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 1018 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) SID2 2nd exposure

– ----------------------------= -------------------------– mAs (first exposure) SID2 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

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