Digital Radiography: Exposure Factor Selection and ALARA

Andrew Woodward M.A., R.T.(R)(CT)(QM) Assistant Professor The University of North Carolina at Chapel Hill School of Medicine Department of Allied Health Sciences: Division of Radiologic Science

DISCLAIMER • Exposure data listed is based upon

simulations performed within a laboratory setting using anthropomorphic phantoms.

• Application of the concepts contained within this presentation should be done under the guidance of a Radiologist and/or Medical Physicist.

Why this topic? • “Dose Creep”

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What do we know?

Film – Intensifying Screen • Self regulating system – limited dynamic range

▪ The receptor response ▴ Screen speed ▴ Film H&D curve ▴ Processing

▪ Selecting appropriate exposure factors assures optimal OD ▴ Over exposed = dark image ▴ Correct exposure = correct OD ▴ Under exposed = light image

Relative Speed

6

Eo

E -

E +

E++

Exposure

Res

pons

e

Dynamic Range

The ability of the receptor to respond to change in exposure.

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Correct exposure

Under exposed -mAs - kVp - RS -Chemistry + Grid ratio + SID

Over exposed +mAs + kVp + RS +Chemistry

- Grid ratio - SID

Film-Screen and the Radiographer • The radiographer immediately recognized

when an exposure error occurred.

• Viewbox and Repeat / Reject Analysis were gate keepers for dose related issues.

Digital Imaging Systems • The receptor and “processing” have changed. • The radiographer’s responsibility to the patient

remains unchanged. ▪ Produce diagnostic images with minimal radiation

exposure.

• As Low As Reasonably Achievable

10

Res

pons

e - O

D

Exposure

Dynamic Range Film - Screen vs. Digital Detector

Digital Systems • CR • DR • DDR • ??????????

• The acronyms are essentially

meaningless.

• Cassette based

• Cassette-less

• Photostimulable Storage Phosphors

• Flat Panel with Thin Film Transistor ▪ Amorphous Selenium

▴ No scintillator required ▪ Amorphous Silicon

▴ Requires a scintillator

• Charged Couple Device ▪ Requires a scintillator

• Complimentary Metal Oxide

Semiconductor ▪ Requires a scintillator

Eo E -

E +

E++

Exposure

Res

pons

e

Dynamic Range Saturation

The ability of the receptor to respond to change in exposure.

Digital Systems • Loss of visual cues

▪ Automatic rescaling ▪ Image processing

• Relationships between radiographic factors and image appearance are decoupled.

▴ “Controlling” factors don’t have same impact on image. mAs ≠ Density / brightness kVp ≠ Contrast

• Application of imaging physics foundation is more critical

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How do you ensure ALARA with the digital receptor you have now?

• Establish protocols with a clearly defined range of exposure indicators for each exam. The exposure indicator values are then “audited” to ensure technologist compliance with ALARA and image quality.

• Consider use of higher kVp levels in comparison to what was used with film-screen.

• Consider use of a SID greater than 40”/101.6cm. ▪ 44”/111.76 cm ▪ 48”/121.92cm

• Replace older grid designs with grids made from materials that attenuate less of the primary beam.

“Defined” Exposure Indicator • The radiologist(s) should be asked to

determine when the noise level present in an image prevents them from providing an accurate interpretation of the image.

• Upon identifying that noise level, the range of exposure indicator values may be established.

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Effect of Automatic Rescaling

Dose and Image Noise are related

67 mR Less noise

34 mR More noise

“The American Association of Physicists in Medicine (AAPM), ………………..….common exposure indices and deviation indices to be implemented across all digital radiography detector types and across all manufacturers and vendors of such equipment. The document explains a method for placing standardized exposure information and content in the DICOM metadata in each image associated with the imaging study. While the details are left to the interested reader [1, 2], it is the manufacturer’s responsibility to calibrate the imaging detector according to a detector-specific procedure, to provide methods to segment pertinent anatomical information in the relevant image region and to generate an exposure index (EI) that is linearly proportional to detector exposure.”

Pediatr Radiol. 2011 May; 41(5): 573–581. Published online 2011 April 14. doi: 10.1007/s00247-010-1954-6

PMCID: PMC3076558 The standardized exposure index for digital radiography: an opportunity for optimization of radiation dose to the pediatric population J. Anthony Seibert 1 and Richard L. Morin2

Higher kVp = Reduced Entrance Skin Exposure

• kVp values for film-screen imaging were chosen based upon a “desired” level of radiographic contrast.

• Image processing associated with digital allows the creation of the image using higher kVp values while letting us maintain a desired image contrast.

kVp • The following images were obtained using AEC and an

anthropomorphic phantom. • The kVp was incrementally increased based upon the

generator controls and the AEC controlled the mAs value used for each exposure.

• There was a 10% difference between the highest and lowest exposure indicator.

• ESE mR values were obtained using an ionization chamber device placed at the beam entrance level to the skin.

60 kVp @ 7.4 mAs 42.5 mR ESE

64.5 kVp @ 5.67 mAs 36.9 mR ESE

18% Reduction in ESE

70 kVp @ 4.29 mAs 32.6 mR ESE

23% Reduction in ESE

81 kVp @ 2.74 mAs 27.3 mR ESE

35% Reduction in ESE

60 kVp @ 7.4 mAs 42.5 mR ESE

81 kVp @ 2.74 mAs 27.3 mR ESE

35% Reduction in ESE

70 kV @ 15.2 mAs 138 mR ESE

81 kV @ 8.94 mAs 108 mR ESE 22% Reduction in ESE

70 kV @ 15.2 mAs 138 mR ESE

81 kV @ 8.94 mAs 108 mR ESE 22% Reduction in ESE

Table 3. Mean effective dose data at varying kVp. Projection kVp Mean effective dose in mSv AP Pelvis 63 0.78 66 (standard technique) 0.6 70 0.51 73 0.42 77 0.32 81 0.28 85 0.25 90 0.22 96 0.27

Radiation Protection Dosimetry (2004), Vol. 108, No. 2, pp. 123---132 DOI: 10.1093/rpd/nch015

Grondin, Y., et al.,(2004) DOSE-REDUCING STRATEGIES IN COMBINATION OFFERS SUBSTANTIAL POTENTIAL BENEFITS TO FEMALES REQUIRING X-RAY EXAMINATION

Exposure Field Size • The judicious use of collimation results in

improved image quality. Why? ▪ Reduces the amount of scatter radiation produced

and therefore less scatter striking the image receptor. • It also reduces the total volume of tissue

irradiated and therefore a reduction in exposure to the patient.

A distinct lack of “collimation”

Collimation

14” x 17” = 238 square inches

14” x 14” = 196 square inches = 18% less

11” x 14” = 154 square inches = 36% less

10” x 12” = 120 square inches = 50% less

• Somatic* ▪ Skin ▪ Bone Marrow (Red)

▴ Skull ▴ Sternum ▴ Scapula ▴ Pelvis ▴ Vertebrae ▴ Epiphyseal ends of long bones

SID and ESE • Increasing SID could be used to reduce the

entrance skin exposure. • Potential to increase spatial resolution. • Possible issue with grid cut-off if focal range of

grid is not matched.

81 kVp @ 8.94 mAs 40” SID 108 mR ESE

81 kVp @ 11.0 mAs 48” SID 87.9 mR ESE 28% Reduction in ESE

Table 4. Mean effective dose data at varying FFDs. Projection FFD in cm Mean effective dose in mSv AP Pelvis

100 (standard technique) 1.15 110 1.08 120 0.85 130 0.81

Grondin, Y., et al.,(2004) DOSE-REDUCING STRATEGIES IN COMBINATION OFFERS SUBSTANTIAL POTENTIAL BENEFITS TO FEMALES REQUIRING X-RAY EXAMINATION

Radiation Protection Dosimetry (2004), Vol. 108, No. 2, pp. 123---132 DOI: 10.1093/rpd/nch015

Filtration • The purpose of adding addition filtration is to

increase the overall energy of the beam with the result being a decrease in patient entrance skin exposure.

Copper Filtration • The literature documents the potential for a

dose reduction of 15 to 35% with the addition of copper filtration.

Anti - scatter grid A core of very thin lead alloy foil strips, separated by a radiolucent inter-space material of aluminum, cellulose fiber, or carbon fiber. It is encased in a sturdy, but radiolucent material. Possibly aluminum alloy or carbon fiber.

Protective wrapper

Interspace Lead foil strip

Grid Construction

• Newer fiber inter-space materials have the potential to reduce exposure 10 to 40%.

Thank you