chapter 10 xray techniques

Chapter 10: Formulating X-ray Techniques

Learning Objectives

• Read and use an x-ray technique chart

• List methods for obtaining and/or creating an x-ray technique chart

• Accurately measure a body part using an x-ray caliper

• Compare fixed kilovolts peak (kVp) technique charts with variable kVp technique charts and state which is preferable

Continued Learning Objectives

• Explain what is meant by optimum kVp and how this value is determined

• Select an appropriate milliamperage station for a given set of circumstances

• Take appropriate steps when the technique chart fails to provide an appropriate exposure

Additional Learning Objectives

• Calculate exposure adjustments for changes in patient/part size

• Determine the technique change required when radiographs are too dark or too light

• Suggest appropriate technique changes for increasing or decreasing the scale of contrast

• Calculate technique changes for variation in source–image receptor distance (SID)

Technique Charts

• Contents of Technique Charts:

  • Type of examination and projection

  • Patient measurement in centimeters (cm)

  • Digital exposure index

  • Source–Image receptor distance (SID)

  • Kilovolts peak (kVp)

  • Milliamperes (mA)

  • Exposure time

  • Grid usage

• Usage of Technique Charts:

  • Technique charts are used to select exposure factors.

  • Each chart is unique to a particular x-ray machine.

  • Technique charts can be generated by:

  • Film manufacturers

  • Technologists

Adjusting an Existing Technique Chart

• If the technique chart does not produce images with appropriate contrast and density, take the following steps:

  • Check the x-ray machine calibration and digital processor systems.

  • Ensure all personnel are using the chart correctly.

  • Make changes to the technique chart only after evaluating all factors that could affect the techniques.

Patient Measurement

• Accurate patient measurement is essential:

  • Sandwich the body part between the lower and upper caliper jaws.

  • Place the jaws directly against the body part without compressing it.

  • Measure the body part in the exposure position.

Fixed and Variable kVp Technique Charts

• Fixed kVp:

  • Optimum kVp for each projection is determined and remains constant.

  • mAs is adjusted according to patient thickness.

  • Results in:

  • Greater exposure latitude

  • Lower image contrast

  • Reduced patient exposure

  • Most frequently used method.

• Variable kVp:

  • mAs remains the same for each body part.

  • Adjustments of 2 to 3 kVp for each cm change in patient size.

  • Results in:

  • Higher image contrast

  • More fine detail in images.

Anatomically Programmed Technique Charts

• A computerized control that allows exposure factors from a conventional technique chart to be retrieved from within the computer.

Optimum kVp

• Definition: Optimum kVp is the kVp that produces appropriate image contrast.

• Determination:

  • Takes a series of phantom images while varying kVp and mAs.

  • Use the 15% rule to adjust kVp:

  • When the kVp is decreased by 15%, double the mAs to maintain density.

Milliamperage Selection

• Understanding mA and time combinations:

  • Different combinations can produce the same mAs.

  • Example calculation:

  • $(300 ext{ mA} imes 0.1 ext{ sec} = 30 ext{ mAs})$

  • $(150 ext{ mA} imes 0.2 ext{ sec} = 30 ext{ mAs})$

  • Factors affecting choice of mA:

  • Tube rating

  • Focal spot size

  • Exposure time

  • Available mA settings.

Technique Adjustment for Density

• Insufficient Density:

  • Increase mAs by 100%.

• Excessive Density:

  • Decrease mAs by 50%.

Technique Adjustment for Contrast

• Too Low Contrast:

  • Decrease kVp by 15%.

  • Increase mAs by 100% to maintain density.

• Too High Contrast:

  • Increase kVp by 15%.

  • Decrease mAs by 50% to maintain density.

Technique Adjustment for SID Variations

• Variations in SID affect density.

• Adjustment Formula:

  • Use the following relationship:

  • $ext{mAs}_1 = ext{Original mAs}$

  • $ext{mAs}_2 = ext{New mAs}$

  • $ext{SID}_1 = ext{Original distance}$

  • $ext{SID}_2 = ext{New distance}$

• Adjust mAs:

  • Increase mAs when SID is increased.

  • Decrease mAs when SID is decreased.

Compensating Filters

• Purpose: Placed between the x-ray tube and patient to achieve consistent radiographic density for body parts with varying thickness, such as the thoracic spine.

Special Considerations

• Pathologic considerations (e.g. tumors, fractures).

• Age: Pediatric patients may require different techniques.

• Body Mass Index (BMI): Obese patients may require specialized adjustments.

Summary

• Technique charts provide sets of exposure factors based on body part and patient size.

• Changes to technique charts should be made after evaluating all influencing factors.

• Accurate measurements for creating technique charts are obtained using calipers.

Summary (Continued)

• Types of Technique Charts: Fixed or variable kVp.

• Optimum kVp must be determined for producing appropriate image contrast.

• mA Selection criteria include tube rating, focal spot size, exposure time, and available mA settings.

Summary (Continued)

• Exposure Time Effects:

  • To increase density, increase mAs when patient size increases or density is insufficient.

  • To decrease density, decrease mAs when patient size decreases or density is excessive.

• Contrast Management:

  • Decrease kVp for shorter scale or higher contrast.

  • Increase kVp for lower or longer scale of contrast.

• SID Effects: Increase mAs when SID is increased to maintain density.

• Consistent Radiographic Density: Compensating filters are employed between the x-ray tube and patient for body parts varying in thickness.