Methodology
CBCT scanning
An anthropomorphic phantom representing an adult male (CIRS ATOM Max 711-HN) was used. The phantom consists of tissue-equivalent materials and provides an anatomically accurate representation of the bones, teeth, soft tissues and air cavities. The phantom was scanned using the 3D Accuitomo 170 CBCT (J. Morita, Kyoto, Japan) using the FOVs indicated in Table 1. The tube voltage (90 kV) and exposure time (17.5 s) were fixed for all scans. The tube current was set at 5 mA, which is the standard setting for adults.
Table 1. Fields of view and phantom positioning
Field of view size (cm)
|
Position
|
‘Dental’ scans
|
17x12
|
Dentomaxillofacial
|
14x10
|
Dentomaxillofacial
|
10x10
|
Both jaws
|
10x5
|
Upper jaw
|
|
Lower jaw
|
8x8
|
Both jaws
|
6x6
|
Upper jaw, anterior region
|
|
Lower jaw, molar region
|
Other scans
|
14x10
|
Sinuses
|
17x5
|
Temporomandibular joint (dual)
|
6x6
|
Temporomandibular joint (single)
|
Image analysis
The RAW data, i.e. a TIF file comprising a series of ‘dark’ projections and a series of approx. 525 two-dimensional radiographic projections covering a full 360° rotation of X-ray tube and detector were analyzed using ImageJ (NIH, Bethesda, MD, USA). A region of interest (ROI) was defined, covering the central area of the projection, with a width and height equal to half of that of the projection. A custom macro was written in ImageJ to automatically determine ROI size and position relative to the dimensions of the projection, ensuring perfect measurement reproducibility. The detector signal (MDS) was measured for 40 dark projections (representing the background signal transmitted by the detector in the absence of X-rays) and 514 radiographic projections (omitting the first and last few projections in each series in order to remove overlapping angles). The former was subtracted from the latter to yield a corrected signal for each radiographic projection.
Next, TCM schemes were derived based on the angular behaviour of the signal. The radiation dose of each TCM scheme was compared to that of the fixed-current situation by means of the average tube current. The image quality was compared using a figure of merit (FOM), equal to the square root of the signal-to-noise ratio.
Results
Detector signal vs. projection angle
A constistent behaviour of the detector signal throughout the rotation of tube/detector was found for all dental scans. A sinusoidal relation between tube angle and relative detector signal was seen, with the highest signal found for lateral projections and the lowest signal for anteroposterior and posteroanterior projections (Fig. 2). The average ratio between the highest and lowest signal per scan was 4.6.
The same sinusoidal behaviour was found for the sinus scan and the dual-TMJ scan; for the small-FOV single-TMJ scan, however, the angular signal variation did not show a distinct pattern (Fig. 3).
Proposed TCM schemes
The new TCM schemes compensate for the high variation in detector signal by altering the tube current in a sinuisodal pattern, yet with a varying amplitude (Fig. 4). The dose reduction compared with a fixed-current scan is 4.8%, 9.6% and 14.4% for TCM1, TCM2 and TCM3, respectively. Comparing the image quality of the TCM modes with the fixed-current exposure mode, the FOM was within 5% of the mean for all modes.