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Abstract:
The MR sequence is the essential tool to measure MR properties in tissues,
increase contrast between these and quantify relevant data. Besides furnishing
the desired tissue-derived MR signal, the sequence must allow spatial
encoding to take place as efficiently as possible to enable patient compliance,
and high-quality MRI at the same time. Starting with a brief introduction regarding MR tissue properties and spatial encoding in k-space, I will describe
the fundamental MR toolkit, based on single echo spin-echo (SE) and gradientecho
(GE) and show how these are extended to different multiple-echo
regimes. I will describe the basic image contrast and how imaging parameters,
repetition time, TR; echo time, TE; flip angle, FA; inversion delay, diffusion
gradients, etc., influence contrast. The possibility to further enhanced imaging
by adequate spin-preparation, inversion or magnetization transfer pulses, etc.,
will also be discussed. Imaging speed can be achieved while remaining in the
single echo regime by faster RF pulsing giving valuable information linked with
magnetic susceptibility as in the GE-based FLASH method, based on spoiling
of unwanted echoes. Another possibility is to use subsequent (spin) echoes to
read out different k-space lines, without the cost of greater saturation that
comes with shorter TRs, as in the SE-based method RARE. An alternative
possibility that further extends available image contrast is to retain the full
magnetization in GE-based sequences and acquire images in the different
steady-state free-precession regimes. The latter techniques have gained momentum through the advent of magnetic fingerprinting and are a fundamental part of the standard MRI toolkit.
