All posts by TheTraumaPro

How Much Radiation Exposure In Imaging Studies?

Everyone knows that CT scans deliver more radiation than conventional x-ray. But how much does each test really deliver? And how significant is that?

Let me try to put it all into perspective. First, how much radiation are we exposed to just living outside the hospital? Background radiation is everywhere. It consists of radioactive gases (argon) in the air we breathe, radiation from the rocks and other things around us, and cosmic rays blasting through us from space.

In the United States, the average background radiation each of us is exposed to is about 3.1 milliSieverts (mSv). I’ve compiled a table to show the approximate dose delivered by some of the common radiographic studies ordered by trauma professionals. And to keep it real, I’ve calculated how much extra background radiation we would have to absorb, in units of time, to have an equivalent exposure.

Read and enjoy! Remember, doses may vary by scanner, settings, and dose reduction measures used.

Test Dose (mSv) Equivalent background
radiation
Chest x-ray 0.1 10 days
Pelvis x-ray 0.1 10 days
CT head 2 8 months
CT cervical spine 3 1 year
Plain c-spine 0.2 3 weeks
CT chest 7 2 years
CT abdomen/pelvis 10 3 years
CT T&L spine 7 2 years
Plain T&L spine 3 1 year
Millimeter wave
scanner (that hands
in the air TSA thing at
the airport)
0.0001 15 minutes
Scatter from a chest
x-ray in trauma bay
when standing one 
meter from the
patient
0.0002 45 minutes
Scatter from a chest
x-ray in trauma bay
when standing three 
meters from the
patient
0.000022 6 minutes

How Much Radiation is the Trauma Team Really Exposed To?

Previously, I posted about “other people” wearing perfectly good lead aprons lifting them up to their chin during portable xrays in the trauma bay. Is that really necessary, or is it just an urban legend?

Lead apron fly

After hitting the medical radiation physics books (really light reading, I must say), I’ve finally got an answer. Let’s say that the xray is taken in the “usual fashion”:

  • Portable technique in your trauma bay
  • Tube is approximately 5 feet above the xray plate
  • Typical chest settings of 85kVp, 2mAs, 3mm Al filtration
  • Xray plate is 35x43cm

The calculated exposure to the patient is 52 microGrays. Most of the radiation goes through the patient onto the plate. A very small amount reflects off their bones and the table itself. This is the scatter we worry about.

So let’s assume that the closest person to the patient is 3 feet away (1 meter). Remember that radiation intensity diminishes as the square of the distance. So if the distance doubles, the intensity decreases to one fourth. By calculating the intensity of the small amount of scatter at 3 feet from the patient, we come up with a whopping 0.2 microGrays. Since most people are even further away, the dose is much, much less for them.

Let’s put it perspective now. The background radiation we are exposed to every day (from cosmic rays, brick buildings, etc) amounts to about 2400 microGrays per year. So 0.2 microGrays from chest xray scatter is less than the radiation we are exposed to naturally in about 44 minutes!

The bottom line: unless you need to work out you shoulders and pecs, don’t bother to lift your lead apron every time the portable xray unit beeps. It’s a waste of time and effort, unless you are dealing with xray imaging on a very regular basis! And that 52 microGrays the patient absorbed? That’s 8 days worth of background radiation.

Wear The Damn Lead Gown!

Pet peeve time. All trauma team members must wear personal protective equipment (PPE) when they attend a trauma activation. It’s for their own protection as well as our patients’. When I am the primary faculty at any trauma activation, I quickly scan all the other team members to ensure they are wearing it. If not, I give them a “gentle reminder” that they need to go get dressed properly.

This is all well and good. But recently I’ve noticed a trend when it comes time to shoot the basic x-rays needed for assessment (chest and/or pelvic images). When the radiology tech calls out to clear the torso and make sure someone else’s head or hands are not over the patient, half the team goes running out of the room. They are missing one key component of their gear:

Yes, their lead gown! Now granted, the amount of radiation exposure is not huge as I’ve documented in previous posts. But it is cumulative and for safety reasons, x-ray exposure must be limited.

But is running out of room the best way to decrease exposure? I think not! This is very disruptive to the way the team should function and interrupts patient care. Ideally, everyone in the room within 2-3 meters of the x-ray tube should be shielded in some way. And the most effective way to do this is to wear the damn lead gown!

Bottom line: I’ve adjusted my scan when the trauma team assembles. I now look for a lead gown underneath the usual PPEs. And if I don’t see it, I remind the offenders that, if they leave the room when the x-rays are taken, I’m not letting them back in. It’s been very effective at reversing this troubling trend.

In my next two posts, I’ll detail how much radiation the team is exposed to, and how much our patients receive from the studies we order.

Serial Lab Testing: Worthwhile or Worthless?: Final Answer

In my last two posts, I detailed the serum sodium measurements in a hypothetical patient two ways. The first was a listing of daily values, and the second provided values obtained every six hours or so. It also showed the sodium supplementation that was ordered based on those values. (I’ve included the table at the bottom of this post)

What did you think? Did the extra determinations help you decide what, if any, treatment was needed? Did the therapies ordered help?

Here are my thoughts:

  • Overall, there was not a huge or rapid decline in sodium values. Given the initial values, I would not have started a saline infusion on day 1, just watched a few daily values and the patients physical exam. The infusion only provided 3gm of salt per day, and the serum Na remained fairly stable for the first 3 days.
  • There was a significant amount of intra-day variation seen on the six hour table. You need to know the normal “within-person ” variation for any lab test you order. If two assays on specimens drawn at the same time can vary by 5%, you must factor this in to your decision making. If the value is 3% lower than the previous draw, the difference could represent normal variation. Obtaining more frequent assays exacerbates the amount of variation you see and my be confusing.
  • From day 5 to 6, the sodium appeared to be rising without any salt supplementation! But then a higher dose was given, and one of the intra-day values dropped to 124. What’s up with that? More variation?!
  • Is the morbidity of frequent blood draws worth it if there is no clinical change in the patient’s exam? What morbidity, you ask? Sleep disturbances, with all the cascading problems like delirium, sundowning, administration of additional meds to compensate, and on and on. Unnecessary medication or interventions. Plus it does not promote patient or family satisfaction at all.

Bottom line: Unless your patient has a clinical problem that may deteriorate rapidly, serial lab determinations are probably not of much value. The example patient was many days out from a TBI with some extra-axial blood. So yes, he could develop hyponatremia, but it would have probably surfaced earlier. Know your within-person  variability, which for sodium is roughly +2 meq. Is your new value within that limit? Then it is statistically the same as the first value unless you see a trend over several measurements. And as always, if you note a marked change in just one value, repeat it immediately before beginning any more drastic interventions.

Reference: Biological variation of laboratory analytes based on the 1999-2002 national health and nutrition examination survey. Natl Health Statistic Reports 21:March 1, 2010.

Day/Time Na Treatment NaCl per day
Day 1 18:30 131
Day 1 22:54 132 0.9% NS @ 125/hr 3G
Day 2 05:59 133 continues 3G
Day 2 12:19 129 continues
Day 2 17:50 129 continues
Day 3 07:18 127 continues
Day 3 12:09 127 continues
Day 3 17:58 126 continues
Day 3 23:53 126 continues
Day 4 07:45 125 continues
Day 4 11:38 122 2% NS @ 25/hr 6G
Day 4 15:25 125 continues
Day 4 19:31 125 continues
Day 5 00:06 122 continues 6G
Day 5 04:04 126 continues
Day 5 08:01 122 continues
Day 5 11:50 132 stop
Day 5 16:14 126
Day 5 19:26 127
Day 6 00:20 129 9.2G
Day 6 04:42 127 2% NS @ 40/hr
Day 6 08:30 124 continues
Day 6 12:29 127 stop
Day 6 16:16 127 Salt tabs 2G tid
Day 6 20:28 132 continues
Day 7 05:22 134 Salt tabs 2G qid 8G
Day 7 12:33 135 continues
Day 8 07:02 131 stop None
Day 8 13:33 136

Serial Lab Testing: Worthwhile or Worthless? Part 2

Yesterday, I posted a series of sodium levels that were drawn daily. There was no change in clinical status as the levels varied from 131 to 125 and back up.

Now let me give you a bit more information. The patient was actually getting serial checks every 6 hours (or more)! Here’s the updated chart:

Day/Time Na Treatment NaCl per day
Day 1 18:30 131
Day 1 22:54 132 0.9% NS @ 125/hr 3G
Day 2 05:59 133 continues 3G
Day 2 12:19 129 continues
Day 2 17:50 129 continues
Day 3 07:18 127 continues
Day 3 12:09 127 continues
Day 3 17:58 126 continues
Day 3 23:53 126 continues
Day 4 07:45 125 continues
Day 4 11:38 122 2% NS @ 25/hr 6G
Day 4 15:25 125 continues
Day 4 19:31 125 continues
Day 5 00:06 122 continues 6G
Day 5 04:04 126 continues
Day 5 08:01 122 continues
Day 5 11:50 132 stop
Day 5 16:14 126
Day 5 19:26 127
Day 6 00:20 129 9.2G
Day 6 04:42 127 2% NS @ 40/hr
Day 6 08:30 124 continues
Day 6 12:29 127 stop
Day 6 16:16 127 Salt tabs 2G tid
Day 6 20:28 132 continues
Day 7 05:22 134 Salt tabs 2G qid 8G
Day 7 12:33 135 continues
Day 8 07:02 131 stop None
Day 8 13:33 136

Confused? Me, too! This poor person had 30 blood draws in 8 days, with 6 per day for two of those days. Carefully look at the amount of salt given in each 24 hour period, and look at the sodium levels for that day.

See the variability, even when getting high doses of sodium chloride? What does this tell you? Was the salt administration helpful? Was seeing the lab value every 4-6 hours valuable?

Tell me what you think. Leave comments or tweet your opinions. Next, I’ll discuss the known variability of the serum sodium assay, and give you my opinion on the value of serial testing.