Intramyocardial Injections to De-Stiffen the Heart: A Subject-Specific in Silico Approach.

We hypothesized that minimally invasive injections of a softening agent at strategic locations in stiff myocardium could de-stiffen the left ventricle (LV) globally. Physics-based finite element models of the LV were created from LV echocardiography images and pressures recorded during experiments in four swine. Results confirmed animal models of LV softening by systemic agents. Regional de-stiffening of myocardium led to global de-stiffening of LV. The mathematical set up was used to design LV global de-stiffening by regional softening of myocardium. At an end diastolic pressure of 23 mmHg, when 8 ml of the free wall was covered by intramyocardial injections, end diastolic volume (EDV) increased by 15.0%, whereas an increase up to 11 ml due to intramyocardial injections in the septum and free wall led to a 26.0% increase in EDV. Although the endocardial intramyocardial injections occupied a lower LV wall volume, they led to an EDV (44 ml) that was equal compared to intramyocardial injections in the mid-wall (44 ml) and larger compared to intramyocardial injections in the epicardium (41 ml). Using an in silico set up, sites of regional myocardium de-stiffening could be planned in order to globally soften overly stiff LV in heart failure with preserved ejection fraction. This novel treatment is built on subject-specific data. Hypothesis-testing of these simulation findings in animal models is warranted.

Efficacy of intramyocardial injection of Algisyl-LVR for the treatment of ischemic heart failure in swine.

BACKGROUND: Progressive thinning and dilation of the LV due to ischemic heart failure (IHF) increases wall stress and myocardial oxygen consumption. Injectable biopolymers implanted in the myocardial wall have been used to increase wall thickness to reduce chamber volume, decrease wall stress, and improve cardiac function. We sought to evaluate the efficacy of a biopolymer (Algisyl-LVR) to prevent left ventricular (LV) remodeling in a swine model of IHF. METHODS: IHF was induced in 11 swine by occluding the marginal obtuse branches of the left circumflex artery. Eight weeks later, Algisyl-LVR was injected into the LV myocardial free wall in five of the 11 animals. Echocardiographic examinations were done every 2weeks for 16weeks. RESULTS: Within eight weeks of treatment, the ejection fraction increased from 30.5%±7.7% to 42.4%±3.5% (treated group) vs. 37.3%±3.8% to 34.3%±2.9% (control), p<0.01. Stroke volume increased from 18.5±9.3mL to 41.3±13.3mL (treated group) vs. 25.4±2.3mL to 31.4±5.3mL (control), p<0.05. Wall thickness in end-diastole of the infarcted region changed from 0.69±0.06cm to 0.81±0.13cm (treated group) vs. 0.73±0.09cm to 0.68±0.11cm (control), p<0.05. Sphericity index remained almost unchanged after treatment, although differences were found at the end of the study between both groups (p<0.001). Average myofiber stress changed from 16.3±5.8kPa to 10.2±4.0kPa (treated group) vs. 15.2±4.8kPa to 17.9±5.6kPa (control), p<0.05. CONCLUSIONS: Algisyl-LVR is an effective strategy that serves as a micro-LV assist device to reduce stress and hence prevent or reverse maladaptive cardiac remodeling caused by IHF in swine.

Partial LVAD restores ventricular outputs and normalizes LV but not RV stress distributions in the acutely failing heart in silico.

PURPOSE: Heart failure is a worldwide epidemic that is unlikely to change as the population ages and life expectancy increases. We sought to detail significant recent improvements to the Dassault Systèmes Living Heart Model (LHM) and use the LHM to compute left ventricular (LV) and right ventricular (RV) myofiber stress distributions under the following 4 conditions: (1) normal cardiac function; (2) acute left heart failure (ALHF); (3) ALHF treated using an LV assist device (LVAD) flow rate of 2 L/min; and (4) ALHF treated using an LVAD flow rate of 4.5 L/min. METHODS AND RESULTS: Incorporating improved systolic myocardial material properties in the LHM resulted in its ability to simulate the Frank-Starling law of the heart. We decreased myocardial contractility in the LV myocardium so that LV ejection fraction decreased from 56% to 28%. This caused mean LV end diastolic (ED) stress to increase to 508% of normal, mean LV end systolic (ES) stress to increase to 113% of normal, mean RV ED stress to decrease to 94% of normal and RV ES to increase to 570% of normal. When ALHF in the model was treated with an LVAD flow rate of 4.5 L/min, most stress results normalized. Mean LV ED stress became 85% of normal, mean LV ES stress became 109% of normal and mean RV ED stress became 95% of normal. However, mean RV ES stress improved less dramatically (to 342% of normal values). CONCLUSIONS: These simulations strongly suggest that an LVAD is effective in normalizing LV stresses but not RV stresses that become elevated as a result of ALHF.

Moderate Ischemic Mitral Regurgitation After Posterolateral Myocardial Infarction in Sheep Alters Left Ventricular Shear but Not Normal Strain in the Infarct and Infarct Borderzone.

BACKGROUND: Chronic ischemic mitral regurgitation (CIMR) is associated with poor outcome. Left ventricular (LV) strain after posterolateral myocardial infarction (MI) may drive LV remodeling. Although moderate CIMR has been previously shown to affect LV remodeling, the effect of CIMR on LV strain after posterolateral MI remains unknown. We tested the hypothesis that moderate CIMR alters LV strain after posterolateral MI. METHODS: Posterolateral MI was created in 10 sheep. Cardiac magnetic resonance imaging with tags was performed 2 weeks before and 2, 8, and 16 weeks after MI. The left and right ventricular volumes were measured, and regurgitant volume indexed to body surface area (regurgitant volume index) was calculated as the difference between left ventricle and right ventricle stroke volumes divided by body surface area. Three-dimensional strain was calculated. RESULTS: Circumferential strain (Ecc) and longitudinal strain (Ell) were reduced in the infarct proper, MI borderzone, and remote myocardium 16 weeks after MI. In addition, radial circumferential (Erc) and radial longitudinal (Erl) shear strains were reduced in remote myocardium but increased in the infarct and borderzone 16 weeks after MI. Of all strain components, however, only Erc was affected by regurgitant volume index (p = 0.0005). There was no statistically significant effect of regurgitant volume index on Ecc, Ell, Erl, or circumferential longitudinal shear strain (Ecl). CONCLUSIONS: Moderate CIMR alters radial circumferential shear strain after posterolateral MI in sheep. Further studies are needed to determine the effect of shear strain on myocyte hypertrophy and the effect of mitral repair on myocardial strain.

Implantation of 3D-Printed Patient-Specific Aneurysm Models into Cadaveric Specimens: A New Training Paradigm to Allow for Improvements in Cerebrovascular Surgery and Research.

AIM: To evaluate the feasibility of implanting 3D-printed brain aneurysm model in human cadavers and to assess their utility in neurosurgical research, complex case management/planning, and operative training. METHODS: Two 3D-printed aneurysm models, basilar apex and middle cerebral artery, were generated and implanted in four cadaveric specimens. The aneurysms were implanted at the same anatomical region as the modeled patient. Pterional and orbitozygomatic approaches were done on each specimen. The aneurysm implant, manipulation capabilities, and surgical clipping were evaluated. RESULTS: The 3D aneurysm models were successfully implanted to the cadaveric specimens' arterial circulation in all cases. The features of the neck in terms of flexibility and its relationship with other arterial branches allowed for the practice of surgical maneuvering characteristic to aneurysm clipping. Furthermore, the relationship of the aneurysm dome with the surrounding structures allowed for better understanding of the aneurysmal local mass effect. Noticeably, all of these observations were done in a realistic environment provided by our customized embalming model for neurosurgical simulation. CONCLUSION: 3D aneurysms models implanted in cadaveric specimens may represent an untapped training method for replicating clip technique; for practicing certain approaches to aneurysms specific to a particular patient; and for improving neurosurgical research.

Human Cardiac Function Simulator for the Optimal Design of a Novel Annuloplasty Ring with a Sub-valvular Element for Correction of Ischemic Mitral Regurgitation.

Ischemic mitral regurgitation is associated with substantial risk of death. We sought to: (1) detail significant recent improvements to the Dassault Systèmes human cardiac function simulator (HCFS); (2) use the HCFS to simulate normal cardiac function as well as pathologic function in the setting of posterior left ventricular (LV) papillary muscle infarction; and (3) debut our novel device for correction of ischemic mitral regurgitation. We synthesized two recent studies of human myocardial mechanics. The first study presented the robust and integrative finite element HCFS. Its primary limitation was its poor diastolic performance with an LV ejection fraction below 20% caused by overly stiff ex vivo porcine tissue parameters. The second study derived improved diastolic myocardial material parameters using in vivo MRI data from five normal human subjects. We combined these models to simulate ischemic mitral regurgitation by computationally infarcting an LV region including the posterior papillary muscle. Contact between our novel device and the mitral valve apparatus was simulated using Dassault Systèmes SIMULIA software. Incorporating improved cardiac geometry and diastolic myocardial material properties in the HCFS resulted in a realistic LV ejection fraction of 55%. Simulating infarction of posterior papillary muscle caused regurgitant mitral valve mechanics. Implementation of our novel device corrected valve dysfunction. Improvements in the current study to the HCFS permit increasingly accurate study of myocardial mechanics. The first application of this simulator to abnormal human cardiac function suggests that our novel annuloplasty ring with a sub-valvular element will correct ischemic mitral regurgitation.

A Novel Method for Quantifying Smooth Regional Variations in Myocardial Contractility Within an Infarcted Human Left Ventricle Based on Delay-Enhanced Magnetic Resonance Imaging.

Heart failure is increasing at an alarming rate, making it a worldwide epidemic. As the population ages and life expectancy increases, this trend is not likely to change. Myocardial infarction (MI)-induced adverse left ventricular (LV) remodeling is responsible for nearly 70% of heart failure cases. The adverse remodeling process involves an extension of the border zone (BZ) adjacent to an MI, which is normally perfused but shows myofiber contractile dysfunction. To improve patient-specific modeling of cardiac mechanics, we sought to create a finite element model of the human LV with BZ and MI morphologies integrated directly from delayed-enhancement magnetic resonance (DE-MR) images. Instead of separating the LV into discrete regions (e.g., the MI, BZ, and remote regions) with each having a homogeneous myocardial material property, we assumed a functional relation between the DE-MR image pixel intensity and myocardial stiffness and contractility--we considered a linear variation of material properties as a function of DE-MR image pixel intensity, which is known to improve the accuracy of the model's response. The finite element model was then calibrated using measurements obtained from the same patient--namely, 3D strain measurements-using complementary spatial modulation of magnetization magnetic resonance (CSPAMM-MR) images. This led to an average circumferential strain error of 8.9% across all American Heart Association (AHA) segments. We demonstrate the utility of our method for quantifying smooth regional variations in myocardial contractility using cardiac DE-MR and CSPAMM-MR images acquired from a 78-yr-old woman who experienced an MI approximately 1 yr prior. We found a remote myocardial diastolic stiffness of C(0) = 0.102 kPa, and a remote myocardial contractility of T(max) = 146.9 kPa, which are both in the range of previously published normal human values. Moreover, we found a normalized pixel intensity range of 30% for the BZ, which is consistent with the literature. Based on these regional myocardial material properties, we used our finite element model to compute patient-specific diastolic and systolic LV myofiber stress distributions, which cannot be measured directly. One of the main driving forces for adverse LV remodeling is assumed to be an abnormally high level of ventricular wall stress, and many existing and new treatments for heart failure fundamentally attempt to normalize LV wall stress. Thus, our noninvasive method for estimating smooth regional variations in myocardial contractility should be valuable for optimizing new surgical or medical strategies to limit the chronic evolution from infarction to heart failure.

CT-reconstructed three-dimensional printed models of the right subclavian artery and aorta define age-related changes and facilitate benchtop catheter testing.

BACKGROUND: Severe tortuosity of the right subclavian artery (RSCA) encountered during transradial cardiac catheterization can lead to longer procedures, increased fluoroscopy time, inability to engage the coronary artery ostia, and potentially procedural failure. Increasing age is strongly correlated with subclavian tortuosity; however, the magnitude and direction of age-related changes in aortic and subclavian artery anatomy have not been defined. METHODS: Chest computed tomography (CT) angiograms of 14 patients (6 age <45 years and 8 age ≥75 years) were evaluated for RSCA tortuosity. Measurements were taken along the midline of the vessel and compared to the straight distance traveled (index of tortuosity = straight distance/midline length). One normal and one tortuous subclavian were selected for three-dimensional printing and various catheters were benchtop tested on both models. RESULTS: The older group had longer (11.95 cm vs 9.6 cm; P<.01) and more tortuous subclavian arteries (lower index of tortuosity, 0.65 vs 0.76; P<.01) with more posterior unfolding (distance to most posterior aspect, 3.74 ± 0.77 cm vs 1.76 ± 0.58 cm; P<.001). Engagement of the coronary arteries of the normal model was significantly easier, with successful engagement of one or both coronaries with every catheter (n=7). Only 2 of 7 catheters (Radial Brachial and Extra Backup 3.0) were able to engage the coronary arteries in the tortuous model. CONCLUSION: Age is associated with elongation, tortuosity, and posterior unfolding of the RSCA. Three-dimensional printing of normal and tortuous arteries is feasible and shows potential to test differences between catheters.

Highly accelerated aortic 4D flow MR imaging with variable-density random undersampling.

PURPOSE: To investigate an effective time-resolved variable-density random undersampling scheme combined with an efficient parallel image reconstruction method for highly accelerated aortic 4D flow MR imaging with high reconstruction accuracy. MATERIALS AND METHODS: Variable-density Poisson-disk sampling (vPDS) was applied in both the phase-slice encoding plane and the temporal domain to accelerate the time-resolved 3D Cartesian acquisition of flow imaging. In order to generate an improved initial solution for the iterative self-consistent parallel imaging method (SPIRiT), a sample-selective view sharing reconstruction for time-resolved random undersampling (STIRRUP) was introduced. The performance of different undersampling and image reconstruction schemes were evaluated by retrospectively applying those to fully sampled data sets obtained from three healthy subjects and a flow phantom. RESULTS: Undersampling pattern based on the combination of time-resolved vPDS, the temporal sharing scheme STIRRUP, and parallel imaging SPIRiT, were able to achieve 6-fold accelerated 4D flow MRI with high accuracy using a small number of coils (N=5). The normalized root mean square error between aorta flow waveforms obtained with the acceleration method and the fully sampled data in three healthy subjects was 0.04±0.02, and the difference in peak-systolic mean velocity was -0.29±2.56cm/s. CONCLUSION: Qualitative and quantitative evaluation of our preliminary results demonstrate that time-resolved variable-density random sampling is efficient for highly accelerating 4D flow imaging while maintaining image reconstruction accuracy.

Distribution of normal human left ventricular myofiber stress at end diastole and end systole: a target for in silico design of heart failure treatments.

Ventricular wall stress is believed to be responsible for many physical mechanisms taking place in the human heart, including ventricular remodeling, which is frequently associated with heart failure. Therefore, normalization of ventricular wall stress is the cornerstone of many existing and new treatments for heart failure. In this paper, we sought to construct reference maps of normal ventricular wall stress in humans that could be used as a target for in silico optimization studies of existing and potential new treatments for heart failure. To do so, we constructed personalized computational models of the left ventricles of five normal human subjects using magnetic resonance images and the finite-element method. These models were calibrated using left ventricular volume data extracted from magnetic resonance imaging (MRI) and validated through comparison with strain measurements from tagged MRI (950 ± 170 strain comparisons/subject). The calibrated passive material parameter values were C0 = 0.115 ± 0.008 kPa and B0 = 14.4 ± 3.18; the active material parameter value was Tmax = 143 ± 11.1 kPa. These values could serve as a reference for future construction of normal human left ventricular computational models. The differences between the predicted and the measured circumferential and longitudinal strains in each subject were 3.4 ± 6.3 and 0.5 ± 5.9%, respectively. The predicted end-diastolic and end-systolic myofiber stress fields for the five subjects were 2.21 ± 0.58 and 16.54 ± 4.73 kPa, respectively. Thus these stresses could serve as targets for in silico design of heart failure treatments.

Feasibility of asymmetric stretch assessment in the ascending aortic wall with DENSE cardiovascular magnetic resonance.

BACKGROUND: Vessel diameter is the principal imaging parameter assessed clinically for aortic disease, but adverse events can occur at normal diameters. Aortic stiffness has been studied as an additional imaging-based risk factor, and has been shown to be an independent predictor of cardiovascular morbidity and all-cause mortality. Reports suggest that some aortic pathology is asymmetric around the vessel circumference, a feature which would not be identified with current imaging approaches. We propose that this asymmetry may be revealed using Displacement Encoding with Stimulated Echoes (DENSE). The objective of this study is to investigate the feasibility of assessing asymmetric stretch in healthy and diseased ascending aortas using DENSE. METHODS: Aortic wall displacement was assessed with DENSE cardiovascular magnetic resonance (CMR) in 5 volunteers and 15 consecutive patients. Analysis was performed in a cross-sectional plane through the ascending aorta at the pulmonary artery. Displacement data was used to determine the wall stretch between the expanded and resting states of the aorta, in four quadrants around the aortic circumference. RESULTS: Analysis of variance (ANOVA) did not only show significant differences in stretch between groups of volunteers (p<0.001), but also significant differences in stretch along the circumference of the aorta (p<0.001), indicating an asymmetric stretch pattern. Furthermore, there is a significant difference in the asymmetry between volunteers and different groups of patients (p<0.01). CONCLUSIONS: Evaluation of asymmetric stretch is feasible in the ascending aorta with DENSE CMR. Clear differences in stretch are seen between patients and volunteers, with asymmetric patterns demonstrated around the aortic circumference.

Left ventricular pressure gating in ovine cardiac studies: a software-based method.

Cardiac imaging using magnetic resonance requires a gating signal in order to compensate for motion. Human patients are routinely scanned using an electrocardiogram (ECG) as a gating signal during imaging. However, we found that in sheep the ECG is not a reliable method for gating. We developed a software based method that allowed us to use the left ventricular pressure (LVP) as a reliable gating signal. By taking the time derivative of the LVP (dP/dt), we were able to start imaging at both end-diastole for systolic phase images, and end-systole for diastolic phase images. We also used MR tissue tagging to calculate 3D strain information during diastole. Using the LVP in combination with our digital circuit provided a reliable and time efficient method for ovine cardiac imaging. Unlike the ECG signal the left ventricular pressure was a clean signal and allowed for accurate, nondelay based triggering during systole and diastole.

Left ventricular myocardial contractility is depressed in the borderzone after posterolateral myocardial infarction.

BACKGROUND: Contractility in the borderzone (BZ) after anteroapical myocardial infarction (MI) is depressed. We tested the hypothesis that BZ contractility is also decreased after posterolateral MI. METHODS: Five sheep underwent posterolateral MI. Magnetic resonance imaging (MRI) was performed 2 weeks before and 16 weeks after MI, and left ventricular (LV) volume and regional strain were measured. Finite element (FE) models were constructed, and the systolic material parameter, Tmax, was calculated in the BZ and remote myocardium by minimizing the difference between experimentally measured and calculated LV strain and volume. Sheep were sacrificed 17 weeks after MI, and myocardial muscle fibers were taken from the BZ and remote myocardium. Fibers were chemically demembranated, and isometric developed force, Fmax, was measured at supramaximal [Ca(2+)]. Routine light microscopy was also performed. RESULTS: There was no difference in Tmax in the remote myocardium before and 16 weeks after MI. However, there was a large decrease (63.3%, p = 0.005) in Tmax in the BZ when compared with the remote myocardium 16 weeks after MI. In addition, there was a significant reduction of BZ Fmax for all samples (18.9%, p = 0.0067). Myocyte cross-sectional area increased by 61% (p = 0.021) in the BZ, but there was no increase in fibrosis. CONCLUSIONS: Contractility in the BZ is significantly depressed relative to the remote myocardium after posterolateral MI. The reduction in contractility is due at least in part to a decrease in contractile protein function.

The effect of mitral annuloplasty shape in ischemic mitral regurgitation: a finite element simulation.

BACKGROUND: Undersized mitral annuloplasty (MA) is the preferred surgical treatment for chronic ischemic mitral regurgitation. However, the preferred shape of undersized MA is unclear. METHODS: A previously described finite element model of the left ventricle with mitral valve based on magnetic resonance images of a sheep with chronic ischemic mitral regurgitation after posterolateral myocardial infarction was used. Saddle-shape (Edwards Physio II) and asymmetric (IMR ETlogix) MA rings were digitized and meshed. Virtual annuloplasty was performed using virtual sutures to attach the MA ring. Left ventricular diastole and systole were performed before and after virtual MA of each type. RESULTS: Both types of MA reduced the septolateral dimension of the mitral annulus and abolished mitral regurgitation. The asymmetric MA was associated with lower virtual suture force in the P2 region but higher force in P1 and P3 regions. Although both types of MA reduced fiber stress at the left ventricular base, fiber stress reduction after asymmetric MA was slightly greater. Neither type of MA affected fiber stress at the left ventricular equator or apex. Although both types of MA increased leaflet curvature and reduced leaflet stress, stress reduction with saddle-shape MA was slightly greater. Both MA types reduced stress on the mitral chordae. CONCLUSIONS: The effects of saddle-shape and asymmetric MA rings are similar. Finite element simulations are a powerful tool that may reduce the need for animal and clinical trials.

Moderate mitral regurgitation accelerates left ventricular remodeling after posterolateral myocardial infarction.

BACKGROUND: Chronic ischemic mitral regurgitation (MR) is associated with poor outcome. However, the effect of chronic ischemic MR on left ventricular (LV) remodeling after posterolateral myocardial infarction (MI) remains controversial. We tested the hypothesis that moderate MR accelerates LV remodeling after posterolateral MI. METHODS: Posterolateral MI was created in 10 sheep. Cardiac magnetic resonance imaging was performed 2 weeks before and 2, 8, and 16 weeks after MI. Left ventricular and right ventricular volumes were measured, and regurgitant volume was calculated as the difference between LV and right ventricle stroke volumes. RESULTS: Multivariate mixed effects regression showed that LV volumes at end diastole and end systole and LV sphericity were strongly correlated with both regurgitant volume (p < 0.0001, p = 0.0086, and p = 0.0007, respectively) and percent infarct area (p = 0.0156, p = 0.0307, and p < 0.0001, respectively). Conversely, whereas LV hypertrophy (LV wall volume) increased from 2 weeks to 16 weeks after MI, there was no effect of either regurgitant volume or percent infarct. CONCLUSIONS: Moderate MR accelerates LV remodeling after posterolateral MI. Further studies are needed to determine whether mitral valve repair is able to slow or reverse MI remodeling after posterolateral MI.

First finite element model of the left ventricle with mitral valve: insights into ischemic mitral regurgitation.

BACKGROUND: Left ventricular remodeling after posterobasal myocardial infarction can lead to ischemic mitral regurgitation. This occurs as a consequence of leaflet tethering due to posterior papillary muscle displacement. METHODS: A finite element model of the left ventricle, mitral apparatus, and chordae tendineae was created from magnetic resonance images from a sheep that developed moderate mitral regurgitation after posterobasal myocardial infarction. Each region of the model was characterized by a specific constitutive law that captured the material response when subjected to physiologic pressure loading. RESULTS: The model simulation produced a gap between the posterior and anterior leaflets, just above the infarcted posterior papillary muscle, which is indicative of mitral regurgitation. When the stiffness of the infarct region was reduced, this caused the wall to distend and the gap area between the leaflets to increase by 33%. Additionally, the stress in the leaflets increased around the chordal connection points near the gap. CONCLUSIONS: The methodology outlined in this work will allow a finite element model of both the left ventricle and mitral valve to be generated using noninvasive techniques.

Phase-contrast magnetic resonance imaging measurements in intracranial aneurysms in vivo of flow patterns, velocity fields, and wall shear stress: comparison with computational fluid dynamics.

Evolution of intracranial aneurysms is known to be related to hemodynamic forces such as wall shear stress (WSS) and maximum shear stress (MSS). Estimation of these parameters can be performed using numerical simulations with computational fluid dynamics (CFD), but can also be directly measured with magnetic resonance imaging (MRI) using a time-dependent 3D phase-contrast sequence with encoding of each of the three components of the velocity vectors (7D-MRV). To study the accuracy of 7D-MRV in estimating these parameters in vivo, in comparison with CFD, 7D-MRV and patient-specific CFD modeling was performed for 3 patients who had intracranial aneurysms. Visual and quantitative analyses of the flow pattern and distribution of velocities, MSS, and WSS were performed using the two techniques. Spearman's coefficients of correlation between the two techniques were 0.56 for the velocity field, 0.48 for MSS, and 0.59 for WSS. Visual analysis and Bland-Altman plots showed good agreement for flow pattern and velocities but large discrepancies for MSS and WSS. These results indicate that 7D-MRV can be used in vivo to measure velocity flow fields and for estimating MSS and WSS. Currently, however, this method cannot accurately quantify the latter two parameters.

MR imaging during endovascular procedures: an evaluation of the potential for catheter heating.

MRI in catheterized patients is considered unsafe due to the potential for focal heating. This concern arises from the continuous metallic braid that is incorporated into catheters to provide their desired physical properties. The potential for catheter heating during MR scanning was assessed in an in vitro model simulating a patient undergoing a neurovascular procedure in which MR scans of the brain will be performed. Heating adjacent to endovascular devices was assessed with fluoroptic temperature probes in a polyacrylamide gel. The effect of variable immersion lengths, lateral and longitudinal offsets, position along the endovascular device, physical MR system, and specific absorption rate (SAR) level were studied to determine their effect on catheter heating. A rapid temperature rise was evident next to endovascular devices during MR scanning and varied moderately with immersed length, position within the bore, measurement point on the device, and MR system used. Peak heating rates were less than 1 degree C/min with maximal SAR exposure and anatomically realistic geometries. Heating scaled linearly with SAR and SAR values below 0.2 W/kg produced negligible heating near catheters. For the evaluated application, substantial SAR restrictions, coupled with limited imaging durations, are proposed as sufficient to permit MRI without concern for thermal injury.