LARGE-ANGLE SPIN-ECHO IMAGING

A study was undertaken to assess the use of excitation flip angles gre ater than 90 degrees for T-1 weighted spin-echo (SE) imaging with a si ngle 180 degrees refocusing pulse and short TR values, Theoretical pre dictions of signal intensity for SE images with excitation pulse angle s of 90-180 degrees were calculated based on the Bloch equations and t hen measured experimentally from MR images of MnCl2 phantoms of variou s concentrations, Liver signal-to-noise ratios (SNR) and liver-spleen contrast-to-noise ratios (CNR) were measured from breathhold MR images of the upper abdomen in 16 patients using 90 and 110 degrees excitati on flip angles. The theoretical predictions showed significant improve ments in SNR with excitation flip angles >90 degrees, which were more pronounced at small TR values. The phantom studies showed reasonably g ood agreement with the theoretical predictions in correlating the exci tation pulse angle with signal intensity, In the human imaging studies , the 110 degrees excitation pulse angle resulted in a 7.4% (p < .01) increase in liver SNR and an 8.2% (p = .2) increase in liver-spleen CN R compared to the 90 degrees pulse angle at TR = 275 ms. Increased sig nal intensity resulting from the use of large flip angle excitation pu lses with a single echo SE pulse sequence was predicted and confirmed experimentally in phantoms and humans,