Tattoo tomography: Freehand 3D photoacoustic image reconstruction with an optical pattern

Abstract

Purpose: Photoacoustic tomography (PAT) is a novel imaging technique that can spatially resolve both morphological and functional tissue properties, such as vessel topology and tissue oxygenation. While this capacity makes PAT a promising modality for the diagnosis, treatment, and follow-up of various diseases, a current drawback is the limited field of view provided by the conventionally applied 2D probes.

Methods: In this paper, we present a novel approach to 3D reconstruction of PAT data (Tattoo tomography) that does not require an external tracking system and can smoothly be integrated into clinical workflows. It is based on an optical pattern placed on the region of interest prior to image acquisition. This pattern is designed in a way that a single tomographic image of it enables the recovery of the probe pose relative to the coordinate system of the pattern, which serves as a global coordinate system for image compounding.

Results: To investigate the feasibility of Tattoo tomography, we assessed the quality of 3D image reconstruction with experimental phantom data and in vivo forearm data. The results obtained with our prototype indicate that the Tattoo method enables the accurate and precise 3D reconstruction of PAT data and may be better suited for this task than the baseline method using optical tracking.

Conclusions: In contrast to previous approaches to 3D ultrasound (US) or PAT reconstruction, the Tattoo approach neither requires complex external hardware nor training data acquired for a specific application. It could thus become a valuable tool for clinical freehand PAT.

Keywords: 3D; Optical pattern; Optoacoustic; Photoacoustic; Tomography.

Conflict of interest statement

The authors declare that they have no conflict of interest.

Figures

Fig. 1

Tattoo tomography The approach to 3D image reconstruction comprises four steps. (1) Prior to image acquisition, the optical pattern is placed on the region of interest. (2) Next, a sequence of images is acquired, each showing a part of the pattern as well as the target region. (3) The image features corresponding to the pattern (here: three points) are extracted from each slice and used to recover the pose of the probe relative to the pattern coordinate system. (4) Once all acquired slices have been transformed (T) into a common coordinate system (C), the 3D volume is compounded

Fig. 2

2D representation of the tattoo containing all relevant variables (left) and corresponding photoacoustic image (right). It shows the pattern (green), and the intersection of the image slice with the pattern (blue line). The image clearly shows three points representing the part of the image that intersects with the optical pattern (l, c, r). Based on the distances dl and dr between neighboring points, the pose of the intersection line represented by a blue line can be unambiguously determined

Fig. 3

Phantom used for the quantitative validation of reconstruction accuracy. The optical markers (silver spheres) enable the comparison to a baseline 3D reconstruction method. The red box (right: zoomed in) highlights the optical pattern placed on top of the N-wires. The wires have a diameter of 0.4mm, and the holes of the frame are 5mm apart from each other

Fig. 4

The experimental setup and results of the feasibility demonstration experiment. The IPCAI logo printed on paper is placed underneath a gel pad on top of which the optical pattern is fixed and some ultrasound gel is added (a). A freehand PA scan is acquired (b), which results in a fully image-based and clear reconstruction of the IPCAI logo

Fig. 5

Good and poor reconstruction of a target region (here the text IPCAI, right) corresponding to a relatively slow and careful image acquisition (top) and a fast and careless acquisition (bottom). The 2D images represent an intersection of the image stack at a fixed y-position before (left) and of the 3D volume after (center) Tattoo reconstruction

Fig. 6

The 3D model of the N-wire phantom (white) is displayed together with the optical pattern compounded volumes of a single scan (blue, left) and all three scans (blue, red, green, right) after applying an ICP algorithm to register the Tattoo point cloud on the N-wire model (cf. Table 1)

Fig. 7

Left: Analysis of an in-plane flat surface constraint violation. a Photoacoustic image of a healthy volunteer wrist showing the encoded pattern information. b Distances of the pattern absorption points assuming a flat surface. c Approximation of the real distances via a Bézier curve with two control points. Right: Potential hardware setup to overcome the flat surface constraint