Observation of radiation damping effects in a birdcage resonator with hyperpolarized 3He at 1.5T

K. Teh, N. de Zanche, J. M. Wild Academic Radiology, University of Sheffield, Sheffield, Yorkshire, United Kingdom, ETH, Zurich, Switzerland Introduction and Background Theory Radiation damping is a non-linear effect caused by an interaction between the sample magnetization and the tuned RF resonator [1]. It can be described as the action of a ‘counter’ field produced by the current induced in the coil, which exerts a torque opposite to that of B1 + to ‘push back’ the sample magnetization towards the longitudinal direction. In a quantum description it is described as a spontaneous emission or maser effect [2]. In experiments with short T2 * or imaging experiments where gradients cause rapid dispersion, radiation damping is less likely as the interaction between the transverse magnetization vector and the coil is rapidly quenched. In the original analysis [1], the radiation damped FID was given by sech(t/τrd) where τrd =(2πηM0Qγ) is the damping time constant, Mo is the longitudinal magnetization vector, Q is the quality factor, and η is the filling factor. Gueron et al [3] gave the damping time in SI units as τrd =(QπηγM0μ0/2), with Mo the longitudinal magnetization density. The effect has been described previously in low field hyperpolarized (HP) gas NMR [4], where T2*’s are very long, using small pick-up coils with high Q, and high η. Birdcage resonators have spatially homogeneous B1 profile [5], important in HP gas MRI which is very sensitive to flip angle by virtue of the non-renewable polarization. The accurate definition and calculation of η for a birdcage resonator, and its relationship with Q is the subject of discussion [6,7], indeed it has been suggested that sensitivity to radiation damping may be a means of measuring η [6]. In this work we probe radiation damping effects with HP He samples at 1.5T, using a volume optimised birdcage coil with high Q and good homogeneity. Methods Measurements were conducted on a 1.5T whole body MRI system (Eclipse-Philips Medical System). The system was fitted with a transmit-receive circuit for He at 48.5 MHz. The He gas (Spectra Gases) was polarized on site by optical pumping with Rb spin exchange apparatus (GE). Studies were performed using two gas phantoms: i) 1 litre Tedlar plastic bags and ii) a 270 ml spherical glass cell. The He polarization, P, was measured at 27% for the bag experiments and 38% for the cell experiments. The magnetization density of the samples was calculated from M0 = μPN/2, where μ is the nuclear magnetic moment of He and N is the number of He atoms per unit volume. In the bag experiments, 80 ml He was mixed with 920 ml N2 at 1 atm giving M0=6.4x10 -3 JTm which is comparable to the M0 of a 1-l sample of H2O at 1.5T (M0=4.8x10 -3 JTm). In the cell experiments, 260 ml of He at 1.2 atm pressure (P=38%) was used, giving M0=0.13 JT m. The dissimilar sample volumes allow the investigation of the variation of η. RF coil: A quadrature T/R birdcage was purpose built –Fig.1. The low-pass birdcage had 15 cm inner diameter, 22 cm shield diameter and twelve 19 cm legs. Measured Q with the coil matched inside the magnet was 250 at 48.5 MHz. Isolation between the two orthogonal ports was -26dB. A 2-D B1 mapping sequence using spoiled gradient echo imaging was used to characterise the coil homogeneity with a 1 l bag phantom (< 15% variation across bag). Calculation of η is an area of debate [6,7], however an upper limit is the ratio of the sample and coil volumes: η20 mins) and constant TR used throughout. The received He signal was then fitted to mn= m0(cosα)sinα, where mn is the detected transverse magnetization after the nth excitation and mo is the initial transverse magnetization. This is an established means of calibrating α in HP gas NMR [3] –for a constant α the log curve, ln(mn/m0) should yield a linear slope of cosα.. The procedure was then repeated on the spherical He cell.