Over recent years open surgery has been replaced by minimally invasive surgery for many procedures. Here, instead of large incisions to ‘open’ the patient up to reach the surgical site, only small incisions are made, like a keyhole. Tools and devices are then passed through these incisions and used to treat the disease. This type of surgery possesses many advantages including, reduced scarring and patient discomfort since only small cuts are made to the patients’ skin, and hospital stays can be shorter. However, there is one major limitation, without opening the patient up, the surgeon has no direct line-of-sight of the disease and the tools during surgery. As such, imaging technologies are required to provide a close-up view of the surgery.
One group of surgeries commonly carried out in a minimally invasive manner are ablation procedures. In these procedures a device is used to heat (or cool) diseased tissue, effectively cooking it and causing the tissue to die. This can be used to treat conditions including atrial fibrillation (where someone has an irregular heart beat) or to destroy tumours. Devices for this ablation include radiofrequency devices or, as in the focus of our research, lasers. With a laser, the laser light is directed at the tissue from the end of an optical fibre and the light absorbed within the tissue causes it to heat up. Once above a certain temperature the cells breakdown and die.
A current issue with using lasers in this context is the inability to monitor the progression of the ablation and to guide the procedure. This means that the disease can be under or over-treated, leading to the need for follow-up procedures or damage to surrounding healthy tissue. Several technologies have been used to mitigate this, such as MRI or traditional ultrasound probes, however, each has its limitations. For example, MRI, whilst providing a good spatial resolution, cannot provide real-time feedback as the image acquisition takes too long. In this work we demonstrate a potential solution to this problem using a new imaging modality, all-optical ultrasound, which can be integrated alongside the laser to provide high-resolution guidance in real-time.
This imaging modality, all-optical ultrasound, is an emerging imaging technology that uses light to generate and receive ultrasound. This means, much like with the internet provided to your home, electric cables can be replaced with optical fibres. This enables the fabrication of highly miniaturised devices which can fit within the surgical tools used for minimally invasive surgery. Further, by generating and receiving ultrasound using light there is the potential for very high resolution imaging and a high sensitivity. This type of optical fibre device provides a view directly ahead of the device, so by putting it alongside the fibre which provides the laser light for ablation it can give an ultrasound image of the diseased tissue directly as it is being ablated. These M-mode ultrasound images display sequentially acquired pulse-echo signals, giving the operator a view of changes occurring within the tissue over time.
In our work we developed a device for this purpose, which consisted of a console with a display for real-time M-mode ultrasound image display and an ultrasound/ablation device (Figure 1). The device combined three optical fibres, one to generate ultrasound, one to receive ultrasound, and one to provide laser light for tissue ablation. The console was used to interrogate the ultrasound transducer and to deliver the laser light for tissue ablation. The total device diameter was less than 1 mm, making it well-suited to integration in surgical devices such as catheters. Further, a segmentation algorithm was implemented to track the ablation depth visible in the M-mode ultrasound images, providing feedback on ablation lesion size.
To test the device, two different tissues were used, heart and liver tissue, both from pigs. These were chosen due to their clinical interest for atrial fibrillation and tumour resection. Additionally, two different ablation protocols were followed, namely, contact and non-contact. For the non-contact regime the ablation fibre was held at a fixed distance of 1 mm from the tissue surface. Whilst for contact the fibre was put in contact with the tissue surface. A series of ablation experiments were carried out using different laser ablation powers and durations. During ablation M-mode ultrasound images were acquired and displayed. Subsequently, after the procedure was complete the ablation lesion was cut along its centre and imaged under a microscope to obtain an independent measure of the ablation depth.
The progress of the ablation was clearly visualised in the M-mode ultrasound images (Figure 2), demonstrating details such as the onset of tissue carbonisation and gas ejection from the tissue. These are important bits of information for the operator during procedures. Further, the ablation boundary was tracked throughout the ablation for all experiments and correlated well with the microscope measurements taken after (Figure 3). This demonstrates the potential for this technology to provide accurate monitoring and feedback during procedures from a highly miniaturised device.
This proof-of-concept demonstrates the possibility for OpUS imaging to be integrated into therapeutic techniques to provide real-time guidance. Further, the ability to accurately track the ablation lesion depth is highly promising for improving guidance during surgeries. The all-optical device had a small form factor and could be readily integrated into a catheter in future iterations. It is expected that this will progress towards in vivo studies and device developments to expand the application to Laser Interstitial Thermal Therapy (LITT) and side-viewing devices for ablation in blood vessels. This work is the first step along the path to the clinic and by providing real-time feedback it is hoped that devices like this could improve the accuracy and outcomes of future medical interventions.
For more details please see the original manuscript published in Communications Engineering here: https://www.nature.com/articles/s44172-022-00020-9
To see our other work with all-optical ultrasound, and the work of our colleagues please visit: https://www.interventionaldevices.org/