MOD 8 - Technical Factors

Technical factor adjustment considerations:

  • Tissue density

  • Tissue thickness

  • PT's age, gender

 

Why adjust?

  • due to Dose Optimization (dose just enough for diagnostic purposes)

    • PT age

    • disease process

    • equipment issues

Tissue Density

  • Dense strcts include matter with high atomic numbers  and the probability of photoelectric effect is proportional to the atomic number cubed

  • However, tissue density does not affect compton scatter but rather increased kV slightly decreases it; oppositely, PE are very dependent on beam energy

 

Optimum kV Range

= provided by manufacturers to ensure processing algorithms are able to produce images with the desired level of contrast

 

Tissue Thickness

  • Increased SID and increased TT will increase attenuation

  • X-rays are attenuated at an exponential rate as tissue thickness increases (5cm TT increase = 50% loss of total x-ray beam)

 

General X-ray Factor Adjustments

  • 5cm TT increase = 100% mAs or 15% kVp

  • 115cm to 100 = decrease 1/4 mAs (0.75x)

  • 5:1 Grid = 2x mAs or 1.15x kVp

  • 8:1 Grid = 4x mAs or 1.15x2 kVp

  • 12:1 Grid = 5x mAs or 1.15x2.5 kVp

 

Imaging Systems

DR Systems and IQ

= much more sensitive systems and accommodates under/over exposures

 

Exposure Indices

 

Exposure Index

= a exposure feedback given to ensure the exposure values remain in the ideal range

 

DR System disadvantage related to PT dose

  • The wide latitude which is the ability for the computerized system to correct the image regardless of exposure, often contributing to excess patient exposure

  • Dose Creep: the practice of techs failing to assess the EI and applying excessive exposures since the image is diagnostic

 

Accurate EI calculations require

  • a minimum of 30% receptor exposure

 

Comparing EIs between manufacturers and systems

  • Are not possible as manufacturers determine system EIs based on receptor response calibrations using unique beam qualities

 

Fuji / CR Systems

  • Uses sensitivity numbers (S#) to express the exposure index

  • S# is inversely proportional to the receptor exposure

 

Siemens / DR Systems

  • Uses EXI numbers

  • Directly proportional

 

Other Factors that affect the S#

  • More scatter higher S#

 

Exposure Factor and Patient Dose

 

Measurement of Radiation Exposure

  • Measurement of energy and expressed in Grays (Gy), but typically mGy in radiography since the dose is very small

  • Exposure calculation in air

    • conducted to assess equipment output, to ensure the accuracy of settings as well as to compare equipment

    • mGya to express ionization and excitation, also known as Air KERMA (Kinetic Energy Released in Matter)

 

Exposure

=  meaning refers to this measure of air ionizing energy

 

Dose

=  refers to the energy deposited in tissue, implies a measure of the potential for biological effects

  • dose measurements:

    • absorbed dose: J/kg = Gyt → doesn’t calculate risk increasing factors, just the energy deposited in tissues

    • effective dose: Sv → calculates risk within a population NOT an individual

    • ESE: most common dose estimate in radiography due to its simplicity, but does not consider pt variables such as age, gonadal tissue or DAP therfore the tech must factors these considerations

  • DAP (Dose Area Product)

    = excessive tissue included (inappropriate collimation) increasing pt dose

Technique Optimization for Dose

 How to reduce pt dose while maintain the interaction of photons at the receptor

  • decreasing mA or mAs with a corresponding increase in kV

  • this will optimize the quality and quantity of the x-ray beam so that the detector signal is relatively unchanged yet the overall exposure to the patient is reduced

  • increase kVp by 7.5% and decreases mAs by ¼ HOWEVER, ensure kVp is maintained within the recommended range

Reading EI Numbers

optimal EI#: mid number of range (if range is 125-500, optimal is 250)

  • so if the EI# comes out to be 125, we can double the exposure to reach the 250 optimum and vice versa

Effects of adjusting the Techniques

double mAs →

  • brems and characteristic effects DOUBLED

  • controls quantity of xrays

increase kVp →

  • increase brems interactions

  • controls quality and the quantity of xrays to some point

How to do the Math

1. Determine ↑ or ↓ in technique needed


2. Decide on what technical factor(s) to modify, considering that:

  • a. kVp to remain within recommended range

  • b. High kVp, low mAs techniques reduce patient dose

  • c. SID should remain consistent for the same body part amongst whole department


3. Calculate new exposure value(s) using the following:

  • a. Doubling or halving mAs

  • b. Changing kVp by 15%

  • c. Exposure maintenance formula/direct square law

  • d. Tissue thickness general rule double rad exposure when increase 5cm thickness

A general rule for factors and conversions: Start hard – finish easy
• Start with harder formulas first
• Do your direct square law if you are changing your distance first
• You can then use this new mAs to continue your adjustments
• Then grids: adding, changing or taking away grids
• Use the GCF formula to help you in your next conversion
• Finish easy with your simple multiplications and divisions
• Converting kV up and down
• Changes in body part thickness (or body area with similar tissue type)
• Increase/decrease of mAs

a 15% change in a 50-80 kVp is roughly a 10kVp

a 15% change in a 85-100 kVp is roughly a 15kVp