## 2 Materials and Methods  
The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Institutional Ethics Committee of Ziekenhuis Oost-Limburg Genk, Belgium (eudract/B-nr B371201630400--approved 17 January 2017). All patients with a planned MI-PLIF surgery from January 2017 until May 2019 were asked to participate. Patients younger than eighteen years old and patients with fusion of more than two levels were excluded from the study. Informed consent was obtained from every participant. The MI-PLIF procedures were performed by three neurosupgeons with eleven (EB--surgeon 1), sixteen (TD--surgeon 2) and nineteen (DP--surgeon 3) years of experience respectively. The assistant imaging techniques during the surgery were either biplanar OEC Fluorostar 7900 series C-arm (GE Healthcare, Little Chalfont, UK) or AIRO(r) iCT (Mobius Imaging, Shirley, MA, USA) in combination with a lateral OEC Fluorostar 7900 series C-arm (GE Healthcare); this will be referred to as C-arm fluoroscopy and iCT, respectively (Figure 1) The operating surgeon chose which imaging techniques were used during the procedures. Peak skin dose (PSD) and effective dose (ED) of the patient, surgeon, operating nurse, and anesthesiologist were measured during MI-PLIF procedures with iCT and C-arm fluoroscopy. The radiation exposure of the neurosurgeon, anesthesiologist, and operating nurse was quantified by the ED. This dosage was measured using personal dosimeters (DoseAware system, Phillips, 56,884 PC Best, The Netherlands). Personal dosimeters were allocated before the start of the surgery and were always positioned on top of the lead apron. The Hp(10) value (personal dose equivalent) was obtained using the Dose View software (DoseAware system, Phillips) by marking the start and end time of the operation. This value represents an estimate of the radiation dose at 10 mm depth in soft tissue and was used as the ED. To evaluate the radiation exposure of the patient during the surgery, the PSD and the ED of the patient were quantified. The ED of the patient was calculated using the parameters of the imaging devices and a conversion factor. The parameters of iCT were collected after each scan and included kilovolt (kV), milliampere (mA), milliampere-second (mAs),computed tomography dose index (CTDI), and dose length product (DLP). Parameters of the lateral C-arm were collected at the end of the surgery. These consisted of kV, mA, dose area product (DAP), entrance dose, and fluoroscopy time. All collected parameters provide indirect information about the X-rays and the radiation dose produced by the devices. A conversion factor was calculated with a Monte Carlo simulation; this simulation computes a 3D dose distribution for a geometrical model of a fictive patient by tracking the trajectory and energy deposition of X-ray photons through the geometrical model (9). Thereafter, the ED derived from iCT was computed by multiplying the conversion factor with the sum of all collected DLPs during surgery. The ED produced by the lateral C-arm was calculated by multiplying the conversion factor with the entrance dose. Because the placement of the graft cages in the iCT group is assisted by a lateral C-arm, the total ED for these patients was obtained by a summation of the ED of the iCT and the ED of the lateral C-arm. In order to measure the localized radiation dose on the frontal and lateral skin, the PSD was quantified with GafchromicTM films (Ashland Advanced Materials, Bridgewater, NJ, USA). To evaluate the time efficiency of the surgeries, the duration of pedicle screw placement and the total operation time was measured. The total operation time length includes the time between the first incision and the last suture. The duration of pedicle screw placement was measured from narcosis to decompression. Normally distributed or not-normally distributed, continuous variables are expressed as mean \(\pm\) SD or as median (interquartile range), respectively. Normality was assessed by the Shapiro-Wilk statistic.