Johannes Petrus Groen

Fast Field Echo imaging: An overview and contrast calculations

Current fast imaging techniques are based on gradient echo sequences with reduced flip angle excitation pulses and very short repetition times TR. Practical T2 values may be of the order of TR or longer. In this situation, a different image contrast can be obtained, depending on details of the sequence. Four essentially different versions of the basic Fast Field Echo (FFE) sequence can be distinguished and are described systematically in this article. For these sequences, image contrast formulas are presented. Practical imaging should tolerate small field inhomogeneities. This requirement can be satisfied by only three of the four versions. Numerical simulations are used to study the influence of a modified phase alternation scheme on image contrasts of two of the remaining sequences. The results of the calculations are verified by phantom studies on a 1.5-T whole-body imager. Implications for contrast in clinical images are discussed in relation to head images obtained on the same machine.

Very fast MR imaging by field echoes and small angle excitation.

An MR imaging technique has been developed producing head and body images of diagnostic quality in only a few seconds acquisition time. The Fourier type imaging technique uses excitation with relatively small excitation angels, echoes produced by gradient inversion, and extremely fast profile repetition. A typical result at 0.5 T is an artifact-free head image of 128 x 128 resolution, 10 mm slice thickness in an acquisition time of 2 seconds.

MR angiography with pulsatile flow

To achieve acceptable scan times, current multiple thin slice and 3D MR angiography (MRA) methods usually are based on continuous data acquisition, without ECG-synchronization. The purpose of this work is to study consequences of pulsatile blood flow for the 2D inflow method. Arterial blood flow and blood signal intensity versus cardiac phase were studied by a 2D phase based method with retrospective cardiac synchronization. Such studies were performed in different parts of the body and with different excitation flip angles. As expected, a clear relation between intensity enhancement and time dependent flow can be demonstrated. The raw data of these multiphase studies was used to simulate alternative inflow MRA data acquisition strategies to improve image quality, without the excessive increase in scan time implied by standard cardiac triggering. The alternatives investigated were data collection during part of the cardiac cycle and cardiac-ordered phase encoding. Simulation results indicate that the best results are obtained by a combination of both strategies. This method was implemented on Philips Gyroscan systems to compare it with standard nontriggered 2D inflow in practical MRA studies. For highly pulsatile flow, much better MR angiograms were obtained in this way.