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Radiographic exposure and image quality
 

Radiographic exposure and image quality

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    Radiographic exposure and image quality Radiographic exposure and image quality Presentation Transcript

    • Radiographic Exposure
      • Exposure Factors influence and determine the quantity and quality of the x-radiation to which the patient is exposed.
      • Radiation quantity refers to the radiation intensity referred to as mR or mR/ mAs.
      • Radiation Quality refers to the beam penetrability and measured in HVL.
    • Radiographic Exposure
      • The radiographic exposure factors are under the control of the operator except for those fixed by the design of the x-ray machine.
      • There are two choices for focal spot.
      • With the exception of compensating filters, added filtration is fixed.
      • The type of high voltage power is also fixed.
    • Exposure Factors Controlled by the Operator
      • kVp
      • mA times Exposure Time = mAs
      • Determines the quality and quantity of the exposure
      • SID, Focal Spot and Filtration are secondary factors
    • kVp
      • As we have discussed in the laboratory, kVp controls radiographic contrast.
      • kVp determines the ability for the beam to penetrate the tissue.
      • kVp has more effect than any other factor on image receptor exposure because it affects beam quality.
    • kVp
      • To a lesser extent it also influences the beam quantity.
      • As we increase kVp, more of the beam penetrates the tissue with higher energy so they interact more by the Compton effect.
      • This produces more scatter radiation which increases image noise and reduces contrast.
    • kVp
      • 50 kV 79% is photoelectric, 21% Compton, < 1% no interaction
      • 80 kVp 46% is photoelectric, 52% Compton 2% no interaction
      • 110 kVp 23% photoelectric, 70% Compton, 7% no interaction
      • As no interaction increases, less exposure is needed to produce the image so patient exposure is decreased.
    • mA
      • 1 Ampere = 1 C/s = 6.3 x 10 18 electrons/ second.
      • The mA selected for the exposure determines the number of x-rays produced.
      • The number of x-rays are directly proportional to the mA assuming a fixed exposure time.
      • 100 mA produced half the x-ray that 200 mA would produce.
    • mA
      • Patient dose is also directly proportional to the mA with a fixed exposure time.
      • A change in mA does not affect kinetic energy of the electrons therefore only the quantity is changed.
    • mA
      • Many x-ray machines are identified by the maximum mA or mAs available.
      • A MP 500 has a maximum mAs of 500 mAs.
      • A Universal 325 has a maximum mA of 300 and maximum kVp of 125
    • mA
      • More expensive three phase machines will have a higher maximum mA.
      • A General Electric MST 1050 would have 1000 mA and 150 kVp.
    • Exposure Time
      • The exposure time is generally always kept as short as possible.
      • This is not to reduce patient exposure but to minimize motion blur resulting from patient movement.
      • This is a much greater problem with weight bearing radiography.
    • Exposure Time
      • Older machine express time as a fraction.
      • Newer machines express exposure time as milliseconds (ms)
      • It is easy to identify the type of high voltage generation by looking at the shortest exposure time.
    • Exposure Time
      • Single phase half wave rectified fasted exposure time is 1/60 second 17 ms.
      • Single phase full wave rectified fastest exposure time is 1/120 second or 8 ms
      • Three phase and high frequency can provide exposure time down to 1 ms.
    • mAs
      • mA and exposure time is usually combined and used as one factor expressed as mAs.
      • mAs controls radiation quantity, optical density and patient dose.
      • mAs determine the number of x-rays in the beam and therefore radiation quantity.
      • mAs does not influence radiation quality.
    • mAs
      • Any combination of mA and time that will give the same mAs should provide the same optical density on the film. This is referred to as the reciprocity law .
      • As noted earlier for screen film radiography, 1 ms exposure and exposure longer than 1 seconds do not follow this rule.
    • mAs
      • On many modern machines, only mAs can be selected. The machine automatically gives the operator the highest mA and shortest exposure time.
      • The operator may be able to select mA by what is referred to as Power level.
    • mAs
      • mAs is one way to measure electrostatic charge. It determines the total number of electrons.
      • Only the quantity of the photons are affected by changes in the mAs.
      • Patient dose is therefore a function of mAs.
    • mAs
      • If we know the mR/mAs, multiply that figure times the mAs. or
      • If we know the mR for a given exposure at a given kVp, we can divide the exposure by the mAs to get the mR/ mAs.
      • To compute exposure we need to know what the mR/mAs is for the kVp used and the SID.
    • Distance
      • Distance affects the exposure of the image receptor according to the inverse square law.
      • Distance affects the intensity of the x-ray beam at the film but has no effect on radiation quality.
    • Inverse Square Law
        • mAs (second exposure) SID 2 2nd exposure
        • ----------------------------= -------------------------
        • mAs (first exposure) SID 2 1st exposure
    • Distance
      • The most common source to image distances are 40” (100 cm) and 72”(182 cm)
      • Since SID does not impact the quality of the beam, adjustments to the technical factors are made with the mAs.
      • To go from 40” to 72” increase the mAs 3.5 time.
    • Distance
      • Increasing the distance will impact the geometric properties of the beam.
      • Increased SID reduces magnification distortion and focal spot blur.
      • With the need to increase the mAs 3.5 times for the 72” SID, tube loading becomes a concern.
    • Distance
      • 72” SID is used for Chest radiography and the lateral cervical spine to reduce magnification.
      • 72” SID used for the full spine to get a 36” beam.
    • Imaging System Characteristics
      • Operator has limited control.
      • The following will impact the technical factors based upon the type of machine.
        • Focal Spot Size
        • Filtration
        • High-voltage Generation
    • Focal Spot Size
      • Most machines limited to two focal spot sizes.
      • Common office focal spots are 1.0 mm for the small and 2.0 mm for large.
      • Highly detailed radiography such as mammography use micro-focus tubes with 0.1 mm and 0.3 mm focal spot sizes.
    • Focal Spot Size
      • The focal spot size limits the tube’s capacity to produce x-rays. The electrons and resulting heat are placed on a smaller portion of the x-ray tube.
      • The mA is therefore limited for the small focal spot. This results in longer exposure times with greater chance of patient movement.
    • Focal Spot Size
      • For single phase machines, the small focal spot use is limited to extremities and the cervical spine.
      • With high frequency, most views can be done on the small focal spot except for larger patient and ones that cannot hold still.
      • My limit is exposure times less than 1/2 s.
    • Focal Spot Size
      • If the mA is properly calibrated, the focal spot will have no impact on the quantity or quality of the beam.
    • Filtration
      • All x-ray beams are affected by the filtration of the tube. The tube housing provides about 0.5 mm of filtration.
      • Additional filtration is added in the collimator to meet the 2.5 mm of aluminum minimum filtration required by law.
      • 2.5 mm is required for 70 kVp.
    • Filtration
      • 3.0 mm is required for at 100 kVp.
      • 3.2 mm is required for operations at 120 kVp.
      • Most machines now are capable of over 100 kVp operation.
      • We have no control on these filters.
    • Filtration
      • Chiropractic radiography is a leader in the use of compensating filters. We have total control over compensating filtration.
      • In areas of the body with high subject contrast or wide differences in density, compensating films improve image quality and reduce patient exposure.
    • High-voltage Generation
      • You will determine the type of high-voltage generation when you purchase your x-ray machine.
      • The type of generator will determine the efficiency of the generator or the amount of ripple in the wave form.
      • Single phase has 100% ripple.
    • Three Phase Generation
      • Three phase has a 14% so it is significant improvement in efficiency increasing both quality and quantity of the beam.
      • More x-rays per mAs with higher energy.
      • Cost to provide 3 phase power is very high so not practical in office.
    • High Frequency Generation
      • Virtually no ripple ( less than 1%.)
      • Inexpensive and can use normal incoming power.
      • Provides significant reduction is mAs or kVp compared to single phase. Reduction of mAs by 50% compared to single phase techniques.
    • Chapter 19 Radiographic Quality
      • Radiographic Quality refers to the fidelity with which the anatomic structures being examined are images on the film.
      • Three main factors:
        • Film Factors
        • Geometric Factors
        • Subject Factors
    • Radiographic Quality
      • Characteristic of radiographic quality:
        • Spatial Resolution (Recorded Detail)
        • Contrast Resolution (Visibility of Detail)
        • Noise (Visibility of Detail)
        • Artifacts
    • Spatial Resolution
      • Spatial Resolution is the ability to image small structures that have high subject contrast such as bone-soft tissue interface.
      • When all of the factors are correct, conventional radiography has excellent spatial resolution.
    • Contrast Resolution
      • Contrast resolution is the ability to distinguish structures with similar subject contrast such as liver-spleen, fat-muscle.
      • Computed tomography and MRI have excellent contrast resolution. Convention radiology is fair to poor.
    • Noise
      • Noise is an undesirable fluctuation in optical density of the image. Two major types:
        • Film Graininess- no control over
        • Quantum Mottle- some control over
    • Film Graininess
      • Film graininess refers to the distribution in size and space of the silver halide grains in the film emulsion.
      • Similar to photographic film. 400 ASA film is more graininess than 100 ASA film.
      • Similar to structure mottle that refers to the size and shape of the phosphors in the intensifying screens.
    • Quantum Mottle
      • Quantum mottle refers to the random nature of how the x-rays interact with the image receptor.
      • It is the primary form of radiographic noise.
      • The use of high mAs and low kVp reduced quantum mottle.
    • Quantum Mottle
      • Very fast screens have higher quantum mottle because it takes fewer x-rays to make the image.
    • Speed
      • Resolution and noise are intimately connected with speed.
      • While the speed of the images receptor is not apparent on the image, it influences both resolution and noise.
    • Radiographic Quality Rules
      • Fast Image receptors have high noise and low spatial and contrast resolution.
      • High spatial and contrast resolution require low noise and slow image receptors.
      • Low noise accompanies slow image receptors with high spatial and contrast resolution.
    • Film Factors of Quality
      • Characteristic curve
        • Density
        • Contrast
        • Latitude
      • Processing
        • Time
        • Temperature
    • Sensitometry
      • Sensitometry is the study of the relationship between the intensity of exposure of the film and the blackness after the film is processed.
      • Unexposed film is clear with a blue tint after processing.
      • Exposed film is black after processing.
    • Sensitometry
      • Two principles involved.
        • Exposure of the film
        • Amount of light transmitted through the processed film of optical density.
      • Used to describe the relationship of radiation exposure and blackness or density on the film.
    • Characteristic Curve
      • This relationship is called the characteristic curve or H & D curve of the film.
      • H & D stands for Hurter and Driffield.
    • Parts of the Characteristic Curve
      • Toe and shoulder where large changes in exposure results in small changes in OD.
      • Very high and very low variations of exposure make very small changes in density.
    • Parts of the Characteristic Curve
      • The straight line or intermediate area is where very small changes in exposure results in large changes in density.
      • This is the important part of the curve in radiography.
    • Log Relative Exposure (LRE)
      • X-ray films responds to a wide range of exposure from 1 mR to 1000 mR.
      • Exposure is represented on logarithmic manner.
    • Optical Density Range
      • The optical density range is from 0.0 for no density to 4.0 for absolute black.
      • Useful range in general radiography is from 0.5 to 2.25.
      • Image range is 0.5 to 1.25 OD
    • Base fog or base density
      • The tint of the base of the film and the inadvertent exposure of the during processing.
      • Range is from 0.1 to 0.3. Should be never above 0.30 most is .21 OD
    • Items that Impact Base Fog
      • Film storage
      • Film exposure to wrong spectrum of light or light intensity.
      • Chemical contamination.
      • Improper processing.
      • High Base fog levels reduce contrast.
    • Contrast
      • Radiographic Contrast is the combined result of image receptor contrast and subject contrast .
      • Image receptor contrast refers to the contrast inherent in the film and influenced by the processing of the film.
    • Contrast
      • Subject contrast is determined by the size, shape and x-ray attenuating characteristics of the subject being examined and the energy (kVp) of the x-ray beam.
    • Image Receptor Contrast
      • Inherent to the film and screen combination but is influenced by:
        • Range of Optical Density
        • Film Processing Technique
      • Film type is determined by the type of intensifying screens used but many dealers sell off brands of film.
    • Image Receptor Contrast
      • The slope of the straight line portion of the H & D curve is the receptor contrast.
      • The average gradient is a straight line drawn between the densities of 0.25 and 2.00 + base fog.
    • Average Gradient
      • The average gradient is a straight line drawn between 0.25 OD and 2.0 OD above base plus fog.
      • This is the normal range of density in a radiograph
    • Speed
      • Speed is the ability of the receptor to respond to low x-ray exposure.
      • The H & D curse is useful in comparing speed when selecting film or screens.
    • Speed
      • A relative number of 100 given to Par Speed Calcium Tungstate Screens.
      • High Speed Calcium Tungstate has a speed of 200. Half of the exposure is needed to produce the same image.
      • Rare earth screen film combinations range is speed from 80 to 1600.
    • Speed
      • By knowing the Speed, sometimes referred to as the Relative Speed Value, it is easy to convert the technical factors for one speed to another speed.
    • LATITUDE
      • Latitude can be observed on the H & D curve.
      • Latitude refers to the range of exposure that will produce a diagnostic range OD.
    • Latitude
      • Latitude and Contrast are inversely proportional.
      • Wide latitude has a wide gray scale or low contrast. (B)
      • Narrow latitude has a short scale or high contrast. (A)
    • Latitude
      • Latitude is designed into some screen and film combinations. With wide latitude, the error factor in technique is wider.
      • Latitude can also be impacted by the technical factors.
    • Film Processing
      • Radiographic Quality is impacted by film processing parameters.
      • The developer must be at the proper concentration and at the correct temperature.
    • Film Processing
      • The film must also spend the correct amount of time in the developer.
      • This is the time & temperature relationship .
    • Processing
      • Speed and base fog increase with the temperature.
      • Contrast will increase to a point and then drop with the base fog increase.
      • Manufactures set processing parameters to optimize speed, contrast and low base fog.
    • Processing
      • In 9th Quarter we will discuss processor quality control in detail.
    • End of Lecture Return to Lecture Index Return to Physics Homepage