available, multiplanar (MPR) reconstruction has been widely used in the ... Over a period of 3 months we examined ten patients. (six men, four women) prior to ...
Dentomaxillofacial Radiology (1997) 26. 327- 331 © 1997 Stockton Press All rights reserved 0250-832X/97 $15.00
Use of spiral computed tomography for multiplanar dental reconstruction L Preda', EM Di Maggio', R Dore l , A La Fianzal, M Solcia', MR Schifino', R Campani' and EG Preda' 'Istituto di Radiologia, "Cltnica Odontoiatrica, fRCCS Policlinico S Matteo, Universiui di Pavia, Pavia, Italy
Objective: To compare spiral with conventional CT for multiplanar reconstruction (MPR) prior to dental implant placement. Methods: Ten patients underwent conventional and then Spiral CT at 1 mm slice thickness. In six patients (Group A) the pitch was 1:1; the other four (Group B) it was 2:1. Image quality and clinical features were evaluated separately on axial and reconstructed images by two experienced radiologists who scored each parameter from 1 (poor, non-diagnostic) to 3 (good, diagnostic). Results: Loss of spatial resolution with spiral CT was not significant and the diagnostic yield poorer only for trabecular bone structure. The MPRs were better and depiction of the mandibular canal more reliable. Conclusions: We recommend the use of spiral CT instead of conventional CT for dental MPR because examination time is shorter and patient comfort is improved. Use of a pitch of 2:1 permits a marked reduction in X-ray dose with no loss of image quality. Keywords: tomography, X-ray computed; dental implantation; image processing, computerassisted Introduction
Materials and methods
Since the late 1980s, when dedicated software became available, multiplanar (MPR) reconstruction has been widely used in the pre-operative assessment of dental implant patients.':" Recently, some possible applications in orthodontics have been reported.":" The main limitations of conventional CT are first, the high Xray dose, particularly considering that the diseases imaged are benign and patients often young, and second the need to apply prolonged restraint.l"-" Motion artifacts, which can be caused even by simple swallowing, may degrade a MPR to the extent that the whole examination has to be repeated, thus doubling the risk. Spiral CT, in which patient translation and X-ray exposure are simultaneous, reduces the examination time and the risk of accidental movement in comparison with conventional CT. 7 The aim of this study was to determine if spiral CT could be applied to dental MPR, using a dedicated commercial software Dental CT (Siemens, Erlangen, Germany), with no loss in diagnostic quality.
Over a period of 3 months we examined ten patients (six men, four women) prior to implant placement with a Somatom Plus/Spiral CT scanner (Siemens, Erlangen, Germany) equipped for spiral scanning with dedicated Dental CT software permitting panoramic and para-axial reconstructions. Nine of the patients examined were over 60-years-old (mean 71.5 years, range 60- 84) and gave their informed consent. One 18-year-old patient was included because he had moved during conventional Dental CT and therefore the examination had to be repeated. The maxilla was scanned in six patients and the mandible in four. All patients were scanned with both techniques at the same examination. Scan parameters are shown in Table 1. We used 1mm slice thickness and 120 kV for both conventional and spiral Dental CT. Between 24 and 35 contiguous axial images were stained with conventional CT. The mAs for conventional Dental CT was 170 and 210 for Spiral Dental CT. Six patients (Group A) underwent a spiral scan with pitch = 1:1 and four patients with pitch = 2:1 to reduce acquisition time. The images were reconstructed with a bone detail Ultrahigh algorithm. Both axial and reconstructed images were evaluated separately on hard copies printed at the same window values by two radiologists with about 3 years
Correspondence to: Dr L Preda, Istituto di Radiologia, IRCCS Policlinico S Matteo, p.le Golgi, 27100 Pavia, Italy Received 3 December 1996; accepted 23 May 1997
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We considered the visibility of the mandibular canal separately from that of the nasopalatine canal among clinical parameters because this structure is particularly important for preimplant planning. Since there was no significant difference (using Wilcoxon matched pairs test) between the scores of the two observers, the results were expressed as mean of each value.
Table 1 Scanning parameters for conventional and spiral CT. In group A the pitch was I : I and in Group B 2: I Conventional CT
Spiral CT (group A)
Spiral CT (group B)
Total scan time (s) 312" 24 15b I I I Slice thickness (mm) 170 210 mAs 210 I mm I mm/s 2 mm/s Table feed/speed I I Increment (mm) I UltraHigh UltraHigh UltraHigh Reconstruction algorithm "Scan time/slice was 2 sec. bThe range was 10 to 16 s
Results The results for the evaluation of image quality and clinical features are reported in Tables 2 and 3. Spatial resolution (as expressed by cortical outline and trabecular bone) was inferior in spiral Dental CT images: however this parameter was always scored at least '2', even in the patients examined at a pitch of 2:1. MPRs with spiral CT had poorer cortical outline and trabecular bone structure, while those for both spiral Dental CT groups had similar or higher scores than conventional Dental CT images for the depiction of mandibular canal and nasopalatine duct and continuity of the MPRs. There were no significant differences between the results for Group A and B (Wilcoxon test). Four patients in Group A had impacted teeth which scored higher than with conventional CT. Conventional axial Dental CT defined the contours of the mandibular canal better, but with spiral Dental CT MPRs, the contour of the canal was continuous, and both contours seem better depicted (Figure la.b,c). The bone area calculated at the implant site was the same with the two methods (Figure 2a and b). Spiral Dental CT had a higher SNR.
experience of dental radiology (LP and EMDM). They scored images on a three point scale (l = poor, 2 = fair, 3 = good). The following features were used to assess image quality: (a) cortical outline; (b) trabecular bone structure; (c) depiction of mandibular canal and nasopalatine duct; (d) continuity of reconstructed images, expressed as presence/absence of step artifacts; (e) signal/noise ratio (SNR). The clinical parameters considered were: (ex) thickness of alveolar ridges; (fJ) height of alveolar ridges; (y) bone mineral content; ( I mm to reconstruct overlapping images." Z-axis definition was improved even with I mm collimation and a non-overlapping recon-
struction index (RI) of 100% of slice thickness. We believe that the better resolution of the Z-axis with spiral Dental CT may depend not only on the use of a lower RI, but also on the geometric properties of spiral scanning since this gain was not influenced by variations in pitch (Figure lc). We used a higher tube current with spiral CT to balance the greater noise derived from 180 linear interpolation." This together with the use of a more effective algorithm may explain in part the improvement in SNR with spiral CT. Considering that CT is frequently used to image benign disease in relatively young patients, it is very important to minimize the dose. The total mAs for each examination can be used as an indicator of relati ve patient exposures when the scanning volume and collimation remains the same. Therefore, by use of a pitch of 2: I in Group B we halved the dose, acquiring the same volume without affecting image quality." If we compare Group B with conventional Dental CT, the former received 25% less total mAs, consequently reducing the total dose. In conclusion, we believe that spiral CT can replace conventional CT for maxillofacial imaging. On 0
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b
Figure 3 (a) Male, 79-years-old: Conventional axial Dental CT clearly depicts a small radicular cyst at the apex of the first upper left premolar (arrowhead) , associated with loss of continuity of the buccal plate (arrows) . (b) Spiral axial Dental CT: the root is more difficult to see (arrowheads) , although the cystic area is clearly demonstrated (arrows)
the basis of the results presented here, we now use routinely spiral Dental CT with a pitch of 2:I prior to
implant placement, with optimal diagnostic results and to the satisfaction of the referring clinicians ,
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