General limitations of measuring bone density of the vertebra in the PA projection include the confounding presence of osteophytes, calcification of paraspinal ligaments, aortic calcification and vertebral scoliosis. Additionally, existing vertebral compression fractures may be interpreted as increased bone density.
Lateral views eliminate the potential artifacts of osteophytes or aortic calcifications.
However, limitations of lateral views include the increased amount of soft tissue in this projection, and the overlap of the ribs and pelvis. This, in turn, decreases the number of vertebrae that can be analyzed, affecting the precision of the measurements and, thus, reducing the usefulness of lateral measurements for following the response to therapy.