A Coregistered Ultrasound and Photoacoustic Imaging Protocol for the Transvaginal Imaging of Ovarian Lesions

Summary

We report a coregistered ultrasound and photoacoustic imaging protocol for the transvaginal imaging of ovarian/adnexal lesions. The protocol may be valuable to other translational photoacoustic imaging studies, especially those using commercial ultrasound arrays for the detection of photoacoustic signals and standard delay-and-sum beamforming algorithms for imaging.

Abstract

Ovarian cancer remains the deadliest of all the gynecological malignancies due to the lack of reliable screening tools for early detection and diagnosis. Photoacoustic imaging or tomography (PAT) is an emerging imaging modality that can provide the total hemoglobin concentration (relative scale, rHbT) and blood oxygen saturation (%sO2) of ovarian/adnexal lesions, which are important parameters for cancer diagnosis. Combined with coregistered ultrasound (US), PAT has demonstrated great potential for detecting ovarian cancers and for accurately diagnosing ovarian lesions for effective risk assessment and the reduction of unnecessary surgeries of benign lesions. However, PAT imaging protocols in clinical applications, to our knowledge, largely vary among different studies. Here, we report a transvaginal ovarian cancer imaging protocol that can be beneficial to other clinical studies, especially those using commercial ultrasound arrays for the detection of photoacoustic signals and standard delay-and-sum beamforming algorithms for imaging.

Introduction

Photoacoustic imaging or tomography (PAT) is a hybrid imaging modality that measures the optical absorption distribution at US resolution and depths far beyond the tissue optical diffusion limit (~1 mm). In PAT, a nanosecond laser pulse is used to excite biological tissue, causing a transient temperature rise due to optical absorption. This leads to an initial pressure rise, and the resultant photoacoustic waves are measured by US transducers. Multispectral PAT involves the use of either a tunable laser or multiple lasers operating at different wavelengths to illuminate the tissue, thereby enabling the reconstruction of optical absorption maps at multiple wavelengths.....

Protocol

All the research performed was approved by the Washington University Institutional Review Board.

1. System configuration: Optical illumination (Figure 1)

1. Use an Nd:YAG laser pumping a pulsed, tunable (690-890 nm) Ti-sapphire laser at 10 Hz.
2. Expand the laser beam by first diverging the beam with a plano-concave lens and then collimating the beam with a plano-convex lens. Use two mirrors to direct the beam onto a beam splitter (described below).
3. Split the expanded laser beam into four beams with equal energy by splitting the original beam into two ....

Results

Here, we show examples of malignant and normal ovarian lesions imaged by USPAT. Figure 3 shows a 50 year old premenopausal woman with bilateral multicystic adnexal masses revealed by contrast-enhanced CT. Figure 3A shows the US image of the left adnexa with the ROI marking the suspicious solid nodule inside the cystic lesion. Figure 3B shows the PAT rHbT map superimposed onto the US and shown in red. The rHbT showed extensive diffus.......

Discussion

Optical illumination
The number of fibers used is based on two factors: light illumination uniformity and system complexity. It is critical to have a uniform light illumination pattern at the skin surface to avoid hot spots. It is also important to keep the system simple and robust with a minimal number of fibers. The use of four separate fibers has previously been shown to be optimal for creating uniform illumination at depths of several millimeters and beyond. Additionally, the light coupling to .......

Materials

Name	Company	Catalog Number	Comments
Clinical US imaging system	Alpinion Medical Systems	EC-12R	Fully programmable clinical US system
Dielectric mirror	Thorlabs	BB1-E03	Used to reflect light along the optical path
Endocavity US transducer	Alpinion Medical Systems	EC3-10	Transvaginal ultrasound probe
Laser power meter	Coherent	LabMax TOP	Used to measure laser energy
Multi-mode optical fiber	Thorlabs	FP1000ERT	Couple laser light to the endocavity ultrasound probe
Non-polarizing beam splitter plate	Thorlabs	BSW11	For splitting laser beam into sensors to measure energy
Plano-concave lens	Thorlabs	LC1715	For laser beam expansion
Plano-convex lens	Thorlabs	LA1484-B	For laser beam collimation
Plano-convex lens	Thorlabs	LA1433-B	Used to focus light into four optical fibers
Polarizing beam splitter cube	Thorlabs	PBS252	For splitting laser beam into four beams
Protective probe shealth	Custom 3D printed		Hold and protect the four optical fibers at the tip of the ultrasound probe
Right angle prism mirror	Thorlabs	MRA25-E03	Used to reflect light along the optical path
Tunable laser system	Symphotic TII	LS-2145-LT50PC	Light source for multispectral PAT
USPAT control software	Custom developed in C++		Controls acquisition parameters of the ultrasound machine and the laser wavelength
USPAT image display software	Custom developed in C++		Displays the US/PAT B-scans and sO2/rHbT maps in real time