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Radiation Exposure Requirements for Digital Radiography

Digital Imaging Summit and Workshop for Veterinary Radiologists San Luis Obispo, California May 29-31, 2008

J.A. Seibert, Ph.D. Department of Radiology UC Davis Medical Center Sacramento, California

Pick a speed, contrast and latitude according to the imaging task

4 Optical Density 3 2 1 S=800 S=400 S=200 S=100

Analog Screen-Film

Noisy Less detail Lower dose

Smooth More detail

1 10 100 1000 Air Kerma (Gy)

0.1

Higher dose

Characteristic Curve:

Response of screen/film and CR / DR

Useless

4

Film Optical Density

3 2 1 0 0.01

Overexposed

1,000 100

Correctly exposed Underexposed Useless

10 1

0.1

1

10

100

Exposure, mR

Relative intensity

Film-screen (400 speed)

10,000 CR plate

Data conversion

Exposure into digital number

Relative PSL

102 101 100 10-1

- gain + gain

Grayscale transformation

Input to output digital number 1,000

Output digital number 800 600 400 200 200 600 1,000 Raw Input digital number 0

511 1023 10-1 100 101 102 103 0 Raw Digital Output Exposure input

Histogram

minmin max max

Overexposure

1. Find the signal

2. Scale to range

3. Create film look-alike

Screen / Film

CR

8 mAs

16 mAs

32 mAs

8 mAs

16 mAs

32 mAs

Recognizing bad images

· Screen-film: under and over exposures easily recognized by appearance

­ Repeat with higher or lower technique based on OD

· Digital Radiography: under and over exposures are not so easily recognized

­ Adequate images with a much wider exposure range ­ Excellent dynamic range may have downsides

Screen-film versus digital response

Contrast Limited Response

S/F

CR

SNR Limited Response

Dose Contrast Detail

0.5 X

1X

2.5 X

5X

Digital radiography examples

Underexposed

Overexposed?

What determines necessary dose?

· Required SNR / CNR of examination · Patient thickness (pediatric vs. adult!) · X-ray acquisition techniques (kVp, mAs, distance) · Detector absorption and conversion efficiency · Detector electronic and stationary noise · Detective Quantum Efficiency (DQE) · Antiscatter grid, air gap · Pre and post processing algorithms

How much dose is necessary?

Contrast Detail Phantom

Limiting Resolution

Lower Dose Higher Dose Limiting Contrast Noiseless Noisy Contrast-Detail

Dependent on patient, type of examination, type of detector....

SNR: quantum statistics plus other noise!

Good flat-field pre-processing

Inadequate flat-field pre-processing

object in

characterizing image spatial resolution (the signal)

imaging system

Line Spread Function (LSF)

image out

input signal amplitude

output signal amplitude

f = 1.5 cycles / mm

large objects

Ideal Performance

smaller objects

characterizing image spatial resolution (signal)

LSF(x)

characterizing image noise RMS noise ()

2

characterizing image noise Noise Power Spectrum: NPS(f)

Fourier Transform

Noise Power Spectrum: NPS(f)

large objects

small objects

2

contrast resolution (NPS)

spatial resolution (MTF)

Detective quantum efficiency (DQE)

DQE is a measure of information transfer efficiency

Detective Quantum Efficiency

For general radiography devices

· High DQE doesn't guarantee good image quality · No substitute for appropriate radiographic technique and proper image processing · Acquisition technique kV, mAs, SID, filters, grid · Using similar technique and grid, constant SNR requires dose proportional to DQE-1

0.8 CsI - TFT 0.6 DQE( f ) a-Se - TFT 0.4 CCD CR "dual-side" S/F 0.2 CR Conventional

0.0 0.0 0.5 1.0 1.5 2.0 2.5 Spatial Frequency (cycles/mm)

Why is incident detector exposure important?

· Is proportional to the image SNR (for given DQE) · Signal to Noise Ratio "image quality"

· Is indirectly related to patient exposure · Image appearance and detector exposure are not linked as with screen-film receptors · Exposure indicators can assist the technologist in identifying appropriate "equivalent speed"

How much dose is used? ....Exposure Index for digital radiography

· Estimated incident exposure to the detector; gives an indication of detector speed class · Gives a manufacturer-dependent value to be used as feedback for "verification" of proper technique for patient size and exam requirements

­ ­ ­ ­ Fuji "S" number.... S 200 / Exposure (mR) Agfa: LgM value... LgM = 2.22 + log E + log(SC/200) Kodak: Exposure Index... EI 1000×log (Exposure, mR)+ 2000 Many other manufacturers with distinct methods

· Inconsistencies in determining exposure index values as well as non-standard EI algorithms are current problems

Sensitivity number, S

(Fuji CR)

· Estimate of the incident exposure on the IP · Comparable to screen-film "speed" · Based on amplification required to map median histogram value to 511 (0 to 1023) · Dependent on histogram shape, segmentation, and examination selected

"L" value

1023 Q1 Output Digital Number

Fuji Example

Image Histogram

511

200 S exposure (mR)

Q2 0

Median value of "useful range" is mapped to middle of output range (511) via +/- applied gain

Median value

S2

SK

S1

Log stimulated luminescence of IP 2.3 0.3 4.3 0.01 mR 100 mR 1 mR

How do you get the data?

· System dependent · In some cases offered by vendor with optional QC package · Can export data into excel spreadsheet

What can you do with the data?

· Illustrate usage and exposure trends · Determine detector speed class values · Generate data for technologist feedback · Document CQI process for digital radiography · Use as a tool for education and training

Use of exposure index as feedback

Adult portable chest exposures

Target exposure range

600 500

38.3%

53.9%

7.8%

600 500

23.1%

73.5%

3.4%

#exams

400 300 200 100 300 200 400 100 500

Q1 Q2

400 300 200 100 <50 500 400 300 100

Q3 Q4

Equivalent speed (S #) Low Incident Exposure First half, 1994: 4572 exams High Low

Equivalent speed (S #) Incident Exposure High

Second half, 1994: 4661 exams

<50

200

0

0

Guidelines for QC based on Exposure

typical adult chest exam at UCDMC, Fuji CR

System "speed" Exposure Indication

· >1000 · 600 - 1000 · 300 - 600 · 150 - 300 · 75 -150 · 50 - 74 · <50

<0.2 mR 0.3-0.2 mR 1.0-0.3 mR 1.3-1.0 mR 2.7-1.3 mR 4.0-2.7 mR >4.0 mR

· Underexposed: repeat · Underexposed: QC exception · Underexposed: QC review · Acceptable range · Overexposed: QC review · Overexposed: QC exception · Overexposed: repeat

Indications from Chuck Willis Ph.D. M.D. Anderson Cancer Center

Dose Considerations

· Exposure Creep

­ Underexposure: noise higher, poor visibility ­ Overexposure: noise lower, good quality, high patient dose ­ Radiologists complain about noise, not overexposure ­ Technologists learn to avoid criticism, and use higher techniques; exposures creep up

April 1 - 17, 1996 Adult Portable Chest

Number of examinations

180 160 140 120 100 80 60 40 20 0

"Exposure Creep"

Grid technique without a grid

>1 00 60 0 069 50 9 054 40 9 044 35 9 037 30 4 03 25 24 027 20 4 022 15 4 017 10 4 012 4 50 -7 4

Sensitivity number

Exposure Index variability

· X-ray beam spectrum (kV and filtration)

­ "EI" number varies with beam hardness (calibration required)

· Collimation (beware of "electronic" collimation) · Delay time from exposure to readout

­ Causes significant variation in EI number due to spontaneous fluorescence ­ Effect is greatest within first 5 minutes

Exposure Index variability

· Image processing algorithms

­ Histogram shape specific ..... a wrist histogram a lot different than a chest!! ­ Segmentation can fail due to inappropriate algorithm

· Ability to change index number interactively?!

­ Defeats purpose of Exposure Index ­ Changing value can compromise image quality ­ Most low S numbers (overexposure) are shifted higher, which saturates signals in output image

Why not change "S" number?

S=42

5x overexposure! But image appears OK! and is OK... But.....

Changing index shifts histogram!

"L" value

1023 Output Digital Number Q1

S=167 S=42

511

Q2 0 S2 SK S1 Saturated!

Median value

Exposure OK?..... NO!! Database is "OK"

What the exposure index doesn't directly tell you......

· Patient dose

­ Dose is dependent on patient size, attenuation properties, technique & setup

· Image quality

­ Quality is mainly based upon SNR and patient positioning; a target exposure index value does not guarantee IQ

Detector exposure index:

Do's and Don'ts

· Do a periodic review of exposure logs · Do look for outliers and repeat offenders · Do use as a feedback tool · Do ensure proper detector/reader calibration · Do NOT place too much importance on value · Do NOT allow arbitrary value adjustment

Standardization Effort in progress

· American Association of Physicists in Medicine Task Group 116 · Collaborative effort

­ ­ ­ ­ Manufacturers Vendors Physicists IEC (international standards organization)

· Develop common "Exposure Indices" across detectors and manufacturers · Provide means for placing in DICOM header

Dose Considerations

Approximate Exposure Indicator Values vs. Receptor Exposure

Manufacturer Canon (Brightness =16, contrast = 10 IDC (ST = 200) Philips Fuji, Konica Kodak (CR, STD) Siemens Symbol REX F# EI S EI EI 50 Gy 50 -1 200 400 1700 500 100 Gy 100 0 100 200 2000 1000 200 Gy 200 1 50 100 2300 2000

Dose Considerations

Approximate Exposure Indicator Values vs. Receptor Exposure

Manufacturer Canon (Brightness =16, contrast = 10 IDC (ST = 200) Philips Fuji, Konica Kodak (CR, STD) Siemens Symbol REX f# EI S EI EI 50 Gy 50 -1 200 400 1700 500 100 Gy 100 0 100 200 2000 1000 200 Gy 200 1 50 100 2300 2000

AAPM TG116

· Recommendation for standard detector exposure index for all digital radiography · Index (KIND) is the air Kerma that the detector should have received under standard beam conditions for the same raw pixel value · Standard beam

­ 70 ± 3.5 kV ­ 0.5 mm Cu + [0 ­ 3] mm Al filtration ­ 6.8 mm Al HVL

· Calculated for every image from the median pixel value in the VOI recognized by the system

"Relative" Index

fREL = 10 × log10 {KIND / KTGT (b,v)}

· KTGT(b,v) is a table of target KIND values stored by body part (b) and view (v) · fREL = 0 is a perfect exposure fREL = ±1 means exposure was high or low by about 28% (one density or mAs step) · KTGT tables to be customized for each site · Both indices saved in the DICOM header (tag is to be determined) · Both change with VOI modification by the tech

Dose Considerations

Both indices change with VOI modification by the technologist

KIND and fREL calculated from this pixel value

KIND = KTGT fREL = 0.0

Number of pixels

Values of Interest

Pixel Value

Dose Considerations

VOI recognition algorithm fails · Gonadal shields, prosthetics, etc. · False fREL reported

Number of pixels

KIND = KTGT KIND ffREL= -1.3 REL = 0.0

KIND andKIND and fREL fREL incorrectly calculated from from this pixel valuevalue this pixel

Correct Values of Interest Pixel Value Incorrect Values of Interest

Dose Considerations

Tech returns VOI to proper position manually

KIND andKIND and fREL fREL incorrectly calculated from from this pixel valuevalue this pixel

KTGT KIND = KTGT -1.3 fREL = 0.0

Number of pixels

Correct Values of Interest Pixel Value Incorrect Values of Interest

Rules for repeats (AAPM TG116)

· Target is -2.0 to +2.0 · Check for noise. Consult with radiologist on need for repeat if KIND is 63% of target (fREL -2) · Investigate cause (do not repeat) if KIND is between 160% and 200% of target (+2.0 fREL +3.0) · Consult with radiologist (check for saturation) on need for repat and counsel of technologist if KIND is 200% of target (fREL +3.0)

Exposure Index monitoring

· Collect EI for every image and analyze

­ ­ ­ ­ ­ ­ By technologist Technique factors X-ray system Plate scanning unit (CR) Processing unit (CR) Anatomical view

· Longitudinal studies

­ ­ ­ Track performance over time Mean and Standard Deviation Watch for trends upward (Creep) and incr. S.D.

Exposure Index Monitoring

· Many systems have no index at all · Many systems do not log the indices · For those that do

­ Multiple machines with logs are not accessible over the network ­ Inconsistent file formats

· Must be performed manually

­ ­ ­ Paper logs Manual electronic log download Manual conversion to common file format

Impractical, and too much work!

What should the manufacturers provide?

· Standardized method to report incident exposure · A method to visibly display the exposure estimate · Audible alert when an "out of range" situation occurs · Implement an exposure "target", specific to each exam · Interface to x-ray systems to get kVp, mA, time data for determination of entrance exposure (usually standard for DR and automatic CR)... & adopt DICOM DX object

CR / DR: Radiation Exposure

· CR & DR systems exhibit wide dynamic range · CR & DR provide ability to reduce retakes due to inappropriate radiographic techniques · The ability to lower radiation dose depends on detector efficiency (DQE) and required SNR · CR system ~200 speed for general applications DR system speed depends on DQE (efficiency) · Exposure indices MUST be monitored

Patient Exposure Reduction

· Employ lowest exposure for diagnostic images

­ Highest "speed class" practical ­ Accurate technique charts and calibrated AEC

· Limit volume irradiated

­

Collimation ­ Effective dose Higher kVp, additional filtration, grid optimization 44" ­ 48" SID Potential histogram analysis error

· Use appropriate techniques

­

· Set up longer distance (within grid focal range)

­

· Employ patient lead shielding when practical

­

Increase kVp within reasonable limits

Technical Factors vs. ESE

AP knee - 8:1 / 103 Al interspace grid

mAs / kVp

15 / 75 9 / 80 7 / 85

ESE

72 61 50

% Change

- 15 % - 30 %

ESE ­ Entrance Skin Exposure in mR

Added Filtration with ESE Reduction

0.10 mm Cu 0.30 mm Cu 0.050 mm Mo Sandwich 0.030 mm Cu 1.0 mm Al - 25% - 35%

- 35%

Note: Increased tube loading, and mAs / kV settings "appear" high

Tradeoffs of Image Quality and Dose

· Subject Contrast

­ ­ ­ Low kVp.... High SNR .. Use Grid.... dose dose dose dose dose dose dose dose dose

· Patient motion

­ High kVp Low mAs ­ Positioning aids, beam attn.

· Detector

­ ­ ­ ­ High DQE CsI phosphor Flat-field Calibration? Cost (CR-$; DR-$$) Patient Positioning

· Spatial Resolution

­ Small pixel ... ­ Low fill-factor..

· Technique

­ ­ ­ ­ Tube filtration.. kVp .... FOV ... High DQE ....

· Processing

­ Optimization for exam ­ Contrast enhancement ­ Frequency enhancement

Conclusions: CR/DR and Exposure

· Flexibility is a double-edged sword with CR/DR

­ reduced retakes ­ variable speed (tailor exposure to exam).... but ­ more difficult to correctly use

· All digital systems should log and monitor detector exposure indices as part of the QC program · Good image quality and appropriate SNR are more important than low radiation dose · Continuous retraining and feedback are necessary!

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