CMR-related definitions

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

• Acquisition: The process of measuring and storing image data
• Apex of the Heart: The blunt rounded tip of the heart forming the left ventricle. The apical portion of the ventricular myocardium is thinner than other portions.
• Artifact: A spurious structure of feature in an image or data produced by an imaging technique or instrument rather than by the imaged object itself.
• Axial: A 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 scanener.
  • B1: the conventional symbol used for identifying the radio frequency (RF) magnetic field

• Base of the Heart: The region formed by the atrium and roots of the great vessels; thus the "top" of the heart, located opposite the apex of the heart.
• Cine: 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.
• Contrast: 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.
• Coronal: 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.
• Echo Planar Imaging (EPI): 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 Train: A series of 180º RF rephasing pulses and their corresponding echoes for a Fast Spin Echo (FSE) pulse sequence.
• Fast Spin Echo (FSE): A fast spin echo sequence is characterized by a series of rapidly applied 180º rephasing pulses and multiple echoes, changing the phase encoding gradient for each echo.
• Fat Saturation (Fat-Sat): 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 Suppression: 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. See also STIR.
• Field of View (FOV): Defined as the size of the two or three dimensional spatial encoding area of the image. Usually defined in units of cm².
• Field Echo (FE) (also known as Gradient Echo): 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
• Flip Angle (FA): 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 Flip Angle is used to defined the angle of excitation for a Field Echo pulse sequence.
• Fourier Transform (FT): A mathematical procedure used in MRI scanners to analyze and separate amplitude and phases of the individual frequence components of the complex time varying signal. Fourier transform analysis allows spatial information to be reconstructed from the raw data.
• Gadolinium (Gd): Gadolinium is a non-toxic paramagnetic contrast enhancement agent utilized in MR imaging. When injected during the scan, gadolinium will tend to change signal intensities by shortening T1 in its surroundings.
• Gating: Timing the acquisition of MR data to physiological motion in order to minimize motion artifacts (e.g., cardiac gating, respiratory gating).
• Image (Data) Acquisiton Time: 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 Reconstruction: The mathematical process of converting the composite signals obtained during the data acquisition phase into an image.
• Inversion Recovery: An imaging sequence that involves successive 180É and 90É pulses, after which a heavily T1-weighted signal is obtained. The inversion recovery sequence is specified in terms of three parameters: inverstion time (TI); repetition time (TR); and echo time (TE).
• Inversion Time: The time period between the 180º inversion pulse and the 90º excitation pulse in an Inversion Recovery pulse sequence.
• K-Space: A data acquisition matrix containing raw image data prior to image processing. In 2DFT, a line of data corresponds to the digitized NMR signal at a particular phase-encoding level.
• Larmor Equation: An equation that states that the frequency of precession of the nuclear magnetic moment is directly proportional to the product of the magnetic field strength (Bo) and the gyromagnetic ratio (g). This is stated mathematically as å = g Bo.
• Larmor Frequency: The frequency at which magnetic resonance in a nucleus can be excited and detected. The frequency varies directly with magnetic field strength, and is normally in the radio frequency (RF) range.
• Magnetic Resonance: The absorption or emission of energy by atomic nuceli in an external magnetic field after the application of RF excitation pulses using frequencies which satisfy conditions of the Larmor equation.
• Magnetic Resonance Angiography (MRA): MR image visualization of selected vascular structures, such as the Circle of Willis or the carotid arteries.
• Maximum Intensity Projection (MIP): A processing method for MRA images. A MIP is a record of a maximum intensity ray (generated through a mathematical algorithm) as it passes through an angiographic volume. Each point in an MIP represents the highest intensity experienced in that location on any partition within the imaging volume.
• MR Imaging: The use of magnetic resonance principles in the production of diagnostic views of the human body where the resulting image is based upon three basic tissue parameters (proton density, T1 relaxation time, T2 relaxation time) and flow characteristics.
• Noise: An undesirable background interference or disturbance that affects image quality.
• Oblique: A plane or section not perpendicular to the xyz coordinate system, such as the long and short axis views of the heart.
• Orthogonal: A plane or section perpendicular to the xyz coordinate system
• Phase Contrast: An MRA technique utilizing the change in the phase shifts of the flowing protons in the region of interest to create an image.
• Phase Encoding: The process of locating an MR signal by altering the phase of spins in one dimension with a pulsed magnetic field gradient along that dimension prior to the acquisition of the signal. As each signal component has experienced a different phase encoding gradient pulse, its exact spatial reconstruction can be specifically and precisely located by the Fourier transformation analysis. Spatial resolution is directly related to the number of phase encoding levels (gradients) used.
• Pixel: Acronym for a picture element, the smallest discrete two-dimensional part of a digital image display.
• Planar Imaging: Comparatively slow gyration of the axis of a spinning body so as to trace out a cone. Caused by the application of a torque tending to change the direction of the rotation axis and continuously directed at right angles to the plane of the torque. The magnetic moment of a nucleus with the spin will experience such a torque when inclined at an angle to the magnetic field, resulting in precession at the Larmor frequency.
• Pulse Sequence: A preselected set of defined RF and gradient pulses, usually repeated many times during a scan, wherein the time interval between pulses and the amplitude and shape of the gradient waveforms will control NMR signal reception and affect the characteristics of the MR images.
• Receiver: The portion of the MRI equipment that detects and amplifies the RF signals picked up by the receiver coil. Includes a preamplifier, NMR signal amplifier, and demodulator.
• Receiver Coil: A coil, or antenna, positioned within the imaging volume and connected to the receiver circuitry that is used to detect the NMR signal. In certain applications, the same coil can be used for both transmission and reception. Receiver coil types include: solenoidal, planar, volume, quadrature and phased array coils.
• Reconstruction: The mathematical process by which the displayed image is produced from the raw k-space data obtained from the receiver circuitry, typically utilizing Fourier transformation and selective filtering.
• Region of Interest (ROI): The area of anatomy being scanned that is of particular importance in the image.
• Relaxation Time: Afer excitation the spins will tend to return to their equilibrium distribution in which there is no transverse magnetization and the longitudinal magnetization is at its maximum value and oriented in the direction of the static magnetic field. After excitation the transverse magnetization decays toward zero with a characteristic time constant T2, and the longitudinal magnetization returns toward equilibrium with a characteristic time constant T1.
• Repetition Time: The amount of time that exists between successive pulse sequences applied to the same slice. It is delineated by initiating the first RF pulse of the sequence then repeating the same RF pulse at a time t. Variations in the value of TR have an important effect on the control of image contrast characteristics. Short values of TR (< 1000 ms) are common in images exhibiting T1 contrast, and long values of TR (> 1500 ms) are comon in images exhibiting T2 contrast. TR is also a major factor in total scan time.
• Resonance: A large amplitude vibration in a mechanical or electrical system caused by a relatively small periodic stimulus with a frequency at or close to a natural frequency of the system. The exchange of energy at a particular frequency between two systems.
• Sagittal: A plane, slice or section of the body cutting from the front to back through the sagittal suture of the skull, and continued down through the body in the same direction, dividing it into two parts, then turning one half to view it from its cut surface.
• Saturation Recovery: A little-used pulse sequence that generates a predominately proton density dependent signal, basically employing a 90º RF excitation pulse, with a very long repetition time. This procedure allows the saturated spins to return to equilibrium before the next pulse is activated.
• Selective Excitation: Controlling the frequency spectrum (bandwidth) of an RF excitation pulse while imposing a gradient magnetic field on spins so that only a desired region will have a suitable resonant frequency to be excited.
• Scan Time: A description of the total time required to acquire all the data needed to produce the programmed image.
• Signal Averaging: A signal-to-noise improvement method that is accomplished by taking the average of several FIDs made under similar conditions. This is also referred to as the number of excitations (NEX) or the number of acquisitions. The approximate amount of improvement in signal-to-noise. (S/N) ratio is calculated as the square root of number of excitations.
• Signal-to-Noise Ratio (S/N, SNR): The ratio between the amplitude of the received signal and background noise, which tends to obscure the signal. SNR, and hence image quality, can be improved by such factors as increasing the number of excitations, increasing the field of view, increasing slice thickness, etc. SNR also depends on the electrical properties of the patient being studied and the type of receiving coil used.
• Slice: The term describing the planar region or the image slice selection region.
• Slice Thickness: The thickness of an imaging slice. Since the slice profile is not sharply edged, the distance between the points at half the sensitivity of the maximum (full width at half maximum) is used to determine thickness.
• Spatial Resolution: The ability to define minute adjacent objects/points in an image, generally measured in line pairs per mm (lp/mm).
• Spin-Echo (SE): Re-appearance of the NMR signal after the FID has apparently died away, as a result of the effective reversal (rephasing) of the dephasing spins by techniques such as specific RF pulse sequences or pairs of field gradient pulses, applied in tiem shorter than or on the order of T2. Proper selection of the TE time of the pulse sequence can help control the amount of T1 or T2 contrast present in the iamge. Also, a pulse sequence type that usually employs a 90° pulse, followed by one or more 180° pulses.
• Steady-State Free Precession: The name for any field echo or gradient echo sequence in which a non-zero steady state develops for both transverse and longitudinal components of magnetization. If the RF pulses are close together, the MR signal will never completely decay, implying that the spins in the transverse (x-z) plane never completely dephase.
• STIR: The acronym for Short T1 Inversion Recovery. A specialized application of Inversion Recovery pulse sequence that sets the inversion time (TI) of the sequence at 0.69 times the T1 of fat, thereby suppressing the fat in the image.
• Surface Coil: A type of receiver coil which is placed directly on or over the region og interest for increased magnetic sensitivity. These coils are specifically designed for localized body regions, and prove improved signal-to-noise ratios by limiting the spatial extent of the excitation or reception.
• T1: Spin-lattice longitudinal relaxation time. The characteristic time constant for spins to realign themselves with the external magnetic field after excitation.
• T1 Weighted: An image created typically by using short TE and TR times whose contrast and brightness are predominately determined by T1 signals.
• T2: Spin-spin or transverse relaxation time. The time constant for loss of phase coherence among spins oriented at an angle to the static magnetic field due to interactions between the spins. Results in a loss of transverse magnetization and the MRI signal.
• T2* ("T-two-star"): The time constant for loss of phase coherence among spins oriented at an angle to the static magnetic field due to a combination of magnetic field inhomogeneities and the spin-spin relaxation. Results in a rapid loss of transverse magnetization and the MRI signal. T2* < T2.
• T2 Weighted: An image created typically using longer TE and TR times whose contrast and brightness are predominately determined by T2 signals.
• TE (Echo Time): Represents the time in milliseconds between the application of the 90° pulse and the peak of the echo signal in Spin Echo and Inversion Recovery pulse sequences.
• Tesla (T): The preferred unit of magnetic flux density. One Tesla is equal to 10,000 gauss. The Tesla unit value is defined as a field strength of 1 Weber per meter², where 1 Weber represents 1 x 108 (100,000,000) flux lines.
• Time of Flight (TOF): An MRA technique relying solely on the flow of unsaturated blood into a magnetized presaturated slice. The difference between the unsaturated and presaturated spins creates a bright vascular image without invasive use of contrast.
• TR (Repetition Time): The amount of time that exists between successive pulse sequences applied to the same slice.

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