The steadily increasing use of computed tomography (CT) in medical imaging in the last decades [1-4] has raised concerns about its potential associated cancer risk. Recent epidemiological studies have suggested an association between CT exposure and cancer risk in pediatric patients [5-7]. Following the ALARA (as low as reasonably achievable) principle, the radiologic community, and CT vendors have made substantial efforts to decrease the radiation burden to patients by establishing imaging campaigns (e.g. image wisely [8] and image gently [9]) as well as by improving CT scanner technology [10-14]. However, these technologies can only reach their full potential if certain scan principles are considered.

One of the most important principles is correct patient positioning. Former studies have reported a negative impact of vertical patient off-centering on radiation dose and image quality [15-26]. An increase in the surface radiation dose of up to 51% [19] and an increase in image noise of up to 41% [16] have been observed with an off-centering of 6 cm. Proper vertical patient centering is particularly important if automatic tube current modulation (ATCM) is applied. In ATCM, the tube current is modulated based on the attenuation profile of the patient’s scout image to account for attenuation differences of body regions. ATCM aims to maintain constant image quality throughout the scan. Patient off-centering can lead to a magnification of the scout image and consequently alter the estimated patient size and modulation curve of ATCM [17; 18; 25-27].

In addition to ATCM, patient positioning also affects organ dose if bowtie filters are used [16]. The function of the bowtie filter is to compensate for differences in x-ray attenuation of the human body according to the patient’s shape. Therefore, patient off-centering can lead to misalignment of the thinner parts of the patient with thicker parts of the bowtie filter and vice versa, resulting in undesired dose depositions in the patient.

Despite a considerable number of studies investigating the impact of vertical patient positioning on radiation dose, there is a lack of knowledge in regards to its effect on the pediatric population. This is of high relevance, considering the radiosensitivity and extended life-span after radiation exposure of pediatric patients as compared to adults. Kaasalainen et al. quantified the effect of patient centering in two studies using pediatric phantoms [20; 21], with one study not applying ATCM [20] and both being limited to the chest [21]. It is important to also consider other body regions such as the head and abdomen that contain highly radiosensitive organs. In head CT, the eye lenses are one of the most radiosensitive organs for which the threshold doses for early and late deterministic effects, such as radiation-induced cataract, have been recently lowered [28; 29].

The purpose of our study was to quantify the impact of patient positioning on organ radiation doses for head and thoracoabdominal CT using dosimetry in a phantom simulating a 5-year old child.