MRI in Practice

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Terms in this set (...)

Molecule
Two or more atoms arranged together
Hydrogen
Most abundant atom in the body
What is hydrogen most commonly found in
Water and fat
Three types of motion present in the atom
Electrons spinning on their own axis, electrons orbiting the nucleus, the nucleus itself spinning on its own axis
What type of spinning does MRI rely on
Individual spins of protons and neutrons within the nucleus
MRI active nuclei
Nuclei with odd mass numbers where the number of neutrons is slightly more or less than the number of protons, spin directions are not equal and opposite so the nucleus it's self has a net spin or angular momentum
MRI active nuclei
Have a tendency to align their axis of rotation to an applied magnetic field, occurs because they have angular momentum or spin, as they contain positively charged protons, they possess electrical charge
Law of electromagnetic induction, Faraday 1833
Refers to three individual forces: motion, magnetism and charge; if two are present then the third is automatically induced. MRI active nuclei that have a net charge and are spinning a.k.a. motion, automatically acquire a magnetic moment and can align with an external magnetic field
-Name the Founder and year
7 Important MR active nuclei
Hydrogen, carbon, nitrogen, oxygen, fluorine, sodium, phosphorus
Protium
Isotope of the hydrogen nucleus that is the MRI active nucleus used in clinical MRI, contains a single proton, has an atomic and mass number of 1
Why is protium used
Because hydrogen is so abundant in the body and because it's solitary proton gives it a relatively large magnetic moment
Why is protium used
The characteristics enable utilization of the maximum amount of available magnetization in the body
Law of electromagnetism
States that a magnetic field is created when a charged particle moves
How does the hydrogen nuclei act as a small magnet
The hydrogen nucleus contains one positively charged proton that spins or moves, therefore the hydrogen nuclei has a magnetic field induced around it
What does the magnet of each hydrogen nucleus contain
A north and south pole of equal strength
What is used in the classical theory of the principles of MRI
The north and south axis of each nucleus is represented by a magnetic moment
What are vector properties?
The magnetic moment of each nucleus has vector properties meaning it has size and direction and is denoted by an arrow
What does the direction and length of the vector designate
The direction of the vector designates the direction of the magnetic moment and the length of the vector designates the size of the magnetic moment
B0
The main magnetic field measured in Tesla
What happens to hydrogen in the absence of an applied magnetic field
The magnetic moments of the hydrogen nuclei are randomly oriented
What happens to the magnetic moments of hydrogen when placed in a strong external magnetic field
The magnetic moments of the hydrogen nuclei align with this magnetic field
Some of the hydrogen nuclei align ________ with the magnetic field while a smaller number of nuclei align ________
Parallel, anti-parallel
Spin up nuclei
Low energy nuclei that align their magnetic moments parallel to the external magnetic field
Spin down nuclei
High energy nuclei that align their magnetic moments in the anti-parallel direction
What determines which hydrogen nuclei align parallel and which align anti-parallel with B0
The strength of the external magnetic field and the thermal energy level of the nuclei
Net magnetization vector or NMV
The net magnetic moment of the patient
B0
The static external magnetic field
The interaction of the NMV with _______ is the basis of MRI
B0
The unit of B0
Tesla or gauss
1 Tesla equals how many gauss
10,000
Precession
Additional spin or wobble of the magnetic moments of hydrogen around B0, causes the magnetic moments to follow a circular path around B0
Precessional path
Path that the magnetic moments of hydrogen follow around B0
Precessional frequency
Speed at which the magnetic moments of hydrogen wobble around B0
Unit of precessional frequency
Megahertz
1 hertz equals
One cycle or rotation per second
1 MHz equals
1 million cycles or rotations per second
Gyromagnetic ratio
Expresses the relationship between the angular momentum and the magnetic moment of each MRI active nucleus
Gyromagnetic ratio
It is constant and is expressed as the precessional frequency of a specific MRI active nucleus at T1, the unit of gyromagnetic ratio is therefore megahertz per Tesla
Gyromagnetic ratio of hydrogen
42.57 MHz per Tesla
Net magnetization vector or NMV
The net magnetic moment of the patient
B0
The static external magnetic field
The interaction of the NMV with _______ is the basis of MRI
B0
The unit of B0
Tesla or gauss
1 Tesla equals how many gauss
10,000
Precession
Additional spin or wobble of the magnetic moments of hydrogen around B0, causes the magnetic moments to follow a circular path around B0
Precessional path
Path that the magnetic moments of hydrogen follow around B0
Precessional frequency
Speed at which the magnetic moments of hydrogen wobble around B0
Unit of precessional frequency
Megahertz
1 hertz equals
One cycle or rotation per second
1 MHz equals
1 million cycles or rotations per second
Gyromagnetic ratio
Expresses the relationship between the angular momentum and the magnetic moment of each MRI active nucleus
Gyromagnetic ratio
It is constant and is expressed as the precessional frequency of a specific MRI active nucleus at T1, the unit of gyromagnetic ratio is therefore MHz per Tesla
Gyromagnetic ratio or precessional frequency of hydrogen
42.57 MHz per Tesla
Precessional frequency of hydrogen in a 1.5 Tesla magnet
63.86 MHz per Tesla, multiply the precessional frequency of hydrogen or 42.57 by the strength of the magnetic field, or 1.5 Tesla
Precessional frequency is also known as
Larmor frequency because it is determined by the Larmor equation
Why do MRI active nuclei precess at different frequencies when exposed to the same field strength
They all have their own gyromagnetic constant
How can hydrogen be specifically imaged while other MRI active nuclei are ignore?
Hydrogen precesses at different frequencies than other MRI active nuclei
If B0 increases why does the Larmor frequency also increase?
The gyromagnetic ratio is a constant of proportionality, B0 is proportional to the Larmor frequency, therefore if one increases the other increases and vice versa
____________ energy nuclei do not possess enough energy to oppose the magnetic field in the anti parallel direction
Low thermal
________________ energy nuclei do possess enough energy to oppose the magnetic field
High thermal
As the strength of the magnetic field increases, _________ nuclei have enough energy to oppose it
Fewer
Thermal equilibrium
The thermal energy of a nucleus is mainly determined by the temperature of the patient, in clinical applications this can not be significantly altered and is not important
What determines the relative quantities of spin up and spin down nuclei?
The strength of the external magnetic field or B0
In _______________________there are always fewer high energy nuclei than low energy nuclei, therefore the magnetic moments of the nuclei aligned parallel to the magnetic field cancel out the smaller number of magnetic moments aligned anti parallel
Thermal equilibrium
_______________ is the relative balance between spin up and spin down nuclei
Net magnetization vector or NMV
Why are non hydrogen magnetic moments not used in clinical MRI?
They do not exist in the body in enough abundance to be imaged adequately because their net magnetic moments are very small
The net magnetic moment of _____________ produces a significant magnetic vector that is used in clinical MRI
Hydrogen
What constitutes the NMV of the patient?
When the patient is placed in the bore of the magnet the magnetic moments of hydrogen nuclei within the patient align parallel and anti parallel to B0, a small excess line up parallel to B0 and constitute the NMV of the patient
The ______________ difference between the two populations increases as B0 increases
Energy
At _________ field strengths _____________ nuclei have enough energy to join the high energy population and align their magnetic moments in opposition to the stronger B0 field
High, fewer
The magnitude of the NMV being larger at high field strengths than at low field strengths results in what?
Improved signal
The interaction of the __________ with B0 is the basis of MRI
NMV
What is the unit of B0?
Tesla or gauss
1 Tesla equals how many gauss?
10,0000
It is the ___________ of the hydrogen nuclei that align with B0, not the hydrogen nuclei themselves
Magnetic moments
Magnetic moments are only capable of aligning with B0 in what two directions?
Parallel or anti parallel
Why are the magnetic moments of hydrogen only capable of aligning in two directions?
They represent the only two possible energy states of hydrogen
The hydrogen __________ itself does not change director but merely spins on its axis
Nucleus
B0 is what plane?
Longitudinal
The plane at 90° to B0 is what?
Transverse
How are the nuclei given sufficient energy to move the NMV into the transverse plane?
A 90° flip angle is used
How does the NMV rotate in the transverse plane?
The same as the longitudinal plane, at the Larmor or precessional frequency
When a ________ less than 90° is used only a portion of the NMV is transferred to the transverse plane
Flip angle
When a flip angle __________ than 90° is used this represents a larger number of high energy spins to low energy spins, the NMV merely reflects the balance between the spin up and spin down populations
Greater
What is the position of each magnetic moment on the precessional path around B0?
Phase
Magnetic moments that are ______________ are in the same place on the precessional path around B0 at any given time
In phase or coherent
Magnetic moments that are ________________ are not in the same precessional path
Out of phase or incoherent
When ______________ occurs all of the magnetic moments move the same position on the precessional path and are in phase
Resonance
For resonance of hydrogen to occur RF or radio frequency at exactly the _____________ of hydrogen must to be applied
Larmor or precessional frequency
The result of resonance is magnetization in the ____________ plane that is in phase or coherent
Transverse
In phase or coherent transverse magnetization processes at the _______________ frequency
Larmor or precessional frequency
States that if a receiver coil or any conductive loop is placed in the area of a moving magnetic field a voltage is induced in the receiver coil, example: the magnetization precessing in the transverse plane
Faraday's Law of electromagnetic induction
The MRI signal is produced when coherent, in phase ________________cuts across the coil
Magnetization
The coherent moving transverse magnetization produces ______________ fluctuations inside the coil that induce an electrical voltage in the coil, this voltage constitutes the MRI signal
Magnetic field
The frequency of the signal is the same as the Larmor or precessional frequency, the magnitude of the signal depends on the amount of ____________ present in the transverse plane
Magnetization
The phenomenon that occurs when an object is exposed to an oscillating disturbance that has a frequency close to its own natural frequency of oscillation
Resonance
When a nucleus is exposed to an ____________ that has a frequency close to its own natural frequency, the nucleus gains energy from the external source
external disturbance
If energy is delivered at a different _____________ to that of the Larmor or precessional frequency of the nucleus, resonance does not occur
Frequency
Band of the electromagnetic spectrum, corresponds to energy at the precessional frequency of hydrogen at all fields strengths in clinical MRI
Radio frequency or RF
For resonance of hydrogen to occur an RF pulse of energy at exactly the __________ frequency of hydrogen must be applied
Larmor
Other MRI nuclei that have aligned with B0 do not resonate because their _________ are different than hydrogen, this is because their gyromagnetic ratios are different than hydrogen
Precessional frequencies
The application of an RF pulse that causes resonance to occur
Excitation
The absorption of RF energy causes an increase in the number of ____________ hydrogen nuclei populations as some of the spin up nuclei gain energy via resonance and become high energy nuclei
spin down
The energy difference between the two corresponds to the energy difference required to produce resonance via
excitation
As the ___________ increases, the energy difference between the two increases so that more energy or higher frequencies are required to produce resonance
field strength
NMV moves out of alignment away from B0, this occurs because some of the low energy nuclei are given enough energy via ____________ to join the high energy population
resonance
The _________ reflects the balance between the low and high energy populations, resonance causes the NMV to no longer lie parallel to B0, but instead at an angle
NMV
The angle to which the NMV moves out of alignment
flip angle
________ of the flip angle depends on the amplitude and duration of the RF pulse
magnitude
A typical flip angle is how many degrees?
90
What does it mean when the NMV has a flip angle of 90°
It has been given enough energy by the RF pulse to move 90° relative to B0, moving it into the transverse plane
Since the NMV is a vector, or quantity, even if flip angles other than 90° are used, there is always a component of magnetization in a plane perpendicular to B0
...
The process by which hydrogen loses the energy given to it by the RF pulse
relaxation
When the RF pulse is stopped, the NMV is again influenced by B0 and tries to realign with it, to do so the hydrogen nuclei must lose the energy given to them by the RF pulse
...
When the amount of magnetization in the longitudinal plane aka B0 gradually increases
recovery
The amount of magnetization in the transverse plane gradually decreases, this happens at the same time but independent of recovery
decay
Induction of reduced signal, the magnitude of the transverse magnetization decreases and so does the magnitude of the voltage induced in the receiver coil
free induction decay signal or FID
During _________ hydrogen nuclei give up absorbed RF energy and the NMV returns to B0
relaxation
At the same time, but independently, the ________ of hydrogen lose coherency and become out of phase due to the dephasing
magnetic moments
_________ results in the recovery of the magnetization in the longitudinal plane and decay of the magnetization in the transverse plane
relaxation
The recovery of longitudinal magnetization is caused by ________ recovery
T1
The decay of transverse magnetization is caused by a process termed _____________
T2 decay
T1 recovery is caused by the nuclei giving up their energy to the surrounding environment or lattice and is termed ____________
spin lattice relaxation
Energy released to the _____________ causes the magnetic moments of the nuclei to recover their longitudinal relaxation
surrounding lattice
The rate of recovery is an exponential process, with a recovery time constant called the _________ relaxation time
T1
The T1 relaxation time is the time it takes _________ of the longitudinal magnetization to recover in the tissue
63%
T2 decay is caused by the ______________ of neighboring nuclei interacting with each other
magnetic fields
T2 decay is termed __________ and results in decay or loss of coherent transverse magnetization
spin spin relaxation
T2 decay is an exponential process, the _____________ of a tissue is its time constant of decay
T2 relaxation time
The T2 relaxation time is the time it takes __________ of the transverse magnetization to be lost
63%
____________ results in the recovery of longitudinal magnetization due to the energy dissipation to the surrounding lattice
T1 relaxation
_____________ results in the loss of coherent transverse magnetization due to interactions between the magnetic fields of adjacent nuclei
T2 relaxation
A ___________ is only induced in the receiver coil if there is enough coherent magnetization in the transverse plane, aka in phase
signal or voltage
The magnitude and timing of the RF pulses form part of ______________ which are the basis of contrast generation in MRI
pulse sequences
A simplified pulse sequence is a combination of RF pulses, signals and intervening periods of recovery
...
The application of RF pulses at certain repetition times and the receiving of signals at predefined echo times is what produces contrast in MRI images
...