**2. Materials & methods**

Randomly chosen 44 extracted and unrestored premolars and molars teeth with appoximal carious lesions were embedded in silicone (Optosil- Bayer Dental, Leverkusen, FRG) blocks in sets of 4 (2 premolars and 2 molars in contact), simulating as far as possible their presumed anatomical relationships. Extra 3 posterior teeth (One of them with a 1 cm metal wire fixed on occlusal, the others drilled in depth of 3 mm round hole in proximal contacts) were mounted for the calibration and control.

D (Agfa Dentus M 2D, Germany) and E (Cea Dent, Ced D, Sweden) – speed radiographic films were used to take x-ray. Exposures were standardized at 18 impulses (0.3 s) from a model Kodak C6 320 FFD (70 kVp, 8 mA) radiation source (Kodak-Trophy ETX, France) that is also then integrated with the imaging system *(Kodak RVG 5000)* with distance of 20 cm holder device (short cone) (Dentsply Gendex Rinn, Milano, Italy). Films were processed immediately after exposure in fresh Kodak Readymatic processing solutions by means of a Velopex automatic temperature controlled processor (Extra-x, London, England). Processed radiographs were scanned to the computer for the measurement of carious lesions depths linearly in software. For the digital imaging, the sensor was fixed with the holder and Trophy dental unit was exposured for the 0.18 s.

The NewTom (NewTom 9000; Quantitative Radiology, Verona, Italy) CBCT machine was used for the volumetric CT imaging. The teeth blocks were inserted to the centre of a water filled glass model have a volume to simulate a human head. Exposure was; 110 kVp, 5,2 mA for 36 s.

The depth of carious lesions was measured linearly in software (NewTom 3G, generation 2).

For the validation criteria, the roots of teeth were discarded and the crowns were sectioned as mesiodistally by a low speed diamond saw (Isomet 100 precission saw, Buchler, Germany) for histological measurements under the stereomicropcopy (Leica MZ 75, Heerbrugg, Germany).

Bland-Altman plot test was used to describe agreement between software and histologic measurements of approximal carious lesion depths.
