Answers to common questions and problems
Counting high backgrounds? Have a question about a product? Receive new calibrators and your efficiency are now way off? No problem, here are some answers to the most frequently asked questions. Check back with us soon, this page updates frequently.
Why is a Multi-Wiper different than regular gamma counter?
Gamma counters have historically been designed primarily for RIA and research involving dose-response curves. Over the years, larger nuclear medicine departments have used them in order to count large numbers of wipes more efficiently than with a single well wipe test counter. While this worked to reduce counting time, it is far from an ideal solution.
Per NRC and most state regulations, wipes need to be reported in DPM. The agencies don’t care how many counts your instrument “sees”, they want to know how much activity is present in a wipe. RIA counters only report in CPM. All wipes must be converted to DPM. Furthermore, an RIA counter is designed to count within a discreet window. It cannot tell what is happening outside that window. So if you are counting in a window for Tc99m, and you have contamination from I131, you would never know it. An alternative is to count in a wide window. This allows you to see any counts present, but then you have no idea what isotope or at what energy the counts are at. Not an ideal solution.
With the Multi-Wiper, we use a 4096 channel analyzer that scans all wells simultaneously. With the wipe test software, you can count in a wide window, but the system can track the energy of each count. Referencing the Isotope Library, it places the counts into the appropriate window and reports what the counts were for each isotope in the library. Further, it flags any isotope that exceeds trigger (user selectable), and reports pass fail. To make things even easier, all counts are automatically converted to DPM, so the final results are exactly that – final results. No conversion, no post counting calculations. The wipe test report that is printed is the final word. Just count and print, you’re done.
What is a Multi-Well Gamma Counter?
A multi-well gamma counter is an instrument which measures gamma radiation, usually for the purpose of clinical or research analysis, RIA (radioimmunoassay) or other tests utilizing gamma producing elements as marking tags. A multi-well counter employs more than one well-type detector to count multiple samples simultaneously. Multi-well gamma counters such as the Genesys Series are the worldwide standard for the clinical evaluation of clinical RIA tests.
But I can get a used gamma counter cheaper…
Absolutely you can get a gamma counter cheaper used than new. But first and foremost, you need to ask the question: “how much will it cost me in the long run”? There are a lot of old, used gamma counters on the market today. But what happens if you buy a used counter from a company that is no longer in business or doesn’t make the product anymore. Sure, you can get a warranty from the seller, and of course they’ll promise you that they will support it for many years. But how old is the counter really?
- The Iso-Data 20/20 series went out of production in 1986
- The Iso-Data 100 and 500 series haven’t been made for at least 15 years
- The Packard Cobra and Riastar ceased to exist about 10 years ago
Any repair parts available are going to be at least that old (remember that’s how long ago they went out of production, not necessarily when the parts you get were made). Grant it, a well made gamma counter should last in excess of 10 years, but that is the age you are getting it from the start! What do you really think the life expectancy is going to be?
Buying a new Genesys may cost more up front, but coupled with the 5 year warranty option, you will come out far ahead cost-wise and save a lot of money in the long run. No worries, no hidden costs. And with the new I125 Auto-Match feature, you won’t even need to buy those expensive sets of matched calibrators every 6 months.
Now that’s real cost saving and true thrift.
…but when the count finishes, they are all zero
The most likely reason is that there are very large background values stored in the instrument and being subtracted from the assay counts. What probably happened is that the background was run at the time you checked the efficiency as a daily instrument Q.C. The calibrators were probably put into the wells during the background count. When you see counts happening on the screen, they are “raw”. In other words, they are not corrected for background or normalization factor. That is the reason why if you pause the counting right before the cycle is done, look at the counts for a particular well, then compare that to the print out, they will not match exactly. Background is subtracted and normalization factors are applied once counting is completed.
Our Gamma-1, Genii, Wiper and Multi-Wiper counters all have a background trigger (user selectable) that alerts you in situations where the background is too high. If you mistakenly left your calibrators in the wells during a background count, you would get a warning message saying background level has been exceeded.
I just entered my new set of Multi-Calibrators, now my efficiency is all wrong!
This is a common problem that users have, at least once a week, we receive a call about efficiency problems. The good new is most of the time it is a simple data entry error. Multi-Calibrators are a set of matched reference sources, usually I125 that have a known decay rate. On any day the included log book will give you a correct DPM ( Disintegrations per minute) at midnight PST. Using this DPM the Genesys will compare it against the CPM ( counts per minute) seen by the gamma counter, giving you the efficiency of your Genesys gamma counter.
Think of it as a way to see how much of the gamma emissions you are collecting. This is a good way to monitor the health of your Genesys gamma counter.
Every set of I125 Multi-Calibrators comes with a log book that has daily DPMs for 26 weeks, after that the I125 has undergone so many half lives it is effectively dead. At least for our purposes, check with your Radiation Safety Officer (RSO) on proper disposal procedures. This also means that every LOT you receive will have different DPM numbers. Because the Genesys automatically decays the DPM it is easy to overlook the need to enter the new Lot information. After all you only do this twice a year.
For every new set of Multi-Calibrators you receive, use the following procedure:
- Check the date and time of your Genesys, if it is not correct, change it now.
- If you are using a Genii or Gamma 1, go to the Isotope Library and edit the isotope. If you are using a Genesys 5000 go to the daily counter checks and select your isotope, then select enter new lot.
- Enter the Lot number, DPM for today. If using a Genii or Gamma 1 also enter the DPM date and time (this would be today and 00:00:00 for the time). This information can be found in the Multi-Calibrators MultiLog logbook.
- Confirm that the Lot information you just entered is correct.
- Run the Efficiency check.
I125 efficiency for a new unit is above 80% typically the efficiency of a healthy unit will be between 76%-86%. If you efficiency is out of this range you may want to contact LTI Technical support to see about having a certified LTI Service Engineer take a look at your Genesys.
High backgrounds counts in one well
Possibly due to contamination. If you normally count background with a sample tray in place, remove it and count the background without the tray. If the background is normal, then you may have a contaminated sample tray. If the background is still high, then remove the plastic well liner and count again. If the background without the well liner is normal, then clean the well liner.
**NOTE** Never place a test tube or any other object into the well unless the well liner is in place. Placing something directly into the well may cause damage and may perforate the thin aluminum inside the well. If that happens, the crystal will hydrate and the detector will have to be replaced. This is a very costly mistake.
If you clean the well liner, use either a solution of Bind-It (for I125, I123, I131) or a 25% bleach solution. Be sure the plastic well liner is rinsed clean and DRY before you place it back into the well.
If the background is still high after you remove the tray and well liner, try cleaning the well. Keep in mind this well is lined with very thin aluminum, NEVER use any harsh chemicals or sharp instruments. These items will damage the well and create a costly repair. To clean the well use either a solution of Bind-It (for I125, I123, I131) or alcohol on a cotton cue-tip, gently clean the interior of the well and allow to dry before turing the unit back on. NEVER POUR ANYTHING DIRECTLY INTO THE WELL!
If the background is still high, then the well may be out of adjustment. This is a simple procedure with the Genii, Gamma-1, Multi-Wiper or Wiper. Simply run an automatic gain adjust procedure (also called Automatic calibration.)
Still need help? At LTI we stand behind our products with personalized support. You can always talk directly to an applications or technical support person. We offer intelligent, friendly advice and help you find quick answers to your questions. Your satisfaction is our goal. We want satisfied customers who tell their friends and colleagues about us. Give us a call or send us an email, we are here to help.
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