Radiographic Technique and Interpretation in Periodontics
by Dinh X. Bui, D.D.S., M.S.

Radiographic Techniques and Interpretation in Periodontics
Successful therapy required accurate diagnosis, which in turn required complete medical/dental history and record. The major data pertinent to diagnostic information can be subdivided into two parts: the clinical record and the radiographic record. Radiographic record allows the clinician to diagnose hard tissue problem related to the tooth itself and the hard tissue component of the periodontium. One of the challenge of the radiographic technique is the ability of the clinician to obtain three dimensional information in a two dimensional representation. Skillful clinician will be able to pick up limited soft tissue information as well. Various issues of radiographic technique must be mastered to achieve the most informations on the film. They are standardization of the radiograph in term of density and position of the object of interest. Standardization of the density reflected in the grayness of the film allowed the clinician to detect minute change as reflected by the degree of radiolucency and/or radioopacity. Standardizationof the position of the object of interest allowed the consistent reference point to be established. Establishing the reference point on the radiograph is necessary to allow consistent viewing angle of the object and for purpose of comparison. Accurate and masterful interpretation of radiograph depends greatly on the amount of information captured in the radiographs. Thus there is a direct interrelationship between radiographic techiques and radiographic interpretation.

The activity or progression of periodontal disease can not be diagnosed by cross sectional study. Similarly, the outcome of periodontal therapy cannot be confirmed without the successive radiographic evaluation over a period of time. Dental radiographs are the traditional method used to assess the destruction of alveolar bone associated with periodontitis. They provided accurate inforation about the bony characteristic in term of density, pathology, and allow the clinician to assess bone destruction/or synthesis. In the past, information in term depth or z axis of the object (information of the object parallel to the beam of the x-ray) cannot be accurately reflected on the radiograph, i.e. the buccolingual information when the beam positioning buccolingually. However, today various radiographic technique allowed the clinician to capture the cross sectional data and also the data is also reproducible. The use of CT scan and tomogram allowed vast radiographic information to be organized, rearanged, and output on the monitor or a film prior to viewing. Convensional tomography techniques contain the common components of an x ray tube, film, and a rigid connecting bar, which rotates around a fixed fulcrum. The type of the tube motion dictates the technique as linear tomography or pluridirectional tomography. Conventional radiograph provide useful information on the interproximal bone levels. Regardless of the type of radiograph, standardization of the radiograph in term of density and position of the beam or object must be carried out especially in research or in the case where critical and/or sensitive comparison must be established. Various radiographic variables affecting radiographic technique are the mechanical control variables: the x-ray film, x-ray source, the exposure and developing time, the amount of radiation used; and the technique control variables such as the x-ray angulation, the x-ray beam direction, and the positioning of the focal point. The mechanical control variables can be easily standardized and duplicated. However, the control variable depends on the knowlege and the skill of each clinician or the person performing the task.

Firstly, positioning of the film must be standardized. There are two techniques, the parallel technique and the bisecting angle technique. In the parallel technique, the radiograph should be taken with the beam perpendicular to the object and the plane of the image to be measured (facial plane of the teeth). This is to minimize distortion and allow for uniform magnification. A study of the compartive efficiency of a bisection and the parallel techniques found that the number of undiagnostic radiographs was reduced by more than half with the paralleling technique. This study uses the Rinn XCP instrument. The bisecting angle involved placement of the film at an angle with the plane of the object and the x-ray beam placed perpendular to the imaginary line bisecting the angle. This technique will require more patient exposure and allowed for nonuniform distortion. However, this technique is useful when the patient has shallow palatal vault which precluded placement of the X ray parallel and behind the facial plane of the teeth. The bisecting angle technique is used mostly on the maxillary arch, whereas whenever possible the paralleling technique should be used.

Next, duplication of the position of the film must be established to allow continuous monitoring of an area of interest over a period of time using a common reference point. Radiographs can be obtained in a constant and reproducible plane using film holders with a template containing some kind of impression material, which as placed in a constant position on a group of teeth or single teeth, and an extension arm that can be precisely attached to both the film holder and the x-ray tube and used to standardize the technique so that the bone mass can be measure. This allowed for standardization of the x-ray beam angulation, x-ray beam direction, and the positioning of the x-ray object. Rosling et al. introduced the radiographic method to obtain reproducible radiographs of several group of teeth in the same jaw using the acrylic splint with film hoder. The tract for the films are paired for 5 different positions on the film. This technique fulfil the high demans of precision in assessing the morphological changes in the alveolar bone crest.

Next, the density radiograph is also must be standardized. Henrikson in 1967 develop a nonradiographic method to study bone mass with high degree accuracy and precision. His method was termed I 125 absorptiometry. It is based on the absorption by bone of a low energy gamma beam, originating from a radioactive source of iodine isotope I 125. This method has been used as the standard for comparing the sensitivity of this system. Another method of study the bone density is the photodensitometric analysis technique. It is based on absorption of the image of an aluminum scale, and transformation of the density reading into milliliters of aluminum equivalent. This transformation is accomplish by the microdensitometer linked to a microcomputer. This technique requires a paralellilzation technique to obtain accurate superimposable radiographs. It enables the clinician not only to detect and recognize variations that cannot be detected by visual inspection but also to quantify the bone changes. Another method which involved the uses of computer is the subtraction radiograph. This is a well established technique in medicine. A serial of radiographs isconverted into digital image, using the computer. Standardization of the radiolucent/radioopacity can be done via computer equilibration or using the step wedge. The images can then be superimposed and the resultant composite viewed on a video screen. Changes in the density and/or volume of bone can be detected as lighter area (bone gain) or dark areas (bone loss). Pixelization can be done on the computer to obtain 3D visualization of bone density. Quantitative changes in comparison with the baseline images can be detected using an algorithm for gray scale levels. Again, this technique requires parallization and also standardization of the object positioning and radioopacity/radiolucency. Studies using this technique have shown high degree of correlation between changes in alveolar bone determined by subtraction radiography and attachment level changes in periodontal patient after therapy and increased detectability of small osseous lesions compared with the conventional radiographs from which the image are produced. Grohdahl and colleagues, using subtraction radiographic analysis, showed nearly perfect radiography at a lesion depth corresponding to .49 mm of compact bone, where as the lesion must be at least 3 times larger to be detectable with a conventional radiology technique. This technique has shown a degree of sensitivity to the I 125 absorptionmetry. It can detect a change in bone as little as 5%. Bragger and Pasquali in 1989 evaluated the influence of the image processing of digital subtraction images on inter and intra-examiner aggreement relative to the detection of alveolar bone change on the radiograph. The images were displayed as digital subtraction images, contrast enhanced subtraction images and as color converted digital subtraction images. The result indicated that image processing of subtraction images using the pseudocolor display of density changes might improve the intra and inter-examiner agreement. The color conversion of certain grey level ranges within digital subtraction images added some quantitative information to the images, i.e. the color of the area on the image represent the degree of bone density. This will facilitate recognition and visualization of the radiographic data. Finally, Computer Assisted densitometric image analysis can be utilized. In this system, a video camera measure the light transmitted through a radiograph, and the signal from the camera is converted to the grayscale level. The camera is interfaced with the image processor and a computer which allow the storage and mathematical manipulation of the images. This system has shown higher sensitivity and a high degree of reproducibility an accuracy. Finally, the CT scan and MRI technology can also be useful in evaluating bone changes. However, these methods are costly and thus only available in special occasion.

Finally, establishing a reference for the purpose of comparison must be established. In general, all methods required a fixed reference point from which to obtain a reproducible measurements. The CEJ, the edge of a stent, root notch, or restoration have all been utilized. Small metal ball (bb’s) has been incorporated into the stint for the purpose of magnification measurement also was used. The CEJ offers the advantage of an already available, natural landmark. However, it does not provide an accurate detection in the buccolingual direction. Another disavantage that it may be obscured in the radiograph due to calculus formation in the successive radiograph. Also the movement of teeth as in many patient with periodontal disease may change the position of the CEJ. The stent is another method. The stent, if constructed over many teeth, can eliminate the problem with movement of teeth. Root notch has been used, especially when histological studies will ensue. Finally, restoration can be placed. However, this method is invasive and cannot be used in patient when the chosen reference point does not require a restoration.

Concerning radiographic diagnosis, two important issues must be discuss are the methods to assess bone destruction and the methods to assess disease progression. Caranza and Newman reported that a substantial volumes of alveolar bone must be destroyed before the loss is detectable in radiographs; more than 30% of the bone mass at the alveolar crest must be lost for a change in bone height to be recognized on the radiograph. The radiograph will not revealed bone loss until more than 30% of bone density has lost. Thus , radiograph are not sensitive, but they may be specific. For the purpose of comparison to obtain the rate of bone destruction, a set of successive radiograph must be taken in the same position, setting, and same degree of radioopacity/radiolucency distribution. Replicate measurements must be performed on the standardize radiograph. Analysis of the differences in the radiographic images can thus provide the measure of the rate of destruction. In the past, linear measurement has been done in numerous studies. Linear measurement only provided data in the two dimensional analysis,i.e., the bone height most of the time. Today, with the help of computer aided subtraction radiography and the use of computer to provide nonlinear measurement, clinician can perform the non-linear analysis for much more accurate comparison of the radiograph. Subtraction radiography has been applied to longitudinal clinical studies. Hausmann et al. detect differences in crestal bone height of .87mm with reliability. Jeffcoat et al. have shown a strong relationship between probing attachment loss detected using sequential measurements made with an automated periodontal probe and the bone loss detected with subtraction radiography. Grondahl et al. in their evaluation of influence of variation in projection geometry on the bone lesion reveals that subtraction technique may offer increased possibilities for the detection of changes in the marginal alveolar bone even under nonideal imaging condition (3 degree angulation of x-ray beam realtive to reference projection). The detectability of small periodontal defects was significantly better when a subtraction technique was used than when conventional radiographic technique was used under optimal conditions. Computer assist densitometric image analysis system (CADIA) also has been used in longitudinal sudy. Deas et al. demonstrated that the prevalence of progressing lesions in periodontitis (38% of the sites per patient), as detected by CADIA method, may be much higher than previously thought. Bragger and Lee Pasquali and Kornman also use CADIA to study the remodelling of interdental alveolar bone after periodontal flap procedure and found that CADIA can assessed the differences in tissue changes in the healing phase following periodontal surgical procedures, which were not detected by the clinical variables applied (Plaque index, gingival index, probing depth, and attachment loss). Finally, Nuclear medicine technique which involved the use of nuclear medicine to detect change in bone metabolism that may precede the architectural changes. Nuclear medicine technique involved using the bone seeking pharmaceutical, such as diphosphonate compound, which is labeled with a radionuclide, Tc99m. This compound injected intravenously, and after a waiting period to allow bony uptake and clearance of the substance, uptake by the bone is meausred with a miniaturized semiconductor probe radiation detector. This semiconductor probe placed directly buccal to the tooth. This technique is used to detect alterations in bone metabolism in disease of bone resorption as well as disease of bone formation. In periodontal diseases, measurement of the bone seeking radiopharmaceutical uptake may be indicative of the rate of bone loss. This technique has exhibited a specificity of 87%, with a total predictive value of 84%.

Samuel Lynch in 1992 reported in the review of methods most widely accepted for evaluation of regenerative procedures. These methods must provide the mean of evaluate attachment level and also evaluation of the changes in alveolar bone height.

To fully interpret and extracting information from the radiograph, the clinician must have a firm understanding of histology and morphology characteristic of the object he or she is looking at. Familiarization of the “normal” representation and recognization of the “abnormal” representation must be mastered by the skillful clinician. Understanding the disease process and the result expected at the time frame, and distinguishing the wound healing and repair will allow him or her to extract much more pertinent information concerning the patient. One thing we must keep in mind that radiography is a tool, and more than one tool must be used prior to make a final and definite diagnosis.

Question still must be answer prior to using X-ray as the primary diagnostic tool in predicting disease progression. As mention above, bone loss must be sufficient prior to the information be presented on the radiograph. Attachment loss, gingival inflammation, probing attachment, violation of the junctional epithelium will not predictably show up on the radiograph. Goodson et al. in 1984 show that loss of probing attachment preceded bone loss by 6 to 8 months. Therefore, conventional radiograph, even the well standardized ones, have a low predictive values for detecting the disease progression. Radiography is the 2 dimensional representation of the 3 dimensional object. At least two films must be taken to achieve a three dimensional analysis of an object. Furthermore, at least two films must be taken with a period in between to diagnose disease progression. Finally, there is low predictive value of radiograph in term of bone metabolism since we cannot predict whether the radiolucency as an active lesion or a healing lesion, or a completely heal defect. Incorporation of successive radiographic images and meticulous medical records thus is recommended to establish a successful diagnosis and therapeutic verification.

____________________________________

References

1. Bean, LR. Comparison of bisecting angle and the paralleling methods of intraoral radiology. J. Of Dent. Educ, 33: 441, 1969.

2. Goaz, P. , White, S. Oral Radiology. Principles and Preventions. C.V. Mosby Company, 1987.

3. Carranza, Newman. Textbook of clinical periodontology, W.B. Saunders, 1996.

4. Goodson, JM, Haffajee, AD., Socransky, SS. The relationship between the attachment level loss and the alveolar bone loss. Journal of Clinical Periodontology, 11: 348, 1984.

5. Payot P., Bickel M., Cimasoni, G. Longitudinal quantitative radiodensitometric study of treated and untreated lower molar furcation involvement. J. of Clinical Periodontology, 14: 8, 1987.

6. Grondahl, HG., Grondahl, K. Subtraction radiography for the diagnosis of periodontal bone lesions. Oral Surg, 55: 208, 1983.

7. Lynch, S. Methods for evaluation of regenerative procedures. Journal of Periodontology, 63: 1085, 1992.

8. Bragger, U., Pasquali, L. The color conversion of alveolar bone density changes in digital subtraction images. Journal of clinical periodontology, 16: 209, 1989.

9. Bragger, U., Pasquali L, Kornman, K. Remodelling of interdental alveolar bone after periodontal flap procedures assessed by means of computer assisted densitometric image analysis (CADIA). Journal of Clinical Periodontology, 15: 558, 1988.

10. Grohdahl, K., Grohndahl, H., Webber, RL. Influence of variations in projection geometry on the detectability of periodontal bone lesions. A comparison between subtraction radiography and conventional radiographic technique. Journal of Clinical Periodontology, 11: 411, 1984.

11. Rosling, B., Hollender, L., Nyman, S., Olson, G. A radiographic method for assessing the changes in alveolar bone height following periodontal therapy. Journal of Clinical Periodontology, 2: 21, 1975.

12. Miles, D., Vandis, M. Advances in dental imaging. The dental clinics of north america. Volume 37, number 4, October 1993.