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British Journal of Radiology (1992) 65, 931-934
© 1992 British Institute of Radiology
doi: 10.1259/0007-1285-65-778-931
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Single energy quantitative computed tomography: the effects of phantom calibration material and kVp on QCT bone densitometry

R. W. Whitehouse, BSc, MB, ChB, FRCR and J. E. Adams, MB, BS, FRCP, FRCR

Department of Diagnostic Radiology, University of Manchester, Oxford Road, Manchester M13 9PT, UK

This excerpt was created in the absence of an abstract.

Bone mineral densitometry by quantitative computed tomography (QCT) was introduced in 1976 (Isherwood et al 1976), and is now a clinically established and useful technique. It is usually performed with a reference phantom scanned with the patient. The reference phantom is required to correct for variations that may occur in measured CT numbers due to CT scale drifts or changes in effective beam energy (Cann and Genant 1980). The measured CT number in a region of interest is expressed in mineral density equivalents of phantom reference material.

A saturated solution of dipotassium hydrogen phosphate (K2HPO4) in water has been used in dosimetry as a cortical bone equivalent material for many years (Witt and Cameron, 1969) and dilutions of this material are popular for QCT reference phantoms used in trabecular bone mineral densitometry (Cann and Genant, 1980). Several potential sources of error have been identified with reference phantoms containing aqueous solutions of K2HPO4, including changes in concentration of the solute with time and displacement effects (Rao et al, 1987). These problems have stimulated searches for alternative reference materials, the most popular of which has been calcium hydroxyapatite. This has been regarded as preferable to K2HPO4 as calcium hydroxyapatite is a true constituent of bone. Calcium hydroxyapatite is not water soluble and has to be embedded in a water equivalent plastic. The attenuations of K2HPO4 and calcium hydroxyapatite alter with X-ray energy in similar, but not identical, rates to that of bone (Hubbell, 1969; ICRU, 1989).

Key Words: QCT mineral densitometry • Reference phantom • Effective energy

Received for publication November 21, 1991. Revision received February 24, 1992. Accepted for publication March 26, 1992.






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