Robot-assisted needle placement in open MRI: system architecture, integration and validation.

In prostate cancer treatment, there is a move toward targeted interventions for biopsy and therapy, which has precipitated the need for precise image-guided methods for needle placement. This paper describes an integrated system for planning and performing percutaneous procedures with robotic assistance under MRI guidance. A graphical planning interface allows the physician to specify the set of desired needle trajectories, based on anatomical structures and lesions observed in the patient's registered pre-operative and pre-procedural MR images, immediately prior to the intervention in an open-bore MRI scanner. All image-space coordinates are automatically computed, and are used to position a needle guide by means of an MRI-compatible robotic manipulator, thus avoiding the limitations of the traditional fixed needle template. Automatic alignment of real-time intra-operative images aids visualization of the needle as it is manually inserted through the guide. Results from in-scanner phantom experiments are provided.

Needle artifact localization in 3T MR images.

This work explores an image-based approach for localizing needles during MRI-guided interventions, for the purpose of tracking and navigation. Susceptibility artifacts for several needles of varying thickness were imaged, in phantoms, using a 3 tesla MRI system, under a variety of conditions. The relationship between the true needle positions and the locations of artifacts within the images, determined both by manual and automatic segmentation methods, have been quantified and are presented here.

Surgical navigation in the open MRI.

The introduction of MRI into neurosurgery has opened multiple avenues, but also introduced new challenges. The open-configuration intraoperative MRI installed at the Brigham and Women's Hospital in 1996 has been used for more than 500 open craniotomies and beyond 100 biopsies. Furthermore the versatile applicability, employing the same principles, is evident by its frequent use in other areas of the body. However, while intraoperative scanning in the SignaSP yielded unprecedented imaging during neurosurgical procedures their usage for navigation proved bulky and unhandy. To be fully integrated into the procedure, acquisition and display of intraoperative data have to be dynamic and primarily driven by the surgeon performing the procedure. To use the benefits of computer-assisted navigation systems together with immediate availability of intraoperative imaging we developed a software package. This "3D Slicer" has been used routinely for biopsies and open craniotomies. The system is stable and reliable. Pre- and intraoperative data can be visualized to plan and perform surgery, as well as to accommodate for intraoperative deformations, "brain shift", by providing online data acquisition.

3D visualisation of the middle ear and adjacent structures using reconstructed multi-slice CT datasets, correlating 3D images and virtual endoscopy to the 2D cross-sectional images.

The 3D imaging of the middle ear facilitates better understanding of the patient's anatomy. Cross-sectional slices, however, often allow a more accurate evaluation of anatomical structures, as some detail may be lost through post-processing. In order to demonstrate the advantages of combining both approaches, we performed computed tomography (CT) imaging in two normal and 15 different pathological cases, and the 3D models were correlated to the cross-sectional CT slices. Reconstructed CT datasets were acquired by multi-slice CT. Post-processing was performed using the in-house software "3D Slicer", applying thresholding and manual segmentation. 3D models of the individual anatomical structures were generated and displayed in different colours. The display of relevant anatomical and pathological structures was evaluated in the greyscale 2D slices, 3D images, and the 2D slices showing the segmented 2D anatomy in different colours for each structure. Correlating 2D slices to the 3D models and virtual endoscopy helps to combine the advantages of each method. As generating 3D models can be extremely time-consuming, this approach can be a clinically applicable way of gaining a 3D understanding of the patient's anatomy by using models as a reference. Furthermore, it can help radiologists and otolaryngologists evaluating the 2D slices by adding the correct 3D information that would otherwise have to be mentally integrated. The method can be applied to radiological diagnosis, surgical planning, and especially, to teaching.

A functional magnetic resonance imaging study of auditory mismatch in schizophrenia.

OBJECTIVE: Previous research has noted functional and structural temporal lobe abnormalities in schizophrenia that relate to symptoms such as auditory hallucinations and thought disorder. The goal of the study was to determine whether the functional abnormalities are present in schizophrenia at early stages of auditory processing. METHOD: Functional magnetic resonance imaging activity was examined during the presentation of the mismatch stimuli, which are deviant tones embedded in a series of standard tones. The mismatch stimuli are used to elicit the mismatch negativity, an early auditory event-related potential. Ten patients with schizophrenia and 10 comparison subjects were presented the mismatch stimuli condition and a control condition in which only one tone was presented repeatedly. RESULTS: The superior temporal gyrus showed the most prevalent and consistent activation. The superior temporal gyrus showed less activation in the schizophrenic subjects than in the comparison subjects only during the mismatch stimuli condition. CONCLUSIONS: This result is consistent with those of mismatch negativity event-related potential studies and suggests that early auditory processing is abnormal in chronic schizophrenia.

Intraoperative diffusion imaging on a 0.5 Tesla interventional scanner.

Intraoperative line scan diffusion imaging (LSDI) on a 0.5 Tesla interventional MRI was performed during neurosurgery in three patients. Diffusion trace images were obtained in acute ischemic cases. Scan time per slice was 46 seconds and 94 seconds, respectively, for diffusion tensor images. Diagnosis of acutely developed vascular occlusion was confirmed with follow-up scans. White matter tracts were displayed with the principal eigenvectors and provided guidance for the tumor surgery. In all cases, the diagnostic utility of LSDI was established. J. Magn. Reson. Imaging 2001;13:115-119.

Motion robust imaging for continuous intraoperative MRI.

The sensitivity of MR imaging to motion and susceptibility normally requires that the physician using intraoperative MRI cease surgical activity while image data sets are acquired. We demonstrate that line scan imaging allows the physician to continue operating without the delays caused by imaging. Consequently, patient anesthesia, surgery, and operating room time can be reduced. J. Magn. Reson. Imaging 2001;13:158-161.

Intraoperative MR systems. Midfield approaches.

Intraoperative MR imaging provides an unrestricted view of intracranial structures and lesions that has revolutionized the way that neurosurgery is performed in the authors' institution. Intraoperative imaging allows the practitioner to update and adjust the approach to intracranial lesions continuously. With this system, important anatomic and vascular structures can be successfully avoided; boundaries of low-grade tumors can be accurately defined, and foci of possible higher grade within these lesions can be identified; foci of high-grade astrocytomas can be differentiated from radiated brain; hyperacute hemorrhage or infarction during and after procedures can be determined; and the possible communication of cystic collections with CSF can be ascertained. These advantages provide a level of comfort to the surgeon and a presumptive margin of safety to the patient that is unattainable during conventional surgical approaches, and given the choice, the authors' neurosurgeons would prefer to operate in the interventional magnet. Preliminary reports concerning the efficacy and usefulness of MR-guided navigational tools for the performance of neurosurgery are encouraging, as noted earlier, Wirtz et al have shown that the more extensive removal of glioblastomas afforded by intraoperative MR leads to significantly prolonged patient survival compared with conventional surgery. Further outcomes analysis must be performed, however, to determine whether these new techniques significantly decrease overall long-term morbidity or increase survival in those patients who have low-grade astrocytomas.

Hyperpolarized (129)Xe T (1) in oxygenated and deoxygenated blood.

The viability of the new technique of hyperpolarized (129)Xe MRI (HypX-MRI) for imaging organs other than the lungs depends on whether the spin-lattice relaxation time, T(1), of (129)Xe is sufficiently long in the blood. In previous experiments by the authors, the T(1) was found to be strongly dependent upon the oxygenation of the blood, with T(1) increasing from about 3 s in deoxygenated samples to about 10 s in oxygenated samples. Contrarily, Tseng et al. (J. Magn. Reson. 1997; 126: 79-86) reported extremely long T(1) values deduced from an indirect experiment in which hyperpolarized (129)Xe was used to create a 'blood-foam'. They found that oxygenation decreased T(1). Pivotal to their experiment is the continual and rapid exchange of hyperpolarized (129)Xe between the gas phase (within blood-foam bubbles) and the dissolved phase (in the skin of the bubbles); this necessitated a complicated analysis to extract the T(1) of (129)Xe in blood. In the present study, the experimental design minimizes gas exchange after the initial bolus of hyperpolarized (129)Xe has been bubbled through the sample. This study confirms that oxygenation increases the T(1) of (129)Xe in blood, from about 4 s in freshly drawn venous blood, to about 13 s in blood oxygenated to arterial levels, and also shifts the red blood cell resonance to higher frequency.

MR imaging-guided percutaneous cryotherapy of liver tumors: initial experience.

PURPOSE: To describe the cryoablation of liver tumors by using a percutaneous approach and intraprocedural magnetic resonance (MR) imaging monitoring and to assess the feasibility and safety of the procedure. MATERIALS AND METHODS: Fifteen hepatic tumors (mean diameter, 2.9 cm) in 12 patients were treated (18 total cryoablations). Fourteen were metastases and one was a hemangioma; all were proved at biopsy. By using a 0.5-T open MR imaging system, cryoneedles were placed and lesions ablated by using real-time monitoring. Clinical signs and symptoms were assessed and laboratory tests performed. Intraprocedural depictions of iceballs were compared with contrast material-enhanced MR imaging-based estimates of cryonecrosis that were obtained 24 hours after cryoablation. RESULTS: MR imaging-guided percutaneous cryotherapy resulted in no serious complications and no clinically important changes in serum liver enzymes or creatinine or myoglobin levels. Intraprocedural MR imaging demonstrated iceballs as sharply marginated regions of signal loss that expanded and engulfed tumors. The maximal iceball size was 4.9 x 2.2 x 2.2 cm with the use of one cryoneedle and 6.0 x 5.6 x 4.9 cm with three cryoneedles. Intraprocedural iceball depictions correlated well with postprocedural cryonecrosis estimates. CONCLUSION: MR imaging-guided percutaneous cryotherapy of liver tumors is feasible and safe. MR imaging can be used to estimate cryotherapy effects and guide therapy intraprocedurally.

Sensitivity profiles from an array of coils for encoding and reconstruction in parallel (SPACE RIP).

A new parallel imaging technique was implemented which can result in reduced image acquisition times in MRI. MR data is acquired in parallel using an array of receiver coils and then reconstructed simultaneously with multiple processors. The method requires the initial estimation of the 2D sensitivity profile of each coil used in the receiver array. These sensitivity profiles are then used to partially encode the images of interest. A fraction of the total number of k-space lines is consequently acquired and used in a parallel reconstruction scheme, allowing for a substantial reduction in scanning and display times. This technique is in the family of parallel acquisition schemes such as simultaneous acquisition of spatial harmonics (SMASH) and sensitivity encoding (SENSE). It extends the use of the SMASH method to allow the placement of the receiver coil array around the object of interest, enabling imaging of any plane within the volume of interest. In addition, this technique permits the arbitrary choice of the set of k-space lines used in the reconstruction and lends itself to parallel reconstruction, hence allowing for real-time rendering. Simulated results with a 16-fold increase in temporal resolution are shown, as are experimental results with a 4-fold increase in temporal resolution. Magn Reson Med 44:301-308, 2000.

Interventional and intraoperative magnetic resonance imaging.

The goal of the Image Guided Therapy Program, as the name implies, is to develop the use of imaging to guide minimally invasive therapy. The program combines interventional and intraoperative magnetic resonance imaging (MRI) with high-performance computing and novel therapeutic devices. In clinical practice the multidisciplinary program provides for the investigation of a wide range of interventional and surgical procedures. The Signa SP 0.5 T superconducting MRI system (GE Medical Systems, Milwaukee, WI) has a 56-cm-wide vertical gap, allowing access to the patient and permitting the execution of interactive MRI-guided procedures. This system is integrated with an optical tracking system and utilizes flexible surface coils and MRI-compatible displays to facilitate procedures. Images are obtained with routine pulse sequences. Nearly real-time imaging, with fast gradient-recalled echo sequences, may be acquired at a rate of one image every 1.5 s with interactive image plane selection. Since 1994, more than 800 of these procedures, including various percutaneous procedures and open surgeries, have been successfully performed at Brigham and Women's Hospital (Boston, MA).

Image-guided therapy and intraoperative MRI in neurosurgery.

Computer-assisted 3D planning, navigation and the possibilities offered by intra-operative imaging updates have made a large impact on neurological surgery. Three-dimensional rendering of complex medical image information, as well as co-registration of multimodal sources has reached a highly sophisticated level. When introduced into surgical navigation however, this pre-operative data is unable to account for intra-operative changes, ('brain-shift'). To update structural information during surgery, an open-configured, intra-operative MRI (Signa SP, 0.5 T) was realised at our institution in 1995. The design, advantages, limitations and current applications of this system are discussed, with emphasis on the integration of imaging into procedures. We also introduce our integrated platform for intra-operative visualisation and navigation, the 3D Slicer.

T(1) of (129)Xe in blood and the role of oxygenation.

In previous experiments by the authors, in which hyperpolarized (129)Xe was dissolved in fresh blood samples, the T(1) was found to be strongly dependent on the oxygenation level, the values increasing with oxygenation: T(1) was about 4 s in deoxygenated samples and about 13 s in oxygenated samples. C. H. Tseng et al. (1997, J. Magn. Reson. 126, 79-86), on the other hand, recently reported extremely long T(1) values using hyperpolarized (129)Xe to create a "blood foam" and found that oxygenation decreased T(1). In their experiments, the continual and rapid exchange of hyperpolarized (129)Xe between the gas phase (within blood-foam bubbles) and the dissolved phase (in the skin of the bubbles) necessitated a complicated analysis to extract the effective blood T(1). In the present study, the complications of hyperpolarized (129)Xe exchange dynamics have been avoided by using thermally polarized (129)Xe dissolved in whole blood and in suspensions of lysed red blood cells (RBC). During T(1) measurements in whole blood, the samples were gently and continuously agitated, for the entire course of the experiment, to avert sedimentation. Oxygenation was found to markedly increase the T(1) of (129)Xe in blood, as originally measured, and it shifts the RBC resonance to a higher frequency. Carbon monoxide has a similar but somewhat stronger effect.

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

PURPOSE: To review preliminary experience with an open-bore magnetic resonance (MR) imaging system for guidance in intracranial surgical procedures. MATERIALS AND METHODS: A vertically oriented, open-configuration 0.5-T MR imager was housed in a sterile procedure room. Receive and transmit surface coils were wrapped around the patient's head, and images were displayed on monitors mounted in the gap of the magnet and visible to surgeons. During 2 years, 200 intracranial procedures were performed. RESULTS: There were 111 craniotomies, 68 biopsies, 12 intracranial cyst evaluations, four subdural drainages, and five transsphenoidal pituitary resections performed with the intraoperative MR unit. In each case, the intraoperative MR system yielded satisfactory results by allowing the radiologist to guide surgeons toward lesions and to assist in treatment. In two patients, hyperacute hemorrhage was noted and removed. The duration of the procedure and the complication rate were similar to those of conventional surgery. CONCLUSION: Intraoperative MR imaging was successfully implemented for a variety of intracranial procedures and provided continuous visual feedback, which can be helpful in all stages of neurosurgical intervention without affecting the duration of the procedure or the incidence of complications. This system has potential advantages over conventional frame-based and frameless stereotactic procedures with respect to the safety and effectiveness of neurosurgical interventions.

Intraoperative dynamic MRI: localization of sites of brain tumor recurrence after high-dose radiotherapy.

In patients with malignant astrocytomas or metastatic brain disease treated with high-dose radiotherapy, conventional imaging methods may not adequately distinguish recurrent tumor from radiation change. We used a fast spoiled gradient refocusing technique in the open-configuration intraoperative MR system to assess the rate of regional enhancement of the treated tumor bed and to localize specific sites for pathologic sampling to determine whether gadolinium uptake correlated with histologic data. Twenty-four patients were studied. Fourteen of 15 patients with areas of early enhancement had recurrent tumor present in histologic samples, and 8 of the remaining 9 patients had only reactive changes. Dynamic MRI was predictive of recurrent tumor (P < .0005, Fisher exact test and P < .002, Student t test). We conclude that dynamic MRI in the open-bore magnet is a promising method for localizing potential sites of active tumor growth in patients treated for malignant astrocytomas and metastatic brain lesions.