In vivo demonstration of reflection artifact reduction in photoacoustic imaging using synthetic aperture photoacoustic-guided focused ultrasound (PAFUSion)

Abstract

Reflection artifacts caused by acoustic inhomogeneities are a critical problem in epi-mode biomedical photoacoustic imaging. High light fluence beneath the probe results in photoacoustic transients, which propagate into the tissue and reflect back from echogenic structures. These reflection artifacts cause problems in image interpretation and significantly impact the contrast and imaging depth. We recently proposed a method called PAFUSion (Photoacoustic-guided focused ultrasound) to identify such reflection artifacts in photoacoustic imaging. In its initial version, PAFUSion mimics the inward-travelling wavefield from small blood vessel-like PA sources by applying ultrasound pulses focused towards these sources, and thus provides a way to identify the resulting reflection artifacts. In this work, we demonstrate reduction of reflection artifacts in phantoms and in vivo measurements on human volunteers. In view of the spatially distributed PA sources that are found in clinical applications, we implemented an improved version of PAFUSion where photoacoustic signals are backpropagated to imitate the inward travelling wavefield and thus the reflection artifacts. The backpropagation is performed in a synthetic way based on the pulse-echo acquisitions after transmission on each single element of the transducer array. The results provide a direct confirmation that reflection artifacts are prominent in clinical epi-photoacoustic imaging, and that PAFUSion can strongly reduce these artifacts to improve deep-tissue photoacoustic imaging.

Keywords: (170.3880) Medical and biological imaging; (170.5120) Photoacoustic imaging; (170.7170) Ultrasound.

Fig. 1

(a) Illustration of inward (P_i) and outward-propagating (P_O) PA transients, for the special case where they are generated by optical absorption in the skin melanin layer. (b) Time-inversion and backpropagation of the PA signal from interval t = [0, T], towards (c) mimicking the inward-travelling wavefield P_i* and identification of resulting echoes.

Fig. 2

Illustration of the experimental setup.

Fig. 3

Scheme of the phantom and experimental setup.

Fig. 4

Epi-mode PA/US setup used for the in vivo measurements.

Fig. 5

Phantom experiment a) photoacoustic image, b) ultrasound image, c) PAFUSion image with identified reflection artifacts, d) corrected photoacoustic image.

Fig. 6

In vivo measurement 1 a) photoacoustic image, b) ultrasound image, c) PAFUSion image with identified reflection artifacts, d) corrected photoacoustic image after subtraction of reflection artifacts, e) photoacoustic image with features marked for reference. The initial photoacoustic image followed by the corrected photoacoustic image after subtraction of the PAFUSion image is shown in Visualization 1.

Fig. 7

In vivo measurement 2 a) photoacoustic image, b) ultrasound image, c) PAFUSion image with identified reflection artifacts, d) corrected photoacoustic image, e) photoacoustic image with features marked for reference. The initial photoacoustic image followed by the corrected photoacoustic image after subtraction of the PAFUSion image is shown in Visualization 2.