Control of intravascular catheters using an array of active steering coils.

PURPOSE: To extend the concept of deflecting the tip of a catheter with the magnetic force created in an MRI system through the use of an array of independently controllable steering coils located in the catheter tip, and to present methods for visualization of the catheter and/or surrounding areas while the catheter is deflected. METHODS: An array of steering coils made of 42-gauge wire was built over a 2.5 Fr (0.83 mm) fiber braided microcatheter. Two of the coils were 70 turn axial coils separated by 1 cm, and the third was a 15-turn square side coil that was 2 x 4 mm2. Each coil was driven independently by a pulse width modulation (PWM) current source controlled by a microprocessor that received commands from a MATLAB routine that dynamically set current amplitude and direction for each coil. The catheter was immersed in a water phantom containing 1% Gd-DTPA that was placed at the isocenter of a 1.5 T MRI scanner. Deflections of the catheter tip were measured from image-based data obtained with a real-time radio frequency (RF) spoiled gradient echo sequence (GRE). The small local magnetic fields generated by the steering coils were exploited to generate a hyperintense signal at the catheter tip by using a modified GRE sequence that did not include slice-select rewinding gradients. Imaging and excitation modes were implemented by synchronizing the excitation of the steering coil array with the scanner by ensuring that no current was driven through the coils during the data acquisition window; this allowed visualization of the surrounding tissue while not affecting the desired catheter position. RESULTS: Deflections as large as 2.5 cm were measured when exciting the steering coils sequentially with a 100 mA maximum current per coil. When exciting a single axial coil, the deflection was half this value with 30% higher current. A hyperintense catheter tip useful for catheter tracking was obtained by imaging with the modified GRE sequence. Clear visualization of the areas surrounding the catheter was obtained by using the excitation and imaging mode even with a repetition time (TR) as small as 10 ms. CONCLUSIONS: A new system for catheter steering is presented that allows large deflections through the use of an integrated array of steering coils. Additionally, two imaging techniques for tracking the catheter tip and visualization of surrounding areas, without interference from the active catheter, were shown. Together the demonstrated steerable catheter, control system and the imaging techniques will ultimately contribute to the development of a steerable system for interventional MRI procedures.

Magnetic resonance imaging-guided renal artery stent placement in a Swine model: comparison of two tracking techniques.

BACKGROUND: Magnetic resonance (MR)-guided interventions have evolved from a pure research application to a preclinical method over the last decade. Among the device-tracking techniques, susceptibility artifact-based tracking relies on the contrast between the surrounding blood and the device, and radiofrequency coil-based tracking relies on the local gradient field amplification in a resonating circuit attached to the interventional device. PURPOSE: To evaluate the feasibility and precision of susceptibility artifact-based and microcoil-based MR guidance methods for renal artery stent placement in a swine model. MATERIAL AND METHODS: MR imaging-guided renal artery stent placements were performed in six fully anesthetized pigs using a 1.5T short-bore MR scanner. Susceptibility artifact-based tracking with manual scan-plane adjustments and microcoil tracking with automatic scan-plane adjustments were used for renal artery stent placements in three pigs in each group. With both methods, near real-time steady-state free-precession (SSFP) imaging was used. Differences between the two tracking approaches on stenting time, total procedure time, and stent position were measured. RESULTS: The microcoil-based approach yielded a shorter mean procedure time (17 vs. 23 min). There was no relevant difference for the mean stenting time (12 vs. 13 min). The mean stent deviation from the aortic wall with the susceptibility approach was larger than with the microcoil approach (10 vs. 4.0 mm). CONCLUSION: For MRI-guided renal artery stent placement, the microcoil-based technique had a shorter procedure time and a higher stent placement precision than the susceptibility artifact-based approach.

Modified block uniform resampling (BURS) algorithm using truncated singular value decomposition: fast accurate gridding with noise and artifact reduction.

The block uniform resampling (BURS) algorithm is a newly proposed regridding technique for nonuniformly-sampled k-space MRI. Even though it is a relatively computationally intensive algorithm, since it uses singular value decomposition (SVD), its procedure is simple because it requires neither a pre- nor a postcompensation step. Furthermore, the reconstructed image is generally of high quality since it provides accurate gridded values when the local k-space data SNR is high. However, the BURS algorithm is sensitive to noise. Specifically, inaccurate interpolated data values are often generated in the BURS algorithm if the original k-space data are corrupted by noise, which is virtually guaranteed to occur to some extent in MRI. As a result, the reconstructed image quality is degraded despite excellent performance under ideal conditions. In this article, a method is presented which avoids inaccurate interpolated k-space data values from noisy sampled data with the BURS algorithm. The newly proposed technique simply truncates a series of singular values after the SVD is performed. This reduces the computational demand when compared with the BURS algorithm, avoids amplification of noise resulting from small singular values, and leads to image SNR improvements over the original BURS algorithm.

A method for fast 3D tracking using tuned fiducial markers and a limited projection reconstruction FISP (LPR-FISP) sequence.

This work demonstrates the feasibility of using wireless, tuned fiducial markers with a limited projection reconstruction-fast imaging with steady-state free precession sequence (LPR-FISP) to accurately obtain tracking information necessary for interactive scan plane selection in magnetic resonance imaging (MRI). The position and orientation of a rigid interventional device can be uniquely determined from the 3D coordinates of three fiducial markers mounted in a known configuration on the device. Three fiducial markers were tuned to the proton resonant frequency in a 0.2T open MR scanner and mounted to the surface of a cylindrical water phantom. An LPR-FISP sequence was developed to suppress the water phantom signal while preserving that of the fiducial markers through a nonselective low-tip-angle excitation and a dephaser gradient applied prior to data acquisition. A localization algorithm was developed to accurately calculate the 3D coordinates of the fiducial markers using four LPR-FISP projections in two orthogonal scan planes. The sequence repetition time (TR = 21 msec) and the limited projection set resulted in fast LPR-FISP coordinate acquisition times of approximately 170 msec with an accuracy (max error) of 3 mm on a 0.2T MR system. This fast, accurate tracking method provides the fundamental technology for interactive MRI scan plane definition for rigid interventional devices without the need for stereotactic cameras or reference frames.

Segmented k-space and real-time cardiac cine MR imaging with radial trajectories.

The authors developed and evaluated two cine magnetic resonance (MR) imaging sequences with a radial rather than a rectilinear k-space coordinate frame: segmented k space and real-time true fast imaging with steady-state precession, or FISP. The two radial k-space segmentation (or view sharing) techniques, which were interleaved or continuous, were compared, and the feasibility of their application in cardiac cine MR imaging was explored in phantom and volunteer studies. Images obtained with the radial sequences were compared with those obtained with two-dimensional Fourier transform, or 2DFT, sequences currently used in cine MR imaging. Temporal resolution of 55 msec was achieved with the real-time radial sequences, which allowed acquisition of almost 19 high-quality images per second.

A multielement RF coil for MRI guidance of interventional devices.

Accurate localization of minimally invasive devices is critical to the success of interventional procedures. Device orientation and tip position are two of the most important pieces of information needed to define device location for magnetic resonance imaging (MRI)-guided interventional procedures. While a single one-element micro coil incorporated into an interventional device has proven to be effective in some applications, it can only supply tip position information. However, multiple positions on the device are necessary to also determine its orientation. For this purpose, a novel single micro coil design with three separate winding elements that provides both the device orientation and tip position is described in this study. Definition of MR scan planes, by using the device orientation and the target tissue location, permits automatic tracking of the insertion of the device. Furthermore, devices that include this coil design are permitted to bend to a limited extent. This makes the micro coil design appropriate for many flexible interventional devices. Reliable near-real-time tracking of three points on an interventional device is demonstrated on a 0.2T MRI system with modest gradient performance. Phantom and in vivo animal experiments are used to demonstrate the utility of this new coil design.

Perfusion-modulated MR imaging-guided radiofrequency ablation of the kidney in a porcine model.

OBJECTIVE: This study was performed to test the hypothesis that temporary renal ischemia will result in increased thermal lesion size during radiofrequency thermal ablation in the kidney. MATERIALS AND METHODS: Twelve kidneys were treated in six pigs that were placed under general anesthesia in the MR suite, using a 0.2-T open C-shaped MR imaging system. A 4-cm-long, 14-mm-diameter balloon catheter was placed into the aorta using a transfemoral approach, and the balloon was positioned proximal to the renal arteries via guidance with MR imaging. A 2-cm exposed-tip MR-compatible 17-gauge radiofrequency electrode was placed into one kidney under MR fluoroscopy using fast imaging with steady-state free precession (FISP) sequences. Thermal ablation was performed with the electrode tip temperature maintained at 90 +/- 2 degrees C for 10 min. This procedure was repeated in the contralateral kidney. The balloon was inflated during one ablation. Postablation images were obtained, the pigs were sacrificed, and both kidneys of each animal were harvested for pathologic correlation. RESULTS: Technical success was achieved in all animals. The lesion measured 14.2 +/- 2.2 mm (mean +/- standard deviation) for the ischemic kidney versus 8.0 +/- 2.6 mm in the normally perfused kidney (p = 0.00002). No significant complications were noted. In all images, thermal lesions displayed low signal intensity with a sharp rim of high signal intensity best visualized using short tau inversion recovery (STIR) sequences with a mean accuracy of 1.3 +/- 1.2 mm when compared with pathologic findings and a mean contrast-to-noise ratio of 4.9 +/- 2.5. CONCLUSION: We accept the hypothesis that temporary renal ischemia leads to a significantly increased radiofrequency ablation lesion size. We conclude that catheter-based balloon perfusion reduction is feasible, that the procedure does not lead to major complications, and that it can be performed using MR imaging as the sole imaging modality.

A rapid look-up table method for reconstructing MR images from arbitrary K-space trajectories.

Look-up tables (LUTs) are a common method for increasing the speed of many algorithms. Their use can be extended to the reconstruction of nonuniformly sampled k-space data using either a discrete Fourier transform (DFT) algorithm or a convolution-based gridding algorithm. A table for the DFT would be precalculated arrays of weights describing how each data point affects all of image space. A table for a convolution-based gridding operation would be a precalculated table of weights describing how each data point affects a small k-space neighborhood. These LUT methods were implemented in C++ on a modest personal computer system; they allowed a radial k-space acquisition sequence, consisting of 180 views of 256 points each, to be gridded in 36.2 ms, or, in approximately 800 ns/point. By comparison, a similar implementation of the gridding operation, without LUTs, required 45 times longer (1639.2 ms) to grid the same data. This was possible even while using a 4 x 4 Kaiser-Bessel convolution kernel, which is larger than typically used. These table-based computations will allow real time reconstruction in the future and can currently be run concurrently with the acquisition allowing for completely real-time gridding.

MR imaging-guided radiofrequency thermal ablation in the porcine brain at 0.2 T.

The aim of this study was to test the hypotheses that (a) MR imaging-guided radiofrequency (RF) thermal ablation is safe and feasible in porcine brain using an open C-arm-shaped low-field MR system, and that (b) induced thermal lesion size can be predicted using low-field MR imaging. Magnetic resonance-guided RF ablation was performed in the cerebral frontal lobes of six pigs. An 18-G monopolar RF electrode was inserted into the porcine brain using MR image guidance and RF was then applied for 10 min. After post-procedure imaging (T2-weighted, T1-weighted before and after gadodiamide administration), the pigs were killed and the brains were used for pathologic examination. Successful RF electrode placement was accomplished in all cases without complications; total magnet time ranged from 73 to 189 min. The thermal lesion size varied from 10 to 12 mm perpendicular to the electrode track and was easily visualized on T2-weighted and enhanced T1-weighted images. Enhanced T1-weighted imaging demonstrated the highest brain-to-RF thermal lesion contrast-to-noise ratio with an average of 1.5 +/- 1.6. Enhanced T1-weighted imaging never underestimated pathologic lesion diameter with a mean difference of 2.3 +/- 1.0 mm and a radiologic/pathologic correlation of 0.69. Magnetic resonance imaging-guided RF thermal ablation is feasible and safe in the porcine brain using an open MR low-field system. Induced-thermal lesion size can best be monitored using enhanced T1-weighted images. In the future, RF ablation under low-field MR guidance may offer an alternative treatment option for primary and secondary brain tumors.

[Comparison of susceptibility artefacts of different radiofrequency electrodes at O.2 T. Influence of electrode positioning, pulse sequence and image reconstruction methods]. / Vergleich der Suszeptibilitätsartefakte unterschiedlicher Radiofrequenzelektroden bei 0,2 T. Einfluss von Elektrodenlegierung, Pulssequenz und Bildrekonstruktionsmethode.

PURPOSE: Interventional MRI procedure monitoring requires small but accurate susceptibility artifacts of the instruments used. In this investigation, susceptibility artifacts of different RF-electrode designs were compared using a variety of pulse sequences and k-space acquisition methods. METHODS: 4 different 18-gauge RF-electrodes (with three single electrodes made of stainless steel, copper, inconal, and a triple-clustered electrode configuration made of inconal) were placed in a 0.2 T MR-scanner perpendicular to the main magnetic field. Pulse sequences used included: TSE T2, FISP, true-FISP, PSIF, and a temperature sensitive ES-GRE sequence. In addition to the 2D Cartesian k-space trajectory with Fourier transformation (2DFT), projection reconstruction (PR) was used with the FISP, true-FISP and PSIF sequences. RESULTS: The best tip accuracy was achieved with the combination of inconal electrodes and TSE T2. The usefulness of the tested sequences was found to be: TSE T2 > PSIF > FISP/true-FISP > ES-GRE. In general 2DFT provided better or equal tip accuracy than PR. The apparent shaft width was smaller using the copper electrode compared to the inconal electrode. However, the "match shaped" tip artifact of the copper probe led to a higher error in tip accuracy. CONCLUSIONS: TSE-T2 sequences and Cartesian 2DFT acquisitions should be used for accurate tip positioning at 0.2 T. Further, artifact size of the electrode shaft prevents the use of inconal for temperature sensitive sequences. Copper electrodes can be used for these purposes, although copper is not considered to be biocompatible at present.

Two-step navigatorless correction algorithm for radial k-space MRI acquisitions.

A new way to correct magnetic resonance image artifacts resulting from view-dependent phase variations and view-dependent variations in rigid body object translation is presented by exploiting basic properties of the trajectory of radial k-space acquisitions. Simulations, phantom studies, and in vivo experiments are used to demonstrate the feasibility and the utility of this method. While somewhat analogous to navigator echo correction, in which special gradients are interleaved into the imaging sequence so echoes at the center of k-space can be acquired prior to or after collection of the image data, the current method does not require additional new gradient structures within the pulse sequence or increases in scan time. The new method uses the phase information from all collected radial k-space data points rather than only the navigator echo, which permits correction of multiple sources of view-dependent phase variation in the image data. The resultant effect is improved image quality in radial MRI acquisitions. Magn Reson Med 45:277-288, 2001.

How does alteration of hepatic blood flow affect liver perfusion and radiofrequency-induced thermal lesion size in rabbit liver?

The purpose of this study was to test the hypothesis that decreasing liver perfusion in rabbits results in an increase in thermal lesion size and that these effects can be accurately monitored using magnetic resonance imaging (MRI). We additionally tested the hypothesis that the increase in thermal lesion size would depend on the particular vessel or vessels occluded (hepatic artery, portal vein, or both). Using an Institutional Animal Care and Use Committee approved protocol, 20 New Zealand white rabbits were randomly assigned to four treatment groups (five in each group): control and ligation of portal vein (PV), hepatic artery (HA), or both PV and HA (HAPV). Surgical ligation of the appropriate vessel was performed under general anesthesia. Immediately after ligation, the rabbits were placed in a 0.2-T open MR system, and an 18-G copper radiofrequency (RF) electrode with a 2-cm exposed tip was inserted into the liver. RF was applied for 10 minutes with the tip temperature maintained at 90 degrees +/- 2 degrees C. Before and after ablation, perfusion data were obtained for 90 seconds using 30 3-second sequential single oblique-slice fast imaging with steady-state progression (FISP) acquisitions after injection of gadolinium-diethylene triamine pentaacetic acid (Gd-DTPA) via the inferior vena cava. Postablation scanning included axial and oblique turbo spin-echo (TSE) T2-weighted (T2w), STIR, and Gd-enhanced T1w sequences. Lesion size was determined perpendicular to the RF electrode using software calipers on the imager. The rabbits were sacrificed after completion of the post-therapy scans, and their livers were harvested for histologic analysis. The liver showed a mean increase in signal amplitude (SA) of 76% 24 seconds after Gd contrast injection in the control group. After contrast injection, the SA increased to a mean of only 66% in the group with ligated hepatic arteries, with no difference in the time to peak compared with the control group. No significant SA increase over baseline could be found in the groups with ligated PV or ligated PV and HA. T2-weighted images demonstrated the highest lesion-to-liver contrast-to-noise ratios (CNRs; mean -5.5) on postprocedure images, followed by STIR images (mean -2.2) in the control group. The lesions were poorly delineated on the Gd-enhanced images. Average maximum lesion sizes (mean +/- 95% confidence interval) were 22 +/- 4.3 mm after ligation of PV, 22 +/- 2.6 mm after ligation of both PV and HA, 14 +/- 2.0 mm after ligation of HA, and 13 +/- 1.9 mm in the control group. We accept the hypothesis that the diameter of the region of coagulation necrosis achieved by standardized RF ablation in the liver increases with reduced organ perfusion and that this effect can be accurately monitored using MRI. The major factor influencing the size of the coagulation area is the portal venous flow. Occlusion of the hepatic artery alone does not significantly increase lesion size. T2w sequences are best suited for postprocedure imaging due to the high lesion-to-liver CNR in rabbits with normal hepatic perfusion. J. Magn. Reson. Imaging 2001;13:57-63.

Radial keyhole sequences for low field projection reconstruction interventional MRI.

Interventional magnetic resonance imaging (IMRI) is a rapidly emerging application for MRI in which diagnostic and therapeutic procedures are performed with MR image guidance. Real-time or near-real-time image acquisition and relative insensitivity to motion are essential for most intraoperative, therapeutic, and diagnostic procedures performed under MR guidance. The purpose of this work was to demonstrate the development and utility of two alternative rapid acquisition strategies during IMRI that are analogous to computed tomography fluoroscopy or keyhole MRI in a radial rather than rectilinear coordinate frame. The two strategies discussed here, interleaved projection reconstruction and continuous projection reconstruction, are compared and the feasibility of their application in experimental interventional applications is studied. J. Magn. Reson. Imaging 2001;13:142-151.

Applying the uniform resampling (URS) algorithm to a lissajous trajectory: fast image reconstruction with optimal gridding.

Various kinds of nonrectilinear Cartesian k-space trajectories have been studied, such as spiral, circular, and rosette trajectories. Although the nonrectilinear Cartesian sampling techniques generally have the advantage of fast data acquisition, the gridding process prior to 2D-FFT image reconstruction usually requires a number of additional calculations, thus necessitating an increase in the computation time. Further, the reconstructed image often exhibits artifacts resulting from both the k-space sampling pattern and the gridding procedure. To date, it has been demonstrated in only a few studies that the special geometric sampling patterns of certain specific trajectories facilitate fast image reconstruction. In other words, the inherent link among the trajectory, the sampling scheme, and the associated complexity of the regridding/reconstruction process has been investigated to only a limited extent. In this study, it is demonstrated that a Lissajous trajectory has the special geometric characteristics necessary for rapid reconstruction of nonrectilinear Cartesian k-space trajectories with constant sampling time intervals. Because of the applicability of a uniform resampling (URS) algorithm, a high-quality reconstructed image is obtained in a short reconstruction time when compared to other gridding algorithms.

Thermal lesion conspicuity following interstitial radiofrequency thermal tumor ablation in humans: a comparison of STIR, turbo spin-echo T2-weighted, and contrast-enhanced T1-weighted MR images at 0.2 T.

The purpose of this study was to compare the contrast between radiofrequency (RF) thermal liver lesions and surrounding tissue in T2-weighted turbo spin-echo sequences (TSE T2), short TI inversion recovery techniques (STIR), and contrast-enhanced (CE) T1-weighted spin-echo images. Nineteen RF thermal ablations were performed on eight patients with metastatic liver tumors. After ablation, contrast-to-noise ratios (CNRs) were calculated between mean signal amplitudes from three regions of interest (ROI) (lesion, surrounding edema, and normal tissue) using TSE T2-weighted, STIR, and contrast-enhanced T1-weighted (CE T1) sequences for each lesion. CNRs between the thermal lesion and normal liver tissue for both TSE T2-weighted (mean 0.9) and STIR (2.0) images were significantly lower than for CE T1-weighted (8.4) images (t-test, alpha = 0.05). However, CNRs between edema rim and the core of the thermal lesion for both TSE T2-weighted (8.1) and STIR images (7.2) were not significantly different (t-test, alpha = 0.05) from CNRs between lesion and normal tissue for CE T1-weighted images (8.4), nor was the CNR between edema rim and normal tissue for both TSE T2-weighted (10.3) and STIR (9.8) images. Although the edema was not visible on CE T1-weighted images, 18 of 19 lesions (94.7%) were surrounded by a hyperintense rim on TSE T2-weighted or STIR images. Both TSE T2-weighted and STIR sequences represent valid techniques for repeatable assessment of RF thermal lesions.

An optical system for wireless detuning of parallel resonant circuits.

A new optical method of detuning parallel resonant circuits is described. This method involves the integration of a photoresistor in parallel with the inductor and capacitor of a parallel resonant circuit, in this case a magnetic resonance imaging (MRI) receiver coil. A fiberoptic cable extending the length of the interventional device is used in conjunction with an external light source to deliver light to the photoresistor. Exposing the photoresistor to light changes its bulk resistance and greatly lowers the Q of the parallel resonant circuit, effectively detuning it. By combining this optical detuning scheme with inductive coupling of the interventional device-mounted microcoils to a standard MRI coil, a completely wireless device for active device tracking has been created. This new device improves on current technology by simplifying device complexity and reducing patient risk by eliminating the need for electrical connections between the device-mounted microcoils to the MR receiver channel.

Active MR guidance of interventional devices with target-navigation.

This project incorporated a novel inductive coupling structure of three micro coils into an invasive device tip to determine both its tip position and orientation. Moreover, with the introduction of a new target-navigation technique the MR scan plane was defined automatically by the invasive device orientation and target tissue location. A time domain multiplexing technique was applied for simultaneous MR imaging and device tracking. Using these techniques, the acquired MR images always showed both the invasive device and its target tissue. Thus, roadmap images and their potential misregistration errors were avoided. A graphical user interface (GUI) was also designed to assist interventional physicians in monitoring and guiding the insertion of the interventional device. Ex vivo phantom and in vivo animal experiments were performed to test this new technique. The methods developed in this project provide a new active technique for interventional device guidance using MRI. Magn Reson Med 44:56-65, 2000.

Semiautomatic 3-D image registration as applied to interventional MRI liver cancer treatment.

We evaluated semiautomatic, voxel-based registration methods for a new application, the assessment and optimization of interventional magnetic resonance imaging (I-MRI) guided thermal ablation of liver cancer. The abdominal images acquired on a low-field-strength, open I-MRI system contain noise, motion artifacts, and tissue deformation. Dissimilar images can be obtained as a result of different MRI acquisition techniques and/or changes induced by treatments. These features challenge a registration algorithm. We evaluated one manual and four automated methods on clinical images acquired before treatment, immediately following treatment, and during several follow-up studies. Images were T2-weighted, T1-weighted Gd-DTPA enhanced, T1-weighted, and short-inversion-time inversion recovery (STIR). Registration accuracy was estimated from distances between anatomical landmarks. Mutual information gave better results than entropy, correlation, and variance of gray-scale ratio. Preprocessing steps such as masking and an initialization method that used two-dimensional (2-D) registration to obtain initial transformation estimates were crucial. With proper preprocessing, automatic registration was successful with all image pairs having reasonable image quality. A registration accuracy of approximately equal to 3 mm was achieved with both manual and mutual information methods. Despite motion and deformation in the liver, mutual information registration is sufficiently accurate and robust for useful applications in I-MRI thermal ablation therapy.