INTRODUCTION

Digital imaging and image processing have a key position in the future of dental radiography. The main advantages of the digital systems are the immediate availability of the image, the elimination of the chemical darkroom process, and a reduced radiation dose. Image contrast, brightness, magnification, and other features of the image can be adjusted right to the user's needs by digital image processing. The measurement of distances and angles is facilitated.

Basic investigations into the diagnostic value of digital cephalometric radiographs have been performed by several authors in the last decade. In all of these papers, the digital cephalograms were obtained either by using storage phosphor plates from general radiology or by digitizing conventional radiographs.

In 1992, Calderazzi et al1 used storage phosphor plates to demonstrate that conventional and digital cephalometry did not differ relative to bone structure representation. The digital imaging technique provided better visualization of soft tissue structures. In 1998, Geelen et al2 found no clinically relevant differences in locating landmarks on conventional and digital radiographs. Three research groups showed independently that a dose reduction between 50% and 75% is possible with storage phosphor plates.3–5 Despite these encouraging results, storage phosphor plates were not available in dentistry for a long time because of high costs and protection by patent. In 1998, the “DenOptix combo,” a scanner for storage phosphor plates in the format of cephalograms, was introduced (Gendex/Dentsply International Inc., Haywood, Calif.).

In 1996, Forsyth et al6,7 mounted conventional radiographs on a light box to capture the digital images by a video camera connected to a computer. The resulting images had a resolution of 512 × 512 pixels, which was quite low compared with today's standards. They found that the random error associated with angular or linear measurements and landmark identification tended to be greater with the digital images than with conventional radiographs. They concluded that, for digital imaging of cephalometric radiographs, a pixel matrix larger than 512 × 512 is required. This requirement is easily met by scanning conventional radiographs with modern image scanners, but the main advantages of direct-digital radiography, ie, time saving and dose reduction, are lost.

In 1996, a device for direct-digital cephalometric radiography was introduced by Sirona (formerly Siemens Dental, Bensheim, Germany). This unit is equipped with a charge-coupled device (CCD) sensor chip for image acquisition and can be used for rotational panoramic radiography and lateral cephalography as well. In the cephalographic mode, the unit works with a narrow beam linear scanning process called a “slot technique.” The patient's head is scanned in lines with a flat, fan-shaped x-ray beam. During the scanning process, which takes about 15.8 seconds, the patient must stay motionless. Figure 1 sketches the principles of both conventional cephalography and this kind of slot-technique digital cephalography.

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Although the total exposure time for slot-technique direct-digital cephalography is much longer than for conventional radiography, a plain dose reduction is expected because of the greater sensitivity and the lower overall system noise of the CCDs. At any given time during the scanning process, only a small linear slice of the skull is exposed to the x-rays. Under the presumption that all other exposure parameters are identical, a dose reduction by a factor of 2 can be expected for this type of digital radiographic device.

The theoretical estimation of dose reduction is comprehensive, but an experimental verification remains necessary. As part of an extensive survey of patient exposure by dental radiography, we have performed dose measurements on conventional and direct-digital cephalography.

MATERIALS AND METHODS

RESULTS

The absorbed doses at the sites under investigation are summarized in Table 1. With both techniques, the highest absorbed doses were recorded at the skin of the left parotid region, at the left parotid gland, and at the skin below the left eye. These measuring sites had the shortest distance to the focal spot of the x-ray tube.

TABLE 1.  Absorbed Doses From Lateral Cephalography in μGy

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At the critical organs of the head and neck, the absorbed dose from conventional radiography was approximately 2-fold higher than for the digital radiographic unit. On the side of the head directed toward the tube, we measured 81 vs 34 μGy at the lens of the eye, 103 vs 45 μGy at the parotid gland, 53 vs 34 μGy at the submandibular gland, and 3 vs 2 μGy at the thyroid gland. Figure 2 shows a direct comparison of the doses to the critical organs in terms of median boxplots with arithmetic means. The effective doses were 2.3 μSv for conventional and 1.1 μSv for direct-digital cephalometric radiography.

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We recorded distinctly higher exposures for the digital device only at the skin of the nasion, philtrum, and labiomental sulcus. These measurement sites are located right in the median line of the face. Small deviations in positioning the phantom can result either in direct irradiation of the TLDs or in shielding effects by the phantom. It is likely that minor positioning differences are the cause of the deviation in the measurement values in the median line of the face. When both sides of the head are compared, the attenuation of the radiation by the tissue or the tissue-equivalent phantom materials are evident. The absorbed doses were about 9 times lower in the side of the head toward the film than in the side toward the x-ray tube.

DISCUSSION

Our results with conventional cephalography are in agreement with the dosimetric data from the recent literature, which are shown in Table 2. The fact that our data are in the lowest range described so far can be explained with the use of a sensitive film-screen system of a relative speed of 400. The relations between the different measuring sites are concurrent with the data published so far.

TABLE 2.  Dosimetric Data on Conventional Cephalography From the Literature and Results of This Study in Chronological Order

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The measurements of the absorbed dose and the calculation of the effective doses demonstrate that direct-digital cephalography yields an average dose reduction by a factor of 2. This corresponds closely with the theoretical estimate of dose reduction: from the total height of the scanned area (210 mm) and the height of the fan-shaped x-ray beam in the plane of the sensor (3.6 mm), the ratio of the total to the effective exposure time can be calculated: 210/3.6 = 58.3. Therefore, a total time of 15.8 seconds for digital image acquisition is equivalent to an effective exposure time (15.8/58.3 seconds) of 0.27 seconds for each point of the head. Conventional cephalography with a sensitive film-screen system requires a typical exposure time of 0.5 seconds for an adult person. Under the presumption that all other exposure parameters are constant, the absorbed dose is proportional to the effective exposure time. Thus, the theoretical estimation yields a dose reduction by a factor of 2. In clinical practice, the relatively long exposure time of 15.8 seconds for the direct-digital cephalographs causes no problems; blurred exposures are rare.

The inevitable exposure to ionizing radiation from natural and man-made sources is about 4 mSv/y.8 Compared with this, the effective dose by lateral cephalometric radiographs in the range of 1–2 μSv is small. Nevertheless, the radiological ALARA principle (as low as reasonably achievable) should always be kept in mind. The sensitivity to radiation detriment is age dependent. Children and adolescents have a much higher risk than adults. This is illustrated in a dramatic manner by the follow-up data from 2 incidents in the early years of therapeutic x-ray use. In the years between 1949 and 1960, about 10,000 children in Israel were exposed to intensive x-rays for treatment of tinea capitis, a disease of the scalp. In Rochester, NY, about 2600 children were irradiated for treatment of thymic enlargement. Both groups had a significantly higher rate of malignant as well as benign head and neck tumors compared with control groups.23,24 Many other studies, such as the follow-up of Hiroshima, the case-control studies on thyroid or parotid cancer, or the Beaver Dam eye study indicate that radiation protection is clearly an important issue.^8,11,25–27 Even with a high level of radiation protection, further technical achievements for dose reduction in cephalography, such as storage phosphor plates or direct-digital radiography, are appropriate.

SUMMARY

The use of direct-digital cephalometry can reduce patient exposure by a factor of 2 when compared with conventional technique with a film-screen combination of a relative speed of 400. From a radiation protection point of view, direct-digital cephalography is superior to conventional radiography.