Introduction

Cone beam computed tomography (CBCT) was developed for dental imaging aims in the 1990s [1].

It is an imaging technique, also known as digital volumetric tomography (DVT), consisting of X-ray computed tomography, which offers high-quality, fast and three-dimensional images of the body with a low radiation dose. From a technical point of view, this tool consists of a radiation source delivering a conic X-rays beam and a flat panel detector on the opposite side. The pursued body area, representing the field of view (FOV) of interest, is located between the emitter and the detector.

CBCT produces volumetric images during a single 360° rotation of both X-ray beam and detector around the stationary patient, unlike MDCT, in which the patient moves forward in the scanner while X-ray source and detectors rotate around him.

This technology is different from a traditional MDCT scanner, which produces a helical two-dimensional (2D) fan shape beam while the volume body of interest moves through the scanner, creating a number of axial slices which can then be collected, reconstituted and formatted to generate the three-dimensional acquisition. As a result, on MDCT, each slice requires a single scan.

The advantage of CBCT over MDCT is that more information can be collected from the area of interest with a single rotation, this allowing lower radiation doses. Moreover, since the patients remain stationary during the acquisition, CBCT generates a reduced image distortion and less artifacts compared to MDCT.

The obtained data are examined and reformatted by a computer software to produce volumetric images made of three-dimensional voxels.

The 3D voxels producing images data have the same dimensional length in all three directions, which means that they are isotropic. As result, the isotropic 3D image data can be processed with volume reconstruction algorithms to obtain multiplanar reconstructions.

CBCT in musculoskeletal imaging

Application of dedicated CBCT for diagnostics in musculoskeletal field is quite recent.

Imaging modalities that are common in our daily practice comprise X-Rays, MDCT, ultrasound (US) and magnetic resonance imaging (MRI). Each one has its own advantages and limits.

  • X-rays represent the first level and most frequently used imaging tool. They are easily available in most medical settings and imply a low radiation exposure to patients.

  • MDCT provides for cross-sectional imaging that defines bones as well as vascularized soft tissues, the latter thanks to the addition of contrast enhancement. However, MDCT scanners imply the disadvantage of being large and expensive, as well as to expose the patient to a high radiation dose.

  • US is a nonionizing imaging technique, so resulting safe for the patient. It can be extremely helpful in providing dynamic real time informations of superficial soft tissue structures. However, US is less useful for imaging bone, and its diagnostic power depends on the skill of the physician performing the examination.

  • MRI is a nonionizing imaging technique, less harmful for the patient and providing detailed cross-sectional imaging. However, its widespread use is limited by size of the equipment, management costs and time to perform the scan.

CBCT is a technique producing low radiation dose and 3D cross-sectional images. The possibility to scan the patient in the upright, weight-bearing position has driven increasing interest within orthopedic practice. Resolution and image quality are variable depending on the system. The best systems offer high-quality images of the fingers, wrist, elbow, foot, ankle and knee and are also able to image long bones of the extremities, even when osteosynthesis bone plates and screws are present [2]. Since all this is achieved at a lower radiation dose than on MDCT, hence, CBCT is gradually replacing MDCT.

For all the above-mentioned reasons, in musculoskeletal field, CBCT has shown to play a role in the evaluation of orthopedic surgery of the limbs and is now recognized to be at least equal to MDCT scan for investigating bone, as it provides very good spatial resolution, making multiplanar reconstructions possible, where the plane of interest can be moved within each section, allowing the entire volume of information to be scanned in the three planes [3].

The CBCT software allows to view the bone segment under examination in three interactive views, coronal, sagittal and axial, and the plane of interest can be moved within each section, allowing the entire volume of information to be scanned in the three planes.

Detection of fractures which are invisible or doubtful on conventional X-rays, verification of healing fractures and nonunion especially when osteosynthesis material is present, assessment of benign tumors and degenerative changes of the bony structures and arthrography could be the main applications [4].

In particular, CBCT is indicated for the pre-operative planning of fractures as well as for the evaluation of the post-surgical treatment and the healing processes, even in the presence of internal metal fixation systems.

Spatial resolution is essential for CBCT imaging where depiction of fine details is crucial for reliable diagnosis [5].

Using a conical X-ray beam, the quality of the CBCT image is superior to that of MDCT, since it has a higher spatial resolution, corresponding to 80–130 microns compared to 500–750 microns of MDCT; it is always isotropic, and it exposes to a lower dose of ionizing radiation, which results 2 to 7 times less than MDCT [6].

After obtaining the authorization for clinical use by both the American FDA (Food and Drug Administration) and the EU with CE labeling [2], and based on these considerations, CBCT has recently been used also in emergency contexts for the search for “hidden” fractures [7].

Failure to identify fracture is the most common diagnostic error, which may account for 41–80% of diagnostic errors in the ED. Missed or late diagnosis of skeletal injuries, especially those of the appendicular skeleton, statistically account for the mainstream of claims in radiography malpractice suits. The main cause of diagnostic error in the ED is the failure to correctly interpret radiographs: some of the fractures are subtle, whereas in other situations the fractures are observed but misinterpreted as normal variants or old injuries [8]. Leading factors contributing to fractures missed on radiographs are the lack of relevant clinical information, inappropriate or insufficient radiographs performed, multiple fractures and severe osteoporosis. Misinterpretation of fractures may determine a delayed treatment and poor outcome for patients treated in the ED [8, 9].

The first experiences document excellent performance of CBCT in emergency radiology departments, especially thanks to transverse imaging in trauma of the extremities. The introduction of CBCT also appears to accelerate turnover in ER trauma.

CBCT, by easily identifying occult cortical fractures and using a lower dose of radiation, is proposed as an alternative or supplement to direct radiograms, optimizing the cost-effectiveness ratio and limiting the number of unnecessary immobilizations [10].

CBCT reduces false negatives, i.e., non-diagnosed fractures, the complications of which subsequently cause complications that can later lead to costly surgery, hospitalization and time off work.

In a recent experience, radiologists progressively even come to prefer CBCT over MDCT, especially for the search of fractures of the ankle, foot, wrist or hand both for quicker access to a dedicated equipment compared to a MDCT, which is always committed to other emergencies of other specialties, and for greater ease of patient positioning [11].

CBCT therefore seems to find a primary indication as a first investigation in the search for occult fractures in locations difficult to investigate with direct radiographic examination both in orthogonal and in detail projections and subject to frequent false negatives on standard radiography such as in carpal bones (Fig. 1) and of the tarsus (Fig. 2).

Fig. 1
figure 1

Importance of multiplanar reconstructions. Carpal X-ray in posterior–anterior (a) projection and oblique (b) projection: no evidence of fractures. In c, same patient than in (a): coronal view of carpal bones on CBCT. In this case, no fractures are evident too. In (d), visibility of coronal fracture at the level of capitatum bone (white arrow)

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Fig. 2
figure 2

Importance of multiplanar reconstructions. In (a), ankle–foot X-ray in lateral projection: dubious fracture (arrow). In (b), same patient than in (a): anterior view of ankle on CBCT. In this case, a dubious fracture is evident (arrow). In (c), visibility of coronal fracture at the level of astragalus bone (white arrow)

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Metal artifacts, synthetic plates or intramedullary nails are significantly reduced in CBCT compared to MDCT, thanks to artifact reduction algorithms, MAR (Metal Artifact Reduction) [10].

CBCT can find its use in young patients thanks to the delivery of a lower dose, in elderly patients in case of fractures on osteoporotic bones and in the evaluation of the callus apposition in the follow-up of surgical repairs of fractures (Fig. 3).

Fig. 3
figure 3

Control after 30 days of a metacarpal fracture. In (a), X-ray evaluation shows quite wide edge of the fracture (white arrow) with poor visibility of callus. In (b) and (c), same patient and same day control than in (a): compared to X-ray, the CBCT gives a better visualization of callus with more narrowed of margin fracture (white arrows)

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In conclusion, CBCT is a promising imaging modality in the investigation and management of bone fractures and could gradually replace conventional X-rays and MDCT studies thanks to the following reasons:

  • Superior and fine spatial resolution for diagnosing occult bone fractures.

  • Reduced radiation dosages both with respect to MDCT and X-rays that usually need more than two orthogonal radiograms.

  • Shorter scanning time than X-rays.

  • Easily access in current practice as first tool in emergency department for trauma in younger people, in older patient with osteoporotic bone, in follow-up post-orthopedic surgery.

  • Optimized cost-effectiveness ratio by limiting the number of needless immobilizations.

However, a variety of limitations can be identified to CBCT.

The first is the investigation time, which can take up to 35 s (range 18–26 s) for image acquisition, compared to the MDCT which only asks 0.3 s; this makes it more sensitive to movement artifacts than a standard CT scan [12].

A second is the small diameter of the field of view compared to the MDCT (77 cm versus 90 cm) [13].

A third is the difficult immobilization and positioning of the patient.

A fourth limitation is that CBCT does not image soft tissues. CBCT has limited low contrast resolution due to various physical and technical factors which bring limitations for soft tissue.

The evaluation of highly scattered radiation during image acquisition will adversely affect the contrast of the projection data and the final reconstructed images.