In this in vitro study, two fresh sheep's heads (one day after death) were used. First, an initial scan of the two heads was carried out to rule out the existence of any FBs or anomalies. The following five different materials, known to be common FBs in head and neck traumas, were selected: wood (dry toothpick), metal (iron), glass, tooth (both enamel and dentin), and stone (small pebbles).(Fig. 1) The materials were prepared in three sizes (0.5, 1, and 2 mm) and placed in three different locations (soft tissue, an air-filled space, and bone surface). Placement techniques are outlined as follows:

After placing the FBs in the six locations of the sheep's head randomly, they were scanned by three imaging devices (panoramic radiography, CBCT, and US) simultaneously.

The panoramic views were scanned by a Papaya (Genoray, Seongnam, Korea) device with exposure settings of 65 kVp, 7 mA, and 12 seconds.

The utilized CBCT device was a GALILEOS Comfort (Sirona, Bensheim, Germany) with settings of 85 kVp, 35 mA, large field of view (15×15 cm), and Vol 1 HC (high resolution and high contrast).

The US machine was a ECUBE7 (Alpinion, Seoul, Korea) device with a frequency of 3 to 12 MHz and a linear intraoral probe (18 mm footprint). This modality could not be used for air-filled spaces.

Ultimately, 16 images from each modality were obtained. (Fig. 2) The images were analyzed by three interpreters (one oral and maxillofacial radiologist, one postgraduate oral and maxillofacial radiology student and one postgraduate oral and maxillofacial surgery student). The observers were well trained and calibrated. A four-point scoring scale ranging from 0 to +4 was used for evaluating accuracy of the modalities for detecting FBs10.(Table 1)

Data was analyzed using IBM SPSS Statistics ver. 21 (IBM Co., Armonk, NY, USA) using Kruskal-Wallis, Mann-Whitney U-test, Friedman and Wilcoxon signed-rank tests. P<0.05 was considered statistically significant.

The accuracy of the three FB detection modalities is demonstrated in Fig. 3. Accuracy was significantly different between the three modalities for all types of FBs. For wooden materials, US was more accurate than CBCT and panoramic radiography (P=0.005 and P=0.004, respectively). For other materials, CBCT was the best detection modality (P<0.05). Metal was the most visible FB in all modalities.

Panoramic radiography was significantly more accurate for detecting metal than for detecting any other material (P<0.001). Detection accuracy for wooden FBs in CBCT was significantly lower than for other materials (P<0.001).

The detection accuracy of the modalities based on FB size is shown in Fig. 4. There was a significant difference between the accuracy of the three techniques for all sizes of FBs. CBCT was significantly more accurate for detecting all three sizes of materials (P<0.05).

The accuracy of all modalities increased with an increasing FB size, but this increase was statistically significant only for US (P<0.05).

The detection accuracy of the three techniques based on FB location is demonstrated in Fig. 5. The difference in detection accuracy between the three modalities was significant for all the three locations. CBCT was the most accurate modality for each location. US was significantly better than panoramic radiography in soft tissue (P=0.001), while panoramic radiography was more accurate than US at the bone surface. However, this difference was not statistically significant (P=0.071).

The highest FB detection accuracy was in the air-filled space for panoramic radiography, at the bone surface for CBCT and in soft tissue for US. Both CBCT and panoramic radiography exhibited the least accuracy in detection of FBs in soft tissue. The differences were statistically significant only for US (P<0.001).

In this study, three imaging modalities (panoramic radiography, CBCT, and US) were evaluated and compared. Panoramic radiography and CBCT are based on x-ray, and US is based on ultrasound waves. CBCT was the most accurate technique for detection of FBs, while US was better than the panoramic technique. Only two studies have assessed CBCT, and they compared this technique with CT scan. Valizadeh et al.2 recommended US as the first choice imaging modality for suspected FBs located in the superficial soft tissues. They recommended CBCT and then CT for FBs penetrating into deeper tissues or beneath bone. In the study by Kaviani et al.9, CBCT scan by NewTom was a more effective technique for visualizing radiopaque FBs in air, compared to conventional CT. Both CT and CBCT can detect even the smallest FBs (0.5 mm) in muscle tissue and adjacent to bone.

CBCT has higher spatial and contrast resolution than panoramic radiography because of the small voxel size (about 0.3 mm) and wide gray scale (12-bit)17. The three-dimensional images of CBCT can also improve its ability to detect and localize FBs better than the panoramic technique18. The panoramic technique has a single image layer (focal trough), and objects out of this trough are blurred and cannot be visualized. US's ability to detect FBs is affected by the acoustic impedance of the object, the depth at which it is located, and the surrounding tissues (objects in the air-filled areas and behind the bone cannot be detected by US)213. Of note, the penetration depth of the high frequency US device we used is lower than that of lower frequency devices.

We compared five different materials, including wood, metal, tooth, glass and stone. For wooden materials, US was the most accurate technique, while the other materials were best seen on CBCT. Metal was the most visible FB in all three modalities. In a study by Aras et al.3, conventional radiography, CT, and US techniques best detected metal materials. Wood was invisible on conventional radiography and had the least visibility in CBCT, while US had good accuracy in detecting wooden materials, as also found in our study. In the study by Kaviani et al.9, both CBCT devices exhibited good accuracy in detecting high-density materials (metal, glass, stone, and tooth), but the detection accuracy for wooden materials was lowest with both CBCT devices. Javadrashid et al.10 concluded that CT and conventional radiography have the highest accuracy for metallic objects and good accuracy for tooth, stone and glass materials. However, wood was invisible with both modalities. US exhibited the highest accuracy for wooden objects and was acceptable for the other materials.

The visibility of FBs on x-ray–based devices depends on the radiodensity of the materials. The higher the density, the higher the attenuation of the x-ray beam and the better the differentiation from surrounding structures18. According to previous studies, metal has the highest radiopacity, and wooden materials are radiolucent16192021. Contrast resolution is the ability to distinguish anatomical structures of objects with similar contrast22. The radiodensity of wooden materials is similar to that of muscles and soft tissue, and thus, contrast resolution is important for detection accuracy of x-ray–based devices15. The contrast resolution of CBCT is lower than that of CT because of greater scatter radiation13.

US can image objects that have different acoustic impedance compared to surrounding tissues due to echo production at their interfaces13. Therefore, it does not have the limitation of x-ray devices in detecting low-density objects.

In this study, FBs were prepared in three sizes (0.5, 1, and 2 mm). Detection accuracy of all imaging modalities increased with an increasing the size of the materials. CBCT was significantly the most accurate modality for detection of all the three sizes of the materials. Materials 0.5 mm in size were least visible on US, and the largest materials had the lowest visibility on panoramic radiography. Kaviani et al.9 showed that two CBCT devices can detect metal, stone, glass and tooth fragments in all the locations at the smallest sizes (0.5 mm), except for 0.5 mm tooth fragments at the bone surface. Wooden objects were not visible with either device except in air-filled spaces with sizes larger than 0.5 mm.

An important factor that determines the ability of any imaging modality to visualize smaller sized objects is spatial resolution. Spatial resolution refers to the ability to image small objects that have high subject contrast22. CBCT has higher spatial resolution than even CT because of lower voxel size9. US with an intraoral probe is high-frequency and therefore has higher spatial-resolution compared to conventional US13.

In the present study, FBs were placed in three locations of sheep's heads (bone surface, soft tissue, and air-filled space). CBCT was the most accurate modality for all the three locations. US was better than panoramic radiography in soft tissues, and panoramic radiography was more accurate than US at the bone surface. The panoramic technique had the highest FB detection accuracy in air-filled space, while accuracy was greatest at the bone surface for CBCT and in soft tissue for US. Both CBCT and panoramic radiography had the lowest accuracy for detecting FBs in soft tissue. Valizadeh et al.2 showed that the accuracy of CBCT and CT was not statistically affected by the location of FBs but that US was able to visualize objects in superficial tissues and unable to detect deeper FBs and objects beneath bone. In the study by Kaviani et al.9, the detection accuracy of the two CBCT devices as not affected by the location of radiopaque objects, but non-radiopaque materials were affected by the surrounding structures to a higher degree. In the study by Javadrashid et al.10, CT had the highest detection accuracy in all three locations, which is reflected by CBCT in our study. US was more accurate than conventional radiography in soft tissues, and conventional radiography was better than US at the bone surface. These results were consistent with the present study.

The prioritization of US in detection of FBs in soft tissue rather than on the bone surface is probably due to the lower penetration of ultrasound waves into tissues13. The lower accuracy of CBCT and panoramic techniques in the detection of FBs in soft tissues demonstrates that the average density of the materials was closer to the density of soft tissues rather than that of bone or air.

Future studies can evaluate these modalities in real patients. In addition, the measurement of the density and the acoustic impedance of FBs and the surrounding tissues can allow for better interpretation of results. In addition, the effect of the depth of FBs on detection accuracy can be assessed.