Superconducting open-configuration MR imaging system for image-guided therapy

MR systems for image-guided therapy

The use of MRI to guide and monitor interventional procedures requires the merging of surgical and MRI environments. The ideal magnet shape for homogeneity and efficiency is spherical, but this design provides no access. Opening the sphere to provide both patient and surgeon access suggests cylindrical or biplanar magnets. Cylindrical magnets have poor surgical access but provide good imaging capabilities, which can be used in conjunction with a neighboring but distinct surgical environment. Biplanar magnets provide more and better approaches to the patient, but generally with lower field strength. Vertical biplanar systems allows surgical approaches from above but reduce the access of support staff to the patient. A hybrid magnet design, which combines the benefits of both cylindrical and biplanar magnets, can provide increased access with simultaneous approach from two sides of the patient. Application-specific magnets can target a smaller region, leading to compact magnet designs that greatly expand access for both surgical intervention as well as patient support. As the field of interventional MRI matures, the suitability of each design to specific applications will be better understood, leading to more integrated system designs tailored to the needs of image-guided therapy.

Target definition and trajectory optimization for interactive MR-guided biopsies of brain tumors in an open configuration MRI system

We present an imaging strategy for planning and guiding brain biopsies in an open configuration MR system. Preprocedure imaging was performed in a 1.5-T MR system and was designed to provide, in a clinically efficient manner, high resolution anatomical and functional/physiologic information for precise definition and tissue characterization of the target, aiming at optimization of the biopsy trajectory for planning a safe and accurate procedure. The interventions were performed in a .5-T open bore magnet, and imaging was optimized to provide the imaging quality and temporal resolution necessary for performing the procedure interactively in near real time. Brain biopsies of 21 patients were performed in a 10-month period. Segmentation and surface rendering analysis of the lesions and vascular structures and dynamic MR perfusion and cortical activation studies provided an efficient and comprehensive way to appreciate the relationship of the target to surrounding vital structures, improved tissue characterization and definition of the tumor margins, and demonstrated the location of essential cortex, allowing appropriate placement of the burr hole and choice of optimal trajectory. Interactive protocols provided good visualization of the target and the interventional devices and offered the operator real-time feedback and control of the procedure. No complications were encountered. Advanced methods of image acquisition and processing for accurate planning of interventional brain procedures and interactive imaging with MR guidance render feasible the performance of safe and accurate neurointerventional procedures.

Monitoring of radio frequency tissue ablation in an interventional magnetic resonance environment. Preliminary ex vivo and in vivo results

RATIONALE AND OBJECTIVES

The authors evaluate the feasibility of monitoring radio frequency (RF) ablation in an interventional, open-configuration, 0.5-tesla magnetic resonance (MR) environment.

METHODS

Ex vivo and in vivo RF coagulation necrosis were induced in porcine paraspinal muscle tissue using a 300 kHz monopolar RF generator applying 5 to 20 W over 3 to 9 minutes. Images were acquired simultaneous to RF application, after RF application, and in an intermittent mode (60 seconds of RF followed by 15 seconds of MR imaging). Temperature changes were monitored based on amplitude (ex vivo) and phase alterations (in vivo) of a T1-weighted graded refocused echo (GRE) sequence enabling an update every 2.5 seconds. A standardized color-coded subtraction technique enhanced signal changes. Additionally, T2- and T1-weighted spin echo (SE) images were acquired with and without intravenous contrast. Macroscopic coagulation size was compared with lesion size seen on MR images.

RESULTS

Although lesion diameters were related directly to applied RF power, the application mode had no significant impact on coagulation size (P > 0.05). As could be expected, MR imaging during RF ablation resulted in major image distortion. Radio frequency effects were seen on images acquired in the continuous and intermittent modes. Coagulation size seen on GRE images correlated well with macroscopy both ex vivo (r = 0.89) and in vivo (r = 0.92). Poorer correlation was found with postinterventional SE sequences (r = 0.78-0.84).

CONCLUSIONS

Magnetic resonance monitoring of RF effects is feasible both ex vivo as well as in vivo using temperature-sensitive sequences in an open-configuration MR environment.

The interventional magnetic resonance unit--the minimal access operating theatre of the future?

Interventional magnetic resonance units give the surgeon the potential to use intraoperative imaging to guide the surgical procedure. The advantages of magnetic resonance (MR) over other intraoperative imaging modalities include excellent soft tissue resolution, lack of ionizing radiation and the ability to reconstruct images in any desired plane. Postulated advantages include the ability to confirm adequate tumour resection, reduction in procedure magnitude and complication rate, shortened inpatient stay and the development of novel minimally invasive techniques including the use of thermal energy to destroy lesions. Fully MR compatible anaesthetic and patient monitoring equipment is available. However, before the MR-guided minimally invasive surgery can become a reality, much work is required in the assessment and development of MR compatible surgical instrumentation and equipment. This review describes the testing and development of instruments and equipment for MR image-guided surgery that we have undertaken. We describe the techniques we employ for open and minimal access surgery within this unique environment. The difficulties of operating within such an environment and the safety issues that this engenders are discussed. The current applications of intraoperative MR in the main surgical specialities are reviewed, and possible future areas of development for MR-guided minimally invasive surgery described.

Interventional magnetic resonance imaging

The development of minimally invasive surgical and interventional techniques has created a need for more accurate and sensitive image guidance and monitoring. Magnetic resonance imaging, with its superior soft tissue discrimination and multiplanar facilities, seems the obvious choice for an ideal image-guidance tool. Until recently, the employment of MRI in this role has been prevented by the physical constraints of conventional, closed-configuration machines. The problem has now been overcome by the development of an open design allowing both horizontal and vertical access to the patient in the scanner so that procedures can be performed concurrent with image acquisition. This configuration, together with the use of fast gradient echo sequences which can scan at speeds close to real time, means that a wide range of interventional procedures can be performed with on-line image guidance and monitoring. In addition, the versatility of the open design means that patients can assume physiological positions to allow dynamic joint imaging to be performed. This opens up a whole new field in the understanding of joint pathophysiology. This review article discusses these recent technological developments and their clinical applications. In particular, the potential role in guidance of biopsies, monitoring of thermal ablation techniques and applications in endoscopic surgery is outlined.

MRI developments in perspective

Following Paul Lauterbur's seminal 1973 paper in Nature, considerable work was needed to overcome a number of physical, engineering and technical problems before the new technique of magnetic resonance imaging (MRI) could be applied clinically. Much of that pioneering work was done in the UK. Since the first head and whole-body images were obtained in the late 1970s, MRI has become a widely used clinical imaging modality capable of yielding tomographic images of excellent spatial resolution and tissue contrast. This review outlines the historical development of MRI in the context both of the technical problems which had to be overcome, and of the clinical uses of MRI. Current areas of research, such as the use of MRI to map brain function, the measurement of physiological parameters such as tissue perfusion, and the use of open-access real-time MRI to guide interventional procedures, are briefly discussed.

Magnetic resonance-guided laparoscopic interstitial laser therapy of the liver

BACKGROUND

There is a necessity for an imaging method during laparoscopy to get a three-dimensional access to the target. In this study we evaluated laparoscopic interstitial laser therapy of the liver under magnetic resonance imaging guidance.

METHODS

Five domestic pigs underwent laparoscopy in an open-configuration magnetic resonance system. Under simultaneous real-time magnetic resonance imaging interstitial laser therapy was applied to the liver. Magnetic resonance images, macroscopic aspects of the lesions, and light microscopic findings were compared.

RESULTS

The interventions could be safely performed. There was no image artifact caused by instruments or by the carbon dioxide. Dynamic gadolinium-enhanced imaging proved to significantly predict the macroscopic volume of the laser lesions.

CONCLUSIONS

Magnetic resonance-guided laparoscopic interstitial laser therapy of the liver combines the advantages of minimal invasive surgery and magnetic resonance imaging. Further development should focus on laparoscopic instruments and temperature sensitive sequences.

Development and implementation of intraoperative magnetic resonance imaging and its neurosurgical applications

OBJECTIVE

We describe the development and implementation of a new open configuration magnetic resonance imaging (MRI) system, with which neurosurgical procedures can be performed using image guidance. Our initial neurosurgical experience consists of 140 cases, including 63 stereotactic biopsies, 16 cyst drainages, 55 craniotomies, 3 thermal ablations, and 3 laminectomies. The surgical advantages derived from this new modality are presented.

METHODS

The 0.5-T intraoperative MRI system (SIGNA SP, Boston, MA), developed by General Electric Medical Systems in collaboration with the Brigham and Women's Hospital, has a vertical gap within its magnet, providing the physical space for surgery. Images are viewed on monitors located within this gap and can also be acquired in conjunction with optical tracking of surgical instruments, establishing accurate intraoperative correlations between instrument position and anatomic structures.

RESULTS

A wide range of standard neurosurgical procedures can be performed using intraoperative MRI. The images obtained are clear and provide accurate and immediate information to use in the planning and assessment of the progress of the surgery.

CONCLUSION

Intraoperative MRI allows lesions to be precisely localized and targeted, and the progress of a procedure can be immediately evaluated. The constantly updated images help to eliminate errors that can arise during frame-based and frameless stereotactic surgery when anatomic structures alter their position because of shifting or displacement of brain parenchyma but are correlated with images obtained preoperatively. Intraoperative MRI is particularly helpful in determining tumor margins, optimizing surgical approaches, achieving complete resection of intracerebral lesions, and monitoring potential intraoperative complications.

Real-time interactive MRI on a conventional scanner

A real-time interactive MRI system capable of localizing coronary arteries and imaging arrhythmic hearts in real-time is described. Non-2DFT acquisition strategies such as spiral-interleaf, spiral-ring, and circular echo-planar imaging provide short scan times on a conventional scanner. Real-time gridding reconstruction at 8-20 images/s is achieved by distributing the reconstruction on general-purpose UNIX workstations. An X-windows application provides interactive control. A six-interleaf spiral sequence is used for cardiac imaging and can acquire six images/s. A sliding window reconstruction achieves display rates of 16-20 images/s. This allows cardiac images to be acquired in real-time, with minimal motion and flow artifacts, and without breath holding or cardiac gating. Abdominal images are acquired at over 2.5 images/s with spiral-ring or circular echo-planar sequences. Reconstruction rates are 8-10 images/s. Rapid localization in the abdomen is demonstrated with the spiral-ring acquisition, whereas peristaltic motion in the small bowel is well visualized using the circular echo-planar sequence.

MR-guided cholecystostomy: assessment of biplanar, real-time needle tracking in three pigs

PURPOSE

To demonstrate the feasibility of magnetic resonance (MR)-guided cholecystostomy using active, real-time, biplanar MR tracking in animal experiments.

METHODS

Experiments were performed on three fully anesthetized pigs in an interventional MR system (GE open). The gallbladder was displayed in two orthogonal planes using a heavily T2-weighted fast spin-echo sequence. These "cholangio roadmaps" were displayed on LCD monitors positioned in front of the interventionalist. A special coaxial MR-tracking needle, equipped with a small receive-only coil at its tip, was inserted percutaneously into the gallbladder under continuous, biplanar MR guidance. The MR-tracking sequence allowed sampling of the coil (needle tip) position every 120 msec. The position of the coil was projected onto the two orthogonal "cholangio roadmap" images.

RESULTS

Successful insertion of the needle was confirmed by aspiration of bile from the gallbladder. The process of aspiration and subsequent instillation of Gd-DTPA into the gallbladder was documented with fast gradient-recalled echo imaging.

CONCLUSION

Biplanar, active, real-time MR tracking in combination with "cholangio roadmaps" allows for cholecystostomies in an interventional MRI environment.

Intraoperative diagnostic and interventional magnetic resonance imaging in neurosurgery

OBJECTIVE

The benefits of intraoperative magnetic resonance (MR) imaging for diagnostic and therapeutic measures are as follows: 1) intraoperative update of data sets for navigational systems, 2) intraoperative resection control of brain tumors, and 3) frameless and frame-based on-line MR-guided interventions. The concept of an intraoperative MR scanner in the sterile environment of operating theater is presented, and its advantages, disadvantages, and limitations are discussed.

METHODS

A 0.2-tesla magnet (Magnetom Open; Siemens AG, Erlangen, Germany) inside a radiofrequency cabin with a radiofrequency-shielded sliding door was installed adjacent to one of the operating theaters. A specially designed patient transport system carried the patient in a fixed position on an air cushion to the scanner and back to the surgeon.

RESULTS

In a series of 27 patients, intraoperative resection control was performed in 13 cases, with intraoperative reregistration in 4 cases. Biopsies, cyst aspirations, and catheter placements (mainly frameless) were performed under direct MR visualization with fast image sequences. The MR-compatible equipment and the patient transport system are safe and reliable.

CONCLUSION

Intraoperative MR imaging is a safe and successful tool for surgical resection control and is clearly superior to computed tomography. Intraoperative acquisition of data sets eliminates the problem of brain shift in conventional navigational systems. Finally, on-line MR-guided interventional procedures can be performed easily with this setting. As with all MR systems, individual testing with phantoms, application of correction programs, and determination of the optimal amount of contrast media are absolute prerequisites to guarantee patient safety and surgical success.

Interventional magnetic resonance. Initial clinical experience with a 1.5-tesla magnetic resonance system combined with c-arm fluoroscopy

RATIONALE AND OBJECTIVES

The authors evaluate the feasibility of performing magnetic resonance (MR) procedures on a 1.5-tesla (T) system combined with conventional c-arm fluoroscopy.

METHODS

A 1.5-T MR imaging system was combined with a conventional c-arm fluoroscopy unit in one room. The two systems were connected via a floating table top. Twenty-six interventional procedures (biopsies, MR-portography, percutaneous alcohol injection, laser ablation, fluid aspiration, and breast marking) were performed in 22 patients under MR, fluoroscopic control, or both. For MR guidance, fast gradient echo sequences were used, initiated from a panel at the front of the magnet. Images were displayed on an liquid crystal display screen positioned on the magnet.

RESULTS

All MR-guided procedures were performed successfully without complications. The addition of c-arm fluoroscopy was useful for bone interventions and MR-portography. All diagnostic biopsies yielded sufficient amounts of tissue for histologic diagnosis. In breast lesions, the target identified on dynamic MR imaging was marked correctly in each case. In interstitial laser thermotherapy the laser effect could be visualized, and in percutaneous ethanol injection the distribution of the alcohol could be seen. Both imaging systems worked without image distortions and high-quality MR images were obtained.

CONCLUSIONS

The combination of a 1.5-T MR imager with a c-arm fluoroscopy system seems to be a promising technical solution for performing interventional MR procedures.

Magnetic resonance imaging of temperature changes during interstitial microwave heating: a phantom study

Changes in magnetic resonance (MR) signals during interstitial microwave heating are reported, and correlated with simultaneously acquired temperature readings from three fiber-optic probes implanted in a polyacrylamide gel phantom. The heating by a MR-compatible microwave antenna did not interfere with simultaneous MR image data acquisition. MR phase-difference images were obtained using a fast two-dimensional-gradient echo sequence. From these images the temperature-sensitive resonant frequency of the 1H nuclei was found to decrease approximately by 0.008 ppm/ degree C. The method and results presented here demonstrate that noninvasive MR-temperature imaging can be performed simultaneously with interstitial microwave thermal treatment.

MR appearance and spectral features of injected ethanol in the liver: implication for fast MR-guided percutaneous ethanol injection therapy

PURPOSE

Our goal was to evaluate several fast MR strategies for monitoring ethanol distributions so that percutaneous ethanol injection might be guided with MRI.

METHOD

Fast RF spoiled GRE sequences (SPGR) and T2-weighted rapid acquisition with relaxation enhancement (RARE) sequences with and without spectroscopic-quality water suppression techniques were assessed for their ability to depict the distribution of injected ethanol in ex vivo pig liver. A line scan Carr-Purcell-Meiboom-Gill spectroscopic imaging sequence was used to validate observations and measure spectral relaxation characteristics of the ethanol signal in liver. Injected deuterated ethanol was also tested as an alternative possibility to depict the distribution of ethanol.

RESULTS

The water-suppressed T2-weighted RARE sequence depicted the distribution of ethanol better than other sequences. Deuterated ethanol appeared as a signal void on all sequences.

CONCLUSION

Water-suppressed T2-weighted RARE sequences could be useful to rapidly monitor MR-guided PEI.

Ceramic surgical instruments: ex vivo evaluation of compatibility with MR imaging at 1.5 T

The purpose of this study was to evaluate the MR compatibility of ceramic instruments. Nine different ceramic instruments were tested with respect to ferromagnetism, heating, and artifacts using previously described techniques. There was no magnetic field attraction, temperature increases were < or = 1 degree C, and artifacts involved a signal void similar to the size and shape of each instrument. The ceramic instruments were demonstrated to be compatible with MR imaging and acceptable for use during intraprocedural MR imaging.

MR imaging of the female pelvic floor in the supine and upright positions

The goal of this study was to determine whether a .5-T open configuration magnet system could be used to evaluate the female pelvic floor support structures and their functional changes in the upright and supine positions. We evaluated five normal volunteers with full bladders in the supine and sitting positions. Multiple measurements were obtained, including distance between symphysis and urethra, bladder neck to fixed pubococcygeal line, and posterior urethrovesical angle. The pelvic floor was evaluated for integrity of the urethra, vagina, and supporting ligaments. High quality, interpretable images were obtained for all five patients in both positions. Most of the pelvic floor structures were stable, with the exception of the posterior urethrovesical angle, which increased in the sitting position. We conclude that the vertically open configuration magnet system shows promise for evaluation of the female pelvic floor, including urinary stress incontinence and prolapse.

Biopsy needle susceptibility artifacts

Understanding the appearance of thin metallic structures in magnetic resonance imaging is important for evaluating the potential role of MRI in guiding and monitoring percutaneous interventions. As most MR compatible instruments are made from materials with a susceptibility different from water, their visibility is enhanced beyond what is expected on the grounds of displaced water alone. Unfortunately, this artifactually enhanced visibility is not constant, but instead depends on a variety of factors. This article presents computer simulations of the image distortion resulting from magnetic susceptibility differences between a needle and the surrounding tissue. The simulations show not only an artifact size that is dependent on needle composition, orientation, and pulse sequence, but also a corresponding shift of the artifact center away from the actual center of the needle. These effects place limits on the accuracy of MRI-guided needle tip placement.

Functional characterization of brain tumors: an overview of the potential clinical value

Early detection and characterization are still challenging issues in the diagnostic approach to brain tumors. Among functional imaging techniques, a clinical role for positron emission tomography studies with [18F]-fluorodeoxyglucose and for single photon emission computed tomography studies with [201Tl]-thallium-chloride has emerged. The clinical role of magnetic resonance spectroscopy is still being defined, whereas functional magnetic resonance imaging seems able to provide useful data for presurgical localization of critical cortical areas. Integration of morphostructural information provided by computed tomography and magnetic resonance imaging, with functional characterization and cyto-histologic evaluation of biologic markers, may assist in answering the open diagnostic questions concerning brain tumors.

Image-guided surgery in a new magnetic resonance suite: preclinical considerations

Surgical procedures require correct identification of exposed anatomy with concomitant localization amidst contiguous structures. In endoscopic procedures the surgeon is provided a real-time endoscopic view and is prepared with radiologic images. Here we present an overview of a methodology of localization using intraoperatively acquired magnetic resonance (MR) images in preparation for magnetic resonance imaging-guided endoscopic sinus surgery. The methodology centers around a unique prototype imaging device and operating environment. An "open" 0.5 Tesla MR unit has been created that allows complete access to the patient's head and neck while concomitant images are obtained. Illustrative examples of localization technique from cadaver experiments are presented, as well as insights into the host of concerns for anesthesia, equipment, surgical instrumentation, communications, and documentation.

Multi-modal volume registration by maximization of mutual information

A new information-theoretic approach is presented for finding the registration of volumetric medical images of differing modalities. Registration is achieved by adjustment of the relative position and orientation until the mutual information between the images is maximized. In our derivation of the registration procedure, few assumptions are made about the nature of the imaging process. As a result the algorithms are quite general and can foreseeably be used with a wide variety of imaging devices. This approach works directly with image data; no pre-processing or segmentation is required. This technique is, however, more flexible and robust than other intensity-based techniques like correlation. Additionally, it has an efficient implementation that is based on stochastic approximation. Experiments are presented that demonstrate the approach registering magnetic resonance (MR) images with computed tomography (CT) images, and with positron-emission tomography (PET) images. Surgical applications of the registration method are described.