Main characteristics of image formation by x-ray techniques

Different X-ray techniques applied to the same anatomical region can produce images showing the same structures. However, the projection phenomena in two-dimensional images and the reconstruction processes in three-dimensional images affect the quality and performance of the images obtained. Therefore, it is crucial to unders-tand the key aspects of image formation in the X-ray techniques used in dentistry. This knowledge will help select the technique that provides the most comprehensive information based on diagnostic needs.

Two-dimensional images

Intraoral radiographic techniques

Intraoral radiographic techniques consist, mainly, of periapical and interproximal or bitewing radiographs. Periapical radiographs are useful for observing the entire tooth, from the apex to the crown, as well as the alveolar bone and the alveo-lar cortex. These radiographs can be obtained using either parallel or bisecting technique. When using parallel technique, periapical radiographs provide an accurate evaluation of the proximal coronal region. However, since the central X-ray beam does not strike this area perpendicularly, the effectiveness of this technique for evaluating enamel on the proximal surfaces and the dentin-enamel junction at the occlusal level is diminished.

Bitewing radiographs are useful for examining dental crowns, roots, and the supporting structures up to the middle third of the root. This technique enables a more accurate assessment of the interproximal surfaces and the distance between the cementoenamel junction and the crestal bone, thanks to the perpendicular angle of the X-ray beam directed at the dental crowns and alveolar crests. It allows for the detection of inter-proximal and secondary carious lesions, as well as the assessment of restorations. Additionally, it can reveal some occlusal carious lesions.

Both techniques provide high spatial resolution and contrast and can be performed in a dental office at a relatively low cost (¹⁴,^{15 16}).

To achieve optimal performance with intraoral radiographic techniques, it is preferable to use image receptor-holders. These devices help maintain the parallelism between the longitudinal axis of the teeth and the image receptor, resulting in less distortion of structures in the images. Additionally, they enhance the reproducibility of the technique, which is essential for monitoring changes in structures over time. This also reduces the likelihood of retakes due to technical errors, ultimately leading to a lower radiation dose for patients (¹⁴,¹⁷).

In contrast, the bisecting-angle and adhesive techniques are less advisable. The bisecting-angle technique tends to produce more distortion errors, such as shortening and elongation, as well as issues with the centralization of the localizing cylinder. The adhesive bitewing technique complicates both vertical and horizontal angulation of the image receptor, often leading to increased overlap of proximal surfaces and excessive vertical angulation, which can impair diagnostic accuracy.

Extraoral radiographic techniques

The most common extraoral radiographic techniques in dentistry are panoramic radiography and lateral cephalometric teleradiography.

Panoramic radiography provides an overall view of the teeth, alveolar bone, and surrounding structures in the maxillofacial area. However, it has lower spatial resolution compared to periapical radiographs. In addition to the general limitations of two-dimensional imaging -such as overlap-ping structures, distortion, and magnification-panoramic radiography has a particular drawback: vertical distortion (¹⁶). This occurs because of the negative angulation of the X-ray beam during the imaging process, which affects the vertical relationships of the structures in the image and can obscure lesions on the crowns of the teeth. Along with horizontal distortion, positioning errors and artifacts further compromise the reliability of structural measurements (¹⁶).

While panoramic radiography has specific applications that are discussed in this article, it is crucial to note that using it routinely for evaluating new patients or at regular intervals is not justified.

Lateral cephalometric or profile teleradiography is a technique that allows for the visualization of the bones of the face and skull, as well as the profile of facial soft tissue. This standardized method is particularly useful for patients with skeletal discrepancies, as it helps evaluate the skeletal pattern (¹⁷).

Three-dimensional images

Cone beam computed tomography (CBCT)

In CBCT, the X-ray tube produces a cone- or pyramid-shaped beam, generating three-dimensional images of the dentoalveolar or maxillofacial region. This technique addresses the limitations of two-dimensional images, although it has a variable spatial resolution compared to periapical radiographs (¹⁸).

The size of the tomographic volume is known as the field of view or FOV, which is measured in diameter and height and varies from small, to medium, or large, depending on each equipment. The resolution of CBCT examinations is influenced by several factors, one of which is the size of the voxel, the smallest unit of the tomographic image. Smaller voxel sizes lead to higher-resolution images. Additionally, the acquisition parameters, such as tube voltage and current, can be adjusted based on the patient's characteristics and the specific diagnostic requirements of the image (³). Therefore, a CBCT study should be customized for each individual case.

CBCT has specific indications that will be discussed later in this article. However, current evidence does not support its use for certain diagnoses because of the artifacts inherent to the technique. These artifacts can reduce diagnos-tic quality and complicate the evaluation of the coronal area of the teeth.

Criteria for the recommendations in this article

This article aims to justify the use of radio-graphic images based on specific needs. However, it's important to remember that the diagnostic approach should be comprehensive. After conducting a clinical examination, any necessary radio-graphs should only be defined if the required information cannot be obtained through clinical assessment. Once the radiographs are obtained, they should be thoroughly analyzed. Figure 1 presents a flowchart outlining the process for selecting radiographic images in dentistry.

Figure 1.  Flowchart of radiographic image selection in dentristry. 

The patient's record should include the clinical examination, symptoms and signs, the reason for the radiographic examination, the techni-que used, the date and number of radiographs taken, and the written radiological interpretation. Relevant information for specific needs should also be documented. Diagnostic errors occur when an imaging study is not thoroughly assessed, when findings are not reported, or are reported inaccurately (¹⁹, ²⁰).

Diagnostic errors can lead to adverse outco-mes for both patient and clinician, such as late diagnosis of serious diseases, unnecessary or harmful treatments, patient mortality (²¹,²²,²³), malpractice lawsuits, and damage to the clinician's reputation (²⁴). Factors associated with these errors include a lack of clinical history, nondiagnostic radiographic images obtained with poor technique, poor knowledge update on the part of the interpreter, observation and lighting conditions (²⁰,²⁵).

The dentist must provide the maxillofacial radiologist with the patient's complete clinical history and be prepared to supply previous images or additional information if required for interpreting the images (¹³, ¹⁵, ²⁰). When the dentist takes and interprets the images, he or she is responsible for the entire process, which must be documented in the patient's record.

Radiographic diagnosis of caries lesions

The request for radiographs to diagnose caries is based on evaluating caries risk. This evaluation also determines the interval for taking radiographs, which is personalized for each patient.

The date of the risk assessment, the risk level (low, moderate, high), and the date of the next assessment must be documented in the patient's record (²⁶). The intervals between subsequent radiographic examinations must be reassessed periodically, along with the evaluation of caries risk (¹⁵).

The use of bitewing radiographs with adhesive bitewings or periapical radiographs is not recommended due to previously mentioned limitations. Panoramic radiographs and CBCT are likewise not indicated (⁶, ¹⁵) (Table 1).

Diagnosis in endodontics

In endodontics, it is essential to use diagnostic tests such as palpation, mobility, percussion, periodontal evaluation, occlusal examination, and sensitivity (²⁷).

The sensitivity tests for the diagnostic stage (²⁷), the preoperative evaluation, the working length, the assessment of the final treatment, the presence of material extrusion, the type of restorative material, among others, must be recorded in the patient's record (¹⁵, ²⁷).

Paralleling technique is the recommended technique for evaluating teeth in endodontics. The bisecting-angle technique and panoramic radiography are not recommended for the reasons previously described regarding posible vertical distortion in the obtained images (²⁸). Routine use of CBCT is not indicated since its use has been found to lead to overdiagnosis and overtreatment (²⁹, ³⁰). Never-theless, there are specific cases in which CBCT must be used (Table 2).

Radiographic diagnosis in periodontal evaluation

The goal of diagnosing the periodontal disease is to identify its stage and severity (¹⁵, ³¹, ³²), considering the depth of the probe, the loss of insertion, the extent of bone loss, the presence of bone defects, and furcation lesions (³²). There is no evidence to establish the periodic acquisition radiographs for periodontal evaluation (¹⁵).

Panoramic radiographs, the bisecting technique, and the adhesive bitewing technique are not recommended due to due to their inherent technical limitations (¹⁵, ³²).

Although panoramic radiographs and the complete set of periapical radiographs have been used for periodontal evaluation, it is currently recommended to avoid this practice due to the limitations of panoramic radiographs and the fact that it involves a higher dose of radiation to the patient (¹⁵, ³²) (Table 3).

Radiographic diagnosis in surgery

Surgical treatment requires appropriate diagnostic tests, which include radiographic imaging (³⁴,³⁵,³⁶,³⁷). Minor procedures are detailed below. Major surgeries are not considered, as they are performed by specialists in a hospital setting (Table 4).

Radiographic diagnosis of implants

X-rays are crucial for assessing the quantity and quality of bone, as well as their relation to the position and design of prosthetic rehabilitation (⁶, ¹⁵, ³⁴).

While CBCT is indicated for treatment planning, the entire acquired tomographic volume must be evaluated and its findings documented in the patient's record (Table 5).

Radiographic diagnosis of edentulous bone ridge

The request for radiographic evaluation is based on the clinical signs or symptoms and the treatment to be performed. In the absence of clini-cal signs and symptoms, taking x-rays is not justified (¹⁵, ²⁶) (Table 6).

Radiographic diagnosis of temporomandibular disorders (TMD)

The initial evaluation of TMD is based on the clinical examination of the TMJ region and the masticatory system (³⁸). The request for a radio-graphic examination is based on the result of the clinical evaluation, when the evaluation of the bone component is required (¹⁵, ³⁸) (Table 7).

Radiographic diagnosis of the development of the dentition

Figure 2, Figure 3, Figure 4 detail speci-fic procedures for developing dentition, and do not include orthodontic treatments performed by specialists (¹⁵, ¹⁷) (Table 8).

Figure 2.  Radiographic image selection in early and late mixed dentition. Adaptedfrom Horner K. (¹⁵) y Isaacson K. (¹⁷).  

Figure 3 Radiographic image selection for a persistent second primary molar in late mixed dentition. Adapted from Horner K. (15) y Isaacson K. (17).  

Figure 4.  Radiographic image selection for the assessment of the permanent canine in the late mixed dentition. Adapted from Horner K.(15) y Isaacson K. (17).