Contrast in Rapid MR Imaging

Partial saturation (PS) is an imaging technique that is useful in applications that require rapid image acquisitions (imaging time <1 min). Image contrast in PS imaging, as in other magnetic resonance methods, depends on the often conflicting effects of differences in proton density, T1, and T2. Previous analyses of pulse sequence optimization to maximize image contrast have assumed 90° pulses and examined the effects of varying repetition times (TR) and echo times (TE). In this paper we present theoretical calculations and images made with a 0.6 T imager to show that the radiofrequency pulse tip angle a, and not the pulse sequence timing parameters, is the most important parameter for producing image contrast. For large tip angles (α ≥= 60°), contrast is primarily determined by differences in T1, but for small tip angles (α ± 25°), contrast is primarily due to differences in T2. The T2-weighted images can be produced as quickly as T1-weighted images by using a small pulse angle and a long TE; it is not necessary to use a long TR to reduce the effects of T1 differences. Optimum pulse angles are calculated, and the potential advantages and disadvantages of T2-weighted and T1-weighted PS imaging are discussed.