Cone beam computed tomography

During dental/orthodontic imaging, the CBCT scanner rotates around the patient's head, obtaining up to nearly 600 distinct images. For interventional radiology, the patient is positioned offset to the table so that the region of interest is centered in the field of view for the cone beam. A single 200 degree rotation over the region of interest acquires a volumetric data set. The scanning software collects the data and reconstructs it, producing what is termed a digital volume composed of three-dimensional voxels of anatomical data that can then be manipulated and visualized with specialized software.^[2][3] CBCT shares many similarities with traditional (fan beam) CT however there are important differences, particularly for reconstruction. CBCT has been described as the gold standard for imaging the oral and maxillofacial area.

History

Oral and maxillofacial radiology

In the late 1990s, Dr Yoshinori Arai in Japan and Dr Piero Mozzo in Italy independently developed Cone Beam Computed Technology for oral and maxillofacial radiology.^[4] The first commercial system (the NewTom 9000) was introduced in the European market in 1996 and into the US market in 2001, by Italian company Quantitative Radiology.^[2][5]

• 
  Axial image obtained from the first Cone-Beam 3D Scan performed on July 1, 1994^[6]
• 
  Axial image obtained from the first Cone-Beam 3D Scan performed on July 1, 1994
• 
  Axial image obtained from the first Cone-Beam 3D Scan performed on July 1, 1994
• 
  Original notes about the first Cone-Beam 3D Scan performed on July 1, 1994

Radiotherapy

Cone beam CT using kilovoltage

While CBCT with X-ray image intensifiers was experimented with in the late 1990s, it was not until the adoption of flat-panel X-ray detectors, with improved contrast and spatial resolution, that CBCT became practical for clinical use in interventional radiology procedures.^[10][11] Many fixed, and even mobile, C-arm fluoroscopy systems are now capable of CBCT acquisitions, in addition to traditional planar fluoroscopy.^[12][13] CBCT aids image guidance during interventional radiology procedures treating various medical conditions including knee osteoarthritis, benign prostatic hyperplasia, and hepatocellular carcinoma.^{[14][15][16][17]}

Applications

Endodontics

The most significant advantage of the CBCT in Endodontics is that it can show critical root canal anatomical features that conventional intraoral or panoramic images cannot.^[18]

According to the American Association of Endodontics, there are numerous specific situations in which 3D images produced by CBCT enhance diagnosis and influence treatment, and its use cannot be disputed over conventional intraoral radiology based on ALARA principles.^[19]

Implantology

A dental cone beam scan offers useful information when it comes to the assessment and planning of surgical implants. The American Academy of Oral and Maxillofacial Radiology (AAOMR) suggests cone-beam CT as the preferred method for presurgical assessment of dental implant sites.^[20]

Orthodontics

As a 3D rendition, CBCT offers an undistorted view of the dentition that can be used to accurately visualize both erupted and non-erupted teeth, tooth root orientation and anomalous structures, that conventional 2D radiography cannot.^[21]

Processing example using x-ray data from a tooth model:

• 
  single sampled (noisy) image
• 
  several samples overlay
• 
  joined images to panoramic
• 
  algorithmic reconstruction
• 
  in-vivo image

Orthopedics

The CBCT scanner offers undistorted views of the extremities. One advantage of orthopedic CBCT is the ability to take weight bearing images of the

Image-guided radiation therapy is a form of external beam radiotherapy where the patient is positioned with the organs to be treated accurately matched in position to the treatment field, to reduce the dose to nearby organs which are not being treated. Many organs inside the body move by millimeters relative to the external skin surfaces, and a CBCT scanner mounted on the head of the radiotherapy unit is used immediately before treatment (and sometimes again during treatment) to ensure the patient's organs are in exactly the right position to match the treatment field, and to adjust the position of the treatment table if necessary. The images may also be used to check for other requirements of some types of treatment, such as full or empty bladder, empty rectum, etc.^[8][27] The same cone beam beam source and detector can alternatively be used to take simple X-ray positioning images if the organ shows particularly well on X-ray or if Fiducial markers have been inserted into the organ.^[28]

Interventional radiology

The CBCT scanner is mounted on a C-arm

• Chemoembolization for Hepatocellular Carcinoma: CBCT with contrast confirms that the proper artery is selected to deliver the therapy. The contrast enhances the parenchyma supplied by the selected artery and therefore reveals if the vasculature also supplies the tumor. Post treatment noncontrast CBCT confirms lipiodol staining of the tumor, which improves operator confidence of complete tumor coverage or further treatment.^[29]
• benign prostatic hypertrophy: CBCT provides the soft tissue detail needed to visualize prostatic enhancement, identify duplicated prostatic arteries, and avoid nontarget embolization. CBCT is superior to DSA for this therapy since the enhancement patterns on DSA can be difficult to discern due to the overlapping pelvic structures and variable arterial anatomy.^[30]
• Abscess drainage: CBCT confirms needle tip location after placement under ultrasound and confirms drain placement by revealing contrast injection into the desired location.
• Adrenal Vein sampling for an adenoma: contrast enhanced CBCT shows perfusion of the adrenal gland to confirm catheter placement for obtaining a satisfactory sample.^[31]
• intracranial and extracranial stents compared to conventional DSA and digital radiography by providing a better depiction of the relationship of the stents to nearby structures (i.e. vascular walls and aneurysm lumen).^[32]
• Lung
  needle biopsy: CBCT guides needle placement and demonstrated a diagnostic accuracy, sensitivity, and specificity of 98.2%, 96.8%, and 100%, respectively. Diagnostic accuracy was unaffected by technically challenging conditions.^[33]
• Vascular Anomalies: After correction of
  infarcts
  in tissue that has been "sacrificed" during the procedure to prevent further shunting. The infarcted tissue appears as a small area of contrast retention.
• Peripheral Vascular Interventions
• Biliary
  Interventions
• Spinal Interventions
• Enterostomy Interventions

Industrial applications

Cone beam CT is used for material analysis, metrology, and nondestructive testing in the manufacturing sector. Cone beam CT is also inspect and detect defects of tiny sizes, such as internal pitting corrosion or cracks of an object in quality control.^[34]

Reconstruction

Cone beam reconstruction algorithms are similar to typical

Oral and maxillofacial radiology

Total radiation doses from 3D dental CBCT exams are 96% lower than conventional CT exams, but deliver 5-16x more radiation than standard dental 2D x-ray (OPG). The time of exposure in CBCT is also comparatively less when compared to conventional CT. ^{[36][37][38][39][40]}

CBCT use is only lightly regulated in the US. The recommended standard of care is to use the smallest possible field of view (FOV), the smallest

There are a number of drawbacks of CBCT technology over that of CT scans, such as increased susceptibility to movement artifacts (in first generation machines) and to the lack of appropriate bone density determination.[43]

Bone density and the Hounsfield scale

The

Although some authors have supported the use of CBCT technology to evaluate bone density by measuring HU,[47]^[48] such support is provided erroneously because scanned regions of the same density in the skull can have a different grayscale value in the reconstructed CBCT dataset.^[49]

X-ray attenuation of CBCT acquisition systems currently produces different HU values for similar bony and soft tissue structures in different areas of the scanned volume (e.g. dense bone has a specific image value at the level of the menton, but the same bone has a significantly different image value at the level of the cranial base).^[43]

Dental CBCT systems do not employ a standardized system for scaling the grey levels that represent the reconstructed density values and, as such, they are arbitrary and do not allow for assessment of bone quality.^[50] In the absence of such a standardization, it is difficult to interpret the grey levels or impossible to compare the values resulting from different machines. While there is a general acknowledgment that this deficiency exists with CBCT systems (in that they do not correctly display HU), there has been little research conducted to attempt to correct this deficiency.^[51]

With time, further advancements in CBCT reconstruction algorithms will allow for improved area detectors,^[52] and this, together with enhanced postprocessing, will likely solve or reduce this problem.^[44] A method for establishing attenuation coefficients with which actual HU values can be derived from CBCT "HU" values was published in 2010 and further research is currently underway to perfect this method in vivo.^[51]

Interventional radiology

While the practicality of CBCT fosters its increasing application in IR, technical limitations hinder its integration into the field. The two most significant factors that affect successful integration are image quality and time (for set up, image acquisition, and image reconstruction). Compared to

• Computed tomography
• Cone beam reconstruction

References