 4-mm thickness on the phantom surface. The phantom used for this experiment was 7.5 cm in diameter and made of 12% porcine skin gelatin (G2500, SIGMA-ALDRICH, Missouri) mixed with 0.4% cellulose (S5504, SIGMA-ALDRICH, Missouri). Three graphite rods, with 2-mm thickness, were placed horizontally inside the phantom in three layers [Fig. 2(b)]. The ring beam was adjusted on each cross-sectional slice by translating the phantom in the vertical direction. The results from this experiment demonstrated the uniqueness of the all-reflective PAT system in creating co-registered PA-US tomographic images with significant depth and will be discussed further in the results section.

Fig. 2

(a) The setup for the full-ring illumination and full-ring acquisition experiments. The cone-shaped mirror and the parabolic reflector create the ring-shaped beam for the scanned cross section. The ring beam is positioned 10 mm below targeted cross-sectional slice for the discussed experiments. (b) The diagram shows the dimension of the tissue-mimicking phantom and the position of the graphite rods from the bottom slice (slice 1) to the top slice (slice 3).

3.1.

Photoacoustic Results Using Full-Ring Illumination with a Linear Array Transducer

Figure 3 plots the PA image for the diagonal graphite absorber in gelatin for the experimental setup shown in Fig. 1(a). A clear PA signal is detected within the field of view of the linear array US transducer, and a depth of 25 mm inside the phantom is imaged. As anticipated, the PA signal intensity is the largest superficially and is attenuated with horizontal depth [Fig. 3(a)] due to absorption and scattering. The normalized PA amplitude versus horizontal depth for this image is plotted in Fig. 3(b), demonstrating the depth dependence of the fluence and the relative uniformity of the PA signal within the middle portion of the phantom. The uniform PA signal between 10 and 20 mm underscores the effectiveness of omnidirectional ring illumination in providing a uniform fluence map within a targeted cross section. In an ideal PA platform, the fluence is depth-independent, and the proposed ring illumination system (Sec. 3.3) can enhance the PAT imaging by providing a more uniform illumination map. The PA signal after 25 mm disappears as the angled optical absorber moves outside of the selected illuminated cross-sectional area.

Fig. 3

(a) The PA signal distribution across the illuminated cross-sectional slice of the gelatin phantom. (b) The graph demonstrates a normalized PA amplitude versus depth. A uniform PA signal amplitude is seen between the depths of 10 and 20 mm.

Figure 4 plots the results from the second experiment, which used two graphite absorbers in separate planes, as shown in Fig. 1(c). Since the linear array transducer has a limited field of view, only a section of the horizontal graphite absorber is shown in Fig. 4(a). As anticipated, both objects can be seen in the US image, while in the PA image, the targeted layer shows a high PA signal which demonstrates the selectivity of the full-ring illumination mode in imaging a targeted cross-sectional slice. The full-ring illumination is also able to deliver sufficient energy to the core of the 70-mm tissue-mimicking phantom [Fig. 4(b)]. This finding highlights the benefit of the ring-illumination system which allows for a lower fluence and a greater cross-sectional illumination depth. The limited field of view of the linear transducer used in these experiments would also not be a problem for the full-ring US transducer, where all objects would be visualized regardless of the location.

Fig. 4

(a) The US image (left) showing the graphite absorbers in two different planes. The interface seen in the picture is an artifact of the phantom-making process. The PA image (right) shows the top and the bottom graphite absorbers, with a more visible top object. (b) The plot of the normalized PA signal amplitude across the top and bottom graphite absorbers. The targeted, top graphite absorber has a larger PA signal amplitude.

It is important to mention that there was an observed PA signal from the middle layer of the phantom (at a depth of 30 mm) which happened due to two reasons. First, the phantom was created in two parts by initially pouring the first layer and then allowing it to set so it could support the graphite rod, after which a second layer of the same material was poured to finish the phantom. The setting process of the first layer and the possible settling of the cellulose scattering material could have created the PA signal seen at the interface. The second reason is related to the incident position of the ring beam on the interface surface between two layers of gelatin phantom which is located at 20 mm above the bottom graphite.

3.2.

Large-Scale Omnidirectional Illumination for Full-Ring Photoacoustic Tomography Experiment

Figure 5 shows the UST and PAT images of the full-ring tomography system shown in Fig. 2(b). The UST images are reconstructed using the waveform method,³⁴ and PAT images are reconstructed using the backprojection method.³⁵ For the backprojection reconstruction, the RF signals received at each transducer across multiple acquisitions of the same slice are averaged together to increase the signal-to-noise ratio (SNR). In Fig. 5, the top and bottom cross-sectional slices, which are separated by 4 cm of gelatin, are shown. In general, the results demonstrate the vertical depth-independent imaging capability of the full-ring illumination system utilizing a ring US transducer.

Fig. 5

(a) UST and (b) PAT images of slices 1 and 3, showing the 8-cm graphite absorber in the tissue-mimicking phantom. The full-ring illumination is able to visualize the whole objects in all slices.

The normalized PA amplitude as a function of the horizontal depth for the three different cross sections is shown in Fig. 6. As anticipated, all three slices showed high PA signal close to the outer surface, and the signal degraded slightly as the horizontal depth increased. In general, the results once again showed the deposited energy uniformity within the phantom, irrespective of vertical imaging depth.

Fig. 6

Normalized PA amplitude across the 8-cm graphite object for all three cross-sectional slices, demonstrating the uniformity of the PA signal across the targeted cross-sectional slice.

To increase the fluence in the central part of the scanned slice, it is important to improve the incident angle of the ring beam and the incident location of the ring beam with respect to the targeted cross-sectional area. In the previous experiment, the ring beam was positioned 10 mm below the targeted cross-sectional slice. Future works will examine the effects of target illumination as a function of beam position below a cross-sectional area and the resulting PA image quality.

3.3.

Future Work: An Optimal, Adjustable, All-Reflective, Full-Ring Illumination Photoacoustic Tomography System

For the full-ring illumination and US transducer system, the single cone mirror and parabolic reflector imaging system is able to create a ringed beam with a 39-deg angle with respect to the phantom surface. To improve the efficiency of the illumination, it is important to enhance the incident angle of the ring beam. The proposed three mirror system (Fig. 7) will have a tissue incidence angle of 66-deg with respect to the object surface, as opposed to 90-deg or normal incidence, which is anticipated to improve the PAT results. This system consists of a cone-shaped reflector and two conical reflectors which can be adjusted independently for scanning the length of a cylindrical object. As shown in Fig. 7, the collimated beam from the laser source is directed normally to the cone-shaped reflector, and upon reflection, a circular beam is directed toward the stationary conical ring reflector. The stationary conical ring reflector then transmits the cylindrical-shaped beam to the mobile conical mirror which focuses the beam onto the targeted slice of the object. The cone-shaped reflector and the first stationary mirror are external to the water tank that houses the second mobile mirror and the ring US transducer.

Fig. 7

(a) Cross-sectional view of the all-reflective, full-ring illumination and full-ring US transducer system, with three reflective elements in the imaging tank. (b) Three-dimensional view of the system showing a ring US-transducer, a pulsed laser, a cone-shaped reflector, and two conical reflectors.

During the scanning, the mobile conical reflector moves synchronously with the ring US transducer. Since the mobile reflector is placed below the transducer level, the breast tissue coverage will be identical to the UST and imaging areas close to the chest wall is feasible. The size and profile of the breast are defined by the UST images, which will be used to find the optimal position of the mobile reflector and adjust it to match the illumination and the acoustic acquisition planes. The design is verified by using ray tracing, which showed that it could illuminate breast tissue of 140 mm in diameter.