Diagnostic Imaging Flashcards
what is an MRI?
a diagnostic tool that utilizes a strong magnetic field and radio waves to produce an image
what is the focus of the procedure with MRI?
the focus is the hydrogen proton. Hydrogen is a primary constituent of water. Body tissue has a high ratio of water, therefore there is a ready supply of protons that can be manipulated.
MRI units
.05-3.0 Tesla (ex: 1.5 T MRI = magnetic field strength 30,000x stronger than the earth’s gravitational pull)
What is a tesla?
A unit of measure associated with magnetic fields
step 1 of MRI
Positively charged hydrogen protons spin in random directions
step 2 MRI
protons wobble in alignment with MF’s of varying intensity
step 3 MRI
a brief radio signal knocks the protons out of alignment
step 4 MRI
protons snap back into alignment with the MF when the radio signal ceases. this emits a radio signal of their own that announces the presence of a specific tissue
radio frequency info
as the protons move back into alignment after the radio wave stops, the protons transmit an energy wave that is unique to a certain type of tissue
“T” in MRI reports
based on the time it takes for protons to realign or reach states of equilibrium
T1
related to the realignment with the imposed magnetic fields
T2
related to the transverse energy pulses that are delivered and the spin dephasing that occurs
T1 and T2 weighted images differ based on what 2 parameters?
1) Time to Echo (TE)
2) Time to Repetition (TR)
define TE
time to echo
- the time from the application of a pulse (RF) to when the spin signal is picked up
define TR
time to repetition
- the time between pulse (RF) sequences
T/F T1 weighted images have a short TE and TR
true, meaning tissue that emits energy more quickly tends to have a brighter image
T/F tissues that are slower to release energy will be darker
true
these 3 things pick up images of tissues that are slow to release energy
- T2
- longer TE and TR times
- tissue with high water content
on T1, what appears bright? what appears dark? (1 thing each)
fat appears bright
water appears dark
on T2, what appears bright and what appears dark? (1thing each)
fat generates a low signal and will appear dark for T2
water has a high T2 signal and appears bright
6 steps to MRI (summary)
- pt placed in scanner
- MF aligns protons w/ pt
- EM pulse is sent into scanner and causes protons to reorient
- pulse turns off and protons relax
- as protons relax, they emit a RF signal that is picked up by the antenna in the scanner
- signal processed by the computer and software creates images
T2 is moderately sensitive for detecting what 2 things?
pathology and edema
disadvantage of T1
poor demonstration (image) of pathology/edema
advantage of T1
provides the best anatomic detail
disadvantage of T2
decreased soft tissue detail
STIR stands for
short TI inversion recovery
what is STIR
a technique used to suppress the signal from fat. it allows improved imaging of inflammatory or neoplastic tissues
FLAIR stands for
fluid-attenuated inversion recovery
what is FLAIR
allows fluid signals to be nullified. this is useful in looking at pathologies of the CNS such as MS
define gadolinium
a non-toxic paramagnetic contrast enhancement agent used in MRI. when injected it will change signal intensities by shortening T1
define T1 weighted
an image created by using short TE and TR times whose contrast and brightness are determined by T1 signals
define T2 weighted
an image created by longer TE and TR times whose contrast and brightness are determined by T2 signals
define hyperintense
more intense/brighter
define hypointense
less intense/darker
define isotense
the same intensity as the ____ (words missing on the PP)
indications of MRI
- tumors
- stroke
- epilepsy
- demyelination
- infection
- CN palsy
- chronic headache
- dementia
advantage of MRI (related to tissue)
- exquisite soft tissue contrast between normal tissue (white/gray matter) and pathologic tissue
advantage of MRI (image)
customize imaging techniques to answer specific questions
advantage of MRI (3 more)
- no ionizing radiation
- directly image in any plane
- ability to do functional MRI
5 disadvantages of MRI
- high cost, limited access
- difficult for unstable pts
- claustrophobia
- absolute contraindications
- not great for bony detail
what does MEG stand for
magnetoencephalography
MEG
- non invasive technique
- measures cellular activity of the brain based on the detections of MF changes
- good temporal/spatial resolution
MEG is used for what in the clinic and in research?
in cognitive research but is useful clinically for evaluating seizures
when MEG is combined with MRI what is it used for
pre-operative brain mapping
cost of MEG?
high, therefore accessibility is limited
EEG stands for
electroencephalography
EEG does what
records the electoral activity of the brain using electrodes attached to the scalp
EEG captures what
the electrical currents generated by activated neurons
EEG
- good temporal resolution but spatial resolution not as good as MEG
- EEGs detect superficial activity
EEG vs MEG clinically
EEG used more often because they cost less
PEt stands for
positron emission tomography
SPET stands for
single photon emission tomography
PET measures what
the metabolism of the tissue. radionuclide is injected into the body
PET + CT help with what
improve the localization of a lesion. assists with differentiating normal from pathologic tissue
fMRI is a measure of what
it is an indirect measure of neural activity
fMRI based on what
the hemodynamic response related to neural activity
fMRI uses MRI imaging physics to trace a contrast medium. what does it track
the ratio of oxygenated to deoxygenated blood via BOLD contrast
what is bold contrast
blood oxygen level dependent contrast - uses T2 weighted process
advantages of BOLD contrast with fMRI
the patient is not exposed to radiation. provides anatomic and function in one procedure and __ (slides cut off again…)
