RT202-LECTURE-6.pdf

PRINCIPLES OF IMAGING SHENNA MARIE G. FORRO, RRT, MSRT JB JAMES A. LAO, RRT, MSRT TECHNICAL FACTORS IN RADIOGRAPHY  TECHNICAL FACTORS IN RADIOGRAPHY Objectives: Define "technique." List the four technical factors used to produce a radiographic image. Define: kilovoltage (kV), m...

PRINCIPLES OF IMAGING SHENNA MARIE G. FORRO, RRT, MSRT JB JAMES A. LAO, RRT, MSRT TECHNICAL FACTORS IN RADIOGRAPHY  TECHNICAL FACTORS IN RADIOGRAPHY Objectives: Define "technique." List the four technical factors used to produce a radiographic image. Define: kilovoltage (kV), milliampere (mA), milliampereseconds (mAs), time, and distance. Explain the function of kilovoltage in the production of x-rays. Discuss how kilovoltage affects wavelength and penetration of the x-ray photons. Define kilovoltage peak (kVp). Explain how kilovoltage affects the energy of the primary x-ray beam. Describe the 15% rule, and explain how it will affect exposure to the film. Describe the criteria to evaluate a radiograph for adequate penetration. Explain how changes in kVp and mAs can be used to control the amount of exposure to the film. Explain how mA and mAs contribute to the production of x-rays. (RECIPROCITY LAW)  Explain how kilovoltage influences the production of scatter radiation. Discuss mA and mAs as quantitative terms in x-ray production. Define "focal spot blooming." Calculate mAs with mA and time values. Describe the relationship between mA and time. Calculate mathematical problems using mA and time formula. Describe the reciprocity law. Explain how distance (FFD) affects exposure to the film. Define the inverse square law. Calculate mathematical problems using the inverse square law and mAs-distance formulas. Differentiate between the inverse square law formula and mAs-distance formula and explain how each is used by the radiographer. TECHNIQUE IN RADIOGRAPHY Radiographers operate equipment to produce x-rays. "technique" is necessary for producing good radiographs. "Technique" refers to the systematic procedure used to produce high-quality radiographs. TECHNIQUE IN RADIOGRAPHY The systematic procedure includes selecting appropriate factors to: Produce an x-ray beam that adequately penetrates the body part. Provide the appropriate level of blackening (density). Ensure proper subject contrast on the radiograph.  Kilovoltage SIGNIFICANT Milliamperes FACTORS TO PRODUCE RADIOGRAPHS Exposure time Distance SIGNIFICANT FACTORS TO PRODUCE RADIOGRAPHS Radiographers must know how each factor: Functions in the production of x-rays. Penetrates human body parts. Is absorbed by body tissue. Produces changes in film density and subject contrast. Kilovoltage  Kilovoltage Defined as the force applied to accelerate (push) the electrons from the cathode to the anode during exposure. Most significant factor in the production of x-rays and radiographs. Affects the speed of electron travel from the cathode to the anode in the x-ray tube. The force behind the electron stream determines the speed at which electrons "slam" into the focal spot. Kilovoltage  Kilovoltage Effect on Photon Quality Kilovoltage significantly affects the quality of photons in the x-ray beam. Energy of the x-ray beam is determined by the kilovoltage peak (kVp) selected at the control panel. Kilovoltage Voltage applied between the cathode and anode can be graphed as a wave pattern Single-phase x-ray generator wave pattern shows voltage reaching its peak and then falling to zero. Only a portion of this waveform is useful for producing a radiograph. The highest level of energy, or crest of the waveform, represents the x-ray photon energy, referred to as the kilovoltage peak (kVp). Kilovoltage X-ray photons travel through matter in wave-like fashion Wavelength describes one full wave pattern or cycle Wavelength is measured from the crest (or peak) of one wave to the crest of the next wave. Increased energy of x-ray photons results in crests closer together, producing shorter wavelengths. Increased kilovoltage produces x-ray photons with shorter wavelengths.  Determines the wavelength of x-ray photons Higher kilovoltage produces shorter Kilovoltage wavelength x-ray photons with greater Selection ability to penetrate body tissue. Kilovoltage determines the energy of photons in the beam and the penetrating power of the beam.  Kilovoltage affects the intensity of the beam. Increased kilovoltage results in Kilovoltage increased beam intensity. and Intensity Higher kVp results in more photons with higher energy levels. Kilovoltage Increase in kVp leads to: Increase in kVp increases beam intensity and penetrating ability. Higher photon energy and beam intensity result in more radiation reaching the film. Increased radiation produces more blackening on the film.  Practical Understanding Important to note that increased kVp does not produce more x-rays but increases the energy of x-ray photons. Kilovoltage Higher energy allows more x- rays to penetrate the body part. This penetration results in increased blackening on the film. 15% Rule in Radiography A 15% increase in kilovoltage results in doubling the amount of blackening on the film Radiographers use the 15% rule to control overall blackening levels when adjusting kVp.  The 15% rule is not linear or proportional, but it is a practical tool for radiographers. It helps control the amount of exposure to the film when changes in kVp are necessary. 15% Rule in Radiography 15% increase: results in doubling the amount of blackening on the film 15% decrease: reduce the blackening of the film by 1/2  Radiographers must be cautious when using kVp to decrease blackening on the film. Significant reduction in kVp may result in inadequate photon energy to penetrate Caution in the body part. Reducing kVp Selected kilovoltage must be adequate to ensure proper penetration of the part. THICKNESS (cm) X 2 + MACHINE CONSTANT: 74 kvp Penetration of the part Refers to selecting a kVp adequate to X-ray photons must produce x-ray emerge as remnant photons with sufficient radiation to strike the energy to pass film. through the part.  Minimum kVp for Penetration Selections below 60 kVp may not adequately penetrate the part. Inadequate penetration can obscure fine markings of skeletal tissue, such as bones of the knee. Hairline fractures could be missed with insufficient penetration.  Controlling Film Blackening kVp may not always be the best factor for controlling blackening on the film. Once adequate penetration is achieved for the average body part thickness, kVp remains consistent. mA (milliamperes) or mAs (milliampere-seconds) are the primary controlling factors for film blackening. Radiographers must adapt to suboptimal conditions and use alternative technical factors.  Kilovoltage Peak and Milliampereseconds Adjustment of Technique Factors Decrease in kVp can be compensated by an increase in milliampereseconds (mAs), and vice versa. General rule: Decrease in kVp by 15% can be compensated by doubling the mAs. Increase in kVp by 15% can be compensated by decreasing the mAs by half mAs and Penetration mAs increase Increases in mAs cannot do not increase compensate for the penetration of inadequate kVp the x-ray beam. selection.  Understanding Penetration and Overpenetration High kVp does not cause "overpenetration." Penetration means x-ray photons break through the object. X-ray photons that pass through the object have penetrated it. Insufficient energy results in photons being absorbed by the object, leading to underpenetration. Adequate penetration occurs when x-ray photons pass through the object. Excessive blackening on the film indicates "overexposure"  Kilovoltage affects the production of scatter radiation. Kilovoltage and Scatter Radiation As x-ray photons strike the body part, two predominant interactions occur: Compton Photoelectric scattering effect  Compton Scattering Probability of Compton interactions increases with higher kVp. Results in an x-ray photon changing direction. Scattered x-ray photon may strike another object if it has sufficient energy. Photon may exit the body part traveling in a different direction from its original path. Radiation produced by Compton interactions is called scatter  Characteristics of Scatter Radiation Exits the body traveling in many different directions and with different energies. Scatter radiation is dangerous to both the patient and the radiographer. Scatter radiation detracts from film quality. The object being imaged becomes the source of scatter radiation. Radiographers must be shielded from the patient during exposures to protect against scatter radiation. Reduction in Kilovoltage and Compton Interactions Lowering With lower energy kilovoltage reduces x-ray photons, the the probability of photoelectric effect Compton becomes more interactions. prominent.  Photoelectric Effect Occurs when lower energy x-ray photons are present. X-ray photon does not have enough energy to penetrate the body part and is absorbed by body tissue. An atomic interaction produces a change in the electron configuration at the atomic level. Results in a free electron and the production of a low-energy x-ray photon called characteristic radiation.  Characteristics of Photoelectric Effect The patient becomes the source of characteristic radiation. Characteristic radiation has little effect on film quality as it is primarily absorbed by the patient. An increase in photoelectric interactions leads to a higher amount of absorbed radiation by the patient.  Characteristic and Scatter Radiation Produced as x-rays interact with body tissue. Radiographers protect themselves from scatter radiation by standing behind walls or glass containing lead. Lead absorbs x-rays, serving as a protective device. Accessories and techniques to control scatter radiation effects will be discussed in later chapters. Objectives in Radiography Minimize patient exposure. Achieve the highest quality radiograph.  Impact of kVp Selection Affects the amount of scatter and characteristic radiation produced. Influences the amount of radiation absorbed by the patient. Increase kvp, penetration inreases, absorp less, scatter increase Low kvp, penetration dec, aborp inc, photoelec inc Higher Reduce the amount of x-ray photons absorbed by the patient. kVp Increase the amount of scatter radiation exposing the settings: film. Produce scatter radiation with higher energy, increasing the chance it will exit the patient and reach the film. Lower Increase the amount of radiation absorbed by the patient. kVp settings:  Effects of kVp on Scatter Radiation and Contrast Increased kVp: Produces more scatter radiation. Adversely affects film quality by increasing fog on the film. Reduces radiographic contrast. Some scatter is necessary for adequate film blackening. Scatter photons may account for 50% to 80% of the photons exposing the film. Effects of kVp on Scatter Radiation and Contrast Decreased kVp Increases the Increases probability of Results in less radiographic photon fog on the film. contrast. absorption. Film Quality KILOVOLTAGE PEAK (KVP) IS THE CONTROLLING FACTOR OF RADIOGRAPHIC CONTRAST. Milliamperes  MILLIAMPERES Defined as the current flow through the cathode filament during exposure. The amount of mA selected controls the current flow through the cathode filament. Increase in mA: Increases current flow. Raises the temperature of the filament. Increases the number of electrons released (thermionic emission).  Effect of Increased mA Produces a greater number of electrons in the space charge for x-ray production. Increases the number of x-ray photons produced at the anode target. Increase or decrease in mA is a quantitative factor. Does not affect the energy of the photons produced. Purpose is to increase the number of electrons traveling from cathode to anode, thus producing more x-ray photons.  Evaluating Radiographs Proportional relationship between film blackening and mA selected. Doubling mA (e.g., from 200 to 400) doubles the amount of blackening on the film (Fig. 7-10). Halving mA (e.g., from 200 to 100) reduces the blackening by half. Subtle changes in blackening require significant adjustments. A change of at least 30 to 35% is needed to produce a visible change in blackening.  mA is the factor of choice for controlling the amount of blackening on the film. Controlling Can be adjusted without significant Film changes in scatter radiation. Blackening Increasing mA generally increases the amount of exposure absorbed by the patient.  Focal Spot "Blooming" Impact of Milliampere and Kilovoltage on Focal Spot Blooming Effect: An increase in tube current (mA) can cause the actual focal spot size to increase. Significant increases in mA may lead to focal spot blooming. Kilovoltage (kVp): An increase in kVp slightly decreases the actual focal spot size.  Focal Spot Size and Radiograph Quality Focal spot size is crucial for producing high- quality radiographs. Blooming caused by high tube currents can adversely affect radiograph quality. Reducing Focal Spot Blooming: Lower mA settings. Higher kVp settings. EXPOSURE TIME  TIME Exposure Time (S) Sets the length of the time for an exposure. Combined with mA, determines the exposure rate. Formula: Milliamperesx Time = Milliampereseconds (mAs) mA x S = mAs Relationship Between mA and Time mAs is considered a single quantitative factor. Increase in mAs increases the amount of x-ray photons exposing the film. mA and time relationship is inversely proportional: An increase in mA requires a decrease in exposure time to maintain the same exposure value and amount of blackening on the film.  Reciprocity Law Arthur Fuchs States that blackening on the film remains constant as long as the total energy exposing the film is constant. Proper calibration of equipment is necessary for dependable results. Fast film-screen systems (e.g., rare earth) affect results.  Reciprocity Law Limitations The law fails with screen exposures of less than 10 milliseconds and more than 6 or 7 seconds. Considered a quantitative factor. Short exposure times prevent body motion during exposure. Longer exposures increase the risk of motion, detrimental to film quality. DISTANCE  Distance (Focal-Film Distance/FFD/SID) Standardized in diagnostic departments: General radiography: 40 to 42 inches. Thoracic cavity radiography: 72 inches. Distance represents the length of space from the focal spot to the recording medium. Intensity of the x-ray beam is affected by changes in FFD: Similar to visible light, intensity decreases with distance as x- rays spread over a larger area. Intensity and Distance X-rays produced at the focal spot exit the tube through the tube port. Intensity decreases and x-rays diverge from the point of origin as they travel to the film.  Inverse square law The inverse square law describes how the intensity of radiation decreases with increasing distance from the source. Specifically: Inverse Square Law: When the distance from the radiation source is doubled, the intensity of radiation at any given point decreases to one-fourth of its original intensity. This is because the same amount of radiation (photons) is spread out over a larger area as the distance increases.  Inverse square law Inverse Square Law and FFD: When the FFD (or SID - Source- to-Image Distance) is doubled, the same number of x-ray photons is spread over an area four times larger than the original area. This results in the intensity of the x-ray beam at any given point being reduced to one-fourth of its original intensity. PRINCIPLES OF IMAGING SHENNA MARIE G. FORRO, RRT, MSRT JB JAMES A. LAO, RRT, MSRT LABORATORY ACTIVITY: Provide a definition of the keywords listed below pattern 1. 15% rule 13. Distance (FFD) 37. Qualitative factor 50. Time (seconds) 26. kVp-density 2. Anode heel effect 14. Distortion 38. Quality radiograph 51. Tissue density relationship 3. Attenuation 15. Equipment calibration 39. Quantitative factor 52. Underpenetration 27. mAs-distance 4. Blackening on the film 16. film factors relationship 40. Quantum mottle 53. Wavelength 5. Contrast 17. Film graininess 28. mAs-distance 41. Reciprocity law relationship 6. Contrast media 18. Filters Compensating 42. Scatter radiation density filters 29. mA-time relationship 43. Scatter radiation 7. Definition 19. Focal spot blooming 30. Milliamperes 44. Screen conversion 8. Densitometer 20. Foreign body density 31. Milliampereseconds factor 9. Density 21. geometric factor 32. Opaque 45. Sine wave 10. Density formula 22. Inverse relationship 33. Optical density range 46. Structure mottle 11. Density-distance 23. Inverse square law 34. Overexposure 47. subject factors relationship 24. Kilovoltage 35. Pathology 48. Technical factors 12. Direct relationship 25. Kilovoltage wave 36. Penetration 49. Technique