Chapter 4: Gradient echo Flashcards
Gradient echo pulse sequences
- uses variable RF excitation pulse flip angles less than 90 degrees
- uses gradients to rephase magnetic moments to form echoes
Purpose of the gradient echoes
shorter TRs which means shorter scan times
less than 90 degrees
equals smaller flip angle
- easier to achieve full relaxation
rephasing/ rewinding
FID occurs due to the inhomogeneities in the magnetic field (T2* decay)
dephasing/spoliing
gradient is applied and some magnetic moments slow down or speed up
incoherent
out of phase
gradient echoes are created by
bipolar gradient
- one positive and one negative
frequency encoding gradient
applied negatively
- increases dephasing and eliminates FID
factors that affect weighting of gradient echoes
- extrinsic parameters (TR,TE. PD)
- steady state
residual transverse magnetization
T1 contrast maximized
flip angle + TR causes saturation
T1 contrast minimized
flip angle + TR does not cause saturation
steady state
TR is shorter than T1 and T2 relaxation of tissue
coherent/ rewound
creates T2* weighted images in a very short amount of time
- reduces flow artifacts
coherent/ rewound parameters
F/A= 30-40 degrees
TR- 20-50ms
Long TE T2* - 10-15ms
incoherent/ spoiled
2 spoiling methods
- radio frequency spoiling
- gradient spoiling
radio frequency spoiling
Rf pulse transmitted at a certain frequency and specific phase
incoherent/ spoiled parameters
F/A- 30-45 degrees
TR- 20-50ms
Short TE (T1) - 5-10ms
reverse echo
used in brain and joint studies with both 2D and 3D volumetric acquisitions
- replaced TSE
Balanced gradient
uses alternating RF pulses with alternating excitation pulses to prevent saturation and maintain steady state
- used in cardiac, great vessel, and spinal imaging
fast gradient echo
multiple images acquired in a single breath hold and permits dynamic imaging of lesions and
echo plannar imaging (EPI)
rapid acquisition that begins the sequence of one or more RF pulses followed by series of gradient echoes
T1 weighed gradient echo
F/A- large
TR- short
TE- long
T2* weighted gradient echo
F/A- small
TR- long
TE- short
PD weighted gradient echo
F/A- small
TR- long
TE- short
Ernest angle
flip angle the highest signal intensity in a tissue with a given T1 recovery time in a given TE
GRASE
gradient echo and spin echo
