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Stereotaxic localization of intracranial targets
Abstract
We report on a useful clinical method for precisely locating intracranial targets. Utilizing the BRW system, the technique is currently used in stereotaxic irradiation of arteriovenous malformations. An intracranial localizer box, with four radio-opaque markers on each face, surrounds the patient's head and is attached to the BRW Head Ring. Two localization films are required. One film includes the target and the eight anterior and posterior markers, whereas the other film includes the target and the eight right and left markers. There are no constraints that the films be orthogonal or parallel to the box faces, only that the target and radio-opaque markers appear on the films. In addition, knowledge of the source-image and source-target distances are not required. Analysis of the projected target and radio-opaque markers gives both the target location and magnification. Simulation with the BRW Phantom Base demonstrates that point targets can be located with respect to the BRW system to within 0.3 mm and magnification determined to within 0.5%.
... First, the frontal and lateral x-ray images were registered to stereotactic space using a localizer box attached to the stereotactic head frame. 18 Lead tip and entry point were manually segmented and the location of the lead in the frame coordinate system was determined. In combination with a freely selectable orientation angle, this information defined a virtual directional marker in stereotactic space. ...
... x-ray imaging with a localizer box in this study. 18 Alternatively, frameless methods could be used based on 2D/3D registration methods. 14,17 It turned out that the algorithm for image comparison is sensitive, even to small inaccuracies in the assumed marker position in the camera system. ...
Purpose:
Orientating the angle of directional leads for deep brain stimulation (DBS) in an axial plane introduces a new degree of freedom that is indicated by embedded anisotropic directional markers. Our aim was to develop algorithms to determine lead orientation angles from computed tomography (CT) and stereotactic x-ray imaging using standard clinical protocols, and subsequently assess the accuracy of both methods.
Methods:
In CT the anisotropic marker artifact was taken as a signature of the lead orientation angle and analyzed using discrete Fourier transform of circular intensity profiles. The orientation angle was determined from phase angles at a frequency 2/360° and corrected for aberrations at oblique leads. In x-ray imaging, frontal and lateral images were registered to stereotactic space and sub-images containing directional markers were extracted. These images were compared with projection images of an identically located virtual marker at different orientation angles. A similarity index was calculated and used to determine the lead orientation angle. Both methods were tested using epoxy phantoms containing directional leads (Cartesia(™) , Boston Scientific) with known orientation. Anthropomorphic phantoms were used to compare both methods for DBS cases.
Results:
Mean deviation between CT and x-ray was 1.5° ± 3.6° (range: -2.3° to 7.9°) for epoxy phantoms and 3.6° ± 7.1° (range: -5.6° to 14.6°) for anthropomorphic phantoms. After correction for imperfections in the epoxy phantoms, the mean deviation from ground truth was 0.0° ± 5.0° (range: -12° to 14°) for x-ray. For CT the results depended on the polar angle of the lead in the scanner. Mean deviation was -0.3° ± 1.9° (range: -4.6° to 6.6°) or 1.6° ± 8.9° (range: -23° to 34°) for polar angles ≤40° or >40°.
Conclusions:
The results show that both imaging modalities can be used to determine lead orientation angles with high accuracy. CT is superior to x-ray imaging, but oblique leads (polar angle >40°) show limited precision due to the current design of the directional marker. This article is protected by copyright. All rights reserved.
... Orientation Determination Based on 2D X-Ray Projection Images For this algorithm, two registered orthogonal X-ray projection images are needed (shown in Fig. 2). For registration, we used the stereotactic frame and X-ray opaque localizers [15], although different 2D and 3D registration techniques are possible [16]. For estimating orientation, we generated virtual images of the marker in a full range of orientations. ...
Introduction:
With recent advancements in deep brain stimulation (DBS), directional leads featuring segmented contacts have been introduced, allowing for targeted stimulation of specific brain regions. Given that manufacturers employ diverse markers for lead orientation, our investigation focuses on the adaptability of the 2017 techniques proposed by the Cologne research group for lead orientation determination.
Methods:
We tailored the two separate 2D and 3D X-ray-based techniques published in 2017 and originally developed for C-shaped markers, to the dual-marker of the Medtronic SenSight™ lead. In a retrospective patient study, we evaluated their feasibility and consistency by comparing the degree of agreement between the two methods.
Results:
The Bland-Altman plot showed favorable concordance without any noticeable systematic errors. The mean difference was 0.79°, with limits of agreement spanning from 21.4° to -19.8°. The algorithms demonstrated high reliability, evidenced by an intraclass correlation coefficient of 0.99 (p < 0.001).
Conclusion:
The 2D and 3D algorithms, initially formulated for discerning the circular orientation of a C-shaped marker, were adapted to the marker of the Medtronic SenSight™ lead. Statistical analyses revealed a significant level of agreement between the two methods. Our findings highlight the adaptability of these algorithms to different markers, achievable through both low-dose intraoperative 2D X-ray imaging and standard CT imaging.
... Stereotactic x-ray consisted of two orthogonal images in the frontal and lateral orientation taken with fiducial markers mounted onto the stereotactic frame. Marker locations in both projections determined the x-ray focus and image plane, which allowed calculating three-dimensional stereotactic coordinates of each electrode using an in-house software module (STVX 3), which implements the well-established algorithm of Siddon and Barth (1987). Coordinates were handed over to the planning system and projected onto the preoperative MRI. ...
Objective
To create probabilistic stimulation maps (PSMs) of deep brain stimulation (DBS) effects on tremor suppression and stimulation-induced side-effects in patients with essential tremor (ET).
Method
Monopolar reviews from 16 ET-patients which consisted of over 600 stimulation settings were used to create PSMs. A spherical model of the volume of neural activation was used to estimate the spatial extent of DBS for each setting. All data was pooled and voxel-wise statistical analysis as well as nonparametric permutation testing was used to confirm the validity of the PSMs.
Results
PSMs showed tremor suppression to be more pronounced by stimulation in the zona incerta (ZI) than in the ventral intermediate nucleus (VIM). Paresthesias and dizziness were most commonly associated with stimulation in the ZI and surrounding thalamic nuclei.
Discussion
Our results support the assumption, that the ZI might be a very effective target for tremor suppression. However stimulation inside the ZI and in its close vicinity was also related to the occurrence of stimulation-induced side-effects, so it remains unclear whether the VIM or the ZI is the overall better target. The study demonstrates the use of PSMs for target selection and evaluation. While their accuracy has to be carefully discussed, they can improve the understanding of DBS effects and can be of use for other DBS targets in the therapy of neurological or psychiatric disorders as well. Furthermore they provide a priori information about expected DBS effects in a certain region and might be helpful to clinicians in programming DBS devices in the future.
Radiosurgery, a term coined in 1951 by the Swedish neurosurgeon Lars Leksell [1], was first practiced using an orthovoltage x-ray apparatus, then a particle accelerator and finally a Co-60 isotope unit. The majority of Leksell’s clinical work was carried out using the latter device, known as the Gamma Knife. With this dedicated tool Leksell was able to pioneer many radiosurgical procedures. By the mid-1980s only a handful of GammaKnife’s existed, limiting the application of this new technique.
Radiosurgery is the preferred treatment for arteriovenous malformations (AVM) which are situated in deep brain locations such as close to the brain stem, or near critical areas such as the visual cortex1. The technique is commonly planned using multiple modalities, such as magnetic resonance (MR), computed tomography (CT) and digital subtraction angiography (DSA). T1 weighted MR images and CT images provide anatomical information but without additional contrast media, these techniques fail to provide adequate vascular information. For this reason, DSA images, which are obtained after contrast injection, are employed for target localisation. However, since these images are 2D projections of 3D structures, they may be inadequate for the 3D definition of large, complexly shaped AVMs2. Since the AVMs treated by radiosurgery are usually close to organs at risk, the dose must conform to the target and therefore accurate 3D target delineation may help achieve this goal.
The word “stereotactic” (or “stereotaxic”) is compounded from the Greek words “stereos,” meaning “solid” or “three-dimensional,” and “taxis,” meaning “arrangement” or “positioning.” Stereotactic brachytherapy refers to the accurate placement of radioactive sources with the aid of a special mechanical frame. Such frames allow a user to specify target points as precise three-dimensional (3-D) sets of coordinates, and to guide a probe along a variety of trajectories to hit a target with great accuracy. A high-resolution 3-D imaging technique, such as computed tomography (CT) or magnetic resonance imaging (MRI), is usually required for identifying the targets.
Radiosurgery or stereotactic radiosurgery (SRS) is defined as the irradiation of intracranial lesions with a single fraction of focused small ionizing radiation beams, such as x-rays or gamma rays, eliminating the need for conventional invasive surgery. Stereotactic radiation therapy (SRT) is the treatment of intracranial lesions with the stereotactic apparatus and multiple fractions. A stereotactic frame allows for rigid immobilization of the patient and accurate localization of the target. The goal of radiosurgery is to locate and define the intracranial lesion and deliver single or multiple doses to the target with small x-ray beams without exceeding the radiation tolerance of normal tissues adjacent to the target volume. Radiosurgery was initiated in 1950 by Lars Leksell to treat dysfunctional intracranial abnormalities originally using orthovoltage x-rays in conjunction with a stereotactic frame of his own design (Leksell 1951).
Photons produced by linear accelerators are now generally available in most radiotherapy centers. There exists a large body of experience with them and they represent an alternative to charged particles and gamma radiation for radiosurgery. In principle, there is no fundamental or decisive difference in the physical properties of accelerator-produced photons compared to the photons emitted by the 60Co sources of the Gamma Knife. The technical and practical aspects of production, however, have led to quite different and specific solutions to the scope of stereotactic radiosurgery.
Two methods for projecting 3D CT data onto plane X-ray radiographs for the use in stereotactic neurosurgery are discussed. Both methods deal with completely arbitrary projection configurations and do not require additional computer equipment. Simulation tests have proven the reliability and necessary accuracy for operational purposes.
A small field irradiation technique to deliver high doses of single fraction photon radiation to small, precisely located volumes (0.5 to 8 cm3) within the brain has been developed. Our method uses a modified Brown-Roberts-Wells (BRS), CT-guided, stereotactic system and a 6 MV linear accelerator equipped with a special collimator (diameters of 12.5 mm to 30.0 mm projected to isocenter) located 23 cm from isocenter. Target localization via planar angiography has been added. Treatment consists of a series of arcing beams using both gantry and couch rotations. During treatment, the patient's head is immobilized independently of the radiotherapy couch and is precisely positioned without reference to room lasers or light field. A precise verification of alignment precedes each treatment. Extensive performance tests have shown that a target, localized by CT, can be irradiated with a positional accuracy of 2.4 mm in any direction with 95% confidence. If angiography is used for localization, the results are better. The dose 1.0 cm outside the target volume is less than 20% of the prescribed dose for a medium sized collimator.
A method for the determination of stereotaxic coordinates in radiography, e.g. angiography, pneumoencephalography or digital vascular radiography, is described. A special localization frame containing radiopaque structures and scales defines a diagnostic coordinate system. This frame is fixed to the X-ray-table prior to the radiographic procedure and two projections are obtained at arbitrary angles to each other. The focus-film distances do not how to be fixed. The target coordinates are then determined either by a simple graphical procedure or with the use of a digitizing x-y-table, by a computer. With the computer method the films are placed on the digitizing table and the target and a few reference points are marked using a cursor. From the relative positions the computer calculates the coordinates. With the special head fixation system, coordinates of structures visualized in radiographic examinations can be transferred to various therapeutic or diagnostic stereotaxic devices.
We have developed a new isocentric two-film reconstruction algorithm for brachytherapy seed and needle implants. The algorithm has no requirements that the two films be orthogonal, symmetric, or even be taken in a transverse plane. In addition, there is no requirement that the two films even have the same number of images. We have found removal of these usual constraints useful for head and neck implants where images are often obscured by patient anatomy. The inherent image matching ambiguities associated with traditional two-film techniques are minimized by considering the image end points, rather than just the image centroids. For two films, the new algorithm, which considers all image combinations at one time, matches all the end-point images on one film with those on the other, and then reconstructs the end-point positions of the seeds. The algorithm minimizes the difference between the actual images and the projected images from the reconstructed seeds. The new two-film image matching problem is shown to be equivalent to the well-known assignment problem. For an implant of N seeds, this equivalence allows the two-film problem to be solved by an algorithm (ACM algorithm 548) that scales with a polynomial power of N, rather than N! as is usually assumed. An implant of N seeds can be matched and reconstructed in approximately (N/20)2s on a VAX 11/780.
A simple approach to stereotaxic radiography, including a new method for determination of target coordinates, is described. The proposed technique may be used in combination with conventional equipment for roentgen examinations available in X-ray departments and operating rooms; the only additional outfit needed is a coordinate frame that can be attached to the patient's head. Determination of the coordinates is easily made graphically, or from a simple equation using a pocket calculator or a mini-computer.
Siddon, A new software package for the microcomputer based Small field radiation therapy for ar-BRW stereotactic system: integrated stereoscopic views of
- W M Saunders
- K R Winston
- P R L Kijewski
- G K Svensson
Saunders, W.M., Winston, K.R., Kijewski, P., Siddon, A new software package for the microcomputer based R.L., Svensson, G.K.: Small field radiation therapy for ar-BRW stereotactic system: integrated stereoscopic views of