Arno Buecker

In Vivo Magnetic Resonance Imaging of Coronary Thrombosis Using a Fibrin-Binding Molecular Magnetic Resonance Contrast Agent

Background—The advent of fibrin-binding molecular magnetic resonance (MR) contrast agents and advances in coronary MRI techniques offers the potential for direct imaging of coronary thrombosis. We tested the feasibility of this approach using a gadolinium (Gd)-based fibrin-binding contrast agent, EP-2104R (EPIX Medical Inc), in a swine model of coronary thrombus and in-stent thrombosis. Methods and Results—Ex vivo and in vivo sensitivity of coronary MR thrombus imaging was tested by use of intracoronarily delivered Gd-DTPA–labeled fibrinogen thrombi (n=6). After successful demonstration, in-stent coronary thrombosis was induced by x-ray–guided placement of thrombogenic-coated, MR-lucent stents (n=5). After stent placement, 60 &mgr;mol of EP-2104R was injected via the left main coronary artery. Free-breathing, navigator-gated 3D coronary MR angiography and thrombus imaging were performed (1) before and after stent placement and (2) before and after EP-2104R. Thrombi were confirmed by x-ray angiography and autopsy. Fibrinogen thrombi: 5 of 6 intracoronarily delivered Gd-labeled fibrinogen clots (≈250 &mgr;mol/L Gd) were visible on MRI and subsequently confirmed by x-ray angiography. In-stent thrombi: in-stent thrombosis was observed in all stents after EP-2104R. Four of 5 thrombi were confirmed by x-ray angiography. Chemical analysis of 2 thrombi demonstrated 99 to 147 &mgr;mol/L Gd. Conclusions—We demonstrate the feasibility of MRI of coronary thrombus and in-stent thrombosis using a novel fibrin-binding molecular MR contrast agent. Potential applications include detection of coronary in-stent thrombosis or thrombus burden in patients with acute coronary syndromes.

Molecular Magnetic Resonance Imaging of Coronary Thrombosis and Pulmonary Emboli With a Novel Fibrin-Targeted Contrast Agent

Background—The differential diagnosis of acute chest pain is challenging, especially in patients with normal ECG findings, and may include coronary thrombosis or pulmonary emboli. The aim of this study was to investigate the novel fibrin-specific contrast agent EP-2104R for molecular targeted MR imaging of coronary thrombosis and pulmonary emboli. Methods and Results—Fresh clots were engineered ex vivo from human blood and delivered in the lungs and coronary arteries of 7 swine. Subsequent molecular MR imaging was performed with a navigator-gated free-breathing and cardiac-triggered 3D inversion-recovery black-blood gradient-echo sequence before and after systemic administration of 7.5 &mgr;mol/kg EP-2104R. Two swine served as the control group. MR images were analyzed by 2 investigators, and contrast-to-noise ratio and gadolinium concentration in the clots were assessed. Before contrast media application, no thrombi were visible. After contrast administration, all 32 pulmonary emboli, 3 emboli in the right heart, and 5 coronary thrombi were selectively visualized as white spots with a mean contrast-to-noise ratio of 32±19. The average gadolinium concentration from all 3 types of thrombi was 144±79 &mgr;mol/L. Conclusions—Molecular MR imaging with the fibrin-targeted contrast-agent EP-2104R allows selective visualization of acute coronary, cardiac, and pulmonary thrombi.

Coronary artery stents in multislice computed tomography: in vitro artifact evaluation.

Rationale and ObjectiveThe aim of this study was to systematically compare the ability to assess the coronary artery lumen in the presence of coronary artery stents in multislice spiral CT (MSCT). MethodsTen different coronary artery stents were examined with 4- and 16-detector row MSCT scanners. For image reconstruction, a standard and a dedicated convolution kernel for coronary artery stent visualization were used. Images were analyzed regarding lumen visibility, intraluminal attenuation, and artifacts outside the stent lumen. Results were compared using repeated-measure analysis of variance. ResultsDepending on stent type, scanner hardware, and convolution kernel, artificial lumen narrowing ranged from 20% to 100%. The convolution kernel had the most significant influence on the visibility of the stent lumen. Artificial lumen narrowing and intraluminal attenuation changes decreased significantly using the dedicated convolution kernel. In general, most severe artifacts were caused by gold or gold-coated stents. ConclusionsIndependent of the scanner hardware or dedicated convolution kernels, routine evaluation of most coronary artery stents is not yet feasible using MSCT.

Molecular magnetic resonance imaging of atrial clots in a swine model.

Background—The detection and differentiation of intracardiac masses is still challenging and may include neoplasms and thrombi. The aim of this study was the investigation of a targeted, fibrin-specific contrast agent (EP-2104R) for molecular targeted magnetic resonance imaging (MRI) of left atrial clots. Methods and Results—Chronic human thrombi were surgically implanted in the left atrial appendage of 5 swine. Molecular MRI was performed with a navigator-gated, free-breathing, cardiac-triggered 3D inversion-recovery, black-blood, gradient-echo sequence before and after systemic administration of 4 &mgr;mol/kg EP-2104R. MR images were analyzed by 2 investigators, and the contrast-to-noise ratio was calculated. Location of clots was confirmed by autopsy, and the gadolinium concentration in the clots was assessed. Before contrast agent administration, thrombi were not visible on black-blood MR images. After contrast administration, all atrial clots (n=5) were selectively visualized as white spots with a high contrast-to-noise ratio (clot/blood, 29.7±8.0). The gadolinium concentration in the clots averaged 74±45 &mgr;mol/L. Conclusions—The fibrin-specific MR contrast agent EP-2104R allows for selective and high-contrast visualization of left atrial clots by means of molecular targeted MRI.

Magnetic resonance-guided placement of atrial septal closure device in animal model of patent foramen ovale

Background—Percutaneous closure of the patent foramen ovale (PFO) is usually performed under x-ray in combination with ultrasound guidance. We tested the feasibility of applying magnetic resonance (MR) guidance for percutaneous closure of PFO in an animal model, thus avoiding the disadvantage of ionizing radiation. Methods and Results—Real-time MRI with radial or spiral k-space filling (15 frames per second) on an interventional 1.5-T high-field whole-body system was exploited to examine the feasibility of MR-guided closure of the PFO in 7 piglets weighing ≈14 kg. A specially designed prototype nonmagnetic closure device was introduced via the femoral vein. The short bore of the magnet and in-room monitors allowed for visualization and steering of the catheter with the loaded occluder. Catheterization of the left atrium and, finally, correct placement of the device was possible in all animals. Deployment of the device was depicted by real-time MR, and initial misplacement, which occurred in 2 animals, was easily detected and corrected. Conclusions—Real-time MR guidance of PFO closure, without the use of ionizing radiation, is feasible in an animal model.

A feasibility study of contrast enhancement of acute myocardial infarction in multislice computed tomography: comparison with magnetic resonance imaging and gross morphology in pigs.

Introduction:Late enhancement magnetic resonance imaging (MRI) of myocardial infarction (MI) is clinically established. There are no reports on MI assessment using state-of-the-art multislice CT technology. For this reason, animal experiments were conducted to examine the applicability of contrast-enhanced ECG-gated multislice computed tomography (MSCT) for the detection of acute MI. The results were correlated with MRI and postmortem tissue staining. Material and Methods:Acute MI was induced in 14 pigs by balloon occlusion of the LAD. In 8 animals, the LAD was reperfused after 45 minutes. In 6 animals, the LAD was permanently blocked. MR imaging was performed 15 minutes after the administration of 0.2 mmol Gd-DTPA/kg/bodyweight. Subsequently, 16-slice MSCT was performed at various timepoints after injecting 120 mL of iodinated contrast medium. 2,3,5-Triphenyltetrazolin-chloride (TTC) staining was acquired for all hearts investigated. Correlation analysis was applied to compare the area of MI derived from MRI, MSCT, and TTC. The reperfused infarcts were compared with the nonreperfused infarcts using an unpaired t test. Results:Mean infarct area as measured by TTC staining was 18.3% ± 7.8% of the left ventricular area. Good correlation of the spatial extent of the infarcted area was found for TTC and MRI as well as for TTC and MSCT data obtained 5 minutes postcontrast injection. MSCT imaging demonstrated a significant difference in density (P < 0.001) between nonreperfused (47.0 ± 6.6 HU) and reperfused (116.4 ± 19.8 HU) infarction. Conclusion:In our pilot study, contrast-enhanced MSCT was feasible to assess myocardial viability in pigs. MSCT also affords differentiation of nonreperfused and reperfused acute MI. MI sizes derived from MSCT imaging correlate well to those obtained with MRI and TTC.

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.

Artifact-Free In-Stent Lumen Visualization by Standard Magnetic Resonance Angiography Using a New Metallic Magnetic Resonance Imaging Stent

Background—Metallic stents cause susceptibility and radiofrequency artifacts on MR images, which, up to now, have not allowed for complete visualization of the stent lumen by MR angiography. The aim of this study was to investigate the potential of a new dedicated renal MRI stent for artifact-free in-stent lumen visualization in vitro and in a swine model. Methods and Results—In vitro investigations were performed with prototypes of balloon-expandable Aachen Resonance Renal MRI Stents dilated to diameters of 3 to 6 mm and placed in an aqueous gadolinium solution (1:25). Phase-contrast and contrast-enhanced T1-weighted gradient echo images were acquired. Renal MRI stents (n=12) were deployed in the renal arteries of 6 pigs. Renal arteries were examined with phase-contrast angiography and with flow measurements before and after stent placement in the stented area, respectively. Additionally, a contrast-enhanced, T1-weighted, spoiled-gradient echo sequence after administration of 0.2 mmol gadolinium-DTPA/kg body weight was performed after stent placement. The visibility of artifacts was analyzed on in vitro and in vivo images by two investigators who knew the stent positions. Stent positions were determined visually (in vitro) or by x-ray angiography (animal experiments). No artifacts were detected independent of the applied imaging sequence and the stent orientation to the main magnetic field. Conclusion—The examined prototypes of fully MR-compatible MRI stents allow artifact-free visualization of the stent lumen with phase-contrast and contrast-enhanced T1-weighted angiography, as well as phase-contrast flow measurements in the stented area.

MR‐guided radiofrequency ablation of hepatic malignancies at 1.5 T: Initial results

To assess the feasibility of magnetic resonance (MR)‐guided radiofrequency ablation (RFA) of hepatic malignancies using a high‐field MR scanner.

Artifact-Free Coronary Magnetic Resonance Angiography and Coronary Vessel Wall Imaging in the Presence of a New, Metallic, Coronary Magnetic Resonance Imaging Stent

Background—Coronary in-stent restenosis cannot be directly assessed by magnetic resonance angiography (MRA) because of the local signal void of currently used stainless steel stents. The aim of this study was to investigate the potential of a new, dedicated, coronary MR imaging (MRI) stent for artifact-free, coronary MRA and in-stent lumen and vessel wall visualization. Methods and Results —Fifteen prototype stents were deployed in coronary arteries of 15 healthy swine and investigated with a double-oblique, navigator-gated, free-breathing, T2-prepared, 3D cartesian gradient-echo sequence; a T2-prepared, 3D spiral gradient-echo sequence; and a T2-prepared, 3D steady-state, free-precession coronary MRA sequence. Furthermore, black-blood vessel wall imaging by a dual-inversion-recovery, turbo spin-echo sequence was performed. Artifacts of the stented vessel segment and signal intensities of the coronary vessel lumen inside and outside the stent were assessed. With all investigated sequences, the vessel lumen and wall could be visualized without artifacts, including the stented vessel segment. No signal intensity alterations inside the stent when compared with the vessel lumen outside the stent were found. Conclusions—The new, coronary MRI stent allows for completely artifact-free coronary MRA and vessel wall imaging.