Abstracts-part IV (Continue in Part V)

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Introduction. Arterial spin labeling allows the measurement of brain perfusion using an endogenously generated spin tracer .lI2. Conventional use of this ...
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A Multislice Arterial Spin Labelling Sequence implemented with a Single Volumetric RF coil. H. D. Morris, Deepak Khuntia, and P. C. Lauterbur Biomedical Magnetic Resonance Laboratory, College of Medicine, University of Illinois at Urbana-Champaign, Urbana IL, 61801

Introduction. Arterial spin labeling allows the measurement of brain perfusion using an endogenously generated spin tracer . l I 2 Conventional use of this technique yields a single slice 2 0 image erfusion map. The construction of the pe usion map requires 2 images , 1) arterial spin labelled, and 2) a magnetization transfer contrast subtraction image. Several slices can be acquired but the necessary time is 2n where n is the number of slices. We present a modifiedsequence that allows for several slices to be obtained rapidly in a time n+l . The pulse sequence takes advantage of the transit time of the labeled spins from the inversion origin (in the carotid arteries) to the brain capillary bed. This transit time has been shown to vary based on pC02 and is estimated to be 300 ms for the normoxic rat3 It is during the transit time that a single phase encode is acquired for all the slices. After this rapid acquisition of slice data a labeling period on the order.of the TI is allowed to restore the steady-state concentration of the arterial saturation. Conventional imaging systems with only a single RF channel may make use of this technique to speed acquisition perfusion data by nearly a factor of 2. Up to 10 2D slices may be acquired during the arterial spin transit time period. The pulse sequence timing diagram shown in Figure 1 illustratesthis point.

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Methods. Male Sprague-Dawley rats (300-4009) were maintained on a mixture of air and Oxygen and COP levels halothane(l%) . were varied to produce changes in cerebral blood flow. NMR experiments were performed on a SISCO/Varian 200/330 imaging spectrometer using home-built RF coils. Pulse sequence timing parameters were TR/TE=2000/30 and 5 slices were obtained during the perfusion transit window for a combined slice acquisition time of 210, milliseconds. Five control images were obtained after the spin labeled run for each set of physiologicalconditions. Results. Perfusion maps were generated from rats under normoxic and hypocapnic conditions. The image etfusion maps show a general reduction in per usion r from 1.4 to 1.O ml -1 min-1 on changing the breathlng mixture The response is rom air to oxygen. consistent with the literature values on rodent No perfusion with variation of pC02. 1 additional artifacts were found in the rapid multislice images when compared to the conventional 2D data. If this technique were

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used to investigate anoxic rodents the shorter transit times for the arterial spins would need to be taken into account by using a much shorter acquisition time. This could be accomplished b either acquiring fewer slices or changing the E of the sequence.

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Conclusion. The multislice arterial spin labelling experiment can be performed with a single volume RF coil. The time required for image acquistion is reduced by (n+l)/2n , where n is the number of slice images. The MTC control images must still be obtained serially, which accounts for the persistent factor of n. This experiment can be performed on a single channel MRI system and does not need any additional hardware. Acknowledgements. This work was partially supported by NIH-BRTR Grant PHS 5 P42 RR 05964, NSF-STC Grant 89-20133, and the Servants United Foundation. References. 1.) D.S. Williams, J.A. Detre, J.S. Leigh, and A.P. Koretskv. .. Proc. of Nat‘l Acad. Sci. 9,212, 1992. 2.) A.C. Silva, W. Zhang, D.S. Williams, and A.P. Koretsky, Proc. of the 12th annual meeting of SMRM,641, 1993. 3.) W. Zhana. D.S. Williams. J.A. Detre. and ’ A.P. Korgtsky, Mag. Res. Med., 25,’362, 1992

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Figure 1. Pulse sequence timin diagram. The spin labeling pulse is execute only once per phase encode step. The slices are acquired in rapid succession without an intervening delay.

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