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 images often have excellent
  appearance
• Low noise

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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

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

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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
50
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
50
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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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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 Exposure Index

• 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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The End
Any questions, you varmints?