Alternatively, for short TR T1-weighted sequences, such as fast 3-dimensional (3D) T1-weighted imaging, the flip angle can be reduced slightly to maintain similar T1-weighting as 1.
phasing) Reduced B0 homogeneity Add shim Reduced B1 homogeneity Consider 3D rather than 2D imaging Increased SAR Decrease flip angle Longer T1 (potential Increase TR increase in scan time) SNR = signal-to-noise ratio; BOLD = blood-oxygen-level-dependent; 3D = 3-dimensional; 2D = 2-dimensional; SAR = specific absorption rate; TR = repetition time.
20) In addition, B1 inhomogeneity and associated spatial flip angle variations have been reported to cause localized areas of reduced contrast, particularly in the medial right breast, (7) which reduce overall sensitivity of contrast-enhanced 2-dimensional imaging at 3T.
Manipulations traditionally used to limit SAR include reducing acquisition flip angle (eg, from 180[degrees] on fast spin-echo [FSE] and ~40[degrees] on gradient-recalled echo [GRE]), which could potentially affect image contrast.
Lowering the flip angle reduces SAR, and thus acquisition time.
For each patient, axial plane, unenhanced and enhanced T2-weighted FLAIR with flip angle 90, TR/TE range 9000/130-150 were obtained respectively.
T1-weighted spin echo: Axial plane, flip angle 90, TR/TE 450/10-20.
As the T1-relaxation time of intravascular contrast is shorter than that of a parenchymal organ, the 3-dimensional spoiled gradient-echo sequence would use a larger flip angle for MR angiography and a smaller flip angle for parenchymal organ imaging.
Spoiled gradient-echo sequences can be modified for a variety of applications by adjusting the flip angle.
0]) and the square of the flip angle ([alpha]) and is directly proportional to the duty cycle (D) and the volume of the patient imaged (V).
As SAR is proportional to the square of the flip angle, decreases in the flip angle utilized at 3T can significantly decrease energy deposition.
The nonselective RF pulse permits ultra-short echo spacing and the variable flip angle
results in a pseudo steady-state with maintained relative tissue contrast (Figure 5) as well as a reduction of blurring associated with high ETL.