Open Magnetic Resonance Imaging System Research Articles

The assessment of cervical spine kinematics: A comparison of iMR and conventional radiographic techniques

Objective: To develop and appraise a non-invasive method of determining disruptions to cervical segment mechanics. Design: A modified open magnetic resonance imaging system and post-process analysis tool were used to determine relative cervical vertebrae rotations and translations in a cohort of symptomatic patients, the results of which were compared to validated precision measures obtained from lateral radiographs. Methods: The cervical motion of 14 subjects with chronic neck symptoms were investigated in neutral, full flexion and full extension positions under voluntary neuromuscular control using open magnetic resonance imaging and lateral rontgenograms. Planar rotations and translations were calculated from the MRI images using the method of Dupuis et al. [8], and compared for agreement with the precision sagittal plane measurement technique of Frobin and colleagues [12, taken from the subjects lateral rontgenograms. Results: The interventional Magnetic Resonance method produced rotational results of up to 10° above and 9°c (SE 1.22) below the method of Frobin et al. [12], and translational results of 18 mm above or below (SE 2.12). The error of repeatability in the interventional Magnetic Resonance method was not significant (rotation P = 0.68, Translation P = 0.96). Conclusions: The disparity between methods limits the potential of Magnetic Resonance Imaging in clinical trials particularly those investigations into pseudarthrosis and instability.

Renal Tumors: MR Imaging–guided Percutaneous Cryotherapy—Initial Experience in 23 Patients

To evaluate the initial clinical experience of magnetic resonance (MR) imaging-guided percutaneous cryotherapy of renal tumors. Twenty-six renal tumors (diameter range, 1.0-4.6 cm; mean, 2.6 cm) in 23 patients were treated with 27 cryoablation procedures by using a protocol approved by the human subjects committee at the authors' institution. The study complied with the Health Insurance Portability and Accountability Act. Written informed consent was obtained from each patient. There were 17 men and six women with an average age of 66 years (range, 43-86 years). Of 26 masses, 24 were renal cell carcinoma, one was a transitional cell carcinoma, and one was an angiomyolipoma. By using a 0.5-T open MR imaging system and general anesthesia in patients, one to five (mean, 2.4) needlelike cryoprobes were placed and lesions were ablated by using real-time MR imaging for intraprocedural monitoring of ice balls. Tumors were considered successfully ablated if they demonstrated no contrast enhancement at follow-up computed tomography or MR imaging (mean, 14 months; range, 4-30 months). Twenty-four of 26 tumors were successfully ablated, 23 of which required only one treatment session. Two complications occurred in a total of 27 cryoablations: one hemorrhage, which required a blood transfusion, and one abscess, which was treated successfully with percutaneous catheter drainage. MR imaging-guided percutaneous cryotherapy of renal tumors shows promise for the treatment of selected small renal tumors, and MR imaging can be used to monitor the treatment intraprocedurally. This technique may prove useful for ablation of renal tumors completely in one session, but long-term follow-up is needed.

Radiofrequency thermal ablation: correlation of hyperacute MR lesion images with tissue response.

To investigate the hypothesis that the outer boundary of the hyperintense region observed in hyperacute (several minutes post-ablation) T2 and gadolinium contrast-enhanced (CE) T1-weighted magnetic resonance (MR) lesion images is an accurate predictor of eventual cell death from radiofrequency (RF) thermal ablation. A low-field, open MR imaging system was used to guide an ablation electrode into a thigh muscle of five rabbits and acquire in vivo T2 and CE T1-weighted MR volumes. Ablation occurred by applying RF current for two minutes with the electrode's temperature maintained at 90 degrees +/- 2 degrees C. After fixation, we sliced and photographed the tissue at 3 mm intervals, using a specially designed apparatus, to obtain a volume of tissue images. Digital images of hematoxylin and eosin (H&E) and Masson trichrome-stained histologic samples were obtained, and distinct regions of tissue damage were labeled using a video microscopy system. After the MR and histology images were aligned using a three-dimensional registration method, we compared tissue damage boundaries identified in histology with boundaries marked in MR images. The lesions have distinct zones of tissue damage histologically: a central zone of necrotic cells surrounded by an outer zone with cells that appeared non-viable and associated with marked interstitial edema. In 14 histology images from five lesions, the inner and outer boundaries of the outer zone were compared with the boundaries of a hyperintense rim that surrounds a central hypointense region in the T2 and CE T1-weighted MR images. For T2 and CE T1-weighted MR images, respectively, the mean absolute distance was 1.04 +/- 0.30 mm (mean +/- SD) and 1.00 +/- 0.34 mm for the inner boundaries, and 0.96 +/- 0.34 mm and 0.94 +/- 0.44 mm for the outer boundaries. The mean absolute distances for T2 and CE T1-weighted MR images were not sufficiently different to achieve statistical significance (P = 0.745, 0.818, for the inner and outer boundary, respectively). In hyperacute T2 and CE T1-weighted MR lesion images, observations strongly suggest that the outer boundary of the hyperintense rim corresponds to the region of eventual cell necrosis within a distance comparable to our ability to measure. This is good evidence that during RF ablation procedures, MR lesion images can be used to accurately localize the zone of irreversible tissue damage at the lesion margin.

Three-dimensional comparison of interventional MR radiofrequency ablation images with tissue response†

Solid tumors are being treated using radiofrequency (RF) ablation under interventional magnetic resonance imaging (MRI) guidance. We are investigating the ability of MRI to monitor ablation treatments by comparing MR images of thermal lesions to histologically assayed tissue damage.An open MRI system was used to guide an ablation electrode into five rabbit thigh muscles and acquire post-ablation MR image volumes. We developed a methodology using a 3D computer registration to make spatial correlations. After MR and histology images were registered with an accuracy of 1.32+/-0.39 mm (mean+/-SD), a boundary of necrosis identified in the histology was compared with the outer boundary of the hyperintense region in MR images.For 14 T2-weighted MR images, the absolute distance between boundaries was 0.96+/-0.34 mm (mean+/-SD). Since the small discrepancy between boundaries is comparable to our registration accuracy, the boundaries may match exactly. Similar correlations to histology were obtained with a deformable model segmentation method.This is good evidence that MR thermal lesion images can be used during RF ablation treatments to accurately localize the zone of necrosis at the lesion margin.

Three-dimensional comparison of interventional MR radiofrequency ablation images with tissue response

Objective: Solid tumors are being treated using radiofrequency (RF) ablation under interventional magnetic resonance imaging (MRI) guidance. We are investigating the ability of MRI to monitor ablation treatments by comparing MR images of thermal lesions to histologically assayed tissue damage.Material and Methods: An open MRI system was used to guide an ablation electrode into five rabbit thigh muscles and acquire post-ablation MR image volumes. We developed a methodology using a 3D computer registration to make spatial correlations. After MR and histology images were registered with an accuracy of 1.32 ± 0.39 mm (mean ± SD), a boundary of necrosis identified in the histology was compared with the outer boundary of the hyperintense region in MR images.Results: For 14 T2-weighted MR images, the absolute distance between boundaries was 0.96 ± 0.34 mm (mean ± SD). Since the small discrepancy between boundaries is comparable to our registration accuracy, the boundaries may match exactly. Similar correlations to histology were obtained with a deformable model segmentation method.Conclusions: This is good evidence that MR thermal lesion images can be used during RF ablation treatments to accurately localize the zone of necrosis at the lesion margin.

Minimally invasive neurosurgery: Anesthetic implications

late to new anesthettc challen~,es could be restated as New surgical procedures demand new anesthetic techniques. Neurosurgery has come a long way from the days of Harvey Cushing and Walter Dandy. Cushing was the pioneer of modern neurosurgery and, in the words of Sir William Osler, ° he opened a new chapter in the field of surgery. Cushing followed the principles of brain imaging, but it was based on his knowledge of neurophysiology and the patient's clinical features. Today, the scope of the available techniques in neuroimaging is vast. Apart from computed tomography (CT) and magnetic resonance imaging (MRI), there are other imaging modalities, such as magnetic resonance spectroscopy, magnetic resonance angiography, single photon emission contrast tomography, and positron emission tomography, to name a few. These techniques, besides providing precise neuroanatomical visualization of intracranial lesions, also have functional imaging capabilities, which give us physiological information about cerebral blood flow/metabolism, outline functional areas like the speech center and motor center and, in chronic pain patients, make it possible to visualize hyperactivity of the pain center in the thalamus, thus differentiating the psychological component from the somatic component of pain. Simultaneously, the field of endoscopy has come a long way, with the availability of video endoscopes and miniature cameras. Tools for endoscopic surgery have permitted surgical access to lesions previously considered inaccessible because of their location. Therefore, minimally invasive neurosurgery is the fulfillment of a long-felt need to develop surgical techniques with minimum potential for damage to the normal brain and a sequel to various technological innovations. The principle of primum nor nocire (do no harm), although important in medicine as a whole, has always been the guiding philosophy in neurosurgery. Preserving the integrity of normal tissue while maximizing the resection of a brain tumor is more effectively possible with the combination of available imaging techniques and a minimally invasive approach (endoscopic surgery). The availability of open MRI systems 2 allows real-time imaging intraoperatively, adding more precision to tumor localization and resection. Besides this, modern imaging technology is also being used for nonsurgical approaches to some of the central nervous system disease states, such as Parkinsonism, chronic pain, acoustic neurinoma, and arteriovenous malformation (AVM). This discussion will focus on 3 aspects of minimally invasive neurosurgery--endoscopic neurosurgery, brain imaging, and gamma knife surgery. NEUROENDOSCOPY History

MR image-guided endovascular procedures with the ultrasmall superparamagnetic iron oxide SH U 555 C as an intravascular contrast agent: study in pigs.

To evaluate the feasibility of using the ultrasmall superparamagnetic iron oxide (USPIO) SH U 555 C as an intravascular contrast agent for magnetic resonance (MR) image-guided vascular procedures with an open MR imaging system. All experiments were performed with MR imaging at 0.2 T. MR image-guided interventions were performed in USPIO-enhanced vessels in four pigs. With near real-time MR image guidance (acquisition time, 0.64 second per section), the splenic and renal arteries were consecutively catheterized by using a susceptibility artifact-based catheter-guide wire combination. Angioplasty and stent implantation were performed four times in the renal artery and twice in the iliac artery. Intraaortal signal intensity (SI) was measured during the interventions. After administration of SH U 555 C (40 micromol of iron per kilogram of body weight), a three-dimensional MR angiographic sequence was performed that allowed visualization of the abdominal and pelvic vessels that were as small as 2 mm in diameter. Catheterization, angioplasty, and stent implantation were successfully guided in the USPIO-enhanced vasculature. Sixty minutes after contrast agent injection, the mean aortic SI was 70% of the maximum measured enhancement levels. One intravenous injection of SH U 555 C enabled long, continuous intravascular SI enhancement at MR angiography, and, in combination with susceptibility artifact-based device tracking, the injection allowed the performance of MR imaging-guided intravascular interventions in an open MR imaging system.

Interactive magnetic resonance imaging-guided management of intracranial cystic lesions by using an open magnetic resonance imaging system.

The authors present their experience with neurosurgical procedures requiring real-time imaging feedback such as aspiration of a cystic structure or abscess cavity, decompression of hydrocephalic ventricles, management of arachnoid cysts, and installation of permanent or temporary drainage conduits, in which interactive magnetic resonance (MR) imaging guidance was used to monitor structural alterations associated with the procedure. Drainage of eight intraparenchymal brain abscesses in seven patients, decompression of space-occupying cystic or necrotic brain tumors in four patients, and endoscopic management of hydrocephalus associated with arachnoid cysts in three patients were performed using MR imaging-guided frameless stereotaxy in an open-configuration 0.5-tesla superconducting MR imaging system. Intraoperative MR imaging guidance provided accurate information on the course of the surgical procedure and associated intraoperative changes in tissue position, such as the degree of cyst aspiration, the presence or absence of hemorrhage or induced swelling, and changes associated with decompression of adjacent brain parenchyma and the ventricular system. No clinically significant complications were encountered in any patient. There were no targeting errors, and procedural objectives were accomplished in all cases. Drainage of brain abscesses, punctures of cystic or necrotic intracranial lesions with subsequent aspiration, and management of hydrocephalus can be performed safely and accurately by monitoring the procedure using real-time MR imaging to obtain immediate feedback on associated dynamic tissue changes.

An in vivo technique for determining 3D muscular moment arms in different joint positions and during muscular activation – application to the supraspinatus

Objective. To develop a 3D in vivo technique for determining the moment arm and insertion angle of muscles in different joint positions and under muscular activity. Design. An open magnetic resonance imaging system and 3D processing techniques were used for determining the moment arm and insertion angle of the supraspinatus in healthy volunteers. Background. Muscular moment arms are important parameters for the computation of joint forces, however, so far in vivo measurements have not considered the influence of muscle activity and were restricted to two dimensions. Methods. 10 healthy shoulders were investigated in 30–150° abduction with and without abducting muscular activity, using an open magnetic resonance imaging. The minimal distance between the midpoint of the humeral head and the line of action of the supraspinatus was determined in 3D. The insertion angle was derived by calculating the angle between the humerus and the supraspinatus. Results. During elevation a significant ( P<0.001) increase of the supraspinatus moment arm was observed (19.9 mm (SD, 2.3 mm) at 30°; 23.3 mm (SD, 2.5 mm) at 150°), being significantly larger at 90° and 120° under muscle activity. The insertion angle also increased significantly during elevation ( P<0.001), the values becoming larger under muscle activity at 60° ( P<0.01) and at 90° ( P<0.05). Conclusions. This 3D technique permits to determine the moment arm and insertion angle of muscles in vivo not only in various joint positions but also under in vivo muscle activity. For the supraspinatus, we observed a constant increase of both parameters during arm abduction. Relevance The technique and these data can be used for improved computation of joint forces in biomechanical model, and for precise diagnostics in patients with altered scapulohumeral motion patterns.

MR-guided interstitial laser-induced thermotherapy of hepatic metastasis combined with arterial blood flow reduction: technique and first clinical results in an open MR system.

The feasibility and safety of percutaneous laser-induced thermotherapy (LITT) of liver metastases in an open low-field magnetic resonance imaging (MRI) system combined with microsphere-modulated blood flow reduction were tested. Nd:YAG laser therapy with an internally cooled laser applicator was performed under local anesthesia on 20 patients with 34 liver metastases. To increase the effectiveness of LITT, degradable starch microspheres were injected into the proper hepatic artery through an MR-visible catheter initially inserted under fluoroscopy. Near real-time imaging was used for positioning the laser applicator. A T1-weighted gradient-echo breath-hold sequence was used for catheter localization and temperature monitoring. The volumes of the liver metastases and the thermonecroses were determined. MRI-guided LITT could be performed in all patients with no clinically relevant complications. Intraprocedural imaging underestimated the extent of thermonecrosis. In conclusion, percutaneous LITT of liver metastases after injection of starch microspheres is both technically feasible and safe in an open MRI system. J. Magn. Reson. Imaging 2001;13:31-36.

Changes in posterior disc bulging and intervertebral foraminal size associated with flexion-extension movement:: a comparison between L4–5 and L5–S1 levels in normal subjects

Background context: No previous study has used magnetic resonance imaging (MRI) to evaluate changes of posterior disc bulging and intervertebral foraminal size in the normal spine with flexion-extension movement, comparing L4–5 versus L5–S1 intervertebral levels. Purpose: To determine changes in posterior disc bulging and intervertebral foraminal size with flexion-extension movement, comparing L4–5 versus L5–S1 intervertebral levels. Study design: An in vivo study of magnetic resonance kinematics with spine flexion extension. Methods: Spines of three volunteers with no history of low back pain were scanned in neutral, flexion, and extension positions in a vertically open MRI system. MRI was repeated after 6 hours of normal activity and an additional 4 hours of heavy activity with a weighted vest. Posterior bulging of the intervertebral disc and the size of intervertebral foramen were measured at the L4–5 and L5–S1 levels. Results: With spine flexion, posterior bulging of the discs increased at L4–5 in eight of nine measurements (three different spine-loading states for each of three subjects) and L5–S1 discs in six of nine measurements. In most cases, posterior bulging decreased with extension. No significant difference was noted in the degree of disc bulge between levels. Foraminal size at L4–5 increased with flexion and decreased with extension, and the extent of these changes was greater at the L4–5 level than at L5–S1. Conclusions: This pilot study demonstrates two distinct behavior characteristics of the normal spine with flexion-extension movement.

Brain tumor surgery with the Toronto open magnetic resonance imaging system: preliminary results for 36 patients and analysis of advantages, disadvantages, and future prospects.

Frameless navigation systems represent a huge step forward in the surgical treatment of intracranial pathological conditions but lack the ability to provide real-time imaging feedback for assessment of postoperative results, such as catheter positions and the extent of tumor resections. An open magnetic resonance imaging system for intracranial surgery was developed in Toronto, by a multidisciplinary team, to provide real-time intraoperative imaging. The preliminary experience with a 0.2-T, vertical-gap, magnetic resonance imaging system for intraoperative imaging, which was developed at the University of Toronto for the surgical treatment of patients with intracranial lesions, is described. The system is known as the image-guided minimally invasive therapy unit. Between February 1998 and March 1999, 36 procedures were performed, including 21 tumor resections, 12 biopsies, 1 transsphenoidal endoscopic resection, and 2 catheter placements for Ommaya reservoirs. Three complications were observed. All biopsies were successful, and the surgical goals were achieved for all resections. Problems included restricted access resulting from the confines of the magnet and the imaging coil design, difficulties in working in an operating room that is less spacious and familiar, inconsistent image quality, and a lack of nonmagnetic tools that are as effective as standard neurosurgical tools. Advantages included real-time imaging to facilitate surgical planning, to confirm entry into lesions, and to assess the extent of resection and intraoperative and immediate postoperative imaging to confirm the extent of resections, catheter placement, and the absence of postoperative complications. Intraoperative magnetic resonance imaging has great potential as an aid for intracranial surgery, but a number of logistic problems require resolution.

Brain Tumor Surgery with the Toronto Open Magnetic Resonance Imaging System: Preliminary Results for 36 Patients and Analysis of Advantages, Disadvantages, and Future Prospects

Brain Tumor Surgery with the Toronto Open Magnetic Resonance Imaging System: Preliminary Results for 36 Patients and Analysis of Advantages, Disadvantages, and Future Prospects

A New Artifact-free Device for Frameless, Magnetic Resonance Imaging-guided Stereotactic Procedures

A new, artifact-free, nonobstructive device was required for planning, guiding, and performing stereotactic procedures in an open magnetic resonance imaging (MRI) system (Signa SP; General Electric, Milwaukee, WI). We devised an MRI-compatible instrument set that was adapted to an optical triangulation system (FlashPoint System; Image Guided Technologies, Boulder, CO) and consisted of a Pathfinder for planning burr holes and trajectories and a Snapper-Stereoguide for guiding minimally invasive instruments such as biopsy cannulae, endoscopes, and laser fibers. The instruments were composed of biocompatible plastic (polyacetal). During evaluation, special attention was paid to safety, accuracy, operation times, and susceptibility artifacts of the system. The first 20 stereotactic procedures performed in the open MRI scanner included 15 biopsies, 1 cyst and 3 abscess evacuations, and 1 endoscopic procedure for treatment of multiseptate hydrocephalus. There were no adverse outcomes, and all biopsies were diagnostic. The frameless stereotactic system in combination with the FlashPoint System proved to be accurate, with a mean error of 1.5 mm. The biopsy devices did not cause any susceptibility artifacts. Highly vascularized structures were clearly visible and were included in the trajectory planning. With experience, the procedure time was reduced to less than 1 hour. This device was found to be quite helpful for planning and guiding stereotactic procedures in the open MRI scanner. Nearly real-time planning and monitoring of stereotactic procedures and the positional accuracy of the system make the open MRI system a definite improvement, compared with conventional stereotactic systems.

Magnetic resonance-guided vascular catheterization: feasibility using a passive tracking technique at 0.2 Telsa in a pig model.

The objective of this study was to demonstrate in an animal model the feasibility of a passive tracking technique for catheter visualization of magnetic resonance (MR)-guided endovascular procedures. All experiments were performed in a 0.2 Tesla open MR system. Susceptibility-based catheters and guide wires were introduced into the aorta and were advanced selectively into the splenic and renal arteries under MR guidance. Based on a previously acquired contrast-enhanced magnetic resonance angiography (MRA) data set, the catheter positioning was performed by using a single-slice true fast imaging with steady state precession (FISP) sequence with a frame rate of 1.3 seconds. Contrast-enhanced MRA was performed in all animals. All catheters were advanced without complications into the aorta and were introduced into the proximal parts of the right renal and splenic arteries under MR guidance. Catheter manipulations were more difficult in the distal parts of these vessels due to the more complex anatomy. Passive catheter tracking is a valuable and technically robust alternative to active tracking methods, because it does not require additional hardware and, thus, can be implemented and used easily with any open MR imaging system. J. Magn. Reson. Imaging 1999;10:841-844.