![]() 11 The results were used as inputs for an in-house dose calculation software and the computed percent depth-dose curves and transverse axis profiles in the homogeneous and heterogeneous block phantoms generally agreed with the measurements on the unit. characterized a kV x-ray source on a Varian On-Board Imager with in-air relative dose and HVL measurements. 9 Randazzo and Tambasco 10 presented a technique to rapidly characterize the CT x-ray beam in terms of fan beam angle-dependent HVL and fluence, using concentric cylindrical aluminum (Al) filters in conjunction with the Boone’s method. confirmed that the above method can also be used with an intermediate size real-time dosimeter (active volume 0.6 cm 3) and also developed a method to account for the finite size of the dosimeter. further validated the method using a real-time dose probe (active volume 0.14 cm 3) on a dedicated breast CT scanner with a custom-designed teflon bow-tie. 7 In his method, F( θ)/ F(0 ∘) can be extracted using the inverse square law and sophisticated analysis of the resultant waveform data. Alternatively, Boone proposed a method to evaluate relative bow-tie attenuation profile from radiation measurements at an off-isocenter position near the scan FOV edge in rotational CT scans. ![]() Thus, the total number of CT scans can exceed 200 when four tube voltages and two bow-tie filters are considered. 6 Such bow-tie attenuation measurement needs about 25 CT scans for a scan field-of-view (FOV) of 50 cm for every tube voltage/bow-tie combination. demonstrated that the above method can also be applied in kilovoltage (kV) cone-beam CT in radiation therapy. Because the measurements could account for changes in the scanner conditions such as x-ray tube aging, their study revealed that simulations using the source models attained high accuracy and even might have advantages over the use of the CT manufacturer-provided source data of the scanner models. 5 With the x-ray tube parked at the 3 or 9 o’clock position and an ion chamber attached to the patient table, relative bow-tie attenuation across a fan beam was cleverly measured by adjusting the table height. presented a noninvasive method to generate equivalent energy spectra and filtration models based on the half-value layer (HVL) and bow-tie attenuation measurements on multidetector CT scanners. To assist users to construct CT scanner source models in Monte Carlo simulations, Turner et al. Its importance is illustrated by the large variations of the central to peripheral CTDI 100 ratio with different CT scanner models and bow-tie filters, as shown in Tables II–V of Li et al. 3 To be successful in Monte Carlo based CT dosimetry, the attenuation profile of the bow-tie filter applied in CT scanner has to be determined accurately. The first issue may be gradually resolved in the future, with the improving performance of computing power and the adoption of graphics processing unit (GPU) or the Intel many integrated core (MIC) processor. 2 For these reasons, Monte Carlo simulation remains the gold standard for patient-specific CT dose evaluation.Īlthough Monte Carlo simulation has been widely used in medical physics, its application in patient-specific CT dose evaluation is still limited, because of limited computing speed and inaccessibility of the manufacturer’s proprietary scanner information. 1 In vivo CT dose measurement in patients is impractical. CT dose index (CTDI) is useful for characterizing CT scanner radiation output, but does not represent patient dose. Radiation dose from a CT scan is associated with multiple scanner and subject related factors, such as CT scanner radiation output, scan length, subject size, and body habitus. The results were used in the geant4 based simulations of the point doses measured using six thimble chambers placed in a human cadaver with abdomen/pelvis CT scans at 100 or 120 kV, helical pitch at 1.375, constant or variable tube current, and distinct x-ray tube starting angles. Each readout signal was corrected for the detector background offset and signal-level related nonlinear gain, and the ratio of the two exposures gave the bow-tie attenuation. Two radiation exposures were made with and without the bow-tie in the beam path. To measure the body bow-tie attenuation on a GE Lightspeed Pro 16 CT scanner, the x-ray tube was parked at the 12 o’clock position, and the detector was centered in the scan field at the isocenter height. The sampling time was as short as 0.24 ms. A scintillator based x-ray detector of 384 pixels, 307 mm active length, and fast data acquisition (model X-Scan 0.8c4-307, Detection Technology, FI-91100 Ii, Finland) was used to simultaneously detect radiation levels across a scan field-of-view.
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