Physics Chapter 1 Flashcards
Atoms are organized into…
Molecules
Most abundant molecule
Hydrogen
_____ orbit Atoms / spin around nucleus
Electrons
_____ within atoms contain all of atoms mass
Nucleus
Nucleons are subdivided into
Protons and neutrons
Positive nucleons
Protons
No charge nucleons
Neutrons
Negative nucleons
Electrons
Sum of protons in the nucleus
Atomic number
Sum of protons and neutrons in nucleus
Atomic weight/mass number
Atoms with same number of protons but different number of neutrons
Isotopes
Electron position not predictable and depends on the energy of an individual electron at any given time
Heisenberg‘s uncertainty principle
Equal amount of negative charged electrons and positive charged protons are…
Stable atoms
Applying energy to knock out electrons and alter balance causes…
Electrical instability
Atoms knocked out by electrical instability are called
Ions
Process of knocking out electrons
Ionization
What are the three motions within atoms
Electrons spin on own axis,
Electrons orbit nucleus,
Nucleus spins on own axis
MRI relies on…
Spinning of specific nuclei in body tissues
Atomic mass and numbers affect amount of…
Spin values
Nucleus has no spin if…
It has even atomic and mass numbers
(6 protons + 6 neutrons = mass of 12)
Why does a nucleus with an even atomic number and mass number have no net spin?
Half spin in one direction and half spin in the opposite direction thus canceling spins
If there is an odd number of protons and an odd number of neutrons or an odd number of both protons and neutrons the spin directions…
Are not equal and are opposite so the nucleus has a net spin
If a nucleus has a net spin this is called…
Angular momentum
Nuclei with an odd mass number or atomic weight are…
MR – active nuclei
Align their axis of rotation to an applied field
MR active nuclei
Refers to connection of electric and magnetic fields and motion. Determines a moving electric field produces a magnetic field and vice versa
Faradays law
_____ Have an electrical charge and are spinning therefore acquire a magnetic field
MR active nuclei
_______ Of each nucleus has size and direction
Magnetic moment/ Magnetic field
__________ Of the nucleus is the vector sum of all the magnetic moments of protons in the nucleus
Total magnetic moment
Isotope of hydrogen is…
Protium
Most common MR active nucleus used is…
Protium
Protium has a mass and atomic number of…
1 (nucleus has a single proton and no neutron)
A solitary proton has a _______ magnetic moment
Large
The spinning of the one positively charged proton in protium induces a magnetic field around it acting as
A small magnet
The magnet of each hydrogen nucleus has…
Equal strength north and south poles
In the absence of applied magnetic fields, orientation of hydrogen nuclei is…
Randomly aligned producing no magnetic field
When placed in a strong static magnetic field (B0) the magnetic moment…
Align with the magnetic field
Alignment of magnetic moments in the same direction as the main B0 field
Parallel Alignment/ Spin-Up (low energy)
Alignment of magnetic moments in the opposite direction to B0 field
Anti-parallel Alignment/ Spin-down (high energy)
Protons of hydrogen nuclei couple with external magnetic field B0
Zeeman interaction
The 2 energy states for hydrogen are…
Low energy nuclei and High energy nuclei
Not enough energy to oppose B0 field. Spin up or parallel alignment
Low energy nuclei
Do you have enough energy to oppose B0 field. Spin down or anti-parallel alignment
High energy nuclei
Predicts the number of spins in each energy level
Boltzman equation
A patient’s temperature determines whether a spin is…
In the high or low energy population
A patient’s temperature is usually similar inside and out of B0. This is called…
Thermal equilibrium
In _______ there are more magnetic moments in spin up then in spin down
Thermal equilibrium
As more spins are aligned to B0, there is a small excess that produces a ________
Net magnetic moment (NMV)
_________ reflects the balance between spin up and spin down nuclei
NMV
The sum of all magnetic moments of excess spin up nuclei and is measurable
NMV
Aligns parallel to main magnetic field in the longitudinal plane (Z axis)
NMV
Number of spins depends on…
The number of molecules per gram of tissue and the strength of B0
As B0 increases more magnetic moments are…
Parallel
As field strength increases fewer spins have…
Enough energy to align in opposition to B0
When low energy population increases in size and high energy population decreases in size…
The NMV increases
For every million protons at 1.5 T the NMV is about…
4.5 million
For every million protons at 3 T the NMV is about…
10 million
SNR increases at higher field strengths because of…
Increased NMV
______ produces an additional spin to hydrogens nucleus spin
B0
Additional spin produced by B0 is called
Precession
________ causes magnetic moments to spin around B0
Precession
The course that hydrogen nucleus spins around B0 is called a
Precessional path
The speed which hydrogen nuclei spin is called
Precessional frequency
Precessional frequency is known as…
Larmour equation
_______ Is one cycle of rotation per second
1 Hz
______ is 1 million cycles per second
1 MHz
Larmour equation is represented by
W0=y*B0
W0 represents
Precessional frequency in MHz
Y represents
Gyromagnetic ratio (MHz/T)
B0 represents
Strength of external field (T)
Relationship between angular momentum and the magnetic moment of each MR active nucleus
Gyromagnetic Ratio
The Gyromagnetic ratio of hydrogen expresses at 1 T is…
42.58 MHz/T
Gyro magnetic ratio of hydrogen at 1.5 T is
63.87 MHz
Gyro magnetic ratio of hydrogen at 0.5 T is…
21.29 MHz
Gyro magnetic ratio of hydrogen at 3 T is…
127.74 MHz
The equation for a Gyro magnetic ratio is…
42.58 MHz * _______ T
_____________ fall into the radiofrequency band of the electromagnetic spectrum
Precessional frequencies
The position of magnetic moments on their professional path
Phase
The unit of phase is…
Radian
A magnetic moment travels through 360 rad or 360° during…
One rotation
Frequency being the rate of change of phase of magnetic moments is defined as
How quick the phase position of a magnetic moment changes over time
Magnetic moments are at different places on the precessional path
Out of phase/ Incoherent
Magnetic moments are at the same place on precessional path
In phase/ Coherent
When B0 is the only influence, nuclei are________
Out of phase with each other
When nuclei are out of phase the NMV…
Does not precess
Occurs when an object is exposed to an oscillating perturbation(a movement disturbance) that has a frequency close to its own natural frequency of oscillation(movement)
Resonance
When a nucleus is exposed to an external force with similar oscillation to the natural frequency of its magnetic moment, the nucleus…
Gains energy from the external source
If energy is delivered at a different frequency, resonance…
Does not occur and the nucleus does not gain energy
For resonance to occur, ________is applied at the Larmour frequency of hydrogen
An RF pulse
Resonance is achieved by transmitting a….
RF pulse/RF excitation pulse
An RF excitation pulse is produced by a….
Transmit coil (ex: body, head, extremity coils)
Consists of electric and magnetic fields that spread in waves at 90° to each other
RF excitation pulse
The RF excitation is only from the component (coil) and produces an oscillating magnetic field called
B1
The B1 field is applied at ____ to B0 at a narrow range or bandwidth of frequencies centered around a central frequency (transmit bandwidth)
90°
Magnetic field of____ is very weak compared to_____
B1; B0
Represented in the transverse plane/ x-y axis
B1 field
Application of B1 in the 90° plane to B0 causes magnetic moments of the spins to…
Precess around transverse/ x-y axis rather than at the longitudinal or Z axis
As the B1 field associated with RF excitation is weak, the magnetic moments of spins precess at a much lower frequency than when aligned with B0 resulting in…
A spiral downward motion of the NMV from longitudinal to transverse plane
Downward spiral of NMV is called
Nutation
_________ Is caused by 2 precessional motions that happen simultaneously (precession around B0 and a slower precession around B1)
Nutation
RF excitation moves magnetic moments of spin up and spin down…
Into phase with each other
RF pulse application is called…
Excitation
Excitation means…
Energy giving
RF excitation gives energy to hydrogen, increasing the…
Number of high energy, spin down nuclei
__________________ absorb energy from RF excitation pulse and move into high energy
Low energy, spin up nuclei
At the same time of the RF excitation, spin down, high energy nuclei are stimulated to…
Release energy and return to the low energy state
Because there are more low energy spins, the net affect is of…
Energy absorption
If the right amount of energy is absorbed, the NMV lies in the…
Transverse plane at 90° to B0
When the right amount of energy is absorbed and the NMV lies in the transverse plane it has moved through a…
Flip angle or tip angle of 90°
The frequency required to cause resonance is related to the difference in…
Energy between high energy and low energy populations and the strength of B0
As B0 increases, the energy difference between the two populations_______
Increases
__________ depends on amplitude and magnitude of RF excitation
Flip angle
The flip angle is usually______
90°
When a flip angle less than 90° is used….
Only a portion of NMV is transferred to the transverse plane
A flip angle of 180° is caused by…
RF excitation of twice the magnitude
____________ produces an inversion of spin populations called saturation
Flip angle of 180°
__________ is caused when spins are unable to absorb more energy or to be stimulated and release more energy
Saturation
Because of resonance, in phase (coherent) magnetization processes in the…
Transverse plane
The change of magnetic flux through a closed circuit induces a
Electromotive force (EMF) in the circuit
______ Is the energy available from a unit of charge traveling once around the loop of wire
EMF
 The movement of electrons caused by a changing magnetic field is called a
Current
If a receiver coil is placed in a moving magnetic field, a _________ generated by current is induced in the receiver coil
Voltage
Voltage generated in a receiver coil is called
Signal
_________ is produced when coherent (in- phase) magnetization cuts across the coil
Signal
When RF excitation is switched off…
NMV is only influenced by B0 and tries to realign with it
When hydrogen nuclei lose energy from RF
Relaxation
During relaxation, the NMV…
Realigns with B0
Simultaneously and independently during relaxation, hydrogen magnetic moments lose coherency due to
Dephasing
_________ is due to inhomogeneities in the B0 field and due to interactions between the spins in the patient’s tissue
Dephasing
The induction of decaying voltage is called the
FID signal
FID signal is named FID because
Spins FREELY precess influenced only by B0
Signal DECAYS with time
Magnetic moments INDUCE a current in the receiver coil
The magnitude and timing of the RF pulses formed part of_____________which are the basis of contrast generation in MRI
Pulse sequences
A simple pulse sequence combines…
RF pulses, signals, and interleaving periods of relaxation
A pulse sequence consists of several time periods. The two main periods are
TR & TE
TR is also known as
Repetition time
TE is also known as
Echo time
Time from the application of one RF excitation pulse to the next for each slice.
Measured in milliseconds.
Determines the amount of longitudinal relaxation between the end of one RF pulse and start of the next
TR
TR determines the amount of______ relaxation that occurs when signal is read
T1
Time from the application of the RF excitation pulse to the peak of signal induced in the receiver coil.
Measured in milliseconds.
Determines how much decay of transverse magnetization occurs.
TE
TE controls the amount of_______relaxation that has occurred when signal is read
T2
