Intraoperative MR imaging guidance for intracranial neurosurgery: experience with the first 200 cases

Intraoperative MRI: A Review of Applications Across Neurosurgical Specialties

Intraoperative MRI (iMRI) made its debut to great fanfare in the mid-1990s. However, the enthusiasm for this technology with seemingly obvious benefits for neurosurgeons has waned. We review the benefits and utility of iMRI across the field of neurosurgery and present an overview of the evidence for iMRI for multiple neurosurgical disciplines: tumor, skull base, vascular, pediatric, functional, and spine. Publications on iMRI have steadily increased since 1996, plateauing with approximately 52 publications per year since 2011. Tumor surgery, especially glioma surgery, has the most evidence for the use of iMRI contributing more than 50% of all iMRI publications, with increased rates of gross total resection in both adults and children, providing a potential survival benefit. Across multiple neurosurgical disciplines, the ability to use a multitude of unique sequences (diffusion tract imaging, diffusion-weighted imaging, magnetic resonance angiography, blood oxygenation level-dependent) allows for specialization of imaging for various types of surgery. Generally, iMRI allows for consideration of anatomic changes and real-time feedback on surgical outcomes such as extent of resection and instrument (screw, lead, electrode) placement. However, implementation of iMRI is limited by cost and feasibility, including the need for installation, shielding, and compatible tools. Evidence for iMRI use varies greatly by specialty, with the most evidence for tumor, vascular, and pediatric neurosurgery. The benefits of real-time anatomic imaging, a lack of radiation, and evaluation of surgical outcomes are limited by the cost and difficulty of iMRI integration. Nonetheless, the ability to ensure patients are provided by a maximal yet safe treatment that specifically accounts for their own anatomy and highlights why iMRI is a valuable and underutilized tool across multiple neurosurgical subspecialties.

Automated instrument-tracking for 4D video-rate imaging of ophthalmic surgical maneuvers

Intraoperative image-guidance provides enhanced feedback that facilitates surgical decision-making in a wide variety of medical fields and is especially useful when haptic feedback is limited. In these cases, automated instrument-tracking and localization are essential to guide surgical maneuvers and prevent damage to underlying tissue. However, instrument-tracking is challenging and often confounded by variations in the surgical environment, resulting in a trade-off between accuracy and speed. Ophthalmic microsurgery presents additional challenges due to the nonrigid relationship between instrument motion and instrument deformation inside the eye, image field distortion, image artifacts, and bulk motion due to patient movement and physiological tremor. We present an automated instrument-tracking method by leveraging multimodal imaging and deep-learning to dynamically detect surgical instrument positions and re-center imaging fields for 4D video-rate visualization of ophthalmic surgical maneuvers. We are able to achieve resolution-limited tracking accuracy at varying instrument orientations as well as at extreme instrument speeds and image defocus beyond typical use cases. As proof-of-concept, we perform automated instrument-tracking and 4D imaging of a mock surgical task. Here, we apply our methods for specific applications in ophthalmic microsurgery, but the proposed technologies are broadly applicable for intraoperative image-guidance with high speed and accuracy.

Intraoperative Imaging for High-Grade Glioma Surgery

This article discusses intraoperative imaging techniques used during high-grade glioma surgery. Gliomas can be difficult to differentiate from surrounding tissue during surgery. Intraoperative imaging helps to alleviate problems encountered during glioma surgery, such as brain shift and residual tumor. There are a variety of modalities available all of which aim to give the surgeon more information, address brain shift, identify residual tumor, and increase the extent of surgical resection. The article starts with a brief introduction followed by a review of with the latest advances in intraoperative ultrasound, intraoperative MRI, and intraoperative computed tomography.

Identification of tumor residuals in pituitary adenoma surgery with intraoperative MRI: do we need gadolinium?

OBJECTIVE

To evaluate the diagnostic accuracy of high-resolution T2w intraoperative magnetic resonance imaging (iMRI) for detecting pituitary adenoma remnants compared to contrast-enhanced T1-weighted images.

METHODS

42 patients underwent iMRI-guided resection of large pituitary macroadenomas and fulfilled the inclusion criteria for this retrospective analysis. Intraoperative and postoperative imaging evaluation of tumor residuals and localization were assessed by two experienced neuroradiologists in a blinded fashion. The diagnostic accuracy of T2w and contrast-enhanced T1w images were evaluated.

RESULTS

The diagnostic accuracy for detecting tumor residuals of high-resolution T2w images showed highly significant association to contrast-enhanced T1w images (p < 0.0001). Furthermore, identification rate of tumor remnants in different compartments, e.g., cavernous sinus, was comparable. In total, coronal T2w images provided a diagnostic sensitivity of 97.7% and specificity of 100% compared to the gold standard of contrast-enhanced T1w images. The postoperatively expected extent of resection proved to be true in 97.6% according to MRI 3 months after resection.

CONCLUSIONS

High-resolution T2w intraoperative MR images provide excellent diagnostic accuracy for detecting tumor remnants in macroadenoma surgery with highly significant association compared to T1w images with gadolinium. The routine-use and need of gadolinium in these patients should be questioned critically in each case in the future.

Review of first clinical experiences with a 1.5 Tesla ceiling-mounted moveable intraoperative MRI system in Europe

High-field intraoperative MRI (iMRI) systems provide excellent imaging quality and are used for resection control and update of image guidance systems in a number of centers. A ceiling-mounted intraoperative MRI system has several advantages compared to a conventional iMRI system. In this article, we report on first clinical experience with using such a state-of-the-art, the 1.5T iMRI system, in Europe. A total of 50 consecutive patients with intracranial tumors and vascular lesions were operated in the iMRI unit. We analyzed the patients' data, surgery preparation times, intraoperative scans, surgical time, and radicality of tumor removal. Patients' mean age was 46 years (range 8 to 77 years) and the median surgical procedure time was 5 hours (range 1 to 11 hours). The lesions included 6 low-grade gliomas, 8 grade III astrocytomas, 10 glioblastomas, 7 metastases, 7 pituitary adenomas, 2 cavernomas, 2 lymphomas, 1 cortical dysplasia, 3 aneurysms, 1 arterio-venous malformation and 1 extracranial-intracranial bypass, 1 clival chordoma, and 1 Chiari malformation. In the surgical treatment of tumor lesions, intraoperative imaging depicted tumor remnant in 29.7% of the cases, which led to a change in the intraoperative strategy. The mobile 1.5T iMRI system proved to be safe and allowed an optimal workflow in the iMRI unit. Due to the fact that the MRI scanner is moved into the operating room only for imaging, the working environment is comparable to a regular operating room.

iMRI During Transsphenoidal Surgery

A variety of intraoperative MRI (iMRI) systems are in use during transsphenoidal surgery (TSS). The variations in iMRI systems include field strengths, magnet configurations, and room configurations. Most studies report that the primary utility of iMRI during TSS lies in detecting resectable tumor residuals following maximal resection with conventional technique. Stereotaxis, neuronavigation, and complication avoidance/detection are enhanced by iMRI use during TSS. The use of iMRI during TSS can lead to increased extent of resection for large tumors. Improved remission rates from hormone-secreting tumors have also been reported with iMRI use. This article discusses the history, indications, and future directions for iMRI during TSS.

Initial experience of using high field strength intraoperative MRI for neurosurgical procedures

We report our initial experience to optimize neurosurgical procedures using high field strength intraoperative magnetic resonance imaging (IOMRI) in 300 consecutive patients as high field strength IOMRI rapidly becomes the standard of care for neurosurgical procedures. Three sequential groups (groups A, B, C; n=100 each) were compared with respect to time management, complications and technical difficulties to assess improvement in these parameters with experience. We observed a reduction in the number of technical difficulties (p<0.001), time to induction (p<0.001) and total anesthesia time (p=0.007) in sequential groups. IOMRI was performed for neuronavigation guidance (n=252) and intraoperative validation of extent of resection (EOR; n=67). Performing IOMRI increased the EOR over and beyond the primary surgical attempt in 20.5% (29/141) and 18% (11/61) of patients undergoing glioma and pituitary surgery, respectively. Overall, EOR improved in 59.7% of patients undergoing IOMRI (40/67). Intraoperative tractography and real time navigation using re-uploaded IOMRI images (accounting for brain shift) helps in intraoperative planning to reduce complications. IOMRI is an asset to neurosurgeons, helping to augment the EOR, especially in glioma and pituitary surgery, with no significant increase in morbidity to the patient.

Application of intraoperative magnetic resonance imaging in large invasive pituitary adenoma surgery

OBJECTIVE

To investigate the clinical application value of intraoperative magnetic resonance imaging (iMRI) in large invasive pituitary adenoma surgery.

METHODS

A total of 30 patients with large pituitary adenoma underwent microscopic tumor resection under the assistance of an iMRI system; 26 cases received surgery through the nasal-transsphenoidal approach, and the remaining four cases received surgery through the pterion approach. iMRI was performed one or two times depending on the need of the surgeon. If a residual tumor was found, further resection was conducted under iMRI guidance.

RESULTS

iMRI revealed residual tumors in 12 cases, among which nine cases received further resection. Of these nine cases, iMRI rescanning confirmed complete resection in six cases, and subtotal resection in the remaining three. Overall, 24 cases of tumor were totally resected, and six cases were subtotally resected. The total resection rate of tumors increased from 60% to 80%.

CONCLUSION

iMRI can effectively determine the resection extent of pituitary adenomas. In addition, it provides an objective basis for real-time judgment of surgical outcome, subsequently improving surgical accuracy and safety, and increasing the total tumor resection rate.

History and evolution of brain tumor imaging: insights through radiology

This review recounts the history of brain tumor diagnosis from antiquity to the present and, indirectly, the history of neuroradiology. Imaging of the brain has from the beginning held an enormous interest because of the inherent difficulty of this endeavor due to the presence of the skull. Because of this, most techniques when newly developed have always been used in neuroradiology and, although some have proved to be inappropriate for this purpose, many were easily incorporated into the specialty. The first major advance in modern neuroimaging was contrast agent-enhanced computed tomography, which permitted accurate anatomic localization of brain tumors and, by virtue of contrast enhancement, malignant ones. The most important advances in neuroimaging occurred with the development of magnetic resonance imaging and diffusion-weighted sequences that allowed an indirect estimation of tumor cellularity; this was further refined by the development of perfusion and permeability mapping. From its beginnings with indirect and purely anatomic imaging techniques, neuroradiology now uses a combination of anatomic and physiologic techniques that will play a critical role in biologic tumor imaging and radiologic genomics.

Time window for postoperative reactive enhancement after resection of brain tumors: less than 72 hours

Object

Early postoperative MRI within 72 hours after brain tumor surgery is commonly used to assess residual contrast-enhancing tumor. The 72-hour window is commonly accepted because previous 1.5-T MRI studies have not found confounding postoperative reactive contrast enhancement in this time frame. The sensitivity to detect contrast enhancement increases with the field strengths. Therefore, the authors aimed to assess whether the 72-hour window is also appropriate for the MRI scanner with a field strength of 3 T.

Methods

The authors retrospectively analyzed findings on early postsurgical MR images acquired in 46 patients treated for high-grade gliomas. They performed 3-T MRI within 7 days before surgery and within 72 hours thereafter. The appearance of enhancement was categorized as postoperative reactive enhancement or tumoral enhancement by comparison with the pattern and location of presurgical enhancing tumor.

Results

Postoperative reactive enhancement was present in 15 patients (32.6%). This enhancement, not seen on presurgical MRI, had a marginal or leptomeningeal/dural pattern. In 13 patients (28.3%) postsurgical enhancement was found within the first 72 postoperative hours, with the earliest seen 22:57 hours after surgery. Subsequent MR scans in patients with postoperative reactive enhancement did not reveal tumor recurrence in these regions.

Conclusions

Postoperative reactive enhancement earlier than 72 hours after brain tumor surgery can be expected in about one-third of the cases in which a 3-T scanner is used. This might be due to the higher enhancement-to-brain contrast at higher field strengths. Therefore, the time window of 72 hours does not prevent reactive enhancement, which, however, can be recognized as such comparing it with presurgical enhancing tumor.

Comparing deflection measurements of a magnetically steerable catheter using optical imaging and MRI

PURPOSE

Magnetic resonance imaging (MRI) is an emerging modality for interventional radiology, giving clinicians another tool for minimally invasive image-guided interventional procedures. Difficulties associated with endovascular catheter navigation using MRI guidance led to the development of a magnetically steerable catheter. The focus of this study was to mechanically characterize deflections of two different prototypes of the magnetically steerable catheter in vitro to better understand their efficacy.

METHODS

A mathematical model for deflection of the magnetically steerable catheter is formulated based on the principle that at equilibrium the mechanical and magnetic torques are equal to each other. Furthermore, two different image based methods for empirically measuring the catheter deflection angle are presented. The first, referred to as the absolute tip method, measures the angle of the line that is tangential to the catheter tip. The second, referred to the base to tip method, is an approximation that is used when it is not possible to measure the angle of the tangent line. Optical images of the catheter deflection are analyzed using the absolute tip method to quantitatively validate the predicted deflections from the mathematical model. Optical images of the catheter deflection are also analyzed using the base to tip method to quantitatively determine the differences between the absolute tip and base to tip methods. Finally, the optical images are compared to MR images using the base to tip method to determine the accuracy of measuring the catheter deflection using MR.

RESULTS

The optical catheter deflection angles measured for both catheter prototypes using the absolute tip method fit very well to the mathematical model (R(2) = 0.91 and 0.86 for each prototype, respectively). It was found that the angles measured using the base to tip method were consistently smaller than those measured using the absolute tip method. The deflection angles measured using optical data did not demonstrate a significant difference from the angles measured using MR image data when compared using the base to tip method.

CONCLUSIONS

This study validates the theoretical description of the magnetically steerable catheter, while also giving insight into different methods and modalities for measuring the deflection angles of the prototype catheters. These results can be used to mechanically model future iterations of the design. Quantifying the difference between the different methods for measuring catheter deflection will be important when making deflection measurements in future studies. Finally, MR images can be used to reliably measure deflection angles since there is no significant difference between the MR and optical measurements.

Application of desorption electrospray ionization mass spectrometry imaging in breast cancer margin analysis

Distinguishing tumor from normal glandular breast tissue is an important step in breast-conserving surgery. Because this distinction can be challenging in the operative setting, up to 40% of patients require an additional operation when traditional approaches are used. Here, we present a proof-of-concept study to determine the feasibility of using desorption electrospray ionization mass spectrometry imaging (DESI-MSI) for identifying and differentiating tumor from normal breast tissue. We show that tumor margins can be identified using the spatial distributions and varying intensities of different lipids. Several fatty acids, including oleic acid, were more abundant in the cancerous tissue than in normal tissues. The cancer margins delineated by the molecular images from DESI-MSI were consistent with those margins obtained from histological staining. Our findings prove the feasibility of classifying cancerous and normal breast tissues using ambient ionization MSI. The results suggest that an MS-based method could be developed for the rapid intraoperative detection of residual cancer tissue during breast-conserving surgery.

Modern intraoperative imaging modalities for the vascular neurosurgeon treating intracerebral hemorrhage

This paper reviews the current intraoperative imaging tools that are available to assist neurosurgeons in the treatment of intracerebral hemorrhage (ICH). This review shares the authors' experience with each modality and discusses the advantages, potential limitations, and disadvantages of each. Surgery for ICH is directed at blood clot removal, reduction of intracranial pressure, and minimization of secondary damage associated with hematoma breakdown products. For effective occlusion and safe obliteration of vascular anomalies associated with ICH, vascular neurosurgeons today require a thorough understanding of the various intraoperative imaging modalities available for obtaining real-time information. Use of one or more of these modalities may improve the surgeon's confidence during the procedure, the patient's safety during surgery, and surgical outcome. The modern techniques discussed include 1) indocyanine green-based video angiography, which provides real-time information based on high-quality images showing the residual filling of vascular pathological entities and the patency of blood vessels of any size in the surgical field; and 2) intraoperative angiography, which remains the gold standard intraoperative diagnostic test in the surgical management of cerebral aneurysms and arteriovenous malformations. Hybrid procedures, providing multimodality image-guided surgeries and combining endovascular with microsurgical strategies within the same surgical session, have become feasible and safe. Microdoppler is a safe, noninvasive, and reliable technique for evaluation of hemodynamics of vessels in the surgical field, with the advantage of ease of use. Intraoperative MRI provides an effective navigation tool for cavernoma surgery, in addition to assessing the extent of resection during the procedure. Intraoperative CT scanning has the advantage of very high sensitivity to acute bleeding, thereby assisting in the confirmation of the extent of hematoma evacuation and the extent of vascular anomaly resection. Intraoperative ultrasound aids navigation and evacuation assessment during intracerebral hematoma evacuation surgeries. It supports the concept of minimally invasive surgery and has undergone extensive development in recent years, with the quality of ultrasound imaging having improved considerably. Image-guided therapy, combined with modern intraoperative imaging modalities, has changed the fundamentals of conventional vascular neurosurgery by presenting real-time visualization of both normal tissue and pathological entities. These imaging techniques are important adjuncts to the surgeon's standard surgical armamentarium. Familiarity with these imaging modalities may help the surgeon complete procedures with improved safety, efficiency, and clinical outcome.

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.

Intraoperative magnetic resonance imaging during surgery for pituitary adenomas: pros and cons

Surgery for pituitary adenomas still remains a mainstay in their treatment, despite all advances in sophisticated medical treatments and radiotherapy. Total tumor excision is often attempted, but there are limitations in the intraoperative assessment of the radicalism of tumor resection by the neurosurgeon. Standard postoperative imaging is usually performed with a few months delay from the surgical intervention. The purpose of this report is to review briefly the facilities and kinds of intraoperative magnetic resonance imaging for all physician and surgeons involved in the management of pituitary adenomas on the basis of current literature. To date, there are several low- and high-field magnetic resonance imaging systems available for intraoperative use and depiction of the extent of tumor removal during surgery. Recovery of vision and the morphological result of surgery can be largely predicted from the intraoperative images. A variety of studies document that depiction of residual tumor allows targeted attack of the remnant and extent the resection. Intraoperative magnetic resonance imaging offers an immediate feedback to the surgeon and is a perfect quality control for pituitary surgery. It is also used as a basis of datasets for intraoperative navigation which is particularly useful in any kind of anatomical variations and repeat operations in which primary surgery has distorted the normal anatomy. However, setting up the technology is expensive and some systems even require extensive remodeling of the operation theatre. Intraoperative imaging prolongs the operation, but may also depict evolving problems, such as hematomas in the tumor cavity. There are several artifacts in intraoperative MR images possible that must be considered. The procedures are not associated with an increased complication rate.

Advances in pituitary imaging technology and future prospects

There have been substantial advances in pituitary imaging in the last half-century. In particular, magnetic resonance imaging is now established as the imaging modality of choice, providing high quality images of the hypothalamic-pituitary axis and adjacent structures. More recent technological advances, such as the emergence of 3 Tesla MRI, are already being widely incorporated into imaging practice. However, other advanced techniques, including a variety of potential imaging biomarkers, still require further research to evaluate their potential and define their precise role. The recent development of intraoperative MRI appears promising and may have the potential to improve the outcome of pituitary surgery. Modern high quality imaging inevitably leads to the discovery of incidental lesions, including those within the pituitary gland, although it also plays a central role in their subsequent evaluation and management.

MR fluoroscopy in vascular and cardiac interventions (review)

Vascular and cardiac disease remains a leading cause of morbidity and mortality in developed and emerging countries. Vascular and cardiac interventions require extensive fluoroscopic guidance to navigate endovascular catheters. X-ray fluoroscopy is considered the current modality for real time imaging. It provides excellent spatial and temporal resolution, but is limited by exposure of patients and staff to ionizing radiation, poor soft tissue characterization and lack of quantitative physiologic information. MR fluoroscopy has been introduced with substantial progress during the last decade. Clinical and experimental studies performed under MR fluoroscopy have indicated the suitability of this modality for: delivery of ASD closure, aortic valves, and endovascular stents (aortic, carotid, iliac, renal arteries, inferior vena cava). It aids in performing ablation, creation of hepatic shunts and local delivery of therapies. Development of more MR compatible equipment and devices will widen the applications of MR-guided procedures. At post-intervention, MR imaging aids in assessing the efficacy of therapies, success of interventions. It also provides information on vascular flow and cardiac morphology, function, perfusion and viability. MR fluoroscopy has the potential to form the basis for minimally invasive image-guided surgeries that offer improved patient management and cost effectiveness.

Development of stereotactic mass spectrometry for brain tumor surgery

BACKGROUND

Surgery remains the first and most important treatment modality for the majority of solid tumors. Across a range of brain tumor types and grades, postoperative residual tumor has a great impact on prognosis. The principal challenge and objective of neurosurgical intervention is therefore to maximize tumor resection while minimizing the potential for neurological deficit by preserving critical tissue.

OBJECTIVE

To introduce the integration of desorption electrospray ionization mass spectrometry into surgery for in vivo molecular tissue characterization and intraoperative definition of tumor boundaries without systemic injection of contrast agents.

METHODS

Using a frameless stereotactic sampling approach and by integrating a 3-dimensional navigation system with an ultrasonic surgical probe, we obtained image-registered surgical specimens. The samples were analyzed with ambient desorption/ionization mass spectrometry and validated against standard histopathology. This new approach will enable neurosurgeons to detect tumor infiltration of the normal brain intraoperatively with mass spectrometry and to obtain spatially resolved molecular tissue characterization without any exogenous agent and with high sensitivity and specificity.

RESULTS

Proof of concept is presented in using mass spectrometry intraoperatively for real-time measurement of molecular structure and using that tissue characterization method to detect tumor boundaries. Multiple sampling sites within the tumor mass were defined for a patient with a recurrent left frontal oligodendroglioma, World Health Organization grade II with chromosome 1p/19q codeletion, and mass spectrometry data indicated a correlation between lipid constitution and tumor cell prevalence.

CONCLUSION

The mass spectrometry measurements reflect a complex molecular structure and are integrated with frameless stereotaxy and imaging, providing 3-dimensional molecular imaging without systemic injection of any agents, which can be implemented for surgical margins delineation of any organ and with a rapidity that allows real-time analysis.

Clinical and economic outcomes of low-field intraoperative MRI-guided tumor resection neurosurgery

PURPOSE

To compare low-field (0.15 T) intraoperative magnetic resonance imaging (iMRI)-guided tumor resection with both conventional magnetic resonance imaging (cMRI)-guided tumor resection and high-field (1.5 T) iMRI-guided resection from the clinical and economic point of view.

MATERIALS AND METHODS

We retrospectively compared 65 iMRI patients with 65 cMRI patients in terms of hospital length of stay, repeat resection rate, repeat resection interval, complication rate, cost to the patient, cost to the hospital, and cost effectiveness. In addition, we compared our low-field results with previously published high-field results.

RESULTS

The complication rate was lower for iMRI vs. cMRI in patients presenting for their initial tumor resection (45 vs. 57 complications, P = 0.048). The iMRI repeat resection interval was longer for this cohort (20.1 vs. 6.7 months, P = 0.020). iMRI was more cost-effective than cMRI for patients who had repeat resections ($10,690/RFY vs. $76,874/RFY, P < 0.001). We found no other clinical or economic differences between iMRI- and cMRI-guided tumor resection surgeries. Overall, we did not find the advantages to low-field iMRI that have been reported for high-field iMRI.

CONCLUSION

There is no adequate justification for the widespread installation of low-field iMRI in its current development state.

Recent advances in pituitary tumor management

PURPOSE OF REVIEW

Advances in the neurosurgical management of pituitary tumors have included the refinement of surgical access and significant progress in navigation technology to help further reduce morbidity and improve outcome. Similarly, stereotactic radiosurgery has evolved to become an integral part in pituitary tumors not amenable to medical or surgical treatment.

RECENT FINDINGS

The evolution of minimally invasive surgery has evolved toward endoscopic versus microscopic trans-sphenoidal approaches for pituitary tumors. Debate exists regarding each approach, with advocates for both championing their cause. Stereotactic and fractional radiosurgery have been shown to be a safe and effective means of controlling tumor growth and ensuring hormonal stabilization, with longer-term data available for GammaKnife compared with CyberKnife.

SUMMARY

The advances in trans-sphenoidal surgical approaches, navigation technological improvements and the current results of stereotactic radiosurgery are discussed.

Intraoperative MR imaging in a case of a cervical spinal cord lesion

The aim of this article is to describe the feasibility of performing intraoperative MR imaging in patients with spinal cord lesions and the potential value of this technique. The authors report a case involving a 28-year-old man who presented with chronic cervical pain and pain along the ulnar side of the forearms during neck flexion. Findings on clinical examination were normal, but MR imaging revealed a multicystic cervical spinal cord lesion. Surgery was undertaken to open the cysts, evacuate old blood, and search for pathological tissue. Intraoperative MR imaging showed that the caudal cyst was not opened, and surgery was therefore continued. The caudal cyst was fenestrated and a suspected small cavernous malformation was removed. Electrophysiological monitoring was performed both before and after the intraoperative MR imaging. The use of intraoperative MR imaging changed the strategy of the procedure and helped the surgeon to safely enter all the cysts in the cervical cord.

Multimodality intraoperative MRI for brain tumor surgery

Intraoperative MRI has already fundamentally changed the way current brain tumor surgery is performed. The ability to integrate high-field MRI into the operating room has allowed intraoperative MRI to emerge as an important adjunct to CNS tumor treatment. Furthermore, the ability of MRI to successfully couple with molecular imaging (PET and/or optical imaging), neuroendoscopy and therapeutic devices, such as focused ultrasound, will allow it to emerge as an important image-guidance modality for improving brain tumor therapy and outcomes.

From vision to reality: the origins of intraoperative MR imaging

Intraoperative MR imaging has become one of the most important concepts in present day neurosurgery. The brain shift problem with navigation, the need for assessment of the degree of resection and the need for detection of early postoperative complications were the three most important motives that drove the development of this technology. The GE Signa System with the "double doughnut" design was the world's first intraoperative MRI. From 1995 to 2007 more than 1,000 neurosurgical cases were performed with the system. The system was used by several different specialties and in neurosurgery it was most useful for complete resection of low-grade gliomas, identification and resection of small or deep metastases or cavernomas, recurrent pituitary adenomas, cystic tumors, biopsies in critical areas and surgery in recurrent GBM cases. Main superiorities of the system were the ability to scan without patient movement to get image updates, the ability to do posterior fossa cases and other difficult patient positioning, the easiness of operation using intravenous sedation anesthesia and the flexibility of the system to be used as platform for new diagnostic and therapeutic modalities.

Intraoperative imaging in neurosurgery: where will the future take us?

Intraoperative MRI (ioMRI) dates back to the 1990s and since then has been successfully applied in neurosurgery for three primary reasons with the last one becoming the most significant today: (1) brain shift-corrected navigation, (2) monitoring/controlling thermal ablations, and (3) identifying residual tumor for resection. IoMRI, which today is moving into other applications, including treatment of vasculature and the spine, requires advanced 3T MRI platforms for faster and more flexible image acquisitions, higher image quality, and better spatial and temporal resolution; functional capabilities including fMRI and DTI; non-rigid registration algorithms to register pre- and intraoperative images; non-MRI imaging improvements to continuously monitor brain shift to identify when a new 3D MRI data set is needed intraoperatively; more integration of imaging and MRI-compatible navigational and robot-assisted systems; and greater computational capabilities to handle the processing of data. The Brigham and Women's Hospital's "AMIGO" suite is described as a setting for progress to continue in ioMRI by incorporating other modalities including molecular imaging. A call to action is made to have other researchers and clinicians in the field of image guided therapy to work together to integrate imaging with therapy delivery systems (such as laser, MRgFUS, endoscopic, and robotic surgery devices).

Advanced imaging in brain tumor surgery

The advanced imaging techniques outlined in this article are only slowly establishing their place in surgical practice. Even a low risk of false information is unacceptable in neurosurgery, thus decision-making is necessarily conservative. As more validation studies and greater experience accrue, surgeons are becoming more comfortable weighing the quality of information from functional imaging studies. Advanced imaging information is highly complementary to established surgical "good practice" such as anatomic planning, awake craniotomy, and electrocortical stimulation; its greatest impact is perhaps on how neurosurgery is planned and discussed before the patient is ever brought to the operating room. Access to functional magnetic resonance (MR) imaging, diffusion tractography, and intraoperative MR imaging can influence neurosurgical decisions before, during, and after surgery. However, the widespread adoption of these techniques in neurosurgical practice remains limited by the lack of standardized methods, the need for validation across institutions, and the unclear cost-effectiveness particularly for intraoperative MR imaging. Before advanced imaging results can be used therapeutically, it is incumbent on the neurosurgeon and neuroradiologist to develop a working understanding of each technique's strengths and weaknesses, positive and negative predictive values, and modes of failure. This content presents several imaging methods that are increasingly used in neurosurgical planning. As these techniques are progressively applied to surgery, radiologists, medical physicists, neuroscientists, and engineers will be necessary partners with the treating neurosurgeon to bridge the gap between the experimental and the therapeutic.

Application of intraoperative high-field magnetic resonance imaging in pediatric neurosurgery

OBJECT

Over the past decade, the use of intraoperative MR (iMR) imaging in the pediatric neurosurgical population has become increasingly accepted as an innovative and important neurosurgical tool. The authors summarize their experience using a mobile 1.5-T iMR imaging unit with integrated neuronavigation with the goal of identifying procedures and/or pathologies in which the application of this technology changed the course of surgery or modified the operative strategy.

METHODS

A database has been prospectively maintained for this patient population. The authors reviewed the hospital charts and imaging results for all patients in the database. This review revealed 105 neurosurgical procedures performed in 98 children (49 male and 49 female) between March 1998 and April 2008. Intradissection (ID) and/or quality assurance images were obtained at the discretion of the surgeon.

RESULTS

The median age at surgery was 12 years (4 months-18 years). One hundred intracranial and 5 spinal procedures were performed; 22 of these procedures were performed for recurrent pathology. Surgical planning scans were obtained for 102 procedures, and neuronavigation was used in 93 patients. The greatest impact of iMR imaging was apparent in the 55 procedures to resect neoplastic lesions; ID scans were obtained in 49 of these procedures. Further surgery was performed in 49% of the procedures during which ID scans had been obtained. A smaller proportion of ID scans in the different cranial pathology groups (5 of 21 epilepsy cases, 4 of 9 vascular cases) resulted in further resections to meet the surgical goal of the surgeon. Two ID scans obtained during 5 procedures for the treatment of spinal disease did not lead to any change in surgery. Postoperative scans did not reveal any acute adverse events. There was 1 intraoperative adverse event in which a Greenberg retractor was inadvertently left on during ID scanning but was removed after the scout scans.

CONCLUSIONS

The application of iMR imaging in the pediatric neurosurgical population allows, at minimum, the opportunity to perform less invasive surgical exposures. Its potential is greatest when its high-quality imaging ability is coupled with its superior neuronavigation capabilities, which permits tracking of the extent of resection of intracranial tumors and, to a lesser extent, other lesions during the surgical procedure.

Integrated MR-laparoscopy system with respiratory synchronization for minimally invasive liver surgery

PURPOSE

The laparoscope has been invaluable in minimally invasive surgery, but provides only a surface view of target tissue; therefore it is lacking internal tissue information. In combination with the laparoscope for visualizing the cross-sectional view of the tissue, MRI is superior to ultrasonography or X-ray CT, because of its high soft-tissue contrast, arbitrary slice orientation and lack of radiation properties. Thus, we propose an integrated MR-laparoscopy system with a respiratory-synchronized navigation.

METHODS

A transmit/receive RF coil for localized MR imaging with a 0.5 T open-MRI was mounted onto the tip of an MR-compatible laparoscope. The signal detection of the coil was examined with an excised porcine liver sample, an agar phantom and the abdominal wall of a healthy volunteer. A real-time navigation system to compensate for respiratory motion was developed, and examined with a healthy volunteer.

RESULTS

The SNRs of the local MR images were 112, 62, and 62 in the liver sample, phantom, and volunteer. The navigation system successfully displayed the scope view, scope location and orientation, and MR images with respiratory-synchronized real time operation.

CONCLUSIONS

The MR-imaging and synchronization function of the proposed system seemed to be helpful for laparoscopic surgery.

Intraoperative MRI: safety

Intraoperative and interventional MR have opened a new chapter in neurosurgery bringing both new therapeutic opportunities and creating unique safety challenges for the MR operating room. The presence of a large magnetic field in the otherwise familiar environment of the operating room necessitates site-specific comprehensive policies for safety, staff training, infection control, and MR compatibility. Intraoperative MRI also creates unique MR image interpretation challenges that are of paramount significance for patient safety. These safety concerns are reviewed with particular reference to the nine years experience of the authors.

Impact of intraoperative high-field magnetic resonance imaging guidance on glioma surgery: a prospective volumetric analysis

OBJECTIVE

To determine the impact of intraoperative magnetic resonance imaging (iMRI) on the decision to proceed with additional glioma resection during surgery and to maximize extent of resection (EOR).

METHODS

Patients who underwent craniotomy for glioma resection with high-field iMRI guidance were prospectively evaluated between September 2006 and August 2007. Volumetric analysis and EOR were assessed with iMRI, using postcontrast T1-weighted images for tumors showing contrast enhancement and T2-weighted images for nonenhancing tumors.

RESULTS

Forty-six patients underwent resection using iMRI guidance, with iMRI being used to evaluate the EOR in 44 patients and for reregistration in 2 patients. Surgery was terminated after iMRI in 23 patients (52%) because gross total resection was achieved or because of residual tumor infiltration in an eloquent brain region. Twenty-one patients (47%) underwent additional resection of residual tumor after iMRI. For enhancing gliomas, the median EOR increased significantly from 84% (range, 59%-97%) to 99% (range, 85%-100%) with additional tumor removal after iMRI (P < 0.001). For nonenhancing gliomas, the median EOR increased (from 63% to 80%) with additional tumor removal after iMRI, but not significantly, owing to the small sample size (7 patients). Overall, the EOR increased from 76% (range, 35%-97%) to 96% (range, 48%-100%) (P < 0.001). Gross total resection was achieved after additional tumor removal after iMRI in 15 of 21 patients (71%). Overall, 29 patients (65%) experienced gross total resection, and in 15 (52%), this was achieved with the contribution of iMRI.

CONCLUSION

High-field iMRI is a safe and reliable technique, and its use optimizes the extent of glioma resection.

The predictive value of low–field strength magnetic resonance imaging for intraoperative residual tumor detection

Object

Neurosurgeons have been utilizing intraoperative MR (iMR) imaging to evaluate the extent of tumor resection since the 1990s. A low–field strength (0.12 T) MR imaging unit (PoleStar N20, Medtronic) is a practical and relatively inexpensive iMR imaging system that has found increased use in neurosurgery. The gold standard for postoperative detection of residual tumor has been high-strength MR imaging performed within 48 hours of resection. The object of this study was to determine the predictive concordance of low-strength iMR imaging with standard high-strength MR imaging for detection of residual tumor.

Methods

The authors retrospectively evaluated the MR images from 74 intracranial tumor resections, comparing the intraoperative images obtained using a 0.12-T iMR imaging unit to the immediate postoperative images obtained using a standard 1.5-T MR imaging unit within 48 hours after surgery.

Results

The sensitivity of low-field MR imaging for detection of residual tumor was 0.74 (95% CI 0.58–0.86), and its specificity was 0.97 (95% CI 0.83–1). When only glial tumors (42 of the 74 lesions) were analyzed, the sensitivity was 0.82 (95% CI 0.59–0.94) and the specificity was 0.95 (95% CI 0.73–1).

Conclusions

These data could assist the neurosurgeon who has to decide intraoperatively whether the observed iMR images show residual tumor or not.

TRANSSPHENOIDAL PITUITARY MACROADENOMAS RESECTION GUIDED BY POLESTAR N20 LOW-FIELD INTRAOPERATIVE MAGNETIC RESONANCE IMAGING

OBJECTIVE

To evaluate the applicability of low-field intraoperative magnetic resonance imaging (iMRI) during transsphenoidal surgery of pituitary macroadenomas.

METHODS

Fifty-five transsphenoidal surgeries were performed for macroadenomas (modified Hardy's Grade II–IV) resections. All of the surgical processes were guided by real-time updated contrast T1-weighted coronal and sagittal images, which were acquired with 0.15 Tesla PoleStar N20 iMRI (Medtronic Navigation, Louisville, CO). The definitive benefits as well as major drawbacks of low-field iMRI in transsphenoidal surgery were assessed with respect to intraoperative imaging, tumor resection control, comparison with early postoperative high-field magnetic resonance imaging, and follow-up outcomes.

RESULTS

Intraoperative imaging revealed residual tumor and guided extended tumor resection in 17 of 55 cases. As a result, the percentage of gross total removal of macroadenomas increased from 58.2% to 83.6%. The accuracy of imaging evaluation of low-field iMRI was 81.8%, compared with early postoperative high-field MRI (Correlation coefficient, 0.677; P &lt;0.001). A significantly lower accuracy was identified with low-field iMRI in 6 cases with cavernous sinus invasion (33.3%) in contrast to the 87.8% found with other sites (Fisher's exact test, P &lt;0.001).

CONCLUSION

The PoleStar N20 low-field iMRI navigation system is a promising tool for safe, minimally invasive, endonasal, transsphenoidal pituitary macroadenomas resection. It enables neurosurgeons to control the extent of tumor resection, particularly for suprasellar tumors, ensuring surgical accuracy and safety, and leading to a decreased likelihood of repeat surgeries. However, this technology is still not satisfying in estimating the amount of the parasellar residual tumor invading into cavernous sinus, given the false or uncertain images generated by low-field iMRI in this region, which are difficult to discriminate between tumor remnant and blood within the venous sinus.

MRI-guided brain tumor cryoablation in a rabbit model

PURPOSE

To report the results of an animal trial exploring the feasibility of minimally invasive MR imaging-guided rabbit brain tumor cryoablation with an argon-based cryo-unit.

MATERIALS AND METHODS

VX2 tumor segments (0.96x0.96x50 mm) were implanted into parietal lobes of 26 New Zealand white rabbits. Seventeen rabbits were treated with cryoablation. Six rabbits were not treated and formed a control group. Cryoablation was achieved with a MR-compatible cryoablation unit using 1.47-mm cryoprobes in an open 0.23 Tesla (T) MRI scanner. The therapeutic response was evaluated with follow-up MRI and the corresponding histopathology. A 3.0 T MRI scanner was used for preoperative and postoperative follow-up imaging. Posttreatment imaging with subsequent endpoints was performed at 3, 7, 14, and 60 days after the treatment.

RESULTS

Of 17 rabbits, 6 died on the first postoperative day. Histopathology revealed coagulation necrosis in samples from the 3rd, 7th, and 14th day, and reactive changes at 60 days, these findings were consistent with MRI. The longest survival time in the treatment group, 60 days (n=2), was considerably longer than the survival times in control group.

CONCLUSION

Successful rabbit brain tumor cryoablation can be achieved using argon-based cryodevice under MRI guidance. The complication rate in this series was relatively high.

Intraoperative 3T MR imaging for spinal cord tumor resection: feasibility, timing, and image quality using a "twin" MR-operating room suite

We assessed feasibility, safety, and timing of an original intraoperative MR procedure in 3 cases of resection of spinal cord glioma by using a clinical 3T MR system connected to an adjacent operating room in a design being coined "twin" or "dual" MR-operating room suite.

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.

Intraoperative magnetic resonance imaging in neurosurgery: the Brigham concept

The resection of brain tumors is limited by the surgeon's ability to precisely define margins. To overcome this problem, various neuronavigational tools have been used. The development of image-guided navigation systems represents a substantial improvement in the microsurgical treatment of various intracranial lesions. However, a major drawback of this technology is that they use images acquired preoperatively, on which the surgical planning and intraoperative performance is based. As the intracranial anatomy dynamically changes during a neurosurgical procedure, only intraoperatively acquired images can provide the neurosurgeon with the information needed to perform real-time, image-guided surgery. Because magnetic resonance imaging best delineates the soft-tissue extent of most tumors, it currently remains the superior method for intraoperative image guidance. In this review, we outline the development as well as current and possible future applications of the intraoperative MRI (iMRI) unit at the Brigham and Women's Hospital, Boston, MA.

Role of magnetic resonance tractography in the preoperative planning and intraoperative assessment of patients with intra-axial brain tumours

PURPOSE

This study was conducted to assess the possibility of identifying precise white matter tracts situated in proximity to intracranial tumours, to define the anatomical and topographical relations between the same white matter tracts and the tumour, to verify the possibility of integrating tractographic images in the context of a package of three-dimensional anatomical images to send to the neuronavigation system, to assess the impact of this information on surgical planning, and to analyse, both pre-and postoperatively, the patient's clinical conditions as an index of the functional integrity of the fibres themselves.

MATERIALS AND METHODS

Twenty-five patients underwent diffusion tensor study prior to neurosurgery. With the use of dedicated software, relative colour maps were obtained and the trajectories of the white matter tracts adjacent to the tumour were reconstructed in three dimensions. These were then processed for preoperative planning. Planning, which was performed with the neuronavigator, was based on analysis of the location of the course of the main white matter tracts adjacent to the lesion (pyramidal tract, optic radiation and arcuate fasciculus). Two neurosurgeons were asked whether the tractography images had modified the access and/or intraoperative approach to the tumour. All patients were clinically assessed both pre-and postoperatively 1 month after the procedure to define the presence of symptoms related to the involvement of the white matter tracts studied and therefore to assess the integrity of the fibres after the operation.

RESULTS

In one patient, the tumour was situated away from all the tracts studied and did not compress them in any way. Overall, 40/75 tracts studied had no anatomical relation with the tumour, were not displaced by the tumour or could not be visualised in their entire course. Analysis of the remaining 35 white matter tracts led to an a priori change in the surgical approach for corticotomy in four patients (16%), with no disagreement between the two neurosurgeons and an impact on the extent of resection during surgery in 17 (68%), thus an overall impact on the surgical procedure in 80% of cases. Eight patients showed no symptoms related to the involvement of the white matter tracts studied. In the remaining 17 patients, the symptoms were related to involvement of the pyramidal tract, arcuate fasciculus or optic radiation. At 1-month follow-up, one previously asymptomatic patient reported a speech disorder (transcortical sensory dysphasia); in the remaining 24, symptoms remained unchanged, with a tendency to improvement in 14/17 with symptoms related to involvement of white matter tracts studied.

CONCLUSIONS

Magnetic resonance (MR) tractography offers the neurosurgeon an anatomical panoramic view that can improve surgical planning for the resection of intracranial tumours. Despite the high incidence of cases in which the lesion is responsible for changes that hinder the reconstruction of white matter tracts, the technique can change the surgical approach for corticotomy, defines the extent of resection and leads to some change in the procedure in 80% of cases. The improvement of pre-existing symptoms and the absence of new symptoms in the postoperative phase, in our opinion, confirms the value of the technique.