Intraoperative MR at 1.5 Tesla--experience and future directions.

The objective of this report is to present and contrast the development of the different intraoperative MR systems that are currently in use. The manuscript focuses on the design and clinical experience of a 1.5 Tesla MR system, based on a movable magnet. This configuration is similar to the operating microscope and other surgical adjuncts, with MR technology moved to and from the patient as needed. The system has been used to monitor 294 neurosurgical procedures. including CNS neoplasia. epilepsy, cervical spine disorders, arteriovenous malformations, cavernomas and aneurysms. In many cases the surgical procedure was significantly altered by intraoperatively acquired MRI. Future developments include the construction of a 3 Tesla intraoperative MR system and an ambidextrous MR-compatible robot. This seamless integration of robotic technology into an intraoperative MR environment may well revolutionize neurosurgery.

Transoral resection of axial lesions augmented by intraoperative magnetic resonance imaging. Report of three cases.

Transoral decompression of the cervicomedullary junction may be compromised by a narrow corridor in which surgery is performed, and thus the adequacy of surgical decompression/resection may be difficult to determine. This is problematic as the presence of spinal instrumentation may obscure the accuracy of postoperative radiological assessment, or the patient may require reoperation. The authors describe three patients in whom high-field intraoperative magnetic resonance (MR) images were acquired at various stages during the transoral resection of C-2 disease that had caused craniocervical junction compression. All three patients harbored different lesions involving the cervicomedullary junction: one each of plasmacytoma and metastatic breast carcinoma involving the odontoid process and C-2 vertebral body, and basilar invagination with a Chiari I malformation. All patients presented with progressive myelopathy. Surgical planning MR imaging studies performed after the induction of anesthesia demonstrated the lesion and its relationship to the planned surgical corridor. Transoral exposure was achieved through placement of a Crockard retractor system. In one case the soft palate was divided. Interdissection MR imaging revealed that adequate decompression had been achieved in all cases. The two patients with carcinoma required placement of posterior instrumentation for stabilization. Planned suboccipital decompression and placement of instrumentation were averted in the third case as the intraoperative MR images demonstrated that excellent decompression had been achieved. Intraoperatively acquired MR images were instrumental in determining the adequacy of the decompressive surgery. In one of the three cases, examination of the images led the authors to change the planned surgical procedure. Importantly, the acquisition of intraoperative MR images did not adversely affect operating time or neurosurgical techniques, including instrumentation requirements.

Intraoperative magnetic resonance imaging-augmented transoral resection of axial disease.

OBJECT: Because transoral decompression of the cervicomedullary junction is compromised by a narrow surgical corridor, the adequacy of decompression/resection may be difficult to determine. This is problematic as spinal hardware may obscure postoperative radiological assessment, or the patient may require reoperation. The authors report three patients in whom high-field intraoperative magnetic resonance (MR) images were acquired at various stages during the transoral resection of C-2 lesions causing craniocervical junction compression. METHODS: In all three patients the lesions involved the cervicomedullary junction: one case each of plasmacytoma and metastatic breast carcinoma involving the odontoid process and C-2 vertebral body, and one case of basilar invagination with a Chiari type I malformation. All three patients presented with progressive myelopathy. Surgery-planning MR imaging studies, performed after the induction of anesthesia, demonstrated the lesion and its relationship to the planned surgical corridor. Transoral exposure was achieved through placement of a Crockard retractor system. In one case the soft palate was divided. Interdissection MR imaging revealed that adequate decompression had been achieved in all cases. In the two patients with carcinoma, posterior instrumentation was placed to achieve spinal stabilization. Planned suboccipital decompression and fixation was averted in the third case because MR imaging demonstrated that excellent decompression had been achieved. CONCLUSIONS: Intraoperatively acquired MR images were instrumental in determining the adequacy of surgical decompression. In one patient the MR images changed the planned surgical procedure. Importantly, the acquisition of intraoperative MR images did not adversely affect operative time or neurosurgical techniques, including the instrumentation procedure.

Neurosurgical suite of the future. III.

The neurosurgical suite of the future will continue to incorporate developing technologies so that lesion localization, surgical dissection, electrode placement, and cell/drug delivery will be optimized. In this article it was shown that MR technology can be placed into a neurosurgical operating room and used as a surgical adjunct in such a way that surgical, anesthetic, and nursing techniques are not [figure: see text] compromised. Essential to the success was the high magnetic field strength and system mobility. It is now hypothesized that MR technology, coupled to advances in molecular therapies, endovascular techniques, and transplantation, will continue to improve neurosurgical outcome not only for benign pathology but also malignant neoplastic and degenerative CNS disorders. Although these technologies are intriguing and ultimately will improve neurosurgical outcome, it is likely that for the foreseeable future, neurosurgery will continue to require sound clinical judgment and surgical dexterity.

Advances in mobile intraoperative magnetic resonance imaging.

OBJECTIVE: The goal was to enhance a mobile magnetic resonance imaging system developed for neurosurgery. Components of the system included an actively shielded, 1.5-T superconducting magnet, a titanium operating room table, a radiofrequency (RF) head coil that could be disassembled, and local RF shielding. METHODS: The system was designed and implemented by the Division of Neurosurgery, University of Calgary (Calgary, Alberta, Canada), in collaboration with the National Research Council of Canada Institute for Biodiagnostics (Winnipeg, Manitoba, Canada). The ceiling-mounted, 1.5-T magnet was moved into and out of the surgical field as required. After initial success in monitoring the resection of various intracranial and cranial base lesions, significant modifications to the system were made by Innovative Magnetic Resonance Imaging Systems, Inc. (Winnipeg, Manitoba, Canada), and BrainLAB (Heimstetten, Germany). These modifications included the design and construction of a shorter magnet with a larger bore and stronger gradients, widening of the titanium operating room table, modification of the RF coil housing to allow vertical movement and incorporation of a three-pin head-clamp, construction of a transparent, copper-impregnated RF shield, and integration with a surgical navigation system. RESULTS: The movable intraoperative imaging system has now been used for 101 neurosurgical procedures, including the previously reported cases. CONCLUSION: The modifications to the system have enhanced its integration with established neurosurgical techniques and have improved patient safety. The larger magnet bore size, together with the ability to move the RF coil vertically, allows placement of patients in prone or lateral positions. Surgical navigation has been successfully integrated with the intraoperatively acquired high-resolution images. The ability to identify and resect residual lesions before wound closure remains a tremendous immediate advantage of this technology.

Utility of a moveable 1.5 Tesla intraoperative MR imaging system.

OBJECTIVE: This study demonstrates the utility of a newly-developed moveable 1.5 Tesla intraoperative MR imaging system using a case report of a multi-lobulated parafalx meningioma. CLINICAL PRESENTATION: A 43-year-old female presented with progression of a multi-lobulated anterior parafalx meningioma several years following resection of a large left frontal convexity meningioma. INTERVENTION AND TECHNIQUE: Surgical excision of the lesion was undertaken. Following apparent total resection, intraoperative MR imaging revealed two residual dumbell shaped lobules. Using these updated MR images, the tumour was readily identified and removed. CONCLUSION: The moveable 1.5 Tesla intraoperative MR system used in the present case provides rapid, high resolution MR images during neurosurgical procedures. Moving the magnet out of the surgical field during surgery permits the use of all standard neurosurgical instruments. The ease of use and quality of images combined with minimal interference on well-established surgical techniques makes this system a valuable adjunct in the neurosurgical treatment of intracranial disease.

A mobile high-field magnetic resonance system for neurosurgery.

OBJECT: The authors' goal was to place a mobile, 1.5-tesla magnetic resonance (MR) imaging system into a neurosurgical operating room without adversely affecting established neurosurgical management. The system would help to plan accurate surgical corridors, confirm the accomplishment of operative objectives, and detect acute complications such as hemorrhage or ischemia. METHODS: The authors used an actively shielded 1.5-tesla magnet, together with 15 mtesla/m gradients, MR console computers, gradient amplifiers, a titanium, hydraulic-controlled operating table, and a radiofrequency coil that can be disassembled. The magnet is moved to and from the surgical field by using overhead crane technology. To date, the system has provided unfettered access in 46 neurosurgical patients. In all patients, high-definition T1- and/or T2-weighted images were rapidly and reproducibly acquired at various stages of the surgical procedures. Eleven patients underwent craniotomy that was optimized after preincision imaging. In four patients who harbored subtotally resected tumor, intraoperative MR imaging aided the surgeon in removing the remaining tumor. Interestingly, the intraoperative administration of gadolinium demonstrated a dynamic expansion of enhancement beyond the preoperative contrast contour in patients with malignant glioma. These zones of new enhancement proved, on examination of biopsy samples, to be tumor. CONCLUSIONS: The authors have demonstrated that high-quality MR images can be obtained in the operating room within reasonable time constraints. Procedures can be conducted without compromising or altering traditional neurosurgical, nursing, or anesthetic techniques. It is feasible that within the next decade intraoperative MR imaging may become the standard of care in neurosurgery.

Hypothermia: depression of tricarboxylic acid cycle flux and evidence for pentose phosphate shunt upregulation.

OBJECT: Hypothermia is used in neurosurgery and other surgical disciplines to reduce tissue injury, but the mechanism of such protection remains elusive. The authors have endeavored to delineate the mechanism of neural protection afforded by hypothermia through a study of glucose metabolism. METHODS: Nuclear magnetic resonance spectroscopy was used to follow the carbon-13 label from [1-13C]glucose as it was metabolized through the glycolytic and tricarboxylic acid pathways. Male Sprague-Dawley rats were maintained at either 37.5 degrees C or 31 degrees C and infused with labeled glucose for 10, 30, 60, 100, or 200 minutes (five rats were used for each time point and for each temperature). At the end of the infusion period, the rats' brains were subjected to rapid freeze-funnel fixation. Water-soluble metabolites were extracted from samples of the neocortex and hippocampus by using perchloric acid extraction. The fractional enrichment of these metabolites was used to calculate the reaction rate constant of formation and steady-state enrichment for a number of metabolites. Hypothermia resulted in a 30 to 40% depression of metabolism (p < 0.0001) in both the neocortex and hippocampus. Steady-state fractional enrichment of metabolites was also decreased by 20 to 25% with hypothermia (p < 0.0001), implying a loss of label during metabolism. CONCLUSIONS: The results of this study suggest that an increased fraction of glucose metabolism was shunted through the pentose phosphate pathway in the presence of hypothermia.

Human cerebral neoplasms studied using MR spectroscopy: a review.

Of primary central nervous system tumors treated each year, the majority are glioma, followed by meningioma and then pituitary adenoma. While the use of magnetic resonance (MR) and computed tomographic imaging is well established in the diagnosis and management of such tumors, these techniques have a limited role in determining the metabolic state, either prior to or following therapy. Multinuclear MR spectroscopy, on the other hand, provides information on tumor metabolism and the effect of therapy on tumor viability. This paper reviews MR spectroscopic studies performed on patients with central nervous system tumors and discusses the impact that such studies have had on tumor diagnosis and management.

Basal versus apical dendritic long-term potentiation of commissural afferents to hippocampal CA1: a current-source density study.

Current-source density analysis was used to estimate the magnitude of the synaptic excitation at the basal and apical dendritic synapses of CA1 following commissural stimulation in the urethane-anesthetized rat, before and after a theta-frequency patterned primed burst tetanus. Stimulation of the contralateral CA3 or the contralateral CA1 stratum oriens excited both the basal and apical dendrites in CA1 about equally. However, primed burst tetanization of the contralateral CA3 or CA1 stratum oriens resulted in significant long-term potentiation (LTP) only at the basal dendrites and not at the apical dendrites. Stimulation of the contralateral CA1 stratum radiatum excited the apical dendrites more than the basal dendrites of CA1, but tetanization of this contralateral site gave little change in the apical or basal dendritic excitation. Tetanization of the contralateral CA1 stratum radiatum after an intraventricular administration of bicuculline, a GABAA antagonist, however, resulted in significant LTP at both the apical and basal dendrites. It was concluded that, in the intact hippocampus in vivo, the threshold for LTP at the commissural apical dendritic synapse was high in comparison to that at the basal dendritic synapse and this high threshold may be partly caused by inhibitory interneurons that predominantly synapsed on the apical dendrites. Thus, the basal and apical dendrites of the CA1 pyramidal cells are not equal in their propensity for long-term plasticity.

Long-term potentiation induced by patterned stimulation of the commissural pathway to hippocampal CA1 region in freely moving rats.

In urethane-anesthetized rats, stimulation of the contralateral hippocampal CA1 region resulted in activation of the homotopic CA1 region. Current-source-density analysis revealed that both basal and apical dendrites were activated. However, alveolar and stratum oriens stimulation in CA1 gave about equal peak excitation of the basal and apical dendrites while CA1 stratum radiatum/moleculare and CA3c stimulation gave stronger apical than basal dendritic excitation. In chronically implanted and freely moving rats, tetanic patterned stimulation of the contralateral CA1, irrespective of depth, resulted in a robust long-term potentiation of the ipsilateral CA1 basal dendritic synapse. The population basal dendritic excitatory postsynaptic potential was initially potentiated to greater than 200% of the baseline and decayed with a 3 h time constant; it lasted at least two days. Patterned stimulation of the commissural inputs at 2 x threshold stimulus intensity seldom potentiated the apical dendritic synapse in CA1; rather, long-term depression was sometimes observed. After tetanic stimulations at 3 x threshold, a small potentiation of the apical dendritic excitation was seen in about half of the experiments. The average apical dendritic potentiation peaked at about 25% and persisted to at least one day. This study provides original evidence that the properties of long-term potentiation are different at the commissural basal dendritic and apical dendritic synapses in CA1 of the behaving rat. Basal dendritic potentiation is low-threshold, high-amplitude and decayed rapidly in the first 3 h. Apical dendritic potentiation is high-threshold, low-amplitude and not rapidly decaying. A long-lasting enhancement of synaptic transmission has been postulated as a physiological correlate of memory. This paper reports properties of this synaptic enhancement for two different types of synapses on the same cells in the behaving animal. The basal dendritic synapse on hippocampal pyramidal cells readily increased their efficacy, up to at least two days, after a brief, patterned stimulation. In the same preparation, it was difficult to obtain a long-lasting increase in the apical dendritic excitation, in contrast to studies on isolated hippocampal slices in vitro.

Radial maze performance following hippocampal kindling.

The relation between hippocampal epileptiform activity and 8-arm radial maze performance was assessed following repetitive afterdischarges (ADs) evoked by stimulation of the hippocampal CA1 region (kindling). Hippocampal kindling, whether to stage V (generalized) convulsions or to a preconvulsive stage, induced deficits in radial maze performance, evaluated by correct arm entries in 8 choices or total maze run (trial) time. The deficits persisted at least until 21 days after the last AD. Hippocampal interictal spikes (ISs) were induced by kindling, but the rate of ISs declined to near zero in a few days. The rate or presence of ISs was not related to maze performance.