A novel alignment device for cone beam computed tomography: principle and application

Abstract

Objectives: the aim of this investigation was to optimize the positioning and size of the scanned field of view (FOV) in cone beam CT (CBCT) scanners using a practical external alignment device fitted in the patient's mouth in order to train radiographers and reduce radiation dose to the patient. This is particularly challenging when using small FOVs to cover small volumes of interest.

Methods: test objects were positioned and scanned using the aligner to show that the design and geometry were correct and help the radiographer to superimpose the scanner and the volume of interest axis of rotation. An in vivo study was then undertaken comparing the accuracy of patient positioning when using the aligner, instead of scouts, to position the patient for small FOV (cylinders of 4-8 cm height and 4-8 cm diameter) dental scans. The scanners used were the Accuitomo F170 CBCT scanner (Morita, Kyoto, Japan) and the iCAT Next Gen CBCT scanner (Imaging Sciences, Hatfield, PA).

Results: there was no significant difference in positioning the patient when using the aligner compared with the scout images.

Conclusions: it is possible to rely on the aligner for patient positioning for a small volume scan and therefore spare the radiation dose associated with scout imaging.

Figure 1

(a) The centre of the scanned volume is determined by the position of the laser illuminators on the patient's face; (b) the scanned volume in relation with the planes defined by the laser illuminators

Figure 2

Screenshot of computer aided design front and side views showing the “bite” form and targets

Figure 3

(a) The relative position of the patient and the illuminators (bold lines) to perform a scan centred in the middle of the dental arch is such that the anterior illuminator falls on the line between the two “1” marks on the anterior target, and the side illuminator falls on the line next to the mark “6” on the side target. (b) The relative position of the patient and the illuminators to perform a 4 cm diameter scan centred on the patient's tooth located above or below the intersection of the red lines is such that the anterior illuminator falls on the line between the marks “1” and “2” on the anterior target, and the side illuminator falls on the line between the mark “1” and “2” on the side target

Figure 4

The aligner positioned in the iCAT scanner for an 8 cm diameter field of view scan centred where the bold lines of Figure 3a intersect

Figure 5

The aligner in a test object in a Accuitomo F170 scanner. (a) The positioning of the area of interest for a 4 or 6 cm diameter by 4 or 6 cm height cylindrical field of view scan can be based on the scout images or (b) the illuminators

Figure 6

The co-ordinates of the centre of rotation of the scanner can be recorded directly from the Accuitomo F170 scanner control panel

Figure 7

(a) Axial and (b) sagittal slices through the aligner positioned as per Figure 4 in the iCAT scanner with an 8 cm diameter field of view. Measurements are 39 mm

Figure 8

(a) Coronal and (b) axial slices through the aligner positioned as per Figure 4 in the iCAT scanner with a 5 cm diameter by 4 cm height field of view. The measurements on the coronal slice are both 20 mm and the measurement on the axial slice measures 25 mm

Figure 9

Differences in the lateral position (X) of the Accuitomo F170 scanner centre of rotation (mean±2SD = 0.3±8.8 mm)

Figure 10

Differences in the anterior–posterior position (Y) of the Accuitomo F170 scanner centre of rotation (mean±2SD = 0.4±14.4 mm)

Figure 11

Differences in the vertical position (Z) of the Accuitomo F170 scanner centre of rotation (mean±2SD = 0.1±11.4)

Figure 12

Patient positioned in the iCAT scanner for a small field of view examination of the upper left first molar