MRI image formation and artefacts pt 2 Flashcards
define spatial resolution + unit
- ability of MRI system to distinguish between 2 near objects
-mm
what are the 2 types of spatial resolution in MRI
in-plane (pixel resolution)
through-plane resolution (slice thickness)
what is the matrix / how do you calculate matrix value
matrix = (number of frequency encoding readout (axis)) x (number of phase encoding (axis))
how do you calculate resolution from an image
resolution = FOV / matrix size
field of view (dimension of image)
matrix ( number of pixels on x axis times pixels on y axis)
a larger pixel size will be unable to resolve 2 near-by structures as compared to small pixel size
what is the difference between the dimensions of a pixel for in-plane and through plane resolution
in-plane resolution = 2D pixel
through plane accounts for slice thickness = voxel (pixel size x width) (3D)
3D volumes are composed of stacks of 2D slices, like a load of bread
each slice has thickness = more hydrogen = more signal
which has higher resolution thicker or thinner slices
thinner
what are the 2 benefits of having gaps between slices
- reduces RF interference
- allows fewer slices to cover whole region of interest
where does the signal and noise come from in the signal to noise ratio
signal = intensity from tissue
noise = intensity from background
know that noise is random in time and occurs at every frequency
what 3 main factors does signal depend on
- number of proton spins
- sequence parameters
- RF coil type
how is signal to noise ratio calculated
To calculate SNR in an image they can take a square sample of the area of interest in the MRI picture (signal) and a square sample of the noise in the background outside the area of interest in the image and divide them.
how does signal to noise ratio relate to bandwidth
- RF bandwidth is the range of sampled frequencies during readout
- the smaller the bandwidth, the less sampled noise relative to signal
SO SNR increases when bandwidth is decreased
what percentage will SNR increase if you half the bandwidth
30%
define contrast
- signal intensity difference between 2 adjacent regions (voxels) of an image
what is the difference in why t1 and t2 contrast is used
t1 = anatomical investigation
t2 = pathological investigations
compare the speed, and detail of t1 and t2 contrast
t1 = fast to acquire, excellent structural detail
t2 = slower to acquire, lower resolution
how does CSF and fat appear on t1 and t2 weighted image
t1 = bright fat, dark CSF
t2 = dark fat, bright CSF
Contrast and be quantified, 1 = high contrast, 0 = low contrast.
how is this calculated
contrast = (signal a - signal b) / (signal a + signal b)
what is the average spatial resolution in MRI
1-2 mm
define contrast to noise ratio
difference between the signal to noise ratio of 2 tissues (a and b)
how do you calculate contrast to noise ratio
CNR = (SI a - SI b) / N
what are the 3 artefacts due to system faults and hardware limitations
- continuous RF interference (zipper)
- instantaneous RF interference (RF spike)
- receiver RF coil non-uniformity
what is the cause of the continuous RF interference / zipper artefact
- continuous RF radiation from an external source is picked up by the RF coils during acquisition of ALL raw data
what is the characteristics of the continuous RF / zipper artefact and why is it like this
- line(s) of bright and dark pixels that look like a zip across the image in the phase encoding direction
- line of pixels alternating of high and low signal going through image
- because interference happens during acquisition of raw data in the phase encoding direction, the artefact appears going left to right across the image (in the y axis)
what are 3 common sources of interference in zipper artefact
- equipment not designed for MRI room
- faulty RF shield (RF enters scanner room and causes interference)
- RF door open (RF enters scanner causing interference)
how can you prevent zipper artefact
- ensure MRI door is shut
- unplug equipment e.g devices on patient
- clean conductive seal around RF door
- RF cage may be faulty (final option)
what causes the instantaneous RF inference/ RF spike artefact
RF radiation pulses from external source is picked up by coils during acquisition of SINGLE raw data point
- bursts of RF that cause spikes are often caused by electrical discharge from static build-up or faulty equipment
describe the apperance of the RF spike artefact and what it looks like compared to a single spike and multiple spikes
- (single spike) regular spaced stripes across entire image in single or multiple slices
(also known as HERRINGBONE OR CORDUROY artefact)
- if there are multiple spikes during acquisitions, then multiple spacial frequencies will over lay the image (looks extremely fuzzy all over image)
what are common sources of the instantaneous RF spike artefact
- faulty lighting/blow bulbs
- conductive debris in RF coil plugs
- discharge from patient clothing
- loose connections coil/scanner
- faulty component in RF coil
what are resolutions for RF spike artefact
- run scan again may be one off
- clean RF coil plugs, turn off light and replace
- if patient is issue, change them into MRI safety gown
- call engineer to fix if issues persist
what causes the RF coil non-uniformity artefact
- inability for RF coil to detune during RF transmission (coil de-coupling failure)
- faulty coil elements causing signal void (putting the patient in the other way will show if artefact is the element or patient)
what does a receiver RF coil non-uniform artefact look like
area of darkness ‘ void ‘
what are 3 types of motion artefacts
- gross patient motion
- physiological (non-flow)
- physiological (flow)
what is the cause of gross patient motion
physical motion occurs over seconds, where as frequency encoding takes milli seconds so motion gets frozen
what does gross patient motion artefact look like
image replication throughout the image in the y axis (PE)
periodic motion makes few replications
random motion takes many replications
how do you migrate gross patient motion artefacts through programs
radial k-space (propeller, blade, multilane)
- of k-space trajectory is radial, artefact will spread itself in all directions and result in better image quality but lower resolution
how do you migrate gross patient motion in general
- reduce scan time
- calm patient, relax them
- sedate if needed
what causes physiological (non-flow) motion
Flow artifacts in MRI refer to distortions or signal changes in the images caused by the movement of fluids, such as cerebrospinal fluid (CSF) and blood flow. These artifacts can result in blurring, ghosting, or signal loss in the images, making it challenging to accurately interpret the anatomical structures.
what are the 3 most common (physiological) flow artefacts
- inflow effect (In GE sequence)
- outflow / flow void (in spin echo)
- velocity induced phase-effects
what is the inflow effect in gradient-echo sequence
bright signal (white blood) as fresh, unsaturated spins in blood flow into a slice between time of repetition
what is the outflow/flow void effect in spin echo sequence
dark signal for fast flowing blood because some spins may enter or leave the selected slice between 90 and 180 degree pulses
what is the velocity induces phase effect
- reduces blood signals and creates a ghost in phase direction ( y axis)
- slow moving fluid e.g CSF create ghosts (bright shadowing around structure) but more apparent with high signalh
how can you migate in flow artefact
- use spatial saturation (REST) slabs just outside imaging slice where blood is in flowing and null the blood so it doesnt contribute to signal
how can you migrate ghosting artefact
switch FE and PE direction so flow ghosting artefact isnt over area of intrest
how can you migrate velocity induced artefact
- gradient moment nulling (to compensate for the differences in phase between static and flowing spins in slice)
in general to correct physiological motion artefacts :
- change parameters e.g thicker slices, lower TE
- use other sequences/planes
spin echo outflow artefact = black blood (vessels)
gradient echo inflow artefact = white blood
what is magnetic susceptibility and what causes magnetic susceptibility artefact
- measure of how much a material will become magnetized in an applied magnetic field
- geometric distortion due to susceptibility difference at air-tissue interface
what does magnetic susceptibility artefact look like
signal loss and distortion (blurry)
- bending/warping of edge of structure and black void
how to migrate magnetic susceptibility artefact
- use spin echo (as its less affected by susceptibility artefact than GE as it has refocusing pulse)
- keep TE as short as possible
- use higher bandwdth
- increase matrix / smaller voxels
know that the susceptibility difference of foreign body is much larger than difference susceptibility between tissue and air so the artefact looks much severe
what is a truncation / GIBBS ringing artefact caused by
sharp boundaries require higher spatial frequencies to resolve them correctly.
when resolution is insufficient ‘ Gibbs’ ringing occurs
what does truncation/ Gibbs ringing artefact look like
wavelike ripple of signal / ringing around edge of structures
how does the strength of Gibbs ringing artefact change with matrix size
increases with decreasing matrix
how do you migate Gibbs ringing artefact
- increase matrix
- image processing that reduces resolution slightly by blurring image (smoothing sharp edges) (can increase/decrease strength of this filter)
what causes the phase wrap around artefact
covered object size is larger than the FOV, part outside the FOV are folded over the image
what does wrap around artefact look like
- anatomy outside FOV is wrapped in and appears on other side of image
how do you migate the wrap around artefact
- phase oversampling
- re-centre FOV
- use local surface transit coil to not excite tissue outside FOV
What is phase oversampling and how is it used in migating wrap around artefacts
- extends the FOV in the phase encoding direction to encompass all tissue then throw away extra I,age coverage before final image is formed to get smaller final FOV
