Neurodiagnostics, Pt. 2 Flashcards
What is MRI based on? How does it work?
magnetic properties of living tissue, primarily spinning hydrogen atoms (protons)
- patient is placed in a magnetic field to produce a net magentization
- radiofrequency (RF) pulses are then administered to displace protons out of the z axis
- as protons relax back to their original energy states, RF energy is released at variable intensities, recorded by a receiver coil, and converted into images
What is phase coherence?
the degree to which precessing (wobbling) nuclear spins are synchronous
What is the difference between T1 and T2 relaxation?
T1 (longitudinal) = time which determines the rate at which excited protons return to equilibrium, measurement of the time taken for spinning protons to realign with the external magnetic field = spin-lattice relaxation
T2 (transverse) = time which determines the rate at which excited protons reach equilibrium or go out of phase with each other, measurement of time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field = spin-spin relaxation
What is echo time (TE)? Repetition time (TR)?
time between the delivery of the RF pulse and the receipt of the echo signal by receiver coils —> time the tissue is being imaged to release energy for collection by MRI coils
amount of time between successive pulse sequences applied to the same slice —> time the tissue is being imaged to relax back toward the direction of the magnetic field before excited again
What is the utility in using different sequences in MRIs?
- grey vs white matter
- water-rich or not
- fatty structure
- free fluid vs tissue-bound fluid
- breaks in BBB
What is the differenct between T1, T2, and intermediate-weighted images?
T1 = water dark, fat bright, white matter brighter than gray matter
T2 = water bright, fat gray, gray matter brighter than white matter
intermediate = fluid-attenuation where free water is suppressed (CSF, cysts) —> FLAIR most common
What are the 2 major advantages to using MRI?
- superior soft tissue detail
- better discernment of different tissue types
What makes up the magnetic vector (M) of patient tissue? What will it align with? How is its energy distinguished from stronger magnetic fields?
numerous spinning protons
strong magnetic fields inside the MRI gantry along the z axis
it must be separated from along the z axis
What is Larmor frequency?
the rate of precession (non-zero spin) of the magnetic moment of the proton around the external magnetic field
What is T1 relaxation?
spin-lattice / longitudinal - transfer of energy from the main magnetic vector of the patient back to the surrounding environment (magnetic field)
What is T2 relaxation? What causes it?
spin-spin / transverse - loss of energy between individual spinning protons within the main tissue magnetic vector —> energy released is less and less useful for signal generation and image formation the faster it is
inhomogeneities within the magnetic field caused by loss of coherence and poor signal formation
What is magnetic field inhomogeneity? What patient factors also cause this?
magnetic field within the MRI gantry is close to uniform, but not close enough to prevent loss of phase coherence
- water content
- hemorrhage
- air-soft tissue interfaces
What are the 3 basic spin echo sequences?
- T1-weighted images
- T2-weighted images
- proton density-weighted images
What is the point of manipulating echo times and repetition times?
manipulated in basic spin echo sequences to emphasize certain tissues based on T1 and T2 relaxation times (that occur concomitantly)
What are 3 characteristics of T1-weighted images? What tissues provide the most signals?
- short echo time
- short repetition time
- 90 degree radiofrequency pulses
tissues with short T1 relaxation - fullest potential for energy transfer, release the most energy when short echo times are used —> fat > other soft tissues > water
How do tissues show up on T1-weighted images?
- fat = bright (hyperintense)
- water/CSF = very dark (hypointense)
- parenchyma = intermediate, white matter brighter than grey matter
What contrast is commonly used with T1-weighted images? What does it do?
gadolinium - paramagnetic compound administered IV
shortens T1 relaxation times in tissues where it localizes, making a very hyperintense signal —> inflammatory foci, tumors, etc.
How do tissues show up on T2-weighted images?
- fat = bright, but not as bright as T1
- water (CSF) = very bright (hyperintense)
- parenchyma = imtermediate, gray matter brighter than white matter
T1 vs T2-weighted images:
T1 = fat hyperintense, CSF hypointense, white matter brighter
T2 = fat bright, CSF hyperintense, gray matter brighter
What are the 2 major characteristics of proton density-weighted images? What does this result in? What information does it give?
long relaxation time, short echo time = minimized T1 and T2 effects
based upon the amount of hydrogen protons available in a tissue, providing more tissue contrast (white vs gray matter, bone vs parenchyma)
T1 vs T2 vs PD:
T1 = fat hyperintense, CSF hypointense, white matter brighter
T2 = fat bright, CSF hyperintense, gray matter brighter
PD = more tissue contrast
What image is produced by FLAIR?
- free fluid (CSF) suppressed
- tissue bound fluid (edema) is hyperintense
FLAIR:
How is short time (Tau) inversion recovery (STIR) used? How does it work?
suppress signals from fat (short T1 relaxation time)
- 180 degree refocusing pulse is applied
- short inversion time is allowed before the 90 degree inversion pulse
- this results in the magnetic vector from fat constantly being displaced to the negative z axis, nullifying its signal
What are gradient (T2*) images? What is it usually used for?
T2-weighted sequence without the refocusing pulse, allowing it to occur much more quickly than T2 relaxation
hemorrhage (signal void)
(very susceptible to inhomogeneities)
What is FATSAT-chemical selective saturation?
tissue-specific pulse frequency is applied prior to the standard pulse, resulting in the saturated target tissue no contributing to image formation
What are diffusion-weighted images based on? What images are produced?
- diffusivity or random Brownian motion of water molecules
- internal thermal energy of water molecules
cytotoxic edema impairs diffusion, increasing signal intensity of that area
