A Pulse Sequence for Flow Evaluation Based on Self-Refocused RF and Interleaved Spiral Readout

✍ Scribed by Atsushi Takahashi; Tie-Qiang Li; Hans Stødkilde-Jørgensen

• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 590 KB
• Volume: 126
• Category: Article
• ISSN: 1090-7807
• DOI: 10.1006/jmre.1997.1128

✦ Synopsis

Recently, the use of the phase-contrast method based on during slice selection. The acquired phase shifts for irregularly interleaved spiral k-space acquisition has been demonstrated moving spins in the presence of long gradient and RF pulses by Pike et al. (1) for flow measurements in vivo. Gradientare difficult to calculate and compensate (7). In the present recalled spiral imaging is a fast nuclear-magnetic-resonance study, we present a pulse sequence based on the combination imaging technique. The advantages of the spiral-imaging of a self-refocused RF pulse (8-11) with spiral readout graditechnique rests with the fact that without stringent requireents. This shortens the slice selection by about 12 ms. Furtherments for the amplitude and slew rate of the gradient system, more, the self-refocused RF pulse dephases the residual transduring only a few free-induction decays, a set of k-space verse magnetization while it excites the longitudinal component data can be collected with sufficient density and extent for (12). With this pulse sequence, we performed velocity-profile reconstructing an image (2, 3). Furthermore, it has recently measurements for a stenosis model with 75% area reduction been demonstrated that spiral imaging is less sensitive to in the cross section. The experimental results clearly demondistortion and ghosting artifacts from flowing material (4), strate that, using this method, velocity profiles for complex since the first and higher order moments of the spiral readout flow can be rapidly and accurately measured with a convengradients are zero at the origin of k space and increase only tional gradient system. slowly and smoothly with k space. However, in practice,

The pulse sequence is schematically presented in Fig. 1. there are several factors which limit the actual performance As shown, the slice selection was achieved by an amplitudeof this technique. The first is static field inhomogeneities, modulated band-selective RF pulse with uniform response causing variation of the point-spread function of the image and pure phase. The pulse duration is 3.2 ms with a selective as a function of offset frequency, leading to geometric distorbandwidth of 1.25 kHz. The selective RF pulse were detion affecting the quality of the image. The second factor is signed to focus the transverse magnetization along the real the eddy currents produced when switching the gradients. axis in the rotating reference frame. The system hardware These may be overcome by measuring the actual k-space has a gradient slew rate and gradient amplitude limits of 17 trajectory modified by the eddy currents and compensating mT/m/ms and 10 mT/m, respectively. The design of the for these during the reconstruction procedure (5). Another interleaved spiral readout gradients was based on the variable factor is that the large signal bandwidth necessary for fast rate method proposed by Hardy and Cline (13). The desired data acquisition reduces the achievable signal-to-noise ratio. in-plane spatial resolution is 0.84 mm/pixel. Five turns of the The fourth factor is the off-resonance artifacts from the fat. spiral trajectory could be achieved with a readout window of A long spectral-spatial radiofrequency pulse, typically 15 19.2 ms, yielding a field-of-view of 215 mm in 20 interms, is usually used to suppress the artifacts from the fat leaves. The first 7.3 ms of the waveforms was limited by tissues (6).

the maximum slew-rate constraint, whereas the remainder In the applications of spiral imaging to quantitative flow was limited by the gradient amplitude constraint. The constudies of unsteady and turbulent flow, it is very important to stant-gradient-amplitude portion of the spiral waveforms keep the slice selection RF and gradient pulses as short as produces a relatively uniform sampling density function; possible in order to eliminate the flow-induced phase artifacts therefore, some correction was required for only the slewrate limited portion. At the end of the spiral-readout gradient,