Chapter 1 Flashcards
What are the two main ways to explain MRI principles?
Classical Theory (Newtonian): Uses mass, spin, and angular momentum.
Quantum Theory (Planck, Einstein, Dirac): Explains MRI at the subatomic level, focusing on energy levels of protons, neutrons, and electrons.
Why is classical theory used more often in MRI explanations?
Because it is easier to understand than quantum mechanics for describing large-scale MRI concepts.
What are the two ways to characterize an atom?
Atomic number = Number of protons in the nucleus (defines chemical identity).
Mass number (atomic weight) = Sum of protons + neutrons in the nucleus.
What are isotopes?
Atoms with the same number of protons but a different number of neutrons.
What are the charges of atomic particles?
Protons = Positive charge (+1).
Neutrons = No charge (neutral).
Electrons = Negative charge (-1).
What are ions?
Atoms that have lost or gained electrons, becoming electrically unstable.
What are the three types of atomic motion?
- Electrons spinning on their own axis.
- Electrons orbiting the nucleus.
- Nucleus spinning about its own axis.
What determines whether an atomic nucleus has spin?
Even atomic and mass number → No net spin (e.g., Carbon-12).
Odd atomic/mass number → Has spin (e.g., Hydrogen-1, Oxygen-17).
What are MR-active nuclei?
Nuclei that have angular momentum (spin) and a net electrical charge, allowing them to interact with a magnetic field.
What law explains how MR-active nuclei acquire a magnetic field?
Faraday’s Law of Electromagnetic Induction – A moving electric charge creates a magnetic field.
What are examples of MR-active nuclei?
¹H (Hydrogen), ¹³C (Carbon), ¹⁵N (Nitrogen), ¹⁷O (Oxygen), ¹⁹F (Fluorine), ²³Na (Sodium).
Why is hydrogen used in MRI?
Most abundant element in the human body.
Single proton gives it a large magnetic moment.
What is Faraday’s Law of Electromagnetic Induction?
A moving electric charge (like a spinning hydrogen proton) creates a magnetic field.
How is the hydrogen nucleus represented in MRI diagrams?
As an arrow called the magnetic moment:
Length = Magnitude of magnetic field.
Direction = Alignment with the external magnetic field (B₀).
What happens to hydrogen nuclei when placed in a strong external magnetic field (B₀)?
Their magnetic moments align with the field, a process called alignment.
What are the two types of alignment?
Parallel alignment (spin-up) → Low-energy state (aligned with B₀).
Antiparallel alignment (spin-down) → High-energy state (opposed to B₀).
What determines the number of energy states a nucleus can have?
Equation: Number of energy states = 2S + 1 (S = Spin quantum number).
For hydrogen (S = ½): 2(½) + 1 = 2 energy states (parallel and antiparallel).
The spin quantum number can only be two numbers:
- +½ → This means the electron is spinning up (↿).
- -½ → This means the electron is spinning down (⇂).
What is Zeeman Interaction?
The process where protons couple with the external magnetic field (B₀), resulting in two distinct energy states.
spin up or spin down
What is the net magnetic vector (NMV)?
The sum of all magnetic moments in the body, representing the small excess of spin-up nuclei in alignment with B₀.
there are always more magnetic moments parellel than anti parallel
Why does the NMV align with B₀?
Because there are always more spin-up nuclei than spin-down nuclei, creating a measurable net magnetization in the longitudinal (z-axis) direction.
What is the most abundant element in the body?
Hydrogen.
Which nuclei are used in MRI?
Nuclei with a net spin.
Why do spinning nuclei create a magnetic field?
Due to Faraday’s Law (a moving charge induces a magnetic field).
How is a nucleus’s magnetic field represented?
By a magnetic moment (arrow).
According to Avogadro’s law, how many molecules are present per gram of tissue?
6 × 10²³ molecules per gram of tissue
How many excess spins are present per gram of tissue?
6 × 10¹⁷ excess spins per gram of tissue
How does increasing B₀ affect the number of spin-up (low-energy) and spin-down (high-energy) nuclei?
Increasing B₀ increases the number of spin-up (low-energy) nuclei and decreases the number of spin-down (high-energy) nuclei
What is the equation for the ratio of spin-up (N⁺) to spin-down (N⁻) populations?
N⁺ / N⁻ = e^(-ΔE / kT)
What does the variable ΔE represent in the Boltzmann equation?
The energy difference between high- and low-energy populations (in Joules)
What is the value of Boltzmann’s constant (k)?
1.381 × 10⁻²³ J/K
Why does SNR increase at higher B₀ values?
Higher B₀ increases the NMV, leading to a greater signal-to-noise ratio (SNR)
What is precession?
The wobbling motion of magnetic moments around B₀ due to its influence
What is the Larmor frequency (ω₀)?
The rate at which magnetic moments precess around B₀
What is the equation for Larmor frequency?
ω₀ = γB₀
What does the gyromagnetic ratio (γ) represent?
The relationship between angular momentum and the magnetic moment of MR-active nuclei
What are the units of Larmor frequency?
Megahertz (MHz)
What is the gyromagnetic ratio of hydrogen?
42.58 MHz/T
What is the precessional frequency of hydrogen at 1.5T?
63.87 MHz (42.58 × 1.5)
What is the precessional frequency of hydrogen at 1.0T?
42.58 MHz (42.58 × 1.0)
What is the precessional frequency of hydrogen at 0.5T?
21.29 MHz (42.58 × 0.5)
What is the precessional frequency of hydrogen at 3.0T?
127.74 MHz (42.58 × 3.0)
What is the gyromagnetic ratio of ¹³C (carbon-13)?
10.71 MHz/T
What is the precessional frequency of carbon at 1.5T?
16.06 MHz (10.71 × 1.5)
What is the gyromagnetic ratio of ¹⁵N (nitrogen-15)?
4.32 MHz/T
What is the precessional frequency of nitrogen at 1.5T?
6.48 MHz (4.32 × 1.5)
Why is hydrogen (¹H) primarily used in MRI instead of other MR-active nuclei?
It has the highest abundance in the body and a large magnetic moment
What does phase refer to in MRI?
The position of magnetic moments on their precessional path at any moment in time
What does out of phase (incoherent) mean?
Magnetic moments of hydrogen are at different places on the precessional path
What does in phase (coherent) mean?
Magnetic moments of hydrogen are at the same place on the precessional path
What happens to precessional frequency when B₀ is increased?
Larmor frequency increases proportionally
How does the Larmor equation explain the specificity of MRI?
Different MR-active nuclei have unique gyromagnetic ratios, allowing MRI to selectively image hydrogen while ignoring other nuclei
What is the equation for calculating the ratio of spin-up to spin-down nuclei?
N⁺ / N⁻ = e^(-ΔE / kT)
What happens to the NMV when the external magnetic field (B₀) increases?
Increases
What is the gyromagnetic ratio of nitrogen-15?
4.32 MHz/T
Which of the following is NOT a correct description of precession?
It occurs due to thermal energy transfer
What is the precessional frequency of hydrogen at 1.5T?
63.87 MHz
What is resonance in MRI?
A phenomenon where a nucleus absorbs energy when exposed to an RF pulse at its Larmor frequency.
What happens if the applied RF pulse is not at the Larmor frequency of hydrogen?
Resonance does not occur, and the nucleus does not absorb energy.
Why do other MR-active nuclei not resonate with the RF pulse used for hydrogen imaging?
Their gyromagnetic ratios are different, leading to different precessional frequencies.
What is B₁ in MRI?
The weak oscillating magnetic field created by the RF excitation pulse, applied perpendicular to B₀.
How is B₁ different from B₀?
B₀ is a strong, static magnetic field, while B₁ is a weak, oscillating magnetic field applied at 90° to B₀.
According to classical theory, what happens when an RF excitation pulse is applied?
The NMV spirals from the longitudinal plane to the transverse plane (nutation).
What is nutation?
A spiral motion caused by the NMV transitioning from the longitudinal to the transverse plane.
According to quantum theory, what does RF excitation do?
It increases the number of high-energy (spin-down) hydrogen nuclei.
Why does the net effect of RF excitation result in energy absorption?
Because there are initially more low-energy spins, causing an overall shift toward high-energy states.
What happens to the phase of magnetic moments after RF excitation?
They become in-phase (coherent), leading to a detectable MRI signal.
What is Planck’s equation for energy absorption?
E = hω₀
What does the variable h represent in Planck’s equation?
Planck’s constant (6.626 × 10⁻³⁴ J/s).
How does increasing B₀ affect the energy required for resonance?
A stronger B₀ increases the energy difference (ΔE) between spin-up and spin-down states.
What happens to ω₀ (Larmor frequency) as B₀ increases?
ω₀ increases proportionally.
What is the relationship between B₀ and resonance frequency?
Higher B₀ leads to a higher resonance frequency, requiring a higher RF pulse frequency for excitation.
What is the typical flip angle for a 90° RF pulse?
π/2 radians.
What is the typical flip angle for a 180° RF pulse?
π radians.
How is flip angle calculated?
θ = γB₁τ
What happens to magnetization at a 90° flip angle?
The NMV is completely transferred into the transverse plane.
What happens at a 180° flip angle?
The NMV is completely inverted, saturating the system.
What is the principle behind MR signal generation?
Faraday’s Law of Electromagnetic Induction.
What does Faraday’s Law state?
A changing magnetic field induces an electromotive force (emf) in a closed circuit.
What equation describes electromagnetic induction?
ε = -N (dΦ/dt)
In the equation ε = -N (dΦ/dt), what does Φ represent?
Magnetic flux.
How is MR signal detected?
By placing a receiver coil in the path of precessing transverse magnetization.
What happens when the RF excitation pulse is turned off?
The NMV begins to realign with B₀, causing relaxation.
What is the Free Induction Decay (FID) signal?
A decaying MR signal induced in the receiver coil due to dephasing of spins.
What causes the loss of transverse magnetization after RF excitation?
Dephasing due to inhomogeneities in B₀ and spin-spin interactions.
Why does MR signal fade over time after excitation?
Due to transverse relaxation and spin dephasing.
What happens to the NMV as relaxation occurs?
It gradually returns to the longitudinal axis.
What must be true for resonance to occur in MRI?
The RF excitation pulse must match the Larmor frequency.
What is the role of B₁ in MRI?
It provides the oscillating field needed for resonance.
According to quantum theory, what happens during RF excitation?
The number of low-energy spins decreases.
What equation describes the energy difference between spin-up and spin-down states?
ΔE = hγB₀.
What happens to magnetization at a 180° flip angle?
The NMV is inverted, fully saturating the system.
What is the equation for calculating the flip angle?
θ = γB₁τ.
What happens when an RF pulse is applied at the Larmor frequency?
Resonance occurs, and hydrogen nuclei absorb energy, moving into a high-energy state.
What is excitation in MRI?
The net absorption of RF energy, causing a shift in spin populations and creating transverse magnetization.
What does the orientation of the Net Magnetic Vector (NMV) depend on?
The balance between spin-up (low-energy) and spin-down (high-energy) nuclei.
What happens to the NMV when there are equal numbers of spin-up and spin-down nuclei?
It moves completely into the transverse plane (90° to B₀).
Why do hydrogen nuclei need to be in-phase after excitation?
Coherent transverse magnetization is required to generate an MRI signal.
How is an MR signal detected?
By placing a receiver coil in the transverse plane, where the rotating transverse magnetization induces a voltage.
What happens to transverse magnetization when the RF pulse is turned off?
The spins begin to dephase, leading to Free Induction Decay (FID).
What is Free Induction Decay (FID)?
The signal decay that occurs when transverse magnetization dephases after the RF pulse is turned off.
What two factors contribute to FID?
B₀ inhomogeneities and spin-spin interactions (T₂ decay).
Why does the MR signal fade over time after the RF pulse is removed?
Because of dephasing of spins, leading to a loss of coherent transverse magnetization.
What does the rate of FID decay depend on?
T₂ relaxation time and magnetic field homogeneity.
Why is FID not used directly for image formation?
It decays too quickly, making it unsuitable for capturing stable signals.
What is a pulse sequence in MRI?
A series of RF pulses, signal acquisitions, and relaxation periods that generate an image.
What does TR stand for in MRI?
Repetition Time – the time between successive RF excitation pulses for a slice.
What does TR control?
The amount of T₁ relaxation that occurs before the next RF pulse is applied.
What does TE stand for in MRI?
Echo Time – the time from the RF excitation pulse to the peak of the detected signal.
What does TE control?
The amount of T₂ relaxation that has occurred when the signal is read.
What unit are TR and TE measured in?
Milliseconds (ms).
If TR is too short, what happens to T₁ contrast?
T₁ contrast increases because tissues do not fully recover before the next RF pulse.
If TE is long, what kind of contrast is emphasized?
T₂-weighted contrast, highlighting tissues with long T₂ relaxation times.
What is the purpose of an RF excitation pulse in MRI?
c) To induce resonance and create transverse magnetization.
How is MR signal detected?
b) By the movement of transverse magnetization cutting through a receiver coil.
Which of the following factors contributes to Free Induction Decay (FID)?
a) Inhomogeneities in B₀.
What does repetition time (TR) determine?
b) The amount of longitudinal relaxation before the next RF pulse.
If TE is long, what type of image contrast will be emphasized?
c) T₂-weighted contrast.
What does TE control?
a) The amount of T₂ relaxation occurring before the signal is read.
What is required for resonance to occur in MRI?
b) The applied RF pulse must match the Larmor frequency of hydrogen.
What happens to the NMV when a 90° RF pulse is applied?
b) It shifts completely into the transverse plane.
What happens when the RF pulse is turned off?
b) Spins gradually return to thermal equilibrium.
Why is Free Induction Decay (FID) not used for image formation?
b) It decays too quickly.
In what year did MRI receive FDA approval?
1985
When did Felix Bloch and Edward Purcell receive the Nobel Prize for Physics for developing nuclear induction?
1952
What was Raymond Damadian’s first human-sized MRI machine called?
Indomitable
Who is credited with the first human body image in 1977?
Raymond Damadian
One of the original names for MRI technology was NMR. What does NMR stand for?
Nuclear Magnetic Resonance
Which two physicists first won a Nobel Prize in Physics for their development of nuclear induction?
Bloch and Purcell
Define relaxation in MRI.
The process by which spins lose energy.
The longitudinal plane is in which axis?
z-axis
What is NMV?
Net Magnetic Vector
Precessional frequency in MRI is measured in what unit?
MHz
What is the Larmor equation used for?
To calculate precessional frequency
What is the Larmor equation?
ω₀ = γB₀
What is the gyromagnetic ratio of hydrogen at 3T?
127.74 MHz
What does the NMV do as the main magnetic field strength increases?
It gets larger.
According to Avogadro’s law, how many molecules exist per gram of tissue?
6 × 10²³ molecules per gram
In MR imaging, the term “spin” refers specifically to the spinning of what?
Nuclei
What is the small magnetic field created by the hydrogen proton called?
Magnetic moment
Parallel alignment occurs when:
Protons are introduced into a magnetic field.
What happens during the ‘excitation’ phase in an MRI scan?
The RF excitation pulse gives energy to nuclei and causes a net increase in high-energy, spin-down spins.
What happens if there is a moving magnetic field?
Electricity is created.
If there is a moving electrical field, what is created?
Magnetism
The potential signal-to-noise ratio (SNR) does what as field strength increases?
Increases
When is Free Induction Decay (FID) created?
After any RF excitation pulse.
Describe Free Induction Decay (FID).
Induction of rapidly decreasing signal.
What is thermal equilibrium in MRI?
When all protons that are going to align with the field have done so in the low and high-energy states.
The application of an RF pulse that causes resonance to occur is called:
Excitation
Which state of alignment is more abundant in MRI?
Parallel
What controls the amount of T1 contrast in an image?
TR (Repetition Time)
What controls the amount of T2 contrast in an image?
TE (Echo Time)
What is the time from the RF pulse to the peak of the signal called?
Echo Time (TE)
What is TR?
Repetition Time – the time from one RF excitation pulse to the next.
What is TE?
Echo Time – the time from the RF pulse to the peak of the signal detected.
What does TR control?
The amount of T1 relaxation that occurs before the next RF pulse is applied.
What does TE control?
The amount of T2 dephasing that occurs when the signal is read.
Which particle of an atom has a negative charge?
Electron
Which particle of an atom has a positive charge?
Proton
What happens to NMV when field strength increases?
It gets larger.
What does frequency represent in MRI?
Speed of precession, in MHz
The time constant of the precessional frequency of a specific MR active nucleus at 1 Tesla is called:
Gyromagnetic ratio
The hydrogen proton induces a strong magnetic field because it is:
Charged and spinning
In what year did MRI receive its FDA approval?
1985
Define relaxation.
Process by which spins lose energy
The longitudinal plane is in the _____________ axis.
z-axis
What is the gyromagnetic ratio of hydrogen at 3T?
127.74 MHz
In MR imaging, the term “spin” refers specifically to the spinning:
Nuclei
What is the Larmor equation used for?
To calculate precessional frequency
The principles of MRI rely on -
Spinning motion of specific nuclei present in biological tissues
What is NMV?
Net Magnetic Vector (NMV)
Precessional frequency in MRI is measured in:
MHz
If there is a moving magnetic field, what could be created?
Electricity
In what year did Felix Bloch and Edward Purcell receive the Nobel Prize for physics for developing nuclear induction?
1952
Raymond Damadian’s first human-sized machine, now in the Smithsonian, is named:
Indomitable
When is FID created?
After any RF excitation pulse
What type of motion is letter A?
Precession
What is the Larmor equation used to calculate?
Precessional frequency
Which particle of an atom has a negative charge?
Electron
Which particle of an atom has a positive charge?
Proton
The tiny magnetic field of the hydrogen proton that is induced by spin and charge and has a north and south pole is called:
Magnetic moment
Parallel alignment is created when -
The protons are introduced into a magnetic field
What controls the amount of T1 that will be in an image?
TR
Frequency is -
Speed of precession, in MHz
One of the original names for the technology, NMR, stood for what?
Nuclear Magnetic Resonance
The time constant of the precessional frequency of a specific MR-active nucleus at 1 Tesla in MHz/T is called:
Gyromagnetic ratio
What happens to NMV as the main magnetic field strength is increased?
It gets larger
What is thermal equilibrium?
When all the protons that are going to line up with the field have done so in the low and high-energy states.
Describe the free induction decay.
Induction of rapidly decreasing signal
The hydrogen proton induces a strong magnetic field because it is:
Charged and spinning
What happens during the ‘excitation’ phase in an MRI scan?
RF excitation pulse gives energy to nuclei and causes a net increase in the number of high-energy, spin-down spins.
If there is a moving electrical field, what is created?
Magnetism
What controls the amount of T1 relaxation that occurs when the signal is read?
TR
What equation is represented by ‘omega 0 = gamma B0’?
Larmor equation
The time from the application of the RF pulse to the peak of the signal induced in the coil is termed the:
Echo time (TE)
What controls the amount of T2 that will be in an image?
TE
Which two physicists first won a Nobel Prize in Physics for their development of nuclear induction?
Bloch and Purcell
The application of an RF pulse that causes resonance to occur is termed:
Excitation
Which state of alignment is more abundant?
Parallel
The potential signal-to-noise ratio (SNR) -
Increases as the field strength increases
Who is credited with the first human body image in 1977?
Raymond Damadian
According to Avogadro’s law -
There are about 6 × 10^23 molecules per gram of tissue.
What controls the amount of T2 dephasing that occurs when the signal is read?
TE
What is letter B?
Net magnetic spin
Why is the hydrogen atom used in clinical MRI?
Due to its abundance in the body and its solitary proton giving a large magnetic moment.
The symbol for the primary static field is:
Bₒ
MR-active nuclei are:
Nuclei that possess an odd mass number.
Out of phase means:
Magnetic moments of H are at different places on the precessional path at a moment in time.
One reason the hydrogen atom is used in clinical MR imaging is its abundance in the body and:
Its solitary proton creates a large magnetic moment.
How many electrons does the hydrogen atom have?
1
What is the gyromagnetic ratio of hydrogen at 1.2T?
51.09 MHz
What are the two branches of physics used to describe the basic principles of MRI?
Classical and quantum theories.
The relative balance between spin-up and spin-down nuclei is called:
Net magnetic vector.
What happens to the hydrogen protons when they are placed in a magnetic field?
They align parallel or anti-parallel.
What prompted the name change from NMR to MRI prior to FDA approval?
To prevent patients from thinking it was radiation.
What determines the relative quantities of spin-up and spin-down nuclei?
The strength of the main magnetic field.
Which of the following is the most abundant atom in the human body?
Hydrogen.
The nucleus of a hydrogen atom contains:
One proton.
Phase is:
A position at a given time.
Spin of MR active nuclei, which depends on the balance between number of protons and neutrons in the nucleus, is defined as:
Angular momentum.
TE determines how much decay of _______________ magnetization is allowed to occur.
Transverse.
What is the most common excitation ‘flip angle’?
90°.
What were the early NMR machines used to study, and what was the name of this technique?
Chemicals, spectroscopy.
Who was the first person to demonstrate a radio signal from the nucleus of an atom?
Isidor Rabi.
What is termed B1?
RF field.
What conditions are necessary for resonance?
Energy applied at the precise frequency and at a right angle.
Energy is most effectively transferred from one system to another when the systems are at:
Resonance.
Whose law states that ‘a moving magnetic field induces a voltage in a conductor coil’?
Faraday’s Law.
Antiparallel is the:
High-energy state.
The number of protons in a nucleus is termed:
Atomic number.
What are the two most common molecules in the human body, where hydrogen is found?
Lipids and water.
Amplitude and duration of an RF pulse will determine:
The flip angle.
How many protons does the hydrogen atom have?
1.
With regard to energy, protons in parallel alignment are in the _________ state, whereas protons in anti-parallel alignment are in the __________ state.
Low-energy, high-energy.
What is ‘TR’?
Time to repeat RF excitation.
TR determines the amount of ____________ relaxation that occurs between the end of one RF pulse and the application of the next.
Longitudinal.
How is the MR signal produced?
Signal is produced when in-phase transverse magnetization cuts across a receive coil.
The Larmor precessional frequency of hydrogen in a 1.5T magnet is:
63.86 MHz.
