Computer-assisted surgical planning for cerebrovascular neurosurgery

Investigation of blood flow in the external carotid artery and its branches with a new 0D peripheral model

BACKGROUND

Patient-specific modelling in clinical studies requires a realistic simulation to be performed within a reasonable computational time. The aim of this study was to develop simple but realistic outflow boundary conditions for patient-specific blood flow simulation which can be used to clarify the distribution of the anticancer agent in intra-arterial chemotherapy for oral cancer.

METHODS

In this study, the boundary conditions are expressed as a zero dimension (0D) resistance model of the peripheral vessel network based on the fractal characteristics of branching arteries combined with knowledge of the circulatory system and the energy minimization principle. This resistance model was applied to four patient-specific blood flow simulations at the region where the common carotid artery bifurcates into the internal and external carotid arteries.

RESULTS

Results of these simulations with the proposed boundary conditions were compared with the results of ultrasound measurements for the same patients. The pressure was found to be within the physiological range. The difference in velocity in the superficial temporal artery results in an error of 5.21 ± 0.78 % between the numerical results and the measurement data.

CONCLUSIONS

The proposed outflow boundary conditions, therefore, constitute a simple resistance-based model and can be used for performing accurate simulations with commercial fluid dynamics software.

Neuronavigation in the surgical management of brain tumors: current and future trends

Neuronavigation has become an ubiquitous tool in the surgical management of brain tumors. This review describes the use and limitations of current neuronavigational systems for brain tumor biopsy and resection. Methods for integrating intraoperative imaging into neuronavigational datasets developed to address the diminishing accuracy of positional information that occurs over the course of brain tumor resection are discussed. In addition, the process of integration of functional MRI and tractography into navigational models is reviewed. Finally, emerging concepts and future challenges relating to the development and implementation of experimental imaging technologies in the navigational environment are explored.

Magnetic resonance angiography image guidance for the microsurgical clipping of intracranial aneurysms: a report of two cases

To describe the integration of magnetic resonance angiography (MRA) in neuronavigation procedures for microsurgery of intracranial aneurysms. MRA was combined with standard magnetic resonance image (MRI) acquisition in the image-guided planning for the microsurgical clipping of a saccular aneurysm in two patients (one 3-mm large middle cerebral artery and one 8-mm large pericallosal artery aneurysm, diagnosed by catheter angiography in both patients) using two different neurosurgical navigation systems. Conventional 3-D T1-weighted MRI with gadolinium and MRA pulse sequences were acquired in frameless stereotactic conditions the day before surgery and thereafter registered, allowing the definition a minimally invasive straight trajectory to the aneurysm neck. MRA-guided neurosurgery allowed a direct approach to the aneurysms at their proper location, reducing the invasiveness of the approach by tailoring the bone opening and reducing the duration and extension of brain retraction. The technique also avoided unnecessary dissection and exposure of the main trunks and collateral vessels. The aneurysms were successfully eradicated without complication. Integration of MRA in the planning and neuronavigation procedure for intracranial aneurysms may minimize the morbidity related to the surgical approach. This technique may be applicable more routinely using standard neuronavigation equipment.

Cortical vessel imaging and visualization for image guided depth electrode insertion

To avoid intracranial hemorrhage during minimally invasive depth electrode insertion without craniotomy for epilepsy surgery, precise in vivo imaging of cortical vessel and relevant rendering methods are critical, and should be used in preoperative planning. In this study, a non-invasive phase contrast MR angiography (PC-MRA) method was chosen for cortical vessel imaging. After image pre-processing (registration and segmentation), three visualization methods were implemented to optimize the vessel imaging and brain tissue rendering for surgical planning. The processed results were evaluated by comparing with intraoperative photographs. The results showed occurrences of missing vessels between imaging and photos (18.3%, 6 cases), but these could be compensated by realistic sulci visualization methods. The results showed 3D texture mapping to be the most suitable cortex visualization method for use in surgical navigation. Based on the methods and evaluations, a new surgical planning system and criteria of usage were developed with input from the surgeons' experience using the prototype system. This system could greatly help reduce the risk of the intracranial hemorrhage during electrode insertion and also avoid potential risks caused by contrast agent injections for contrast enhanced MRA or CTA.

Three-dimensional reconstructed computed tomography-magnetic resonance fusion image-based preoperative planning for surgical procedures for spinal lipoma or tethered spinal cord after myelomeningocele repair

Surgical procedures for spinal lipoma or tethered spinal cord after myelomeningocele (MMC) repair are often difficult and complicated, because the anatomical structures can be deformed in complex and unpredictable ways. Imaging helps the surgeon understand the patient's spinal anatomy. Whereas two-dimensional images provide only limited information for surgical planning, three-dimensional (3D) reconstructed computed tomography (CT)-magnetic resonance (MR) fusion images produce clearer representations of the spinal regions. Here we describe simple and quick methods for obtaining 3D reconstructed CT-MR fusion images for preoperative planning of surgical procedures using the iPlan(®) cranial (BrainLAB AG, Feldkirchen, Germany) neuronavigation software. 3D CT images of the vertebral bone were combined with heavily T(2)-weighted MR images of the spinal cord, lipoma, cerebrospinal fluid (CSF) space, and nerve root through a process of fusion, segmentation, and reconstruction of the 3D images. We also used our procedure called "Image Overlay" to directly project the 3D reconstructed image onto the body surface using an LED projector. The final reconstructed 3D images took 10-30 minutes to obtain, and provided the surgeon with a representation of the individual pathological structures, so enabled the design of effective surgical plans, even in patients with bony deformity such as scoliosis. None of the 19 patients treated based on our 3D reconstruction method has had neurological complications, except for CSF leakage. This 3D reconstructed imaging method, combined with Image Overlay, improves the visual understanding of complicated surgical situations, and should improve surgical efficiency and outcome.

Benefits and limitations of image guidance in the surgical treatment of intracranial dural arteriovenous fistulas

BACKGROUND

Despite major advances in endovascular embolization techniques, microsurgical resection remains a reliable and effective treatment modality for dural arteriovenous fistulas (DAVF). However, intraoperative detection of these lesions and identification of feeding arteries and draining veins can be challenging. In a series of 6 patients who were not candidates for definitive treatment by endovascular embolization we evaluated the benefits and limitations of computer-assisted image guidance for surgical ablation of DAVF.

METHODS

Of the 6 patients, 5 presented with haemorrhage and one with seizures. Diagnosis of DAVF was made by conventional angiography and dynamic contrast enhanced MR angiography (CE-MRA). All patients were surgically treated with the assistance of a 3D high resolution T1-weighted MR data set and time-of-flight MR angiography (MRA) obtained for neuronavigation. Registration was based on cranial fiducials and image-guided surgery was performed with the navigation system.

FINDINGS

Four of the 6 patients suffered from DAVF draining into the superior sagittal sinus, one fistula drained into paracavernous veins adjacent to the superior petrosal sinus and one patient had a pial fistula draining in the straight sinus. DAVF diagnosed with conventional angiography could be located on CE-MRA and MRA prior to surgery. MRI and MRA images were combined on the neuronavigation workstation and DAVF were located intraoperatively by using a tracking device. In 4 out of 6 cases neuronavigation was used for direct intraoperative identification of DAVF. Brain shift prevented direct tracking of pathological vessels in the other 2 cases, where navigation could only be used to assist craniotomy. Microsurgical dissection and coagulation of the fistulas led to complete cure in all patients as confirmed by angiography.

CONCLUSIONS

Neuronavigation may be used as an additional tool for microsurgical treatment of DAVF. However, in this small series of 6 cases, surgical procedures have not been substantially altered by the use of the neuronavigation system. Image guidance has been beneficial for the location of small, superficially located DAVF, whereas a navigated approach to deep-seated lesions was less accurate due to the familiar problem of brain shift and brain retraction during surgery.

Multimodal image fusion in ultrasound-based neuronavigation: improving overview and interpretation by integrating preoperative MRI with intraoperative 3D ultrasound

OBJECTIVE

We have investigated alternative ways to integrate intraoperative 3D ultrasound images and preoperative MR images in the same 3D scene for visualizing brain shift and improving overview and interpretation in ultrasound-based neuronavigation.

MATERIALS AND METHODS

A Multi-Modal Volume Visualizer (MMVV) was developed that can read data exported from the SonoWand neuronavigation system and reconstruct the spatial relationship between the volumes available at any given time during an operation, thus enabling the exploration of new ways to fuse pre- and intraoperative data for planning, guidance and therapy control. In addition, the mismatch between MRI volumes registered to the patient and intraoperative ultrasound acquired from the dura was qualified.

RESULTS

The results show that image fusion of intraoperative ultrasound images in combination with preoperative MRI will make perception of available information easier by providing updated (real-time) image information and an extended overview of the operating field during surgery. This approach will assess the degree of anatomical changes during surgery and give the surgeon an understanding of how identical structures are imaged using the different imaging modalities. The present study showed that in 50% of the cases there were indications of brain shift even before the surgical procedure had started.

CONCLUSIONS

We believe that image fusion between intraoperative 3D ultrasound and preoperative MRI might improve the quality of the surgical procedure and hence also improve the patient outcome.

3D volume segmentation of MRA data sets using level sets: image processing and display

In this article, we use a level set-based segmentation algorithm to extract the vascular tree from magnetic resonance angiography (MRA) data sets. The classification approach depends on initializing the level sets in the 3D volume, and the level sets evolve with time to yield the blood vessels. This work introduces a high-quality initialization for the level set functions, allowing extraction of the blood vessels in 3D and elimination of non-vessel tissues. A comparison between the 2D and 3D segmentation approaches is made. The results are validated using a phantom that simulates the MRA data and show good accuracy.

Role of frameless stereotaxy in the surgical treatment of cerebral arteriovenous malformations: technique and outcomes in a controlled study of 44 consecutive patients

OBJECTIVE

To describe a frameless stereotactic technique used to resect cerebral arteriovenous malformations (AVMs) and to determine whether frameless stereotaxy during AVM resection could decrease operative times, minimize intraoperative blood losses, reduce postoperative complications, and improve surgical outcomes.

METHODS

Data for 44 consecutive patients with surgically resected cerebral AVMs were retrospectively reviewed. The first 22 patients underwent resection without stereotaxy (Group 1), whereas the next 22 patients underwent resection with the assistance of a frameless stereotaxy system (Group 2).

RESULTS

The patient characteristics, AVM morphological features, and percentages of preoperatively embolized cases were statistically similar for the two treatment groups. The mean operative time for Group 1 was 497 minutes, compared with 290 minutes for Group 2 (P = 0.0005). The estimated blood loss for Group 1 was 657 ml, compared with 311 ml for Group 2 (P = 0.0008). Complication rates, residual AVM incidences, and clinical outcomes were similar for the two groups.

CONCLUSION

Frameless stereotaxy allows surgeons to 1) plan the optimal trajectory to an AVM, 2) minimize the skin incision and craniotomy sizes, and 3) confirm the AVM margins and identify deep vascular components during resection. These benefits of stereotaxy were most apparent for small, deep AVMs that were not visible on the surface of the brain. Frameless stereotaxy reduces the operative time and blood loss during AVM resection.

Automated segmentation of MR images of brain tumors

An automated brain tumor segmentation method was developed and validated against manual segmentation with three-dimensional magnetic resonance images in 20 patients with meningiomas and low-grade gliomas. The automated method (operator time, 5-10 minutes) allowed rapid identification of brain and tumor tissue with an accuracy and reproducibility comparable to those of manual segmentation (operator time, 3-5 hours), making automated segmentation practical for low-grade gliomas and meningiomas.

Depth-buffer targeting for spatially accurate 3-D visualization of medical images

During interactive image-guided surgery (IIGS), a surgeon uses data from medical images to help guide the surgical procedure. At Vanderbilt University, an IIGS software system called Orion has been developed which is capable of displaying up to four 512 x 512 images and the current surgical position using an active optical tracking system. Orion is capable of displaying data from any tomographic image volume and from any NTSC video image. An additional display module has been implemented to display three-dimensional information as well as the tomographic slices. This provides the surgeon with valuable anatomical information that is not readily obtained from the tomographic slices alone. Before the surgery, a set of rendered images is created, each with a different angular view of the tomographic volume in order to surround the site of surgical interest. The major objectives of the display module are to display the appropriate rendered image from the set, identify the current probe position on the selected image, and provide an indication of distance between the probe and the physical point of the anatomy indicated on the image. This can provide the surgeon with vital information such as distance to blood vessels, tumors, or other critical structures.

Gray-scale skeletonization of small vessels in magnetic resonance angiography

Interpretation of magnetic resonance angiography (MRA) is problematic due to complexities of vascular shape and to artifacts such as the partial volume effect. We present new methods to assist in the interpretation of MRA. These include methods for detection of vessel paths and for determination of branching patterns of vascular trees. They are based on the ordered region growing (ORG) algorithm that represents the image as an acyclic graph, which can be reduced to a skeleton by specifying vessel endpoints or by a pruning process. Ambiguities in the vessel branching due to vessel overlap are effectively resolved by heuristic methods that incorporate a priori knowledge of bifurcation spacing. Vessel paths are detected at interactive speeds on a 500-MHz processor using vessel endpoints. These methods apply best to smaller vessels where the image intensity peaks at the center of the lumen which, for the abdominal MRA, includes vessels whose diameter is less than 1 cm.

Computer-assisted resection of cerebral arteriovenous malformations

OBJECTIVE

A series of 22 patients with arteriovenous malformations (AVMs) were surgically treated using computer-assisted image guidance. The value of image guidance for nidus definition and detection of feeding arteries and draining veins was assessed.

METHODS

Seven of the 22 patients presented with hemorrhage. The sizes of the AVMs ranged from 1 to 8 cm. Six patients underwent preoperative embolization. For 18 patients (81.8%), the AVMs were located in highly eloquent areas. A passive-marker-based neuronavigation system (BrainLab, Heimstetten, Germany) was used for intraoperative image guidance. Segmentation of the pathological vessels was performed preoperatively, on the basis of 2-mm helical computed tomographic angiographic slices, to obtain three-dimensional reconstructions of the AVMs. Temporary clips were initially placed on all identifiable feeding arteries, for intranidal pressure reduction before AVM dissection. Dissection of the AVMs was then performed along the main draining veins, as identified by neuronavigation. Patient follow-up monitoring ranged from 3 to 16 months (median, 7 mo).

RESULTS

The computer-calculated registration accuracy ranged between 1.1 and 3.1 mm (median, 1.4 mm). Exact nidus definition was possible for all 22 patients. The principal draining veins were also identified for all patients. Feeding arteries could be detected after the segmentation process when the vessels were at least 3 mm in diameter (19 patients). Complete collapse of the AVMs was achieved with initial clip application for 3 patients; partial intranidal pressure reduction was observed for 12 patients. No significant decompression by feeder clipping was possible for pre-embolized AVMs. Perioperative mortality and morbidity rates were 0 and 14%, respectively.

CONCLUSION

This image-guided technology allows observation of the relationship between AVMs and adjacent brain structures, increasing spatial orientation during surgery. Definition of an optimal surgical approach and early localization of feeding arteries for temporary occlusion minimize tissue manipulation and enhance the safety of direct dissection along the draining veins, which is necessary in eloquent areas.

Computer-based imaging and interventional MRI: applications for neurosurgery

Advances in computer technology and the development of open MRI systems definitely enhanced intraoperative image-guidance in neurosurgery. Based upon the integration of previously acquired and processed 3D information and the corresponding anatomy of the patient, this requires computerized image-processing methods (segmentation, registration, and display) and fast image integration techniques. Open MR systems equipped with instrument tracking systems, provide an interactive environment in which biopsies and minimally invasive interventions or open surgeries can be performed. Enhanced by the integration of multimodal imaging these techniques significantly improve the available treatment options and can change the prognosis for patients with surgically treatable diseases.

Functional magnetic resonance (fMR) imaging of a rat brain tumor model: implications for evaluation of tumor microvasculature and therapeutic response

Functional MR (fMR) imaging techniques based on blood oxygenation level dependent (BOLD) effects were developed and applied to a rat brain tumor model to evaluate the potential utility of the method for characterizing tumor growth and regression following treatment. Rats bearing 9L brain tumors in situ were imaged during inhalation of room air and after administration of 100% oxygen + acetazolamide (ACZ) injected 15 mg/kg intravenously. Pixel-to-pixel fMR maps of normalized signal intensity change from baseline values were calculated from T2 weighted spin echo (SE) images acquired pre- and post- oxygen + ACZ administration. Resultant fMR maps were then compared to gross histological sections obtained from corresponding anatomical regions. Regions containing viable tumor with increased cellular density and localized foci of necrotic tumor cells consistent with hypoxia were visualized in the fMR images as regions with decreased signal intensities, indicating diminished oxyhemoglobin concentration and blood flow as compared to normal brain. Histological regions having peritumor edema, caused by increased permeability of tumor vasculature, were visualized in the fMR images as areas with markedly increased signal intensities. These results suggest that fMR imaging techniques could be further developed for use as a non-invasive tool to assess changes in tumor oxygenation/hemodynamics, and to evaluate the pharmacologic effect of anti-neoplastic drugs.

Three-dimensional image reconstruction for low-grade glioma surgery

Three-dimensional image reconstruction for preoperative surgical planning and intraoperative navigation for the resection of low-grade gliomas was performed in 20 patients. Thirteen of these surgeries were performed while the patient received a local anesthetic to allow for cortical mapping. Ninety percent of the patients were functionally intact postoperatively. The authors propose that the combination of the three-dimensional image reconstruction and surgical navigation, in conjunction with intraoperative cortical mapping, provides an additional means for surgeons to improve the safety and precision of the procedures.