Purpose: To characterize the standard modes of Varian on board imaging (OBI) v1.4 system and identify techniques to further optimize imaging parameters, in particular, for breast treatment setup.
Methods: A male anthropomorphic torso phantom was used for image quality assessment and a simpler thorax phantom for dose measurements. Both phantoms had artificial breasts attached. Doses were measured with an ion chamber in seven locations in the thorax and the breast. Evaluation of image quality was performed in terms of contrast-to-noise ratio (CNR) and in combination with the dose-to-contrast-to-noise (CNRD) parameter. The effect of kVp and mAs on the image quality, dose, and CNRD parameter was analyzed. In addition, image geometry with noncentral isocenter location with start and stop imaging angles adjusted for greater sparing of the contralateral breast was evaluated in terms of image quality and dose.
Results: The measurements showed doses between 0.02 and 1.6 cGy for the three full-fan modes and 0.6-3.2 cGy for half-fan modes. This is a reduction of over 80% and 30%-50% compared to OBI v.1.3 modes for full-fan and half-fan modes, respectively. The CNRD is the highest for both low dose modes (low dose thorax and low dose head). Optimal ranges for an averaged sized thorax are tube voltages not higher than 100 kVp and current-time products between 100 and 400 mAs. For the contralateral breast and lung, a dose less than 0.03 cGy per scan was measured for the optimized image geometry with the noncentral isocenter location.
Conclusions: The OBI v1.4 system allows for imaging with a larger variety of imaging parameters compared to previous OBI v1.3 systems. The largest doses (up to 4 cGy) were measured in a phantom when OBI v1.4 system was used for imaging with half-fan modes. Using full-fan modes resulted in the doses less than 1.6 cGy. Further decrease in dose may be achieved by reducing mAs while preserving acceptable image quality. Organ specific sparing (e.g., contralateral breast) may be achieved by proper selection of the start and stop angles. For thorax imaging, the use of Low Dose Thorax mode is recommended.