You have probably all heard of them but do you know exactly what they are? One of the most important functions of a cone beam computed tomography (CBCT) system is to reproduce a three-dimensional structure and represent that structure as an accurate two-dimensional cross-section on a monitor.
There are several characteristics that affect how well a CBCT system performs this task. Spatial resolution, contrast resolution, linearity, noise and artefacts are the primary characteristics that affect image quality in CBCT. Enhancing or suppressing any of these characteristics depends on the imaging interests and the region of interest being scanned.
Cone beam images provide undistorted and accurate dimensional views of the dental anatomy. Panoramic images, by contrast, are magnified and distorted. Magnification by itself is not a problem, as long as one knows or can calculate the magnification factor. Distortion, on the other hand, is the unequal magnification of different parts of the same image.
A CBCT image is basically a volume of information, usually cylindrical, called the field of view (FOV), this volume is then broken down into smaller units called voxels.
A voxel is a three-dimensional pixel and isotropic in nature; the smaller the voxel the higher the resolution. However, as with all technology, this simple approach can be misleading.
CBCT image quality is dependent upon balancing these characteristics and parameters to produce the best possible image for the anatomical region being scanned. Image noise and artefacts are the two biggest enemies of CT image quality.
Noise is considered to be the number one limiting factor of CT image quality. Noise is the portion of a signal that contains no information. Noise is characterised by a grainy appearance of the image. If a quality assurance phantom is comprised of a known material like water, when the phantom is used to evaluate the quality of the image, it is expected that every portion of that phantom would have the CT number zero.
Due to the statistical fluctuation in every scan it is impossible for this to occur. In the case of too little radiation, too few photons reach the detectors. As a result, the variance of CT numbers pixel-to-pixel is quite large. When an ROI is selected and the average CT number is displayed, the standard deviation is quite large. The level of noise in an image is recorded as the standard deviation in an ROI measurement. The larger the standard deviation, the less accurate the average CT number of the ROI.
Metallic objects such as dental fillings or prosthesis cause a streaking effect on an image. The appearance mimics a streaking effect from the middle of the structure towards the edges of the object scanned. The primary reason streaks occur from metal objects is because they exceed the attenuation values that a CT system can faithfully image. The raw data when captured on the CBCT scanner can be processed through special algorithms within the software and many of the streak artefacts can now be removed, producing an image with better diagnostic value.
The first scanners were large machines, often requiring a large FOV to be imaged. While offering unparalleled diagnostic images, they often included many anatomical structures unrelated to the treatment. The person requesting the image is responsible for reporting on the image in full, it is not acceptable to simply look at the region of interest; the whole scan should be considered radiologically and fully reported upon. This will generally require the services of a dental radiologist.
The latest machines available now provide the clinician with the means to generate a small, flexible FOV, targeting only the region of interest with obvious benefits to the patient, with reduced dose, along with a lower workload for the clinician.
When choosing a CBCT device, it is important to understand the capabilities of the device, there is a plethora of unfamiliar jargon to understand. Voxels, pixels, volumes, resolution ... the list goes on and careful consideration should be given to, and advice sought on, how best to proceed.