Coherence-Weighted Synthetic Focusing Applied to Photoacoustic Imaging Using a High-Frequency Annular-Array Transducer

Abstract

This paper presents an adaptive synthetic-focusing scheme that, when applied to photoacoustic (PA) data acquired using an annular array, improves focusing across a greater imaging depth and enhances spatial resolution. The imaging system was based on a 40-MHz, 5-element, annular-array transducer with a focal length of 12 mm and an 800-μm diameter hole through its central element to facilitate coaxial delivery of 532-nm laser. The transducer was raster-scanned to facilitate 3D acquisition of co-registered ultrasound and PA image data. Three synthetic-focusing schemes were compared for obtaining PA A-lines for each scan location: delay-and-sum (DAS), DAS weighted with a coherence factor (DAS + CF), and DAS weighted with a sign-coherence factor (DAS + SCF). Bench-top experiments that used an 80-μm hair were performed to assess the enhancement provided by the two coherence-based schemes. Both coherence-based schemes increased the signal-to-noise ratio by approximately 10 dB. When processed using the DAS-only scheme, the lateral dimension of the hair in a PA image with 20 dB dynamic range was between 300 μm and 1 mm for imaging depth ranging from 8 to 20 mm. In comparison, the DAS + CF scheme resulted in a lateral dimension of 200 to 450 μm over the same range. The DAS + SCF synthetic focusing further improved the smallest-resolvable dimension, which was between 150 and 400 μm over the same range of imaging depth. When used on PA data obtained from a 12-day-old mouse embryo, the DAS + SCF processing improved visualization of neurovasculature.

Optical imaging of post-embryonic zebrafish using multi orientation raster scan optoacoustic mesoscopy

“…The latter operates with optical diffraction resolution using focused light beams that is a much higher resolution than optoacoustic mesoscopy using linear arrays 15 , 16 . The combination of advanced ultrasound detection techniques, including annular detector arrays 27 , could possibly improve the SNR and the focusing ability of the detector used.…”

Fiber-optic annular detector array for large depth of field photoacoustic macroscopy

“…Finally, the contributions of the signals of all rings are summed up. To reduce imaging artifacts originating from the off-axis-sensitivity, coherence weighting is used [18] , [17] . By scanning the sample, the obtained A-scans can be arranged to form two-dimensional or three-dimensional images.…”

“…To suppress these aberrations, arrays of ring-shaped transducers with varying diameters can be used. Annular detector arrays for photoacoustic microscopy were, e.g., presented in [14] , [15] , [16] , [17] . A piezoelectric annular detector array for large DOF photoacoustic imaging was demonstrated by Passler et al [18] .…”

Enhancement of in vivo cardiac photoacoustic signal specificity using spatiotemporal singular value decomposition

“… 17 These factors contribute to reduced signal specificity in the myocardial wall rendering cardiac PAI interpretation difficult. Adaptive beamforming algorithms such as spatial and spatiotemporal coherence weighting, 17 – 20 short-lag spatial coherence weighting, 21 delay-multiply and-sum, 22 and multiple DAS with Enveloping (multi-DASE) 23 have been employed to suppress incoherent clutter signals. However, these methods may also undesirably suppress the myocardial wall PA signals during clutter suppression leading to reduced signal specificity.…”