100

 

Terminology

TERMINOLOGY

Echo Time (TE): Time for absorbed radiofrequency energy to be released from the protons. This parameter is
a property of the MRI machine and generally ranges between 80ms and 20ms.

Hyperintense: bright appearance relative to surroundings.

Hypointense: dark appearance relative to surroundings.

Pulse Sequence: A series of radiofrequency pulses used in conjunction with gradient magnetic fields to produce
magnetic resonance images. Different pulse sequences are sued to emphasize difference in either T1 relaxation
times (producing a T1 weighted image) or T2 relaxation times (producing a T2 weighted image).

Relaxation- A characteristic time that it takes the spin system (protons) to return to equilibrium after being
perturbed by a radiofrequency pulse. Relaxation rates are the measured relaxation values of specific tissues
which play an important role in providing image contrast.

Repetition Time (TR): Time allowed for protons to align with the direction of the main magnetic field (time
between radiofrequency pulses). This parameter is a property of the MRI machine and generally ranges between
2000ms and 500ms.

Spin (Proton) Density Images:These images are created with a long TR to eliminate the effect of T1 and a short
TE to eliminate the effect of T2. The result is that the image contrast is based only on the nmber of hydrogen nuclei
per mm3 in the different tissues being examined.

STIR Sequence: STIR is an acronym for "Short T1 Inversion Recovery." The short T1 inversion time highlights
differences between fat and water due to fat's short T1 relaxation time.

T1 (Time One) Relaxation: A time constant expressing the effect of the environment on the protons. It is the rate
at which the protons realign along the main magnetic field. Fluid, CSF, and ventricles are hypointense, white matter is
hyperintense, fat is hyperintense, and air and cortical bone are hypointense. This time constant is a property of the tissue
being examined as opposed to TE and TR set by the operator.

T2 (Time Two) Relaxation: A time constant expressing the effects of protons upon each other. It is the rate at which protons
lose synchronization with each others spins. Fluid is hyperintense, white matter is less intense than gray matter, fat is
hyperintense, and air and cortical bone are hypointense. This time constant is a property of the tissue being examined as
opposed to TE and TR set by the operator.

T1-Weighted Images:Images produced by using a short TE to eliminate the effect of T2 Relaxion and a short TR in order not
to eliminate the effect of T1 Relaxation. Practically speaking neither T1 nor T2 components can be solely depicted, hence the
term weighted is used as T1 properties are emphasized.

T2 -Weighted Images: Images produced by using a long TR to eliminate the effect of T1 Relaxion and a long TE in order not
to eliminate the effect of T1 Relaxation. Practically speaking neither T1 nor T2 components can be solely depicted, hence the
term weighted is used as T2 properties are emphasized.

The above properties are on a continuum and depending on the TR and TE used by the operator,
and the section being studied, the resultant image may be placed anywhere along this spectrum.

 

 
Copyright © 2006 - University of Pennsylvania School of Veterinary Medicine, All rights reserved.
Faculty: Dr. Alexia McKnight
Student:Charles Bradley 2009