Читать книгу Bone and Soft Tissue Augmentation in Implantology - Группа авторов - Страница 55

In medical hard tissue diagnosis, computer tomography (CT) is the gold standard for three-dimensional (3D) imaging. Medical CT offers 3D imaging that allows for a distortion-free, metrically correct spatial diagnosis and planning, including the anatomical structures. If the CT scan is performed with a radiopaque setup of the prosthetic wax-up, the implant planning can even be performed from a prosthetic point of view. In CT, the x-ray beam is picked up by a line-shaped detector, the layers being obtained by rotating the radiation source and the detector around the patient. In the initial CT, the patient is pushed further through the gantry, so that the number of slices and the distance of the scan determine its resolution.³⁹

Today, a continuous feed generates a 3D volume on spiral CTs, which is then visualized in layers. Various protocols exist for performing a low-dose scan to reduce the primary dose.⁹¹ For medical indications, the visualization of hard- and low-contrast structures (soft tissue) is important. Therefore, a high energy is necessary, which limits it for routine use in the dental office.

Due to the scattered radiation caused by metallic prosthetic structures, it is important to position the patient carefully, depending on whether one is scanning the maxilla or mandible, so that the jaw can be aligned parallel to the beam path. If the patient is not properly positioned, metal scattering may limit the diagnostic value of the image in determining anatomical structures. Extensive prosthetic restorative reconstructions, such as a fixed partial prosthesis made of zirconia ceramic, may impair the diagnostic accuracy due to scattered radiation. By altering the gantry inclination of the spiral CT scan, the direction of the metal scatter can be changed, resulting in the improved usability of the scan. However, this requires a well-trained radiologist who is familiar with dental details. Due to the high initial cost and the fact that a radiologist is required to take the scan, this diagnostic tool is offered exclusively in centers for radiology (Fig 2-17a and b).

In addition to the reconstructions in the classical radiologic layers, special dental CT reconstructions are also possible, which offer sections perpendicular to a panoramic curve (Fig 2-17c to h). For dental diagnostics, the images are exported in DICOM format so that they can be read in the various viewing and planning software.¹ In addition to a precise representation of the bony structures, the spiral CT is particularly distinguished in the imaging of soft tissue (Fig 2-18a to e). Although radiation exposure has been reduced in modern devices, the spiral CT still presents a higher radiation exposure compared with digital volume tomography devices (CBCT).⁵⁶

Therefore, if CBCT is available, spiral CT should be avoided for preimplantation diagnostics or for proposed 3D surgical guides or navigation procedures.^21,23,72

Fig 2-17a Panoramic view with suspected cystic findings in the right sinus.

Fig 2-17b CT scan with clarification of the findings: maxillary cyst with concomitant mucocele in the maxillary right sinus.

Fig 2-17c CT images of the mandible in the context of augmentation planning: the layers are 1 mm in thickness.

Fig 2-17d CT images of the maxilla with the same thickness.

Fig 2-17e Multiple slices from the posterior region of the mandible.

Fig 2-17f Multiple slices from the posterior region of the maxilla.

Fig 2-17g Important septa in the basal area of both maxillary sinuses.

Fig 2-17h The same septa in the middle of both maxillary sinuses.