1. Lecture 4(Unit II): Gas-Filled Detector Quality Control
"Enrico Fermi," Microsoft® Encarta® Online Encyclopedia © Microsoft Corporation. All Rights Reserved.

2. Lecture 5 Objectives
Describe the quality control (QC) procedures for gas filled detectors including their frequency and regulatory limits
Discuss limitations of gas-filled detectors

3. Quality Control for a Dose Calibrator
Quality Control (QC) involves a system of routine measurements that ensure the proper functioning of equipment.
QC Type:
Geometry
Accuracy
Linearity
Constancy
When:
At installation
After repair or move
After syringe-type change
At Installation
Annually
Installation
Quarterly
Daily
(And also weekly)

4. Quality Control for a Dose Calibrator
Geometry
Checks the affect of changes in sample volume have on the dose calibrator’s activity reading
Different volumes in a vial:
1 mCi Tc-99m in 1 mL in 30 mL vial
Assay
Increase volume to 4 mL
Repeat for 8, 10, 15, & 25 mL
Assay after each
Different volumes in a syringe
About 1 mCi Tc-99m in 0.5 mL in 3 cc syringe.
Assay
Increase volume by 0.5 mL.
Repeat for 1, 1.5, and 2 mL
Assay after each
Select a volume as a standard
Variations should be less than ± 10%--if not:
Apply correction factors based on:
Standard reading / Volume sample reading

5. Quality Control for a Dose Calibrator
Accuracy
Testing to see if a mCi is a mCi on commonly used settings.
Uses a set of standard NIST-verified sealed radioactive sources with varying energy level emissions to see if the dose calibrator reads true.
Commons sources: 57Co, 137Cs, & 133Ba.
Activity should be at least 50μCi and preferably 200 or greater μCi.
At least one source has an emission energy between 100 & 500 keV.
Read net activity should be compared with decay activity calculated for the standard source used.
NRC wants the DC reading to be within 10% of the calculated standard activity.
Textbook states that readings are to the within 5% for the device to be considered “accurate.”

6. Quality Control for a Dose Calibrator
Linearity
Determines how accurately the DC measures doses over the entire possible range of patient doses used.
Range of doses: highest used (about 150mCi) down to 30μCi.
Method 1—approx 150 mCi of 99mTc measured 2 to 3 times each day for several days until its activity drops below 30μCi.
Method 2—Uses set of lead DC inserts that have been calibrated to determine expected absorption of activity for each insert. The measure starts with a large dose (150mCi or more) of 99mTc, which is sequentially shielded by the inserts until it measures less than 30μCi. A reading is made for each insert.
Two Methods Used:
For Method 1, the readings are plotted on a semi logarithmic graph to compare the actual readings to the expected values through decay.
For Method 2, a special worksheet is used, but the NRC recommends mapping these to a semilog graph.
(We will practice both in lab).
For the NRC, a variation of more than + or – 10% means the instrument must be serviced.

7. Figure 09A: Activity non-linearity due to saturation

8. Figure 09B: Activity non-linearity due to recombination

9. Calicheck Linearity Sleaves

10. Quality Control for a Dose Calibrator
Constancy
A daily test with a long T1/2 radionuclide source to see if the DC’s readings are consistent with the slowly decaying source activity.
Commonly uses sealed vials of 57Co and 137Cs.
Often see 57Co used for a daily measurement on 99mTc channel, and 137Cs used for a weekly measurement on all commonly used channels.
The daily and weekly readings can then be plotted on a semi logarithmic graph.
The source dose is calculated over time according to decay, and two lines are plotted on the graph based on + and – 10% of the calculated dose.
The dose readings should fall between the lines.
NRC allows a variation of + or – 10% from the calculated activity of the source.
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 80.

11. Quality Control for Survey Meters
Quality Control (QC) for both ionization chamber and Geiger-Mueller survey meters is the same.
QC Type:
Calibration (Accuracy)
Constancy
When:
Before first use
Annually
After repair
Daily
After battery change
After any maintenance
After calibration

12. Quality Control for Survey Meters
Calibration—to set the meters to give reliable readings for exposure rates based on a known NIST traceable source.
The instrument is calibrated based on readouts of the source at about 1/3 and 2/3 the instrument’s full scale.
An attenuator may be used to create different exposure rates from the source.
Most nuclear medicine departments send the survey meter to a special lab that has the sources and equipment needed for calibration
Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 151.

13. Quality Control for Survey Meters
Constancy—the “source check.” A measurement of the survey meter’s continued accuracy based on a recorded reading of the check source at the time of calibration.
Uses a designated sealed source as a check source to check if the reading of the meter continues to be accurate.
The check source is usually attached to body of the GM meter. It is usually a separate vial for the ionization chamber survey meter.
The rate of the check source reading at time of calibration is posted on the meter.
The NRC specifies that the source check reading should not be more than 10% off of the reading posted on the meter.
If the reading is off by more than 10%, than the meter should be recalibrated.

14. Limitations of Gas-filled Detectors
Poor efficiency, particularly with high energy photons
Particulate radiation difficult to detect—difficult to differentiate between energy levels
DC relies on Bremmstrahlung on pure beta emitters—can be inaccurate
Dead Time

15. Next time…
UNIT II TEST !!!
Picture from Neatorama.com available at accessed 10 Oct 2017