Frameless stereotactic ultrasonography: method and applications

Serial intraoperative magnetic resonance imaging of brain shift

OBJECTIVE

A major shortcoming of image-guided navigational systems is the use of preoperatively acquired image data, which does not account for intraoperative changes in brain morphology. The occurrence of these surgically induced volumetric deformations ("brain shift") has been well established. Maximal measurements for surface and midline shifts have been reported. There has been no detailed analysis, however, of the changes that occur during surgery. The use of intraoperative magnetic resonance imaging provides a unique opportunity to obtain serial image data and characterize the time course of brain deformations during surgery.

METHODS

The vertically open intraoperative magnetic resonance imaging system (SignaSP, 0.5 T; GE Medical Systems, Milwaukee, WI) permits access to the surgical field and allows multiple intraoperative image updates without the need to move the patient. We developed volumetric display software (the 3D Slicer) that allows quantitative analysis of the degree and direction of brain shift. For 25 patients, four or more intraoperative volumetric image acquisitions were extensively evaluated.

RESULTS

Serial acquisitions allow comprehensive sequential descriptions of the direction and magnitude of intraoperative deformations. Brain shift occurs at various surgical stages and in different regions. Surface shift occurs throughout surgery and is mainly attributable to gravity. Subsurface shift occurs during resection and involves collapse of the resection cavity and intraparenchymal changes that are difficult to model.

CONCLUSION

Brain shift is a continuous dynamic process that evolves differently in distinct brain regions. Therefore, only serial imaging or continuous data acquisition can provide consistently accurate image guidance. Furthermore, only serial intraoperative magnetic resonance imaging provides an accurate basis for the computational analysis of brain deformations, which might lead to an understanding and eventual simulation of brain shift for intraoperative guidance.

Novel, compact, intraoperative magnetic resonance imaging-guided system for conventional neurosurgical operating rooms

OBJECTIVE

Preliminary clinical experience with a novel, compact, intraoperative magnetic resonance imaging (MRI)-guided system that can be used in an ordinary operating room is presented.

DESCRIPTION OF INSTRUMENTATION

The system features an MRI scanner integrated with an optical and MRI tracking system. Scanning and navigation, which are operated by the surgeon, are controlled by an in-room computer workstation with a liquid crystal display screen. The scanner includes a 0.12-T permanent magnet with a 25-cm vertical gap, accommodating the patient's head. The field of view is 11 x 16 cm, encompassing the surgical area of interest. The magnet is mounted on a transportable gantry that can be positioned under the surgical table when not in use for scanning, thus rendering the surgical environment unmodified and allowing the use of standard instruments. The features of the integrated navigation system allow flap planning and intraoperative tracking based on updated images acquired during surgery.

OPERATIVE TECHNIQUE

Twenty patients with brain tumors were surgically treated using craniotomy or trans-sphenoidal approaches. One patient underwent conscious craniotomy with cortical mapping, and two underwent electrocorticography.

EXPERIENCE AND RESULTS

Planning was accurate. Resection control images were obtained for all patients during surgery, with precise localization of residual tumor tissue. There were no surgical complications related to the use of the system.

CONCLUSION

This intraoperative MRI system can function in a normal operating room modified only to eliminate radiofrequency interference. The operative environment is normal, and standard instruments can be used. The scanning and navigation capabilities of the system eliminate the inaccuracies that may result from brain shift. This novel type of intraoperative MRI system represents another step toward the introduction of the modality as a standard method in neurosurgery.

In Vivo Analysis of Heterogeneous Brain Deformation Computations for Model-Updated Image Guidance

Neurosurgical image-guidance has historically relied on the registration of the patient and preoperative imaging series with surgical instruments in the operating room (OR) coordinate space. Recent studies measuring intraoperative tissue motion have suggested that deformation-induced misregistration from surgical loading is a serious concern with such systems. In an effort to improve registration fidelity during surgery, we are pursuing an approach which uses a predictive computational model in conjunction with data available in the OR to update the high resolution preoperative image series. In previous work, we have developed an in vivo experimental system in the porcine brain which has been used to investigate a homogeneous finite element rendering of consolidation theory as a tissue deformation model. In this paper, our computational approach has been extended to include heterogeneous tissue property distributions determined from an image-to-grid segmentation scheme. Results produced under two different loading conditions show that heterogeneity in the stiffness properties and interstitial pressure gradients varied over a range of physiologically reasonable values account for 1-3% and 5-8% of the predicted tissue motion, respectively, while homogeneous linear elasticity is responsible for 60-70% of the surgically-induced motion that has been recoverable with our model-based approach.

Quantification of, visualization of, and compensation for brain shift using intraoperative magnetic resonance imaging

OBJECTIVE

Modern neuronavigation systems lack spatial accuracy during ongoing surgical procedures because of increasing brain deformation, known as brain shift. Intraoperative magnetic resonance imaging was used for quantitative analysis and visualization of this phenomenon.

METHODS

For a total of 64 patients, we used a 0.2-T, open-configuration, magnetic resonance imaging scanner, located in an operating theater, for pre- and intraoperative imaging. The three-dimensional imaging data were aligned using rigid registration methods. The maximal displacements of the brain surface, deep tumor margin, and midline structures were measured. Brain shift was observed in two-dimensional image planes using split-screen or overlay techniques, and three-dimensional, color-coded, deformable surface-based data were computed. In selected cases, intraoperative images were transferred to the neuronavigation system to compensate for the effects of brain shift.

RESULTS

The results demonstrated that there was great variability in brain shift, ranging up to 24 mm for cortical displacement and exceeding 3 mm for the deep tumor margin in 66% of all cases. Brain shift was influenced by tissue characteristics, intraoperative patient positioning, opening of the ventricular system, craniotomy size, and resected volume. Intraoperative neuronavigation updating (n = 14) compensated for brain shift, resulting in reliable navigation with high accuracy.

CONCLUSION

Without brain shift compensation, neuronavigation systems cannot be trusted at critical steps of the surgical procedure, e.g., identification of the deep tumor margin. Intraoperative imaging allows not only evaluation of and compensation for brain shift but also assessment of the quality of mathematical models that attempt to describe and compensate for brain shift.

Three-dimensional optical flow method for measurement of volumetric brain deformation from intraoperative MR images

A three-dimensional optical flow method to measure volumetric brain deformation from sequential intraoperative MR images and preliminary clinical results from five cases are reported. Intraoperative MR images were scanned before and after dura opening, twice during tumor resection, and immediately after dura closure. The maximum cortical surface shift measured was 11 mm and subsurface shift was 4 mm. The computed deformation field was most satisfactory when the skin was segmented and removed from the images before the optical flow computation.

Three-dimensional ultrasound imaging of the rotator cuff: spatial compounding and tendon thickness measurement

Three-dimensional (3-D) volume reconstructions of the shoulder rotator cuff were generated from freehand ultrasound (US) scans acquired with a magnetic tracking system. Image stacks acquired with lateral overlap from multiple acoustic windows were spatially compounded to provide an extended representation of the rotator cuff tendons. A semiautomated technique was developed for measuring rotator cuff thickness from the 3-D compound volumes. Scans of phantoms and volunteer subjects were used to evaluate the accuracy and repeatability of the thickness measurements. For an in vitro phantom with known thickness, the mean difference between the true value and the automatic measurements was 0.05 +/- 0.28 mm. Thickness measurements made manually from 2-D images and automatically from 3-D volumes were different by 0.03 +/- 0.44 mm in vitro and -0.06 +/- 0.36 in vivo. Repeated thickness measurements in vivo differed by 0.06 +/- 0.36 mm. The 3-D measurement technique offers a promising method for evaluating rotator cuff tendons.

Intraoperative ultrasound for guidance and tissue shift correction in image-guided neurosurgery

We present a surgical guidance system that incorporates pre-operative image information (e.g., MRI) with intraoperative ultrasound (US) imaging to detect and correct for brain tissue deformation during image-guided neurosurgery (IGNS). Many interactive IGNS implementations employ pre-operative images as a guide to the surgeons throughout the procedure. However, when a craniotomy is involved, tissue movement during a procedure can be a significant source of error in these systems. By incorporating intraoperative US imaging, the target volume can be scanned at any time, and two-dimensional US images may be compared directly to the corresponding slice from the pre-operative image. Homologous points may be mapped from the intraoperative to the pre-operative image space with an accuracy of better than 2 mm, enabling the surgeon to use this information to assess the accuracy of the guidance system along with the progress of the procedure (e.g., extent of lesion removal) at any time during the operation. Anatomical features may be identified on both the pre-operative and intraoperative images and used to generate a deformation map, which can be used to warp the pre-operative image to match the intraoperative US image. System validation is achieved using a deformable multi-modality imaging phantom, and preliminary clinical results are presented.

In vivo quantification of a homogeneous brain deformation model for updating preoperative images during surgery

Clinicians using image-guidance for neurosurgical procedures have recently recognized that intraoperative deformation from surgical loading can compromise the accuracy of patient registration in the operating room. While whole brain intraoperative imaging is conceptually appealing it presents significant practical limitations. Alternatively, a promising approach may be to combine incomplete intraoperatively acquired data with a computational model of brain deformation to update high resolution preoperative images during surgery. The success of such an approach is critically dependent on identifying a valid model of brain deformation physics. Towards this end, we evaluate a three-dimensional finite element consolidation theory model for predicting brain deformation in vivo through a series of controlled repeat-experiments. This database is used to construct an interstitial pressure boundary condition calibration curve which is prospectively tested in a fourth validation experiment. The computational model is found to recover 75%-85% of brain motion occurring under loads comparable to clinical conditions. Additionally, the updating of preoperative images using the model calculations is presented and demonstrates that model-updated image-guided neurosurgery may be a viable option for addressing registration errors related to intraoperative tissue motion.

Endoscopic sinus surgery using intraoperative computed tomography imaging for updating a three-dimensional navigation system

OBJECTIVES

The use of three-dimensional navigation systems provides information on the structures surrounding the field of operation and thereby reduces the risk of iatrogenic damage. The computed tomography (CT) data conventionally used are provided by preoperative scanning procedures, which means that tissue changes coming about during surgery are not seen on the screen. An intraoperative CT scanning procedure being able to update the CT data could provide a solution.

STUDY DESIGN

Endoscopic sinus operations using an intraoperative CT updating the three-dimensional navigation system were performed on six persons to find out, whether the above is true.

METHODS

Different parameters, advantages, and disadvantages in the cases of these six patients were compared with a group of 22 patients who underwent conventional endoscopic sinus surgery with different three-dimensional navigation systems without updating the CT data set.

RESULTS

The intraoperative CT for updating the three-dimensional navigation system provides useful information for the surgeon.

CONCLUSION

Balancing its advantages against its disadvantages, the updating of the CT data set with intraoperative CT cannot be recommended for conventional standard endoscopic sinus surgery.

Three-dimensional spatial compounding of ultrasound scans with weighting by incidence angle

A three-dimensional (3D) ultrasound imaging system has been used to study spatial compounding of images acquired with different scanhead positions and orientations. A compounding algorithm has been developed that assigns regional weights depending on the local incidence angle of the ultrasound beam. Compound scans were performed of bones in vitro and the shoulder rotator cuff in volunteer subjects. Border measurements (peak value and width) were compiled as a function of ultrasound beam incidence angle and compared for single views and for maximum, mean and weighted mean compounding techniques. The weighted mean produces less variability than that of the maximum and mean for both intensity and border width. The weighted method also demonstrates less blurring of borders than the maximum and mean methods. Surfaces derived from the weighted reconstructions exhibited fewer gaps and fewer spurious connections between surfaces, which could be of particular importance for automated image analysis.

Interactive 3-D registration of ultrasound and magnetic resonance images based on a magnetic position sensor

The use of stereotactic systems has been one of the main approaches for image-based guidance of the surgical tool within the brain. The main limitation of stereotactic systems is that they are based on preoperative images that might become outdated and invalid during the course of surgery. Ultrasound (US) is considered the most practical and cost-effective intraoperative imaging modality, but US images inherently have a low signal-to-noise ratio. Integrating intraoperative US with stereotactic systems has recently been attempted. In this paper, we present a new system for interactively registering two-dimensional US and three-dimensional magnetic resonance (MR) images. This registration is based on tracking the US probe with a dc magnetic position sensor. We have performed an extensive analysis of the errors of our system by using a custom-built phantom. The registration error between the MR and the position sensor space was found to have a mean value of 1.78 mm and a standard deviation of 0.18 mm. The registration error between US and MR space was dependent on the distance of the target point from the US probe face. For a 3.5-MHz phased one-dimensional array transducer and a depth of 6 cm, the mean value of the registration error was 2.00 mm and the standard deviation was 0.75 mm. The registered MR images were reconstructed using either zeroth-order or first-order interpolation. The ease of use and the interactive nature of our system (approximately 6.5 frames/s for 344 x 310 images and first-order interpolation on a Pentium II 450 MHz) demonstrates its potential to be used in the operating room.

Model-updated image guidance: initial clinical experiences with gravity-induced brain deformation

Image-guided neurosurgery relies on accurate registration of the patient, the preoperative image series, and the surgical instruments in the same coordinate space. Recent clinical reports have documented the magnitude of gravity-induced brain deformation in the operating room and suggest these levels of tissue motion may compromise the integrity of such systems. We are investigating a model-based strategy which exploits the wealth of readily-available preoperative information in conjunction with intraoperatively acquired data to construct and drive a three dimensional (3-D) computational model which estimates volumetric displacements in order to update the neuronavigational image set. Using model calculations, the preoperative image database can be deformed to generate a more accurate representation of the surgical focus during an operation. In this paper, we present a preliminary study of four patients that experienced substantial brain deformation from gravity and correlate cortical shift measurements with model predictions. Additionally, we illustrate our image deforming algorithm and demonstrate that preoperative image resolution is maintained. Results over the four cases show that the brain shifted, on average, 5.7 mm in the direction of gravity and that model predictions could reduce this misregistration error to an average of 1.2 mm.

Model-Updated Image-Guided Neurosurgery Using the Finite Element Method: Incorporation of the Falx Cerebri

Surgeons using neuronavigation have realized the value of image guidance for feature recognition as well as for the precise application of surgical instruments. Recently, there has been a growing concern about the extent of intraoperative misregistration due to tissue deformation. Intraoperative imaging is currently under evaluation but limitations related to cost effectiveness and image clarity have made its wide spread adoption uncertain. As a result, computational model-guided techniques have generated considerable appeal as an alternative approach. In this paper, we report our initial experience with enhancing our brain deformation model by explicitly adding the falx cerebri. The simulations reported show significant differences in subsurface deformation with the falx serving to damp the communication of displacement between hemispheres by as much as 4 mm. Additionally, these calculations, based on a human clinical case, demonstrate that while cortical shift predictions correlate well with various forms of the model (70-80% of surface motion recaptured), substantial differences in subsurface deformation occurs suggesting that subsurface validation of model-guided techniques will be important for advancing this concept.

A low-cost PCI-bus-based ultrasound system for use in image-guided neurosurgery

A low-cost PCI-bus-based ultrasound sub-system has been developed and integrated into the image-guided neurosurgery system currently in use at the Cleveland Clinic. Two software applications have been developed that integrate real-time ultrasound images with preoperative MR and CT data sets. By tracking the position of the ultrasound probe during surgery, it is possible to display a real time ultrasound image and the corresponding (preoperative) oblique CT or MR slice. This provides immediate positional feedback to the neurosurgeon during the surgical procedure.

Intraoperative imaging techniques in the treatment of brain tumors

Despite significant advances in medical imaging techniques and their routine preoperative use, real-time intraoperative information regarding anatomy remains of indisputable importance to neurosurgeons. Intraoperative displacement of the brain tissue caused by surgical retraction or the resection cavity itself, as well as shift caused by cerebrospinal fluid leakage, may result in alteration of the surgical anatomy of the lesion and surrounding structures. Neurosurgical navigation methods are beneficial in providing accurate intraoperative information regarding the anatomy of the surgical field. Furthermore, interactive image guidance may decrease incision lengths, operating times, and postoperative morbidity. This review focuses on recent developments in neurosurgical navigational techniques that enable real-time anatomic visualization during brain tumor surgery.

Intraoperative brain shift and deformation: a quantitative analysis of cortical displacement in 28 cases

OBJECTIVE

A quantitative analysis of intraoperative cortical shift and deformation was performed to gain a better understanding of the nature and extent of this problem and the resultant loss of spatial accuracy in surgical procedures coregistered to preoperative imaging studies.

METHODS

Three-dimensional feature tracking and two-dimensional image analysis of the cortical surface were used to quantify the observed motion. Data acquisition was facilitated by a ceiling-mounted robotic platform, which provided a number of precision tracking capabilities. The patient's head position and the size and orientation of the craniotomy were recorded at the start of surgery. Error analysis demonstrated that the surface displacement measuring methodology was accurate to 1 to 2 mm. Statistical tests were performed to examine correlations between the amount of displacement and the type of surgery, the nature of the cranial opening, the region of the brain involved, the duration of surgery, and the degree of invasiveness.

RESULTS

The results showed that a displacement of an average of 1 cm occurred, with the dominant directional component being associated with gravity. The mean displacement was determined to be independent of the size and orientation of the cranial opening.

CONCLUSION

These data suggest that loss of spatial registration with preoperative images is gravity-dominated and of sufficient extent that attention to errors resulting from misregistration during the course of surgery is warranted.

Computed intraoperative navigation guidance--a preliminary report on a new technique

OBJECTIVE

To assess the value of a computer-assisted three-dimensional guidance system (Virtual Patient System) in maxillofacial operations.

DESIGN

Laboratory and open clinical study.

SETTING

Teaching Hospital, Austria.

SUBJECTS

6 patients undergoing various procedures including removal of foreign body (n=3) and biopsy, maxillary advancement, and insertion of implants (n=1 each).

INTERVENTIONS

Storage of computed tomographic (CT) pictures on an optical disc, and imposition of intraoperative video images on to these. The resulting display is shown to the surgeon on a micromonitor in his head-up display for guidance during the operations.

MAIN OUTCOME MEASURES

To improve orientation during complex or minimally invasive maxillofacial procedures and to make such operations easier and less traumatic.

RESULTS

Successful transferral of computed navigation technology into an operation room environment and positive evaluation of the method by the surgeons involved.

CONCLUSIONS

Computer-assisted three-dimensional guidance systems have the potential for making complex or minimally invasive procedures easier to do, thereby reducing postoperative morbidity.

Performance of a miniature magnetic position sensor for three-dimensional ultrasound imaging

A miniature magnetic position sensor used for three-dimensional ultrasound imaging was tested for precision and accuracy in vitro. The sensor alone was able to locate points with root-mean-square (rms) uncertainty of 1.7 mm and accuracy of 0.05 +/- 0.62 mm over its specified operating range of 50 cm. With an ultrasound imaging system, a point was located from arbitrary viewing windows with 2.4-mm rms uncertainty and 0.06 +/- 0.68 mm accuracy. If viewing windows were limited to those representative of a typical ultrasound examination, the system could achieve rms uncertainty in point location of < 1 mm. Performance was not affected by operation of the imaging system when the sensor was mounted on an ultrasound scanhead. Sensitivity to metals in the operating environment was also measured.

Use of a biocompatible fiducial marker in evaluating the accuracy of computed tomography image registration

RATIONALE AND OBJECTIVES

Accurate registration of computed tomography (CT) images to the patient is crucial for computer-assisted surgery. Markers used in roentgen stereo-grammetric analysis (RSA) can be located in a CT scan using a novel approach and also can be located physically. Roentgen stereo-grammetric analysis data act as "ground truth" for the three-dimensional marker locations.

METHODS

Two foam-bone phantoms were marked. The markers were scanned seven times with RSA, three times with axial CT, and contacted four times with a coordinate measuring arm. Root-mean-square (RMS) errors were derived for the registrations.

RESULTS

Computed tomography and RSA data register to 0.15 mm RMS error. Computed tomography and arm data register to 0.25 mm. The markers are biocompatible, and the coordinate measuring arm is usable in an operating room.

CONCLUSIONS

Typical in vitro registration errors are approximately 2 mm. The authors have developed a marker that provides superior registration, and a procedure that can be used for in vivo studies.

Three-dimensional imaging with stereotactic ultrasonography

Stereotactic ultrasonography is a technique for determining the position and orientation of B-mode ultrasound images in a reference coordinate system. A technique for constructing three-dimensional (3D) image volumes has been developed that uses this new technology. Given several registered images, a 3D volume is constructed either by a "nearest-neighbor" or a "closest-points" interpolation approach. The resulting volume can be rendered using 3D rendering software. In addition, the voxels in the volume are at known positions allowing determination of position for structures in the volume. Results are shown for various test cases, and applicability to medical imaging applications and stereotactic neurosurgery is discussed.