Differences between x-ray, CT and MR angiography:
|Image signal strength reflects intraluminal density of contrast material - contrast required||Image signal strength due to intrinsic properties of the blood and the flow - no contrast required.
The contrast agent itself is not imaged but the effect it has on the surrounding water
|Selective study - contrast injected into specific vessels||Contrast injected intravenously and phase images acquired determines vessels imaged||Non-selective|
|2D data set||Can obtain 3D data-set|
It is first useful to go through some of the flow artefacts created during MR imaging as these are exploited when performing MR angiography.
Time of flight effects
Time of flight effects include:
- Flow-related enhancement
- High velocity signal loss or "washout"
Occurs in gradient echo imaging and is a result of magnetic saturation
GE involves exposing the tissue to multiple short TRs. Stationary tissue will be subjected to many TRs, reducing the transverse magnetisation. However, fresh blood flowing into the imaging slice will not have been exposed to these TRs and will have a lot more transverse magnetisation than the surrounding tissue and will return a larger signal.
Occurs in spin-echo imaging.
The signal from spin echo depends on the tissue receiving both a 90° and 180° RF pulse to generate the echo. If tissue in a slab exposed to the 90° pulse then moves out of the slab it will not receive the 180° RF pulse to be able to generate an echo. Similarly, tissue moving into the slab exposed to the 180° RF pulse will not have been exposed to the 90 ° RF pulse beforehand.
The time between the 90 and 180° RF pulses is TE/2. Therefore, if the tissue moves faster than TE/2 all the material exposed to the initial 90° pulse will not be exposed to the 180° pulse. If, however, it moves slightly slower than TE/2, then some material will be exposed to both and generate a small signal, depending upon how much material remains.
Spin phase effects
Remember from phase encoding that the gradient applied affects the phase of the precessing protons and magnetic gradients are used to use this effect to localise the location of a signal. A proton that is moving along a gradient will alter its phase according the length of time the gradient is applied, the magnitude of the gradient and the velocity of the proton. We know the time and magnitude of the gradient and so we can calculate the velocity of a proton.
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MR angiogram techniques
The types of MR angiograms can be broadly separated out into two types: dark blood and bright blood. The bright blood techniques are then further subdivided according to whether they use gadolinium or not.
The main ones used are time-of-flight, phase contrast and contrast-enhanced techniques.
Time of flight (TOF)
This is a gradient echo (GE) sequence that uses flow-related enhancement. It has a short repetition time (TR) to ensure that all stationary spins will have their signal saturated out. Only spins that then move into the imaging field, that have not experienced the saturating RF pulses, will yield a high signal. It can either be a 2D or 3D study.
Pre-saturating bands are used to reduce the signal from blood flowing into the imaging field from a certain direction e.g. apply it distal to the imaging field to saturate out returning venous flow but ensure high signal from outgoing arterial flow.
- Can be used for venous (2D, good for low velocities) or arterial imaging (3D, good for high velocities)
- Very sensitive to flow
- Saturates out all background signal
- 3D TOF is very high resolution (1mm)
- Flow voids due to:
- In-plane saturation
- Post-stenotic turbulence distal to the stenosis
- Slow flow
- Can exaggerate the length of occlusion and stenosis
- Long imaging time (minutes to hours)
- Sensitive to metal artefact
- Very high T1 signal from stationary nuclei will be visible (e.g. haemorrhage)
- Retrograde arterial flow may be obscured if venous saturation bands have been applied
Phase contrast (PC)
Exploits differences in transverse magnetisation i.e. spin phase
- Contrast agent not used
- Can reconstruct the data in any plane as usually acquired using 3D method
- Good background suppression
- Insensitive to T1 effects
- Can control the velocity dependent phase shift to alter sensitivity to different flow velocities
- Velocity can be quantified as well as the direction unlike ToF MRA which is just bright or not
- Takes 4x as long as TOF as image required in three orthogonal directions to create image
- No in-plane flow voids
- More sensitivity to turbulence
Contrast enhanced (CE)
Uses Gadolinium Chelate agents (cause shortening of the T1 relaxation of blood compared with background tissue leading to a high signal intensity of blood on T1-weighted sequences). The area of interest is imaged in the first pass of the contrast to ensure the best signal.
- More accurate
- Faster scan so can image at different phases e.g. pre-contrast, arterial, venous
- Fewer flow-related artefacts
- Not flow-sensitive