Purpose:To demonstrate the feasibility of a four-dimensional phase contrast (PC) technique that permits spatial and temporal coverage of an entire three-dimensional volume, to quantitatively validate its accuracy against an established time resolved two-dimensional PC technique to explore advantages of the approach with regard to the fourdimensional nature of the data. Materials and Methods:Time-resolved, three-dimensional anatomical images were generated simultaneously with registered three-directional velocity vector fields. Improvements compared to prior methods include retrospectively gated and respiratory compensated image acquisition, interleaved flow encoding with freely selectable velocity encoding (venc) along each spatial direction, and flexible trade-off between temporal resolution and total acquisition time. Results:The implementation was validated against established two-dimensional PC techniques using a well-defined phantom, and successfully applied in volunteer and patient examinations. Human studies were performed after contrast administration in order to compensate for loss of inflow enhancement in the four-dimensional approach. Conclusion:Advantages of the four-dimensional approach include the complete spatial and temporal coverage of the cardiovascular region of interest and the ability to obtain high spatial resolution in all three dimensions with higher signal-to-noise ratio compared to two-dimensional methods at the same resolution. In addition, the four-dimensional nature of the data offers a variety of image processing options, such as magnitude and velocity multi-planar reformation, three-directional vector field plots, and velocity profiles mapped onto selected planes of interest.

The Ross operation provides excellent survival in adults and children willing to accept a risk of reoperation. Male sex and a primary diagnosis of aortic insufficiency had a negative effect on late results.

Fully balanced steady state free precession (SSFP) imaging (TrueFisp, FIESTA, Balanced FFE) has recently gained increased importance due to its high signal-to-noise ratio (SNR) and high blood-tissue contrast-to-noise ratio. These properties make it particularly useful for cardiovascular applications such as two-or three-dimensional cine MRI (1-28).Little has been reported, however, on the influence of flow on the formation of the SSFP signal and the subsequent steady state. In-flow of fresh, unsaturated spins can strongly influence the transition into the steady state and the steady-state signal itself or, in case of pulsatile flow, even result in periodically varying signal intensity that never reaches a steady state. As a result, signal intensities may be significantly altered and give rise to in-flow enhancement as compared to static structures of the same material and flow artifacts.The purpose of this article is to demonstrate the effects of in-flow on SSFP signal formation. Different-flow related-contributions to the SSFP signal were explored and used to simulate the effects of flow on SSFP signal intensities. The results of computer simulations were validated in phantom experiments using steady flow of different rates as well as pulsatile flow waveforms.The effects of imperfect excitation profiles and signal contributions from flowing "out-of-slice" spins had to be taken into account to fully understand and model the measured SSFP signal. The results show that SSFP imaging has considerable flow-related signal changes which strongly depend on off-resonance effects such as imperfect shim, eddy currents, or susceptibility effects. As a result, the excitation profile is replaced by a spatially nonuniform effective slice thickness that is flow-dependent. Implications for SSFP imaging in humans therefore include frequency offset-dependent in-flow enhancement with a concomitant sensitivity to flow artifacts. MATERIALS AND METHODS Theory and SimulationsThe temporal evolution of the magnetization (as a function of the number n of RF-excitations) is calculated using a matrix formalism similar to methods discussed by Hargreeves et al. (19) and Scheffler et al. (30). A train of equally spaced RF-pulses (constant TR) with constant relaxation times T 2 and T 1 was assumed for all simulations.Based on the Bloch equations, it can be shown that the magnetization M ជ n ϩ directly after the n th RF-pulse is given by the following recursive expression (see also Appendix):where R x (Ϯ␣) and D z (⌬ TR ) represent rotations of the magnetizations around x and z, respectively. R x (Ϯ␣) describes RF-excitation with flip angle ␣ and alternation of RF-phase which was realized by alternating the sign of ␣ for consecutive RF-excitations. D z (⌬ TR ) corresponds to a phase shift of the magnetization vector by ⌬ TR over the TR interval. E ជ 1 (TR,T 1 ) and E 2 (TR,T 2 ,T 1 ) represent T 1 -relaxation and T 2 -decay. The SSFP signal intensity M xy ϩ and phase ЄM xy ϩ immediately after the RF-pulse can be computed from the transverse com...

To characterize gradient field nonuniformity and its effect on velocity encoding in phase contrast (PC) MRI, a generalized model that describes this phenomenon and enables the accurate reconstruction of velocities is presented. In addition to considerable geometric distortions, inhomogeneous gradient fields can introduce deviations from the nominal gradient strength and orientation, and therefore spatially-dependent first gradient moments. Resulting errors in the measured phase shifts used for velocity encoding can therefore cause significant deviations in velocity quantification. The true magnitude and direction of the underlying velocities can be recovered from the phase difference images by a generalized PC velocity reconstruction, which requires the acquisition of full three-directional velocity information. The generalized reconstruction of velocities is applied using a matrix formalism that includes relative gradient field deviations derived from a theoretical model of local gradient field nonuniformity. In addition, an approximate solution for the correction of one-directional velocity encoding is given. Phase contrast MRI (PC-MRI) is widely used to assess blood flow and tissue motion. This technique relies on the measurement of changes in the signal phase due to flow or motion in the presence of known linear magnetic gradient fields (1-5).Although it is well known that nonuniformity in magnetic field gradients can cause significant image warping and require correction, little has been reported about the impact of spatial gradient field distortions on velocity encoding.In PC-MRI, these imperfections introduce errors in velocity measurements by affecting the first moments used to encode flow or motion. Any error in strength or direction of the local gradient from its ideal value is directly reflected in the strength or direction in the first-order gradient moments and thus in the velocity encoding. Therefore, gradient field distortions can lead to considerable deviations between the designed encoding and the actual encoding in PC-MRI. The true gradient field demonstrates deviations not only from the nominal gradient strength but also from the intended gradient direction, and thus affects not only the magnitude of encoded velocities but also the velocity encoding direction. In some recent high-performance gradient systems, the coil size was reduced to limit dB/dt and amplifier power. As a result, gradient uniformity has become even more degraded.Currently, based on the deviations of the actual gradient from the desired uniform gradient, the spatial image distortions in magnitude and phase images are retrospectively corrected by image unwarping algorithms, e.g., with algorithms included in the image reconstruction software (7-18). The velocity-encoded information is moved to its correct location, but the error in velocity encoding due to the local field deviation still persists.In this work we demonstrate the effect of imperfect gradient fields on velocities measured with PC-MRI. Observed errors are directly...

The velocity field within scale models of branching coral Stylophora pistillata colonies was measured using magnetic resonance velocimetry (MRV). The models were based on digital representations of real coral skeletons derived using X-ray computed tomography (CT) and constructed using rapid-prototype manufacturing. Two morphologies of S. pistillata from the Red Sea grown in different flow regimes were used. To simplify visualization of the data, velocities were parsed into a series of spherical shells, giving the velocity distributions as functions of distance from coral center for both morphologies. The low-flow morphology distributed flow velocity relatively evenly throughout the interior. In contrast, the high-flow morphology showed a wider range of velocities with regions of flow channeling and flow stagnation.