John G. Stears; Joel P. Felmlee; Joel E. Gray (September 1986), "cf. , Half-Value-Layer Increase Owing to Tungsten Buildup in the X-ray Tube: Fact or Fiction", Radiology, 160 (3): 837–838, doi:10.1148/radiology.160.3.3737925, PMID3737925
John G. Stears; Joel P. Felmlee; Joel E. Gray (September 1986), "cf. , Half-Value-Layer Increase Owing to Tungsten Buildup in the X-ray Tube: Fact or Fiction", Radiology, 160 (3): 837–838, doi:10.1148/radiology.160.3.3737925, PMID3737925