Rolf W. Guenther

Hydrodynamic Thrombectomy of Hemodialysis Fistulas: First Clinical Results

PURPOSE A hydrodynamic thrombectomy system was used for the treatment of recent dialysis shunt thrombosis. PATIENTS AND METHODS Sixteen shunt thromboses in 14 patients were included in the study. There were seven polytetrafluoroethylene grafts and nine native arteriovenous fistulas. Occlusion time ranged from 6 to 48 hours, and thrombus length ranged from 4 to 40 cm. RESULTS Thrombectomy was technically successful in 15 of 16 instances. No significant residual thrombus was found in 15 cases. In one case, half of the thrombus remained in the vessel and the procedure failed technically. One embolus to the radial artery occurred after balloon dilation following hydrodynamic thrombectomy and was removed percutaneously. Early rethrombosis within 24 hours occurred in five shunts; four more rethrombosed within 2 weeks to 3 months. Eleven shunts were available for follow-up. Cumulative patency was 41% after 6 months. CONCLUSION Hydrodynamic thrombectomy is a promising concept for declotting of both hemodialysis grafts and native shunts and may offer an alternative to thrombolysis and surgical thrombectomy.

Real-time MR fluoroscopy for MR-guided iliac artery stent placement

The purpose of this study was to test the feasibility of real‐time magnetic resonance (MR) guidance of iliac artery stent placement. Radial scanning together with the sliding window reconstruction technique was implemented on a 1.5 T magnet, yielding a frame rate of 20 images per second. Seven prototype nitinol ZA stents were deployed in iliac arteries of living pigs under MR control. All stents were well visualized on the radial MR images, allowing depiction of the mounted stents as well as stent deployment without anatomy‐obscuring artifacts. Stent placement was sucessful in all cases and took 6 minutes on average. The position of the stents was correctly visualized by real‐time radial MR scanning, as proved by digital subtraction X‐ray angiography. Combined radial scanning and the sliding window reconstruction technique allow real‐time MR‐guided stent placement in iliac arteries. J. Magn. Reson. Imaging 2000;12:616–622.

In vivo safe catheter visualization and slice tracking using an optically detunable resonant marker

The purpose of this study was to test the in vivo feasibility of safe automatic catheter tracking based on an optically detunable resonant marker installed on the catheter tip, and also to test the compatibility of this approach with guidewire materials. The design of the resonant marker and the integration into the real‐time MR environment is described. The catheter was used for real‐time MR‐guided catheterization of the aorta, left ventricle, and carotid in two swine. For in‐plane visualization, the marker was repeatedly detuned. For automatic slice tracking, a projection difference measurement including detuning was interleaved with the imaging sequence. In vitro experiments were conducted to investigate the RF‐safety of the marker and the effect of the guidewires on the signal intensity. For all orientations the marker provided excellent in vivo contrast using a radial steady‐state free‐precession sequence. Flashing of the marker by repetitive tuning/detuning further improved the in‐plane visualization. Automatic slice tracking during real‐time imaging was successfully performed. The plastic guidewires did not interfere with the marker, and detuning by guidewires containing nitinol could be compensated. In conclusion, automatic slice tracking as well as excellent in‐plane visualization can be achieved with this approach and it is safe with respect to RF transmission. Magn Reson Med 52:860–868, 2004.

MR evaluation of stent patency: in vitro test of 22 metallic stents and the possibility of determining their patency by MR angiography.

Meyer JMA, Bucker A, Schurmann K, et al. MR evaluation of stent patency: In vitro tests of 22 metallic stents and the possibility of determining their patency by MR angiography. Invest Radiol 2000;35:739–746. RATIONALE AND OBJECTIVES.To determine the extent to which visualization of intrastent anatomy in stents of different composition and design is possible by using contrast-enhanced MR angiography. METHODS.Twenty-two MR-compatible stents, most of which had a diameter of 8 mm, were positioned in a phantom filled with aqueous gadolinium solution. Coronal and axial spoiled three-dimensional gradient-echo sequences were performed. Images were acquired with stents positioned at varying angulations to the main magnetic field B0. Profiles orthogonal to the stent axis allowed measurement of artifact sizes independent of window width and center. RESULTS.Oriented along B0, the Cragg, Corvita, Passager, Wallstent, Strecker, Impag, Perflex, and ZA stents allowed visualization of more than 48% of the lumen. The Memotherm, Smart, and Jostent SelfX stents showed a prominent reduction of the inner lumen to below 41%. The lumina of the covered Jostent, Palmaz, Sinus, and Symphony stents were completely obscured. The Impag, Perflex, and Strecker tantalum stents showed growing artifact sizes and a lumen reduction of at least 40% with increasing angulation to B0. CONCLUSIONS.Evaluation of the inner stent lumen by applying contrast-enhanced, three-dimensional gradient-echo sequences is not possible for the majority of stents because of their large artifacts. These depend on the stent type and orientation to B0. Even stents made of nitinol and cobalt alloys only allow qualitative patency assessment but no quantification of stenosis.

MR imaging of the knee: three-dimensional volume imaging combined with fast processing.

A new examination protocol for patients with suspected meniscal or hyaline cartilage disorders is presented. Knees were imaged with a fast gradient echo imaging sequence with subsequent three-dimensional transformation. Data were transferred to an image processing system that allows reconstruction in any plane desired. Approximately 200 interactively positioned reconstructions are displayed and reviewed in approximately 10 min. Thirty-five patients with suspected meniscal or hyaline cartilage disorders were examined prospectively. All underwent subsequent arthroscopy. The magnetic resonance (MR) diagnosis showed an accuracy of 91.4% compared with arthroscopic findings. Owing to the short imaging time, the proposed procedure permits screening of knee joint disorders with MR.

Dual-source computed tomography for assessing cardiac function: a phantom study.

Purpose:To investigate the influence of heart rate and temporal resolution on the assessment of global ventricular function with dual-source computed tomography (DSCT). Materials and Methods:A dynamic cardiac phantom was repeatedly scanned with a DSCT scanner applying a standardized scan protocol at different heart rates, ranging from 40 to 140 bpm. Images were reconstructed with monosegmental and bisegmental algorithms using data from a single source and from both sources. Ventricular volumes and ejection fraction (EF) were computed by semiautomated analysis. Results were compared with the phantoms real volumes. Interscan, intraobserver, and interobserver variability were calculated. Results:For single-source data reconstruction temporal resolution was fixed to 165 milliseconds, whereas dual-source image reconstructions resulted in a temporal resolution of 83 milliseconds (monosegmental) and 67.7 ± 14.2 milliseconds (bisegmental), respectively. In general, deviation from the phantoms real volumes was less with dual-source data reconstruction when compared with single-source data reconstruction. Comparing dual-source data reconstruction with single-source data reconstruction, the percent deviation from the phantoms real volumes for EF was 0.7% (monosegmental), 0.7% (bisegmental), and 4.3% (single source), respectively. There was no correlation between heart rate and EF for dual-source data reconstruction (r = −0.168; r = −0.157), whereas a relevant correlation was observed for single-source data reconstruction (r = −0.844). Interscan, intraobserver, and interobserver variability for EF were 1.4%, 0.9%, and 0.3%, respectively. Conclusions:DSCT allows reliable quantification of global ventricular function independent of the heart rate. Multisegmental image reconstruction is not needed for DSCT assessment of global ventricular function.

Time-Resolved 3D MR Angiography of the Foot at 3 T in Patients with Peripheral Arterial Disease

OBJECTIVE The objective of our study was to prove the feasibility and clinical relevance of fast contrast-enhanced time-resolved 3D MR angiography (MRA) with submillimeter spatial resolution at a high magnetic field strength. SUBJECTS AND METHODS Twenty-one patients (five women, 16 men; mean age +/- SD, 65 +/- 14 years) were examined on a 3-T whole-body MR system with an 8-element phasedarray coil for preoperative evaluation of the pedal arterial system and assessment of the visualized vessels to serve as a graft touch-down site in pedal bypass surgery. Time-resolved 3D MRA of the foot was performed after automatic injection of 0.2 mmol/kg of gadobenate dimeglumine using a sagittal gradient-echo T1-weighted sequence (TR/TE, 4.2/1.6; flip angle, 30 degrees ; field of view, 290 mm; matrix, 352; 120 slices; slice thickness, 0.8 mm) with a spatial resolution of 0.8 x 0.8 x 1.6 mm reconstructed to 0.6 x 0.6 x 0.8 mm and a temporal resolution of 3.9 seconds using keyhole and sensitivity-encoding (SENSE) technology (SENSE factors: 4 in anteroposterior direction and 2 in right-left direction). Dynamic subtractions and rotating maximum intensity projections were calculated. The original image data sets were transferred to a dedicated workstation for objective signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) analysis of the arteries. Subjective image analysis regarding image quality and diagnostic findings was performed by two radiologists in consensus. RESULTS In all patients, images of diagnostic quality were obtained. Despite the known limitations regarding signal intensity measurements in images acquired with the use of parallel imaging technique, SNR and CNR proved to be excellent, with mean +/- SD values of 294 +/- 158 and 248 +/- 144, respectively. Although most of the patients had diabetic foot syndrome with arteriovenous shunting, the arteries and the potential vessel for bypassing could be clearly separated from the veins in each case due to the temporal information given by our study. The ability to reliably discriminate arteries from veins is of high clinical relevance in planning pedal bypass surgery. CONCLUSION Fast contrast-enhanced time-resolved 3D MRA of the foot at 3 T is feasible and of high clinical value for the preoperative evaluation of the arterial supply of the foot.

Iliocaval stenosis and iliac venous thrombosis in retroperitoneal fibrosis : percutaneous treatment by use of hydrodynamic thrombectomy and stenting

A case of bilateral iliac stenosis and caval stenosis due to retroperitoneal fibrosis was treated by caval stenting and iliac balloon angioplasty, but was complicated by subsequent iliac thrombosis. Venous thrombectomy was successfully achieved by hydrodynamic thrombectomy, and iliac patency was stabilized by bilateral stent insertion.

Effects of lithotripsy on rat kidney: evaluation with MR imaging, histology, and electron microscopy

Magnetic resonance (MR) imaging at 1.5 T was used to evaluate the effects of extracorporeal shock wave lithotripsy (ESWL) in 30 rats and the findings on T1– and T2-weighted (spin echo 600/22, 1,600–2,000/90) images were compared with histology and scanning microscopy. The observed pathologic changes increased in severity with the number of shock waves given (500–5,000 15 kV). Post-ESWL MR findings in 54 kidneys included perirenal and subcapsular fluid (n = 30), diffuse loss of corticomedullary junction definition (n = 28), intrarenal foci of increased (n = 7) or decreased (n = 6) signal intensity, focal indentation of the renal contour (n = 5), and loss of distinction between the renal, splenic, or hepatic contour (n = 7). The subcapsular and intrarenal findings corresponded pathologically to areas of hemorrhage and hematoma formation—the contour changes to foci of renal scarring or perirenal adhesions. Electron microscopy demonstrated marked alterations of the renal tubules and vasculature. The study shows the feasibility of assessing the nature and chronology of renal damage post-ESWL in a rat model by MR.

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

Abstract 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.