Answer to Question #4269 Submitted to "Ask the Experts"

I am looking for reference material for fetal exposure based on chest fluoroscopy; more specifically, attenuation of internal scatter through abdominal tissue based on a point scatter measurement. I realize that there are many situational variables, but any reference material would be appreciated.

There are several difficulties with estimating fetal exposure from chest fluoroscopy. Unlike plain radiography, many of the variables needed for the calculation are likely unknowable (unless they were recorded at the time of the procedure) and therefore must be estimated. Fluoroscopy is performed with a physician or technologist in the room administering the radiation while watching the "video" image in real time. From one patient to another there are typically large variations in both patient positioning and the total amount of time of radiation. Almost never are these values recorded at the time of the study. In addition, it is quite common for additional plain radiographs to be obtained at the time of the fluoroscopic study, but the dose attributable to these can be calculated separately as for any plain film and then be added to the total.

The easiest approach is to calculate a "dose per minute of fluoroscopy." Because the specific geometry of any given study is usually not known (and may well vary quite a bit during a single procedure) the best that can usually be done is to assume a particular geometry that is likely to approximate a typical study. For this calculation, I will assume that the study was performed with the source and detector positioned similar to that of a full-field view of the thoracic spine. The x-ray source is 102 cm anterior to the patient and the detector is directly posterior. The x-ray beam is collimated to include the entire thorax (36 x 44 cm field of view).

Besides the geometry, one must also know the effective energy of the x-ray beam for the tube voltage that is used. Typically this information is available for any particular fluoroscopy unit. Specifically, what is needed is the effective half-value layer (HVL), the units of which are millimeters of aluminum (mm Al). With this value, the fetal dose per entrance skin exposure (in mGy/R) can be determined at different stages of pregnancy from the following table (note that this table is specific to the thoracic spine geometry that was assumed):

The only other value that is needed is the amount of exposure, at the point of skin entrance, delivered per unit of time. This value is usually given in R/min and is usually available for any particular fluoroscopy unit.

Sample Calculation:
A woman gets a fluoroscopic procedure of her chest to evaluate whether the movement of the diaphragm is symmetrical. She later discovers that she is pregnant. At the time of the study she was near the end of her first trimester (three months). She has had several imaging studies, including the fluoroscopy study, and wishes to know the total radiation dose her fetus received. Looking at the periodic quality control logs for the particular unit involved, it is discovered that HVL for the settings typically used for fluoroscopy is 3.1 mm Al. In addition, that machine, at this setting, delivers 1.3 R/min. Using the closest values on the table, it can be determined that a three-month fetus will receive about 0.027 mGy/R. Taking that value and multiplying it by the exposure rate

0.027 mGy/R × 1.3 R/min = 0.0351 mGy/min

gives the dose to the fetus per unit of time that the x-ray machine is on.

The only missing variable is the actual amount of time that the machine was delivering x rays during the study. This value can be estimated by talking to the physician or technologist who was in the room during the study. In this case, the radiologist reports that the entire procedure lasted about 10 minutes, but the x rays were on for about three minutes only, and he is certain that the value is less than five minutes, because every five minutes a buzzer on the machine goes off, and in this case the buzzer never went off.

0.0351 mGy/min × 3 min = 0.1053 mGy.

0.0351 mGy/min × 5 min = 0.1755 mGy.

So, in this case, the best estimate of the fetal dose from this study is 0.1 mGy, and you can feel confident that it is less than 0.18 mGy (the amount from five minutes of fluoroscopy). These values can be added to whatever dose the fetus may have received from the other studies the patient had.

Edwin Donnelly, MD, PhD