|Year : 2021 | Volume
| Issue : 4 | Page : 435-441
Detection of mandibular canal in human dry mandibles with cone beam computed tomography using 270° and 360° protocols under continuous and pulse modes – A comparative study
Karishma1, AG Annaji1, N Rakesh2, L Upasana1, MS Abhinetra1
1 Department of Oral Medicine and Radiology, V S Dental College and Hospitals, Bengaluru, Karnataka, India
2 Department of Oral Medicine and Radiology, MS Ramaiah Dental College, Bengaluru, Karnataka, India
|Date of Submission||30-Jul-2021|
|Date of Decision||30-Sep-2021|
|Date of Acceptance||01-Nov-2021|
|Date of Web Publication||27-Dec-2021|
Department of Oral Medicine and Radiology, V S Dental College and Hospital, VV Puram, Bengaluru, Karnataka - 560 004
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Background: Cone beam computed tomography (CBCT) enables practitioner to accurately localize and mark the nerve canal. The rotation of the X-ray tube and the detector around the patient's head produces multiple projection images. A high number of projections is associated with increased radiation dose to the patient, higher spatial and contrast resolution. Shorter scan arc results in reduced scan time, reduced dose and smaller file sizes. The pulsed X-ray beam reduces the actual exposure time than the scanning time and the patient radiation exposure. Aim: (1) To assess and compare the diagnostic efficacy between pulse mode and continuous mode (2) To assess and compare the diagnostic efficacy between 270° and 360° CBCT rotational protocols Subjects and Methods: A total of 30 Intact Dentate or Partially dentate dry human mandibles consisting first, second, and third molars were subjected to a total of 4 CBCT scans: 270° rotation scans under continuous and pulse mode, 360° rotation scans under continuous and pulse mode using two CBCT units. Distance from the inferior cortical border of mandible (ICBM) to the roof of the inferior alveolar nerve canal (IANC) was measured at four locations: mental foramen, first, second and third molar. The results were then compared. Results: Comparison of the IANC length measurements for the two modes revealed no significant difference. Conclusion: Shorter scan arc and pulse mode can be adopted in patients finding it difficult to remain stable for prolonged periods of time, while maintaining appreciable clarity and quality of image. This could apply to geriatric patients, children, mentally challenged, in patients with trauma, neurological diseases, anxiety, claustrophobia, wherein the patient's movements might need re-scanning, which would lead to an extra radiation dose to the patient.
Keywords: Arc of rotation, CBCT, continuous mode, inferior alveolar nerve canal, pulse mode
|How to cite this article:|
Karishma, Annaji A G, Rakesh N, Upasana L, Abhinetra M S. Detection of mandibular canal in human dry mandibles with cone beam computed tomography using 270° and 360° protocols under continuous and pulse modes – A comparative study. J Indian Acad Oral Med Radiol 2021;33:435-41
|How to cite this URL:|
Karishma, Annaji A G, Rakesh N, Upasana L, Abhinetra M S. Detection of mandibular canal in human dry mandibles with cone beam computed tomography using 270° and 360° protocols under continuous and pulse modes – A comparative study. J Indian Acad Oral Med Radiol [serial online] 2021 [cited 2022 May 21];33:435-41. Available from: https://www.jiaomr.in/text.asp?2021/33/4/435/333869
| Introduction|| |
Cone beam computed tomography (CBCT) is an advanced digital imaging technique that allows the operator to generate multiplanar slices of a region of interest and to reconstruct a 3D image by using a cone-shaped rotating x-ray beam via a series of mathematical algorithms. The rotation of the X-ray tube and the detector around the patient's head produces multiple projection images. The total number of acquired projections depends on the rotation time, frame rate (number of projections acquired per second) and on completeness of the trajectory arc. A high number of projections is associated with higher spatial resolution and greater contrast resolution but at the expense of increased radiation dose to the patient. Reducing the number of projections, while maintaining a clinically acceptable image quality, results in patient dose reduction through a reduction in tube current-exposure time product. Most CBCT acquisition protocols have a 360° rotation around the subject. Some models of CBCT equipment offer the opportunity to perform partial rotations i.e. 180° or 270° instead of the standard 360°. Most CBCT units have fixed scan arcs; however, some provide a choice of manual controls to reduce the scan arcs. Shorter scan arc results in reduced scan time, reduced dose,, smaller file sizes which facilitate image transfer and archiving.
Shorter scan time is desirable in patients finding it difficult to remain stable for prolonged periods of time for example geriatric patients, children, mentally challenged, in patients with trauma, neurological diseases, anxiety, claustrophobia and hence reduce motion artefacts.,
X-ray generation may be continuous or pulsed. The pulsed X-ray beam is preferred to the continuous X-ray beam as the actual exposure time is markedly less than the scanning time and reduces the patient radiation exposure considerably.
This study was done to to assess and compare the diagnostic efficacy between pulse mode and continuous mode in 270° and 360° CBCT rotational protocols in determining the location of the mandibular canal.
| Subjects and Methods|| |
Institutional ethical clearance was obtained prior to the study (Ref No. KIMS/IEC/D14/2018). The study was conducted by following all the protocols and principles under the purview of the Helsinki Declaration (1964 and later). Dentate or partially dentate dry human mandibles consisting of first, second and third molars with no identifiable markers such as age, sex and ethnicity were included in this study. Completely edentulous mandible and those with primary and mixed dentition were excluded from this study.
Unbroken and intact thirty dentate or partially dentate dry human mandibles were sequentially numbered from 1 to 30. A customized trough was fabricated using acrylic to seat and to provide a stable platform for each of the mandibles used in the study. Acrylic was chosen because it attenuates the beam in a similar fashion, to the presence of the oral soft tissues. The mandibles were submerged in water because water is a good approximation of human tissues [Figure 1].
The acrylic trough with mandible submerged in water was mounted within the field of view of both CBCT setup. It was placed on a tripod for NewTom Giano CBCT machine and was positioned on the scanning platform of KODAK CS 9300 Premium CBCT machine [Figure 2] and [Figure 3]. Initially, CBCT images of all 30 mandibles were scanned with NewTom Giano CBCT unit with 360° rotational protocol. This unit is Auto-Adaptative with intensity modulation during rotation and most of the exposure parameters are fixed. The scans were performed at 90 kVp, 4 mA, 9 s, FOV- 11×5 cm, voxel size-0.15mm in continuous mode and at 90 kVp, 3 mA, 3.6 s, FOV 11×5 cm and voxel size- 0.3mm in pulse mode. Following this, CBCT scans were acquired using KODAK CS 9300 Premium CBCT scanner with 270° rotational protocol at the following exposure parameters: 90 kVp, 4 mA, 11.30 s, FOV-17×13.5 cm, voxel size-0.3mm in continuous mode and with 90 kVp, 4 mA, 6.40s, FOV-17×11cm, voxel size-0.25mm in pulse mode.
|Figure 2: The acrylic trough positioned on KODAK CS9300 Premium CBCT unit|
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|Figure 3: Acrylic trough placed on a tripod for NewTom Giano CBCT machine|
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For each individual mandibles, a total of four CBCT scans were acquired (a) 360° rotation scans under continuous and pulse modes, (b) 270° rotation scans under continuous and pulse modes. Two oral and maxillofacial radiologists with more than 10 years of experience randomly evaluated 20% of scans in each of the 4 groups. This was caliberated in terms of inter examiner reliability. It was found to be at 0.9 and was considered good. The images were viewed and analyzed using reconstruction software, NNT Viewer version 8.0 and CS 3D Imaging v3.6.2 in NewTom Giano and KODAK CS 9300 Premium CBCT machines respectively. Multiple cross-sectional images were generated with 2 mm slice intervals and 10 mm slice thickness. For each CBCT scan the cross-section that marks the middle of the site was selected at the following four locations: (a) mental foramen (b) first molar (c) second molar (d) third molar. The distance from the inferior cortical border of the mandible (ICBM) to the roof of the inferior alveolar nerve canal (IANC) at the previously mentioned four cross sectional locations were measured [Figure 4], [Figure 5], [Figure 6], [Figure 7].
|Figure 4: Measurement procedures done for pulse mode using CBCT reconstruction software CS 3D Imaging v3.6.2 at M2M and M3M|
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|Figure 5: Measurement procedures done for continuous mode using CBCT reconstruction software CS 3D Imaging v3.6.2 at M2M and M3M|
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|Figure 6: Measurement procedures done for continuous mode using CBCT reconstruction software NNT Viewer version 8.0 at mental foramen (MF)|
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|Figure 7: Measurement procedures done for pulse mode using CBCT reconstruction software NNT Viewer version 8.0 at mental foramen|
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Sample size estimation
At 95% CI and 80% power of the study, based on the results of a previous study a minimum of 15 subjects were required in each group, totalling to 30 subjects recruited for this study:
where n = Sample size, d = 2.3, σ1 = SD of group 1 = 1.24, σ2 = SD of group 2 = 1.26, Z1-α/2 = 1.96, and Z1-β = 0.84
Substituting the values:
where n = 15 mandibles per group, total number of groups = 2, and N = 15 × 2 = 30 mandibles
| Results|| |
In the 360° protocol, the mean lengths from the roof of IANC to the ICBM were found to be 9.67 (±1.90) at M1M, 9.80 (±1.99) at M2M, 11.83 (±2.27) at M3M, 12.37 (±1.4) at MF in continuous mode and 9.47 (±1.67) at M1M, 9.53 (±1.99) at M2M, 11.87 (±2.36) at M3M, 12.43 (±1.38) at MF in pulse mode. In the 270° protocol, the mean lengths from the roof of IANC to ICBM were 9.57 (±2.17) at M1M, 10.07 (±2.22) at M2M, 12.80 (±2.51) at M3M, 12.83 (±1.36) at MF in continuous mode and were 9.47 (±1.75) at M1M, 9.80 (±2.34) at M2M, 12.83 (±2.56) at M3M, 12.90 (±1.58) at MF in pulse mode [Table 1].
|Table 1: Descriptive statistics of 360° and 270° protocol in continuous and pulse modes|
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[Table 2] shows P values for the comparison of mean lengths from roof of IANC to ICBM under continuous and pulse modes in 360° and 270° protocols respectively.
|Table 2: Comparison of continuous mode and pulse mode in 360° and 270° protocol|
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In the 360° protocol, the P values of M1M, M2M, M3M, and MF were 0.816, 0.629, 0.952, and 0.939, respectively, and were considered to be statistically nonsignificant.
In the 270° protocol, the p- values of M1M, M2M, M3M, and MF were 0.988, 0.670, 0.976, and 0.940, respectively, and were considered to be statistically nonsignificant.
[Table 3] shows P values for the comparison of mean lengths from roof of IANC to ICBM under 360°and 270° protocol in continuous mode at M1M, M2M, M3M and MF were 0.764, 0.713, 0.151 and 0.203 respectively and are not statistically significant. In the pulse mode the P values of M1M, M2M, M3M and MF were 0.970, 0.703, 0.179 and 0.232 respectively and are not statistically significant.
|Table 3: Comparison of 360° protocol and 270° protocol in continuous and pulse modes|
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| Discussion|| |
Three-dimensional presurgical assessment is often necessary to identify vital anatomical structures which will maximize the potential for success of the treatment. IANC localization is important for implant treatment planning, implant placement and any surgical procedures involving the mandible. The present study was set out to assess and compare the diagnostic efficacy between pulse and continuous modes each in 360° and 270° CBCT scan protocols by measuring the distance of IANC from the inferior cortical border of human dry mandibles at 4 specific sites.
Several studies have documented the reliability of the 180° CBCT protocol compared with the 360° CBCT protocol in clinical applications. Tadinada et al. compared 180° and 360° rotational CBCT protocols for the measurements of the IANC throughout its course in the mandible and found that the 180° and 360° protocols were identical. In a study by Yadav et al. to evaluate the arthritic changes in the TMJ it was demonstrated that the 180° rotation protocol was as effective as the 360° rotation protocol in detecting small and large defects in the mandibular condyle. In a study evaluating the detection of simulated periapical lesions in human dry mandibles by Al-Nuaimi et al. with standard and dose reduction modes, there was no significant difference in diagnostic accuracy of CBCT scans taken with a 180° protocol and 360° protocol.
Increasing the degree of rotation of the CBCT scanners means an increased number of basis images with a potential improvement in diagnostic yield. However, with higher degrees of rotation comes longer scan time, longer reconstruction time but at the expense of increased radiation exposure to the patient. Brown et al. (2009) have shown that increasing the number of projections does not influence the linear accuracy of CBCT.
Some studies suggest that, for certain clinical applications on specific CBCT equipment, partial rotations can be used while maintaining acceptable diagnostic accuracy and image quality. Reduction of scan time is another way to lessen the likelihood of patient movement, thereby avoiding motion artifacts, which otherwise may compromise image quality, misdiagnosis may occur or a re-exposure of the patient may be decided., However, excessive reduction in the number of projections significantly deteriorates image quality and diagnostic performance. Though, rotation cycle of 1800 is found to be adequate for most diagnostic purposes, a rotation cycle of 3600 is preferred in endodontic cases as it provides higher resolution, detail and clarity with minimal artefacts. Thus, rotation cycle should be chosen according to the diagnostic need. If the machine offers reduced dose protocol it would be an advantage.
In the previous similar studies different arc of rotations were compared within the same machine. In the present study due to nonavailability of such setup, two CBCT machines with different arcs of rotation, were compared.
X ray beam can be pulsed or continuous. The pulsed X-ray beam is preferred to the continuous X-ray beam as the actual exposure time is up to 50% less than the scanning time and hence reduces the patient radiation exposure considerably. Also pulsed X-ray systems may exhibit improved spatial resolution owing to reduced motion effect. In this study the scans exposure time was reduced from 9 s to 3.6 s in 360° scans and from 11.30 s to 6.40 s in 270° scans when changed from continuous mode to pulse mode. The total radiation exposure reduced from 272.04 mGycm2 to 115.38 mGycm2 in 360° scans and from 1585 mGycm2 to 898 mGycm2 in 270° scans when changed from continuous to pulse modes. Shorter scan arc used here did not reduce radiation dose due to the selection of different FOV & voxel size which influences patient radiation exposure. There is a clear trend for smaller FOVs to offer lower doses.
Although there was slight drop in the images quality when changed from continuous to pulse mode, this however did not diminish the diagnostic yield of the images produced. Indeed, in this study, the IANC was clearly visualized whether the arc of rotation was 360° or 270° in Continuous or pulse modes. Diagnostic difficulties were encountered however, when analyzing the samples with pre-existing, sparse trabecular patterns.
It was recently reported in a systematic review of the literature regarding CBCT applications and technical factors that there is still a lack of uniformity in the technical specifications and design of the models produced from different manufacturers. There are a wide variety of CBCT units, each with specific settings (mA, kVp, imaging mode, scan mode, voxel size, FOV), all of which are factors that may influence diagnosis., Parameters in dental CBCT equipment is either fixed or can be varied depending on the CBCT unit. Fixed parameters limit the options for further optimization.
Limitations and Future Prospects
This study was limited in that two different CBCT systems with different scan settings and technical specifications were used. There was heterogeneity of devices, some parameters and softwares used in the present study, which makes a direct comparison impossible.
| Conclusion|| |
From analysis and contemplation of the results obtained from this study the following conclusions can be made;
- Decreasing scan time can benefits patients, not considered ideal for scans of longer duration. It also reduces motion artefact which otherwise would be prevalent in a longer duration scan.
- Patient effective radiation was much less in pulse mode than in continuous mode.
- Although there was slight drop in the images quality when changed from continuous to pulse mode, this however did not diminish the diagnostic yield of the images produced.
- Shorter scan arc and pulse mode can be adopted in patients finding it difficult to remain stable for prolonged periods of time, while maintaining appreciable clarity and quality of image. This could apply to geriatric patients, children, mentally challenged, in patients with trauma, neurological diseases, anxiety, claustrophobia, wherein the patient's movements might need re-scanning, which would lead to an extra radiation dose to the patient.
- Much like the previous studies which were performed on 180° and 360° arcs of rotation. The present study documented the reliability of the 270° CBCT protocol compared with the 360° rotational scan protocols.
- Within the limitations of this study, the present results demonstrated no significant differences in the diagnostic performance in detecting and measuring Inferior alveolar nerve canal from inferior cortical border of mandible under pulse and continuous modes between CBCT machines with 360° and 270° arcs of rotation.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2], [Table 3]