ºÝºÝߣ

ºÝºÝߣShare a Scribd company logo

Copy of Osric ¡¤ ºÝºÝߣsCarnival.pdf

?
?
L
Louie Robertson

Optical density (OD) refers to the degree of blackening on a radiographic film. It has a numeric value and can range from completely black (no light transmission) to almost clear. The primary factor that controls OD is the milliampere-seconds (mAs) exposure setting, as OD increases directly with mAs. Doubling the mAs will double the OD. Other factors like distance, kilovoltage peak (kVp), intensifying screen, film processing and emulsion can also affect OD. The document discusses these technical factors in radiography and their relationship to achieving the proper optical density on the radiographic image. It also covers topics like the inverse square law, square law, and reciprocity

1 of 36
Download to read offline
Film Screen Image Acquisition
X-ray film sensitive to radiation ? Density: Overall blackening of film ? Density depends on amount of radiation reaching film ? Film gets DARKER as exposure increase What is DENSITY? 2
Lets Talk about optical density 3
¡° Optical density is the degree of blackening of the finished radiograph. OD has a numeric value and can be present in varying degrees, from completely black, in which no light is transmitted through the radiograph, to almost clear. 4
In medical imaging, many problems involve an image being ¡°too dark¡± or ¡°too light.¡± A radiograph that is too dark has a high OD caused by overexposure. This situation results when too much x-radiation reaches the image receptor.
A radiograph that is too light has been exposed to too little x-radiation, resulting in underexposure and a low OD. 6
Check which is under/over exposed 7
FACTORS mAs and KVP 8
mas ? When distance is fixed, however, as is usually the case, the mAs value becomes the primary variable technique factor used to control OD. OD increases directly with mAs, which means that if the OD is to be increased ? on a radiograph, the mAs setting must be increased accordingly.Optical density can be affected by other factors, but the mAs value becomes the factor of choice for its control 9
¡° A wise penguin once said 10
? A change in mAs of approximately 30% is required to produce a visible change in OD. As a general rule, when only the mAs setting ischanged, it should be halved or doubled). If a significant change is not required, a repeat examination probably is not required.
I A C I B I 12 Optical density is determined principally by the mAs value, as shown by these phantom radiographs of the abdomen taken at 70 kVp. A, 10 mAs. B, Plus 25%, 12.5 mAs. C, Plus 50%, 15 mAs.
OPTICAL DENSITY - OD has a precise numeric value that can be calculated if the level of light incident on a process ?lm (Io) and the level of light transmitted through that ?lm (It) are measured. 13 100% LIGHT PHOTON (Io) 100% LIGHT PHOTON (Io)
DENSITOMER ER
FACTORS AFFECTING RADIOGRAPHIC DENSITY INFLUENCING FACTOR FOR DENSITY¡¯ ? Focal film distance ? kVp ? Intensifying screen ? Film processing ? Film emulsion ? Heel effect ? Pathology 15
X-RAY QUANTITY X-RAY INTENSITY ? The intensity of the x-ray beam of an x-ray imaging system is measured in milligray in air (mGya) [formerly milliroentgen (mR)] and is called the x-ray quantity. ? Another term, radiation exposure, is often used instead of x-ray intensity or x-ray quantity. All have the same meaning, and all are measured in mGya (mR). ? The mGya (mR) is a measure of the number of ion 16 ? Ionization of air increases as the number of x-rays in the beam increases. The relationship between the x-ray quantity as measured in mGya (mR) and the number of x-rays in the beam is not always one to one. Some small variations are related to the effective x-ray energy.
17
18 Milliampere Seconds (mAs). X-ray quantity is directly proportional to the mAs. When mAs is doubled,the number of electrons striking the tube target is doubled, and therefore the number of x-rays emitted is doubled. mAs is directly proportional to the density. When Density is extremly increases it reduce contrast, then if its extremly decrease, contrast also decrease Quantity = mAs mAs = density
MAS When distance is fixed, however, as is usually the case, the mAs value becomes the primary variable technique factor used to control OD. OD increases directly with mAs, which means that if the OD is to be increased on a radiograph, the mAs setting must be increased accordingly. 19
20 Changes in mAs value have a direct effect on optical density (OD). A, Original image. B, Decrease in OD when the mAs value is decreased by half. C, Increase in OD when the mAs value is doubled.
21 TECHNIQUE /DENISTY CHANGES
22 Normal chest radiograph taken at 100 cm source-to-image receptor distance (SID). B, If the exposure technique factors are not changed, a similar radiograph at 90 cmSID (A) will be overexposed, and at 180 cm SID (C), will be underexposed. Optical density can be controlled in radiography by two major factors: mAs and SID. A significant number of problems would arise if the SID were continually changed. Therefore, SID usually is fixed at 90 cm for mobile examinations, 100 cm for table] studies, and 180 cm for upright chest examinations. Figure 13-7 illustrates the change in OD that occurs at these SIDs when other exposure technique factors remain constant.
23 90 cm 100 cm 180 cm A B C 120kVp 5mAs 120kVp 5mAs 120kVp 5mAs
DISTANCE ? X-ray intensity varies inversely with the square of the distance from the x-ray tube target. ? This relationship is known as the INVERSE SQUARE LAW 24 where I1 and I2 are the x-ray intensities at distances d1 and d2, respectively.
When SID is increased, mAs must be increased by SID2 to maintain constant exposure to the image receptor. ? Compensating for a change in SID by changing mAs by the factor SID2 is known as the SQUARE LAW, a corollary to the inverse square law.
KILOVOLT PEAK (KVP) ? Kilovolt Peak (kVp). X-ray quantity varies rapidly with changes in kVp. The change in x-ray quantity is proportional to the square of the ratio of the kVp; in other words, if kVp were doubled, the x-ray intensity would increase by a factor of 4. ? Mathematically, this is expressed as follows: 26
27 KVP AND INTENSITY RELATIONSHIP ? In practice, a slightly different situation prevails. Radiographic technique factors must be selected from a relatively narrow range of values, from approximately 40 to 150 kVp. Theoretically, doubling the x-ray intensity by kVp manipulation alone requires an increase of 40% in kVp. ? This relationship is not adopted clinically because as] kVp is increased, the penetrability of the x-ray beam is increased, and relatively fewer x-rays are absorbed in the patient. More x-rays go through the patient and interact with the image receptor. Consequently, to maintain a constant exposure of the image receptor, an increase of 15% in kVp should be accompanied by a reduction of one half in mAs.
28 A B C Normal chest radiograph taken at 70 kVp (B). If the kilovoltage is increased by 15% to 80 kVp (A), overexposure occurs. Similarly, at 15% less, 60 kVp (C), the radiograph is underexposed.
29 ? Technique changes involving kVp become complicated. A change in kVp affects penetration, scatter radiation, patient radiation dose, and especially contrast. ? Image contrast is affected when kVp is changed to adjust OD. This makes it much more difficult to optimize OD with kVp. It takes the eye of an experienced radiologic technologist to determine whether OD is the only factor to be changed or if contrast also should be changed to optimize the radiographic image.
KVP AND INTENSITY RELATIONSHIP 30 ? Note that by increasing kVp and reducing mAs so that image receptor exposure remains constant, the patient dose is reduced significantly. ? The disadvantage of such a technique adjustment is reduced image contrast when screen film is the image receptor. There is no change in contrast when using digital image receptors.
RECIPROCITY LAW: ? One would think that the OD on a radiograph would depend strictly on the total exposure (mAs) and would be independent of the time of exposure. This, in fact, is the reciprocity law. Whether a radiograph is made with short exposure time or long exposure time, the reciprocity law states that the OD will be the same if the mAs value is constant. ? The reciprocity law holds for direct exposure with x-rays, but it does not hold for exposure of film by the visible light from radiographic intensifying screens. Consequently, the reciprocity law fails for screen-film exposures at exposure times less than approximately 10 ms or longer than approximately 2 s. 31
32 DENSITY .25 TO -2.5 The straight line of the H&D curve
33 The angle of the target can cause a density variation up to 45% between the cathode and the anode. Density is always greater at the cathode end. It is more pronounced as: Short SID¡¯s Large image receptor/open wide collimator X-ray tubes having small target angle (12o or less) ANODE HEEL EFFECT
34 FILM EMULSION Speed. Screen-film IRs are available with different speeds. Speed is the sensitivity of the screen-film combination to x-rays and light. Usually, a manufacturer offers several different IRs of different speeds that result from different film emulsions and different intensifying screen phosphors. For direct-exposure film, speed is principally a function of the concentration and the total number of silver halide crystals. For screen film, silver halide grain size, shape, and concentration are the principal determinants of film speed.
35 FILM EMULSION Compared with earlier technology, current emulsions contain less silver yet produce the same optical density (OD) per unit exposure. This more efficient use of silver in the emulsion is called the covering power of the emulsion.
36 I.S. Intensifying screen amplify the action of x-rays through their property of fluorescence. Fluorescent light is responsible for more than 98% of the image density. All other factors remaining constant, an increase in screen speed will result in an increase in optical density. Screen speed and optical density are directly proportional. Screen speed and patient dose are inversely proportional. Screen speed and x-ray tube heat production are inversely related. Compensations for changes in relative speed can be made by adjusting mAs:

More Related Content

Copy of Osric ¡¤ ºÝºÝߣsCarnival.pdf

  • 2. X-ray film sensitive to radiation ? Density: Overall blackening of film ? Density depends on amount of radiation reaching film ? Film gets DARKER as exposure increase What is DENSITY? 2
  • 3. Lets Talk about optical density 3
  • 4. ¡° Optical density is the degree of blackening of the finished radiograph. OD has a numeric value and can be present in varying degrees, from completely black, in which no light is transmitted through the radiograph, to almost clear. 4
  • 5. In medical imaging, many problems involve an image being ¡°too dark¡± or ¡°too light.¡± A radiograph that is too dark has a high OD caused by overexposure. This situation results when too much x-radiation reaches the image receptor.
  • 6. A radiograph that is too light has been exposed to too little x-radiation, resulting in underexposure and a low OD. 6
  • 7. Check which is under/over exposed 7
  • 9. mas ? When distance is fixed, however, as is usually the case, the mAs value becomes the primary variable technique factor used to control OD. OD increases directly with mAs, which means that if the OD is to be increased ? on a radiograph, the mAs setting must be increased accordingly.Optical density can be affected by other factors, but the mAs value becomes the factor of choice for its control 9
  • 10. ¡° A wise penguin once said 10
  • 11. ? A change in mAs of approximately 30% is required to produce a visible change in OD. As a general rule, when only the mAs setting ischanged, it should be halved or doubled). If a significant change is not required, a repeat examination probably is not required.
  • 12. I A C I B I 12 Optical density is determined principally by the mAs value, as shown by these phantom radiographs of the abdomen taken at 70 kVp. A, 10 mAs. B, Plus 25%, 12.5 mAs. C, Plus 50%, 15 mAs.
  • 13. OPTICAL DENSITY - OD has a precise numeric value that can be calculated if the level of light incident on a process ?lm (Io) and the level of light transmitted through that ?lm (It) are measured. 13 100% LIGHT PHOTON (Io) 100% LIGHT PHOTON (Io)
  • 15. FACTORS AFFECTING RADIOGRAPHIC DENSITY INFLUENCING FACTOR FOR DENSITY¡¯ ? Focal film distance ? kVp ? Intensifying screen ? Film processing ? Film emulsion ? Heel effect ? Pathology 15
  • 16. X-RAY QUANTITY X-RAY INTENSITY ? The intensity of the x-ray beam of an x-ray imaging system is measured in milligray in air (mGya) [formerly milliroentgen (mR)] and is called the x-ray quantity. ? Another term, radiation exposure, is often used instead of x-ray intensity or x-ray quantity. All have the same meaning, and all are measured in mGya (mR). ? The mGya (mR) is a measure of the number of ion 16 ? Ionization of air increases as the number of x-rays in the beam increases. The relationship between the x-ray quantity as measured in mGya (mR) and the number of x-rays in the beam is not always one to one. Some small variations are related to the effective x-ray energy.
  • 17. 17
  • 18. 18 Milliampere Seconds (mAs). X-ray quantity is directly proportional to the mAs. When mAs is doubled,the number of electrons striking the tube target is doubled, and therefore the number of x-rays emitted is doubled. mAs is directly proportional to the density. When Density is extremly increases it reduce contrast, then if its extremly decrease, contrast also decrease Quantity = mAs mAs = density
  • 19. MAS When distance is fixed, however, as is usually the case, the mAs value becomes the primary variable technique factor used to control OD. OD increases directly with mAs, which means that if the OD is to be increased on a radiograph, the mAs setting must be increased accordingly. 19
  • 20. 20 Changes in mAs value have a direct effect on optical density (OD). A, Original image. B, Decrease in OD when the mAs value is decreased by half. C, Increase in OD when the mAs value is doubled.
  • 22. 22 Normal chest radiograph taken at 100 cm source-to-image receptor distance (SID). B, If the exposure technique factors are not changed, a similar radiograph at 90 cmSID (A) will be overexposed, and at 180 cm SID (C), will be underexposed. Optical density can be controlled in radiography by two major factors: mAs and SID. A significant number of problems would arise if the SID were continually changed. Therefore, SID usually is fixed at 90 cm for mobile examinations, 100 cm for table] studies, and 180 cm for upright chest examinations. Figure 13-7 illustrates the change in OD that occurs at these SIDs when other exposure technique factors remain constant.
  • 23. 23 90 cm 100 cm 180 cm A B C 120kVp 5mAs 120kVp 5mAs 120kVp 5mAs
  • 24. DISTANCE ? X-ray intensity varies inversely with the square of the distance from the x-ray tube target. ? This relationship is known as the INVERSE SQUARE LAW 24 where I1 and I2 are the x-ray intensities at distances d1 and d2, respectively.
  • 25. When SID is increased, mAs must be increased by SID2 to maintain constant exposure to the image receptor. ? Compensating for a change in SID by changing mAs by the factor SID2 is known as the SQUARE LAW, a corollary to the inverse square law.
  • 26. KILOVOLT PEAK (KVP) ? Kilovolt Peak (kVp). X-ray quantity varies rapidly with changes in kVp. The change in x-ray quantity is proportional to the square of the ratio of the kVp; in other words, if kVp were doubled, the x-ray intensity would increase by a factor of 4. ? Mathematically, this is expressed as follows: 26
  • 27. 27 KVP AND INTENSITY RELATIONSHIP ? In practice, a slightly different situation prevails. Radiographic technique factors must be selected from a relatively narrow range of values, from approximately 40 to 150 kVp. Theoretically, doubling the x-ray intensity by kVp manipulation alone requires an increase of 40% in kVp. ? This relationship is not adopted clinically because as] kVp is increased, the penetrability of the x-ray beam is increased, and relatively fewer x-rays are absorbed in the patient. More x-rays go through the patient and interact with the image receptor. Consequently, to maintain a constant exposure of the image receptor, an increase of 15% in kVp should be accompanied by a reduction of one half in mAs.
  • 28. 28 A B C Normal chest radiograph taken at 70 kVp (B). If the kilovoltage is increased by 15% to 80 kVp (A), overexposure occurs. Similarly, at 15% less, 60 kVp (C), the radiograph is underexposed.
  • 29. 29 ? Technique changes involving kVp become complicated. A change in kVp affects penetration, scatter radiation, patient radiation dose, and especially contrast. ? Image contrast is affected when kVp is changed to adjust OD. This makes it much more difficult to optimize OD with kVp. It takes the eye of an experienced radiologic technologist to determine whether OD is the only factor to be changed or if contrast also should be changed to optimize the radiographic image.
  • 30. KVP AND INTENSITY RELATIONSHIP 30 ? Note that by increasing kVp and reducing mAs so that image receptor exposure remains constant, the patient dose is reduced significantly. ? The disadvantage of such a technique adjustment is reduced image contrast when screen film is the image receptor. There is no change in contrast when using digital image receptors.
  • 31. RECIPROCITY LAW: ? One would think that the OD on a radiograph would depend strictly on the total exposure (mAs) and would be independent of the time of exposure. This, in fact, is the reciprocity law. Whether a radiograph is made with short exposure time or long exposure time, the reciprocity law states that the OD will be the same if the mAs value is constant. ? The reciprocity law holds for direct exposure with x-rays, but it does not hold for exposure of film by the visible light from radiographic intensifying screens. Consequently, the reciprocity law fails for screen-film exposures at exposure times less than approximately 10 ms or longer than approximately 2 s. 31
  • 32. 32 DENSITY .25 TO -2.5 The straight line of the H&D curve
  • 33. 33 The angle of the target can cause a density variation up to 45% between the cathode and the anode. Density is always greater at the cathode end. It is more pronounced as: Short SID¡¯s Large image receptor/open wide collimator X-ray tubes having small target angle (12o or less) ANODE HEEL EFFECT
  • 34. 34 FILM EMULSION Speed. Screen-film IRs are available with different speeds. Speed is the sensitivity of the screen-film combination to x-rays and light. Usually, a manufacturer offers several different IRs of different speeds that result from different film emulsions and different intensifying screen phosphors. For direct-exposure film, speed is principally a function of the concentration and the total number of silver halide crystals. For screen film, silver halide grain size, shape, and concentration are the principal determinants of film speed.
  • 35. 35 FILM EMULSION Compared with earlier technology, current emulsions contain less silver yet produce the same optical density (OD) per unit exposure. This more efficient use of silver in the emulsion is called the covering power of the emulsion.
  • 36. 36 I.S. Intensifying screen amplify the action of x-rays through their property of fluorescence. Fluorescent light is responsible for more than 98% of the image density. All other factors remaining constant, an increase in screen speed will result in an increase in optical density. Screen speed and optical density are directly proportional. Screen speed and patient dose are inversely proportional. Screen speed and x-ray tube heat production are inversely related. Compensations for changes in relative speed can be made by adjusting mAs: