MR-guided Interventional Procedures Research Articles

Real-Time Control of Active Catheter Signals for Better Visual Profiling During Cardiovascular Interventions Under MRI Guidance

In MR-guided interventional procedures, RF coils can be attached to the instruments to provide a positive MR signal for device tracking. The signal from these coils can vary strongly over the procedure and mask the surrounding anatomy. The purpose of this study is to introduce and demonstrate a low-cost, vendor- and device-independent interface circuit that allows the interventionalist to adjust the active device signal intensity. In this work a variable attenuator circuit was constructed to control the tip signal of an active coronary artery catheter in real-time from within the MR scanner room. Performance of the attenuator circuit and the active catheter was characterized on the test bench, in a phantom model, and in vivo. The system was used in a pig model at 3T during the introduction of the catheter into the left coronary artery. The circuit could attenuate the amplitude of the tracking coil signal by up to 20 dB. Without attenuation, the tracking coil signal intensity was masking anatomical details of the coronary ostium making it impossible to reliably introduce the catheter into the artery. After interactive adjustment, which was performed in a few seconds by the interventionalist, the improved visualization of the vascular anatomy enabled a rapid insertion of the catheter into the coronary ostium. The vendor-independent variable attenuator provides real-time control of the catheter signal without interrupting the image acquisition. Even though most MRI systems can control the individual signal levels from coils by software, the attenuator hardware is advantageous as it can be integrated into any MR-system, and it provides a direct interface for the interventionalist at the magnet.

Autoregressive moving average modeling for spectral parameter estimation from a multigradient echo chemical shift acquisition

The authors investigated the performance of the iterative Steiglitz-McBride (SM) algorithm on an autoregressive moving average (ARMA) model of signals from a fast, sparsely sampled, multiecho, chemical shift imaging (CSI) acquisition using simulation, phantom, ex vivo, and in vivo experiments with a focus on its potential usage in magnetic resonance (MR)-guided interventions. The ARMA signal model facilitated a rapid calculation of the chemical shift, apparent spin-spin relaxation time (T2*), and complex amplitudes of a multipeak system from a limited number of echoes (< or equal 16). Numerical simulations of one- and two-peak systems were used to assess the accuracy and uncertainty in the calculated spectral parameters as a function of acquisition and tissue parameters. The measured uncertainties from simulation were compared to the theoretical Cramer-Rao lower bound (CRLB) for the acquisition. Measurements made in phantoms were used to validate the T2* estimates and to validate uncertainty estimates made from the CRLB. We demonstrated application to real-time MR-guided interventions ex vivo by using the technique to monitor a percutaneous ethanol injection into a bovine liver and in vivo to monitor a laser-induced thermal therapy treatment in a canine brain. Simulation results showed that the chemical shift and amplitude uncertainties reached their respective CRLB at a signal-to-noise ratio (SNR) > or =5 for echo train lengths (ETLs) > or =4 using a fixed echo spacing of 3.3 ms. T2* estimates from the signal model possessed higher uncertainties but reached the CRLB at larger SNRs and/or ETLs. Highly accurate estimates for the chemical shift (<0.01 ppm) and amplitude (<1.0%) were obtained with > or =4 echoes and for T2*(<1.0%) with > or =7 echoes. We conclude that, over a reasonable range of SNR, the SM algorithm is a robust estimator of spectral parameters from fast CSI acquisitions that acquire < or =16 echoes for one- and two-peak systems. Preliminary ex vivo and in vivo experiments corroborated the results from simulation experiments and further indicate the potential of this technique for MR-guided interventional procedures with high spatiotemporal resolution approximately 1.6 x 1.6 x 4 mm3 in < or =5 s.

Real‐time geometric distortion correction for interventional imaging with echo‐planar imaging (EPI)

Many MR-guided interventional procedures rely on fast imaging sequences for providing images in real-time with a precise relation between the target position in the image and its true position. Echo-planar imaging (EPI) methods are very fast but prone to geometric distortions. Here, we propose a correction method designed for real-time conditions, adapting existing approaches based on dual EPI acquisition with varying echo times. The method is demonstrated with MR-thermometry for guiding thermal therapies. The proposed approach imposes a small penalty in acquisition speed but adds negligible latency to data processing, an important element for interventions of mobile organs.

Keeping current with the latest advances in cardiovascular imaging

The past decades have seen the evolution of growing information in correlative and comparative noninvasive imaging studies involving the structure and function of the heart. Noninvasive imaging can now assess virtually every aspect of cardiac function: anatomy, physiology, biochemistry, innervation, and its vascular system. With the development of these advanced imaging technologies, the focus of medical care has steadily shifted from clinical history and physical examination to increased reliance on image-guided diagnosis and treatment. Although there are many scientifi c journals and textbooks to read and symposia to attend, it is extremely diffi cult for clinicians to distill the literature into cohesive information for the day-to-day management of patients. Thus, a journal in which experts present clear and concise reviews about the latest advances in cardiovascular imaging should be welcomed and readily accepted. The launch of this new journal, Current Cardiovascular Imaging Reports , represents a step toward fulfi lling this unmet clinical need of keeping current. The benefi ts derived from the rapid technologic developments in imaging have revolutionized the entire cardiovascular fi eld. As a result, new disciplines such as molecular imaging have been created in cardiovascular imaging. The evolution of fused or integrated imaging systems, such as hybrid positron emission tomography (PET)/CT, PET/MR, and single-photon emission CT (SPECT)/CT, has opened a whole world of new opportunities for cardiovascular imaging; it allows the combination of a highly informative anatomic localization of a lesion or disease process with its functional correlates in the form of perfusion, metabolism, or receptor activation. For example, the stimulus for plaque rupture and thrombus formation leading to acute myocardial infarction is not well defi ned. Even when culprit lesions are identifi ed by coronary angiography, they may not necessarily represent the lesion that ultimately will rupture. Contrarily, plaque rupture occurs frequently in nonculprit or insignifi cantly narrowed coronary arteries. The application of hybrid PET/CT or PET/MR systems may potentially identify the vulnerable plaque within a coronary artery lumen by colocalizing infl ammatory cells (using radionuclide techniques) to the specifi c plaque (using CT or MR) that is vulnerable to rupture. Beyond diagnosis of a disease process, these techniques are rapidly transitioning to help guide therapy, as in PET/CT-guided ablation therapy and MR-guided interventional procedures. Further development and refi nement of the instrumentations and shorter examination times will most likely cause image-guided interventional procedures to become clinically widespread. Current Cardiovascular Imaging Reports will place the entire area of cardiovascular imaging in its proper perspective by establishing the indications and limitations of each imaging technique and by summarizing recent advances. Authors will present review articles examining timely topics and the genesis of great ideas that have led the scientifi c fi eld in recent years. Each of the six issues per year will be dedicated to one particular imaging technique: echocardiography, nuclear imaging, cardiac MR, cardiac CT, intravascular imaging, or molecular imaging. Coherent and unifi ed messages of latest advances will be provided in each issue. The articles will be abundantly illustrated with highquality images. Distinguished section editors with special interest and personal achievements will make every effort to maintain a current and evidence-based journal and to limit speculative material. This journal will serve as an invaluable resource to readers. Beyond the primary objective of making Current Cardiovascular Imaging Reports clinically relevant, we will also aim to break down artifi cial barriers among those who are called clinicians, radiologists, physiologists, biochemists, physicists, and pharmacologists in the study of the heart. Together, we look forward to fulfi lling our goals to the best of our ability by providing growth of ideas, advancement of knowledge, and excellent scientifi c contributions. This issue marks the start of an orderly procession of scholarly publications for many years to come.

Magnetic Resonance-Guided Musculoskeletal Interventional Radiology

As an imaging modality, magnetic resonance (MR) guidance has great potential to direct diagnostic and therapeutic procedures performed in the musculoskeletal region and influence patient management. MR-guided interventional procedures involving bone, soft tissue, intervertebral discs, and joints are safe and sufficiently effective for use in clinical practice. This article discusses and illustrates the procedural characteristics and techniques when performing MR-guided musculoskeletal interventions. Biopsy procedures are similar to other modalities for bone and soft tissue lesions. MR guidance is advantageous if the lesion is not visible by other modalities and for regions adjacent to hardware and implants, subselective targeting, intra-articular locations, and periarticular cyst aspiration. MR guidance has also been used for a host of spine injections and pain management procedures such as sacroiliac joint injections, discography, transforaminal epidural injection, selective nerve block, sympathetic block, celiac plexus block, and facet joint cryotherapy neurotomies. Future directions of clinical applications include tumor ablation and multimodality procedure suites. MR-guided musculoskeletal procedures will continue to be a growth area particularly for the diagnosis and treatment of bone and soft tissue neoplasia.

MR-guided interventional procedures: a review

Magnetic resonance imaging (MRI) has emerged as a potential guidance tool for a variety of procedures. Diagnostic and therapeutic procedures using either open surgical or percutaneous access are performed. They span from simple lesion targeting and biopsy to complex applications requiring multiple tasks performed simultaneously or in rapid succession. These tasks include instrument guidance and therapy monitoring as well as procedural follow-up. The interventional use of MRI (IMRI) is increasing steadily. This article reviews the prerequisites, systems, and clinical interventional procedures of IMRI.

リアルタイムMR画像ガイドにおけるオフセット型穿刺補助器具の開発

In the MR-guided interventional procedures, an optical tracking system for the scan plane control plays an important role. Two types of new assistive devices for the system were developed, in which the needle guide was placed away from the center of the hand piece or its direction was changed. The system was modified to determine the image planes with the new path of the needle similar to the conventional setting. Since the needle guides were off-center, surgical instruments, endoscope or surgeons' hands did not disturb the image plane determination even during the procedures. One of them (torch type) enabled punctures from the sides of the patient, which were previously impossible. The errors caused by these devices were negligible. They were quite helpful to accomplish more accurate MR image guidance and to increase the availability of the MR-guided interventional procedures.

Interventional musculoskeletal procedures performed by using MR imaging guidance with a vertically open MR unit: assessment of techniques and applicability.

To evaluate the safety of and time required for a broad range of musculoskeletal interventional procedures performed by using magnetic resonance (MR) imaging guidance with a vertically open 0.5-T unit. Sixty-three MR imaging-guided procedures were performed. A vertically open MR unit equipped with in-room display monitors allowed interactive freehand MR guidance predominantly with fast spin-echo and gradient-echo sequences. Each procedure was classified in terms of the anatomic location, procedure type, and tissue type involved. The procedures were evaluated for success of needle placement, adequacy of tissue sampling, total procedural time, needle time, number of needle passes, and complications. Procedures consisted of tissue sampling with core-needle (n = 6) or fine-needle aspiration (n = 20) biopsy, corticosteroid or contrast agent injection (n = 19), joint cyst aspiration (n = 7), and drainage (n = 11). Successful needle placement was achieved in all 63 cases. Cytologic and histologic tissue samples were sufficient for pathologic diagnosis in 24 of 26 cases. In two cases, complications occurred: transient local bleeding and a brief vasovagal episode. The mean total procedural time was 64.8 minutes; the mean needle time, 26.2 minutes; and the mean number of needle passes per patient, 1.6. With use of a vertically open MR unit, MR-guided interventional procedures involving bone, soft tissue, intervertebral disks, and joints are safe and sufficiently rapid for use in clinical practice.

Real-time magnetic resonance imaging: diagnostic and interventional applications.

The advent of ultra-fast imaging techniques has extended the utility of magnetic resonance imaging (MRI) from a static and purely diagnostic status to an imaging modality ideally suited for a number of therapeutic applications. These advances--along with the recent development and refinement of miniature intravascular imaging catheters and MRI-compatible guidewires, balloon catheters, and radiofrequency ablation catheters--have created an exciting forum of novel approaches for detecting and treating both acquired and congenital cardiovascular disease. This review covers the current state of the art in fast cardiovascular MRI, catheter-tracking techniques for MR fluoroscopy, and currently available interventional MRI systems. Early diagnostic and therapeutic applications, such as high-resolution intravascular and intracardiac imaging, balloon angioplasty, stent placement, and radiofrequency ablation techniques, are discussed and extended to several potential approaches specific to pediatric cardiac therapeutic catheterization. Lastly, safety aspects of MR-guided interventional procedures are presented.

Temperature measurement using echo-shifted FLASH at low field for interventional MRI.

We investigated the feasibility of using echo-shifted fast low-angle shot (FLASH) for temperature-monitored thermo-therapeutic procedures in a 0.2 T interventional magnetic resonance (MR) scanner. Based on the proton resonance frequency shift technique, modified echo-shifted FLASH has sufficiently high signal-to-noise ratio to provide accurate temperature maps with short scan times, i.e., 5 seconds in phantoms (TR = 20.5 msec; effective TE = 30 msec; one echo shift; NSA = 2) and ex vivo experiments (TR = 19.4 msec; effective TE = 28.9 msec; one echo shift; NSA = 2) and 3 seconds (TR = 19.4 msec; effective TE = 28.9 msec, one echo shift; NSA 1) for an in vivo case. The proton resonance frequency shifts with temperature observed in a 0.2 T MR scanner using this sequence were -0.0072 ppm/degrees C (temperature uncertainty = +/-2.5 degrees C) for polyacrylamide phantoins and -0.0086 ppm/degrees C (temperature uncertainty = +/- 1 degrees C) for ex vivo bovine liver. These experiments demonstrated that echo-shifted FLASH is a viable method for low-field temperature monitoring despite the decreased signal and decreased phase sensitivity compared with its counterpart in a 1.5 T MR imaging system. The improved temporal resolution of temperature images, now possible in low-field interventional MR systems using echo-shifted FLASH, will allow clinicians more accurate monitoring of interstitial ablation in MR-guided interventional procedures.

Design, clinical suitability and future aspects of a 0.5 T MRI special system for interventional use

To develop a superconducting magnetic resonance (MR) imager that provides direct access to the patient and permits interactive MR-guided interventional procedures. A 0.5-T superconducting magnet that allows a region of vertical access to the patient was designed and constructed. This magnet was integrated with newly designed shielded gradient coils, flexible surface coils, nonmagnetic displays, position-monitoring probes and device-tracking instrumentation. The magnet homogeneity was 12.3 ppm or better, and the gradient field was linear to within 1% over an imaging region of 30 cm in diameter. The signal-to-noise ratio was 10% higher than in a comparable 0.5-T superconducting imager. Images were obtained in several anatomic regions with use of routine pulse sequences. Interactive image plane selection and near real-time imaging, with use of fast gradient-recalled echo sequences were demonstrated at a rate of one image every 1.5 s. More than 240 procedures (including open brain surgery) have been successfully performed within this environment. MR-guided interventional and surgical procedures can be performed with full patient access using an open-configuration, superconducting MR magnet with near real-time imaging and interactive image plane control.

Intraoperative diagnostic and interventional magnetic resonance imaging in neurosurgery.

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. 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. 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. 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.

Intraoperative Diagnostic and Interventional Magnetic Resonance Imaging in Neurosurgery

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 the operating theater is presented, and its advantages, disadvantages, and limitations are discussed. A 0.2-tesla magnet (Magnetom Open; Siemens AG, Erlangen, Germany) inside a radio frequency cabin with a radio frequency-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. 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. 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.

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

To develop a superconducting magnetic resonance (MR) imager that provides direct access to the patient and permits interactive MR-guided interventional procedures. A 0.5-T superconducting magnet that allows a region of vertical access to the patient was designed and constructed. This magnet was integrated with newly designed shielded gradient coils, flexible surface coils, and nonmagnetic displays and with position-monitoring probes and device-tracking instrumentation. The magnet homogeneity was 12.3 ppm, and the gradient field was linear to within 1% over an imaging region 30 cm in diameter. The signal-to-noise ratio was 10% higher than in a comparable 0.5-T superconducting imager. Images were obtained in several anatomic regions with use of routine pulse sequences. Interactive image plane selection and near real-time imaging, with use of fast gradient-recalled echo sequences, were demonstrated at a rate of one image every 1.5 seconds. MR-guided interventional procedures can be performed with full patient access with use of an open-configuration, superconducting MR magnet with near real-time imaging and interactive image plane control.