Simultaneous near-infrared diffusive light and ultrasound imaging

Abstract

We have constructed a near-real-time combined imager suitable for simultaneous ultrasound and near-infrared diffusive light imaging and coregistration. The imager consists of a combined hand-held probe and the associated electronics for data acquisition. A two-dimensional ultrasound array is deployed at the center of the combined probe, and 12 dual-wavelength laser source fibers (780 and 830 nm) and 8 optical detector fibers are deployed at the periphery. We have experimentally evaluated the effects of missing optical sources in the middle of the combined probe on the accuracy of the reconstructed optical absorption coefficient and assessed the improvements of a reconstructed absorption coefficient with the guidance of the coregistered ultrasound. The results have shown that, when the central ultrasound array area is in the neighborhood of 2 cm x 2 cm, which corresponds to the size of most commercial ultrasound transducers, the optical imaging is not affected. The results have also shown that the iterative inversion algorithm converges quickly with the guidance of a priori three-dimensional target distribution, and only one iteration is needed to reconstruct an accurate optical absorption coefficient.

Fig. 1

Schematic arrangement of NIR source and detector fibers on the probe. Small solid circles are the source fibers and larger solid cycles are the detector fibers.

Fig. 2

Picture of an experimental probe. An ultrasound array of 8 × 8 = 64 transducers occupies the central 3 cm × 3 cm area, and 12 dual-wavelength source fibers and 8 detector fibers are deployed at the periphery.

Fig. 3

Schematic of the NIR frequency-domain imaging system. The modulation frequency is 140 MHz. The 12 dual-wavelength source channels are switched on sequentially by a PC, and 8 detector channels receive signals in parallel. BPF, bandpass filter; OSC, oscillator.

Fig. 4

(a) Log(ρ_αβ ²A_αβ) versus distancev ρ_αβ after calibration. (b) Phase ϕ_αβ versus distance ρ_αβ after calibration.

Fig. 5

Schematic of our ultrasound scanner. We connected 64 ultrasound transducers to 64 parallel transmission and reception channels. The transmission part consists of 64 high-voltage pulsers, which can be controlled by computer-generated delay profiles. The reception part consists of 64 two-stage amplifiers and A/D converters. CH, channel.

Fig. 6

Ultrasound subarray scanning configuration.

Fig. 7

Picture of our combined system. NIR system (top) and ultrasound system (bottom) are mounted on a hospital cart.

Fig. 8

Reconstructed NIR images of deeper targets (2.5 cm in depth, 1 cm in diameter, and the fitted background μ_a and μ_s′ are 0.015 and 5.36 cm⁻¹, respectively). The left column corresponds to images of a high-contrast target (μ_a = 0.25 cm⁻¹) obtained from different probe configurations, and the right column corresponds to images of a low-contrast target (μ_a = 0.1 cm⁻¹). Each row is related to a specific hole size: (a) and (b) no hole, (c) and (d) 2 cm × 2 cm, (e) and (f) 3 cm × 3 cm.

Fig. 9

Reconstructed NIR images for shallow targets (1.5 cm in depth, 1 cm in diameter, and the fitted background μ_a and μ_s′ are 0.015 and 5.36 cm⁻¹, respectively). The left column corresponds to images of a high-contrast target (μ_a = 0.25 cm⁻¹), and the right column corresponds to images of a low-contrast target (μ_a = 0.1 cm⁻¹). Each row is related to a specific hole size: (a) and (b) no hole, (c) and (d) 2 cm × 2 cm, (e) and (f) 3 cm × 3 cm.

Fig. 10

Deep target (2.5 cm in depth, 1 cm in diameter) of low optical contrast (μ_a = 0.10 cm⁻¹ and fitted background μ_a and μ_s′ are 0.02 and 5.08 cm⁻¹, respectively). (a) A-scan line of the reflected ultrasound pulse-echo signal indicating the target depth. (b) Absorption image of the low-contrast target obtained from optical-only reconstruction. (c) Ultrasound-guided reconstruction at target depth.

Fig. 11

Simultaneously obtained ultrasound and NIR absorption images. The fitted background μ_a and μ_s′ are 0.017 and 4.90 cm⁻¹, respectively. (a) Ultrasound and (b) NIR absorption image of two high-contrast targets (target μ_a = 0.25 cm⁻¹). (c) Ultrasound and (d) NIR image of two low-contrast targets (target μ_a = 0.10 cm⁻¹). In both high- and low-contrast cases, the two targets were located at 2.5 cm in depth.

Fig. 12

(a) and (c) −6-dB contour plots of ultrasound images shown in Figs. 11(a) and 11(c). The outer contour is −6 dB from the peak, and the contour spacing is 1 dB. (b) and (d) Corresponding NIR absorption maps reconstructed in target regions specified by ultrasound.