MRI sequences and weighting Flashcards
T/f: only nuclei in the lower energy spin up state generate MR signal
True
T/F: Only atoms with an odd number of protons or neutrons exhibit magnetic resonance
true
What is 1T equivalent in Mhz and in Gauss? Whaat is the earth magnetic field?
-1T = 42.6Mhz = 10 000 gauss
-Earth magnetic field = 0.5 gauss
How does field strength (T) affect T1 and T2 signal?
As field strength increases, there is increased T1 signal strength. T2 is unaffected by magnet strength.
What is the use of a shim coil
Shim coils adjust the uniformity of the magnetic field strength.
What are gradient coils used for
gradient coils used for: coils used for PEG, SSG and FEG. Switching on and off makes the noise.
T/f: spatial resolution is better with smaller coils
False: SR is independent of the physical size of coil elements
What is the difference between resistive and superconductive magnets in terms of strength and when should be shut off
-Resistive magnets: designed to be turned off at end of day, provide strength up to 0.3 or 0.5T
-Superconductors: should never be switched off and provide up to 10T
T2: give timings of the sequence + what is bright on T2 weighting?
-TR 1000-2000ms and TE 90-140ms
-Bright on T2: CSF, urine, amniotic fluid and water have long T2
Why does water have a long T2 (and bright signal?)
Relaxation of water occurs slowly , where molecules are moving around very rapidly, dipole –
dipole interactions are very brief, making T2 relaxation less efficient,
leading to a long T2.
What determines T2 weighting?
TE time
-What time determines the T1 time? Give the TE and TR of a T1 sequence
-TR time
-TR time 300-800ms and TE 15ms
*T1 weighted images have a short TR and a short TE. T2 is always shorter than T1.
Explain the differences in T1 in tissues. What is bright on T1?
-Fat and large molecules such as proteins in fluid are effective at removing
energy in spin–lattice relaxation; this shortens T1. However, in solids,
where water is more tightly bound, T1 relaxation becomes less efficient
and T1 lengthens.
- Fat is bright on T1, while water and CSF are dark.
What are the axis of the 3 important spatial encoding steps?
-Slice selection: Z
-Frequency encoding: X axis
-Phase encoding: Y axis
Slice selection (SSG) -T/F: slice selection is applied simultaneously with the initial RF excitation
True: slice select gradient together with the RF bandwidth determines the thickness of the slice selected.
SSG: How can slice thickness be reduced?
-Decreasing RF bandwidth for each slice
-Increasing the gradient of the RF pulse (steeper)
*thinner slices produce more anatomical detail but have a lower SNR.
Frequency encoding gradient (FEG): how does this work?
-Along X axis
-Apply initial dephasing gradient followed by a rephasing gradient.
-Applied during signal acquisition (TE) –> once FEG is applied, the protons of interest will precess with frequency that varies according to their position along the gradient in the x axis. Spatial localisation is then achieved by identifying particular frequencies.
Name 1 artefact that occurs in the FEG and one in the PEG direction
PEG: Aliasing
FEG: Chemical shift
how does aliasing occur?
-Aliasing: occurs if signal is not sampled regularly enough and leads to underestimation of frequency (and misplacing of the data) - Nyquist limit.
- A band-pass filter only allows through a certain range of frequencies can help reduce this.
How can chemical shift be reduced?
- a steeper FEG gradient
- a wider receiver bandwidth
- higher bandwidth per pixel
- lower field strength
What does the PEG do?
-Provides information along the Y axis – each line fills a line of K space
T/F: in a standard spin echo sequence, all of the information for a single slice is obtained within 1 TR
-False: Each step in the PEG ecoding takes 1 TR with eg 256 steps, the time to image each slice is 256 x TR
What sequence can acquire a single slice in 1 TR?
Single shot techniques (eg EPI) can an entire slice be acquired in 1 TR
PEG - T/F: in a standard SE sequence – the image can be built line by line during acquisition
-False: raw data from each TR is stored in K space –> encodes for spatial frequency domain. then do Fourier transform.
Timings: what is faster acquiring PEG or FEG data?
-FEG is faster
-PEG takes a lot longer to accumulate – traditionally each PEG steg needed a new TR but in modern techniques can allow collection of several PEGs in each TR.
*PEG is still the time limiting factor in MR compared to FEG.
K space - What does the periphery vs centre of data contain in K space?
-Periphery contains data relating to spatial resolution (higher frequency waves)
-Centre contains data for high signal intensity and contrast (lower frequency waves)
What will a low pass filter than only includes the centre of K space produce?
-Very smooth image but lacks edges and detail
What will a high pass image that only includes the peripheral of K space produce?
-An image with lots of details and edges but no low contrast features.
What is the equation for scan time
scan time = TR x number of PEGs x number of signal averages (NEX)
Sequences: How is a proton density sequence made?
-PD doesn’t display the magnetic characteristics of hydrogen (unlike T1 and T2) but instead the number of nuclei in that image. Want to minimise the contributions of T1 and T2
-Cortical bone and air appear dark on all MR weighting as all have very few protons.
-Short TE (negates T2 eg at 15ms) and long TR (negates T1 eg 1000-3000ms) gives PD weighted image.
Sequences: Spin echo - how do these work?
Spin echo sequences begin with a 90° pulse which rotates the magnetisation vector into the transverse plane. A rephasing pulse is then applied using a 180° RF pulse at the time TE/2. The rephasing pulse generates an echo at time TE with multiple echoes being produced by repeating the rephrasing 180° pulse.
Sequences: Spin echo - explain timings for T1 and T2 weighting
For spin echo sequences, TR and TE are short for T1 weighting and TR and TE are long for T2 weighting.
Sequences: Spin echo - Does spin echo sample T2 or T2*?
-T2
-Application of 180 pulse allows for sampling of T2 (unlike in gradient echo which samples T2* as it does not account for local field inhomogeneities).
Spin echo advantage (how does it affect SNR, T2 weighting) vs disadvantages (scan times and power)
-Advantages: high SNR, true T2 (not T2* weighting).
-Disadvantages: long scan times, uses more RF power than a gradient echo.
What is the time limiting factors in a SE sequence?
SE: Phase encoding. Can reduce time of SE by simultaneously recording multiple PEGs, which results in acquisition time being shortened.
SE: How does fast turbo spin echo work?
-FSE uses several refocusing 180 RF pulses to rephase and produced extra echoes at different PEGs for each excitation.
-This greatly reduced the time of the scan by filling multiple lines of K space
SE -What is an ETL and how does this related to scan length?
echo train length – number of echoes acquired in a given TR interval. The higher the ETL, the faster the scan
What is fast turbo spin echo used in?
-Very fast t/f good for MR angiography where need fast scan times.
-Can greater two images of 2 different contrasts eg PD and T2 by using different echo times and filling 2 k spaces.
What are disadvantages of Fast spin echo?
-Only able to achieve heavily T2 weighted images.
What is fast advanced spin echo (HASTE) used?
-MRCP study: use the turbo spin echo and fill an entire K space in 1 cycle. Very very fast – will every K space row in 1 cycle.
-Full up ½ of the K space and use a half Fourier imaging to extrapolate the other half.
Gradient ECHO (GE) - how does this work?
-Gradient echo works better for sequences with a short TR, eg T1 weighted scans. Forgoes the 180 RF pulse and instead uses a gradient to rephase the spins.
-Sequence: apply RF pulse –> SSG + PEG + FEG applied.
-FEG: apply negative FEG followed by positive FEG –> spins rephase until a signal is created (gradient echo).
Does GE produce T2 or T2* images?
-T2*
-Rephases spins faster than SE through an initial reverse of the FEQ signal however magnet inhomogeneities are not countered by this so the image is T2* weighted and doesn’t reach equivalent rephasing when compared to the 180 pulse of SE
Are GE sequences fast? how does this related to weighting?
-Gradient Echo sequences are fast b/c uses a reduced strength RF pulse and a short tip angle allows for a short TR.
-Short TR is good for T1 weighted images
What is inversion recovery? How does it work?
-Inversion recovery is a variant of spin echo sequences.
-Inversion recovery sequences can reduce the signal from a certain type of tissue by timing a 90 degree pulse to occur when the mZ is 0 for that tissue type.
What is STIR and FLAIR? Does it enhance tissue boundaries?
-STIR uses this to suppress the signal from fat
fluid attenuated inversion recovery
-FLAIR uses this to supposes the signal from water.
*STIR doesn’t enhance tissue boundaries.
T/F: inversion recovery is used to accentuated subtle differences in T1 weighting between tissues
-True: the standard SE is preceded by a 180 pulse
-This allows more time for differences in the longitudinal relaxation of spins to become apparent.
How does DWI work?
-Uses spin echo base sequence (Echo planar imaging.)
-In DWI the tissue oedema produces a high signal –> DWI shows high signal from tissues which have abnormal proton movement within tissue water.
-2 diffusion gradients are applied to either side of the 180 RF pulse. Stationary spins return a high signal as they have been exposed to both the dephasing and the rephasing gradients.
-Ischaemic/infarcted tissue has lost cell integrity, with resultant oedema impairing diffusion.
DWI: what is EPI? How is this useful for DWI?
-fast technique where an entire slice can be obtained in under 100ms. A single excitation undergoes multiple rephasing of material. Resolution is lower eg 64 x 64 matrix but speed is useful for functional imaging.
-EPI minimises the effect of patient motion as it is very quick –> important in DWI b/c small motion of molecules will be masked by an macroscopic body motion.
DWI: what is the B value?
-The degree of diffusion weighting is represented by the B value – the more sensitive the DWI sequence is to molecular motion, the higher the B value (but also more noise and less signal).
DWI: how is the B value increased?
B value is increased by larger diffusion gradients (increase amplitude or duration) or increased time between dephasing and rephasing of diffusion gradients.
DWI - What is the apparent diffusion coefficient used for?
-DWI images have T2 weighting: t/f a lesion that has shown bright on DWI may be due to restricted diffusion or due to inherent high T2 signal.
-ADC map is used to remove the effects of inherent T2 signal.
-Restricting lesions will appear dark on ADC map.
What is anisotropic diffusion?
When diffusion is not equal in every direction
DWI artefacts - describe T2 shine through, T2 dark through and metal artefacts
-T2 shine through: intrinsic high T2 signal shows bright on DWI – ADC removes that effect.
-T2 dark-through: intrinsic low T2/T2* signal shows as low signal on DWI
-Metal artifact: DWI very susceptible to artifact created by metal and blood products.
