NMR signal and MRI contrast
Objetivos de aprendizaje
- To describe how the NMR signal is recorded
- To define the free induction decay
- To explain differences between T2 and T2*
- To describe the role of the 180° rephasing pulse
- To state the stages of a spin echo sequence
- To understand the relation between TR, TE and T1-, T2-, and DP signal weightings
- To give common values of T1, T2, TR and TE times
- Coils are only sensitive to variations of transverse magnetization vector. After a 90° RF pulse, the Free Induction Decay (FID) signal is oscillating at resonance frequency and signal enveloppe is a decay curve described as an exponential curve, depending on tissue-specific spin-spin relaxation and static field inhomogeneities. This decay is characterized by time constant T2*. T2* is always shorter than T2.
- The 180° RF pulse reverses dephasing due to static field inhomogeneities (T2* effects) but not random spin-spin relaxation (T2 effects, tissue-specific). Spin Echo sequence requires an excitation pulse (90° RF pulse) and a 180° rephasing pulse. Time between 90° pulse and 180° pulse is TE/2. MR Signal is acquired at echo time TE, when signal of the echo is the strongest. The signal enveloppe joining maximums of echos after 180° RF pulses is corresponding to the pure T2 decay curve.
- The 90° - 180° RF pulses sequence must be repeated as many times as the number of lines in the data matrix. The time between each 90° RF pulse (excitation pulse) is called Repetition Time (TR).
- You must keep in mind that tranverse relaxation (transverse magnetization decay, producing MR signal) and longitudinal relaxation (longitudinal magnetization recovery) are simultaneous. The longer the TR is, the more longitudinal magnetization will recover.
- With a spin echo sequence :
- TR modifies T1-weighting : the longer is the TR, the less T1-weigthed the image is
- TE modifies T2-weighting : the shorter is the TE, the less T2-weigthed the image is
- A short TR and a short TE give a T1-weighted image.
- A long TR and a long TE give a T2-weighted image.
- A long TR and a short TE give a PD-weighted image.
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