Time of flight
MRI
- General
- Assesses flow within a vessel
- Saturates signal from stationary tissues.
- Blood flow within arteries is fast flowing.
- Moving spins have a large magnitude of magnetisation.
- Advantages
- MRI TOF used for follow up treated aneurysms (devices with minimal metal artefact).
- Provides a good estimation of residual aneurysm filling.
- No ionising radiation.
- Does not require additional contrast material.
- Limitations
- Slow flow would in signal drop-out.
- Relatively long acquisition.
- Reduced sensitivity for identifying AVMs/dAVFs compared with time resolved MRI and DSA.
- Poor specificity in assessing for obliteration of AVM post treatment.
MRA
- TOF MRA is based on the principle of flow- related enhancement, which occurs when unsaturated blood enters a region whose background is suppressed by multiple RF pulses
- 2D and 3D acquisition modes possible
- Mechanism
- Stationary tissues in an imaged volume become magnetically saturated by multiple repetitive RF-pulses that drive down their steady-state magnetization levels. "Fresh" blood flowing into the imaged volume has not experienced these pulses and thus has a high initial magnetization. The signal from inflowing blood thus appears paradoxically bright compared to background tissue.
- TOF angiography method is based around a conventional 2D or 3D gradient echo (GRE) sequence with optional gradient-moment nulling.
- Is by far the most frequently used due to its superior spatial resolution, higher sensitivity to slow flow, and lower acquisition times.
Time resolved MRA techniques
- Time Resolved Imaging of Contrast KineticS (TRICKS) and Time-resolved angiography With Stochastic Trajectories (TWIST) are time-resolved MRA techniques using view-sharing and under sampling of the k-space periphery
- Allows rapid acquisition of multiple images during passage of the contrast bolus
- 20 images/sec
- Trade-off exists between spatial and temporal resolution
- Pros
- Time-resolved MRA sequences are widely used wherever circulation is rapid (carotids, cardio-pulmonary system) or unpredictable (extremities).
- The method is particularly useful for evaluating collateral or retrograde flow around stenoses and in the work up of arteriovenous malformations.
- Accurate timing of bolus arrival is not required;
- The technologist simply starts the sequence and runs it until the contrast has passed through the vascular system.
- Much smaller doses of contrast can also be used than with conventional CE-MRA.
- Fluoroscopic triggering methods used in conventional CE-MRA are low-resolution, 2D versions of these time-resolved sequences
Phase contrast MRA
- Can be used to measure
- CSF flow: aqueductal stenosis
- Blood flow
- Method
- Bipolar Gradients
- PC-MRA uses bipolar gradients, which are pairs of gradients with equal magnitude but opposite direction12. These gradients encode the velocity of moving blood into the phase of the MRI signal
- Phase Shift
- Stationary spins (protons) experience no net phase shift after the gradients are applied, while moving spins (such as those in blood) experience a phase shift proportional to their velocity
- Stationary objects will have no net phase shift
- Moving objects will have a net phase shift
- Two spins flowing at the same speed but in opposite directions will have equal but opposite phase shifts.
- By measuring changes in phase, therefore, velocity can be computed.
- Velocity Encoding
- By applying these gradients along different directions (x, y, and z), the velocity components of blood flow can be measured
- Image Creation
- The phase information is used to create images that highlight blood vessels and flow patterns
- PC-MRA is particularly useful for evaluating blood flow in arteries and veins, detecting abnormalities such as
- Stenosis (narrowing),
- Occlusions (blockages),
- Aneurysms (vessel wall dilatations)
- An axis (X, Y or Z) is supplied with a pair of bipolar gradient
Contrast-enhanced MRA
- T1 weighted
- Manually triggered, automated (Smart-prep) or fixed time delay.
- Higher chance of venous contamination than other imaging modalities.
- Quicker than TOF.
- Poor spatial resolution – not usually first line for diagnosing aneurysms,
- Useful for follow-up of treated aneurysms – specifically stent/FD assisted
- Gadolinium…
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