MRA can be prone to artifacts
Phase Contrast MRA
Makes use of phase differences caused by motion to determine signal.
User must choose VENC (velocity encoding) to determine sensitivity of the scan to a range of flow. This is the maximum velocity that will register properly. If the VENC is too large, flow may not register correctly (a form of aliasing). If the VENC is too small say 30 cm/sec, flow of 31 or 61 cm/sec may show up as flow of 1 cm/sec. Ideally VENC should be set slightly higher than the highest velocity you would expect to encounter in the patient.
There are cases where phase contrast MRA missed an aneurysm that TOF MRA clearly shows. The speaker noted I know what the phase contrast advocates are thinking, "What VENC did you use?, The answer is "The wrong VENC. Each run with a different VENC take about 15 minutes to acquire, so its important to start out with the right VENC.
Explain MPR vs. MIP vs. Shaded Surface Display.
MPR is a reformatted slice prone to the same issues as routine axial slice, such as partial volume averaging
MIP refers to Maximum Intensity Projection. see Radiographics reference below
Shaded Surface Display: A surface is defined by a threshold density. This surface is illuminated with a virtual light source and the resulting image is created.Â
Q: What are the key technical factors for creating a batch of MIP images?
A: Interval and slab thickness.
Q: What is the key technical factor for creating a Shaded Surface Display rendering?
A: Threshold
Reference:
Radiographics