Multiangle Long-Axis Lateral Illumination Photoacoustic Imaging Using Linear Array Transducer

Abstract

Photoacoustic imaging (PAI) combines optical contrast with ultrasound spatial resolution and can be obtained up to a depth of a few centimeters. Hand-held PAI systems using linear array usually operate in reflection mode using a dark-field illumination scheme, where the optical fiber output is attached to both sides of the elevation plane (short-axis) of the transducer. More recently, bright-field strategies where the optical illumination is coaxial with acoustic detection have been proposed to overcome some limitations of the standard dark-field approach. In this paper, a novel multiangle long-axis lateral illumination is proposed. Monte Carlo simulations were conducted to evaluate light delivery for three different illumination schemes: bright-field, standard dark-field, and long-axis lateral illumination. Long-axis lateral illumination showed remarkable improvement in light delivery for targets with a width smaller than the transducer lateral dimension. A prototype was developed to experimentally demonstrate the feasibility of the proposed approach. In this device, the fiber bundle terminal ends are attached to both sides of the transducer's long-axis and the illumination angle of each fiber bundle can be independently controlled. The final PA image is obtained by the coherent sum of subframes acquired using different angles. The prototype was experimentally evaluated by taking images from a phantom, a mouse abdomen, forearm, and index finger of a volunteer. The system provided light delivery enhancement taking advantage of the geometry of the target, achieving sufficient signal-to-noise ratio at clinically relevant depths.

Keywords: Monte Carlo; illumination scheme; in vivo; linear array; mouse; photoacoustic imaging.

Figure 1

(a) Standard dark-field illumination scheme to acquire photoacoustic (PA) image in reflection mode; a rectangular optical fiber terminal illuminates the surface of the target. (b) Bright-field illumination where the laser beam and the acoustic field are coaxially aligned. (c) Proposed long-axis lateral illumination architecture; the variation of light incidence angle provides wide illumination to the surface of the target.

Figure 2

(a) Three-dimensional model and (b) prototype of the multiangle long-axis lateral illumination device attached to a linear array ultrasound transducer.

Figure 3

Depiction of the experimental setup used to acquire the multiangle long-axis lateral illumination PA images. The distance of 19.5 mm between the transducer and the phantom surface forces the focal illumination region to be at the phantom surface for ${\theta }_i$ = 0°.

Figure 4

(a) Photographs of the index finger and forearm of the volunteer. The dashed lines indicate the position and orientation of the transducer. (b) Depiction of the experimental setup used to acquire in vivo PA images of Balb/C mouse.

Figure 5

Normalized fluence maps obtained by Monte Carlo simulation for targets larger (a–c) and smaller (e–g) than the image width. An intermediate situation representing a mouse torso was also considered (i–k). All cases were simulated for the bright-field, standard dark-field, and long-axis lateral illumination schemes. Average fluence values were estimated within regions of interest (ROIs) A, B, and C for all cases (d,h,l).

Figure 6

Comparison between the standard dark-field illumination scheme and the multiangle long-axis lateral illumination. For targets larger than the image width with a nonflat surface, the long-axis lateral and the bright-field illumination schemes have the advantage of delivering light within the imaging plane. For targets smaller than the transducer width, the long-axis lateral illumination scheme can deliver light to the sides of the target.

Figure 7

PA subframes of the homogeneous phantom for increasing illumination angles in the range 0°–18°. Each subframe is an average of the phantom’s elevational dimension (i.e., 3.8 cm). Blue arrows indicate the generation of PA signals beyond the laser focal region.

Figure 8

Average PA signal as a function of the illumination angle, along the axial direction at (a) central (ROI-1) and (b) peripheral ROIs (ROI-2 and ROI-3); (c) average PA signal magnitude along lateral direction for depths ranging from 0 mm to 2 mm.

Figure 9

(a) Mean square root of PA signal and (b) mean depth of PA signal as a function of illumination angle.

Figure 10

(a) Final PA image of a single image slice of the phantom, (b) PA signal profile of this PA image shows the contribution of all illumination angles to generate PA signal at depths greater than 10 mm, and (c) SNR as a function of image depth.

Figure 11

In vivo multiangle long-axis lateral illumination PA and B-mode images. (a) PA image and (b) B-mode image of the human forearm, anatomical structures such as tendons and subcutaneous blood vessels can be identified; (c) PA image and (d) B-mode image of the human index finger, the cylindrical shape allows light delivery by the laterals promoting the visualization of the palmar digital artery at depth of 10.3 mm. (e) PA image and (f) B-mode image of the Balb/Cmouse abdomen, PA signal from an aortic branch at 10.5 mm of depth can be visualized.