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**f** → Frequency, usually n or (preferred) ν (ν; see Greek Symbols).
**FADE (Fast Acquisition, Double Echo:** a fast imaging technique which observes both components of the SSFP signal in separate acquisition periods during a single interpulse interval.
**Faraday shield:** electrical conductor designed so as to block out unwanted electric fields.
**Fast Fourier transform (FFT):** an algorithm which greatly speeds up the computation of Fourier transforms.
**Fast imaging** → rapid imaging; check Abbreviations and Acronyms.
**Fat suppression:** the proton signal consists of water and fat (lipid) components. The fat component can be removed using one of several techniques: → presaturation.
**Ferromagnetic:** a substance, such as iron, that has a large positive magnetic susceptibility.
**FFT** → Fast Fourier transform.
**FID** → Free induction decay.
**Field cycling relaxometry** → Relaxometry.
**Field echo** → Gradient echo.
**Field gradient** → Gradient magnetic field.
**Field lock:** a feedback control used to maintain the static magnetic field at a constant strength, usually by monitoring the resonance frequency or a line in the spectrum of a reference sample.
**Field strength** → magnetic field.
**Filling factor:** a measure of the geometrical relationship of the RF coil and the body. It affects the efficiency of irradiating the body and detecting NMR signals, thereby affecting the signal-to-noise ratio and, ultimately, image quality. Achieving a high filling factor requires fitting the coil closely to the body, thus potentially decreasing patient comfort.
**Filtered back projection:** mathematical technique used in reconstruction from projections to create images from a set of multiple projection profiles. It essentially involves 'correcting' the projection profiles by convolving them with a suitable mathematical filter and then back-projecting the filtered projections into image space. Widely used in conventional computed tomography (x-ray CT).
**FISP (Fast Imaging with Steady State Free Precession):** a fast imaging sequence which attempts to combine the signals observed separately in the FADE sequence, generally rendered impracticable by the effects of main field inhomogeneities and imperfections in the gradient waveforms. Confusingly now often used to refer to a refocused FLASH type sequence.
**FLASH (Fast Low Angle SHot imaging):** a rapid partial saturation technique using a low flip angle to improve the S/N ratio. The refocused FLASH and spoiled. FLASH sequences are modifications which incorporate or remove the effects of transverse coherences respectively.
**Flip angle:** amount of rotation of the macroscopic magnetization vector produced by an RF pulse, with respect to the direction of the static magnetic field.
**Flow:** (blood) volume per time: cm^{3}/s.
**Flow encoding:** the use of phase encoding or spin tagging techniques to obtain information on the direction and velocity of flowing material.
**Fourier transform (FT):** a mathematical procedure to separate the frequency components of a signal from its amplitudes as a function of time, or vice versa. The Fourier transform is used to generate the spectrum from the FID in pulse NMR techniques and is essential to most imaging techniques.
**Fourier transform imaging:** magnetic resonance imaging technique in which at least one dimension is phase encoded by applying variable gradient pulses along that dimension before 'reading out' the NMR signal with a gradient magnetic field perpendicular to the variable gradient. The Fourier transform is then used to reconstruct an image from the set of encoded NMR signals. The spin warp imaging version is the most practical for this technique.
**Free induction decay (FID):** if transverse magnetization of the spins is produced, e.g. by a 90° pulse, a transient NMR signal will result that will decay toward zero with a characteristic time constant T2 (or T2*); this decaying signal is the FID. In practice, the first part of the FID is not observable due to residual effects of the powerful exciting RF pulse on the electronics of the receiver.
**Free induction signal (FIS)** → Free induction decay.
**Frequency (Greek: ν, sometimes also: f):** the number of repetitions of a periodic process per unit time, measured in Hertz, abbreviated Hz. It is related to angular frequency, ω, by ν = ω /2π.
**FT** → Fourier transform.
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