MRI Flashcards

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
Horizontal = frequency encoding
frequency encoding results in a column of protons which have identical frequencies APPLIED at SAME TIME AS READOUT
26
TR =
time BETWEEN 90 RF's
27
When is SSG applied?
at same time as 90 degree RF's
28
**Modifications** Table time
Time = TR x Phase matrix x # of excitations Nex = number of times each set of phase encoding steps is repeated
29
**Modifications** **when does time not follow normal equation?**
Fast spin echo Acquisition time is proportional to 1/echo train length
30
3D scan time?
TR x phase matrix x NEX x **#Slices**
31
Modifications Spatial res primary factor
voxel size
32
Voxel = ?
slice thickness x (fov phase/matrix size phase x fov read/matrix size read)
33
**Modifications - spatial res** **FOV**
SMALLER FOV = better spatial res
34
Modifications - Spatial res Matrix size
Larger matrix = smaller pixels, better res pixel = FOV/Matrix
35
Modifications - Spatial res Gradient
higher amplitude, more intense gradient = better spatial res
36
Modifications - Spatial res Slice thickness how to change
thicker slice = increased transmit RF pulse or decrease (less steep) slice selection gradient THINNER SLICE = BETTER SPATIAL RES
37
SUMMARY FOR BETTER SPATIAL RES 4 things
Small FOV Big matrix THIN slices (steep/large select gradient, thin transmit RF bandwidth) Small voxel
38
Modifications - Signal to Noise Voxel
OPPOSITE OF SPATIAL RES BIGGER VOXEL = BETTER SNR
39
Modifications - SNR voxel factors
BIGGER BETTER SNR LARGER FOV SMALLER MATRIX THICKER SLICES (increased RF pulse, decreased SSG)
40
Modifications - SNR Field strength
Stronger field = BETTER SNR
41
Modifications - SNR coils
Smaller, surface coils improve signal
42
Modifications - SNR excitations per slice (number of averages) trade - off?
More excitations = more signal = BETTER SNR INCREASED imaging time
43
Modifications - SNR Receiver bandwidth
Fat bandwidth, more noise picked up (noise constant in time), lower SNR NARROW BANDWIDTH = BETTER SNR
44
SNR summary BETTER, 9 things...
Stronger magnet Long TR Short TE BIG FOV SMALL Matrix THICK slices (weaker gradient, thick transmit BW) MORE NEX SMALL BANDWIDTH APPROPRIATE COILS
45
Receiver vs transmit bandwidth
Receiver bandwidth, bigger = more noise = WORSE SNR Transmit bandwidth, bigger = THICKER SLICE = BETTER SNR
46
PD and SNR
Good way to remember LONG TR SHORT TE **BOTH GOOD WAYS TO IMPROVE SNR**
47
Tradeoffs Field strength
Better SNR INCREASED T1 time and therefore **longer acquisition time via TR** **more fielf, more Signal, longer time**
48
Tradeoffs NEX
NEX directly related to signal (better SNR) but only by factor of root-2 acquisition time goes up linearly
49
Spin echo basics
90 - PE, FE - 180 - echo, FE - 90 SSG with 90 180 at 1/2 TE, flanked by cancelling SSG, produces echo
50
FSE basics ?echo train length
Idea is to reduce TR (major contributor to length) ## Footnote **MULTIPLE 180'S** **TR = time between 90's with bunch of 180's between** **Echo train length = echoes in same TR**
51
Fat signal and Fast spin echo
Fast repetition of 180 pulses cause T2 of fat to lengthen J coupling
52
Acquisition time in FSE
approx proportional to **1/ETL**
53
Inversion recovery examples and times
**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
GRE
No 180, flip angle less than 90 (no 180 means T2\* not T2, so more susceptibility) Lower SAR (less heat)
55
GRE echo in GRE called? Effect of shortened TR on transverse mag Spoiled vs Refocused GRE
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
Echo Planar imaging aka
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
EPI artifacts
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
DWI Two gradients base sequence
NON-moving molecules get hit twice, dephased then rephased ----\> HIGH signal MOVING molecules Hit once, move, second gradient misses original protons, LOW SIGNAL
59
B factor
Higher B factor = greater diffusion weighting
60
B Zero
No contribution from diffusion, usually long TR and TE, B zero basically a T2
61
fMRI depends on ?
blood flow results in local reduction of deoxyhem deoxyhem is paramagnetic, alters T2\* fMRI depends on T2\*
62
2D TOF MRA
GRE saturation pulse to null venous or arterial flow SMALL VOXEL SIZE
63
3D TOF MRA
higher SNR than 2D, good for high flow like COW shorter imaging time
64
Phase contrast MRA
bipolar gradients to create contrast from flow High VENC arterial, low venous QUANTITATIVE IMAGE - can measure mean blood flow velocity and direction
65
Fat sat technique that's not STIR
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
Out of phase timing
1.5T magnet out 2.2 msec in 4.4 3T magnet out 1.1 sec in 2.2 sec
67
Does it matter to miss 2.2 msec and start with in-phase?
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
Type 1 chemical shift (fat) artifact looks like which sequences
Bright rim on one side, dark on the other Spin echo or GRE
69
Type 2 chemical shift (fat) artifact Looks like what sequence?
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
Type 1 better/worse 3 things
**Field strength** - **stronger = more** chem shift **Gradient strength** - **increased gradient = less** chem shift **Readout Bandwidth** - **Fatter = less** chem shift
71
MRI contrast 2 main types
positive agents - shorten T1 negative agents - magnetic inhomog from susceptibility, T2 shortening (darker)
72
Gad charge chelated to
+3 DTPA chelated
73
How does Gd work
Gd has seven unpaired electrons, causes LOCAL augmentation of magnetic field
74
Gd and T2 effect
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
**MRI artifacts - image process** Aliasing cause direction fix
wrapping of anatomy from under sampled portions PHASE ENCODE direction increase FOV or Change PE direction
76
**MRI artifacts - chemical shift** **direction** **types**
FREQUENCY type 1 bright - dark type 2 dark all the way around
77
**MRI artifacts - image process** Truncation/Gibbs cause look direction fix/penalty for fixing
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"** ## Footnote **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
MRI artifacts - image process partial volume
same as CT, different signal intensities overlapping in same voxel ---\> intermediate signal fix with smaller pixels
79
MRI artifacts - Patient related Motion
created differences between FE and PE **mainly seen in PE**
80
MRI artifacts - patient related Flow SE vs GRE
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
MRI artifacts - Patient related Magic angle sequence?
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
MRI artifacts - RF related Cross Talk
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
MRI artifacts - Zipper cause look
Stray RF pulses, inadequate shielding **zipper, 1 to 2 pixels in width running across, typically PE**
84
MRI artifacts - External field Inhomo fat supp
local inhomo's allow certain areas of fat to resist suppression, mimic edema fix = STIR, especially with metal
85
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.*
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
Shimming goal Types
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
MRI artifacts - Gradient related Eddy Currents cause look sequences fix
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
MRI artifacts - Errors in data Dielectric/standing waves cause look fix
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
MRI artifacts - errors in data Crisscross/herringbone look cause fix
Obliquely oriented stripes throughout data processing or recon Fix by reconstructing again
90
Cardiac MRI Bright blood - type of sequence
Gradient
91
Cardiac MRI Dark blood - sequence?
Spin echo Double inversion recovery
92
Inversion recovery normal time
around 330msec nulled myocardium like flair or stir
93
Breast MRI features of implant rupture sequence
fat and water saturated sequence (silicon will be bright)
94
Breast MRI common artifact
chemical shift fix = increase bandwidth
95
Breast MRI Directions
Phase run side to side so pulsation stays out of tits
96
Breast MRI Signal flair
breast to close to coil element, won't fat sat out correctly reposition patient
97
Quench
venting cooling liquid helium out of the room resistance rapidly increases with temperature and magnet fails
98
When to quench
someone pinned by something metallic Life threatening fire
99
Code in the scanner?
not a reason to quench
100
5G line
5 Gauss exclusion zone risk to implanted devices line
101
Noise comes from?
Gradient coils rapid changes in currrent
102
FDA noise maximum
140 dB (99dB for patients with hearing protection)
103
Neurostim
induced electrical currents, primarily in extremities painful usually high bandwidth readouts and rapid gradient switching fix = reduce bandwidth, increase TR
104
SAR what is? formula? basic depends ons
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
Higher SAR sequences?
Spin echo (higher flip angles)
106
SAR limits
no temp increase greater than 1C ## Footnote **FDA LIMIT 4W/kg**
107
MRI zones
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
Code in zone 4?
Techs do CPR in zone 4, **move to zone 2 for code team**
109
black hole over left hemiliver worse with fix
Dielectric effect 'local eddy currents' worse with stronger fields worse with fat ascitic people fix- use dielectric pads, parallel RF transmission (SENSE)
110
Magic angle better with
Better with longer TE (T2 sequences) 55 degrees BETTER at higher field strengths
111
Examples of 'long, short' TR and TE
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
concentric rings on CT
Ring artifact defective detector 3rd generation (old) scanners fix, recalibrate, replace
113
Lateral res does best with? effect of gain?
a narrow beam worse with cranked up gain (widens beam)