Novel methods of measuring single scan dose profiles and cumulative dose in CT.

Computed tomography dose index (CTDI) is a conventional indicator of the patient dose in CT studies. It is measured as the integration of the longitudinal single scan dose profile (SSDP) by using a 100-mm-long pencil ionization chamber and a single axial scan. However, the assumption that most of the SSDP is contained within the chamber length may not be valid even for thin slices. We have measured the SSDPs for several slice widths on two CT scanners using a PTW diamond detector placed in a 300 mm x 200 mm x 300 mm water-equivalent plastic phantom. One SSDP was also measured using lithium fluoride (LiF) TLDs and an IC-10 small volume ion chamber, verifying the general shape of the SSDP measured using the diamond detector. Standard cylindrical PMMA CT phantoms (140 mm length) were also used to qualitatively study the effects of phantom shape, length, and composition on the measured SSDP. The SSDPs measured with the diamond detector in the water-equivalent phantom were numerically integrated to calculate the relative accumulated dose D(L)(0)calc at the center of various scan lengths L. D(L)(0)calc reached an equilibrium value for L > 300 mm, suggesting the need for phantoms longer than standard CT dose phantoms. We have also measured the absolute accumulated dose using an IC-10 small volume ion chamber, D(L)(0)SV, at three points in the phantom cross section for several beamwidths and scan lengths. For one CT system, these measurements were made in both axial and helical scanning modes. The absolute CTDI100, measured with a 102 mm active length pencil chamber, were within 4% of D(L)(0)SV measured with the small volume ion chamber for L approximately 100 mm suggesting that nonpencil chambers can be successfully used for CT dosimetry. For nominal beam widths ranging from 3 to 20 mm and for L approximately 250 mm, D(L)(0)SV values at the center of the water-equivalent phantom's elliptic cross section were approximately 25%-30% higher than the measured CTDI100. For small beamwidths, the difference in D(L)(0)SV for L approximately 250 mm and L approximately 14 x beamwidth (CTDI14nT) reached up to 50%. Peripheral point doses at 70 mm depth along the major axis of the phantom for L approximately 250 mm were up to 22% higher than for L approximately 100 mm. The differences between CTDI100 and D(L)(0)SV for L approximately 250 mm were in good agreement with the predictions made from the numerical integration of the measured SSDPs. Due to the considerable dose measured beyond the length of standard CT phantoms, CT dosimetry for longer body scan series should be performed in longer phantoms. Measurements could be made as we have shown, using a small volume chamber translating through the beam using multiple scans.