Digital volume tomography

The digital volume tomography (DVT) is a three-dimensional imaging tomography method with the use of X-rays , which especially in the Otorhinolaryngology , the oral and maxillofacial surgery and dentistry is used. The origin of DVT in Germany lies in dentistry. There it was originally referred to as “dental volume tomography”. The devices of that time differ considerably in terms of recording technology and display as well as radiation exposure from today's modern ones. A few years ago, through improvement and further development, it was possible to introduce "digital volume tomography" into ENT medicine. Modern, Hounsfield-calibrated devices can be referred to as “cone beam CTs” (see computer tomography ), as is customary in the Anglo-American literature . This technology opens up completely new avenues, so that the assessment of soft tissue structures as well as a so-called "virtual endoscopy" is possible, which Ramming and Waller presented scientifically for the first time in Germany in the field of ear, nose and throat medicine.

Imaging

Similar to computed tomography (CT) or magnetic resonance tomography (MRT), DVT is also used to generate cross-sectional images. DVT is a digital recording technique in which a three-dimensional (3D) beam is used in combination with a flat detector. The beam is faded in either in a cone shape (image intensifier) ​​or in a pyramid shape (flat panel detectors). Compared to the image intensifiers, the flat panel detectors are characterized by lower distortion and greater detail sharpness and spatial resolution. They are particularly suitable for the large-area sensors commonly used in ENT and should be preferred to image intensifiers. A large number of projection recordings are generated on a circular path, from which a 3D volume of the imaged region is calculated directly by means of back projection. Typical for the method is an isometric spatial resolution in the volume in all three spatial directions and the concentration on the display of high contrast, i.e. on hard tissue. Compared to classic (single-line) computed tomography (CT), DVT is technically differentiated by the use of a three-dimensional useful beam and a two-dimensional image receptor.

CBCT devices generate their volume data sets using a mathematical process (back projection) from usually several hundred individual X-ray projection images. Like all technical measurements, the latter are prone to errors. The calculated 3D reconstructions based on these measurements and simplified physical assumptions contain these errors as so-called "artifacts". Erasure and hardening artifacts caused by high-density structures (e.g. metallic restorations) in the direction of the beam path are typical. These can make the assessment of directly adjacent structures (e.g. approximal spaces in caries diagnosis) impossible, sometimes pathological structures can also be simulated (e.g. dark peri-implant zones around implant images). Aliasing artifacts can also occur (so-called moiré patterns, i.e. repeating patterns or stripes in the image). Due to the relatively long cycle times of several seconds, shake artifacts also occur, which for technical reasons increase with higher spatial resolution. - Compared to conventional, two-dimensional methods, three-dimensional x-ray imaging offers the fundamental advantage of being able to reproduce the naturally present three-dimensionality of anatomical structures without any loss of dimensions. In contrast to two-dimensional x-rays, where the information in the direction of the beam path is greatly reduced, three-dimensional x-rays like DVT enable the depicted anatomical structures to be displayed in all spatial directions. This leads to an increased directional information content of three-dimensional recordings (see figure). The spatial assignment of anatomical structures is often only possible in three dimensions. Since this is a relatively new procedure, there is so far no evidence for many questions to what extent this additional information provides an increased diagnostic benefit or a clinical benefit for the patient. However, this can be clearly affirmed from routine clinical diagnostics.

The devices on the market differ mainly in the design as DVT / CBCT or hybrid device (combination of DVT, OPG and CEPH), scan angle (200 ° -360 °), the size of the field of view (FOV) , Type of patient positioning and fixation (standing, sitting or lying) and the type of sensor used (CMOS, ASi flat panel detector or image intensifier). Particular attention is paid to the type of tube used (high frequency, pulsed, not pulsed), tube voltage (80–120 kV), focal point (0.3–0.7 mm) and the scan duration (exposure time). The following parameters have a particularly positive effect on image quality with low patient exposure: pulsed HF tubes, small focal point, high electrical voltage and short effective exposure time. Furthermore, it is advantageous that the total exposure time is also kept short. This reduces the likelihood of distortion or camera shake artifacts. It is also possible to limit (display) the possible FOV using lead screens (collimators) with some devices. Due to the collimation, the beam cone is only aligned to the anatomical (partial) region that is required after the acquisition to calculate the 3D volume - this avoids unnecessary irradiation. Only this real collimation is used for radiation protection. Unfortunately, most of the devices of the older and, incomprehensibly, the newer generation can only perform a collimation mathematically, i.e. H. without reducing the realistic exposure values.

As a rule, devices that are designed as pure DVT devices have different mechanical properties than hybrid devices. This is due to the design of the suspension of the U-arm. In the case of hybrid devices, it is necessary to expose the jaw arch for OPG images in an approximately parabolic path. As a result, the U-arm of the hybrid device has to perform both a radial and an XY movement. In the case of pure DVT devices, on the other hand, only a stable central bearing of the axle shaft of the U-arm is required. The suspension of the U-arm in pure DVT solutions is usually designed to be more massive, as the devices usually map a larger focus-object distance. It is also important to bring the detector as close as possible to the object; this results in an asymmetrical suspension of the U-arm, which is compensated by weights in the U-arm. In simplified terms, increasing the focus-object distance and reducing the detector-object distance make the beam cone angle flatter and the image of the objects in the beam path - due to less distortion on the detector - sharper (Iluma, Whitefox). The following rule is known from general X-ray theory: The focus-object distance must be as large as possible and the object-film distance as small as possible.

This is why these hybrid DVT devices, which were primarily developed for dental use, are unsuitable for ENT medicine. The latest generation of DVT devices also has a Hounsfield calibration. Here the values ​​of different X-ray densities are converted into standardized Hounsfield units (HUs). These are between −1000 HU for air and 500 to 3000 HU for bones. This means that soft tissue can also be clearly assigned and displayed with the help of a mathematical calculation process. This technology opens up completely new avenues, so that the assessment of soft tissue structures as well as a so-called "virtual endoscopy" is now possible. Thorough computer tomographic soft tissue diagnostics are not yet possible here, but the surface can be displayed photo-realistically due to the high contrast change between air and mucous membrane, for example in the main / paranasal sinus.

Comparison with other radiological imaging modalities

The data acquisition and computation of the image data in digital volume tomography is most similar to rotational angiography . The image intensifier of an angiography system or that of a C-arm is used to acquire the raw data.

A DVT generates two-dimensional images as a data set to calculate three-dimensional structures, while the imaging of a computer tomograph was originally based on one-dimensional detection on a single-line detector. Today, however, the difference is becoming increasingly blurred, since current computer tomographs have up to (2 ×) 320 lines and thus also work with a two-dimensional projection, i.e. with a cone beam and no longer with a fan beam.

Effective dose - radiation exposure DVT and CT

The effective dose to which a patient is exposed and the fact that there is no limit value are decisive for the radiation exposure of organs. In this context, it is also important that radiation exposure at a younger age is associated with a higher risk and that the risk decreases with increasing age. This is of immense importance when examining children. The radiation risk in children under 10 years of age is 6 times higher than in a 30 to 50 year old with the same dose. The calculation of the effective doses is based on a proposal by the International Commission on Radiological Protection (ICRP, www.icrp.org). These ICRP guidelines are available in the version from 1990, 2005 and 2007. The respective guideline must always be given with the year for comparability, since the values ​​determined for the effective dose vary considerably depending on the ICRP version even with the same device and identical measurement protocol can distinguish (4). In general, therefore, only values ​​according to ICRP should be compared with the question of radiation exposure.

The most important studies on this topic show a significant reduction in radiation in favor of DVT compared to conventional CT. In the area of ​​the petrous bones, the effective radiation dose in a DVT is below 8 µSv. This corresponds to just 1% of the radiation exposure of a spiral CT for this region. It is not only the radiation exposure of the relatively insensitive bone structures that has to be taken into account. The lens of the eye is radiated through, especially when performing x-ray diagnostics of the paranasal sinuses. This tissue is one of the most radiation-sensitive structures in the head and neck area, especially in children. According to the S1 recommendation of Dt. Society for Dentistry, Oral and Maxillofacial Medicine (DGZMK guideline, www.dgzmk.de) the effective dose according to ICRP publication 103 (8) from 2007 of a DVT is 221 + -275 µSv compared to 788 + -334 µSv for a CT . In the current guideline (33) of Dt. Society for Implantology (S2.K guideline), which also includes the Working Group for Radiology (ARö) as well as the National Association of Statutory Health Insurance Dentists (KZBV) and the DT. Companion for ZMK-Heilkunde (DGZMK), effective doses are given for DVT between 11 µSv and 674 µSv, for CT the values ​​of 180 µSv and 2100 µSv are more than significantly higher. In the documentation "evidence-based guidelines" of the European Commission (Radiation Protection N. 172) concerning the Cone Beam CT for the dental and maxillofacial surgery area (Cone Beam CT for dental and maxillofacial radiology) from 2012 for the dento-alveolar area Values ​​between 11 and 674 µSv and for the craniofacial X-ray examination using CBCT from 30 to 1073 indicated. This contrasts with values ​​of 280 to 1410 µSv for conventional multi-slice CT for images of the maxillo-mandibular area. The work by Ludlow and Ivanvic shows that the radiation exposure of the DVT is 1.5 to 12.3 times lower than that of the CT. With Loubele, the difference in radiation exposure is between 97% and 80% less with a DVT than with a conventional CT. Effective doses of 37 to 126 mSv (ICRP 1990) and 46 to 157 mSv (ICRP 2007) were determined for the ILUMA® device (26). The German Federal Office for Radiation Protection specifies a typical effective dose of 1.7 to 2.3 mSv for CT of the skull.

However, no imaging by means of a DVT currently possible below the head, so that the DVT for use in dentistry , of the oral and maxillofacial surgery and the Otorhinolaryngology limited (paranasal sinuses, middle ear, and TMJ) is. Here, however, there is a broad spectrum for the application of DVT. However, the DVTs should not be used routinely for orthodontic patients . Although all conventional orthodontic X-ray documents can be replaced with a DVT, a complete set of conventional documents with 36 µSv effective radiation exposure means significantly less radiation than a comparable DVT with 132 µSv.

According to the current X-ray ordinance , the operation of such a device (in contrast to a CT) in a dental practice by the dentist is permitted in Germany if the dentist has a correspondingly expanded specialist knowledge in accordance with the X-ray ordinance and the specialist guidelines. The same applies to oral and maxillofacial surgeons and ear, nose and throat specialists.

The first devices for the application of digital volume tomography as a replacement for a conventional CT are already being developed and for some time they have been used routinely in numerous private practices in addition to clinics.

Areas of application

The DVT was mainly used in dentistry to plan operations and place implants . It is now also used in traumatology, oral and maxillofacial surgery, endodontics (root canal treatments), temporomandibular joint treatment and periodontics (gum treatment). So z. B. the exact positional relationships of complicated retained (wisdom) teeth, such as the mandibular canal or the maxillary sinus, can be precisely determined. In dental surgery, DVT is mainly used to diagnose bony, dento-maxillo-facial structures. Possible indications are alveolar process fractures and bony pathological changes such as B. odontogenic tumors and larger periapical lesions.

Dental volume tomography of an maxillary sinus with accompanying sinusitis (*) after acute dental nerve inflammation in the upper jaw

In ENT medicine, it is also used for diagnostics and before operations in the area of ​​the paranasal sinuses or the ears (petrous bone).

DVT can also be used to differentiate between odontogenic (originating from the tooth) sinusitis and rhinogenic (originating from the nasal mucosa) sinusitis. The maxillary sinus, which is the focus of attention here, forms the interface between dentistry and ear, nose and throat medicine.

Thanks to new devices, DVT (CBCT) are also used in human and veterinary medicine.

Device technology and software for dental volume tomographs

Dental volume tomography of the lower jaw, rendered illustration with the lower jaw nerve

In the meantime, more and more large dental companies have shifted to the manufacture or sale of dental volume tomographs. Due to the falling acquisition costs for DVT devices, they are now also of interest to general dental practices. In particular, so-called combination devices with additional sensors (for orthopantomogram and lateral x-ray image ) are ideal for a general dental practice. Currently, there is also a trend towards devices with volume-specific, freely adjustable examination fields ( field of view) for further radiation reduction. The data formats are more and more standardized, but not all manufacturers provide the DICOM standard for archiving and exchanging digital tomographs between doctors. In some cases, completely manufacturer-specific file formats are selected, which makes the unhindered exchange between doctors more difficult. There are a few devices that have been specially developed for the requirements in the ENT area.

DVT devices in ear, nose and throat medicine

In ear, nose and throat medicine, DVT devices have not yet found their way into diagnostics that often, also due to the much smaller number of doctors working in this field. The DVT devices required in this area must necessarily map large volumes and are therefore significantly more expensive than many dental volume tomographs.

DVT devices in orthopedics

New CBCT devices also offer the possibility of using a gantry up to 59 cm and a patient table to display extremities and other orthopedic issues with a low dose and high resolution. Another advantage over multi-slice spiral CT (MSCT) is lower metal artifact noise.

Further areas of application

Apart from medical technology, the process is also used in a slightly different form for material testing. Larger sensors with changed sensitivity, longer exposure times, higher X-ray doses and more penetrating X-rays (higher voltage of the X-ray tube; for heavier chemical elements such as iron or copper) are used.

literature

• PA Ehrl: 3-D diagnostics in dentistry - current. In: ZWP. Volume 4, 2009, pp. 48–53 (PDF file; 269 kB).
• German Society for Dentistry, Oral and Maxillofacial Medicine: Guidelines S1 recommendation for dental volume tomography (DVT). (PDF; 1.3 MB).
• Jonathan Fleiner, Nils Weyer, Andres Stricker: DVT diagnostics, dental volume tomography. The most important cases in everyday clinical practice as an image atlas. Systematized reporting, diagnosis, therapy. Verlag 2einhalb, 2013, ISBN 978-3-9815787-0-6 .
• J. Ramming, T. Waller, M. Ramming: The digital volume tomography (DVT) in the ENT practice.
  • Part 1: Basics and legal requirements. In: ENT forum. 15, 2013, pp. 113-122.
  • Part 2: Clinical Applications, Diagnostics of the Nose and Paranasal Sinuses. In: ENT forum. 15, 2013, pp. 148-154.
  • Part 3: Clinical applications, diagnosis of the temporal bones and other structures. In: ENT forum. 15, 2013, pp. 198-208.
  • Part 4: Practical questions, economy, discussions and controversies. In: ENT forum. 15, 2013, pp. 252-261.

Individual evidence

1. ^ J. Ramming, T. Waller, M. Ramming: DVT and virtual endoscopy. Lecture, symposium of Dt. Society for digital volume tomography, Kiel 2011.
2. ↑ J. Ramming, T. Waller, M. Ramming: The digital volume tomography (DVT) in the ENT practice: devices, indications and application spectrum. In: ENT forum. 15, 2013, pp. 54-61.
3. ^ J. Ramming, T. Waller, M. Ramming: Digital volume tomography (DVT) in ENT practice - Part 1: Basics and legal requirements. In: ENT forum. 15, 2013, pp. 113-122.
4. ↑ Reiner Koppe et al: 3-D rotation angiography (3-D-RA) in neuroradiology. In: Clinical Neuroradiology. v13 n2, June 2003, pp. 55-65. (Springerlink)
5. ↑ R. Schulze: Current status of digital X-ray technology . In: Dentistry. Volume 96, No. 6, March 16, 2006, pp. 42-48.
6. ↑ JB Ludlow, LE Davis-Ludlow, SL Brooks, WB Howerton: Dosimetry of 3 CBCT devices for oral and maxillofacial radiology: CB Mercuray, NewTom 3G and i-CAT. In: Dentomaxillofac Radiol. 35, 2006, pp. 219-226.
7. ↑ M. Loubele, R. Bogaerts, E. Van Dijck et al: Comparison between effective radiation dose of CBCT and MSCT scanners for dentomaxillofacial applications. In: European Journal of Radiology. 71 (3), 2009, pp. 461-468.
8. ↑ J. Vassileva, D. Stoyanov: Quality control and patient dosimetry in dental cone beam CT. In: Radiat Prot Dosimetry. 139 (1-3), 2010, pp. 310-312. Quoted from Sebastian Berthold, Maximilian Patzelt: Determination of the effective dose, the dose area product and a correlation coefficient in various dental digital volume tomographs. Inaugural dissertation . Albert Ludwig University, Freiburg im Breisgau 2010.
9. ↑ Federal Office for Radiation Protection, Radiation Topics, June 2012, www.bfs.de.
10. ↑ Luca Signorelli, Raphael Patcas, Timo Peltomäki, Marc Schätzle: Radiation dose of cone-beam computed tomography compared to conventional radiographs in orthodontics . In: Journal of Orofacial Orthopedics / Advances in Orthodontics . tape 77 , no. 1 , January 2016, ISSN  1434-5293 , p. 9-15 , doi : 10.1007 / s00056-015-0002-4 .
11. ↑ R. Schulze: DVT diagnostics in implantology: Basics - pitfalls. on: zmk-aktuell.de , February 17, 2011.
12. ↑ J. Voßhans et al.: Exact determination of the position of the lower eight before operation. In: zm. 95, No. 2, January 16, 2005, pp. 32-36.
13. ↑ MA. Geibel: DVT Compendium. Self-published, 2011, ISBN 978-3-88006-300-6 .
14. ↑ Godbersen: Digital Volume Tomography, Diagnostic Opportunities in ENT Medicine. In: ENT news. 6-2009, pp. 46-53.
15. ↑ M. Bremke, R. Leppek, JA Werner: The digital volume tomography in ENT medicine. In: ENT. Volume 58, Number 8, 2010, pp. 823-832.
16. ↑ Kaßner, Hörmann: Use of 3D volume tomography in the clinical routine of ENT medicine. In: Digital Dental News. 4th year, October 2010, pp. 28–31.
17. ↑ M. Jungehülsing: The sinus lift from the point of view of the ENT doctor. Part 1 to 3, on: zmk-aktuell.de , July 14, 2010.
18. ↑ R Patcas, G Markic, L Müller, O Ullrich, T Peltomäki: Accuracy of linear intraoral measurements using cone beam CT and multidetector CT: a tale of two CTs . In: Dentomaxillofacial Radiology . tape 41 , no. 8 , December 2012, ISSN  0250-832X , p. 637–644 , doi : 10.1259 / dmfr / 21152480 ( birpublications.org [accessed February 28, 2019]).

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