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MRI Flashcards

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1
Q

Larmor

Precession frequency = ?

A

frequency = w = gamma x field strength

gamma = proportionality constant = gyromagnetic ratio

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2
Q

T1 term

A

spin lattice relaxation

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3
Q

T1 time

A

Time at which longitudinal magnetization is 63% of its final value

Greater field strength = longer T1

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4
Q

T2 fancy term

A

spin spin relaxation

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5
Q

T2 time

A

time at which signal has decayed to 37 % of its original value

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6
Q

what causes T2 decay (loss of transverse mag)

A

external field inhomogeneity

inhomogeneities in local field, within tissues. This is why pure things (water) take longer to decay and are therefore bright!

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7
Q

which is faster T2 or T2*

A

T2* decays faster

Tissue spin interaction + field inhomogeneity

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8
Q

TR =

A

time between initiation of two successive RF pulses

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9
Q

T2 or T2* decay (downslope)

180 degree RF

A

FID free induction decay

NO FID with a 180 pulse, just inverts longitudinal mag

otherwise all flip angles cause them

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10
Q

180 RF, T2 and T2*

A

after a 90 produces transverse mag, as T2 decay is happening, hit with a 180 degree RF pulse

this second, 180 pulse 1 clears out inhomos turning T2* into T2 and creates an echo

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11
Q

180 RF generates what?

A

turns T2* to T2 and generates echo

echo is some time after the 180 where transverse mag signal has refocused and peaks again in uniformity, then trails off again

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12
Q

When to deliver 180 RF

A

180 RF given at 1/2 time to echo

1/2 TE = 180 RF

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13
Q

Rough long and short TR

A

Long TR = > 2000ms

Short TR = 250-700 ms

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14
Q

long and short TE

A

Long TE = >60ms

Short TE = 10-25ms

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15
Q

Proton density times

A

MINIMIZE BOTH T1 and T2 effects

Long TR and Short TE

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16
Q

K space trivia

what’s at the center

what’s at the periphery

A

Center = information about gross form and contrast

Periphery = information about spatial resolution

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17
Q

Spatial encoding

A

slice select

encode spatial information along rows

encode spatial information along columns

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18
Q

localizing gradients

A

identical properties, applied at different times and different directions

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19
Q

Selecting desired slice

A

SSG

placed perpendicular to desired slice plane

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20
Q

Selective pulse

A

on top of a slice select gradient

RF pulse applied at same frequency as protons in slice being sampled, only protons in this plane will be affected

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21
Q

effect of 180 RF on gradient in spin echo

A

180 RF pulse applied after the 90 fucks up the field, so before and after the 180RF, you place two identical gradients to cancel each other out and correct for errors

gradients on either side of 180 RF

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22
Q

phase encoding

A

encoding spatial info in vertical direction

phase encode gradient causes protons in same row perpendicular to the gradient to have same phase. All protons will have same frequency at this point

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23
Q

phase vs frequency encoding

A

phase encoding is much longer. This is why it’s done on the thinner portion

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24
Q

DURATION of a 2d imaging sequence

phase or frequency encoding

A

DEPENDS ON PHASE ENCODING

Duration = TR x Phase encode steps x number of excitations

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25
Q

Horizontal = frequency encoding

A

frequency encoding

results in a column of protons which have identical frequencies

APPLIED at SAME TIME AS READOUT

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26
Q

TR =

A

time BETWEEN 90 RF’s

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27
Q

When is SSG applied?

A

at same time as 90 degree RF’s

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28
Q

Modifications

Table time

A

Time = TR x Phase matrix x # of excitations

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

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29
Q

Modifications

when does time not follow normal equation?

A

Fast spin echo

Acquisition time is proportional to 1/echo train length

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30
Q

3D scan time?

A

TR x phase matrix x NEX x #Slices

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31
Q

Modifications

Spatial res

primary factor

A

voxel size

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32
Q

Voxel = ?

A

slice thickness x (fov phase/matrix size phase x fov read/matrix size read)

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33
Q

Modifications - spatial res

FOV

A

SMALLER FOV = better spatial res

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34
Q

Modifications - Spatial res

Matrix size

A

Larger matrix = smaller pixels, better res

pixel = FOV/Matrix

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35
Q

Modifications - Spatial res

Gradient

A

higher amplitude, more intense gradient = better spatial res

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36
Q

Modifications - Spatial res

Slice thickness

how to change

A

thicker slice = increased transmit RF pulse or decrease (less steep) slice selection gradient

THINNER SLICE = BETTER SPATIAL RES

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37
Q

SUMMARY FOR BETTER SPATIAL RES

4 things

A

Small FOV

Big matrix

THIN slices (steep/large select gradient, thin transmit RF bandwidth)

Small voxel

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38
Q

Modifications - Signal to Noise

Voxel

A

OPPOSITE OF SPATIAL RES

BIGGER VOXEL = BETTER SNR

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39
Q

Modifications - SNR

voxel factors

A

BIGGER BETTER SNR

LARGER FOV

SMALLER MATRIX

THICKER SLICES (increased RF pulse, decreased SSG)

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40
Q

Modifications - SNR

Field strength

A

Stronger field = BETTER SNR

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41
Q

Modifications - SNR

coils

A

Smaller, surface coils improve signal

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42
Q

Modifications - SNR

excitations per slice (number of averages)

trade - off?

A

More excitations = more signal = BETTER SNR

INCREASED imaging time

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43
Q

Modifications - SNR

Receiver bandwidth

A

Fat bandwidth, more noise picked up (noise constant in time), lower SNR

NARROW BANDWIDTH = BETTER SNR

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44
Q

SNR summary

BETTER, 9 things…

A

Stronger magnet

Long TR

Short TE

BIG FOV

SMALL Matrix

THICK slices (weaker gradient, thick transmit BW)

MORE NEX

SMALL BANDWIDTH

APPROPRIATE COILS

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45
Q

Receiver vs transmit bandwidth

A

Receiver bandwidth, bigger = more noise = WORSE SNR

Transmit bandwidth, bigger = THICKER SLICE = BETTER SNR

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46
Q

PD and SNR

A

Good way to remember

LONG TR

SHORT TE

BOTH GOOD WAYS TO IMPROVE SNR

47
Q

Tradeoffs

Field strength

A

Better SNR

INCREASED T1 time and therefore longer acquisition time via TR

more fielf, more Signal, longer time

48
Q

Tradeoffs

NEX

A

NEX directly related to signal (better SNR) but only by factor of root-2

acquisition time goes up linearly

49
Q

Spin echo basics

A

90 - PE, FE - 180 - echo, FE - 90

SSG with 90

180 at 1/2 TE, flanked by cancelling SSG, produces echo

50
Q

FSE basics

?echo train length

A

Idea is to reduce TR (major contributor to length)

MULTIPLE 180’S

TR = time between 90’s with bunch of 180’s between

Echo train length = echoes in same TR

51
Q

Fat signal and Fast spin echo

A

Fast repetition of 180 pulses cause T2 of fat to lengthen

J coupling

52
Q

Acquisition time in FSE

A

approx proportional to 1/ETL

53
Q

Inversion recovery

examples and times

A

START WITH 180

wait for relaxation time of thing you want to null (TI), hit with 90, this tissue gives no signal

STIR - 120-160 ms

FLAIR - 2000 ms

STIR- best fat supp sequence for metal or field inhomos

NO Gd with STIR (Gd has similar TI to fat)

IR tradeoff = increased acquisition time (longer TR)

54
Q

GRE

A

No 180, flip angle less than 90

(no 180 means T2* not T2, so more susceptibility)

Lower SAR (less heat)

55
Q

GRE

echo in GRE called?

Effect of shortened TR on transverse mag

Spoiled vs Refocused GRE

A

Echo in GRE = ‘Field echo’

Shortened TR means permanent residual transverse mag

Spoiled (incoherent) GRE = gradients or RF pulses used to get rid of transverse mag

Refocused (coherent) GRE = rewind gradient. example = SSFP

refocused = T2* weighted

56
Q

Echo Planar imaging

aka

A

Single shot

single RF pulse

can be done with spin echo OR GRE

FASTEST SEQUENCE

turning the phase and freq encoding gradients on and off rapidly, very fast filling of K space

57
Q

EPI artifacts

A

VERY susceptible to susceptibility (improved with segmented sequences)

Ghosting (gradient imperfections fuck with spatial encoding)

Chemical shift - narrow readout bandwidth used (fat bandwidth improves chemical shift)

58
Q

DWI

Two gradients

base sequence

A

NON-moving molecules get hit twice, dephased then rephased —-> HIGH signal

MOVING molecules Hit once, move, second gradient misses original protons, LOW SIGNAL

59
Q

B factor

A

Higher B factor = greater diffusion weighting

60
Q

B Zero

A

No contribution from diffusion, usually long TR and TE, B zero basically a T2

61
Q

fMRI

depends on ?

A

blood flow results in local reduction of deoxyhem

deoxyhem is paramagnetic, alters T2*

fMRI depends on T2*

62
Q

2D TOF MRA

A

GRE saturation pulse to null venous or arterial flow

SMALL VOXEL SIZE

63
Q

3D TOF MRA

A

higher SNR than 2D, good for high flow like COW

shorter imaging time

64
Q

Phase contrast MRA

A

bipolar gradients to create contrast from flow

High VENC arterial, low venous

QUANTITATIVE IMAGE - can measure mean blood flow velocity and direction

65
Q

Fat sat technique that’s not STIR

A

Selective pulse

Instead of using inversion time, works based on resonance diff bt protons in different microenvironments

PRECESSION FREQUENCY DIFFERENCE, more noteable with stronger fields

FAT SAT example- narrow RF pulse dropped only at resonance of fat, followed by SPOILER GRADIENT

ONLY dephases protons primed by initial narrow RF pulse (fat), so they can’t contribute to signal

Proceed as normal first RF pulse delivered immediately after spoiler

66
Q

Out of phase timing

A

1.5T magnet

out 2.2 msec

in 4.4

3T magnet

out 1.1 sec

in 2.2 sec

67
Q

Does it matter to miss 2.2 msec and start with in-phase?

A

Yes. IRON liver

Iron related T2* effect gets worse, darker, with time and second image will be darker with Fe present

Done correctly

Iron liver bright on 2.2 msec out of phase, darker on 4.4 msec in phase

Done wrong

Iron liver brighter on 4.4 sec in phase, darker on 6.6 out of phase, could wrongly assume it’s 2/2 fat

68
Q

Type 1 chemical shift (fat) artifact

looks like

which sequences

A

Bright rim on one side, dark on the other

Spin echo or GRE

69
Q

Type 2 chemical shift (fat) artifact

Looks like

what sequence?

A

Black boundary, India Ink

Type 2 goes all the way around

fat-water interface on GRE if a voxel is 50/50 fat water, signals cancel

Spin echo will eliminate

Type 2 only with GRE

70
Q

Type 1 better/worse

3 things

A

Field strength - stronger = more chem shift

Gradient strength - increased gradient = less chem shift

Readout Bandwidth - Fatter = less chem shift

71
Q

MRI contrast

2 main types

A

positive agents - shorten T1

negative agents - magnetic inhomog from susceptibility, T2 shortening (darker)

72
Q

Gad

charge

chelated to

A

+3

DTPA chelated

73
Q

How does Gd work

A

Gd has seven unpaired electrons, causes LOCAL augmentation of magnetic field

74
Q

Gd and T2 effect

A

at most concentrations, T1 effect dominates (bright)

High concentrations (bladder), T2 takes over (DARK)

pseudolayer in bladder = layering dark in dependent bladder where concentration is high enough for T2 to dominate

75
Q

MRI artifacts - image process

Aliasing

cause

direction

fix

A

wrapping of anatomy from under sampled portions

PHASE ENCODE direction

increase FOV or

Change PE direction

76
Q

MRI artifacts - chemical shift

direction

types

A

FREQUENCY

type 1 bright - dark

type 2 dark all the way around

77
Q

MRI artifacts - image process

Truncation/Gibbs

cause

look

direction

fix/penalty for fixing

A

K space fourier’d to a block of data, ripples occur at abrupt tissue interfaces with appearance of lines (csf/cord –> fake syrinx) “limited sampling of free induction decay”

More commonly in phase encoding direction (usually smaller PE matrix than FE)

Better = more matrix (decreasing bandwidth, pixel size, less FOV)

fixing means increased time and worse SNR

78
Q

MRI artifacts - image process

partial volume

A

same as CT, different signal intensities overlapping in same voxel —> intermediate signal

fix with smaller pixels

79
Q

MRI artifacts - Patient related

Motion

A

created differences between FE and PE

mainly seen in PE

80
Q

MRI artifacts - patient related

Flow

SE vs GRE

A

Ghosting in PE direction

GRE more susceptible

SE flow is DARK (GETS HIT BY 90 THEN MISSED BY 180, NO ECHO)

GRE flow is BRIGHT

fix = apply a sat band (90 followed by crusher gradient)

81
Q

MRI artifacts - Patient related

Magic angle

sequence?

A

MSK seen with tendons

SHORT TE SEQUENCES (T1, PD, GRE)

focus forms an angle of 55 degrees with main field

NOT seen on T2 (long TE)

82
Q

MRI artifacts - RF related

Cross Talk

A

Overlap in RF and FT pulses, excitation of neighboring section more than once in a single repetition

Leads to partial separation and LOWER SIGNAL

3D images not susceptible to this

83
Q

MRI artifacts - Zipper

cause

look

A

Stray RF pulses, inadequate shielding

zipper, 1 to 2 pixels in width running across, typically PE

84
Q

MRI artifacts - External field

Inhomo fat supp

A

local inhomo’s allow certain areas of fat to resist suppression, mimic edema

fix = STIR, especially with metal

85
Q

MRI artifacts - magnetic susceptibility

dia, para, ferromagnetic

most and least severe in which seqs

Susceptibility artifact due to metallic artifact can be reduced by increasing bandwidth, orienting the frequency encoding direction parallel to the long axis of the metal, increasing the field of view, decreasing the voxel size, and maximizing the echo train length. Decreasing slice thickness and echo time will also help.

A

Most severe with GRE

Less with SE (180 RF minimizes T2*)

T2* worsening with time (Fe liver getting darker on in phase)

also seen at tissue interfaces (bone and muscle, bone and air)

fixes - Use SE, swap directions, align metal with field, wider bandwidth

86
Q

Shimming

goal

Types

A

Improve field homogeneity

Passive - phantom scanned and shims plates adjusted until field is homogenous. DONE AT INSTALL

Active - using electromag coil, can be done after each patient (or sequence)

87
Q

MRI artifacts - Gradient related

Eddy Currents

cause

look

sequences

fix

A

Generated when gradients are turned on and off rapidly

occur in magnet, cables, wires or patient

looks like distortion, shift/shear

WORST WITH DWI (smeared frontal lobes)

fix by optimizing sequence of gradient pulses

88
Q

MRI artifacts - Errors in data

Dielectric/standing waves

cause

look

fix

A

reduction of wavelength bc of dielectric constant..?

local eddy currents

WORSE with stronger magnet, large bellies with ascites

Dark signal in central abdomen over LEFT LOBE OF LIVER

fix with dielectric pads bt patient and anterior body coil

parallel RF transmission, RF pulses from a set of coils with independent RF pulse (longer pulse)

89
Q

MRI artifacts - errors in data

Crisscross/herringbone

look

cause fix

A

Obliquely oriented stripes throughout

data processing or recon

Fix by reconstructing again

90
Q

Cardiac MRI

Bright blood - type of sequence

A

Gradient

91
Q

Cardiac MRI

Dark blood - sequence?

A

Spin echo

Double inversion recovery

92
Q

Inversion recovery

normal time

A

around 330msec

nulled myocardium like flair or stir

93
Q

Breast MRI

features of implant rupture sequence

A

fat and water saturated sequence (silicon will be bright)

94
Q

Breast MRI

common artifact

A

chemical shift

fix = increase bandwidth

95
Q

Breast MRI

Directions

A

Phase run side to side so pulsation stays out of tits

96
Q

Breast MRI

Signal flair

A

breast to close to coil element, won’t fat sat out correctly

reposition patient

97
Q

Quench

A

venting cooling liquid helium out of the room

resistance rapidly increases with temperature and magnet fails

98
Q

When to quench

A

someone pinned by something metallic

Life threatening fire

99
Q

Code in the scanner?

A

not a reason to quench

100
Q

5G line

A

5 Gauss exclusion zone

risk to implanted devices line

101
Q

Noise comes from?

A

Gradient coils

rapid changes in currrent

102
Q

FDA noise maximum

A

140 dB (99dB for patients with hearing protection)

103
Q

Neurostim

A

induced electrical currents, primarily in extremities

painful

usually high bandwidth readouts and rapid gradient switching

fix = reduce bandwidth, increase TR

104
Q

SAR

what is?

formula?

basic depends ons

A

ability to cook a patient via energy of RF pulse

Bo^2 x alpha^2 x duty cycle

Double field strength, quadruple SAR

alpha = flip angle, double = quadruple SAR

Duty cycle inversely related to TR, so double TR = half SAR

105
Q

Higher SAR sequences?

A

Spin echo (higher flip angles)

106
Q

SAR limits

A

no temp increase greater than 1C

FDA LIMIT 4W/kg

107
Q

MRI zones

A

I - outside building

II - No restriction - waiting room - screening takes place here

III - Restricted room - control room, where the techs work.

SHOULD BE A LOCK BETWEEN 2 and 3

IV - Scanner

108
Q

Code in zone 4?

A

Techs do CPR in zone 4, move to zone 2 for code team

109
Q

black hole over left hemiliver

worse with

fix

A

Dielectric effect ‘local eddy currents’

worse with stronger fields

worse with fat ascitic people

fix- use dielectric pads, parallel RF transmission (SENSE)

110
Q

Magic angle

better with

A

Better with longer TE (T2 sequences)

55 degrees

BETTER at higher field strengths

111
Q

Examples of ‘long, short’ TR and TE

A

Spin Echo:

  • Short TR < 700ms - Long TR > 2000 ms
  • Short TE < 25ms - Long TE > 60ms
  • For GRE:
  • Short TR < 50ms - Long TR > 100 ms
  • Short TE < 5 ms - Long TE > 10ms
112
Q

concentric rings on CT

A

Ring artifact

defective detector

3rd generation (old) scanners

fix, recalibrate, replace

113
Q

Lateral res does best with?

effect of gain?

A

a narrow beam

worse with cranked up gain (widens beam)

physics factoids (8 decks)

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