Mr Imaging, Flow and Motion

The present work is intended as a nonmathematical review of the role of flow and motion in nuclear magnetic resonance (MR) imaging. A historical review of MR flow measurement techniques is given, followed by a short overview of flow models in vitro and in vivo. The theory behind the influence of motion on the modulus and phase MR signal information is discussed and effects such as washin/washout, flow-induced signal void, phase offset, and phase dispersion are defined. A simple approach to the concept of MR angiography is given, and methods for quantitative flow measurements such as the phase mapping technique, are surveyed. Aspects of the measurement of diffusion and mirocirculation are given, and finally, an overview of the role of MR flow imaging in present and future clinical application is given.

MR evaluation ex vivo and in vivo of a covered stent-graft for abdominal aortic aneurysms: ferromagnetism, heating, artifacts, and velocity mapping

Magnetic resonance imaging (MRI) safety was evaluated at 1.5 T in a covered nickel titanium stent-graft (Vanguard) used for endovascular treatment of abdominal aortic aneurysms (AAAs). Imaging artifacts were assessed on MRI with contrast-enhanced (CE) three-dimensional (3D) MR angiography (MRA) and spiral computed tomography (CT) in 10 patients as well as ex vivo. Velocity mapping was performed in the suprarenal aorta and femoral arteries in 14 patients before and after stent-graft placement. For comparison it was also performed in six healthy volunteers. No ferromagnetism or heating was detected. Metal artifacts caused minimal image distortion on MRI/MRA. The artifacts disturbed image evaluation on CT at the graft bifurcation and graft limb junction. No significant differences in mean flow were found in patients before and after stent-graft placement. Our study indicates that MRI at 1.5 T may be performed safely in patients with the (Vanguard) stent-graft. MRI/MRA provides diagnostic image information. Velocity mapping is not included in our routine protocol.

MR flow quantification with cardiovascular applications: a short overview

We present a short overview of the potential of magnetic resonance imaging (MRI) for quantification of flow in the cardiovascular system. The most widespread method for creation of MRI flow information utilized flow-sensitizing magnetic field gradients. Objects that move in the varying magnetic field introduced by such gradients change their precession frequency and therefore obtain a velocity-dependent offset phase angle. The exact phase behaviour for different types of gradients and motion patterns can be calculated and a very simple linear relationship is predicted by theory as well as confirmed in experiments between constant velocity and phase angle. Phase-sensitive flow MRI (velocity mapping) is frequently performed as a two-dimensional gradient-echo technique with flow sensitivity (flow encoding) in the through-plane direction, but other encoding directions are possible. Parameters that can be determined from a velocity map are linear velocity in each voxel, vessel area and flow rate. In the case of a stenotic vessel, the trans-stenotic pressure gradient can also be estimated. The velocity mapping method has been extensively used for cardiac flow studies in adult patient groups, e.g. for indirect measurements of coronary artery flow and for the study of aortic valve diseases. In children, the method has recently been used to determine shunt volumes in congenital heart disease. We conclude that flow investigation with MRI may in the future present a good alternative for the clinical evaluation of cardiovascular disorders.