Image formation 2: Basic Pulse Sequence Flashcards
What is it difficult to reconstruct?
Artefact free images from projections
What is technically difficult to measure?
MR signal immediately after a 90 degree pulse
- can be overcome by collecting a spine echo (SE)
What is pulse sequence?
Succession of commands that the computer gives the scanner to obtain desired signal, hence image
What is spin echo?
Regeneration of spin phase information apparently lost during decay of FID
– Insensitive to magnetic field B0 inhomogeneity, but longer scan times
What is Gradient echo?
- Applying external dephasing gradient field across the specimen or tissue
- calibrated change in local magnetic field and hence alter resonance frequencies slightly across the specimen
- Accelerated dephasing of FID
What is the first step in the formation of a spin echo?
90˚ pulse is applied along +x’ which rotates the longitudinal magnetization MS into the transverse plane along y’ axis
What is the second step in the formation of a spin echo?
180˚ RF pulse is applied again along the postive x’ axis
-This reverses the direction of the magnetisation so now it is along the -y’ axis
What is the third step in the formation of a spin echo?
The spins continue to move in the same direction as they were travelling before the 180˚ pulse
What is the fourth step in the formation of a spin echo?
There is a time point (the ‘spin echo’) where the isochromats rephase and the magnetisation of the entire spin ensemble reaches a maximum again
What is the fifth step in the formation of a spin echo?
The centre of the spin echo (where the signal will reach a maximum) occurs at a time referred to as the “echo time” or TE of the sequence – The sampling or acquisition window will be centred around this time point • Finally the isochromats begin to dephase again
What happens after the spin . echo has formed?
The magnetisation will continue to precess and therefore dephase
What is the basis of spin echo Imaging?
Combining the effect of a field gradient (to localise the signal) with the concept of refocusing transverse magnetisation
What is Spin Echo?
Add another pulse to generate an ‘‘echo’’ that occurs later in time and happens to reverse the T2* effects to leave us with T2 only
Why do we rotate the protons with a 180-degree pulse?
They continue precessing in the same direction but will refocus along the -y axis
What is the spin dephasing effect due to?
Dispersion in the resonance frequency produced
What is the rephase lobe on the gradient needed?
Counteract the dephasing of spins during the application of pulse
What is the k-space sampling?
- Apply a 90 degree RF pulse (produces an FID)
- Apply large, negative phase encode gradient along y direction
- Apply a positive dephase gradient along the x axis
- At a time TE/2 after the 90 degree pulse w eapply a 180 degree pulse
- Apply a readout gradient and start measuring the echo signal
What will produce different degrees of contrast between tissues?
Different combinations of TR and TE