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• MRI Terms

- P -

• PARAMAGNETIC SUBSTANCE

A substance with weak magnetic properties due to its unpaired electrons. Researchers are developing certain paramagnetic materials, such as gadolinium, as MRI invasive contrast media.



• PARTIAL VOLUMING

A loss of resolution due to excessively large voxels, typically caused by slices that are too thick.



• PERMANENT MAGNET

A magnet design that utilizes blocks of ferromagnetic materials (permanent magnets) to generate a magnetic field between the two poles of the magnet. There is no requirement for additional electrical power or cooling, and the iron-core structure of the magnet leads to a limited fringe field and no missile effect. Due to weight considerations, permanent magnets are usually limited to maximum field strengths of 0.3T.



• PHANTOM

An artificial object of known dimensions and properties that is used to test or monitor an MRI systems homogeneity, imaging performance and orientation aspects.



• PHASE

An angular relationship describing the degree of synchronism between two sinusoidal waveforms of the same frequency.



• PHASE COHERENCE

A term describing the degree to which precessing nuclear spins are synchronous.



• PHASE CONTRAST

An MRA technique utilizing the change in the phase shifts of the flowing protons in the region of interest to create an image.



• PHASE ENCODING

The process of locating an MR signal by altering the phase of spins in one dimension with a pulsed magnetic field gradient along that dimension prior to the acquisition of the signal. As each signal component has experienced a different phase encoding gradient pulse, its exact spatial reconstruction can be specifically and precisely located by the Fourier transformation analysis. Spatial resolution is directly related to the number of phase encoding levels (gradients) used.



• PIXEL

Acronym for a picture element, the smallest discrete two-dimensional part of a digital image display.



• PLANAR IMAGING

A method of scanning in which the data is collected simultaneously from an entire layer.



• PRECESSION

Comparatively slow gyration of the axis of a spinning body so as to trace out a cone. Caused by the application of a torque tending to change the direction of the rotation axis and continuously directed at right angles to the plane of the torque. The magnetic moment of a nucleus with spin will experience such a torque when inclined at an angle to the magnetic field, resulting in precession at the Larmor frequency.



• PRESATURATION (PRE-SAT)

A specialized technique employing repeated RF excitation of structures adjacent to the ROI for the purpose of reducing or eliminating their phase effect artifacts.



• PROTON

A positively charged particle located in the nucleus of an atom. The number of protons in the nucleus governs the chemical properties of that element.



• PROTON DENSITY

The concentration of mobile Hydrogen atoms within a sample of tissue. See also Hydrogen Density.



• PROTON DENSITY WEIGHTED IMAGE

An image produced by controlling the selection of scan parameters to minimize the effects of T1 and T2, resulting in an image dependent primarily on the density of protons in the imaging volume.



• PULSE PROGRAMMER

The computer-controlled component of the MRI scanner that determines the timing of the pulse sequence parameters of the scan, such as echo time, pulse amplitude, phase and frequency.



• PULSE SEQUENCE

A preselected set of defined RF and gradient pulses, usually repeated many times during a scan, wherein the time interval between pulses and the amplitude and shape of the gradient waveforms will control NMR signal reception and affect the characteristics of the MR images.