MRI TERMINOLOGIES Flashcards
is a continuously changing flow of electrons that alternates its polarity at a periodic rate.
AC
the process of measuring and storing image data.
Acquisition
the total number of independent data samples in the frequency (f) and phase (f) directions.
Acquisition matrix
the period of time required to collect the image data. This time does not include the time necessary to reconstruct the image. ADC - analog-to-digital converter
Acquisition time
the phenomenon resulting from digitizing fewer than two samples per period in a periodic function. __ can occur in MR imaging whenever the area of anatomy extends beyond the field of view. These areas extending beyond the field of view boundaries are aliased back into the image to appear at artifactual locations.
Aliasing (wrap around artifact)
current that continuously changes in magnitude and direction. In the US the current changes at a frequency of 60 Hz.
Alternating current
the signal height. The greater the amplitude of the signal, the larger the number of protons in the image and the brighter it will appear.
Amplitude
being continuous, or having a continuous range of values.
Analog
a system that receives analog input data and produces digital values at its output. Used by the MRI scanner to convert the received signal into a format more compatible with the computer systems.
Analog-to-digital converter (ADC)
a device that enables the sending and/or receiving of electromagnetic waves.
Antenna
the storage of image and patient data for future retrieval.
Archiving
a dedicated computer system used to perform Fourier transformations to accelerate the processing of the received numerical data relative to the MR imaging process.
Array processor
See signal averaging
Averaging
plane, slice or section made by cutting the body or part of it at right angles to the long axis. If the body or part is upright, the cut would be parallel to the horizon. B or Bo - a conventional symbol for the constant magnetic field produced by the large magnet in the MR scanner. B1 - the conventional symbol used for identifying the radio frequency (RF) magnetic field.
Axial
an all-inclusive term referring to the preselected band or range of frequencies which can govern both slice select and signal sampling.
Bandwidth (BW)
a variation in the nominal Larmor frequency for a particular isotope within the imaging volume. The amount of shift introduced is directly proportional to the strength of the magnetic field, and is specified in parts per million (ppm) of the resonant frequency.
Chemical shift
a series of rapidly recorded multiple images taken at sequential cycles of time and displayed on a monitor in a dynamic movie display format. This technique can be used to show true range of motion studies of joints and parts of the spine.
Cine
a large network of interconnecting blood vessels at the base of the brain that when visualized resembles a circle.
Circle of willis
a psychological reaction to being confined in a relatively small area.
Claustrophobia
the ratio of signal intensity differences between two regions,scaled to image noise. Improving CNR increases perception of the distinct differences between two clinical areas of interest.
Contrast-to-noise ratio
the act of maintaining a constant phase relationship between oscillating waves or rotating objects.
Coherence
the relative difference of signal intensities in two adjacent regions of an image. Image contrast is heavily dependent on the chosen imaging technique (i.e., TE, TR, TI), and is associated with such parameters as proton density and T1 or T2 relaxation times.
Contrast
an image phenomenon where the darks become bright, and the brights become dark. This is usually most prevalent in sequences utilizing an extended TR.
Contrast reversal
a plane, slice or section made by cutting across the body from side to side and therefore parallel to the coronal suture of the skull.
Coronal
an artifact introduced into images by interference between adjacent slices of a scan. This artifact can be eliminated by limiting the minimum spacing between slices.
Crosstalk
a cooling agent, typically liquid helium or liquid nitrogen used to reduce the temperature of the magnet windings in a superconducting magnet. dB/dt - The rate of change of the magnetic field.This shows the ratio between the amount of change in amplitude of the magnetic field (dB) and the time it takes to make that change (dt). The value of dB/dt is measured in Tesla per second (T/s). DC -direct current.
Cryogen
the fanning out or loss of phase coherence of signals within the transverse plane. See also T2.
Dephasing
a magnetic field characterized by its own north and south magnetic poles separated by a finite distance.
Dipole
a continuous current that flows in only one direction.
Direct current
the total number of pixels in the selected matrix, which is described by the product of its phase and frequency axis.
Display matrix
a theory of magnetism which assumes that groups of atoms produced by movement of electrons align themselves in groups called”domains” in magnetic materials.
Domain theory
Diethylenetriaminepentaacetic acid - Gadolinium chelating (chemical bonding) agent that solves the problem of toxicity
Dtpa
the utilization of rapid gradient reversal pulses of the readout gradient resulting in a series of gradient echo signals to reduce fast dephasing or signal loss.
Echo planar imaging (EPI)
See TE
Echo time
a series of 180° RF rephasing pulses and their corresponding echoes for a Fast Spin Echo (FSE) pulse sequence.
Echo train
Meaning of ETL?
Echo train length
an induced spurious electrical current produced by time-varying magnetic fields.__ can cause artifacts in images and may seriously degrade overall magnet performance.
Eddy current
a type of magnet that utilizes coils of wire, typically wound on an iron core, so that as current flows through the coil it becomes magnetized.
Electromagnet
the response of electrons to electromagnetic radiation and magnetic fields at discrete frequencies.
Electron spin resonance
a state of balance that exists between two opposing forces or divergent forms of influence.
Equilibrium
delivering (inducing, transferring) energy into the “spinning” nuclei via radio-frequency pulse(s), which puts the nuclei into a higher energy state. By producing a net transverse magnetization an MRI system can observe a response from the excited system.
Excitation
an electrically conductive screen or shield that reduces or eliminates interference between outside radio waves and those from the MRI unit.
Faraday shield
a specialized technique usually associated with short TR, reduced flip angle and repeated 180° rephasing pulses.
Fast scanning
a __ sequence characterized by a series of rapidly applied 180° rephasing pulses and multiple echoes, changing the phase encoding gradient for each echo.
Fast spin echo
A specialized technique that selectively saturates fat protons prior to acquiring data as in standard sequences, so that they produce negligible signal. The pre-saturation pulse is applied prior to each slice selection. This technique requires a very homogeneous magnetic field and very precise frequency calibration.
Fat saturation
the process of utilizing specific parameters , commonly with STIR (short TI inversion recovery) sequences, to remove the deleterious effects of fat from the resulting images.
Fat suppression
the United States Food and Drug Administration
FDA
defined as the size of the two or three dimensional spatial encoding area of the image. Usually defined in units of cm2.
Field of view
a particularly fast and efficient computational method of performing a Fourier Transform, which is the mathematical process by which raw data is processed into a usable image.
Fast fourier transform
echo produced by reversing the direction of the magnetic field gradient to cancel out the position- dependent phase shifts that have accumulated due to the gradient.
Field echo ( gradient echo)
FLuid Attenuated Inversion Recovery
Flair
Fast Low-Angle Recalled Echoes
Flare
the angle to which the net magnetization is rotated or tipped relative to the main magnetic field direction via the application of an RF excitation pulse at the Larmor frequency. The __ is used to define the angle of excitation for a Field Echo pulse sequence.
Flip angle
a function of specific pulse sequences, i.e., CRISP¿ (Complex Rephasing Integrated with Surface Probes) spin echo, wherein the application of strategic gradient pulses can compensate for the objectionable spin phase effects of flow motion.
Flow compenstation
invisible lines of force that extend around a magnetic material. The greatest density is at the two poles of the magnet.
Flux
the number of lines of force per unit area of a magnetic material.
Flux density
a mathematical procedure used in MRI scanners to analyze and separate amplitude and phases of the individual frequency components of the complex time varying signal. __ analysis allows spatial information to be reconstructed from the raw data.
Fourier transform
if transverse magnetization of the spins is produced, e.g., by a 90É RF pulse, a transient MR signal at the Larmor frequency results that decays toward zero with a characteristic time constant of T2*. This decaying signal is the FID.
Free induction decay
the number of cycles or repetitions of any periodic wave or process per unit time. In electromagnetic radiation, it is usually expressed in units of hertz (Hz), where 1 Hz = 1 cycle per second.
Frequency
the process of locating an MR signal in one dimension by applying a magnetic field gradient along that dimension during the period when the signal is being received.
Frequency encoding
a term usually relating to the extents of the magnetic field surrounding the magnet. Safety requirements dictate that the distances of particular field strengths from the magnet must be known, and that potentially unsafe areas must be indicated with appropriate warning signs. Access to areas with field strengths of 5 gauss and higher must be strictly controlled. Gx, Gy, Gz - the conventional symbols for the three orthogonal magnetic gradients. The subscripts designate the conventional spatial direction of the gradient.
Fringe field
is a non-toxic paramagnetic contrast enhancement agent utilized in MR imaging. When injected during the scan, __ will tend to change signal intensities by shortening T1 in its surroundings.
gadolinium
timing the acquisition of MR data to physiological motion in order to minimize motionartifacts (e.g., cardiac gating, respiratory gating).
Gating
a unit of magnetic field strength that is approximately the strength of the earth’s magnetic field at its surface (the earth’s field is about 0.5 to1G).
Gauss
three paired orthogonal current- carrying coils located within the magnet which are designed to produce desired gradient magnetic fields which collectively and sequentially are superimposed on the main magnetic field (Bo) so that selective spatial excitation of the imaging volume can occur.
Gradient coils
A small linear magnetic field applied in addition to (superimposed on) the large static magnetic field in an MRI scanner.
Gradient magnetic field
a constant for any given nucleus that relates the nuclear MR frequency and the strength of the external magnetic field.
Gyromagnetic ratio
the standard unit of frequency equal to 1
cycle per second.
Hertz
uniformity of the main magnetic field.
Homogeneity
the concentration of Hydrogen atoms in water molecules or in some groups of fat molecules within tissue. Initial MR signal amplitudes are directly related to H+ density in the tissue being imaged.
Hydrogen density
the time required to gather a complete set of image data. The total time for performing a scan must take into consideration the additional image reconstruction time when determining how quickly the image(s) may be viewed.
Image data acquisition
the mathematical process of converting the composite signals obtained during the data acquisition phase into an image.
Image reconstruction
