Today begins decade #3 at work. 20 years ago today, I started working here. To say that there’s been a lot of changes at work since I started would be a bit of an understatement.
I’ve gone from having everything all nice and contained within the main campus to now having multiple locations across the tri-county area with imaging equipment that I need to visit. The amount of imaging equipment I lay hands on has also gone up by about 3x since I started. It definitely keeps me busy, but doesn’t leave much time for working on other things like I used to have.
Lots of interesting things happening now, and there’s going to be a lot of new equipment arriving over the next few years: new hospital buildings, new clinic sites, new imaging technology. With any luck, we’ll be able to get a diagnostic imaging residency program started in the near future too.
A video created by instructors in the Radiologic Technology program at St. Johns River State College and shared over on the Radiology subreddit, reminds technologists to wear lead aprons when doing portable radiography.
In the video, they use what appears to be a Geiger-Mueller (GM) survey meter to show that even when standing far away from the portable unit or behind a wall, technologists are exposed to scatter radiation that is greatly reduced when wearing a lead apron.
Conventional wisdom for portable radiography tells people to stand at least 6 feet away (about 180 cm) during exposures and that the amount of scatter radiation that far away is pretty low and insignificant. Most portable units have the exposure switch on a pretty long stretchy cord, so getting 10 feet away (about 305 cm) isn’t that difficult. However, due to room/area constraints, it might not be possible for other patients/staff to get that far away.
One could definitely argue about the appropriateness of using a GM survey meter to measure scatter radiation, but for demonstration purposes it’s a reasonable instrument to use. To quantify how much scatter radiation technologists are exposed to, an ionization chamber is a much more appropriate instrument to use. Prompted by the video and spurred on by my own curiosity, I decided to have a quick look at the amount of scatter radiation. Armed with my Radcal meter and 10×6-1800 large volume ionization chamber, I did some quick and dirty measurements to investigate.
To simulate a maximal scatter situation, I used two 32 cm CTDI phantoms as my large “patient” and a 35×43 cm field. Source-detector (SID) distance was set to 100 cm. The center of the ionization chamber was positioned 225 cm away from the center of the field (the farthest away I could reasonably get in the room I was in) and 94 cm above the floor (approximately waist height for an average sized person).
Three exposures at each of 60, 80, 100, and 120 kV were acquired and averaged. To ensure a decent amount of exposure at the chamber, 50 mAs was used for each exposure. The table below gives the average scatter exposure recorded at the chamber in nGy/mAs and the scatter exposure normalized to a distance of 100 cm.
Portable Radiography Scatter Exposure
(nGy/mAs) @ 1 m
Plotted on a graph, it looks like this.
A second order polynomial fits the data pretty nicely: Scatter (nGy/mAs) = 0.0117kV2- 0.3279kV - 5.0002
Consider an abdominal radiograph performed at 80 kV and 40 mAs. From the graph, scatter exposure is about 40 nGy/mAs. At a distance of 225 cm, the scatter exposure would be about 1.6 μGy. At a distance of 10 feet, inverse square correction (a reasonable approximation) puts the scatter exposure at around 0.87 μGy. At a distance of 6 feet, it would be a little higher at around 2.4 μGy.
This data only represents one unit, one measurement location, and a maximal scatter setup, but still illustrates that while scatter is detectable, the exposure to surrounding people is still fairly low.
How applicable are these numbers generally? For radiographic units (fixed and portable), it turns out that there’s not as much variation in radiation output as one might think. The amount of scatter exposure will vary with location around the source (lower behind the portable unit because of shielding by the portable) but should be fairly symmetric. It probably wouldn’t be too unreasonable to use the data here to get ballpark figures on how much scatter exposure technologists and other personnel would be exposed to. Remember, the data presented here represents kind of a worst case scenario with a large patient and large field, so any estimates based on these numbers should be considered as upper limits.
If you’re a technologist who does a lot of portable radiographs, wearing a lead apron and keeping your distance probably isn’t a bad idea.
If you’ve ever wondered how wide the x-ray beam for a Hologic bone mineral density (BMD) scanner is, I can now tell you it’s not very wide. They’re considerably thinner than I expected in fact.
I placed a strip of Gafchromic XRCT film on top of the housing surrounding the x-ray tube and ran the scanner through its various scan modes, moving the strip between each scan.
Each of the vertical stripes represents the width of the beam at about 40 cm from the focal spot and about 10 cm below where the x-ray beam would enter the patient. From left to right are the beam widths for the fastest to the slowest scan modes. The scale on Gafchromic strip is marked off in millimeters.
The beam width for the fastest scan mode is 2 mm. The next two modes have a beam width of 1 mm, and the slowest scan mode uses a beam width that looks like about 0.2 mm. With a source to detector distance of a little over 100 cm, the beam width at the detector ranges from about 5 mm to 0.5 mm.
Just about every medical physicist has a collection of old test gear, phantoms, test objects ,meters and the like.
A few years ago, while rummaging through the equipment cabinet in our store room/library/lab, I came across a variant of a mammography phantom that I hadn’t seen before. Instead of the normal pink wax insert, this one had 16 wax squares of different colours.
Aside from the curved bit of plastic at one end of the phantom (a test object, not a ghostly apparition), it’s the same size as the conventional ACR accreditation phantom. Reminds me of one of those sliding number/picture puzzles where you have to slide the squares around to reconstruct the image.
I let it sit on my book shelf along with some of the other pieces in the collection. A few months ago, I decided it was time to have a look and see what the inside of the wax blocks looked like.
Looks like at some point in its history, the pieces got a little scrambled and reinserted a bit randomly. I was expecting that each colour block would represent a different density. Instead there are the usual fiber, speck, and mass groups, but not nearly as uniformly placed as in the accreditation phantom.
I don’t know how old this phantom is or what time frame it might have been used at work. The only mammography phantom I was familiar with before this one was the pink one, so possibly before 1996 at least. Definitely pre-1999.
If anybody out there happens to know anything about this style of mammography phantom, let me know.
Development on my equipment database has slowed down a bit partly because of being busy at work and partly because the database files on my home computer keep getting corrupted for some reason and I haven’t bothered to figure out why or fix it yet.
The equipment tracking part does pretty much everything I need now (still a few things to take care of), so my latest efforts have been on trying to get the test data locked in my spreadsheets into the database.
I started off using the PhpSpreadsheet package (which is still under development), but I found a lot of what was in the documentation wouldn’t work. I ended up going to the older PHPExcel package instead. Using this made it relatively easy to create some LaravelArtisan commands that pick out the test datafrom my spreadsheets and stick them into the database. Now I can batch add data to the database using a simple shell script. One problem with the current commands is that they won’t work with older version of my spreadsheets yet because the locations of some of the data has changed over time. Not sure I’m too worried about that yet. They also don’t handle problems very gracefully yet. Something to work on later perhaps.
The DB schema for the test data is still being worked on, but I think I’ve got something that will let me pick out data for an individual survey, as well as show a time series from a specific test for a given machine.
Current works in progress are views to display the test data. I’ve got a few done, but still have a bunch more to do.
I love how easy doing all of this has been with Laravel.