CT, MR angiography

Question 1

what is the CT angiography technique?

Answer 1

helial CT acquisition (multi detector spiral ct)

IV injection of iodinated contrast. timing important

Question 2

what is post-processing of CTA

Answer 2

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

Question 3

advantages of CTA

Answer 3

less costly than MR
rapid - limits motion artefact
patient tolerance
good resolution
no flow related artefacts
calcification well seen 
whole head an neck circulation

Question 4

disadvantages of CTA

Answer 4

ionising radiation, iodinated contrast, streak artefacts, lower resolution, acquisition limit evaluation of flow

Question 5

what are pitfalls of CTA

Answer 5

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

Question 6

what can windowing do

Answer 6

cause calcification around the pic

Question 7

what are MRA techniques

Answer 7

time of flight
contrast enhanced (CEMRA)
phase contrast
asl

Question 8

advantages of MRA

Answer 8

no ionising radiation
combined with other sequences
post-processing

Question 9

disadvantages

Answer 9

susceptible to artefacts, long acquisition times, patient compliance, MR safety, expensive

Question 10

how is MRA TOF done

Answer 10

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

Question 11

what are principles of TOF. is flow suppressed?

Answer 11

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

Question 12

what is good/bad with saturation and intravoxel dephasing in mra tof signal loss

Answer 12

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

Question 13

how is 2d tof mra acquired, what does it image

Answer 13

slice by slice

images long vascular segments

Question 14

how is 3d tof mra acquired

what does it image

Answer 14

isotropic volumetric images

images relatively small area and vessels running in various orientations,

Question 15

advantages of 2D TOF

Answer 15

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

Question 16

disadvantages of 2D TOF

Answer 16

large voxels, long TE intravoxel dephasing
relatively poor resolution
poor suppression of short T1 tissues

Question 17

3d TOF advantages

Answer 17

high spatial resolution
small voxels - minimises intravoxel dephasing
short TE (less dephasing)
high SNR (3D)
isotropic voxels - multiplanar reformations

Question 18

3d TOF disadvantages

Answer 18

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

Question 19

what is MOTSA

Answer 19

multiple overlapping thin slab acquisition - reduce flow saturation, use both methods of TOF

Question 20

what is the use of CEMRA and how

Answer 20

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

Question 21

advantages of CEMRA

Answer 21

low intravoxel dephasing - short TR
low saturation - Gadolinium
short acquisition - low motion
less artefacts

Question 22

disadvantages of CEMRA

Answer 22

gadolinium 
cannot be repeated
synchonisation
short acquisition - resolution challenge
subtraction - reduced SNR

Question 23

advantage of 2D TOF vs CEMRA

Answer 23

sometimes can’t see abnormality without contrast

can suppress venous flow anything that comes from opp. side

Question 24

what is MRA phase contrast

Answer 24

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

Question 25

what is ASL mra used for

Answer 25

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

Question 26

what are ASL MRA advantages

Answer 26

covers entire vasculature
non-invasive
no contrast agents
no ionising radiation
vessel selection
can be quantitative

Question 27

what are ASL MRA limitations?

Answer 27

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