WHAT IS A FLIP ANGLE (FA)?
by Joanne Santos
By definition a FLIP ANGLE: is the rotation of the Net Magnetic Vector or NMV by a radiofrequency relative to the main magnetic field (Bo).
So what does that mean?
Hydrogen protons (or the nucleus) align with the main magnetic field (Bo) also known as longitudinal magnetization. In order to acquire a signal from the body’s tissue the protons must be stimulated; a RF pulse (SE), or a gradient pulse (GRE) is used to excite the protons and move them toward the transverse plane or xy plane. A NMV is used to measure the angle at which the protons are ‘flipped’. This angle along with TR and TE will determine the contrast or weighting that differentiates one image from another i.e. PD, T1, T2, and T2*.
Spin Echo Imaging uses a 90° flip angle, with a 180° refocusing pulse applied by using a radiofrequency or RF pulse. TR’s are much longer thus making SE sequences longer. Traditional SE sequences rely on TR, TE and the amplitude of 90° to make one or more echoes to create images.
PD WEIGHTING: T1 WEIGHTING: T2 WEIGHTING:
TR- Long TR- Short TR- Long
TE- Short TE- Short TE- Long
Due to the application of greater amplitude RF pulses in SE sequences the body can warm to a point that would exceed the body’s core temperature, this is measured by Specific Absorption Rate (SAR). Reducing the flip angle will keep the SAR down at appropriate FDA levels. However, reducing the flip angle may greatly affect image contrast.
Gradient Echo Imaging uses small flip angles of less than 90°. This means that the TR will be shorter which reduces scan time. That is why GRE imaging is faster than SE imaging.
PD WEIGHTING: T2* WEIGHTING:
FA – <40° FA- <40°
TR- Long TR, reduces T1 TR-Long TR, reduces T1
TE-Short TE, reduces T2*, increases PD and T1 TE- Long TE,enhances T2*
TR- Short TR, enhances T1
TE- Short TE, reduces T2*
VARIABLE FLIP ANGLES: Result in TOF imaging: The flip angle can be increased slowly during the data acquisition of an image. It starts by using a small flip angle that increases with each line of data acquisition.
WHAT IS INVERSION TIME (TI)?
By definition Inversion Time: The time period between the initial 180° inversion pulse and the 90° excitation pulse in an IR pulse sequence
An Inversion Recovery Pulse Sequence acquires different contrast or weighting by adjusting the following parameters: TR, TE, and TI (inversion time). The TR has to be long to allow for full longitudinal recovery before the next pulse. It can yield strong contrast between tissues that vary in T1 relaxation times. An IR pulse sequence can also suppress fluid or fat. Inversion times can also be measured as long or short. A long TI is identified as 1700-2200ms, and a short TI is 100-180ms. STIR sequences null the fat signal, and FLAIR sequences null the fluid signal.
T1 Weighting: STIR: FLAIR:
TR- Long TR- Long TR- Long
TE- Short TE- Long TE- Long
TI- Medium (300-700ms) TI- Short TI- Long
Lawrence N.T. MD, FACR (2005). Edison Imaging-JFK Medical Center, New Jersey Neuroscience Institute, Edison, NJ; Seton Hall School of Graduate Medical Education, South Orange, NJ. Retrieved from: http://www.medscape.com/viewarticle/498295_2
McRobbie, D.W., et al (2007). MRI from picture to proton 2nd ed. Cambridge University Press.
Westbrook, C. (2008) Handbook of MRI technique. 3rd ed. Wiley-blackwell Publishing, U.K.