Cone-beam CT Radiography for Paediatric Oral Assessment

Radiographs are a valuable diagnostic tool, as an adjunct to clinical examination in the diagnosis of dental diseases. Two dimensional periapical and panoramic radiographs are routinely used in dental practice. However, there are certain limitations of two-dimensional radiographs, which can be overcome by three-dimensional, imaging techniques such as cone beam computed tomography.

On 8 November 1895 Wilhelm Conrad Röntgen accidentally discovered X-rays. The first original dental roentgenogram from a portion of a glass imaging plate was taken by Dr Otto Walkhoff in January 1896 in his mouth for an exposure time of 25 min. Since then, dental imaging has seen tremendous progress and its applications in various fields of dentistry. Broadly, imaging techniques used in Dentistry can be categorized as intraoral and extraoral, analogue and digital, ionizing and non-ionizing imaging, and two-dimensional (2-D) and three-dimensional (3-D) imaging.

2-D Conventional radiographs provide excellent images for most dental radiographic needs. Their primary use is to supplement the clinical examination by providing insight into the internal structure of teeth and supporting bone to reveal caries, periodontal and periapical diseases, and other osseous conditions. A significant constraint of conventional radiography is the superimposition of overlying structures, which obscures the object of interest. Eventually, it results in collapsing 3-D structural information onto a 2-D image, which leads to loss of spatial information in the third dimension.

In this review, the following points will be discussed

  • Background on CBCT
  • Considerations in the use of CBCT in paediatric patients including barriers and precautions.
  • Indications for use of CBCT in paediatric dentistry

Background

Cone-beam computed tomography (CBCT) was developed in the early 90s in two countries independently. Arai et al. (1) in Japan and Mozzo et al. (2) in Italy.

The term cone beam is derived from the cone-shaped beam sent through the region of interest and is subsequently received by a 2-D flat panel detector (3). The projections will be converted into a 3-D image (in axial, coronal and sagittal planes) by a modification of the original cone-beam developed by Feldkamp et al. (4).

CBCT can produce a 3-D dataset of the region of interest but does not lose the ability to create a 2-D image if needed. CBCT creates real size data with isotropic voxel size.

Considerations in the use of CBCT in paediatric patients

CBCT should only be used when the question for which imaging is required can not be answered adequately by lower dose conventional radiography (5). Although CBCT has its place in paediatric dentistry, its use needs to be justified on a case by case basis and the benefits need to outweigh the risks.

CBCT as a method of oral and maxillofacial radiology in children has become more popular in the last decades but its use is controversial due to radiation dose and greater susceptibility to radiation damage in children than adults due to immature anatomical and biological structures.

Ionizing radiation can have deterministic and stochastic damaging effects. A deterministic effect is characterized by a threshold below which the effect does not occur and the severity of the effect increases with increasing exposure. High doses of radiation cause the killing of the cells. A stochastic effect refers to the potential possibility of carcinogenesis secondary to radiation-induced DNA damage. Although CBCT does not have deterministic effects, it does have the potential to cause stochastic effects.

Children are more vulnerable to ionizing radiation and are at a higher risk of developing cancer. This is due to more rapid growth in their tissues and cells, making the DNA more vulnerable to damage. A child also has a longer time to allow for a tumour to develop compared to an adult when exposed to radiation at a young age (6). Ionizing radiation cumulative effects should also be considered in taking X Rays in young children.

According to ICRP, there is a nominal probability coefficient for all radiation-induced fatal cancers averaged over a whole population to be 5% per sievert, which means the risk associated with CBCT to be between 1 in 1:00,000 and 1 in 350,000 (6).

When using CBCT as the first line of investigation, it is important to keep in mind that the doses from CBCT are low compared with conventional CT but significantly higher than conventional dental radiography techniques. The main advantage of CBCT is a 3-D reconstruction from a single scan which will result in a much lower effective dose to a patient compared to a medical CT.

 (Haridas et al. 2016)(7)

The field of view and the selected resolution can have a substantial effect on the effective dose. Increasing the voxel size accuracy of the scan from 400 to 200 μm doubles the effective dose because twice as many projections need to be made (6). FOV limitation reduced the radiation dose effectively. 

Three basic principles of radiation protection need to be taken into account when using CBCT in children as for any other x-ray (5)

  1. Justification:  Taking radiographs is only indicated if there are no other means of obtaining the necessary information. If the patient cannot cope with the procedure, no radiographs should be taken.
  2. Limitation principle: Radiation dose to a patient should stay as low as reasonably achievable (ALARA). 
  3. Optimisation principle: Obtain the best diagnostic images possible, with both previous principles in mind. 

In a recent article Kühnisch et al.  provided a very useful diagram of the workflow during prescription on dental radiographs in children and adolescence, taking into consideration the 3 basic principles of radiation protection (8).

Kühnisch et al. 2020

A study by Theodorakou et al. 2012 looked at doses from 5 different CBCT machines for a 10-year-old child and an adolescent. They reported that the doses were equal to those in adults. In the same study, they found that the thyroid gland seemed to receive four times more radiation in a 10-year-old compared to an adolescent (9).

There has been great emphasis on the importance of radiation protection in children (10). A few radiation protection devices have been reported to reduce the dose. Correct use of a  thyroid collar can reduce the localised effective dose to the thyroid gland and the oesophagus by approximately 50-40% respectively but doesn’t reduce the total effective dose (11). In another study by Prins et al, it was reported that using lead glasses in paediatric patients can reduce the dose by 67% (12).

It is important to assess patients’ cooperation before prescribing CBCT especially in younger children and children with special needs. The patient needs to be able to stand motionless for a prolonged period for an accurate image and to prevent motion artefact (13).

The risk of movement is reported to be 11 times higher in children 15 years old or younger compared to 31-year-old patients (13).

An important disadvantage of CBCT are artifacts that may be caused by:

  • Patient movement 
  • Metallic objects
    • Implants
    • Amalgam filling
    • Endodontic obturation material 

Although there have been attempts form manufacturing companies to alter the algorithm to improve the artifact on CBCT images, they have not been able to get rid of all artefacts and especially movement artefacts. 

Beam hardening or streaming artefacts can impede the diagnostic value of an image greatly and should be considered before exposing a child to CBCT (6).

Other artefacts such as motion artefacts which may be caused by patients movement involving breathing, heartbeat, muscular movement and tremor, will result in a blurry image, have stripes or ring-like artefacts and double contours. This can result in a geometric error in the reconstruction process (13).

Recommendations on the use of protective precautions while prescribing CBCT in children (8):

  • The smallest FOV for the given indication should be used on an individually based level. This is especially important considering the height of the FOV. 
  • Depending on the treatment needs the largest voxel size should be chosen on an individual and indication-based level.
  • Although it is important to maintain sufficient therapeutic value of the image, the radiation dose should be kept as low as possible. Change in image settings such as ultra-low dose settings, shorter exposure time, a lower amount of projections (resolution), lower beam intensity (mA), reduction of the potential (kV), and the use of automated exposure control can reduce the radiation dose.
  • Thyroid shields should be used at all times in the paediatric patient except when the area of interest is structures below, or very close to, the axial level of the top of the shield, in which situation artefacts from the shield might affect the quality.  When using tube current modulation during the scan, giving real-time feedback from the detector to the exposure control, thyroid shields should not be used. In the case of using automatic exposure control based on scout images, the thyroid shield should be positioned only after the scout images have been taken.
  • Informed consent is needed before a dental radiograph is carried out. The potential risk of using ionizing radiation should be discussed before exposing the patient to ionizing radiation. 
  • It is important to consider alternatives and X-ray free techniques during the decision-making process of exposing a patient to x rays. Repeat examinations should be avoided to prevent extra exposures.
  • The type of performed x-ray, dose and diagnosis should be recorded in the patient’s clinical record.
  • The paediatric dentist needs to be trained and experienced to evaluate dental radiographs. It may be required for a CBCT interpretation to be done by a maxillofacial radiologist.
  • The use of CBCT for incidental findings are not justified.
  • Use of CBCT in children with disabilities, learning difficulties,  less or non-cooperative children and adolescents who are unable to keep still during the radiographic exposure may not be indicated.

Indications of using CBCT in paediatric dentistry.

A classification of uses of CBCT adapted from the European Guidelines is listed below (14).

More recently an overview of potential clinical- based indications for dental radiographs in children and adolescents was published by EAPD which demonstrates the clinical use of CBCT as well as 2D imaging (8).

In a systematic review by De Vos et al. CBCT was mostly used in maxillofacial surgery (41%), followed by dentoalveolar issues (29 %), orthodontics (16 %) and dental implantology (11 %). CBCT was less commonly used in Endodontology, periodontology, general dentistry and forensic dentistry.

Caries

There have been several studies comparing the efficacy of detecting proximal caries using conventional radiography and CBCT.

It is important to keep in mind most of these studies were done on the permanent dentition. 

Senel et al. reported there was no significant difference in the detection of proximal caries in permanent teeth using visual inspection, film, CCD, PSP plates and CBCT (15).

Akdeniz et al. compared the accuracy of proximal caries depth measurements between limited CBCT and conventional 2-D imaging using storage phosphor and film radiography. Although the study concluded LCBCT seems to offer advantages over conventional 2-D techniques for determining the depth of small proximal carious lesions it is important to note that none of the teeth in the study were restored or in contact with restored teeth. Restorations particularly of dense material such as amalgam can cause artifacts and compromise the final diagnosis(16).

In an in-vitro study 257, non-filled approximal surfaces from human permanent premolars and molars were recorded by two intraoral digital receptors, a storage phosphor plate (Digora Optime, Soredex) and a solid-state CMOS sensor (Digora Toto, Soredex), and scanned in a cone

beam CT unit (3D Accuitomo FPD80, Morita) with a FOV of 4 cm and a voxel size of 0.08 mm.

A significantly higher sensitivity was obtained by all observers with CBCT which was not compromised by a lower specificity. The authors concluded CBCT was much more accurate in the detection of surface cavitation in approximal surfaces than intraoral receptors. A CBCT examination performed for other reasons should also be assessed for approximal surface cavities in teeth without restorations (17).

In a recent systematic review by Horner et al. it was reported most studies relating to CBCT and caries detection are based on in Vitro research most of which showed little difference in diagnostic accuracy when CBCT imaging was used compared with intraoral radiography. All existing guidelines do not recommend using CBCT as a standard tool for caries detection (18).

Dental Trauma

Most studies have looked at trauma to permanent dentition and no solely paediatric studies have been conducted on primary teeth for evaluation of CBCT efficacy for diagnosis of root fracture as a consequence of trauma. The level of accuracy in identifying a root fracture in a non-endodontically treated tooth has been reported much higher than a periapical radiograph (19–24).

Bornstein et al. compared intraoral occlusal and periapical radiographs vs limited CBCT in diagnosing root-fractured permanent teeth in 38 patients with 44 permanent teeth with horizontal root fractures. They found that fracture location on the palatal surface of the root was more coronally placed on CBCT than on radiographs. In particular, a cervical fracture was more commonly seen on CBCT, potentially influencing management (25)

There is insufficient evidence to recommend the standardized use of CBCT for any type of acute dental or dentoalveolar trauma. However, it could be considered on a case-by-case basis in severely traumatized permanent teeth, e.g., teeth with multiple fractures, root or crown-root fractures with mobile coronal tooth fragments (8).

Orthodontics

In recent years many studies have looked at the use of CBCT for orthodontic treatment planning. The focus is on the recognition of orthodontic landmarks through CBCT compared with cephalometric two-dimensional imaging. Although many studies have been conducted in vitro the conclusion is that two-dimensional is still the preferred method and CBCT may only be used in selected cases (6).

As mentioned below CBCT allows for more accurate localisation of the maxillary canine with an eruption disturbance than conventional radiography. 

It should be noted that the majority of orthodontic patients are children and young adults who are most sensitive to ionizing radiation.

Surgical Planning

Accurate measurements are essential for surgical planning such as implant surgery and autotransplantation of unerupted teeth. Sakabe et al. investigated the accuracy of the measurements of the tooth crown width of unerupted teeth using limited CBCT. Images of impacted supernumerary teeth in the medium maxilla were taken before extraction and it was concluded from their study CBCT measurements are accurate enough for surgical planning (26)

CBCT imaging reduces the risk of damage to the surrounding anatomical structures as they are in close association with the cortical bone (27).

2D imaging such as OPG, horizontal tube shift (28), and vertical tube shift (29) have also been used to localise the position of a supernumerary tooth. Panoramic radiographs are unreliable for identifying supernumerary teeth (30) and the vertical tube shift is more successful in localization of the unerupted supernumerary tooth (31).

After the third molars, canines are the most common impacted teeth and are usually palatally located. It is important to identify the correct location of the impacted canine and its relationship and proximity to the adjacent structures. Diagnosis and treatment planning using 2-D imaging can be difficult due to distortion project errors, blurred images and complex maxillofacial structures projecting onto a 2-D radiograph.  Computed radiography can overcome the limitations of conventional radiography and increase the detection rate of root resorption by 50% and is superior for localization of the impacted canines(6).

CBCT may also be of high value in cases of autotransplantation of teeth to produce replica donor teeth and surgical guides (32).

Pathological Conditions

Periodontal disease in children requiring regenerative treatment is rare. A review of the literature has not found any evidence for use of CBCTfor periodontal treatment in children. 

A consequence of  The introduction of CBCT has made it possible to acquire 3-D information that can lead to improved detection rates of external root resorption (33).

Although many of the studies on root resorption are in vitro it is reasonable to suggest a 3-D radiograph will be a superior technique compared to 2-D imaging particularly on buccal and lingual surfaces (18).

A different pathological condition that can be diagnosed radiographically is external cervical resorption (ECR). Goodell et al compared treatment plans for  ECR developed by CBCT and periapical radiographs for 56 teeth with ECR. in 56.7% of cases the treatment plan developed from CBCT scans were different from periapical radiographs (28).

Although CBCT can be used for diagnosis of calcified tissue, it is not a suitable technique for the diagnosis of pathology in soft tissue. CBCT may be used if there is concern about the hard tissue of the TMJ. In these cases, CBCT may be used adjunct to MRI which has more superior diagnostic values for soft tissue pathology (34).

CBCT is not the best method to assess soft tissue pathologies such as cysts, tumours or benign conditions. However, it may be indicated to identify the position and integrity of lesion margins and its relationship with adjacent structures (8).

Dental Anomalies

There is very little diagnostic efficacy evidence for use of CBCT for dental anomalies such as den invaginatus, fusion, dilaceration, gemination, and other morphological anomalies. CBCT may be indicated if conventional radiographs do not provide adequate anatomical information which can aid in the management (8).

Developmental Disorders

There have not been many studies looking at the use of CBCT in specific craniofacial syndromes. Horner et al. have reviewed the literature and found that CBCT was mostly used for imaging cleft lip and palate patients as it would provide a volumetric assessment of the defect before bone grafting. The radiation dose used in CBCT is lower than a  CT scan.

Acute Dental Infection

A systematic review by Horner et al. 2020 did not find any diagnostic efficacy evidence for using CBCT. Evidence from observational studies suggests that CBCT may identify more periapical inflammatory lesions but with the increased risk of reporting false-positive results (18).

Constricted Airways

In recent years, there has been more attention to airway morphology and its relationship to malocclusion and sleep-disordered breathing and the effects of orthodontic treatment on sleep apnea (35). Although Lenza et al. claim that in patients with enlarged tonsils,  CBCT facilitates analysis and measurement of the airway volume and cross-sections compared to the use of 2-D radiography technique (36), studies using CBCT imaging to test this hypothesis have had conflicting results due to lack of standardized protocol during imaging acquisition(37,38).  

Forensic Identification

CBCT may be used for age estimation, forensic facial reconstruction, analysis of bite-marks, sex determination and frontal sinus pattern (39).

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