Registration of Functional and Anatomical MRI: Accuracy Assessment and Application in Navigated Neurosurgery

Recent Trends, Technical Concepts and Components of Computer-Assisted Orthopedic Surgery Systems: A Comprehensive Review

Computer-assisted orthopedic surgery (CAOS) systems have become one of the most important and challenging types of system in clinical orthopedics, as they enable precise treatment of musculoskeletal diseases, employing modern clinical navigation systems and surgical tools. This paper brings a comprehensive review of recent trends and possibilities of CAOS systems. There are three types of the surgical planning systems, including: systems based on the volumetric images (computer tomography (CT), magnetic resonance imaging (MRI) or ultrasound images), further systems utilize either 2D or 3D fluoroscopic images, and the last one utilizes the kinetic information about the joints and morphological information about the target bones. This complex review is focused on three fundamental aspects of CAOS systems: their essential components, types of CAOS systems, and mechanical tools used in CAOS systems. In this review, we also outline the possibilities for using ultrasound computer-assisted orthopedic surgery (UCAOS) systems as an alternative to conventionally used CAOS systems.

Radiation-induced liver damage: correlation of histopathology with hepatobiliary magnetic resonance imaging, a feasibility study

PURPOSE

Radiotherapy of liver malignancies shows promising results (radioembolization, stereotactic irradiation, interstitial brachytherapy). Regardless of the route of application, a certain amount of nontumorous liver parenchyma will be collaterally damaged by radiation. The functional reserve may be significantly reduced with an impact on further treatment planning. Monitoring of radiation-induced liver damage by imaging is neither established nor validated. We performed an analysis to correlate the histopathological presence of radiation-induced liver damage with functional magnetic resonance imaging (MRI) utilizing hepatobiliary contrast media (Gd-BOPTA).

METHODS

Patients undergoing local high-dose-rate brachytherapy for whom a follow-up hepatobiliary MRI within 120 days after radiotherapy as well as an evaluable liver biopsy from radiation-exposed liver tissue within 7 days before MRI were retrospectively identified. Planning computed tomography (CT)/dosimetry was merged to the CT-documentation of the liver biopsy and to the MRI. Presence/absence of radiation-induced liver damage (histopathology) and Gd-BOPTA uptake (MRI) as well as the dose applied during brachytherapy at the site of tissue sampling was determined.

RESULTS

Fourteen biopsies from eight patients were evaluated. In all cases with histopathological evidence of radiation-induced liver damage (n = 11), no uptake of Gd-BOPTA was seen. In the remaining three, cases no radiation-induced liver damage but Gd-BOPTA uptake was seen. Presence of radiation-induced liver damage and absence of Gd-BOPTA uptake was correlated with a former high-dose exposition.

CONCLUSIONS

Absence of hepatobiliary MRI contrast media uptake in radiation-exposed liver parenchyma may indicate radiation-induced liver damage. Confirmatory studies are warranted.

In vivo assessment of catheter positioning accuracy and prolonged irradiation time on liver tolerance dose after single-fraction 192Ir high-dose-rate brachytherapy

BACKGROUND

To assess brachytherapy catheter positioning accuracy and to evaluate the effects of prolonged irradiation time on the tolerance dose of normal liver parenchyma following single-fraction irradiation with 192Ir.

MATERIALS AND METHODS

Fifty patients with 76 malignant liver tumors treated by computed tomography (CT)-guided high-dose-rate brachytherapy (HDR-BT) were included in the study. The prescribed radiation dose was delivered by 1 - 11 catheters with exposure times in the range of 844 - 4432 seconds. Magnetic resonance imaging (MRI) datasets for assessing irradiation effects on normal liver tissue, edema, and hepatocyte dysfunction, obtained 6 and 12 weeks after HDR-BT, were merged with 3D dosimetry data. The isodose of the treatment plan covering the same volume as the irradiation effect was taken as a surrogate for the liver tissue tolerance dose. Catheter positioning accuracy was assessed by calculating the shift between the 3D center coordinates of the irradiation effect volume and the tolerance dose volume for 38 irradiation effects in 30 patients induced by catheters implanted in nearly parallel arrangement. Effects of prolonged irradiation were assessed in areas where the irradiation effect volume and tolerance dose volume did not overlap (mismatch areas) by using a catheter contribution index. This index was calculated for 48 irradiation effects induced by at least two catheters in 44 patients.

RESULTS

Positioning accuracy of the brachytherapy catheters was 5-6 mm. The orthogonal and axial shifts between the center coordinates of the irradiation effect volume and the tolerance dose volume in relation to the direction vector of catheter implantation were highly correlated and in first approximation identically in the T1-w and T2-w MRI sequences (p = 0.003 and p < 0.001, respectively), as were the shifts between 6 and 12 weeks examinations (p = 0.001 and p = 0.004, respectively). There was a significant shift of the irradiation effect towards the catheter entry site compared with the planned dose distribution (p < 0.005). Prolonged treatment time increases the normal tissue tolerance dose. Here, the catheter contribution indices indicated a lower tolerance dose of the liver parenchyma in areas with prolonged irradiation (p < 0.005).

CONCLUSIONS

Positioning accuracy of brachytherapy catheters is sufficient for clinical practice. Reduced tolerance dose in areas exposed to prolonged irradiation is contradictory to results published in the current literature. Effects of prolonged dose administration on the liver tolerance dose for treatment times of up to 60 minutes per HDR-BT session are not pronounced compared to effects of positioning accuracy of the brachytherapy catheters and are therefore of minor importance in treatment planning.

Radiobiological restrictions and tolerance doses of repeated single-fraction hdr-irradiation of intersecting small liver volumes for recurrent hepatic metastases

BACKGROUND

To assess radiobiological restrictions and tolerance doses as well as other toxic effects derived from repeated applications of single-fraction high dose rate irradiation of small liver volumes in clinical practice.

METHODS

Twenty patients with liver metastases were treated repeatedly (2 - 4 times) at identical or intersecting locations by CT-guided interstitial brachytherapy with varying time intervals. Magnetic resonance imaging using the hepatocyte selective contrast media Gd-BOPTA was performed before and after treatment to determine the volume of hepatocyte function loss (called pseudolesion), and the last acquired MRI data set was merged with the dose distributions of all administered brachytherapies. We calculated the BED (biologically equivalent dose for a single dose d = 2 Gy) for different alpha/beta values (2, 3, 10, 20, 100) based on the linear-quadratic model and estimated the tolerance dose for liver parenchyma D90 as the BED exposing 90% of the pseudolesion in MRI.

RESULTS

The tolerance doses D90 after repeated brachytherapy sessions were found between 22 - 24 Gy and proved only slightly dependent on alpha/beta in the clinically relevant range of alpha/beta = 2 - 10 Gy. Variance analysis showed a significant dependency of D90 with respect to the intervals between the first irradiation and the MRI control (p < 0.05), and to the number of interventions. In addition, we observed a significant inverse correlation (p = 0.037) between D90 and the pseudolesion's volume. No symptoms of liver dysfunction or other toxic effects such as abscess formation occurred during the follow-up time, neither acute nor on the long-term.

CONCLUSIONS

Inactivation of liver parenchyma occurs at a BED of approx. 22 - 24 Gy corresponding to a single dose of ~10 Gy (alpha/beta ~ 5 Gy). This tolerance dose is consistent with the large potential to treat oligotopic and/or recurrent liver metastases by CT-guided HDR brachytherapy without radiation-induced liver disease (RILD). Repeated small volume irradiation may be applied safely within the limits of this study.

Brain injury and recovery following binge ethanol: evidence from in vivo magnetic resonance spectroscopy

BACKGROUND

The binge-drinking model in rodents using intragastric injections of ethanol (EtOH) for 4 days results in argyrophilic corticolimbic tissue classically interpreted as indicating irreversible neuronal degeneration. However, recent findings suggest that acquired argyrophilia can also identify injured neurons that have the potential to recover. The current in vivo magnetic resonance (MR) imaging and spectroscopy study was conducted to test the hypothesis that binge EtOH exposure would injure but not cause the death of neurons as previously ascertained postmortem.

METHODS

After baseline MR scanning, 11 of 19 rats received a loading dose of 5 g/kg EtOH via oral gavage, then a maximum of 3 g/kg every 8 hours for 4 days, for a total average cumulative EtOH dose of 43 +/- 1.2 g/kg and average blood alcohol levels of 258 +/- 12 mg/dL. All animals were scanned after 4 days of gavage (post-gavage scan) with EtOH (EtOH group) or dextrose (control [Con] group) and again after 7 days of abstinence from EtOH (recovery scan).

RESULTS

Tissue shrinkage at the post-gavage scan was reflected by significantly increased lateral ventricular volume in the EtOH group compared with the Con group. At the post-gavage scan, the EtOH group had lower dorsal hippocampal N-acetylaspartate and total creatine and higher choline-containing compounds than the Con group. At the recovery scan, neither ventricular volume nor metabolite levels differentiated the groups.

CONCLUSIONS

Rapid recovery of ventricular volume and metabolite levels with removal of the causative agent argues for transient rather than permanent effects of a single EtOH binge episode in rats.

Assessment of the tolerance dose of the hepatic reticulo-endothelial system (RES) after single fraction HDR-irradiation: an in-vivo study employing SSPIO

PURPOSE

To prospectively assess a dose-response relationship for the hepatic reticulo-endothelial system (RES) after small volume single fraction irradiation of liver parenchyma in vivo.

MATERIALS AND METHODS

Twenty-five liver tumors were treated by computed tomography (CT)-guided interstitial brachytherapy. Magnetic resonance imaging (MRI) was performed 1 day before and 3 days, 6, 12 and 24 weeks after therapy. MR-sequences included T2-w Turbo Spin Echo (TSE) enhanced by hepatic RES targeted Standard Superparamagnetic Iron Oxide (SSPIO). All MRI data sets were merged with three dimensional (3D) dosimetry data and evaluated by two radiologists. We estimated the threshold dose for either edema or function loss as the D90. A match-pair analysis was performed with another 25 liver tumors, which were treated the same but had MRI follow-up using the hepatocyte specific MRI contrast media Gadobenate dimeglumine (Gd-BOPTA).

RESULTS

Three days post brachytherapy the D90 for hepatic RES function loss reached the 18.3 Gray (Gy) isosurface (Standard Deviation (SD) 7.7). At 6 weeks, the respective zone had increased significantly to the 12.9 Gy isosurface (SD 4.4). After 12 and 24 weeks, the dysfunction of liver volume decreased significantly to the 15 Gy and 20.4 Gy isosurface respectively (SD 7.1 and 10.0). Comparison to the hepatocyte function loss indicates a higher minimal threshold dose of the hepatic RES.

CONCLUSION

Hepatic RES demonstrated a high regenerative capacity and a higher minimal threshold dose than hepatocytes. Temporary function loss was found from the 13 Gy isosurface.

Real-time 3T fMRI data of brain tumour patients for intra-operative localization of primary motor areas

INTRODUCTION

In patients with tumours in or near the motor cortex reliable intra-operative identification of the precentral gyrus can be difficult due to anatomical dislocation. Maps of functional magnetic resonance imaging (fMRI) based on the blood oxygen level dependent (BOLD) effect are used to localize eloquent functional areas of the brain but require postprocessing for reduction of false positive activations. We set the focus of this study on the evaluation of feasibility and clinical usefulness of using real-time fMRI t-maps without postprocessing for pre-operative planning and intra-operative localization of functional motor areas.

METHODS

Real-time fMRI t-maps from a 3-T MRI scanner were co-registered with MRI data. Ten patients were operated under general anaesthesia using 3D neuronavigation with integrated real-time fMRI t-maps. Surgical and functional outcome was compared to results of 12 patients who previously underwent wake surgeries.

RESULTS

Good neurological outcome was achieved in all treated patients. Main activation clusters on fMRI real-time maps were easily identified. Co-registered real-time fMRI data without additional postprocessing were useful in planning the surgical approach. However, due to brain shift and large voxel size of BOLD contrast signals on t-maps exact localization of borders between tumours and functional areas was not possible intra-operatively.

CONCLUSION

Our method is very simple to use and effective in guiding the neurosurgeon safely through minimally invasive craniotomies to tumours in eloquent areas without setting lesions to functional areas. Furthermore, the neurosurgeon is more independent when tumour location requires acquisition of fMRI data for pre-operative planning and intra-operative navigation.

MR-guided biopsy: a review of current techniques and applications

Biopsy has become a cornerstone of modern medicine and most modern biopsies are performed percutaneously using image guidance, typically computed tomography or ultrasound. MR-guided biopsy offers many advantages over these more traditional modalities, and the recent development of interventional MR imaging techniques has made MR-guided percutaneous biopsies and aspirations a clinical reality. As the field of MR-guided procedures continues to expand and to attract more attention from radiologists, it is important to understand the concepts, techniques, applications, advantages, and limitations of MR-guided biopsy/percutaneous procedures. Radiologists should also recognize the need for their significant involvement in the technical aspects of MR-guided procedures, since several user-defined parameters can alter device visualization in the MR imaging environment and affect procedure safety. This article reviews the prerequisites, systems, and applications of MR-guided biopsy.

Brain tumor perfusion: comparison of dynamic contrast enhanced magnetic resonance imaging using T1, T2, and T2* contrast, pulsed arterial spin labeling, and H2(15)O positron emission tomography

OBJECTIVES

Different techniques for measuring of perfusion are clinically available, but these are usually applied to healthy brain tissue.

MATERIAL AND METHODS

Five different techniques were used here in 12 patients with brain tumors to investigate the impact of tumor vascularization on the perfusion signal: three qualitative dynamic contrast-enhanced/susceptibility-contrast magnetic resonance imaging (DCE-MRI/DSC-MRI) techniques exploiting T(1), T(2), T(2)(*) contrast, and two quantitative techniques, pulsed arterial spin labeling (PASL) and H(2)(15)O positron emission tomography (H(2)(15)O-PET).

RESULTS

In a first approximation, a linear correlation was found between all five imaging modalities regarding the perfusion signal of both, normal brain tissue and tumor. The estimated values for tumor perfusion differed significantly between the techniques (1=methodical mean in arbitrary units): PASL: 0.83, H(2)(15)O-PET: 0.62, T(1)-DCE: 1.73, T(2)-DCE: 0.69, T(2)(*)-DSC: 0.89.

CONCLUSIONS

The tumor perfusion values, determined with different techniques are not comparable. The T(2)(*)-DSC, here applied with contrast agent presaturation of extravascular space, and PASL depict median perfusion most reliably.

Gamma knife surgery for arteriovenous malformations in the brain: integration of time-resolved contrast-enhanced magnetic resonance angiography into dosimetry planning

Object

The aim of this study was to develop an algorithm for the integration of time-resolved contrast-enhanced magnetic resonance (MR) angiography into dosimetry planning for Gamma Knife surgery (GKS) of arteriovenous malformations (AVMs) in the brain.

Methods

Twelve patients harboring brain AVMs referred for GKS underwent intraarterial digital subtraction (DS) angiography and time-resolved MR angiography while wearing an externally applied cranial stereotactic frame. Time-resolved MR angiography was performed on a 1.5-tesla MR unit (Achieva, Philips Medical Systems) using contrast-enhanced 3D fast field echo sequencing with stochastic central k-space ordering. Postprocessing with interactive data language (Research Systems, Inc.) produced hybrid data sets containing dynamic angiographic information and the MR markers necessary for stereotactic transformation. Image files were sent to the Leksell GammaPlan system (Elekta) for dosimetry planning.

Results

Stereotactic transformation of the hybrid data sets containing the time-resolved MR angiography information with automatic detection of the MR markers was possible in all 12 cases. The stereotactic coordinates of vascular structures predefined from time-resolved MR angiography matched with DS angiography data in all cases. In 10 patients dosimetry planning could be performed based on time-resolved MR angiography data. In two patients, time-resolved MR angiography data alone were considered insufficient. The target volumes showed a notable shift of centers between modalities.

Conclusions

Integration of time-resolved MR angiography data into the Leksell GammaPlan system for patients with brain AVMs is feasible. The proposed algorithm seems concise and sufficiently robust for clinical application. The quality of the time-resolved MR angiography sequencing needs further improvement.

Electromagnetic Tracking–Guided Percutaneous Intrahepatic Portosystemic Shunt Creation in a Swine Model

Electromagnetic tracking potentially may be used to guide percutaneous needle-based interventional procedures. This brief report describes our initial experience with an electromagnetic guidance system featuring real-time tracking of respiratory-related target motion using an internal fiducial. An algorithm based on a "freehand" needle puncture technique was employed to puncture the right portal vein and a right hepatic vein for percutaneous intrahepatic portosystemic shunt in a swine model. Preoperative computed tomographic images registered with the electromagnetically tracked needle position were displayed for guidance. Successful puncture was confirmed angiographically. Simultaneous percutaneous transhepatic puncture of the targeted veins was accomplished on the initial attempt.

Correlative imaging strategies implementing CT, MRI, and PET for staging of childhood Hodgkin disease

BACKGROUND

The value of different correlative imaging strategies with F18-fluorodeoxyglucose positron emission tomography (FDG-PET) and conventional imaging modalities (CIM) for initial staging of pediatric Hodgkin disease (HD) was assessed.

METHODS

Thirty-three patients (age, 4 to 18 y) with histologically proven HD underwent initial staging with computed tomography (thorax), magnetic resonance imaging (neck, abdomen, pelvis), and FDG-PET in a prospective study. Image fusion (PET-CIM) was performed using a semiautomatic voxel-based algorithm. Analysis of separate, side-by-side (SBS) and fused PET and CIM was performed evaluating 21 nodal and 6 extranodal regions per patient for presence of lymphoma, applying a 5-point confidence scale. The reference data was clinical follow-up (>12 mo).

RESULTS

Concerning lymph node regions above and below the diaphragm the accuracy of CIM, PET, SBS, and image fusion was 86%, 89%, 94%, 97%, and 94%, 94%, 97%, 98%. In extranodal regions, the accuracy was 96%, 96%, 100%, and 100%. The reviewers' confidence was improved significantly by image fusion. Staging and therapy assignment on the basis of CIM was correctly modified by SBS in 5 and 4, by image fusion in 7 and 5 patients.

CONCLUSIONS

Combined reading of FDG-PET and CIM is crucial for accurate staging in pediatric HD. Image fusion improves the observers' confidence and has impact on the therapeutic management.

Percutaneous Biopsy from Blinded to MR Guided: An Update on Current Techniques and Applications

The advent of interventional MR imaging techniques as well as their adoption to guide percutaneous biopsies and aspirations has served as a further step along a series of technical refinements that commenced with the implementation of image-guided approaches for tissue sampling. Nowadays, the practice of and the expectations from these procedures are quite different from those of the blind percutaneous thrusts performed in the late nineteenth and early twentieth centuries. As the field of interventional MR imaging continues to flourish and to attract more radiologists who realize the many opportunities that this technology can offer to their patients, there is a need for a full comprehension of the concepts, techniques, limitations, and cost-effectiveness of MR imaging guidance to present this service to clinical partners in the appropriate setting. Radiologists should also recognize the need for their significant involvement in the technical aspects of MR-guided procedures, because several user-defined parameters and trajectory decisions can alter device visualization in the MR imaging environment and hence affect procedure safety.

In vivo assessment of the tolerance dose of small liver volumes after single-fraction HDR irradiation

PURPOSE

To prospectively assess a dose-response relationship for small volumes of liver parenchyma after single-fraction irradiation.

METHODS AND MATERIALS

Twenty-five liver metastases were treated by computed tomography (CT)-guided interstitial brachytherapy. Magnetic resonance imaging was performed 1 day before and 3 days and 6, 12, and 24 weeks after therapy. MR sequences included T1-w gradient echo (GRE) enhanced by hepatocyte-targeted gadobenate dimeglumine. All MRI data sets were merged with 3D dosimetry data and evaluated by two radiologists. The reviewers indicated the border of hyperintensity on T2-w images (edema) or hypointensity on T1-w images (loss of hepatocyte function). Based on the total 3D data, a dose-volume histogram was calculated. We estimated the threshold dose for either edema or function loss as the D(90), i.e., the dose achieved in at least 90% of the pseudolesion volume.

RESULTS

Between 3 days and 6 weeks, the extension of the edema increased significantly from the 12.9 Gy isosurface to 9.9 Gy (standard deviation [SD], 3.3 and 2.6). No significant change was detected between 6 and 12 weeks. After 24 weeks, the edematous tissue had shrunk significantly to 14.7 Gy (SD, 4.2). Three days postbrachytherapy, the D(90) for hepatocyte function loss reached the 14.9 Gy isosurface (SD, 3.9). At 6 weeks, the respective zone had increased significantly to 9.9 Gy (SD, 2.3). After 12 and 24 weeks, the dysfunction volume had decreased significantly to the 11.9 Gy and 15.2 Gy isosurface, respectively (SD, 3 and 4.1).

CONCLUSIONS

The 95% interval from 7.6 to 12.2 Gy found as the minimal hepatocyte tolerance after 6 weeks accounts for the radiobiologic variations found in CT-guided brachytherapy, including heterogeneous dose rates by variable catheter arrays.

Combination of Functional Magnetic Resonance Imaging-guided Neuronavigation and Intraoperative Cortical Brain Mapping Improves Targeting of Motor Cortex Stimulation in Neuropathic Pain

OBJECTIVE:

To evaluate, regardless of the clinical results, the contribution of combining functional magnetic resonance imaging (fMRI) with intraoperative cortical brain mapping (iCM) as functional targeting methods for epidural chronic motor cortex stimulation (MCS) in refractory neuropathic pain.

METHODS:

Eighteen neuropathic pain patients (central stroke in six; trigeminal neuropathy in six; syrinx or amputation in six) who underwent operations for epidural MCS were studied with preoperative fMRI and iCM. fMRI investigated motor tasks of hands (as well as foot and tongue, when painful). fMRI data were analyzed with Statistical Parametric Mapping99 software (University College London, London, England; initial analysis threshold corresponding to P &lt; 0.001), registered in a neuronavigation system, and correlated during surgery with iCM. The primary aim of this study was to improve the topographical precision of MCS. Matching of fMRI and iCM specifically was examined.

RESULTS:

Correspondence between the contour of the fMRI activation area and iCM in precentral gyrus (mean distance, 3.8 mm) was found in 17 (94%) of 18 patients. Eleven of them showed correspondence for more restrictive values of the analysis threshold (P &lt; 0.0001); in six patients, the quality of the iCM was reduced by somatosensory wave attenuation and general anesthesia. In this group of six patients, a combination of both techniques was used for the final targeting. Correspondence was not found in one patient as the result of image distortion and residual motion artifact. At follow-up (4–60 mo), MCS induced significant pain relief in a total of 11 patients (61%).

CONCLUSION:

This study confirms the functional accuracy of fMRI guidance in neuropathic pain and illustrates the usefulness of combining fMRI guidance with iCM to improve the functional targeting in MCS. Because appropriate targeting is crucial to obtaining pain relief, this combination may increase the analgesic efficacy of MCS.

Application of a non-linear image registration algorithm to quantitative analysis of T2 relaxation time in transgenic mouse models of AD pathology

Transgenic mouse models have been essential for understanding the pathogenesis of Alzheimer's disease (AD) including those that model the deposition process of beta-amyloid (Abeta). Several laboratories have focused on research related to the non-invasive detection of early changes in brains of transgenic mouse models of Alzheimer's pathology. Most of this work has been performed using regional image analysis of individual mouse brains and pooling the results for statistical assessment. Here we report the implementation of a non-linear image registration algorithm to register anatomical and transverse relaxation time (T2) maps estimated from MR images of transgenic mice. The algorithm successfully registered mouse brain magnetic resonance imaging (MRI) volumes and T2 maps, allowing reliable estimates of T2 values for different regions of interest from the resultant combined images. This approach significantly reduced the data processing and analysis time, and improved the ability to statistically discriminate between groups. Additionally, 3D visualization of intra-regional distributions of T2 of the resultant registered images provided the ability to detect small changes between groups that otherwise would not be possible to detect.

Evaluating requirements for spatial resolution of fMRI for neurosurgical planning

The unambiguous localization of eloquent functional areas is necessary to decrease the neurological morbidity of neurosurgical procedures. We explored the minimum spatial resolution requirements for functional magnetic resonance imaging (fMRI) data acquisition when brain mapping is used in neurosurgical planning and navigation. Using a 1.5 Tesla clinical MRI scanner, eight patients with brain tumors underwent fMRI scans using spatial resolution of approximately 4 x 4 x 4 mm(3) to map the eloquent motor and language areas during the performance of cognitive/sensorimotor tasks. The fMRI results were then used intra-operatively in an open MRI system to delineate eloquent areas. Retrospectively, activation patterns were visually inspected by a neurosurgeon to determine qualitatively whether ambiguity with respect to the activation boundaries, due to low spatial resolution, could be of potential significance for surgical guidance. A significant degree of ambiguity in both the extent and shape of activation was judged to be present in data from six of the eight patients. Analysis of fMRI data at multiple resolutions from a normal volunteer showed that at 3 mm isotropic resolution, eloquent areas were better localized within the gray matter although there was still some potential for ambiguity caused by activations appearing to cross a sulcus. The data acquired with 2-mm isotropic voxels significantly enhanced the spatial localization of activation to within the gray matter. Thus, isotropic spatial resolution on the order of 2 x 2 x 2 mm(3), which is much higher than the resolutions used in typical fMRI examinations, may be needed for the unambiguous identification of cortical activation with respect to tumors and important anatomical landmarks.

Cerebral ischemia in aneurysmal subarachnoid hemorrhage: a correlative microdialysis-PET study

BACKGROUND AND PURPOSE

Cerebral microdialysis (MD) is discussed as a technique for detection of cerebral ischemia in subarachnoid hemorrhage; however, clinical data on cerebral blood flow (CBF) are limited in these patients. The main objective of this study was to investigate whether pathological MD parameters reflect a reduced regional CBF (rCBF) determined by 15O-H2O PET.

METHODS

Thirteen subarachnoid hemorrhage patients (age, 48.7+/-15.0 years; World Federation of Neurological Surgeons grade 1 to 5) were studied. Extracellular glucose, lactate, lactate/pyruvate (L/P) ratio, glutamate, and glycerol levels were analyzed hourly. rCBF was determined in the volume of interest of the MD catheter and all vascular territories. MD values were correlated to rCBF on the day of PET. Then, MD concentrations of asymptomatic versus ischemic phases (3-day medians) were analyzed.

RESULTS

In symptomatic patients (n=10), rCBF was significantly lower compared with controls (n=3, P=0.048). Glutamate correlated best with rCBF (r=-0.66; P=0.014), followed by glycerol (r=-0.62; P=0.021). The L/P ratio was most sensitive (0.82) and specific (1.0) in indicating symptoms of ischemia, but only during longer periods of ischemia.

CONCLUSIONS

rCBF correlates best with glutamate, followed by glycerol, whereas the L/P ratio is sensitive only after longer periods of ischemia. Clinically relevant regional metabolic derangements occur already above an rCBF of 20 mL x 100 g(-1).min(-1). Future research should focus on identifying alternative causes of metabolic derangement in subarachnoid hemorrhage patients and optimal treatment management in these patients.

Intraoperative magnetic resonance imaging and magnetic resonance imaging-guided therapy for brain tumors

Since their introduction into surgical practice in the mid 1990s, intraoperative MRI systems have evolved into essential, routinely used tools for the surgical treatment of brain tumors in many centers. Clear delineation of the lesion, "under-the-surface" vision, and the possibility of obtaining real-time feedback on the extent of resection and the position of residual tumor tissue (which may change during surgery due to "brain-shift") are the main strengths of this method. High-performance computing has further extended the capabilities of intraoperative MRI systems, opening the way for using multimodal information and 3D anatomical reconstructions, which can be updated in "near real time." MRI sensitivity to thermal changes has also opened the way for innovative, minimally invasive (LASER ablations) as well as noninvasive therapeutic approaches for brain tumors (focused ultrasound). Although we have not used intraoperative MRI in clinical applications sufficiently long to assess long-term outcomes, this method clearly enhances the ability of the neurosurgeon to navigate the surgical field with greater accuracy, to avoid critical anatomic structures with greater efficacy, and to reduce the overall invasiveness of the surgery itself.