In vivo ultrasound modulated optical tomography with a persistent spectral hole burning filter

Abstract

We present in vivo ultrasound modulated optical tomography (UOT) results on mice, using the persistent spectral hole burning (PSHB) effect in a Tm³⁺:YAG crystal. Indocyanine green (ICG) solution was injected as an optical absorber and was clearly identified on the PSHB-UOT images, both in the muscle (following an intramuscular injection) and in the liver (following an intravenous injection). This demonstration also validates an experimental setup with an improved level of performance combined with an increased technological maturity compared to previous demonstrations.

Fig. 1.

Experimental setup for PSHB-UOT. Amp.: Optical amplifier. (P)BS: (Polarizing) beam splitter. AOD(M): Acousto-optic deflector (modulator). The use of a deflector allows for a large acousto-optic diffraction angle, which leads to an efficient isolation of the different orders of diffraction. Multi. Fiber: Multimode optical fiber. HWP: Half-wave plate. US transducer: Ultrasonic transducer. Cryo: Cryostat. S.: Shutter. APD: Avalanche photodiode. Acq. Card: Acquisition card.

Fig. 2.

a) SHB scheme using only one sideband of tagged photons. b) Experimental chronogram of pump-probe stage, along with the shutter status. Time scale is not respected. The cycle period $T_{cycle}$ is 127 ms, and the delay $T_{US}$ between two consecutive US pulses is 70 µs. An additional delay $T_{delay}$ of 8 µs is added between each pair of US and acquisition card trigger, to match the beginning of each line acquisition with the start of US emission in the medium. c) The spectral hole contrast profile (see definition in text) measured with a chirped probe signal centered at the pump frequency, with and without magnetic field. The full width at half maximum (FWHM) is 2.1 MHz and 2.0 MHz., respectively.

Fig. 3.

Schematic presentation of PSHB-UOT setup for a) intramuscular injection of ICG in the thigh muscle (transmission configuration) and for b) intravenous injection of ICG to detect ICG accumulated in the liver (reflection configuration). A zoom on the contact between the agarose gel and the thigh muscle is shown in the inset of a). The front piece of the mechanical adapter (which hosts the US transducer and the agarose gel) and that of the mechanical support for the mouse body (in transmission configuration), along with the mechanical clamps for the multimode fiber, the liquid core fiber and the Hamilton pump syringe are not shown. The mouse body has a gray colour instead of white, for visualization purpose. c), d) Photos of the experimental setups for ICG c) intramuscular injection and d) intravenous injection PSHB-UOT tests, respectively. Only the coupler of the multimode fiber is shown, and the liquid core fiber is clamped on a mechanical holder (black piece with blue scotch glued upon on the image). The mouse is positioned on its side in the intramuscular injection test, and on its back in the intravenous test.

Fig. 4.

a) PSHB-UOT image of the Intralipid phantom, with b) the vertical profile cut through both optical absorbers and c) the horizontal profile cut through the lower optical absorber. Raw data are shown in black and red dots, while gaussian fits are shown in orange and blue lines, respectively. d) Photo of the two absorbers in the Intralipid phantom. e) Maximum signal of PSHB-UOT images at different pump powers (magenta points), and the temperature of Tm³⁺:YAG crystal at different pump powers (dark yellow curve).

Fig. 5.

a) PSHB-UOT images before (i) and after (ii) ICG injection (20 µL) into the mouse thigh muscle. The signal of each image is normalized. The white boxes help locating the intensity drop due to the optical absorption of ICG b) Vertical profiles at x = 20 mm before and after ICG injection, showing a clear intensity drop at y = 28 mm. c) US image alone (i), and US image with UOT image (ii) (after ICG injection) projected on it. The needle tip and the mouse thigh are marked in orange contours. In the US image, the diameter of the needle appears larger than its real diameter ( $\sim$ 1 mm compared to 0.4 mm). d) Bright field image (i) and whole body NIR-II fluorescence images of mouse (side position) acquired before (ii) and after ICG injection (iii), with the latter showing a clear fluorescence signal of ICG trace in the thigh muscle

Fig. 6.

a) Photo of PSHB-UOT imaging setup for ICG injection into bovine tissue (10 mm thick). b) PSHB-UOT image taken before ICG injection, showing the presence of the needle (faint blue line) inside the tissue c) PSHB-UOT image taken after the injection of ICG (10µL), showing the presence of the needle and the ICG trace held on its apex. The signal of each UOT image is normalized.

Fig. 7.

a) PSHB-UOT images taken for a vertical section of the mouse liver before (i) and 45 min after (ii) the intravenous injection of ICG. The area of strong artefact signal is marked with dashed white boxes in both UOT images. b) Vertical profiles at x = 19 mm before and 45 min after intravenous ICG injection, showing a strong intensity drop from y = 27 mm onwards. c) US image alone (i), and US image with the projected UOT image (45 min after ICG injection) (ii). The mouse liver is marked in orange contour. The liver is present between the lung (left side of the image) and the intestinal tract (right side of the image) d) Bright field image (i) and whole body NIR-II fluorescence images of mouse (ventral position) acquired before (ii) and after the injection (iii), showing a clear accumulation of ICG in the liver. Note that these fluorescence images show the entire liver of the mouse, whereas the UOT images show only a cut section of the liver.