Nils Krämer

Magnetic Resonance–Guided Cardiac Interventions Using Magnetic Resonance–Compatible Devices: A Preclinical Study and First-in-Man Congenital Interventions

Background— Percutaneous cardiac interventions are currently performed under x-ray guidance. Magnetic resonance imaging (MRI) has been used to guide intravascular interventions in the past, but mainly in animals. Translation of MR-guided interventions into humans has been limited by the lack of MR-compatible and safe equipment, such as MR guide wires with mechanical characteristics similar to standard guide wires. The aim of the present study was to evaluate the safety and efficacy of a newly developed MR-safe and compatible passive guide wire in aiding MR-guided cardiac interventions in a swine model and describe the 2 first-in-man solely MR-guided interventions. Methods and Results— In the preclinical trial, the new MR-compatible wire aided the performance of 20 interventions in 5 swine. These consisted of balloon dilation of nondiseased pulmonary and aortic valves, aortic arch, and branch pulmonary arteries. After ethics and regulatory authority approval, the 2 first-in-man MR-guided interventions were performed in a child and an adult, both with elements of valvar pulmonary stenosis. Catheter manipulations were monitored with real-time MRI sequence with interactive modification of imaging plane and slice position. Temporal resolution was 11 to 12 frames/s. Catheterization procedure times were 110 and 80 minutes, respectively. Both patients had successful relief of the valvar stenosis and no procedural complications. Conclusions— The described preclinical study and case reports are encouraging that with the availability of the new MR-compatible and safe guide wire, certain percutaneous cardiac interventions will become feasible to perform solely under MR guidance in the future. A clinical trial is underway in our institution.Background—Percutaneous cardiac interventions are currently performed under x-ray guidance. Magnetic resonance imaging (MRI) has been used to guide intravascular interventions in the past, but mainly in animals. Translation of MR-guided interventions into humans has been limited by the lack of MR-compatible and safe equipment, such as MR guide wires with mechanical characteristics similar to standard guide wires. The aim of the present study was to evaluate the safety and efficacy of a newly developed MR-safe and compatible passive guide wire in aiding MR-guided cardiac interventions in a swine model and describe the 2 first-in-man solely MR-guided interventions. Methods and Results—In the preclinical trial, the new MR-compatible wire aided the performance of 20 interventions in 5 swine. These consisted of balloon dilation of nondiseased pulmonary and aortic valves, aortic arch, and branch pulmonary arteries. After ethics and regulatory authority approval, the 2 first-in-man MR-guided interventions were performed in a child and an adult, both with elements of valvar pulmonary stenosis. Catheter manipulations were monitored with real-time MRI sequence with interactive modification of imaging plane and slice position. Temporal resolution was 11 to 12 frames/s. Catheterization procedure times were 110 and 80 minutes, respectively. Both patients had successful relief of the valvar stenosis and no procedural complications. Conclusions—The described preclinical study and case reports are encouraging that with the availability of the new MR-compatible and safe guide wire, certain percutaneous cardiac interventions will become feasible to perform solely under MR guidance in the future. A clinical trial is underway in our institution.

A Concept for Magnetic Resonance Visualization of Surgical Textile Implants

Purpose:To develop a method for visualizing surgical textile implant (STI) with superparamagnetic iron oxides (SPIO), using magnetic resonance imaging (MRI). Therefore, positive-contrast inversion-recovery with on-resonant water suppression (IRON) was applied and its properties were evaluated in vitro. Materials and Methods:STI with different concentrations of SPIO integrated into the base material were produced. Imaging was performed on a clinical 1.5 Tesla scanner, using conventional balanced gradient echo sequences (SSFP), T2*-weighted sequences, and IRON-imaging. In vitro experiments were conducted in an agarose phantom. On MR-images, contrast-to-noise-ratios, and the dimensions of the implant were assessed. Results:Conventional MRI exhibited SPIO-loaded STI as signal voids. Using IRON, the mesh was clearly exhibited hyperintensely with suppression of on-resonant background signals with a distinct differentiation to other sources of off-resonances. Concentrations of approximately 9 mg/g led to best positive contrast and highest contrast-to-noise-ratios using IRON. Depending on B0-orientation, phase encoding direction and the STIs SPIO-load, the IRON-signal showed a characteristic pattern and an overestimation of STI size up to 4.6 mm. Conclusion:The integration of SPIOs into the base material combined with IRON is a feasible approach to visualize STI with MRI. This method could help to identify mesh-related problems in time and to reduce the need for surgical revision.

Magnetic Resonance–Guided Cardiac Interventions Using Magnetic Resonance–Compatible DevicesClinical Perspective

Background— Percutaneous cardiac interventions are currently performed under x-ray guidance. Magnetic resonance imaging (MRI) has been used to guide intravascular interventions in the past, but mainly in animals. Translation of MR-guided interventions into humans has been limited by the lack of MR-compatible and safe equipment, such as MR guide wires with mechanical characteristics similar to standard guide wires. The aim of the present study was to evaluate the safety and efficacy of a newly developed MR-safe and compatible passive guide wire in aiding MR-guided cardiac interventions in a swine model and describe the 2 first-in-man solely MR-guided interventions. Methods and Results— In the preclinical trial, the new MR-compatible wire aided the performance of 20 interventions in 5 swine. These consisted of balloon dilation of nondiseased pulmonary and aortic valves, aortic arch, and branch pulmonary arteries. After ethics and regulatory authority approval, the 2 first-in-man MR-guided interventions were performed in a child and an adult, both with elements of valvar pulmonary stenosis. Catheter manipulations were monitored with real-time MRI sequence with interactive modification of imaging plane and slice position. Temporal resolution was 11 to 12 frames/s. Catheterization procedure times were 110 and 80 minutes, respectively. Both patients had successful relief of the valvar stenosis and no procedural complications. Conclusions— The described preclinical study and case reports are encouraging that with the availability of the new MR-compatible and safe guide wire, certain percutaneous cardiac interventions will become feasible to perform solely under MR guidance in the future. A clinical trial is underway in our institution.Background—Percutaneous cardiac interventions are currently performed under x-ray guidance. Magnetic resonance imaging (MRI) has been used to guide intravascular interventions in the past, but mainly in animals. Translation of MR-guided interventions into humans has been limited by the lack of MR-compatible and safe equipment, such as MR guide wires with mechanical characteristics similar to standard guide wires. The aim of the present study was to evaluate the safety and efficacy of a newly developed MR-safe and compatible passive guide wire in aiding MR-guided cardiac interventions in a swine model and describe the 2 first-in-man solely MR-guided interventions. Methods and Results—In the preclinical trial, the new MR-compatible wire aided the performance of 20 interventions in 5 swine. These consisted of balloon dilation of nondiseased pulmonary and aortic valves, aortic arch, and branch pulmonary arteries. After ethics and regulatory authority approval, the 2 first-in-man MR-guided interventions were performed in a child and an adult, both with elements of valvar pulmonary stenosis. Catheter manipulations were monitored with real-time MRI sequence with interactive modification of imaging plane and slice position. Temporal resolution was 11 to 12 frames/s. Catheterization procedure times were 110 and 80 minutes, respectively. Both patients had successful relief of the valvar stenosis and no procedural complications. Conclusions—The described preclinical study and case reports are encouraging that with the availability of the new MR-compatible and safe guide wire, certain percutaneous cardiac interventions will become feasible to perform solely under MR guidance in the future. A clinical trial is underway in our institution.

Unmasking pedal arteries in patients with critical ischemia using time-resolved contrast-enhanced 3D MRA

OBJECTIVE To test the diagnostic relevance of fast Gadobenate dimeglumine (Gd-BOPTA) enhanced, time-resolved, three-dimensional magnetic resonance angiography (t3D MRA) of distal calf and pedal vasculature in critical limb ischemia in a prospective comparison with conventional selective digital subtraction angiography (DSA) and high-resolution duplex ultrasound (US) scan. METHODS From April 2007 to June 2008, 34 feet of 29 consecutive patients suffering from limb-threatening ischemia underwent diagnostic US scan, DSA, and t3D MRA before treatment. The investigations took place within 3 days. A t3D MRA was performed using a 3 Tesla whole-body magnetic resonance (MR) system with an eight-element phased-array coil. Image quality and diagnostic findings were subjectively analyzed by two radiologists and one vascular surgeon. Each distal calf and foot was divided into six arterial segments for DSA and t3D MRA, and four segments were investigated by US scan. Patency or occlusion was studied with all the techniques, whereby DSA and t3D MRA were additionally evaluated in patients having greater or less than 50% stenosis. Finally, images were visually assessed by the three observers by applying a six-point grading scale. The acquired data was statistically analyzed using McNemars test and Wilcoxons matched-pairs signed-rank sum test. The P values of less than an alpha level of .05 were considered to be statistically significant. RESULTS We achieved MRA images of diagnostic quality in all patients. Significantly more patent pedal arteries were identified by applying t3D MRA than DSA (P < .001) and US scan (P < .02). For estimating the degree of stenosis, no technique proved to be superior (P > .28). Overall image quality was rated best for t3D MRA. Additionally, potential bypass target vessels could be clearly discriminated from pedal veins due to the temporal resolution. CONCLUSION In our prospective study, t3D MRA has been proven to be superior to DSA and US scan in pedal vasculature imaging in critical limb ischemia. This is a valuable, noninvasive method for detecting potential pedal bypass target arteries.

Abbreviated Biparametric Prostate MR Imaging in Men with Elevated Prostate-specific Antigen

Purpose To determine the diagnostic accuracy for clinically significant prostate cancer achieved with abbreviated biparametric prostate magnetic resonance (MR) imaging in comparison with full multiparametric contrast material-enhanced prostate MR imaging in men with elevated prostate-specific antigen (PSA) and negative transrectal ultrasonography (US)-guided biopsy findings; to determine the significant cancer detection rate of biparametric versus full multiparametric contrast-enhanced MR imaging and between-reader agreement for interpretation of biparametric MR imaging. Materials and Methods In this institutional review board-approved retrospective review of prospectively acquired data, men with PSA greater than or equal to 3 ng/mL after negative transrectal US-guided biopsy findings underwent state-of-the-art, full multiparametric contrast-enhanced MR imaging at 3.0-T including high-spatial-resolution structural imaging in several planes, diffusion-weighted imaging at 0, 800, 1000, and 1400 mm2/sec, and dynamic contrast-enhanced MR imaging, obtained without endorectal coil within 34 minutes 19 seconds. One of four radiologists with different levels of expertise (1-9 years) first reviewed only a fraction of the full multiparametric contrast-enhanced MR images, consisting of single-plane (axial) structural imaging (T2-weighted turbo spin-echo and diffusion-weighted imaging), acquired within 8 minutes 45 seconds (referred to as biparametric MR imaging), and established a diagnosis according to the Prostate Imaging Reporting and Data System (PI-RADS) version 2; only thereafter, the remaining full multiparametric contrast-enhanced MR images were read. Men with PI-RADS categories 3-5 underwent MR-guided targeted biopsy. Men with PI-RADS categories 1-2 remained in urologic follow-up for at least 2 years, with rebiopsy (transrectal US-guided or transperineal saturation) where appropriate. McNemar test was used to compare diagnostic accuracies. To investigate between-reader agreement, biparametric MR images of 100 patients were read independently by all three radiologists. Results A total of 542 men, aged 64.8 years ± 8.2 (median PSA, 7 ng/mL), were included. Biparametric MR imaging helped detect clinically significant prostate cancer in 138 men. Full multiparametric contrast-enhanced MR imaging allowed detection of one additional clinically significant prostate cancer (a stage pT2a, intermediate-risk cancer with a Gleason score of 3+4) and caused 11 additional false-positive diagnoses. Diagnostic accuracy for detection of clinically significant cancer of biparametric MR imaging (89.1%, 483 of 542) was similar to that of full multiparametric contrast-enhanced MR imaging (87.2%, 473 of 542). Between-reader agreement of biparametric MR imaging interpretation was substantial (κ = 0.81). Conclusion Biparametric MR imaging allows detection of clinically significant prostate cancer missed by transrectal US-guided biopsy. Biparametric prostate MR imaging takes less than 9 minutes examination time, works without contrast agent injection, and offers a diagnostic accuracy and cancer detection rate that are equivalent to those of conventional full multiparametric contrast-enhanced MR imaging protocols.

Assessment of myocardial edema by computed tomography in myocardial infarction.

OBJECTIVES The aim of this study was to analyze whether cardiac computed tomography (CT) permits the assessment of myocardial edema in acute myocardial infarction (MI). BACKGROUND Several studies proved the value of detecting myocardial edema from T2-weighted cardiac magnetic resonance (CMR) for differentiating acute from chronic MI. Computed tomography is suited for depicting MI, but there are no data on CT imaging of myocardial edema. We hypothesized that areas of reduced attenuation in acute MI may correspond to edema. METHODS In 7 pigs (55.2 +/- 7.3 kg), acute MI was induced using a closed chest model. Animals underwent unenhanced arterial and late-phase dual source computed tomography (DSCT) followed by T2-weighted and delayed contrast-enhanced CMR. Animals were sacrificed, and the excised hearts were stained with 2,3,5-triphenyltetrazolin chloride (TTC). Size of MI, contrast-to-noise ratio, and percent signal difference were compared among the different imaging techniques with concordance-correlation coefficients (rho(c)), Bland-Altman plots, and analysis of variance for repeated measures. RESULTS Infarction was transmural on all slices. On unenhanced, arterial, and late-phase DSCT, mean sizes of MI were 27.2 +/- 8.5%, 20.1 +/- 6.9%, and 23.1 +/- 8.2%, respectively. Corresponding values on T2-weighted and delayed enhanced CMR were 28.5 +/- 7.8% and 22.2 +/- 7.7%. Size of MI on TTC staining was 22.6 +/- 7.8%. Best agreement was observed when comparing late-phase CT (rho(c) = 0.9356) and delayed enhanced CMR (rho(c) = 0.9248) with TTC staining. There was substantial agreement between unenhanced DSCT and T2-weighted CMR (rho(c) = 0.8629). Unenhanced DSCT presented with the lowest percent signal difference (46.0 +/- 18.3) and the lowest contrast-to-noise ratio (4.7 +/- 2.0) between infarcted and healthy myocardium. CONCLUSIONS Unenhanced DSCT permits the detection of myocardial edema in large acute MI. Further studies including smaller MI in different coronary artery territories and techniques for improving the contrast-to-noise ratio are needed.

Preclinical evaluation of a novel fiber compound MR guidewire in vivo.

Purpose:Interventional magnetic resonance imaging requires dedicated and MR-compatible devices. The guidewire is a key item for intravascular interventions. Mechanical stability, good visibility during real-time imaging, and RF safety are essential. A novel fiber-compound MR guidewire (GW) was evaluated in different MR-guided interventional scenarios. Materials and Methods:The GW (diameter 0.032”) consists of a fiber-compound produced using a micropultrusion technique doped with iron particles and a 10-cm Nitinol tip. Several iron splints are additionally attached at regular distances to visualize GW-movement. A protective polymer jacket with hydrophilic coating covers the core material. As approved by the government committee on animal investigations, the GW was evaluated in 5 pigs. Under complete MR-guidance, catheterization of the carotid and renal arteries, segmental arteries of the kidneys, the contralateral inguinal artery, and the left ventricle was performed using real-time gradient echo sequences in a 1.5 Tesla scanner. Different interventional applications including balloon dilatation, stent deployment, and embolization of small vessels were investigated. The time to probe the vessels under magnetic resonance imaging guidance and visibility of the GW are assessed. Handling and visibility under fluoroscopy were compared with a standard Nitinol guidewire as a benchmark. Results:On real-time magnetic resonance imaging, the iron-induced artifacts enabled a distinct visualization of the GW shaft and of its markings with a mean size of 2.6 mm and 5.4 mm, respectively. This facilitated fast navigation to the target vessels (averages: renal arteries 16 seconds, carotid artery 5 seconds, and contralateral inguinal artery 42 seconds.) with an exact depiction of the respective vessel. All interventional procedures were performed successfully. No GW-related side effects as kinking or breakage of the wire or GW induced blood-clotting were observed. All interventionalists assessed handling of the GW to be nearly equal in terms of stiffness, flexibility, and guidance compared with a standard Nitinol guidewire. X-ray visibility was less distinct but still diagnostically good. Conclusion:With the aid of the GW, different fully real-time MR-guided endovascular interventions become feasible.

Magnetic resonance imaging of the cardiac venous system and magnetic resonance-guided intubation of the coronary sinus in swine: a feasibility study.

Objectives:To visualize the coronary sinus using magnetic resonance (MR), and to demonstrate the feasibility of MR-guided intubation of the cardiac venous system (CVS) in swine. Materials and Methods:A total of 6 pigs were investigated. All experiments were performed using an interventional 1.5-Tesla MRI system. The CVS was visualized using an inversion-recovery navigator-gated whole-heart steady-state free-precession sequence after administration of gadobenate dimeglumine contrast agent. The coronary sinus was then intubated under MR-guidance with a passive MR-compatible guidewire modified by incorporation of iron oxide markers for improved visualization and a nonbraided Cobra-catheter. MR-guided interventions were monitored using a steady-state free-precession real-time imaging sequence. Time needed was measured for MR-guided intubation of the CVS and compared with the time needed for fluoroscopy guided intubation of the CVS. Results:Visualization and intubation of the coronary sinus and its site branches was feasible in all cases. Time spent for MR-guided intubation of the CVS was comparable to time spent for fluoroscopy-guided intubation (8.2 ± 2 minutes vs. 8.3 ± 1.3 minutes; P = 0.85). Conclusions:MR-visualization and MR-guided intubation of the coronary sinus and its side branches is feasible. The feasibility of MR-guided intubation of the CVS might have relevance for procedures like cardiac resynchronization therapy and percutaneous transcatheter mitral annuloplasty, requiring improved 3-dimensional knowledge about cardiac vein anatomy in the near future.

Mapping of proton relaxation near superparamagnetic iron oxide particle-loaded polymer threads for magnetic susceptibility difference quantification.

ObjectivesConventional radiological methods, including magnetic resonance imaging (MRI), fail to visualize polymeric surgical mesh implants because of small thread dimensions and material characteristics. For MRI delineation of such meshes, superparamagnetic iron oxide particles (SPIOs) are integrated in the mesh polymer. Usually, if SPIOs are used as an intravenous contrast agent, they increase the R1 and R2 of adjacent protons. It can be assumed that embedding SPIOs in polymers alters their molecular dynamics. The aim of this study was to investigate the influence of SPIO integration in polymer on the relaxation of adjacent protons. Materials and MethodsPolymer threads were placed in an agarose phantom. At 1.5 T, R1, R2, and R2* maps were calculated from multi inversion-recovery spin echo, multi–spin echo, and multi–gradient echo images, respectively. The threads were aligned parallel or orthogonal to B0. ResultsNo impact of SPIO on proton R1 and R2 was observed. R2* was increased by the SPIO-loaded threads. R1 and R2 amplitude maps showed a magnetic susceptibility difference of 0.97 ppm/(mg SPIO/g polymer) around SPIO-loaded threads. ConclusionsIn contrast to SPIO in aqueous solutions, polymer-embedded SPIO do not affect proton R1 and R2. However, embedded SPIO generate strong local static magnetic field gradients. Thus, SPIO integration is suitable to control the magnetic susceptibility of polymer threads. This can be exploited to visualize implanted polymer-based meshes in MRI using R2* susceptible sequences. Because no impact on R1 and R2 of adjacent protons by SPIO embedded in mesh threads was observed, structures adjacent to implanted meshes will be observable in R1 and R2 maps.

Rapid Right Ventricular Pacing with MR-compatible Pacemaker Lead for MR-guided Aortic Balloon Valvuloplasty in Swine

PURPOSE To assess the feasibility and effectiveness of rapid right ventricular pacing with a magnetic resonance (MR)-compatible pacemaker lead during MR-guided aortic valvuloplasty. MATERIALS AND METHODS This study was approved by the institutional animal research committee. Seven pigs were investigated. All experiments were performed with an interventional 1.5-T MR system. Interventions were monitored with a steady-state free precession real-time imaging sequence. An MR-compatible pacemaker lead was placed in the right ventricular apex with MR guidance before valvuloplasty. After positioning the balloon in valve position, valvuloplasty was performed with rapid right ventricular rapid pacing at a heart rate of 180 beats per minute to minimize cardiac output. RESULTS Positioning of the pacemaker lead with MR guidance was feasible in all swine (sensing, 6 mV +/- 1; threshold, 1 V +/- 0.5). The lead could be seen on steady-state free precession images without inducing any artifacts. Rapid right ventricular pacing was feasible in all swine, and balloon stability at the time of inflation was achieved with no balloon movement. Aortic valvuloplasty was successfully accomplished in all experiments. CONCLUSION Rapid right ventricular pacing with an MR-compatible pacemaker lead is feasible and effective.