Biomedical subjects

Jonathan S Lewin

Publications and source records attributed to Jonathan S Lewin.

At least 55 records · Page 3Linked to original sources

PubMed2004

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.

Animals↗

PubMed2003

Targeted overexpression of luteinizing hormone causes ovary-dependent functional adenomas restricted to cells of the Pit-1 lineage.

The majority of pituitary adenomas in humans are nonmetastasizing, monoclonal neoplasms that occur in approximately 20% of the general population. Their development has been linked to a combination of extrinsic factors and intrinsic defects. We now demonstrate with transgenic mice that targeted and chronic overexpression of LH causes ovarian hyperstimulation and subsequent hyperproliferation of Pit-1-positive cells that culminates in the appearance of functional pituitary adenomas ranging from focal to multifocal expansion of lactotropes, somatotropes, and thyrotropes. Tumors fail to develop in ovariectomized mice, indicating that contributions from the ovary are necessary for adenoma development. Although the link between chronic ovarian hyperstimulation and PRL-secreting adenomas was expected, the involvement of somatotropes and thyrotropes was surprising and suggests that multiple ovarian hormones may contribute to this unusual pathological consequence. In support of this idea, we have found that ovariectomy followed by estrogen replacement results in the expansion of lactotropes selectively in LH overexpressing mice, but not somatotropes and thyrotropes. Collectively, these data indicate that estrogen is sufficient for the formation of lactotrope adenomas only in animals with a hyperstimulated ovary, whereas the appearance of GH- and TSH-secreting adenomas depends on multiple ovarian hormones. Together, our data expand current models of pituitary tumorigenesis by suggesting that chronic ovarian hyperstimulation may underlie the formation of a subset of pituitary adenomas containing lactotropes, somatotropes, and thyrotropes.

Adenoma↗

PubMed2003

Variation correction algorithm: analysis of phase suppression and thermal profile fidelity for proton resonance frequency magnetic resonance thermometry at 0.2 T.

PURPOSE: To develop and analyze the performance of the variation correction algorithm (VCA), a phase correction technique that mitigates the contribution of background phase variations by combining accurate alignment of echoes, K-space-based phase correction (as opposed to spatial polynomials), and extraction of alias-free phase difference images. MATERIALS AND METHODS: A series of echo-shifted gradient-recalled echo (GRE) images was processed with K-space alignment and phase corrected with increasing sizes of M x M masks of central K-space coefficients. The extent of background phase variation suppression due to magnet field drift was assessed. Further, a simulated thermal profile was superimposed on the same data in a related experiment. Residual errors in reconstructed simulated thermal profiles were quantitatively characterized to estimate algorithm performance. RESULTS: Using a 3 x 3 K-space mask, the VCA was able to 1) maintain the typical mean background error in a 35 x 35 pixel region of interest (ROI) at -0.1 degrees C; and 2) reconstruct, relative to the applied thermal profile, a phase-corrected profile that typically contains a 1.7 degrees C underestimation of peak temperature difference and a mean error along the 60 degrees C line of -0.8 degrees C. CONCLUSION: The results suggest that thermal profiles can be accurately reconstructed at 0.2 T using the VCA, even in the presence of over 1 ppm spatially and temporally dependent field drift over a 1-hour time frame.

Algorithms↗

PubMed2003

Keyhole Dixon method for faster, perceptually equivalent fat suppression.

PURPOSE: To reduce the acquisition time associated with the two-point Dixon fat suppression technique by combining a keyhole in-phase (Water + Fat) k-space data set with a full out-of-phase (Water - Fat) k-space data set and optimizing the keyhole size with a perceptual difference model. MATERIALS AND METHODS: A set of keyhole Dixon images was created by varying the number of lines in the keyhole data set. Off-resonance correction was incorporated into the image reconstruction process to improve the homogeneity of the fat suppression. A perceptual difference model (PDM) was validated with human observer experiments and used to compare the keyhole images to images from a full two-point Dixon acquisition. The PDM was used to determine the smallest keyhole width required to obtain perceptual equivalence to images obtained from the full two-point Dixon method. RESULTS: In experimental phantom studies, the keyhole Dixon image reconstructed from 96 of 192 Water + Fat k-space lines and 192 Water - Fat k-space lines was perceptually equivalent to the full (192 + 192) two-point Dixon images, resulting in a 25% reduction in scan time. Clinical images of a volunteer's knee, orbits, and abdomen created from the smallest, perceptually equivalent keyhole width resulted in a 27%-38% reduction in total scan time. CONCLUSION: This method improves the temporal efficiency of the conventional two-point Dixon technique and may prove especially useful for high-field systems where specific absorption rate (SAR) limits will constrain radiofrequency (RF)-based fat suppression techniques.

Adipose Tissue↗

PubMed2003

Three-dimensional method for comparing in vivo interventional MR images of thermally ablated tissue with tissue response.

PURPOSE: To investigate the ability of magnetic resonance (MR) to monitor radio-frequency (RF) ablation treatments by comparing MR images of thermal lesions to histologically assayed cellular damage. We developed a new methodology using three-dimensional registration for making spatial correlations. MATERIALS AND METHODS: A low-field, open MRI system was used to guide an ablation probe into rabbit thigh muscle and acquire MR volumes after ablation. After fixation, we sliced and photographed the tissue at 3-mm intervals, using a specially designed apparatus, to obtain a volume of tissue images. Histologic samples were digitized using a video microscopy system. For our three-dimensional registration method, we used the tissue images as the reference, and registered histology and MR images to them using two different computer alignment steps. First, the MR volume was aligned to the volume of tissue images by registering needle fiducials placed near the tissue of interest. Second, we registered the histology images with the tissue images using a two-dimensional warping technique that aligned internal features and the outside boundary of histology and tissue images. RESULTS: The MR and histology images were very well aligned, and registration accuracy, determined from displacement of needle fiducials, was 1.32 +/- 0.39 mm (mean +/- SD), which compared favorably to the MR voxel dimensions (0.70 mm in-plane and 3.0 mm thick). A preliminary comparison of MR and tissue response showed that the region inside the elliptical hyperintense rim in MR closely corresponds to the region of necrosis as established by histology, with a mean absolute distance between MR and histology boundaries of 1.17 mm, slightly smaller than the mean registration error. The MR region slightly overestimated the region of necrosis, with a mean signed distance between boundaries of 0.85 mm. CONCLUSION: Our results suggest that our methodology can be used to achieve three-dimensional registration of histology and in vivo MR images. In MR lesion images, the inner border of the hyperintense region corresponds to the border of irreversible cell damage. This is good evidence that during RF ablation treatments, iMRI lesion images can be used for real-time feedback.

Animals↗

PubMed2003

Sub-acute changes in lesion conspicuity and geometry following MR-guided radiofrequency ablation.

PURPOSE: To evaluate MR signal and lesion zone volume evolution through the sub-acute phase following image-guided radiofrequency (RF) thermal ablation. MATERIALS AND METHODS: For many tissues, including muscle and liver, thermal lesions that result from RF heating have a characteristic two-boundary appearance featuring an inner core (zone I) surrounded by a hyper-intense margin (zone II) and normal tissue (zone III), found in both T(2) and contrast enhanced (CE) T(1)-weighted MR images, both immediately post-ablation and four days later. First, we compared corresponding points between manually segmented zone boundaries apparent in T(2)- and CE T(1)-weighted images. Second, we examined the contrast-to-noise ratio (CNR) between all zone combinations. Third, we quantified the volume of zone I, zone II, and the entire lesion using a three-dimensional lesion geometry model fitted to segmented images. RESULTS: On a slice-by-slice basis, no statistically significant differences were found between zone boundaries apparent in T(2) and CE T(1)-weighted images. The contrast to noise ratio (CNR) of zone I vs. zone II, zone I vs. background muscle, and zone II vs. background muscle was always equal or greater for T(2)-weighted images than for CE T(1)-weighted images. In addition, by day four, zone II significantly increased in intensity compared to background muscle. The median Zone I volume increase was 44.2% (42.6%) using T(2) weighted images and 55.5% (68.7% interquartile range) using CE T(1)- weighted images. This expansion likely corresponds to an enlargement of the ablated, coagulative necrosis, region. The median Zone II volume increase was 15.0% (42.6%) using T(2)- weighted images 1.5% (38.8%) using CE T(1)-weighted images. CONCLUSIONS: 1) There are no significant differences between the apparent zone boundaries in T(2)- and CE T(1)-weighted images; 2) CNR is equal or greater for T(2)-weighted images as compared to CE T(1)-images; and 3) both the inner and outer lesion zone volumes typically increase several days post-ablation.

Animals↗

PubMed2003

Radio-frequency-induced thermal lesions: subacute magnetic resonance appearance and histological correlation.

PURPOSE: To investigate the relationship between subacute magnetic resonance (MR) images of radio-frequency (RF) ablation lesions and tissue viability as determined from histological tissue samples. MATERIALS AND METHODS: We generated lesions (N = 5) in a rabbit thigh model. Four days later, we obtained in vivo T(2)- and contrast-enhanced (CE) T(1)-weighted images and ex vivo histological samples approximately perpendicular to the electrode path. Using three-dimensional registration and warping, we spatially compared manually segmented boundaries apparent on MR images to boundaries separating distinct histological zones determined from hematoxylin and eosin (H&E) and Masson trichrome (MT) stains, as well as birefringence studies. RESULTS: Lesions have a characteristic MR appearance: an outer hyperintense margin (M2) separating background tissue (M3) from an inner core (M1), in both T(2) and CE T(1) images. Histologically, there are two zones of damage: an outer zone of likely nonviable cells (H2) separating background tissue (H3) from an inner core of coagulated nonviable cells (H1). We measured distances between automatically computed correspondence points along histological and MR boundaries. For T(2) and CE T(1) images, respectively, M1 vs. H1 distances were 0.72 +/- 0.99 mm (mean +/- SD) and 0.10 +/- 0.95 mm, while outer M2 vs. H2 boundary distances were 0.26 +/- 1.16 mm and 0.05 +/- 1.08 mm. The discrepancy between histological and MR boundaries was larger than the variability in segmenting MR images, but probably within registration error. There were no significant differences between T(2) and CE T(1) boundaries. CONCLUSION: Lesion boundaries apparent in both T(2)- and CE T(1)-weighted MR scans, performed several days postablation, similarly predict the histological response. That is, the lesion core (M1) corresponds to nonviable coagulated cells (H1), while the hyperintense margin (M2) corresponds to likely nonviable cells undergoing necrotic changes (H2).

Animals↗

PubMed2003

Determining and optimizing the precision of quantitative measurements of perfusion from dynamic contrast enhanced MRI.

PURPOSE: To examine the sensitivity of quantitative dynamic contrast enhanced MRI (DCE-MRI) perfusion maps to errors in the various source images and to determine optimal imaging parameters for reducing this sensitivity. MATERIALS AND METHODS: A detailed analysis of the precision of a DCE-MRI protocol was performed using the "propagation of errors" technique to investigate the effect of errors in the source images on errors in K(trans). Optimal parameter values and interactions between parameters were examined. The propagation of errors analysis was validated by Monte-Carlo simulations. RESULTS: The precision of K(trans) was found to be most sensitive to artifacts in the tissue portion of the baseline images and least sensitive to noise in the arterial portion of the dynamic images. The tip-angle strongly affected the precision, with the optimum being a function of tissue T1(0). CONCLUSION: Protocol optimization requires matching the tip-angle to the anticipated T1(0) of the tissue of interest; however such optimization yields a relatively small improvement. Future developmental efforts would be most productively focused on minimizing the artifact level.

Artifacts↗

PubMed2003

Real-time catheter tracking and adaptive imaging.

PURPOSE: To evaluate the performance of a real-time MR system for interventional procedures that adjusts specific image parameters in real time based on a catheter's speed of insertion. MATERIALS AND METHODS: The system was implemented using only the hardware provided with a standard short-bore 1.5 T scanner (Siemens Magnetom Sonata) (with the exception of small tracking markers affixed to the catheter). The system tracks the position of an MR microcoil-instrumented catheter and automatically updates the scan plane's position and orientation, as well as other features, including, but not limited to, field of view, resolution, tip angle, and TE. A real-time feedback loop continuously localizes the tracking markers, updates the scan plane position and orientation, calculates the catheter's speed, adjusts the value of specific image parameters, then collects new image data, reconstructs an image, and provides it for immediate display. The system was evaluated in phantom and in vivo porcine experiments. RESULTS: The system is able to accurately localize a moving catheter in the abdominal aorta, calculate the device speed, and respond by adjusting specified image parameters 98% of the time, with precision of approximately 2 mm and 1.5 degrees. CONCLUSION: Simply slowing the speed of the catheter allows the clinician to adjust predetermined image parameters. This work also has the potential to build a degree of intelligence into the scanner, enabling it to react to changes in the clinical environment and automatically optimize specific image parameters.

Animals↗

PubMed2003

Novel interleaved spiral imaging motion correction technique using orbital navigators.

Although spiral imaging seldom produces apparent artifacts related to flow, it remains sensitive to rapid object motion. In this article, a new correction method is presented for rapid rigid body motion in interleaved spiral imaging. With this technique, an identical circular navigator k-space trajectory is linked to each spiral trajectory. Data inconsistency due to both rotation and translation among spiral interleaves can be corrected by evaluating the magnitudes and phases of the data contained in the navigator "ring." Further, it is difficult to create a frequency field map for off-resonance correction when an object moves during a scan, because there is motion-dependent misregistration between the two images acquired with different TEs. However, this difficulty can be overcome by combining the motion-correction method with a recently proposed technique (off-resonance correction using variable-density spirals (ORC-VDS)), thereby enabling both motion compensation and off-resonance correction with no additional scanning.

Artifacts↗

PubMed2003

Block regional off-resonance correction (BRORC): a fast and effective deblurring method for spiral imaging.

One primary disadvantage of spiral imaging is blurring artifact due to off-resonance effects. The conventional frequency segmented off-resonance correction method that is performed over the entire image is computationally intense due to the large number of fast Fourier transforms (FFTs) required. Here, a new fast off-resonance correction method, block regional off-resonance correction (BRORC), is presented. In this method, off-resonance correction proceeds block-by-block through the reconstructed image with FFTs performed on matrices that are smaller than the full image matrix. The BRORC algorithm is typically several times more computationally efficient than the conventional off-resonance correction algorithm. Additional computational reductions can be expected for the BRORC if only specific image regions require deblurring. The newly proposed off-resonance correction method offers significant speed advantages and equivalent image quality when compared to conventional off-resonance correction methods.

Algorithms↗

PubMed2003

Radial alternating TE sequence for faster fat suppression.

This study describes a steady-state sequence that uses a radial k-space trajectory and alternating echo times (TEs) between even and odd k-space views. The sequence generated a single data set that was used to reconstruct images with inherent fat suppression. This fat suppression results from the fat phase variation in alternate echoes giving rise to cancellation in the central portion of k-space. This new fat-suppression method provides inherent fat suppression in half the acquisition time relative to the radial two-point Dixon method. The improvement in k-space sampling efficiency is demonstrated in phantom and clinical images, and through measured point-spread functions (PSFs). As a result, the radial alternating TE sequence offers improved temporal resolution over a radial version of the two-point Dixon sequence by requiring fewer total projections to obtain the same effective resolution in water-based tissues.

Adipose Tissue↗

PubMed2003

Dixon techniques in spiral trajectories with off-resonance correction: a new approach for fat signal suppression without spatial-spectral RF pulses.

Spiral imaging has recently gained acceptance in MR applications requiring rapid data acquisition. One of the main disadvantages of spiral imaging, however, is blurring artifacts that result from off-resonance effects. Spatial-spectral (SPSP) pulses are commonly used to suppress those spins that are chemically shifted from water and lead to off-resonance artifacts. However, SPSP pulses may produce nonuniform fat signal suppression or unwanted water signal suppression when applied in the presence of B(0) field inhomogeneities. Dixon techniques have been developed as methods for water-fat signal decomposition in rectilinear sampling schemes since they can produce unequivocal water-fat signal decomposition even in the presence of B(0) inhomogeneities. This article demonstrates that three-point and two-point Dixon techniques can be extended to conventional spiral and variable-density spiral data acquisitions for unambiguous water-fat decomposition with off-resonance blurring correction. In the spiral three-point Dixon technique, water-fat signal decomposition and image deblurring are performed based on the frequency maps that are directly derived from the acquired images. In the spiral two-point Dixon technique, several predetermined frequencies are tested to create a frequency map. The newly proposed techniques can achieve more effective and more uniform fat signal suppression when compared to the conventional spiral acquisition method with SPSP pulses.

Adipose Tissue↗

PubMed2003

Intraoperative MR imaging.

Intraoperative MR imaging has become a safe and effective technology that has revolutionized the way neurosurgery is performed. Benefits include the ability to update data sets for navigational systems, to monitor tumor resections, to adjust the approach to intracranial lesions, and to guide functional and drug or cell delivery procedures. Use of this technique can help avoid inadvertent injury of important anatomic and vascular structures. In addition, complications such as ischemia or hemorrhage can be detected early. Intraoperative MR imaging is particularly useful for ensuring that brain biopsies yield diagnostic tissue and for assessing the completeness of tumor resection. As is true for any new technology, the benefits of intraoperative MR imaging must be examined carefully to guarantee appropriate use. Many neurosurgical procedures do not require real-time image guidance and can be performed safely using current surgical techniques, including microsurgical methods and frameless and frame-based stereotaxy. Other tumor resections, tumor biopsies, and surgical and interventional procedures distinctly benefit from the sophisticated information provided by intraoperative imaging techniques. In surgery for low-grade gliomas, intraoperative MR imaging has found general acceptance, whereas its usefulness to monitor the resection of high-grade gliomas remains controversial. The economic issues related to intraoperative MR imaging cannot be overlooked. The acquisition of an intraoperative MR imaging system is associated with considerable expense, and its performance increases the cost of equipment and the operating time. Despite these additional expenses, intraoperative MR imaging can lead to a potential overall cost reduction in the treatment of certain patients if long-term cure can be achieved, repeat resection can be avoided, or procedure-associated morbidity can be reduced. Although intraoperative MR imaging techniques hold tremendous potential, the definition of their appropriate role in the delivery of successful and cost-effective medical care awaits further study.

Brain↗

PubMed2003

Endoscopically inserted endoluminal receiver coil for high-resolution magnetic resonance imaging of the pancreas: Initial results in an animal model.

BACKGROUND: This study assessed the feasibility of high-resolution magnetic resonance imaging of the pancreas by means of an endoscopically inserted endoluminal magnetic resonance receiver coil. METHOD: A 0.032-inch diameter internal magnetic resonance imaging receiver coil was endoscopically inserted into the pancreatic duct in 4 pigs through the accessory channel of a standard duodenoscope to obtain high-resolution magnetic resonance images by using T1- and T2-weighted sequences. RESULTS: The pig anatomy precluded the usual transoral approach; however, transgastric access allowed endoscopic transpapillary insertion of a receiver coil into the pancreatic duct in all animals without the need for sphincterotomy. The small swine pancreas could then be visualized by magnetic resonance imaging with a 0.3 x 0.3-mm in-plane resolution. CONCLUSION: High-resolution pancreas magnetic resonance imaging is feasible by using an endoscopically inserted endoluminal receiver coil. The smaller stomach and larger pancreatic duct diameter in humans will facilitate clinical application of the imaging procedure.

Animals↗

PubMed2003

Slice-to-volume registration and its potential application to interventional MRI-guided radio-frequency thermal ablation of prostate cancer.

In this study, we registered live-time interventional magnetic resonance imaging (iMRI) slices with a previously obtained high-resolution MRI volume that in turn can be registered with a variety of functional images, e.g., PET, SPECT, for tumor targeting. We created and evaluated a slice-to-volume (SV) registration algorithm with special features for its potential use in iMRI-guided radio-frequency (RF) thermal ablation of prostate cancer. The algorithm features included a multiresolution approach, two similarity measures, and automatic restarting to avoid local minima. Imaging experiments were performed on volunteers using a conventional 1.5-T MR scanner and a clinical 0.2-T C-arm iMRI system under realistic conditions. Both high-resolution MR volumes and actual iMRI image slices were acquired from the same volunteers. Actual and simulated iMRI images were used to test the dependence of SV registration on image noise, receive coil inhomogeneity, and RF needle artifacts. To quantitatively assess registration, we calculated the mean voxel displacement over a volume of interest between SV registration and volume-to-volume registration, which was previously shown to be quite accurate. More than 800 registration experiments were performed. For transverse image slices covering the prostate, the SV registration algorithm was 100% successful with an error of <2 mm, and the average and standard deviation was only 0.4 mm +/- 0.2 mm. Visualizations such as combined sector display and contour overlay showed excellent registration of the prostate and other organs throughout the pelvis. Error was greater when an image slice was obtained at other orientations and positions, mostly because of inconsistent image content such as that from variable rectal and bladder filling. These preliminary experiments indicate that MR SV registration is sufficiently accurate to aid image-guided therapy.

Algorithms↗

PubMed2003

Volume registration using needle paths and point landmarks for evaluation of interventional MRI treatments.

We created a method for three-dimensional (3-D) registration of medical images (e.g., magnetic resonance imaging (MRI) or computed tomography) to images of physical tissue sections or to other medical images and evaluated its accuracy. Our method proved valuable for evaluation of animal model experiments on interventional-MRI guided thermal ablation and on a new localized drug delivery system. The method computes an optimum set of rigid body registration parameters by minimization of the Euclidean distances between automatically chosen correspondence points, along manually selected fiducial needle paths, and optional point landmarks, using the iterative closest point algorithm. For numerically simulated experiments, using two needle paths over a range of needle orientations, mean voxel displacement errors depended mostly on needle localization error when the angle between needles was at least 20 degrees. For parameters typical of our in vivo experiments, the mean voxel displacement error was < 0.35 mm. In addition, we determined that the distance objective function was a useful diagnostic for predicting registration quality. To evaluate the registration quality of physical specimens, we computed the misregistration for a needle not considered during the optimization procedure. We registered an ex vivo sheep brain MR volume with another MR volume and tissue section photographs, using various combinations of needle and point landmarks. Mean registration error was always < or = 0.54 mm for MR-to-MR registrations and < or = 0.52 mm for MR to tissue section registrations. We also applied the method to correlate MR volumes of radio-frequency induced thermal ablation lesions with actual tissue destruction. In this case, in vivo rabbit thigh volumes were registered to photographs of ex vivo tissue sections using two needle paths. Mean registration errors were between 0.7 and 1.36 mm over all rabbits, the largest error less than two MR voxel widths. We conclude that our method provides sufficient spatial correspondence to facilitate comparison of 3-D image data with data from gross pathology tissue sections and histology.

Anatomy, Cross-Sectional↗

PubMed2003

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

PURPOSE: 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. MATERIALS AND METHODS: 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. RESULTS: 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. CONCLUSION: 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.

Animals↗