midterms 2 Flashcards
are loops of wire or thin conductive sheets on a cylindrical shell lying just inside the bore of an MR scanner. When current is passed through these coils a secondary magnetic field is created
Gradients
slightly distorts the main magnetic field in a predictable pattern, causing the resonance frequency of protons to vary in as a function of position
gradient field
primary purpose of the gradient system in MRI is to create ___ in the magnetic field. This enables the MRI scanner to localize signals from specific locations within the body and create detailed images.
spatial variations
Types of gradients:
- X-Gradient (Slice selection)
- Y-Gradient (Frequency encoding)
- Z-Gradient (Phase encoding)
critical component of the MRI system that plays a key role in generating the RF pulses used to excite the protons in the body.
Radio frequency (RF) transmitter
- a specific type of receive RF coil.
- typically placed directly on or near the region of the body being imaged to capture the RF signals emitted by the protons.
- highly sensitive to the tissue immediately beneath them, allowing for high signal-to-noise ratio (SNR) and better image quality in localized areas.
Surface coil
process of reducing magnetic field inhomogeneities.
Magnetic shimming
2 main types of shimming:
passive and active (sometimes both may be used)
uses Shim coils, which have electric currents directed through them to generate their own magnetic field cancelling the remaining field gradients in the main magnetic field
active shimming
uses ferromagnetic pellets or pieces of sheet metal attached to the MRI scanner bore
passive shimming
used to cool the superconducting magnets that generate the strong magnetic fields necessary for MRI imaging
cryogens
Main Cryogens Used in MRI:
Liquid Helium (He-4):
Liquid Nitrogen (N₂):
- most commonly used cryogen in MRI systems, particularly for cooling the superconducting magnets.
- Boiling point of 4.2 K (-452.0°F / -268.9°C), which is low enough to keep superconducting magnets at the temperatures required for them to operate in a superconducting state.
Liquid Helium (He-4):
- used as a secondary cooling agent, typically for pre-cooling the magnet before the liquid helium is introduced
- boiling point of 77 K (-195.8°F / -196.2°C), which is significantly higher than that of liquid helium but still cold enough to help cool the superconducting magnet.
Liquid Nitrogen (N₂):
- Used to control the computer.
- computer initiates the appropriate radio-wave transmissions and then receives and analyzes the data.
- independent diagnostic workstation may be used to perform the same function as those of the operator’s console, depending on system configuration. However, usually only the operator’s console can control the actual imaging process.
Console
houses the electronics necessary for transmitting the radio-wave pulse sequences and for receiving and analyzing the MRl signal. The raw data and the computer-constructed images can be stored on a computer di k temporarily but are usually transferred to a magnetic tape or an optical disk for permanent storage and retrieval.
Computer Room
influences the quality and clarity of the resulting images. It refers to the ratio of the desired signal (the true information, such as tissue details) to the background noise (unwanted random variations that reduce image clarity)
Signal-to-noise Ratio
represents the strength of the information coming from the tissues being imaged. It is directly related to factors like tissue properties, magnetic field strength, and the pulse sequence used.
Signal (S)
represents random variations in the signal that are not related to the tissue properties but come from various sources, including thermal noise in the MRI equipment, electronic noise, or even patient movement.
Noise (N)
