The protocol included an axial T1-weighted turbo spin echo (TSE) sequence with fat suppression in the transverse plane with the following parameters: repetition time, TR = 625 ms, echo time, TE = 12 ms, flip angle, [alpha] = 150[degrees], and voxel size, 0.
These maps were used to calculate the dynamic change in relaxation rate from the dynamic scan using a flip angle method proposed by Li et al.
The Ernest angle (5) is the flip angle in spoiled gradient-echo imaging where signal intensity is maximized:
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.
It is apparent that the flip angle
, important as it is, it does not directly show deviation from uniformity.
Increasing the flip angle
also results in better suppression of stationary tissue.
For each patient, axial plane, unenhanced and enhanced T2-weighted FLAIR with flip angle
90, TR/TE range 9000/130-150 were obtained respectively.
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).
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.
7) This effect, known as the dielectric effect, causes the flip angle
and signal measured in the breast to be nonuniform (Figure 2).
The reduction from a flip angle
of 180[degrees] (to typically approximately 120[degrees]) has been well tolerated thus far without any noticeable deleterious effect on image contrast.
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.