Effects of coregistration of MR to CT images on MR stereotactic accuracy

A multimodal imaging-guided software for access to primate brains

Background

Imaging-guided access to the brain has become a routine procedure for various research and clinical applications, including drug administration, neurophysiological recording, and sampling tissue. Therefore, open-source software is required to handle such datasets in these specific applications.

New methods

Here, we proposed an open-source tool utilizing different imaging modalities for automating the steps to access the brain. This tool provides means for easily calculating the coordination of the area of interest concerning a specific point of entry. The source and documentation are available at this link.

Results

We have used this software for three different applications: electrophysiological recording, drug infusion in the nonhuman primate brain, and guided biopsy procedure in the human brain. We performed a neural recording of two monkeys' prefrontal cortex and inferior temporal cortex using this software in submillimeter resolution. We also applied our procedure for infusion in the putamen and caudate nuclei in both hemispheres of another group of rhesus monkeys with histological proof in one animal. More so, we validated this software in the human subjects that underwent biopsy surgery with the commercial software used in human biopsy surgery.

Comparison with existing methods

Our software uses different imaging modalities by co-registering them. This will provide structural details of the skull and brain tissue. We can calculate each brain region's coordination at the point of entry by re-slicing the images. Atlas-based image segmentation were implemented in our software. Three mentioned applications of our software in neuroscience will be further discussed in this paper.

Conclusion

In our procedure, working with different imaging modalities provides a precise estimation of the specific region in the brain related to the location of implants or stereotaxic frames. There is no limitation to using metal implants in this procedure.

Fast Automated Stereo-EEG Electrode Contact Identification and Labeling Ensemble

INTRODUCTION

Stereotactic electroencephalography (SEEG) has emerged as the preferred modality for intracranial monitoring in drug-resistant epilepsy (DRE) patients being evaluated for neurosurgery. After implantation of SEEG electrodes, it is important to determine the neuroanatomic locations of electrode contacts (ECs), to localize ictal onset and propagation, and integrate functional information to facilitate surgical decisions. Although there are tools for coregistration of preoperative MRI and postoperative CT scans, identification, sorting, and labeling of SEEG ECs is often performed manually, which is resource intensive. We report development and validation of a software named Fast Automated SEEG Electrode Contact Identification and Labeling Ensemble (FASCILE).

METHODS

FASCILE is written in Python 3.8.3 and employs a novel automated method for identifying ECs, assigning them to respected SEEG electrodes, and labeling. We compared FASCILE with our clinical process of identifying, sorting, and labeling ECs, by computing localization error in anteroposterior, superoinferior, and lateral dimensions. We also measured mean Euclidean distances between ECs identified by FASCILE and the clinical method. We compared time taken for EC identification, sorting, and labeling for the software developer using FASCILE, a first-time clinical user using FASCILE, and the conventional clinical process.

RESULTS

Validation in 35 consecutive DRE patients showed a mean overall localization error of 0.73 ± 0.15 mm. FASCILE required 10.7 ± 5.5 min/patient for identifying, sorting, and labeling ECs by a first-time clinical user, compared to 3.3 ± 0.7 h/patient required for the conventional clinical process.

CONCLUSION

Given the accuracy, speed, and ease of use, we expect FASCILE to be used frequently for SEEG-driven epilepsy surgery. It is freely available for noncommercial use. FASCILE is specifically designed to expedite localization of ECs, assigning them to respective SEEG electrodes (sorting), and labeling them and not for coregistration of CT and MRI data as there are commercial software available for this purpose.

CT and MRI Image Fusion Error: An Analysis of Co-Registration Error Using Commercially Available Deep Brain Stimulation Surgical Planning Software

INTRODUCTION

During deep brain stimulation (DBS) surgery, computed tomography (CT) and magnetic resonance imaging (MRI) scans need to be co-registered or fused. Image fusion is associated with the error that can distort the location of anatomical structures. Co-registration in DBS surgery is usually performed automatically by proprietary software; the amount of error during this process is not well understood. Here, our goal is to quantify the error during automated image co-registration with FrameLink™, a commonly used software for DBS planning and clinical research.

METHODS

This is a single-center retrospective study at a quaternary care referral center, comparing CT and MR imaging co-registration for a consecutive series of patients over a 12-month period. We collected CT images and MRI scans for 22 patients with Parkinson's disease requiring placement of DBS. Anatomical landmarks were located on CT images and MRI scans using a novel image analysis algorithm that included a method for capturing the potential error inherent in the image standardization step of the analysis. The distance between the anatomical landmarks was measured, and the error was found by averaging the distances across all patients.

RESULTS

The average error during co-registration was 1.25 mm. This error was significantly larger than the error resulting from image standardization (0.19 mm) and was worse in the anterior-posterior direction.

CONCLUSIONS

The image fusion errors found in this analysis were nontrivial. Although the estimated error may be inflated, it is sig-nificant enough that users must be aware of this potential inaccuracy, and developers of proprietary software should provide details about the magnitude and direction of co-registration errors.

Gamma Knife® icon CBCT offers improved localization workflow for frame-based treatment

Object

The purpose of this study was to compare two methods of stereotactic localization in Gamma Knife treatment planning: cone beam computed tomography (CBCT) or fiducial. While the fiducial method is the traditional method of localization, CBCT is now available for use with the Gamma Knife Icon. This study seeks to determine whether a difference exists between the two methods and then whether one is better than the other regarding accuracy and workflow optimization.

Methods

Cone beam computed tomography was used to define stereotactic space around the Elekta Film Pinprick phantom and then treated with film in place. The same phantom was offset known amounts from center and then imaged with CBCT and registered with the reference CBCT image to determine if measured offsets matched those known. Ten frameless and 10 frame‐based magnetic resonance imaging (MRI) to CBCT patient fusions were retrospectively evaluated using the TG‐132 TRE method. The stereotactic coordinates defined by CBCT and traditional fiducials were compared on the Elekta 8 cm Ball phantom, an anthropomorphic phantom, and actual patient data. Offsets were introduced to the anthropomorphic phantom in the stereotactic frame and CBCT's ability to detect those offsets was determined.

Results

Cone beam computed tomography defines stereotactic space well within the established limits of the mechanical alignment system. The CBCT to CBCT registration can detect offsets accurately to within 0.1 mm and 0.5°. In all cases, some disagreement existed between fiducial localization and that of CBCT which in some cases was small, but also was as high as 0.43 mm in the phantom domain and as much as 1.54 mm in actual patients.

Conclusion

Cone beam computed tomography demonstrates consistent accuracy in defining stereotactic space. Since both localization methods do not agree with each other consistently, the more reliable method must be identified. Cone beam computed tomography can accurately determine offsets occurring within stereotactic space that would be nondiscernible utilizing the fiducial method and seems to be more reliable. Using CBCT localization offers the opportunity to streamline workflow both from a patient and clinic perspective and also shows patient position immediately prior to treatment.

CT versus MR Imaging in Estimating Cochlear Radiation Dose during Gamma Knife Surgery for Vestibular Schwannomas

BACKGROUND AND PURPOSE

Leksell stereotactic radiosurgery is an effective option for patients with vestibular schwannomas. Some centers use a combination of stereotactic CT fused with stereotactic MR imaging to achieve an optimal target definition as well as minimize the radiation dose delivered to adjacent structures that correlate with hearing outcomes. The present prospective study was designed to determine whether there is cochlear dose variability between MR imaging and CT.

MATERIALS AND METHODS

Fifty consecutive patients underwent stereotactic radiosurgery for vestibular schwannomas. Dose-planning was performed using high-definition fused stereotactic MR imaging and stereotactic CT images. The 3D cochlear volume was determined by delineating the cochlea on both CT and T2-weighted MR imaging. The mean radiation dose, maximum dose, and 3- and 4.20-Gy cochlear volumes were identified using standard Leksell Gamma Knife software.

RESULTS

The median mean radiation dose delivered to the cochlea was 3.50 Gy (range, 1.20-6.80 Gy) on CT and 3.40 Gy (range, 1-6.70 Gy) on MR imaging (concordance correlation coefficient = 0.86, r ² = 0.9, P ≤ .001). The median maximum dose delivered to the cochlea was 6.7 Gy on CT and 6.6 Gy on MR imaging (concordance correlation coefficient = 0.89, r ² = 0.90, P ≤ .001). Dose-volume histograms generated from CT and MR imaging demonstrated a strong level of correlation in estimating the 3- and 4.20-Gy volumes (concordance correlation coefficient = 0.81, r ² = 0.82, P ≤ .001 and concordance correlation coefficient = 0.87, r ² = 0.89, P ≤ .001).

CONCLUSIONS

Both MR imaging and CT provide similar cochlear dose parameters. Despite the reported superiority of CT in identifying bony structures, high-definition MR imaging alone is sufficient to identify the radiation doses delivered to the cochlea.

Validation of a semi-automatic co-registration of MRI scans in patients with brain tumors during treatment follow-up

There is an expanding research interest in high-grade gliomas because of their significant population burden and poor survival despite the extensive standard multimodal treatment. One of the obstacles is the lack of individualized monitoring of tumor characteristics and treatment response before, during and after treatment. We have developed a two-stage semi-automatic method to co-register MRI scans at different time points before and after surgical and adjuvant treatment of high-grade gliomas. This two-stage co-registration includes a linear co-registration of the semi-automatically derived mask of the preoperative contrast-enhancing area or postoperative resection cavity, brain contour and ventricles between different time points. The resulting transformation matrix was then applied in a non-linear manner to co-register conventional contrast-enhanced T1 -weighted images. Targeted registration errors were calculated and compared with linear and non-linear co-registered images. Targeted registration errors were smaller for the semi-automatic non-linear co-registration compared with both the non-linear and linear co-registered images. This was further visualized using a three-dimensional structural similarity method. The semi-automatic non-linear co-registration allowed for optimal correction of the variable brain shift at different time points as evaluated by the minimal targeted registration error. This proposed method allows for the accurate evaluation of the treatment response, essential for the growing research area of brain tumor imaging and treatment response evaluation in large sets of patients. Copyright © 2016 John Wiley & Sons, Ltd.

Investigating the function of deep cortical and subcortical structures using stereotactic electroencephalography: lessons from the anterior cingulate cortex

Stereotactic Electroencephalography (SEEG) is a technique used to localize seizure foci in patients with medically intractable epilepsy. This procedure involves the chronic placement of multiple depth electrodes into regions of the brain typically inaccessible via subdural grid electrode placement. SEEG thus provides a unique opportunity to investigate brain function. In this paper we demonstrate how SEEG can be used to investigate the role of the dorsal anterior cingulate cortex (dACC) in cognitive control. We include a description of the SEEG procedure, demonstrating the surgical placement of the electrodes. We describe the components and process required to record local field potential (LFP) data from consenting subjects while they are engaged in a behavioral task. In the example provided, subjects play a cognitive interference task, and we demonstrate how signals are recorded and analyzed from electrodes in the dorsal anterior cingulate cortex, an area intimately involved in decision-making. We conclude with further suggestions of ways in which this method can be used for investigating human cognitive processes.

Image coregistration: quantitative processing framework for the assessment of brain lesions

The quantitative, multiparametric assessment of brain lesions requires coregistering different parameters derived from MRI sequences. This will be followed by analysis of the voxel values of the ROI within the sequences and calculated parametric maps, and deriving multiparametric models to classify imaging data. There is a need for an intuitive, automated quantitative processing framework that is generalized and adaptable to different clinical and research questions. As such flexible frameworks have not been previously described, we proceeded to construct a quantitative post-processing framework with commonly available software components. Matlab was chosen as the programming/integration environment, and SPM was chosen as the coregistration component. Matlab routines were created to extract and concatenate the coregistration transforms, take the coregistered MRI sequences as inputs to the process, allow specification of the ROI, and store the voxel values to the database for statistical analysis. The functionality of the framework was validated using brain tumor MRI cases. The implementation of this quantitative post-processing framework enables intuitive creation of multiple parameters for each voxel, facilitating near real-time in-depth voxel-wise analysis. Our initial empirical evaluation of the framework is an increased usage of analysis requiring post-processing and increased number of simultaneous research activities by clinicians and researchers with non-technical backgrounds. We show that common software components can be utilized to implement an intuitive real-time quantitative post-processing framework, resulting in improved scalability and increased adoption of post-processing needed to answer important diagnostic questions.

Standardization of terminology in stereotactic radiosurgery: Report from the Standardization Committee of the International Leksell Gamma Knife Society: special topic

OBJECT

This report has been prepared to ensure more uniform reporting of Gamma Knife radiosurgery treatment parameters by identifying areas of controversy, confusion, or imprecision in terminology and recommending standards.

METHODS

Several working group discussions supplemented by clarification via email allowed the elaboration of a series of provisional recommendations. These were also discussed in open session at the 16th International Leksell Gamma Knife Society Meeting in Sydney, Australia, in March 2012 and approved subject to certain revisions and the performance of an Internet vote for approval from the whole Society. This ballot was undertaken in September 2012.

RESULTS

The recommendations in relation to volumes are that Gross Target Volume (GTV) should replace Target Volume (TV); Prescription Isodose Volume (PIV) should generally be used; the term Treated Target Volume (TTV) should replace TVPIV, GTV in PIV, and so forth; and the Volume of Accepted Tolerance Dose (VATD) should be used in place of irradiated volume. For dose prescription and measurement, the prescription dose should be supplemented by the Absorbed Dose, or DV% (for example, D95%), the maximum and minimum dose should be related to a specific tissue volume (for example, D2% or preferably D1 mm3), and the median dose (D50%) should be recorded routinely. The Integral Dose becomes the Total Absorbed Energy (TAE). In the assessment of planning quality, the use of the Target Coverage Ratio (TTV/ GTV), Paddick Conformity Index (PCI = TTV2/[GTV · PIV]), New Conformity Index (NCI = [GTV · PIV]/TTV2), Selectivity Index (TTV/PIV), Homogeneity Index (HI = [D2% –D98%]/D50%), and Gradient Index (GI = PIV0.5/PIV) are reemphasized. In relation to the dose to Organs at Risk (OARs), the emphasis is on dose volume recording of the VATD or the dose/volume limit (for example, V10) in most cases, with the additional use of a Maximum Dose to a small volume (such as 1 mm3) and/or a Point Dose and Mean Point Dose in certain circumstances, particularly when referring to serial organs. The recommendations were accepted by the International Leksell Gamma Knife Society by a vote of 92% to 8%.

CONCLUSIONS

An agreed-upon and uniform terminology and subsequent standardization of certain methods and procedures will advance the clinical science of stereotactic radiosurgery.

Criteria for deep-brain stimulation in Parkinson’s disease: review and analysis

Deep-brain stimulation is currently the most effective surgical treatment for advanced Parkinson's disease. The relevant targets to date are the subthalamic nucleus and the globus pallidus internus, although the thalamus (ventralis intermedius nucleus) is preferred in tremor-dominant, aged Parkinson's disease patients. Long-term benefit in cardinal parkinsonian signs, motor fluctuations and dyskinesia has been reported in 5-year follow-up studies of subthalamic nucleus deep-brain stimulation. However, some psychiatric consequences have raised important issues and emphasized the need for an experienced deep-brain stimulation surgical team. This team should be multidisciplinary and involve movement disorder neurologists, neurosurgeons, neuropsychologists and psychiatrists. The recent observation that deep-brain stimulation of the pedunculopontine nucleus improves axial signs, possibly even in those less responsive to levodopa, brings new hope to the management of advanced Parkinson's disease.

Image fusion pitfalls for cranial radiosurgery

Stereotactic radiosurgery requires imaging to define both the stereotactic space in which the treatment is delivered and the target itself. Image fusion is the process of using rotation and translation to bring a second image set into alignment with the first image set. This allows the potential concurrent use of multiple image sets to define the target and stereotactic space. While a single magnetic resonance imaging (MRI) sequence alone can be used for delineation of the target and fiducials, there may be significant advantages to using additional imaging sets including other MRI sequences, computed tomography (CT) scans, and advanced imaging sets such as catheter-based angiography, diffusor tension imaging-based fiber tracking and positon emission tomography in order to more accurately define the target and surrounding critical structures. Stereotactic space is usually defined by detection of fiducials on the stereotactic head frame or mask system. Unfortunately MRI sequences are susceptible to geometric distortion, whereas CT scans do not face this problem (although they have poorer resolution of the target in most cases). Thus image fusion can allow the definition of stereotactic space to proceed from the geometrically accurate CT images at the same time as using MRI to define the target. The use of image fusion is associated with risk of error introduced by inaccuracies of the fusion process, as well as workflow changes that if not properly accounted for can mislead the treating clinician. The purpose of this review is to describe the uses of image fusion in stereotactic radiosurgery as well as its potential pitfalls.

[Stereotactic and diagnostic imaging in radiosurgery]

Constant progress in medical imaging and particularly magnetic resonance imaging has profound impact in planning for stereotactic radiosurgery and radiotherapy. The purpose of this paper is to discuss the integration of medical imaging modalities in the planning process. Principles of generic algorithms to calculate stereotactic coordinates are treated for tomographic imaging and digital substraction angiography, and their accuracies are analyzed in a review of the literature. The algorithmic foundations and performance of automatic intermodality co-registration methods are developed. Finally, the MRI sequences useful in planning and follow-up are discussed and the role of MR angiographic sequences compared to conventional X-ray angiography in the particular case of the arteriovenous malformation planning.

Comparison of registration accuracy of skin- and bone-implanted fiducials for frameless stereotaxis of the brain: a prospective study

The registration accuracy of skin- and bone-implanted fiducials using a frameless stereotactic system were analyzed prospectively.Twenty-eight patients underwent resection of intra-axial neoplasmas after both skin- and bone-implantable fiducial markers were placed. Both sets of fiducials were independently co-registered to a magnetic resonance imaging data set acquired preoperatively using the ISG Viewing Wandtrade mark. Root mean square errors were recorded as an objective measure of registration accuracy of the two types of fiducials.Root mean square errors of bone-implanted fiducials registration were lower than those of skin fiducials; however, this difference was not statistically significant (p = 0.206).The registration accuracy of skin- and bone-implanted fiducials appears to be similar. Still, bone-implanted fiducials may be advantageous compared to skin fiducials when re-registration of the patient-image space is desired intraoperatively such as during major drift in the patient's position or after surgical repositioning.

Image-guided surgery and medical robotics in the cranial area

Surgery in the cranial area includes complex anatomic situations with high-risk structures and high demands for functional and aesthetic results. Conventional surgery requires that the surgeon transfers complex anatomic and surgical planning information, using spatial sense and experience. The surgical procedure depends entirely on the manual skills of the operator. The development of image-guided surgery provides new revolutionary opportunities by integrating presurgical 3D imaging and intraoperative manipulation. Augmented reality, mechatronic surgical tools, and medical robotics may continue to progress in surgical instrumentation, and ultimately, surgical care. The aim of this article is to review and discuss state-of-the-art surgical navigation and medical robotics, image-to-patient registration, aspects of accuracy, and clinical applications for surgery in the cranial area.

In vivo accuracy of image guidance performed using optical tracking and optimized registration

Object

Image guidance systems involving the use of frameless referencing of surgical space to compile volumetric imaging data sets recently have come into widespread use. Few studies have addressed the true intraoperative surgical accuracy (that is, the application accuracy) of these systems except in a subjective manner. Calculated accuracies given by the systems do not necessarily reflect true intraoperative accuracy.

Methods

To objectively assess the stereotactic accuracy of a frameless image guidance system using optical spatial referencing, the author analyzed postoperative magnetic resonance (MR) images after placement of depth electrodes for the investigation of epilepsy. Preoperative planning for the treatment of seven patients included implanting skull fiducial screws and obtaining computed tomography/MR fusion images by using ImMerge image fusion software on the StealthStation (Medtronic, Inc.). A total of 42 electrodes were placed. Postoperative volumetric MR images were fused with preoperative study images. The difference between the planned electrode trajectories and targets and the visualized electrodes was measured in stereotactic space.

Conclusions

The mean distance between the distal electrode contact and the distal end of the planned trajectory for the 42 targets was 3 ± 1.5 mm. The most common error was in depth. The author’s technique did not involve rigid skull fixation of electrodes because they were subsequently tunneled subcutaneously and later removed at the bedside of the patient. Errors in depth were known to be due to traction at the time of tunneling and not due to stereotactic factors. Correcting for depth along the electrode trajectory, the mean accuracy was found to be 2.4 ± 1 mm.

New radiofrequency coil integrated with a stereotactic frame for intraoperative MRI-controlled stereotactically guided brain surgery

Mislocalization errors caused by MR image distortions or brain shift are one of the main causes of complications after stereotactically guided neurosurgical procedures. A special device, which could ameliorate such effects and provide intraoperative acquisition of MR images of sufficient diagnostic accuracy, was developed. It is composed of a radiofrequency receiver coil integrated with a modified Komai stereotactic frame and a Sugita four-pin head holder. Clinical testing revealed that the use of the device during stereotactically guided procedures under the control of low magnetic field strength (0.3 T) intraoperative MRI ameliorates the effects of brain shift, and permits to obtain informative tissue samples and to perform aggressive removal of the lesion without any damage of the eloquent cerebral structures. The possibility of an easy shift from a stereotactically guided to a routine microneurosurgical procedure represents an additional benefit of the developed device.

Technical note: the design of a stereotactic frame for direct MRI-anatomical correlation of the brachial plexus

The purpose of this study was to identify optimal magnetic resonance imaging (MRI) conditions to visualize discrete alterations of brachial plexus components, as part of a biomechanical study of minor nerve compression syndromes. A method was developed allowing direct comparison between the MRI image and the subsequently obtained matching anatomic section of the same specimen. We designed a stereotactic frame to obtain the precise orientation of the MRI plane with reference to the specimen and adapted a vertical band saw for multiplanar sectioning of cadaveric specimens. Two cadaveric upper quadrants were examined by MRI (TR 450 ms, TE 13 ms, pixel matrix 512 x 512 and FOV 23-26 cm) and anatomical slices were produced. One specimen was sectioned axially, while the second specimen was sectioned in an oblique plane corresponding to the natural longitudinal axis of the upper part of the brachial plexus. MR images and the corresponding slices exhibited a strong correlation. This correlation was checked by using vitamin A pearls as landmarks. MR images revealed more detail after the correlating anatomical slices were analyzed. The present study shows that the method is suited for direct MRI-anatomic comparison of the brachial plexus and is also proposed for application to other topographical regions.

MR-guided stereotactic neurosurgery—comparison of fiducial-based and anatomical landmark transformation approaches

For application in magnetic resonance (MR) guided stereotactic neurosurgery, two methods for transformation of MR-image coordinates in stereotactic, frame-based coordinates exist: the direct stereotactic fiducial-based transformation method and the indirect anatomical landmark method. In contrast to direct stereotactic MR transformation, indirect transformation is based on anatomical landmark coregistration of stereotactic computerized tomography and non-stereotactic MR images. In a patient study, both transformation methods have been investigated with visual inspection and mutual information analysis. Comparison was done for our standard imaging protocol, including t2-weighted spin-echo as well as contrast enhanced t1-weighted gradient-echo imaging. For t2-weighted spin-echo imaging, both methods showed almost similar and satisfying performance with a small, but significant advantage for fiducial-based transformation. In contrast, for t1-weighted gradient-echo imaging with more geometric distortions due to field inhomogenities and gradient nonlinearity than t2-weighted spin-echo imaging, mainly caused by a reduced bandwidth per pixel, anatomical landmark transformation delivered markedly better results. Here, fiducial-based transformation yielded results which are intolerable for stereotactic neurosurgery. Mean Euclidian distances between both transformation methods were 0.96 mm for t2-weighted spin-echo and 1.67 mm for t1-weighted gradient-echo imaging. Maximum deviations were 1.72 mm and 3.06 mm, respectively.

Stereotactic radiotherapy for treatment of cavernous sinus meningiomas

PURPOSE

To assess the safety and efficacy of stereotactic radiotherapy (SRT) using a linear accelerator equipped with a micromultileaf collimator for cavernous sinus meningiomas.

METHODS AND MATERIALS

Forty-five patients with benign cavernous sinus meningiomas were treated with SRT between November 1997 and April 2002. Sixteen patients received definitive treatment on the basis of imaging characteristics of the cavernous sinus tumor. Twenty-nine patients received SRT either as immediate adjuvant treatment after incomplete resection or at documented recurrence. Treatment planning in all patients included CT-MRI image fusion and beam shaping using a micromultileaf collimator. The primary tumor volume varied from 1.41 to 65.66 cm(3) (median, 14.5 cm(3)). The tumor diameter varied from 1.4 to 7.4 cm (median, 3.8 cm). Tumor compressed the optic chiasm or optic nerve in 30 patients. All tumors were treated with a single isocenter plus a margin of normal parenchyma varying from 1 to 5 mm (median, 2.5 mm). The prescribed dose varied from 4250 to 5400 cGy (median, 5040 cGy). The prescription isodose varied from 87% to 95% (median, 90%). The maximal tumor dose varied from 5000 to 6000 cGy (median, 5600 cGy). The follow-up varied from 12 to 53 months (median, 36 months).

RESULTS

The actuarial 3-year overall and progression-free survival rate was 100% and 97.4%, respectively. One patient (2%) developed local relapsed at 18 months. A partial imaging response occurred in 18% of patients, and the tumor was stable in the remaining 80%. Preexisting neurologic complaints improved in 20% of patients and were stable in the remainder. No patient, tumor, or treatment factors were found to be predictive of imaging or clinical response. Transient acute morbidities included headache responsive to nonnarcotic analgesics in 4 patients, fatigue in 3 patients, and retroorbital pain in 1 patient. No treatment-induced peritumoral edema, cranial neuropathy, endocrine dysfunction, cognitive decline, or second malignancy occurred. One patient had an ipsilateral cerebrovascular accident 6 months after SRT.

CONCLUSION

Stereotactic radiotherapy is both safe and effective for patients with cavernous sinus meningiomas. Field shaping using a micromultileaf collimator allows conformal and homogeneous radiation of cavernous sinus meningiomas that may not be amenable to single-fraction stereotactic radiosurgery because of tumor size or location. Additional clinical experience is necessary to determine the position of SRT among the available innovative fractionated RT options for challenging skull base meningiomas.

Virtual Reality Neurosurgery: A Simulator Blueprint

OBJECTIVE

This article details preliminary studies undertaken to integrate the most relevant advancements across multiple disciplines in an effort to construct a highly realistic neurosurgical simulator based on a distributed computer architecture. Techniques based on modified computational modeling paradigms incorporating finite element analysis are presented, as are current and projected efforts directed toward the implementation of a novel bidirectional haptic device.

METHODS

Patient-specific data derived from noninvasive magnetic resonance imaging sequences are used to construct a computational model of the surgical region of interest. Magnetic resonance images of the brain may be coregistered with those obtained from magnetic resonance angiography, magnetic resonance venography, and diffusion tensor imaging to formulate models of varying anatomic complexity.

RESULTS

The majority of the computational burden is encountered in the presimulation reduction of the computational model and allows realization of the required threshold rates for the accurate and realistic representation of real-time visual animations.

CONCLUSION

Intracranial neurosurgical procedures offer an ideal testing site for the development of a totally immersive virtual reality surgical simulator when compared with the simulations required in other surgical subspecialties. The material properties of the brain as well as the typically small volumes of tissue exposed in the surgical field, coupled with techniques and strategies to minimize computational demands, provide unique opportunities for the development of such a simulator. Incorporation of real-time haptic and visual feedback is approached here and likely will be accomplished soon.

Radiosurgery performed with the aid of a 3-mm collimator in the subthalamic nucleus and substantia nigra of the vervet monkey

OBJECT

Radiosurgery for functional neurosurgery performed using a linear accelerator (LINAC) has not been extensively characterized in preclinical studies. In the present study, the properties of a newly designed 3-mm-diameter collimator were evaluated in a dedicated LINAC, which produced lesions in the basal ganglia of vervet monkeys. Lesion formation was determined in vivo in three animals by examining magnetic resonance (MR) images to show the dose-delivery precision of targeting and the geometry and extent of the lesions. Postmortem immunohistochemical studies were conducted to determine the extent of lesion-induced radiobiological effects.

METHODS

In three male vervet monkeys, the subthalamic nucleus (STN; one animal) and the pars compacta of the lateral substantia nigra (SN; two animals) were targeted by a Novalis Shaped Beam Surgery System that included a 3-mm collimator and delivered a maximum dose of 150 Gy. Magnetic resonance images obtained 4, 5, and 9 months posttreatment were reviewed, and the animals were killed so that immunohistological characterizations could be made.

CONCLUSIONS

The generation of precise radiosurgical lesions by a 3-mm collimator was validated in studies that targeted the basal ganglia of the vervet monkey. The extent of the lesions created in all animals remained restricted in diameter (< 3 mm) throughout the duration of the studies, as assessed by reviewing MR images. Histological studies showed that the lesions were contained within the STN and SN target areas and that there were persistent increases in glial fibrillary acidic protein immunoreactivity. Increases in immunoreactivity for tyrosine hydroxylase, the serotonin transporter, and the GluR1 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate glutamate receptor in penumbral regions of the lesion were suggestive of compensatory neuronal adaptations. This radiosurgical approach may be of particular interest for the induction of lesions of the STN and SN in studies of experimental parkinsonism, as well as for the development of potential radiosurgical treatments for Parkinson disease.

A phantom study of the geometric accuracy of computed tomographic and magnetic resonance imaging stereotactic localization with the Leksell stereotactic system

OBJECTIVE

To assess the spatial accuracy of magnetic resonance imaging (MRI) and computed tomographic stereotactic localization with the Leksell stereotactic system.

METHODS

The phantom was constructed in the shape of a box, 164 mm in each dimension, with three perpendicular arrays of solid acrylic rod, 5 mm in diameter and spaced 30 mm apart within the phantom. In this study, images from two different MRI scanners and a computed tomographic scanner were obtained using the same Leksell (Elekta Instruments, Stockholm, Sweden) head frame placement. The coordinates of the rod images in the three principal planes were measured by using a tool provided with Leksell GammaPlan software (Elekta Instruments, Norcross, GA) and were compared with the physical phantom measurements.

RESULTS

The greatest distortion was found around the periphery, and the least distortion (<1.5 mm) was present in the middle and most other areas of the phantom. In the phantom study using computed tomography, the mean values of the maximum errors for the x, y, and z axes were 1.0 mm (range, 0.2-1.3 mm), 0.4 mm (range, 0.1-0.8 mm), and 3.8 mm (range, 1.9-5.1 mm), respectively. The mean values of the maximum errors when using the Philips MRI scanner (Philips Medical Systems, Shelton, CT) were 0.9 mm (range, 0.4-1.7 mm), 0.2 mm (range, 0.0-0.7 mm), and 1.9 mm (range, 1.3-2.3 mm), respectively. Using the Siemens MRI scanner (Siemens Medical Systems, New York, NY), these values were 0.4 mm (range, 0.0-0.7 mm), 0.6 mm (range, 0.0-1.0 mm), and 1.6 mm (range, 0.8-2.0 mm), respectively. The geometric accuracy of the MRI scans when using the Siemens scanner was greatly improved after the implementation of a new software patch provided by the manufacturer. The accuracy also varied with the direction of phase encoding.

CONCLUSION

The accuracy of target localization for most intracranial lesions during stereotactic radiosurgery can be achieved within the size of a voxel, especially by using the Siemens MRI scanner at current specifications and with a new software patch. However, caution is warranted when imaging peripheral lesions, where the distortion is greatest.

An image fusion study of the geometric accuracy of magnetic resonance imaging with the Leksell stereotactic localization system

A special acrylic phantom designed for both magnetic resonance imaging (MRI) and computed tomography (CT) was used to assess the geometric accuracy of MRI-based stereotactic localization with the Leksell stereotactic head frame and localizer system. The acrylic phantom was constructed in the shape of a cube, 164 mm in each dimension, with three perpendicular arrays of solid acrylic rods, 5 mm in diameter and spaced 30 mm apart within the phantom. Images from two MR scanners and a CT scanner were obtained with the same Leksell head frame placement. Using image fusion provided by the Leksell GammaPlan (LGP) software, the coordinates of the intraphantom rod positions from two MRI scanners were compared to that of CT imaging. The geometric accuracy of MR images from the Siemens scanner was greatly improved after the implementation of a special software patch provided by the manufacturer. In general, much better accuracy was achieved in the transverse plane where images were acquired. Most distortion was found around the periphery while least distortion was present in the middle and most other parts of the phantom. For most intracranial lesions undergoing stereotactic radiosurgery, accuracy of target localization can be achieved within size of a voxel, especially with the Siemens scanner. However, extra caution should be taken for imaging of peripheral lesions where the distortion is the greatest.

A prospective comparison between three-dimensional magnetic resonance imaging and ventriculography for target-coordinate determination in frame-based functional stereotactic neurosurgery

OBJECT

The purpose of this prospective study was to compare stereotactic coordinates obtained with ventriculography with coordinates derived from stereotactic computer-reconstructed three-dimensional magnetic resonance (3D-MR) imaging in functional stereotactic procedures.

METHODS

In 15 consecutive patients undergoing functional stereotactic procedures, both preoperative frame-based stereotactic 3D-MR imaging and intraoperative ventriculography were performed. Differences between 3D-MR imaging and ventriculography in X, Y, and Z coordinates of the anterior commissure (AC), posterior commissure (PC), and target area were calculated, as well as the 3D distance between the position of AC, PC, and target within stereotactic space as obtained using both methods. The position of the stereotactic MR imaging fiducial markers measured using 3D-MR imaging compared well with the markers' known position embedded in the software (mean error 0.4 mm, maximal error for an individual slice 1.2 mm). For the individual coordinates, only for Y-PC was a difference found between 3D-MR imaging and ventriculography that significantly exceeded half the size of a pixel, the theoretical limit of precision when using a digitized imaging technique. However, the mean difference was smaller than 1 mm. The mean 3D distance between the 3D-MR imaging- and ventriculography-derived coordinates was 1.09 mm for AC, 1.13 mm for PC, and 1.29 mm for the targets.

CONCLUSIONS

With these data it is shown that there is sufficient agreement between ventriculography-derived and 3D-MR imaging-derived stereotactic coordinates to justify the use of 3D-MR imaging target determination in frame-based functional stereotactic neurosurgery.

Development of a unique phantom to assess the geometric accuracy of magnetic resonance imaging for stereotactic localization

OBJECTIVE

To test the spatial accuracy of coordinates generated from magnetic resonance imaging (MRI) scans, using the Brown-Roberts-Wells head frame and localizer system (Radionics, Inc., Burlington, MA).

METHODS

An anthropomorphic head phantom, consisting of a two-dimensional lattice of acrylic spheres (4-mm diameter) spaced 10 mm apart and embedded in a brain tissue-mimicking gelatin-agar gel, was constructed. The intersphere distances for the target lattice positions in MRI and computed tomographic scan sets were compared. The data sets were fused, and differences in fiducial marker and intraphantom target positions were measured.

RESULTS

Intersphere distances were identical for the MRI and computed tomographic scan sets (10 +/- 0.1 mm). Differences in fiducial marker positions [maximal lateral difference, 0.97 mm; mean absolute lateral difference, 0.69 +/- 0.22 mm; maximal anteroposterior (AP) difference, 1.99 mm; mean absolute AP difference, 1.29 +/- 0.67 mm] were correlated with differences in intraphantom target positions (maximal lateral difference, 0.83 mm; mean absolute lateral difference, 0.28 +/- 0.24 mm; maximal AP difference, -1.97 mm; mean absolute AP difference, 1.63 +/- 25 mm; maximal vertical difference, -0.73 mm; mean absolute vertical difference, 0.34 +/- 0.21 mm). This suggested that improper fiducial rod identification and the subsequent transformation to stereotactic coordinate space were the greatest sources of spatial uncertainty.

CONCLUSION

With computed tomographic data as the standard, these differences resulted in maximal and minimal composite uncertainties of 2.06 and 1.17 mm, respectively. The measured uncertainties exceed recommended standards for radiosurgery but allow the possible use of MRI-based stereotactic treatment planning for certain intracranial lesions, if the errors are corrected using appropriate software. Clinicians must recognize that error magnitudes vary for different systems, and they should perform systematic, scheduled, institutional error analyses as part of their ongoing quality assurance processes. This phantom provides one tool for measuring such variances.

Evaluation of the spatial accuracy of magnetic resonance imaging-based stereotactic target localization for gamma knife radiosurgery of functional disorders

PURPOSE

This study was undertaken to determine the impact of geometric distortions on the spatial accuracy of magnetic resonance imaging (MRI)-guided stereotactic localization for gamma knife functional radiosurgery.

METHOD

The spatial accuracy of MRI was evaluated by comparing stereotactic coordinates of intracranial targets, external fiducials, and anatomic structures defined by computed tomographic and MRI studies of the Radionics skull phantom (Radionics, Inc., Burlington, MA), the Rando head phantom, and 11 patients who underwent gamma knife functional radiosurgery. The distortion in MRI was assessed from computed tomographic and MRI fusion studies for these patients, as well as from MRI studies acquired by swapping the direction of the magnetic field gradients for five patients who underwent gamma knife radiosurgery and three patients who underwent MRI-guided frameless surgery. A follow-up program to compare the location of the created lesion with the intended target complemented the analysis.

RESULTS

The average difference between computed tomographic and MRI stereotactic coordinates of external fiducials, intracranial targets, and anatomic landmarks was of the order of 1 pixel size (0.9 x 0.9 x 1 mm3) along the x, y, and z axes. The average linear scaling along these axes as determined by fusion studies was approximately 0.8% and consistent with a single pixel. The follow-up studies, available for seven patients, revealed good agreement between the location of the created lesion and the intended target.

CONCLUSION

The spatial accuracy of an MRI-based localization system can be comparable to computed tomography-based localization with the added benefit of MRI resolution. Both machine- and object-related MRI distortions can be reduced to an acceptable level with contemporary scanners, optimized scanning sequences, and distortion-resistant stereotactic instruments.

Electrophysiological versus image-based targeting in the posteroventral pallidotomy

OBJECTIVE

To study the functional accuracy of stereotactic targeting for the posteroventral pallidotomy (PVP), comparing targets chosen on magnetic resonance images (MRI), and fused MRI to computed tomographic (CT) images, with electrophysiologically refined anatomical targets. METHDOS AND MATERIALS: For each of the 10 pallidotomies three sets of targets were collected, beginning with the MRI targets. The second target set was measured on images generated by nonlinear volumetric fusion of MRI images with CT using Image Fusion (Radionics, Inc.). The anatomical target site was then determined electrophysiologically with intraoperative microelectrode recording and macroelectrode stimulation guidance.

RESULTS

Magnetic resonance imaging or MRI-CT fused images alone would not have been sufficiently accurate to preclude visual or motor complications in the posteroventral pallidotomy, based on our target located within 1 mm of the optic tract and within 2 mm of the internal capsule. In 2 of the 10 cases of either MRI or fused images, the targets were dangerously close to the optic tract. Two of 10 of the fused targets were within the internal capsule. The fusion of MRI with CT did not functionally improve the targeting accuracy of MRI, since the means of the MRI targets and the fused targets were statistically the same. Individually, however, the MRI target was different from the fused target in each case by an average radial distance of 3.5 +/- 2.3 mm, but such corrections were not statistically or surgically significant.

CONCLUSIONS

Image-based targeting including MRI or fused MRI-CT data may not be sufficiently accurate to prevent capsular or visual deficits in the posteroventral pallidotomy, necessitating electrophysiological refinement. In this report, the functional accuracy of MRI was not improved by fusion with CT.

Ventricular volume following third ventriculostomy

OBJECT

Ventricular size often shows no obvious change following third ventriculostomy, particularly in the early postoperative period, making postoperative evaluation difficult without expensive and often invasive testing in patients with equivocal clinical responses. The authors hypothesized that performing careful volumetric measurements would show decreases in size within the first 3 weeks after surgery.

METHODS

Volumetric measurements were calculated from standard 3 x 3-mm axial computerized tomography (CT) scans obtained immediately before and 3 and 21 days after surgery. Two independent investigators measured third ventricular volume in a series of 16 patients and lateral ventricular volume in 10 of the patients undergoing stereotactically guided endoscopic third ventriculostomy for noncommunicating hydrocephalus. Fifteen patients were symptomatically improved at the time the follow-up scan was obtained. Third ventricular volume decreased in all patients by a mean of 35% (range 7.8-95.1%) and lateral ventricular volume decreased in all patients by a mean of 33% (range 4.5-80.3%). The degree of change correlated with the length of preoperative symptoms (p < 0.005). The one patient who experienced no improvement showed no decrease in third ventricular volume. In seven of 10 patients, the decrease in third ventricular volume exceeded the decrease in lateral ventricular volume. Repeated measurements indicated that the 95% confidence interval for the authors' calculations varied around the mean by 2.5% for third ventricular volume and 1.2% for lateral ventricular volume. Long-term outcome was excellent, with only one case of delayed failure. The mean follow-up duration was 12 months.

CONCLUSIONS

Volumetric measurements calculated from standard CT scans will show a demonstrable decrease in ventricular volume soon after successful third ventriculostomy and can be helpful in assessing patients postoperatively. Although the third ventricle may exhibit a greater decrease, the lateral ventricular measurements are more accurate. Patients with more indolent symptoms show the smallest change.

Optimizing accuracy in magnetic resonance imaging-guided stereotaxis: a technique with validation based on the anterior commissure-posterior commissure line

OBJECT

Some of the earliest successful frame-based stereotactic interventions directed toward the thalamus and basal ganglia depended on identifying the anterior commissure (AC) and posterior commissure (PC) in a sagittal ventriculogram and defining the intercommissural line that connects them in the midsagittal plane. The AC-PC line became the essential landmark for the localization of neuroanatomical targets in the basal ganglia and diencephalon and for relating them to stereotactic atlases. Stereotactic/functional neurosurgery has come to rely increasingly on magnetic resonance (MR) imaging guidance, and methods for accurately determining the AC-PC line on MR imaging are being developed. The goal of the present article is to present the authors' technique.

METHODS

The technique described uses MR sequences that minimize geometric distortion and registration error, thereby maximizing accuracy in AC-PC line determinations from axially displayed MR data. The technique is based on the authors' experience with the Leksell G-frame but can be generalized to other MR imaging-based stereotactic systems. This methodology has been used in a series of 62 stereotactic procedures in 47 adults (55 pallidotomies and seven thalamotomies) with preliminary results that compare favorably with results reported when using microelectrode recordings. The measurements of the AC-PC line reported here also compare favorably with those based on ventriculography and computerized tomography scanning.

CONCLUSIONS

The methodology reported here is critical in maintaining the accuracy and utility of MR imaging as its role in modern stereotaxy expands. Accurate parameters such as these aid in ensuring the safety, efficacy, and reproducibility of MR-guided stereotactic procedures.

Functional magnetic resonance imaging localization of ictal onset to a dysplastic cleft with simultaneous sensorimotor mapping: intraoperative electrophysiological confirmation and postoperative follow-up: technical note

INTRODUCTION

Although technically challenging to obtain, ictal functional magnetic resonance imaging has been used to localize ictal onset zones in a small number of patients. We used this technique to demonstrate the inherent epileptogenicity of dysplastic cortex.

METHODS

We present a 16-year-old female patient with intractable left-sided sensorimotor seizures and a congenital dysplastic cleft lying along the right rolandic fissure. Preoperative functional magnetic resonance imaging (blood oxygen level-dependent sequence, 1.5 T) localized the motor and sensory cortices to the anterior border of the cleft. During a speech activation run, the patient experienced a 20-second seizure. Initial activation was seen within the dysplastic cortex along the deep posterior margin of the cleft. Intraoperative median nerve stimulation produced a distinct N20/P20 wave inversion over the dysplastic cleft. Stimulation mapping performed with the patient awake confirmed the location of the sensorimotor cortex on the anterior border of the cleft, and preresection electrocorticography identified abundant interictal spikes along the posterior border after opening the cleft.

RESULTS

After surgical resection of the dysplastic cortex, the patient exhibited transient minimal weakness and mild neglect, which resolved within 1 week. Two years after surgery, she was neurologically intact and seizure-free.

CONCLUSION

This study used functional magnetic resonance imaging to demonstrate the inherent epileptogenicity of dysplastic cortex and to simultaneously map ictal and functional cortex. The N20 wave inversion can be a useful intraoperative tool for identifying the central sulcus (or its equivalent), even in the presence of abnormal cortical architecture.

[Stereotactic localization in medical imaging. Technical and methodologic aspects]

Stereotactic neurosurgery and stereotactic radiation therapy require the three-dimensional localization of lesions for biopsy or for treatment planning. The aim of this paper is the description of methods used in the different imaging modalities: x-ray teleradiography, digital subtracted angiography, computed tomography, and nuclear magnetic resonance imaging. The simple pin-target locating techniques are distinguished from those serving to the definition of volumes target necessary to treatment planning. Performances and difficulties of these techniques are emphasized. The specific methodology developed in Lille is described as an example. Organizational aspects and necessary quality controls for a good progress of the entire procedure, from imaging to treatment, are also discussed.

The registration of multiple medical images acquired from a single subject: why, how, what next?

This paper reviews some of the recent techniques which have been used to register multiple images of the same patient. Image registration is a problem which has been receiving significant attention from the medical image processing community in recent years. A successful image registration can aid in patient diagnosis, treatment assessment, image guided interventions, surgery planning and surgery. At present the majority of work has focused on rigid body transformations of images. We shall discuss some of the approaches used and outline a key automatic method in detail. In order to allow image registration of parts of the body which do not remain rigid, either due to patient movement or a change in pathology, nonlinear deformation techniques are being developed. We shall talk of the history of these methods before explaining deformations using landmarks and a recent extension to allow the definition of rigid structures in such warps in more detail. Validation of these methods is of great importance and we shall discuss work which has already been carried out on this topic for rigid body registrations as well as ideas for the validation of deformation algorithms.

Frameless 3D volume registration of MR data sets for stereotactic pallidotomy

Frameless 3D volume registration of Magnetic Resonance (MR) and computed (CT) data sets has been described by Kummar et al. [11]. Its use in 3D volume registration for stereotactic planning in patients undergoing pallidotomy is presented. Pre-operative examinations with the stereotactic frame and postoperative examinations without the stereotactic frame can be co-registered and reviewed for accuracy of planned and lesional coordinates.

Pallidotomy: a survey of current practice in North America

Twenty-eight centers completed a survey about their current practice of pallidotomy. This sample represents a non-exhaustive survey of the current practice of pallidotomy in North America and is not a study of outcomes. 1015 patients underwent 1219 pallidotomies: 811 (80%) unilateral, 72 (7%) staged bilateral, and 132 (13%) simultaneous bilateral. Pallidotomy has long been an accepted procedure and the indications for this surgery, in the opinion of the responding centers, were rated on a scale of 1 (poor) to 4 (excellent) and demonstrated dyskinesia as the best indication (median = 4); on-off fluctuations, dystonia, rigidity, and bradykinesia as good indications (median = 3); and freezing, tremor and gait disturbance as fair indications (median = 2). Most centers used MRI alone (50%) or in combination with CT scan (n = 6) or ventriculopathy (n = 5) to localize the target. The median values of pallidal coordinates were: 2 mm anterior to the midcommissural point 21 mm lateral to the midsagittal plane and 5 mm below the intercommissural line. Microrecording was performed by half of the centers (n = 14) and half of the remaining centers were considering starting it (n = 7). Main criteria used to define the target included the firing pattern of spontaneous neuronal discharges (n = 13) and the response to joint movement (n = 10). Most centers performed motor (n = 26) and visual (n = 23) macrostimulation. Twenty four centers performed test lesions using median values of 55 degrees C temperatures for 30 s. Final lesions consisted of 3 permanent lesions placed 2 mm apart, each lesion created with median values of 75 degrees C temperatures for 1 minute. Median hospital stay was 2 days.