Slip ring :

A component of the gantry transferring data or electrical energy to or from the stationary (fixed) part of the scanner to the rotating part of the gantry, allowing continuous rotation of the gantry. Low-voltage slip rings can supply current to an onboard generator, high-voltage slip rings allow the generator to be separate from the gantry and supply high kV to the tube.

   Spatial resolution :

Ability to resolve structures according to size. Formally assessed by calculation of the modulation transfer function. Influenced on CT by choice of field of view (targeting), mAs (radiation dose) and reconstruction algorithm (bone or high spatial frequency algorithm).

   Spin echo (SE) :

The reappearance of an MR signal, after the initial signal (FID) has disappeared following a 90° RF pulse, by the application of a refocusing 180° RF pulse. This refocusing pulse results in an effective reversal of the dephasing of the spins by eliminating the effects of inhomogeneity in the main magnetic field.

   Stochastic effects :

Stochastic effects are effects that occur on a random basis with its effect being independent of the size of dose. The effect typically has no threshold and is based on probabilities, with the chances of seeing the effect increasing with dose. Cancer is a stochastic effect.

   Superparamagnetic Substances :

This describes substances (e.g. haemosiderin, superparamagnetic iron oxide liver, lymph node and oral bowel contrast agents) with magnetic susceptibilities 100-1000 times stronger than paramagnetic substances. These substances produce marked shortening of the T2 relaxation time, producing areas of signal void.

   T1 relaxation time :

Also called the longitudinal or spin-lattice relaxation time, referring to the time taken for the spins to give the energy obtained from the initial RF pulse back to surrounding lattice (environment) and return to equilibrium.

   T2 relaxation time :

This is termed transverse or spin-spin relaxation time, and is the characteristic time constant for loss of phase coherence among spins orientated at an angle to the static main magnetic field. This time constant arises from interaction between the spins (hence the term “spin-spin”).

   TE :

This refers to the echo time, i.e. the time between the centre of the excitation pulse and the peak of the echo.

   TI :

This refers to the inversion time and represents the time after the middle of 180° RF inverting pulse in the IR sequence to the middle of the 90° read pulse. This time monitors the amount of longitudinal magnetization and can be varied in the IR sequence e.g. as in STIR and FLAIR.

   Time-of-flight (TOF) angiography :

Also referred to as in-flow angiography. It is used in 2D or 3D mode based on the principle of flow-related enhancement, i.e. blood flowing into a slice appearing brighter than surrounding tissue because it has not become saturated by previous RF pulses.

   Voxel :

An acronym for volume element, being a 3D region of an imaged object represented in 2D by a pixel.

   X-rays :

X Rays are electromagnetic waves or photons not emitted from the nucleus, but normally emitted by energy changes in electrons. These energy changes are either in electron orbital shells that surround an atom or in the process of slowing down such as in an X-ray machine.