MRI sequences and weighting

Question 1

T/f: only nuclei in the lower energy spin up state generate MR signal

Answer 1

True

Question 2

T/F: Only atoms with an odd number of protons or neutrons exhibit magnetic resonance

Answer 2

true

Question 3

What is 1T equivalent in Mhz and in Gauss? Whaat is the earth magnetic field?

Answer 3

-1T = 42.6Mhz = 10 000 gauss
-Earth magnetic field = 0.5 gauss

Question 4

How does field strength (T) affect T1 and T2 signal?

Answer 4

As field strength increases, there is increased T1 signal strength. T2 is unaffected by magnet strength.

Question 5

What is the use of a shim coil

Answer 5

Shim coils adjust the uniformity of the magnetic field strength.

Question 6

What are gradient coils used for

Answer 6

gradient coils used for: coils used for PEG, SSG and FEG. Switching on and off makes the noise.

Question 7

T/f: spatial resolution is better with smaller coils

Answer 7

False: SR is independent of the physical size of coil elements

Question 8

What is the difference between resistive and superconductive magnets in terms of strength and when should be shut off

Answer 8

-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

Question 9

T2: give timings of the sequence + what is bright on T2 weighting?

Answer 9

-TR 1000-2000ms and TE 90-140ms
-Bright on T2: CSF, urine, amniotic fluid and water have long T2

Question 10

Why does water have a long T2 (and bright signal?)

Answer 10

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.

Question 11

What determines T2 weighting?

Answer 11

TE time

Question 12

-What time determines the T1 time? Give the TE and TR of a T1 sequence

Answer 12

-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.

Question 13

Explain the differences in T1 in tissues. What is bright on T1?

Answer 13

-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.

Question 14

What are the axis of the 3 important spatial encoding steps?

Answer 14

-Slice selection: Z
-Frequency encoding: X axis
-Phase encoding: Y axis

Question 15

Slice selection (SSG) -T/F: slice selection is applied simultaneously with the initial RF excitation

Answer 15

True: slice select gradient together with the RF bandwidth determines the thickness of the slice selected.

Question 16

SSG: How can slice thickness be reduced?

Answer 16

-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.

Question 17

Frequency encoding gradient (FEG): how does this work?

Answer 17

-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.

Question 18

Name 1 artefact that occurs in the FEG and one in the PEG direction

Answer 18

PEG: Aliasing
FEG: Chemical shift

Question 19

how does aliasing occur?

Answer 19

-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.

Question 20

How can chemical shift be reduced?

Answer 20

• a steeper FEG gradient
• a wider receiver bandwidth
• higher bandwidth per pixel
• lower field strength

Question 21

What does the PEG do?

Answer 21

-Provides information along the Y axis – each line fills a line of K space

Question 22

T/F: in a standard spin echo sequence, all of the information for a single slice is obtained within 1 TR

Answer 22

-False: Each step in the PEG ecoding takes 1 TR  with eg 256 steps, the time to image each slice is 256 x TR

Question 23

What sequence can acquire a single slice in 1 TR?

Answer 23

Single shot techniques (eg EPI) can an entire slice be acquired in 1 TR

Question 24

PEG - T/F: in a standard SE sequence – the image can be built line by line during acquisition

Answer 24

-False: raw data from each TR is stored in K space –> encodes for spatial frequency domain. then do Fourier transform.

Question 25

Timings: what is faster acquiring PEG or FEG data?

Answer 25

-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.

Question 26

K space - What does the periphery vs centre of data contain in K space?

Answer 26

-Periphery contains data relating to spatial resolution (higher frequency waves) -Centre contains data for high signal intensity and contrast (lower frequency waves)

Question 27

What will a low pass filter than only includes the centre of K space produce?

Answer 27

-Very smooth image but lacks edges and detail

Question 28

What will a high pass image that only includes the peripheral of K space produce?

Answer 28

-An image with lots of details and edges but no low contrast features.

Question 29

What is the equation for scan time

Answer 29

scan time = TR x number of PEGs x number of signal averages (NEX)

Question 30

Sequences: How is a proton density sequence made?

Answer 30

-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.

Question 31

Sequences: Spin echo - how do these work?

Answer 31

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.

Question 32

Sequences: Spin echo - explain timings for T1 and T2 weighting

Answer 32

For spin echo sequences, TR and TE are short for T1 weighting and TR and TE are long for T2 weighting.

Question 33

Sequences: Spin echo - Does spin echo sample T2 or T2*?

Answer 33

-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).

Question 34

Spin echo advantage (how does it affect SNR, T2 weighting) vs disadvantages (scan times and power)

Answer 34

-Advantages: high SNR, true T2 (not T2* weighting). -Disadvantages: long scan times, uses more RF power than a gradient echo.

Question 35

What is the time limiting factors in a SE sequence?

Answer 35

SE: Phase encoding. Can reduce time of SE by simultaneously recording multiple PEGs, which results in acquisition time being shortened.

Question 36

SE: How does fast turbo spin echo work?

Answer 36

-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

Question 37

SE -What is an ETL and how does this related to scan length?

Answer 37

echo train length – number of echoes acquired in a given TR interval. The higher the ETL, the faster the scan

Question 38

What is fast turbo spin echo used in?

Answer 38

-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.

Question 39

What are disadvantages of Fast spin echo?

Answer 39

-Only able to achieve heavily T2 weighted images.

Question 40

What is fast advanced spin echo (HASTE) used?

Answer 40

-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.

Question 41

Gradient ECHO (GE) - how does this work?

Answer 41

-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).

Question 42

Does GE produce T2 or T2* images?

Answer 42

-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

Question 43

Are GE sequences fast? how does this related to weighting?

Answer 43

-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

Question 44

What is inversion recovery? How does it work?

Answer 44

-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.

Question 45

What is STIR and FLAIR? Does it enhance tissue boundaries?

Answer 45

-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.

Question 46

T/F: inversion recovery is used to accentuated subtle differences in T1 weighting between tissues

Answer 46

-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.

Question 47

How does DWI work?

Answer 47

-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.

Question 48

DWI: what is EPI? How is this useful for DWI?

Answer 48

-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.

Question 49

DWI: what is the B value?

Answer 49

-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).

Question 50

DWI: how is the B value increased?

Answer 50

B value is increased by larger diffusion gradients (increase amplitude or duration) or increased time between dephasing and rephasing of diffusion gradients.

Question 51

DWI - What is the apparent diffusion coefficient used for?

Answer 51

-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.

Question 52

What is anisotropic diffusion?

Answer 52

When diffusion is not equal in every direction

Question 53

DWI artefacts - describe T2 shine through, T2 dark through and metal artefacts

Answer 53

-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.