Optimizing CBCT Imaging Quality Protocol
There are different factors affecting Cone Beam CT image quality. I’m not gonna go into details on how CBCT works or the image acquisition principles behind it, rather we will keep it simple and go over things that a practitioner can do to get the best image quality out of their CBCT machine (bang for the buck). So without further ado let’s get started!
Field of View (FOV)
The field of view determines the size of the image volume. Small FOV means you are focusing on few teeth, large FOV means you’re looking at the maxillofacial region.
As a rule of thumb, smaller is better here. The way CBCT imaging works is by generating multiple cone (image intensifier systems) or square (flat panel type) projections around the patient’s head. Large FOV means you’re exposing large area of the patient’s head. This leads to increase in scattered radiation and that radiation increases noise in the final image as noise information is useless. This increase in noise lowers your image quality.
Smaller is better.
Scanning time represents the total time from pressing the exposure button to start the acquisition sequence till the CBCT machine is done scanning. This can be anyway between few seconds to several. (~ 9 secs – 30 secs). During the scan, any patient movement will lead to motion artifacts such as ghosting or blurriness (loss of sharpness). The final volume may still be useful but the image quality maybe suboptimum or sometimes completely useless. Scanning time is determined by the CBCT machine and how fast it can scan the patient. Other factors such as normal scan vs high resolution scan settings affect scanning time as high resolution scan requires more projections around the patient head to increase SNR (signal to noise ration).
Shorter is better
Normal vs High Resolution
As the CBCT machines rotates around the patient’s head it pluses X-radiation to acquire multiple projections around the head. The higher the number of projections the more information contributing to the final image will be collected. This leads to better image quality but more radiation dose to patients. For the most part, normal mode is the most widely used. Also high resolution mode generates larger CBCT data set file and it takes longer to process and view the image volume. High resolution is good for cases of vertical root fractures or small pathology that the normal scan couldn’t detect but the practitioner is suspecting something based on clinical findings.
High resolution is better. (Normal should be fine for most cases)
It is critical to get the patient to stay still in the machine. While patients can continue breathing, thank goodness, they are not allowed to swallow. Longer scanning time is more prone to patients’ movement. Table top machines are the best, patients seated on a chair are second, and third are machines where patients stand. Machines that use chin support and head strap are better than machines with a chin support only.
Patient can breathe but not allowed to swallow. No motion is a Must.
Try your best to avoid metal restorations and prosthesis if you’re using a smaller volume FOV. Most of the time those cannot be avoided. Metal restorations cause many artifacts that limit visibility and simulate false lesions in the mouth.
Some manufacturers have added a metal reduction algorithm to the CBCT reconstruction software. This can be helpful.
Less metal is better
Voxel, a volumetric pixel or 3D pixel, represents a single element in the image volume. It has x,y,z dimensions and they are equal. Voxel size can be anywhere between 400 microns down to 75 microns. Depending on what type of information you need from the CBCT scan, a 200-300 maybe sufficient (i.e. implants planning) or 75 microns maybe needed (endodontics).
Smaller means sharper image but more noise, larger means less sharpness and less noise.
Get it right the first time
Use the scout view to determine where your image volume starts and ends. Train your staff properly, attend CE courses, ask questions and get good viewing stations and subdued ambient light while viewing the volume.
Hopefully the information here helps practitioners who have a CBCT or are considering getting one.
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