chap 8 Flashcards
MRA techniques include
- PC- MRA
- TOF - MRA
- CE - MRA
what is the equation for available imaging time ?
= R-R interval - (trigger window + trigger delay)
the parameters used in gating for T1 weighting
1 R-R interval, short TE
the parameters used in gating for PD weighting
2-3 R-R intervals, short TE
parameters used in gating for T2 weighting
2-3 R-R intervals, long TE
the vascular signal, produced on PC-MRA, relies on
velocity - induced phase shifts
remedy for Moire artifact
keep arms/shoulders or other anatomy within the FOV, use SE
remedy for Magic angle artifact
move the body part or change the TE
remedy for aliasing or “wrap” artifact
use “no phase wrap” software or increase the FOV
remedy for chemical misregistration artifact
select a TE that matches the periodicity of fat and water, & use an SE rather than a GRE
remedy for zipper artifact
check for RF leak, call an engineer
remedy for cross excitation artifact
gap between the slices, alternate slices
remedy for chemical shift
use lower field strengths, smaller FOV, & lower rBW
remedy for truncation artifact
increase the number of phase encoding steps and not undersampling of data. Square matrix will also avoid
remedy for Magnetic susceptibility
remove metal, use SE, decrease TE, lower field strengths
remedy for ghosting artifact
swap phase & frequency, pre sat, respiratory compensation, concise patient directions
technique for black blood imaging
SE with pre sats, inversion recovery
technique for bright blood imaging
GRE, GMN, contrast enhancement
how much do patients heart rate vary during the scan?
10-20%
what causes ghosting artifact
anatomy moving along the phase direction during the pulse sequence
what causes aliasing artifact
anatomy outside the FOV is mapped inside the FOV
what causes chemical shift artifact
different chemical environments of fat & water
what causes truncation artifact
under sampling of data
what causes cross excitation artifact
RF pulse not square, adjacent slices get energy pulse from neighbors
what causes a zipper artifact
leak in the RF shielding
what causes magic angle artifact
collagen or other tightly bound structure of fibers is 55 degree angle to Bo
what causes shading artifact
inhomogeneities in the main magnetic field
MRA techniques: 2 T2 data sets, one during systolic and one during diastolic (fresh blood imaging)
digital subtraction MRA
MRA techniques: Enhancement related to the flow of blood
TOF MRA
MRA techniques: Signal relies on velocity induced phase shifts , takes advantage that flowing nuclei moving along a bipolar gradient have a higher signal than stationary nuclei
PC MRA
MRA techniques: T1 3D gradient echo following gadolinium and dynamic imaging
contrast enhanced MRA
when using MRA to evaluate extracranial vascular flow, such as that within common carotid arteries, a recommended technique is
2D time of flight MRA
lead wires on the patients chest, used when imaging the chest, heart, & great vessels
ECK, EKG
light sensor attached to patients finger to detect pulses through capillaries, not as accurate
peripheral gating
bellows around the chest, signal corresponds to max and min motion of the chest, TR may be reduced
respiratory compensation
the signal from flowing blood within the vessels in MRI and MRA relies on
first order motion
scan is triggered from beats of the heart, uses R-R, signal during diastole
prospective gating
post process after the scan is completed, continuously throughout all the cardiac phases
retrospective gating
waiting time before each R-R wave
trigger window
waiting period after each R-R
trigger delay
MOTSA
maintain signal & improve coverage with large volume
projects maximum intensity
MIP
surface is presented on the reformatted data as though illuminated by a directional light source
SSD
the direction in which ghosting & truncation artifacts happen in
phase only
the direction in which aliasing, magnetic susceptibility, & magic angle artifacts happen in
both phase & frequency
the direction in which chemical shift & zipper artifacts happen in
frequency only
first small wave on ekg
p wave , atrial systole
last wave on ekg
T-wave , ventricular diastole, may be elevated due to magnetohemodynamic effect
chemical shift occurs in what direction
frequency only
the appearance of phase mismapping artifact (ghosting)
produces replications of moving anatomy across the image
the appearance of chemical shift artifact
dark edges at edge between fat & water
appearance of magnetic susceptibility artifact
large signal voids , worse on 3T than 1.5T
appearance of cross excitation & cross talk artifact
black line over cord, adjacent slices in different image contrasts
appearance of zipper artifact
dense line across the image
in tissues that contain collagen at high signal intensity
magic angle artifact
aka wrap, anatomy outside FOV is folded on top of anatomy inside FOV
aliasing
black & white bending at the edge of the FOV
Moire artifact
aka gibbs , from under sampling, banding at the interfaces of high & low signal, low intensity band running through high intensity area
truncation artifact
optimizes scan time - the operator witnesses the arrival of the contrast
fluoro triggering
optimizes scan time - tracker pulse measures signal from the lumen
bolus tracking
optimizes scan time - small injection made to determine the exact time to begin scanning
test bolus
