Biomedical subjects

Gunnar Brix

Publications and source records attributed to Gunnar Brix.

14 recordsLinked to original sources

PubMed2006

Tracer kinetic analysis of signal time series from dynamic contrast-enhanced MR imaging.

Rapid magnetic resonance imaging (MRI) makes it possible to detect the fast kinetics of tissue response after intravenous administration of a paramagnetic contrast medium (CM), reflecting the status of tissue microcirculation. In this paper, the basic physical and tracer kinetic principles of dynamic relaxivity and susceptibility contrast-enhanced MRI are reviewed. Quantitative analysis of data acquired is broken up into an MR-specific part, in which the signal variation observed is related to the CM concentration in the tissue, and an MR-independent part, in which the computed concentration time series are analyzed by tracer kinetic modeling to estimate well-defined physiological tissue parameters. The clinical application of dynamic MRI techniques is demonstrated by two representative studies.

Algorithms↗

PubMed2005

Biodistribution and pharmacokinetics of the (19)F-labeled radiosensitizer 3-aminobenzamide: assessment by (19)F MR imaging.

3-Aminobenzamide (3-ABA) is a potent radiosensitizer that inhibits the repair of DNA strand breaks. The aim of this study was to monitor the biodistribution and pharmacokinetics of a fluorinated 3-ABA derivative in tumor-bearing rats by magnetic resonance imaging (MRI). To this end, 3-ABA was labeled with fluorine-19 by trifluoroethylation [3-amino-N-2,2,2-trifluoroethylbenzamide (3-ABA-TFE)], which only slightly increased the cytotoxicity of the compound as demonstrated by colony-forming assays. After intraperitoneal injection of 400 mg/kg BW 3-ABA-TFE to nine Copenhagen rats with Dunning prostate adenocarcinoma, (19)F MR images were acquired at a whole-body MR system with a spatial sampling of 10 x 10 x 15 mm(3). While 3-ABA-TFE was observed in all major organs and the muscular system, only a small and heterogeneous signal could be detected in the adenocarcinoma. Serial MR measurements yielded maximum tissue signals about 2 days after 3-ABA-TFE administration. At this time point, the mean muscle-to-liver and tumor-to-liver signal ratio was 0.31+/-0.07 and 0.11+/-0.04, respectively. Application of the (19)F MRI strategy makes it possible to measure the biodistribution and pharmacokinetics of 3-ABA-TFE in individual animals in a longitudinal manner. The results obtained for the prostate adenocarcinoma indicate that delivery of 3-ABA-TFE to solid tumors may be seriously hampered by tumor-specific factors and that the intratumoral uptake of the substance may be lower than in normal tissues. Therefore, the development of effective carrier systems is mandatory to improve tumor-selective delivery.

Adenocarcinoma↗

PubMed2005

Improved vascular opacification in cerebral computed tomography angiography with 80 kVp.

PURPOSE: We sought to intraindividually compare computed tomography angiographies (CTAs) acquired at 80 kVp and 120 kVp with respect to vessel contrast, noise level, and radiation dose. MATERIAL AND METHODS: CTA was performed on a single-slice CT scanner using tube voltages of 80 kVp and 120 kVp in 29 patients with arteriovenous malformations. Mean Hounsfield Units (HU) were evaluated for different vessels and brain parenchyma. To determine contrast-to-noise ratios (CNRs), noise levels were estimated from phantom measurements. RESULTS: The calculated effective dose to male/female patients was 0.4/0.5 mSv for 80 kVp and 0.7/0.8 mSv for 120 kVp. CT density in blood vessels was between 297 and 458 HU for 80 kVp and 152 and 229 HU for 120 kVp (P<0.0001). Despite an increased noise level in the low-voltage images, the CNR was 26-59% higher at 80 kVp than at 120 kVp (P<0.05). CONCLUSION: The use of a reduced tube potential leads to improved CNR in CTA of the cerebral vasculature and a markedly reduced radiation exposure to patients.

Adult↗

PubMed2005

Classification of signal-time curves obtained by dynamic magnetic resonance mammography: statistical comparison of quantitative methods.

OBJECTIVE: This study compares the performance of quantitative methods for the characterization of signal-time curves acquired by dynamic contrast-enhanced magnetic resonance mammography from 253 females. MATERIALS AND METHODS: Signal-time curves obtained from 105 parenchyma, 162 malignant, and 91 benign tissue regions were examined (243 lesions were histopathologically validated). A neural network, a nearest-neighbor, and a threshold classifier were applied to either the entire signal-time curve or pharmacokinetic and descriptive parameters calculated from the curves to differentiate between 2 (malignant or benign) or 3 tissue classes (malignant, benign, or parenchyma). The classifiers were tuned and evaluated according to their performance on 2 distinct subsets of the curves. RESULTS: The accuracy determined for the neural network and the nearest-neighbor classifiers was nearly identical (approximately 80% in case of 3 tissue classes, and approximately 76% in case of the 2 classes). In contrast, the accuracy of the threshold classifier applied to the discrimination of 3 classes was low (65%). CONCLUSION: Quantitative classifiers can support the radiologist in the diagnosis of breast lesions.

Breast↗

PubMed2005

Radiation exposure of patients undergoing whole-body dual-modality 18F-FDG PET/CT examinations.

UNLABELLED: We investigated radiation exposure of patients undergoing whole-body 18F-FDG PET/CT examinations at 4 hospitals equipped with different tomographs. METHODS: Patient doses were estimated by using established dose coefficients for 18F-FDG and from thermoluminescent measurements performed on an anthropomorphic whole-body phantom. RESULTS: The most relevant difference between the protocols examined was the incorporation of CT as part of the combined PET/CT examination: Separate low-dose CT scans were acquired at 2 hospitals for attenuation correction of emission data in addition to a contrast-enhanced CT scan for diagnostic evaluation, whereas, at the other sites, contrast-enhanced CT scans were used for both purposes. Nevertheless, the effective dose per PET/CT examination was similar, about 25 mSv. CONCLUSION: The dosimetric concepts presented in this study provide a valuable tool for the optimization of whole-body 18F-FDG PET/CT protocols. Further reduction of patient exposure can be achieved by modifications to the existing hardware and software of PET/CT systems.

Body Burden↗

PubMed2004

Microcirculation and microvasculature in breast tumors: pharmacokinetic analysis of dynamic MR image series.

The purpose of this study was to quantify microcirculation and microvasculature in breast lesions by pharmacokinetic analysis of Gd-DTPA-enhanced MRI series. Strongly T1-weighted MR images were acquired in 18 patients with breast lesions using a saturation-recovery-TurboFLASH sequence. Concentration-time courses were determined for blood, pectoral muscle, and breast masses and subsequently analyzed by a two-compartment model to estimate plasma flow and the capillary transfer coefficient per unit of plasma volume (F/VP, KPS/VP) as well as fractional volumes of the plasma and interstitial space (fP, fI). Tissue parameters determined for pectoral muscle (fP = 0.04 +/- 0.01, fI = 0.09 +/- 0.01, F/VP = 2.4 +/- 1.3 min(-1), and KPS/VP = 1.2 +/- 0.5 min(-1)) and 10 histologically proven carcinomas (fP = 0.20 +/- 0.07, fI = 0.34 +/- 0.16, F/VP = 2.4 +/- 0.7 min(-1), and KPS/VP = 0.86 +/- 0.62 min(-1)) agreed reasonable well with literature data. Best separation between malignant and benign lesions was obtained by the ratio KPS/F (0.35 +/- 0.17 vs. 1.23 +/- 0.65). The functional imaging technique presented appears promising to quantitatively characterize tumor pathophysiology. Its impact on diagnosis and therapy management of breast tumors, however, has to be evaluated in larger patient studies.

Breast Neoplasms↗

PubMed2004

Quantification of pulmonary blood flow and volume in healthy volunteers by dynamic contrast-enhanced magnetic resonance imaging using a parallel imaging technique.

RATIONALE AND OBJECTIVES: We sought to optimize the dosage of a paramagnetic contrast medium (CM) for the quantification of pulmonary blood flow and volume by contrast-enhanced dynamic magnetic resonance imaging (MRI) using a parallel imaging technique and to prove the feasibility of the approach in healthy volunteers. METHODS: In a phantom study, the dependency of signal increase on different concentrations of the CM gadodiamide was evaluated by means of an ultra-fast MRI sequence with a generalized autocalibrating partially parallel acquisition technique (acceleration factor = 2). Using the same sequence, measurements were performed in a healthy volunteer after administration of different CM dosages for contrast dosage optimization in vivo. Finally, perfusion measurements were performed in 16 healthy volunteers after the administration of the optimal CM dose. Signal-time curves were evaluated from the pulmonary artery and from predefined regions of the lung. Pulmonary regional blood volume (RBV) and flow (RBF) were estimated using an open 1-compartment model. RESULTS: Phantom studies yielded a linear signal increase up to a concentration of 5.0 mmol/L gadodiamide. Results of contrast dosage optimization in vivo showed that the maximum CM dose providing a linear relationship between signal increase and CM concentration in the pulmonary artery of a healthy volunteer was approximately 0.05 mmol/kg-bw. Quantification of pulmonary blood volume and flow was reproducible in healthy volunteers, yielding mean values for the upper lung zones of 7.1 +/- 2.3 mL/100 mL for RBV and 197 +/- 97 mL/min/100 mL for RBF and for lower lung zones, 12.5 +/- 3.9 mL/100 mL for RBV and 382 +/- 111 mL/min/100 mL for RBF. CONCLUSIONS: If an adequate amount of gadodiamide and fast MR sequences are used, quantification of pulmonary blood flow and volume is feasible.

Adult↗

PubMed2003

Quantification of renal perfusion using an intravascular contrast agent (part 1): results in a canine model.

In this work absolute values of regional renal blood volume (rRBV) and flow (rRBF) are assessed by means of contrast-enhanced (CE) MRI using an intravascular superparamagnetic contrast agent. In an animal study, eight foxhounds underwent dynamic susceptibility-weighted MRI upon injection of contrast agent. Using principles of indicator dilution theory and deconvolution analysis, parametric images of rRBV, rRBF, and mean transit time (MTT) were computed. For comparison, whole-organ blood flow was determined invasively by means of an implanted flow probe, and the weight of the kidneys was evaluated postmortem. A mean rBV value of 28 ml/100 g was found in the renal cortex, with a corresponding mean rBF value of 524 ml/100 g/min and an average MTT of about 3.4 s. Although there was a systematic difference between the absolute blood flow values determined by MRI and the ultrasonic probe, a significant correlation (r(s) = 0.72, P < 0.05) was established. The influence of the arterial input function (AIF), T(1) relaxation effects, and repeated measurements on the precision of the perfusion quantitation is discussed.

Animals↗

PubMed2003

[Acquisition and assessment of patient exposure in diagnostic radiology and nuclear medicine].

The regular annual monitoring of patient exposure in radiation diagnostics, as performed by the Federal Office for Radiation Protection (BfS), plays an important role in evaluating the awareness of radiological quality and safety in Germany and the risk-benefit optimization for patients. For the reporting year 1997, X-ray diagnostics resulted in a mean effective dose of 2 +/- 0.5 mSv per head of population. The underlying frequency of medical X-ray examinations was approximately 136 million, i.e., 1.7 examinations annually per head of population. In terms of nuclear medicine diagnostics, the patients exposure amounted to approximately 0.15 mSv effective dose per head of population. In this case, the number of examinations amounted to approximately 4 million, corresponding to a frequency of approximately 0.05 examinations annually per head of population. The paper discusses factors influencing the calculation of exposure, as well as the lack of an internationally accepted protocol to evaluate patient exposure.

Environmental Exposure↗

PubMed2002

Fast implementation of the single scatter simulation algorithm and its use in iterative image reconstruction of PET data.

In positron emission tomography (PET), scatter correction is usually performed prior to image reconstruction using a more or less exact model of the scatter processes. These models require estimates of the true activity and object density distributions of the imaged object. The problem is that these estimates are computed from measured data and, therefore, already contain scattered events. The purpose of this work was to overcome this problem by incorporating scatter characteristics directly into the process of iterative image reconstruction. This could be achieved by an optimized implementation of the single scatter simulation (SSS) algorithm, which results in a significant speed-up of the scatter estimation procedure. The scatter simulation was then included in the forward projection step of maximum likelihood image reconstruction. The results demonstrate that this approach leads to a more exact estimation of the scatter component which cannot be obtained by a simple sequential data processing strategy.

Algorithms↗

PubMed2002

A survey of PET activity in Germany during 1999.

Positron emission tomography (PET) is the most powerful molecular imaging technique currently available for clinical use. The aim of this study was to provide public health information on PET procedures carried out in Germany - a country with a very high number of PET installations. To this end, all facilities that in 1999 were running at least one dedicated PET system were contacted and requested to provide information in a questionnaire on the radiopharmaceuticals applied, the total number and age distribution of patients and volunteers examined, the main diagnostic applications and the range of administered activities. Based on the information provided by 48 of the 60 PET facilities in Germany, an annual frequency of about 0.34 PET procedures per 1,000 inhabitants was estimated, associated with an annual per capita effective dose of about 1.9 micro Sv. Averaged over all PET procedures, the mean effective dose to patients was 5.6 mSv. The age distribution of patients and volunteers was skewed markedly towards higher ages; only a very small fraction (<3%) of patients were children younger than 15 years while older patients, and especially those in the age group between 41 and 65 years, were overrepresented relative to the general population. In total, 28 different PET radiopharmaceuticals were used, with only half of these having been administered to more than 20 patients each. The most frequently applied radiotracer was the glucose analogue 2-[(18)F]fluoro-2-deoxy- D-glucose (FDG), which was utilised in more than 84% of all PET procedures. For this tracer, the median value for activities applied for examinations in the three-dimensional (3D) acquisition mode was only half of that used for two-dimensional (2D) measurements. Based on a statistical analysis of the distribution of mean FDG activities administered to patients in the 48 PET facilities who responded to our inquiry, diagnostic reference levels of 370 and 200 MBq are proposed for the 2D and the 3D mode, respectively.

Age Distribution↗

PubMed2002

Neural network-based segmentation of dynamic MR mammographic images.

The usefulness of neural networks for the classification of signal-time curves from dynamic MR mammography was recently demonstrated by our group. The multi-layer perceptron under study consists of 28 input, 4 hidden, and 3 output nodes, and was trained to classify signal-time curves into three tissue classes: "carcinoma," "benign lesion," and "parenchyma." Extending this approach, it was the aim of the present study to evaluate the performance of the developed network in the segmentation of dynamic MR mammographic images in comparison to a pixel-by-pixel two-compartment pharmacokinetic analysis. The population investigated in this pilot study comprised 15 women with suspicious lesions in the breast, which were confirmed histologically after the MR examination. The neural network classified the same areas as malignant as those which were marked as being highly suspicious by the pharmacokinetic mapping approach but with the advantage that no a priori knowledge on tissue microcirculation was needed, that computation proved to be much faster, and that it yielded a unique classification into just three tissue classes.

Breast↗

PubMed2002

Estimation of heat transfer and temperature rise in partial-body regions during MR procedures: an analytical approach with respect to safety considerations.

In order to assess thermal response to RF exposure during MR procedures at the tissue level, simple analytical solutions to the non-stationary Pennes' bio-heat equation were obtained using the Green's function approach. Two thermal models appropriate for partial-body exposure were analyzed: In the first model, the temperature field at the periphery of an idealized volume RF resonator was modeled. The analytical solution reveals that tissue response to RF heating is characterized by an equilibration time and length. Both parameters are inversely related to tissue perfusion and vary for the soft-tissues considered between 0.27-25 min and 1.5-12 mm, respectively. None of the tissues investigated increase in temperature more than 0.5 degrees C for each W/kg of power dissipated. Secondly, a homogeneous tissue solution was derived that predicts the temperature-time course to an MR examination with time-varying specific absorption rates (SAR). Since SAR limits indicated in current MR safety standards relate to running SAR averages computed over an appropriate period of time, an expression was formulated that gives an upper limit for the temperature rise averaged over the same period of time, as a function of both the upper limit of running SAR averages and the duration of the MR examination. The analysis revealed that the partial-body SAR limits indicated in the IEC standard may not guarantee under all circumstances compliance with the basic restrictions concerning temperature rise.

Body Temperature↗

PubMed2002

Dynamic MR mammography: three-dimensional real-time visualization of contrast enhancement in virtual reality.

RATIONALE AND OBJECTIVES: In view of the increasing use of breast magnetic resonance (MR) imaging to supplement x-ray mammography. the authors developed a method for fast and efficient analysis of dynamic MR images of the female breast. MATERIALS AND METHODS: The MR image data sets were acquired with a saturation-recovery turbo fast low-angle shot sequence to detect the kinetics of the contrast agent concentration in the whole breast at a high temporal and spatial resolution. A morphologic three-dimensional fast low-angle shot data set was also acquired. The dynamic image data sets were analyzed with tracer kinetic modeling to describe the physiologic processes underlying the contrast enhancement in mathematical terms and enable the estimation of functional tissue-specific parameters, which reflect the status of microcirculation. To display morphologic and functional tissue information simultaneously, the authors developed a multidimensional real-time visualization system (with three-dimensional texture mapping), which enables a practical and intuitive human computer interface in virtual reality. RESULTS: The spatially differentiated representation of the computed functional tissue parameters superimposed on the anatomic information offers several possibilities: (a) more discernible contrast enhancement, (b) inspection of the data volume in three-dimensional space by means of rotation and transparency variation, (c) location of lesions in space and thus faster and more natural recognition of topologic coherencies, and (d) fast and efficient overview in compressed form. CONCLUSION: A feasibility study demonstrated that multidimensional visualization of contrast enhancement in virtual reality is a practicable idea. Detection and location of multiple breast lesions may be an important application.

Breast↗