Improved SE-SPI for T₂ Mapping in Porous Media

The SE-SPI (Spin Echo Single Point Imaging) pulse sequence [1] is employed to measure T₂ distributions spatially resolved of fluids in porous solids. The method employs a single phase encode gradient for 1D images (profiles), with phase imported to a series of spin echoes. The first echo time is limited by the gradient switch time. Subsequent echoes may have very short echo times, usually 300-400 microseconds. However, this sequence has a few limitations:
(1) The first echo time is still long (about 1.6 ms), so very short T₂ components are not observed. Our goal is to reliably observe T₂s as short as 1 ms.
(2) Composite RF pulses have been used to improve the quality of the echo train. However, these pulses significantly increase the RF duty cycle.
Further improvements of this T₂ mapping pulse sequence are presented herein:
(1) For SE-SPI, rapidly switched gradients are desired and they must be truly zero before the first refocusing pulse is applied. A recently developed pre-equalization technique [2] was employed to calculate an input gradient waveform that yields a desired gradient waveform in the sample space, based on measurements of the system impulse response. This technique allows us to reduce the first echo time to as short as 600 microseconds.
(2) The XY-16 phase cycle currently employed may be replaced by a shorter 8-step phase cycle with composite RF pulses replaced by ordinary RF pulses.
(3) The number of points acquired for each echo is maximized and the digital filter width is correlated with the natural NMR lineshape width, to maximize the signal-to-noise ratio.

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