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
fat has a _____ T1 than does free water
shorter
gives off the least signal on T1 image and appears black
free water
images produced by long TE
T2 weighted image
- those tissues that hold their transverse magnetization longest give off the strongest and brightes signal
gives off strongest T2 signal and appears bright white on T2 image
free water
“water image”
T2-weighted image
free water has a _ T2 than does fat
longer
T1 TR and TE range
short TR (100-1000 ms) short TE (20-30ms)
T2 TR and TE range
long TR (1600-3000 ms) long TE ( 70-100 ms)
proton-density imaging
- long TR and short TE
compliments a T2 image
tissues with high intensity signal
short T1
long T2
high proton density
tissues with low intensity signal
long T1
short T2
low proton density
inversion recovery
used to generate a T2 image where you want the fat suppressed and want max highlighting of fluid
- 180s followed by 90s
- aka STIR
- super T2 image
gradient echo
- uses only one RF excitation pulse, at less than or equal to 90
- use when you need to cut down on T2 imaging time
- good for ligaments and articular cartilage and diagnosing pigmented vilonodular synovitis
most commonly used to identigy fluid filled lesions on T1 by bright rim sign surrounding lesion
IV gadolinium
- contrast material used to suppress fat on T2 image (like STIR)
fast spin echo/ turbo spin echo
- acquires several samples in the time one sample is obtained with conventional SE technique
- cuts down on imaging time
imaging techniques that will highlight inflammation include ..
- IV gadolinum
- T2
- STIR
the shorter the T1 of a tissue, the brighter the image of _____
the longer the T2 of the tissue, the brighter the image of _______
- fat, methemoglobin, mucus
- water, CSF
what substances are always black
calcium air flowing blood hemosiderin tendons and ligaments cartilage and cortical bone
skeletal m appears ____ at all pulse sequences
gray
marrow appears ____ on T1 and ____ on T2
- bright; dark
due to presence of fat
structures that appear dark on T1: tendon bone marrow cortical bone fat fluid
tendons
cortical bone
fluid
“magic angle” ***
- on images with short TE (T1 images). when cartilage or tendon (tissue with highly structured collagen fibers) is orientated 55 degrees from the plane of magnetic fiels, will see areas of increased signal intensity in these structures
- artifact, false pathology on T1
- need to compare structures to T2 images to determine if true pathology
can i get an MRI with a metal screw in foot?
yes - orthopedic implants are safe because they do not exhibit ferromagnetism
- will cause obscuring of surrounding tissue on image to to halo effect
which of these are contraindications to performing an MRI?
- pacemaker
- orthopedic metal hardware
- retained metal from gunshot in Iraq war veteran
- metal shavings in eye
- retained bullet fragments from DSM police gun
1.
3.
4.
which image should you read first?
T2
also read film in same order (coronal, followed by transverse, followed by sagittal)
also always start from proximal to distal on coronal, inferior to superior on transverse, lateral to medial on sagittal
anatomical image
pathological image
T1
T2
muscles under the 5th met plantarlly
FDMB
T of ADM
M of ADM
muscles under 4th and part of 3rd met plantarlly
interossei
m under the medial 3rd met up to cortex of 1st met
oblique head of ADH
muscles under the 1st met
FHB
T of FHL
M of ABH
m under lateral half of calcaneus
ABM
m under the medial half of calcaneus
QP
FDB
even more medially past the calcaneus is the ABH
when you see FHL tendon on MRI, what is neurovasculature structure closest to it?
always tibial n.
most lateral interossei
3rd plantar interossei
