In continuing the terms you need to know in CBCT and digital imaging in general. All of these terms will be important to you to familiarize yourself, so, when you attend training and seminars about digital imaging, these terms will be familiar to you and what it means when you are asking your patient to have digital imaging taken and when you are reading your results.

F
 
Focus-Skin Distance Or FSD

The distance from the focal spot to the x-ray beam entrance side of the patient.

In better understanding, we assume a body consisting of homogeneous muscle tissue. In the following figure, lines are drawn to divide the body into HVLs (Half-Value Layer). The exposure is reduced by a factor of one half each time it passes through 1 HVL. The thickness of 1 HVL depends on the photon-energy spectrum. However, for the immediate discussion, we assume that 1 HVL is equivalent to 4 cm of tissue. A 20-cm thick body section consists of 5 HVLs. Therefore, the exposure decreases by one half as it passes through each 4 cm of tissue. At the exit surface, the exposure is a small fraction of the entrance surface exposure.

H

Half-Value Layer or HVL
It is the thickness of the material at which the intensity of radiation entering it is reduced by one half. HVL can also be expressed in terms of air kerma rate (AKR), rather than intensity meaning the half-value layer is the thickness of specified material that, "attenuates the beam of radiation to an extent such that the AKR is reduced to one-half of its original value.

Hounsfield Scale
A quantitative scale for describing radiodensity.

Hounsfield Unit Or HU
A unit used in medical imaging (CT or MRI scanning) to describe the amount of x-ray attenuation of each voxel (volume element) in the three-dimensional image. The voxels are normally represented as 12-bit binary numbers and therefore have 212 = 4096 possible values. These values are arranged on a scale from -1024 HU to +3071 HU, calibrated so that -1024 HU is the attenuation produced by air and 0 HU is the attenuation produced by water. Tissue and bone then produce attenuations in the positive range. The HU of common substances:

The reading in Hounsfield Units is also called the CT number.
The unit is named for the British engineer Godfrey Hounsfield, who demonstrated the first CT scanner in 1972. For this invention, he received the Nobel Prize in medicine in 1979.

I

IAN
Also Related to Mandibular Nerve. Inferior Alveolar Nerve Identification. Also called the inferior dental nerve. The IAN is the part of the mandibular nerve branch that enters the mandible through the mandibular foramen. Disturbances of the IAN will predominantly give sensitivity symptoms in the soft tissue of the lower lip and chin.

Image Box
Used to view axial slides continuously.

Impaction
Impaction occurs when there is prevention of complete eruption into a normal functional position of one tooth by another, due to lack of space (in the dental arch), obstruction by another tooth, or development in an abnormal position.

Isotropic
Identical in all directions; invariant with respect to direction. This means your measurement are the same in any direction.

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