Flashcards in Module 6 Deck (31)
What are the two purposes of a RF pulse ?
1. Rephasing in spin echo sequences
2. Excitation purposes
What are the main types of pulse sequences ?
1. Spin Echo
2. Gradient Echo
What is the main difference between spin echo and gradient echo?
The spin echo is rephased with a 180 rephasing pulse and gradient echo rephases with a gradient
How would T1 contrast be generated with conventional spin echo with one echo?
- one echo
- short TE
What types of contrast are generated with two echos?
T2 or Proton Density
True or False - T1 images are the best images for showing pathology.
False - T1 images are best for showing anatomy
Which type of contrast has higher SNR?
T1 has higher SNR but lower CNR
In fast or turbo spin echo what does the number of pulses performed per TR correspond to ?
- number of echos produced
- number of lines of k space filled
True or False : if the echo train is too long the pulses will get smaller and eventually run out of signal
What is the effective TE?
This is what is used to weight the image even though the data has different weightings. Phase encoding slopes will be applied to the data to achieve this weighting
If there is a steep phase encoding gradient how does that affect the amplitude of the resulting echo?
They reduce the amplitude of the resulting echo
How are shallow and steep slopes placed ?
Shallow slopes (max signal) - centre over effective TE
Steep slopes - Away from effective TE
What is the result of placing steep slopes away from the effective TE?
They fill the outer lines of k-space so they have more effect on resolution less effect on contrast
Why does FSE have limited use in the chest and abdomen?
Breathing artefacts are worse
Why is fat bright on T2 weighted FSE images ?
The extra RF pulses causes fat spin spin interaction effects to be reduced
True or False : The repeated 180 degree pulses causes magnetization transfer effects to increase and muscle to appear lighter
False it cause muscles to appear darker
How can blurring be reduced at the edges of tissues with different T2 values?
Reducing the echo train length
Which of the following is not an advantage of FSE:
a. Excellent image quality
b. Reduced artefacts
c. Increased T2 information
d. Can use multiple NEX and high resolution matrix
b. is not an advantage there is actually more flow and motion artefacts in FSE
How are the lines of k-space filled in Single Shot Fast Spin Echo ?
They are filled all at once
Why is there a possibility of not getting enough coverage in single shot fast spin echo?
High SAR because of the amount of 180 degree pulses which can cause heat to generate which can be dangerous
What does a reverse flip angle excitation pulse do? Why?
Moves remaining transverse magnetization into longitudinal plane
this allows it to be available for next TR
What is unique about the resultant image of Driven Equilibrium Fourier transform?
Signal intensity of water is increased
What is the correct sequence for inversion recovery?
180 degree inversion pulse ---> inversion time ----->90 degree excitation pulse ----> FID is rephased by 180 degree pulse to produce a spin echo
What is the main factor for image contrast with inversion recovery ?
How is a T1 weighted image acquired ?
90 degree pulse applied after NMV has relaxed through transverse plane
Why does the 180 degree inverting pulse cause large differences in contrast ?
They recover from full saturation rather than from transverse plane
More time for differences in T1 weighting to show up
What is done for pathology weighting?
Longer TE is used so that long T2 tissues are bright therefore pathology appears bright
What is a disadvantage of inversion recovery ?
Long scan times is a disadvantage
What are the two types of fast inversion recovery?
STIR and FLAIR
How is the TI chosen for a STIR sequence ?
It is chosen so it matches fat is fully recovered to the transverse plane this nulls the signal from fat