February 2015
Image guidance is a central component of minimally invasive fetal surgery for treatment of twin-to-twin transfusion syndrome (TTTS). The gold standard for treatment involves laser photocoagulation of anastamosing vessels on the fetal side of the placenta. In current practice, anastomosing vessels are identified with vessels along the equator using a fetoscope. Due to the limitations of this modality, there is a risk that sub-surface vessels that are small and those that are at the periphery of the placenta are missed, so that treatment is incomplete. Ultrasound (US) imaging with a probe positioned at the external surface of the mother provides inadequate visualization for small placental vessels. Visualization with US can be particularly poor when the placenta is on the posterior of the uterus, at a large distance from the imaging probe so that low US frequencies are required.
Photoacoustic (PA) imaging has strong potential to provide guidance information during TTTS that is complementary to fetoscopy and external US imaging. With PA imaging, pulsed or temporally modulated excitation light is absorbed in tissue, which causes temperature rises and subsequent generation of US waves. These US waves can be received with an imaging probe at the surface of a patient and reconstructed to form 2D or 3D images with contrast for tissue chromophores. Multispectral PA images, which are acquired by varying the wavelength of excitation light, can be used to provide quantitative information about chromophore concentrations.
Conventional implementations of PA imaging, in which excitation light is delivered at the surface of the patient, may be suboptimal in the context of TTTS. As placental vessels typically lie more than five centimeters below the surface, the PA signals may be very low or undetectable due to the prominence of scattering and absorption of excitation light. In this study, light is delivered directly to the placental surface using an optical fiber that is sufficiently small to be inserted into the instrument channel of a fetoscope. The generated PA signals were detected by a commercial linear array US imaging probe.
The PA imaging system in this study was extended to allow for real-time in-plane ultrasonic tracking of the optical fiber that delivers excitation light. Ultrasonic tracking was implemented with a fiber-optic hydrophone that was positioned alongside the delivery fiber, which received transmissions from the ultrasound imaging probe. The received transmissions were used to reconstruct an image of the hydrophone that is co-registered to the PA and US images. Multispectral PA imaging and ultrasonic tracking were performed on a human placenta ex vivo, and the measured photoacoustic spectrum from a vein was compared with the optical spectra of oxy- and deoxy-haemoglobin.
Wenfeng Xia