Bone mineral densitometer beam widths

If you’ve ever wondered how wide the x-ray beam for a Hologic bone mineral density (BMD) scanner is, I can now tell you it’s not very wide. They’re considerably thinner than I expected in fact.

I placed a strip of Gafchromic XRCT film on top of the housing surrounding the x-ray tube and ran the scanner through its various scan modes, moving the strip between each scan.

BMD beam width
BMD beam width

Each of the vertical stripes represents the width of the beam at about 40 cm from the focal spot and about 10 cm below where the x-ray beam would enter the patient. From left to right are the beam widths for the fastest to the slowest scan modes. The scale on Gafchromic strip is marked off in millimeters.

The beam width for the fastest scan mode is 2 mm. The next two modes have a beam width of 1 mm, and the slowest scan mode uses a beam width that looks like about 0.2 mm. With a source to detector distance of a little over 100 cm, the beam width at the detector ranges from about 5 mm to 0.5 mm.