MRI: A General Overview of the Field

Transcription

1 Interventional and Intraoperative MRI: A General Overview of the Field I Ferenc A. Jolesz, MD Abbreviations: LED - light-enutttng diode. 311 = threr-dlmensional THE DEVELOPMENT AND application of MRI techniques for intraoperative guidance has followed the ongoing trend in interventional radiology to apply all imaging technololgy for reducing the invasiveness of surgical diagnosis and therapy. At the same time, the emergence of interventional MRI has paralleled the introduction of various image guidance methods to surgery. These techniques became available due to the rapid progress in introducing new imaging modalities in radiology combined with the continuously improving high performance computing technology. IMAGE-GUIDED PROCEDURES The integration of imaging with computers has affected the field of diagnostic and interventional radiology and has influenced the field of surgery. This field was unaffected because the role of imaging in guiding therapeutic procedures had already been accepted by both interventional radiology and some surgical fields, including neurosurgery, head and neck surgery, and endoscopic or minimally invasive surgeries. The reason for applying images to guide procedures is the limitation of direct visualization of anatomy. Within the constraint of percutaneous interventional approaches or the minimally invasive (endoscopic or laparoscopic) surgical access routes, there is not enough visible clues to appreciate the entire anatomy. The invisibility through needles and the limited visibility through small craniotomies, short incisions, and the keyholes of endoscopes clearly necessitates some kind of intraoperative image guidance. By applying computer technology, intraoperative image guidance based on previously acquired images is readily available (1-3). Using this technology, one can navigate through the three-dimensional (3D) image data and thus fulfill the need for enhanced visibility. Nevertheless, preoperative images cannot represent the changes occurring intraoperatively. Unavoidable shifts and deformations of soft tissues can be recognized and accounted for only by intraoperative imaging. In addition, intraoperatively acquired images can assist the radiologist and surgeon in localizing the lesions, defining their margins, and targeting them for biopsy or surgical resection. From the Division of MRI. Brighain and Women s Hospital, 75 Francis Street. Boston. MA Received Orlober : acceptedoctober 16.Address reprint requests to F A J ISMRM Image-guided procedures mandate unique data acquisition, processing, and display methods. These also require the integration of an imaging modality with high performance computers and algorithmic tools (automated segmentation and registration, virtual reality displays) (4,5). A simulated procedural environment is critical for creating an image-based virtual reality environment for performance of simulated procedures. These can be used not only for training and planning but for real-time monitoring and guiding various interventional and surgical procedures. The role of surgical planning is to define the safest possible approach before the actual performance of the invasive procedure (6). In this optimization process, alternative methods are tested and analyzed using a preoperative model. Further expansion of surgical planning and simulation beyond some minimally invasive brain and head and neck applications requires more complex methods such as elastic warping, which accounts for tissue deformations and organ shifts. Intraoperative tissue distortions, shifts, and displacements cause substantial errors. Modeling of elastic deformations and correction of the images is possible, but within a limited range, beyond which intraoperative imaging becomes necessary. By merging the procedures with 3D images of anatomy or function, we will be able to integrate all the available information intraoperatively (7). Near-real-time imaging during procedures provides frequent updates about changes in anatomy or organ motion, depicts the position of instruments, and establishes the relationship between the patient and the images. This type of interactive visualization is sufficient for most percutaneous biopsy and intravascular interventional procedures but is not satisfactory for intraoperative image guidance when high soft tissue contrast and volumetric visualization is necessary. Advances in imaging technology, particularly in 3D ultrasound, high speed CT, and especially MRI combined with high performance computing, permit the integration of real-time volumetric imaging and interactive localization. For intraoperative image guidance, in most cases, positional sensing and tracking are essential for establishing a relationship between the operator s movements and the image-based information. The various tracking technologies with single or multiple sensors permit the use of instruments such as pointers or tracking tools (virtual needles or pointers) and enable the physician to plan or define alternative trajectories in real time. Position sensors can be combined with any cross-sectional imaging modality to provide interactive scanning. Virtual endoscopic display of CT and MRI data can be visualized at the exact position of a sensor in the real endoscope or at the tips of endoscopic instruments. 3

2 An interactive display of these virtual images improves both diagnosis and orientation during therapeutic endos- COPY (8). MRI-GUIDED INTERVENTIONS As compared with other imaging modalities, MRI is most attractive for guiding, monitoring, and controlling therapy ( ). Most features of a conventional closed-mri system are already consistent with the requirements of image guidance (relatively high spatial, temporal, and superior contrast resolution, directly acquired multiplanar scan plane, temperature, and flow sensitivity, etc.). The major limitation has been the closed configuration of the magnets used for diagnostic imaging, which does not allow access to the patient during imaging. Nevertheless, high field (1.5-T) systems can be used for intewentional procedures that use percutaneous access. It has obvious advantages in image quality, which is especially important for intravascular interventions. The combination of MRI systems with fluoroscopy and ultrasound may provide additional ways to improve image guidance and complement MRI-based targeting or monitoring. Although a closed-configuration MRI system can be applied for image-guided procedures, the ideal interventional MR unit allows almost unlimited access to the patient from all sides while enabling the operators to obtain high resolution images in any desired plane in real time. The system should emulate the advantages of other competing image guidance systems (fluoroscopy, CT, or ultrasound) by providing high resolution near real-time images. In addition, the same standards of sterility and patient monitoring must be met as in conventional operating rooms. Full access to MRI-compatible anesthesia, monitoring, and life support equipment must be available. Open configuration magnets permit restricted (horizontal gap configuration) or full (vertical gap configuration) access to the patient. Whereas many companies developed open MRI systems (they range from low to midfield strength), which can be used for (some) interventional procedures, their horizontal access limits the performance to percutaneous procedures, which can be monitored within the imaging volume. So far, only one intraoperative MRI system offers a vertically open midfield at.5 T. This MR unit was designed specifically for interventional and intraoperative imaging with interactive features integrating the MRI with anesthesia and therapy systems (12). If the intraoperative MRI systems are equipped with sensors or tracking devices, they can provide an interactive multiplanar scanning environment for diagnostic biopsies, percutaneous therapeutical procedures, endoscopies, or other minimally invasive interventions and for open surgeries (13). The ability to interactively use multiplanar MRI to localize first and then to target and eventually monitor the procedure is an essential feature. Although the ideal imaging for intraoperative guidance is in real time, the temporal resolution requirements of interventional MRI are quite variable. The definition of realtime imaging or dynamic image update is relative and depends on the time constraints of the processes being imaged. For interactive image guidance, the images must be generated and displayed without disrupting or slowing down the procedure. Several novel imaging techniques offer improved temporal resolution by less redundant spatial encoding and without considerably affecting spatial resolution and signal-to-noise ratio. MR fluoroscopy and adaptive d-ynamic imaging approaches can use preexist- ing information for encoding changes in image data (14-16). For MR-guided biopsies, a variety of techniques have been used, ranging from freehand to stereotactic grid systems. The most important recent development is related to the use of near-real-time needle guidance methods using optical-tracking-based frameless stereotaxy systems. This system allows tracking of a biopsy probe with a 3D optical digitizer system using three video sensors to localize two or three infrared light-emitting diodes (LEDs) mounted on a handheld probe (13). A biopsy needle or other devices can be attached to this probe. Similar to diagnostic ultrasound, the operator uses the position of the probe to interactively select any image planes, but unlike in ultrasound, multiple orthogonal planes can be acquired in relationship to the probe s position. The one-step localization and targeting using MRI-based image guidance is essential for brain biopsy and has all the characteristic features of frameless stereotaxy. When compared with ultrasound, CT, and fluoroscopy, MR-guided biopsy at present probably compares unfavorably in regard of procedure cost. This results primarily from the high initial capital investment for the installation of interventional MRI systems. Nevertheless, some lesions are only visualized with MRI, and MR guidance obviously is the method of choice to biopsy those lesions that are difficult to access using standard imaging guidance techniques. Interactive MR guidance for brain biopsy is a safer and faster procedure than frame-based stereotaxy. The procedure time is significantly shorter than with standard frame-based biopsies (2 versus 4 hours). Localizing and targeting with a virtual needle track and near-real-time imaging of the needle advancement in multiple planes provides preprocedural planning of the optimal trajectory and appreciation of anatomy around the needle path (17,18). One other area in which MH-guided biopsies have been shown to be advantageous over CT and ultrasound guidance are skull base lesions in which beam-hardening artifacts can make detection of lesions difficult and visualization by ultrasound is limited. MRI has been found to be an ideal imaging method because of its ability to freely choose an imaging plane and visualize blood vessels in the brain. The application of interventional MRI for sinus endoscopy has similar advantages (19-21). MRI is particularly useful if the lesion can be difficult to access with CT (because of limited angle of the gantry) or with ultrasound guidance, ie, lesions in the dome of the liver. In the liver, real-time interactive scan plane selection and trajectory planning in an open system provided an obvious progress from previous cross-sectional biopsy methods (10.13). MRI of the breast is evolving as a useful modality in certain clinical situations. Occasionally, lesions may be seen only on MRI but not with any other imaging method. Several guiding devices and breast immobilization techniques have been described to optimize MR-guided breast interventions (22). As early as biopsies in MR were investigated (9,23,24), interest in drainage procedures was aroused. Early reports described clinical experiences with a wire-sheath system for biopsies and drainages (23.24). Further experiences in performing an MR-guided nephrostomy, an MR-guided cholecystostomy (25), and an MR-guided drainage and shunting of brain cysts (17,18) were reported recently. Although image guidance by MRI was first applied to biopsies, it was justified by the superior tissue charac- 4 JMRl January/February 1998

3 terization of MRI, which explains its high sensitivity for lesion detection. This original application was followed by the recognition of the potential of MRI to monitor thermal ablations. Using temperature-sensitive MRI sequences, various thermal ablations with image-based control of energy delivery can be performed. It was conceived that MRI-guided interstitial laser therapy, cryotherapy, and radiofrequency-induced thermal ablative treatment can replace some open tumor surgeries (26-32). For the control of energy deposition, temperature-sensitive imaging is necessary to avoid the heating of normal structures (33-36). Computerized techniques can predict the spatial extent of temperature changes and end power deposition before it reaches critical structures. MRI allows continuous monitoring of heating or freezing of various soft tissue tumors. One key question, however, remains unanswered: whether the MRI detection of tissue phase transitions induced by heat or cold is predictive of cell death. No scientific evidence is available yet that any of the MR parameters or their combination represent irreversible definite cell death. Heat is generated by high intensity acoustic waves, which can be focused and targeted. Focused ultrasound treatment avoids tissue damage outside the focal volume, and it is potentially superior to other more invasive thermal tumor ablations (36-39). Early clinical trials with breast tumor treatment are very promising (40). This noninvasive ablative method may be applicable not only for tumor treatment but also for functional neurosurgery. Most recently, occlusion of blood vessels was demonstrated using MRI-guided focused ultrasound (4 1). Image-guided percutaneous procedures can develop into new therapies such as targeted drug delivery, which include installation of chemicals, chemotherapeutic agents, or high energy isotopes (brachytherapy). The use of focused ultrasound or other directed energy sources for targeted drug delivery or gene therapy requires control of energy deposition by MRI. Alteration of cells, cell membranes, and barriers (blood-brain bamer) allows the delivery of larger molecules (drugs, peptides, proteins, genetic material) into cells or organ compartments that are otherwise not accessible (42). Because of the high magnetic fields in MR scanners, conventional ferromagnetic percutaneous access tools or surgical instruments cannot be used in these systems. They are subject to acceleration and spatial dislocation, thus potentially endangering both the patient and the operators. Even nonferromagnetic metals cause disturbing image artifacts by inducing magnetic field inhomogeneities. Standard medical-grade stainless steel cannot be used because of artifact induction. Therefore, several alloys and ceramic materials have been developed that can be used in MR systems. Passive visualization techniques may take advantage of some residual device-induced susceptibility artifacts, generally sufficient for recognizing needles during biopsy and drainage procedures. Passive visibility of these devices has been investigated at different field strengths. For intravascular interventional procedures, active tracking techniques are under development, basically implementing miniature radiofrequency coils into the catheters or endoscopes. The position of these mini coils can then be localized with MRI in three dimensions and they can be tracked in near real time (43.44). 0 INTRAOPERATIVE MRI The vision of combining the resources of an operating room with MRI technology and high performance computing is relatively new (45.46). It became possible to ful- fill this vision because the simultaneous combination of direct vision and imaging is possible within a unique environment of interventional MRI. The intraoperative MRI incorporates both the operating room and an imaging system such as MRI. This integrated system allows accurate localization and targeting, allows definition of tumor margins or the extent of disease, and defines anatomy even as it is changing during surgeries. This application of MRI guidance may improve clinical outcome and reduce complication rates by decreasing invasiveness. By merging MRI with frameless stereotaxy, navigational tools, and multimodality image fusion, the combination of all available information with image update can revolutionize minimally invasive therapy and will result in new treatment strategies and approaches. With the availability of MR-compatible surgical devices and instruments, including the intraoperative microscope, the first open brain surgery using intraoperative MRI was performed in Currently, craniotomies using MR guidance are performed routinely at several institutions, and lesions treated include intracranial hemorrhages and cysts, as well as malignant and benign brain tumors, cavernous hemangiomas, and arteriovenous malformations. During open surgery, the surgeon usually cannot see beyond the visible surface, so it is very helpful to have intraoperative imaging to scan the operational volume before the craniotomy and during the intracranial surgery. Nevertheless, the major advantage of intraoperative imaging is obvious during tumor resections when the residual tumor is visualized and the intraoperative MRI ensures the completeness of resection ( 17.18). Intraoperative MR guidance also can been applied for spinal surgery. It is anticipated that the ability to position patients in the scanners with vertical gap in a sitting position might open new avenues for spinal evaluation and possible dynamic spinal surgery ( 17). Intraoperative MRI facilitates the procedures in several aspects. Initial imaging at the onset of the procedure allows evaluation of the position of the lesion in relation to the position of the patient on the operating table. Realtime intraoperative MRI or frequent volumetric image updates allows the surgeon to correct and optimize the approach during the surgical procedure. This includes planning of the skin incisions and craniotomies as well as trajectories to the lesion through the parenchyma. Therefore, surgical morbidity can be reduced by minimizing injury and maximal preservation of normal tissue. Intraoperative, interactive MR guidance has been shown to enable the detection of intraprocedural complications such as hemorrhage. It can be superior to the surgeon s eye (even if assisted by a microscope) in its ability to detect abnormal tissue. This is especially true in detecting gliomas in the brain and malignant tumors in the breast. The high sensitivity of MRI in detecting tumors can be further enhanced by the intravenous application of gadolinium, which helps in evaluating the extent as well as completeness of removal of the lesion. Therefore, the lesions margins and complete removal of the lesion are monitored with serial intraprocedural imaging. All these features make the system highly superior to conventional surgery and to other imaging modalities. CONCLUSION Since its initial introduction, interventional and intraoperative MRI has evolved from an experimental research tool into a complex clinically applied technique that offers significant promise for minimally invasive surgery and in- Volume 8 - Number 1 * JMRl - 5

4 terventional radiology both in regard to procedural ease and safety. Already, several MRI scanners suitable for some percutaneous procedures or even open surgeries are being marketed. In areas such as neurosurgery and endoscopic surgery, these adjunctive imaging and guidance systems may revolutionize treatments by offering safer options than conventional direct llsualizationbased techniques. Furthermore, a potentially larger impact is anticipated to stem from the expanding role in guiding and monitoring of interstitial thermal therapies. However, cost constraints and the change in the national attitude toward cost-intensive, high-tech medicine may have negative effects on future development. Therefore, this promising technology, more than any other in the past, will have to prove its cost-effectiveness in the new health care environment before it becomes more widely used. The concept of image guidance has resulted in a strategic shift in medical imaging from diagnosis to treatment. Radiologists, interventionalists, and surgeons with computer scientists and engineers will expedite technology development. which will result in improved safety, efficacy, and cost-effectiveness. Collaboration between the imaging experts, clinical fields, and applied sciences will eventually change the interventional and the surgical fields and may result in the creation of a new medical field. References 1. Zamorano L, Jiang 2. Kadi AM. Computer-assisted neurosurgery system: Wayne State University hardware and software configuration. Comput Med Imaging Graph 1994; 18: Galloway RL, Maciunas RJ, Latimer JW. The accuracies of four stereotactic frame systems: an independent assessment. Biomed Instrum Technol 1991: 25: Zinreich JS, Tebo SA. Frameless stereotactic integration of CT imaging data: accuracy and initial applications. Radiology 1993; 188: Wells WM. Grimson WEL, Kikinis R, Jolesz FA. Adaptive segmentation of MRI data. IEEE Trans Med Imaging 1996; 15: Holman BL. 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5 45. Jolesz FA, Shtem F. The operating room of the future: report 46. Silverman SG. Jolesz FA, Newman RW, et al. Design and imof the National Cancer Institute Workshop. Invest Radio1 1992: 27: plementation of an interventional MR imaging suite. Am J Roentgen : 168: Board Certification in MRI Physics by Faiz M. Khan, Ph.D. Minneapolis, MN The American Board of Medical Physics (ABMP) has decided to start a board certification program in magnetic resonance imaging physics. The members of the inaugural MRI Examination Panel are: Wad Sobol (Chair), Jerry Allison, Stewart Bushong, Geoffrey Clarke, Dick Drost, John Hazle, Ronald Price, Perry Sprawls, and Balasubramanian Rajagopalan. The first written examination in MRI Physics will be held on August 8th and 9th, The deadline for receipt of applications for the Written Exam is January 15th Candidates are required to pass Part I (General Medical Physics) and Part I1 (MRI Physics) exams before being admitted to Part I11 (Orals) the following year. ABMP diplomates in other subspecialities who apply for MRI Physics certification are not required to take the Part I examination. For detailed information about the ABMP certification exam and its requirements please contact: American Board of MedicaI Physics, c/o Credentialing Services, Inc., P.O. Box 1502, Galesburg, Illinois , phone: (309) , fax: (309) Volume 8 - Number 1 JMRl - 7