How does the flip angle affect MRI?
The flip angle is an MRI phenomenon by which the axis of the hydrogen proton shifts from its longitudinal plane (static magnetic field B0) Z axis to its transverse plane XY axis by excitation with the help of radiofrequency (RF) pulses.
How does flip angle affect contrast?
Relatively low flip-angle values produce PD contrast. As the flip angle is reduced within this region, there is a significant decrease in magnetization and the resulting signal intensity.
Which flip angle is commonly used in MR imaging?
Advances in magnetic resonance imaging (MRI) Larger flip angles are used to produce predominantly T1WI and smaller flip angles for predominantly T2WI. Images produced are susceptible to magnetic field inhomogeneities due to lack of the 180 degrees refocusing pulse leading to signal loss.
How does flip angle affect SAR?
Since SAR is proportional to the square of the flip angle, sequences with the highest SAR levels tend to be those containing a large number of 180º refocusing-pulses applied over a short period of time.
What is a flip angle in MRI?
What affects MRI SAR?
SAR is proportional to the square of the RF frequency (ω). So, all other things being equal, SAR depends on the square of Bo. The same pulse sequence in the same patient imaged at 1.5T would therefore generate 4x the SAR if performed at 3.0T.
How does flip angle affect scan time?
A flip angle lower than 90° (partial flip angle) decreases the amount of magnetization tipped into the transverse plane. The consequence of a low-flip angle excitation is a faster recovery of longitudinal magnetization that allows shorter TR/TE and decreases scan time.
Does flip angle affect SNR?
Other parameters affecting the SNR are the sequence used, echo time (TE), repetition time (TR), and the flip angle. The SNR increases with the TR but the T1 effect is also lost at longer TRs. Conversely, the SNR decreases as the TE increases.
How can I lower my MRI SAR?
Precautions to reduce the SAR to patients can include:
- taking breaks between high SAR sequences.
- alternating between low SAR and high SAR sequences.
- reducing the flip angle.
- reducing slice numbers.
- reducing pulse number and duration.
- reducing pulse frequency.
- ensuring the patient is lightly dressed.
What factors affect flip angle?
It is affected significantly by the radiofrequency pulse shape used (e.g., Gaussian, sinc, truncated-sinc), as well as slice-selection gradients, off-resonance excitations, and Bo field inhomogeneities. These effects become particularly problematic as field strengths are increased to 3.0T and above.
Why do you use flip angle in MRI?
T2 contrast requires us to eliminate T1 contrast from our image. Typically we wait a long period of time for all our tissues to recover completely (long TR). The use of a flip angle allows us to produce this affect quicker. This can be done by using a short flip angle.
What is the flip angle of a RF pulse?
Flip Angle. In a situation where we are using a short TR and a 90 degree RF pulse, we will be reducing the amount of signal seen tissues with longer relaxation times. By using a flip angle (a RF pulse less than 90 degrees) we will be able to tilt hydrogen part way to the transverse plane and signal can be preserved.
Why do we use a 90 degree flip angle?
In a situation where we are using a short TR and a 90 degree RF pulse, we will be reducing the amount of signal seen tissues with longer relaxation times. By using a flip angle (a RF pulse less than 90 degrees) we will be able to tilt hydrogen part way to the transverse plane and signal can be preserved.
What happens to MR signal when flip angle is reduced?
Stimulated echoes from these stored longitudinal components make progressively larger contributions to the MR signal, which now depends on both T1 and T2. It is these stimulated echoes, acting along multiple extra coherence pathways, that allow a higher than expected signal to be obtained when flip angle is reduced.
