Answer to Question #11603 Submitted to "Ask the Experts"

I am of childbearing age. Recently, I underwent three knee x rays in an orthopedic clinic. I had three views: lateral, anterior-posterior, and patella sunrise view. They had a new radiology tech who just graduated, and it was his first day on the job. I was supine, and in both cases of lateral and patella sunrise (skyline) views, I had to hold the screen tablet with my hands.

My first question is: what would be the estimated exposure to the arm holding the screen on the side?

My second question, which concerns me most, is about the sunrise patella view. I was lying on my back with my knees bent and holding the screen on my belly with my hands. The tube was aimed at what seemed a straight horizontal line just at the level below my knees. It looked like the x rays would go through my entire body, especially my hips and head (elevated so practically unobstructed by the rest of my body), after hitting the x-ray plate. It seems like my radiation dose would be tremendously high since my whole body was in the direct line of the focused x-ray beam. I would like an educated guess on the amount of radiation delivered to my entire body from this one x ray.

You prefaced your questions with the statement that you are of childbearing age. From this, I gather that you are concerned for some future pregnancy. Rest assured that the doses associated with the x rays of your knee will have absolutely no effect on your ability to get pregnant or on the health of any future child, nor would there be any genetic damage.

You also indicated that this was the first day for a recently graduated technologist. All radiological technology schools have a very intensive clinical program where students perform x rays under the supervision of an experienced technologist and are graded on their positioning skills. So even though it was his first day, it was not the first set of x rays he has taken.

Your first question is in regard to the dose to your hands and forearms holding the image receptor. Your hands and forearms would receive less exposure than your knee received.

Your second question was on the sunrise view of the patella. (As an aside, it’s called this because the patella appears to be rising over the horizon as in Figure 1.)

Figure 1. Sunrise view of a patella

You are concerned because it appeared as if your entire body was in the primary x-ray beam. But let's take a look at the positioning from a different viewpoint (Figure 2). Notice that the x-ray machine is at a slight upward angle. The radiograph is taken with the x-ray beam tangential to the patella and parallel to the long axis of the lower extremity.

Figure 2. Patient position for a patella sunrise view x ray

Figure 3 provides another perspective. The light field is used for positioning, and it matches the x-ray field. As you can see, the x-ray field is almost parallel to the lower leg, and since the lower leg is bent, the x-ray beam is actually heading away from the rest of the body. I also included this picture (Figure 3) to demonstrate that your hands were probably not in the primary x-ray beam.

Figure 3. X-ray field for a patella sunrise view x ray

If you go back to the first image (Figure 4), the white area below the red line I drew is the lower leg bone. This further demonstrates that the x-ray machine is inclined to be nearly parallel to the lower leg.

Figure 4. Lower leg bone in a patella sunrise view x ray

All of this means that your hips, pelvis, abdomen, and chest should not have been in the primary beam and would have been exposed to scattered radiation only. Even if this was not the case, your effective dose would be less than from a routine chest x ray and would be a fraction of a millisievert (mSv).

Kent Lambert, CHP