Interventional MR Research Articles

Dynamic Interventional MR Imaging Using Hyperpolarized C-13

The para-hydrogen method has been used to polarize the 13C atoms in selected water-soluble molecules. The degree of polarization, achieved by recently developed methods, is in the order of 35% which is more than a factor of 105 above the thermal equilibrium polarization at 1.5 T. Earlier work has shown the potential of hyperpolarized 13C substance as a general ECF MR contrast medium. The aim of the present work has been to demonstrate the potential of using 13C CM during interventional MRI procedure aiming for real time visualization of catheter movements and the coronary arteries.

Magnetresonanzgesteuerte Kortikosteroid-Infiltration der Sakroiliakalgelenke: Schmerztherapie der Sakroiliitis bei Patienten mit Spondylitis ankylosans

To evaluate the efficacy and specific properties of MR imaging-guided corticosteroid infiltration of the sacroiliac (SI) joints in the treatment of therapy-refractory sacroiliitis in patients with ankylosing spondylitis. In this study, 26 patients were prospectively included. Inclusion criteria were AS with therapy refractory acute sacroiliitis and inflammatory back pain > or = 6 months. The intervention was performed using an open low-field MR-scanner. Inflammatory back pain was assessed on a visual analog scale (VAS). Success of the therapy was defined as an absolute reduction of the VAS score </= 5, a relative reduction of the VAS score > or = 35 % and persisting improvement > or = 2 months. The grade of sacroiliitis was documented using high-field MR imaging. Variables were compared using McNemar test and Wilcoxon test. The mean remission time was calculated using a Kaplan-Meier analysis. A p-value < 0.05 was considered statistically significant. The intervention was technically successfully performed in all patients. Following MR imaging-guided corticosteroid infiltration of the SI joints, the VAS score improved from 8 (5 - 10) points to 4.5 (0 - 8) points (- 44 %) in all patients (n = 26), which was statistically significant (p < 0.001). Of 26 patients, 22 (85 %) fulfilled the predefined criteria for successful therapy. This group had a statistically significant (p < 0.01) improvement of the VAS score from 8 (6 - 10) to 3 (0 - 5) (- 63 %). Improvement was seen after 7 (1 - 30) days. There was a marked reduction of the subchondral bone marrow edema (- 38 %). The mean remission time was 12 (4 - 18) months. MR imaging-guided corticosteroid infiltration of the SI joints proved to be an effective therapy of inflammatory back pain in patients with therapy refractory AS. With the ability of multiplanar imaging, precise localization of the bone marrow edema and the lack of ionizing radiation, interventional MR imaging currently represents the superior method for the treatment of the predominantly young patient group presenting with ankylosing spondylitis. Owing to short intervention times, open MR-scanners are ideally suited for MR imaging-guided infiltration of the SI joints.

Interventional magnetic resonance angiography with no strings attached: Wireless active catheter visualization

Active instrument visualization strategies for interventional MR angiography (MRA) require vascular instruments to be equipped with some type of radiofrequency (RF) coil or dipole RF antenna for MR signal detection. Such visualization strategies traditionally necessitate a connection to the scanner with either coaxial cable or laser fibers. In order to eliminate any wire connection, RF resonators that inductively couple their signal to MR surface coils were implemented into catheters to enable wireless active instrument visualization. Instrument background to contrast-to-noise ratio was systematically investigated as a function of the excitation flip angle. Signal coupling between the catheter RF coil and surface RF coils was evaluated qualitatively and quantitatively as a function of the catheter position and orientation with regard to the static magnetic field B0 and to the surface coils. In vivo evaluation of the instruments was performed in interventional MRA procedures on five pigs under MR guidance. Cartesian and projection reconstruction TrueFISP imaging enabled simultaneous visualization of the instruments and vascular morphology in real time. The implementation of RF resonators enabled robust visualization of the catheter curvature to the very tip. Additionally, the active visualization strategy does not require any wire connection to the scanner and thus does not hamper the interventionalist during the course of an intervention.

Interventional and intraoperative MR: review and update of techniques and clinical experience.

The concept of interventional magnetic resonance imaging (MRI) is based on the integration of diagnostic and therapeutic procedures, favored by the combination of the excellent morphological and functional imaging characteristics of MRI. The spectrum of MRI-assisted interventions ranges from biopsies and intraoperative guidance to thermal ablation modalities and vascular interventions. The most relevant recently published experimental and clinical results are discussed. In the future, interventional MRI is expected to play an important role in interventional radiology, minimal invasive therapy and guidance of surgical procedures. However, the associated high costs require a careful evaluation of its potentials in order to ensure cost-effective medical care.

Atherosclerotic Vascular Disease Conference: Writing Group IV: imaging.

The goals of the imaging writing group were to define the role of imaging and to recommend important programmatic research and advocacy initiatives in atherosclerotic peripheral vascular disease for the American Heart Association. It should be noted that this is not a practice guideline initiative; therefore, the writing group purposely avoided recommending imaging modalities for specific circulatory beds. This report discusses the following imaging modalities: duplex ultrasound, MRI and MR angiography (MRA), CT imaging and angiography (CTA), and digital intra-arterial angiography. The requirements for proficiency in all of these modalities include physician oversight, quality assurance programs, and standardization for acquisition of images, interpretation of the study, postprocessing procedures, workstations, and reporting. The roles of imaging for each modality such as screening for subclinical disease, diagnosis, and treatment planning as an adjunct to invasive therapies (interventional MR and CT) and follow-up and monitoring are reviewed.

Real‐time modulation of acoustic gradient noise in interventional MRI

AbstractAcoustic noise is inherent to magnet resonance imaging (MRI) and poses safety risks to patients and interventional radiologists. Although a spectrum of noise reduction techniques has been implemented, new developments further increase sound levels. Silent pulse sequences based on derating gradient currents have been proposed but are limited to slow pulse sequences not suitable for interventional MRI (iMRI). However, iMRI does not continuously require fast imaging with high image update rates. Therefore, the aim of this study was to develop a tool that could remotely derate the gradient currents in real time, operated by the interventionalist. The duration of the derated gradients is increased to retain image resolution [this extends imaging time and echo time (TE)]. The experiment setup consisted of a custom‐built remote controller located in the interventional MR room. A fast spoiled gradient recalled echo (FSPGR) sequence was tested and acoustic recordings were made with a microphone in the magnet bore. An optical link sent data from the remote controller to a computer that was connected to a 1.5‐T cardiovascular MR scanner over ethernet. A software server installed on the MR system received the requests and passed these on to the integrated pulse generator in order to control the real‐time imaging. Slew rate adjustments were accomplished by calculating instruction period changes and adjusted waveform amplitudes accordingly. The experimental setup reduced the slew rate up to 16‐fold, which resulted in a considerable acoustic noise reduction of 21.1 dBA. The remote controller performed well and image quality showed no substantial qualitative changes. We made use of the previously described idea of silent pulse sequence designing. Although derating is at the expense of the image refresh rate, our real‐time slew rate reduction is compatible with the fact that iMRI procedures do not continuously require fast image update rates. © 2004 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 20B: 34–39, 2004.

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.

Stereotactic Target Localization Accuracy in Interventional Magnetic Resonance Imaging

Objective: To compare stereotactic target determination, based on images obtained from interventional MRI (iMRI), conventional closed MR and CT. Methods: Stereotactic coordinates for 55 targets in an artificial scull were derived from iMRI scans and compared using CT as the standard. Stereotactic coordinates were also derived from iMRI scans in a series of patients and compared using iMRI fused with CT as the standard. Results: The mean difference between targets in the skull phantom determined from iMRI and CT images was 0.90 ± 0.28 mm, with a maximum difference of 1.57 mm. The mean difference between targets in the patients derived from iMRI alone and interventional MR fused with CT was 1.39 ± 0.54 mm, with a maximum difference of 2.47 mm. Discussion: The results indicate that iMRI can be used for stereotactic target localization.

Interventional MRに新たな可能性を提案するX-MR装置 : 超伝導1.5テスラMR装置とフルデジタルX線Angio装置の組み合わせによるNew Intervention(&lt;特集&gt;MRの治療応用と機器)

Interventional MRに新たな可能性を提案するX-MR装置 : 超伝導1.5テスラMR装置とフルデジタルX線Angio装置の組み合わせによるNew Intervention(&lt;特集&gt;MRの治療応用と機器)

Software Tools for Interventional MR Guided Navigation for Thermal Ablation Procedure

Software Tools for Interventional MR Guided Navigation for Thermal Ablation Procedure

Simultaneous real-time visualization of the catheter tip and vascular anatomy for MR-guided PTA of iliac arteries in an animal model.

To examine the feasibility of simultaneous MR real-time active tip tracking and near real-time depiction of the vascular anatomy for percutaneous angioplasty of iliac arteries under MR guidance. Nine surgically created stenoses of external iliac arteries in pigs were dilated with MR-compatible balloon catheters (Cordis, Roden, The Netherlands). These catheters were equipped with a microcoil for active tracking of the catheter tip with an in-plane update rate of 10 positions per second. The procedures were performed on an interventional 1.5 T Gyroscan ACS-NT scanner (Philips, Best, The Netherlands). Real-time calculation of images acquired by radial k-space filling was performed on a specially designed backprojector exploiting the sliding window reconstruction technique (Philips Research Laboratories, Hamburg, Germany). The image update rate was 20 frames per second using a radial gradient-echo technique (TR = 12 msec, TE = 3.3 msec, 300 radials). MR angiography and X-ray digital subtraction angiography on the X-ray system positioned in-line next to the interventional MR system served as control for the angioplasty results. Real-time guidance and positioning of the balloon catheters was possible. The actual position of the catheter tip was indicated in the MR images without any time delay for the reconstruction of the anatomical MR images, which were updated with a rate of 20 frames per second. This yielded a combination of a roadmap and fluoroscopy image, in which the catheter position and the anatomical background image both were continuously updated in real time. Six out of nine stenoses were successfully dilated. The effects of the angioplasty could be visualized by the real-time MR technique, as was proven by X-ray digital subtraction angiography. Active tip tracking simultaneous with MRI of the vascular anatomy-both in real time-is possible with the applied technique, enabling MR-guided percutaneous dilatation (PTA) of iliac arteries.

MR imaging-detected breast lesions: histopathologic correlation of lesion characteristics and signal intensity data.

The aim of our study was to differentiate benign from malignant breast lesions that had been detected exclusively on MR imaging by analyzing qualitative and quantitative lesion characteristics. We performed 51 MR imaging-guided breast interventions (41 preoperative lesion localizations and 10 large-core needle biopsies) in 45 patients with exclusively MR imaging-detected lesions. All patients had previously undergone diagnostic dynamic contrast-enhanced MR imaging of the breast with a double breast coil at 1.0 T (n = 36) or 1.5 T (n = 15). The diagnostic MR images were evaluated on a workstation. Lesion morphology (size, shape, margin type, enhancement pattern), signal intensity parameters (time to peak enhancement, maximum slope of enhancement curve, washout, relative water content), and scores analogous to the Breast Imaging Reporting and Data System (BI-RADS) categories were correlated with histology. Histology revealed malignancy in 37.3% (19/51) of the lesions. The positive predictive value for malignancy of exclusively MR imaging-detectable lesions increased as the analogous BI-RADS category increased. Late inhomogeneous contrast enhancement was the only morphologic criterion that was statistically significantly correlated with malignancy. Malignant and benign lesions did not differ significantly in any of the quantitatively evaluated signal intensity parameters. Carcinomas showed a tendency toward faster and stronger enhancement and stronger washout. The classification of exclusively MR imaging-detectable breast lesions according to a combination of morphologic and perfusion parameters including the late enhancement pattern helps identify the lesions for which interventional MR imaging is required. Quantitative signal intensity data alone do not suffice.

Sicherheitsaspekte bei der interventionellen MRT

Because of its high soft-tissue contrast, Magnetic Resonance Imaging (MRI) is used increasingly for guidance and control of minimal invasive and neurological surgical procedures. Besides common precautions during an MRI investigation, special attention has to be paid to the consequences of MR compatibility, accuracy of localisation of interventional tools and geometrical distortions. As a new application of interventional MR intravascular procedures are developing that involve the introduction of guidewires, catheters and miniaturized coils with their leads into the blood vessels. Resonant currents and high electric fields can develop at the conductor ends, possibly causing burning lesions.

Iterative Multimodal Computer-Assisted Navigation Using Pre- and Intraoperative Image Data

Summary Computer-assisted navigation shall reliably guide the surgeon to a planned target position. Various medical image modalities are required for a safe diagnosis and intervention planning. For treatment forms like LITT, therapy simulation should be part of a planning scenario. In soft tissues, the intervention usually will render preoperative images inappropriate if tissue shifts occur. In these cases intraoperative images are necessary to show the current state, to update the preoperative data, and to register preoperative images to the current situation. The update is an iterative process triggered by situational changes during the ongoing intervention. This paper presents our concept of iterative multimodal navigation that has been implemented in the LOCALITE Navigator for interventional MR and ultrasound.

Metallic neurosurgical implants: evaluation of magnetic field interactions, heating, and artifacts at 1.5-Tesla.

The purpose of this study was to use ex vivo testing to determine the magnetic resonance imaging (MRI) safety aspects for seven different metallic neurosurgical implants in association with the 1.5-T MR environment. Ex vivo testing was performed using previously-described techniques for the evaluation of magnetic field interactions (deflection angle and torque), heating (gel-filled phantom and fluoroptic thermometry; 15 minutes of MRI at a specific absorption rate [SAR] of 1.4 W/kg), and artifacts (using T1-weighted, spin-echo and gradient-echo pulse sequences). None of the metallic implants displayed interactions with the magnetic field. The highest temperature change was +0.6 degrees C for the representative implant that was evaluated. Artifacts were relatively minor. The lack of magnetic field interactions and negligible heating indicate that MR procedures may be conducted safely in patients with these neurosurgical implants using MR systems with static magnetic fields of 1.5-T or less. Furthermore, these implants may be considered for use in interventional MR procedures insofar as the MR safe qualities and relatively small artifacts would likely be desirable for such procedures.

MR Imaging-Guided Biopsy and Therapeutic Intervention in a Closed-Configuration Magnet

The purpose of the study was to evaluate practical experience with interventional MR imaging in a closed-bore 1.5-T imaging system. A total of 361 MR-guided biopsies were performed in 250 patients and analyzed retrospectively. The technique comprised four steps: localization; planning; action (cutting or aspiration biopsy, or instillation of a therapeutic agent), with verification in two perpendicular planes; and obtaining control scans. The mean duration of a biopsy was 21 min; there were no major complications. Image contrast, signal, matrix options, and visibility of needle track and tip position permitted uncomplicated orientation. The interventional MR technique could be applied in any puncture setting.

Enhanced CAS in head and neck-surgery: evaluation of intraoperative data modality iMRI and sonography

Purpose: Computer-assisted surgical procedures are well established in head/neck- and neurosurgery. Most systems are based on preoperative CT or MR data. To visualize the surgical change and re-registration, there is a need for intraoperative imaging. Currently, only few imaging methods are available to supply up-to-date information on the surgical progress, e.g., inter-/intraoperative MRI, interoperative CT, and sonography/ultrasound. The purpose of this study is to compare procedures and algorithms for intraoperative co-registration of iMRI and sonography in head/neck- and neurosurgery. Methods: Intraoperative imaging was performed by interventional MR (GE Signa SP) and high-resolution ultrasound (Siemens Sienna). iMRI: Localite Navigator Software (enhanced CAS) allows integration of preoperative data to intraoperative imaging of MR, fMR and CT. The co-registration was based on fiducial markers. The implementation was designed as a multimode real-time navigation system supplying images of bony structures/contours (derived from CT data) fused into real-time MR images. A total of 10 markers (three forehead, three temporal scalp, three mastoid, and one occipital scalp) were used in this study to define reproducible fixation points. Sonography: Based on neuronal networks, a recognize-algorithm for co-registration was developed using sonography data sets of the neck. Results: The statistical point-to-point accuracy (marker difference) of co-registration in the open MR was determined to be 1.2±0.1 mm. The co-registration CT preop and iMRI are reliable and nowadays established in selected surgical procedures of the skull base, e.g., biopsy of the petroclival region. The system yields a significantly better image quality and higher repetition rate than the standard real-time images. Typical for any marker-based registration, the matching accuracy for the registered data sets depends on the accuracy of locating the markers in the respective images, and the inherent displacement of the fiducial markers. The markerless co-registration Sono preop/Sono intraop based on neuronal networks is still less reliable with a positive recognition rate of 46/51 of structures with a maximal volume of 2.110 cm 3. Recognized structures can be used for co-registration with different data modalities, e.g., MR, CT or for single-modality navigation. Conclusions: Both interventional MRI as well as sonography allows intraoperative data update. The marker-based co-registration CT preop/iMRI is still more reliable than Sono preop/Sono intraop based on neuronal networks. Further developments tend towards the improvement of mutual information algorithm and marker-like landmarks to improve the recognition rate of integrated sonography. Low costs and less surgical tools expenditure are the advantages of sonography, and it will be a serious possibility for intraoperative CAS-update. Otherwise, the study demonstrates the possibility of combining intraoperative MR and sonography with other modalities for enhanced CAS in ORL and neurosurgery.

An integrated visualization system for surgical planning and guidance using image fusion and an open MR.

A surgical guidance and visualization system is presented, which uniquely integrates capabilities for data analysis and on-line interventional guidance into the setting of interventional MRI. Various pre-operative scans (T1- and T2-weighted MRI, MR angiography, and functional MRI (fMRI)) are fused and automatically aligned with the operating field of the interventional MR system. Both pre-surgical and intra-operative data may be segmented to generate three-dimensional surface models of key anatomical and functional structures. Models are combined in a three-dimensional scene along with reformatted slices that are driven by a tracked surgical device. Thus, pre-operative data augments interventional imaging to expedite tissue characterization and precise localization and targeting. As the surgery progresses, and anatomical changes subsequently reduce the relevance of pre-operative data, interventional data is refreshed for software navigation in true real time. The system has been applied in 45 neurosurgical cases and found to have beneficial utility for planning and guidance. J. Magn. Reson. Imaging 2001;13:967-975.

MR-guided percutaneous drainage of abdominal fluid collections in combination with X-ray fluoroscopy: initial clinical experience.

The aim of this study was to examine the feasibility of a hybrid interventional MR system, which combines a closed bore magnet with a C-arm fluoroscopy unit for percutaneous drainage of abdominal fluid collections. During the past 2 years, we have performed four drainage procedures in four patients (mean age 47 years). Three patients had abscesses (psoas muscle, kidney, subphrenic location) and the fourth patient had a recurrent splenic cyst. All procedures were performed on an interventional MR system consisting of a 1.5-T ACS-NT scanner combined with a specially shielded C-arm. The drainages were guided by T1-weighted fast gradient-echo images, T2-weighted single-shot turbo spin-echo images or both. A standard 18 G (1.2 mm) nonferromagnetic stainless steel needle with a Teflon sheath was used for the punctures following which a 0.89 mm nitinol guidewire was inserted into the fluid collection. Thereafter, the patient was positioned in the immediate adjacent fluoroscopy unit and a drainage catheter was placed under fluoroscopic control. All drainage catheters were successfully placed into the fluid collections, as proven by fluid aspiration and resolution of the collection. The mean time needed for the entire drainage procedure (MR and fluoroscopy) was 110 min. No procedure-related complications occurred. It is feasible to perform drainage procedures on a closed-bore MR scanner. The multiplanar imaging capabilities of MR are particularly helpful for fluid collections in the subphrenic location.