MRI Pulse Sequence Flashcards
the combination of the characteristics and timing of the radio pulses and magnetic field gradients
pulse sequence
Each sequence will have a number of ____
parameters
Multiple sequences are grouped together into an
MRI protocol
Extrinsic contrast parameters
Time to Repetition/Repetition Time (TR)
Time to Echo/Echo Time (TE)
Flip Angle
time from the application of an excitation pulse to the application of the next pulse
repetition time (TR)
It determines how much longitudinal magnetization recovers between each pulse
repetition time (TR)
Repetition time (TR) is measured in
milliseconds
time between the application of radiofrequency excitation pulse and the peak of the signal induced in the coil
Echo time (TE)
Echo time (TE) is measured in
milliseconds
Time of signal acquisition
Echo time (TE)
Pulses are characterized by their
flip angles
Intrinsic contrast mechanisms
T1 recovery T2 decay Proton density flow apparent diffusion coefficient (ADC)
Relaxation process could:
Transversal relaxation
Longitudinal relaxation
Time that it takes for the longitudinal magnetization to recover, to go back to its original value, is described by
longitudinal relaxation time / T1 Recovery or Spin-Lattice relaxation
Nuclei/Proton lose precessional coherence or dephase and the NMV decays in the transverse plane, this dephasing relaxation process is called
Transverse Relaxation times or T2 decay or spin-spin energy transfer
It is caused by the exchange of energy nuclei to their surrounding environment or lattice
T1 recovery
T1 time of fat is
very short
Fat is able to absorb energy quickly, therefore it is
efficient to absorb energy
T1 time of water is
quite long
Water is
inefficient at receiving energy from nuclei
Do not permit full longitudinal recovery in either fat or water so that there are different longitudinal components in fat and water
Short TRs
As the NMV does not recover completely to the positive longitudinal axis, they are pushed beyond the transverse plane by the succeeding 90 degrees RF pulse
Saturation
Allow full recovery of the longitudinal components in fat and water
Long TRs
It is caused by the exchange of energy from one nucleus to another
T2 decay
occurs as a result of the spins of adjacent nuclei interacting with each other and exchanging energy
T2 decay in fat and water
Fat’s T2 time
very short compared with that of water
Do not permit full dephasing in either fat or water so their transverse components are similar
Short TEs
Allow dephasing of the transverse components in fat and water
Long TE
Water/Liquids have
a long T1 and long T2
It is one where the contrast depends predominantly on the differences in the T1 times between tissues
T1 weighted image
To achieve T1 weighting, the TR must be ______ so that neither fat nor water has sufficient time to fully return to B0
short enough
Tissue with short T1 relaxation times such as fat, are
bright (high signal)
Tissue with long T1 relaxation times such as water, are
dark (low signal
Typical parameters for T1 weighting
TR 300-600 ms
TE 10-30 ms
It is one where the contrast depends predominantly on the differences in the T2 times between tissues
T2 weighting
controls the amount of T2 decay that is allowed to occur before the signal is received
TE
To achieve T2 weighting, the TE must be ______ to give both fat and water time to decay
long enough
Tissue with short T2 decay times such as fat, are
dark (low signal)
Tissue with long T2 decay times such as water, are
bright (high signal)
Typical parameters for T2 weighting
TR 2000 ms +
TE 70 ms +
differences in the proton densities must be demonstrated
proton density image
Proton density
Long TR, Short TE
The heart of MRI
Pulse sequence mechanisms
They are the computer software executing a series of commands to apply an rf pulses, gradients, data sampling windows
Pulse sequence mechanisms
Enable to control of the way in which the system applies RF pulses and gradients
Pulse sequence mechanisms
Main purposes of pulse sequences
to rephase spins and remove inhomogeneity effects …
to enable manipulation of the TE and TR
Spins are rephased in two ways
by using 180 RF pulse
using a gradient
are used to produce T1, T2 or proton density weighted images and are one of the most basic pulse sequences used in MRI
Conventional spin echo pulse sequence (SE)
