3.2.1 Dental Radiograph Generators [28–30]

Radiographic exposure is controlled by three components: kVp (kilovolt peak), mA (milliamperage), and exposure time. KVp controls the power of each particular X‐ray particle, which controls the penetration of the beam through tissues. This equates to the “quality” of the X‐ray beam. The “quantity” of the exposure is determined by the combination of mA and time of exposure. The higher the mA, the more X‐rays produced over a given time period.

Since there is minimal variation of tissues within the oral cavity, the kVp and mA are constant on most dental radiology units; the only variable factor is time. Exposure is typically measured in seconds (or parts thereof), but some units use pulses. Most dental radiology units have a digital control for the exposure, which is set by the operator. Recently, however, veterinary specific machines have become available that have a computer that sets the exposure based on the size of the patient, the type of film used (or digital system), and the object tooth/teeth.

Finally, hand‐held devices are currently available. These are particularly valuable in practices with minimal space as well as mobile practices. The reader should note that they are not approved in all states/countries and the reader is directed to their local government for direction.

3.2.2 Dental Radiographic Film [8, 29–31]

There are three speeds of dental film in common use: “D,” “E,” and “F.” The speed is dependent on the size of the silver halide crystals and therefore the amount of radiation required. “E” speed film requires approximately ½ the amount of radiation for exposure than “D” speed film, and “F” speed is ¼–½ of “E” [33, 34]. There is a slight decrease in resolution with faster films due to the larger crystal size, but according to most experts, the difference is negligible [34, 35]. It is critical to note that different speed films require different safe light colors (see below).

There are different sizes of dental film available (4, 3, 2, 1, and 0). The most common sizes used in veterinary medicine are 4, 2, and 1. Size 4 film is the largest available and is generally used for full mouth radiographs and major maxillofacial surgery. Size 2 is the most commonly used film size for single tooth radiographs. It should be noted, however, that the canines of large breed dogs cannot be completely imaged on a size 2 film. Size 1 or 0 are used for small dogs and cats, especially for mandibular views.

3.2.3 Digital Dental Radiography [36]

3.2.3.2 Direct Systems – Digital Radiography (DR)

These systems employ solid‐state sensors. The two major types of solid‐state sensors are the charge‐coupled device (CCD) and complementary metal oxide semiconductor (CMOS). These systems convert the energy from X‐ray photons into electronic signals. A scintillation layer is placed on top of the sensor in order to turn X‐ray photons into light photons, which are subsequently absorbed by the chip.

3.2.3.3 Creation of the Digital Image

Regardless of the digital system, the images are created in a similar fashion. The plates/sensors measure the intensity of the photons following passage through the oral tissues, typically on a 256 unit gray scale. Zero corresponds to the maximum measurable radiation (or black) and 255 corresponds to no exposure (or white).

Intensity measurements are performed over a number of very small regions of the sensor called pixels (which stands for picture element). Resolution of a digital dental image is determined by the pixels, specifically the size, number, and color depth. Each pixel is 15–40 square micrometers (μm) in size, equating to thousands of pixels on a size 2 sensor. After the intensity is measured, the score of each individual pixel is transferred to the computer for image creation. The computer assigns a gray‐scale value to each pixel and places it in the correct location. The computer then generates the “raw” image one pixel at a time.

All digital dental software programs have the ability to manipulate this numerical information, thus changing the appearance of the image. This is performed by subjecting the information to mathematical equations called algorithms. A simple algorithm is reversing the gray scale (white becomes black) to create a “negative.” However, there are numerous highly complicated algorithms that can improve contours and/or contrast, and can even adjust an image that is over‐ or underexposed. These changes can be done manually, although all systems have the ability to either “optimize” the image automatically or provide the user with a group of pre‐set settings to choose from. This is responsible for much of the differences in the appearance of the images between manufacturers.

3.2.3.4 CR versus DR

Numerous studies have compared DR to CR produced images. The majority of these studies report that DR systems may have a higher resolution [37, 38]. These same studies report, however, that PSP systems will produce quality images over a wider exposure range (see below) [37–39]. In contrast, one study reported that PSP plates had superior image quality to solid‐state (DR) images [39]. The 2008 and 2010 sensor shootouts showed that the PSP system compared favorably to the sensor systems. Finally, a good technique to determine the quality of the digital system is to compare line pairs.

3.2.3.5 Advantages of Digital Radiography [36, 40–44]

The main advantage of digital radiography is decreased radiation exposure and anesthesia time. With DR technology, the image is available in seconds. Additionally, since the image is produced while the sensor and tube‐head are still in place, minor technique adjustments can be performed without starting again. The scanning of the CR phosphor plates takes about 10 seconds, and the plate must be removed from the patient’s mouth. This negates some of the speed advantage; however, the larger plates that are available (size 4 and even larger) can make up for this time shortcoming. Experienced practitioners can actually be faster with CR systems, but for the novice, the ease of retakes with DR improves the learning curve. In addition, CR plates have increased latitude of exposure than either DR or standard film, which limits retakes.

Digital images have other advantages over analog films, including [45]:

• Large size of the image projected on the computer screen combined with the ability to manipulate the image allows for easier interpretation.
  
• The ability to “mark” the images facilitates client communication.
  
• Digital images are permanent (if properly stored) and therefore do not degrade over time.
  
• The use of toxic chemicals is avoided, as well as the hassle of proper disposal.
  
• Digital images save storage space in the clinic and staff time searching for previous films.
  
• Digital images can be quickly uploaded to telemedicine sites or e‐mailed to specialists for consultation.
  
• DR sensors have an additional advantage of creating far less radiographic exposure than standard film.

It should be noted, however, that one study (admittedly older) actually showed an increase in exposure with a digital system using a dosimeter [46]. These properties can make dental radiographs a very effective marketing tool.

3.2.3.6 Disadvantages of Digital Radiography

The main argument against digital radiographs was that they provided less detail than standard radiographic film [47–51]. This is controversial at this time as other studies report similar quality, with the difference depending on several factors [43, 52–55]. When image quality in general is studied, digital radiographs are typically rated inferior to standard film. However, some studies report that digital images are overall superior to standard film [56]. Additionally, when particular pathology or procedural aspects are studied, digital images can be superior to plain film. At this point they are at least equal to and in some instances superior to standard film [57–60]. The one point of agreement in almost every study was that enhanced images were superior to the raw images [55, 60, 61]. Further, when digital systems are compared to analog film, less exposure errors were created [60, 61]. The biggest drawback to DR systems is the lack of a size 4 sensor plate [36]. This is mostly a problem in large breed dogs as some teeth cannot be completely imaged on one film. In addition, exposing full‐mouth radiographs in large and giant breed dogs requires significantly more exposures.

Most DR sensors have a very narrow exposure range to create an acceptable image, and consequently this may increase retakes. There is one exception that has a wide latitude of exposure (Sopix Digital Dental Sensor, Dental FocusTM, LLC, Neshanic Station, NHJ), but it is recommended that the lowest possible exposure setting should be used.

The CR system negates many of the above issues. First, it has a size 4 plate and the individual plates are generally under $100. Therefore, replacement of damaged plates is not a huge expense. However, these plates reach “wear out” faster than direct digital systems and can easily be scratched, therefore requiring more frequent replacement.

The major concern with CR systems is that their optimum radiographic exposure is higher than Ektaspeed film. In addition, the wide latitude of dose levels allows a diagnostic radiograph to be created with exceedingly high exposure. Practitioners must be very careful and use the lowest possible setting.

The additional disadvantage with digital radiography is the initial setup cost. However, over time the savings in film and development costs (not to mention time) will more than pay for the system. The only other disadvantage to digital systems is the need for consistent backup of the computer information. If a computer fails, the information will be lost. Backup to a mirrored hard drive, CDS, or preferably a web‐based storage location is mandatory.

3.2.4 Recommendations [36]

Standard (analog) film is certainly still acceptable for intraoral dental radiography. It is of similar quality to digital and is initially much less expensive. Therefore, it is certainly a great first step. However, over time, the cost of film and chemicals will negate this. This does not count the decreased time enjoyed with dental systems.

For most general practices, a size 2 DR sensor is the best choice. This is because in general it is faster than CR or standard film systems. This is especially true when inexperienced technicians are taking images, as retakes are much faster. For technicians who are confident in their sills, the size 4 phosphor plate can make full mouth radiographs faster (especially in large breed dogs). Feline‐only practices would benefit from a size 1 sensor, as its smaller size better fits the feline anatomy, but this size would not be useful for extra oral radiography of the temporomandibular joint (TMJ).

Any practice can benefit from the variations in size offered by CR systems. However, those practices performing major maxillofacial surgery greatly benefit from the larger images available from these systems. Further mixed practices who may see equine patients should consider a CR system, especially one that can take even larger films than a size 4. Finally, PSP plates are excellent for small mammal and reptile radiographs. If you are still confused as to which system is best for your practice, a pre‐purchase consult can be extremely valuable.

A final thought to consider when purchasing a digital dental system is your monitor. The fine resolution, depth of color, and algorithms in high‐end digital dental systems are not likely to be appreciated on inexpensive monitors. This is particularly true of laptop computer screens, which are what the majority of clinics use. Make sure to purchase a monitor that justifies your investment.

3.3.1 Step 1: Patient Positioning

Position the patient so that the area of interest is convenient to the radiographic beam. In general, this is where the arcade to be imaged is “up.” When imaging the mandibular canines and incisors the patient should be in dorsal recumbency. For mandibular cheek teeth, the patient can stay in dorsal, or be placed in lateral recumbency with the affected side “up.”

Positioning for maxillary teeth is controversial, with some dentists recommending ventral and others lateral recumbency. While it is easier to visualize angles in ventral recumbency, rolling patients into this position for intra‐ and/or post‐op images is arduous and time consuming. In addition, this typically displaces the monitoring leads, adding time to the procedure. Finally, it can be traumatic to the spine of older pets as well as the hips of large breed dogs. For these reasons, in our practice virtually all maxillary radiographs are exposed in lateral recumbency (Figure 3.4a,b).

3.3.2 Step 2: Film Placement [11, 29, 62–64]

When utilizing standard film, there is an embossed dot on the film packet that should be placed toward the X‐ray beam. In most films, this side is pure white and the opposite or “back” side of the film is colored. The dot should also be positioned away from the structures to be imaged to avoid interference.

For DR sensors, the cord will exit on the “back” side of the sensor and this side goes away from the tube head. For PSP plates, the side with writing goes away from the tube head.

The film should be placed as near as possible to the teeth and oral mucosa to minimize distortion. Position the film/sensor in the mouth so that the entire tooth (crown and entire root surface) is covered by the film/sensor (if possible). If the target tooth is larger than the size 2 sensor (which is common in large breed dogs), it is recommended to:

• Position the sensor apically enough to expose the apex of the tooth and 3 mm beyond. This will not expose the crown, but in many cases the crown is not a critical piece of information.
  
• Expose two images (one of the root and one of the crown).

3.3.3 Step 3: Positioning the Beam Head [11, 28–30, 32, 63–66]

There are two major techniques for positioning the beam head in veterinary patients, both of which are used on a daily basis.

3.3.3.1 Parallel Technique

This is where the film is placed parallel to the object being radiographed and the beam placed perpendicular to both the film/sensor and the tooth/root. This provides the most accurate image, but is only useful in the mandibular molars and caudal premolar teeth. The maxillary teeth cannot be imaged due to the fact that dogs and cats do not have an arched palate, which interferes with parallel placement. Similarly, the symphysis interferes with parallel placement for the rostral mandibular teeth (including the first and second premolar in dogs and occasionally third premolar in cats).

3.3.3.2 Bisecting Angle Technique

This is the most common type of dental radiograph taken in veterinary patients. This is the most scientifically correct way to image veterinary patients, but is very cumbersome and time consuming. This technique utilizes the theory of equilateral triangles to create an image that accurately represents the tooth and roots.

1. Place the film/sensor as parallel as possible to the tooth root.
   
2. Measure the angle between the tooth root and film.
   
3. Cut the measure angle in half.
   
4. Place the beam placed to the bisected angle.
   
5. This gives the most accurate representation of the root.

If this angle is incorrect, the radiographic image will be distorted. This is because the X‐ray beam will create an image that is longer or shorter than the object imaged.

The best way to conceptualize this technique is to imagine a building and the sun. The building creates a 90° (right) angle to the ground. The bisecting angle in this case is 45°.

Early and late in the day, the sun is at an acute angle to the ground and casts a long shadow. In radiography, this occurs when the angle of the beam to the sensor/film is too small and is known as elongation. At some point in the late morning and early afternoon, the sun is at a 45° angle to the building, which is the bisecting angle, which gives an accurate representation of the building height. As the sun continues up, the shadow shortens. This occurs in veterinary radiography when the angle of the beam to the sensor/film is too great and is known as foreshortening. Finally, at noon, the sun is straight up from the building, which gives no shadow.

3.3.3.3 Simplified Technique

This technique [28], developed by Dr. Tony Woodward, does not utilize direct measurement of any angle, instead relying on approximate angles to create diagnostic images. There are only three angles used for all radiographs in this system, 20, 45, and 90.

As above, the mandibular molars and caudal premolars are exposed at a 90° angle (parallel technique).

Maxillary premolars and molars have their roots straight up from the crowns, with the sensor essentially flat across the palate. This creates a 90° angle and thus a 45° bisecting angle is used.

Canines and incisors curve backward significantly (approaching a 40° angle to the palate) and therefore are imaged with a 20° angle rostrocaudally.

There are only four conditions where this technique may not be sufficient.

1. Maxillary canines [67]. The roots of the maxillary canines are directly dorsal to the maxillary first and second premolars in dogs and the second and occasionally third premolar in cats. Therefore, an additional rotation to 20° lateral is necessary to avoid superimposition of the canine and these teeth. This will image the root over the nasal cavity.
   
2. Mesial mandibular premolars [32, 65]. The apices of these teeth are often not imaged on films using the parallel technique. This is because the symphysis interferes with the ventral placement of the film/sensor. On occasion this can be alleviated by simply slightly rotating the PID ventrally to foreshorten the radiograph. If this is insufficient, the bisecting angle technique is utilized. To perform the bisecting angle technique for these roots, place the film/sensor in position for the canines/incisors and place the PID 45° laterally.
   
3. Maxillary cheek teeth in cats [32, 65]. When using the standard intraoral bisecting angle technique, the zygomatic arch may interfere with good visualization of the maxillary fourth premolar/first molar as well as the distal root of the maxillary third premolar. This author does not feel that this significantly affects interpretation with digital systems. However, if the practitioner wishes to view these teeth without interference, the extraoral technique can be utilized.
   
   
   
   1. Place the film/sensor on the table.
      
   2. Position the cat on the sensor with the arcade to be imaged down.
      
   3. Gently hold the jaws apart with a radiolucent mouth gag.
      
   4. Angle the beam through the mouth to create the bisecting angle (approximately 30°). **Since this image was created extraorally, it will be opposite the arcade determined by the techniques presented later in this chapter.
   
   
4. Mesial roots of the maxillary fourth premolar [66]. The straight lateral 45° bisecting angle gives a good representation of the mesial roots but they will be overlapped. In this author’s opinion, this is sufficient for a general practitioner in the vast majority of cases. It is exceedingly rare to see a periapical lucency on only one root of this tooth and periodontal disease can be easily seen on the straight lateral. Therefore, in most cases extra images are not necessary. However, if the practitioner desires to view these roots separately, an additional angle is necessary. The PID is angled in both the vertical and horizontal planes. The horizontal shift can either be in the mesial or distal direction and is called the tube shift technique [63].

To perform this technique:

1. Position the PID in position for the straight lateral image:
   
   
   
   1. 45° in the vertical plane.
   
   
2. Rotate the PID approximately 30° in the horizontal plane. This can either be done distally or mesially.

Once the roots are split, it is imperative to know which root is which. The classic way of determining the mesiobuccal from the mesiopalatal root is to determine it via the SLOB (same lingual/opposite buccal) technique [30, 63]. This means that the root that is more lingual (or palatal) will be imaged in the same direction the tube is shifted and the buccal root will be imaged in the opposite direction. Therefore, with a distal tube shift, the palatal root will move distally in comparison to the buccal root. With a mesial tube shift, the palatal root will move mesial in relation to the buccal.

However, there is a much simpler way to determine which root is which. If the PID has been shifted mesially, the distal root of the fourth premolar will be imaged over the first molar. In this case, the buccal root is in the middle. When the PID is shifted distally, the distal root is well visualized, away from the first molar and the palatal root is in the middle. Since the whole tooth cannot be effectively evaluated with the mesial tube shift technique, this author recommends that only the distal tube shift technique be used, thus creating a quality image of the entire tooth. If the distal root is imaged well, the palatal root is in the middle.

3.3.4 Step 4: Setting the Exposure [29]

If your machine has manual settings, you will need to determine the correct exposure time. In general, cats require only two settings, one for the maxilla and one for the mandible. For dogs, there will be about five settings regardless of size of tooth or patient.

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