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MRI TERMS
BASIC TERMINOLOGY
| Question | Answer |
|---|---|
| symbol used to represent the static main magnetic field of the magnetic resonance (MR) imaging system; the strength of the magnetic field is expressed in units of tesla (T). | B0 — (pronounced “B zero”) |
| symbol used to represent the radiofrequency (RF) field in the MR system; the RF coils, or transmitter coils, at the Larmor frequency produce the B1 field. | B1 — |
| summarizes the influence of the gradients in diffusion-weighted imaging; the higher the b-value the stronger the diffusion weighting. | b-value — |
| phenomenon caused by protons resonating at different frequencies in a magnetic environment. | Chemical shift — |
| the process of maintaining a constant relationship between the rotations of hydrogen protons; loss of phase coherence of the nuclear spins results in a decrease in transverse magnetization and decrease in MR signal. | Coherence — |
| single or multiple loops of wire that produce a magnetic field when current flows through them, or that detect a changing magnetic field by voltage induced in the wire. | Coil — |
| after a radiofrequency (RF) pulse is applied, phase differences appear between precessing spins; the resulting decay in spin-spin interaction occurs in the transverse plane. | Dephasing — |
| a substance that has a magnetic susceptibility of less than 0 because it has no unpaired orbital electrons; examples include silver, copper and mercury. | Diamagnetic — |
| the result of radiofrequency (RF) wavelengths shortening inside the body at higher field strengths. | Dielectric effect — |
| interval during the repetition time (TR) that the gradient is permitted to be at maximum amplitude. | Duty cycle — |
| time period from the middle of one echo to the middle of the next echo. | Echo spacing — |
| time in milliseconds between the 90-degree pulse and the peak of the echo signal; TE is the primary factor controlling T2 relaxation. | Echo time (TE) — |
| electric current induced in a conductor when that conductor is exposed to a changing magnetic field. | Eddy current — |
| state of balance that exists between two opposing forces or divergent forms of influence | Equilibrium — |
| the flip angle for a particular spin that provides maximum signal in the least amount of time when the signal is averaged over many transients. | Ernst angle — |
| a brief radiofrequency (RF) pulse that distorts the equilibrium of the spins in the magnetic field; the RF pulse transfers energy to the spinning nuclei, placing the nuclei in a higher energy state; scanner then collects the signal from the excited nuclei | Excitation pulse — |
| parameters that can be manipulated; extrinsic parameters include repetition time (TR), echo time (TE), inversion time and flip angle. | Extrinsic parameters — |
| if a receiver coil or any conductive loop is placed in the area of a moving magnetic field, voltage is induced in the receiver coil; this moving magnetic field voltage is the MR signal. | Faraday law of induction — |
| a substance that demonstrates a positive magnetic susceptibility greater than 1; these substances are highly attracted to a magnetic field and retain their magnetism even after the magnetic field is removed; include iron, steel, nickel and cobalt. | Ferromagnetic — |
| area of the anatomy being imaged; increasing the field of view decreases echo space and resolution. | Field of view (FOV) — |
| algorithm used to convert raw scan data from waveform to digital form. | Fourier transform — |
| signal induced by radiofrequency (RF) excitation of nuclear spins that decrease exponentially because of T2 relaxation. | Free induction decay (FID) — |
| the angle to which the net magnetization is rotated or tipped relative to the main magnetic field direction when a radiofrequency (RF) excitation pulse is applied at the Larmor frequency; MR imaging frequently uses 90-degree and 180-degree flip angles. | Flip angle — |
| the number of repetitions of a process over a unit of time (eg, hertz). | Frequency — |
| a unit measuring magnetic field strength; 1 tesla = 10,000 gauss. | Gauss (G) — |
| a linear slope of magnetic field strength across the scanning volume in a particular direction; gradients change the magnetic field along the slope by adding or subtracting magnetic field strength. | Gradient — |
| the strength of the gradient. | Gradient amplitude — |
| the time it takes for gradients to reach their maximum strengths or amplitudes; gradient rise time is measured in millitesla per meter (mT/m) or gauss per centimeter (G/cm). | Gradient rise time — |
| ratio of the magnetic moment (field strength) to the angular moment (frequency); the gyromagnetic ratio of hydrogen is a constant and can vary slightly. The MR Basics series uses a gyromagnetic ratio for hydrogen of 42.58 megahertz per tesla (MHz/T). | Gyromagnetic ratio — |
| a magnetic field is homogeneous when it has the same field strength across the entire field; homogeneity is an important criterion for image quality. | Homogeneity — |
| standard unit of frequency equal to one cycle per second; the larger unit megahertz (MHz) = 1,000,000 Hz. | Hertz — |
| difference in signal strength between tissues; contrast is affected by pulse sequences and other factors chosen by the MR technologist. | Image contrast — |
| imaging parameter that cannot be changed; intrinsic parameters include T1 relaxation, T2 relaxation and proton density. | Intrinsic parameter — |
| area that serves as the mathematical repository for the Fourier transform; in general scan time is the amount of time needed to fill k-space. | k-space — |
| rate at which the nuclear spins precess around the direction of the magnetic field; the rate depends on the type of nuclei and the strength of the magnetic field. | Larmor frequency — 1 OF 3 |
| states that the precessional frequency (ω) of the nuclear magnetic moment is directly proportional to the product of the magnetic field strength (B0) and the gyromagnetic ratio (γ) of hydrogen; | Larmor frequency — 2 OF 3 |
| stated mathematically, the equation reads ω = γB0 . The gyromagnetic ratio of hydrogen is a constant and may vary based on the MR technologist’s training. The MR Basics series uses a gyromagnetic ratio for hydrogen of 42.58 megahertz per tesla (MHz/T). | Larmor frequency — 3 OF 3 |
| magnetic environment where the nuclei exchange energy during longitudinal relaxation. | Lattice — |
| portion of the magnetization vector in the direction of the z-axis, that is, along the main magnetic field; after excitation by a radiofrequency (RF) pulse, longitudinal magnetization returns to equilibrium within a characteristic time constant T1. | Longitudinal magnetization — |
| Return to equilibrium of longitudinal magnetization after excitation; longitudinal relaxation is due to the energy exchange between the spins and surrounding lattice, also called spin-lattice relaxation. | Longitudinal relaxation — |
| tissue-specific time constant that describes the return of longitudinal magnetization to equilibrium; after the time period of T1, longitudinal magnetization increases to approximately 63% of its end value; a tissue parameter that determines contrast. | Longitudinal relaxation time — |
| space surrounding a magnet (or a conductor with current flowing through it) that has special characteristics; every one exercises a force on magnetizable parts aligned along a primary axis (magnetic north or south pole). | Magnetic field — |
| strength of the magnetic field force on magnetizable parts. In physics, the effect is called magnetic induction; in MR, it is referred to as magnetic field strength. | Magnetic field strength — |
| point in the center of the magnet where x, y and z equal 0. | Magnetic isocenter — |
| a measure of magnitude and direction of an object’s magnetic properties that causes the object to align with the B0 field and create its own field. | Magnetic moment — |
| the integration of all the individual nuclear magnetic moments that have a positive magnetization value at equilibrium vs. those in a random state. | Magnetization vector — |
| fundamental property of all matter related to moving electrons. | Magnetism — |
| absorption or emission of electromagnetic energy by atomic nuclei in a static magnetic field after excitation by electromagnetic radiofrequency (RF) radiation at a resonance frequency. | Magnetic resonance — |
| degree of magnetism that exists within any substance or the ability of a material to become magnetized. There are four types of magnetic susceptibility: diamagnetic, paramagnetic, superparamagnetic and ferromagnetic. | Magnetic susceptibility — |
| abel given to a piece of equipment or medical device that is considered to be MR safe. | MR conditional — |
| electromagnetic signal in the RF range; the signal is produced by the precession of transverse magnetization created by a variable voltage in a receiver coil. | MR signal — |
| the sum of the magnetic moments of unpaired nuclei after pairs of low-energy and high-energy nuclei cancel each other out. | Net magnetization vector — |
| see number of excitations. | NEX — |
| see number of signals averaged. | NSA — |
| number of times the image is sampled, also referred to as number of signals averaged (NSA). | Number of excitations (NEX) — |
| number of times the image is sampled, also referred to as number of excitations (NEX). | Number of signals averaged (NSA) — |
| a substance with a magnetic susceptibility between 0 and 1 because it has unpaired orbital electrons; examples include tungsten, platinum and gadolinium. | Paramagnetic — |
| the degree to which precessing nuclear spins are synchronous. | Phase coherence — |
| the motion of net magnetization as is it “wobbles” around the main magnetic field of the MR scanner; precession measurement is the signal produced by the wobbling protons. | Precession — |
| equation stating that the Larmor frequency (ω) is equal to the product of the gyromagnetic ratio of hydrogen (γ) and the strength of magnet (B0), or ω = γB0. | Precessional frequency calculation — |
| positively charged particle located in the nucleus of an atom. | Proton — |
| In MR, a 4-line diagram representing all pulse sequences used for a scan. | Pulse sequence diagram — 1 of 3 |
| 1st represents the timing of the radiofrequency(RF)pulse. The 2nd(Gz) represents a gradient pulse used for slice selection. | Pulse sequence diagram — 2 of 3 |
| The 3rd(Gy) represents a gradient pulse used for phase encoding. The 4th(Gx) represents a gradient pulse used for frequency encoding. | Pulse sequence diagram — 3 of 3 |
| frequency required to excite hydrogen nuclei to resonate; MR uses frequencies in the megahertz (MHz) range. | Radiofrequency (RF) — |
| dynamic, physical process in which a system returns from a state of imbalance to equilibrium; MR imaging consists of two types of relaxation: longitudinal and transverse. | Relaxation — |
| time period between the beginning of a pulse sequence and the beginning of the succeeding, identical pulse sequence. | Repetition time (TR) — |
| exchange of energy between two systems at a specific frequency; resonance occurs when an object is exposed to a precessional frequency the same as its own. | Resonance — |
| frequency at which resonance occurs; the frequency for the radiofrequency (RF) pulse matches the Larmor frequency. | Resonance frequency — |
| a gradient that rephases and creates a coherent gradient echo. | Rewinder gradient — |
| coils or antennas used in MR to transmit radiofrequency (RF) pulses or receive RF signals. | RF coils — |
| when the same amount of nuclear spins are aligned against and with the magnetic field | Saturation — |
| the process of creating a uniform, or homogeneous, magnetic field. | Shimming — |
| the strength of the gradient over distance; slew rate is calculated by dividing the gradient amplitude by the gradient rise time. | Slew rate — |
| a measure of radiofrequency (RF) energy absorbed by the body and expressed in watts per kilogram (W/kg); SAR characterizes the increased heating of tissue due to RF exposure. | Specific absorption rate (SAR) — |
| the property exhibited by atomic nuclei that contain either an odd number of protons or neutrons, or both. | Spin — |
| a substance that has ferromagnetic properties in bulk; similar to paramagnetic substances except each individual atom is independently influenced by an external magnetic field. example of a superparamagnetic substance is an iron-containing contrast agent. | Superparamagnetic — |
| a measure of magnetic field strength of the MR scanner; 1 tesla = 10,000 gauss. | Tesla — |
| when the application of a radiofrequency (RF) pulse causes the net magnetization vector to flip into the transverse plane; the magnetization vector as measured in the x-y plane. | Transverse magnetization — |
| decay of transverse magnetization through the loss of phase coherence between precessing spins due to spin exchange; also known as spin-spin relaxation. | Transverse relaxation — |
| tissue-specific time constant that describes the decay of transverse magnetization in an ideal, homogeneous magnetic field; after the time T2, transverse magnetization loses 63 percent of its original value; a tissue parameter that determines contrast. | Transverse relaxation time — |
Created by:
zach1710