Optimisation of the combined gradient echo/spin echo (GESE) sequence for the measurement of T2 at 7.0 T

The combined gradient echo/spin echo (GESE) sequence has been used to measure T2 and T2* simultaneously in the brain at 7T in both healthy volunteers and patients . GESE is particularly useful at 7T as it is insensitive to RF pulse errors since errors in either of the pulse flip angles will cause constant attenuation of all the echoes, leading to a simple reduction in SNR, and it has a low SAR. It is also insensitive to T1 saturation (weighting) since the time between the refocusing pulse and next 90 pulse is constant. Other groups have used variants of this sequence in the past at lower field. Aims: To determine the best approach for fitting GESE data, and to optimise the acquisition parameters for SNR in the measured T2 and T2′ in terms of TE, M and ΔΤ. Optimization GESE consists of a series of M gradient echoes (GE) (i.e. an EPI readout gradient) superimposed on a spin echo (SE) (figure 1). This is repeated N times, where N is the number of phase encoding steps, with a phase encoding gradient applied before the start of the readout. (In practice this was implanted using ‘EPI test mode’ on a Philips scanner). Figure 2 shows typical T2 maps obtained using this sequence. Two methods for calculating the relaxation parameters were investigated: Method A: The signal obtained at each gradient echo is modelled as