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Photoacoustic and Ultrasonic Imaging for Minimally-Invasive Guidance

Ultrasound imaging is frequently used for guiding minimally invasive medical procedures such as peripheral nerve blocks, tumour biopsy, and fetal blood sampling. Accurate and efficient identification of the procedure target and the needle is of paramount importance to ensure the efficacy and safety of the procedures. However, ultrasound imaging suffers from intrinsically low soft tissue contrast that sometimes results in insufficient visibility of critical tissue structures such as nerves and small blood vessels. Moreover, visibility of clinical needles with ultrasound imaging is strongly dependent on the insertion angle and depth of the needle. Loss of visibility of tissue targets or the needle can cause significant complications.

At PURL, we investigate the combination of ultrasound and photoacoustic imaging for guiding minimally invasive medical procedures by offering complementary information to each other, with ultrasound imaging providing tissue structural information and photoacoustic imaging identifying critical tissue targets and interventional devices such as clinical metallic needles.


The imaging depths of surface-illumination-based photoacoustic imaging systems have been suffering from rapid attenuation of light with depth, which limits the clinical applicability of photoacoustic imaging. To address this challenge, we developed an interventional multispectral photoacoustic imaging system for guiding minimally invasive procedures (Xia et al, 2015), PA excitation light was delivered inside the tissue through an optical fibre placed within the cannula of an injection needle. Ultrasound signals were detected at the tissue surface using a clinical Ultrasound imaging probe. The feasibility of the system for guiding fetal surgical procedures was demonstrated by visualising surface vasculature a freshly excised human placenta (Xia et al, 2015). 

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Laser diodes and light emitting diodes (LEDs) have shown promise as an alternative to commonly used solid-state laser excitation sources with their compact size and low cost, further benefiting the clinical transition of PA imaging (Xia et al, 2018). However, the pulse energy of LEDs is much lower than that of the laser, leading to degraded PA images with lower signal-to-noise (SNR) ratio.


To enhance the image performance of LED (low-fluence) based PA/US imaging system, with a particular focus on blood vessels and interventional devices such as metallic needles:

1.     We proposed a spatiotemporal singular vector decomposition (STSVD) based denoising algorithm dedicated to light sources that typically have low fluence and high pulse repetition rates (Shi et al, 2022). STSVD leverages both spatial and temporal correlations between radiofrequency (RF) data frames. It demonstrated an SNR improvement by 187.4% using in vivo data compared to the noisy data and a fast-processing time of 50 μs per frame, making it potentially helpful for real-time clinical applications. 

2.     We demonstrated robust enhancements of needle visualisation with LED-based PA system (Shi et al, 2022). This was achieved by combing extracorporeal and interstitial illuminations with nanocomposite coatings.  The visualisation of both the needle shaft and tip was substantially enhanced, with a maximum visibility of 3.8 cm (maximum displaying range of the system) using porcine tissues ex vivo.


3. Image artefacts can be observed with comparable PA intensities to those of interventional devices. Therefore, we developed a deep learning (DL)-based framework based on semi-synthetic dataset for further enhancing PA visualisation of clinical metallic needles (Shi et al, 2021). With in vivo inference using human fingers, the proposed U-Net based model outperformed traditional image processing algorithm such as Hough Transform and exhibited a 5.8 times improvement in SNR compared to conventional reconstruction.


The PA image quality for both the blood vessels and clinical metallic needles was improved with the proposed methods, which would be of great help on improving the outcomes of PA/US-guided minimally invasive medical procedures in the future.

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