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MRI physics Flashcards

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1
Q

Which MRI parameter change decreases metallic susceptibility artifact?

A

Increased receiver bandwidth.

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2
Q

Name the artifact.

  • What causes it?
A

Gibbs phenomenon (aka ringing artifact): multiple, regularly spaced bands parallel to one another.

  • Typically present as alternating bright and dark bands, which slowly fade as the distance from the source of the artifact increases.
  • They often occur at sharp boundaries.
  • Caused by insufficient sampling of high frequencies which may occur at sharp discontinuities in signal.
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3
Q

What happens when you place an MRI receiver coil farther than normal from an area of interest?

A

Decreased SNR

  • The signal will be lower.
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4
Q

What pulse sequence was used to obtain these images?

A

Echo-planar imaging using single-shot acquisitions.

  • It is extremely fast & used for DWI, perfusion & functional MR (BOLD).
  • Speed is important for DWI as it attempts to measure subvoxel Brownian motion.
  • The disadvantage to echo-planar imaging is that it is exceptionally prone to susceptibility artifacts.
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5
Q

What does an ADC map represent?

A

A pixel-by-pixel map of the slope of ln(signal intensity) measured across different acquired b-values.

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6
Q

Along what direction is the frequency-encoding gradient here?

A

Superior to inferior

  • Frequency encoding is always horizontal & done at the time of the echo read.
  • The kidneys demonstrate chemical shift artifact, 1st kind.
    • This only occurs along fat-water interfaces.
    • The width of the artifact increases w/increasing field strength & lower receiver bandwidth.
  • Chemical shift artifact only occurs along the frequency-encoded axis.
  • The black line where the fat pixels were shifted away from the cyst wall are superior; the white line into which fat pixels were shifted from fat into the cyst are inferior.
  • This indicates that the frequency encoding direction increases from superior to inferior–from black to white.
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7
Q
  1. In the MRI coordinate system, what designates the direction of the magnetic field?
A

Z-axis (longitudinal)

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8
Q

What is the Larmor equation & what does it mean?

A
  • It states that the precession frequency of the protons is proportional to the field strength of the magnet, multiplied by some constant for that atom, in this case, H.
  • Essentially, the stronger the magnet, the higher the precessional frequency.
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9
Q

Name the 2 functions of the RF pulse

A
  1. To synch the protons & get them to precess similarly (in phase).
  2. To decrease/move them out of longitudinal magnetization into the transverse direction of magnetization.
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10
Q

When is the only time that you can measure MRI signal?

A

When the protons are out of their longitudinal direction.

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11
Q

T1:

  1. In terms of the MR coordinate system, what does this represent?
  2. Do the protons relax or decay to return to full longitudinal magnetization?
  3. What kind of energy transfer does T1 relaxation involve?
A
  1. Relaxation of the protons back to full longitudinal (Z) magnetization, i.e., in the direction of Z, the magnetic field.
  2. They relax. T2 decays!
  3. Heat.
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12
Q

What is the definition of times (per their graphs) of T1 & T2?

A

T1 = time to 63% of longitudinal relaxation.

T2 = time to 63% of phase decay.

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13
Q

How do T1 & T2 times differ re: signal brightness?

A
  • T1: the shorter the time, the brighter the signal.
  • T2: the shorter the time, the darker the signal.
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14
Q

What is the difference b/w T2 and T2*?

A
  • T2* includes the random decay found in T2 + any local field inhomogeneities that would contribute to decay.
  • B/c there is additional field forces, T2* decay is faster than T2 alone.
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15
Q

Define TR

A
  • Repetition time.
  • Time in between successive RF pulses.
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16
Q
  1. Why is a 180 pulse given?
  2. When is this pulse given?
  3. What is the echo?
A
  1. To get rid of T2* inhomogeneities.
  2. At 1/2 the time to echo.
  3. It’s the best/cleanest time to collect signal.
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17
Q

What are the typical TR/TE times for a typical spin-echo T1 vs. T2-W vs. PD image?

A

T1: 340 / 13 msec

T2: 3500 /1200 msec

PD: 2000 / 15 msec

(Recall, for T1W images, you want a short TR & TE; for T2W, you want both to be long)

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18
Q
  1. What does a Fourier transform do?
  2. What coordinate system is this done within?
A
  1. Takes data from the time domain & converts it into the frequency domain.
  2. K-space!
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19
Q

What is the center of K-space made of, and the periphery?

A

Center: basic contrast.

Periphery: spatial resolution, i.e., edges & detail.

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20
Q

What is the super-important equation for table time?

A

time = (TR) x (phase matrix) x (NEX)

TR = time between repetitions (ms)

Phase matrix = the data the system collects from each phase encoding step

NEX = number of times each set of phase encoding steps is repeated

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21
Q

How are STIR sequences affected by magnetic susceptibility (metal) & field inhomogeneity?

A
  • They’re far less susceptible relative to other fat suppression techniques.
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22
Q
  1. Can STIR be used with Gad?
  2. If not, why not?
A
  1. NO!
  2. B/c STIR nulls out short T1, i.e., fat and gad (gad has a similar T1 to fat).
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23
Q

How does acquisition time for inversion recovery sequences compare to conventional spin-echo?

A

STIR/FLAIR are longer as they start off with the extra 180 pulse, which adds the time to inversion, which increases the total time to repetition.

24
Q

Quenching:

  1. What does quenching do?
  2. 2 instances in which the magnet can be quenched.
  3. 1 in which it cannot.
A
  1. Vents the liquid helium out of the magnet & room.
  2. a) if a pt is pinned; b) if there’s a fire.
  3. Code blue in the scanner–DO NOT QUENCH!
25
Q

The 5G line:

  1. What does this mean?
  2. Clinically, what’s its significance-2?
  3. What 2 factors influence the distance of the 5G line from the machine.
A
  1. 5 - Gauss exclusion zone.
  2. Line denoting the risk to implanted devices.
    1. Also, ferromagnetic materials can become projectile.
  3. 2 factors re: distance:
    1. Magnet strength.
    2. Shielding used.
26
Q

MRI noise:

  1. Where does most of it come from?
  2. Which sequences are the loudest?
  3. What’s the FDA limit for MR operation?
A
  1. Gradient coils.
  2. Those that use gradient coils a lot, i.e., echo planar (gradients are switched on/off rapidly), i.e., DWI.
  3. 140dB (my ideal weight!).
27
Q

SAR (specific absorption rate):

  1. Essentially, what is this?
  2. What’s the equation for it?
  3. Which has a higher SAR, spin echo or gradient & why?
A
  1. Heating of tissue by the MR.
  2. SAR = (magnet strength)2 x (flip angle)2 x (duty cycle)

where duty cycle = 1/TR

  1. Spin echo, especially inversions, as they have larger flip angles (180).
28
Q
  1. What is the min GFR required to give gad?
A
  1. >30, else the risk of NSF.
29
Q

ACR MRI safety zones:

  1. What are they and what’s in them?
  2. What should be stipulated b/w zones 2 & 3/4?
  3. What is required to enter zones 3 & 4?
  4. What is the ideal process if a pt codes in the MR?
A
  1. Zones:
    1. No restriction: outside the building.
    2. No restriction: waiting room & change room.
    3. Restricted: control room.
    4. Restricted: magnet room.
  2. A lock on the door to the MR control room.
  3. Screening forms for metal.
  4. The MR techs should start CPR, then move the pt out to zone 2, ideally, where the code team can work & don’t need screening clearance.
30
Q

How does voxel size relate to spatial resolution?

A

The smaller the voxel, the better the spatial resolution.

31
Q

State the important equation for factors that determine voxel size

A

voxel size = (slice thickness) x (FOV) / (matrix size)

  • So as slice thickness increases, so does voxel size, so worse spatial rez.
  • As FOV increases, so does voxel size, so worse spatial rez.
  • As matrix size increases, voxel size decreases, so better spatial rez.
32
Q

How do voxel, spatial rez & SNR relate to each other?

A
  • As voxels decrease, spatial rez increases.
  • As voxels increase, SNR increases, so larger voxels = better SNR.
33
Q

What’s the Larmor frequency of protons in a 1.5T magnetic field?

A

63MHz

34
Q
  1. What is the T1 (ms) of fat at 1.5T?
  2. CSF/fluid?
A
  1. fat = 250ms
  2. fluid = 2000-3000ms
35
Q

Name that sequence

A

T1 & T2 fast spin echo (FSE, or turbo spin echo)

  • Commonly used for T1/T2 of brain/spine (has supplanted spin-echo)
  • Fast acquisition: ~10s
  • Low magnetic susceptibility artifact
  • B/c echoes are generated using the 180-degree inversion pulses, this corrects for external magnetic field inhomogeneity (T2*).
36
Q

Name that sequence

A

Fast spin echo (FSE or turbo spin echo)

  • 90-degree RF pulse, followed by successive, 180-degree pulses in the echo train.
37
Q

Name that sequence

A
  • Conventional spin echo
38
Q

Name that sequence

A

Conventional T1W spin echo sequence

39
Q

Define TI

A

Inversion time

  • It’s the first time period (TI) used in inversion recovery sequences.
  • Measures the time between the initial 180 pulse, and the first 90 pulse.
  • Will differ depending on the material you want to sat out.
40
Q

What’s the difference in TI times for fat vs. water & in what sequences are these important?

A
  • TI = inversion recovery sequences (STIR, FLAIR).
  • The TI times are those needed to suppress a material (fat or water).
  • STIR: fat-suppressed, short TI = 100-180 msec.
  • FLAIR: water-suppressed, long TI = 1700-2200 msec.
41
Q

Name that sequence

A

Inversion recovery

  • Initial, 180-degree pulse & TI time.
  • Used in STIR or FLAIR.
42
Q

Name that sequence

A

STIR

43
Q

Name the sequence

A

Gradient echo (GRE)

  • Note: no initial 90 or 180-degree pulse–it’s the only one that doesn’t have it.
  • Reason being, you want to capture T2* field inhomogeneities (hemosiderin, calcium).
44
Q

Name the sequence

A

Gradient echo (GRE)

45
Q

Name the sequence

A

Echo-planar imaging

  • Used for DWI
  • The fastest sequence available: so less motion artifact (see below compared to conventional SE).
46
Q

Name the sequence

A

Echo planar imaging (EPI)

  • One initial RF pulse followed by multiple on/off phase/frequency encoding gradients.
  • Base sequence for DWI.
47
Q

Which blood product produces a T1W hyperintense signal?

A

Methemoglobin: a subacute blood product that is paramagnetic.

Other paramagnetics: gadolinium chelates, melanin, manganese, free radicals.

48
Q

List 6 substances that are T1 bright.

A

Fat

Subacute blood: methemoglobin

Melanin

Manganese

Gad

Protein

49
Q

Describe the antenna effect.

A
  • Occurs when an electric field couples w/a wire to create heat.
  • Can happen w/abandoned intracardiac pacemaker leads.
  • This is why they are contraindicated.
  • Temporary epicardial leads are safe (see below).
50
Q

What kind of MRI is this and what is used as contrast?

A

fMRI

  • Deoxyhemoglobin is the contrast agent.
  • fMRI depends on T2* effects.
  • Measures increased blood flow to vessels that accompany neural activity, which reduces deoxyHgb.
51
Q

Which sequence is used to generate 2D-TOF MRA images?

A

Gradient echo

52
Q

What does ppm stand for in MRS imaging?

A

Parts per million (%) of the Larmor frequency.

53
Q

In MSK MRI, which sequence should you use to decrease metallic artifact?

A

STIR

  • But you can’t use Gad with it.
54
Q
  1. Name the sequence?
  2. What is the timing of out of phase in 1.5 & 3T magnets?
A
  1. In/out of phase: spoiled GRE.
  2. Out of phase: 1.5T = 2.2ms; 3T = 1.1ms.
55
Q

What is the difference in appearance of types 1 & 2 chemical shift artifacts?

A
  • Type 1: white on one side, dark on the frequency encoding direction side.
  • Type 2: if a voxel has 50% fat/water, the India ink line will surround this area.
56
Q

Name the factor that increases type 1 chemical shift artifact & the 2 that decrease it.

A
  • Increase: field strength.
  • Decrease: larger gradient strength & wider readout bandwidth.
57
Q

Generally, what is the phase-encoding direction for: head, body, spine?

A
  • Generally, the shorter distance in each:
  • Head: side to side.
  • Body: front to back.
  • Spine: sagittally.

Rads physics (6 decks)

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