What is CBCT? Is it CT?

Oral problems may be something that none of us can avoid. Toothache is usually caused by dental pulp (dental nerve) infection caused by dental caries (tooth decay), gingivitis, periodontitis, pulpitis, or fractured teeth.

When mild, only oral anti-inflammatory drugs or simple treatment;

In severe cases, dental fillings, root canal treatment, etc. may be required. It is necessary to borrow a dental film machine to obtain a clear image of the crown and root to determine the location, scope and severity of local dental lesions;

In severe cases, the tooth may need to be extracted and replanted, which requires oral CT, scientific name: CBCT.

So, is CBCT CT?

Yes and no!

1. Dental film machine—panoramic machine—CBCT

X-ray imaging technology plays a very important role in the field of stomatology, initially as “pure dental radiology”, and then gradually evolved into “oral and maxillofacial medical imaging”.

As a vertical field of X-ray imaging, the development of dental imaging has lasted for more than a hundred years, and has gone through stages such as dental film machine, film panoramic machine, digital dental film machine, digital panoramic machine, and CBCT.

01. Tooth film machine

In 1896, German dentist Walkhoff took a picture of his own teeth after undergoing 25 minutes of X-ray exposure. Ten years later, the world’s first commercial dental X-ray machine “REKORD” was born, we generally call it “dental X-ray machine”.

In 1982, French Trophy introduced the first intraoral X-ray detector (Radio-Visio-Graphy, RVG) for intraoral X-ray photography. This is the first generation of DR equipment, and the radiation dose is only one-tenth of the traditional film dose, marking the entry of dental and maxillofacial X-ray imaging into the digital age.

The dental film machine is used for 2D high-definition imaging of 1~3 teeth in the mouth. It is widely used in dental clinical fields such as tooth body, endodontics and periodontal disease. However, due to the limited imaging field of view, the dental film machine cannot obtain Images of the outside of the tooth are generally not used for orthodontic or implant-related diagnosis and treatment.

02. Panorama machine

Dental film machines can usually only observe a certain tooth or a certain part, but in many cases we are not sure which tooth caused the toothache, which requires a large field of view curved surface X-ray machine, that is, a panoramic machine .

In 1961, the world’s first commercial analog panoramic camera was used in clinical practice; in 1996, Sirona, Germany applied CCD technology to panoramic cameras and launched digital panoramic cameras.

The panoramic camera is designed based on the principle of tomography, and its imaging logic is very similar to breast DBT. During panoramic photography, according to the anatomical characteristics of the oral and maxillofacial region, the X-ray tube is selected to surround the human head at about 120°, from the temporomandibular joint on one side to the temporomandibular joint on the other side, and finally obtain a 2D image of the whole mouth at one time , so that the tomographic photography of the jaw bone and the whole mouth presents a plane view that expands left and right on one picture.

The panoramic machine has the advantages of comprehensive observation, simple operation, and easy acceptance by children and elderly and weak patients. However, the disadvantages of panoramic cameras are also very obvious:

1) Compared with dental film machines, the clarity and details of the interior of teeth in panoramic imaging are obviously inferior;
2) Due to the curved tomography, the image has serious distortion and distortion, and image overlap is also inevitable.

Therefore, the panoramic camera is mainly used to observe the shape and position of all teeth and the inside of the jaw to provide image basis for orthodontic correction and tooth restoration. It is not suitable for fields that require very high definition of tooth structure imaging.

03. CBCT

For dental diseases, dental films and panoramic films have a high diagnostic effect; but for endodontic diseases, 2D imaging is prone to overlapping images, and cannot further clarify the three-dimensional information of root canals and other surrounding tissues, which is prone to Missed or misdiagnosed cases of root fissures. This requires three-dimensional tomography, or CBCT.

Take the following picture as an example. The figure on the left is equivalent to a panoramic flat film. We can only see the image of the object in her right hand, but it cannot reflect the object held by the real person’s chest; while the CBCT in the right picture has three-dimensional imaging As a result, you can not only see the imaging of the object in your right hand, but also the imaging of the object held in front of your chest, which belongs to stereoscopic imaging.

In 1998, the world’s first commercial CBCT: NewTom 9000 came out. Two years later, it was formally produced and applied to dental clinics. CBCT is a revolutionary advancement in the field of dentistry, realizing the leap from two-dimensional to three-dimensional.

CBCT not only provides 3D images of full-mouth teeth composed of multi-plane 2D images, but also performs multi-slice observations including coronal, sagittal, and cross-sectional views, and can intuitively display three-dimensional structures, providing doctors with Diagnosis of oral and maxillofacial diseases provides an important clinical basis.

In addition, CBCT can also be used as an alternative to dental film machines and panoramic machines. It can automatically generate panoramic films without overlapping ghosts, and can obtain “small dental films” of single or multiple teeth through image segmentation. Not only the image is clearer, More detail and the ability to rotate in a 3D plane. Objectively speaking, due to its high radiation dose, CBCT may not necessarily be able to replace dental film machines, but it is the general trend to replace panoramic machines.

In recent years, “three-in-one CT” and “four-in-one CT” have appeared in the field of dentistry. The so-called “three-in-one CT” refers to the integration of CBCT, panorama, and head side three functions; “four-in-one CT” means the combination of CBCT, panorama, head side and intraoral photography (dental film). The emergence of multifunctional CT meets all the needs of oral clinical diagnosis.

2. CBCT V.S. CT

In 1989, the world’s first spiral CT was officially born, which was the first leap in CT technology. However, it is still a single-row CT, which we call Fan Beam CT (Fan Beam CT). Subsequently, by setting multiple rows of detectors on the Z-axis, multiple tomographic images can be obtained by making the gantry rotate one revolution, which we call cone beam CT (Cone Beam CT).

Therefore, multi-detector CT also belongs to the category of CBCT. However, we usually refer to it as (general) CT and apply it to whole body diagnosis. Corresponding to it is specialized CT, such as CBCT.

Usually, CBCT refers to the equipment based on flat panel detector to achieve three-dimensional imaging, the most famous of which is oral CBCT, so that CBCT has become synonymous with oral CT.

CBCT, the whole “cone-beam computed tomography (CBCT)”, consists of a tube and a flat-panel detector. Unlike the closed-loop circular hole design of general practice CT, CBCT adopts an open structure to maintain its flexibility.

Different from the high-kV, high-mAs, multi-turn high-speed scan of CT, CBCT is a low-kV, low-mAs, single-turn slow scan, which performs a single rotation scan of 180°~360° around the patient’s head to obtain the patient’s images from all angles. Hundreds of two-dimensional projections are then obtained through cone-beam CT reconstruction algorithms (such as FDK) to obtain isotropic three-dimensional images. Compared with conventional CT, CBCT has the following advantages:

1) The radiation dose is lower. The radiation dose of head CT is usually 2000µSv, while the radiation dose of CBCT is 20~500µSv (radiation dose is different for different fields of view), which is much lower than that of CT;

2) The spatial resolution is higher, the CT scanning thickness is about 0.5 mm~1cm, while the CBCT layer thickness can reach 80~400 µm, which greatly improves the image accuracy and can capture more anatomical details.

3. Low-dose CBCT

We know that the radiation dose of CBCT is only a few tenths of that of conventional CT, which is relatively safer. However, dental treatment is more complicated and often requires multiple shots. Taking orthodontics as an example, a treatment cycle often requires 7 or 8 CBCT examinations. It is quite “scary” to add up, especially many children need orthodontics. Compared with adults, children are 2 to 3 times more sensitive to X-rays than adults, and are more likely to be injured. Therefore, CBCT with lower radiation dose is needed.

Given the smaller field of view, the lower the radiation dose. We often make the FOV as small as possible to reduce the radiation dose without compromising the diagnosis. In recent years, with the advancement of technology, CBCT manufacturers have innovated in hardware:

1) Hybrid pulse tube

Today, CBCT generally uses a hybrid pulse tube, covering both pulsed and continuous scanning modes. Pulse scanning, the actual exposure time of the tube is much shorter than the scanning time, not only the life of the tube is longer, but also the radiation dose can be doubled; however, the cooling system of continuous exposure scanning is relatively complete, which is suitable for clinics with high frequency of use .

2) CMOS detector

For X-ray equipment, the detector is the core of the core. At present, CBCT mainly uses two types of amorphous silicon/IGZO detectors and CMOS detectors (please refer to: An article to understand X-ray detectors (Part 1): a technology in full bloom). Compared with amorphous silicon/IGZO detectors, since the substrate of CMOS detectors is single crystal silicon, the electron mobility is much higher, which can make CMOS detectors have higher signal-to-noise ratio, higher spatial resolution, and faster acquisition speed. Faster, lower dose DQE is higher.

Today, hybrid pulse tubes and CMOS detectors have become standard configurations for high-end CBCTs.

In addition to oral CBCT, specialized CBCTs such as breast CT, KV-CBCT, and CBCT-DR have also been developed.

For example, conventional CT has high radiation dose and low spatial resolution, which is not friendly to breast examination. The dedicated breast CT not only lowers the radiation dose, but also has an imaging resolution of less than 100 microns;

For example, tumor radiotherapy technology has entered a new era of precise radiotherapy represented by image-guided three-dimensional conformal radiotherapy and intensity-modulated radiotherapy. The development of image-guided radiation therapy (IGRT) technology, especially KV-CBCT The use of imaging systems has greatly improved the accuracy of radiation therapy.

For another example, humans are erect animals, and filming in the standing position can best reflect the cause of the patient’s daily bone pain and the degree of bone deformity. Conventional CT cannot achieve 3D imaging in the weight-bearing position. CBCT-DR can well reflect the state of the patient’s joint force changes in the weight-bearing position, and has extremely high clinical application value.