12 Mri Flashcards
Xxxx is an 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
Xxxx 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
Superconductive magnets with field strengths of __________ are common for clinical systems, and 4 to 7 T clinical large bore magnets are currently used for research
applications, with possible future clinical use.
1.5 to 3 T
Superconductive magnets with field strengths of xx to xx T are common for clinical systems, and 4 to 7 T clinical large bore magnets are currently used for research
applications, with possible future clinical use.
Superconductive magnets with field strengths of 1.5 to 3 T are common for clinical systems, and 4 to 7 T clinical large bore magnets are currently used for research
applications, with possible future clinical use.
Xxxx are 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
Air core magnets
In addition to the main magnet system, other components
are also necessary. Xxx interact with the main magnetic field to improve homogeneity (minimal variation of the magnetic flux density) over the volume used for patient imaging.
Xxx exist within the main bore of the magnet to transmit energy to the patient as well as to receive returning signals.
Xxx are contained within the main bore to produce a linear variation of magnetic field strength across the useful magnet volume.
Shim coils
Radiofrequency (RF) coils
Gradient coils
Xxx describes the extent to which a material becomes magnetized
when placed in a magnetic field.
Xxx opposes the external magnetic field and lowers the local magnetic field surrounding the material.
On the other hand, the xxx can form in the same direction as the
applied magnetic field, and increase the local magnetic field
Magnetic susceptibility
Induced internal magnetization
internal magnetization
Magnetic properties are influenced by spin and
charge distributions intrinsic to the proton and neutron.
A magnetic dipole is created
for the proton, with a positive charge equal to the electron charge but of opposite sign,
due to nuclear “spin.”
Overall, the neutron is electrically uncharged, but subnuclear
charge inhomogeneities and an associated nuclear spin result in a magnetic field of
opposite direction and approximately the same strength as the proton.
nuclear magnetic moment
Magnetic characteristics of the nucleus are described by xxx
vector indicating magnitude and direction
Typical magnetic field strengths for MR systems range from xxx .
For protons, the precessional frequency is xxx , and increases or decreases with an
increase or decrease in magnetic field strength
- 3 to 4.0 T
42. 58 MHz/T
Xxx corresponds to the
energy separation between the protons in the parallel and antiparallel directions.
The xxxx considers the RF energy as photons (quanta)
instead of waves.
Protons oriented parallel and antiparallel to the external magnetic
field, separated by an energy gap, DE, will transition from the low- to the highenergy level only when the RF pulse is equal to the precessional frequency
resonance frequency
quantum mechanics model
Xxx 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.
is applied on Mz, rotating it
from the longitudinal direction into the transverse plane
Flip angles
torque
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
T2 relaxation time
Xxx is the term describing
the release of energy back to the lattice (the molecular arrangement and structure of
the hydration layer), and the regrowth of Mz.
Spin-lattice relaxation
The xxx
is the time between the excitation pulse and the appearance of the peak amplitude of an induced echo, which is determined by applying a 180-degree RF inversion pulse
or gradient polarity reversal at a time equal to TE/2.
time of echo (TE)
