of Radiographic Interpretation

Fig. 2.1

Periapical radiographic images obtained by intraoral solid-state sensors, used for diagnosis, odontometry, and follow-up

2.2 Digital Radiography

Digital intraoral receptors have several advantages compared to conventional film-based imaging, for example, the lower dose per exposure, the ability to perform image quality enhancements, task-specific image processing, an almost instantaneous availability of images, and a greater reduction in working time. An additional environmental advantage is the elimination of processing chemicals [4].

Digital images are fundamentally composed of numeric information that can be differentiated from each other based on two characteristics: pixel spatial distribution and gray values attributed to them. The pixel size and the bit depth are important features, which determine the spatial and contrast resolutions for the acquisition of digital radiographs (Table 2.1). Presently, the pixel size is about 19–50 μm in intraoral digital radiographic systems, similar to conventional films, which allows a theoretical maximum spatial resolution of about 25 line pairs per mm [4, 5]. High image resolution appears to improve the accuracy for the diagnosis of root fractures and the determination of file lengths [6, 7].

Table 2.1

Some commonly used terms

Pixel

It is the foundation block of digital imaging and is the smallest complete sample of an image. It is a unit of digital resolution

Bit depth

This indicates the shades of gray used to define each pixel. It is commonly measured as “number of bits”

Spatial resolution

This refers to the ability of the imaging modality to differentiate two objects

With respect to the contrast resolution, image acquisition software offers various bit depths, allowing 8, 10, 12, and 16 bits, the last generating images with 65,536 different shades of gray per pixel. It is important to note that the higher the bit depth, the greater the contrast resolution will be. Thus, it enables a more detailed visualization of subtle differences which may increase the diagnostic accuracy, such as for the determination of file lengths, in which high contrast resolution is recommended [6, 8]. Regarding the detection of subtle radiographic density differences, 12-bit image or higher is needed for accurate determination of endodontic file position [8]. However, it is important to remember that the human is capable to perceive a limited number of shades of gray.

2.3 Digital Image Acquisition and Receptors

Digital image acquisition can be performed by two distinct ways: (1) indirect acquisition by means of digitizing films using flatbed scanners with a transparency adapter, slide scanners, or digital cameras to convert an existing analog radiograph into a digital image and (2) direct acquisition. In direct acquisition, the conventional film could be replaced by a solid-state sensor with/without a cord (charged coupled devices or complementary metal-oxide semiconductors) or by a photostimulable storage phosphor (PSP) plate.

Intraoral solid-state sensors (CCD and CMOS) are composed by silicon chips wrapped in a rectangular, rigid, 5–10.5 mm thick plastic pack [9]. Most intraoral solid-state sensors are connected to the computer by a cable, and the image is displayed almost immediately on a computer monitor after exposure [9, 10]. Usually, the cable is 0.40–3 m long, which can be extended on some digital systems. There are few direct digital systems that use wireless technologies for data transmission. Wireless digital systems can either use common radiofrequency waves or Bluetooth data transmission [9]. Intraoral solid-state sensor systems could be efficient aids in endodontic procedures, in which an additional image effortlessly can be acquired from a different angle with the sensor in the same position. This might make these sensors the elected digital receptor in endodontic practice [10].

The intraoral solid-state sensor’s disadvantages include (1) smaller active areas of image receptors when compared to films and phosphor plate—which limits the visualized structures and increases the number of radiographic exams needed; (2) the reduced dynamic scale resulting in overexposed images and requiring several repeats; and (3) solid-state image receptors which are rigid and thicker than others, causing discomfort to the patient and difficulty in the placement of the sensor for intraoral radiographs for posterior teeth. The cable used to connect the sensor to the computer also brings discomfort to the patient.

PSP plate systems use plates covered with phosphor crystals. A wide variety of intraoral imaging plate’s sizes are available, with dimensions compatible to conventional film sizes 0, 1, 2, 3, and 4. The PSP plates are as comfortable as radiography film [10]. The substantial dose reduction compared with film is one of the advantages of PSP plates, and no repeats are required when overexposing phosphor plates [11]. The main disadvantage of PSP-based digital systems is the need of phosphor plate continual reposition, since they can present image artifacts as small or big scratches and stains with a related deterioration of image quality as time progresses and by usage. Handling the plates with gloves and wiping the soiled plates with a soft cloth should increase the reusability of plates. Vigorous rubbing is not recommended as it produces scratches on the plates [12].

In contrast with the conventional film, an intraoral digital radiographic receptor is projected to be used for several patients. However, they should not be sterilized by the methods commonly available in dental practice. Thus, the most viable method to prevent the transfer of microorganisms to the digital receptor is to use barrier envelopes as the plastic covers. When phosphor storage plates are used, the use of a second plastic barrier is recommended to minimize contamination by microorganisms from the oral microbiota. Also, the barrier should then be disinfected before the plate is removed from the barrier and placed in the scanner [12].

2.4 Limitations of Bidimensional Radiography

As mentioned previously, the periapical radiographs are currently used for management of endodontic problems, from diagnosis until follow-up assessment. Although these images present high detail and the image receptors show high spatial resolution, some limitations are evident, such as the compression of three-dimensional anatomy (Fig. 2.2), geometry distortion, and anatomical noise, which may impair the diagnosis. Additionally, it must be emphasized that all radiographs taken during diagnosis, treatment, posttreatment, and follow-up should be standardized with respect to alignment between the X-ray beam, tooth, and the receptor, avoiding under- or overestimation of the disease and the healing process [13].

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Fig. 2.2

Limitation of the periapical radiograph. (a) Nonvisualization of the root canal bifurcation on the third apical of the orthoradial incidence. (b) A variation of the horizontal angulation. (c) The presence of the filling material allowed its precise identification

2.5 Cone Beam Computed Tomography in Endodontics

Cone beam computed tomography (CBCT) is a diagnostic imaging modality that provides an accurate three-dimensional image of dento-maxillofacial structures. This imaging technology has been growing rapidly in recent years in the field of dentistry, replacing medical computed tomography (CT), due to its superior image quality of dental hard tissue and bone, in addition to low radiation dose. While CT images present anisotropic voxels, CBCT data are isotropic, which allows geometrically accurate measurements [13, 14]. Other advantages include faster scan time and low economic cost. The main disadvantage of CBCT is its inability to assess soft tissue lesions.

Several CBCT systems are available, with different parameters of exposure, scan volume, spatial resolution, and image quality. Among them, the size of the field of view (FOV) can vary from 3 × 4 cm (limited) to up to 20 cm (large) and the voxel size from 0.076 to 0.4 mm. Some equipments were developed specifically to scan limited volumes, such as a particular tooth, and it’s desired that the optimal resolution does not exceed 0.2 mm for endodontic evaluations [14]. In general, the smaller the scan volume, the smaller the radiation dose and higher is the spatial resolution [14], which is essential for endodontic purposes. The choice for dose-reduced protocols should be preferable, considering the ALARA (as low as reasonably achievable) principle, whenever possible.

The criteria for referring patients to CBCT endodontics exams should be prudently selected, due to the higher radiation dose employed, in comparison with two-dimensional dental radiographs. Three-dimensional images are not recommended as a usual method for diagnosis [15]. A request for CBCT images should consider the impact both on the diagnosis and on its ability to change the treatment planning. Thus, after a careful clinical examination, only complex diagnosis tasks must be referred [16].

There are many indications of CBCT in endodontics, such as canal morphology evaluation, periapical lesions, dental trauma, identification of broken instrument, extent of extruded root canal material, root resorptions, vertical root fracture, nonhealing root canals, etc., among others. Each case should be individually assessed, keeping in mind the radiation risk and the high economic cost involved in CBCT exams.

2.5.1 Canal Morphology

The success of an endodontic treatment depends on the precise identification of root canal morphology. Periapical radiography is the usual diagnostic tool to evaluate the morphology of the root canals. Intentional variation in X-ray beam angulation may provide additional information not readily available from the orthoradial images. The horizontal beam angulation for number identification and morphology of the root canals depends on the separation and divergence between the canals, and a horizontal beam angulation between 20 and 40° is recommended. Although radiography may reveal the characteristics, it is unlikely to show the complexities of the canal’s anatomy.

Three-dimensional imaging allows a better identification of accessory canals, root curvatures, and root canal anomalies in teeth with complex morphology when intraoral conventional radiograph fails to demonstrate the real anatomy (Fig. 2.3). A superior performance is notable, especially in multi-rooted teeth and in the identification of second mesiobuccal canal in upper first molars [17]. However, CBCT should be avoided as a standard method for the screening of root canal anatomy.

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Fig. 2.3

Axial views. Right maxillary second molar with buccal root in “C” with the absence of endodontic material, verified after a CBCT exam to investigate possible cause of apical lesion

2.5.2 Periapical Pathosis

The periapical inflammatory lesion is one of the most common diseases affecting the jaws. The diagnosis is performed based on clinical signs and symptoms and complemented by radiographic exams.

Frequently, the graduation of the periapical lesion is underestimated by periapical radiography. However, it usually shows healthy periapical conditions. To increase the possibility of a correct diagnosis, the patient could be submitted to an intraoral radiographic examination using different projections.

As stated earlier, high-resolution CBCT images allow earlier diagnosis with better accuracy than periapical radiographs [18, 19], especially in small lesions. When evaluating the impact of three-dimensional imaging on planning decisions, considering the size of lesion, some evidences point out no differences between periapical radiographs and volumetric acquisitions [20], whereas for preoperative assessment, additional informations provided by CBCT directly influence the treatment plan in approximately 62% compared to periapical conventional radiographs [18]. In fact, limited high-resolution CBCT scans should be indicated in selected cases, particularly in those with positive clinical signs and symptoms and negative finding in bidimensional radiographs [16] (Figs. 2.4 and 2.5 ).

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Oct 21, 2018 | Posted by in Endodontics | Comments Off on of Radiographic Interpretation
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