PHYSICS - MRI Flashcards
Ferromagnetic substances
dramatic increase in local magnetic field, large increase in susceptibility; e.g. metal
Paramagnetic substances
small increase in local magnetic field, small increase in susceptibility; e.g. deoxyhemoglobin, gadolinium
Diamagnetic substances
small decrease in local magnetic field, small decease in susceptibility; e.g. tissues, calcium
Requirement for an atom to have net magnetism
odd mass number (protons + neutrons)
Net magnetization in the absence of an external magnetic field
no net magnetization; protons are randomly oriented
Larmor frequency is proportional to…
magnetic field strength
Net longitudinal magnetization (Mz) is proportional to…
magnetic field strength; parallel vs. antiparallel orientation
Larmor frequency for H+ at 1 Tesla
42 MHz; which means the gyromagnetic ratio of H+ is 42 MHz/Tesla
Transverse magnetization (Mxy) immediately after external magnetic field applied
none; phase is random
Is the parallel or antiparallel orientation a lower energy state?
parallel orientation is a lower energy state (preferred)
Susceptibility definition
extent to which matter becomes magnetized when placed in an external magnetic field; causes spin dephasing resulting in signal loss
Flip angle definition
angle of net magnetization vector relative to the Z-axis
For resonance to occur, the RF pulse must be…
RF pulse must be at the Larmor frequency and perpendicular to the Z-axis
Free induction decay signal
voltage detected by coils, which is induced by the rotating transverse magnetization vector; voltage oscillates at the Larmor frequency
Relationship between magnetic field strength and FID signal
directly proportional; increased field strength => increased FID signal
T1 relaxation time increases or decreases with increased field strength?
increases (longer T1 relaxation time); energy exchange with the lattice is less efficient
Short T1 relaxation time - bright or dark
bright; tissue recovers signal quickly
Long T1 relaxation time - bright or dark
dark; tissue recovers signal slowly
Short T2 relaxation time - bright or dark
dark; tissue loses signal quickly
Long T2 relaxation time - bright or dark
bright; tissue loses signal slowly
Causes of loss of phase coherence
spin-spin interactions and magnetic field inhomogeneities (may be external or local)
T2 relaxation time increases or decreases with increased field strength?
neither; T2 relaxation is independent of magnetic field strength
T1 or T2 relaxation times are longer? (generally)
T1 relaxation times are much longer
Faraday’s Law of Induction
a moving magnetic field with induce a current within a coil
T1 time constant
time at which 63% of Mz has formed
T2 time constant
time at which Mxy has decayed to 37% of its original value
How many T1’s for recovery of full net longitudinal magnetization?
4 T1’s (~99%); same for T2 decay (4 T2’s)
How does an increase in magnetic field strength by 4x affect T1 relaxation?
2x increases in T1 relaxation time
T2 signal when longitudinal magnetization has fully recovered
none; not possible to have T2 signal (transverse magnetization) when longitudal magnetization has fully recovered
T2* decay
dephasing due to spin-spin interactions and magnetic field inhomogenties (local or external)
Formula for T1 contribution to signal
1 - e^(-t/T1), where t is the TR and T1 is tissue specific
Formula for T2 contribution to signal
e^(-t/T2), where t is the TE and T2 is tissue specific
Short TR
<500 msec
Long TR
> 2000 msec
Short TE
<30 msec
Long TE
> 80 msec
TR and TE for proton density
long TR, short TE
Spin density sequence
a.k.a. proton density
Spin echo: 180 degree pulse timing
TE/2
T2 vs. T2* decay
T2 decay is the result of spin-spin interactions, while T2* decay is the result of spin-spin interactions + field inhomogeneities
Relationship between magnetic field strength and SNR
directly proportional; 2x field strength => 2x SNR (noise does not change with field strength)
Gradient echo pulse sequence
very short TR; <90 degree pulse => bipolar (dephasing and rephasing) gradients
How to: increase T1-weighting on GRE
increase flip angle
How to: increase T2-weighting on GRE
increase TE
Effect of gadolinium on T1 and T2 relaxation
gad increases T1 and T2 relaxation (shortening) => T1 bright, T2 dark
Standard dose of gadolinium
0.1 mmol/kg
Risk of gadolinium administration in CKD
nephrogenic systemic fibrosis (widespread tissue fibrosis)
Contraindications to gadolinium
pregnancy, GFR <30
Gadolinium agents with no known association with NSF
macrocyclic gadolinium agents
Timing of slice select gradient
applied during RF pulse (RF pulse determines which “slice” of tissue is excited); during ALL RF pulses, not just initial
How to: obtain thinner slices
decrease transmit bandwidth (of RF pulse), increase slice select gradient strength
Timing of frequency encoding gradient
applied at TE (during sampling)
Echo sampling rate
number of times each echo is sampled; corresponds to frequency encode matrix size
Determinants of matrix size
number of times each echo is sampled in the frequency encoding direction, number of phase encoding gradients in the phase encoding direction
Timing of phase encoding gradient
between RF pulse and echo; different phase encoding gradient is applied for each acquired echo
Center of k-space
center represents low spatial frequencies (large structures/”contrast”)
Periphery of k-space
periphery represents high spatial frequencies (small features and edges/”details”)
Gradient applied across the widest dimension (generally)
frequency encoding gradient
Fast spin echo (FSE)
fill multiple rows of k-space within a single TR (multiple TEs in each TR); longer TR required
Shimming
used to correct small inhomogeneities in the external magnetic field => improved uniformity
Transmit bandwidth
range of frequencies emitted in an RF pulse
Relationship between receiver bandwidth and noise
directly proportional; greater receiver bandwidth => increased noise
Echo train length (ETL)
number of echoes acquired in a single TR; FSE or turbo spin echo sequences
256 x 128 matrix - which is FE and which is PE direction?
longer dimension is typically the FE direction (so 256 in this example)
TOF in head
3-D TOF GRE
TOF in neck
2-D TOF GRE
Advantages of GRE
short TR (faster acquisition)
Disadvantages of GRE
lower signal (smaller flip angles), more noise; echoes formed by rephasing gradients are relatively weak
TI (in inversion recovery)
between 180 and 90 degree pulses; TR in IR sequences includes both TI and TE
Effect of increased magnetic field strength on TI
increased field strength => increased TI (T1 relaxation is longer)
Inversion recovery + gad
STIR is not used with gad; T1 shortening caused by gad creates a null point similar to fat
Dielectric artifact
a.k.a. standing wave artifact; RF pulse wavelength approximates dimension of patient; worse with higher field strengths
Dark signal in central abdomen over left lobe of liver
dielectric artifact
3-D MRI
2 phase-encoding gradients (y and z-axes); better Z-axis resolution, but longer study time (motion)
Echo planar imaging (EPI)
rapidly switching gradients results in numerous echoes generated within a single TR; fast, less motion, but increases susceptibility
BOLD (acronym)
blood oxygen level dependent (oxy-Hb); fMRI technique to detect areas of increased blood flow related to localized brain activity; heavily T2*-weighted EPI sequence
Contrast in MRI is determined by…
tissue properties (T1, T2, T2*)
How to: increase SNR
increase voxel size, increase magnetic field strength, decrease receiver bandwidth, increase number of acquisitions/excitations per slice, smaller coil (surface coil), increase TR/decrease TE
Type 1 chemical shift artifact
differences in precessional frequencies at fat-water interfaces results in misregistration; seen in all sequences
How to: fix type 1 chemical shift artifact
increase receiver bandwidth, increase strength of frequency encoding gradient (steeper slope), or use fat sat
Effect of increased magnetic field strength on type 1 chemical shift artifact
increased type 1 chemical shift artifact
Type 2 chemical shift artifact
in-phase and OOP sequences detetermined by differences in precessional frequencies of fat and water protons; GRE only
Pulse sequence for in and out-of-phase images
GRE pulse sequence
India ink artifact
voxels at fat-organ interfaces contain portions of both fat and water resulting in signal dropout
Dixon W
sum of in and out-of-phase images; result is a fat-saturated T1 image
Truncation artifact
a.k.a. Gibbs or ringing; occurs at sharp edges; may mimic syrinx
How to: fix truncation artifact
increase matrix size (# of phase-encoding steps), use smoothing filter
Wrap around artifact
a.k.a. aliasing; caused by small FOV
How to: fix wrap around artifact
increase FOV, phase oversampling, switch PE and FE directions, apply saturation bands outside FOV
Magic angle artifact
occurs in tendons; 55 degree angle to Z-axis; disappears on T2
Fat saturation techniques for an inhomogeneous field
STIR or Dixon W
Fat saturation techniques for post-gad imaging
chemical fat sat (FSFS) or Dixon W
Artifact(s) occurring in frequency-encoding direction
type 1 chemical shift
Spike artifact
a.k.a. herringbone; electromagnetic spike during filling of k-space
How to: fix spike artifact
remove bad data point or re-scan patient
Requirements for MR scanning in pregnancy
must document necessity of information, cannot be achieved by ultrasound, and cannot wait until after delivery
SAR limits
relates to heating; 3 W/kg per 15 min for head and 4 W/kg per 15 min for body
Who sets SAR limits?
FDA
Effect of increasing receiver bandwidth
decreased SNR, decreased type 1 chemical shift, decreased TR/TE, decreased scan time
Difference between k-space matrix size and image matrix size
no difference in terms of size; both have the same dimensions
Effect of partial k-space acquisition
decreased SNR, decreased scan time
How to: decrease susceptibility
decrease TE, use SE instead of GRE, decrease field strength, metal suppression sequence (if from metal)
How to: prevent muscle twitching or paresthesias
tell patient to not cross legs or join hands; due to rapidly switching gradients (also create acoustic noise)
MR conditional
safe for specific MR environments (e.g. magnet strength, SAR)
MR zone 1
waiting area
MR zone 2
patient survey area
MR zone 3
control room; restricted access
MR zone 4
magnet room; restricted access
Zipper artifact
may appear as an irregular line across image or scattered dots
How to: fix zipper artifact
shut scanner room door, remove cell phone, check RF shielding of room
How to: fix incomplete fat suppression
shimming, use STIR instead (non-post gad only)
Signal of fat on FSE
increased T2 relaxation time of fat => fat is bright on T2 (J-coupling)
Relationship between coil size and SNR
increase coil size => more noise => decreased SNR
Five-gauss line
defines controlled access area around the MRI scanner; 0.5 mT
When to use half-dose of gad?
GFR 30-40
Advantages of FSE
shorter study, more time for larger FOV, less motion => higher resolution
Disadvantage of FSE
decreased contrast (less signal from each subsequent TE), worse T1 (because longer TR)
Chemical fat suppression
a.k.a. frequency-selective fat saturation; prepatory pulse => spoiler gradient => normal sequence; longer acquisition because of extra prepatory pulse
Susceptibility artifact based on sequence
EPI > GRE > SE > FSE
SAR based on sequence
FSE > SE > GRE; GRE has a small flip angle and single RF pulse
Determinants of spatial resolution
slice thickness, FOV, matrix size
BLADE or PROPELLER
redundant sampling of the center of k-space => less motion artifact, slower acquisition
Half-fourier acquisition
faster acquisition, lower SNR
Timings of in and out-of-phase sequences
TE 2.2 ms for out-of-phase, TE 4.4 ms for in-phase (at 1.5 T)
How to: decrease SAR
use fewer RF pulses, increase TR, smaller flip angles, lower field strength
Fastest MR sequence
EPI (single shot)
Readout gradient
a.k.a. frequency encoding gradient
Bright blood (cardiac MRI)
SSFP (GRE based); may obtain cine imaging
Dark blood (cardiac MRI)
SE based; “new” blood not excited by 90 degree pulse, thus generating no signal
Ghosting artifact
due to motion; may also be called smearing, pulsation, or motion artifact; occurs in phase-encode direction
Gadolinium should be administered at what temperature?
room temperature (72 degrees F)
How to: fix dilelectric artifact
use a lower field strength (1.5T magnet), drain ascites
Determinants of SAR
field strength, flip angle, TR, frequency of RF pulses; 2x field strength or flip angle => 4x SAR
Increase in core body temperature should not exceed… (adults)
1 C
Increase in core body temperature should not exceed… (infants)
0.5 C
Sequence with highest SNR
proton density (but has poor tissue contrast)
SAR (acronym)
specific absorption rate
Short or long inversion times for fat and fluid
fat has a short TI and fluid has a long TI (based on their T1 properties)
DWI pulse sequence
90 degree pulse => dephasing gradient => 180 degree pulse => rephasing gradient; EPI-based pulse sequence
Difference between b-0 and b-1000 (DWI)
b-0 has no diffusion gradients; b-1000 has diffusion gradients; b-0 and b-1000 are used to compute the ADC
Flow-related enhancement
a.k.a. TOF; GRE-based; fresh blood in plane is not saturated by short TRs => high signal
Causes of signal loss on TOF
slow flow, turbulent flow, no flow, flow parallel to imaging plane
Phase contrast MRA
bipolar gradients (positive and negative) applied between excitation and readout; stationary spins are cancelled out, while mobile spins only experience one of the gradients which generates signal; gray = stationary tissues
Benefit of phase contrast MRA
quantitive measurement of velocity
Majority of MRI-related adverse events
RF burns (bone screws, tattoos, EKG leads)
Diffusion tensor imaging
quantifies extent to which water molecules are restricted in various directions; can infer path of white matter tracts
2D Fourier transform
k-space => image; 3D images required a 3D Fourier transform
