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FRCR physics > MRI sequences and weighting > Flashcards

MRI sequences and weighting Flashcards

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1
Q

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

A

True

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2
Q

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

A

true

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3
Q

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

A

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

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4
Q

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

A

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

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5
Q

What is the use of a shim coil

A

Shim coils adjust the uniformity of the magnetic field strength.

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6
Q

What are gradient coils used for

A

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

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7
Q

T/f: spatial resolution is better with smaller coils

A

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

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8
Q

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

A

-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

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9
Q

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

A

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

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10
Q

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

A

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.

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11
Q

What determines T2 weighting?

A

TE time

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12
Q

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

A

-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.

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13
Q

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

A

-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.

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14
Q

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

A

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

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15
Q

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

A

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

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16
Q

SSG: How can slice thickness be reduced?

A

-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.

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17
Q

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

A

-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.

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18
Q

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

A

PEG: Aliasing
FEG: Chemical shift

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19
Q

how does aliasing occur?

A

-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.

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20
Q

How can chemical shift be reduced?

A
  • a steeper FEG gradient
  • a wider receiver bandwidth
  • higher bandwidth per pixel
  • lower field strength
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21
Q

What does the PEG do?

A

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

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22
Q

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

A

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

23
Q

What sequence can acquire a single slice in 1 TR?

A

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

24
Q

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

A

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

25
Q

Timings: what is faster acquiring PEG or FEG data?

A

-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.

26
Q

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

A

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

27
Q

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

A

-Very smooth image but lacks edges and detail

28
Q

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

A

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

29
Q

What is the equation for scan time

A

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

30
Q

Sequences: How is a proton density sequence made?

A

-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.

31
Q

Sequences: Spin echo - how do these work?

A

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.

32
Q

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

A

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

33
Q

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

A

-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).

34
Q

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

A

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

35
Q

What is the time limiting factors in a SE sequence?

A

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

36
Q

SE: How does fast turbo spin echo work?

A

-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

37
Q

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

A

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

38
Q

What is fast turbo spin echo used in?

A

-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.

39
Q

What are disadvantages of Fast spin echo?

A

-Only able to achieve heavily T2 weighted images.

40
Q

What is fast advanced spin echo (HASTE) used?

A

-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.

41
Q

Gradient ECHO (GE) - how does this work?

A

-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).

42
Q

Does GE produce T2 or T2* images?

A

-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

43
Q

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

A

-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

44
Q

What is inversion recovery? How does it work?

A

-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.

45
Q

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

A

-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.

46
Q

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

A

-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.

47
Q

How does DWI work?

A

-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.

48
Q

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

A

-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.

49
Q

DWI: what is the B value?

A

-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).

50
Q

DWI: how is the B value increased?

A

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

51
Q

DWI - What is the apparent diffusion coefficient used for?

A

-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.

52
Q

What is anisotropic diffusion?

A

When diffusion is not equal in every direction

53
Q

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

A

-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.
 

FRCR physics (13 decks)

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