ACT's intrinsic insensitivity to interference from electromagnetic fields renders it suitable for clinical CMR.

The proposed MR-stethoscope presents a promising alternative to currently available techniques for cardiac gating of (ultra)high field MRI. Its intrinsic insensitivity to interference from electromagnetic fields renders it suitable for clinical imaging because of its excellent trigger reliability, even at 7.0 Tesla.

Velocity-driven adiabatic fast passage (AFP) is commonly employed for perfusion imaging by continuous arterial spin labeling (CASL). The degree of inversion of protons in blood determines the sensitivity of CASL to perfusion. For this study, a computer model of the modified Bloch equations was developed to establish the optimum conditions for velocity-driven AFP. Natural variations in blood velocity over the course of the cardiac cycle were found to result in significant variations in the degree of inversion. However, the mean degree of inversion was similar to that for blood moving at a constant velocity, equal to the time-averaged mean, at peak velocities and heart rates within normal ranges. A train of RF pulses instead of a continuous RF pulse for labeling was found to result in a highly nonlinear dependence of the degree of inversion on RF duty cycle. This may have serious implications for the quantification of perfusion.Magn The development of arterial spin labeling (ASL) MRI has made it possible to study microvascular blood flow completely noninvasively, offering a means of investigating tissue perfusion in numerous functional and pathological states. Much work has been performed to assess and improve the accuracy of perfusion values from ASL (1-3). However, its inherently low sensitivity remains one of the most significant limitations of the technique, and the labeling process is a key factor in determining the sensitivity.Velocity-driven adiabatic fast passage (AFP) was applied initially for MR angiography (4). It is now widely regarded as the preferred method for continuous ASL (CASL) because it provides a continuous supply of blood that is labeled by inversion. However, the degree of inversion is limited by several factors, including the amplitudes of the magnetic field gradient and radiofrequency (RF) pulse used for AFP; the velocity of blood, which varies in arteries over the course of the cardiac cycle; and the spin relaxation in blood. In addition, the duration of RF pulses is often limited by RF amplifiers, especially on some clinical scanners, and the relatively long pulse required for labeling in CASL experiments (2 or 3 s) must be divided into a series of short pulses. It has been assumed that the effectiveness of AFP is proportional to the duty cycle of the resulting train of RF pulses (2,5).A computer modeling approach was adopted for this study in order to investigate the influences of several parameters on velocity-driven AFP individually, in a controlled manner. The aims were to establish optimal RF pulse parameters, to determine the effect of pulsatile variations in the velocity of blood, and to study the dependence of the degree of inversion on the duty cycle of a discontinuous RF pulse during labeling. The modeling was performed with one set of parameters for imaging humans in a clinical scanner, and a second set of parameters for imaging rodents in a small-bore system for animals. THEORYTo invert moving spins by AFP, an RF pulse, B 1 , is applied in the presence of a magnetic field gr...

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