Proceedings of the meeting - Department of Physics - University of ...

Slice profile effects in variable flip angle HP 3 He MRIK. Teh, K.J. Lee, J.M. WildAcademic Radiology, University Of Sheffield, Sheffield, UKP29Introduction Unlike thermally polarised NMR,hyperpolarised (HP) M o is non-recoverable and is progressivelydiminished after application of each RF pulse. Hence HP gasMRI is very sensitive to flip angle deviations and differentmethods have been implemented (1, 2) to maximise theutilisation of the non-constant M 0 . To minimise blurring throughk-space filtering (2) a constant transverse magnetisation can becreated using the variable flip angle scheme (1): θ(n)=tan -1 (1/√(N-n)) Eq.[1] where N is the total number of excitations, nis the nth excitation pulse and θ is the flip angle. In this work theperformance of the variable flip angle scheme was investigated insingle slice and multi-slice 2-D spoiled gradient echo imagingexperiments with HP 3 He gas. The slice profile in HP gas MRIhas previously been shown to be variable from RF view – RFview as a result of the non-ideal flip angle distribution causingdifferential rates of magnetisation depletion across the slice. Thiscan result in a “rabbit ear” profile shape as described previously(2). A strategy to compensate for non-uniformity inmagnetisation response caused by this non-ideal view dependentslice profile by using a variable slice select gradient betweenviews, is presented here.Methods Measurements were conducted on a 1.5Twhole body MRI system (Eclipse-Philips Medical System).Studies were performed using a mixture of (100ml 3 He and900ml N 2 ) contained in a 1 litre Tedlar plastic bag. The 3 He gas(Spectra Gases) was polarised on site to around 20% with opticalpumping rubidium spin exchange apparatus (GE Health). Aquadrature high quality factor (Q) T/R birdcage was used for allexperiments with homogeneous flip angle distribution, importantin experiments sensitive to flip angle changes. An interleavedsequence was used with N=4 pulses. Eq.[1] was used todetermine the theoretical amplitude of RF pulses needed per viewto maintain constant transverse magnetisation without any sliceselection scaling. Scaling for non-uniform slice profile effects,introduced by the 2D slice selection process, was thenimplemented by determining the scale factor for RF pulseamplitude. To do this the transverse magnetisation, M xy (z) wassummed over a) the whole FOV, this represents the single 2-Dexperiment and b) by summing M xy (z) over the FWHM limitswhich represents the multi slice experiment - see Fig.[1]. Thelimits of ± FWHM/2 represent the scenario in multi-slicing dueto magnetisation burnout on either sides of the excited slicethrough multi slicing. By taking limits of FWHM as shown inFig.[1] it was assumed that above and below the limits ofFWHM the sum of the magnetisation is zero. Diffusion effectsbetween slices are negligible in these low N experiments due tothe short TR. The sequence was run with 256 samples, 13 mmslice thickness and a TR of 80ms.-FOV/2-FWHM/2FWHM/2FWHMFig.[1]- limits of integration of M xy (z) for scenarios a) and b).Results Fig.[2a] shows the plot of normalised magnetisationversus pulse number, n, as calculated by summing M xy (z) overFOV limits for the theoretical flip angles (30°,35°,45°,90°) asgiven by Eq.[1]. It is obvious that a non-steady magnetisationresponse is found despite using the variable flip angle scheme –ZFOV/2we attribute this to view dependent slice profile effects (2).Fig.[2b] has the same magnetisation response plotted over theFWHM limits of integration. The plot enables determination ofthe slice selection scaling to achieve constant M o . From this datathe slice thickness was scaled per view as illustrated in Fig.[3a]where the slice profile changes with n. Notice changes in themeasured slice profile width due to slice selection scaling.Fig.[3b] is the corresponding curve depicting magnetisationchanges as a function of n with an integration limit of the wholeFOV. Despite the variable flip angle and slice select scaling anon-steady magnetisation response is still seen. Fig.[4a] showsplot of slice profile, M xy (z), over the limits of the FWHM andFig.[4b] shows corresponding normalised magnetisationresponse from integrating between these limits –note the almostflat response in this case.Normalised MoMoMo1.41.351.31.251.21.151.11.05FOV / 2∑−FOV/ 2M xy( z)11 2 3 4n pulsesFig.[2a]2.5 x 105 21.5-6.5mm10.50-FOV/2ZFig.[3a]14 x 104121086420-∆Z/2ZFig[4a]6.5mmFOV/2∆Z/2Normalised MoNormalised MoNormalised Mo1.21.181.161.141.121.11.081.061.041.020.980.960.940.920.90.8811 2 3 4n pulses1Fig.[2b]0.861 2 3 4n pulses1.041.0210.980.960.940.920.90.88FWHM / 2FWHM / 2Fig.[3b]0.861 2 3 4n pulsesFig[4b]Conclusion In this work it was demonstrated that variableflip angle techniques previously used in 2D HP gas MRI areprone to error and can provide a non-ideal non-constant magnetisationresponse caused by the non-uniform distribution of flipangles across the slice (2). By implementing a variable slice selectgradient in conjunction with variable flip angle techniquesenables us to maintain constant M o throughout the multi-pulseexperiment. The results presented here are directly applicable tothe high acceleration factor parallel imaging techniques that canbe achieved with HP gas using low numbers of RF excitations(3). Future experiments will include implementation in 2D imagingsequences with more pulses (higher N), and also the possibilityof tailoring the slice profile of each pulse in the sequence.References[1] Zhao L et al ,J Magn Reson Ser B 1996;113:179-183[2] Wild JM et al, Magn Reson Med 2002;47:687-695[3] Lee R et al Magn Reson Med 2006;55:1132-1141.Acknowledgement EPSRC #GR/S81834/01(P), GE Health,Spectra Gases, Aerosol Society UK∑−FWHM/ 2FOV / 2∑−FOV/ 2∑−FWHM/ 2M xy( z)M xy( z)M xy( z)