227Day diagnostic radiology (PDF) | Diagnostic radiology ...
Quality Control in Diagnostic Radiology
description
Transcript of Quality Control in Diagnostic Radiology
Quality Control in Diagnostic Radiology
Factors driving Q.C.Why do we do it?
Legal RequirementsAccreditation
JCAHOACR
Clinical improvementequipment performanceimage quality
Q.C. Goals
Minimize dose topatientsstaff
Optimize image qualityEstablish baselines
More on this in a moment
Why is Q.C. Important?
Without a QC program the only way to identify problems is on
patient images. And some problems don’t show up on images.
Yeah, that’s what I always say.
QC can detectMalfunctionsUnpredictability
may be hard to isolate clinically
Inefficient use of Radiationhigh fluoroscopic outputs
Radiation not reaching receptorinadequate filtrationoversized collimation
Goals of a Q.C. Program
Obtain acceptable image with least possible radiation exposure topatientsstaff
Attempt to identify problems before they appear on patient filmswithout QC problems only detected on
patient films
“Acceptable” Image
Image containing information required by radiologist for correct interpretation
goal: minimize exposure while maintaining acceptability
high exposure images often have excellent appearanceLow noise
Q.C. & BaselinesBaselines
quantitative data on equipment obtained during normal operations
Baselines useful for troubleshootingisolating problem component, for example
generator processor
Allows efficient use of engineering / repair personnel
X-Ray Quality Control
FiltrationFocal Spot SizeCollimationMaximum Fluoroscopic OutputCalibration VerificationPhototimer Performance
Why is Filtration Important?
Tube emits spectrum of x-ray energiesFiltration preferentially attenuates low
energy photonslow energy photons expose patients
do not contribute to image low penetration
Half Value Layer (HVL)We don’t measure filtrationWe measure HVLHVL: amount of absorber that reduces beam
intensity by exactly 50%
Half Value Layer
Depends uponkVpwaveform
(single/three phase)inherent filtration
Minimum HVL regulated by law
Maximum HVL regulated only in mammography
kVp HVL (mm Al) 30 0.3 40 0.4 49 0.5 50 1.2 60 1.3 70 1.5 71 2.1 80 2.3 90 2.5100 2.7110 3.0120 3.2130 3.5140 3.8150 4.1
Georgia State Rules & Regulations for X-Ray
Radiographic HVL Setup
R
Filter
Tabletop
Radiographic
Checking HVL Compliance(Radiographic)
How much aluminum must be placed in beam to reduce intensity by exactly 50%?
R
Filter
Tabletop
Radiographic
filter mR(mm Al)------------------- 0 2502.5 133
filter mR(mm Al)------------------- 0 2502.5 125
filter mR(mm Al)------------------- 0 2502.5 111
90 kVp Measurements; 2.5 mm Al minimum HVL
AcceptableHVL > 2.5 mm
MarginalHVL = 2.5 mm
UnacceptableHVL < 2.5 mm
OK! Must add Al to reduce beam to exactly 50%
Not OK! Must remove Al to reduce beam to exactly 50%
Checking HVL Compliance(Radiographic)
Is this machine legal?2.5 mm Al minimum filtration at
90 kVp
R
Filter
Tabletop
Radiographic
filter mR(mm Al)------------------- 0 4502.5 205
90 kVp Measurements
Fluoroscopic HVL Setup
R
ImageTube
Tabletop
Filt er
Fluoroscopic Tube Filtration(Half Value Layer)
Absorber(to protectImage Tube)
Fluoroscopic HVL
Set desired kilovoltage manually
measure exposure rates instead of exposure
Move absorbers into beam as needed
R
ImageTube
Tabletop
Filt er
Fluoroscopic Tube Filtration(Half Value Layer)
Absorber(to protectImage Tube)
Focal Spot Size
We measure apparent focal spot
Trade-offsmaller spot
reduces geometric unsharpness
larger spot improves heat ratings
ApparentFocal Spot
ActualFocal Spot
Focal Spot Size (cont.)
Focal spot size changes with techniqueStandard technique required
75 kV (typical)50% maximum mA for focal spot at kV used direct exposure (no screen)
NEMA Standardsdefines tolerances
Nominal Size Tolerance------------------------------------->1.5 mm 30%>0.8 and <=1.5 mm 40%<0.8 mm 50%
Focal Spot Measuring ToolsDirect Measurement
Pin Hole CameraSlit Camera
Indirect Measurement of Resolving PowerStar Test PatternBar Phantom
Direct Focal Spot Measurement
Measure focal spot directly in each direction
Use triangulation to correct for distancesformula corrects for finite
tool sizetwo exposures required for
slit
PinholeCamera
SlitCamera
Star Test Pattern
Measures resolving powerinfers focal spot sizeDependent on focal spot energy
distributionmeasure
largest blur diameter (in each direction)magnification
use equation to calculate focal spot size
Bar PhantomMeasures
resolving powerFind smallest
group where you can count three bars in each direction
Radiographic Collimation
X-Ray / Light Field AlignmentBeam Central Axis
should be in center of x-ray beamCollimator field size indicatorsPBL (automatic collimation)
field automatically limited to size of receptor
Bucky AlignmentUsing longitudinal bucky light &
transverse detent, x-ray field should be centered on bucky film
X-Ray / Light Field Alignment
Mark light field on table top with pennies
Tabletop
X-ray / Light Field AlignmentSlight misalignmenton this edge
X-Rays
Tabletop
Light
Lamp
Radiographic X-Ray / Light Field Alignment
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
Collimator Test Tool Template
ImageTube
Tabletop
Film
Fluoroscopic Collimation
Fluoroscopic Collimation
Maximum Fluoro Outputput chamber in beam on
tabletopblock beam with lead
above chamberfools generator into
providing maximum output10 R/min. limit for ABS
fluoro R
Lead
ImageTube
Tabletop
Maximum Fluoro OutputLead
Calibration Performance Parameters
Timer AccuracyRepeatabilityLinearity/ReciprocityKilovoltage accuracymA
must be measured invasively
Calibration
mR/mAs should stay constant for all combinations of mA & kVp at any particular kVp
mA time mAs mR mR / mAs (msec)------------------------------------------------------100 .1 10 240 24200 .05 10 ? ? 50 .2 10 ? ?
120 kVp
Constant mAs
Calibration
mR/mAs should stay constant for all combinations of mA & time at any particular kVp
mA time mAs mR mR / mAs (msec)-----------------------------------------------------100 .1 10 240 24200 .1 20 ? ?100 .4 40 ? ?
120 kVp
Double mAs
Double mAsagain
Phototiming(check with output or film)
ReproducibilityDensity ControlsField PlacementField Balance
Phototiming Operation should be Predictable
R
Tablet op
R
Phototimer Density Control Settings
Density Control-2 -1 0 1 2
41 49 62 76 96
Phototiming Density Steps
should be predictable & approximately
even
% Step to Step Change
0.0
10.0
20.0
30.0
0 -2 -1 0 1 2 0 0Density Control Setting
Ch
ang
e
Phototimer Field Placement / Balance
Placementcover desired field
with leadselect field as
indicated
Balanceno fields coveredselect field as
indicated
R
Tablet op
Measurement of Photot imerField Placement / Balance
R
Lead for checking field placement
Phototimer Field Placement / Balance
Field Placement Field BalanceField Selected 15 cm Lucite, 81 kVp
Left Center Right L & R Field mR
Left 355 23.2 29 51 Left 6.6
Field Center 26.9 242 25.4 25.6 Center 4.9
Covered Right 29.9 24.3 610 56.6 Right 6.3
L & R 578 18.3 266 354 L & R 6.3
Above readings in mR "Disc" units
Phototimingchecked with Exposure Index
kV Responsephototimer pick-up attenuation may
vary with kVphototimer must track kV response of
rare-earth film
Rate ResponseCheck with varying
phantom (lucite) thickness mA
kV/Rate ResponsekV
70 81 90
Lucite 17.5 4.5 4.9 5.2
Depth 12.5 4.7
(cm) 7.5 4.7
Thickness Tracking
Lucite Thickness
Opt
ical
Den
sity
0
2
4
17.5 12.5 7.5
kV Response
kilovoltage
Opt
ical
Den
sity
0
2
4
70 81 90
Tabletop
Measurement of PhototimerkV / Rate Response
Film
Any questions,
you varmints?