Handheld probe for quantitative micro-elastography

Abstract

Optical coherence elastography (OCE) has been proposed for a range of clinical applications. However, the majority of these studies have been performed using bulky, labbased imaging systems. A compact, handheld imaging probe would accelerate clinical translation, however, to date, this had been inhibited by the slow scan rates of compact devices and the motion artifact induced by the user's hand. In this paper, we present a proofof-concept, handheld quantitative micro-elastography (QME) probe capable of scanning a 6 × 6 × 1 mm volume of tissue in 3.4 seconds. This handheld probe is enabled by a novel QME acquisition protocol that incorporates a custom bidirectional scan pattern driving a microelectromechanical system (MEMS) scanner, synchronized with the sample deformation induced by an annular PZT actuator. The custom scan pattern reduces the total acquisition time and the time difference between B-scans used to generate displacement maps, minimizing the impact of motion artifact. We test the feasibility of the handheld QME probe on a tissue-mimicking silicone phantom, demonstrating comparable image quality to a benchmounted setup. In addition, we present the first handheld QME scans performed on human breast tissue specimens. For each specimen, quantitative micro-elastograms are co-registered with, and validated by, histology, demonstrating the ability to distinguish stiff cancerous tissue from surrounding soft benign tissue.

Handheld volumetric manual compression‐based quantitative microelastography

“…To enable QME measurements, we add a silicone bilayer (see Figure A inset) between the imaging window and the sample to measure surface stress. As described in previous work , the bilayer comprises two sections: a top, transparent section (Wacker P7676 A and B at 1:1 mixing ratio, ~250 μm thick, local Young's modulus = 18.7 ± 1.2 kPa at 20% axial strain) and a bottom, scattering section (Wacker P7676 A and B at 1:1 mixing ratio, mixed with 0.3 mg/mL TiO 2 scattering particles, ~250 μm thick, local Young's modulus = 18.7 ± 1.2 kPa at 20% axial strain). The two sections are cured together and the large variation in scattering light intensity between the layers results in high OCT contrast that is used to calculate the axial strain of the top section, which is equivalent to the axial strain of the bottom section, as the two sections use the same compound material.…”

“…This method helps to reduce image artifacts introduced by the edge detection failure due to poor edge contrast at the layer‐tissue interface reported in previous work . The scattering bottom section is used to estimate the local strain of the layer between consecutive B‐scans, at the measured axial strain . This local strain is then used to calculate the local stress in the sample, knowing the precharacterized stress‐strain curve of the layer material and assuming uniaxial stress distribution throughout the sample.…”

“…To date, QME has been primarily implemented in bulky benchtop setups , limiting its compatibility with clinical deployment. To overcome this limitation, we have recently developed a prototype handheld QME probe which holds promise for intraoperative use . In that work, the probe was customized with an annular piezoelectric actuator, providing step‐wise mechanical loading to the tissue, demonstrating rapid volumetric QME acquisition and high image contrast between stiff tumor and soft benign tissue regions.…”

“…First, the piezoelectric actuator complicates the optical and mechanical design of the probe, increasing the complexity and cost of the QME imaging system. For instance, in the handheld QME probe used in Reference , to allow the beam to propagate through the annular piezoelectric actuator, the inner and outer diameter of the actuator were customized to be 14 and 30 mm, respectively, resulting in a bulky front section of the probe, which may not be compatible with some clinical applications where a compact probe is required. Also, as piezoelectric actuators use delicate materials and additional apparatus, such as a high voltage amplifier, fabricating the actuator and incorporating it in the OCT imaging system can be expensive.…”

“…Second, piezoelectric actuators induce relatively small local strains to the sample. For instance, imparting 1 milli‐strain (mε) to a 5 cm thick sample, corresponding to 50 μm stroke from the actuator, requires a 70 mm stack of the piezoelectric material used in . This further complicates the design of the probe and may preclude QME in applications where scans of thick tissue are required.…”