Modified Carr-Purcell-Meiboom-Gill sequence for NMR fourier imaging applications

In the clinical application of NMR imaging the observation of significant image contrast arising from T2 differences between tissues has led to the frequent use of multiple spin-echo acquisition sequences.However, it has been noted that a significant loss of signal results with multiecho NMR imaging, leading to inaccurate TZ measurement and image artifacts, particularly when selective 180" pulses are used (1-3).Solutions to this problem include the use of composite pulses to improve the accuracy of the 180" selective pulses.However, for compensation of the effects of nonideal pulses with the Fourier method of imaging (4, 5), the use of the well-known Carr-Purcell (CP) (6) or Cat-r-Purcell-Meiboom-Gill (CPMG) (7) pulse sequences is no longer satisfactory.This is a consequence of the phase encoding which is applied prior to observation of the multiple spin-echo response.In this case a modified pulse sequence can be used to provide a significant improvement in the ability to compensate for nonideal refocusing pulses.The CP or CPMG sequences assume a given phase relationship between the spin response following excitation and the phases of the subsequent 180' pulses.When phase encoding of the NMR response is performed in Fourier imaging, this fixed phase relationship no longer exists.In this situation these pulse sequences perform as predicted for the "in-phase" component of the refocused spin magnetization, but no longer provide any compensation of pulse errors for the "out-of-phase" component, where the phase is referred to the real and imaginary parts, respectively, of the FID following the initial excitation and phase encoding.Figure la shows the result of a calculation of the amplitudes of a number of spin ethos following the application of the CP and CPMG pulse sequences for the case where the refocusing pulse is incorrectly set and provides a 170" rotation only.The simulation used comprised of a vector rotation for in phase and out-of-phase magnetization vectors, M(x, y, z) = [ 1, 0, 01 and M'(x, y, z) = [0, 1, 0] respectively, with appropriate rotation matrices for the x and y refocusing pulses.The resultant magnetization values were then modified to include the effect of the refocused spins being distributed over a range of angles where improper refocusing takes place.The out-of-phase component of the magnetization is assumed to have been created following the application of magnetic field gradients for phase OO22-2364186 $3.00