Flip Angle and Inversion Time

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.

SMALL FLIP ANGLES:  Result in PD and T2* weighting, by reducing T1 weighting     

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*

 LARGE FLIP ANGLES:  Result in T1 imaging, by enhancing T1 weighting          

T1 WEIGHTING:

FA-          >50°                                                                                                       

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

  

REFERENCES

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.

http://fonar.com/glossary.htm

http://www.mr-tip.com/serv1.php?type=ldir

http://www.wikidoc.org/index.php/Basic_MRI_Physics

http://en.wikibooks.org/wiki/Basic_Physics_of_Nuclear_Medicine/MRI_%26_Nuclear_Medicine

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