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Pencil ionization chambers are employed for the direct determination of the computed tomography dose index (CTDI), which is used for patient dosimetry in CT examinations. The chambers are calibrated in air kerma length in a reference x-ray field. The precision of calibration is influenced by several factors which are investigated in the present work. It is shown that the best irradiation length is 50% of the chamber rated length. The reading must be corrected for a residual signal due to the irradiation of the sensitive chamber volume by scattered radiation. The origin of this radiation is identified mainly at the edges of the diaphragm limiting the useful field. The residual signal is determined by a linear fit of measurements performed at minimum of three diaphragm apertures. The optimum geometric conditions are also discussed.

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