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MRI Related Terms

- T -

  • T1 RELAXATION

  • See Longitudinal Relaxation Time.

  • 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 WEIGHTED

  • An image created typically by using longer TE and TR times whose contrast and brightness are predominately determined by T2 signals. TAU (t) - the interpulse times (time between the 90 and 180 pulse, and between the 180 pulse and the echo) used in a spin echo pulse sequence. 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.

  • 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.

  • 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 meter 2, where 1 Weber represents 1 x 108 (100, 000, 000) flux lines.

  • THREE DIMENSIONAL IMAGING (3DFT)

  • A specialized imaging technique that uses computer processing to combine individual slice acquisitions together to produce an image that represents length, width and height. TI (Inversion Time) - the time between the initial (inverting) 180 pulse and the 90 pulse used in inversion recovery pulse sequences.

  • TIME OF FLIGHT (TOF)

  • And 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 the invasive use of contrast media.

  • TIP ANGLE

  • Angle between the net magnetization vector before and after an RF excitation pulse. Small tip angles allow a decrease in TR, which is used to decrease scan time in Field Echo pulse sequences. See Flip Angle.

  • TR (Repetition Time)

  • The amount of time that exists between successive pulse sequences applied to the same slice. See also Repetition Time.

  • TRANSAXIAL

  • A plane perpendicular (rotated 90) to the long axis of the human body. See also Axial.

  • TRANSCEIVER COIL

  • An MRI surface coil that acts as both transmitter and receiver.

  • TRANSMITTER

  • The portion of the MR scanner that produces the RF current and delivers it to the transmitting coil (antenna). The RF signal produced by the transmitter is used to excite the protons in the imaging volume.

  • TRANSVERSE MAGNETIZATION

  • Component of the net magnetization vector at right angles to the main magnetic field. Precession of the transverse magnetization at the Larmor frequency is responsible for the detectable NMR signal. In the absence of externally applied RF energy, the transverse magnetization will decay to zero with a characteristic time constant of T2, or more strictly T2*.

  • TRANSVERSE RELAXATION TIME

  • The time constant, T2, which determines the rate at which excited protons reach equilibrium, or go out of phase with each other. A measure of the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field due to interaction between spins, resulting in a reduction in the transverse magnetization. The transverse magnetization value will drop from maximum to a value of about 37% of its original value in a time of T2.

  • TUNING

  • The process of adjusting the transmitter and receiver circuitry so that it provides optimal signal performance at the Larmor frequency. A properly tuned scanner will produce images with a higher signal- to-noise ratio, and therefore improved diagnostic versatility.

  • TWO-DIMENSIONAL IMAGING (2DFT)

  • The Fourier transformation process reconstructs the detected frequency and phase encoded image information (which are rotated 90 from each other) into a usable image.


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