Glossary Terms  Home

P

Parallel State

The low-energy state in which an atomic spin precesses around an axis that is parallel to that of the main magnetic field.

Paramagnetic

Having the property of being attracted to a magnetic field, though with less concentration of magnetic flux than ferromagnetic objects.

Peak

The maximal amplitude of the hemodynamic response, occuring typically bout 4 to 6 s following short-duration event.

Phantom

An object used for testing MR systems. Most phantoms are filled liquids or gels with known properties, so that problems with the scanner system can be readily identified.

Phase

Accumulated change in angle.

Phase-Encoding Gradient

A gradient turned on before the data acquisition period, so that spins can accumulate differential phase offset over space.

Phased Array

A method for arranging multiple surface detector coils to improve spatial coverage while maintaining high sensitivity.

Phrenologists

Adherents to the belief that bumps and indentations on the skull provided information about the magniturde of some trait supported by the underlying brain region.

Pixel

A two-dimensional picture element

Positron Emission Tomography (PET)

A functional neuroimaging technique that creates images based upon the movement of injected radioactive material.

Precession

The gyroscopic motion of a spinning object, in which the axis of spin itself rotates around a central axis, like a spinning top.

Preprocessing

Computational procedures that are applied to fMRI data following image reconstruction but before statistical analysis. Preprocessing steps are intended to reduce variability in the data taht is not associated wit hthe experimental task and to prepare the data for statistical testing.

Principle of Reciprocity

The rule stating that the quality of an electromagnetic coil for transmission is equivalent to its quality for reception (if it can generate a homogenous magnetic field at excitation, it can also receive signals uniformly).

Projectile Effect

The movement of an untethered ferromagnetic object through the air toward the bore of the MRI scanner.

Pulse Sequence

A series of changing magnetic field gradients and oscillating electromagnetic fields that allows the MRI scanner to create images sensitive to a particular physical property.