Original study - ZZI 03/2012

Comparison of different cone-beam computed tomography scans of a human head preparation created using the 3D eXam (KaVo) for analysis of the image quality

H. Neller1, M.-A. Geibel2

Introduction: Differences in the picture quality and accuracy of digital representations of a human head preparation are to be assessed by visual analysis and comparison of distance measurements and measurements of Hounsfield Units.

Methods: 55 records of a human head preparation are created with the 3D eXam (KaVo) and are evaluated using the 3D eXam Vision software (KaVo).

Results: All records have reproducible results. The analysis of the cone-beam computed tomography scans shows an improving image quality and detail with increasing resolution. The comparison of scans shows slight differences in the values of distance measurements and measurements of Hounsfield Units.

Conclusion: Different cone-beam computed tomography scans show an improved image quality and detail with increasing resolution, but provide slightly different values for distance measurements and measurements of Hounsfield Units. The slightly occurring differences are negligible since they also occur in the same order of magnitude in the comparison of records created in the exactly identical recording mode.

Keywords: cone-beam computed tomography; resolution; distance measurement; Hounsfield unit; image quality

Introduction

As structures in the beam path of two-dimensional images are superimposed because of summation effects, anatomical details are often not clearly visible. In addition, a large number of oral diseases require a second representation level for successful diagnosis and treatment. A wide range of traumatological events and image-based surgical procedures also require three-dimensional imaging.

Three-dimensional radiological diagnosis of the oral region therefore plays an increasingly important role in the assessment of anatomical conditions and diagnosis of pathological processes in the oral-maxillofacial region. Computer tomography (CT) is a standardised procedure for three-dimensional imaging. Because of new technological developments in dental radiology, digital cone-beam computed tomography (CBCT) has become established as an alternative method to computer tomography.

The 3D eXam (Kaltenbach & Voigt Dental GmbH, Bismarckring 39, 88400 Biberach/Riss, Germany) used in the study operates according to the cone-beam principle. The X-ray source – with the detector unit positioned opposite – rotates around the head of the patient. The patient sits in the 3D eXam with the head fixed in position by a neck strap, chin support and forehead brace, while the data record is created. The field of view is adjusted by motorised collimation of the beam path and has selectable preset levels. The size of the detector is 20 x 25 cm. It consists of amorphous silicon and provides for a pyramid-shaped X-ray beam, which produces a cylindrical reconstruction volume. Depending on the field of view (FOV), the detector is adjusted horizontally or vertically. Collimation fades in the field of view to the region of interest (ROI), which achieves a reduction of the radiation dose as well as shorter scan and reconstruction times. The 3D eXam operates using an X-ray source with 90–120 kV, 3–8 mA, a focus of 0.5 mm and the field of view enables exposure with voxel sizes of 0.4 mm, 0.3 mm, 0.25 mm, 0.2 mm to 0.125 mm.

The required size and exposure of the imaging volume are different depending on the specialist field. This produces many different regions of interest (ROIs), which always depend on the indication [3].

It is important in this context to define precisely the size of the region of interest (ROI) on the image in order to derive the correct size of the required field of view (FOV).

The setting parameters before data acquisition are crucial for producing the image quality of the digital cone-beam computed tomography required for diagnosis.

 

Aim

Possible differences in the image quality between digital cone-beam computed tomography scans of a human head preparation, which differ crucially in the field of view settings and exposure, are to be shown by visual analysis of individual image details and comparison of distance measurements and measurements of Hounsfield Units.

Material and methods

For data acquisition a human head preparation on a wooden frame is fixed securely in position in the 3D eXam (Fig. 1) so that its position cannot change during the creation of 55 different data records.

There are critical differences in the field of view and exposure setting parameters of the 55 data records (Tab. 1). The presets of the 3D eXam are used for fading in the field of view (Tab. 1).

The 3D eXam acquisition software will be used for data acquisition and the 3D eXam Vision processing software will be used for analysing the data on the diagnostic monitor (both Kaltenbach & Voigt Dental GmbH, Bismarckring 39, 88400 Biberach/Riss, Germany).

Relevant anatomical points will be marked in the sagittal, axial and coronal view in the 3D eXam Vision software (Fig. 2–4) and these marks will be used to take distance measurements and measurements of Hounsfield Units in the sagittal, axial and coronal planes as parameters for the image quality. The measurements will be used to analyse the mean as well as the minimum and maximum.

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