Basic Principles of X-ray and Panoramic Radiography

Basic Principle of X-ray

• Ionization: when a neutral atom loses an electron, it becomes a positive ion, and the free electron becomes a negative ion.
• Ionization requires sufficient energy to overcome the electron binding energy.
• X-ray machine consists of an X-ray tube and a power supply.
• X-ray tube is composed of a cathode and an anode situated within an evacuated glass envelop or tube.

X-ray Tube

• The filament is the source of electrons within the X-ray tube.
• Filaments typically contain about 1% thorium, a weakly radioactive metal.
• The filament is mounted between two stiff support wires lead through the glass envelop and connect to both high-voltage and the low-voltage electrical sources.
• Focusing cup is a negatively charged concave reflector that electrostastically focuses the electrons emitted by the filament into a narrow beam directed at a small rectangular area on the anode called the focal spot.
• The X-ray tube is evacuated to prevent collision of the fast-moving electrons with gas molecules, which would significantly reduce their speed and also to prevent oxidation of the filament.

Anode

• Consists of a tungsten target embedded in a copper stem.
• The purpose of the target is to convert the kinetic energy of the colliding electrons into X-ray photons.
• Tungsten has several characteristics of an ideal target material, including high atomic number (74), high melting point (3422℃), high thermal conductivity, and low vapor pressure.
• The Focal spot is the area on the target to which the focusing cup directs the electrons and from which X-rays are produced.

Production of X-rays

• Most high-speed electrons traveling from the filament to the target interact with target electrons and release their energy as heat.
• Occasionally, these electrons convert their kinetic energy into photons by the formation of bremsstrahlung radiation and characteristic radiation.

Interactions of X-ray with Matter

• Attenuation results from absorption of individual photons in the beam by atoms in the absorbing tissues or photons being scattered out of the beam.
• Types of interactions include coherent scattering, photoelectric absorption, and Compton scattering.

Basic Principle of Panoramic Radiology

• Panoramic imaging (pantomography) is a technique for producing a single image of the facial structures of both maxillary and mandibular dental arches and their supporting structures.
• Panoramic images are most useful clinically for diagnostic problems requiring broad coverage of the jaws.

Principle of Panoramic Image Formation

• Panoramic radiography, an X-ray source and an image receptor rotate around the patient's head and create a curved image layer (focal trough), a zone in which the included objects are displayed clearly.
• Objects in front of or behind this image layer are unclear and largely not seen.
• The panoramic machine creates an image layer through the dentition and adjacent structures.

Focal Trough

• The focal trough is a three-dimensional curved zone, or "image layer", where the structures lying within this zone are reasonably well defined on the final panoramic image.
• Images are most clear in the middle and become less clear further from the central line.
• Objects outside the image layer are unclear, magnified, or reduced in size, and are sometimes distorted to the extent of not being recognizable.

Indications of Panoramic Radiography

• Overall evaluation of dentition
• Examine for intraosseous pathology, such as cysts, tumors, or infections
• Gross evaluation of temporomandibular joints
• Evaluation of position of impacted teeth
• Evaluation of eruption of permanent dentition
• Dentomaxillofacial trauma
• Developmental disturbances of maxillofacial skeleton

Advantages of Panoramic Imaging

• Broad coverage of facial bones and teeth
• Ease of technique
• Can be used in patients with trismus or patients who cannot tolerate intraoral radiography
• Useful visual aid in patient education and case presentation

Disadvantages of Panoramic Imaging

• Lower resolution images that do not provide the fine details
• Magnification across image is unequal (distortion)
• Image is superimposition of real, double, and ghost images
• Requires accurate patient positioning
• Difficult to image both jaws when patient has severe maxilla-mandibular discrepancy