CT, MR angiography Flashcards
what is the CT angiography technique?
helial CT acquisition (multi detector spiral ct)
IV injection of iodinated contrast. timing important
what is post-processing of CTA
axial source data
isotropic voxels
multiplanar reformats - axial/coronal/sagittal, change thickness of images (if vessel want thin)
max intensity projections - find voxel with high attenuation or density and make them stick out in volume
3d volume rendering
curved planar reformats
advantages of CTA
less costly than MR rapid - limits motion artefact patient tolerance good resolution no flow related artefacts calcification well seen whole head an neck circulation
disadvantages of CTA
ionising radiation, iodinated contrast, streak artefacts, lower resolution, acquisition limit evaluation of flow
what are pitfalls of CTA
windowing - calcium blooming (on workstation can change contrast)
suboptimal opacification
vessel segment hidden by bone at skull base
too much post processing - loss of data due to over smoothing, cropping of small vessels by bone removal
what can windowing do
cause calcification around the pic
what are MRA techniques
time of flight
contrast enhanced (CEMRA)
phase contrast
asl
advantages of MRA
no ionising radiation
combined with other sequences
post-processing
disadvantages
susceptible to artefacts, long acquisition times, patient compliance, MR safety, expensive
how is MRA TOF done
technique to visualise flow within vessels without the need for contrast
2D or 3D gradient echo sequence
based on flow related enhancement on imaging slice
manipulates MR environment so only spins within flowing blood generate signal
stationary spins saturated - low MR signal
fresh unsaturated spins carried in by flowing blood
what are principles of TOF. is flow suppressed?
slice is perpendicular to flow
blood flow in perpendicular to radiofrequency pulses and because they are moving they are not suppressed.
repetitive rf pulses suppress on the slice.
flow is not suppressed so if vein crossing from other side not suppressed. use saturation pulse beyond the slice to saturate anything that comes from distal part
what is good/bad with saturation and intravoxel dephasing in mra tof signal loss
saturation –> background tissue suppression (good)
blood in slice taking too long
worse with thick slabs
intravoxel dephasing - different velocities within voxel - turbulent flow, stenosis, corners - wlll lose signal
susceptibility induced - field inhomogeneities, transitions - lose signal
worse with big voxels
how is 2d tof mra acquired, what does it image
slice by slice
images long vascular segments
how is 3d tof mra acquired
what does it image
isotropic volumetric images
images relatively small area and vessels running in various orientations,
advantages of 2D TOF
individual slices - minimal saturation of blood. coverage area expandable by adding more slices
sensitivity to slow flow
sat band prior to each slice- good venous saturation
disadvantages of 2D TOF
large voxels, long TE intravoxel dephasing
relatively poor resolution
poor suppression of short T1 tissues
3d TOF advantages
high spatial resolution
small voxels - minimises intravoxel dephasing
short TE (less dephasing)
high SNR (3D)
isotropic voxels - multiplanar reformations
3d TOF disadvantages
slab saturation - slab boundary artefact less effective venous suppression poor suppression of short T1 tissues motion artefact corrupt all slices slightly longer acquisition than 2D
what is MOTSA
multiple overlapping thin slab acquisition - reduce flow saturation, use both methods of TOF
what is the use of CEMRA and how
for extracranial MRA
done by a 3D spoiled gradient echo sequence - flip angle 25-50 degrees
Gadolinium injection - shorter T1 - blood high signal
rapid acquisition
arterial phase - timing is acquisition is important - automated
synchronised with first pass
images in long axis of vessel - not reliant on inflow or new spins, more efficient data acquisition
subtraction
advantages of CEMRA
low intravoxel dephasing - short TR
low saturation - Gadolinium
short acquisition - low motion
less artefacts
disadvantages of CEMRA
gadolinium cannot be repeated synchonisation short acquisition - resolution challenge subtraction - reduced SNR
advantage of 2D TOF vs CEMRA
sometimes can’t see abnormality without contrast
can suppress venous flow anything that comes from opp. side
what is MRA phase contrast
to visualise moving fluids
main use: visualise vein or moving CSF, MRV
no contrast required
spins passing along the direction of the magnetic field gradient acquire phase change
measures phase change
proportional to distance travelled along gradient
provide quantitative and directional data
what is ASL mra used for
in research only. flowing blood is tagged with RF pulses to have high mag and stationary tissue is suppressed
assess cerebral vasculature and hemodynamics
conditions exhibiting delayed arterial transit time and collateral flow pathways
what are ASL MRA advantages
covers entire vasculature non-invasive no contrast agents no ionising radiation vessel selection can be quantitative
what are ASL MRA limitations?
require high arterial velocities, presumes the flow direction is known which is not always the case. potential partial labelling of vessels in close proximity to selected vessel
small signal variations within vessels affect assessment of partial stenosis
