A study on the ability of panoramic, CT, Cone-beam CT, MRI and ultrasonography in detecting different foreign-bodies in the maxillofacial region (an in-vitro study)

Background and aim: Entrapment of foreign-bodies is a common phenomenon in traumatic events occurring in the maxillofacial region. The purpose of this study was to evaluate the efficiency of panoramic, CT, Cone-beam CT, MRI and ultrasonography in detecting different foreign-bodies in the maxillofacial region. Methods: Four different materials were used as foreign-body including metal, glass, rubber and wood. These particles were prepared in four different sizes from 2*2*2 mm to 5*5*5 millimeters. The foreign-bodies were then implanted into a sheep’s head in the infraorbital part of maxilla and mandibular buccal region. The panoramic, CT, Cone-beam CT, MRI and ultrasonography were obtained from the model and the images were blindly observed and analyzed by two radiologists. A four-point scale was used to interpret the visibility of found foreign-bodies. Results: CT had the best efficiency in detecting different foreign-bodies. Cone-Beam CT was the next useful technique. The ability of differentiating the foreign-bodies from the adjacent structures were poor in MRI and ultrasonography. As expected, the panoramic was only efficient in detecting metallic bodies. Conclusion: CT-scan can be introduced as the best imaging modality in detecting different foreign-bodies especially non-metallic ones. CBCT is also acceptable for metal, glass and rubber particles.


INTRODUCTION
Entrance of foreign-bodies into the soft-tissues is one of the most common issues in traumatic events occurring at the maxillofacial region.These foreign-bodies are originated from different kinds of materials.Inflammation, infection and pain are the most prevalent outcomes of these events.Accurate localization of a foreign-body helps the surgeon to remove it with minimum adverse effect on the adjacent tissues and reduce the harm to vital organs near the foreignbody (1).
The factors such as size, type and location of foreign-body could make the removal process challenging.Pre-operative radiographic imaging plays an important role in selecting the proper surgical approach (2).
Although metallic foreign-bodies are often detected in radiographs readily, but detection of non-metallic ones requires more comprehensive assessment.The panoramic view, CT, Cone-beam CT (CBCT), MRI and ultrasonography (US) are the most common imaging modalities that are used in maxillofacial foreign-body cases; each has some advantages and disadvantages (3,6).
The panoramic imaging has limitations in precise localization since it only depicts the foreign-body's adjacent structures in a two-dimensional view.Metallic artefacts could severely reduce the diagnostic efficiency when examining metallic foreign-bodies with CT-scan or MRI.The risk of displacement of metallic bodies limits the usage of MRI in this field.CBCT has also evident shortages in illustrating soft-tissues accurately (7).
According to the advantages and disadvantages of each imaging modality, we decided to assess the diagnostic efficiency of panoramic.CT, CBCT, MRI and US in detecting different sizes of foreign-bodies (especially non-metallic ones) in the maxillofacial region.

MATERIALS AND METHODS
In this in-vitro study, we designed and prepared four different materials including metal (steel), glass (car windshield), wood (used in the interior decoration of cars) and condensed rubber (from tires) in 4 different sizes including 2*2*2 mm, 3*3*3mm, 4*4*4 mm and 5*5*5 mm (Figure 1).A sheep's head was used in this study.The whole experiment was done one day after its death and all images were taken on the same day.Foreign-bodies were implanted in the infraorbital and maxillary and mandibular buccal regions on the bone periost using a scalpel and the skin was then sutured (Figure 2).
The panoramic radiograph, CBCT, CT-scan, MRI and ultrasonography was acquired from the sheep head.
The panoramic image was obtained with PaX-I (Vatech, South Korea) with the mA=8, kVp=60 and time=10 s.The image was exported from Easydent ver.1.0.1 software (Vatech, South Korea).The MDCT (Sensation 16 Speed 4D, Siemens, Forchheim, Germany) captured the CT images.Since the metallic foreign-bodies caused severe artefacts on CT and MRI images in the pilot study, they were removed before getting CT and MRI images.The CBCT slices were provided by PaX-i3D (Vatech, Korea) with the optimum exposure factors and EzDent3D ver.4.0 software (Vatech, Korea).The MRI scanner The obtained images were evaluated on a Lenovo L27q monitor in a normal illumination status by two radiologists.They were unaware of existence of any foreign-body and were asked to record a score for the visibility of possible bodies.A four-point scale was used to interpret the visibility of found foreign-bodies (Table 1) (8).

RESULTS
The interobserver agreement kappa coefficient was good between the observers (0.81).The ability of each imaging modality is presented separately (Tables 2 to 5) (Figures 3, 4).
Panoramic was acceptable for detecting metallic foreign bodies while CBCT performed better on both metallic and non-metallic ones.MRI and ultrasonography could not differentiate well the non-metallic bodies.CT did the best amongst other imaging modalities.

DISCUSSION
Whenever a foreign-body unintentionally enters into the maxillofacial area, it might be located in any region.This study investigated four different types and sizes of foreign-bodies in an in-vitro sheep head model.Various foreignbodies expose different objective physical characteristics when displayed with different visualization systems.A type of foreign-body could be readily detected on a radiograph while it might be untraceable with another imaging system (9).
In general, choosing the appropriate imaging modality to trace a foreign-body depends on its location, size, physical properties and adjacent structures (10).
Radiologists use different imaging systems like CT, CBCT, MRI, ultrasonography or plain radiology in order to visualize accurately the entrapped foreign-body.CT is suggested as a decent technique for envisioning and localizing foreign bodies since the physician could precisely determine the size and shape of enmeshed material (11).However, metallic   artefacts are troublesome when tracing foreign-bodies with CT scans and could obscure the observer vision.The present survey is consistent with previous studies about the efficiency of CT.When a foreign-body is located in the superficial tissues, ultrasonography could be a useful technique.On the other hand, it might be inefficient in assessing deep structures or inside the aerated cavities (12).In the current study, ultrasonography were not much successful and was only reliable in large metal foreign bodies' detection.
Although a radiopaque foreign-body is conveniently visualized on even plain radiographs, non-radiopaque ones do no generate a significant image on x-ray images.Therefore, it is the physical properties that arbitrate the visibility of a foreign-body on an image.Hence, a type of foreign-body might be missed with one imaging approach and successfully traced with another one.A foreign body can be revealed on an image when its radiodensity (beam attenuation) vary sufficiently from that of the adjacent structures.However, when the size of foreign body gets smaller, the spatial resolution of imaging system also affects its detection (13)(14)(15).
CBCT has the advantage of less metal artefacts and was accredited for localizing foreign bodies in cases studies by Stuehmer et al.Our study, CBCT exhibited well results in detecting all types of foreign bodies except wood (16, 17).
Krimmel studied wooden objects using CT and MRI and found out that CT would be the first diagnostic choice.Wooden foreign bodies were initially missed in plain radiographs and those with longer presence in soft-tissue had higher visibility due to higher density which can be caused by inflammatory reaction around the objects or absorbing water from the surrounding tissues that can increase the radiodensity of wood (18,19).
According to previous surveys results and our findings, MRI should not be introduced as the first imaging modality for visualizing foreign-bodies because of high metal artefact, risk of foreign-body displacement and being timeconsuming.Furthermore, different radiologists might have very different interpretations on MR images (13).The current study demonstrated that MRI could detect glass and rubber but since the most of these materials appear as low-signal or signal-void on MR images, the differentiation of different kinds of foreign-bodies is difficult.In the current study, CT-scan had the best efficiency in detecting different foreign-bodies.Cone-Beam CT was the next useful technique.The ability of differentiating the foreign-bodies from the adjacent structures were poor in MRI (T1-weighted, T2-weighted, FLAIR and fat-saturated images) and ultrasonography.As expected, the panoramic was only efficient in detecting metallic materials.
Detecting non-metallic non-radiopaque objects might be a challenging issue.CT and CBCT offer hope for illustrating probable non-metallic foreign-bodies in the maxillofacial region.

CONCLUSIONS
CT-scan can be introduced as the best imaging modality in detecting different foreign-bodies especially non-metallic one.CBCT is also acceptable for metal, glass and rubber bodies.

Figure 3 :
Figure 3: The panoramic view

Table 1 :
Basic criteria used for image interpretation

Table 2 :
The ability of different imaging modalities in detecting metal

Table 3 :
The ability of different imaging modalities in detecting glass

Table 4 :
The ability of different imaging modalities in detecting rubber