History
The application of lasers in the brain started in 1965, when a pulsed ruby laser was used on the cranium of mice and guinea pigs, leading to immediate death. The cause of death was sudden increased intracranial pressure due to the explosive interaction of the laser and the brain tissue in a closed cranium. To overcome the explosive interaction of the laser with tissue and to obtain more accurate and controlled laser energy, a continuous-wave laser with an improved delivery system was used. In 1966, a CO2 laser, which is a high-power continuous-wave laser, was used to vaporize a recurrent glioma. Although the procedure was precise and controlled, it was a time-consuming procedure and consequently impractical. In 1983, Bown used the Nd:YAG laser to induce focal tissue coagulation in an experimental brain model, which led to the development of Laser interstitial thermal therapy (LITT). After several clinical trials, in 1990, Sugiyama et al. reported the clinical application of LITT to treat 5 patients with brain tumours; the ablation procedure was performed under CT guidance.
Recent advances in MRI equipment, thermal imaging sequences, software, and laser delivery techniques and equipment enabled the prediction and accurate control of tissue temperatures which renewed the use of the Nd:YAG laser. This reintroduced the laser as a promising minimally invasive alternative for management of several intracranial pathologies.
Since 2010, ‘Laser Interstitial Thermal Therapy’ (LITT) (a minimally invasive and cytoreductive neurosurgical technique) has gained significant momentum. Several technological enhancements such as MRI thermometry and improved laser probe design have enabled feasibility and improved the safety of LITT procedures. Numerous reports have been published describing the treatment of lesions ranging from tumours to epileptogenic foci, but the indications for LITT continue to evolve. Numerous technical advancements in laser probe design and MRI have led to the development of the modern stereotactic LITT neurosurgical procedure.
MRI-guided stereotactic laser therapy
MRI-guided laser interstitial thermal therapy (LITT) is the selective ablation of a lesion or a structure using heat liberated from a laser. Stereotactic laser interstitial thermal therapy (LITT) is an emerging, minimally invasive, and cytoreductive neurosurgical technique for a variety of central nervous system (CNS) lesions ranging from tumours to epilepsy foci.
Although the technique was first described in 1983, enthusiasm for its use was limited in large part due to the inability to monitor tissue temperature during laser treatment and thus control the extent of ablation. Renewed interest in LITT has been sparked by advances in intraoperative magnetic resonance imaging (MRI) techniques that now enable real-time thermometry using T1-weighted 2D images obtained during the ablation procedure. Use of stereotactic LITT with MR thermometry has since been described for treatment of primary and metastatic brain tumours, radiation necrosis, and epilepsy foci. Investigation of new uses and the limitations of LITT is currently a highly active area of research.
The LITT system - comprises a laser system, workstation, and MRI.
The laser system comprises a laser light source, laser fibres, applicator, sheath and diffusion tip. The laser is generated by the source and then transmitted from the source to the tumour through optical fibres. During transmission of the laser, part of the energy can be lost and is absorbed by the transmitting fibres, which can eventually damage the fibres. Laser fibres can be optical or sapphire. Sapphire fibres are better, as they are heat resistant and transmit lasers with minimal energy absorption, making them more durable and efficient. The laser fibres are flexible and are carried to the centre of the tumour by an applicator. An optical diffusing tip modifies the laser beam to a spherical emission, hence achieving a homogenous and symmetric distribution of energy into a sphere of tissue.
MRI images obtained before and during the LITT procedure are sent from the MRI scanner to a linked workstation. The workstation provides real-time thermal maps for monitoring the procedure and estimates tissue necrosis.
Successful thermal ablation requires accurate targeting of the tumour and maintenance of a sufficient temperature level while excluding damage to the adjacent structures. MRI is used to identify the lesion and plan the trajectory for the laser probe. More importantly, it is used to visualize and quantify heat deposition within and surrounding the area of ablation, a process called magnetic resonance thermometry. MR thermometry provides a non-invasive, real-time temperature monitoring during the procedure and assesses target cell death. MRI thus is essential to the safety and efficacy of the procedure.
Sources:
https://www.ncbi.nlm.nih.gov/books/NBK545597/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7023945/
https://cancerimagingjournal.biomedcentral.com/articles/10.1186/s40644-019-0250-4
https://www.england.nhs.uk/2022/10/nhs-launches-laser-beam-brain-surgery-to-treat-epilepsy/
https://optics.org/news/9/8/35
https://www.monteris.com/healthcare-professionals/neuroblate-system/
https://www.bbc.co.uk/news/health-63345796
