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• MRI Terms

- F -

• FARADAY SHIELD (Faraday Cage)

An electrically conductive screen or shield that reduces or eliminates interference between outside radio waves and those from the MRI unit.



• FAST SCANNING

A specialized technique usually associated with short TR, reduced flip angle and repeated 180� rephasing pulses.



• FAST SPIN ECHO (FSE)

A fast spin echo pulse sequence characterized by a series of rapidly applied 180� rephasing pulses and multiple echoes, changing the phase encoding gradient for each echo.



• FAT SATURATION (FAT-SAT)

A specialized technique that selectively saturates fat protons prior to acquiring data as in standard sequences, so that they produce negligible signal. The pre-saturation pulse is applied prior to each slice selection. This technique requires a very homogeneous magnetic field and very precise frequency calibration. See also Fat Suppression.



• FAT SUPPRESSION

The process of utilizing specific parameters, commonly with STIR (short TI inversion recovery) sequences, to remove the deleterious effects of fat from the resulting images. See also STIR.



• FDA

The United States Food and Drug Administration FID - see Free Induction Decay



• FFT (Fast Fourier Transform)

A particularly fast and efficient computational method of performing a Fourier Transform, which is the mathematical process by which raw data is processed into a usable image.



• FIELD ECHO (FE) (also known as GRADIENT ECHO)

Echo produced by reversing the direction of the magnetic field gradient to cancel out the position-dependent phase shifts that have accumulated due to the gradient.



• FIELD OF VIEW (FOV)

Defined as the size of the two or three dimensional spatial encoding area of the image. Usually defined in units of cm2.



• FLAIR

FLAIR FLuid Attenuated Inversion Recovery



• FLARE

Fast Low-Angle Recalled Echoes



• FLIP ANGLE (FA)

The angle to which the net magnetization is rotated or tipped relative to the main magnetic field direction via the application of an RF excitation pulse at the Larmor frequency. The Flip Angle is used to define the angle of excitation for a Field Echo pulse sequence.



• FLOW COMPENSATION

A function of specific pulse sequences, i.e., CRISP (Complex Rephasing Integrated with Surface Probes) spin echo, wherein the application of strategic gradient pulses can compensate for the objectionable spin phase effects of flow motion.



• FLUX

Invisible lines of force that extend around a magnetic material. The greatest density is at the two poles of the magnet.



• FLUX DENSITY

The number of lines of force per unit area of a magnetic material.



• FOURIER TRANSFORM (FT)

A mathematical procedure used in MRI scanners to analyze and separate amplitude and phases of the individual frequency components of the complex time varying signal. Fourier transform analysis allows spatial information to be reconstructed from the raw data.



• FOV

See Field Of View.



• FREE INDUCTION DECAY (FID)

If transverse magnetization of the spins is produced, e.g., by a 90� RF pulse, a transient MR signal at the Larmor frequency results that decays toward zero with a characteristic time constant of T2*. This decaying signal is the FID.



• FREQUENCY

The number of cycles or repetitions of any periodic wave or process per unit time. In electromagnetic radiation, it is usually expressed in units of hertz (Hz), where 1 Hz = 1 cycle per second.



• FREQUENCY ENCODING

The process of locating an MR signal in one dimension by applying a magnetic field gradient along that dimension during the period when the signal is being received.



• FRINGE FIELD

A term usually relating to the extents of the magnetic field surrounding the magnet. Safety requirements dictate that the distances of particular field strengths from the magnet must be known, and that potentially unsafe areas must be indicated with appropriate warning signs. Access to areas with field strengths of 5 gauss and higher must be strictly controlled. FSE - See Fast Spin Echo. Gx, Gy, Gz - the conventional symbols for the three orthogonal magnetic gradients. The subscripts designate the conventional spatial direction of the gradient.