Radiographic Technique for Archaeological Human Dry Bones: a scoping review

were included but were limited to studies involving human dry bone and written in the English language. Of 244 potential studies, results identified seventeen journal articles and four academic textbooks with direct recommendations or guidance for radiographic technique. The primary reason for exclusion was the omission of methodological detail. The majority of included texts addressed the identification of pathologies, cortical thickness or detection of Harris lines. While recommendations exist, gaps in the knowledge include dedicated guidelines for specific anatomy and the integration of photography during radiographic imaging.


Introduction
Radiography has been used in the analysis of archaeological human skeletal remains to characterise or assist diagnosis of unknown pathologies, estimate age at death, demonstrate traumatic injuries and provide indications of biological stress (Mays 2007;Leo et al. 2013;Licata et al. 2019). In short, radiography facilitates the reconstruction of the biological profile of the deceased, although cultural modifications have also been explored (Ramírez-Salomón et al. 2018). In comparison with advanced imaging modalities such as computed tomography (CT), radiography is relatively inexpensive, more accessible and with lower logistical and training burdens (Garvin and Stock 2016;Vallis 2017). The advantages of CT cannot be dismissed though, with better visualisation of overlapping structures and generation of volumetric data allowing image reconstruction and interrogation (Beckett 2014). However, the lack of access to CT in commercial practice or academia is commonplace, with current British guidelines for recording human remains advocating radiography as a viable alternative (Mitchell and Brickley 2017). This study investigates the availability of literature guiding human dry bone radiography, primarily concerning the technical recommendations and workflow processes. In parallel with clinical use of radiography, this is termed radiographic technique. For the purposes of this study, archaeological human dry bones can be defined as complete or fragmented osteological remains lacking soft tissue typically recovered through excavation.
Nevertheless, the imaging of archaeological human dry bones presents unique challenges requiring a bespoke approach. For example, excavated remains may be incomplete and impregnated with soil or other debris (Elliott in press). Furthermore, disarticulated bones require an additional osteology skillset for accurate identification of laterality and orientation that may be unfamiliar to clinical radiographers (Elliott in press). In order to overcome these challenges and develop a standardised approach for future research, an evidence-based solution based upon high-quality research is required. It is hoped that a unified approach formulated upon a robust methodology will allow comparable datasets to improve research potential.

Review aims
This review sought to map existing literature related to human dry bone radiography within archaeology. Specifically, the aim of the study was to quantify and characterise current knowledge and recommendations related to radiographic technique or imaging workflow. Literature purely related to the interpretation of radiographic images fell outside the scope of this study. Radiography within this study relates to planar imaging involving the production of two-dimensional radiographic images (also known as 'plain film radiography'). This encapsulates digital systems, computed radiography or chemical film processing but excludes CT reconstructions or fluoroscopy.

Method
A scoping review was adopted for this study owing to the heterogeneous nature of available literature, not restricted to time period of study (ancient-historic remains), research question or radiographic equipment deployed. The Arksey and O'Malley (2005) framework for scoping review methodology was used to inform the design of this study. Scoping reviews do not involve quality assessment of the literature as with systematic reviews, rather their goal is to map existing literature and answer broader research questions (Peters et al. 2015). Furthermore, the purpose of a scoping review is not to synthesise current knowledge to answer a specific question but to present themes and incidences of occurrence, often in graphical form. A comprehensive protocol with predefined objectives and methodology is required and outlined below.

Search strategy
Literature searches of JSTOR, PubMed and Science Direct were conducted using the search terms archaeology and radiography within the title or abstract, and paleoradiography, paleoimaging, or paleoradiology in any field. Publications were limited to academic textbooks or peer-reviewed journal articles written in the English language and published between 2001-2021. No geographical limitations were imposed; research was accepted from any country of origin. Relevant reference lists were also hand searched for additional literature. Searches were performed for each database on 1 November 2021.

Study selection
Study selection followed a three-stage process beginning with an assessment of title and abstract against eligibility criteria (Table 1). A broad inclusion of any study investigating or providing specific guidance for radiography of human dry bones were considered. Those involving non-human or mummified remains were excluded unless presenting information or results alongside human or disarticulated remains. The second stage involved reading the full text and applying a scoring system (Table 2) for progression onto stage three. Owing to the diversity in literature and iterative nature of scoping reviews some flexibility in methodology is acceptable (Peters et al. 2015).
A scoring system was created in order not to risk excluding those articles attaining near-fulfilment of inclusion criteria despite clear application to human dry bones.
Using this system, studies were either eliminated from the review, ascribed partial relevance or total relevance to the review aims (scoring one-three respectively). The scrutiny of study materials, methodology and concluding remarks acted as a failsafe check for inclusion prior to data charting. A threshold level for inclusion involved basic descriptions of technical radiographic details (exposure values, positioning) and/or specific commentary on bones selected for imaging. Any literature with a score of three was automatically progressed onto data charting. Radiography of human dry bones from any time period or geographical location of archaeological investigation Guidance or methodological assessment of radiographic technique relating to: Anatomy selected for imaging Radiographic views deployed Exposure factor selection Imaging workflow or concurrent activities Any medium of acquisition: computed, digital or wet-film processing

Non-human remains
Solely mummified remains Purely describing interpretation of radiographic imaging Below threshold for basic radiographic parameters: Exposure factors and radiographic views undertaken  Radiogrammetry process 2, 3, 21 Photodensitometry process 8 Advocates clinical radiographic views and positioning 4, 9

Data charting
Data charting refers to the extraction of pertinent information from included articles to answer the review aim. The following information was extracted: type of literature, relevance to scoping review, application of radiography and recommendations specific to archaeological human dry bone. At its simplest, literature was divided between academic textbooks/peer-reviewed journal articles, with a further division between those articles that identified as primary data collection studies and guidance or protocol literature. Relevance to the scoping review included guidance or evaluation of technical specification for radiography (exposure values, specimen positioning or equipment setup), imaging workflow (bone selection, role division or concurrent activities) and quantitative analysis of bone (photodensitometry or radiogrammetry). The application of radiography within studies related to how imaging was used to answer specific research question(s) or objective(s). This was not always possible as academic textbooks covered a wide range of applications, and so were assigned as 'broad spectrum usage'. Lastly, included literature was scoured for specific recommendations for technical radiographic details or imaging workflow processes. For instance, the radiographic views undertaken (positioning of the specimen during imaging) or bone selection for pathologies. A predefined set of criteria for recommendations were not followed; instead an evolving list was generated in reaction to emerging publication themes.

Synthesis of results
Data were presented as a narrative thematic synthesis to quantify and characterise current research in human dry bone radiography. Critical appraisal of literature was not performed, as per the purpose of a scoping review.  Figure 1). Of the 244 unique journal articles or academic books identified using the search terms, 31 were obtained for full reading of text for assessment eligibility, with 21 publications subsequently satisfying the inclusion criteria (Appendix). A literature map is presented in Figure 2, outlining the current research and academic textbooks for archaeological human dry bone radiography based upon the search results of this study.

Type of literature and relevance to scoping review
Peer-reviewed journal articles (n = 17) were found to be more prevalent than academic textbooks (n = 4). Of those articles, primary research (n = 14) were more common than guidance or protocol publications (n = 3). While categorising type of literature was simple, relevance to the scoping review was less clear, with some literature contributing to multiple facets. The majority of publications (n = 17) were relevant to technical aspects of radiography, with nine providing recommendations for imaging workflow. A small portion of the literature provided direct procedural guidance for quantitative analysis of archaeological human dry bones (n = 3), including details on bone selection and/or radiographic views necessary.

Application of radiography upon human dry bones
A myriad of research objectives were identified during the scoping review, as shown in Figure 2. Nevertheless, the primary focus was upon the investigation of pathologies (n = 7), cortical thickness as an indicator of bone loss (n = 3) and Harris Line investigation (n = 3). Academic textbooks lacked a singular research objective per se, instead providing the foundational knowledge required for the application of radiography. For instance, Beckett and Conlogue (2010) and Conlogue and Beckett (2020) include radiographic theory alongside practical examples of application. These case studies present unique challenges, solutions and recommendations (particularly for field applications). Not all case studies involved dry human bone though, with a preponderance of mummy examples; however the core concepts were relevant.

Recommendations for radiographic technique
Of the 21 studies included within this review, only three self-identified in their title as guidance or protocol for radiographic imaging. The remainder either offered a general overview of the topic, as with academic textbooks, or specific recommendations due to primary data collection findings. An excellent example of the latter includes Primeau et al. (2016), with their investigation into Harris Lines leading to evidencebased recommendations for radiographic views. In contrast, other primary research studies provided detailed methodological accounts of radiography but with an absence of specific recommendations in the conclusion or elsewhere. Such studies hold value though, as with Biehler-Gomez et al. (2019), whose extensive photographic and radiographic figures may serve as indicative examples of technique for future studies. Table 3 provides a break-down of recommendations for radiography of archaeological human dry bone. Recommendations for X-ray exposure factors, radiographic views and bone selection for imaging were most prevalent whereas specialist procedures such as radiogrammetry or photodensitometry were lacking. Overall, there were more publications addressing radiographic technical details than imaging workflow, but these addressed the wider interaction with affiliated disciplines such as anthropology, osteoarchaeology (both for macroscopic inspection) and radiology (for image interpretation).

Scope of literature
Although sparse, the results of this scoping review demonstrate the existence of literature specifically catering for radiographic technique with archaeological human dry bone. Furthermore, a plethora of applications have been identified, albeit in a relatively small number of publications. It should be reiterated that the results do not simply represent the use of radiography within archaeological literature, but the evaluation or recommendations for its application. A full account of all literature involving radiographic analysis would be of little value, except perhaps validating its use in archaeological practice. A variety of publications directly addressed radiography as an imaging modality in archaeology but did not offer practical advice and were therefore excluded from the review (Chhem 2006;Beckett 2014;Licata et al. 2019;Wanek et al. 2021).
In these examples the authors offer well-informed evaluations of radiography as an imaging modality or are dedicated to the interpretation of trauma or pathologies on radiographs but do not provide recommendations, as shown in Table 3. The inclusion of photogrammetry or radiogrammetry articles within the results may come under scrutiny, as they align closer to image interpretation than acquisition. However, the procedural nature and practical advice concerning bone selection and/or radiographic views warranted their acceptance. For instance, Manifold (2014) suggests excluding bones with disease, trauma or soil infiltration during photodensitometry. Other literature was tantalisingly close to inclusion but was relegated because of involvement with mummies (Kristóf et al. 2015;Beckett et al. 2020b), or animal remains (Symmons 2004). The exception was Seiler et al. (2018), who gave an account of both mummified and skeletonised human remains.
A key factor for exclusion was the omission of basic radiographic parameters within methodologies. This was more noticeable with primary research studies where radiography was pivotal to their research goal and yet details were scant. To illustrate the point, an assortment of archaeological investigations for disease, biological stress, bone loss or trauma yielded excellent examples of radiography in practice but lacked adherence to the review aims (Ameen et al. 2005;Dabernat and Crubézy 2009;Beauchesne and Agarwal 2017;Cieślik et al. 2017). Conversely, publications were found that directly addressed radiographic methodology, which may assist standardisation of future practice. Bruwelheide et al. (2001) present a detailed protocol for radiographic and photographic documentation of remains prior to reburial or repatriation. Specifically, the authors provide both visual and written explanations of specimen selection and positioning for imaging, accompanied by examples and recommendations for broad-based documentation. The texts of Beckett and Conlogue (2010) and Beckett et al. 2020a) offer a host of practical considerations, including proformas for recording specimen imaging and example risk assessments. Lastly, Elliott (in press) and Meyer et al. (2020) provide workflow diagrams demonstrating the transit of specimens between team member specialisms (e.g. photography, radiography, osteology) and the documentation process.

Gaps in the knowledge base
Although the results demonstrate a wide range of applications of radiography, the paucity of literature indicates that greater investigation is required to inform practice. The majority of available research concerns the identification of pathologies, leading to the assumption that this represents the foremost area of enquiry within osteoarchaeology. Bone loss, whether volume or density, has seen concerted research efforts (see Mays 2016; Agarwal 2018) but requires greater guidance at a practical level to encourage or facilitate further investigations. The recent article by Gilmour et al. (2021) concerning metacarpal radiogrammetry may serve as an exemplary format. The authors include a detailed radiographic methodology, with accompanying procedural guidelines for quantitative analysis of the resultant imagery. An evidence-based approach using a skeletal collection of known provenance allows for greater reliability and more robust recommendations as a result. Similar studies for other regions of anatomy, especially regarding photodensitometry, would be beneficial.
Interestingly, several studies refuted the value of radiography to identify bone lesions, stating that macroscopic osteological analysis (by eye) is superior (van Schaik et al. 2017;. However, Fatula (2021) clarifies the issue, stipulating that occult lesions that are invisible to the eye may only be identified through radiographic means (if a non-destructive approach is desired). A pragmatic approach would therefore include visual inspection followed by radiographic imaging for confirmation of diagnosis or a skeletal survey to account for other pathological manifestations elsewhere (i.e. metastasis, congenital or metabolic malformations). Bruwelheide et al. (2001) and Biehler-Gomez et al. (2019) provide the strongest arguments for radiography and photography to be used in tandem as complementary methods of recording the deceased. Literature regarding the photography of human dry bone undoubtedly exists, but the integration with radiography requires further investigation.

Limitations
This scoping review was limited to archaeological literature, thereby excluding potentially aligned disciplines that may offer valuable insights. For instance, radiography has been utilised during victim identification in forensic investigations of skeletonised remains to estimate age through dental eruption analysis (Ashifa et al. 2020). Other studies, such as Silva et al. (2013), present the use of radiography with dry bones to match post-mortem and ante-mortem dental and sinus appearances. Although incongruent with the end-purpose of the imaging (i.e. to answer questions of the law), archaeologists may yet learn valuable lessons concerning methodology, logistics and interdisciplinary collaboration. Another limitation concerns the exclusion of non-English language publications, which may have introduced language bias, thereby potentially eliminating valuable literature.
Lastly, the rapid digital access to journal articles was in contrast to a slow process of inter-library loans for textbooks, potentially limiting their inclusion within this study.

Conclusions
A total of 21 publications have been found that provide guidance, protocols or recommendation for radiography of archaeological human dry bones. The majority of these are peer-reviewed journal articles, dominated by primary research studies. Radiography has been applied to a wide variety of research objectives including identification of pathologies, Harris Lines and visualisation of trauma. Literature based upon primary data collection tended to provide recommendations for specific tasks, whereas academic textbooks were found to have a wealth of case studies and details regarding equipment or documentation process. Excluded literature typically lacked the methodological detail or subsequent recommendations to be of value to this review, despite direct relevance to human dry bones. Future research may benefit from a holistic approach to human dry bone remains, incorporating both photography and forensic research to support a standardised approach.