DEXA Dual-Energy X-Ray Absorptiometry  -  Bone Density 6 times
pQCT Peripheral Quantitative Computed Tomography   - Bone Mineral Content 3 times

DEXA (also DXA) measures bone density and can also do full body composition scans.  In our program, we travel to the DEXA machine at the following times:
  • During screening
  • In quarantine while ambulatory
  • Mid-way through spaceflight simulation
  • Immediately after sim ends
  • Six months after recovery period
  • One year after release

We have scans on the spine, hip, leg, heel and wrist... where X-Ray beams are used to measure bone mass.  The DEXASCAN.COM website has an excellent page about DEXA, how it scans in linear fashion, what it looks for, and example results.

The pQCT is another type of X-Ray scan, where the mineral density of the bones is measured, using the lower leg.

Yep, we surrender a limb in there, but the clamps are gentle... you're just "stuck" for awhile.  This machine takes many sweeps around the leg to gather cross-sections for a "Volumetric Model." We are informed prior to each pQCT procedure that we will be exposed to radiation, and all female subjects have pregnancy tests prior to any scanning.  

Radiation over the duration is about 20.34 mRem for males and 44.54 mRem for females.  (For comparison, radiation for living in the city of Houston for a year is 250 mRem.)

mrem = a unit for measuring radiation, equal to the effect that one roentgen of X-rays or gamma-rays would produce in a human being. 

One mrem is one thousandth (10-3) of a “Roentgen Equivalent Man."  A roentgen is a unit of quantity of ionizing radiation, named after the German physicist and discoverer of X-rays Wilhelm Conrad Röntgen (1845-1923).