Thanks But No Thanks!

By Bruno Azevedo, D.D.S., M.S.

Cone Beam Computed Tomography (CBCT) has become an essential imaging diagnostic tool in endodontics. There is an indisputable acceptance that 3D imaging is far superior diagnostically to 2D imaging. CBCT provides more information to the clinician regarding analyzing dental and peripheral anatomy and periapical and root pathologies. CBCT imaging also works as a scaffold for newly developed guided and dynamic navigation surgeries and is strongly recommended before using the new irrigation systems. CBCT imaging is also embraced in academics and is part of the educational curriculum of the next generation of endodontists in all programs in North America.

Interestingly, CBCT imaging popularity follows the same trends with other dental specialties and general dentistry. Dental Specialties such as periodontics, orthodontics, and oral surgery rely on 3D information to better treatment plan surgeries with surgical guides. General dentists and prosthodontists purchase and use CBCT scans to integrate with CADCAM technology. The current scenario where different dental professionals use CBCT imaging to help their patients is favorable and allows for better comprehensive overall treatment plans. However, it raises an interesting question for the endodontic specialty. It has become more common for patients to bring CDs and jump drives to the endodontic practice with the CBCT scan acquired at the general dentist or other specialty offices. Should endodontists accept CBCT scans acquired in another practice for different diagnostic purposes?

Patients expect their current CBCT scan to provide the information needed to render the correct diagnosis and guide treatment. They don’t want to pay for another scan or be irradiated again. The problem is that in most cases, that is not true. Most CBCT scans acquired in the general dentist, and non-endodontic specialty practices don’t have the resolution and the correct acquisition parameters to render high-resolution images necessary to analyze the microanatomy of the pulp and interpret vertical root fractures. Endodontists must familiarize themselves with the technical specifications of CBCT imaging acquisition to differentiate standard scans from Endo Enhanced Advanced Imaging. To help answer this question is essential to focus on the lack of resolution and image quality and be less concerned with one’s ability to open and manipulate software. The justification of an inability to open a scan or lack of knowledge on how to use software might render a negative professional impression about the clinician by the patient.

To better understand the differences between CBCT imaging acquisition, we must differentiate the three types of CBCT scans.  CBCT imaging is classified by the Field Of View (FOV). They are Large, Medium, and Small FOVs. Each FOV has different acquisition parameters, rendering different imaging quality and being used for different diagnostic purposes. Large and medium FOVs ( volumes equal to or larger than 8cm x 8cm) are acquired when the quantity of information is the purpose of the scan. They are typically used when the primary goal of the examination is to analyze the full or partial oral maxillofacial complex, including the airways, TMJs, the cervical spine, and the skull base. Due to the limitations of the imaging sensor and computational reconstruction, Large and Medium CBCT volumes are reconstructed with larger voxels. A voxel is a tridimensional pixel and is a vital parameter to help increase the spatial resolution of CBCT scans. In theory, the smaller the voxel, the better the ability to display detail; Larger and medium FOVs do not have the same ability to show detail as a small FOV scan, even if the larger FOV has the same voxel size.

Contrary to popular belief, the voxel size alone is not solely responsible for the final resolution. Larger and medium FOVs can be acquired using the lateral-offset projection geometry, allowing a smaller sensor to image a larger anatomical area by shifting the sensor position laterally. Lateral offset projection geometry differently than the Aligned projection geometry used in small FOVs leads to more peripheral x-ray beams, which have more acute angles during the acquisition process, cause variations in the reconstruction process, and degrades image quality.  Although some CBCT scans can reconstruct larger FOVs using submillimeter voxels, the final resolution is inferior to the CBCT scans acquired with smaller FOVs and identical voxels sizes.  The small FOVs (volumes equal to or smaller than 6cm x 6cm) are acquired when the quality of the information is more important than the quantity of information. Smaller FOVs have the central beam of radiation aligned to the cbct sensor and provide higher accuracy and spatial resolution during image reconstruction better stable for endodontic diagnosis.

Other CBCT imaging parameters to maximize the chances of correct vertical root fracture diagnosis and the interpretation of the microstructures related to the internal and external dental anatomy must be considered. Tube current (mA) controls the number of photons associated with the primary radiation beam and significantly influences the diagnostic detection of root fractures in CBCT scans. Higher mA (above 8) allows for better visualization of root fractures when compared to scans acquired with lower mA (5 and below). Kilovoltage (kVp) controls the penetration power of the photons. High KVp values (over 90) are indicated for endodontic scans because they can decrease CBCT artifacts and improve overall image quality. Another critical factor is the acquisition time. Acquisition time controls the number of base images acquired during the rotation of the C-arm. The longer the acquisition time, the more base images are obtained, and the higher the CBCT scan resolution will be. Longer acquisition times also lead to significantly better vertical root fracture detection specificity and less beam hardening artifact caused by metal posts and gutta-percha. Another point to consider is the liability associated with larger FOVs which will image areas beyond the diagnostic training of non-radiologists.

Since the CBCT scans acquired in most general dentists and dental specialties offices are not optimized for endodontic purposes, they should not be immediately accepted by the endodontic practice. There is a significant difference between visualizing pathology surrounding a tooth versus interpreting the microanatomy and better understanding the cause of the disease.  Endodontists should inform the referring dentists and patients that a second scan at the endodontic practice will likely be acquired. When asked why should a second scan be acquired, the answer is simple: “Because the previous CBCT scan image quality is not enhanced for endodontic purposes. Endodontic Enhanced Advanced Imaging requires small fovs, small voxel sizes, higher kVp and mA, and longer exposure times when clinically possible. The information acquired from an Endodontic Enhanced CBCT scan far outweighs any biological risk to the patient and therefore is justified.

Please consider calling your referrals and explaining the significant differences between CBCT imaging acquisition protocols. Sometimes it is just easier to say, “Thanks but no thanks!”

Works Cited

1. AAE and AAOMR Joint Position Statement: Use of Cone Beam Computed Tomography in Endodontics 2015 Update. Special Committee to Revise the Joint AAE/AAOMR Position Statement on Use of CBCT in Endodontics. Oral Surg Oral Med Oral Pathol Oral Radiol 2015;120:508-12.
2. Boulos Bechara, C. Alex McMahan, Ibrahim Nasseh, Hassem Geha, Elie Hayek, Georges Khawam, Michel Raad, Marcel Noujeim, Number of basis images effect on detection of root fractures in endodontically treated teeth using a cone beam computed tomography machine: an in vitro study, Oral Surgery, Oral Medicine, Oral Pathology and Oral Radiology, Volume 115, Issue 5, 2013.
3. Gustavo Machado Santaella, Pedro Luiz Rosalen, Polyane Mazucatto Queiroz, Francisco Haiter-Neto, Ann Wenzel and Rubens Spin-Neto, Quantitative assessment of variation in CBCT image technical parameters related to CBCT detector lateral-offset position, Dentomaxillofacial RadiologyVol. 49, No. 2, 2019.
4. Pauwels R, Araki K, Siewerdsen JH, Thongvigitmanee SS. Technical aspects of dental CBCT: state of the art. Dentomaxillofac Radiol. 2015
5. Freitas DQ, Fontenele RC, Nascimento EHL, Vasconcelos TV, Noujeim M. Influence of acquisition parameters on the magnitude of cone beam computed tomography artifacts. Dentomaxillofac Radiol. 2018
6. Tangari-Meira R, Vancetto JR, Dovigo LN, Tosoni GM. Influence of Tube Current Settings on Diagnostic Detection of Root Fractures Using Cone-beam Computed Tomography: An In Vitro Study. J Endod. 2017

Dr. Bruno C. Azevedo is a Board Certified oral & maxillofacial radiologist and is also known as “The Cone Beam Guy”, with his extensive experience in 3D imaging technologies in dentistry.