11 Nov Robot-controlled laser for neurosurgery
The mere thought of brain surgery while awake makes many people shudder. Those affected are confronted with a frightening procedure: During a craniotomy – the opening of the skull – the bone material is removed using mechanical instruments, which literally shakes the patient. The immensely perceived noise and strong vibrations cause severe psychological stress. Awake operations are therefore usually only carried out if only a small opening in the skull is required for the procedure – for example for deep brain stimulation in the case of severe movement disorders.
In the case of tumor removal, the opportunity to interact with patients during the procedure would provide an important control option. Particularly when regions of the brain that are critical for speech and motor function are affected, surgical teams could test at any time whether the removal of tissue triggers functional deficits.
The situation is similar when implanting pacemakers for deep brain stimulation. In order to optimize their effect against severe shaking palsy caused by Parkinson’s disease, for example, the electrodes for brain stimulation must be positioned with high precision in the affected areas of the brain.
Contactless surgery
At the Fraunhofer Institute for Laser Technology ILT in Aachen, a robot-assisted and optically precisely monitored laser procedure has now been developed so that neurosurgical interventions can be performed much more frequently while the patient is awake than before. The bone tissue of the skull is ablated using short-pulse laser radiation. The switch from mechanical instruments to laser technology is intended to make craniotomies virtually silent, vibration-free and therefore gentle. The procedure is also intended to minimize the risk of meningeal injuries during craniotomy and improve the post-operative healing process by sensory control of the laser process.
Nanosecond pulses open the skull
With this goal in mind, the team in the ‘Stella’ project is developing an efficient, safe and largely automated laser cutting process. The core component is a CO2 laser with 120 nanosecond short laser pulses. The short pulses ensure that no carbonization effects occur as a result of heat input at the cutting edges. This is because thermal damage to bone tissue impedes the healing process. Due to the short exposure time, the ns pulses remove the hard tissue without significantly heating the surrounding tissue. According to the researchers, the new laser procedure leaves clean and thermally unaffected incision edges. However, efficiency is also important in everyday clinical practice. “We are currently achieving ablation rates of 1.6 cubic millimetres per second,” reports Dr. Achim Lenenbach, head of the Laser Medical Technology and Biophotonics department at Fraunhofer ILT. For clinical applications, an efficient cutting process requires 2.5 mm³/s, explains the researcher. In order to achieve this, a solid-state laser adapted to the bone cutting process is used.
Switch to solid-state laser
Previously, the CO2 laser beam was guided via an articulated mirror arm. However, in order to increase efficiency, reproducibility and flexibility, the Fraunhofer team has equipped the laser craniotome with a fiber-guided solid-state laser that emits 100 ns short laser pulses in the mid-infrared spectral range around 3 µm. “Light with this wavelength is absorbed very well by bone tissue, can be guided in a fiber and is therefore easier to combine with the robot than CO2 laser radiation,” says the expert. The combination with the robotic arm could also pave the way for further medical applications. Among other things, this is interesting for operations on the spine, which are risky due to the proximity to the spinal cord. The sensor-controlled short-pulse laser process could minimize the risk.
As the short-pulse laser source with a wavelength of 3 µm and a pulse duration of 100 ns that is in demand for the laser craniotome is not commercially available, the Lasers and Optical Systems department at the Fraunhofer ILT is developing it together with industrial partners. This brings the targeted ablation rates within reach without thermal damage to the surrounding hard tissue.
In the future, high-precision laser operations could make surgical procedures on the spine safer. Image: Ralf Baumgarten/Fraunhofer ILT
Sensory monitoring of the laser cutting process
To ensure that the laser beam only removes bone tissue and that the underlying structures such as the meninges or spinal cord remain intact, the laser cutting process is monitored by an OCT (Optical Coherence Tomography) measurement system. An OCT measuring beam superimposed on the cutting beam determines the local cutting depth and residual thickness of the bone. The process stops immediately before the bone is cut. The remaining fine bone lamella can then be removed from the composite with little effort and without risk of injury. The precisely controlled bone removal ensures effective protection of the tissue under the skull or in the spinal canal. “To achieve this, software continuously evaluates the process-synchronously recorded sensor signals and transmits the results to the real-time control of the laser surgical system,” explains Lenenbach. The inline OCT sensor system also shows the surgeons how the removal of the bone tissue is progressing. Once the almost silent cutting process is complete, they can lift off the loosened skull cap to begin the neurosurgical procedure. The bone flap is then reinserted and quickly grows back together with the surrounding tissue thanks to the gentle laser cutting process.
Circular laser cut on a bovine bone with superimposed point cloud from the measurement data of the OCT scan. Image: Fraunhofer ILT
Virtual system model
During the development process, a virtual system model of the laser craniotome enables the team to investigate any technical disruptions in the craniotomy process and to virtually test the influence of individual system components without modifying the hardware. This allowed them to test alternative scanner models, perform the process with an automated stereotactic system or with a collaborative robot, and optimize the virtualized system very efficiently. “Virtualization has become a very important tool for us to design and test laser-based surgical systems and gradually bring them closer to clinical practice,” concludes Lenenbach. Digital prototyping is an important tool for efficient development processes.
Source and image: https://www.ilt.fraunhofer.de
