IMAGING OBJECTIVES

The objectives of diagnostic imaging depend on a number of factors, including the amount and type of information required and the period of the treatment rendered. The decision of when to image along with which imaging modality to use depends on the integration of these factors and can be organized into three phases.

IMAGING MODALITIES

The decision to image the patient is based on the patient’s clinical needs. After a decision has been made to obtain images, the imaging modality is used that yields the necessary diagnostic information related to the patient’s clinical needs and results in the least radiologic risk. The opinion of a radiologist may be required for more complex modalities or for situations in which the attending dentist is less experienced.

Maximizing the ratio of benefit to risk for imaging examinations is a fundamental tenet of radiology. Examinations known to produce this result are not necessarily the examinations that cost the least, are in proximity to the dentist, or produce the lowest radiation exposure.^1–3 However, they enable the dentist to provide the proper care or treatment for the patient.

Many imaging modalities have been reported as useful for dental implant imaging, including devices recently developed specifically for dental implant imaging (Box 3-1).^4,5

These imaging modalities can be described as analog or digital and two- or three-dimensional. Most dentists are more familiar with analog two-dimensional imaging (see Box 3-1). Analog imaging modalities are two- dimensional systems that use radiograph film or intensifying screens as the image receptors. The image quality of these systems is characterized by resolution/modulation transfer function, contrast/H and D curve, noise/Weiner spectrum, and sensitivity.⁶ The clinical performance of these imaging systems is gauged by receiver operator characteristics.^7,8

Digital images also can be produced with each imaging modality (see Box 3-1). A digital two-dimensional image is described by an image matrix that has individual picture elements called pixels. A digital image is described by its width and height and pixels (i.e., 512 × 512). For larger digital images (i.e., 1.2 × 1.2 M, where M is megapixels), the image is alternatively described as a 1.5-M image. Each picture element, or pixel, has a discrete digital value that describes the image intensity at that particular point. The value of a pixel element is described by a scale, which may be as low as 8 bits (256 values) or as high as 12 bits (4096 values) for black-and-white imaging systems, or 36 bits (65 billion values) for color imaging systems.^9–11 Black-and-white digital images are displayed optimally on a dedicated black-and-white monitor. Generally, 8 bits or 256 levels can be displayed effectively on a monitor.

A digital three-dimensional image is described by an image matrix that has individual image/picture elements called voxels. A digital three-dimensional image is described not only by its width and height and pixels (i.e., 512 × 512) but also by its depth/thickness. An imaging volume or three-dimensional characterization of the patient is produced by contiguous images, which produce a three-dimensional structure of volume elements (i.e., computed tomography [CT], magnetic resonance imaging [MRI], and interactive computed tomography [ICT]). Each volume element has a value that describes its intensity level. Typically, three-dimensional modalities have an intensity scale of 12 bits or 4096 values. Box 3-1 identifies the three-dimensional imaging modalities.

PRESURGICAL IMAGING (PHASE 1)

In the field of oral implantology, there exist numerous radiographic imaging modalities available for the presurgical assessment of the dental implant patients. In the past, intraoral radiographs along with panoramic images were used as the sole determinants of implant diagnosis and treatment planning. With the advancement of radiographic technology, various three-dimensional imaging systems are now available to the dental profession allowing the implant team an infinite amount of diagnostic information.

The goal of presurgical radiographic evaluation is to assess the available bone quality and quantity, angulation of bone, selection of potential implant sites, and to verify absence of pathology. However, there exists no ideal radiographic imaging technique in the field of oral implantology that would be acceptable for all patients. All imaging techniques in the field of dentistry have inherent advantages and disadvantages and have been shown to exhibit false-negative and false-positive images.^12–14

When selecting a radiographic modality for preoperative assessment, a careful examination of the available imaging options should be evaluated for selection as per the patient’s needs. In dental and medical radiology, a recommended principle when selecting the appropriate radiographic modality is based on radiation dosage. The “as low as reasonably achievable” (ALARA) principle should always be adhered to that states that the diagnostic imaging technique selected should include the lowest possible radiation dose to the patient. However, patient care and treatment planning should not be jeopardized in response to radiation dose. Studies have shown that an overwhelming number (90%) of dentists prescribe panoramic images as the sole determinant for implant treatment planning.^15,16 In comparison, less than 10% prescribe conventional or CT. On the other hand, the American Academy of Oral and Maxillofacial Radiology guidelines state that all implant site evaluations should be evaluated with a three-dimensional imaging technique such as conventional or computerized tomography.¹⁷

This phase of implant imaging is intended to evaluate the current status of the patient’s teeth and jaws and to develop and refine the patient’s treatment plan. Evaluation of the patient by members of the dental implant team is accomplished with a review of the patient’s history, a thorough clinical examination, and a review of the patient’s radiologic examinations. At this point, the dentist should be able to rule out dental or bone disease and establish a tentative clinical objective that meets the patient’s functional and esthetic needs. If the dentist cannot rule out dental or bone disease, further clinical or radiologic examination is necessary.

The global objective of this phase of treatment is to develop and implement a treatment plan for the patient that enables restoration of the patient’s function and esthetics by the accurate and strategic placement of dental implants. The patient’s functional and esthetic needs can be transformed physically into a three-dimensional diagnostic template, which enables the implant team to identify the specific sites of prospective implant surgery in the imaging examinations. The specific objectives of preprosthetic imaging listed in Box 3-2.

All the modalities identified in Box 3-1 have been used in the first diagnostic phase of treatment.^4,5 However, dental implant cases are inherently three-dimensional problems relating to the final prosthetics, occlusion, and function of the patient’s three-dimensional anatomy. A three-dimensional treatment plan ideally identifies at each prospective implant site the amount of bone width, the ideal position and orientation of each implant, its optimal length and diameter, the presence and amount of cortical bone on the crest, the degree of mineralization of trabecular bone, and the position or relationship of critical structures to the proposed implant sites. Thus the modalities of choice for presurgical implant treatment planning provide high-resolution and dimensionally accurate three-dimensional information about the patient at the proposed implant sites.

The imaging modalities listed in Box 3-1 can be subdivided into planar two-dimensional, quasi—three-dimensional, and three-dimensional imaging modalities. Planar imaging modalities include periapical, bite-wing, occlusal, and cephalometric imaging and are simply two-dimensional projections of the patient’s anatomy. Thus the dentist cannot possibly develop a three-dimensional perspective of the patient’s anatomy with a single image. However, with a number of cleverly oriented projections, development of some useful three-dimensional information is possible.

Quasi—three-dimensional imaging modalities include x-ray tomography and some cross-sectional panoramic imaging techniques. These techniques produce a number of closely spaced tomographic images, and the three-dimensional perspective of the patient’s anatomy is developed by viewing each image and mentally filling in the gaps. Three-dimensional imaging techniques include CT and MRI and enable the dentist to view a volume of the patient’s anatomy. These techniques are quantitatively accurate, and three-dimensional models of the patient’s anatomy can be derived from the image data and used to produce stereotactic surgical guides and prosthetic frameworks.

PERIAPICAL RADIOGRAPHY

Periapical radiographs are images of a limited region of the mandibular or maxillary alveolus. Periapical radiographs are produced by placing the film intraorally parallel to the body of the alveolus with the central ray of the x-ray device perpendicular to the alveolus at the region of interest, producing a lateral view of the alveolus.¹⁸ Periapical radiography provides a high-resolution planar image of a limited region of the jaws.¹⁹ Number 2 size dental film provides a 25 × 40-mm view of the jaw with each image. Periapical radiographs provide a lateral view of the jaws and no cross-sectional information. Even with adjacent periapical radiographs made with limited oblique orientations, three-dimensional information is of little use for the implant imaging (Box 3-3).

Periapical radiographs may suffer from distortion and magnification. The most accurate radiographic technique used for periapical radiology is the paralleling technique that necessitates placing the film or sensor parallel to the long axis of the implant, tooth, or osseous structure in question. These principles in positioning will allow for an intraoral image with minimal distortion and magnification. If improper positioning or the bisecting angle technique is used, vertical and horizontal measurements may be distorted and magnified.

The long cone paralleling technique eliminates distortion and limits magnification to less than 10%. Millimeter radiopaque grids, sometimes used in endodontics, may be superimposed over the film before it is exposed, but are of little quantitative value and provide misleading information because they lie on the film and obfuscate the underlying anatomy and do not compensate for magnification.

Image shape distortion occurs when unequal magnification of the object exists. This will occur when the total area in question (alveolar bone, implant) does not have the same focal spot-to-object distance. When the x-ray beam is perpendicular to the object, but the object is not parallel to the film, foreshortening will occur. If the x-ray beam is oriented perpendicular to the object but not the film, elongation will occur (Figure 3-1). These basic and important concepts will help minimize distortion and magnification when using intraoral radiographs.²⁰

Figure 3-1 Film positioning. A, The central ray is perpendicular to bone and film resulting in no distortion. B, The central ray is perpendicular to film but not to the implant which will result in foreshortening. C, The central ray is perpendicular to the object but not the film and will result in elongation.

Image magnification may be assessed by placing a known-dimension radiographic marker (e.g., 5-mm ball bearing) at the crestal region of the desired implant location. When the marker is elongated, so is the implant site. For example, a ball bearing radiographic measurement of 8 mm relates to a 60% magnification. Therefore the image below the ball bearing may represent a 60% magnification of dimension.

The opposing landmark of available bone in implant dentistry is beyond lingual muscle attachments in the mandible or beyond the palatal vault in the maxilla. As such, the image most often must be foreshortened to visualize the opposing cortical plate. As a result, the actual available bone height may be difficult to determine.

The bone density at the crest is also a factor to evaluate crestal bone loss with radiographic indexes. In D4 bone, no cortical plate is present on the crest, and fine trabecular bone is primarily present. Burnout effects are common when standard kilovolt and milliampere settings are used, making crestal bone loss evaluation with digital intraoral systems of benefit in these situations.^21–23

The dense cortical plates on the lateral aspect of the mandible and palatal aspect of the maxilla make bone quality difficult to assess with a periapical radiograph. In fact, a 40% change in trabecular bone density is necessary before a difference may be observed in the anterior mandible.

In terms of the objectives of presurgical imaging, periapical radiography is:

In the preprosthetic phase, these films most often are used for single-tooth implants in regions of abundant bone width.

DIGITAL RADIOGRAPHY

One of the most recent significant advances in dental radiology is the advent of digital technology that has allowed numerous limitations of conventional intraoral radiography to be reduced. The advantages of digital radiography and the uses in oral implantology are well documented.^24,25 With the use of digital radiography, implant surgical procedures and prosthetics have been simplified with increased efficiency.

Digital radiology is an imaging process wherein the film is replaced by a sensor that collects the data. The analog information received is then interpreted by specialized software, and an image is formulated on a computer monitor. The resultant image can be modified in various ways, such as gray scale, brightness, contrast, and inversion. Color images may be formed to enhance the digital image for better evaluation. Computerized software programs (i.e., DexisImplant) are now available that allow for calibration of magnified images, thus ensuring accurate measurements (Figure 3-2).

Figure 3-2 Digital radiographic system that includes digital sensor and computer.

(Courtesy Dexis, LLC.)

When compared with conventional radiographs, the most current digital systems have significantly less radiation^26,27 with superior resolution.²⁵ However, with respect to oral implantology, the most significant advantage of digital radiography is the instantaneous speed in which images are formed that is highly useful during surgical placement of implants and the prosthetic verification of component placement (Figure 3-3).

Figure 3-3 A, Direct measurements of vital structures may be made directly on the calibrated digital images. Studies have shown that on plain film intraoral radiographs, more than 50% of the time the mandibular canal and mental foramen are indistinguishable. However, this limitation has been drastically reduced with the advent of digital imaging. B, Image magnification can be determined by imaging a known-diameter radiographic marker, and the appropriate size implant (adjusted for magnification) may then be selected and placed into the edentulous area.

A disadvantage of digital radiography is the size and thickness of the sensor and the position of the connecting cord. These features make the positioning of the sensor more difficult in some sites such as those adjacent to tori or a tapered arch form in the region of the canines (Table 3-1).²⁸

Table 3-1 Comparison of Film versus Digital-Based Images

From Park ET, Williamson GF: Digital radiography: an overview, J Contemp Dent Pract 3:1-13, 2002.

OCCLUSAL RADIOGRAPHY

Occlusal radiographs are planar radiographs produced by placing the film intraorally parallel to the occlusal plane with the central x-ray beam perpendicular to the film for the mandibular image and oblique (usually 45 degrees) to the film for the maxillary image. Occlusal radiography produces high-resolution planar images of the body of the mandible or the maxilla.¹⁹ Maxillary occlusal radiographs are inherently oblique and so distorted that they are of no quantitative use for implant dentistry for determining the geometry or the degree of mineralization of the implant site. In addition, critical structures such as the maxillary sinus, nasal cavity, and nasal palatine canal are demonstrated, but the spatial relationship to the implant site generally is lost with this projection (Box 3-4).

Because the mandibular occlusal radiograph is an orthogonal projection, it is a less-distorted projection than the maxillary occlusal radiograph. However, the mandibular alveolus generally flares anteriorly and demonstrates a lingual inclination posteriorly, producing an oblique and distorted image of the mandibular alveolus, which is of little use in implant dentistry. In addition, the mandibular occlusal radiograph shows the widest width of bone (i.e., the symphysis) versus the width at the crest, which is where diagnostic information is needed most (Figure 3-4). The degree of mineralization of trabecular bone is not determined from this projection, and the spatial relationship between critical structures, such as the mandibular canal and the mental foramen, and the proposed implant site is lost with this projection. As a result, occlusal radiographs rarely are indicated for diagnostic presurgical phases in implant dentistry.

Figure 3-4 A, Occlusal radiographs have been postulated to show the width of bone in the anterior region. B, However, occlusal radiographs actually show the widest bucco-lingual distance (red arrows) not in the same plane. Actual width of bone (green arrow).

CEPHALOMETRIC RADIOGRAPHY

Cephalometric radiographs are oriented planar radiographs of the skull. The skull is oriented to the x-ray device and the image receptor using a cephalometer, which physically fixes the position of the skull with projections into the external auditory canal. The geometry of cephalometric imaging devices results in a 10% magnification of the image with a 60-inch focal object and a 6-inch object-to-film distance.¹⁹

A lateral cephalometric radiograph is produced with the patient’s midsagittal plane oriented parallel to the image receptor. This radiograph demonstrates a cross-sectional image of the alveolus of the mandible and the maxilla in the midsagittal plane.²¹ With a slight rotation of the cephalometer, a cross-sectional image of the mandible or maxilla can be demonstrated in the lateral incisor or in the canine regions. Unlike panoramic or periapical images, the cross-sectional view of the alveolus demonstrates the spatial relationship between occlusion and esthetics with the length, width, angulation, and geometry of the alveolus and is more accurate for bone quantity determinations. Implants often must be positioned in the anterior regions adjacent to the lingual plate.

The lateral cephalometric radiograph is useful because it demonstrates the geometry of the alveolus in the mid-anterior region and the relationship of the lingual plate to the patient’s skeletal anatomy (Figures 3-5 and 3-6; Box 3-5). The width of bone in the symphysis region and the relationship between the buccal cortex and the roots of the anterior teeth also may be determined before harvesting this bone for ridge augmentation. Together with regional periapical radiographs, quantitative spatial information is available to demonstrate the geometry of the implant site and the spatial relationship between the implant site and critical structures such as the floor of the nasal cavity, the anterior recess of the maxillary sinus, and the nasal palatine canal. The lateral cephalometric view also can help evaluate a loss of vertical dimension, skeletal arch interrelationship, anterior crown/implant ratio, soft tissue profile, anterior tooth position in the prosthesis, and resultant moment of forces. As a result, cephalometric radiographs are a useful tool for the development of an implant treatment plan, especially for the completely edentulous patient. However, this technique is not useful for demonstrating bone quality and only demonstrates a cross-sectional image of the alveolus where the central rays of the x-ray device are tangent to the alveolus.

Figure 3-5 A limited projection of the mandibular symphysis region is useful for preoperative evaluation of the symphysis.

Figure 3-6 A, An alternative technique to the standard lateral cephalometric unit is the use of an occlusal film (size 2) held by the patient using a standard intraoral radiographic unit. B, Resultant image.

Disadvantages of cephalometric radiographs include cross-sectional information limited to the midline area and difficulty in cephalometric machine accessibility. Any non-midline structure is superimposed on the contralateral side. This radiographic technique is operator technique sensitive and, if improperly positioned, will result in a distorted view. Because lateral cephalometric radiographs use intensifying screens, resolution and sharpness is compromised in comparison to intraoral radiographic techniques.

PANORAMIC RADIOGRAPHY

Panoramic radiography is a curved plane tomographic radiographic technique used to depict the body of the mandible, the maxilla, and the lower half of the maxillary sinuses in a single image. This modality is probably the most used diagnostic modality in implant dentistry. However, for quantitative presurgical implant imaging, panoramic radiography is not the most diagnostic. This radiographic technique produces an image of a section of the jaws of variable thickness and magnification. The image receptor traditionally has been radiograph film but may be a digital storage phosphor plate or a digital charge—coupled device receptor.^23,29,30 Nonetheless, panoramic images offer many advantages (Box 3-6).

The significant limitations of panoramic radiographs can be classified into two categories: (1) distortions inherent in the panoramic system and (2) errors in patient positioning. Panoramic radiography is characterized by a single image of the jaws that demonstrates vertical and horizontal magnification, along with a tomographic section thickness that varies according to the anatomical position. The x-ray source exposes the jaws from a negative angulation and produces a relatively constant vertical magnification of approximately 10%. The horizontal magnification is approximately 20% and variable depending on the anatomical location, the position of the patient and the focus object distance, and the relative location of the rotation center of the x-ray system. Clinical data have shown that nonuniform magnification may be in the range of 15% to 220%.^31–33 Structures of the jaws become magnified more as the object-film distance increases and the object—x-ray source distance decreases.

Structures that are located obliquely in relation to the implant receptor produce aspects of the structures that are magnified more when they are farther from the image receptor and less when they are closer to the image receptor.^34–36 Uniform magnification of structures produces images with distortion that cannot be compensated for in treatment planning. The posterior maxillary regions are generally the least distorted regions of a panoramic radiograph. The tomographic section thickness of panoramic radiography or trough of focus is thick, approximately 20 mm, in the posterior regions and thin, 6 mm, in the anterior region.³⁶

Traditional panoramic radiography is a high-yield technique for demonstrating dental and bone disease. However, panoramic radiography does not demonstrate bone quality/mineralization, is misleading quantitatively because of magnification and because the third-dimension cross-sectional view is not demonstrated, and is of some use in demonstrating critical structures but of little use in depicting the spatial relationship between the structures and dimensional quantitation of the implant site. Because panoramic radiography is such a popular and widely available technique in dentistry, dentists have developed means to compensate for its shortcomings. Implant companies often market magnified overlays with a preset 25% magnification for evaluation of an implant size that are placed on a panoramic film for comparison with vital structure positions.

Because the concept of standardized magnification is impossible and unreliable, most clinical studies have observed the inaccuracies of direct measurements of panoramic radiographs^37,38 (Figure 3-7).

Figure 3-7 A, All panoramic beam angles are approximately at 8 degrees, which gives the image inherent magnification. B, Because of the curvature of the arch, panoramic machines have changing rotational centers.

Inherent magnification is dependent upon patient positioning errors, which results in significant geometric distortion. With knowledge, most errors in patient positioning can be corrected (Table 3-2). However, in a given plane, horizontal distortion cannot be determined and measurements are completely unreliable. The horizontal dimensions are affected by the rotation center of the beam that changes with relation to the object-film distance.²⁶

Table 3-2 Common Panoramic Positioning Problems and Corrections