PubMed Health⌕ Search

Biomedical subjects

G Stamm

Publications and source records attributed to G Stamm.

At least 19 recordsLinked to original sources
PubMed2004

Assessment of a theoretical formalism for dose estimation in CT: an anthropomorphic phantom study.

Dose assessment in computed tomography (CT) is challenging due to the vast variety of CT scanners and imaging protocols in use. In the present study, the accurateness of a theoretical formalism implemented in the PC program CT-EXPO for dose calculation was evaluated by means of phantom measurements. Phantom measurements were performed with four 1-slice, four 4-slice and two 16-slice spiral CT scanners. Firstly, scanner-specific nCTDIw values were measured and compared with the corresponding standard values used for dose calculation. Secondly, effective doses were determined for three CT scans (head, chest and pelvis) performed at each of the ten installations from readings of thermoluminescent dosimeters distributed inside an anthropomorphic Alderson phantom and compared with the corresponding dose values computed with CT-EXPO. Differences between standard and individually measured nCTDIw values were less than 16%. Statistical analysis yielded a highly significant correlation (P < 0.001) between calculated and measured effective doses. The systematic and random uncertainty of the dose values calculated using standard nCTDIw values was about -9 and +/- 11%, respectively. The phantom measurements and model calculations were carried out for a variety of CT scanners and representative scan protocols validate the reliability of the dosimetric formalism considered-at least for patients with a standard body size and a tube voltage of 120 kV selected for the majority of CT scans performed in our study.

Head↗
PubMed2004

[5 years of "concerted action dose reduction in CT" -- what has been achieved and what remains to be done?].

In May 1998, the German "Concerted Action Dose Reduction in CT" was founded by all parties involved in CT. Its intention was to achieve a significant reduction of the radiation exposure caused by CT, a matter that has increasingly been considered a major challenge since the early nineties. As a result of a number of joint efforts, the essential preconditions have been established by now. The fifth anniversary of the Concerted Action gave rise for both retrospection and outlook on the tasks that have already been accomplished and those that still need to be done. For this purpose, a one-day symposium took place in Berlin on November 4, 2003. The contents of a total of 18 contributions will be outlined here in brief.

Adult↗
PubMed2004

[CT-based liver volumetry in a porcine model: impact on clinical volumetry prior to living donated liver transplantation].

PURPOSE: Exact preoperative determination of the liver volume is of great importance prior to hepatobiliary surgery, especially in living donated liver transplantation (LDLT). In the current literature, a strong correlation between preoperatively calculated and intraoperatively measured liver volumes has been described. Such accuracy seems questionable, primarily due to a difference in the perfusion state of the liver in situ versus after explantation. Purpose of the study was to asses the influence of the perfusion state on liver volume and the validity of the preoperative liver volumetry prior to LDLT. METHODS: In an experimental study, 20 porcine livers were examined. The livers were weighted and their volumes were determined by water displacement prior and after fluid infusion to achieve a pressure physiologically found in the liver veins. The liver volumes in the different perfusion states were calculated based on CT-data. The calculated values were compared with the volume measured by water displacement and the weight of the livers. RESULTS: Assessment of calculated CT volumes and water displacements at identical perfusion states showed a tight correlation and differed on average by 4 +/- 5 %. However, livers before and after fluid infusion showed a 33 +/- 8 % (350 +/- 150 ml) difference in volume. CONCLUSION: CT-volumetry acquires highly accurate data as confirmed by water displacement studies. However, the perfusion state has major impact on liver volume, which has to be accounted for in clinical use.

Animals↗
PubMed2003

Radiation exposure in multi-slice versus single-slice spiral CT: results of a nationwide survey.

Multi-slice (MS) technology increases the efficacy of CT procedures and offers new promising applications. The expanding use of MSCT, however, may result in an increase in both frequency of procedures and levels of patient exposure. It was, therefore, the aim of this study to gain an overview of MSCT examinations conducted in Germany in 2001. All MSCT facilities were requested to provide information about 14 standard examinations with respect to scan parameters and frequency. Based on this data, dosimetric quantities were estimated using an experimentally validated formalism. Results are compared with those of a previous survey for single-slice (SS) spiral CT scanners. According to the data provided for 39 dual- and 73 quad-slice systems, the average annual number of patients examined at MSCT is markedly higher than that examined at SSCT scanners (5500 vs 3500). The average effective dose to patients was changed from 7.4 mSv at single-slice to 5.5 mSv and 8.1 mSv at dual- and quad-slice scanners, respectively. There is a considerable potential for dose reduction at quad-slice systems by an optimisation of scan protocols and better education of the personnel. To avoid an increase in the collective effective dose from CT procedures, a clear medical justification is required in each case.

Data Collection↗
PubMed2002

[CT-expo--a novel program for dose evaluation in CT].

CT-Expo is a novel MS Excel application for assessing the radiation doses delivered to patients undergoing CT examinations, based on computational methods that were used to analyze the data collected in the German survey on CT practice in 1999. The program enables the calculation of all dose quantities of practical value, such as axial dose free-in-air, weighted CTDI, dose-length product, effective dose and uterine dose. In contrast to existing programs for CT dose assessment, CT-Expo offers a number of unique features, such as gender-specific dose calculation for all age groups (adults, children, newborns), applicability to all existing scanner models including correction of scanner-specific influences, and the possibility of comparison with the results from the German CT survey on CT practice. Three different application modules offer free and standardized dose calculations as well as a comprehensive benchmarking section including guidance on dose optimization. The program is available as shareware in both German and English version. Additional information and a demo version free of charge can be requested via e-mail from the author's address stamm.georg@mh-hannover. de) or from the web page http://www.mh-hannover.de/kliniken/radiologie/str_04.html.

Adult↗
PubMed2001

Precise determination of aortic length in patients with aortic stent grafts: in vivo evaluation of a thinning algorithm applied to CT angiography data.

The aim of this study was to develop a technique for precise determination of the aortic length using volumetric CT data for potential use prior to endovascular stent-graft placement. The study population consisted of 20 patients (38 measurements) with already performed endoluminal grafting. This allowed for in vivo evaluation of our technique. Its length according to the graft specifications served as a gold standard for our own measurements. The implemented graft length varied between 120 and 195 mm. Computed tomography angiography was performed with 3-mm slice collimation, 5-mm table feed and a reconstruction interval of 2 mm. Following semi-automatic segmentation of the aorta and its large side branches, the median centerline (skeleton) of the vessels was determined employing a modified three-dimensional thinning algorithm. The algorithm was validated by comparing the calculated length of the resulting skeleton with the specifications of the grafts. The calculated length was sufficiently precise despite the limiting reconstruction interval of 2 mm of our CT data which only permitted an assessment of stent length in 2-mm steps. The differences in the measured length and graft length were in the range between 0 and 8 mm (< 5%) with a mean fractional error of 2.46 +/- 2.37 mm. The use of an intelligent region growing algorithm capable of coping with variable arterial enhancement significantly reduced operator post-processing time. The average time necessary for segmentation was 7 min (range 3-10 min). Our algorithm provides a non-invasive method for objective and precise measurement of aortic length apparently even in tortuous vessels. It has the potential to replace angiography for aortic and iliac length measurements with calibrated catheters prior to endovascular intervention.

Algorithms↗
PubMed2000

[Individual planning of liver surgery interventions with a virtual model of the liver and its associated structures].

BACKGROUND: In an interdisciplinary approach, HepaVision (MEVIS, Bremen), a software tool specifically developed for 3D visualization of the liver, was employed for individual planning of extensive liver resections and evaluation of living-relative donations. So far there is experience with more than 50 biphasic spiral CT examinations. RESULTS: The spatial relationship of large tumors to crucial hepatic structures, the demonstration and evaluation of anatomic variants regarding vascular supply and the risk stratification of liver failure by volumetric analysis on the basis of portal venous drainage supported precise indication for surgery. Surgical safety is increased by preoperative planning and simulation of necessary vessel reconstructions. By hiding selective areas of portal venous drainage or applying freely selectable clip planes, segmental as well as non-anatomical resections can be simulated and their effects analyzed. The virtual preoperative situs was confirmed intraoperatively in all 17 patients of our study population who underwent segmental liver resection for either a tumor or living-relative donation.

Adenoma, Liver Cell↗
PubMed2000

[Basic principles of data acquisition and data processing for construction of high quality virtual models].

Creating models for virtual reality subdivides into several steps. The aim of the data acquisition is the extraction of nearly isotropic (same solution in all three axes) data sets with low noise content. An approximate isotropy can be achieved by suitable choice of scan parameters. For raw data reconstruction, the application of high-resolution reconstruction algorithms is prohibited due to increased noise. A missing isotropy can computationally be approximated by interpolation. Further noise suppression is achieved by applying filters. Additionally, the contrast of the object for segmentation can be increased by image processing operators. The correct choice of the segmentation method and the editing tools is essential for a precise segmentation with minimal user interaction. Prior to visualization, smoothing the shape of the segmented model (shape-based or morphological interpolation, polygon reduction of wire frame model) further improves the visual appearance of the 3D model.

Algorithms↗
PubMed1999

[3-dimensional (3-D) visualization of the liver for planning extensive liver resections].

Preoperative planning of liver resections in patients with liver tumors is based on sonography, computed tomography and magnetic resonance imaging. A new three-dimensional (3D) visualization program was developed based on CT data. This visualization program was used for preoperative planning in 6 patients with liver tumors in problematic intrahepatic localizations. In 5 out of 6 patients the liver resection could be performed as preoperatively planned. The intraoperative findings agreed with the 3D visualization in all these patients. 3D Visualization of the liver allowed clear and interactive planning of liver resections and improved the preparation of complex liver resections.

Adenoma, Liver Cell↗
PubMed1998

[Comparison of film-screen combinations in contrast-detail diagram and with interactive image analysis. 3: Trimodal histograms of gray scale distribution in bar groups of lead pattern images].

The following four screen film combinations were compared: a) a combination of anticrossover film and UV-light emitting screens, b) a combination of blue-light emitting screens and film and c) two conventional green fluorescing screen film combinations. Radiographs of a specially designed plexiglass phantom (0.2 x 0.2 x 0.12 m3) with bar patterns of lead and plaster and of air, respectively were obtained using the following parameters: 12 pulse generator, 0.6 mm focus size, 4.7 mm aluminum prefilter, a grid with 40 lines/cm (12:1) and a focus-detector distance of 1.15 m. Image analysis was performed using an Ibas system and a Zeiss Kontron computer. Display conditions were the following: display distance 0.12 m, a vario film objective 35/70 (Zeiss), a video camera tube with a PbO photocathode, 625 lines (Siemens Heimann), an Ibas image matrix of 512 x 512 pixels with a spatial resolution of ca. 7 cycles/mm, the projected matrix area was 5000 micron 2. Maxima in the histograms of a grouped bar pattern were estimated as mean values from the bar and gap regions ("mean value method"). They were used to calculate signal contrast, standard deviations of the means and scatter fraction. Comparing the histograms with respect to spatial resolution and kV setting a clear advantage of the UVR system becomes obvious. The quantitative analysis yielded a maximum spatial resolution of approx. 3 cycles/mm for the UVR system at 60 kV which decreased to half of this value at 117 kV caused by the increasing influence of scattered radiation. A ranking of screen-film systems with respect to image quality and dose requirement is presented. For its evaluation an interactive image analysis using the mean value method was found to be superior to signal/noise ratio measurements and visual analysis in respect to diagnostic relevance and saving of time.

Humans↗
PubMed1997

[Comparison of film-screen combinations with contrast detail diagram and interactive image analysis. 2: Linear assessment of grey scale ranges with interactive image analysis].

The following three screen-film combinations were compared: a) a combination of anticrossover film and UV-light emitting screens, b) a combination of blue-light emitting screens and film, and c) a conventional green fluorescing screen-film combination. Radiographs of a specially designed plexiglass phantom (0.2 x 0.2 x 0.12 m3) with bar patterns of lead and plaster and of air, respectively were obtained using the following parameters: 12 pulse generator, 0.6 mm focus size, 4.7 mm aluminum pre-filter, a grid with 40 lines/cm (12:1) and a focus-detector distance of 1.15 m. Image analysis was performed using an IBAS system and a Zeiss Kontron computer. Display conditions were the following: display distance 0.12 m, a vario film objective 35/70 (Zeiss), a video camera tube with a PbO photocathode, 625 lines (Siemens Heimann), an IBAS image matrix of 512 x 512 pixels with a resolution of 7 lines/mm, the projected matrix area was 5000 microns2. Grey scale ranges were measured on a line perpendicular to the grouped bar patterns. The difference between the maximum and minimum density value served as signal. The spatial resolution of the detector system was measured when the signal value was three times higher than the standard deviation of the means of multiple density measurements. The results showed considerable advantages of the two new screen-film combinations as compared to the conventional screen-film combination. The result was contradictory to the findings with pure visual assessment of thresholds (part I) that had found no differences. The authors concluded that (automatic) interactive image analysis algorithms serve as an objective measure and are specifically advantageous when small differences in image quality are to be evaluated.

Algorithms↗
PubMed1994

Sensitive method for the assay of sertindole in plasma by high-performance liquid chromatography and fluorimetric detection.

A simple and highly sensitive normal-phase HPLC method is described for determining sertindole concentrations in human plasma using fluorimetric detection. A short C8 column was used to extract sertindole and the internal standard from plasma; the column was rinsed with acetonitrile, and the analytes were recovered by elution with methanol. This uncommon selectivity between the two solvents allowed clean extraction and near- quantitative recovery of the analytes (> 89%). Separation was done on a 5-microns silica-gel column and detection was performed by fluorimetry, with emission at 340 nm and excitation at 260 nm. The detection and lower quantifiable limits were 0.01 and 0.025 ng/ml, respectively, with no interference from plasma or potential metabolites.

Antipsychotic Agents↗
PubMed1989

[A new beta 1-receptor blocker in the therapy of essential hypertension and angina pectoris].

In 242 patients with hypertension and/or angina pectoris, a new cardioselective betablocker without ISA, bisoprolol (Concor), was tested. The average mean value of 168/102 mm Hg was lowered in the 174 hypertensive patients by a systolic value of 17 and a diastolic value of 11 mm Hg. A normal diastolic pressure of 95 mm Hg or below was attained within 4 weeks in 73% of patients. Angina pectoris improved from 7 attacks per week before treatment to 3 attacks after 2 weeks; patients with additional hypertension showed a further improvement after another two weeks to an average of 1.7 attacks per week. Side effects were most frequently dizziness, headache and fatigue and also a few patients with gastrointestinal symptoms, an unusual side effect with this treatment. The results show the effective antihypertensive and antianginal action of bisoprolol in a large group of outpatients.

Adrenergic beta-Antagonists↗
PubMed1981

Functional characterization of central alpha-adrenoceptors by yohimbine diastereomers.

Rat occipital cortex slices were preincubated with [3H]noradrenaline and then superfused with medium containing 30 micrometer cocaine. They were stimulated electrically at 3 Hz. Unlabelled noradrenaline (0.1 micrometer), alpha-methyl-noradrenaline (0.01-0.1 micrometer), xylazine (0.1-10 micrometer) and guanabenz (0.01 micrometer) decreased, whereas yohimbine (0.01-1 micrometer), rauwolscine (0.01-1 micrometer), corynanthine (10 micrometers), tolazoline (0.1-10 micrometers) and azapetine (0.1-1 micrometer) increased the stimulation-evoked overflow of tritium without a change in basal outflow. Pseudoyohimbine and prazosin at up to 0.1 micrometer did not change the evoked overflow, and a higher concentrations enhanced the basal outflow of tritium. In vivo, yohimbine 10 mg/kg and rauwolscine 10 mg/kg markedly, and corynanthine 10 mg/kg slightly accelerated the alpha-methyltyrosine-induced disappearance of noradrenaline from rat whole brain. Yohimbine and rauwolscine but not corynanthine also accelerated the alpha-methyltyrosine-induced depletion of dopamine. The results add four compounds to the list of drugs with alpha-adrenoceptor affinity which inhibit (agonists) or facilitate (antagonists) action-potential-evoked release of noradrenaline in rat brain cortex. The presynaptic receptors are of the alpha 2-type. The receptors which control the activity of noradrenaline and dopamine neurons in vivo also appear to be alpha 2.

Animals↗