Velocity Encoding Research Articles

Fast Whole-Brain 4D Contrast-Enhanced MR Angiography with Velocity Encoding Using Undersampled Radial Acquisition and Highly Constrained Projection Reconstruction: Image-Quality Assessment in Volunteer Subjects: Fig 1.

We report on the image quality obtained by using fast contrast-enhanced whole-brain 4D radial MRA with 0.75-second temporal resolution, isotropic submillimeter spatial resolution, and velocity encoding (HYPRFlow). Images generated by HYPR-LR by using the velocity-encoded data as the constraining image were of diagnostic quality. In addition, we demonstrate that measurements of shear stress within the middle cerebral artery can be derived from the high-resolution 3D velocity data.

Cardiovascular Magnetic Resonance

Cardiovascular magnetic resonance (CMR) creates images from atomic nuclei with uneven spin using radiowaves in the presence of a magnetic field. Full details of the physical principles can be found elsewhere.1 For clinical purposes, MR is performed using hydrogen-1, which is abundant in water and fat. Radiofrequency waves excite the area of interest to create tissue magnetization that decays (relaxation) and after a short period is induced to release energy as a radio signal. These echoes are converted with Fourier transformation into images of spatially resolved radio signals. Relaxation is quantified in spatially orthogonal directions as T1 and T2, which allows tissue characterization to serve as a powerful clinical tool. A CMR scanner consists of a superconducting magnet, a radiofrequency transmitter and receiver connected to radio aerials, and gradient coils driven by powerful pulses of electricity to create transient magnetic fields. The imaging computer triggers to the ECG and runs scanning sequences that coordinate the complex processes. The spin-echo sequence yields static images with black blood, whereas the gradient-echo sequence usually acquires multiple images through the cardiac cycle that display cardiovascular function as cine loops. Preparation pulses can be used to display T1 or T2 contrast for tissue characterization. Fast and real-time sequences include fast low-angle shot, steady-state free-precession, spiral, and echo-planar imaging. Velocity and flow can be quantified with gradient-echo sequences with encoding of velocity in the signal phase.2 CMR is extremely safe, which is unequivocally advantageous compared with x-rays, but special cautions apply. Ferromagnetic items become potentially dangerous projectiles in the scanner room. Although most metallic medical implants, including all prosthetic cardiac valves and coronary stents, are MR compatible, care is required with cerebrovascular clips. Pacemakers and defibrillators present problems, in particular related to wire heating, and are a contraindication for CMR, although specialist centers have …

Results of a tissue engineered pulmonary valve in humans assessed with CMR

Methods TEng valves had been implanted into the right ventricular outflow tract of 12 pts (age: 15 ± 8,5 yrs; Matrix-PTM n = 7; Matrix-P-PlusTM n = 5) due to pulmonary stenosis or insufficiency within a median 5 months range (2-7 months) before a comprehensive follow-up by CMR. Echocardiography was suspicious of accelerated blood flow in the main pulmonary artery in 10 of those. The severity of the pulmonary valve stenosis was assessed by gradient echo cine MRI (FOV 270 × 270 mm, voxel size 1.88 × 1.94 × 5 mm, TR/TE/α = 4.4 ms/2.7 ms/15°) and phase-contrast velocity mapping (FOV 270 × 270 mm, voxel size 1.64 × 1.4 × 7 mm, TR/TE/α = 15 ms/2.5 ms/30°, velocity encoding (VENC) = 400 cm/s) were applied. For tissue characterisation late enhancement imaging (Gd 0.1 mmol/kg) using an ECG triggered 3D inversion recovery sequence (TR/TE/α= 3.7 ms/1.8 ms/15°; voxel size 1.17 × 1.27 × 10 mm) and T1 and T2 weighted turbo spin echo sequences were utilized.

Neuronal variability and linear neural coding in the vestibular system

Event Abstract Back to Event Neuronal variability and linear neural coding in the vestibular system Adam Schneider1*, Maurice Chacron1 and Kathleen Cullen1 1 McGill University, Canada Neuronal variability is ubiquitous in the central nervous system. It was originally thought of as an obstacle to overcome by the nervous system as stated by Jon von Neumann, "How can a reliable nervous system be made of unreliable parts?" However, recent studies have suggested more positive roles for neural variability such as increasing information transmission through suprathreshold stochastic resonance. The vestibular system benefits from easily characterized sensory stimuli and has several advantages for studying the effects of neural variability on neural coding. Vestibular afferents have been characterized as either regular (low variability) or irregular (high variability) according to their coefficient of variation (CV). Despite spanning a wide range of CV across the afferent population, they are nonetheless known for their faithful linear encoding of head velocity as a firing rate modulation, as shown by Goldberg and Fernandez (1971). These afferents project to neurons within the medial vestibular nuclei that have also been shown to encode sensory stimuli through modulation of firing rate in vivo. This faithful linear encoding is surprising given that in vitro studies have found that these neurons possess several nonlinearly activated ion channels such as calcium-activated potassium as well as hyperpolarization-activated channels such as I-h. How do those nonlinear conductances interact with in vivo conditions in order to make such a nonlinear system behave linearly? We performed numerical simulations of a detailed Hodgkin-Huxley type model based on in vitro data, originally developed by Av-Ron and Vidal (1999) to model in vitro recordings from central vestibular neurons. In vivo conditions were mimicked by the addition of noise to the model and tuning the noise intensity such that the CV generated by the model matches that found in in vivo recordings. We found that this nonlinear model was capable of various exotic phenomena in the absence of noise such as oscillations and burst firing. These nonlinearities strongly interfered with the model’s ability to encode sinusoidal and noise stimuli through changes in firing rate. Most surprisingly, addition of noise reduced the effects of these nonlinearities significantly and allowed the model to encode stimuli linearly. These results show that variability introduced by synaptic bombardment can significantly attenuate nonlinearities due to voltage-gated ion channels and allow faithful linear encoding of sensory stimuli. Conference: Computational and Systems Neuroscience 2010, Salt Lake City, UT, United States, 25 Feb - 2 Mar, 2010. Presentation Type: Poster Presentation Topic: Poster session II Citation: Schneider A, Chacron M and Cullen K (2010). Neuronal variability and linear neural coding in the vestibular system. Front. Neurosci. Conference Abstract: Computational and Systems Neuroscience 2010. doi: 10.3389/conf.fnins.2010.03.00196 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 03 Mar 2010; Published Online: 03 Mar 2010. * Correspondence: Adam Schneider, McGill University, Montréal, Canada, adam.schneider@mail.mcgill.ca Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Adam Schneider Maurice Chacron Kathleen Cullen Google Adam Schneider Maurice Chacron Kathleen Cullen Google Scholar Adam Schneider Maurice Chacron Kathleen Cullen PubMed Adam Schneider Maurice Chacron Kathleen Cullen Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Relation between age and aortic wall compliance in the Marfan syndrome: evaluation with Velocity-Encoded MRI

MRI was performed on 1.5 T Philips Achieva MRI (Philips Medical Systems, Best, The Netherlands). PC-MRI with through-plane velocity encoding (Venc = 150 cm/s) and free-breathing was performed perpendicular to the ascending and descending aorta at the level transecting the pulmonary trunk. Another acquisition (Venc = 100 cm/ s) was performed at the abdominal aorta. PWV was determined for the aortic arch (AA), distal aorta (DA) and total aorta using the transit-time method described by Grotenhuis.1 Brachial-cuff systolic and diastolic blood pressure (BP) were obtained. Distensibility (= luminal area change/(diastolic luminal area × pulse pressure)) and Stiffness Index (= ln [(BPSystolic/BPDiastolic)/(diameter change/diastolic diameter)]) were determined at the ascending aorta in gradient-echo PC-MRI magnitude images. Reduced wall compliance is expressed as increased PWV and SI and decreased Dist. PWV, Dist and SI were compared in Marfan and controls using paired ttests. Age relation was determined by linear regression.

3D velocity quantification in the heart: Improvements by 3D PC‐SSFP

To test whether a 3D imaging sequence with phase contrast (PC) velocity encoding based on steady-state free precession (SSFP) improves 3D velocity quantification in the heart compared to the currently available gradient echo (GE) approach. The 3D PC-SSFP sequence with 1D velocity encoding was compared at the mitral valve in 12 healthy subjects with 3D PC-GE at 1.5T. Velocity measurements, velocity-to-noise-ratio efficiency (VNR(eff)), intra- and interobserver variability of area and velocity measurements, contrast-to-noise-ratio (CNR), and artifact sensitivity were evaluated in both long- and short-axis orientation. Descending aorta mean and peak velocities correlated well (r(2) = 0.79 and 0.93) between 3D PC-SSFP and 3D PC-GE. At the mitral valve, mean velocity correlation was moderate (r(2) = 0.70 short axis, 0.56 long axis) and peak velocity showed good correlation (r(2) = 0.94 short axis, 0.81 long axis). In some cases VNR(eff) was higher, in others lesser, depending on slab orientation and cardiac phase. Intra- and interobserver variability was generally better for 3D PC-SSFP. CNR improved significantly, especially at end systole. Artifact levels did not increase. 3D SSFP velocity quantification was successfully tested in the heart. Blood-myocardium contrast improved significantly, resulting in more reproducible velocity measurements for 3D PC-SSFP at 1.5T.

Flow Assessment Through Four Heart Valves Simultaneously Using 3-Dimensional 3-Directional Velocity-Encoded Magnetic Resonance Imaging With Retrospective Valve Tracking in Healthy Volunteers and Patients With Valvular Regurgitation

To validate 3-dimensional (3D) 3-directional velocity-encoded (VE) magnetic resonance imaging (MRI) for flow assessment through all 4 heart valves simultaneously with retrospective valve-tracking during off-line analysis in healthy volunteers and in patients with valvular regurgitation. Three-dimensional 3-directional VE MRI was performed in 22 healthy volunteers and in 29 patients with ischemic cardiomyopathy who were suspected of valvular regurgitation and net flow volumes through the 4 heart valves were compared. Furthermore, the analysis was repeated for each valve in 10 healthy volunteers and in 10 regurgitant valves to assess intra- and interobserver agreement for assessment of respectively net flow volumes and regurgitation fraction. In healthy volunteers, the average net flow volume through the mitral valve, tricuspid valve, aortic valve, and pulmonary valve was 85 +/- 20 mL, 85 +/- 21 mL, 83 +/- 19 mL, 82 +/- 21 mL, respectively. Strong correlations between net flow volumes through the 4 heart valves were observed (intraclass correlation coefficients [ICC] 0.93-0.95) and the coefficient of variance (CV) was small (6%-9%). The repeated analysis by the same observer and by a second observer yielded good agreement for measurement of net flow volumes (ICC: 0.93-0.99 and CV: 3%-7%). Strong correlations between the net flow volumes through the 4 heart valves were also observed in the patients with valvular regurgitation (ICC: 0.85-0.95 and CV: 7%-18%). The average net flow volume through the mitral valve, tricuspid valve, aortic valve, and pulmonary valve was 63 +/- 20 mL, 63 +/- 20 mL, 63 +/- 20 mL, 63 +/- 20 mL, respectively. Furthermore, the intra- and interobserver agreement for assessment of regurgitation fraction was good (ICC: 0.86 and 0.85, CV: 12% and 13%). Flow assessment using 3D 3-directional VE MR with retrospective valve-tracking during off-line analysis enables accurate quantification of net flow volumes through 4 heart valves within a single acquisition in healthy volunteers and in patients with valvular regurgitation.

Interventricular Mechanical Dyssynchrony: Quantification with Velocity-encoded MR Imaging

To evaluate the performance of velocity-encoded (VENC) magnetic resonance (MR) imaging, as compared with pulsed-wave echocardiography (PW-ECHO), in the quantification of interventricular mechanical dyssynchrony (IVMD) as a predictor of response to cardiac resynchronization therapy (CRT). The study was approved by the local ethics committee, and all patients provided written informed consent. The study involved the examination of 45 patients (nine women, 36 men; median age, 60 years; interquartile age range, 47-69 years) with New York Heart Association class 2.0-3.0 heart failure and a reduced left ventricular ejection fraction (median, 25%; interquartile range, 21%-32%), with (n = 25) or without (n = 20) left bundle branch block. Aortic and pulmonary flow curves were constructed by using VENC MR imaging and PW-ECHO. IVMD was defined as the difference between the onset of aortic flow and the onset of pulmonary flow. Intraclass correlation coefficient, Spearman correlation coefficient, Bland-Altman, and Cohen kappa analyses were used to assess agreement between observers and methods. Inter- and intraobserver agreement regarding VENC MR imaging IVMD measurements was very good (intraclass r = 0.96, P < .001; mean bias, -3 msec +/- 11 [standard deviation] and 0 msec +/- 10, respectively). A strong correlation (Spearman r = 0.92, P < .001) and strong agreement (mean difference, -6 msec +/- 16) were found between VENC MR imaging and PW-ECHO in the quantification of IVMD. Agreement between VENC MR imaging and PW-ECHO in the identification of potential responders to CRT was excellent (Cohen kappa = 0.94). VENC MR measurements of IVMD are equivalent to PW-ECHO measurements and can be used to identify potential responders to CRT.

Encoding of smooth pursuit eye movement velocity takes place in a complete cortical network

Encoding of smooth pursuit eye movement velocity takes place in a complete cortical network

Self-gated Fourier velocity encoding

Self-gating is investigated to improve the velocity resolution of real-time Fourier velocity encoding measurements in the absence of a reliable electrocardiogram waveform (e.g., fetal magnetic resonance or severe arrhythmia). Real-time flow data are acquired using interleaved k-space trajectories which share a common path near the origin of k-space. These common data provide a rapid self-gating signal that can be used to combine the interleaved data. The combined interleaves cover a greater area of k-space than a single real-time acquisition, thereby providing higher velocity resolution for a given aliasing velocity and temporal resolution. For example, this approach provided velocity spectra with a temporal resolution of 19 ms and velocity resolution of 22 cm/s over an 818 cm/s field-of-view. The method was validated experimentally using a computer-controlled pulsatile flow apparatus and applied in vivo to measure aortic-valve flow in a healthy volunteer.

WE‐E‐210A‐01: Functional Cardiovascular MRI: Assessment, Visualization and Quantification of 3D Blood Flow Characteristics

Magnetic Resonance Imaging (MRI) techniques provide a non‐invasive method for the highly accurate anatomic depiction of the heart and vessels. Most MR‐sequences demonstrate more or less significant sensitivity to flow and motion, which can lead to artifacts in many applications. The intrinsic motion sensitivity of MRI can, however, also be used to image vessels like in phase contrast (PC) MR‐angiography or to quantify blood flow and motion of tissue. Such techniques offer the unique possibility to acquire spatially registered functional information simultaneously with the morphological data within a single experiment. Characterizations of the dynamic components of blood flow and cardiovascular function provide insight into normal and pathological physiology and have made considerable progress in recent yearsTo synchronize flow or motion sensitive measurements with periodic tissue motion or pulsatile flow, data acquisition is typically gated to the cardiac cycle and time resolved (CINE) anatomical images are collected to depict the dynamics of tissue motion and blood flow during the cardiac cycle. Visualization and quantification of blood flow and tissue motion using PC MRI has been widely used in a number of applications. In addition to analyzing tissue motion such as left ventricular function, time‐resolved 2D PC MRI techniques have proven to be useful tools for the assessment of blood flow within the cardiovascular system.Moreover, 3D spatial encoding offers the possibility of isotropic high spatial resolution and thus the ability to measure and visualize the temporal evolution of complex flow and motion patterns in a 3D‐volume. In this context, ECG synchronized and respiration controlled flow sensitive 3D MRI using 3‐directional velocity encoding (also termed ‘flow sensitive 4D MRI’) can be employed to detect and visualize global and local blood flow characteristics in targeted vascular regions (aorta, cranial arteries, carotid arteries, etc.). For the analysis and visualization of complex, three‐directional blood flow within a 3D volume, various visualization tools, including 2D vector‐fields, 3D streamlines and particle traces, have been reported. In addition more advanced data quantification strategies of directly measured (e.g. flow rates) or derived parameters (e.g. pressure difference maps, wall sheer stress, pulse wave velocity, etc.) are promising as new clinical markers for the characterization of cardiovascular disease.This lecture will provide an overview of the MR imaging principles and advanced acquisition methods, data processing, flow visualization and quantification strategies, and clinical applications of flow sensitive MRI imaging.Learning Objectives:1. Understand the basic and advanced methods for flow measurements using MRI2. Understand techniques for 3D flow visualization and quantification of blood flow and derived parameters3. Understand the issues related to clinical applications of flow‐sensitive MRI

Theory and Validation of Magnetic Resonance Fluid Motion Estimation Using Intensity Flow Data

BackgroundMotion tracking based on spatial-temporal radio-frequency signals from the pixel representation of magnetic resonance (MR) imaging of a non-stationary fluid is able to provide two dimensional vector field maps. This supports the underlying fundamentals of magnetic resonance fluid motion estimation and generates a new methodology for flow measurement that is based on registration of nuclear signals from moving hydrogen nuclei in fluid. However, there is a need to validate the computational aspect of the approach by using velocity flow field data that we will assume as the true reference information or ground truth.Methodology/Principal FindingsIn this study, we create flow vectors based on an ideal analytical vortex, and generate artificial signal-motion image data to verify our computational approach. The analytical and computed flow fields are compared to provide an error estimate of our methodology. The comparison shows that the fluid motion estimation approach using simulated MR data is accurate and robust enough for flow field mapping. To verify our methodology, we have tested the computational configuration on magnetic resonance images of cardiac blood and proved that the theory of magnetic resonance fluid motion estimation can be applicable practically.Conclusions/SignificanceThe results of this work will allow us to progress further in the investigation of fluid motion prediction based on imaging modalities that do not require velocity encoding. This article describes a novel theory of motion estimation based on magnetic resonating blood, which may be directly applied to cardiac flow imaging.

Accurate quantification of heart valve regurgitation in all four heart valves simultaneously using 3D velocity-encoded MRI with retrospective valve tracking

In regurgitant heart valves, surgical decision-making is based on the severity of the regurgitation through the particular valve. Conventional two-dimensional (2D) one-directional velocity-encoded (VE) MRI is routinely used for flow assessment, but this technique has been shown to be inaccurate and correlation between the net flow volumes through the valves is weak, even in the absence of regurgitation. 2D one-directional VE MRI is limited because the position and angulation of the acquisition plane cannot be adapted to the valve motion and the direction of the inflow and regurgitant jet.

Shared velocity encoding (SVE): a new method for real-time velocity measurement with high temporal resolution

Conventional ECG-triggered, segmented phase-contrast imaging (PC-MRI) is an accurate and clinically proven technique to characterize blood flow velocity. However, this method requires reliable cardiac gating, regular cardiac rhythm, and either signal-averaging, respiratory gating, or breath-holding to suppress respiratory motion artifacts. Furthermore, the resulting velocity information is a weighted temporal average of information acquired over multiple cardiac and respiratory cycles;short-term hemodynamic variations are lost. Real-time PC-MRI has been previously proposed using GRE-EPI [1] and spiral acquisitions [2], but limited performance has precluded routine clinical application. The aim of the present work is to design and demonstrate a novel method for rapid real-time velocity measurement with sufficient temporal resolution to eliminate the need for ECG synchronization and breath-holding, and to provide beat-to-beat hemodynamic information.

MRT-basierte Flussmessungen im Truncus pulmonalis zur Detektion einer pulmonal-arteriellen Hypertonie in Patienten mit zystischer Fibrose

Development of pulmonary arterial hypertension (PH) is a common problem in the course of patients suffering from cystic fibrosis (CF). This study was performed to evaluate MRI based flow measurements (MR(venc); Velocity ENCoding) to detect signs of an evolving PH in patients suffering from CF. 48 patients (median age: 16 years, range: 10 - 40 years, 25 female) suffering from CF of different severity (mean FEV (1): 74% +/- 23, mean Shwachman-score: 63 +/- 10) were examined using MRI based flow measurements of the main pulmonary artery (MPA). Phase-contrast flash sequences (TR: 9.6 ms, TE: 2.5 ms, bandwidth: 1395 Hertz/Pixel) were utilized. Results were compared to an age- and sex-matched group of 48 healthy subjects. Analyzed flow data where: heart frequency (HF), cardiac output (HZV), acceleration time (AT), proportional acceleration time related to heart rate (ATr), mean systolic blood velocity (MFG), peak velocity (Peak), maximum fow (Fluss(max)), mean flow (Fluss(mitt)) and distensibility (Dist). The comparison of means revealed significant differences only for MFG, Fluss(max) and Dist, but overlap was marked. However, using a scatter-plot of AT versus MFG, it was possible to identify five CF-patients demonstrating definite signs of PH: AT = 81 ms +/- 14, MFG = 46 +/- 11 cm/s, Dist = 41% +/- 7. These CF-patients where the most severely affected in the investigated group, two of them were listed for complete heart and lung transplantation. The comparison of this subgroup and the remaining CF-patients revealed a highly significant difference for the AT (p = 0.000001) without overlap. Screening of CF-patients for the development of PH using MRvenc of the MPA is not possible. In later stages of disease, the quantification of AT, MFG and Dist in the MPA may be useful for the detection, follow-up and control of therapy of PH. MR(venc) of the MPA completes the MRI-based follow-up of lung parenchyma damage in patients suffering from CF.

Effective motion‐sensitizing magnetization preparation for black blood magnetic resonance imaging of the heart

To investigate the effectiveness of flow signal suppression of a motion-sensitizing magnetization preparation (MSPREP) sequence and to optimize a 2D MSPREP steady-state free precession (SSFP) sequence for black blood imaging of the heart. Using a flow phantom, the effect of varying field of speed (FOS), b-value, voxel size, and flow pattern on the flow suppression was investigated. In seven healthy volunteers, black blood images of the heart were obtained at 1.5T with MSPREP-SSFP and double inversion recovery fast spin echo (DIR-FSE) techniques. Myocardium and blood signal-to-noise ratio (SNR) and myocardium-to-blood contrast-to-noise ratio (CNR) were measured. The optimal FOS that maximized the CNR for MSPREP-SSFP was determined. Phantom data demonstrated that the flow suppression was induced primarily by the velocity encoding effect. In humans, FOS=10-20 cm/s was found to maximize the CNR for short-axis (SA) and four-chamber (4C) views. Compared to DIR-FSE, MSPREP-SSFP provided similar blood SNR efficiency in the SA basal and mid-views and significantly lower blood SNR efficiency in the SA apical (P=0.02) and 4C (P=0.01) views, indicating similar or better blood suppression. Velocity encoding is the primary flow suppression mechanism of the MSPREP sequence and 2D MSPREP-SSFP black blood imaging of the heart is feasible in healthy subjects.

Quantitative 2D and 3D phase contrast MRI: Optimized analysis of blood flow and vessel wall parameters

Quantification of CINE phase contrast (PC)-MRI data is a challenging task because of the limited spatiotemporal resolution and signal-to-noise ratio (SNR). The method presented in this work combines B-spline interpolation and Green's theorem to provide optimized quantification of blood flow and vessel wall parameters. The B-spline model provided optimal derivatives of the measured three-directional blood velocities onto the vessel contour, as required for vectorial wall shear stress (WSS) computation. Eight planes distributed along the entire thoracic aorta were evaluated in a 19-volunteer study using both high-spatiotemporal-resolution planar two-dimensional (2D)-CINE-PC ( approximately 1.4 x 1.4 mm(2)/24.4 ms) and lower-resolution 3D-CINE-PC ( approximately 2.8 x 1.6 x 3 mm(3)/48.6 ms) with three-directional velocity encoding. Synthetic data, error propagation, and interindividual, intermodality, and interobserver variability were used to evaluate the reliability and reproducibility of the method. While the impact of MR measurement noise was only minor, the limited resolution of PC-MRI introduced systematic WSS underestimations. In vivo data demonstrated close agreement for flow and WSS between 2D- and 3D-CINE-PC as well as observers, and confirmed the reliability of the method. WSS analysis along the aorta revealed the presence of a circumferential WSS component accounting for 10-20%. Initial results in a patient with atherosclerosis suggest the potential of the method for understanding the formation and progression of cardiovascular diseases.

Mitral Valve and Tricuspid Valve Blood Flow: Accurate Quantification with 3D Velocity-encoded MR Imaging with Retrospective Valve Tracking

To validate flow assessment performed with three-dimensional (3D) three-directional velocity-encoded (VE) magnetic resonance (MR) imaging with retrospective valve tracking and to compare this modality with conventional two-dimensional (2D) one-directional VE MR imaging in healthy subjects and patients with regurgitation. Patients and volunteers gave informed consent, and local medical ethics committee approval was obtained. Patient data were selected retrospectively and randomly from a database of MR studies obtained between July 2006 and July 2007. The 3D three-directional VE MR images were first validated in vitro and compared with 2D one-directional VE MR images. Mitral valve (MV) and tricuspid valve (TV) flow were assessed in 10 volunteers without valve insufficiency and 20 patients with valve insufficiency, with aortic systolic stroke volume (ASSV) as the reference standard. Phantom validation showed less than 5% error for both techniques. In volunteers, 3D three-directional VE MR images showed no bias for MV or TV flow when compared with ASSV, whereas 2D one-directional VE MR images showed significant bias for MV flow (15% overestimation, P < .01). TV flow showed 25% overestimation; however, this was insignificant because of the high standard deviation. Correlation with ASSV was strong for 3D three-directional VE MR imaging (r = 0.96, P < .01 for MV flow; r = 0.88, P < .01 for TV flow) and between MV and TV flow (r = 0.91, P < .01); however, correlation was weaker for 2D one-directional VE MR imaging (r = 0.80, P < .01 for MV flow; r = 0.22, P = .55 for TV flow) and between MV flow and TV flow (r = 0.34, P = .34). In patients (mean regurgitation fractions of 13% and 10% for MV flow and TV flow, respectively), correlation between MV flow and TV flow for 3D three-directional VE MR imaging was strong (r = 0.97, P < .01). Use of 3D three-directional VE MR imaging enables accurate MV and TV flow quantification, even in patients with valve regurgitation.

1086 Accurate quantification of simultaneous mitral and tricuspid blood flow using 3D velocity-encoded MRI with retrospective valve tracking

Introduction In valvular disease, surgical decision-making regarding timing and type of intervention is based on the severity of the regurgitation through the particular atria-ventricular valve. Conventional one-directional (1-dir) velocityencoded (VE) MRI is routinely used for flow assessment over the mitral (MV) and tricuspid valve (TV), but this technique has been shown to be inaccurate and correlation between 1-dir VE MRI MVand TV-flow measurement is weak.

Adaptation of Velocity Encoding in Synaptically Coupled Neurons in the Fly Visual System

Although many adaptation-induced effects on neuronal response properties have been described, it is often unknown at what processing stages in the nervous system they are generated. We focused on fly visual motion-sensitive neurons to identify changes in response characteristics during prolonged visual motion stimulation. By simultaneous recordings of synaptically coupled neurons, we were able to directly compare adaptation-induced effects at two consecutive processing stages in the fly visual motion pathway. This allowed us to narrow the potential sites of adaptation effects within the visual system and to relate them to the properties of signal transfer between neurons. Motion adaptation was accompanied by a response reduction, which was somewhat stronger in postsynaptic than in presynaptic cells. We found that the linear representation of motion velocity degrades during adaptation to a white-noise velocity-modulated stimulus. This effect is caused by an increasingly nonlinear velocity representation rather than by an increase of noise and is similarly strong in presynaptic and postsynaptic neurons. In accordance with this similarity, the dynamics and the reliability of interneuronal signal transfer remained nearly constant. Thus, adaptation is mainly based on processes located in the presynaptic neuron or in more peripheral processing stages. In contrast, changes of transfer properties at the analyzed synapse or in postsynaptic spike generation contribute little to changes in velocity coding during motion adaptation.