General MRI Acronym Definitions

 

Spectroscopy Acronym Directory  General MRI Acronym Directory

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Acronym SNR
Expansion Signal to Noise Ratio
Explanation

(Diagram)

Average pixel intensity (signal) over standard deviation of background signal (noise). Determined by constant K x voxel size x square root of measurements over square root of bandwidth
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Reference 10-1
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Acronym SPECT
Expansion Single Photon Emission Computed Tomography
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Acronym SS
Expansion Slice Select Gradient
Explanation In order to produce spatial localization, a gradient is applied which varies linearly from the center of the magnet. In order to excite a specific slice, an RF pulse is applied at the resonance frequency + an additional frequency offset that corresponds to the magnetic field the desired slice experiences (main field + slice select gradient).
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Related Terms slice profile, spatial localization, PE, FE, FOV
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Acronym STE
Expansion Stimulated Echo
Explanation The spin echo that results from two successive RF pulses is a Hahn echo (90-90, 90-180, etc). If a third RF pulse is applied, a stimulated echo results that is half the size of the Hahn echo.
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Related Terms SE, SSFP
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Acronym T
Expansion Tesla
Explanation Unit of magnetic field. Equal to 10,000 Gauss.
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Related Terms Gauss
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Acronym T1
Expansion T1
Explanation Spin-lattice relaxation. A time constant whereas at time=T1 after an RF pulse, 63.2% of magnetization has recovered alignment with the main field. Energy absorbed by the excited protons is released back into surrounding lattice, re-establishing thermal equilibrium. Varies with field strength
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Related Terms T2
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Acronym T2
Expansion T2
Explanation Spin-spin relaxation. Exponential loss of signal resulting from purely random spin-spin interactions in the transverse (X-Y) plane. Temporary and random interactions between adjacent spins that result in dephasing and loss of signal. At time t=T2 after an RF pulse, 63.2% of the signal amplitude has been lost.
Synonyms transverse relaxation.
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Acronym T2 PEDD
Expansion T2 Proton Electron Dipole Dipole interaction
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Reference SMRM 9, 125
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Acronym T2 PRE
Expansion T2 Proton Relaxation Enhancement
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Acronym TD
Expansion  
Explanation Time between RF spin echo and gradient echo in a GRE sequence
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Related Terms TE, TR, GRE
Reference ACR Glossary of MR Terms
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Acronym TE
Expansion Time to Echo
Explanation In SE imaging, time from middle of 90-degree RF pulse to middle of resulting spin-echo
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Reference SMRM 9, 125
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Acronym TI
Expansion Time following Inversion Pulse
Explanation Defined as the time from the middle of the 180-degree inversion pulse and the middle of the succceeding 90-degree RF pulse in an IR sequence.
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Related Terms IR
Reference ACR Glossary of MR Terms
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Acronym TOF
Expansion Time of Flight
Explanation MRA Technique. Refers to time necessary for blood to flow through an imaging slice. Use of SE sequences results in "black blood", while use of GRE sequences results in "bright blood" due to flow-related enhancement of unsaturated spins moving into imaging plane. Saturation pulse is used to saturate out either arterial or venous spins. 3D TOF techniques achieve higher resolution but are subject to increased scan time, motion artifact, and limited scan volume.
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Related Terms MRA, PC, MOTSA
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Acronym TR
Expansion Time to Repetition
Explanation Time between the beginning of one pulse sequence and the beginning of the next sequence.
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