Percutaneous radiofrequency ablation (RFA) is used to destroy small liver tumors by locally inducing heat. However, there is a high tumor recurrence rate due to insufficient real-time image guidance during the procedure. We studied multi-wavelength photoacoustic imaging for identifying ablated tissue by taking the ratio of the photoacoustic signals at two wavelengths. To realize this, we first simulated the optical penetration in the liver and its influence on the optimal wavelength pair. Finally, the photoacoustic signals of treated and untreated bovine liver tissue were measured between 680 nm to 1100 nm to find candidate wavelength pairs for successful ratio imaging.
The change in the optical properties of tissue during thermal treatment can be potentially used to monitor procedures like Radiofrequency Ablation (RFA). We present key features in the optical absorption and scattering of tissue during the RFA procedure and during post-ablation cooling down to room temperature. We have used time-resolved diffuse optical spectroscopy for the measurement of the optical properties of tissue for the wavelengths from 650 to 1100 nm. Ex vivo experiments were conducted using a clinical RFA system on bovine liver tissue. Measurements were performed for two temperatures (70°C and 105°C). The following features were observed in the optical properties. First, there was a decrease in optical absorption and an increase in scattering during the treatment. With overtreatment, the absorption increased for initial part of the spectrum (until 910 nm) and scattering decreased in comparison to normal treatment. Secondly, a redshift of the hemoglobin peak and blue shift around water peak was observed in the optical absorption. Finally, a new peak around 840 nm and a valley around 920 nm appeared with heating. When the tissue was allowed to cool down, most of the changes in the absorption around the water peak partially reversed including the blue shift and the valley around 920 nm. Additionally scattering decreased with cooling. Results show key features in the optical properties of tissue during RFA, the effect of overtreatment and post-treatment cooling in ex vivo tissue. Insights from this study will help in advancing optical methods in monitoring thermal treatment.
Radiofrequency ablation (RFA) procedures for liver cancer treatment are hindered by high tumor recurrence. This is thought to be due to the intrinsic limitation of the heating mechanism and insufficient real-time feedback from imaging modalities. Most RFA procedures are performed under ultrasound (US) imaging and there are limitations in accurate device guidance and ablation monitoring. We propose photoacoustic (PA) imaging as a potential add-on to US imaging to address these limitations. Specifically, we present two interstitial PA imaging methods. Firstly, an annular fiber probe that can encapsulate an RFA device in its lumen. This device enables RFA device guidance, visualization of major blood vessels and targeting tumor tissue. Secondly, we used a cylindrical diffuser-based interstitial illumination to differentiate coagulated and native tissue. We present our results on RFA device guidance and ablation visualization using these approaches. The contrast provided by PA imaging for RFA needle and multiple electrodes is compared against that of US images. The difference between coagulated and native ex vivo liver tissue using PA imaging is studied. Finally, we propose a protocol to incorporate the minimally invasive PA imaging for the clinical RFA procedures. We would like to conclude with a note on how the proposed approach can potentially improve the outcome of RFA procedures.
Radiofrequency ablation (RFA) is minimally invasive thermotherapy, where a heating source is used to target and kill malignant cells in a tissue. While RFA has tremendous potential in the field of oncology, there is also a need for reliable real-time monitoring of this procedure to avoid over or under treatment. In this work, we investigate the use of timeresolved diffuse optical spectroscopy (DOS) to continuously track the change in optical properties during RFA to monitor the process of ablation. The time evolution of the spectra of the optical properties of the tissue undergoing treatment gives deep insights into the structural and constitutional changes occurring during the RFA treatment.
We present laser-induced ultrasound (LIUS) imaging, using a conventional linear ultrasound probe as a receiver. The LIUS source consists of a 40 μm thick film of Carbon Black-doped PDMS. Illumination of this LIUS transmitter with a 10 ns pulsed Nd:YAG laser with a 10 Hz repetition rate leads to the generation of a short, unipolar ultrasound pulse as a consequence of the photoacoustic effect. Two synthetically focused imaging techniques will be presented: coherently compounded multi-angled plane wave imaging (PWI) and synthetic transmit aperture imaging (SAI) . In the PWI case a planar LIUS transmitter, matched in size to the conventional probe aperture, is used. In the SAI case, the same film is illuminated sequentially at different locations along the aperture by an array of multimode optical fibres. For both PWI and SAI a comparison between conventionally acquired and LIUS images is made, as well as a cross-comparison between PWI and SAI. Images of wire phantoms, speckle analysis and finally images of tissue-mimicking phantoms demonstrate the image quality and advantages offered by LIUS sources. Aside from generating shorter pulses for enhanced resolution, the continuous nature of the absorber and the illumination spot provides a cleaner, more homogeneous plane wave field. The outlook for these unconventional US sources and their relative advantages and disadvantages are discussed.
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