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History, Physiology, Modality Options, and Safety for Diagnostic Imaging of the Oral Cavity

Brenda L. Mulherin

Lloyd Veterinary Medical Center, Iowa State University College of Veterinary Medicine, Ames, IA, USA

CONTENTS

1. History of Diagnostic Imaging
2. Philosophy of Diagnostic Imaging
3. Radiographic Indications
4. Intraoral Dental Radiographic Equipment
5. Radiographic Imaging
6. Radiographic Densities
7. Digital Image Creation
8. Advanced Imaging Modalities
9. Definitions Relating to Imaging Modalities
10. Computed Tomography Window Width and Window Level
11. Other Common Viewing Windows
12. Radiation Safety
13. Radiation Safety Equipment Inspection
14. References

History of Diagnostic Imaging

Discovery of X-rays

8 November 1895 was the extraordinary discovery of Roentgen rays, otherwise known as X-rays [1, 2]. X-rays were discovered by a German physicist named Wilhelm Conrad Roentgen (Figure 1.1). Roentgen published a paper regarding this unique discovery entitled "On a new kind of rays" in Sitzungsberichte der Wurzburger Physik. -Medic. -Gesellschaft. on 28 December 1895 [1, 2]. This date is now considered the true discovery of X-rays [2]. In Roentgen's research, his wife Bertha Roentgen assisted in acquiring the first radiographic image of the human body [3]. She placed her hand on the photographic plate, and the X-ray beam was applied to her hand. This experiment yielded the first X-ray image of the bones and soft tissue of Bertha's hand and her wedding ring [3] (Figure 1.2). Interestingly, during the days of research in his laboratory, Roentgen did not know what kind of radiation he was experimenting with, so he referred to the waves as X-rays, which is how they are still known today [2]. Early in the discovery of radiology, most of the radiographic images produced were taken and created by photographers, or medical experts who had interest in photography [4].

Developing of Safety Measures

Following the discovery of X-rays, Nikola Tesla tried to develop a protection shield from what he perceived the harm that could come from X-ray exposure [2]. Tesla suggested that by placing an aluminum plate between the object of interest and the X-rays, there would be a reduction in the amount of X-ray energy received, hence the concept of the inverse square law [2].

Shortening of Exposure Time

Historically, X-ray energy was continuously emitted to the object of interest anywhere from 15 to 60?minutes at a time. In 1896, Professor Mihajo Idvorski Pupin from Columbia University, also known as Michael Pupin, tried to find a way to shorten the exposure time required to acquire an image [2, 4]. He placed a photographic plate behind a fluorescent screen and then applied the X-ray energy to the object of interest to create an image on the film [2, 4]. This was found to reduce the exposure time to only a few seconds [2, 4].

Figure 1.1 William Conrad Roentgen, the German physicist who first discovered X-rays.

Source: Courtesy of John Wiley & Sons.

Figure 1.2 The first X-ray image of the bones and soft tissue of the hand of Bertha Roentgen.

Source: Courtesy of John Wiley & Sons.

Glass Plates to Film

In 1896, Carl Schleusner manufactured the first glass plates to be used for radiographic image creation [4]. At the time, radiographs that were taken on glass plates were thought to be superior to those taken on film [4]. In 1914, during World War I, glass plates needed to be replaced with film as the glass needed to create the radiographic plates was manufactured in Belgium [4]. Due to the war, the supply of glass decreased, while the demand for radiographic images increased. In 1918, the first radiographic film was produced with high-speed emulsion on both sides, reducing radiation exposure and exposure times [4]. By the 1940s, non-screen radiograph film was introduced, and automatic film processors were becoming available [4].

Progression to Digital

Since the 1980s, digital radiography (DR) has slowly been replacing film in many hospitals and practices [5]. DR was quickly accepted into the veterinary profession as the speed of digital acquisition and ability to read images at computer terminals throughout the hospital allow for efficient interpretation and maximization of patient care.

The main difference between using conventional radiographic film and a digital system is in the viewing of the images. Digital radiographic images are electronically captured and viewed at a computer terminal, whereas conventional radiographic film is viewed with an illuminated view box. Transitioning to digital has significant benefits for diagnostic evaluation compared to conventional radiography. Digital systems allow a radiologist, specialist, or practitioner to evaluate images remotely. It allows for simpler storage, organization, and an easier way to compare images. Many advanced imaging methods including computed tomography (CT), magnetic resonance imaging (MRI), ultrasound, and cone beam computed tomography (CBCT) allow imaging of the body in such detail that 3D reconstructions of organs can be made. This helps to prepare the surgeon in advance for a procedure. Over the years, the application of radiation for visualization within a patient's body without surgical exploration has changed the field of medicine. These different imaging modalities have revolutionized the diagnostic field of medicine [2].

Philosophy of Diagnostic Imaging

Choosing the Appropriate Modality

It is the responsibility of the clinician to choose the imaging modality appropriate to gather the most information possible bearing in mind modality availability, ability for swift interpretation, patient stability, and any financial considerations an owner may have. Ideally, a clinician will choose the imaging modality that is the most cost-effective and readily available yet yielding the necessary information to make a diagnosis. Radiographic imaging is of no use if the images created cannot be accurately interpreted by the clinician or at the very least, quickly distributed to an outside source that can interpret the images in an expedited manner.

Ability to Interpret Findings

The ability to interpret diagnostic images is based on the ability to interpret shadows. A comprehensive grasp of anatomy and the interaction of radiation with different structure densities is imperative to be able to evaluate areas of the body for disease conditions. Unfortunately, not everything is black and white when attempting to interpret radiographic images. Variations in patient confirmation and ranges of normal within the same species can lead the interpreter to difficulties of identifying normal from abnormal even within the same patient.

Diagnostic imaging can allow the clinician to survey an organ system, assess trauma, explore an area for a suspected neoplastic process, or allow for patient follow-up in monitoring disease progression or therapeutic effectiveness. There are many different imaging modalities that can be used to create a diagnostic picture of a specific area of interest. Each modality has its own advantages and disadvantages to their use. It is up to the clinician to decide which imaging modality is best suited to provide the diagnostic picture of what question they would like to answer. Regardless of the modality used to acquire diagnostic images, any findings should be interpreted based on a thorough examination of the patient and how those findings relate to an anesthetized oral examination and the patient's presenting complaint.

Radiographic Indications

Documentation of Disease

Dental radiographic imaging can document the amount of disease that is present within the oral cavity. It can be used to estimate the amount of bone loss that has occurred, evaluate for evidence of endodontic disease and any embedded teeth, or retained tooth roots that may be present, among other things [6].

Value of Full-Mouth Radiography

Taking full-mouth radiographs of canine and feline patients is included in the American Animal Hospital Association Guidelines regarding the dental care for dogs and cats [7]. Full-mouth radiography is defined as a series of images taken of the teeth and bone of the jaw, both dentulous and edentulous portions [6]. The main reason full-mouth radiographs are taken is to establish a baseline to monitor for disease progression as well as determine if there is any existing disease or abnormalities present within the mouth prior to treatment.

American Animal Hospital Association Guidelines Regarding Dental...