Materials and methods.
Original radiographic material.
Eleven patients (21-65 years of age) who exhibited clinical and radiological signs of periapical pathology were selected. The selected teeth had periapical lesions not larger than 6 mm in diameter as seen on the periapical radiographs following root canal treatment. Standardized periapical radiographs were obtained immediately postoperative elyandat 3,6,9 and12 months; at the same times a complete clinical endodontic examination and clinical periapical diagnosis were performed. At each stage standardized periapical radiographs were obtained using individualized acrylic bite blocks and a modification of the Rinn system. Kodak Ekta speed film (Eastman Kodak Company, Rochester, NY, USA) was used. Radiographic exposures were made using a dental X-ray unit (X-Mind de Gotgen, Milan, Italy) operating at 70 kVp, 8 mA and 0.40 s exposure time. The focus to object distance was 45 cm. All exposure conditions, film processing and evaluation procedures were identical. The 12-month radiographs were used to determine whether healing was present or not and these radiographs were considered as the gold standard. None of the patients had uncontrolled systemic disease, patients who needed antibiotic cover for endodontic therapy and pregnant patients were excluded. Acquisition of the digital subtraction images.
To full one of the basic requirements for subtraction radiography, standardized pairs of images of the same object were taken with identical geometry and image processing procedures. The Rinn system (Updegrave 1951) was modified by attaching the film-holder-device rigidly to the tubes. Biting in an acrylic index (CoeTray Plastic) fixed tothe film holder device thepatientwasable to bite into the correct repeatable position. These modifications provided acceptable super imposable images for digital subtraction radiography. Other factors influencing the film density were kept constant. Digital subtraction images.
From a pair of standardized radiographs to be compared densitometrically, digitized pictures were taken using a commercial black-and white CCD camera (Hitachi Ci- 20 p.m., Tokyo, Japan; 734 _580 pixels, specially adapted for picture processing) and a frame grabber hardware card (Matrox MVP/AT, Matrox Electronic Systems Ltd, Qeuebec, Canada H9P 2T4) in a microcomputer (Compaq 386/20, Compaq Computer Co., Houston, TX, USA, with VGA Graphics, 60 MB Hard drive and 4 Mega Extenden RAM). The image-processing system allowed the capture of four frames on board with a resolution of 512 x512x8 pixels.
Before evaluating radiographic density differences between the two standardized radiographs, each follow- up radiograph was aligned to the stored baseline image. This superimposition took place in three steps. To obtain a first alignment, the baseline image was divided into a chessboard format. The follow-up radiograph was then shifted and rotated until the two images were superimposed, showing 50% of the image on the monitor originating from the stored baseline image and the corresponding 50% from the follow-up radiograph. Changing the display from the baseline to the follow- up image in a flickering manner, a second adjustment was performed until the two images were perceived as one stable image on the monitor. For the third adjustment, the real-time subtraction image was displayed on the video monitor, which allowed exact adaptation of the two images. After the best possible superimposition had been achieved using the micrometer screws for axial and rotational alignment, the followup image was finally grabbed, and a coarse intensity adjustment was performed using the frame grabbers electronic gain and offset adjustment.
The superimposition was based on the assumption that the film planes did not change between two exposures for a pair of standardized radiographs. In addition, this method did not compensate for projection errors caused by film bending, but the real-time subtraction allowed for an optimal alignment over the area of interest.
To correct for changes in density caused by different exposure and/or developing conditions, the gray level histograms of the two images were compared and adjusted using a non-parametric contrast correction method. After the gray level adaptation, a subtraction image from a site where absolutely no change in density had occurred showed a perfect cancellation of the structures. An average gray level value of 128 (the middle of the digitizer gray level range set by software) showed up at each pixel. Areas with gray levels less than 128 in the subtraction image (appearing dark against the background of 128) indicated loss in density and areas with gray levels greater than 128 (appearing bright against the background of 128) indicated and increase in density. Contrast-enhanced digital subtraction images.
The original digital subtraction image could theoretically demonstrate gray levels between 0 and 255. Because this method was, however, used to depict small changes in density over time, most of the pixels were expected to demonstrate a gray level near 128 (in the range between100 and156).Therefore, the lookup table was modified interactively with an adjustable ramp stretching an individually chosen range of gray levels in the image. Colour-coded digital subtraction images.
The lookup table could also be changed to display any given intensity in the subtraction image to different colours. The following colour composition was chosen: gray levels between 115 and 0 were depicted in ascending intensities of red; gray levels between 115 and 141 were displayed in one intensity of green; gray levels between 141 and 255 were depicted in descending intensities of blue.
The colour conversion resulted in subtraction images in which the range of gray levels between 115 and 141 appeared green (representing no change in bone density), gray levels from 0 to115 appeared red (representing loss in density), and gray levels from141to 255 appeared blue (representing increase in density). Regions of interest colour-coded image.
Regions of interest in the subtracted colour-coded image to be interpreted were outlined and projected on top of the originally digitized black and white radiograph for better localization of the structures of interest. Interpretation of the images.
Four practitioners interpreted the sets of radiographs. They assessed 59 pairs of images projected at random using a slide projector. On the left side, there was a reference image with no lesion, and on the right an image with or without a lesion. Each reader was asked to rate each pair of images on a three-point scale: yes, absolutely sure that gain or loss was present; uncertain, if there was gain or loss; no, absolutely sure that no gain or loss is present. There was no time limit for each decision. Data analysis.
For the diagnosis of periapical bone density changes at different time points either in conventional pairs of radiographs or using digital subtraction images inter and intra-examiners agreement was analyzed using the kappa-statistic.