Elimination of transverse coherences in FLASH MRI

Abstract

Fast low-angle shot (FLASH) imaging enables T1-weighted scans to be acquired in a few seconds. However, the diagnostic image quality is severely compromised by the appearance of artifactual bands parallel to the frequency encode direction. We show that the band structure arises from differences in the ability of the phase encode gradient to spoil transverse coherences that build up between successive repetition intervals. A theoretical understanding of the mechanisms involved leads to a comparison between various methods of spoiling the unwanted echoes throughout the whole field of view. Spoiler gradients whose amplitudes change linearly with phase encode step number are treated in detail. The theory predicts that the spoilers will rotate and rescale the band structure and these results are confirmed experimentally. The effect of the spoilers at a given location along the gradient is equivalent to the effect on the entire field of view of an incremented phase shift applied to the radiofrequency pulse. An appropriate rf phase shift scheme should therefore provide ideal spoiling characteristics for FLASH imaging.

AFNI: Software for Analysis and Visualization of Functional Magnetic Resonance Neuroimages

“…METHODOLOGY FOR TRANSFORMING FUNCTIONAL ACTIVATION MAPS As mentioned earlier, it is common to gather a whole-brain high resolution anatomical reference data set at the beginning of an FMRI scanning session. With the Spoiled GRASS imaging sequence (13), this takes only 10{15 minutes on a clinical scanner. Subsequent functional activation data sets can then be mapped into stereotaxic coordinates once the mapping for the anatomical data set has been determined.…”

Steady state of gradient echo sequences with radiofrequency phase cycling: Analytical solution, contrast enhancement with partial spoiling

“…Written informed consent was obtained before the measurement. The maxima, which occur at ϭ p ⅐ 360Њ/q, (p, q positive integers) have been explained as the result of an incomplete cancellation of transverse magnetization (2,4,16). Because of y q ϭ 1, we see that in these cases the continued fraction expansion [55] can be closed (⌳ ϩ (q) ϭ ⌳ ϩ (0) ) and can thus be solved in a similar way like the case ϭ 0 (FISP) in Appendix C. Indeed, Fig.…”

“…To this end sufficiently strong crusher gradients are nowadays generally applied in combination with an RF phase cycling condition (RF spoiling) (1,2). The latter means that the differences between the phases k of consecutive excitation pulses are linearly incremented:…”

A magnetization‐driven gradient echo pulse sequence for the study of myocardial perfusion

“…In theory, the approach of longitudinal magnetization to the steady state can be predicted analytically provided that transverse magnetization is perfectly spoiled after each RF pulse. In practice, spoiling of transverse magnetization can be achieved using gradients or RF spoiling algorithms (24,25). We chose to spoil transverse magnetization using RF spoiling, rather than gradient spoiling, to minimize TR and thus reduce imaging time.…”

“…A band-width of ±32 kHz was used, such that readout time was (1/(32,000·2)s)·160 data points = 2.5 ms. The RF phase scheme (24,25) commenced with the dummy pulses and was continued without interruption during the imaging pulses to ensure that no new transients would be introduced. As with standard General Electric SPGR sequences, RF spoiling was accomplished by adding a constant phase increment to the RF during each cycle.…”