Burnt Out

Top:  Anterior feet (detector 1); Bottom:  Posterior feet  (detector 2)

What's wrong with this picture?  

First off, this is an In111-WBC scan to localize infection in the ankle.  The top image is the anterior view (detector 1) and the bottom image is the posterior view (detector 2).  What is interesting is that there is a "bite" taken out of detector 2.  As you know, this is probably not something that you would want in your image, but rather a technical artifact in which a photomultiplier tube has burnt out during the image acquisition.

There are 2 things to discuss here:

1.  White Blood Cell (WBC) scans:  

The decision to use Tc-99m WBC versus In-111 WBC is based on the a decision tree approved by our physician.  It really revolves around whether the patient is diabetic and or has a prosthetic or implant post surgery.  With WBC scans, it's primarily used to determine infection such as osteomyelitis, which for the most part represents 99% of our cases.

a.  Not diabetic or no implant - Tc-99m WBC

b.  Diabetic with (?) foot infection and/or has prosthetic implant - In111-WBC 

All WBC cases have a bone scan first followed by either the Tc-99m or In-111 WBC scan afterwards.  The one main thing to note is that if the In-111 route is taken, they are given Tc-99m sulphur colloid first before the In-111 WBC injection on the same day to determine if there is any bone marrow involvement with the site in question.

Having said this, I have yet to see a positive scan to date.  The current case above was to determine post surgical infection of the right ankle.  The scans for both the sulphur colloid and the In-111 WBC were unremarkable, but there were post surgical changes on the right distal fibula and ankle (image not shown) on the bone scan.

What about gallium?  At our site, gallium is not the isotope of choice unless discitis is indicated.  In-111 WBC's are not as effective in relation to Ga-67 in conjunction with a bone scan, to determine discitis/vertebral osteomyelitis.  Accuracy for WBC's are generally low for this type of imaging (Henkin et al. Part IV p. 1129)

2.  Photomultiplier tubes (PMT)

Basic premise with PMT's are to convert light into an electrical current.  There are a multiple of reasons why PMT's burn out.  Daily quality control and monthly preventative maintenance programs are important to detect these irregularities.  PMT's have been around for many years, but with new solid state cameras like the CZT (Cadmium Zinc Telleride) equipped systems, PMT's may be a thing of the past.  Who knows?