Chapter 13 Common MR Pulse Sequences Flashcards
A pulse sequence can be defined as:
a series of pulse with periods of time in between.
There are 2 major types of pulse sequences:
Spin Echo and Gradient Echo
the spin echo category includes:
conventional spin echo
fast spin echo and inversion recovery sequences - STIR & FLAIR
An Inversion Recovery (IR) sequence is basically a:
spin echo sequence except it begins with a 180 degree inversion pulse
What is Time of Inversion (TI)?
(in an IR sequence)
the time between the initializing 180 degree pulse and the 90 degree pulse
the time of inversion significantly affects MR Image contrast
following a 180 degree inversion pulse, the net magnetization will recover along the longitudinal axis, if one selects an inversion time that is 69% of a tissue’s T1 relaxation time, then at the time of the 90degree pulse application, that tissue would be crossing over the 0 longitudinal point and will in effect be nulled (No signal)
As one changes the TI, one changes the
tissue which is nulled
there are 2 ways in which IR sequences are used clinically:
nulling fat or nulling fluid (CSF)
If a short inversion time (TI) is selected, we can null the signal from:
FAT
what is STIR?
Short Tau Inversion Recovery
nulling fat from signal
the exact TI depends on the
field strength
the appropriate TI to null fat will _____as field strength ______.
increases;increases
TI depends on field strength
what is FLAIR?
Fluid Attenuated Inversion Recovery
nulling CSF from signal
a FLAIR sequence which utilizes a long TR and long TE is referred to as a
T2 FLAIR
the pathologic fluid remains bright
what sequence is MS well visualized on
T2 FLAIR
If one selects a more moderate TR and a short TE (along with TI to null CSF), one can obtain:
T1 weighted images with excellent gray/white matter contrast
important points to remember regarding IR sequences:
- selecting a T1 that is 69% of a tissues T1 relaxation time will null the signal from that tissue.
- the TI will vary not only with tissue but field strength.
- if nulling fat, the sequence is referred to as STIR
- if nulling CSF, the sequence is referred to as FLAIR
the main types of gradient echo sequences are :
steady state
spoiled
balanced gradient echos
echo planar imaging EPI
basic contrast properties unique to gradient echo sequences are:
following an RF pulse, transverse magnetization decays due to several factors:
++ T2 relaxation - this rate varies with tissue’s fat and/or water content
++ T2* - a more rapid decay due to the addition of off resonance effects (T2 prime) which are inhomogeneities in the field and chemical shift
gradient echo sequences do not utilize a
180 degree refocusing pulse prior to the formation of the echo so the transverse magnetization decays at a rate based on T2*
gradient spin echo sequences are _____to inhomogenetities
gradient echo sequences are _____to spin echo sequences with regards to sensitivity to small areas of hemorrhage, particularly in the brain
SUPERIOR
gradient spin echo sequences are _____to inhomogeneities
in a gradient echo sequences there is no
180 degree refocusing pulse prior to the formation and sampling of the echo.
if a voxel contains both water and fat protons, depending on the time at which the signal is sampled, the water and fat protons may either be in phase or out of phase
the ______ the flip angle, the ____the TR require for complete longitudinal recovery to occur.
higher;longer
T or F
the flip angle in addition to TR is a parameter used to control the amount of T1 weighting in a gradient echo sequence
true
_______a flip angle has the same contrast effect on the image as raising the TR
reducing (or lowering)
both would result in reduced saturation
for any tissue, based on it’s T1 relaxation time and the TR selected, there is a flip angle which result in maximum signal. that flip angle is known as the:
Ernst Angle
when the TR is shorter than the T2 (& T2*) of most tissues, after several pulses, longitudinal and transverse magnetization _____. This condition is known as________.
coexist; steady state
