The role of xeroradiography in cephalometric radiology

Journal of Dentistry, 5, No. 1, 1977,pp. 32--38. Printedin GreatBritain

The role of xeroradiography in cephalometric radiology R. Davis, AIST Division o f Physics, Institute of Cancer Research and Royal Marsden Hospital, London W. H. Binnie, BDS, DDS, MScD, FDS, RCPS (Glas) R. A. Cawson, MD, BDS, FDS RCS (Eng) and RCPS (Glas), MRCPath Department o f Oral Medicine and Pathology, Guy's Hospital, London R. T. Reed, BDS, FDS, RCPS (Glas), DOrth RCS (Eng) Department of Orthodontics and Children's Dentistry, Guy's Hospital, London A. J. Stacey, MPhil, MlnstP Department of Medical Physics, Charing Cross Hospital (Fulham), London

ABSTRACT The advantages of xeroradiography in the precise identification of anatomical landmarks for orthodontic analysis are overwhelming. In addition, the soft tissues of the face are shown fully and in detail. The tongue and soft palate can also be visualized. However, there is undue anxiety among clinicians and radiologists about the risk of excessive zadiation exposure to patients. This has hitherto been regarded as unavoidable for the purposes of xerozadiography. This paper sets out in detail the advantages of xeroradiography for cephalometry and also clarifies the pzoblem of patient exposure by presenting detailed measurements of the parameters affecting exposure and of the incident skin exposure. In this way it can be shown that by choice of coizect kilovoltage and adequate filtration the incident skin exposure can actually be reduced when compared with conventional techniques using film. INTRODUCTION Cephalometric radiographs were introduced by Broadbent (1931) and have become accepted as a basic tool o f orthodontists and oral surgeons throughout the world. The recorded information aids diagnosis and treatment planning, provides a means o f assessing changes due to orthodontic treatment or surgery and has uses in the research which is so vital to these fields. A technique which increases the quality and quantity of information in these radiographs is therefore to be welcomed. Xerography was invented by Chester Carlson in 1938 and is the basis o f many commercial document-copying machines. The technique has been applied to radiology by the use of specially prepared plates sensitive to X-radiation, which may be used to replace conventional radiographic ~ m . The process has been discussed by McMaster (1951), Boag et al. (1971, 1972) and Rawls and Owen (1972) and detailed information can be found in these publications. The first dental application of xeroradiography was reported by Hills et al. (1955), who showed a lateral oblique jaw projection with resolution superior to that of conventional fdm. Unfortunately this involved high radiation exposures which were too great for routine dental use. It is now well established, however, that these exposures can be reduced to acceptable levels by employing high kilovoltage techniques (James et al., 1973; Boag et al., 1976).

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Binnie et al. (1975) pointed out that although the dental profession used radiography extensively, only throe papers were available at that time on dental xororadiography (Pogorzelska-Stronczak, 1963; Lapinskas and Lapinskene, 1968; Pawls and Owen, 1972), and that all of these authors agreed that the potential benefits of the technique demanded further research and development. Binnie et al. (1975) also demonstrated the superiority of lateral skull and lateral oblique jaw xororadiographs in comparison with conventional films and concluded that the technique offered very real advantages to the orthodontist and oral surgeon. These assets were also accompanied by a reduction in the radiation exposure to the patient's skin. The clinical advantages of xeroradiography in cephalometric analysis have boon illustrated by Tipnis (1975), Schriver et al. (1975) and Lopez (1976). Tipnis reported that a considerably heavier exposure was required for the particular xeroradiographic techniques which he used than for conventional films. This disagrees with the measured values described by Binnie et al. (1975), Boag et al. (1976) and in this paper. It seems appropriate to report the relevant experience gained by the present authors and to discuss some of the physical parameters which affect both the patient dose and information content in xeroradiography.

MATERIALS AND METHODS Patients were positioned in a true lateral or anteroposterior position at 60 in (152 cm) focus to 'film', i.e. detector, distance by means of a standard craniostat. The X-ray tube head was fitted with a GE 11FP6C light beam diaphragm and the tube (Victor 11086)was energized by a General Electric generator, type GE DXS1350. The relevant parameters for both types of radiography are shown in Table/. This also gives the measured skin exposure values (mR). The 35-ml ionization chamber and electrometer system (Electronic Instruments Limited, Type 37(3) employed for these measurements was calibrated at appropriate photon energies by intercomparison with a standard instru; ment, which had itself been calibrated at the National Physical Laboratories. The measured values of applied tube voltages (Table 1) were obtained by the method of Ardran and Crooks (1968). Table L Parameters employed in the radiological techniques

Technique

Film type

IntendAntilying scatter screens grid

Applied tube voltage (k Vp)

mA

Focus-detector distance (in)

Total filtre- Incident tion skin (ram exposure AI) (mR)

Conventional Dupont radiography* Cronex 4

Ilford Fast Tu ngstate

Ratio: 10/1

70 (nominal) 68.5 (measured)

30

60

1.8¢

88

Xeroradiographyt

None

None

120 (nominal) 105 (measured)

20

60

4.8

82

Automatic | 25 System

* Films developed in a Kodak Automatic processor with 3.5-minute cycle time. t Both modes (positive and negative) were employed. Plus compensating aluminium wedge.

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i Fig. I. Lateral skull xerorediogreph, positive mode.

Fig. 2. Lateral skull xeroradiograph, negative mode.

Fig, 3. Anteroposterior xeroradiogreph, positive mode.

RESULTS Fig. 1 shows a lateral skull xeroradiograph employing the positive mode of development whilst Fig. 2 illustrates the use of the negative mode. Fig. 3 is an example of an anteroposterior projection in the positive mode. No attempt has been made to include comparable conventional radiographs because the difficulties of accurate reproduction put ~ m s at a great disadvantage. Nevertheless, it may be informative to compare these reproductions of xeroradiographs with an actual film to assess the amount o f detail detectable with each method.

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PHYSICAL CONSIDERATIONS It has been mentioned that the physical principles of xeroradiography have been described and discussed extensively in the relevant literature. A brief summary of the process may be presented in the following five steps: 1. An electric charge is deposited on to the surface of a selenium plate; in the absence of radiation (e.g. X-rays or visible light) the charge remains evenly distributed. 2. X-radiation causes the charge to leak away through the plate, approximately in proportion to the quantity of radiation reaching each point of the plate's surface. Thus an incident X-ray pattern is converted to a distribution of electric charge. 3. Powder is sprayed towards the plate surface, forming a cloud of fine particles, some of which adhere to the charge pattern (influenced by the local electrostatic forces) to give a visible image. 4. In the 125 System, used in this work, the picture is obtained as a permanent copy by transfer and heat sealing into the plastic coating of a specially prepared paper. 5. Two types of image can be produced according to the charge imposed on the plate during development. A negative image can be produced which is essentially similar in character to a conventional radiograph. In the positive mode, dense structures appear dark in the image; these differences are shown in Figs. 1-3. The two different image modes have different applications, but for the purpose of indentification of f'me bony landmarks the positive mode is more satisfactory, as discussed later. Two outstanding features of xeroradiography are the 'edge effect' and the very wide exposure latitude of the system. The 'edge effect' is due to the accumulation of powder at discontinuities in the electric charge. Since these discontinuities result from differences in X-ray absorption between adjacent tissues, the ensuing sharpness in the radiograph gives it a remarkable resemblance to an anatomical line drawing. There is an almost complete absence. of the optical density gradients upon which conventional radiography is based. The wide latitude enables tissues of different radiological densities and thicknesses to be recorded in one image and also ensures an acceptable result notwithstanding a variety of exposure conditions; this greatly reduces the need for repeat exposures and is a very significant advantage. It also provides the means to balance optimal image quality and minimal patient dosage by the careful selection of applied tube voltage, added f~tration and variations in focus to detector distance for any particular radiological technique. Table I indicates that, for the xeroradiographs described in this paper, 120 kVp was selected with the maximal available f~tration of 4.8 mm A1. These parameters produced an incident skin exposure slightly less than that involved in the conventional technique. In fact, this nominal kilovoltage was incorrect and measurement showed a lower value. Current work in progress shows that for some anatomical sites, kilovoltages of 120 and above, employed with additional copper filtration, can reduce the radiation exposure to the patient still further without significant loss of image quality (Davis and Stacey, 1977). Indeed, in some instances image quality has been improved. It is apparent, therefore, that first, caution is needed in the selection of exposure parameters to ensure patient dosage is at the lowest practicable levels. Secondly, the optimal conditions for each type of examination should be determined with full understanding~of the physics of the procedure but having regard to the desired clinical result. With so many variables involved in determining the dose delivered to the patient it is sur-

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prising that most current literature (eventltat discussing radiation exposure) provides insufficient information to guide the reader to an adequate choice of exposure parameters.

ORTHODONTIC CONSIDERATIONS Lateral skull cephalometdc radiography is widely used in orthodontics for the pre-treatment assessment of the dental and skeletal abnormality. It is also used to monitor changes taking place during growth and as a result of treatment. For satisfactory clinical use such radiographs must not only be of good individual quality but also reproducible in a standard fashion before, during and after treatment in such a way that the radiographs or tracings taken from them may be superimposed wRh precision. To achieve this end, many anatomical points have been described by various authors. A variety of these landmarks is still widely used, but there is as yet only limited consensus as to the anatomical points which should be chosen to define particular structures. One reason for the multiplicity of landmarks is the difficulty of demonstrating many of these with equal reliability in successive fdms. A bone such as the mandible with its dense cortical plate presents little difficulty since its margins are always dearly visible on a standard radiograph. However, there are considerable problems visualizing the less dense structures, particularly in the mid-facial region, and also the deeply placed midline structures which are obscured by overlying bones. Soft tissues and the very delicate maxillary alveolar and nasal bones are particularly difficult to demonstrate even with the use of aluminium wedges to f'dter the X-ray beam in these areas. The anteroposterior view is less commonly used than the true lateral view. It is of value in the localization of unerupted teeth, particularly upper cuspids, when used in combination with the lateral f'~n. This view is also useful in the investigation of skeletal discrepancies in the coronal plane, such as clefts of the palate and mandibular asymmetry, and to measure changes brought about by arch expansion following surgical or orthodontic treatment. Both positive and negative modes produce better imaging than fdm. Generally speaking, the positive mode is better for bony detail and the negative for soft tissue. The positive mode is therefore the method of choice in cephalometrics. Comparison of the conventional radiograph with the xeroradiograph clearly demonstrates that the latter shows striking improvement, particularly in the simultaneous delineation of detail of the fine structure of both hard and soft tissues. At the same time, the detail of denser structures which are quite readily seen on conventional radiographs is in no way diminished. Particular reference is therefore made to the foilowing points: 1. Soft tissue detail (which is not normally seen on conventional f'dm) is shown more completely and with great clarity. The detail is superior because of the 'edge effect' and the extent is increased because it is not limited to the area covered by an aluminium wedge. The whole facial profde is shown and even such details as the ala of the nose and vermilion border of the lip are clearly demarcated. The lip/incisor relationship is particularly clearly demonstrated. 2. The outline of the nasal bone and point N are clearly defined. 3. The thin labial plate of alveolar bone of the upper incisor teeth and the anterior limit of the anterior nasal spine are exactly demarcated. By contrast, this region is poorly defamed on conventional film. 4. Point A is a widely used anatomical point whose function is to demarcate the anterior border of the maxillary bone. This point can be precisely defined by xeroradiography, but

Daviset al.: Xeroradiographyin cephalometricdentistry

37

on conventional film it is so difficult to locate that wide degrees of error and misinterpretation can result. 5. The precise outlines of the maxillary bones and o f the floor of the anterior cranial fossa are often used to superimpose succeeding films of the same patient on a 'best fit' principle (rather than using a defmed plane such as SN) to follow progress. It is obvious that this can be done more accurately on a xerographic cephalogram. 6. Though not strictly relevant to cephalometrics, lateral skull xeroradiographs also show the size and posture of the tongue, and its relation to the soft palate. This is an incidental bonus in that these tissues cannot easily be visualized on conventional film but may be o f importance in studying the effects of the soft tissues on the occlusion.

Summary of advantages and disadvantages of xeroradiography in cephalometric radiology Advantages 1. Resulting from edge effect: a. Enhanced sharpness of detail. b. Sharp delineation o f anatomical features beneath overlying structures. 2. Resulting from latitude of exposure: c. Allows soft tissue and bone detail to be better visualized on a single radiograph, dispensing with the need for a wedge filter. d. Satisfactory radiographs over a wide range of exposure parameters reduces retakes. e. Precise reproducibility of reference points. 3. Resulting from the use of high kilovoltage technique and increased tube filtration: f Reduction in radiation exposure to the patient. g. Shorter exposure time, reducing risk of movement. 4. Resulting from print form of radiograph: h. No viewing screen needed. i. Facilitates ease of viewing and demonstrating. j. Suitable for tracing or drawing upon. 5. No silver used. 6. The plate is re-usable almost indefinitely.

Disadvantages 1. The Xerox 125 System: a. Available in relatively few dental departments. b. High cost but partly offset by the re-usable plate and low price of other materials. c. Limited to one plate size.

REFERENCES Azdran G. M. and Crooks H. E. (1968) Checl~ng diagnostic X-ray beam quality. Br. J. RadioL 41, 193-198. Binme W. H., Stacey A. ]., Davis R: et al. (1975) AppLications of xeroradiography in dentistry. J. Dent. 3, 99-104'. Boag J. W., Stacey A. J. and Davis R. (1971) Some clinical and experimental applications of xeroradiography. J. Photograph. $cL 19, 45--48.

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Boas J. W., Stacey A. I. and Davis R. (1972)Xerographic recording of mammograms. Br. J. RadioL 45, 633-640. Boag J. W., Stacey A. J. and Davis R. (1976) Radiation exposure to the patient in xeroradiography. Br. d. RadioL 4 9 , 2 5 3 - 2 6 1 . Broadbent B. H. (1931) A new X-ray technique and its application to orthodontia. Angle Orthod. 1, 45-46. Davis R. and Stacey A. J. (1977) High kilovoltage techniques and the choice of optimum filtration in xeroradiography. Br. J. RadioL In the press. Hills T. H., Stanford R. W. and Moore R. D. (1955) Xeroradiography II. The present medical applications. Br. d. Radiol. 28, 545-551. James P., Badderley H., Boas J. W. et al. (1973) Xeroradiography--its use in peripheral contrast medium angiography. Clin. Radiol. 24, 67-71. Lapinskas V. A. and Lapinskene A. V. (1968) Xeroradiography and the prospects of its use in stomatology. Stomatologia (Mosk.) 47, 35-38. Lopez J.jun (1976) Xeroradiography in dentistry. J. Am. Dent. Assoc. 92, 106-110. McMaster R. C. (1951) New developments in xeroradiography. Non-destructive Testing 10, 8-25. Pogorzeiska-Stronczak B. (1963) Xeroradiography in stomatology. Pol. Rev. RadioL Nucl. Med. 27, 266-276. Rawls H. R. and Owen W. D. (1972) The dental prognosis for xeroradiography. Oral Surg. 33, 476--480. Schriver W. R., Swintak E. F. and Darlak J. D. (1975) Xerocephalography. Oral Surg. 40, 705-708. Tipnis A. K. (1975) Xeroradiography for lateral skull radiographs. Br. J. Orthod. 1, 1 8 7 189.