MRI Flashcards
(28 cards)
How does SAR change between a GE and a TSE sequence?
SAR increases with a TSE sequence due to repeated 180 RF pulses
How does the signal level change between a GE and a TSE sequence?
Signal increases with TSE due to multiple 180 RF pulses, each generating a signal within one TR period
How does the flip angle change between a GE and a TSE sequence?
Flip angle increases with TSE because the flip angle in a GE sequence is between 10-80 degrees, whereas the flip angle of a TSE sequence is 90 and 180 degrees
How does the scan time change between a GE and a TSE sequence?
Scan time increases with TSE due to the short TR and smaller flip angles of a GE sequence, but TSE is quicker than a SE
What is an advantage and a disadvantage of a superconducting magnet MRI scanner?
Ability to get high field strengths and able to turn off the magnet if required but need a lot of power and have cryogen hazards
What is an advantage and a disadvantage of a permanent magnet MRI scanner?
It has a lower cost due to not requiring power however they have a lower field strength and cannot turn it off in an emergency
What is an advantage and a disadvantage of a resistive magnet MRI scanner?
The field can be controlled but they are temperature sensitive
What is meant by T2 relaxation?
Dephasing in the xy (transverse) plane resulting in loss of transverse magnetisation due to spin-spin interactions
What is the gyromagnetic ratio of protons?
42.57 MHz/T
How is the tissue magnetisation affected from an 11.7 T scanner to a 1.5 T machine?
Less tissue magnetisation
How is the image uniformity affected from an 11.7 T scanner to a 1.5 T machine?
Less uniform
How is T1 affected from an 11.7 T scanner to a 1.5 T machine?
Longer T1
At what time is the optimum nulling of the white matter tissue?
Describe the effect the static magnetic field has on the spins of protons, and how it relates to the
signal in MRI?
When in an external magnetic field, the protons magnetic moment will either align or anti-align with the field, whilst precessing at the larmor frequency, with more aligned than anti-aligned. These magnetic moments combine to create a net magnetisation vector. This net magnetisation vector can then be used to acquire a signal by flipping it into the transverse plane and observing the signal produced as it recovers back to the longitudinal direction. When this happens the net magnetisation vector is precessing in the transverse plane producing a signal
How does the strength of the external field effect the signal strength?
Stronger B means a greater proportion of spins are aligned than anti-aligned causing a greater net magnetisation vector because it takes more energy to oppositely align meaning they tend to align.
What kind of relaxation causes the shape of the signal due to a single 90 degree RF pulse?
T2* - dephasing of spins due to spin-spin interactions, as well as dephasing due to inhomogeneities in the external field
A spin echo sequence consists of a 90° RF pulse followed by a 180° RF pulse. Explain the purpose of the 180° pulse.
The 180 degree pulse flips/inverts the spins/positions so that they start to return to precess in phase again, resulting in an increase in signal. When you measure the signal, this signal will be much closer to the value you expect due to T2 relaxation (rather than T2*)
Give two reasons why is it advisable to have the TE as low as possible for a T1 weighted
sequence?
Want to minimise the contribution to contrast due to T2 in a T1 weighted image so you need to choose a TE time when the difference in Mxy between tissue is minimal, giving minimal contrast, it also minimises T2 or T2* but not allowing enough time for the spins to dephase due to spin-spin before taking a signal
Additionally a low TE gives a higher signal
Explain what is meant by T1 relaxation.
Recovery of the net magnetisation vector into the longitudinal plane (in the direction of the main magnetic field)
From darkest to brightest, order the following tissues as they appear on T1 weighted brain
images
* Fat
* Cerebro-spinal fluid (CSF)
* White matter
Briefly justify your ordering.
Fat - brightest - quickest T1 - spin lattice interactions due to long chains
White matter
CSF - closest to water - molecules - less interactions so slower T1 recovery
Which two properties of the sequence can be used to control the weighting of the resulting images?
TE and TR (and angle in GE)
Give typical values of for TE, TR and flip angle which could be used to produce a T2-weighted
image using a GE sequence.
TE: 30-80 ms (long)
TR: (long)
angle: 10-40 degrees
Give typical values of for TE, TR and flip angle which could be used to produce a T1-weighted
image using a GE sequence.
TE: 1-15 ms (short)
TR: short
angle: 50-80 degrees
Give typical values of for TE, TR and flip angle which could be used to produce a PD-weighted
image using a GE sequence.
TE: 1-15 ms (short)
TR: long
angle: 10-40 degrees
