How much scatter radiation exposure?

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
kVScatter exposure
(nGy/mAs)
Scatter exposure
(nGy/mAs) @ 1 m
6017.588.8
8043.2218.8
10079.5402.6
120123.9627.2

Plotted on a graph, it looks like this.

Portable Radiography Scatter Exposure
Portable Radiography Scatter Exposure at 225 cm

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.

Bone mineral densitometer beam widths

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.

BMD beam width
BMD beam width

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.

 

Vintage mammography phantom

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.

Old RMI mammography phantom SN 152-1015
Old RMI mammography phantom SN 152-1015

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.

Old RMI mammography phantom SN 152-1015
Old RMI mammography phantom SN 152-1015 side view

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.

Old RMI mammography phantom SN 152-1015
Old RMI mammography phantom SN 152-1015

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.

RadDB: Storing and viewing test data

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 Laravel Artisan commands that pick out the test data from 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.

An array of focal spots

Some time ago, I came across an image acquired using a pinhole array that showed very nicely how the effective focal spot changes across the image receptor due to the x-ray tube anode angle. I don’t recall if it was in a textbook or a paper, but it’s something I’ve been wanting to replicate for myself to include in my teaching file.

I found some ~1 mm thick sheet lead left over from from some past experiments and punched a bunch of holes in it on a 10 mm grid using a push pin.

Pinhole Grid
1 mm pin holes on a 10 mm grid punched into a sheet of lead

After some experimenting to find a decent x-ray technique to use, I ended up with these two images for the large and small focal spots.

Large focal spot 81 kV 1mAs
Large focal spot. Acquired at 81 kV, 1 mAs, 181 cm SID

Small focal spot 81 kV 1mAs
Small focal spot. Acquired at 81 kV, 1 mAs, 181 cm SID

I’ve chosen to invert the grayscale to use a black background instead of the normal white to make the focal spot images easier to see.

The pinholes are a little bit on the large side (~1 mm diameter) so the focal spot images aren’t as well defined as what I’d have gotten using a pinhole camera (which has a ~0.1 mm diameter hole), but these are good enough for demonstration purposes.

What’s going on here?

In all x-ray tubes, the tungsten anode is angled about 12-17° from the perpendicular relative to the anode-cathode direction, as shown in the image below (taken from Review of Radiologic Physics by Walter Huda).

Line focus principle
Diagram illustrating the line focus principle

When most people think about the focal spot of the x-ray tube, they’re thinking about the effective focal spot (F). The focal spot size of a tube is specified along the central axis of the beam perpendicular to the image receptor. If you were to look up from the image receptor to the x-ray tube (along F), you’d see a tiny little rectangle where the x-rays come from.

Now, consider the situation where we move away from the perpendicular to some other location along the image receptor. Now if you look back at the x-ray tube, the effective focal spot size has changed (G and H).

Effective focal spot size from different locations
Effective focal spot size from different locations

The effective focal spot gets larger as you move toward the cathode, and smaller moving toward the anode. In addition, the shape of the focal spot changes as well. This is most easily seen in the large focal spot image above.

This effect has some interesting ramifications when it comes to talking about focal spot blurring. Because the effective focal spot size changes across the image receptor, this means the amount of focal spot blurring also changes across the image receptor.  Fortunately, focal spot blurring is relatively small compared to other sources of blurring in medical imaging, so even though focal spot blurring varies across the image, it’s not a huge thing to worry about.