Synopsis. Presaturation is often used to increase SNR in arterial spin labeling (ASL). The presaturation efficiency of sinc pulses can be improved and B1 ...
Presaturation Efficiency in Pulsed Arterial Spin Labeling in the Presence of B1 Inhomogeneities K. Sidaros1, I. K. Andersen1, T. T. Liu2, E. C. Wong2, R. B. Buxton2 1
Danish Research Centre for Magnetic Resonance, Copenhagen University Hospital, Hvidovre, Denmark, 2Center for fMRI, University of California, San Diego, La Jolla, California, United States
Synopsis Presaturation is often used to increase SNR in arterial spin labeling (ASL). The presaturation efficiency of sinc pulses can be improved and B1 sensitivity reduced by using multiple 90° sinc pulses without increasing SAR to the level of adiabatic pulse trains. The degree and dependence on B1 of static tissue subtraction in ASL is also affected by the number of saturation pulses used.
Introduction In-slice presaturation is often used in pulsed arterial spin labeling (ASL) to suppress static tissue, thereby increasing the SNR of the magnetization difference between the control and tag images, ∆M [1,2]. Presaturation can be achieved with a variety of methods ranging from a single slice-selective 90° sinc pulse to complex pulse trains of adiabatic pulses [3,4]. The latter are resistant to B1 inhomogeneities, but may cause SAR problems, especially at high fields. Sinc pulses suffer from B1 sensitivity, but have lower SAR values. One method of improving the saturation efficiency of sinc pulses is to use multiple 90° pulses separated by crusher gradients. This study shows how the presaturation efficiency, in the presence of B1 inhomogeneities, using sinc pulses in the context of ASL is affected by the number of presaturation pulses used, npsat.
Methods PICORE measurements with and without presaturation were carried out on a phantom (T1/T2=760/45ms) using a Varian 4T scanner. The imaging parameters were TI=200ms, TR=2s, FOV=200mm, 64x64 matrix, nacq=12, one 8mm slice. A 10cm inversion slab was used with varying gaps between the inversion and imaging regions. The RF pulses used were a 15ms adiabatic HS inversion pulse [3] (µ=10,β=800Hz), a 4ms sinc pulse for imaging and 12.8ms sinc pulse(s) for presaturation. In-slice presaturation was applied in a symmetric slab that extended halfway between the edges of the imaging and tagging regions. npsat was varied between 0 and 3. Bloch equation simulations were also carried out with B1 fields of 80%, 100% and 120% of the nominal B1 values to estimate the theoretical effect of B1 inhomogeneities.
Results and Discussion Figure 1A shows the normalized signal in the control experiment, Mcon, with a 10mm gap in three ROIs in the phantom as a function of npsat. The ROIs correspond to areas with different levels of B1. Due to the short TI, the signal is expected to be low for efficient presaturation. It is clear that for npsat=1, there is a large dispersion in the Figure 1: Normalized M at a gap of con intensity due to B1 inhomogeneity. This dispersion decreases with increasing npsat which agrees well with the 10mm. (A) phantom measurements. simulated results shown in figure 1B. The overall saturation efficiency, however, is highest for npsat=2. (B) Simulations Figure 2 shows the normalized ASL signal, ∆M, as a function of the gap between the inversion and imaging regions for npsat=0-3. ∆M should be zero since there is no perfusion in the phantom. The non-zero ∆M therefore reflects incomplete static tissue subtraction also known as an offset [5]. For large gaps, ∆M=0, and is not dependent on B1. For medium and small gaps, ∆M is B1 insensitive for npsat=0 and 3, but not for npsat=1 and 2. Using 3 pulses therefore gives a narrower distribution of offsets than 2 pulses. Small gaps are preferable to large gaps since they decrease the transit delays of tagged blood, but only as long as there is no significant offset. npsat>1 is therefore better than npsat=1 since it gives better static tissue subtraction. Using npsat=3, however, decreases the presaturation efficiency compared to npsat=2 (figure 1). From the above results, it can be deduced that for the RF pulses used in this study, the best number of presaturation pulses is 2 or 3 depending on whether saturation efficiency or B1 insensitivity has the highest priority.
Conclusion It was shown that using multiple sinc pulses separated by crusher gradients for presaturation gives higher saturation efficiency and reduced B1 sensitivity compared to using a single pulse. Furthermore, it was demonstrated that static tissue subtraction in ASL improves when using multiple saturation pulses.
Figure 2: Normalized ∆M as a function of the gap between the inversion and imaging regions for npsat=0-3. Upper row: phantom measurements. Lower row: simulations.
References [1] [2] [3] [4] [5]
Wong, EC, et al.[1997] NMR Biomed. 10:237-49 Mai, V, et al. [1996] Proceedings Sixth ISMRM, p.1215 Silver, M S, et al. [1984] J Magn Reson 59: 347-351 Luo, Y, et al. [2001] Magn Reson Med 45:1095-1102 Sidaros, K, et al. [2002] Proceedings Tenth ISMRM, p.1063
Proc. Intl. Soc. Mag. Reson. Med. 11 (2003)
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