Operation; General Principles Of Operation - GENDEX Oralix AC Manuel De L'utilisateur

2 Operation

2.1

General principles of operation

The tubehead-based dental X-ray system is a simple type of x-ray generator.
Essentially, it consists of a high-voltage transformer that elevates the voltage taken from the mains supply up to an
extremely high value (e.g. 65 kVp = 65000 V peak), and applies it across a x-ray tube. The x-ray tube acts as a
voltage rectifier and lets current pulses (the mA) flow through in correspondence with every half mains phase.
Inside the x-ray tube, the highly energetic electrons during the current pulses hit the tungsten surface of the anode
and cause it to emit x-rays photons of various energies, the highest ones corresponding (in the keV unit of
measurement) to the peak electron-accelerating voltage across the x-ray tube. Note that the higher the average
energy of the x-ray photons, the more penetrating the x-ray beam is.
X-rays are only present during the very moment of the irradiation. They travel along a straight direction and cannot
be deflected or reflected. When they pass through solid matter, or strike against a surface, they momentarily bring
about a certain amount of secondary and less energetic x-rays, respectively called scattered and back scattered
radiation.
The high-voltage transformer and the x-ray tube are soaked in dielectric oil both for electrical insulation and for
convection of the intense heat generated in the x-ray tube. All around the x-ray tube there is lead shielding that
effectively blocks x-rays in all directions, except through a small output window. This window also incorporates an
Aluminum filter to remove the useless, less penetrating photons from the x-ray beam.
In front of this window and rigidly attached to the tubehead, there is an open-end Beam Limiting Device and
Position Indicating Device (BLD/PID), that is a device including a lead diaphragm that collimates the x-ray beam
field to the required shape and size, and means to apply such field to the required position. This device is also
known as a "cone" (short for "coning device").
There are two types of cone that can equip your Oralix tubehead, depending on the option chosen, both providing
a Source Skin Distance (SSD) of 20 cm:
A round BLD, consisting of a near-focus collimator and an open-end cylindrical spacer, with a circular
irradiated field of 6 cm diameter. The tip of the cone includes a lead shielding for protection against back-scattered
radiation.
A rectangular BLD, made out of metal, providing an irradiated field of 3.5 x 4.5 cm. It can be rotated so to align the
x-ray field with the film.
The cone can be removed and interchanged by turning it anti clockwise.
The rectangular BLD requires a more accurate operation than the round one (and re-takes may occasionally be
required). However, the integral imparted dose using rectangular BLD is less than half than with round BLD (due to
the smaller irradiated area). Also, the image quality thus obtained benefits from the less scattered radiation.
Radiation Safety
X-ray equipment may cause injury if used improperly. The instructions contained in this manual must be read and
followed when installing, servicing, or operating the Gendex Oralix AC. The Oralix AC provides a high degree of
protection from unnecessary X-radiation. However, no practical design can provide complete protection, nor
prevent operators from exposing themselves or others to unnecessary radiation. It is important that you become
fully acquainted with applicable government radiation protection regulations.
Printed on: 22 Mär 2012, 09:22:12 am; Printed by: TOBIAS BAUER
GENDEX
6 Oralix AC