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Dose of the cone-beam CT

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Measurement of the absorbed dose (unit: Gray, Gy) of the cone-beam CT is particularly important in case of examination of living objects like humans or living animals. Following small adjustments the same CTDI (Computed Tomography Dose Index) numbers used in third generation human scanners can be applied for this purpose. The CTDI is based on the measurement of the D(z) dose profile along the z-direction when the gantry performs one single rotation. Although the radiation is limited in space by collimators to the active surface of the detector, the dose profile in the z-direction is elongated compare to the ideal rectangular shape. The reasons are the imperfection of the collimation and the scattering of the x-ray photons. Performing scans with multiple rotations generates overlap at the edge of the dose profile; therefore it results in higher dose.

The basis of the dose calculation is the CTDI100 that is defined as the integral of the dose profile along the z-direction over 10 centimetres divided by the axial field of view. The D(z) can be measured easily by a 10 cm long ionization chamber positioned axially and transaxially in the centre:

CTDI_{100}=\frac{1}{AFOV} \int_{-5cm}^{+5cm} D(z) dz

In order to take into account the dose variations inside the object in the transaxial direction the weighted CTDI (CTDIW) can be defined as the weighted sum of the CTDI100,central (measured in transaxially central position) and the CTDI100,peripheral (measured at 3, 6, 9 and 12 o’clock position and averaged):

CTDI_{W}=\frac{1}{3}CTDI_{100, central} + \frac{2}{3}CTDI_{100, peripheral}

In spiral scanning the absorbed dose is inversely proportional to the pitch. In case of overlapping scan (p<1) the dose profiles are piled up, while in acquisitions with gaps (p>1) the profiles are separated. The volumetric CTDI can take into account this effect:

CTDI_{VOL}=\frac{1}{p} \cdot CTDI_{W}

All CTDI values are normalized to a unit length along the z-axis. To assess the absorbed dose for the entire scan range the Dose Length Product (DLP) can be calculated:

DLP=CTDI_{VOL} \cdot L

Based on the DLP value the effective dose (unit: Sievert, Sv) reflecting the biological effect of the radiation can be estimated using specie and organ specific coefficients.

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