NEET MDS Lessons
Radiology
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.
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.