NEET MDS Lessons
Radiology
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.
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.
General guidelines for vertical angulations for common dental radiographs in children:
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: -10 to -20 degrees
- Mandibular Lateral Incisors:
- Vertical Angulation: -10 to -20 degrees
- Mandibular Canines:
- Vertical Angulation: -15 to -25 degrees
Posterior Teeth
- Maxillary Premolars:
- Vertical Angulation: +30 to +40 degrees
- Maxillary Molars:
- Vertical Angulation: +20 to +30 degrees
- Mandibular Premolars:
- Vertical Angulation: -5 to -10 degrees
- Mandibular Molars:
- Vertical Angulation: -5 to -10 degrees
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.
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
Digital Radiology
Advances in computer and X-ray technology now permit the use of systems that employ sensors in place of X-ray ?lms (with emulsion). The image is either directly or indirectly converted into a digital representation that is displayed on a computer screen.
DIGITAL IMAGE RECEPTORS
- charged coupled device (CCD) used
- Pure silicon divided into pixels.
- Electromagnetic energy from visible light or X-rays interacts with pixels to create an electric charge that can be stored.
- Stored charges are transmitted electronically and create an analog output signal and displayed via digital converter (analog to digital converter).
ADVANTAGES OF DIGITAL TECHNIQUE
Immediate display of images.
Enhancement of image (e.g., contrast, gray scale, brightness).
Radiation dose reduction up to 60%.
Major disadvantage: High initial cost of sensors. Decreased image resolution and contrast as compared to D speed ?lms.
DIRECT IMAGING
- CCD or complementary metal oxide semiconductor (CMOS) detector used that is sensitive to electromagnetic radiation.
- Performance is comparable to ?lm radiography for detection of periodontal lesions and proximal caries in noncavitated teeth.
INDIRECT IMAGING
- Radiographic ?lm is used as the image receiver (detector).
- Image is digitized from signals created by a video device or scanner that views the radiograph.
Sensors
STORAGE PHOSPHOR IMAGING SYSTEMS
Phosphor screens are exposed to ionizing radiation which excites BaFBR:EU+2 crystals in the screen storing the image.
A computer-assisted laser then promotes the release of energy from the crystals in the form of blue light.
The blue light is scanned and the image is reconstructed digitally.
ELECTRONIC SENSOR SYSTEMS
X-rays are converted into light which is then read by an electronic sensor such as a CCD or CMOS.
Other systems convert the electromagnetic radiation directly into electrical impulses.
Digital image is created out of the electrical impulses.
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.