This prospective cohort study was carried out in the Department of Otorhinolaryngology at Tampere University Hospital, Finland from 2006 to 2015. The study was approved by the ethical committee of the Pirkanmaa Hospital District (no 96032). Written informed consent was obtained of each participant. A random sample of 59CRS patients, who were evaluated to benefit from sinus CT scans during 2006-2007, was enrolled to this study. Patient data was collected from medical records and by a questionnaire at the time the sinus CT was performed. The follow-up data of sinonasal operations and time of surgery was collected form patient records of Tampere University Hospital or Tampere City Hospital in 2015 in average 9 years after the time of performing sinus CT scans. None of the subjects had used Aspirin desensitization, allergen immunotherapy, or anti IgE therapy prior to or during the sinus CT scans or during the follow-up.

The patients underwent routine sinus multiple detector computed tomography (CT_MD) examinations for clinical purposes. Two different CT_MD machines were used: GE Light Speed 16 (GE Healthcare, Milwaukee, Wisconsin) and Philips Brilliance 64 (Philips, Best, the Netherlands). The patients were imaged in supine position with a kilovoltage of 120 kV and a milliampere second of 100 mAs. In the GE machine, the slice thickness was 0.625 mm with coronal reconstructions at 1.5 mm. In the Philips machine, the slice thickness was 0.9 mm with coronal reconstructions at 0.9 mm. Both were three dimensional (3D) in nature without any gap. In all cases, the imaging was performed using a bone filter technique. The imaging covered the entire sinonasal area both in the axial and coronal directions. The coronal reformations spanned through the entire area, starting from the anterior wall of the frontal sinuses and ending to the level of the posterior wall of the sphenoid sinuses. We performed 3 mm-slice-thick reconstructions (CT_3mm) from coronal, sagittal and axial data sets of CT_MD. The coronal reformations in the CT_MD and CT_3mm spanned through the entire area, starting from the anterior wall of the frontal sinuses and ending at the level of the posterior wall of the sphenoid sinuses.

CT_MD and CT_3mm scans were observed by three independent observers blinded to each other and to the patient history data. The observation of the same patient’s scans occurred at least one week apart. The focus was to compare agreement between CT_MD and CT_3mm scans and between three observers in evaluating CRS-related changes and radiological anatomy of the structures. The observers were an experienced head and neck radiologist (AM), an experienced ENT- and rhinosurgeon (JN), and a fifth year ENT resident (ST-S). They filled in a 49 -item form of sinonasal structures from both scans and from both sides of each patient (Table 3). Similar forms were filled from CT_MD and CT_3mm scans. Evaluation of the same patient’s scans occurred at least 7 days apart. All asked structures had 2 - 5 different choices. Before starting the evaluation of the CT scans, all choices were carefully discussed by the observers. Observers also made a pilot of 15 CT scans in order to make sure that all observers understood how to fill in the forms. The Radiologist did not respond to the questions: “Need for septoplasty” and “Grade of surgeon’s confidence based on images”.

Statistical analysis was carried out by the SPSS Base 15.0 Statistical Software Package (SPSS Inc., Chicago, IL, USA). Cohen’s kappa was used to compare the degree of agreement of CT_MD and CT_3mm scans (e.g. intraobserver agreement); and the inter-observer agreement of CT_3mm scans. The calculation is based on the difference between how much agreement is actually present compared to how much agreement would be expected to be present by chance alone. The established interpretation of Kappa-value is classified into 6 subgroups: Poor ≤ 0.2, Fair 0.21 - 0.4, Moderate 0.41 - 0.6, Good 0.61 - 0.8, and Very Good 0.81 - 1.0. A value under zero means that the agreement is worse than by chance, and the value range is from −1 to +1. Two-tailed P-values of <0.05 were considered statistically significant.

The characteristics of patients are shown in Table 1. The mean (min-max) age was 43 (13 - 77) years by the time of taking the CT scans. 54.2% of patients underwent sinonasal operation within a year after the CT scans were performed (Table 1). Of these patients that were operated at the time of CT scans, 7 (21.9%) underwent revision sinonasal surgery in average 3 years later. 47.5% of patients reported suffering from diseases (other than CRS) with regular need of medications. The most frequent diseases were (number of parients): heart and vascular diseases [11] , hypothyreosis [8] , migraine [3] and arthrosis/arthritis [4] .

Lund-MacKay scores (Table 2) and 43 other structures were evaluated by a radiologist, an ENT surgeon and an ENT resident from both CT_MD and reconstructed CT_3mm scans (Table 2, Table 3). The inter-observer agreement for CT_3mm scans was moderate (kappa 0.4 - 0.6) in the majority of structures (Table 3). In CT_3mm scans, the surgically important structures with greatest disagreement were found in Keros classification (radiologist vs. ENT resident), the location of anterior ethmoidal artery and optic nerve (all observers), prominent ethmoid bulla (ENT resident vs. ENT surgeon), and thickness and contact to middle turbinate of orbital lamina of ethmoidal bone (radiologist vs. ENT surgeon) (Figure 1, Table 3).

We compared the degree of agreement between CT_MD and CT_3mm scans of the Lund-MacKay scores and 43 oth-

Abbreviation: VAS = visual analogue scale (0 - 10).

er structures evaluated by the radiologist. In general, the intra-observer agreement was very good (kappa 0.8 - 1.0) in the majority of structures, such as Lund Mackay scores (Table 4). Yet, several surgically important structures did not show adequate agreement: OMC-area, frontal and spheno-ethmoid recess, lamina papyracae, and optic nerve (Table 4). Anterior ethmoidal artery was responded to be undetectable in 95% of the CT_3mm scans, but only in 13% of the CT_MD scans (Figure 1).

ENT surgeon and ENT resident estimated operative certainty of CT_3mm in comparison to CT_MD. ENT surgeon responded that the operative confidence is good in 69.6% of CT_MD scans, and 64.3% of CT_3mm scans. The intra- observer agreement in the grade of ENT surgeon’s confidence of CT_3mm in comparison to CT_MD was moderate (kappa 0.4, P = 0.005). ENT resident responded that the operative confidence is good in 73.2% of CT_MD scans, but only in 37.5% of CT_3mm scans. The intra-observer agreement in the grade of ENT resident’s confidence of CT_3mm in comparison to CT_MD was thus poor (kappa 0.2, P = 0.035).