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A Universal Definition for CT Irradiated Length

S McKenney

S McKenney1*, D Bakalyar1 , J Boone2 , (1) Henry Ford Health System, Detroit, MI, (2) UC Davis Medical Center, Sacramento, CA


SU-C-12A-6 Sunday 1:00PM - 1:55PM Room: 12A

Purpose: The length of scan shown or calculated from the console is the distance between the center of the starting and ending images. The irradiated length for such a set of images can vary substantially from this distance, depending on the acquisition mode. We propose that the rise to equilibrium function h(λ) be used to uniquely define an irradiated length independent of the details of the scanning process. This method also seamlessly accommodates recent developments in active collimation.

Method: h(λ) is determined along the central axis of a long cylindrical phantom, ideally from integration of the dose spread function. For practical reasons h(λ) may be approximated from helical scans with real-time dose rate measurements or from a series of central dose measurements using finite irradiated length exposures. For a particular scan then, the irradiated length λ is calculated from a central dose measurement of h and then applying the inverse function h(λ)⁻¹.

Results: h(λ) was determined from measurements made in a prototype ICRU/TG200 phantom, 60 cm long and 30 cm in diameter. The resultant λ generally agreed with indicated scan length plus one beam width for axial and indicated scan length for helical scans if overscanning was accounted for. For most scanners, DLP/CTDIvol is a good measure of irradiated length allowing for another index of comparison.

Conclusion: Central dose measurements with a small chamber along with the rise to equilibrium function h(λ) provide us with a robust and consistent means of determining scan length independent of the means of irradiation over a variety of scanners and scanning conditions. The method is limited to λ significantly less than the equilibrium.

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