Characteristics of an MRI sequence
The architecture of a sequence consists of the essential components on one hand, and the various options, on the other. The building blocks of the sequence are radiofrequency pulses and gradients.
The essential components for any imaging sequence are:
- An RF excitation pulse, required for the phenomenon of magnetic resonance
- Gradients for spatial encoding (2D or 3D), whose arrangement will determine how the k-space is filled
- A signal reading, combining one or a number of echo types (spin echo, gradient echo, hahn echo, stimulated echo…) determining the type of contrast (the varying influence of relaxation times T1, T2and T2*).
The options consist of other radiofrequency pulses, gradients or variable reconstruction methods to:
- Either modify the contrast (preparing magnetization by inversion-recovery, fat saturation, magnetization transfer…)
- or accelerate the sequence (partial Fourier plane filling, parallel acquisition, fast magnetization restoration…)
- or to reduce artifacts (flow compensation, synchronisation, presaturation bands …)
Finally, the user must choose the sequence parameters (TR, TE, flip angle, turbo factor, field of view matrix) to find the best compromise between contrast, spatial resolution and speed.
With the exception of inversion-recovery, the optional techniques to modify the contrast of a sequence will be examined in a dedicated chapter. Likewise, the treatment of artifacts and parallel imaging methods will be dealt with in separate chapters.