A ¹⁵³Sm source was used with a dual photon absorptiometry technique in vitro to determine the feasibility of using this isotope as an alternative source to ¹⁵³Gd for further in vivo studies. The source activity was typically .4 Ci (i.e., at the source collimator exit) with a half life of 46.8 hours and photon energies of "103" keV and ¹⁵³Eu X-rays at "42.5" keV. The system was evaluted using phantom measurements of water, aluminum and polyethylene to simulate soft tissue, bone and fat respectively. During stationary measurements, a total absorber thickness between 15 and 22.5 cm. was used in conjunction with an aluminum thickness range of .314 to 1.91 em. (.848-5.16 g/cm²). The coefficient of variation and the accuracy at most total absorber thicknesses was less than 2%. This showed that ¹⁵³Sm could produce precision and accuracy comparable to ¹⁵³Gd. When polyethylene (fat) was present, as the amount of polyethylene in the RST measurement increased, the error due to a difference in the amount of polyethylene between the RST and the BMC measurements increased. For a percent difference in polyethylene thickness between the RST and BMC measurement of less than 50% at 16.3% RST polyethylene content, measurements of aluminum (bone) above 1.8 g/cm²(.662 cm. of aluminum) produced results accurate to within 10% of the true aluminum (BMC) value for this experimental geometry. Motional studies were performed using a modified OHIO NUCLEAR rectilinear scanner. The optimal scan speed was shown to be in the range of 4-6 mm./sec. with a sample space of between .5 to .7 cm.. The worst-case dose using the optimal scan parameters was determined as 6.2 and 1. 4 mRad per scan of approximately 24 minutes for bone and soft tissue respectively. The combination of stationary and motional in vitro studies indicated that ¹⁵³Sm could be an economical and effective alternative to ¹⁵³Gd for clinical studies. A Monte Carlo simulation was implemented to determine the optimal detector collimator entrance size. Simulation of the experimental geometry indicated that the .6 cm. detector collimation used experimentally was near the optimal size to produce efficient results in terms of accuracy and precision. A brief summary of Monte Carlo methods and radiation theory is also included. / Thesis / Master of Engineering (ME)