AIM: Ch 12: Basic MRI Flashcards
It is energy coupling that causes the individual nuclei, when placed in a strong external magnetic field, to selectively absorb, and later release, energy unique to those nuclei and their surrounding environment
Resonance
This is the spectroscopic study of the magnetic properties of the nucleus of the atom
Nuclear magnetic resonance (NMR)
Two factors for which the protons and neutrons of the nucleus have their magnetic field based on
Nuclear spin
Charge distribution
It is a fundamental property of matter; it is generated by moving charges, usually electrons
Magnetism
These are the smallest entities of magnetism
Domains
True or False:
Magnetic properties of materials result from the organization and motion of the electrons in either a random or a nonrandom alignment of magnetic “domains”.
True
True or False:
Most materials do not exhibit overt magnetic properties, but one notable exception is the permanent magnet.
True
Magnetic fields exist as dipoles, where the north pole is the ____ of the magnetic field lines
and the south pole is the ____
Origin, return
Conceptualized as the number of magnetic lines of force per unit area, which decreases roughly as the inverse square of the distance from the source
Magnetic field strenght, B (magnetic flux density)
SI unit for B
Tesla
Earth’s magnetic field is about?
1/20,000 = 0.00005 T = 0.05 mT
1 T in gauss (G)
10,000 G
Rule that describes the direction of the magnetic field based on the sign and direction of the charge in the wire
Right hand rule
State the right hand rule
The fingers point in the direction of the magnetic field when the thumb points in the direction of a moving positive charge (i.e., opposite to the direction of electron
movement)
Two factors in which the magnetic field strength and field density are based on
Amplitude of the current
Number of coil turns
This determines the overall magnitude of the magnetic field strength
Amplitude of the current in the coil
Magnetic field lines extending beyond the concentrated field
Fringe fields
Heart of the MR system
Magnet
Three parameters which determine the performance criteria of any particular magnet type
Field strength
Temporal stability
Field homogeneity
Made of wire-wrapped cylinders of approximately 1-m diameter and greater, over a cylindrical length of 2 to 3 m, where the magnetic field is produced by an electric current in the wires. When the wires are energized, the magnetic field produced is parallel to the long axis of the cylinder.
Air core magnets
Fringe fields: Extensive
Constructed from permanent magnets, a wire-wrapped iron core “electromagnet,” or a hybrid combination. In these designs, the magnetic field runs between the poles of the magnet, most often in a vertical direction.
Solid core magnets
Fringe fields: Contained
It is a characteristic of certain metals (e.g., niobium–titanium alloys) that when maintained at extremely low temperatures (liquid helium; less than 4°K) exhibit no resistance to electric current
Superconductivity
This results when there’s failure in the replenishment of the liquid helium such that the temperature rises above a critical value and hence, loss of superconductivity occurs and resistance heating of wires will boil the helium
Quench
These coils interact with the main magnetic field to improve homogeneity (minimal variation of the magnetic flux density) over the volume used for patient imaging
Shim coils
Exist within the main bore of the magnet to transmit energy to the patient as well as to receive returning signals
Radiofrequency (RF) coils
These are contained within the main bore to produce a linear variation of magnetic field strength across the useful magnet volume.
Gradient coils
True or False:
Magnetic field strength decreases with distance from the center of each coil
True
This describes the extent to which a material becomes magnetized when placed in a magnetic field.
Magnetic susceptibility
Three categories of susceptibility are defined based upon the arrangement of electrons in the atomic or molecular structure:
Diamagnetic, paramagnetic, and ferromagnetic
Category of susceptibility with the following properties:
1. Susceptibility: Slightly negative
2. Oppose the applied magnetic field because of paired electrons in the surrounding electron orbitals
3. Examples: calcium, water, and most organic materials
Diamagnetic
Category of susceptibility with the following properties:
1. Susceptibility: Slightly positive
2. Enhance the local magnetic field, but they have no measurable self magnetism
3. Examples: molecular oxygen, deoxyHb, some blood degradation products such as methemoglobin, and GAD-based contrast agents
Paramagnetic
These are “superparamagnetic” in that they augment the external magnetic field substantially. These materials, containing iron, cobalt, and nickel, exhibit “self-magnetism” in many cases, and can significantly distort the acquired signals.
Ferromagnetic
Two factors that influence magnetic properties intrinsic to the proton and neutron
Spin and charge distributions
A magnetic dipole is created for the proton due to ____.
Nuclear spin
This term describes the magnetic characteristics of the nucleus and is represented as a vector indicating magnitude and direction
Nuclear magnetic moment
For a given nucleus, the nuclear magnetic moment is determined through the pairing of the constituent protons and neutrons.
P + N = even number, NMM = 0
P + N = odd number, NMM (+)
True or False:
Nuclear magnetic moment is determined through the pairing of the constituent protons and electrons.
False
Protons and neutrons
True or False:
If the sum of the number of protons (P) and number of neutrons (N) in the nucleus is even, the nuclear magnetic moment is essentially zero
True
However, if N is even and P is odd, or N is odd and P is even, the resultant noninteger nuclear spin generates a nuclear magnetic moment.
True or False:
A single nucleus can generate a large enough nuclear magnetic moment to be observable, and that the conglomeration of large numbers of nuclei (10^15) arranged in a random orientation also generates an observable nuclear magnetic moment of the sample
False
A single nucleus does not generate a large enough nuclear magnetic moment to be observable, but the conglomeration of large numbers of nuclei (10^15) arranged in a nonrandom orientation generates an observable nuclear magnetic moment of the sample
Element which has the largest magnetic moment and greatest abundance, chiefly in water and fat, and is by far the best element for general clinical utility
Hydrogen
The torque perpendicular to the direction of the applied magnetic field onto the protons causes ____. It occurs at an angular frequency (number of rotations/sec about an axis of rotation) that is proportional to the magnetic field strength B0.
Precession
This equation describes the dependence between the magnetic field, B0, and the angular precessional frequency, w0
Larmor equation
Typical magnetic field strengths for MR systems range from ______.
0.3 to 4.0 T
For protons, the precessional frequency is ____, and increases or decreases with an increase or decrease in magnetic field strength.
42.58 MHz/T
True or False
The differences in the gyromagnetic ratios and corresponding precessional frequencies allow the selective excitation of one element from another in the same magnetic field strength.
True
The applied magnetic field B0 is directed parallel to the _-axis of the three-dimensional Cartesian coordinate axis system and perpendicular to the _ and _ axes
z, x and y
Nice to Know:
The magnetic interactions between precessional frequencies of the tissue magnetic moments with the externally applied RF (depicted as a rotating magnetic field) can be described more clearly using the rotating frame of reference, while the observed returning signal and its frequency content is explained using the laboratory (stationary) frame of reference.
Externally applied RF: rotating frame of reference
Returning signal: laboratory frame of reference
The magnetic field vector components of the sample in the perpendicular direction are randomly distributed and sum to zero. Briefly irradiating the sample with an electromagnetic RF energy pulse tuned to the ____ promotes protons from the low-energy, parallel direction to the higher energy, antiparallel direction, and the magnetization along the direction of the applied magnetic field shrinks.
Larmor (resonance) frequency
Identify the component of the magnetic moment that is:
a. Parallel to B0
b. Perpendicular to B0
a. Longitudinal magnetization
b. Transverse magnetization
It corresponds to the energy separation between the protons in the parallel and antiparallel directions.
Resonance frequency
Displacement of the equilibrium magnetization occurs when the magnetic component of the RF excitation pulse, known as the B1 field, is precisely matched to the precessional frequency of the protons. The resonance frequency corresponds to the energy separation between the protons in the parallel and antiparallel directions.
They represent the degree of Mz rotation by the B1 field as it is applied along the x’-axis (or the y’-axis) perpendicular to Mz
Flip angles
Flip angles are the result of the angular displacement of the longitudinal magnetization vector from the equilibrium position. The rotation angle of the magnetic moment vector is dependent on the duration and amplitude of the B1 field at the Larmor frequency. Flip angles describe the rotation of Mz away from the z-axis.
Elapsed time between the peak transverse signal (e.g., directly after a 90-degree RF pulse) and 37% of the peak level (1/e) is the ____
T2 relaxation time
After a 90-degree RF pulse is applied to a magnetized sample at the Larmor frequency, an initial phase coherence of the individual protons is established and maximum Mxy is achieved. Rotating at the Larmor frequency, the transverse magnetic field of the excited sample induces signal in the receiver antenna coil (in the laboratory frame of reference). A damped sinusoidal electronic signal, known as the ____, is produced
Free induction decay (FID)
