Functional MR imaging of the human brain using FLASH: influence of various imaging parameters

In this study the influence of a large variety of imaging parameters on the signal increase (DeltaS) and the contrast-to-noise ratio (CNR) of functional magnetic resonance imaging experiments was determined using FLASH imaging at 2 T. During visual stimulation of the brain we detected significant variations of DeltaS as a function of the echo time (30 ms: 3.5 +/- 0.4%, 60 ms: 6.8 +/- 0.7%), slice thickness (2.5 mm: 6.8 +/- 0.7%, 10.0 mm: 3.3 +/- 0.3%), and pixel size (4.69 mm: 3.1 +/- 0.3%, 1.88 mm: 5.9 +/- 0.5%). Significant changes of DeltaS with flip angle occurred for TE = 20 ms (15 degrees : 2.1 +/- 0.2%, 60 degrees : 3.2 +/- 0.5%). At TE = 30 ms there still was a slight increase (15 degrees : 3.0 +/- 0.4%, 60 degrees : 3.8 +/- 0.5%), while at TE = 50 ms no changes of DeltaS could be detected with flip angle. Furthermore, DeltaS decreased with the use of first-order flow and motion compensation (off: 5.8 +/- 0.6%, on: 4.5 +/- 0.5%). The purpose of this study was to identify the optimal imaging parameters for blood oxygenation level dependent contrast using FLASH imaging at 2 T. Relying on a time normalized contrast-to-noise ratio (CNR(n)) we found the following parameters to be optimal: TE approximately 40-50 ms, a rather low spatial resolution (slice thickness approximately 5.0-7.5 mm, pixel size approximately 2.3-4.6 mm, matrix size 64 x 48), and flip angles lower than 30 degrees. Flow compensation should not be applied, and a rather low bandwidth of approximately 2.5 kHz is favorable, as it yields a superior signal-to-noise ratio.