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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This study demonstrates the feasibility of applying free-breathing, cardiac-gated, susceptibility-weighted fast spin-echo imaging together with black blood preparation and navigatorgated respiratory motion compensation for anatomically accurate T* 2 mapping of the heart. First, T* 2 maps are presented for oil phantoms without and with respiratory motion emulation (T* 2 ‫؍‬ (22.1 ؎ 1.7) ms at 1.5 T and T* 2 ‫؍‬ (22.65 ؎ 0.89) ms at 3.0 T). T* 2 relaxometry of a ferrofluid revealed relaxivities of R* 2 ‫؍‬ (477.9 ؎ 17) mM ؊1 s ؊1 and R* 2 ‫؍‬ (449.6 ؎ 13) mM ؊1 s ؊1 for UFLARE and multiecho gradient-echo imaging at 1.5 T. For inferoseptal myocardial regions mean T* 2 values of 29.9 ؎ 6.6 ms (1.5 T) and 22.3 ؎ 4.8 ms Emerging cardiovascular magnetic resonance (CVMR) imaging applications include T* 2 relaxation sensitized techniques that are increasingly used in basic research and (pre)clinical imaging. Reports include detection of myocardial ischemia in patients with suspected coronary artery disease (1), detection of perfusion changes after the application of dipyridamole (2,3), detection of scarred myocardium (4), and the assessment of tissue oxygenation related to endothelium-dependent blood flow changes (5). T* 2 mapping has been shown to be of clinical value for the ascertainment of myocardial iron levels (6 -12).The most widely used fast imaging methods for myocardial T* 2 mapping are gradient echo or echo planar imaging (EPI)-based techniques. The relatively strong T* 2 -weighting required to make gradient echo sequences sensitive to changes in magnetic susceptibility necessitates a long evolution time (TE) between the RF excitation and the data acquisition. This generally restricts data acquisition to a single cardiac slice, which is the maximum coverage that can be accommodated in a single breathhold. EPI offers excellent temporal resolution for CVMR and reasonable in-plane resolution but the minimum echo time is limited by hardware so that gradient echo EPI is prone to susceptibility artifacts, which manifest themselves as image distortion and signal loss. This effect is severe at high magnetic field strengths (13), where T* 2 is shorter than at low magnetic fields and is particularly pronounced in regions with poor B 0 homogeneity. Consequently, the disadvantages of T 1 -related saturation effects, artifacts due to ventricular blood flow (5), and image distortion due to intrinsic sensitivity to B 0 -inhomogeneities (1) must be addressed to pave the way for a broader clinical acceptance of myocardial T* 2 imaging/mapping.Fast spin echo imaging techniques are largely free of image distortion related to B 0 inhomogeneity because of the use of RF refocused echoes, which also provide intrinsic signal suppression of fast-flowing blood. Fast spin echo techniques present an alternative particularly at high magnetic field strengths where T 2 is much longer than T* 2 which contains T 2 and susceptibility-related (T 2 Ј) contributions (1/T* 2 ϭ 1/T 2 ϩT 2 Ј). This makes it attractive to use spin echo-based rapid im...

Purpose: To compare k-t BLAST (broad-use linear-acquisition speedup technique)/k-t SENSE (sensitivity encoding) with conventional SENSE applied to a simple fMRI paradigm. Materials and Methods:Blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) was performed at 3 T using a displaced ultra-fast low-angle refocused echo (UFLARE) pulse sequence with a visual stimulus in a block paradigm. Conventional SENSE and k-t BLAST/k-t SENSE data were acquired. Also, k-t BLAST/k-t SENSE was simulated at different undersampling factors from fully sampled data after removal of lines of k-space data. Analysis was performed using SPM5.Results: Sensitivity to the BOLD response in k-t BLAST/ k-t SENSE was comparable with that of SENSE in images acquired at an undersampling factor of 2.3. Simulated k-t BLAST/k-t SENSE yielded reliable detection of activationinduced BOLD contrast at undersampling factors of 5 or less. Sensitivity increased significantly when training data were included in k-space before Fourier transformation (known as ''plug-in'').Conclusion: k-t BLAST/k-t SENSE performs at least as well as conventional SENSE for BOLD fMRI at a modest undersampling factor. Results suggest that sufficient sensitivity to BOLD contrast may be achievable at higher undersampling factors with k-t BLAST/k-t SENSE than with conventional parallel imaging approaches, offering particular advantages at the highest magnetic field strengths.

k-t BLAST is compatible with fMRI acquisitions and opens up possibilities including distortion-free T2*-weighted blood oxygen level dependent fMRI with displaced UFLARE at high magnetic field strengths.

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