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MRI Related Terms

- R -

  • RADIO FREQUENCY

  • An electromagnetic wave with a frequency that is in the same general range as that used for the transmission of radio and television signals. Abbreviated RF. The RF pulses used in MR are commonly in the 1-100 megahertz range, and their principle effect upon a body is potential tissue heating caused by absorption of the applied pulses of RF energy.

  • READOUT GRADIENT

  • Magnetic field gradient applied during the period when the receiver components are on. The application of this gradient, which is active during the period when the echo is being formed, results in the frequency encoding of the object being imaged. See also Frequency Encoding.

  • 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 coils 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

  • After 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 (TR)

  • 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 common in images exhibiting T2 contrast. TR is also a major factor in total scan time. See also TR.

  • REPHASING

  • The process of returning out-of-phase magnetic moments back into phase coherence. Caused either by rapidly reversing a magnetic gradient (Field Echo) or by applying a 180 RF pulse (Spin Echo). In the spin-echo pulse sequence this action effectively cancels out the spurious T2* information from the signal.

  • RESISTIVE MAGNET

  • A common type of magnet that utilizes the principles of electromagnetism to generate the magnetic field. Typically large current values and significant cooling of the magnet coils is required. Resistive magnets fall into two general categories - iron-core and air-core. Iron-core electromagnets provide the advantages of a vertically-oriented magnetic field, and a limited fringe field with little, if-any, missile effects due to the closed iron-flux return path. Air-core electromagnets exhibit horizontally oriented fields, which have large fringe fields (unless magnetically shielded) and are prone to missile effects. Resistive magnets are typically limited to maximum field strengths of approximately 0.6T.

  • 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. ROI - see Region Of Interest.


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MRI Terms


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