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MRI Related Terms

- S -


  • A plane, slice or section of the body cutting from 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.


  • The conversion of analog signals to discreet digital values through a preselected measurement process. SAR - see Specific Absorption Rate.

  • SAR

  • (Specific Absorption Rate), which is defined as the RF power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg).


  • A little-used pulse sequence that generates a predominately proton density dependent signal, basically employing a 90 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.


  • 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. See also Acquisition Time, Image (Data) Acquisition Time.


  • Coils positioned near the main magnetic field that carry a relatively small current that is used to provide localized auxiliary magnetic fields in order to improve field homogeniety. See also Shimming


  • The process of improving field homogeniety by compensating for imbalances in the main magnetic field of an MRI system. This can be accomplished through a combination of passive (mechanical) shimming (e.g., adding or removing steel from the magnets poles) and active shimming (the use of shim coils) to fine-tune the magnetic field.


  • 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 the number of excitations ( ).


  • The ratio between the amplitude of the received signal and background noise, which tends to obscure that 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.


  • The term describing the planar region or the image slice selection region.


  • Relates to the addition of phase encoding steps for 3D volumetric imaging.


  • Exclusive excitation of spins in one slice performed by the coincident combination of a gradient magnetic field and a narrow bandwidth or slice selective RF pulse at a specific Larmor frequency.


  • 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.


  • The acronym for Slice-specific, Multi-Angle, multi-Resolution, multi-Thickness scanning. This function allows the operator to individually customize the thickness, field-of-view and position of each slice in a multi-angle study. SNR - see Signal-To-Noise Ratio


  • The ability to define minute adjacent objects/points in an image, generally measured in line pairs per mm (lp/mm).


  • An RF exposure concern that describes the potential for heating of the patient's tissue due to the application of the RF energy necessary to produce the NMR signal. The RF induced heat load can be directly related to the

  • SPIN

  • The property exhibited by atomic nuclei that contain either an odd number of protons or neutrons, or both.


  • 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 time 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 image. Also a pulse sequence type that usually employs a 90 pulse, followed by one or more 180 pulses.


  • See T1 and Longitudinal Relaxation Time.


  • See T2 and Transverse Relaxation Time.


  • 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 enough 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 TI Inversion Recovery. A specialized application of the 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. See also Fat Suppression.


  • A magnet whose field is generated by current in wires made of a superconducting material such as niobium-titanium, that has no resistance when operated at temperatures near absolute zero(-273C, -459F). Such magnets must be cooled by, for example, liquid helium. Superconducting magnets typically exhibit field strengths of >0.5T and have a horizontal field orientation, which makes them prone to missile effects without significant magnetic shielding. See also Quenching.


  • A type of receiver coil which is placed directly on or over the region of interest for increased magnetic sensitivity. These coils are specifically designed for localized body regions, and provide improved signal-to-noise ratios by limiting the spatial extent of the excitation or reception. T - Tesla T1 - spin-lattice longitudinal relaxation time. The characteristic time constant for spins to realign themselves with the external magnetic field after excitation.

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MRI Terms

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