NEET MDS Lessons
Radiology
DENTAL X-RAY TUBE
The dental X-ray tube is surrounded by a glass envelope that houses a vacuum.
The glass prevents low-grade radiation from escaping. The vacuum insures the protection of the equipment from catastrophic failure. Production of X-rays generates enormous amounts of heat; the vacuum prevents the risk of combustion and ensures the proper environment for conduction of electrons.
There are two separate energy sources, one that powers the energy potential between the cathode ?lament and the anode, and the other being
the controls for the cathode ?lament. The latter essentially is the on and off switch of the X-ray unit.
The cathode ?lament is heated which causes electrons to be emitted.
These electrons are then accelerated by the electrical potential of the circuit.
Between the two points is a tungsten target.
When electrons strike the target, X-rays are produced.
HALF-VALUE LAYER
- Property of a material whereas the thickness (mm) reduces 50% of a monochromatic X-ray beam.
- Half-value layer of a beam of radiation from an X-ray unit is about 2 mm of aluminum (Al).
PRIMARY RADIATION
- Is the main beam produced from the X-ray tube.
SECONDARY RADIATION
- Produced by the collision of the main beam with matter which causes scatter.
RELATIVE RADIO SENSITIVITY OF THE TISSUES
Radiosensitive (2500 r or less kills or seriously injures many cells)
Lymphocytes and lymphoblasts
Bone marrow (myeloblastic and erythroblastic cells)
Epithelium
Germ cells (testes and ovary)
Radioresponsive (2500-5000 r kills or seriously injures many cells)
Epithelium of skin and many appendages.
Endothelium of blood vessels
Salivary glands
Growing bone and cartilage.
Conjunctiva, cornea and lens of eye
Collagen and elastic tissue(fibroblasts themselves are resistant)
Radioresistant (over 5000 r are required to kill or injure many cells)
Kidney
Liver
Thyroid
Pancreas
Pituitary
Adrenal and parathyroids
Mature bone and cartilage
Muscle
Brain and other nervous tissue.
The numbers represent the minimum damaging doses; a gray and a sievert represent roughly the same amount of radiation:
• Fetus--2 grays (Gy).
• Bone marrow--2 Gy.
• Ovary--2-3 Gy.
• Testes--5-15 Gy.
• Lens of the eye--5 Gy.
• Child cartilage--10 Gy.
• Adult cartilage--60 Gy.
• Child bone--20 Gy.
• Adult bone--60 Gy.
• Kidney--23 Gy.
• Child muscle--20-30 Gy.
• Adult muscle--100+ Gy.
• Intestines--45-55 Gy.
• Brain--50 Gy.
Radiographic films used in Dentistry
1. Intraoral Periapical (IOPA) Film
- Size 0:
- Dimensions: 22 x 35 mm
- For: Small children
- MPD (Maximum Permissible Dose) for whole body: 0.1 Rem in 1 year
- Size 1:
- MPD for gonads/bone marrow: 0.5 Rem in 1 year
- Size 2:
- Dimensions: 24 x 40 mm or 32 x 41 mm
- For: Anterior projections and adults
- MPD for gestation period in relation to the fetus: 5 Rem
- MPD for skin: 0.5 Rem in 1 year
- Radiation Exposure:
- Mean exposure from one IOPA: 300 mR
- Mean exposure from improved dental X-ray techniques: as low as 110 mR
2. Bitewing Film
- Size 0:
- For: Very small children
- Size 1:
- For: Children
- Size 2:
- For: Adults
3. Occlusal Film
- Size:
- 3 times larger than size 2 film (57 x 76 mm)
- Used for capturing larger areas of the dental arch.
4. Screen Film
- Size:
- 8 x 10 inches
- Used for extraoral projections in conjunction with an intensifying screen.
Additional Information
- Erythema Dose: The amount of radiation necessary to produce a noticeable skin reaction, typically 300-400 R.
- ALARA Principle: Stands for "As Low As Reasonably Achievable," emphasizing the importance of minimizing radiation exposure.
Radiation Biology
-X- and g -rays are called sparsely ionizing because along the tracks of the electrons set in motion, primary ionizing events are well separated in space.
Alpha-particles and neutrons are densely ionizing because the tracks consist of dense columns of ionization.
X-rays, gamma rays, electrons, and protons are all low LET forms of radiation in that their density of ionization is sparse. In general, they penetrate tissues deeply and result in less intracellular radiation injury.
High LET forms of radiation, such as heavy nuclear particles (e.g. fast neutrons), penetrate tissues less deeply and cause more radiation injury to biologic material.
Cells are most sensitive to Radiation when:
- they are undifferentiated.
Exceptions to this Law:
- lymphocyte
- Oocyte
X-rays and gamma rays show latent injury that is residual tissue damage even after the initial radiation reaction is subsided.
Proteins tend to be more radiosensitive than carbohydrates and lipids.
Most radiosensitive tissue-small lymphocyte
Most radioresistant tissue- brain
Embryonic, immature or poorly differentiated tissues are more easily injured by radiation, but they also show greater recovery properties.
All cells show increased susceptibility to radiation at the time of mitotic division and if the cells are irradiated during the resting phase, mitosis is delayed or inhibited.
- In general, cells are most radiosensitive in late M and G2 phases and most resistant in late S.
- for cells with a longer cell cycle time and a significantly long G1 phase, there is a second peak of resistance late in G1
- the pattern of resistance and sensitivity correlates with the level of sulfhydryl compounds in the cell. Sulfhydryls are natural radioprotectors and tend to be at their highest levels in S and at their lowest near mitosis.
- To produce its effect. Oxygen must be present during the radiation exposure or at least during the lifetime of the free radicals (10-5 sec).
- Mandible is more ssceptible to radiation injury than maxilla due to the denser structure and poorer blood supply.
- Salivary glands though an organ with a low turnover rate, was unusually sensitive to radiation
- Liposarcoma tumors are the most radiosensitive soft tissue tumors
- Exophytic tumors are usually more easily controlled with radiation while infiltrative and ulcerative lesions are more radioresistant.
The infiltrative and ulcerative lesions are more likely to be larger than clinically apparent and contain a larger proportion of hypoxic cells.
Bisecting angle technique
Bisecting angle technique is a method used in dental radiography to obtain radiographs of teeth and surrounding structures. This technique involves positioning the X-ray beam perpendicular to an imaginary line that bisects the angle formed by the long axis of the tooth and the film or sensor. Here are the general guidelines for angulations when using the bisecting angle technique:
Anterior Teeth
- Maxillary Central Incisors:
- Vertical Angulation: +40 to +50 degrees
- Maxillary Lateral Incisors:
- Vertical Angulation: +40 to +50 degrees
- Maxillary Canines:
- Vertical Angulation: +45 to +55 degrees
- Mandibular Central Incisors:
- Vertical Angulation: -15 to -25 degrees
- Mandibular Lateral Incisors:
- Vertical Angulation: -15 to -25 degrees
- Mandibular Canines:
- Vertical Angulation: -20 to -30 degrees
Posterior Teeth
- Maxillary Premolars:
- Vertical Angulation: +30 to +40 degrees
- Maxillary Molars:
- Vertical Angulation: +20 to +30 degrees
- Mandibular Premolars:
- Vertical Angulation: -10 to -15 degrees
- Mandibular Molars:
- Vertical Angulation: -5 to -10 degrees
Key Points
- Positioning: The film or sensor should be placed as close to the tooth as possible, and the X-ray beam should be directed perpendicular to the bisecting line.
- Patient Comfort: Ensure that the patient is comfortable and that the film or sensor is properly stabilized to avoid movement during exposure.
- Technique Variability: The exact angulation may vary based on the individual patient's anatomy, so adjustments may be necessary.
Age Groups and Radiographs
-
Age 2:
- Anterior IOPA's: 2
- Posterior IOPA's: 4
- Bitewings: 2
- Total Films: 12
-
Age 8:
- Anterior IOPA's: 8
- Posterior IOPA's: 4
- Bitewings: 2
- Total Films: 14
-
Age 8 (another entry):
- Anterior IOPA's: 8
- Posterior IOPA's: 8
- Bitewings: 2
- Total Films: 20
Summary of Total Films by Type
-
Anterior IOPA's:
- Age 2: 2
- Age 8: 8
- Age 8 (another entry): 8
- Total Anterior IOPA's: 18
-
Posterior IOPA's:
- Age 2: 4
- Age 8: 4
- Age 8 (another entry): 8
- Total Posterior IOPA's: 16
-
Bitewings:
- Age 2: 2
- Age 8: 2
- Age 8 (another entry): 2
- Total Bitewings: 6
Overall Total Films
- Total Films for Age 2: 12
- Total Films for Age 8 (first entry): 14
- Total Films for Age 8 (second entry): 20
- Grand Total Films: 12 + 14 + 20 = 46
Fractures of the Zygomatic Arch
- Structures: Zygomatic arch, zygomatic bone.
- Best Views:
- Submento-Vertex View: Provides a clear view of the zygomatic arch and helps assess fractures.
- Waters View: Useful for visualizing the zygomatic bone and maxillary sinus.
- Reverse Townes View: Can also be used to visualize the zygomatic arch.
Base of Skull
- Structures: Base of the skull, cranial fossae.
- Best Views:
- Submento-Vertex View: Effective for assessing the base of the skull and related fractures.
Maxillary Sinus
- Structures: Maxillary sinus, zygomatic bone.
- Best Views:
- Waters View: Excellent for visualizing the maxillary sinus and any associated fractures.
Fractures of Zygoma
- Structures: Zygomatic bone, zygomatic arch.
- Best Views:
- Waters View: Good for assessing zygomatic fractures.
- PA View: Provides a frontal view of the zygomatic bone.
- Reverse Townes View: Useful for visualizing the zygomatic arch.
Nasal Septum
- Structures: Nasal septum, nasal cavity.
- Best Views:
- PA View: Useful for assessing the nasal septum and any associated fractures.
Condylar Neck Fractures
- Structures: Mandibular condyle, neck of the condyle.
- Best Views:
- Lateral Oblique View (15°): Good for visualizing condylar neck fractures.
- Transpharyngeal View: Useful for assessing the condylar region.
Medially Displaced Condylar Fractures
- Structures: Mandibular condyle.
- Best Views:
- Lateral Oblique View (30°): Effective for visualizing medially displaced condylar fractures.
Coronoid Process of Mandible
- Structures: Coronoid process.
- Best Views:
- PA View of Skull: Can help visualize the coronoid process.
Fractures of Ramus and Body of Mandible
- Structures: Mandibular ramus, body of the mandible.
- Best Views:
- Lateral Oblique View (15°): Useful for assessing fractures of the ramus and body of the mandible.
Horizontal Favorable and Unfavorable Fractures of Mandible
- Structures: Mandible.
- Best Views:
- Lateral Oblique View (30°): Effective for evaluating horizontal fractures.
Bony Ankylosis of TMJ
- Structures: Temporomandibular joint.
- Best Views:
- CT Scan: Provides detailed imaging of bony structures and ankylosis.
Fibrous Ankylosis of TMJ
- Structures: Temporomandibular joint.
- Best Views:
- CT Scan: Useful for assessing fibrous ankylosis.
Internal Derangement of the Disk
- Structures: TMJ disk.
- Best Views:
- MRI: The best modality for evaluating soft tissue structures, including the TMJ disk.
Disk Perforation
- Structures: TMJ disk.
- Best Views:
- MRI: Effective for diagnosing disk perforation.
Arthrography
- Structures: TMJ.
- Best Views:
- Arthrography: Can be used to assess the TMJ and visualize the disk and joint space.