Image characteristics

Image Characteristics - Image formation and the characteristics of the image formed.

1. IMAGE CHARACTERISTICS PRESENTED BY: BHARGAVI SOOD MODERATOR : DR. ARCHNA NAGPAL

2. INTRODUCTION A diagnostic radiograph, should provide a great deal of accurate anatomical information. T he images exhibit proper density and contrast, are of the same shape and size as the object exposed, and have sharp outlines. It is important to understand the image characteristics, to understand the quality of the radiograph. .

3. IMAGE FORMATION

4. RADIOGRAPHIC DENSITY When a film is exposed by an x-ray beam (or by light, in the case of screen-film combinations) and then processed, the silver halide crystals in the emulsion that were struck by the photons are converted to grains of metallic silver. These silver grains block the transmission of light from a viewbox and give the film its dark appearance The degree of darkening or opacity of an exposed film is referred to as optical density , defined as follows: The overall blackness or darkness of a dental image is termed density. Io- Incident light density It- transmitted light density If, D = 0 {100% light transmitted} D = 1 {10% light transmitted} D = 2 {1% light transmitted}

5. Unexposed film, when processed – shows density. This minimal density is called base plus fog and typically is 0.2 to 0.3. This is due to inherent density of base and added tint. So, basically, Density increases – film is darker; Density decreases – film is lighter It depends upon various factors: Relation to Xray machine Image Receptor Relation to Object Relation to processing

6. Sr. No. Factors affecting Density Effect on Density Reason I. Relation to Xray Machine Kilovoltage peak Increases Increased penetrating power of xray - more photons reach film – increased blackening b. Milliamperage Increases Increased no. of photons – increased density c. Intensity Increases Xray beam will have more remnant energy – exposing more of film – increased blackening d. Exposure time Increases Increased no. of photons e. Source – film distance Decrease As intensity is inversely proportional to distance – density is decreases with distance f. Grid Decrease II. Relation to Image Receptor a. Xray film speed Decreases High speed film – decreased exposure – decreased density b. Intensifying screen Decreases Decreased mA – decrease density

7. III. Relation to Object Type of material Decreases As density of material increases – less radiation reach the film – decreased density b. Subject/Object thickness Decreases IV. Relation to Processing a. Developing (increase time) Increased More silver halide crystals react with developer – darkening of film b. Fixing (increase time) Decreased Removes excess Silver Halide crystals Obese patient – increased soft tissue; increase exposure time Edentulous patient- lack of soft tissue: decrease exposure Child- narrow facial structure, decrease exposure time

8. Also known as – Film Characteristic curve/ Sensitometeric curve/ Density curve/ Hurter – Driffield curve. Discovered in 1890 The curve shows relation of density of film to different exposure time. When other factors are constant: No. of photons reaching film ∝ density of film CHARACTERISTIC CURVE Intensity of Radiation Exposure Time

9. CHARACTERISTIC CURVE TOE: the film has low density, due to low exposure (No or very little contrast) Straight Line: density increases as exposure increases (highest contrast) {the film has optimal density and contrast} Shoulder: High exposure region, very high density (Low contrast) OPTIMAL - 0.6 – 3.0 – optical density unit BASE PLUS FOG

10. Reciprocity Law the Reciprocity L aw states that the OD will be the same if the mA x s value is constant. It does not depend upon the time of exposure. But on the energy imparted to the radiographic film. This is for direct exposure films, but fails for indirect exposure films.

11. RADIOGRAPHIC CONTRAST The difference in densities between light and dark regions on a radiograph High Contrast Film/ Short scale Contrast Low Contrast Film/ Long scale Contrast

12. Sr.NO FACTORS AFFECTING CONTRAST EFFECT ON CONTRAST REASONS I. Relation to Patient [ Subject Contrast] a. Tissue Thickness Decreases with increases subject thickness. Depends upon different degree of beam attenuation. Increase in these factors - more absorption of xray - less remnant beam reaches film – decreased density b. Tissue Density Increased tissue density decreases contrast c. Atomic Number Increased atomic number, increased contrast Increases atomic number- higher beam attenuation more contrast variations Eg. Cephalogram has high contrast II. Relation to Xray Machine a. Kvp Low contrast with increased Kvp Less difference in attenuation between the different parts of the subject due to increased penetrating power b. Exposure rate Exposure rate – increase brightness Increased exposure time – low contrast

13. Sr.NO FACTORS AFFECTING CONTRAST EFFECT ON CONTRAST REASONS I. Relation to Film [ Film Contrast] a. Optical Density Low or High OD exhibit poor contrast Decreased/ Increased number of non-diagnostic photons reaching film b. Film Speed Increased speed, low contrast c. Type of film Direct vs Indirect film Less contrast in indirect film d. Emulsion Double emulsion – less contrast II. Relation to Processing a. Scatter Radiation Compton/ Coherent scatter decreases contrast Less difference in attenuation between the different parts of the subject due to increased penetrating power b. Fog Decreases contrast Developing of unexposed silver halide crystals

14. Minimum contrast – 0.02

15. APPLIED Low Contrast : detection for periodontal and periapical disease. { As high kVp is used, Xrays passing through smaller trabeculae, lamina dura, marrow spaces and periodontal space will carry discernable and diagnostic information} But, very high kVp , will cause loss of details- burn-out effect. High Contrast : detection of caries Grid: reduces scattered radiation reaching film.`

16. RADIOGRAPHIC SPEED Film speed frequently is expressed as the reciprocal of the exposure (in roentgens) required to produce an optical density of 1 above base plus fog. Film speed ∝ 1 roentegens When numbers are used to express speed, all are relative to 100; this is called par speed. Numbers higher than 100 refer to fast or high-speed image receptors. Film speed is controlled largely by the size of the silver halide grains and their silver content Radiographic speed refers to the amount of radiation required to produce an image of a standard density. FAST FILM: LOW EXPOSURE SLOW FILM: LONGER EXPOSURE

17. The Characteristic curve of fast film is placed on the left, more towards Y- Axis, when density and contrast is optimal.

18. FILM LATITUDE Film Latitude 1 contrast APPLICATION: Wide latitude films are useful when both the osseous structures of the skull and the soft tissues of the facial region must be recorded Film latitude is a measure of the range of exposures that can be recorded as distinguishable densities on a film. WIDE LATITUDE – LONG GRAY SCALE NARROW LATITUDE – SHORT GRAY SCALE

19. RADIOGRAPHIC NOISE It is seen on a small area of film as localized variations in density. the appearance of uneven density of a uniformly exposed radiographic film.

20. errors in film handling, such as fingerprints or bends in the film, or errors in film processing, such as splashing developer or fixer on a film or marks or scratches from rough handling mottle may be seen as film graininess , which is caused by the visibility of silver grains in the film emulsion, especially when magnification is used to examine an image. Film graininess is most evident when high-temperature processing is used. by a fluctuation in the number of photons per unit of the beam cross-sectional area absorbed by the intensifying screen. is graininess caused by screen phosphors. FAST SCREENS

21. SHARPNESS AND RESOLUTION Resolution, or resolving power, is the ability of a radiograph to record separate structures that are close together. Sr.NO FACTORS AFFECTING RESOLUTION EFFECT ON RESOLUTION I. Type of Film Direct exposure film Increases b. Indirect exposure film Decreases [ 9 – 10 lp /mm] II. Speed Fast Film Decreases b. Slow Film Increases

22. TESTING:- measured by radiographing an object made up of a series of thin lead strips with alternating radiolucent spaces of the same thickness. measured by radiographing an object made up of a series of thin lead strips with alternating radiolucent spaces of the same thickness. resolving power is measured as the highest number of line pairs (a line pair being the image of an absorber and the adjacent lucent space) per millimeter. panoramic film-screen: 5 lp /mm periapical film: 20 lp /mm

23. SHARPNESS INCLUDES: Geometric unsharpness (including the penumbra effect) Motion unsharpness ( caused by the patient moving during the exposure) Absorption unsharpness (caused by variation in object shape, e.g. cervical burn-out at the neck of a tooth ) Screen unsharpness ( caused by the diffusion and spread of the light emitted from intensifying screens) Sharpness is the ability of a radiograph to define an edge precisely (e.g., the dentin-enamel junction, or a thin trabecular plate Sr.NO FACTORS AFFECTING SHARPNESS EFFECT ON SHARPNESS REASON I. Image Receptor Blurring Film Speed Increased in slow speed Finer crystal size; finer sharpness b. Intensifying screen Decreases in fast screen The light emerging from Intensifying screen; diverges; blurring of radiograph. Fast screen; large phosphor layer and crystals; more dispersion of light, loss of sharpness. c. Parallax Decreases double-emulsion film also causes a loss of image sharpness II. Motion Blurring Decreases on movement Movement of the film, subject, or x-ray source during exposure. III. Geometric Blurring Focal Spot Increases as focal spot decreases L arge focal spot causes xray production from larger area, which causes more penumbra. b. Source-Object Increases as distance increases Reduction in divergence of the xrays . c. Object Receptor Increases as distance decreases Reduction in divergence of the xrays . Minimized by placing screen and film closer

24. THANK YOU

25. REFERNCES Mallya S, White LE. Pharoah's oral radiology: principles and interpretation. Bushong SC, Facmp SF. Radiologic Science for Technologists E-Book: Physics, Biology, and Protection. Mosby; 2020 Dec 2. Whaites E, Drage N. Essentials of dental radiography and radiology. Elsevier Health Sciences; 2013 Jun 20.

26. PARLLAX EFFECT Motion Blurring Movement of xray tube