19/20: MRI Interpretation - Mahoney Flashcards
physical property of MRI
net magnetizaton
do MRIs use ionizing radiation?
no
what is the patient placed inside…
strong homogenous magnetic field [0.5 to 3.0 Tesla
1 tesla T = ______ gauss G
10,000
the magnetic field of earth =
0.5 G
steps of MRI
- radiowaves directed at patient’s body
- hydrogen atoms (protons) interact with radio waves, producing high and low intensity signals
- signals are detected and measured
- image of body is generated containing light and dark areas that correspond to high and low intensity signals, respectively
most common substance in tissue
H20
- H2 atom (proton) is manipulated by MRI to obtain the images we read on film
- could conceivable measure any chemical but H20 is most numerous
when placed in magnetic field, the majority of H2 nuclei aligns themselves …
parallel to field (Bo)
cumualtive magnetic force of H2 nuclei
M vector
resonant frequency
when protons aligned parallel also spin and wobble at a certain frequency unique for H2 = resonant frequency
a radio frequency wave which matches the resonant frequency of H2 is transmitted to pt lying in magnetic field causing 2 effects …
- alters precession h2 nuclei so that they precess in phase
2. flips the M vector away from equilibrium position parallel to Bo (because H2 nuclei enter a higher energy state)
if a 90 degree RF pulse is transmitted, M rotates …
if 180 degree RF …
- 90 degrees from long axis into transverse plane
- 180 degrees and becomes oriented in opposite direction from 90 degree pulse
as the H2 nuclei return to their original state what two things happen
- they release energy
- the precessing in phase becomes more random
as time passes, M becomes increasingly oriented along the long axis (parallel to Bo) and decreasingly oriented in transverse plane)
release of energy cause the longitudinal mag. to ________ until it reaches equilibrium value
increase
loss of phase coherence causes transverse mag. to _____ to its equilibrium value of zero
decay
rate of longitudinal relaxation
T1
rate of transverse relaxation
T2
can T1 or T2 be changed by the operator of MRI unit
NO
- T1, T2, and density of tissue are properties of tissue type that cannot be changed by the operator of MRI unit
amount of time allowed to elapse between successive 90 degree RF pulses
TR repetition time
time interval that elapses between a 90 degree RF pulse and measurement of the first spin echo signal
TE echo time
spin echo imaging
- any time you have a 90 degree pulse followed by a series of 180 degree pulses
- presence of RF signal form tissues is dependent on presence of transverse magnetization, but it decays so rapidly that a signal is difficult to measure unless you prolong the signal by adding additional 180 degree RF pulses
longer TE =
shorter TR =
- better water image
- better fat image
images produced by short TR
T1 images
- those tissues that return to Bo the fastest give off the most energy and appear bright (white) on T1 images
“fat image”
T1 weighted image
- water in fat is closest to RF of pulse, so gives off most energy and appears brightes
