A new portable x-ray unit

A new Carestream DRX Revolution Nano portable x-ray unit arrived at work this week. Normally the arrival of a new portable x-ray unit wouldn’t be a terribly notable event, but the size of the unit and label on the tube head intrigued me.

It’s a pretty compact unit (about the size of a shopping cart), weighing in at just over 100 kg. The x-ray tube is considerably smaller than a normal x-ray tube. I was told by the service engineer that the x-ray tube uses carbon nanotubes for the cathode. I had read about this technology a few years ago, but wasn’t aware that it was being used commercially. This is the first application I’ve heard of in the medical imaging world.

The machine is quiet during the exposures, without the normal sound of an x-ray tube anode spinning up, so I suspect this is a tube with a stationary anode.

The Nano turns out to be a pretty low power unit, even for a portable unit. The x-ray technique maxes out at 110 kV and 12.5 mAs. At 60 and 80 kV, the maximum mAs was 20 and 16 respectively. Not entirely sure if this was just a soft limit based on the imaging protocol I selected, or a hard limit. I still need to go through the documentation and the technical specifications for the unit.

Radiation output in mGy/mAs was pretty similar to a conventional x-ray tube (compared to a Shimadzu portable unit in the graph below).

Radiation output (mGy/mAs) graph
Radiation output for the Carestream Nano (orange diamond) and Shimadzu portable ( blue square)

Exposure times for the Nano were quite a bit longer though, so the tube obviously operates at a considerably lower tube current than a conventional x-ray tube. Pretty clear from the exposure rate graph below that while the mGy/mAs is similar, the Nano tube is spitting out much less radiation.

Exposure rate (mGy/s) graph
Exposure rate (mGy/s) for the Carestream Nano (orange diamond) and the Shimadzu portable (blue square). Multiple values at 80 kV are exposures at different mAs settings.

Crunching a few numbers, I found that the tube current for the Nano goes between 30 – 60 mA, about 1/4 of what I might expect for a regular portable x-ray unit, but about what I’d expect for something with a stationary anode.

kV/mA graph
mA range at different kV settings for the Nano (orange diamond) and Shimadzu portable (blue square)

As far as the kV and exposure rate wave form goes, it’s about as perfect as I’ve ever seen from any x-ray tube. Excuse the small size of the graph. The software for my meter started spitting out tiny images into my spreadsheets instead of the big ones it used to, and I haven’t figured out how to fix it yet.

kV and exposure rate wave forms for the Nano
Exposure rate (green) and kV (red) wave forms from the Nano

It’s a pretty neat little unit. Should be pretty decent for imaging babies and small kids (unless they’re very squirmy), but probably a bit under powered for imaging anything larger than a toddler. I predict the addition of another more conventional portable x-ray unit a few months down the road.

20 years at work!

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.

Next few years should be an interesting time.

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.