Purpose: To compare the approach-to-equilibrium function H(L) measured on four CT scanners from different manufacturers.
Methods: The instantaneous dose rate dD/dt was recorded during full-length (60 cm) helical scans (AAPM Report 111 method) through an ICRU/AAPM CT Phantom using a 0.6 cc ion chamber located in its central axial plane at radii 0, 6.68, and 13.37 cm. This was done on Siemens Force, GE VCT, Philips Brilliance64, and Toshiba Aquilon64 CT scanners at 80, 100, 120 and 140 kV. dD/dt was converted to dD/dz using the table speed where z is axial position. h(L) is dD/dz integrated over length L centered about the central axial plane. h(L>5 cm) assumed the form of increasing exponential decay with asymptotic limit Deq, the equilibrium dose. h(L)/Deq = H(L), where logâ‚‚[1 â€“ H(L>5 cm)] is a line with slope 1/Î».
Results: H(60 cm) was â‰¥ 98.7% across all scanners, energies, and ion chamber locations. Although H(L) curves were qualitatively similar between scanners and over all energies, its shape changed noticeably as a function of ion chamber location; H(L=10 cm) was ~0.75 for the edge ion chamber location and ~0.55 at the center position. Î» ranged from 77 to 105 mm across all scans. For any given scanner, the range of Î» was 20% or less over all ion chamber locations and tube potentials and was 14% or less over all kV for the center dosimeter location.
Conclusion: The phantomâ€™s 60 cm length was sufficient to capture nearly all contributions to the central axial planar dose. For a given scanner, energy, and ion chamber location, Deq was relatively constant, demonstrating a fairly uniform dose distribution over the central plane. However, h(L) at the phantom center approaches Deq more slowly than at the phantom edge; this illustrates the significance of scan length to central organ doses.