Orthodontists prioritise a comprehensive evaluation of the human face and stomatognathic system. Radiological imaging is crucial in orthodontic diagnosis, treatment planning, simulation and progress assessment. As the field of radiology has progressed, it has incorporated additional methods, including non-radiation sources and techniques, to enhance diagnosis. The term radiation comes from the Latin word ‘radius’ and refers to various forms of energy produced by a source that moves in circles. There are two primary types of radiation: particulate and electromagnetic radiation.

Non-radiation methods, such as magnetic resonance imaging (MRI) and ultrasound (US), have also contributed to the science and clinical practice. This chapter will explore the uses of three-dimensional radiation imaging in orthodontics.

Electromagnetic radiation

It is the movement of energy through space as a combination of electric and magnetic fields. X-rays are electromagnetic radiations produced from the interaction of electrons with large atomic nuclei in X-ray machines. They usually have a wavelength of approximately 0.01–10 nm ( Fig. 37.1 ).

Line diagram showing various wavelengths and frequencies of different radiation sources.

Radiation has both direct and indirect effects on the living system. When the energy of the photon or secondary electrons ionises biological macromolecules, the effect is direct.

Alternately, an indirect effect is when a photon may be absorbed by water in an organism, producing free radicals, which produce changes to the biological molecules.

Radiation-related injuries in any organism are primarily of two types.

• 1.

  Deterministic effects: This term implies that the radiation damage occurs and that the damage is dependent on the amount of radiation received. These include changes in the blood count, hair loss, tissue necrosis or cataracts.

• 2.

  Stochastic effects: Here, the probability of suffering from a disease caused by radiation is proportional to the amount received years before. Examples include cancer or leukaemia.

The sources of radiation are of two major types:

• 1.

  Natural radiation: These could be derived from cosmic and terrestrial sources, including radioactivity from soil and radioactive decay products of uranium. The other sources include radon, a decay product in the uranium.

• 2.

  Artificial sources: The bulk of it includes medical diagnostic and treatment sources. The other sources include consumer and industrial products, nuclear science research, mining and nuclear accidents.

Broadbent introduced traditional cephalometry in the USA, while Hofrath did the same in Germany in 1931. Roentgeno-cephalometry is the standardised 2D X-ray widely used as a research tool and clinical investigation for studying craniofacial growth and orthodontic treatment.

Limitations of 2D cephalometry: According to Quintero et al., there are several reasons for the limited validity of 2D cephalometry’s scientific method and its applications. These are :

• 1.

  The conventional 2D head film represents the face and skull skeleton, a three-dimensional (3D) object. This depiction also carries the inherent displacement of structures vertically and horizontally, in direct proportion to their relative distances from the recording plane. It is important to note that this representation is limited to a 2D format and does not fully capture the intricacies of the 3D object.

• 2.

  Cephalometric analyses are based on the assumption of a perfect superimposition of the right and left sides of the mid-sagittal plane. However, this is infrequently observed because facial symmetry is rare and because of relative image displacement of the right and left sides. Hence, accurately assessing structures away from the mid-sagittal plane is an unrealistic goal.

• 3.

  During the process of image acquisition, several radiographic projection errors may arise. These errors are primarily associated with magnification, distortion, errors in patient positioning and projective distortion, which are inherent to the geometric relationships between the film, patient and focus. It is imperative to recognise and understand these errors, as they can have significant implications for the accuracy and reliability of radiographic images. ^,

• 4.

  Manual data collection and processing in cephalometric analyses have been shown to have low accuracy and precision. Manual tracings of anatomic structures used for cephalometric analysis incorporate intra- and inter-observer errors. Conventional analyses utilise linear and angular measurements defined in a coordinate system that varies across patients and in longitudinal assessments.

• 5.

  Errors in landmark identification could be problematic in specific anatomical structures due to a lack of well-defined anatomic outlines and shadows, lack of contrast and overlapping anatomy. Despite its limitations, cephalometric analysis has been widely used in orthodontics, with numerous analysis techniques developed over the years.

Sir Godfrey N. Hounsfield revolutionised the medical field in 1967 by introducing the first clinical CT scanner. This scanner utilised a translate-rotate, parallel-beam geometry for data acquisition. Essentially, pencil beams of X-rays were directed at a detector opposite the source, and the intensity of transmitted photons was measured. By translating and rotating the gantry, X-ray attenuation data were captured. While advancements have since been made in X-ray sources, detectors and acquisition geometries, the core principles behind CT remain unchanged ( Figs 37.2 and 37.3 ).

Rotating type of anode is used in CT and CBCT machines for better dissipation of heat produced.

Comparison of basic functioning difference between (A) CT and (B) CBCT technologies.

There are two types of CT scanners: fan beam and cone beam. These scanners differ in how they capture X-ray images. The fan-beam scanner features a rotating gantry that houses an X-ray source and detector. It captures images of the patient in slices, typically in the axial plane, using a narrow fan-shaped X-ray beam that passes through the patient. The resulting images are interpreted by stacking the slices to create multiple 2D representations. In traditional helical fan-beam CT scanners, multi-detector arrays are used. This allows for the capture of up to 64 slices simultaneously, resulting in the generation of 3D images with significantly lower radiation doses than single detector fan-beam CT arrays.

Current cone beam machines scan patients in three possible positions: (1) sitting, (2) standing and (3) supine. The four components of CBCT image production are :

• 1.

  Acquisition configuration

• 2.

  Image detection

• 3.

  Image reconstruction

• 4.

  Image display

Imaging in CBCT is accomplished by using a rotating gantry to which an X-ray source and detector are fixed. The X-ray source and digital sensor revolve synchronously around the rotation fulcrum fixed within the centre of the region of interest. A divergent cone-shaped source of ionising radiation is directed through the middle of the ‘area of interest’ onto an area X-ray detector [Flat Panel Detector (FPD)/CMOS sensor] on the opposite side. During the rotation, multiple (from 150 to more than 600) sequential planar projection images (base images) of the field of view (FOV) are acquired in a complete (360 degrees) or sometimes partial (180 and 270 degrees), arc ( Fig. 37.4 ). This procedure varies from a traditional medical CT, which uses a fan-shaped X-ray beam in a helical progression to obtain individual image slices of the FOV and then stacks the slices to obtain a 3D representation. In helical CT imaging, each slice requires a separate scan and separate 2D reconstruction.

In CBCT, a complete or partial gantry rotation around subject head produces the multiple sequential base images. From these base images, software programs reconstruct a volumetric dataset composed of isotopic voxels.

The speed at which individual images are acquired is called the frame rate and is measured in frames, that is, projected images per second. The maximum frame rate of the detector and rotational speed determines the number of projections that may be acquired.

The detector is of two types. An image intensifier tube/charge-coupled device combination (CCD) or flat panel (FP) detector. The principal determinants of voxel size in CBCT are the tube focal spot size, X-ray geometric configuration and pixel size of the detector.

Primary reconstruction: After the basic projection frames have been acquired, the creation process of the volumetric data set is called primary reconstruction. The CBCT software handles this process. The display is the critical step, which is the compilation of all available voxels and presentation to the radiologist for interpretation.

Secondary reconstruction: The software generates the volumetric dataset’s orthogonal projection slices [multiplanar reconstruction (MPR) slices, sagittal, axial, coronal], called secondary reconstruction ^{, ,} ( Fig. 37.5 ).

Secondary reconstruction.

The orthogonal projection slices (Multiplanar Reconstruction or MPR slices) of the volumetric dataset is generated by the secondary reconstruction using software. (A) Axial, (B) coronal, (C) sagittal.

There are several types of scanners available in the market. These scanners can be categorised based on the patient positioning and FOV.

Based on the patient positioning ( Fig. 37.6 ):

• 1.

  Supine position

• 2.

  Sitting position

• 3.

  Standing position

Types of CBCT scanners based on patient positioning. (A) Supine position (NewTom 5G). (B) Sitting position (iCat NG). (C) Standing position (NewTom VGi).

Source: Courtesy: (A, C) Newtom, Bologna, Italy (B) Imaging Sciences International, Pennsylvania, USA .

Field of view

Field of view (FOV) could be defined as the area of interest required for study and is exposed to radiation scanning procedures. The size of the FOV depends on the size and shape of the detector, the geometry of the beam and the ability to collimate the beam. Collimation of the beam eliminates the photons outside the intended FOV, limiting the radiation exposure to the region of interest. Selecting an appropriate FOV for each patient is crucial, as it strongly affects the radiation dose and exposure. A large scanned area results in higher exposure, so determining the appropriate FOV based on the patient’s disease representation is vital. FOV selection ensures that necessary diagnostic information is obtained while minimising radiation exposure. The indications of CBCT imaging in orthodontics based on the FOV are given in Table 37.2 . ^{, ,}

TABLE 37.2

Indications of CBCT imaging in orthodontics

S. no.	FOV	Clinical conditions	Examples
1.	Small	Dental structural anomalies	Supernumerary teeth, hypodontia
		Anomalies in dental position	Impacted teeth
2.	Medium	Compromised dento alveolar boundaries	Cleft lip and palate
		TMJ assessment	Condylar positional changes, condylar hyperplasia, hypoplasia or aplasia, previous condylar trauma
		Localised jaw pathology	Infections, benign cysts and tumours
3.	Large	Dentofacial deformities and craniofacial anomalies	Facial asymmetry, Class II and Class III skeletal malocclusions, vertical and transverse deficiencies
		Surgical planning	Orthognathic surgical planning
		Airway pathology	Obstructive sleep apnoea/hypopnoea syndrome

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