(F) The number of cycles of a periodic process per unit time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength. The frequency of for example ultrasound is expressed in units of hertz (Hz), where 1 Hz = 1 cycle per second.
Usual frequencies in medical imaging are e.g., for ultrasound in the range of 1-20 MHz, for x-rays above 3x10¹⁶ Hz and for gamma rays less than 10²⁰ Hz.

The gantry aperture is the opening through which the table with the patient moves for the examination. Typical diameters of the gantry range from 50 to 85 cm. The gantry aperture diameters between 70 and 85 cm are useful for biopsy procedures and obese patients. Larger gantry apertures allow for easier handling of the biopsy equipment.
The scanning field of view in which the CT data are acquired is smaller than the gantry aperture diameter.

Nuclear cardiology has a wide range of techniques that permit the accurate assessment of perfusion, metabolism, sympathetic innervation, and mechanical function of the heart.

Scintigraphic techniques of the heart include:
See also Echocardiography and Cardiovascular Imaging.

In nuclear medicine a hot spot describes a strong activity enrichment in one or more ranges of the organ or body part which is examined.
A pinhole collimator may be used if images of a hot spot and the surrounding with very high resolution are necessary. Zoom magnification or a converging collimator also may be used to improve resolution and assign the hot spot.

(H) The Hounsfield scale displays radiodensity in a linear scale of gray shades expressed in Hounsfield units (HU). The Hounsfield scale is a quantitative transformation of the attenuation coefficient.
The Hounsfield value -1000 is defined as the radiodensity of air, 0 H that of distilled water at standard pressure and temperature, and denser tissues like for example cranial bone can reach 2000 H. The radiation attenuation of dental fillings or artificial implants depends on atomic number of the elements used. Titanium usually has an amount of +1000 HU. Iron steel can have a density greater than the highest range (traditional 3095 H) covered by the standard Hounsfield scale of a CT scanner. Areas with attenuation coefficients that exceed the scale's maximum are white areas in which no detail is visible.
Some CT machines are relatively tolerant, precise representing regions with very high densities. Sometimes, an option is available to select an extended CT number scale.