Conical ring array detector for large depth of field photoacoustic macroscopy

Abstract

Photoacoustic microscopy and macroscopy (PAM) using focused detector scanning are emerging imaging methods for biological tissue, providing high resolution and high sensitivity for structures with optical absorption contrast. However, achieving a constant lateral resolution over a large depth of field for deeply penetrating photoacoustic macroscopy is still a challenge. In this work, a detector design for scanning photoacoustic macroscopy is presented. Based on simulation results, a sensor array geometry is developed and fabricated that consists of concentric ring elements made of polyvinylidene fluoride (PVDF) film in a geometry that combines a centered planar ring with several inclined outer ring elements. The reconstruction algorithm, which uses dynamic focusing and coherence weighting, is explained and its capability to reduce artefacts occurring for single element conical sensors is demonstrated. Several phantoms are manufactured to evaluate the performance of the array in experimental measurements. The sensor array provides a constant axial and lateral resolution of 95 µm and 285 µm, respectively, over a depth of field of 20 mm. The depth of field corresponds approximately to the maximum imaging depth in biological tissue, estimated from the sensitivity of the array. With its ability to achieve the maximum resolution even with a very small scanning range, the array is believed to have applications in the imaging of limited regions of interest buried in biological tissue.

Review of consensus test methods in medical imaging and current practices in photoacoustic image quality assessment

“… 110 While targets were often aligned perpendicular to the image plane, some photoacoustic CT studies used line targets parallel to the plane. 10 , 114 An alternative approach was to image spherical point targets such as 10- to black polyethylene microspheres, 11 , 51 , 55 , 56 , 119 – 123 100- to graphite particles, 124 , 125 or polyamide particles. 126 A few papers evaluated resolution using pairs of adjacent targets such as crossed threads, for instance using Sparrow’s resolution criterion.…”

“…In some PAI articles, sensitivity referred to ultrasonic transducer sensitivity (e.g., responsivity in V/mPa or noise-equivalent pressure in Pa), rather than image sensitivity. 123 , 144 Most sensitivity studies were performed to demonstrate detectability of exogenous contrast agents including dyes, 85 , 112 , 145 – 147 encapsulated-ink microbubbles, 148 and nanoparticles, 59 , 80 , 126 , 149 – 156 although other studies evaluated endogenous chromophores, such as melanoma cells 11 , 157 or blood with varying hematocrit. 128 Some studies used generic absorptive targets such as embedded tubes 48 , 102 , 114 or solid agar inclusions 158 containing colored inks.…”

“…One study used a SMOFLipid-agar phantom containing 0.7-mm diameter graphite rods to evaluate reduction of x-shaped reconstruction artifacts using dynamic focusing and coherence weighting. 123 Another study evaluated a technique to remove reflection artifacts caused by acoustic heterogeneity using a clear gelatin phantom 164 or water bath 165 containing inclusions with different acoustic properties from the background medium. Artifact reduction was quantified using intensity reduction ratio, i.e., the ratio of original to corrected ROI intensity.…”