MEDRADSC 3DE3 Lecture Notes - Lecture 5: Spin Echo, Spin–Lattice Relaxation, Transverse Plane
18 Jun 2018
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What are gradients used for: spatial encoding
Introduction
Pulse sequence: a timed sequence of events to produce a particular type of contrast
oSeries of RF pulses with intertwining periods of rest
oDefines what type of image we’re going to get
oDifferences between them are:
high contrast- has high signal to noise ratio (T2), good for pathology (increase in
vascular/fluid composition)
When we’re looking at t2 images- big contrast between the two vectors because
fat vector loses magnetization in the transverse plane much faster than our
water vector
Add positive gradient: causes spins to speed up
Pulse Sequence
T2* causes rapid loss of coherent transverse magnetization/signal before most tissues have time
to attain their T1 or T2 relaxation times no contrast
Signal needs to be regenerated to measure the relaxation times:
oApply 180 RF pulse (spin echo pulse sequence)
Causes the slower spins to speed up and the faster spins to slow down
rephasing the spins in order to hear the signal
oUsing gradients (gradient echo pulse sequence)
Spin Echo
find more resources at oneclass.com
find more resources at oneclass.com
oRF pulses are applied for excitation purposes and rephasing
oTR controls T1 weighting
oShort TR maximizes T1 weighting
oTE controls T2w or decay in the transverse plane
oLong TE maximizes T… w
o90 degree FA is used
oGold standard of imaging – high SNR (T1); pathology T2
oSE advantages
good image quality
versatile
true t2 w
oTime consuming (-)
Correction of T2* with an SE pulse sequence
1. 90` FA (flip angle) causes NMV to flip to transverse plane
o90-degree RF pulse NMV goes 90 degrees to B0
o180-degree RF pulse NMV goes antiparallel to B0
2. NMV precesses – which induces a signal (voltage) in the
receiver coil (according to Faraday’s Law)
3. 90` RF is removed and T2* dephasing occurs immediately,
generates our FID (immediate loss of signal)
4. 180` RF pulse is used to compensate for dephasing
5. 180` RF pulse has enough energy to move the NMV through 180` - which is now on the other
side but still in the transverse plane
6. Dephasing or relaxing spins have all come back into phase with each other
othe trailing edge begins to catch up with the leading edge
7. Maximum signal can now be induced in the receiver coil (has minimized T2*) signal called a
“spin echo”
oThe spin echo contains both T1 and T2 information, T2* is minimized – more time has
been allowed for tissues to reach their T1 and T2 relaxation times
find more resources at oneclass.com
find more resources at oneclass.com
Spin Echo: Timing Parameters
TR controls T1 weighting (short TR= max t1 weighting, long TR
Tau time: time it takes to rephase after the 180 RF pulse is applied, is half the TE time
otime taken to rephase after the application of the 180 ° RF pulse = the time to dephase
when the 90° RF pulse was withdrawn
Using ONE echo:
oproduces T1 weighted images
Anything that affects T1 of tissue will change its appearance on T1 images
short TR: not allowing recovery fat and water vectors will be far apart
short TE (180 ° RF pulse and subsequent echo occur early, so that only a litt le
T2 decay has occurred)
excellent contrast: fluids dark, fat bright, water-based tissues grey
oOne 90 and then one 180 RF pulse
Using TWO echoes:
oProduces PD and T2 weighted images in the same TR time
1. PD weighted image: first spin echo generated using short TE
1.i. Only a little T2 decay occurs- minimal T2 differences in this echo
1.ii. Long TR: minimize t1/amount of recovery in longitudinal plane/minimize the
contrast between the two vectors
1.ii.1. Need to allow full T1 relaxation time
1.ii.2. little contrast difference in water-based tissues, fluids slightly
darker, fat is bright
1.iii. less contrast than T1 or T2
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
When we"re looking at t2 images- big contrast between the two vectors because fat vector loses magnetization in the transverse plane much faster than our water vector. Add positive gradient: causes spins to speed up. T2* causes rapid loss of coherent transverse magnetization/signal before most tissues have time to attain their t1 or t2 relaxation times no contrast. Signal needs to be regenerated to measure the relaxation times: apply 180 rf pulse (spin echo pulse sequence) Causes the slower spins to speed up and the faster spins to slow down rephasing the spins in order to hear the signal: using gradients (gradient echo pulse sequence) Correction of t2* with an se pulse sequence. Spin echo : the spin echo contains both t1 and t2 information, t2* is minimized more time has been allowed for tissues to reach their t1 and t2 relaxation times. Tr controls t1 weighting (short tr= max t1 weighting, long tr.
