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
What will changing TR change the contrast between?
Tissues with different T1 relaxation times
What will changing TE change the contrast between?
Tissues with different T2 values
What does T1 weighting have?
- Short TE
- Short TR
- PD weighting
What does T2 have?
- Long TE
- long TR
- PD weighting aswell
What generally produces no signal?
- Long TE
2. Short TR
What does fluids have?
- Long T1
2. Long T2
What does water-vased tissues have?
- Mid-range T1
2. Short T2
What does Fat-based tissue have?
- Short T1
2. Short T2
What happens in PD-weighted images?
Tissues with highest PD are brighter
What happens in T2-weighted images?
Tissues with longest T2 are brighter
-Takes longer for transverse magnetisation to decay
What happens in T1-weighted imageS?
Tissues with longest T1 are darker
-Takes longer for longitudinal magnetisation to recover
What are the spin echo tissue signal intensity?
• Short TE - Grey matter is white - White matter is darker - CSF bright - The contrast is from PD • Longer TE - GM still white - WM is still darker - Contrast greater than short TE - CSF is very bright - The contrast is from PD and T2
• A very long TE
- WM and GM are black
- Only CSF remains
What is Dual echo sequence?
Include images with different weightings and/or echo times and are used to obtain both proton density and T2 weighted images without increasing the measurement time
What do different compartment in tissues have?
Different relaxation characteristics
What can be detected by fitting the transverse magnetisation decay curve (TMDC)?
Multi-exponential relaxation
How is a gradient echo formed?
First dephasing and subsequently rephasing the FID so that all isochromat magnetisation vectors are back in phase with one another at the centre of gradient echo
Where does the gradient echo occur at?
the point in time where the net phase is zero, i.e. the amount of dephasing is equal to the amount of rephasing – Signal reaches a maximum at the gradient echo
What is gradient echo signal?
T2* dependent
There is no 180 degree pulse to refocus the dephasing of isochromat due to B0 field inhomogeneities
What is the first step in the formation of a gradient echo?
90° pulse is applied and isochromats are tipped into the transverse plane
What is the second step in the formation of a gradient echo?
Dephasing gradient is applied and the isochromat dephase as a result of this grafient
What is the third step in the formation of a gradient echo?
Equal and opposite gradient is applied (a rephasing gradient)
-isochromat phases are reversed and a gradient echo is formed at the point in time where the amounts of rephasing and dephasing by gradients are exactly equal
What does T2* describe?
Total transverse relaxation due to both spin-spin interaction and B0 field inhomogeneities
What does magnet homogeneity directly relate to?
Image quality and various artefacts
What are the types of problem associated with poor magnet homogeneity?
- Shading
- Spatial distortion
- Blurring
- Intensity loss
- Curved slice profiles
- Zebra banding artefacts
What is inversion recovery pulse sequence?
spin echo pulse sequence preceded by a 180° RF pulse
What does the Inversion recovery pulse sequence do?
inverts longitudinal magnetisation (Mz), namely, it flips Mz to its negative value, -Mz.Tissues regain Mz at different longitudinal (T1) relaxation rates determined by their T1 relaxation times
What are the Inversion recovery contrast?
- Short T1
- GM is white
- WM is darker
- contrast: T1 and PD - Longer T1
- WM is white
- GM is darker - Longer still T1
- WM and GM are iso-intense - Very long T1
- GM is white
- WM darker
- contrast: mainly due to PD
