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?