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Quality Control in Diagnostic Radiology

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Presentation on theme: "Quality Control in Diagnostic Radiology"— Presentation transcript:

1 Quality Control in Diagnostic Radiology

2 Factors driving Q.C. Why do we do it?
Legal Requirements Accreditation JCAHO ACR Clinical improvement equipment performance image quality Medical Physicists at Work

3 Q.C. Goals Minimize dose to Optimize image quality Establish baselines
patients staff Optimize image quality Establish baselines More on this in a moment

4 Yeah, that’s what I always say.
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.

5 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

6 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

7 “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

8 Q.C. & Baselines Baselines Baselines useful for troubleshooting
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

9 X-Ray Quality Control Filtration Focal Spot Size Collimation
Maximum Fluoroscopic Output Calibration Verification Phototimer Performance

10 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

11 Half Value Layer (HVL) We don’t measure filtration We measure HVL
HVL: amount of absorber that reduces beam intensity by exactly 50%

12 Half Value Layer Depends upon Minimum HVL regulated by law
kVp HVL (mm Al) 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

13 Radiographic HVL Setup

14 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) filter mR (mm Al) filter mR (mm Al) Not OK! Must remove Al to reduce beam to exactly 50% OK! Must add Al to reduce beam to exactly 50% Acceptable HVL > 2.5 mm Marginal HVL = 2.5 mm Unacceptable HVL < 2.5 mm

15 Checking HVL Compliance (Radiographic)
Is this machine legal? 2.5 mm Al minimum filtration at 90 kVp 90 kVp Measurements filter mR (mm Al)

16 Fluoroscopic HVL Setup

17 Fluoroscopic HVL Set desired kilovoltage manually
measure exposure rates instead of exposure Move absorbers into beam as needed

18 Focal Spot Size We measure apparent focal spot Trade-off
smaller spot reduces geometric unsharpness larger spot improves heat ratings Apparent Focal Spot Actual

19 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 Standards defines tolerances Nominal Size Tolerance >1.5 mm % >0.8 and <=1.5 mm % <0.8 mm %

20 Focal Spot Measuring Tools
Direct Measurement Pin Hole Camera Slit Camera Indirect Measurement of Resolving Power Star Test Pattern Bar Phantom

21 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

22 Star Test Pattern Measures resolving power measure
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

23 Bar Phantom Measures resolving power
Find smallest group where you can count three bars in each direction

24 Bar Phantom Setup

25 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

26 X-Ray / Light Field Alignment
Mark light field on table top with pennies

27 Radiographic X-Ray / Light Field Alignment

28 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

29 Fluoroscopic Collimation

30 Fluoroscopic Collimation

31 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

32 Maximum Fluoro Output Lead

33 Calibration Performance Parameters
Timer Accuracy Repeatability Linearity/Reciprocity Kilovoltage accuracy mA must be measured invasively

34 Calibration 120 kVp mA time mAs mR mR / mAs (msec)
? ? ? ? Constant mAs mR/mAs should stay constant for all combinations of mA & kVp at any particular kVp

35 Calibration 120 kVp mA time mAs mR mR / mAs (msec)
? ? ? ? Double mAs Double mAs again mR/mAs should stay constant for all combinations of mA & time at any particular kVp

36 Phototiming (check with output or film)
Reproducibility Density Controls Field Placement Field Balance Phototiming Operation should be Predictable

37 Density Control Phototimer Density Control Settings -2 -1 1 2 41 49 62
a b l e t o p Density Control -2 -1 1 2 41 49 62 76 96

38 Phototiming Density Steps should be predictable & approximately even

39 Phototimer Field Placement / Balance
s u r m n f P h i F d c / B Lead for checking field placement Placement cover desired field with lead select field as indicated Balance no fields covered

40 Phototimer Field Placement / Balance

41 Phototiming checked 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

42 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 kV Response kilovoltage Optical Density 2 4 70 81 90 Thickness Tracking 4 Optical Density 2 17.5 12.5 7.5 Lucite Thickness

43 Any questions, you varmints?
The End Any questions, you varmints?

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