Accuracy in endodontic diagnosis has always lacked total objectivity due to

the limitations in both the sensitivity and specificity of our clinical and radiographic special tests (Mejàre, et al., 2015; 2012; Petersson, et al., 2012). The limitations of conventional two-dimensional radiography have been well documented, and the relatively recent introduction of limited volume cone beam computerized tomography (CBCT) in endodontics has significantly increased our ability to accurately visualize periradicular pathology through slices (Venskutonis, et al., 2014; Abella, et al., 2012), without the limitations imposed by surrounding hard tissues.

Further clinical benefits include the ability of CBCT to accurately visualize the anatom-ical complexities of the root canal system (Zhang, et al., 2011), to appreciate the extent of resorptions (Kamburoglu, 2011), to observe the effects of vertical root frac-tures (Metska, et al., 2012), and to under-stand treatment failure through untreated anatomy. These factors significantly enhance diagnosis, treatment planning, and clinical activity.

Using this enhanced knowledge in clinical endodontics can offer endodon-tists opportunities to provide patients with a greater degree of treatment predictability, and aids decision-making at the initial stages without the need for invasive procedures. Extending the use further can allow minimally invasive endodontics to treat challenging problems, such as I described in the article “Endodontics in 3D – a clinical series” (2014). This involves the continuous use of the scan during treatment utilizing head-up treat-ment displays.

Advanced techniques in time-shifted endodontics

Beyond these novel uses of the tech-nology, further derivations can be made using advanced post-processing techniques, fully utilizing the increased information gathered through the scan, and further justifying the increased radiation.

A particular area of difficulty in endo-dontic diagnosis is the comparison of radiographic imaging over time. The ability to determine whether radiolucency is increasing or decreasing in size is constantly in demand to determine the success or failure of endodontic treatment and the diagnosis of pain.

This can translate further to diagnosis and treatment planning in the presence of an asymptomatic radiolucency associated with endodontically treated teeth. This common clinical conundrum can present itself on routine radiography, and the impulse to retreat should be tempered with an initial search for historical imaging of the area to try and determine whether the lesion has changed in size over time.

The difficulties in achieving a precise comparison are well recognized, according to Wu, Shemesh, and Wesselink (2009). Beyond the subjectivity in determining the presence and size of a radiolucency with inter- and intra-observer variation, the

geometry of the image and the physical diffi-culty of lining up the X-ray over an extended time period is more than likely to produce inaccuracies. A radiolucency projected at different horizontal and vertical angulations will appear as different sizes.

Bearing in mind all of these limitations in radiographic comparison, along with the inability to visualize up to 50% of periapical lesions using standard radiography (Vens-kutonis, et al., 2014), it is not surprising that our success-failure statistics are likely to be inaccurate.

Those currently quoted will almost certainly be a victim of the data “garbage in, garbage out” principle (GIGO) (López, et al., 2014).

The thick slice techniqueA single limited volume CBCT scan can

provide a baseline image that can be used both prospectively and retrospectively for comparison to a standard periapical radio-graph taken at any horizontal and vertical projection achieved in the mouth.

Increasing slice thicknessThe process involves the production of

a virtual periapical radiograph from a limited volume CBCT scan by increasing the thick-ness of the coronal slice equal to, or larger than, the hard tissues imaged by a routine

The thick slice technique: a virtual periapical radiograph

Dr. Richard S. Kahan explores the benefits of using advanced CBCT techniques in radiographic comparison and illustrates its use in a clinical case

Richard S. Kahan, BDS, MSc (Lond), LDS RCS (Eng), is a specialist endodontist working in Harley Street, London, and the former director of endodontic courses at UCL Eastman CPD. He has lectured widely on endodontics and technology, and set up the Academy of Advanced Endodontics to teach the fundamentals of endodontic treatment to GPs and master classes for specialists.

Educational aims and objectivesThis clinical article aims to demonstrate an advanced CBCT technique in a clinical case using a time-shifted comparative virtual periapical radiograph created by a thick slice technique.

Expected outcomesEndodontic Practice US subscribers can answer the CE questions on page 39 to earn 2 hours of CE from reading this article. Correctly answering the questions will demonstrate the reader can:• See the use of virtual periapical radiographs from limited volume CBCT scans in

successful endodontic diagnosis.• Realize the potential difficulty in endodontic diagnosis regarding the comparison of radiographic imaging

over time.• Recognize the thick slice technique.

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periapical radiographic projection. This will produce a virtual periapical radiograph (Figures 1A-2B).

Adjustments in the horizontal and vertical planes

Adjustments need to be made to match the angulation of the central X-ray beam in both horizontal and vertical planes (Figures 3A-6B). This can be done through rota-tion in the axial and coronal planes and visual comparison with the true periapical radiograph.

The correct angulations can be judged by the relative positions of two objects to

one another on the radiograph. Tooth and root morphology, restorations, and root filling materials can be effective markers. The resolution of the virtual periapical radio-graph is inferior to standard radiography, and further post processing to increase the contrast (reducing the number of grayscales) is necessary.

Clinical case reportA 41-year-old female patient who had

fractured her jaw 10 years previously was referred by a specialist endodontist for a limited volume CBCT scan in order to help investigate and find the source of nonspecific

pain in her lower left quadrant. Both clinical and radiographic tests were inconclusive.

The LL6 had been endodontically retreated 1 year previously. The tooth had been root treated 2 years previously by a general dental practitioner and had never felt right. The re-root filling appeared satis-factory, but the periapical review radiograph showed apical radiolucencies at both mesial and distal roots. Without a common align-ment of the historical radiographs with the review radiographs, the possible healing signs observed could not provide the diag-nostic confidence to rule out the LL6 as the source of the pain. The LL7 did not respond

Figures 1A and 1B: 4 x 4 cm limited volume CBCT scan slices of an upper molar in the coronal and sagittal planes at 0.5 mm slice thickness

Figures 3A and 3B: Rotating the axial slice of the scan in a clockwise direction creates a distally -angled projection in the horizontal plane with the palatal root moving distally in the sagittal view

Figures 5A and 5B: Rotating the coronal slice of the scan in an counterclockwise direction creates the common effect seen when the X-ray plate is angled along the vault of the palate, creating an upward-angled projection, relatively elongating the palatal root

Figures 6A and 6B: The opposite and clinically rarely seen effect of a downward-angled projection by clockwise rotation of the scan, relatively shortening the palatal root

Figures 2A and 2B:The same molar scan at 30 mm slice thick-ness producing a virtual periapical radiograph

Figures 4A and 4B: Rotating the axial slice of the scan in an counterclockwise direction creates a mesially angled projection in the horizontal plane with the palatal root moving mesially in the sagittal view

Limited volume

CBCT scanning is a

powerful diagnostic tool

with particular benefit

to endodontists.

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to vitality testing but had only a shallow resto-ration. The question was whether the LL6 should be retreated once again, other teeth investigated, or a referral made for atypical facial pain.

Four periapical radiographs were provided, one preoperative (Figure 7), one postoperative (Figure 8), and two review radiographs at different angles (Figures 9A and 9B).

A 4 x 4 cm limited volume CBCT scan of the lower left posterior quadrant was taken using a Morita Veraviewepocs 3D,

imaging the LL5 to the LL7. A metal plate was screwed into the buccal cortical plate below the LL6 and LL7. The LL6 had been root treated with all three canals filled to the radiographic apex. A small radiolucency was associated with the mesial root apex together with widening of the periodontal ligament (PDL) space around the distal root apex. The LL7 had a small occlusal restora-tion with minimal depth. The pulp chamber and the coronal third of the root canals showed significant sclerosis, and there was very definitive widening of the PDL space

around the mesial root apex (Figure 10, arrowed). The LL5 showed no signs of any periradicular pathology.

A virtual periapical was produced using the thick slice technique within the One Volume Viewer imaging software and then compared to the postoperative view taken 1 year previously. Horizontal angulation was judged through the relative positions of the mesiobuccal and mesiolingual root fillings (Figure 11). The vertical angulation was judged using the distance of the distal root apex of the LL7 to the metal plate (Figure 12).

Figure 7: Preoperative periapical radiograph

Figures 9A and 9B: One-year postoperative LL6 review radiographs taken at different angles showing periapical radiolucencies

Figure 8: Postoperative periapical radiograph following endodontic retreatment of the LL6

Figure 10: 4 x 4 cm limited volume CBCT sagittal slice of the lower left quadrant at 1 mm slice thickness showing widening of the PDL spaces below the mesial root of the LL7

Figure 11: 4 x 4 cm limited volume CBCT sagittal slice of the lower left quadrant at 25 mm slice thickness with mesial angulation in the horizontal plane

Figure 12: 4 x 4 cm limited volume CBCT sagittal slice of the lower left quadrant at 25 mm slice thickness with upward angulation in the vertical plane

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A diagnostically acceptable compara-tive virtual periapical radiograph was created (Figure 13). The radiolucencies at both mesial and distal roots certainly appeared to be smaller. Furthermore, the widening seen at the apex of the LL7 appeared more pronounced in the virtual periapical than the radiograph taken a year earlier.

The analysis was able to confirm healing at the LL6 and suggested a deteriorating pulpal issue at the LL7 with severe scle-rosis and PDL widening. Without further clinical evidence and testing, this would not be a definitive diagnosis, but it was able to increase diagnostic confidence.

ConclusionLimited volume CBCT scanning is a

powerful diagnostic tool with particular benefit to endodontists. The increased radiation dose needs to be well justified, and it is up to the clinician to use all of the high-quality data captured for the benefit of the patient. Through careful and thoughtful processing, a single CBCT scan can be used together with standard radiography, not only for direct reporting at one instance in time, but also retrospectively as demonstrated in this article, and prospectively, for reviewing treatment carried out following a scan.

AcknowledgmentThe author would like to thank Dr. Bina Patel for providing the patient radiographs.

REFERENCES

1. Abella F, Patel S, Duran-Sindreu F, Mercadé M, Bueno R, Roig M. Evaluating the periapical status of teeth with irre-versible pulpitis by using cone-beam computed tomog-raphy scanning and periapical radiographs. J Endod. 2012;38(12):1588-1591.

2. Kahan RS. Endodontics in 3D – a clinical series. Dentistry. 2014;May:68-70. Accessed online. http://endodontics.co.uk/wp-content/uploads/2014/12/VertfractMay14.pdf.

3. Kamburoğlu K, Kurşun S, Yüksel S, Oztaş B. Observer ability to detect ex vivo simulated internal or external cervical root resorption. J Endod. 2011;37(2):168-175.

4. López FU, Kopper PM, Cucco C, Della Bona A, de Figueiredo JA, Vier-Pelisser FV. Accuracy of cone-beam computed tomography and periapical radiography in apical periodontitis diagnosis. J Endod. 2014;40(12):2057-2060.

5. Mejàre IA, Axelsson S, Davidson T, Frisk F, Hakeberg M, Kvist T, Norlund A, Petersson A, Portenier I, Sandberg H, Tranaeus S, Bergenholtz G. Diagnosis of the condition

of the dental pulp: a systematic review. Int Endod J. 2012;45(7):597-613.

6. Mejàre IA, Bergenholtz G, Petersson K, Tranæus S. Esti-mates of sensitivity and specificity of electric pulp testing depend on pulp disease spectrum: a modelling study. Int Endod J. 2015;48(1):74-78.

7. Metska ME, Aartman IH, Wesselink PR, Özok AR. Detection of vertical root fractures in vivo in endodontically treated teeth by cone-beam computed tomography scans. J Endod. 2012;38(10):1344-1347.

8. Petersson A, Axelsson S, Davidson T, Frisk F, Hakeberg M, Kvist T, Norlund A, Mejàre I, Portenier I, Sandberg H, Tranaeus S, Bergenholtz G. Radiological diagnosis of peri-apical bone tissue lesions in endodontics: a systematic review. Int Endod J. 2012;45(9):783-801.

9. Venskutonis T, Daugela P, Strazdas M, Juodzbalys G. Accu-racy of digital radiography and cone beam computed tomog-raphy on periapical radiolucency detection in endodontically treated teeth. J Oral Maxillofac Res. 2014;5(2):e1.

10. Wu MK, Shemesh H, Wesselink PR. Limitations of previously published systematic reviews evaluating the outcome of endodontic treatment. Int Endod J. 2009;42(8):656-666.

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Figure 13: A correctly lined up comparison between the postoperative radiograph following endodontic retreatment of the LL6 and a virtual periapical created from the CBCT scan 1 year later, showing a reduction in lesion size at the mesial and distal roots of the LL6 and some increased widening of the PDL space around the mesial root of the LL7

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