Module 1 Fundamentals of MRI Flashcards
What are the basic requirements for MRI?
-strong magnetic field
-external RF energy source
-odd number of nuclear protons
Why is a strong magnetic field required for MRI?
because the patient must be situated within a magnetic environment in order to generate the MR signal that’s used to create images
Why is an external energy source required for MRI?
patient’s tissues must be stimulated with energy before those tissues can generate the MR signal that’s used to create images
Why is an odd number of nuclear protons required for MRI?
for a material to be suitable for MR it must be able to respond appropriately to the external magnetic field and external energy source
what is the particle of interest for MRI?
hydrogen
Why is hydrogen suitable for MRI?
-it is abundant in the body
-it consists of a single proton
what is a proton?
a positively charged particle
what are the 2 orientations of protons in a magnetic field?
parallel and antiparallel
what is antiparallel orientation?
protons orient themselves antiparallel where their spin axes align against the external magnetic field, it is an excited, high energy state.
the north pole of the proton aligns with the north pole of the external magnet
what is parallel orientation?
protons orient themselves parallel which is a relaxed, low energy state where their spin axes align with the main magnetic field. north pole of the protons aligns with the south pole of the external magnet
what is the net magnetic moment?
-the result of all the protons aligning either parallel or antiparallel
–the sum of the magnetic effects of all the protons exposed to the main magnetic field
An overall net magnetization occurs in the _______direction.
parallel
this is also called equilibrium magnetizaton
what is precession?
-a wobbling motion
-the net magnetization wobbles around the main magnetic field once it has been tilted
How is net magnetization measured?
-the net magnetization must be tilted away from its alignment with the magnetic field
-when its tilted away, it forms the actual MR signal
T or F
we need to manipulate the net magnetization of protons in order to generate an electromagnetic signal we can measure
true
a MR signal is:
an oscillating electromagnetic wave at a specific frequency
the MR signals frequency is established by:
the rate of precession of the net magnetization
the precessional frequency is often called the:
resonant frequency
the resonant frequency is determined by the:
strength of the magnetic field as described by the larmor equation
what is the larmor equation?
the resonant frequency of a population of a specific nucleus not only depends on the magnetic field strength that the nuclei are exposed to but also depends on a mathematical constant which is unique for each type of nucleus
what is the gyromagnetic ratio?
the mathematical constant unique for each type of nucleus
defines how the resonant frequency of the net magnetization is affected by the magnetic field strength for a given atomic nucleus
as the magnetic field strength increases, the resonant frequency of the net magnetization:
increases
what is the gyromagnetic ratio of hyrdrogen
42.6 megahertz per tesla (MHz/T)
this means in a 1 Telsa field, the magnetization of protons will precess at a rate of 42.6 million complete rotations per second
larmor equation
the gyromagnetic ratio in MHz/T x the magnetic field in tesla = the resonant or larmor frequency in MHz
it is precisely at the precessional frequency that:
RF energy is transmitted into the patient to disturb or tilt the net magnetization of protons
the protons will generate the MR signal precisely at the:
precessional frequency
the goal in MRI is to:
to differentiate one type of body tissue from another in order to depict normal tissue as well as pathology
To learn about tissues, we must:
disturb the proton’s equilibrium and move their net magnetization out of alignment with the main magnetic field
Once we stop the applied disturbance the magnetization will
move back or relax toward it’s equilibrium through two distinct relaxation processes
it is through ________ that the tissues can be distinguished in an image
relaxation processes
the applied disturbance to make an image comes in the form of a
brief pulse of radio frequency, or RF, energy
what is resonance?
the phenomenon which permits the efficient transfer of energy from one object or system to another
by adding RF energy the protons net magnetization can be
moved out of alignment with the main magnetic field
the transfer of energy will only take place if
the frequency of the energy source is the same frequency of the system to be excited
if we apply energy to the body at the precessional or resonant frequency
we can transfer energy to the protons and detect the energy that comes back to us from the protons. the energy we detect is the MR signal which is used to construct images
the direction of the main magnetic field is referred to as :
the longitudinal direction
the plane which is perpendicular to the longitudinal direction is called the:
transverse plane.
Before receiving any excitation from an RF pulse, the net magnetization is said to be in the ______plane
longitudinal plane aligned with the magnetic field of the MR magnet
as we apply RF energy to the patient the net magnetization tilts out of alignment with the main magnetic field as it continues to precess so that it traverses a:
somewhat spiral path from the longitudinal direction toward the transverse plane
the amount of spiral motion away from alignment with the external field is determined by the power of
the RF pulse. which depends on the amplitude and duration of the RF pulse applied.
the stronger the RF pulse and/or longer it is applied to the net magnetization the:
farther the net magnetization of protons will deflected from alignment with the main field
what is the RF flip angle?
the angle the net magnetization is tilted away from the main magnetic field
(the angle between the starting position of the net magnetization and the resultant position after the delivery of the RF energy)
what does the RF flip angle depend on?
RF amplitude
RF pulse duration
An RF pulse with a 90 degree flip angle deflects:
the entire net magnetization into the transverse plane since the transverse plane is is oriented 90 degrees to the longitudinal direction
an RF pulse with a 90 degree flip angle is often used with what
a spin echo pulse sequence
a gradient echo pulse sequence typically uses a flip angle which is:
less than 90 degrees
a smaller flip angle would only:
move a component or part of the net magnetization into the transverse plane
what is relaxation?
the loss of energy by the protons following RF excitation
what are the 2 relaxation processes?
T1 or spin-lattice
T2 or spin-spin
both occur simultaneously but are completely independent of one another
what is T1 relaxation?
the dissipation of energy from the protons into the surrounding molecular lattice
-the time required for the net magnetization to grow to 63% of it’s final amplitude
T1 relaxation times are responsible for:
determining the unique appearance of each type of tissue in a T1 weighted image
white matter has a very short T1 relaxation time, gray matter has a little longer than white and CSF has a longer time than gray matter
white matter contains nerve fibers which are wrapped in myelin which is mostly fat so it has the shortest T1 time because it has the most fat and will be brighter
gray matter contains nerve cell bodies which are surround in extracellular fluid so it has longer T1relaxation time than white matter but still shorter than CSF
CSF is mostly “water” so it has the longest
ON T1 WEIGHTED
white matter has the _______amplitude.
gray matter has the _______amplitude.
CSF has the _______amplitude.
highest.
next highest.
lowest.
this means white matter is the brightest, gray matter the next brightest, and CSF the darkest
The MR signal is always collected in:
the transverse plane
what is the RF pulse?
synchronizes the precessional phase of proton’s magnetization
deflects the net magnetization into the transverse plane
what is phase coherence
immediately following the RF pulse, the net mag is tilted into the transverse plane and is oriented such that the magnetic contribution from each proton is synchronized in the same direction. this is phase coherence or “in phase”
dephasing
the protons individual moments start to cancel each other out which results in a drop in the transverse net magnetization
Dephasing occurs because:
the phase effects of chemical shift properties
local magnetic field inhomogeneities , magnetic susceptibility
spin spin interactions
Free Induction Decay
FID
the signal decay that occurs as a result of the dephasing of the transverse magnetization
FID represents the amplitude of the precessing MR Signal in the transverse plane during dephasing
in order to collect the MR signal, the protons magnetization needs to be:
at least partially in phase
this is done by rephrasing the dephased magnetization
in a spin echo pulse sequence, the MR signal can be rephrased and measured by:
applying a RF pulse with a 180 degree flip angle. this 180 degree RF pulse completely flips the dephased magnetization 180 degrees about the transverse plane.
-it reverses the dephasing interactions of the protons magnetic moments in the transverse plane causing them to again add up or rephase
Once the spins are rephased, a signal called a _______can be collected
spin echo
-each subsequent 180 degree RF pulse administered causes another echo to be formed
what is a spin-spin interaction
the phenomenon in which energy is transferred from one proton to another
when spin spin interactions occur:
the protons magnetic influence becomes apparently random in nature so they no longer contribute to the magnetization forming the echo
what is T2 relaxation
the reduction of transverse magnetization due to spin-spin interactions
the time at which transverse magnetization has decayed to 37% of it’s full value due entirely to spin spin interactions
tissues with longer T2 relaxation times exhibit a
longer lasting transverse magnetization (dephases more slowly) compared to tissues with a shorter T2 time (dephases more rapidly)
CSF has a _______T2 relaxation time
gray matter has a _______T2 relaxation time
white matter has a _______T2 relaxation time
long & the highest amplitude
intermediate & next highest amp
short & shortest amp
CSF will be the brightest on T2, gray matter next brightest, white matter the darkest
what is echo time
TE
the time from center of the 90 degree RF pulse to the center of the echo
what is repetition time
TR
the time from one 90 degree pulse to the next 90 degree pulse applied to the same population of protons
The TE and TR are parameters which can be varied to:
control the contrast between tissues in the images
TR is the parameter which controls
how much contrast due to T1 relaxation time is seen between the tissues
TE is the parameter which controls
how much contrast due to T2 relaxation times is seen between the tissues
