Bowtie filtration for dedicated cone beam CT of the head and neck: a simulation study

Abstract

Objective: To investigate the influence of bowtie filtration on dedicated cone beam CT (CBCT) of the head and neck.

Methods: A validated hybrid simulation technique was used to model a commercial CBCT system with offset scanning geometry, 90 kV tube potential and 145×75 mm imaging field of view. Three bowtie filters were formulated to produce uniform flux intensity in the projection image of cylindrical objects of diameter 14, 16 and 18 cm. The influence of these simulated filters was compared with the original flat filtration in terms of the output radiation field, the dose delivered to the object, the scatter distribution in projections and the quality of the reconstructed image.

Results: Compared against flat filtration, dose reduction for the bowtie case, examined as a function of radial distance within a 16-cm-diameter water cylinder, varied from 8.7% at the centre to 53.8% at the periphery. Scatter reduction, quantified using scatter-to-primary ratio in projection images, was up to 37.6% for a 14-cm-diameter cylindrical contrast phantom. Using the supplied routine image reconstruction, bowtie filtration resulted in comparable noise appearance, contrast resolution and artefact pattern for computational anatomical phantoms, with <5% difference in contrast-to-noise ratio.

Conclusion: Bowtie filtration can effectively reduce the dose and scatter in CBCT of the head and neck. For better image quality, corresponding modification to the image pre-processing and reconstruction is needed.

Advances in knowledge: The hybrid simulation approach can usefully explore the impact of proposed system component and design changes.

Figures

Figure 1.

Image acquisition geometry of the Scanora 3D cone beam CT system.

Figure 2.

Flowchart of the hybrid simulation technique for cone beam CT.

Figure 3.

Development of bowtie filter for the Scanora system: (a) positions of the original flat copper filters, (b) substitution of the second copper sheet with an aluminium plate, (c) attenuation by a water cylinder and (d) variable bowtie thickness. FAD, focal spot to rotation axis distance; FDD, focal spot to detector distance.

Figure 4.

(a) Bowtie filter dimension calculated with D=16 cm and (b) normalised horizontal intensity profile of the projection image with the presence of the object.

Figure 5.

(a) The quality assurance (QA) phantom and the phantom holder supplied with the Scanora 3D CBCT system (Soredex, PaloDEx Group, Finland) and (b) dimensions of the QA phantom in lateral view. PMMA, polymethyl methacrylate.

Figure 6.

Three-dimensional views of (a) the skin, skull and brain of the Zubal head phantom and (b) the head and neck anatomy of the Plasticboy phantom [16].

Figure 7.

Normalised intensity profile over the horizontal direction for the experimental flood image and the flood images simulated with different filtrations.

Figure 8.

(a) Examples of the photon fluence distribution characterised by the modulation templates for a bowtie filter and (b) width at 75% maximum of the horizontal profile of the modulation templates at different energy bins and for different bowtie dimensions.

Figure 9.

Normalised dose distribution as a function of the radial distance within a 16-cm diameter water cylinder imaged at the rotation centre using different filtrations.

Figure 10.

Profile of scatter distribution, scatter-to-open field ratio (SOFR) and scatter-to-primary ratio (SPR) in the projection image of the quality assurance phantom simulated with different filtrations.

Figure 11.

Mean profile at different sections of the reconstructed axial image of the quality assurance phantom. Experimentally acquired data with flat filtration, simulated data with flat filtration and simulated data with different bowtie filtrations are indicated. PMMA, polymethyl methacrylate.

Figure 12.

A typical reconstructed axial image of the Zubal phantom with (a) the original flat filtration, (b) the bowtie filtration and (c) comparison of a selected line profile across the image. The line in (a) and (b) indicates the position of the profile, and the dashed circles in (c) indicate the selected region for use in contrast-to-noise ratio calculation. Arrows indicate the flat and bowtie filtrations.

Figure 13.

A panoramic view derived from the reconstructed image of the Plasticboy phantom with (a) the original flat filtration, (b) the bowtie filtration and (c) comparison of a selected line profile across the panoramic view. The line in (a) and (b) indicates the position of the profile. Arrows indicate the flat and bowtie filtrations.