A universal method for geometric correction of magnetic resonance images for stereotactic neurosurgery

Distortion Correction Protocol for 3T Stereotactic Magnetic Resonance Imaging: A Clinical Study

BACKGROUND

With application of 3T magnetic resonance imaging (MRI) to functional neurosurgery procedures and given the inherent requirement of millimetric precision, the need to develop a method for correction of geometric image distortion emerged. The aim of this study was to demonstrate clinical safety and practical viability of a correction protocol in patients scheduled to undergo stereotactic procedures using 3T MRI.

METHODS

This prospective study comprised 20 patients scheduled to undergo computed tomography (CT) stereotactic functional procedures or encephalic brain lesion biopsies. The CT images were references for MRI geometric accuracy calculations. For each scan, 2 images were obtained: normal and reversed images. Eight distinct points on CT and MRI were selected summing 152 points that were based on a power analysis calculation value >0.999. One patient was excluded because of the inability to find reliable common landmark points on CT and MRI.

RESULTS

The distortion range was 0-5.6 mm and increased proportionally with stereotactic isocenter distance, meaning the distortion was greater in the periphery. After correction, the minimum and maximum distortion found was 0 mm and 3.5 mm, respectively. There was no significant difference between CT and MRI corrected x-coordinates (P > 0.05).

CONCLUSIONS

The proposed method can satisfactorily correct geometric distortions in clinical 3T MRI studies. Clinical use of the technique can be practical and efficient after software automation of the process. The method can be applied to all spin-echo MRI sequences.

Spatial Distortion in MRI-Guided Stereotactic Procedures: Evaluation in 1.5-, 3- and 7-Tesla MRI Scanners

BACKGROUND

Magnetic resonance imaging (MRI) is replacing computed tomography (CT) as the main imaging modality for stereotactic transformations. MRI is prone to spatial distortion artifacts, which can lead to inaccuracy in stereotactic procedures.

OBJECTIVE

Modern MRI systems provide distortion correction algorithms that may ameliorate this problem. This study investigates the different options of distortion correction using standard 1.5-, 3- and 7-tesla MRI scanners.

METHODS

A phantom was mounted on a stereotactic frame. One CT scan and three MRI scans were performed. At all three field strengths, two 3-dimensional sequences, volumetric interpolated breath-hold examination (VIBE) and magnetization-prepared rapid acquisition with gradient echo, were acquired, and automatic distortion correction was performed. Global stereotactic transformation of all 13 datasets was performed and two stereotactic planning workflows (MRI only vs. CT/MR image fusion) were subsequently analysed.

RESULTS

Distortion correction on the 1.5- and 3-tesla scanners caused a considerable reduction in positional error. The effect was more pronounced when using the VIBE sequences. By using co-registration (CT/MR image fusion), even a lower positional error could be obtained. In ultra-high-field (7 T) MR imaging, distortion correction introduced even higher errors. However, the accuracy of non-corrected 7-tesla sequences was comparable to CT/MR image fusion 3-tesla imaging.

CONCLUSION

MRI distortion correction algorithms can reduce positional errors by up to 60%. For stereotactic applications of utmost precision, we recommend a co-registration to an additional CT dataset.

An image correction protocol to reduce distortion for 3-T stereotactic MRI

BACKGROUND

Image distortion limits application of direct 3-T magnetic resonance imaging for stereotactic functional neurosurgery.

OBJECTIVE

To test the application of a method to correct and curtail image distortion of 3-T magnetic resonance images.

METHODS

We used a phantom head model mounted on a platform with the dimensions and features of a stereotactic frame. The phantom was scanned within the head coil of a Philips Achieva 3T X series (Philips Medical Systems, Eindhoven, the Netherlands). For each scan, 2 images were obtained-the normal and the reversed images. We applied the inverted gradient correction protocol to produce a corrected x, y, and z coordinates. We applied the Cronbach test or coefficient of reliability to assess the internal consistency of the data.

RESULTS

For all analyzed data, the P value was >.05, indicating that the differences among the observers were not statistically significant. Moreover, the data rectification proved to be effective, as the average distortion after correction was 1.05 mm. The distortion varied between 0.7 mm and 3.7 mm, depending on the target location.

CONCLUSION

This study examined a rectifying technique for correcting geometric distortion encountered in magnetic resonance images related to static field inhomogeneities (resonance offsets), and the technique proved to be highly successful in producing consistently accurate stereotactic target registration. The technique is applicable to all routinely used spin-echo MRI.

Fiducial Versus Nonfiducial Neuronavigation Registration Assessment and Considerations of Accuracy

Objective:

For frameless stereotaxy, users can choose between anatomic landmarks (ALs) or surface fiducial markers (FMs) for their match points during registration to define an alignment of the head in the physical and radiographic image space. In this study, we sought to determine the concordance among a point-merged FM registration, a point-merged AL registration, and a combined point-merged anatomic/surface-merged (SM) registration, i.e., to determine the accuracy of registration techniques with and without FMs by examining the extent of agreement between the system-generated predicted value and physical measured values.

Methods:

We examined 30 volunteers treated with gamma knife surgery. The frameless stereotactic image-guidance system called the StealthStation (Medtronic Surgical Navigation Technologies, Louisville, CO) was used. Nine FMs were placed on the patient's head and four were placed on a Leksell frame rod-box, which acted as a rigid set to determine the difference in error. For each registration form, we recorded the generated measurement (GM) and the physical measurement (PM) to each of the four checkpoint FMs. Bland and Altman plot difference analyses were used to compare measurement techniques. Correlations and descriptive analyses were completed.

Results:

The mean of values for GMs were 1.14 mm for FM, 2.3 mm for AL, and 0.96 mm for SM registrations. The mean errors of the checkpoints were 3.49 mm for FM, 3.96 mm for AL, and 3.33 mm for SM registrations. The correlation between GMs and PMs indicated a linear relationship for all three methods. AL registration demonstrated the greatest mean difference, followed by FM registration; SM registration had the smallest difference between GMs and PMs. Differences in the anatomic registration methods, including SM registration, compared with FM registration were within a mean ± 1.96 (standard deviation) according to the Bland and Altman analysis.

Conclusion:

For our sample of 30 patients, all three registration methods provided comparable distances to the target tissue for surgical procedures. Users may safely choose anatomic registration as a less costly and more time-efficient registration method for frameless stereotaxy.

Computer-aided navigation in neurosurgery

The article comprises three main parts: a historical review on navigation, the mathematical basics for calculation and the clinical applications of navigation devices. Main historical steps are described from the first idea till the realisation of the frame-based and frameless navigation devices including robots. In particular the idea of robots can be traced back to the Iliad of Homer, the first testimony of European literature over 2500 years ago. In the second part the mathematical calculation of the mapping between the navigation and the image space is demonstrated, including different registration modalities and error estimations. The error of the navigation has to be divided into the technical error of the device calculating its own position in space, the registration error due to inaccuracies in the calculation of the transformation matrix between the navigation and the image space, and the application error caused additionally by anatomical shift of the brain structures during operation. In the third part the main clinical fields of application in modern neurosurgery are demonstrated, such as localisation of small intracranial lesions, skull-base surgery, intracerebral biopsies, intracranial endoscopy, functional neurosurgery and spinal navigation. At the end of the article some possible objections to navigation-aided surgery are discussed.

Effect of changing patient position from supine to prone on the accuracy of a Brown-Roberts-Wells stereotactic head frame system

OBJECTIVE

Despite the growing popularity of frameless image-guided surgery systems, stereotactic frame systems are widely accepted by neurosurgeons and are commonly used to perform biopsies, functional procedures, and stereotactic radiosurgery. We investigated the accuracy of the Brown-Roberts-Wells stereotactic frame system when the mechanical load on the frame changes between preoperative imaging and the intervention because of different patient position: supine during imaging, prone during intervention.

METHODS

We analyzed computed tomographic images acquired from 14 patients who underwent stereotactic biopsy, deep brain stimulator implantation, or radiosurgery. Two images were acquired for each patient, one with the patient in the supine position and one in the prone position. The prone images were registered to the respective supine images by use of an intensity-based registration algorithm, once using only the frame and once using only the head. The difference between the transformations produced by these two registrations describes the movement of the patient's head with respect to the frame.

RESULTS

The maximum frame-based registration error between the supine and prone positions was 2.8 mm; it was more than 2 mm in two patients and more than 1.5 mm in six patients. Anteroposterior translation is the dominant component of the difference transformation for most patients. In general, the magnitude of the movement increased with brain volume, which is an index of head weight.

CONCLUSION

To minimize frame-based registration error caused by a change in the mechanical load on the frame, stereotactic procedures should be performed with the patient in the identical position during imaging and intervention.

Comparison of frameless stereotactic systems: accuracy, precision, and applications

OBJECTIVE

Frameless stereotactic systems have become an integral part of neurosurgical practice. At our center, we recently introduced for clinical use a small, portable, frameless stereotactic system, namely the Cygnus PFS system (Compass International, Rochester, MN). The purpose of this study was to compare the accuracy of the Cygnus PFS system with that of two larger systems that are also currently in use at our institution, i.e., the SMN system (Zeiss, Oberkochen, Germany) and the ISG viewing wand (ISG Technologies, Toronto, Canada). These systems represent three kinds of frameless stereotactic technologies that are commercially available. Each system uses a different method of spatial localization, i.e., mechanical linkage (ISG system), magnetic field digitization (Cygnus system), or optical technology (SMN system).

METHODS

Using a stereotactic "phantom," we measured the accuracies of all three systems with identical data sets. The errors in localization in three-dimensional space for nine targets were calculated by using 10 magnetic resonance imaging data sets. The precision of each system was also calculated.

RESULTS

With this experimental protocol, the Cygnus system attained a mean accuracy of 1.90 +/- 0.7 mm, the ISG viewing wand system a mean accuracy of 1.67 +/-0.43 mm, and the SMN microscope a mean accuracy of 2.61 +/- 0.99 mm. The precision values were not significantly different among the systems.

CONCLUSION

We observed only small differences in accuracy and precision among these three systems. We briefly review the advantages and disadvantages of each system and note that other factors, such as portability, ease of use, and microscope integration, should influence the selection of a frameless stereotactic system.

Navigation-guided opening of the internal auditory canal via the retrosigmoid route for acoustic neuroma surgery: cadaveric, radiological, and preliminary clinical study

OBJECTIVE

We investigated the usefulness of a microscope-based navigational system (Multi Koordinaten Manipulator; Zeiss, Oberkochen, Germany) for removal of the posterior wall of the internal auditory canal (IAC) via the retrosigmoid route.

METHODS

A cadaveric study was performed to assess the navigational localization error for the retrosigmoid approach to the IAC. Computed tomographic findings for 47 acoustic neuroma cases were divided into three groups, on the basis of the relationship between the labyrinth and the sigmoid-fundus line (medial, on the line, or lateral). Furthermore, the shortest distances between the most medial labyrinthine extension and the resection line were measured. In 20 acoustic neuroma operations, the different features and the practicality of the microscope-based navigational system for opening of the IAC were evaluated.

RESULTS

The mean anatomic localization errors were 0.67 +/- 0.2 mm (95th percentile, 1.32 mm) for navigation to the IAC and 0.71 +/- 0.37 mm (95th percentile, 1.68 mm) for navigation to the posterior semicircular canal. The average distances between the most medial labyrinthine extension and the resection line were 3.65, 3.36, and 2.0 mm for the lateral, on-the-line, and medial groups, respectively. Direct contouring of structures at risk does not take into account the localization error, nor does it provide reliable navigational information. A novel indirect contouring concept that takes into account the localization error (the safety corridor method) was therefore introduced.

CONCLUSION

The value of navigational assistance for opening of the IAC is promising but still limited. Further development is required before the clinical effects of this navigational approach can be evaluated.

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.

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.

Accuracy of true frameless stereotaxy: in vivo measurement and laboratory phantom studies. Technical note

The authors present the results of accuracy measurements, obtained in both laboratory phantom studies and an in vivo assessment, for a technique of frameless stereotaxy. An instrument holder was developed to facilitate stereotactic guidance and enable introduction of frameless methods to traditional frame-based procedures. The accuracy of frameless stereotaxy was assessed for images acquired using 0.5-tesla or 1.5-tesla magnetic resonance (MR) imaging or 2-mm axial, 3-mm axial, or 3-mm helical computerized tomography (CT) scanning. A clinical series is reported in which biopsy samples were obtained using a frameless stereotactic procedure, and the accuracy of these procedures was assessed using postoperative MR images and image fusion. The overall mean error of phantom frameless stereotaxy was found to be 1.3 mm (standard deviation [SD] 0.6 mm). The mean error for CT-directed frameless stereotaxy was 1.1 mm (SD 0.5 mm) and that for MR image-directed procedures was 1.4 mm (SD 0.7 mm). The CT-guided frameless stereotaxy was significantly more accurate than MR image-directed stereotaxy (p = 0.0001). In addition, 2-mm axial CT-guided stereotaxy was significantly more accurate than 3-mm axial CT-guided stereotaxy (p = 0.025). In the clinical series of 21 frameless stereotactically obtained biopsies, all specimens yielded the appropriate diagnosis and no complications ensued. Early postoperative MR images were obtained in 16 of these cases and displacement of the biopsy site from the intraoperative target was determined by fusion of pre- and postoperative image data sets. The mean in vivo linear error of frameless stereotactic biopsy sampling was 2.3 mm (SD 1.9 mm). The mean in vivo Euclidean error was 4.8 mm (SD 2 mm). The implications of these accuracy measurements and of error in stereotaxy are discussed.

Validation of object-induced MR distortion correction for frameless stereotactic neurosurgery

Spatial fidelity is a paramount issue in image guided neurosurgery. Until recently, three-dimensional computed tomography (3D CT) has been the primary modality because it provides fast volume capture with pixel level (1 mm) accuracy. While three-dimensional magnetic resonance (3D MR) images provide superior anatomic information, published image capture protocols are time consuming and result in scanner- and object-induced magnetic field inhomogeneities which raise inaccuracy above pixel size. Using available scanner calibration software, a volumetric algorithm to correct for object-based geometric distortion, and a Fast Low Angle SHot (FLASH) 3D MR-scan protocol, we were able to reduce mean CT to MR skin-adhesed fiducial marker registration error from 1.36 to 1.09 mm. After dropping the worst one or two of six fiducial markers, mean registration error dropped to 0.62 mm (subpixel accuracy). Three dimensional object-induced error maps present highest 3D MR spatial infidelity at the tissue interfaces (skin/air, scalp/skull) where frameless stereotactic fiducial markers are commonly applied. The algorithm produced similar results in two patient 3D MR-scans.

Optimized magnetic resonance image-guided stereotaxis: a technique with validation based on the anterior commissure-posterior commissure line

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 venticulogram 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. Our technique uses MR sequences that minimize geometric distortion and registration error, thereby maximizing accuracy in AC-PC line determinations from axially displayed MR data. The techniques are based on our 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 equivalent or superior to results reported using microelectrode recordings. The measurements of the AC-PC line reported here compare favorably with those based on ventriculography and computerized tomography previously reported. 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.