In SPGR, the transverse magnetization is prevented from forming echoes by using an RF spoiling technique. This is accomplished by linearly incrementing the phase of the RF pulses from one TR interval to the next. Both the RF excitation and readout phase must be incremented. This can be thought of as exciting along the x-axis and reading along x at the first iteration, then rotating the exciter/reader by some angle (phase) so that the previous excitation magnetization is effectively phase-offset from the current signal, and so should contribute minimally. Because SPGR is designed specifically to disrupt the T2 coherence pathways, it is most useful for creating T1-wtd and PD-wtd images.

Use of a short TE decreases the amount of T2*-weighting in this sequence. For short TE, the combination of TR and flip angle together determines the image contrast. With all other parameters equal, low flip angles are used to create more PD-weighting in the image, and higher flip angles yield increased T1-weighting. The flip angle yielding the maximum signal intensity that can be obtained using the SPGR sequence is given by the Ernst angle, which can be calculated from:

cos =exp(-TR/T1) (1.5)

where is the flip angle.

This flip angle also defines the critical flip angle below which the images are predominantly PD-weighted (the farther away from the Ernst angle, the higher the PD-weighting), and above which the images are T1-wtd.

There are useful rules-of-thumb approximations that can be applied to determine what flip angle to choose for a certain type of contrast. These rules-of-thumb stem from a short TR assumption (TR << T1). In this limit, the quantity (exp(-TR/T1)) can be approximated with (1-TR/T1), from which the Ernst angle, _E can be approximated as:

Using the above approximation, the Ernst angle in degrees can be quickly calculated for each tissue based on its TR-to-T1 ratio. For PD weighting, the rule-of-thumb is to choose a flip angle that is lower than the lowest Ernst angle, whereas, the rule-of-thumb is choose a flip angle larger than the largest Ernst angle for T1 weighting.

	SPGR techniques are often used in the evaluation of articular cartilage, but are not useful in the post-operative setting due to the signal losses and distortions associated with metal on GRE sequences. These techniques are sometimes used as input for 3D rendering algorithms (Fig. 1.26), but these require long acquisition times.
(click on image for enlarged view) Figure 1.26 Thin-slice SPGR image of a knee reconstructed from a 3D data set.