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Osteometry of the Mandible Performed Using Dental MR Imaging

Christian J. O. NaelGo,a, Michael Pretterkliebera, Andre Gahleitnera, Christian Czernya, Martin Breitensehera and Herwig Imhofa

a From the Department of Radiology, University of Vienna AKH, Währingergürtel 18–20, A-1090 Vienna, Austria.



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  		FIG 1. A, Panoramic reconstruction from an axial CT scan (effective section thickness of 1 mm) of a severely atrophic mandible. The cut line for this reconstruction was centered on the mandibular canal (arrowheads), which is clearly depicted in terms of well-distinguished bony delineations.

B, Panoramic reconstruction from an axial T1-weighted (6.2/20) MR image (31° flip angle and an effective section thickness of 0.5 mm) of the same mandible as in A also directly shows the course of the neurovascular bundle (arrowheads), which is characterized by moderate signal intensity. Differentiation of nerves and vessels is not possible.



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  		FIG 2. A, A cut line following the course of the mandibular canal was drawn on the dental (axial) CT scans with an effective section thickness of 1 mm. The panoramic and cross-sectional reconstructions were calculated as sections parallel and perpendicular to this line.

B, A cut line for panoramic and cross-sectional reconstructions on an axial T1-weighted (6.2/20) MR image of the same mandible as in A (31° flip angle and an effective section thickness of 0.5 mm) was drawn by following the neurovascular bundle. Note the slight difference of the orientation of the proposed cross sections, which are shown as small lines perpendicular to the cut line. Only exactly parallel cross sections were used for measurement.



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  		FIG 3. A, Cross-sectional reconstruction of a mandible derived from an axial CT scan with an effective section thickness of 1 mm. Localization of the mandibular canal (arrowheads) was not possible with certainty on this section. A comparison with adjacent sections was necessary to determine the exact location of the mandibular canal for the measurements.

B, Cross-sectional reconstruction of the same mandible as in A, in the same location, derived from an axial T1-weighted (6.2/20) MR image (31° flip angle and an effective section thickness of 0.5 mm). The mandibular neurovascular bundle (arrowheads) is easily distinguished on this single section as a moderately hyperintense structure. No additional technique was required for its localization.

C, Cross section for direct osteometry corresponding to cross-sectional reconstructions in A and B. The contents of the mandibular canal were colored with enamel. The location of the mandibular canal (dark area) is the same as that identified on the dental MR cross section. Note that the transition from cortical to spongy bone is not as clear-cut as the reformatted MR and CT images suggest.



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  		FIG 4. On comparable cross sections, defined distances and diameters were measured on dental MR images and CT scans and by direct osteometry. Two sets of measurements were obtained in every quadrant of the mandibles: the first set was taken from the cross-sectional slice at the level through the mental foramen, the second from the cross-sectional slice at a point 15 mm posterior to the mental foramen. The following distances were measured: ABL, the longest axis in the bucco-lingual direction of the cross section; AAB, the longest axis in the apico-basal direction of a cross section; DTT, the distance from the top of the foramen (set 1) or top of the mandibular canal (set 2) to the top of the alveolar ridge; DBB, the distance from the bottom of the foramen (set 1) or bottom of the mandibular canal (set 2) to the base of the mandible; and DBC, the diameter of the bone cortex at the alveolar ridge.



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  		FIG 5. A, Comparison between direct osteometry and dental MR imaging. Measurements obtained by direct osteometry are drawn on the x-axis and corresponding measurements obtained on dental MR images are drawn on the y-axis (diamonds). With a 100% correlation between dental MR and direct osteometry, all dental MR measurements would meet the first median (straight line). Overestimations of distances judged on dental MR images thus lie above and underestimations below the first median. Excellent linear correlation between dental MR imaging and direct osteometry, with nearly all measurements fitting the first median, is shown.

B, The correlation between direct osteometry and dental MR imaging was only moderate in regard to the diameter of the bone cortex at the edge of the alveolar ridge (DBC).

C, Dental CT also shows a strong linear correlation with direct osteometry. The dental CT measurements (squares) nearly meet the first median (straight line).

D, For dental CT measurements, the diameter of the bone cortex at the edge of the alveolar ridge (DBC) was only moderately linearly correlated with that of direct osteometry.

E, Dental CT and MR imaging show the strongest linear correlation. Dental MR measurements are drawn on the y-axis (triangles) and dental CT measurements on the x-axis. Dental MR shows a very slight overestimation of distances (shown as points lying above the expected first median) compared with dental CT measurements.

F, Because on dental CT and MR images the transition between cortical and spongy bone appears to be sharp, correlation was extremely strong for the measurements of the diameter of cortical bone at the edge of the alveolar ridge (DBC), although the correlation of both techniques with direct osteometry was only moderate. Measurements from dental MR images are drawn on the y-axis (dots) and those of dental CT scans on the x-axis. With a 100% correlation, all measurements would lie on the first median (straight line).





Copyright © 2009 by the American Society of Neuroradiology. Print ISSN: 0195-6108 Online ISSN: 1936-959X