MRI Definitions Flashcards
Cryogen
Cooling agent, such as liquid helium or nitrogen. In MR imaging, cooling agents are used to maintain the superductivity of the magnet
Echo Time(TE)
Time between the middle of the (90°) excitation pulse to the center of the echo. TE determines images contrast and is usually measured in milliseconds (ms).
Fourier Transform (FT)
Mathematical technique for MR signal to be decomposed into a sum of sine waves of different frequencies, phases, and amplitudes.
Coronal plane
Orthogonal plane dividing the body into posterior and anterior parts (frontal plane).
Gadolinium-DTPA (Gd-DTPA)
Contrast agent used in MRI; the update of gadolinium-containing contrast agent reduces the T1 and T2 values of tissues, depending on the concentration. The T1 effect is the more relevant in clinical examinations.
Gradient coils
Coils used to generate magnetic gradient fields. Gradient coils are operated in pairs in the magnet, at the same current but of opposite polarities. One of the coils increases the static magnetic-field a certain amount; the opposite coil reduces it by the same amount, changing the magnetic field overall. The change is the linear gradient. According to the coordinate axes (x, y, and z).
Hydrogen nuclei
Most abundant atom in the body; used in MRI for the clinical scanning.
Larmor Frequency
Frequency at which the nuclear spins precess around the direction of the outer magnetic field. The Larmor frequency depending on the type of nucleus and the strength of the magnetic field. At 1.5T, the Larmor frequency of protons is approximately 63 MHz.
Permanent Magnet
Magnet whose magnetic field originates from permanently ferromagnetic materials to generate a magnetic field between two poles of the magnet. There is no requirement for additional electrical power or cooling. Permanent magnets cannot be turned off, even in case of emergency. These magnets are usually low-field magnets (0.4T). 72
Proton Density (PD)
Long repetition time (TR) (reduces T1); short echo time (TE) (minimizes T2). See also Spin density
Radiofrequency (RF)
Portion of the electronic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna. The RF pulses used in MRI are commonly in the 1 to 100 megahertz range. The primary effect on the human body is energy dissipation in the form of heat, usually on the surface of the body. Energy absorption is an important value for establishing safety thresholds.
Radiofrequency (RF) coil
Used for transmitting RF pulses or receiving MR signals.66
Receiver coil
Coil of the RF receiver; it detects the MRI signal.66
Relaxation time
Following excitation, the nuclear spins tend to return to their equilibrium position, in accordance with these time constants, and release excess energy.66 T1 and T2
Resistive magnet
Type of magnet that uses the principles of electromagnetism to generate the magnetic field. Typically, large current values and significant cooling of the magnet coils are required. A resistive magnet does not require cryogens, but it needs a constant power supply to maintain a homogeneous magnetic field and can be expensive to maintain . These magnets are usually low -field magnets (0.61T).
