Quality Control in Diagnostic Radiology

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• Quality Control in Diagnostic Radiology

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Factors driving Q.C.Why do we do it?

• Legal Requirements
• Accreditation
• JCAHO
• ACR
• Clinical improvement
• equipment performance
• image quality

Medical Physicists at Work
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Q.C. Goals

• Minimize dose to
• patients
• staff
• Optimize image quality
• Establish baselines
• More on this in a moment

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Why is Q.C. Important?
Without a QC program the only way to identify
problems is on patient images. And some
problems dont show up on images.
Yeah, thats what I always say.
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QC can detect

• Malfunctions
• Unpredictability
• may be hard to isolate clinically
• Inefficient use of Radiation
• high fluoroscopic outputs
• Radiation not reaching receptor
• inadequate filtration
• oversized collimation

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Goals of a Q.C. Program

• Obtain acceptable image with least possible
  radiation exposure to
• patients
• staff
• Attempt to identify problems before they appear
  on patient films
• without QC problems only detected on patient films

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Acceptable Image

• Image containing information required by
  radiologist for correct interpretation
• goal minimize exposure while maintaining
  acceptability
• high exposure films often have excellent
  appearance
• cardboard cassettes

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Q.C. Baselines

• Baselines
• quantitative data on equipment obtained during
  normal operations
• Baselines useful for troubleshooting
• isolating problem component, for example
• generator
• processor
• Allows efficient use of engineering / repair
  personnel

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X-Ray Quality Control

• Processor Sensitometry
• Filtration
• Focal Spot Size
• Collimation
• Maximum Fluoroscopic Output
• Calibration Verification
• Phototimer Performance

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Photographic Density

• Optical density
• measure of film blackness or opacity
  IoD log --- ItwhereD
  densityIo light incident on filmIt light
  transmitted by film
• D1 gt 1/10 of light transmitted
• D2 gt 1/100 of light transmitted
• D3 gt 1/1000 of light transmitted

Io
Film
It
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H D Curve

• Shows relationship between radiation striking
  film/screen and optical density
• Function of
• film
• screen
• kVp
• developer temp
• chemistry condition

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Sensitometry Parameters

• Base Fog
• O.D. of unexposed portion of film

Base Fog
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Sensitometry Parameters

• Speed
• O.D. of selected step about 1.0 above base fog

O.D.
H D Curve
1.0
Speed
Log Relative Exposure(step)
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Sensitometry Parameters

• Contrast
• slope of H D curve
• Difference between optical densities of two
  selected steps
• Higher step O.D. - Lower step O.D.

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Temperature Problems

• developer (critical)
• affects
• contrast
• speed
• base fog
• must be controlled within about /- .5o
• wash water
• on older model processors, wash water was
  pre-heat for developer
• dryer

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Film Processor Sensitometry

• Sensitometer
• flashes calibrated step wedge on film
• Densitometer
• reads optical density (O.D.) of selected steps
• Plot results
• Notes
• Use Control Film
• Perform before first clinical useof processor

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Why is Filtration Important?

• Tube emits spectrum of x-ray energies
• Filtration preferentially attenuates low energy
  photons
• low energy photons expose patients
• do not contribute to image
• low penetration

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Half Value Layer (HVL)

• We dont measure filtration
• We measure HVL
• HVL amount of absorber that reduces beam
  intensity by exactly 50

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Half Value Layer

• Depends upon
• kVp
• waveform (single/three phase)
• inherent filtration
• Minimum HVL regulated by law
• Maximum HVL regulated only in mammography

Georgia State Rules Regulations for X-Ray
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Radiographic HVL Setup
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Checking HVL Compliance(Radiographic)

• How much aluminum must be placed in beam to
  reduce intensity by exactly 50?

90 kVp Measurements 2.5 mm Al minimum HVL
filter mR (mm Al) -------------------
0 250 2.5 133
filter mR (mm Al) -------------------
0 250 2.5 125
filter mR (mm Al) -------------------
0 250 2.5 111
Not OK! Must remove Al to reduce beam to exactly
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OK! Must add Al to reduce beam to exactly 50
Acceptable HVL gt 2.5 mm
Marginal HVL 2.5 mm
Unacceptable HVL lt 2.5 mm
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Checking HVL Compliance(Radiographic)

• Is this machine legal?
• 2.5 mm Al minimum filtration at 90 kVp

90 kVp Measurements
filter mR (mm Al) -------------------
0 450 2.5 205
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Fluoroscopic HVL Setup
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Fluoroscopic HVL

• Set desired kilovoltage manually
• measure exposure rates instead of exposure
• Move absorbers into beam as needed

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Focal Spot Size

• We measure apparent focal spot
• Trade-off
• smaller spot reduces geometric unsharpness
• larger spot improves heat ratings

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Focal Spot Size (cont.)

• Focal spot size changes with technique
• Standard technique required
• 75 kV (typical)
• 50 maximum mA for focal spot at kV used
• direct exposure (no screen)
• NEMA Standardsdefines tolerances

Nominal Size Tolerance ------------------------
------------- gt1.5 mm
30 gt0.8 and lt1.5 mm 40 lt0.8 mm
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Focal Spot Measuring Tools

• Direct MeasurementPin Hole Camera
• Slit Camera
• Indirect Measurement of Resolving Power
• Star Test Pattern
• Bar Phantom

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Direct Focal Spot Measurement

• Measure focal spot directly in each direction
• Use triangulation to correct for distances
• formula corrects for finite tool size
• two exposures required for slit

Slit Camera
Pinhole Camera
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Star Test Pattern

• Measures resolving power
• infers focal spot size
• Dependent on focal spot energy distribution
• measure
• largest blur diameter (in each direction)
• magnification
• use equation to calculate focal spot size

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Bar Phantom

• Measures resolving power
• Find smallest group where you can count three
  bars in each direction

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Bar Phantom Setup
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Radiographic Collimation

• X-Ray / Light Field Alignment
• Beam Central Axis
• should be in center of x-ray beam
• Collimator field size indicators
• PBL (automatic collimation)
• field automatically limited to size of receptor
• Bucky Alignment
• Using longitudinal bucky light transverse
  detent, x-ray field should be centered on bucky
  film

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X-Ray / Light Field Alignment

• Mark light field on table top with pennies

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Radiographic X-Ray / Light Field Alignment
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Fluoroscopic Collimation

• image field is scale seen on monitor
• expose film on table above scale
• compare visual field (monitor) with x-ray field
  on film
• must check all magnification modes

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Fluoroscopic Collimation
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Fluoroscopic Collimation
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Maximum Fluoro Output

• put chamber in beam on tabletop
• block beam with lead above chamber
• fools generator into providing maximum output
• 10 R/min. limit for ABS fluoro

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Maximum Fluoro Output
Lead
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Calibration Performance Parameters

• Timer Accuracy
• Repeatability
• Linearity/Reciprocity
• Kilovoltage accuracy
• mA
• must be measured invasively

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Non-invasive Calibration Tools

• Fancy
• Ion chambers
• Electronic Black Boxes
• Not as fancy
• Wisconsin Test Cassette
• Pocket Dosimeter
• Spin Top

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Timing Review

• Single Phase
• Full-wave rectified
• 120 pulses / second
• Half-wave rectified
• 60 pulses / second
• Self rectified
• 60 pulses second
• Three Phase / Constant Potential / Medium or High
  Frequency
• continuous output (not pulsed)

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Time Measurement

• Digital Black Box Meter
• Spin Top
• spins manually
• one dot per pulse (single phase)
• can not use with 3 phase
• Synchronous Spin Top
• spins at a set rate
• can use for single or 3 phase
• measure angle for 3 phase

One hole in solid disk
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Spin Top

• If this is a half-wave rectifier, what is
  exposure time?

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Spin Top

• If this is a full-wave rectifier, what is
  exposure time?

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Synchronous Spin Top

• What is this waveform?
• What is the exposure time?

90o
1 revolution / sec.
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kVp Measurement

• Waveform represented by a single number
• Wisconsin Test Cassette
• Digital beam analyzers
• use differential filtration
• Invasive measurements
• dynalyzer

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Calibration
120 kVp
mA time mAs mR mR / mAs
(msec) ------------------------------------------
------------ 100 .1 10 240
24 200 .05 10 ?
? 50 .2 10
? ?
Constant mAs

• mR/mAs should stay constant for all combinations
  of mA kVp at any particular kVp

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Calibration
120 kVp
mA time mAs mR mR / mAs
(msec) ------------------------------------------
----------- 100 .1 10 240
24 200 .1 20 ?
? 100 .4 40 ?
?
Double mAs
Double mAs again

• mR/mAs should stay constant for all combinations
  of mA time at any particular kVp

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Phototiming(check with output or film)

• Reproducibility
• Density Controls
• Field Placement
• Field Balance

Phototiming Operation should be Predictable
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Phototimer Density Control Settings
R
R
T
a
b
l
e
t
o
p
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Phototiming Density Steps should be predictable
approximately even
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Phototimer Field Placement / Balance

• Placement
• cover desired field with lead
• select field as indicated
• Balance
• no fields covered
• select field as indicated

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Phototimer Field Placement / Balance
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Phototimingchecked with film density

• kV Response
• phototimer pick-up attenuation may vary with kV
• phototimer must track kV response of rare-earth
  film
• Rate Response
• Check with varying
• phantom (lucite) thickness
• mA

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kV/Rate Response
kV
70
81
90
Lucite
17.5
4.5
4.9
5.2
Depth
12.5
4.7
(cm)
7.5
4.7
Thickness Tracking
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Optical
Density
2
0
17.5
12.5
7.5
Lucite Thickness
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Phototiming(etc.)

• Minimum response time
• Limitations of phototimer with rare earth systems
• Back-up time function
• Maximum mAs
• 600 mAs (above 50 kVp)
• 2,000 mAs (below 50 kVp)

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The End
Any questions, you varmints?