Answer to Question #10787 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
I work in a large diagnostic imaging department at a large hospital.
Routinely, technologists are taking upright abdominal x rays at 1.83 m instead of the conventional 1.02 m. Their rationale is that they can include more anatomy on the image taken at 1.83 m and have less chance of missing the patient's diaphragms (which is a criteria for the x ray). It is also easier as the x ray is almost always preceded by a chest x ray that is always done at 1.83 m, so the technician does not have to move the x-ray tube into 1.02 m.
The other exam that the technicians do this for is an x-table lateral hip. By convention, it should be done at 1.02 m but they take it at 1.83 m.
As automatic exposure control is used for both of the above exams, the resultant image is always fine, but the mAs that has to be generated by the x-ray tube is massive.
I have had technicians argue that the dose is no different to the patient due to the difference in distance that the primary beam has to travel. I am not sure if this is true. I do believe that this practice shortens the life of the x-ray tube.
So the questions are:
- Does the practice of imaging a patient at 1.83 m versus 1.02 m increase the dose to the patient?
- Why is 1.02 m conventionally used in radiography? Is it to prolong the life of x-ray tubes?
- As imaging at 1.83 m reduces magnification, is it a balance between the two?
With automatic exposure control, the differences in imaging at 1.83 m versus 1.02 m are as follows:
Assuming a patient abdominal thickness of 0.305 m, the ratio of the skin entrance dose (SED) to the dose at the detector would be 1.832/(1.83-0.305)2 = 1.44 for a source to image distance (SID) of 1.83 m and 1.022/(1.02-0.305)2 = 2.04 for an SID of 1.02 m. So the SED might be a bit lower at 1.83 m vs 1.02 m. However the other considerations include:
- As you suggested the x-ray tube output will have to increase due to the longer exposure time necessary to get the same information density at the image receptor. This will increase the tube workload but for general purpose radiography, I would not think it would be too significant.
- Depending on the thickness of the patient there might be a chance of having an underexposed image if the time required to record an adequate exposure on the detector exceeded the backup timer shut-off time.
- The longer exposure time may also increase the chance of motion blurring in the image (slightly).
- There would be somewhat less beam divergence (magnification/distortion) at 1.83 m vs 1.02 m.
- The anatomy recorded will ultimately be limited by the size of the image receptor and on collimation. Depending on the patient size and particular setup, it may be true one could include more anatomy on the image taken at an SID of 1.83 m vs 1.02 m.
The reason why hip and abdomens (or other tabletop exams) are usually shot at 1.02 m rather than 1.83 m is because it's just much easier to use a 1.02 m SID from an ergonomic perspective, plus the tabletop grid is typically focused at that same distance of 1.02 m.
Jerrold T. Bushberg, PhD, DABMP, FAAPM
Clinical Professor, Radiology