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MRI in Practice Chapter 1
Terms in this set (155)
2 ways of explaining the fundamentals of MRI
classically and quantum physics
Two or more atoms arranged together
Most abundant atom in the body
What is hydrogen most commonly found in
Water (H2O) and fat (CH3)
subdivided into protons and neutrons
sum of the protons in the nucleus
sum of the protons and the neutrons in the nucleus
same number of Protons and different number of Neutrons.
what makes a atom electrically stable
having the same number of electrons and protons.
when external energy knocks electrons out electrons form the atom. this causes a deficit in the number of electrons compared with protons and causes electrical instability.
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
how do subatomic particles spin
they automatically spin in opposite directions but at the same rate as their partners. in nuclei that have an even mass number, ex have the same number of protons and neutrons. half spin in direction and half the other. the nucleus itself has no net spin.
the spin of MR active nuclei, which depends on the balance between the number of protons and neutrons in the nucleus
What are 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.
denotes the direction of the north/south axis of the magnet and the amplitude of the magnetic field
MRI active nuclei that have a net charge and are spinning/motion they
automatically acquire a magnetic moment and can align with an external magnetic field
Important MR active nuclei
Hydrogen, carbon, nitrogen, oxygen, fluorine, sodium, phosphorus
what allows the the nucleus to be MR active
although neutrons have no net charge, their subatomic particles are not evenly arranged over the surface of the neutron and this imbalance enables the nucleus in which the neutron is situated to be MR active as long as the mass number is odd
How is alignment measured
as the total sum of the nuclear magnetic moments and is expressed as a vector quantity.
the strength of the total magnetic moment is specific to
every nucleus and determines the sensitivity to magnetic resonance
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. These 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
The main magnetic field measured in Tesla
The static external magnetic field
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 ________
Alignment classical theory
with no magnetic field the magnetic moments of the hydrogen nuclei are randomly oriented. when placed in a strong static external magnetic field the magnetic moments of the H1 nuclei align with the magnetic field
How do the hydrogen align with the magnetic field in classical theory
some of the hydrogen nuclei align parallel with the magnetic field (in the same direction) while a smaller number align anti-parallel to the magnetic field (in the opposite direction)
who discovered Quantum theory
Max Planck in 1900
describes the properties of electromagnetic radiation in terms of discrete quantities of energy called quanta
applying quantum theory to MRI
Hydrogen nuclei possess energy in two discrete quantities or populations termed low and high or spin up and spin down
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
The interaction of the NMV with _______ is the basis of MRI
Net magnetization vector (NMV)
the magnetic vector produced as a result of alignment of excess hydrogen nuclei with B0. and reflects the relative balance between spin-up and spin-down nuclei.
assumes patient's temperature is constant and therefore does not influence the thermal energy of hydrogen during the MR experiment.
Low thermal energy nuclei
do not possess enough energy to oppose the magnetic field and therefor they align parallel to the magnetic field
High thermal energy nuclei
do possess enough energy to oppose the magnetic field and therefor they align anti-parallel to the magnetic field
the stronger the magnetic field
the more nuclei align parallel to the magnetic field because they no longer have the energy to oppose it.
The unit of B0
Tesla or gauss
1 Tesla equals how many gauss
Additional spin or wobble of the magnetic moments of hydrogen around B0, causes the magnetic moments to follow a circular path around B0
Path that the magnetic moments of hydrogen follow around B0
1 hertz equals
One cycle or rotation per second
1 MHz equals
1 million cycles or rotations per second
Expresses the relationship between the angular momentum and the magnetic moment of each MRI active nucleus. 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 (MHz/T)
Net magnetization vector or NMV
The net magnetic moment of the patient
Precessional frequency AKA Larmaor frequency
Speed at which the magnetic moments of hydrogen wobble around B0
ω0 = B0 × λ
ω0 is the precessional frequency
B 0 is the magnetic field strength of the magnet
λ is the gyromagnetic ratio.
the value of the precessional frequency is
governed by the larmor equation
Unit of precessional frequency
Gyromagnetic ratio or precessional frequency of hydrogen
Precessional frequency of hydrogen in a 1.5 Tesla magnet
Precessional frequency of hydrogen in a 1.0 Tesla magnet
Precessional frequency of hydrogen in a .5 Tesla magnet
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
________________ energy nuclei do possess enough energy to oppose the magnetic field
As the strength of the magnetic field increases, _________ nuclei have enough energy to oppose it
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
_______________ 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 are not in enough abundance in the body 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
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
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
The magnitude of the NMV being larger at high field strengths than at low field strengths results in what?
The interaction of the __________ with B0 is the basis of MRI
It is the ___________ of the hydrogen nuclei that align with B0, not the hydrogen nuclei themselves
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
What is the position of each magnetic moment on the precessional path around B0?
Magnetic moments that are ______________ are in the same place on the precessional path around B0 at any given time
In phase or coherent
what happens to Magnetic moments when resonance occurs
all the magnetic moments move to the same position on the precessional path and are then in phase
Magnetic moments that are ________________ are not in the same precessional path
Out of phase or incoherent
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
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 Magnetization cuts across the coil
The coherent moving transverse magnetization produces ______________ fluctuations inside the coil that induce an electrical voltage in the coil, this voltage constitutes the MRI signal
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
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
how dose resonance occur
When a nucleus is exposed to an external perturbation that has a frequency close to its own natural frequency, the nucleus gains energy from the external force. The nucleus gains energy and resonates if the energy is delivered at exactly the same precessional frequency.
If energy is delivered at a different _____________ to that of the Larmor or precessional frequency of the nucleus, resonance does not occur
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 Larmor frequency of hydrogen must be applied
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
The application of an RF pulse that causes resonance to occur
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
The energy difference between the two corresponds to the energy difference required to produce resonance via
As the ___________ increases, the energy difference between the two increases so that more energy or higher frequencies are required to produce resonance
One of the results of resonance
NMV moves out of alignment away from B0, this occurs because some of the low energy nuclei are given enough energy via resonance to join the high energy population
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
The angle to which the NMV moves out of alignment
magnitude of the flip angle depends on
the amplitude and duration of the RF pulse
A typical flip angle is how many degrees?
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
B0 is what plane?
How does the NMV rotate in the transverse plane?
The same as the longitudinal plane, at the Larmor or precessional frequency
The plane at 90° to B0 is what?
when a Flip angle of 90° is used
the nuclei are given sufficient energy so that the longitudinal NMV is completely transferred into a transverse NMV
When a Flip angle less than 90° is used
only a portion of the NMV is transferred to the transverse plane. this represents a smaller number of low-energy spins becoming high energy spins as a result of excitation
When a flip angle Greater 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
How are the nuclei given sufficient energy to move the NMV into the transverse plane?
A 90° flip angle is used
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
process by which the NMV looses energy
process by which hydrogen looses its energy
How dose relaxation happen
when the RF pulse is switched off the MNV is again influenced by B0 and it tries to realign with it. to do so the H1 must lose the energy given to them by the RF pulse.
what happens to the nuclei when relaxation occurs
as relaxation occurs the MNV returns to B0, because some of the high-energy nuclei return to the low-energy population and align their magnetic moments in the spin-up direction
When the amount of magnetization in the longitudinal plane aka B0 gradually increases
The amount of magnetization in the transverse plane gradually decreases, this happens at the same time but independent of recovery
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
At the same time, but independently, the ________ of hydrogen lose coherency and become out of phase due to the dephasing
_________ results in the recovery of the magnetization in the longitudinal plane and decay of the magnetization in the transverse plane
The recovery of longitudinal magnetization is caused by ________ recovery
The decay of transverse magnetization is caused by a process termed _____________
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 Magnetization
The rate of recovery is an exponential process, with a recovery time constant called the
T1 relaxation time
The T1 relaxation time is the time it takes
63% of the longitudinal magnetization to recover in the tissue
T2 decay is caused by the
magnetic fields of neighboring nuclei interacting with each other
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
63% of the transverse magnetization to be lost
results in the recovery of longitudinal magnetization due to the energy dissipation to the surrounding lattice
results in the loss of coherent transverse magnetization due to interactions between the magnetic fields of adjacent nuclei
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
A simplified pulse sequence is a combination of
RF pulses, signals and intervening periods of recovery
Time from the application of one RF pulse to the application of the next RF pulse for each slice, measured in milliseconds
repetition time or TR
Repetition time or TR determines the amount of ______________ that is allowed to occur between the end of one RF pulse and the application of the next
TR determines the amount of ____________ that has occurred when the signal is read
The time from the application of the RF pulse to the peak of the signal induced in the coil, measured in milliseconds
echo time or TE
The echo time or TE determines how much ________ of transverse magnetization is allowed to occur
TE controls the amount of ______________ that has occurred when the signal is read
what produces contrast in MRI images
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
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