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.
Synonyms
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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