| Résumé | The FLASH NMR imaging technique enables the acquisition of 2D NMR images within a few seconds. The speed is achieved by using a very short repetition time (TR), on the order of 20 ms, which is usually much less than 5 T₁ . If TR is less than T₂, transverse coherence created by an excitation pulse lasts for several TR time intervals and may give rise to spin echoes and stimulated echoes. These echoes interfere with the free induction decay signal and complicate the interpretation of the resulting images. They can be suppressed by applying a variable spoiling gradient after each data acquisition or by varying the phase of the excitation pulse from scan to scan. The introduction of a variable gradient or a variable RF phase into the pulse sequence will undoubtedly complicate the sequence. Recently, a prefocused excitation pulse, employing both amplitude and phase modulations, was proposed for use in the FLASH imaging sequence to eliminate the transverse coherence without additional spoiling measures. A prefocused excitation pulse does not require reversing the polarity of the slice gradient following the excitation and therefore reduces the eddy currents. We question, however, the conclusion that it is possible to eliminate the transverse coherence without a variable spoiling gradient and without varying the phase of the excitation pulse. We believe that if an RF pulse, simple or complex, is applied periodically in the presence of a field gradient, the transverse coherence de-focused during one RF pulse period will be, at least partially, refocused during the next application of the same pulse. We show below that if TR is shorter than T₂ and if no variable spoiling gradient is applied, the FLASH imaging sequence will partially refocus the transverse coherence, no matter what RF pulse is used for the excitation. |
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