Stefan Schaller

Individually adapted examination protocols for reduction of radiation exposure in chest CT.

Wildberger JE, Mahnken AH, Schmitz-Rode T, et al. Individually adapted examination protocols for reduction of radiation exposure in chest CT. Invest Radiol 2001;36:604–611. rationale and objectives. To develop a simple directive for the reduction of radiation exposure without loss of diagnostic information in routine chest CT examinations. methods.Two hundred fifty adult patients (164 male, 86 female) were entered into a prospective trial. All examinations were performed with a multislice CT technique (Somatom Volume Zoom, Siemens). Four groups of 50 patients each were scanned with patient-related specific parameters: individual mA-s values were derived from the estimated body weight: kilograms + 10, ± 0, − 10, and − 20 mAs. The results were compared with those of 50 patients who were examined by a standard chest protocol by using the parameters 120 mAs and 140 kV. All other parameters including the tube voltage were kept constant. Subjective image quality was rated on a three-point scale: 1 = excellent, 2 = fair, 3 = nondiagnostic. In addition, objective criteria based on signal-to-noise measurements were assessed by using a region-of-interest methodology. results.Image quality was sufficient in all cases. Mean subjective gradings of image quality, based on soft-tissue window settings, were 1.1 for the 120-mAs protocol, 1.1 for the (body weight [kg] + 10) mAs protocol, 1.1 for the (body weight [kg] ± 0) mAs protocol, 1.3 for the (body weight [kg] − 10) mAs protocol, and 1.2 for the (body weight [kg] − 20) mAs protocol. Objective criteria based on noise measurements showed mean ± standard deviation values of 5.7 ± 0.8 Hounsfield units (HU) for the 120-mAs protocol. For the reduced-dose protocols, values were calculated as 7.6 ± 1.2 HU (group + 10), 7.9 ± 1.3 HU (group ± 0), 8.7 ± 1.2 HU (group − 10), and finally 9.1 ± 1.3 HU (group − 20). The best correlation for an entire subgroup was achieved with the − 10 protocol (body weight [kg] − 10) mAs, with nearly constant noise related to body weight in all patients. conclusions.By deriving mAs values from body weight estimation, an individually adapted protocol for chest CT can be recommended and easily employed in a clinical setting. With an adaptation of the tube current–time product based on the estimated body weight of the patient − 10 (body weight [kg] − 10 mAs), a well-balanced examination without significant loss of information, even in soft-tissue window settings, can be performed with this particular scanner. For this adapted mAs protocol, a mean reduction of radiation exposure of 45% was achievable, compared with the standard protocol. A maximum decrease per case down to 31 mAs was obtained, without relevant loss of image quality. Therefore, for other types of CT scanners, analogous protocols may be adapted.

Adaptive surface data compression

Three-dimensional visualization techniques are becoming an important tool for medical applications. Computer generated 3D reconstructions of the human skull are used to build stereolithographic models, which can be used to simulate surgery or to create individual implants. Anatomy-based three-dimensional models are used to simulate the physical behaviour of human organs. These 3D models are usually displayed by a polygonal description of their surface, which requires hundreds of thousands of polygons. For interactive applications this large number is a major obstacle. We have improved an adaptive compression algorithm that significantly reduces the number of triangles required to model complex objects without losing visible detail and have implemented it in our surgery simulation system. We present this algorithm using human skull and skin data and describe the efficiency of this new approach. Zusammenfassung: Computerbasierte dreidimensionale Visualisierungstechniken haben im letzten Jahrzehnt Einzug in die Medizin gehalten. Aus den computergenerierten dreidimensionalen Rekonstruktionen des Gesichtsschadels werden unter anderem mittels Stereolithographie reale Modelle erstellt, an denen geplante chirurgische Eingriffe simuliert werden konnen , oder aber die 3D-Rekonstruktionen dienen dazu, patientenangepaste Implantate herzustellen . Die Geometrie solch komplexer 3D Modelle wird im allgemeinen mit Hilfe hunderttausender einzelner, planarer Polygone beschrieben. Eine interaktive Darstellung dieser Modelle ist oftmals nicht mehr moglich. In dieser Arbeit beschreiben wir ein erweitertes adaptives Verfahren zur signifikanten Reduzierung von Polygonoberflachen, ohne das damit ein Detailverlust in der Darstellung verbunden ist. Dieses Reduzierungsverfahren wurde in ein Operationsplanungssystem integriert und umfassend verifiziert. An zwei medizinischen Datensatzen, der 3D Rekonstruktion der Hautoberflache und des Gesichtsschadels, wird die Leistungsfahigkeit dieses neuen Verfahrens aufgezeigt.

Spiral CT: medical use and potential industrial applications

Demands on image quality in general and on low-contrast resolution in particular ware very high for medical uses of x-ray computed tomography (CT). Therefore in conventional single-slice CT, perfect planar geometry was strictly adhered to where consistent data over 360 degrees can be acquired. This implied scanning the object slice by slice. Fast volume imaging had been a request in medical imaging for a long time, but results of various approaches like cone-beam CT remained unsatisfactory. Spiral CT, the first non-planar scan mode which gained general acceptance, was first introduced in 1989, 3D volume imaging of 3 to 100 cm body sections within 10 to 60 s by spiral CT is now routine in medical imaging. We will review the spiral scan principle and the different approaches to image reconstruction which are presently implemented. In particular, the type of z- interpolation algorithm can be used to include noise, z-axis resolution and artifact behavior. Aspects of image quality are discussed in detail. There are only subtle differences between conventional single slice CT and spiral volume scanning. Thereby the old paradigm that high quality CT scanning demands perfect planar geometry has become obsolete. The medical use of spiral CT is meanwhile undisputed; but a review of typical applications indicates then need for further improvements in the speed of volume data acquisition, in particular the need for more efficient use of the available x-ray power. This shall lead to new approaches like the combined use of spiral CT and multi-row detectors and cone angle CT based on area detectors. We review respective efforts and their implications for potential industrial applications.

Design, Technique, and Future Perspective of Multislice CT Scanners

The introduction of spiral CT in the early 1990s laid the foundation for a fundamental improvement in CT imaging (Kalender et al. 1990; Crawford and King 1990). For the first time volume data could be acquired without misregistration of anatomical details, which initiated the development of three-dimensional image processing techniques such as multi-planar reformations (MPRs), maximum intensity projections (MIPs), surface-shaded displays (SSPs) or volume-rendering techniques (VRTs). However, as a consequence of increasing clinical demands single-slice spiral CT with 1 s gantry rotation time soon encountered its limitations. The ideal of isotropic resolution, of acquiring image voxels with comparable sizes in all three dimensions, could only be met by a substantial reduction of the scan range (Kalender 1995). The first step towards larger volume coverage and improved transverse resolution was a two-slice CT scanner introduced in 1993 (Elscint TWIN).

Computed tomography-bronchoscopic simulation for guiding transbronchial fine needle aspiration of extramural targets: a phantom study.

Objectives:Extramural paratracheal/-bronchial tumors of the mediastinum and the hilum that cannot be seen in bronchoscopy constitute a particular challenge for transbronchial fine needle aspiration cytology. A software prototype was developed as a guidance tool to visualize extramural targets on computed tomography (CT)-bronchoscopy. A phantom study was conducted to evaluate this guidance tool. Material and Methods:For CT-bronchoscopic simulation extramural targets are visualized behind the semitransparent wall in the endoluminal view. An airway phantom with 16 targets was examined by 3 bronchoscopists. In a first pass the targets were bronchoscopically punctured in the conventional way only with knowledge of axial CT-sections. In a second pass guidance by CT-bronchoscopic simulation was used. A postinterventional CT scan of the phantom was conducted to analyze the spatial relationship between the marked puncture sites and the targets. The punctures were classified in hits and failed punctures due to deviation in distance and angle. Results:The total hit rate of the 3 operators was significantly higher with CT-bronchoscopic simulation (32 of 48) than with the conventional method (14 of 48; P < 0.01). Concerning the failed punctures the deviation in distance and angle was significantly smaller with CT-bronchoscopic simulation (P < 0.01, P < 0.05, respectively). Conclusion:CT-bronchoscopic simulation significantly increased hit rate of bronchoscopic punctures of extramural lesions compared with conventional orientation using axial CT-sections in this phantom study. These results suggest that CT-bronchoscopic simulation might be a valuable tool for increasing yield and accuracy of bronchoscopic transbronchial fine needle aspiration in patients with mediastinal and hilar masses that are invisible for conventional bronchoscopy.

Visualization of Large Image Data Volumes Using PACS and Advanced Postprocessing Methods

Computed tomography has experienced tremendous technological developments since its introduction more than 30 years ago. Acquisition of single images took several minutes on the first scanners in the 1970s. The introduction of spiral CT in the early nineties was one of the milestones of CT history, resulting in fundamental and far-reaching improvements of CT imaging, and opening a spectrum of entirely new applications, like CTA with new visualization techniques like MPR, MIP or even VRT (Kalender et al. 1990; Crawford and King 1990; Kalender 1995). For the first time it became possible to perform volumetric imaging of larger scan ranges in a single breath-hold, effectively avoiding misregistration artifacts. However, the goal of isotropic imaging of large scan ranges within one breath-hold could not be reached. As a result, compromises had to be accepted between scan range, longitudinal resolution and scan time.