Preincision and Anesthesia

All anesthesia medications, lines, and monitors must be MRI safe, and a standard patient MRI safety screening form should be completed. In cases in which ablation is the surgical goal without the need for intraoperative electrophysiology, the patient may undergo general anesthesia in which total intravenous anesthesia is not necessary. Chemical paralysis is used to prevent unwanted movement that can result in errant ablation. An arterial line is not necessary. Bladder decompression is achieved using an indwelling catheter. A surgical team pause is performed, and perioperative antibiotics administered.

Delivering the Laser

MRI-guided LITT is dependent on accurate stereotactic targeting aided by identification and minimization of possible sources of error during all stages of the procedure, including acquisition of image space, defining patient space, coregistration of image and patient space, surgical planning, and performing the surgical implantation of the laser. MRI-guided LITT procedural workflows are dependent on institutional resources and practice patterns, and use frame-based, frameless, and robotic systems to transfer stereotaxic location from the planning system to a bone-anchored bolt that holds the laser. Ablations may be carried out in an operating room (OR) with intraoperative MRI, in a separate MRI suite after stereotactic placement of laser probe in a traditional OR, or entirely in an MRI suite using MR-compatible delivery systems. Transport of the implanted patient to the MRI suite carries the risk of laser dislodgement, but this can be mitigated with the assignment of a surgical chaperone to singularly escort the patient on transit.

Frame-Based Stereotaxy

In general, arc-centered stereotactic frames, including Leksell and CRW, are commonly used and are the gold standard for performing stereotactic/functional neurosurgery. Frame-based stereotaxy requires a working frame and computer software package that allows planning. Limitations to using frame-based stereotaxy include patients of very young age, previous surgical defects, and head-frame volume discordance. For LITT-E, the frame can be placed in the preoperative area or following induction with general anesthesia. Stereotactic imaging with frame in place can be either MRI or computed tomography (CT) with coregistration to preoperative imaging used for surgical planning.

Frameless Stereotaxy

A plethora of frameless stereotactic systems maintain high accuracy while avoiding the need to frame the cranium; examples include use of a AxiEm (Medtronic, Fridley, MN), multiaxial arm stereotaxy (Vertek, Varioguide) (BrainLab, Munich, Germany), bone fixed targeting (STarFix microTargeting system [FHC, Inc. Bowdoin, ME], Clearpoint MRI guided [MRI Interventions, Inc. Irvine, CA], Nexframe [Medtronic, United States]), or robotized stereotactic assistant (ROSA). Frameless systems facilitate the fusion of patient space with MRI space by recapitulating a series of fiducials, or trusted points. We provide a brief summary regarding several of these systems in the following paragraphs.

Medtronic AxiEm is an electromagnetic tracking system that uses a patient tracker that sticks to the face or head in an immobile area. A magnetic field approximately 50 cm wide from the patient tracker is detected by the emitter that allows you to track the tip of adapted instruments. This system obviates the need for cranial fixation, posing obvious benefits in infants, children, or other individuals with contraindications to cranial fixation. However, the head remains unfixed, the system is incapacitated by regional metal, and the system is based on tracking the tip of adapted instruments.

The Medtronic Stealth Treon Vertek and BrainLab Varioguide are multiaxial arms that communicate by optical imaging. The stereotaxy is conferred to the arm based on predefined trajectories in the software package, and that image space is coregistered to the patient space using surface tracking or fiducials. The Varioguide and Vertek arms similarly require cranial fixation using the Mayfield and have multiple degrees of freedom. Each has been reported to have competitive accuracy with frame-based stereotaxy. A retrospective analysis directly comparing the 2 systems over a 5-year period, including 247 consecutive biopsies, resulted in insignificant differences in diagnostic yield of the biopsy, complication rate, or biopsy-related mortality.

STarFix microTargeting system, Clearpoint, and Nexframe are examples of skull-based targeting systems. The STarFix microTargeting system involves the outpatient placement of 3 to 4 fiducials, a 3-dimensional (3D) CT followed by OR for fitting and procedure. A review using this system for 5 patients and biopsy followed by MRI guided laser interstitial thermal therapy (MRIgLITT) resulted in accurate biopsies and less than an hour OR times.

ROSA is another frameless stereotactic system applicable to LITT-E. Robotized assistants are published with submillimeter accuracy and ROSA can be more efficient than frame-based targeting when multiple trajectories are needed; hence, when multiple lasers or simultaneous stereoelectroencephalography (sEEG) electrodes are planned, there may be benefit to using ROSA. In particular, laser ablations for multifocal epilepsy based on sEEG that require transition between electrode and laser placement are facilitated by the ROSA system.

It is clear that many frameless systems exist for stereotaxy, each subtly nuanced. Overall, there appears to be equipoise in accuracy and risk profiles among frameless systems making the local resources the most important factor determining system selection. Although a prospective randomized study between frame-based and frameless (Varioguide) stereotaxy reports similar diagnostic specimen rates, trajectory accuracy and complication rate in stereotactic biopsies, frameless stereotaxy for laser ablation specifically is reported to be associated with higher adverse events, including hemorrhage and neurologic deficits.

Trajectory Planning

Stereotactic trajectories have inherent risks independent of what is passed through the brain. The skull, dura mater, subarachnoid space, parenchyma, sulci, and ventricles all have inherent risk associated with transgression. Imaging considerations when planning a trajectory should take into account the anatomic variance in skull thickness, subarachnoid spaces, blood vessels, ventricular volumes, and lesion target volumes. Recursive planning before surgery can maximally benefit the patient. Ependymal surfaces should be avoided when possible in addition to minimizing the number of pial surfaces to be transgressed. Avascular planning of trajectory widened to 3 mm on infused T1 MRI has resulted in a 1.6% hemorrhage rate in this patient population. Neck flexion, pronation, or any other position that restricts venous outflow should be avoided because it may cause intracranial veins to become more conspicuous and increase the risk of rupture during stereotaxy.

Planning for Heat Sinks

Although there is no quantification technique that exists for the identification of heat sinks a priori near the target, experience has led to strategies to mitigate the effect of heat sink on lesion reduction. Common heat sinks are cisterns, such as the ambient and quadrigeminal cistern in mesial temporal lobe epilepsy (mTLE) or the suprasellar cistern in hypothalamic hamartoma (HH), in which the combination of cerebrospinal fluid and arterial pulsations greatly dissipate applied thermal energy. Ventricles are also heat sinks, especially those that may have been scarred and narrowed by previous surgery, such as the foramen of Monro. Sulci can also house heat sinks, especially those that possess a moderately large artery to increase pulsations. Last, there are considerable iatrogenic heat sinks from previous surgery and partial resections that can limit ablation volume. The most impactful iatrogenic heat sinks are encountered in partial central resection of hypothalamic hamartomas that convert a typically round, isothermic target into 2 thermally isolated lesions requiring 2 or more ablation zones. Occasionally indirect evidence of heat sinks can be encountered on planning imaging as noticeable flow voids on T2 imaging. Generally, we adopt our targeting to compensate for a heat sink by placing the laser as close to the heat sink as possible for the high-magnitude heat sinks (cisterns) and place the laser one-third of the proposed ablation diameter closer to the moderate heat sinks on the oblique planar view. These adjustments should allow the heat to spread in the opposite direction of the heat sink as the power of the laser is increased.

Implanting the Laser

In general, drilling a burr hole for placement of a stereotactic bolt requires attention to technique. Oblique burr hole drilling allows lateral movement of the drill bit and degrades the stereotactic accuracy; whenever possible, a more orthogonal trajectory should be used to minimize this risk. If an oblique trajectory is necessary, serial drilling using a smaller drill bit to create a pilot hole will help with unintentional movement at the bony surface. Rigid guidance of the skull bolt into the bone is optimal, or if this is not possible, screwing of the plastic skull bolt can be done by hand over the stylet guided to target by rigid stereotaxy. Dural penetration is also simplified by orthogonal puncture, with obliquity complicating the haptic feedback and maximizing the length of the dural opening. We apply monopolar electrocautery to an obturator during penetration to reduce the associated hemorrhage with dural penetration, which is anecdotally effective.

Each stereotactic pass involves creating a corridor first with a straight metal obturator to target depth. The obturator must be straight, which can be assessed by the extent of wobble by rolling it on the back table. The obturator is placed to target depth slowly and steadily through the axial plane, focusing on the haptic feedback as the tip passes through structures of variable densities. On some systems, radicles can be used to fluoroscopically confirm accurate targeting, and in MR-compatible systems, ceramic stylets are used to establish the trajectory. Once the skull bolt is secured and the corridor of the trajectory established with a probe, the cannula, followed by the laser fiber (Visualase; Medtronic) or the laser probe (Neuroblate; Monteris, Inc, Plymouth, MN) is placed down to target and secured.

Intraoperative Imaging and Thermometry

Once the laser is delivered to the target, an MRI is obtained to confirm targeting. The imaging prescriptions that support the MR thermography are published elsewhere. Briefly, the imaging protocol begins with the acquisition of 3D T1 MRI from which the treatment planes can be selected. Subtle differences in work flow exist related to the laser ablation system and software that is chosen, described as follows.

Medtronic Visualase

In cases in which the target is in noneloquent cortex, in which completeness of the lesion ablation is the primary objective, a single oblique treatment plane is selected that encompasses the entire trajectory. In lesions abutting eloquent tissue, 2 treatment planes are selected: one obliquely inclusive of the trajectory and one selected to optimally visualize adjacent structures that are to be preserved. Adding treatment planes has practical consequences on the refresh time of the near real-time MR thermography, with the refresh time of a single plane being 3.5 seconds, and 7.0 seconds for 2 planes. Once the planes are established, background images are obtained, the sequence of which is chosen to optimize contrast between the lesion and the surrounding brain, they are fused to a continuous MR thermogram. A test dose of heat, typically 15% of a 15W laser setting on a 10-mm diffuser, or 8% of a 10W laser setting on a 3-mm diffuser, is applied and the depth of the laser fiber within the cannula is adjusted to optimize the application of heat to the center of the lesion.

Use of Low-Limit Markers

When ablating a target abutting vital structures, low-limit markers, designed to automatically turn the laser off when the pixel under that marker reaches 50°C (the default temperature, although this can be modulated), are placed approximately 1 to 2 mm between the structure to be preserved and the heat source. The current software offers 3 of these markers to distribute along the structures to be preserved, but in complex lesion targeting, such as hypothalamic hamartoma, more low-limit markers are needed. In those scenarios, high-limit markers, used to monitor the temperature near the heat source to avoid cannula destruction from overheating, can be borrowed to serve as monitor markers, but they need to be visually monitored for manual shut-off. Also, we have lowered the temperature of the low limit from 50 to 48 for an additional margin of safety. One high-limit is reserved for monitoring the temperature at the heat source.

Once the low-limit markers are in place, the laser is turned on to allow the heat within the lesion to increase until the irreversible damage map, as calculated and projected on the basis of the Arrhenius equation, covers the lesion. This is optimally performed by adjusting the laser wattage to keep the high-limit marker from 85 to 89 to maximize the heat delivery. After the irreversible damage map covers the lesion, the laser is discontinued.

Monteris Neuroblate

The Monteris Neuroblate software automatically monitors the imaging in 3 planes and 5 views: sagittal oblique, coronal oblique, and 3 axials along the ablation trajectory. The thermogram is automatically fused to the background imaging, the sequence of which is selected to optimize lesion contrast. Eight temperature reference points are placed within a focused thermal field to minimize the artifactual drift of the MR thermogram data, and after several iterations, the thermogram is stabilized and the background thermogram color turns green. The laser is then engaged, at only one intensity, and lines of damage, apoptotic, and coagulation damage expand over time. Once the ablation lines encircle the target, the laser is turned off. One can chose diffuse or side-fire laser probes, the latter of which provides directional laser energy offering a theoretic advantage of selectivity in very short ablations.

MR Thermogram Limitations

The MR thermogram can be prone to artifact and signal drift. Artifacts that are frequently encountered are pulsation artifact and artifact from fixation devices. Pulsation artifact is mostly unavoidable, but occasionally can be minimized by scanning in a plane that does not include a pulsatile artery. Fixation devices also can obscure the thermogram, the most impactful of which is the signal void created by titanium skull bolts. Potential solutions to this problem include the use of a less rigidly fixated plastic skull bolt to visualize surface ablations, use of derrick-style frame (Clearpoint, Nexframe, STarFix) that keeps the top of the trajectory free of metal artifact, or an ablation without thermography dosed by the ablation parameters at the deeper, visualized portion of the trajectory. To address thermography signal drift, the Neuroblate (Monteris, Inc) system has stabilization points within a limited field to minimize the effect of drift. In the Visualase system, restarting the thermogram every 8 to 10 minutes may mitigate this artifact.

Confirmatory Imaging

Confirmatory imaging (to show adequacy of the thermal injury) may be obtained in diffusion-weighted imaging (or higher-resolution diffusion tensor imaging) and infused T1 sequences. The laser fiber and cannula are removed, the stab wound is closed, and an additional sequence of FFE (or GRE) is obtained to evaluate for hemorrhage.

Postoperative Care

Postoperative care in stereotactic laser ablation (SLA) is typically quite minimal due to the small size of the incision typically needed to place the laser cannula. Most of the postoperative care concerns are related to the edema created by the thermal ablation. If the thermal ablation is performed adjacent to vital structures, such as in hypothalamic hamartoma or is targets abutting the motor strip, high-dose steroids typically minimize the symptoms related to the severe and rapid onset edema commonly encountered in thermal ablation. Activity restrictions typical of postcraniotomy care are not required.

Special Considerations in Cranial Immobilization

Patients with open cranial vaults (eg, infants), thin bone (eg, hydrocephalus), or open cranial defects (eg, postcraniectomy) are at high risk from cranial fixation. Calvarial molding can occur in patients with open cranial vaults, like infants or patients with cranioplasties, and the application of force vectors from fixation pins cause a time-dependent, pressure-dependent change in head shape. Calvarial molding over time creates an incongruence of imaging space to patient space coregistration and results in accumulation of stereotactic error. Minimizing time spent in cranial fixation can be accomplished, for example, by proceeding past nonsterile registration directly to sterile registration in systems such as the ROSA.

One strategy to limit pin-associated morbidity is to keep the force per pin very low. Low cranial fixation force limits the resistance to movement and increases the chance of the patient slipping in pins. Pin slippage can cause scalp injury, injury to soft tissue structures like the eyes or ears, secondary closed head injury, or cervical spine injury. If using this strategy is necessary, excessive padding around the patient’s head is ideal.

In adult and pediatric patients the bone thickness varies, and thin regions like the squamosal temporal bone are more dangerous to dock skull pins even in adults, with skull fracture and epidural hematoma as known consequences. No clinical decision rule regarding the minimum thickness of bone for safe cranial fixation exists. Age is used as a surrogate indicator for fixation; the mindful neurosurgeon may choose specifically between 3-pin Mayfield (with pediatric or adult pins), a Sugita 6-pin frame (with pediatric or adult pins), or a stereotactic 4-pin frame such as the CRW or Leksell.

One strategy for cranial fixation is based on the premise that very thin bone benefits from spreading the force of fixation across many points; hence, for thin bone, the 6-pin Sugita frame is a better option than the 3-pin Mayfield. In contradistinction, evaluating pin pressure can be an adaptive technique, and the Sugita lacks any pressure indicators. This ultimately makes the eventual force per pin evaluable only by surgeon judgment. The Mayfield head-holder fixation force can be titrated easily by the side indicator (it is important to know when the Mayfield was last calibrated if using the side indicator). The CRW frame also requires physician judgment for fixation force on each post-associated pin, but the frame lends itself to stereotaxy primarily. Finally, many procedures are amenable to serial tightening of the cranial pins throughout a case; however, this must be avoided in stereotactic cases because adjusting the pins after registration will cause a frame shift that introduces stereotactic error.

Finally, cranial fixation is always a balance of minimizing pin force to reduce the risk of skull injury while simultaneously maximizing the amount of fixation. Throughout a case the pins loosen and the skull is at risk of moving. Careful avoidance of force application to the skull during the surgery, throughout the incision, drilling, or anchor bolt fixation phases, should always be used when less than 60 lb of force is used for cranial fixation.

Troubleshooting Transcranial Bolts

In the described operative paradigm, the stereotactic trajectory defined by the frame or robot is transferred to the transcranial bolt, with only the depth to target needed to reach the target point. Therefore, the integrity of the bolt-skull interface is critical. Sometimes, bone-anchored bolts are not possible to place due to craniectomized bone, thin bone, or dislodged skull bolts used for preablation depth electrode studies.