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What are the 2 theories to explain MRI fundamentals?
Classical and quantum
Explain classical theory
It's the mechanical view of how the universe works on a large scale.
How is MRI explained when using classical theory
mass, spin, and angular momentum
explain quantum theory
how the world works on a smaller (subatomic) scale i.e. protons, neutrons, and electrons
The sum of all protons in the nucleus
What is the mass number?
sum of protons and neutrons
Atoms with the same number of protons but a diffrent number of neutrons
How are isotopes used in MRI?
What are the 3 types of motion in an atom?
1. electrons spinning on their own axis
2. electrons orbiting the nucleus
3. the nucleus itself spinning about its own axis
What is angular momentum?
In nuclei with an odd number of protons, an odd number of neutrons, or an odd number of both, the spin directions are not equal and opposite, so the nucleus itself has a net spin
Only nuclei with an ____ mass number or atomic weight are used in MRI.
what is a magnetic moment?
the direction of a nuclei
list 4 common MR active nuclei
Why does MRI use hydrogen (protium)?
It is used because hydrogen is very abundant in the human body and because of its solitary proton, it gives it a relatively large magnetic moment
What are the 3 individual forces of Faraday's law of induction?
2. electrical field
3. magnetic field
Explain Faraday's law of electromagnetic induction
Determines that a moving electric field produces a magnetic field and vice versa
Three individual forces of EMF
Why are hydrogen nuclei used in MRI?
Hydrogen atoms are the most abundant atom in the body. (our bodies are ~60% water). It's nucleus (single proton) has a spin
Explain magnetic alignment with an external magnetic field according to classical theory
parallel alignment: low energy, align with magnetic field, spin up
antiparallel alignment: high energy, align against the magnetic field, spin down
explain the magnetic alignment without an external magnetic field according to classical theory
randomly oriented in space, no overall magnetic effect
Explain the magnetic alignment with an external magnetic field according to quantum theory
low energy nuclei: parallel/spin-up
high energy nuclei: anti-parallel/spin-down
describe the characteristics of low energy nuclei
do not have enough energy to oppose B0 field, align with external magnetic field, parallel/spin up
describe the characteristics of high energy nuclei
have enough energy to oppose the main B0 magnetic field, align against the magnetic field (anti-parallel)
What does parallel alignment mean in reference to alignment to the main magnetic field?
Alignment of magnetic moments in the same direction as the main B0 field (spin-up)
What does antiparallel mean in reference to the main magnetic field?
alignment of magnetic moments in the opposite direction to the main B0 field (spin-down)
Define net magnetic vector (NMV)
the sum of all magnetic moments of teh individual hydrogen nuceli
Describe thermal equilibrium
What is precession?
circular path around B0
describe precession in reference to alignment of hydrogen protons and the main magnetic field
B0 produces an additional spin or "wobble" of the magnetic moments around B0
How is precession affected by the strength of the external magnetic field?
stronger external magnetic field = faster precession???
What is precessional frequency?
the speed at which magnetic moments precess around B0
What is another name for precessional frequency?
What is the Larmor equation?
w0 = yB0
i.e. 42.58 MHz x 1.5T = 63.87 MHz
What is the gyromagnetic ratio of hydrogen?
What is the unit of measurement for precession?
This is the rate of phase change of magnetic moments
What is out of phase or incoherent
magnetic moments of hydrogen are at different places on the precessional path at a moment in time
what is in phase or coherent
magnetic moments of hydrogen are at the same place on the precessional path at a moment in time
When does resonance occur?
resonance occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation
What occurs during an RF excitation pulse?
Flip angle: the amount of rotation the net magnetization experiences during the application of a RF pulse. the angle to which the NMV moves out of alignment.
What are the results of resonance (classical theory)?
application of the B1 field (RF excitation pulse) in a plane at 90 degrees to B0 causes magnetic moments of the spins to precess around this axis rather than about the longitudinal plane. the magnetic moments of spin-up and spin-down nuclei move into phase with each other. All magnetic moments are in-phase.
what are the results of resonance (quantum theory)?
Causes excitation or "energy-giving". RF excitation pulse gives energy to hydrogen nuclei and causes a net increase in the number of high-energy, spin-down nuclei. Low energy, spin-up nuclei absorb the energy from the RF pulse moving into the high energy population. the high energy, spin-down are stimulated to release energy and return to the low energy state.
B1 is also known as the _______ plane or ______ axis.
B0 is also known as the _______ plane or ______ axis
Explain what an RF flip angle is
If the right amount of energy is absorbed the NMV will move 90 degrees to B0
Explain how the magnitude of flip angle is determined
1. amplitude of the RF pulse
2. duration of the RF pulse
enough energy is given to move the NMV completely from the longitudinal plane to the transverse
90 degree flip angle
only a portion of the NMV is transferred into the transverse plan
flip angles less then 90 degrees
RF excitation pulse is twice the magnitude that would produce a 90 degree flip angle
180 degree flip angle
How is MR signal generated?
in-phase (coherent) magnetization is precessing in the transverse plane (changing magnetic field = current) the change in magnetic flux through a closed circuit induces an electromagnetic force (emf) in the circuit.
When is MR signal generated
when coherent (in-phase) magnetization cuts across the coil
Explain how faraday's law of electromagnetic induction contributes to the creation/detection of the MR signal
a changing magnetic field causes movement of charged particles (electrons) this flow of electrons is a current if a receiver coil or any conducive loop is placed in the moving field a voltage (signal) will be induced in the receiver coil
Define FID (free induction decay)
loss of signal due to relaxation
When does FID occur?
immediately after the RF pulse ends
what is happening to the NMV during FID
The NMV is only influenced by B0, the NMV will try and realign with B0. To do this, hydrogen nuclei lose energy given to them by the RF excitation pulse. As relaxation occurs, the NMV returns to align their magnetic moments in the spin-up direction. at the same time, but independently the magnetic moments of hydrogen lose coherency due to rephrasing due to inhomogeneities in the B0 field and due to interactions between spins in the patients tissues.
Explain FID's relationship to the MR signal
FID results in a loss of signal
as hydrogen nuclei try and realign with B0, they need to lose the energy that was given to them by the RF excitation pulse
Define TR pulse time parameters
Repetition Time (TR) is the time from one RF pulse to the application of the next RF pulse for each slice
*determines the amount of T1 relaxation that is allowed to occur
Define TE pulse time parameters
Echo Time (TE) time from the application of the RF pulse to the peak of the induced signal.
*determines the amount of T2 relaxation that has occurred when the signal is read
define image contrast
the difference between adjacent densities
What factors affect image contrast?
intrinsic and extrinsic contrast parameters
what are intrinsic contrast parameters
those that cannot be changed because they are inherent to the body's tissues
what are extrinsic contrast parameters
those that can be changed because they are under our control
T1 recovery time, T2 decay, proton density, flow, and apparent diffusion coefficient (ADC) are all _________ contrast parameters
TR, TE, TI, flip angle, turbo factor/echo train length, and b value are all _______ contrast parameters
Explain T1 recovery
The recovery of longitudinal magnetization due to spin-lattice relaxation after the RF excitation pulse is switched off. Signal will recover 63% of its initial value after the RF pulse has been applied
define spin-lattice energy transfer
process by which energy is given up by spins to the surrounding molecular lattice
explain spin-lattice energy transfer
Energy released by spins to the surrounding molecular lattice causes magnetic moments of hydrogen nuclei to recover their longitudinal magnetization.
explain T2 decay
the decay of coherent transverse magnetization due to spin-spin relaxation after the RF excitation pulse is switched off
define spin-spin energy transfer
process by which interactions between the magnetic fields of adjacent nuclei cause dephasing.
explain spin-spin energy transfer
Caused by one spin transferring energy to another spin rather than into the lattice. Occurs due to hydrogen nuclei being in the same environment and experiencing the same B0 field. Magnetic moments of hydrogen nuclei lose phase coherence because of these interactions
Explain how spin-spin interactions are affected by magnetic field inhomogeneities
If a hydrogen nucleus lies in an area of inhomogenity with higher field strength, the precessional frequency of its magnetic moment increases (speeds up), if it is in an area with lower field strength it decreases (slows down). This relative acceleration and deceleration of magnetic moments due to magnetic field inhomogeneities, and differences in the precessional frequency in certain tissues, causes immediate dephasing o fate magnetic moments of the spins and produces a FID.
What is the molecular structure of fat
Contain hydrogen molecules arranged with carbon and oxygen, closely packed together molecules (lipids), slow molecular tumbling rate. the carbon does not steal hydrogen's electron (protects the nucleus from the effects of the magnetic field)
what is the molecular structure of water
contain 2 hydrogen atoms with 1 oxygen, molecules are spaced apart, fast molecular tumbling rate, oxygen in water steals electrons away from around the hydrogen nucleus (makes the hydrogen more available to the effects of the main magnetic field)
hydrogen in _______ recovers more rapidly along the longitudinal axis than _____ and loses transverse magnetization faster than water.
How does areas of high signal appear on an MRI image?
hyperintense, appears bright/white
how does areas of low signal appear on an MRI image?
hypointense, appears dark/black
how do areas of intermediate signal appear on an MRI image?
shades of gray between white and black
2 main factors that determine T1/T2 relaxation times/rates
1. if tumbling rate matches the Larmor frequency of hydrogen
2. if molecules are closely packed together
How does the molecular makeup of fat affect T1 relaxation?
Fat has a low inherent energy so it can easily absorb energy into its lattice from hydrogen nuclei. Fat has a slow molecular tumbling rate and the recovery process is relatively rapid. The tumbling rate matches the Larmor frequency and allowed efficient energy exchange. Magnetic moments of fat nuclei are able to relax and regain their longitudinal magnetization quickly
T1 time of fat is ______
short; NMV of fat realigns with B0 rapidly
How does the molecular makeup of water affect T1 relaxation
Water has a high inherent energy and does not easily absorb energy into its lattice from hydrogen nuclei. Water has a high molecular tumbling rate and is less efficient due to the tumbling rate not matching the Larmor frequency. There is ineffiencent energy exchange from nuclei to lattice and the magnetic moments of hydrogen take longer to relax and regain longitudinal magnetization
T1 recovery time of water is ________
long; NMV of water takes longer to realign with B0
How does the molecular makeup of fat affect T2 decay
Molecules in fat are packed closely together making spin-spin interactions more likely to occur. Magnetic moments of hydrogen nuclei in fat precess similar to molecular tumbling rate so the magnetic moments dephase quickly and there is a rapid loss of coherent transverse magnetization.
T2 decay time of fat is _________
How does the molecular makeup of water affect T2 decay
Molecules are spaced far apart so spin-spin is less likely to occur. magnetic moments of hydrogen in water precess much faster than molecular tumbling so the magnetic moments dephase slowly, there is a gradual loss of coherent transverse magnetization
T2 decay time of water is _________
What is T2* decay
T2* decay is the decay of the FID following the RF excitation pulse, this decay is faster than T2 decay because it's a combo of T2 decay itself and dephasing due to magnetic field inhomogeneities
When does T2* decay occur?
immediately after the RF pulse is removed
How is T1 contrast created?
Contrast derived from the differences in T1 recovery times of tissues. Likely to occur if the NMV does not fully recover longitudinal magnetization between each RF pulse.
T1 contrast time for fat has a _________
fast T1 recovery time
T1 recovery time for water has a _______
slow T1 recovery time
T1 contrast is controlled by
For good T1 contrast, the TR must be _________
any time the NMV is used beyond 90 degrees
when the NMV is pushed to a full 180 degrees
How is T2 contrast created?
T2 contrast is derived from the differences in the T2 decay times. Likely to occur if vectors dephase and there is a difference in coherent transverse magnetization in each tissue
T2 decay time for fat has a ______
fast T2 decay time
T2 decay time for water has a ________
slow T2 decay time
Fat has a ______ T2 decay time
Water has a _______ T2 decay time
This process depends how closely the molecules are packed together
T2 contrast is controlled by the _______
For good T2 contrast the TE must be
Define proton density contrast
differences in signal intensity between tissues, consequence of their relative number of mobile hydrogen protons per unit volume
tissues with high proton density appear ________ on an MRI image
tissues with a low proton density appear ________ on an MRI image
Must be a large component of coherent magnetization in the transverse plane to induce large signal in the coil
must be a small component of coherent magnetization in the transverse plane to induce a small signal in the coil
On a T1 weighted image fat appears
on a T1 weighted image water appears
on a T2 weighted image water appears
on a T2 weighted image fat appears
define image weighting
To minimize mixed appearance images extrinsic parameters are selected to weight image contrast towards one contrast parameter and away from others
what are the intrinsic imaging parameters associated with T1 weighting
TR, T1 recovery time, ?????????????
T1 weighted image relies on a ______ TR and a ______ TE
What extrinsic parameters affect T1
TR and TE MUST be short
T2 has a _______ TE and a _____ TR
what extrinsic parameters affect T2
TE and TR MUST be long
How is PD weighted achieved via extrinsic parameters
Long TR = fat and water vectors can fully recover longitudinal magnetization (diminishing T1 contrast)
short TE = fat and water vectors do not have enough time to dephase (diminishing T2 contrast)
the movement of molecules in the extracellular space due to random thermal motion
what are common restrictions of diffusion
ligaments, membranes, macromolecules, pathology
What is ADC?
the net displacement of molecules diffusing across an area of tissue per second (intrinsic parameter)
what is happening when a tissue has low ADC (stroke)
tissue is stationary (diffusion is limited/restricted), tissue acquires no phase change after rate gradients are applied which results in a high signal being obtained from low ADC tissues
what is happening when a tissue has high ADC
tissue is free (diffusion is normal), tissue acquires phase change after 1st gradient but do not get rephrased by the 2nd because they are moving which results in low signal obtained from high ADC tissues
which type of ADC is normal tissue?
When is fMRI used?
when we want to see the brain during activity/stimulus and rest
briefly explain the main contrast principle of fMRI
utilizes the differences in oxyhemoglobin and deoxyhemoglobin in blood to acquire image contrast
what do contrast enhancement agents do to affect the image?
they change the relaxation times of certain tissues
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