[Sonography aided computer assisted surgery (SACAS) in orbital surgery]

[Excisional surgery of orbital tumors]

The indications for orbital tumor surgery are an incisional biopsy to confirm the diagnosis or in malignant operable tumors a complete excision or a debulking to avoid complications in large invasively infiltrating tumors. In the case of benign tumors, the indications for surgery depend mostly on the clinical symptoms and cosmetic esthetic disfigurement. In the present article the preoperative examinations as well as surgical access approaches to different orbital regions, endoscopic procedures and methods of intraoperative navigation are presented. Magnetic resonance imaging is the instrument of choice, whereby in many cases computed tomography (CT) adds further information. Depending on the indications, diffusion-weighted sequences, CT angiography and digital subtraction angiography (DSA, catheter angiography) are added to the preoperative diagnostics. For space-occupying lesions located anterior to the bulbar equator, an anterior orbitotomy can be performed transconjunctivally or transpalpebrally. A lateral orbitotomy is used to reach lateral, laterocranial, and lateroinferior orbital segments, whereas transcranial approaches are suitable for processes located far posterior and for those with retro-orbital intracranial extension as well as for processes in the optic foramen/superior orbital fissure. The indications for an endonasal access approach are processes medial to the bulb or optic nerve and up to the orbital apex. A transantral access can be chosen for caudal, mediolateral, and medioinferior space-occupying lesions. Modern orbital surgery is complemented by endoscopic procedures and intraoperative navigation. Orbital tumors belong to the interdisciplinary relevant diseases. Therefore, an optimal management takes place at specialized multidisciplinary centers.

The transnasal approach to the skull base. From sinus surgery to skull base surgery

The indications for endonasal endoscopic approaches to diseases of the skull base and its adjacent structures have expanded considerably during the last decades. This is not only due to improved technical possibilities such as intraoperative navigation, the development of specialized instruments, and the compilation of anatomical studies from the endoscopic perspective but also related to the accumulating experience with endoscopic procedures of the skull base by multidisciplinary centers. Endoscopic endonasal operations permit new approaches to deeply seated lesions and are characterized by a reduced manipulation of neurovascular structures and brain parenchyma while at the same time providing improved visualization. They reduce the trauma caused by the approach, avoid skin incisions and minimize the surgical morbidity. Transnasal endoscopic procedures for the closure of small and large skull base defects have proven to be reliable and more successful than operations with craniotomies. The development of new local and regional vascularized flaps like the Hadad-flap have contributed to this. These reconstructive techniques are furthermore effectively utilized in tumor surgery in this region. This review delineates the classification of expanded endonasal approaches in detail. They provide access to lesions of the anterior, middle and partly also to the posterior cranial fossa. Successful management of these complex procedures requires a close interdisciplinary collaboration as well as continuous education and training of all team members.

[New developments in computer-assisted surgery (CAS). From intraoperative imaging to ultrasound-based navigation]

Ever faster processor capacity is having an impact on computer-assisted or computer-aided surgery (CAS). The fusion of different imaging modalities enables functional data such as PET-CT, for example, to be available in image-guided surgery. Referencing of image data is the key to precise navigation. Intraoperative data acquisition is a new approach to improving accuracy. Thus, intraoperative CT conducted under navigational support enables automatic referencing of up-to-date image data. Alternatively, intraoperative magnetic resonance imaging or intraoperative sonography can be performed. Ultrasound systems have already been successfully integrated in existing navigational systems to compensate for intraoperative tissue shifting. Ultrasound systems may play a role in the future as a single modality in image-guided surgery in soft tissue of the neck and skull bone.

Error analysis for determination of accuracy of an ultrasound navigation system for head and neck surgery

OBJECTIVE

The use of conventional CT- or MRI-based navigation systems for head and neck surgery is unsatisfactory due to tissue shift. Moreover, changes occurring during surgical procedures cannot be visualized. To overcome these drawbacks, we developed a novel ultrasound-guided navigation system for head and neck surgery. A comprehensive error analysis was undertaken to determine the accuracy of this new system.

MATERIALS AND METHODS

The evaluation of the system accuracy was essentially based on the method of error definition for well-established fiducial marker registration methods (point-pair matching) as used in, for example, CT- or MRI-based navigation. This method was modified in accordance with the specific requirements of ultrasound-guided navigation. The Fiducial Localization Error (FLE), Fiducial Registration Error (FRE) and Target Registration Error (TRE) were determined.

RESULTS

In our navigation system, the real error (the TRE actually measured) did not exceed a volume of 1.58 mm(3) with a probability of 0.9. A mean value of 0.8 mm (standard deviation: 0.25 mm) was found for the FRE. The quality of the coordinate tracking system (Polaris localizer) could be defined with an FLE of 0.4 +/- 0.11 mm (mean +/- standard deviation). The quality of the coordinates of the crosshairs of the phantom was determined with a deviation of 0.5 mm (standard deviation: 0.07 mm).

CONCLUSION

The results demonstrate that our newly developed ultrasound-guided navigation system shows only very small system deviations and therefore provides very accurate data for practical applications.

Navigation-supported and sonographically-controlled fine-needle puncture in soft tissues of the neck

In surgery, sonography has been a well-accepted means of orientation for years. The immediate vicinity of many vital structures in the head and neck region calls for a very exact visualization of the surgical instrument in the 2-D ultrasonic picture. We report on the development of a new method for navigation-supported and sonographically-controlled fine-needle puncture in soft tissues of the neck. Our system comprises a navigated ultrasound probe, a navigated fine-puncture needle and a coordinate sensor. A personal computer with specially-developed software assists calibration and surgical application. The applicability test for the system is described. In vitro, a model lymph node of 9 mm in diameter had been hit. It is shown that the target structure can be aimed at very precisely by the navigated puncture needle. An accuracy of 97% and a specificity of 99% could be demonstrated. The development of a very precise and easy-to-handle method for navigation-supported fine-needle puncture in the neck region is presented. The outstanding advantage of this method is that no rigid reference gadget fixed to the patient's body is necessary. That makes this method very suitable for surgery in the neck region. Contrary to other sonographically-supported navigation methods in the head and neck region, preoperative imaging (CT or MRT) is dispensable.

[Navigation-assisted sonography for soft tissues in the head and neck region]

BACKGROUND

In soft tissue surgery of the head and neck region tissue shifts limit the usefulness of conventional CT/MRI-based navigation procedures. Furthermore, changes caused by invasive measures cannot be visualized.

METHODS

A novel navigation device for sonography of soft tissues was developed. This consists of a navigated ultrasound scanner, a navigated surgical instrument, and a personal computer with custom-made software. Its use makes an additional visualization by means of CT or MRI dispensable.

RESULTS

The system deviation (three-dimensional error) of this newly developed prototype was less than 1 mm. The practical application in a model setup showed good handling properties of the system. Orientation and approach of the surgical instrument to the sonographically visualized target structure were rapid and accurate.

CONCLUSION

This new navigation system does not require additional CT or MRI images. The navigated ultrasound probe shows tissue changes in real time. This navigation system is especially suitable for invasive procedures in soft tissues.