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Comparison of techniques to characterise the point spread function of an acoustic-resolution photoacoustic microscope

Shah, A; Ivory, A M; Rajagopal, S; Zeqiri, B (2019) Comparison of techniques to characterise the point spread function of an acoustic-resolution photoacoustic microscope. In: 2019 IEEE International Ultrasonic Symposium (IUS), 6-9 October 2019, Glasgow, UK.

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Abstract

Motivation
This work presents a method to characterize the point spread function (PSF) of an acoustic-resolution photoacoustic microscope (PAM) (easyPAM-400TM, Kibero). In the absence of a point absorber, the conventional lateral and axial PSF characterisation approaches of a PAM involve the line spread function (LSF) measurement of a slanted edge and shift-and-sum, respectively. A comparison is made between these approaches that derive the PSF to that obtained by imaging point targets.
Methods
The PAM, used in this study has a single element ultrasound transducer (centre frequency - 400 MHz, bandwidth - 150 MHz) and a 532 nm pulsed laser with a broad beam illumination. The lateral resolution of the PAM was derived by estimating the edge spread function (ESF) of 15 edges of an UASF-1951 test chart (Fig. 1A). Assuming the acoustic beam profile is Gaussian, the LSF for each edge was calculated by differentiating the ESF. The lateral resolution of the system was defined as the full width-at-half maximum (FWHM) of the LSF (Fig. 1B). The axial resolution, derived using the shift and sum method, was defined as the smallest shifted distance that allowed the resolution of the two peaks in the sum of the enveloped original and shifted A-line signal (Fig. 1C) generated from a 1 µm thick gold film, by an amplitude difference of at least 10% (Fig. 1D). These results were compared to the FWHM of the Gaussian fits to the enveloped photoacoustic signal amplitude, along the axial and lateral directions (Fig. 1E and 1F) of 3 black dyed microspheres (diameter~3 µm, PolybeadTM), dispersed in gelatin.
Results and conclusion
The lateral and axial resolutions of the PAM, derived using the approaches mentioned above were estimated to be 6.43±1.33 µm and 7.6±0.2 µm, respectively. The lateral and axial PSF obtained from the point target were 7.75±0.25 µm and 8.3±0.67 µm, respectively. Comparison indicated that there is good agreement between the approaches that derive the PSF and that obtained by imaging point targets. A minor discrepancy (statistically insignificant) could be attributed to the scattering of light in the gelatin phantom in which the microspheres were dispersed.

Item Type: Conference or Workshop Item (Paper)
Subjects: Acoustics > Ultrasound
Divisions: Medical Physics
Identification number/DOI: 10.1109/ULTSYM.2019.8926150
Last Modified: 03 Jun 2020 14:45
URI: http://eprintspublications.npl.co.uk/id/eprint/8733

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