The resolution and real-time navigation of intraoperative MRI (iMRI) has been leveraged in neurosurgery. We review frameless stereotactic biopsy platforms and focus on emerging technology integrating intraoperative MRI with frameless stereotaxy. Brain biopsy with iMRI allows for an accurate tissue sample with the ability to correct cannula trajectory during surgery, eliminating misdiagnosis secondary to faulty targeting. This technology allows for a percutaneous approach avoiding large incisions, obviates the need for frozen tissue evaluation, has the potential to reduce unnecessary specimen harvesting and operating room time, and optimizes safety of targeting deep brain lesions.
Key points
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Resolution and real-time navigation of intraoperative MRI (iMRI) has been leveraged by different fields in neurosurgery.
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Frameless stereotactic systems that rely on preoperative imaging have become the most common brain biopsy systems being used with relatively good success but have serious drawbacks.
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Brain biopsy with iMRI is an accurate way to sample tissue and correct the trajectory of biopsy cannula during surgery, eliminating misdiagnosis secondary to faulty targeting.
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The ClearPoint Smartframe biopsy platform uses a minimally invasive approach for frameless stereotactic brain biopsy using a percutaneous neuronavigation platform compatible with iMRI.
Introduction
Advancements in medical imaging with the introduction of computed tomography scans and MRI have dramatically influenced diagnostic capabilities in human health. The usefulness of this technology has led to earlier detection and management of patients with neurologic disease. Consequently, it has resulted in the early detection of ambiguous lesions in eloquent brain areas, surrounded by critical vascular structures raising radiographic suspicion for underlying disease. The dilemma to treat or not treat suspicious lesions in asymptotic patients relies often on accurate tissue acquisition and diagnosis. As a result, brain biopsy techniques continue to evolve in providing the safest and most precise methods for obtaining a diagnosis.
The development of image-guided brain biopsy methods began with techniques such as open free-hand methods using data from computed tomography scans for surgical planning to stereotactic frame-based approaches. More recently, frameless stereotactic systems incorporating preoperative imaging have become the most common biopsy systems in use today. This evolution underscores the dynamic development of technology and methods that continue to push stereotactic brain surgery in becoming safer and more effective. Careful examination between frame-based and frameless stereotaxy has compelled most neurosurgeons to conveniently adopt a frameless brain biopsy technique based on comparable diagnostic yield between both approaches, decreased operating room time and reduced the risk of postoperative infection secondary to frame fixture points. A metaanalysis 15 years ago of 7471 frameless biopsies demonstrated a diagnostic yield of 91%, morbidity of 3.5%, and mortality of 0.7%. More contemporary data suggest that modest gains have been made, pushing the diagnostic yield to 93.8% according to a recent metaanalysis, but with a comparable safety profile.
Despite such advances and widespread popularity, both frame-based and frameless stereotactic biopsy rely on preoperative images with intraoperative anatomic or fiducial registration to reach target tissue based on calculated measurements in 3-dimensional space. The primary limitation of this approach is the inability to account for dynamic changes, such as shift of intracranial structures, potentially interfering with the diagnostic accuracy in cases with small lesions. A critical review of the literature has raised concerns that, although reported diagnostic yields remain high, the diagnostic accuracy of stereotaxy with preoperative neuronavigation may be lower than previously reported. In addition, miscalculations or faulty technical misalignment of hardware during surgery can potentially contribute to fatal intraoperative events. These potential pitfalls leave very little room for error when surrounding neurovascular structures are at stake.
The integration of intraoperative MRI (iMRI) with stereotactic frameless biopsy has been the logical next step in the evolutionary refinement of brain biopsy techniques. The resolution and real-time radiographic feedback of iMRI is remarkably accurate in phantom experiments and in a recent case series demonstrated a diagnostic yield in 78 patients of 97.4%. This technology eliminates the possibility of misdiagnosis secondary to faulty targeting by confirmation of accurate positioning of the biopsy cannula. Although a few groups have reported on this technique, low-resolution intraoperative scanners and the lack of a standardized protocol continue to delay its potential for broad clinical application. Here we review various biopsy platforms that rely on preoperative image guidance and focus particularly on the integration of intraoperative imaging and newly developed MRI-compatible biopsy platform with a case illustration. This platform uses a minimally invasive approach for frameless stereotactic brain biopsy using a percutaneous neuronavigation platform, ClearPoint System neuronavigation platform (MRI Interventions, Memphis, TN), compatible with standard 1.5 and 3.0 T scanners, resulting in safe and successful diagnostic yield in 5 patients. The advantages and limitations of this approach are discussed. Although current frameless, stereotactic techniques are capable of providing satisfactory accuracy and safety in a majority of cases, there are specific diagnostic scenarios for which the MRI-guided biopsy technique described here is more ideally suited.
Introduction
Advancements in medical imaging with the introduction of computed tomography scans and MRI have dramatically influenced diagnostic capabilities in human health. The usefulness of this technology has led to earlier detection and management of patients with neurologic disease. Consequently, it has resulted in the early detection of ambiguous lesions in eloquent brain areas, surrounded by critical vascular structures raising radiographic suspicion for underlying disease. The dilemma to treat or not treat suspicious lesions in asymptotic patients relies often on accurate tissue acquisition and diagnosis. As a result, brain biopsy techniques continue to evolve in providing the safest and most precise methods for obtaining a diagnosis.
The development of image-guided brain biopsy methods began with techniques such as open free-hand methods using data from computed tomography scans for surgical planning to stereotactic frame-based approaches. More recently, frameless stereotactic systems incorporating preoperative imaging have become the most common biopsy systems in use today. This evolution underscores the dynamic development of technology and methods that continue to push stereotactic brain surgery in becoming safer and more effective. Careful examination between frame-based and frameless stereotaxy has compelled most neurosurgeons to conveniently adopt a frameless brain biopsy technique based on comparable diagnostic yield between both approaches, decreased operating room time and reduced the risk of postoperative infection secondary to frame fixture points. A metaanalysis 15 years ago of 7471 frameless biopsies demonstrated a diagnostic yield of 91%, morbidity of 3.5%, and mortality of 0.7%. More contemporary data suggest that modest gains have been made, pushing the diagnostic yield to 93.8% according to a recent metaanalysis, but with a comparable safety profile.
Despite such advances and widespread popularity, both frame-based and frameless stereotactic biopsy rely on preoperative images with intraoperative anatomic or fiducial registration to reach target tissue based on calculated measurements in 3-dimensional space. The primary limitation of this approach is the inability to account for dynamic changes, such as shift of intracranial structures, potentially interfering with the diagnostic accuracy in cases with small lesions. A critical review of the literature has raised concerns that, although reported diagnostic yields remain high, the diagnostic accuracy of stereotaxy with preoperative neuronavigation may be lower than previously reported. In addition, miscalculations or faulty technical misalignment of hardware during surgery can potentially contribute to fatal intraoperative events. These potential pitfalls leave very little room for error when surrounding neurovascular structures are at stake.
The integration of intraoperative MRI (iMRI) with stereotactic frameless biopsy has been the logical next step in the evolutionary refinement of brain biopsy techniques. The resolution and real-time radiographic feedback of iMRI is remarkably accurate in phantom experiments and in a recent case series demonstrated a diagnostic yield in 78 patients of 97.4%. This technology eliminates the possibility of misdiagnosis secondary to faulty targeting by confirmation of accurate positioning of the biopsy cannula. Although a few groups have reported on this technique, low-resolution intraoperative scanners and the lack of a standardized protocol continue to delay its potential for broad clinical application. Here we review various biopsy platforms that rely on preoperative image guidance and focus particularly on the integration of intraoperative imaging and newly developed MRI-compatible biopsy platform with a case illustration. This platform uses a minimally invasive approach for frameless stereotactic brain biopsy using a percutaneous neuronavigation platform, ClearPoint System neuronavigation platform (MRI Interventions, Memphis, TN), compatible with standard 1.5 and 3.0 T scanners, resulting in safe and successful diagnostic yield in 5 patients. The advantages and limitations of this approach are discussed. Although current frameless, stereotactic techniques are capable of providing satisfactory accuracy and safety in a majority of cases, there are specific diagnostic scenarios for which the MRI-guided biopsy technique described here is more ideally suited.
Frameless stereotactic biopsy platforms with preoperative image guidance
There are several frameless stereotactic biopsy platforms that are currently available for neurosurgeons. The 2 most commonly used and that recently have been compared head to head are the Medtronic Stealth Treon Vertek (Medtronic Inc., Minneapolis, MN) and BrainLAB Varioguide (BrainLAB, Feldkirchen, Germany). Technical notes and large case series published with these stereotactic biopsy platforms have been described in the literature with relatively good reproducibility and expeditious integration into clinical practice. Both systems rely on a software computer module that allows for preoperative image registration and guidance for preoperative trajectory planning.
Use of the Medtronic Stealth Treon Vertek was described initially in a large case series of 164 consecutive intracranial biopsies. This system relies on a skull-mounted trajectory guide that is affixed over the planned burr hole for the biopsy trajectory. The platform also includes a flexible arm with a navigated aiming device that provides limited degrees of rotation in positioning and targeting with the biopsy needle. The surgeon’s incision and burr hole are planned based on preoperative image guidance and the length of the incision needs to account for the burr hole and the size of the skull mount, which is about to 1 to 2 cm larger than a 14-mm burr hole. The system relies on a biopsy needle with an outer diameter of 2.2 mm and a cutting window of 7 mm that is then inserted through the needle guide through preoperative planned trajectory toward the lesion.
The BrainLAB Varioguide biopsy platform was introduced in 2009 after the Medtronic system, and was used in an initial cohort of 27 patients in a preliminary technical report and then in a larger case series of 102 patients. Unlike the Medtronic biopsy platform, the BrainLAB Varioguide does not rely on a skull-mounted trajectory guide, but rather uses a stereotactic arm with a biopsy needle adaptor that is capable of holding 3 different biopsy needle sizes ( Fig. 1 ). The system is precalibrated for a 1.8-mm biopsy needle with a 1.8-mm diameter and 10-mm cutting window. The surgeon plans an incision for the size of a 14-mm burr hole because no skull mount is required. The stereotactic arm is guided into the surgical field based on a planned preoperative trajectory. Three joints are manipulated using a software wizard on the computer module to aim the needle toward the lesion site. The biopsy needle is then inserted into the adaptor and advanced based on preoperative measurements.
When compared with frame-based systems, these frameless platforms demonstrated a high diagnostic yield. This yield is better than other reported frameless systems and comparable with frame-based systems. When empirically compared with each other, both systems have shown equal efficacy, safety, and comparable operative times. In a retrospective case series, an experience with 247 consecutive brain biopsy procedures performed using both the Medtronic Stealth Treon Vertek and the BrainLAB Varioguide frameless systems, the authors demonstrated an overall diagnostic yield of 94.6% with 11 biopsies being nondiagnostic, 5 (4.9%) with the Medtronic, and 6 (5.9%) with the BrainLAB system. A small difference in the operating time (108 minutes vs 120 minutes) was found between the 2 biopsy methods, with BrainLAB being minimally longer. The symptomatic hemorrhage rate was low (1.2%) and the authors concluded that both biopsy systems were equally safe and reliable.
Despite such equivalency, at our institution we readily use both biopsy systems with equal diagnostic yield and safety. However, we do favor the BrainLAB Varioguide frameless system for several reasons. The lack of a skull mount confines the incision length to that of the size of the bur hole made. In addition, the skull mount designed by Medtronic can be cumbersome to affix to the skull and must be placed carefully to account for the optimum trajectory toward the aperture of the burr hole. Small errors in alignment may result in restriction of the degrees of freedom in the overlying needle guide and this may lead to deflection of the biopsy needle by the bone edge if not aligned properly, ultimately resulting in repositioning of the skull mount. Because of all these careful nuances in affixing the skull mount, we find that our operative time with the BrainLAB Varioguide stereotactic arm is significantly less than that with the Medtronic biopsy platform despite what is reported currently. From a hospital resource perspective, the BrainLAB Varioguide does require more startup capital than the Medtronic platform. However, this is an investment that should be factored by each institution and its resource constraints.
Frameless stereotactic biopsy platforms with intraoperative image guidance
Despite their ubiquitous use, both frame-based and frameless stereotactic biopsies rely on preoperative images with intraoperative anatomic or fiducial registration to reach deep brain targets based on precalculated measurements in 3-dimensional space. The primary limitations of these approaches are the inability to account for shift of intracranial structures (that reduce accuracy of biopsy at target), the inability to adapt to vital structures along the needle trajectory in real time, and the lack of intraoperative imaging confirmation of target biopsy accuracy. Moreover, miscalculations or technical misalignment of hardware during biopsy can result in intraoperative morbidity. Although diagnostic yield maybe high, recent data indicate that the diagnostic accuracy of stereotaxy, using conventional preoperative neuronavigation may be lower than previously reported (accuracy range, 51%–91%).
To overcome these limitations, an MRI-compatible brain biopsy platform has been developed that now permits the integration of iMRI with frameless stereotaxy. The resolution and real-time imaging feedback of this iMRI technology has been demonstrated in phantom experiments, nonhuman primate studies, drug infusion cannula placement, and placement of deep brain stimulator leads. We recently applied its utility and demonstrated its safety and accuracy in stereotactic brain biopsy. Application of this platform to stereotactic brain biopsy could eliminate misdiagnosis secondary to faulty targeting, provide real-time adaptability for needle trajectory, confirm accurate biopsy cannula position, and provide immediate imaging of complications.
The ClearPoint System Neuronavigation Platform
The ClearPoint System (MRI Interventions Inc., Irvine, CA) includes an integrated head fixation frame, a disposable single use ClearPoint SmartFrame Device, a dedicated computer workstation with ClearPoint Software for surgical planning, and an MRI-compatible in-room computer monitor ( Fig. 2 ). In most cases, 2 MRI-compatible cannulas were used during the procedure, a ceramic guide stylet (MRI Interventions Inc.) and a biopsy needle (Ad-Tech, Racine, WI) were used for targeting and tissue collection.
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