MRI INSTRUMENTATION Flashcards
It is the second most powerful fundamental force of nature
Magnetism
o A force created by a magnetic field which behaves similarly to that of an electric field.
Magnetic force
Gravitational force is given by
Newton’s law
Electric force is given by
Coulombs’s Law
Magnetics force is given by
Gaus’s Law
refers to the relative ease with which a material can be made magnetic.
Magnetic susceptibility
Easily magnetized
Ferromagnetic
Very weakly magnetized
Paramagnetism
Unaffected by magnetic field
Diamagnetic
• this occurs when a nuclei has spin directions that are not equal and opposite.
• Also known as “Net Spin”
• Observed in nuclei with odd number of protons, odd number of neutrons or an odd number of proton and neutrons
Angular Momentum
• Refers to the rotational motion of the angular momentum vector of a particle around an external axis or field
Precession
• Precessional frequency of a particle
Larmor Frequency
• Electromagnetic radiation with frequencies of approximately 10 to 200 MHz.
Radiofrequency
refers to the position of the magnetic moments on the precessional path at any moment in time.
Phase
▪ Magnetic moments are on different places on the precessional path at a
moment in time
Out of Phase (In Coherent)
▪ Magnetic moments are on the same place on the precessional path at a
moment in time
In Phase (Coherent)
• Refers to the natural precessional motion of nuclear magnetic moments after an RF pulse
has been applied and subsequently turned off.
Free Precession
Time required for an excited particle (hydrogen), after being applied with an RF pulse and subsequently turned off, to lose its energy.
Relaxation Time
• Also known as Proton Density
• Refers to the number of hydrogen nuclei (protons) per unit volume of tissue that can be magnetized and contribute to MRI signal
Spin Density
• Most popular MRI system and features a tunnel-shaped magnet bore.
Closed Bore
• Longitudinal table movement in these system allows the patient to be positioned with the Region of Interest (ROI) lying at the center of the magnet bore.
Closed Bore
• This system encloses the patient from the front, back and side but still allows limited access.
Closed Bore
• The design also facilitates easy side access to the patient when undertaking
interventional procedures (biopsies).
Open Systems
These scanners permit a degree of sideways table movement
Open Systems
Uses large Permanent Magnets and superconducting solenoids to generate the main magnetic field.
Open Systems
• Are designed to scan limbs and are smaller in size.
Extremity Systems
• Typical design is approx. the size and shape of a washing machine with an aperture in the center large enough to accommodate an arm or a leg.
Extremity Systems
describes the number of flux lines passing through a given area.
FLUX DENSITY
magnetic field strength is measured in
Amperes per meter
Flux density (field strength) is measure in
Tesla
• The law quantifies and states that when an electric current flows through a conductor, it generates a magnetic field around the conductor.
Ampere’s Law:
• This system is equipped with large discs of ferromagnetic alloy.
Permanent Magnets
• Employs copper-wound solenoids that operate just below room temperature.
Resistive Electromagnets
• Introduced to address resistivity issues of Resistive electromagnets.
Superconducting Electromagnets
Also known as coolants; and are used to reduce the temperature of the windings to within 4 degrees of absolute zero (4 Kelvin).
Cyrogen System
• Derived from Greek word meaning “Cold” and “Stable”
CRYOSTAT
• This is the area inside the cylinder of the cryostat.
WARM BORE
reroutes the fringe field away from the outside environment and back
toward the scanner
Passive shielding
• which requires the scanner to be surrounded by large steel plates
Passive Shielding
• which uses additional solenoid magnets.
Active Shielding
uses shims to adjust for large changes in magnetic field homogeneity
Passive shimming
• use electromagnets and is used addition to passive shimming types:
Active Shimming
- can be manipulated at anytime by adjusting the current flow
through windings
resistive
- no electrical power is needed.
superconducting
• create linear slopes along the B0.
• apply a current to the gradient coils that offsets any minor inhomogeneity in the main
magnetic field
Gradient Offset (Dynamic) Shimming
o LEFT AND RIGHT
o SAGITTAL CUTS
o PAIR OF COILS POSITIONED ON EITHER SIDE OF THE CYLINDER
o POSITIONED SO THAT THE GRADIENT MAGNETIC FIELD IS ACROSS THE PX LATERALLY
o PROVIDES SPATIAL LOCALIZATION ALONG THE X-AXIS, PHASE- ENCODING OR FREQUENCY ENCODING
X Gradient Coils
o ANTERIOR/ POSTERIOR
o CORONAL CUTS
o POSITIONED SO THAT THE GRADIENT MAGENTIC FIELD IS ACROSS THE PX VERTICALLY
o PROVIDES SPATIAL LOCALIZATION ALONG THE Y- AXIS, PHASE - ENCODING OR FREQUENCY ENCODING
Y Gradient Coils
o PAIR OF CIRCULAR COILS EACH OF WHICH IS WOUND ON THE CYLINDER AND OPPOSITE ENDS
o MAXWELL COILS OR HELMWOTHS COILS
o ALLOWS SLICE SELECTION ALONG THE AXIS OF THE GRADIENT
o FOR AXIAL AND TRANSVERSE CUTS
Z Gradient Coils
are the receivers, and sometimes also the transmitters, of
radiofrequency (RF) signals in equipment used in magnetic resonance imaging
Radiofrequency coils (RF coils)
are designed to provide a homogeneous RF excitation across a large volume. Most clinical MRI scanners include a built in volume coil to perform whole-body imaging, and smaller volume coils have been constructed for the head and other
extremities.
Volume Coils
are designed to provide a very high RF sensitivity over a small region of interest. These coils are often single or multi-turn loops which are placed directly over the anatomy of interest. The size of these coils can be optimized for the specific region
of interest.
Surface coils
