Neuroimaging Flashcards
Magnetic Resonance Imaging - diagnostic tool that utilizes
a strong magnetic field and radio waves to produce an image
Focus of MRI is
hydrogen proton
Hydrogen is a primary constituent of
water
body tissue has high ratio of water - there is a supply of protons that can be manipulated with MRI
MRI units in a clinical setting are in the ___ to __ range
.5 to 3.0 Tesla
A MRI that is 1.5 T has a magnetic field strength that is how many times stronger than earth’s gravitational pull
30,000
what is a tesla
unit of magnetic field intensity equal to the magnitude of the magnetic field vector needed to produce a force of 1 newton on a charge of one coulomb moving perpendicular to the direction of the magnetic field vector with a velocity of 1 meter per second
Basis steps of what happens during an MRI - step 1 - 4
- hydrogen protons normally spin in random directions
2 protons wobble in alignment with magnetic fields
3 radio signal knocks the protons out of alignment
4 when radio signal ceases, the protons snap back into alignment emitting a radio signal of their own - announces presence of a specific tissue
Basis steps of what happens during an MRI - step 1 - 4
radio signal soundwave frequency matches
the frequency of wobble of certain protons
How do radio frequencies work
radio wave moves protons out of alignment and then when stopped, protons go back and as they align they transmit an energy wave unique to that type of tissue
What is the T you read on an MRI report
based on time it takes for protons to realign or reach a state of equilibrium
T1 is related to
realignment with imposed magnetic fields
T2 is related to
the transverse energy pulses that are delivered and the spin dephasing that occurs
T1 and T2 weighted images differ based on what two parameters
time to echo (TE)
time to repetition (TR)
TE
time to echo - time from the application of a pulse (RF) to when the spin signal is picked up
TR
time to repetition - time between pulse (RF) frequencies
T1 weighted images have a ___ TE and TR
Short
Hence the tissue that emit energy more quickly tend to have a brighter image
Tissues that are slower to release (longer TE and TR) will appear
darker on T1?
T2 has ___ TE and TR times
longer
T2 picks up images of tissues that are slow to release energy - what falls in this category
tissue with high water content
Fat will appear ___ on a T1
bright
Water will appear __ on a T1
dark
On a T2 image fat will generate what kind of signal
a very low signal and will be dark
On a T2 image water will generate what kind of signal
high and will appear bright
Different MRI techniques - short T1 inversion recovery (STIR) - used to
used to suppress the signal from fat
Different MRI techniques - short T1 inversion recovery (STIR) - allows improved imaging of what issues
inflammatory or neoplastic issues
Different MRI techniques - Fluid attenuated inversion recovery (FLAIR) - allows for what
fluid signals to be nullified
Different MRI techniques - Fluid attenuated inversion recovery (FLAIR) - useful in look at pathologies of the
CNS such as MS
Gadolinium (Gd) - define
nontoxic paramagnetic contrast enhancement agent utilized in MRI
when injected will change signal intensities by shortening T1 in its surroundings
T1 weighted - define
an image created typically by using short TE and TR times whose contrast and brightness are predominantly determined by T1 signals
T2 weighted - define
image created typically by using longer TE and TR times whose contrast brightness are predominantly determined by T2 signals
TAU (t) - define
the interpulse times used in a spin echo pulse sequence
interpulse times
time between the 90 and 180 pulse, and between the 180 pulse and the echo
TE define
echo time - time in milliseconds between the application of the 90 degre pulse and the peak of the echo signal in spin ech and inversion recovery pulse sequences
Hyperintese
more intense, brighter
hypointense
less intense, darker
Isointense
the same intensity
MRI indications
Tumors Stroke Epilepsy Demyelination Infection Cranial nerve palsy Chronic HA Dementia
MRI advantages
1 good soft tissue contrast with normal tissue
2 customize imaging technique to answer a specific question
3 no ionizing radiation
4 directly image in any plane
5 ability to do functionl MRI
MRI disadvantages
1 high cost, limited access 2 difficult for unstable pt 3 clausrophobia 4 absolute contraindications 5 not great for bony detail
Functional neuroimaging techniques
MEG
EEG
fMRI
PET
MEG (Magnetoencephalography)
what is it
temporal and spatial resolution
Non invasive
measure cellular activity of the brain based on detection of magnetic field changes secondary to ion flow
Good temporal and spatial resolution
MEG - used primarily with
cognitive research but can be used clinically for evaluation of seizures
MEG combined with MRI
known as magnetic source imaging (MSI)
Used for pre-operative mapping
Cost of MEG
high - limits the accessibility
EEG (electroencephalography)
what is it
Records electrical activity of the brain using electrodes that are attached to the scalp
Captures electrical currents generated by activated neurons
EEG temporal and spatial resolution
temporal is good but spatial is not as good as MEG
Multichannel EEG
improved lacalization of events, particularly the more superficial activity
EEG - cost
less than MEG so are more regularly used in clinical situations
PET (positron emission tomography) and SPET (single photon emission tomography)
PET measures the metabolism of the tissue
as tissue activity inc there is an inc in demand for energy
PET and SPET injects what into the body
radionuclide - 2 fluoro 2 deoxy D glucose (FDG)
This metabolizes and releases a beta particle called a positron
Combination of PET and CT can improve what
localization of a lesion
PET CT fusion imagine assists in differentiation of
pathologic tissue from normal tissue
fMRI
indirect measure of neural activity
fMRI is based on the
hemodynamic response related to neural activity
fMRI makes use of MRI imaging physics to
trace a contrast medium
tracks the ratio of oxygenated to deoxygenated blood
this method is known as blood oxygen level dependent (BOLD) contrast
BOLD contrast fMRI is based on fact that
magentic signal of oxygenated hemoglobin differs from deoxygenated
fMRI BOLD contrast method uses the __ weighted process
T2
Benefits to the BOLD contrast fMRI method
patient is not exposed to radiaion
provides both anatomic and function in one procedure
