Purpose or Case Report: Children have up to 10 times higher sensitivity to ionising radiation, therefore dose reduction is still a hot topic in paediatric computed tomography (CT). One of the most effective dose reduction tools is decreasing tube voltage. Because of the exponential relationship between radiation dose and peak kilovoltage, tube voltage should be adjusted according to age. Since Hounsfield units (HU) are depending on the radiation dose, different computed tomography angiography (CTA) thresholds for different tube voltages should be considered.
This study was designed to demonstrate the alteration of HU depending on varied tube voltage and furthermore to consider the necessity of consequently modified CTA threshold values.
Methods & Materials: Nine different samples (water, air, fat, and six samples with different concentration of contrast agent) were included in a phantom, which was scanned with three different tube voltages (120 kV, 100 kV, 80kV). The same radiation dose (10.4 DLP) was obtained in every scans, therefore the relevant parameters were corresponding modified. HU of different samples were measured in all scans and afterward compared to each other. Measurements at 120 kV were served as reference.
Results: Compared to reference values HU has changed at 100 kV and at 80 kV up to 18.79% and 50.42%, respectively. The lower tube voltage is associated with increased HU.
Corresponding to these data there was found also increased (up to 17.13% at 100 kV and up to 47.08% at 80 kV) HU according the CTA thresholds (100-400 HU).

Conclusions: Decreased tube voltage from 120 kV to 100 kV or to 80 kV is followed by alteration in CT Hounsfield units. Therefore CTA thresholds should be adapted to tube voltage.