MRI: Magnets, RF Coils, and Gradient Coils Flashcards
However, most existing human MRI scanners have strengths up to
3.0 Tesla
The FDA approved what strength of human MRI scanners for clinical diagnosis?
7.0 Tesla
10,000 gauss is equivalent to
1 Tesla
The magnetic field of the earth ranges from ____ to ____ gauss, with an average of ____ gauss
0.25 to 0.65, average of 0.5
What substance is associated with maintaining superconducting magnets that is generally expensive
Helium
What difficulty is associated with the high costs of MRI scans?
Difficulty of making the magnetic field uniform
Type of MRI magnet that has a strength limited to about 1 Tesla. Maintenance costs are negligible
Permanent magnet
Type of MRI magnet that can achieve high magnetic field, has high maintenance costs, and is most popular in hospitals
Superconducting magnet
The main function of a magnet is to cause proton spins to rotate/precess at a frequency proportional to the magnetic field, also known as
Resonance frequency
In Tx RF coils, the transmission of homogenous energy is important, so the transmission is often performed with a
Large RF coil (a body coil premounted on the scanner)
In Tx RF coils, the proton spins that have the same frequency as the frequency in the transmission coil will receive energy and get excited. This is also called
Magnetic resonance
Aside from using a single RF coil, what are the two separate RF coils that also can be used for MR signals?
Transmission (Tx) and reception (Rx) RF coils
MRI is an integration of many ____ _____ that makes it expensive
Hardware equipment
The main magnetic fields are along what direction?
Z direction
How can we get spatial information in MRI?
Gradient coils
Positional information of detectors (RF coils) cannot be used to get these information in MRI except in parallel imaging
Spatial information
High sensitivity to the object is important, so the reception of MR signals is performed with a
Small RF coil (close to the object)
In Rx RF coils, when the transmission RF energy is off, MR signals will be _____ and _____ in the receiver RF coil
Induced and detectable
What makes MRI safe?
No ionizing radiation
These gradient coils are to modulate the main magnetic field strengths along the X and Y but not to generate any fields along X or Y
X and Y gradient coils
Magnetic field induced by a gradient coil is superimposed on the
MRI magnet field
Spatially different precession frequencies allow us to get
Spatial information
Varying magnetic field or gradient causes this to be a function of spatial location
Precession frequency (omega)
Typical MRI has a field strength of up to _____ Tesla, 60,000x greater than the earth’s magnetic field
3.0 Tesla
Unit of SAR
Watts per kilogram (W/kg)
A measure of the rate at which energy is absorbed by the human body when exposed to an RF electromagnetic field.
These have upper limits in clinical scanners for safety
SAR or specific absorption rate
What particular material is not allowed in MRI that involve pacemakers, artificial limbs, or most devices
Metal
Modulate the resonance frequency depending on the spatial location along X, Y, and Z
Gradient coils
Transmits and/or receives signals at the resonance frequency
Radio frequency (RF) coils
Generates magnetic fields that causes protons to precess at a frequency proportional to the strength of the magnetic fields (resonance frequency)
Magnet
A noninvasive biomedical imaging device with a relatively high spatial resolution, various soft tissue contrast, and integration of many different imaging modalities in one single scanner
MRI
