To compare the effects of different arm positions on dose exposure and image quality (IQ) in cervical spine CT after trauma in different patient groups. Patients in standard (STD = 126) and in swimmer’s position (SWIM = 254) were included. Body mass index (BMI subgroup 1 = underweight to subgroup 4 = obese), anterior–posterior diameter (AP), left–right diameter (LR), area of an ellipse (AoE) and angle between the humeral heads (optimal STD < 3°, optimal SWIM > 10°) were used as grouping criteria. Computed tomography dose index (CTDI) was documented. Two radiologists rated the IQ at three levels (CV1/2, CV4/5, CV7/T1) using a semi-quantitative scale (0 = not diagnostic, 1 = diagnostic with limitations, 2 = diagnostic without limitations). The Mann–Whitney U test correlations of grouping criteria with dose effects and intra-class correlation (ICC) were calculated. ICC was 0.87. BMI grouping showed the strongest correlation with dose effects: CTDI of optimal STD versus optimal SWIM positioning was 3.17 mGy versus 2.46 mGy (subgroup 1), 5.47 mGy versus 3.97 mGy (subgroup 2), 7.35 mGy versus 5.96 mGy (subgroup 3) and 8.71 mGy versus 8.18 mGy (subgroup 4). Mean IQ at CV7/T1 was 1.65 versus 1.23 (subgroup 1), 1.27 versus 1.46 (subgroup 2), 1.06 versus 1.46 (subgroup 3), 0.79 versus 1.5 (subgroup 4). Patients with a BMI > 20 kg/m2 benefited from both potential dose reduction and improved image quality at the critical cervicothoracic junction when swimmer’s position was used. • BMI is a useful metric for personalized optimization in CT for the c-spine. • Using swimmer’s position, patients can benefit from dose reduction. • In some patients a superior image quality can be achieved with swimmer’s position. • For swimmer’s positioning an angle of more than 10° is optimal.