Chapter 15 Magnetic Resonance I Flashcards
how is magnetism created?
moving electric charges
how do magnetic fields exist
as dipoles
what is magnetic field strength
tesla
number of magnetic lines per unit area
what nuclei have magnetization
nuclei with odd number of protons or odd number of neutrons
other names for magnetic nuclei
-dipoles
-spins
-magnetic moments
magnetic field strength of protons vs electrons
proton fields are 1000X weaker than electron fields
what do protons do in the absence of an external magnetic field?
orient randomly
no net magnetization
what happens to magnetic nuclei when they are placed in a magnetic field
precess at the larmor frequency
what is larmor frequency proportional to?
magnetic field strength
larmor frequency of protons
42 MHz/T
parrallel vs anti-parrallel proton orientation
parrallel = lower energy level- slghtly more protons oriented this way
what component contributes to MR signal?
z component only
net tissue magnetization
difference between parrallel and anti-parralel magnetization along z-axis
what is magnetic susceptibility
extent to which matter becomes magnetized when placed in an external magnetic field
-local magnetic fields change because of the effect of atomic electrons
diamagnetic materials
small negative susceptability
small decreases of the local magnetic field
tissues and plastic
what happens at tissue interfaces
changes in susceptibility result in changes in local magnetic fields
-likely yields to signal loss due to spin de-phasing
paramagnetic materials
-increase local magnetic field
-caused by magnetism of unpaired atomic electrons
-gadolinium
-deoxyhemoglobin
ferromagnetic materials
-dramatically increase local magnetic field
-large susceptibility
-steel
-some implanted medical devices
net magnetization at 1 T
3 in a million (low SNR)
what is resonance
RF field interacts with net nuclear magnetization
applied RF must be at Larmor frequency
causes Mz to rotate at a rate proportional to RF intensity
what happens after Rf is switched off
Mz has rotated through a flip angle
have longitudinal and perpendicular magnetization
what does doubling RF pulse duration do?
doubles flip angle
free induction decay
-transverse magnetization rotates at Larmor frequency
-detected as induced voltage in coil wrapped around tissues
-voltages detected in coil oscillate at larmor freqneyc = FID
how to increase FID signal frequency
increase magnetic field
does longitudinal magnetization lead to signal?
NO
T1
longitudinal magnetization grows from 0 to Mz exponentially
-after T1, 64% of Mz will have formed
-full Mz forms after 4T1
-spin-lattice interactions
T1 and T2 of bone
molecules are large and slow
long > 1000 ms T1
short < 0.01 ms T2
T1 and T2 of soft tissue
molecules are medium size
moderate 500 ms T1
short 50 ms T2
T1 and T2 of fluid
molcules are small and fast
long > 1000 ms T1
long > 1000 ms T2
how does T1 change with magnetic field strength
increases with increasing strength
quadrupling magnetic field doubles T1 time
what determines T1 and T2?
-molecules that move at the larmor frequency encourage nuclei to return to equlibrium (T1 shorter)
T2
exponential decay of transverse magnetization
-at time T2, FID has decayed to 37% of its original value
-after 4X T2, transverse magnetization is null
-spin-spin interactions dephase transverse M when they experience each other’s magnetic field
how to shorten T2
increase spin-spin interactions
does T2 depend on field strength
no
is T2
Mt cannot be present when Mz fully recovered
what increases T2*
any magnetic field inhomogeneity that increases spin dephasing in the transverse plane
where do inhomogeneities come from
susceptibility diffrrences at tissue boundaries
MR magnets always have some
vicinity of magnetic tissues
what is T2*
dephasing of Mxy from field inhomogeneities
paramagnetic contrast agent
gadolinium DTPA
what does gadolinium do
reduces T1 by increasing spin-lattice interctions
-makes hyperintensity on T1 weighted images
-positive contrast agent
what is Fe3O4
superparamagnetic
-when placed in external magnetic field, develop a strong internal magnetization
paramagnetic contrast agents
disrupt local field inhomogeneity
shorten T2 and T2*
hypointensity on T2 weighted images
negative contrast agent
where is super-paramagnetic iron oxide used?
image liver and reticuloendothelial system
ultrasmall SPIO are used to differentiate begign and inflammatory lesions
describe spin echo
90-180-acquire
180 at TE/2 generates a spin echo at time TE, canceling out T2* dephasing effects
why do the spins spins at different speeds?
slightly higher local field= slightly faster spin
what does signal localization require?
magnetic field gradients
gradient changes Larmor frequenc along the gradient direction
what permits MR signal to be extracted?
fourier analysis
how is the voxel where signal is coming from selected?
use 3 gradients
slice select
frequency encode
phase encode
each echo is obtained with a unique set of combinations of 3 gradients
what is needed to generate 128x128 MR image?
acquire 128 echoes and sample each echo 128 times
bandwidth
narrow range of frequencies included in RF pulse
determines slice thickness
-increasing RF bandwidth results in thicker slices
slice-select gradient strength
mT/m
increasing this strength gives thinner slices for the same BW
frequency vs phase encode
when echoes are formed, a frequency encode gradient is applied to encode spatial info along the x axis. Perpendicular to slice-select direction
a phase encode is used to encode spatial info along the y-axis. Perpendicular to slice-select and frequency-encode direction
what does sampling the echo more do to imaging time?
when echo sampling rate doubles, signals are halved but imaging time is the same
what gradients are on during 90 and 180 degree pulses for spin echo?
slice select
what determines the matrix size in the phase encode direction?
number of phase encode gradients
what does each number in k-space refer to?
spatial frequency
center vs periphery of k-space
center = low spatial frequency
periphery = high spatial frequency
what do low vs high spatial frequencies provide
low = image contrast
high = small features and edges
how are MR images obtained from k-space?
2D FT of k-space
strength of earth’s magnetic field
50 uT
what do MR machines use to generate uniform magnetic field
superconducting MR magnets
-kept cold using liquid helium
is magnetic field always on?
yes for superconducting magnets
magnet quench
wire temperature rises
system loses its superconducting properties
stored magnetic energy is converted to heat
homogeneity of magnetic fields for MRI
a few parts per million
what can elevators and other large metal structures due to MRI machine
disrupt uniformity of main magnetic field and degrade image quality
shimming
used to correct main field to improve uniformity
how do metallic objects show up in spin echo>
bright area (pulse pile) due to magnetic field distortions
missile effect
ferromagnetic objects can be pulled into the magnet
why 3 orthogonal gradient coils?
allows gradients to be oriented in any arbitrary direction
what kind of coils produce z gradients and x/y gradients
hemholtz coils for z
saddle coils for x/y
gradient strenths on 1.5 T scanner
30 mT/m
slew rate
time to achieve required magnetic field amplitude
what do nonuniform gradients cause
image distortions
how fast are gradients switched on and off
< 500 us
eddy currents
generated in coils or metal structures from the switching of the gradients
-eddy currents create image artifacts
-actively shielding gradient coils helps to reduce effects of eddy currents
transmitter vs receiver coils
transmitter: send RF pulses
receiver: detect radio waves from patients
transmit bandwidth
range of emitted frequencies
can some coils do both transmit and receive?
yes they can switch
what is receiver BW proportional to?
-gradient strength
-noise goes up with BW and therefore gradient strength
volume coils
designed to transmit and receive uniform RF signal throughout a volume
surface coil
more sensitive close to coil
signal drops off as you go away from coil
linear vs quadrature coils
linear- receive signal from only x or y of rotating magnetization
quadrature: uses signal in both x and y axes- increases SNR
phased array coils
-used for parrallel imaging
-combination of many surface coils around body part
dieletric artifacts
occurs at high fields because RF waves are short compared to size of body\
-standing wave currents can arise and cause destructive and constructive interference
how do we prevent RF signals from getting into coils and adding background noise?
RF shielding
also prevents RF pulses from interfering with outside electronic equipment
what is the RF shiedling
faraday cage
-copper
-all utility services like electrical must be routed through special filters
what does RF leakage into the MR suite cause?
zipper artifacts along phase encode direction
fringe field
peripheral magnetic field that extends from the magnet
static magnetic shiedling
thick iron plates or layers of special steel sheet metal in the MR magnet room walls
distance of the 0.5 mT line (5 G) from isocenter of 1.5 T magnet
along central bore axis: 12 m unshielded, 4 m shielded
perpendicular to bore: 9.5 m unshielded, 2.5 m shielded
active shielding
-uses coils to make magnetic fields to cancel out ambient field
-take away need for static magnetic shielding
what magnetic fields can deactivate pacemakers
0.5 mT
where is access restricted
inside 5 G (0.5 mT) line
S distortions
magnetic fields may result in S distortions in fluoro images
how does time and RF strength affect flip angle?
flip angle is proportional to time * RF strength
FID frequency is proportional tp?
applied magnetic field
T2* vs T2 vs T1 times
T2 * are 10 times shorter than T2 and 100 X shorter than T1 times
where are inhomogeneities most likely to affect signal intensity?
gradient recalled echoes
SE removes T2* effect
why does fat saturation failure ok?
main magnetic field has non-uniformities
T2* comes from?
inhomogeneities in main magnetic field
this is why we can remove T2* with SE- because the inhomogeneitu is constant in the field
Does T2 depend on field strength?
no
what do contrast agents do to relaxation times?
reduce them
