For early scientific studies of mummies, as they were performed in Europe starting in the 17th century,1 the wrappings of the mummies had to be removed. The examination of their dentition remained limited to a visual inspection through the occasionally opened lips. This consequently led to misjudgments, as in the case of Ramses II. His teeth “are clean and in an excellent state of preservation: they were only slightly worn”, Smith stated in his description of the mummy unwrapped in 1886.2 A later radiographic investigation by Harris corrected this statement: “Ramesses II was in every sense a true dental cripple, suffering from extreme wear of his teeth […], extreme periodontitis […] periapical abscess”.3 The use of radiography for the non-invasive examination of mummies and their dentition then became widely used as a new method of investigation.45 Nonetheless, Moodie, in the first systematic radiographic analysis of a major collection of mummies,6 commented that the identification of caries involved “numerous difficulties, especially the intervention of many different objects producing obscurity of the teeth.”.7 The issue of superposition in planar X-ray imaging was only solved with the introduction of X-ray computed tomography (CT). This imaging modality, with its multiple post-processing capabilities, is considered somewhat of a gold standard in mummy research,8 and CT scans do indeed allow a detailed examination of dental pathologies.9 The possibilities and indications for conventional radiography, however, should not be overlooked.10 Under field conditions outside of the clinical setting, the chemical processing and development of analog X-ray films are especially difficult. The use of Polaroid film for such purposes remained an experiment.11 In this setting, digital conventional X-ray systems offer significant advantages. However, in any form of conventional radiography, the 3-dimensional structures of the skull base and the dentition are projected onto a 2-dimensional image. The resulting superposition of multiple structures on radiographs is known as anatomical noise.12 Different types of projections can be used to avoid or at least to minimize such superposition artifacts, but they require proper positioning of the head.13 When wrapped mummies are examined, especially within their coffins, the orientation of the head may be inconvenient, and compensatory adjustment of the incident angle of the X-ray beam is often limited by the thickness of the coffin walls and the position of the mummy. Furthermore, positioning of the detector plate and the X-ray source can become challenging when space in storage depots is limited. In many cases, superposition of anatomical structures therefore cannot be completely avoided and even the tube-shift technique remains of limited use.13 As a consequence, the interpretation of pathological changes of the dentition or the skeletal system in conventional radiographs of wrapped mummies remains challenging, and even the determination of laterality–that is, distinguishing between the left and right side of the dentition–is not always straightforward. CT scans allowing for later 2-dimensional multiplanar and curved multiplanar reformatting, as well as 3-dimensional volume rendering, can remedy such difficulties.14 However, for this purpose the specimens need to be transported to a facility where a CT scanner is located, at the risk of damaging fragile specimens and generating substantial costs.15 Portable digital X-ray equipment, in contrast, can be used on site and has proven its robustness under harsh conditions–heat, cold, dust, etc.–where the chemical processing of conventional X-ray film would be difficult and time-consuming.16 Furthermore, digital X-ray detector plates provide an excellent spatial resolution perfectly suited for the examination of oral and maxillofacial structures.17 Another advantage of digital X-ray systems is the almost immediate availability of images. This is especially useful under the above-discussed conditions, where it is often necessary to repeat a shot. An unknown position of the head within a coffin, amulets and jewelry obscuring an area of interest, superposition of anatomical structures, and the unknown densities of coffin walls, cartonnages, and their painted surfaces sometimes require repeated adjustments of imaging parameters (voltage and tube current). This can only be accomplished with a system that performs with minimal time consumption and cost.

In this article, the advantages and limitations of different approaches and projections are discussed for planar oral and maxillofacial radiography using portable digital X-ray equipment during archaeological excavations. Furthermore, based on our own experiences supporting anthropological investigations in the Valley of the Kings in Egypt, recommendations are provided regarding projections and sample positioning for examining the dentition of dry skulls and mummies.

Images could be acquired successfully within the expected practical limitations according to our experiences from previous similar expeditions.

Skeletonized skulls comprising the dentition of the upper jaw were mainly radiographed using the lateral, anteroposterior or semi-axial projections, primarily for evaluation of the osseous structures. However, the dentition was only partially assessable with these projections. Lateral projections provided good results in all categories.

The lateral-oblique projection was only applicable for mandibles, while axial projections could be applied for both isolated skulls and mandibles. In addition, the skeletonized upper and lower jaws were accessible for the small, high-resolution dental detector. This procedure was, however, not applicable when perioral soft tissues were preserved, leaving the oral cavity inaccessible. Isolated mummified heads were therefore radiographed using the above-mentioned most common projections as well as the axial projection and the tube-shift technique.

Entire mummies, with the head still attached to the rest of the body, were the most challenging specimens to image. Projections were often difficult to adjust, and several test shots were required to properly achieve the desired projections. Nevertheless, in many situations, not all possible projections were required for optimal diagnostic outcomes. In addition, it must be kept in mind that the investigation of these human remains was also assisted by photographic documentation, without further indication for radiography.

Descriptive statistical data (semi-quantitative data) for both readers (R1 and R2), as well as the mean values of both readers are listed in Tables 1, 2, 3, 4. ICC calculations showed that the 2 readers had good consistency in their ratings of diagnostic image quality, with values of 0.765 for single measures and 0.867 for average measures. Overall, the visibility of diagnostically relevant dental structures was rated highest by both authors on X-ray images acquired using the dental detector (consistent ratings of 4 points on the 4-point Likert scale for the imaged mandibles), the high resolution of which revealed the finest dental and osseous structures, but at the cost of a very small detector size (27×36 mm). Tube-shift projections of the dentition used for the imaging of mummified heads and entire or partial mummies achieved the second-best rating (average over both readers of 3.0±0.4 points on the 4-point Likert scale), with well-identifiable details of tooth rows and periodontal tissues, but with a slightly lower resolution, since a standard-size digital detector was used (358×430 mm). Lateral projections achieved the third-best ratings (average ratings of 2.5±0.4, 2.89±0.5, 2.61±1, and 3.27±0.6 on the 4-point Likert scale) for entire or partial mummies, mummified heads, skeletonized upper jaws, and skeletonized mandibles, respectively, still providing images of acceptable diagnostic quality. Lateral projections were the most used type of projection (in 46 out of 55 specimens).

This article reflects radiographic fieldwork over a relatively long period of time. According to our experiences, we can give some recommendations to facilitate a practicable workflow in similar conditions:

1. Fully skeletonized skulls can be positioned in almost any desired projection. Therefore, even “intraoral” shots with the dental detector placed on the uncovered dentition of the upper and lower jaw are feasible and useful. In such cases, “intraoral” radiography using a small dental detector follows standard positioning and projections, such as the parallel periapical projection or the bisecting- angle periapical projection.20 However, if soft tissues are missing, dental pathologies, such as occlusal, more extended approximal carious lesions or root caries and alveolar bone loss, can be identified immediately by visual inspection. The indications for X-ray imaging may therefore be more restrictive, for example only if hidden carious lesions or intraosseous bone alterations are suspected. A full set of oral radiographs is not initially required. Axial radiographs, using a larger standard-size X-ray detector, give an overview of the whole dentition, possibly impacted teeth, or the cortices of the upper and lower jaw. An anteriorly or laterally shifted axial projection can provide additional information on the frontal or lateral part of the dentition. In such cases, the skull or the mandible can directly be placed on the detector. In addition, the “extraoral,” lateral-oblique projection of the mandible, again using the larger standard-size X-ray detector, provides a good overview of the posterior teeth and the adjacent osseous structures. Therefore, in case of dry skulls, the specified “intraoral” techniques using the dental detector can be combined with an “extraoral” technique using a larger standard-size X-ray detector.

2. Isolated mummy heads with preserved soft tissue, in contrast, may still be freely positioned, but the intraoral space is no longer accessible due to the completely dried and rigid soft tissues. Therefore, lateral, axial, anteroposterior, semi-axial, and lateral-oblique projections have to be performed. The anteroposterior projection should be favored over the usual posteroanterior projection. This way, the fragile soft tissue structures of the face, including the nose, the lips, the chin, and the skin covering the supraorbital region, are at less risk of being damaged, since the head rests on its occipital region. The distortion resulting from the greater distance to the detector plate seems to be an acceptable trade-off in order to prevent damaging the specimen. Such a set of projections provides a good overview of the skull, the skull base, and the oral and maxillofacial structures. In semi-axial projections, defects of the skull base, often created for excerebration, can be identified and localized in the ethmoid or sphenoid region.20 The superposition of the right and the left tooth row can present a further problem, which can be countered by the tube-shift technique. This technique helps to reduce superposition and allows the laterality of the rows of the dentition to be determined. For this, two additional images, one with a more cranial and one with a more caudal positioning of the X-ray tube, are required. The row of teeth nearer to the detector plate will then move “with” the tube, and in this manner, the right and left sides can be distinguished.

3. In cases of an entire mummy, with its head still attached to the rest of the body, the situation is again different. Most the time the head's position is set and can only be slightly moved or may even not be altered at all. Nevertheless, under such conditions the intended projection of an X-ray image should still be directly verified. If the wrappings of a mummy are still preserved or if the mummy rests in a coffin or a cartonnage, a test shot is recommended to obtain information about the posture and the localization of the head. The positioning of the X-ray tube and the detector plate can then be adjusted for further images of the desired projections.

We would also like to mention some of the limitations of our study. Since some of the discussed projections are only applicable for certain types of specimens, the number of evaluated projections per object was small, which has to be considered in addition to the inherent limitations of semi-quantitative grading systems. In addition, the number and type of radiographs acquired mainly depended on the anthropologists' questions. The main objective therefore was not the production of “clinically correct” images, but to answer questions such as determination of the age at death, which explains some of the qualitative limitations of the resulting X-ray images.

In conclusion, radiographic examination of the dentition and its adjacent structures in the context of archeological excavations is a challenging task. New imaging modalities, such as CT and DVT, have substantially improved diagnostic capabilities in routine clinical practice.21 With some limitations, this also applies to mummy research. However, when investigations have to take place under field conditions, conventional planar digital X-ray imaging, due to its ubiquity, remains an excellent method-and often the only practicable one-for examining the skulls and teeth of ancient Egyptian mummies. Despite challenging field conditions, radiographic images of excellent diagnostic quality can be obtained, if an appropriate methodology regarding the selected projections and sample placement is followed.