DENSITY. Density. Principles of Imaging Science II (RAD 120)

Transcription

1 Principles of Imaging Science II (RAD 120) Radiographic Density 1 DENSITY One of the photographic properties that determines visibility of detail Overall blackness or darkness of the entire radiographic image or a specific area When evaluating an image for proper radiographic density, the density of the entire image is considered 2 Density Only the anatomical parts should be evaluated for proper density The collimated border surrounding the structure represents x-ray beam attenuation in air The radiologist evaluates the image for anatomic or pathologic changes that cause a change in normal density 3 1

2 DENSITY CONTROLLING FACTOR: mas (Quantity) mas = ma X time (sec) 25 mas = 100 ma X 0.25 sec (250 ms) 25 mas = 200 ma X sec (125 ms) 25 mas = 400 ma X sec (62.5 ms) ma, time inversely proportional to maintain the same mas or density Why change ma, time when a desired mas is known? 4 Density Controlling Factors ma = mas/time Calculate the ma, given 50 mas, 100 ms 500 ma Calculate the ma, given 100 mas, 100 ms 1000 ma time = mas/ma Calculate the time, given 20 mas, 200 ma 0.1 sec, 100 ms Calculate the time, given 20 mas, 400 ma 0.5 sec, 50 ms 5 Density Evaluation Your initial reaction should determine if the radiograph demonstrates proper density If it is not correct, then it is too dark (overexposed), or too light (underexposed) You must determine the extent to which it is to dark or too light. 6 2

3 Density Evaluation Over and underexposure is generally expressed as either 50% or 100% incorrect If a radiograph is very light or dark, it is 100% over or underexposed Underexposure - correct by multiplying the mas by 2 (40 mas vs 80 mas) Overexposure correct by multiplying the mas by 0.5 (40 mas vs 20 mas) 7 Density Evaluation If a radiograph is minimally light or dark and warrants a repeat, it is 50% over or underexposed Underexposure - correct by multiplying the mas by 1.5 (40 mas vs 60 mas) Overexposure correct by multiplying the mas by 0.75 (40 mas vs 30 mas) Remember that at least a 30% change in mas is needed to demonstrate a visible difference in density. Known as the 30% rule. mas is the factor of choice for density changes 8 mas 9 3

4 Density: Influencing Factors Kilovoltage Controls the energy, penetrability or quality of the x-ray beam. As kvp is increased or decreased, the quantity of x-ray photons produced increases or decreases 15% rule application 15% change in kvp results in 100% change in density 7.5% change in kvp results in 50% change in density 10 Density Evaluation When evaluating density determine if the structure is properly penetrated. This applies to underexposed images. An image cannot be overexposed if it is not adequately penetrated Look for structures behind another If it is penetrated, change the mas; if not change the kvp according to the 15% rule 11 Density: INFLUENCING FACTORS kilovoltage I 1: I 2 : kvp 1: I 1 kvp 1 2 == I 2 kvp 2 2 Beginning Intensity New Intensity Beginning kilovoltage kvp 2 : New kilovoltage 12 4

5 Density: kilovoltage calculations 13 Density: Influencing Factors Source-Image Distance (SID) Distance between the x-ray source(fs) and the image receptor Affects the intensity of the x-ray beam according to the inverse square law If the SID is changed, and density should be maintained, apply the Density Maintenance Formula 14 Density: INFLUENCING FACTORS Source - Image Distance mas 1 D 1 2 == mas 2 D 2 2 mas 2 = mas 1 (D 22 ) (D 12 ) mas 1: mas 2 : D 1: D 2 : Beginning mas New mas Beginning distance New distance 15 5

6 Density: Distance calculations. 16 Density: INFLUENCING FACTORS There is a direct relationship between RSS and density Common RSS range from As RSS increases, density increases As RSS decreases, density decreases Based upon no compensatory changes in mas A radiograph will be dark if you intended to use an extremity cassette and you incorrectly selected a high speed cassette. 17 Density: INFLUENCING FACTORS Film/Screen Combination mas 1 RSS 2 == mas 2 RSS 1 mas 2 = mas 1 (RSS 1 ) (RSS 2 ) mas 1: mas 2 : RSS 1: RSS 2 : Beginning mas New mas Beginning Film/Screen Speed New Film/Screen Speed 18 6

7 Density: RSS calculations 19 Density: INFLUENCING FACTORS Grids are used to absorb scattered photon emitted from the patient before they interact with the image receptor (IR) Placed between the patient and IR When a grid is used, you must adjust mas to compensate for the loss of density that would be produced by scatter 20 Density: INFLUENCING FACTORS Inverse relationship between grid use and density An image would be more dense if no grid were used compared to using a grid Using a grid requires higher mas according to the following: Grid Ratio GCF 5:1 2X 6:1 3X 8:1 4X 10:1 4X 12:1 5X 16:1 6X Non-grid 1X 21 7

8 Density: INFLUENCING FACTORS Grids mas 1 GCF 1 == mas 2 GCF 2 mas 2 = mas 1 (GCF 2 ) (GCF 1 ) mas 1: mas 2 : GCF 1: GCT 2 : Beginning mas New mas Beginning grid conversion factor New grid conversion factor 22 Density: Grid calculations 23 Density: String Formula mas 2 = mas 1 (D 2 ) 2 (GCF 2 ) (RSS 1 ) (D 1 ) 2 (GCF 1 ) (RSS 2 ) 24 8

9 Density: String Formula 25 Density: String Formula 26 Density: String Formula mas 2 = mas 1 (D 2 ) 2 (GCF 2 ) (RSS 1 ) (D 1 ) 2 (GCF 1 ) (RSS 2 ) 27 9

10 Density: Influencing Factors Generator Type An AP projection of the hip is taken using a three phase, 12 pulse high frequency generator. Technical factors are 70 kvp, 24 mas, 400 RSS, 40 SID, 10:1 grid. Using the correct conversion factor(2.0), calculate the mas needed when using a single phase, 2 pulse unit. To maintain image density use the formula mas 2 = mas 1 X conversion factor mas 2 = Generator Type Conversions & Voltage Ripple Voltage waveforms resulting from various power supplies. The ripple of the kilovoltage is indicated as a percentage for each waveform. 29 Density Influencing Factors Beam Restriction (Collimation) Less scatter radiation is produced and density is decreased 14 X 17 to 8 X 10, increase mas 50-60% 14 X 17 to 10 X 12, increase mas 30-40% When collimation is increased, the amount of irradiated tissue decreases 30 10

11 Density: Influencing Factors Filtration Indirect relationship with density Increased filtration = Decreased density Film Processing Direct relationship with density for developer time, temperature, and replenishment rates Focal Spot Size, Anode Heel Effect Minimal effect 31 Anode Heel Effect 32 Density: Influencing Factors Anatomical Part Thickness or Tissue Indirect relationship with density Larger patients attenuate more x-ray photons Higher atomic # tissues attenuate more x-ray photons Pathology Additive Disease - Density decreases Destructive Disease Density increases Contrast Media Positive Contrast Density decreases Negative Contrast Density increases Refer to density chart in Carlton text (mas adjustments are not made) 34 11

12 Radiographic Density Review 85 kvp to 60 kvp A. Increased density B. Decreased density C. No change in density 100 milliseconds to 50 milliseconds 800 ma, 0.04 seconds to 600 ma, 53 ms 40 SID to 100 cm SID Table-top radiography to a 10:1 bucky grid 2.5 mm focal spot size to 1.2 mm focal spot size 90 sec film processing to 3 min film processing 200 ma to 1000 ma Normal tissue to additive pathologic condition 15 cm abdomen patient to 28 cm abdomen patient 35 Greatest Density Calculation Which of the following sets of technical factors will demonstrate the greatest density? ma time SID RSS Grid A ms :1 B ms :1 C ms :1 D ms :1 36 Greatest Density Calculation Practice Which of the following sets of technical factors will demonstrate the greatest density? mas SID RSS Grid A :1 B :1 C :1 D :

13 Digital Image Receptor Systems Density and contrast Controlled through post-processing Still, the image receptor must receive proper exposure Extremely high or low exposures do not produce acceptable image quality 38 Digital Image Receptor Systems kvp based on desired contrast Distance selected based on desired beam geometry mas is adjusted to meet exposure needs 39 Concept Point Check Part II: Matching Identify the effect on radiographic density when a single change is made. A. Radiographic density will increase B. Radiographic density will decrease C. No change in density will result 40 SID to 60 SID 400 ma 20 milliseconds to 800 ma 30 milliseconds 1.0 mm Focal Spot Size to 1.2 mm Focal Spot Size 500 RSS to 100 RSS 50 kvp to 65 kvp 5:1 ratio grid to 16:1 ratio grid 40 13