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.

Charleston Eats: Thai East Fusion

Over by the Tanger Outlet Mall in North Charleston, just a few doors down from Mr. K’s Used Bookstore on International Blvd is a new Thai restaurant, Thai East Fusion.  I learned about it from a recent article in the Post & Courier and we decided to check it out this afternoon.

Rows of benches line the side of the strip mall space, with a row of tables running down the middle.  Inside is comfortable, brightly lit and not overly decorated.

We started off with a dish of kimchi and the ginger salad.  The salad that came out was larger than expected with a nice amount of sweet ginger dressing that wasn’t too gingery.

Thai East Fusion Salad and Kimchi
Thai East Fusion Salad and Kimchi

The kimchi, according to the P&C article, is made in-house using the family’s secret recipe.  Crisp, pungent, spicy, and delicious.  Probably some of the best kimchi I’ve had in the area.

Thai East Fusion Kimchi
Thai East Fusion Kimchi

Entrees come in 5 levels of spiciness: No spice, mild, medium, spicy, and Thai spicy.  For dinner, I chose the  Pad Thai with beef (Thai spicy), and Connie got the Basil Chicken (medium).  Generously laden plates came out, each enough for three or four people (two or three if everybody is really hungry). 

The Pad Thai was pretty tasty with the good flavourful kind of spicy.  A few squirts of Sriracha sauce can be used to bump up the spice level even more.

Thai East Fusion Pad Thai
Thai East Fusion Pad Thai

The Basil Chicken was quite good with a nice coconut-y curry sauce.  The chicken was nicely cooked and very tasty.  The medium spicy-ness was perfect for Connie.  For fellow anti-cilantro types, the cilantro on the Basil was just a garnish and is easily removed (or just ask to leave off the cilantro).

Thai East Fusion Basil Chicken
Thai East Fusion Basil Chicken

Everything we had was delicious.  We quite enjoyed the meal, and are looking forward to going back to try the mango sticky rice, which they unfortunately didn’t have when we were there.

Fossil Grant Collection watch battery

For future reference (and in case anybody else needs to know).

My Fossil watch (Grant Collection, FS 4736) takes a SR621SW type battery.

Naturally, it’s a different type of battery than all the other button cell using devices I have.

Also for future reference, this is a nice cross-reference chart for button cell/watch batteries.

Remembrance Day: Armistice + 100 years

Today is the 100th anniversary of the armistice that ended World War I.  A day to pay respects and honour to the millions that served and died in The Great War and the wars that followed.

Lest We Forget
Lest We Forget

In Flander’s Fields
In Flanders fields the poppies blow
Between the crosses, row on row
That mark our place; and in the sky
The larks, still bravely singing, fly
Scarce heard amid the guns below.

We are the Dead. Short days ago
We lived, felt dawn, saw sunset glow,
Loved and were loved, and now we lie
In Flanders fields.

Take up our quarrel with the foe:
To you from failing hands we throw
The torch; be yours to hold it high.
If ye break faith with us who die
We shall not sleep, though poppies grow
In Flanders fields.

Lt. Col. John McRae (1872-1918)

At 11AM, bells will ring in remembrance across the US.  When you hear them, stop a moment and remember their sacrifice.

US WW I Centennial Commission: Bells of Peace
Bells of Peace