A Literature Review on Bitewing Radiography

Bitewing radiography: A Literature Review

Introduction

 Diagnostic ionising radiation can cause physical damage to target tissues1, therefore a thorough understanding of dosimetry can optimise the quality of treatment and safety to patients. Radiation dose can be measured in terms of absorbed, effective and equivalent dose. Absorbed dose (Gy) is the energy (by ionising radiation) imparted per unit mass in an absorbing medium and is related to the number of ionising events in any biological target applying to all radiation exposures.1  When assessed, the absorbed dose correlates with delayed, cumulative and stochastic effects.1Equivalent dose is the sum of absorbed dose in a tissue from different types of contributing radiation multiplied by their respective radiation qualities.1 The International Commission on Radiological Protection (ICRP, 2007) defines effective dose (Sv) as the sum of the equivalent doses in principle tissues, each weighted by a tissue weighting factor (wT).2  The wT are generalised for all ages and genders, hence they do not apply to any specific individual or radiosensitive subpopulations (children or women).1The revised wT in relation to the head and neck are presented in Table 1, along with the figures derived from data on the risks of heritable disease and cancer initiation.2Note that the lens of the eye is not given a tissue wT by the ICRP.

According to ICRP, effective dose does not provide an individual-specific dose, thus it should not be used for epidemiological studies or as a predictive tool to determine future cancer risk.1, 2Rather, it should be used for dose assessment in planning and optimising radiological protection.1, 2 Potential detriment of an individual should be based on tissue radiation absorbed dose and dose-response relationships.1

Current protocols, such as ALARA (As Low As Reasonably Achievable) and its variations, were last updated in 2014 to better protect patients from excessive and unnecessary radiation exposure. ALARA call for the use of techniques such as collimation, which is the act of restricting the primary beam and scattered radiation. In intraoral imaging, this can be achieved through the use of different geometric open-ended collimators. In extraoral imaging, the size of the primary beam is restricted in accordance to age appropriate settings.

Recent advancements in extraoral imaging has reduced the challenges faced by conventional intraoral imaging such as children, special needs patients and patients with a strong gag reflex.3 Since extraoral bitewing radiography has a high sensitivity and low specificity for certain dental diagnoses, it can be used as an alternative method for intraoral bitewing radiography.4 However, for as far as to the knowledge of the authors of the current review, there is no information available on the patient absorbed radiation doses from EBs compared to conventional IBs.

Literature review

Intraoral and extraoral imaging

Intraoral imaging is the most common technique used in dentistry and it has been used traditionally to evaluate individual teeth (periapical lesions, caries, periodontal bone levels and dental trauma).5 The limitations, however, include superimposition of structures, possible geometric projection errors and lack of the third dimensions.5 Typical effective doses from an intraoral bitewings (IBs) ranges from 0.66-18.47 μSv depending on the x-ray machine configuration and type of sensor.6

The development of extraoral imaging modalities such as dental panoramic radiography, cone-beam computed tomography (CBCT) and cephalometric radiography have widened the scope of possible structures imaged and the possibility of three dimensional imaging. Out of all the dental extraoral imaging options, panoramic tomography is most commonly found and used in private dental practice.6

Panoramic X-ray machines are a type of tomographic machines which produces a curved image layer (focal trough) centred at the contours of the jaw.5 Panoramic radiography has been used to assess dental developmental stage, dental anomalies as well as pathology of the jaw bones.5 However, the radiation dose from panoramic radiographs is a concern due to the higher radiation dose compared to intraoral radiographs and therefore should be used only when deemed necessary.7 Their dose has reduced over time due to manufacturers employing multiple or continuously moving rotation centres, using faster intensifying screen-film systems, digital image detectors and using different imaging modes.8 Further radiation dose reduction can be achieved by lowering the kVp (the energy of the X-rays), milli-Amperage (number of X-rays), acquisition times and patient size appropriate settings.9

A more recent development in the last decade in extraoral imaging is the ability to take bitewings through panoramic machines.4 This imaging option uses the panoramic machine to take only two images from either lateral sides of the patient. Extraoral bitewings (EB) are able to take an image from the lateral incisors to the third molars, including the crown and apical areas and has increased X-ray photon saturation compared to IB resulting in decreased noise10. Although there are many studies looking at dosimetry on panoramic radiography, there is no literature on dose from EB at the time of writing. Chan et al.3 had found that EB could be an attractive alternative to IB as it was found to be able to capture images with more open interproximal contacts and were actually able to detect more interproximal caries. Another study by Walaa Hussein et al. found that the accuracy of IB compared to EB in detecting proximal caries was not significantly different.11 Therefore, the aim of our in-vitro study is to measure the radiation dose associated with EB from a Planmeca® panoramic machine (ProOne) and a Planmeca® CBCT machine with proper panoramic radiography features (Promax 3D Max) and to compare these doses with IB doses.

Tissues at risk

Both conventional intraoral and panoramic radiographs, pose a risk for radiosensitive organs in the head and neck region.12 These structures are prone to scattered radiation which can increase the risk of thyroid cancer and cataracts.13

Shousha et al.’s 2010 study examined the absorbed dose (mGy) in areas of interests from the head and neck region from 14 intraoral periapical films (full-mouth survey) and panoramic radiographs.8An adult male Rando-phantom was used with thermoluminescent dosimeters (TLDs) for average and large-sized adults (i.e. moderate and maximum dose, respectively).8The results reveal that for all doses, the equivalent doses during a full-mouth survey are higher than those from panoramic x-ray for all tissues except skin.8 They hypothesised that the lower equivalent dose for the skin could be due to the fact that the proportion of skin is far less during intraoral radiography.8 The average absorbed dose is also higher for intraoral radiographs, since they are performed without intensifying screens, thus have greater spatial resolution (implying higher radiation dose) than panoramic radiographs.14However, the third cervical vertebrae received a higher dose from the panoramic than the intraoral x-ray, since panoramic imaging involves irradiation of the spin.8 Since the thyroid gland is one of the more radiosensitive organs and is frequently exposed to scattered radiation and the primary x-ray beam, the results show that the thyroid gland received 4.5μGy during intraoral radiography.8

A similar study conducted by Granlund et al., in 2016, examined four IBs and panoramic imaging.31 Salivary glands and the oral mucosa received the highest absorbed intraoral doses of 542μGy, whilst the thyroid gland, bone surfaces, eye, muscle and lymphatic nodes received 8μGy, 8μGy, 4μGy, 4μGy and 4μGy respectively.15  For panoramic imaging, the absorbed doses were the highest for salivary glands and oral mucosa, with doses varying between 348-2887μGy.15  The absorbed doses to other head and neck organs were all less than 230μGy. The effective doses were between 19-57μSv.15Hence, Granlund et al. and Ludlow et al.(2008), suggested that salivary glands and oral mucosa absorbed the highest doses of radiation during radiographic examinations.15, 16However, unlike Shousha et al., Granlund et al. concluded the absorbed dose to the salivary glands during panoramic radiography is 2-3-fold higher than the dose from a full-mouth intraoral examination.15

A 2019 paper by Rivera et al. , also determined the absorbed and effective dose in the head and neck region due to scattered radiation in patients undergoing panoramic radiography.12 Previous studies on both patients and phantom heads report that the dose to the thyroid varies from 34.4-94.7μGy17-21, whereas for the lens of the eye, doses range from 70μGy20 to 110μGy.17 The International Atomic Energy Agency (2010) accepts that in panoramic radiography the typical range for the effective dose of the salivary glands varies from 4-30μSv.22The salivary glands revealed to have the highest average absorbed and effective dose of 3044.3μGy and 30.4μSv respectively due to scattered radiation.12 The lens of the eye closely followed with average effective and absorbed doses of 10.5μSv and 87.4μGy respectively.12 Comparatively the thyroid glands had the least average absorbed and effective dose of 94.7μGy and 3.8μSv respectively.12 Toossi MT reported that the radiation dose specifically from panoramic x-ray to the parotid gland and occipital region was 319-367μGy and 262μGy respectively.23They reasoned that the highest dose in the parotid gland was due to its location within the primary beam in both intra and extraoral radiography.15, 23However, since the lens of the eye is outside the area of primary exposure and due to the higher threshold of the TLDs they found the eye, received no measurable dose.23

It should be noted that although the thyroid absorbed dose is in the same order of magnitude as the lens of the eye in some studies, the effective dose of the lens of eye is larger due to the weighting factor of the organ.12Although the gonads are farther away from the mammary glands in the area of entry of the x-ray beam (maxillary), the absorbed doses are approximately the same, implying that the distance has no effect.12 The most probable explanation for this is that when the x-ray beam hits the maxilla, the scattered x-rays which hit both the gonads and mammary glands have the same dose due to the scattering angle of the photons.12

Collimation

 The aim of any dental radiograph is to give diagnostic value while keeping the radiation exposure dose as low as reasonably achievable (ALARA).24 Because there are very few data regarding the radiation doses from EBs, there is need for an investigation. One of the many recommendations made by the American Dental Association (ADA), ICRP, and the National Commission on Radiation Protection (NCRP) to reduce radiation dose is to use collimators. 2,24-26 The rationale behind collimation is to restrict the primary beam and scattered radiation, which minimises the field of view (FOV) so that the area exposed to radiation is reduced accordingly.27-29 NCRP guidelines state that the x-ray beam should not exceed the receptor by more than 2% of the source-to-image receptor distance.26 This is necessary due to ionisation risk being cumulative, and the high frequency use of dental imaging throughout a patient’s lifetime.30 There are two types of open-ended collimators used in IO radiography – circular and rectangular.27 To gain better insight into the potential of reducing patient radiation dose in IO and EO, their relationship with or without various geometric collimators is investigated. Considering the rapid development of technology regarding dental imaging, it was decided that for this review only papers published in the past 10 years (2009-2019) would be included.

In 2019 a systematic review published by Avdeesh et al.,24 which investigated the evidence available regarding the reduction of radiation dose when rectangular collimators were used compared with circular collimators, or the absence of collimators. From their systematic review, Aps and Scott’s 201429 and Johnson et al.’s 201427 paper, which involved patient skin dosimetry and IBs both in an in vitro and/or virtual reality environment, were discussed.

Aps and Scott reported that, in their study on anthropomorphic phantoms and Monte Carlo calculations, the use of rectangular collimation decreases the patient radiation absorbed dose to 50% compared to circular collimators during IO bitewing imaging.29 Likewise, Johnson et al. reported up to 40% reduction.27 Table 2 summarises the dose reduction percentages based on organ sites that are relevant to this study. Note that the percentages were calculated by the authors of this current paper. The conclusion drawn from these two recent studies are homogenous. Both sets of authors found significant dose reduction at the thyroid, lens of eye, and salivary glands. In particular, Aps and Scott reported significant reduction at the thyroid in adults of up to 83.9%.29 Although not to such extent, Johnson et al. also reported clinically significant reduction of up to 49.2%.27 The relationship between dose reduction and different collimators, specific to IO bitewings in children, was not investigated in Aps and Scott’s study but Johnson et al. reports around 55.2% for this relationship.27

One reason why dose reduction at the thyroid is close to being 3 times higher than other radiosensitive organs in Aps and Scott’s study could be due to the difference in scattered radiation29. As the original authors acknowledge, the anatomical position of the thyroid gland implies that the equivalent dose recorded is purely from scattered radiation compared to that of other radiosensitive organs like the salivary glands, which are directly in the primary beam.29 For this reason, it may be safe to assume that in comparison to circular collimators, rectangular collimation has much higher protective value for other peripheral organs that are not directly in the primary beam pathway, such as the lens of the eye. Currently, there are no studies providing radiation dose for the lens of the eye during IO imaging that are considered recent by the authors of this paper, and it is hoped one can obtain equivalent dose readings on the lens of the eye through skin dosimetry to facilitate more direct comparison of data in future studies.

A 2019 paper by Magill et al. 31, that is obviously not included in the aforementioned systematic review by Avdeesh et al.24 compared radiation dose reduction between three different sized circular collimators and a universal rectangular collimator using kerma area product (KAP). KAP is a measure of the energy imparted by air by ionising radiation over the entire physical area of the x-ray field. Although KAP is not directly comparable to equivalent or effective dose, KAP reduction can be, and has been in this paper, converted to a percentage to allow for comparison with figures from Aps and Scott29, and Johnson et al.’s papers27.

Similar to the aforementioned papers, Magill et al. reports up to 60% radiation dose reduction when using rectangular collimators superimposed on a large (31.7cm2) circular collimator.31 The respective figures for a medium sized (25.7cm2) and a small sized (20.4cm2) circular collimator were around 54% and 40%.31 As can be seen in Table 3, Magill et al. has also reported dose reduction for radiosensitive organs after using rectangular collimators. A significant finding is dose reduction of up to 81% at the thyroid, which is of similar value to 83.9% as reported in Aps and Scott.29, Magill et al.’s31 study was conducted on adult phantom heads only. When comparing these KAP reduction percentages to the dose reduction percentages of adults in Aps and Scott, and Johnson et al.’s papers, the conclusion drawn is homogenous. Rectangular collimators have superior dose reduction ability over circular collimators in IO imaging.27, 29, 31 However, discrepancy in dose reduction of the salivary gland across Johnson et al. and Magill et al.’s papers, and the lack of data regarding lens of the eye, reflects inconsistency in the effects of collimation at the organ specific level.27, 31

 Conclusion

From this narrative literature review, it is clear that further research should be conducted in both IO and EO imaging regarding radiation doses. There is inconsistent data regarding dose reduction at the salivary glands, and the lack of data regarding the lens of the eye calls for more dosimetry experiments to be conducted at these anatomical sites, for both IO or EO imaging. To the best of the authors’ knowledge, radiation doses relevant to EO imaging are centred around conventional panoramic radiography only, whereas no studies have investigated dosimetry in EO bitewings. As a result, the values mentioned above are perhaps an overestimation as the FOV for panoramic bitewings is smaller. Similarly, studies such as Shousha et al8 and Grandlund et al15 analysed the radiation doses from full mouth series rather than single IO bitewings, which further overestimates radiation dose. More dosimetric studies on EO bitewings, with organ specific dosimetry, and also collimation during IO imaging, are needed. The latter will provide clinically relevant information on what the consequences are of EBs with regard to patient radiation dose.

References

1. Fisher DR, Fahey FH. Appropriate use of effective dose in radiation protection and risk assessment. Health physics 2017;113:102.

2. Wrixon AD. New ICRP recommendations. Journal of radiological protection 2008;28:161.

3. Crombie K, Shaikh A, Harnekar S. An alternative extra-oral digital technique for bitewing radiography. South African Dental Journal 2018;73:265-267.

4. Chan M, Dadul T, Langlais R, Russell D, Ahmad M. Accuracy of extraoral bite-wing radiography in detecting proximal caries and crestal bone loss. J Am Dent Assoc 2018;149:51-58.

5. Vandenberghe B, Jacobs R, Bosmans H. Modern dental imaging: a review of the current technology and clinical applications in dental practice. Eur Radiol 2010;20:2637-2655.

6. Mettler Jr FA. Medical Radiation Exposure in the United States: 2006–2016 Trends. Health physics 2019;116:126-128.

7. van der Stelt PF. [Panoramic radiographs in dental diagnostics]. Ned Tijdschr Tandheelkd 2016;123:181-187.

8. Shousha HA, Abd-El Hafez A, Ahmad F. Dosimetric study of the effective doses resulting during dental X-ray and panoramic radiography. Radiation Effects & Defects in Solids 2011;166:67-73.

9. Wahid MA, Choi E, MacDonald DS, Ford NL. Dosimetry analysis of panoramic-imaging devices in different-sized phantoms. J Appl Clin Med Phys 2017;18:197-205.

10. Terry GL, Noujeim M, Langlais RP, Moore WS, Prihoda TJ. A clinical comparison of extraoral panoramic and intraoral radiographic modalities for detecting proximal caries and visualizing open posterior interproximal contacts. Dentomaxillofacial Radiology 2016;45:20150159.

11. Abu El-Ela WH, Farid MM, Mostafa MS. Intraoral versus extraoral bitewing radiography in detection of enamel proximal caries: an ex vivo study. Dentomaxillofac Radiol 2016;45:20150326.

12. Campillo-Rivera GE, Vázquez-Bañuelos J, García-Duran Á, Escalona-Llaguno MI, Arteaga MV, Vega-Carrillo HR. Doses in eye lens, thyroid, salivary glands, mammary glands, and gonads, due to radiation scattered in dental orthopantomography. Applied Radiation and Isotopes 2019;146:57-60.

13. Lee B, Lee J, Kang S, et al. Calculation of patient effective dose and scattered dose for dental mobile fluoroscopic equipment: application of the Monte Carlo simulation. Radiat Prot Dosimetry 2013;153:80-84.

14. Davis AT, Safi H, Maddison SM. The reduction of dose in paediatric panoramic radiography: the impact of collimator height and programme selection. Dentomaxillofac Radiol 2015;44:20140223.

15. Granlund C, Thilander-Klang A, Ylhan B, Lofthag-Hansen S, Ekestubbe A. Absorbed organ and effective doses from digital intra-oral and panoramic radiography applying the ICRP 103 recommendations for effective dose estimations. The British journal of radiology 2016;89:20151052.

16. Ludlow JB, Davies-Ludlow LE, White SC. Patient risk related to common dental radiographic examinations: the impact of 2007 International Commission on Radiological Protection recommendations regarding dose calculation. The journal of the American Dental association 2008;139:1237-1243.

17. Moudi E, Hadian H, Monfared A, Haghanifar S, Deilam G, Bahemmat N. Assessment of radiation exposure of eyes, parotid and thyroid gland during panoramic radiography. World Journal of Medicine and Medical Science Research 2013;1:044-050.

18. Schulze RKW, Cremers C, Karle H, de las Heras Gala H. Skin entrance dose with and without lead apron in digital panoramic radiography for selected sensitive body regions. Clinical oral investigations 2017;21:1327-1333.

19. Wrzesien M, Olszewski J. Absorbed doses for patients undergoing panoramic radiography, cephalometric radiography and CBCT.  2017.

20. Kanzaki T, Takahashi Y, Yarita K. Absorbed dose to the eye lens during dental radiography. Oral Radiology 2017;33:246-250.

21. Jung Y, Dong K-R, Kweon D-C, et al. A study on the Effects of Scattering Dose on Eyes and Thyroid for Panoramagraphy (Focus on TLD and PLD). Journal of Radiation Protection and Research 2010;35:1-5.

22. Organization WH. Radiological Protection for Medical Exposure to Ionizing Radiation. Safety Guide (Spanish ed.).  2010.

23. Toossi MTB, Akbari F, Roodi SB. Radiation exposure to critical organs in panoramic dental examination. Acta Medica Iranica 2012:809-813.

24. Shetty A, Almeida FT, Ganatra S, Senior A, Pacheco‐Pereira C. Evidence on radiation dose reduction using rectangular collimation: a systematic review. International dental journal 2019;69:84-97.

25. Affairs ADACoS. The use of dental radiographs: update and recommendations. The Journal of the American Dental Association 2006;137:1304-1312.

26. Measurement NCoRPa. NCRP report No. 145, radiation protection in dentistry. NCRP 2003.

27. Johnson KB, Ludlow JB, Mauriello SM, Platin E. Reducing the risk of intraoral radiographic imaging with collimation and thyroid shielding. Gen Dent 2014;62:34-40.

28. Aps J, Scott J. Oblique lateral radiographs and bitewings; estimation of organ doses in head and neck region with Monte Carlo calculations. Dentomaxillofacial Radiology 2014;43:20130419.

29. Aps JK, Scott JM. Oblique lateral radiographs and bitewings; estimation of organ doses in head and neck region with Monte Carlo calculations. Dentomaxillofac Radiol 2014;43:20130419.

30. Benchimol D, Koivisto J, Kadesjö N, Shi X-Q. Effective dose reduction using collimation function in digital panoramic radiography and possible clinical implications in dentistry. Dentomaxillofacial Radiology 2018;47:20180007.

31. Magill D, Ngo NJH, Felice MA, Mupparapu M. Kerma area product (KAP) and scatter measurements for intraoral X-ray machines using three different types of round collimation compared with rectangular beam limiter. Dentomaxillofacial Radiology 2019;48:20180183.

This literature review has been written by a student and is published as an example. See our guide on How to Write a Dissertation Literature Review for guidance on writing your own.