IMAGE CHARACTERISTICS
🌟IMPORTANT POINTS TO REMEMBER
🔹Density: It is also called photographic density. It is the term used for describing the degree of film blackening and is measured by densitometer. In diagnostic radiology, the range of optical density is 0.3–2.
🔹Radiolucent areas: The white areas on the radiographic film
🔹Radiopaque areas: The black areas on the radiographic film
🔹Factors affecting the density of film: They consist of factors in relation with X-ray machine (kVp, milliamperage, intensity of X-ray beam, exposure time, source to film distance, filtration, grid), factors in relation with image receptor (X-ray film type, intensifying screen), factors in relation with object (type of material, thickness of material or subject thickness, obese patient, edentulous patient, small patient, internal structure, shape of object) and factors in relation with processing of the film (developing of the film, fixing of film).
🔹Characteristic curve: It is also called Hurter–Driffield curve or H–D curve and is named after two early investigators of film sensitometry. It is a graph showing the variation in optical density (degree of blackening) with different exposures.
🔹Film gamma: Film gamma is maximum gradient or slope of the linear portion of the characteristic curve.
🔹Average gradient: Average gradient is a more useful measurement and is usually calculated between density 0.2 and 2 above background fog density.
🔹Contrast: The difference in optical density between two points on a film that have received different exposures.
🔹Step wedge: It is a device to demonstrate short- and long-scale contrast.
🔹Factors affecting the contrast of the film: They consist of factors in relation to the patient (tissue thickness, tissue density, atomic number), factors in relation to the X-ray machine (kVp, penetration power, exposure rate), factors in relation with film (characteristic curve, optical density, speed of the film, type of film, emulsion) and factors in relation with processing (processing, scatter radiation, fog).
🔹Film speed and film sensitivity: It is the exposure required to produce an optical density of 1.0 above background fog.
🔹Film latitude: It is the range of exposure to which X-ray film responds with densities within the acceptable range.
🔹A film with characteristic curve having long straight-line portion and a shallow slope will have wide latitude.
🔹Exposure latitude: It is the range of exposure of an X-ray film permissible for good diagnostic results.
🔹Resolution: It is the measure of a radiograph to differentiate between different structures that are close together.
🔹Van Allen apparatus: This consists of a slit system formed by two lead alloy blocks mounted on a carriage which is moved laterally by a differential screw.
🔹Sharpness: It is defined as the ability of the X-ray film to define outline of an object.
🔹Geometric unsharpness: It includes the penumbra effect (fuzzy, unclear area that surrounds a radiographic image).
🔹Motion unsharpness: It is caused by the patient’s movement during the exposure.
🔹Absorption unsharpness: It is caused by variation in shape of the object. Absorption might differ between centre and periphery leading to unsharp edges.
🔹Film unsharpness: It is necessary to make the radiograph of a lead plate whose edge is carefully machined. The film is then placed in the microdensitometer, an instrument that records the light transmission of a very restricted area of film.
🔹Parallax: It is seen only in double emulsion film. When viewed from an angle, there may be an appearance of two images.
🔹Screen unsharpness: It is caused by diffusion and spread of the light emitted from the intensifying screen. It is caused by fine size of photosensitive crystals. Film with high intensification factors leads to deterioration.
🔹Reciprocity law failure: There is the failure of reciprocal law in the case of radiation exposure rate with exposure time. It is seen in the case of indirect-action films. At a lower exposure rate, a particular quantity of radiant energy produces less photographic effect than at a higher exposure rate. If the exposure rate of the incident radiation is increased further, then after a particular time the photographic effect diminishes.
🔹Film fog: It is unwanted density present in the unexposed film on development. If the fog level is very high, the film will lack contrast and low-density areas will merge with the fog, thereby losing details.
📌 MULTIPLE CHOICE QUESTIONS (MCQs)
1. The film which transmits 100% of light through it has a density of
a. D = 0
b. D = 1
c. D = 2
d. D = 3
Answer: a
2. The range of optical density used in diagnostic radiology is
a. 0.5–1.0
b. 0.3–2.0
c. 1.5
d. 1.5–2.0
Answer : b
3. Factor which affects the optical density of films includes
a. kVp
b. Filtration
c. Source to film distance
d. All of the above
Answer: d
4. The type of contrast film used in the detection of periodontal and periapical diseases is
a. High-contrast film
b. Low-contrast film
c. Step wedge
d. None of the above
Answer: b
5. Minimum contrast which can be detected visually under the best condition is about
a. 0.02
b. 0.5
c. 1.0
d. 0.6
Answer: a
6. Which of the following statements is true about contrast of film?
a. Contrast of the film is decreased if the thickness of the material is less.
b. Contrast will be more if an atomic number is less.
c. Film consisting of thicker emulsion produces less
contrast.
d. All of the above.
Answer : a
7. Penumbra effect is a type of
a. Motion unsharpness
b. Geometric unsharpness
c. Absorption unsharpness
d. Film unsharpness
Answer: b
8. The unwanted density present in the unexposed filmon development is called
a. Radiographic mottle
b. Radiographic noise
c. Film fog
d. None of the above
Answer: c
9. The type of film fog produced due to unsuitable safelight in a darkroom is
a. Photographic fog
b. Light fog
c. Colour fog
d. Oxidation fog
Answer: b
10. Van Allen apparatus is used to measure
a. Resolution
b. Speed
c. Contrast
d. Density
Answer: a