CONE BEAM CT : NON-DENTAL APPLICATIONS

Initially cone beam CT was almost exclusively used to perform dental radiology. However, the first generation CBCT systems were later increasingly used to study sinuses, facial and nose fractures, temporomandibular joints etc. 3D-cephalometric head and neck studies became possible once CBCT systems were available that allowed scanning of the complete head. For this purpose a double rotation technique with stitching of the resulting two data sets was needed. CBCT systems on which the rotation could be stopped were needed to perform dynamic swallow or pharyngography studies. The advent of more powerful high-end CBCT systems led the way to temporal bone and skull base imaging. Finally, high-end "supine" CBCT systems using a "gantry" made small joint musculoskeletal imaging possible. These non-dental CBCT studies gradually replaced conventional X-rays and CT/MDCT studies because they allowed imaging with higher resolution, lower radiation dose and less metal artifacts. In this paper the most important non-dental CBCT indications will be discussed.

Initially, Cone Beam CT was almost exclusively used to perform dental radiology.The first major application was presurgical planning of implant placement (1).In these patients cross-sectional imaging provided additional information and especially avoided damage to the inferior alveolar nerve during implantation.The second major indication was the evaluation of the relation between impacted wisdom teeth and the inferior alveolar canal prior to extraction.Many other applications followed: tooth impaction, supplementary and surnumerary teeth, tooth agenesis, cleft lip and palate, preoperative evaluation in maxillofacial surgery, dental-jaw trauma, cysts and benign lesions of the jaws, and finally also orthodontic and endodontic problems (2).Before 3D introduction, dental plain films and panoramic radiographs were mostly used for the above indications.However, conventional CT and multidetector CT (MDCT) gradually replaced these imaging modalities.The major drawbacks of these conventional CT systems were the moderate spatial resolution of 500 micrometer (µm) or higher and the higher radiation burden which withheld the clinicians to ask for CT studies especially in younger patients.The advent of Cone Beam CT systems using 10-100 microsievert (µSv) and resolutions down to 200 µm took away most restraints.This was not always justified as some of the early Cone low the most established and new non-dental applications will be discussed.

CBCT 3D-cephalometric analysis
Conventional lateral cephalograms were used for many years for orthodontic assessment, treatment and follow-up.On these 2D projection images both the bony structures and the overlying soft tissues could be evaluated and measured prior to surgery or non-surgical orthodontic treatment.Maxillofacial surgeons however dreamed of 3D-data which would allow them to make 3D-cephalograms and to achieve more exact presurgical assessment and planning (Fig. 1).This became possible on conventional CT and MDCT but the much higher radiation dose and low spatial resolution hampered further use of this technique at that stage.These inconveniences were overcome with CBCT.CBCT 3D-cephalography became the second major CBCT application as dentists and maxillofacial surgeons were the first to have access to CBCT (9,10).This was however not possible before a major technical problem was tackled: the small size of the flat panel detectors.Most CBCT detectors have Beam CT systems used an effective radiation dose of up to 1200 µSv which is an equivalent of 240 panoramic radiographs (3).Nevertheless newer CBCT systems in general use a lower radiation dose than conventional CT's (4)(5)(6) and reach a spatial resolution of 100 µm or less.This latter characteristic gave CBCT the status of a valuable "endodontic" diagnostic technique.It was obvious that the strongest trumps of CBCT, lower radiation dose, higher spatial resolution and less metal artifacts also applied in other head and neck regions (7).The major drawback remains the long acquisition time of 10 to 40 seconds.It is not possible to perfectly immobilize young children and less cooperative especially edentulous elderly people for such a long time and therefore these patients are better studied with fast MDCT.Extensive literature on the CBCT technique and dental CBCT indications (Table I), today accounting for 27% of the studies in our hospital (Table II), is already available (8).Be- diation dose are preferred.Another requirement is that these patients must be scanned in the sitting position in order to evaluate the soft tissues in their natural upright position and therefore CBCT's with a trolley designed to scan patients in the supine position can't be used for this application.It also became gradually possible to superimpose 3D laser acquired data of the soft tissues of the face on top of the bony skull image generated by the CBCT.This was at first only possible with the development and use of dedicated "imagematching" software (Fig. 2).Fully integrated facial scanning only became possible with newer CBCT systems that enable concomitant 3D laser acquisition of the soft tissues of the face during the CBCT acquisition (1), potentially providing more accurate 3D information as both data sets are acquired at the same time with the patient immobilized in the same position.The same CBCT technique can of course also be applied in other patients like those requiring jaw bone or facial bone reconstruction following destruction by a malignant tumor (Fig. 4) etc.Today 3D-cephalometric CBCT studies are the fourth most frequent CBCT indication and account for 10% of all CBCT studies in our department although this is probably somewhat biased by the tube and flat panel detector must be displaced during this double rotation acquisition.Another solution uses vertical rotation of the flat panel detector.This solves the problem of the image height but reduces the image size in the transverse plane making it often difficult to include the nasion and both external auditory canals and overlying soft tissues.Initially these images were predominantly used in young patients for pre-surgical assessment, planning and followup of patients requiring orthodontic (11) (Fig. 2), cleft lip and palate (12,13) and major congenital facial malformation/deformity corrective surgery (Fig. 3).In these young patients repetitive imaging is often needed and therefore CBCT's using a low ra-a height of 13 cm and a width of 23 cm and therefore it is impossible to image the facial structures from the upper larynx to the forehead and to include both external auditory canals as well.To achieve this the "double rotation" technique was introduced.A first CBCT acquisition (full rotation) centered on the lower half of the facial structures is followed by a second CBCT acquisition centered on the upper half of the facial structures.A considerable overlap is used to avoid artifacts at the interface and a final "extended field-of-view (FOV)" image of 17 cm high x 23 cm wide can be achieved by software "stitching" of the two 13 x 23 cm data sets (Fig. 1a).Patient immobilization is crucial as the X-ray  Stitched extended FOV CBCT image, one of the 360 (lateral) views used to reconstruct the 3D CBCT images (A).These 3D CBCT images can provide more accurate measurements than the 2D projection conventional cephalograms.Both the 3D CBCT images and 2D projections (cephalograms) made from these 3D CBCT images can be made from the CBCT data set (B).

A B
to know that the use of a small FOV is one of the easiest ways to further reduce the radiation dose.Good collimation and planning allows imaging of the sinuses using a 8 cm (h) x 8cm (d) FOV instead of a 12 cm (h) x 12 cm (d) FOV which results in a 70% dose reduction.A major advantage of CBCT is that images have the same quality in all possible planes.This allows the ENT surgeon to evaluate the OMU and frontal sinus drainage anatomy in detail prior to acute or chronic sinusitis surgery (Fig. 5, 6).For this indication CBCT with the lowest possible radiation dose can be used and provides all (OMU) in oral implant candidates (14)(15)(16).Many studies demonstrated that similar low dose scanning can be achieved with MDCT and CBCT.Low dose imaging is crucial as inflammatory sinus disease is often recurring and results in repetitive imaging requests.However, at such a low dose MDCT images become very "noisy", are degraded by metal artifacts and still have a moderate spatial resolution in comparison with CBCT.Hence, although the radiation dose can be similar the MDCT image quality is no longer comparable with the quality provided by good and/or high-end CBCT's.It is also important specific high referral pattern of these patients in our hospital (Table I).

CBCT of the sinuses
The first real CBCT application outside the dental and maxilla-facial field was sinus imaging.Several years ago low dose MDCT of the sinuses replaced the conventional Xrays of the sinuses and became the golden standard and it took a couple years again before it was challenged by CBCT.Nevertheless the first experiences were dental again and concerned odontogenic sinusitis and evaluation of the osteomeatal unit

A B C D E
Lateral oblique transcranial (Schuller's), posteroanterior (Towne's) and caudocranial (Hirtz) views were very popular but were not always easy to acquire because of superposition of different structures on these 2D projection images.This explains why condylar fractures were one of the most missed fractures at that time.This problem was solved when CT and later MDCT were used to study the TMJ's.On these 3D-images superposition of bony structures was of course no longer a problem but this was achieved at the cost of a higher radiation dose.With the advent of CBCT this radiation dose ten require repetitive imaging.This explains the fast increase in the number of CBCT "sinus" studies accounting today for almost 38% of all the CBCT studies in our hospital.These CBCT sinus studies substitute the sinus studies on MDCT which increases the availability of these more expensive MDCT systems for more complex CT examinations.

CBCT of the temporomandibular joint
For many years conventional Xray's were used to get a view on the temporomandibular joints (TMJ's).necessary information.Follow-up CBCT can also be justified then as the radiation dose is very low (Fig. 7).Other indications are diagnosis and follow-up of benign sinus and nasal cavity lesions (Fig. 8).The weakness of CBCT is that its "contrast resolution" is bad and that consequently the soft tissues cannot be evaluated.That is why a CBCT should be avoided in patients with a suspect lesion or proven malignant tumor (Fig. 9) or in patients presenting with neurological signs and/or deficits.In these patients the imaging modality of choice is MR.A large population has sinusitis and they of- bleeding, cranial nerve damage, intra orbital or globe lesions etc. can all be overlooked on CBCT and therefore these patients should immediately be referred for a MDCT or MR examination.Another CBCT indication is the evaluation of nose fractures.It is known that nasal fracture lines are missed on lateral and anteroposterior conventional X-ray's in up to 50% of the cases.False positive diagnoses are caused by nasal fissures, false negative diagnoses by lateral wall (Fig. 14) and cartilage fractures which cannot be distinguished on these images.The latter have however consequences as patients with nasal septum damage need an open reduction procedure.
The most frequent reason to use imaging in patients with nose fractures are the medico-legal context (52%), the "diagnostic" reason for the referral (29%), the detection of trauma related foreign bodies etc.But, as mentioned, 2D X-ray's are often false negative and fractures or lesions of the septum and lateral nasal walls very high resolution and low radiation dose make it a better adapted imaging technique than MDCT.Moreover CBCT images have the same high quality in all possible planes, while the quality of the reformatted MDCT images is lower than the quality of the original in-plane axial MDCT images.This advantage of CBCT is best seen in patients with fractures at the level of the cribriform plate, orbita roof (Fig. 12) and orbital floor (Fig. 13) (20) which are easier to detect on coronal and sagittal CBCT images.This also explains why CBCT is very helpful in patients with CSF rhinorrhea where small fracture lines or breaches in the anterior or middle fossa must be detected.On the other hand CBCT has a very bad contrast resolution and therefore blood or soft tissue damage can hardly be evaluated.This explains why CBCT should no longer be used when clinical examination reveals potential intracranial, skull base or cranial nerve damage.Epidural and subdural hematomas, cerebral concussion and could be further reduced.This made CBCT the technique of choice to study bony pathology of the TMJ (17).The most frequent indications today to perform a CBCT of the TMJ are: osteoarthritis (Fig. 10), inflammatory arthritis, synovial chondromatosis, fractures (Fig. 11), ankylosis, developmental abnormalities, benign neoplasms etc. (18).The low radiation dose also made CBCT suited to perform follow-up studies for the evaluation of disease progression or treatment results.CBCT with a 15 cm FOV in the axial plane allows visualization of both TMJ's (19) .However, TMJ pain and internal derangement is most often caused by a disc problem.The soft tissues of the TMJ and the disc cannot be visualized by CBCT and hence internal derangement problems should be studied using (pseudo)dynamic MR.

Maxillofacial and nose trauma
CBCT is well suited to detect maxillofacial and orbital fractures and its Fig. 7. -Repetitive imaging of the sinuses illustrating the need of "low dose" sinus imaging.A: Presurgical CBCT dentascan for planning of implant placement showing an insufficient height of the alveolar ridge on both sides (white arrows).The sinuses are incompletely visualized and the OMU cannot be assessed.B: CBCT of the sinuses performed 6 weeks later after a sinus lifting procedure was performed on both sides.On the left side the procedure is complicated by infection (black arrow) and the complete sinus is inflamed and now the OMU can be evaluated but a clear opening to the middle meatus cannot be recognized (grey arrow).C: Follow-up CBCT of the sinuses after 8 weeks showing that the inflammation cannot be controlled conservatively (black arrow).D: Follow-up CBCT of the sinuses after 26 weeks.The infected material used for the sinus lifting was removed and a bilateral FESS procedure was performed (white arrowheads).There is healing of the left alveolar ridge (white arrow) but there is still partial obliteration of both sinuses which needs treatment before a new attempt of sinus lifting is possible.CBCT is the evaluation of the thickness of the frontoparietal bone.This region is the harvest place for autologous calvarian split thickness bone grafts which are used to perform a "sinus lifting" or "bone augmentation" procedure in patients in whom not enough alveolar bone is available to warrant useful oral implant placement.These patients are scanned with their head and neck in anteflexion.In this position the upper jaw, the sinuses and the frontoparietal bone can be imaged together

Skull base -vertex
Tumoral lesions of the skull base and vertex are best studied on MR.However, CBCT becomes valuable when calcifications inside the lesion must be demonstrated or excluded, and when a preoperative "bony" roadmap is required (Fig. 15, 16).Only the bone must be visible on these images and this explains why low dose high resolution CBCT can replace MDCT for these indications.Another indication to use low dose are more missed than detected (21).3D imaging techniques are able to detect most of these fractures which remain invisible on the 2D X-ray's.Nevertheless the use of conventional CT and MDCT was rejected because the much higher radiation dose did not match the questionable advantages, even in the more severe cases.The use of small FOV low dose 3D CBCT imaging changed this and it replaces today 2D X-ray imaging when imaging is needed or can be justified in these patients.The small displaced segment is extremely well visualized even at the lowest irradiation dose and weakest possible resolution (0.4 mm) that can be achieved on CBCT.For further protection the patient was wearing a radiation protective coat during the examination.results were excellent and it is clear that this technique has the potential to become the imaging technique of choice for sites where videofluoroscopy is not available.The major drawback is that the FOV only allows visualization of the oral cavity, nasopharynx, oropharynx, hypopharynx and larynx region and that the thoracic esophagus is not visualized.Therefore this technique cannot be used when the entire course of the oesophagus must be assessed but is ideally suited to perform pharyngographies.

Temporal bone imaging
Imaging of the temporal bone is challenging.The structures to be studied are extremely small and are located in or are surrounded by extremely dense bone.For many years conventional CT and MDCT were the preferred imaging techniques but a high radiation dose was needed.In-of videofluoroscopy.Videofluoroscopy is today the imaging technique of choice to perform barium/Gastrografin swallow tests and pharyngography but is expensive and unavailable in many hospitals.In these hospitals CBCT might become a good alternative for a videofluoroscopy swallow study or video pharyngography.The dynamic resolution of CBCT remains inferior to that of videofluoroscopy but its spatial resolution is higher (Fig. 18) while the radiation dose, 50 µSv for 18 images/ second is lower than that of the conventional dynamic X-ray techniques which is 75 µSv for 6 images/second.Therefore one must be able to halt the CBCT rotation during the acquisition of the 360 images.This is today only available on some modified CBCT systems on which the feasibility was tested.It goes without saying that only CBCT systems made to study patients in the sitting position can be used for this application.The and hence the radiation dose is kept to a minimum while "dentascan" information about the maxilla, potential sinus problems (sinusitis, problems at the level of the OMU) and the "thickness" of the cranial donor site can all be assessed on one single CBCT study (22).Once the grafts are taken from the donor site the latter is filled up with hydroxyapatite, lyofilised bone, or with pharmaceutical products like Palaco ® or Norian® (Fig. 17)

Swallow test -pharyngography
During a CBCT study 360 images are taken during the routine 20 second 360° rotation of the X-ray tube and flat panel detector around the patient's head.This means that 18 images per second are acquired.This is a lot more than the 6-8 images/sec that can routinely be acquired with standard X-ray equipment and approaches the dynamic resolution  ing the scan as swallowing causes a movement of more than 100 µm and therefore severely degrades image quality.It is clear that small children and edentulous elderly patients or patients who are unable to co-operate enough are better studied using fast MDCT.In addition, more powerful X-ray tubes are needed to study the dense temporal bone structures.This is why today the best results are achieved when CBCT systems are used with an X-ray source which is able to deliver 110 kV and up to 10-20 mA.Even with these more powerful CBCT systems the image quality can still be degraded by noise when one is dealing with a "large or voluminous" head.In these patients it is wiser to go for a lower resolution of 150 µm providing images with a better contrast resolution and less noise.The development of more powerful CBCT systems could therefore again improve the quality of these temporal bone studies at a radiation dose lower or equal to the low dose protocols used today on the best MDCT systems.This would temporal bone (24-25)(Fig.19).The major problem is however that this high resolution on CBCT can only be achieved with a long acquisition time of 40 seconds.This makes the technique vulnerable to movement artifacts and can result in the need for "retakes" resulting in a higher used radiation dose or even in a non-diagnostic study if the patients is not able to cooperate enough .Therefore the patient must be extremely well immobilized.This is more difficult in the sitting or standing position and therefore CBCT systems that can study the patients in the supine position are better suited for temporal bone imaging.The patients must also be instructed not to swallow and to breathe through the nose dur-deed, today temporal bone CT is probably one of the studies with the highest radiation burden.The resolution of these studies was situated between a routine moderate 625 µm and a highest achievable resolution of 230 µm.CBCT of cause had the potential to acquire images with a better spatial resolution of 75 -150 µm at a lower radiation dose (23).Depending on the technique and MDCT system used the radiation dose can be reduced to 50% and in some cases even to 14% when CBCT imaging is used.Another advantage is that CBCT is less sensitive to metal artifacts from metal containing pistons and middle ear implants, cochlear implants or from metal artifacts coming from outside the Hence access to these more powerful systems can then best be restricted to those users who are performing the above mentioned studies that might require a higher radiation dose.It is possible to image both temporal bones together (single rotation with a 15 x 5 cm FOV) or separately (two separated rotations with a 8 x 8 x 5 cm FOV).The radiation dose of both techniques is similar as is the image quality .In our experience the slightly higher resolution that is achieved when the temporal bones are imaged separately does B A Fig. 17. -CBCT scanning protocol for preprosthetic cranial bone grafting of the atrophic maxilla Simultaneous CBCT imaging of the maxillary receptor site, sinuses and cranial donor site.
A: Coronal CBCT through the frontoparietal vertex with the head slightly in anteflexion.This allows simultaneous scanning and visualization of the maxilla receptor site, sinuses and cranial donor site.Harvest place (white arrow).B: Axial CBCT through the parietal skull, harvest place (white arrow).malities and lesions caused by congenital malformation (e.g.aplasia of the stapes or crura of the stapes, aplasia or hypoplasia of the oval window) by chronic otitis media (e.g.tympanosclerosis with calcifications on stapes and/or footplate), etc.Finally, the positive MDCT diagnosis of superior semicircular canal dehiscence or "third window) is often corrected as negative on CBCT as the very thin bony wall of the superior semicircular canal is only detectable on images with a resolution close to 100 µm.The above explains why CBCT has the potential to and is already replacing MDCT.The main reason why CBCT is not used for temporal bone imaging today are availability of a high quality CBCT (today only 3 to 4 CBCT systems have the quality to perform temporal bone studies), the longer acquisition time -computer calculation timeslower and more complicated workstation, and the less user friendly and more time consuming "PACS" connection.From the diagnostic and quality point of view CBCT is ready lowing auto evacuation, etc. can all be seen on CBCT images.The higher resolution of CBCT also makes it easier to detect ossicular fractures, dehiscent facial nerve canals, congenital inner and middle ear malformations etc (Fig. 21).The diagnosis of a congenital middle and/or inner ear malformation frequently has to be made early in life and in these very young patients the CBCT study has to be performed under anesthesia, hence anesthetic gases must be available in the CBCT room.A MR study is most often also required for inner ear malformations which are then performed during the same anesthesia session.The major gain of CBCT over MDCT is in the detection of otosclerosis, stapes -stapes footplate -oval window (Fig. 22), and dehiscent superior semicircular canal lesions (Fig. 23).The subtle changes on the footplate or at the fissula antefenestram in otosclerosis, can sometimes only be detected on CBCT.The use of high resolution CBCT also allows detection of stapes, footplate/oval window abnor-not justify the 100% more time that must be invested.Preliminary results of an ethical committee approved comparative study between CBCT and MDCT showed that the majority of the checked anatomic structures were better or even only seen on CBCT (20) and the superiority of CBCT over MDCT was more pronounced in the coronal plane.This is explained by the fact that the coronal MDCT images are reformatted and this is needed as the radiation dose in the axial plane is already relatively high making additional separate scanning in the coronal plane unacceptable.The higher resolution allows making diagnoses which could not be made or were more difficult to make with MDCT and apply for lesions of the external, middle and inner ear (26).In patients with chronic otitis media the lysis of the lenticular process or long process of the incus (Fig. 20), inflammatory changes between incus and malleus, subtle tympanosclerosis, incudostapedial luxation, the thin walls of the cholesteatoma sac fol-B A

Musculoskeletal imaging
Patients were examined in the sitting or standing position on almost all of the first available CBCT systems.Therefore the only MSK structure that was examined at that time was the cervical spine.However the initial CBCT systems couldn't provide the needed spatial resolution and quality to match the quality of MDCT.Even with the advent of more powerful CBCT systems the visualization of the lower cervical vertebra remained a problem.Even the latest most powerful CBCT systems are unable to deliver X-rays which can visualize the lower cervical vertebra after passing through the shoulders.Also the limited field of view makes visualization of the "entire" cervical spine difficult in larger patients.Nevertheless the newer high-end  Fractures through the scaphoid bone treated with a scaphoid bone screw.Notice the non-union of the fractures (white arrows) and the absence of metal artefacts.

B A
long bones of the extremities, even when osteosynthetic bone-plates and screws are present (28).And above all this is achieved at a lower radiation dose than on MDCT.Hence CBCT is gradually replacing MDCT and MR replaces MDCT when soft tissue information is needed.Detection of fractures which are invisible pine position on a trolley or systems with a ring design through which an arm or leg could be put.Again resolution and image quality were very variable depending on the system.The best systems however offer 100-125 µm high quality images of the fingers, wrist (27), elbow, foot, ankle and knee and are also able to image CBCT systems proved their value in the evaluation of local bone lesions and in difficult cases where metal artifacts on MDCT and MR make evaluation of the vertebra impossible.Musculoskeletal CBCT became possible with the advent of systems that looked like a small CT and that could examine the patient in the su-

B A B A
or doubtful on conventional X-rays, verification of healing fractures and non-union especially when osteosynthetic material is present (29), assessment of benign tumors and degenerative changes of the bony structures, arthrography (30) etc. are some of the excellent CBCT applications in the extremities and the above mentioned joints.Musculoskeletal CBCT is today already the fastest growing new indication and in the future more powerful systems using larger flat panel detectors could even increase the share of this application.

Miscellaneous imaging
A less frequent indication is the diagnosis of salivary gland calculi when ultrasound cannot solve this problem.The high resolution and low radiation dose make CBCT suited to detect these calculi (31).Moreover these calculi often occur in younger patients and in the areas close to the eyes, two reasons to go for the technique with the lowest radiation dose.CBCT sialography is another new possible technique (32).A different less frequent indication is dacryocystography.It is possible to put some Iodine containing contrast material in the eye and to visualize the lacrimal canaliculi and sac with CBCT and to check if the nasolacrimal duct is patent.Again CBCT offers high resolution at a low radiation dose which is important in this area so close to the lens.CBCT is also used for 3D-analysis of the airway in patients with cleft lip and/or palate (33), obstructive sleep apnea (34) and in patients who underwent orthognatic surgery (35).

Endodontics -Periodontology
As described above, CBCT vendors developed high-end systems which enabled sinus, temporal bone and MSK imaging.The very high resolution that can be achieved with these systems in turn opened new possibilities in the endodontic and periodontal field.Visualisation of a fourth molar tooth root canal, tooth ankylosis, subtle dental fractures, evaluation of the dental development stage etc. are only some of the many new indications but this is beyond the scope of this "non-dental applications" paper.

Fig. 1 .
Fig. 1. -Extended field of view (FOV) CBCT acquired with the double rotation technique covering all the necessary structures from the larynx to the forehead.Stitched extended FOV CBCT image, one of the 360 (lateral) views used to reconstruct the 3D CBCT images (A).These 3D CBCT images can provide more accurate measurements than the 2D projection conventional cephalograms.Both the 3D CBCT images and 2D projections (cephalograms) made from these 3D CBCT images can be made from the CBCT data set (B).

Fig. 2 .
Fig. 2. -Three-dimensional analysis of maxillary protraction with intermaxillary elastics to miniplates.A,B: Three-dimensional CBCT reconstruction of the face pre-treatment (A) and after protraction (B) showing the retrognathia (black arrow) and correction after treatment (grey arrow).C,D: Clinical image of pre-treatment maxillary retrognathia (C) (black arrow) and correction after protraction (D) (grey arrow).E: Superimposed 3D laser acquired data on the CBCT acquired 3D reconstruction of the teeth (white arrows).

FFig. 3 .Fig. 4 .Fig. 5 .Fig. 6 .
Fig. 3. -Patient with Gardner's syndrome Three dimensional CBCT with reconstruction of the face and anterior skull showing the multiple osteomas which are typically found in patients with Gardner's syndrome.These patients require repetitive imaging and hence low radiation dose CBCT is the perfect tool to keep the radiation dose low while providing 3D anatomical information.

Fig. 8 .Fig. 10 .Fig. 9 .
Fig. 8. -Maxillary sinus disease of odontogenic origin -keratocyst.A: Axial CBCT of the sinuses showing that the left maxillary sinus is completely opacified.The cortical lining of the lesion inside the walls of the maxillary sinus reveals that the lesion is of odontogenic origin (white arrows).B: Sagittal CBCT images through the left maxillairy sinus showing the cortical wall of the keratocyst (white arrows) and the upward displacement of the molar tooth, squeezed against the orbital floor (black arrow).C: Coronal CBCT images confirming the displacement of the molar tooth (black arrow).

Fig. 11 .
Fig. 11.-Undisplaced fracture of the left condyle.A-D: Axial CBCT image at the level of the upper (A) and lower (B) part of the condyles and coronal (C) and sagittal (D) CBCT image through the left condyle showing the intact condyle on the right and the fractures through the cortex of the left condyle (arrows) without any displacement of the latter.

Fig. 12 .
Fig. 12. -Fracture through the anterior wall of the frontal sinus with extension in the orbital roof.A-B: Sagittal (A) and axial (B) CBCT image showing an impression fracture of the anterior wall of the frontal sinus (white arrows).C: Three-dimensional virtual reality CBCT reconstruction of the face showing the complex impression fracture of the frontal bone with an extension in the left orbital roof (black arrow).

Fig. 13 .
Fig. 13.-Fracture through the left orbital floor in a pregnant woman.A-B: Coronal (A) and sagittal (B) low dose CBCT (32 µSv) showing an inferior displaced segment of the orbital floor (white arrows).The small displaced segment is extremely well visualized even at the lowest irradiation dose and weakest possible resolution (0.4 mm) that can be achieved on CBCT.For further protection the patient was wearing a radiation protective coat during the examination.

Fig. 14 .
Fig. 14. -Nose fractures (two different patients).A: 3D virtual reality CBCT reconstruction of the nose showing 2 fracture lines running through the right lateral wall of the nose (arrows).B: 3D virtual reality CBCT reconstruction of the nose showing a fracture through the bridge (white arrow) and the left lateral wall (black arrow) of the nose.

Fig. 15 .
Fig. 15.-Juvenile angiofibroma with skull base involvement.A: Sagittal Gd-enhanced T1-weighted images showing the enhancing angiofibroma (white arrow) with destruction of the floor of the sphenoid sinus and extension into this sinus (black arrow).B: Sagittal CBCT confirming the bony destruction at the central skull base (black arrow).C: Axial CBCT demonstrating the destruction of the vomer (black arrow) and anteromedial wall of the right vidian canal (white arrow).D: Coronal CBCT illustrating the destruction of the floor of the sphenoid sinus (black arrow).

Fig. 16 .
Fig. 16.-Chondrosarcoma with calcifications inside the tumor.A: Axial T2-weighted images showing a lesion at the level of the right petro-occipital fissure with very high signal intensity (white arrow), compatible with a chondrosarcoma.B: Coronal Gd-enhanced T1-weighted image through the lesions illustrates that it is a solid enhancing lesion (white arrow).C: Low dose axial CBCT shows the presence of calcifications (white arrows), supporting the diagnosis of chondrosarcoma, a diagnosis which was later histologically confirmed.

Fig. 18 .
Fig. 18. -Dynamic CBCT, 18 frames per second, in a patient who underwent hemimandibular resection and reconstruction with a metal plate.The epiglottis shows a normal downward bending during swallowing (grey arrow) but in spite of this normal epiglottic function contrast is flowing down the trachea (white arrows).Contrast in the ventricle of Morgagni (black arrow).

→Fig. 21 .
Fig. 21.-CBCT double oblique reformatted high resolution images (150 µm resolution) of the middle ear in two patients with a congenital middle ear malformation.Stapedial artery (A), congenitally narrowed oval window/footplate (B).A: CBCT image in a patient presenting with tinnitus on the right side, demonstrating the presence of a stapedial artery between the crurae of the stapes (black arrow).Normal thickness of the footplate (white arrow).Notice the presence of a ventilation tube in the thickened drum.B: CBCT image in a patient with congenital conductive hearing loss on the left side.The posterior part of the oval window or footplate is calcified and thickened (white arrow).Only the anterior 2/3 of the oval window and/or footplate are open (black arrow).Capitulum of the stapes with attachment of the stapedius muscle tendon (grey arrow).

Fig. 19 .
Fig. 19.-CBCT used to verify the position of a cochlear implant (Cochlear Nucleus Freedom with 22 electrode-array contact points).A: Parasagittal CBCT reconstruction through the electrode array showing the 22 electrode contacts without any metal artefacts.Normal position inside the cochlea with the first electrode (1) inside the basal turn.B: Axial CBCT image through the basal turn of the cochlea demonstrating the normal position of the electrode array inside the scala tympani (1) with a normal open scala vestibuli (2).

Fig. 20 .
Fig. 20.-CBCT in a patient with conductive hearing loss: lysis of the incudostapedial junction on the right side.A: Axial CBCT of the right middle ear with a spatial resolution of 150 µm showing demineralization of the lenticular process of the incus (white arrow) and stapes capitulum (grey arrow).Surgery confirmed the findings and the ossicular chain was repaired.B: Axial CBCT of the left middle ear showing the normal mineralization of the lenticular process of the incus (white arrow) and stapes capitulum (grey arrow).

Fig. 22 .Fig. 24 .
Fig. 22. -CBCT double oblique reformatted high resolution images (150 µm resolution) of the middle ear in two different patients with otosclerosis.A: Fenestral otosclerosis with a thickened grayish appearance of the footplate (black arrow).The fissula antefenestram is normal.Type 1 (Veillon) otosclerosis.Compare with the normal thickness of the footplate in B. B: Fenestral otosclerosis with a hypodens otospongiosis area in the fissula antefenestram (grey arrow), just anterior to the footplate with a size of less than 1 mm.Type 2a (Veillon) otosclerosis.Normal thickness of the footplate (white arrow).

Fig. 23 .
Fig. 23.-Dehiscent superior semicircular canal imaging: positive on MDCT, negative on CBCT A: Reformatted coronal CT image shows a dehiscent superior semicircular canal (white arrow).In order to reduce the dose the patient was scanned in the axial plane and coronal reformatted images, with some loss of resolution as a consequence, were made.B-C: Coronal (B) an paracoronal CBCT images parallel to the superior semicircular canal (C) with the same resolution (150 µm) and quality as the axial images and which can therefore show the thin bony wall of the superior semicircular canal (black arrow).
and its superiority will only increase with the ongoing technical improvements of CBCT systems.

Fig. 25 .
Fig. 25.-CBCT-arthrography in a patient with osteochondritis dissecans of the elbow.A-B: Coronal (A) and sagittal (B) high resolution 150 µm CBCT images through the elbow showing a loose body and the destructed cartilage at the level of the capitulum of the humerus (white arrow) and a second loose body proximal to the trochlea (black arrow).Bone erosions/defects (grey arrows).

Fig. 26 .
Fig. 26.-Non-union of a scaphoid bone fracture, follow-up after surgery.Fractures through the scaphoid bone treated with a scaphoid bone screw.Notice the non-union of the fractures (white arrows) and the absence of metal artefacts.

Fig. 27 .
Fig. 27.-Follow-up study after arthrodesis procedure of the ankle.Coronal (A) and sagittal (B) high resolution (150 µm) images of the ankle show the presence of three screws which are hardly disturbing the visualization of the surrounding bone.The joint space between the tibia and talus is still open (black arrows) and there is also bone resorption around the screws (white arrows) confirming the clinical doubt about normal evolution to arthodesis.

Fig. 28 .
Fig. 28.-Sialolithiasis in Wharton's duct Axial (A) and coronal (B) CBCT through the floor of the mouth showing a duct stone in Wharton's canal (white arrows).

Table I .
-Most frequent dental indications (based on 899 CBCT studies performed during 2 consecutive months in 2012).

Table II .
-Most frequent cone beam CT indications (based on 899 CBCT studies performed during 2 consecutive months in 2012).