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G Brix

Publications and source records attributed to G Brix.

At least 73 records · Page 4Linked to original sources
PubMed1997

[Morphologic MR imaging with T1-weighted sequences for radiotherapy goal volume definition of intracranial tumors. Comparison with FLASH-Turbo-FLASH and SE sequences].

INTRODUCTION: The goal of this study was to compare contrast-enhanced T1-weighted Flash and Turbo-Flash sequences with conventional spin-echo sequences as a basis for planning high-precision radiotherapy. METHODS: A total of 25 consecutive patients with different intracranial tumors and a disrupted blood-brain barrier were studied. T1-weighted Flash, Turbo-Flash and conventional spin-echo images were evaluated after controlled 30-s infusion of 0.1 mmol/kg body weight of Gd-DTPA. The evaluation of the three sequences included the measurement of the spinal- and contrast-to-noise ratios, the visual inspection of the tumors and artifacts, and the measurement of tumor size. RESULTS: The signal- and contrast-to-noise ratios were significantly (P < 0.05-0.01) lower for Flash and Turbo-Flash than for conventional spin-echo sequences. However, visual inspection of the contrast-enhancing tumors revealed in 23 and 24 of 25 lesions on Flash and Turbo-Flash images, respectively, good or very good tumor visibility when compared with conventional spin-echo images with a reduction of imaging time by a factor of 7-8. Flash and Turbo-Flash sequences were more prone to susceptibility artifacts, conventional spin-echo sequences more to pulsation artifacts in the posterior fossa. Tumor sizes were comparable in all three techniques. CONCLUSION: At present, conventional spin-echo images are superior to fast Flash and ultrafast Turbo-Flash sequences as a basis for accurate target volume definition in high-precision radiotherapy. However, fast Flash and Turbo-Flash images may be a practicable alternative to conventional spin-echo images for tumors in the posterior fossa or in patients unable to tolerate a stereotactic fixation device. Despite some limitations, Turbo-Flash sequences enable fast dynamic MR imaging combined with an acceptable morphology, which may be sufficient to target volume planning in high-precision radiotherapy.

Adult↗
PubMed1997

[Methodological approaches to quantitative evaluation of microcirculation in tissues with dynamic magnetic resonance tomography].

The development of rapid magnetic resonance imaging (MRI) sequences makes it possible to detect the fast kinetics of tissue response after intravenous administration of paramagnetic contrast media (CM), reflecting the status of tissue microcirculation. In this paper, the basic physical and tracer kinetic principles of dynamic relativity and susceptibility contrast MRI techniques are reviewed. The quantitative analysis of the acquired dynamic image data is broken up into an MR specific part, in which the observed signal variations are related to the CM concentration in the tissue, and an MR independent part, in which the computed concentration-time-courses are analyzed by tracer kinetic modeling. The purpose of the applied models is to describe the underlying physiological processes in mathematical terms and thus to enable the estimation of tissue specific parameters from measured dynamic image series. Whereas the capillary permeability can be estimated from dynamic relativity contrast enhanced MRI studies, the regional blood volume as well as the regional blood flow can be determined from dynamic susceptibility contrast enhanced image series. However, since there are no intravascular but only diffusible CM available at present, the application of the susceptibility technique is currently restricted to brain tissues with intact blood brain barrier. The practical realization of both dynamic MRI techniques is demonstrated by case studies.

Blood Flow Velocity↗
PubMed1997

[Quality assurance in roentgen mammography. Comparison of recommended EUREF guidelines with relevant German regulations].

The Program "Europe against Cancer" published the 2nd edition of quality assurance guidelines for breast cancer screening in June 1996. For the enforcement of these guidelines, a European network of reference centres (EUREF) is being established. Although the EUREF protocol contains guidelines for all disciplines involved in breast cancer screening, this article concentrates on the physical and technical aspect. The comparison with the German regulations (DIN Norms) demonstrates the high requirements requested by the EUREF guidelines with its tighter limits and more extensive and more frequent tests.

Breast Neoplasms↗
PubMed1997

Contrast-enhanced magnetization transfer imaging: improvement of brain tumor conspicuity and delineation for radiosurgical target volume definition.

PURPOSE: To assess the contrast-noise-ratio (CNR), and thus tumor conspicuity and delineation, on contrast-enhanced T1-weighted magnetization transfer (MT) images compared to conventional T1-weighted spin echo (SE) images as a strategy to improve definition of the macroscopic boost volume in radiosurgery treatment planning in patients with high grade gliomas or metastatic brain lesions. MATERIALS AND METHODS: Fifty patients (mean age, 51 years) with histologically proven or suspected high grade glioma or cerebral metastases were prospectively examined by MR imaging. Following gadolinium dimeglumine administration (0.1 mmol/kg body weight) the brain was imaged with both a T1-weighted MT-fast low angle shot (FLASH) pulse sequence and with a conventional T1-weighted SE sequence without MT saturation. Lesion conspicuity, size and CNR were compared for both techniques. RESULTS: The mean tumor diameter of malignant gliomas was significantly (P < 0.01) larger when measured on T1-weighted MT-FLASH images compared to T1-weighted SE images and was comparable for metastatic lesions. The mean CNR of enhancing lesions on T1-weighted MT-FLASH was 14 +/- 5 compared to 10 +/- 4 on SE images, representing a significant (P < 0.05) improvement. Lesion conspicuity and delineation was improved in 10 of 20 patients (50%) with high grade gliomas and in 15 of 30 patients (50%) with metastases. Additional contrast enhancing lesions were detected in 8 of 30 patients (27%) with metastases on MT-FLASH images. Lesion conspicuity was markedly improved in the posterior fossa. DISCUSSION: Contrast-enhanced T1-weighted MT-FLASH images improve lesion detection and delineation in the planning process of radiosurgery in patients with intracranial high grade gliomas or metastases and may even alter the treatment approach.

Adult↗
PubMed1997

Intracranial meningeomas: time- and dose-dependent effects of irradiation on tumor microcirculation monitored by dynamic MR imaging.

The purpose of this study was the characterization of the time- and dose-dependent effects of irradiation on tumor microcirculation by means of dynamic MR imaging and correlation of the estimated data with tumor response in patients with meningeomas. Dynamic MR imaging studies were performed in 20 patients with intracranial meningeomas prior to (n = 20) and at 6 (n = 17), 18 (n = 17), and 50 wk (n = 14) after the end of radiotherapy. In seven of these patients, dynamic measurements were also performed during fractionated radiotherapy after approximate 20 Gy and 54 Gy. During and after short-time infusion of gadopentetate dimeglumine, the kinetics of lesion response was resolved using a strongly T1-weighted saturation recovery TurboFLASH (SRTF) sequence. The signal-time courses of the suspected lesions were analyzed using a pharmacokinetic two-compartment model. The calculated parameters amplitude A (reflecting gadopentetate dimeglumine accumulation in the extracellular space) and exchange rate constant k21 (depending on vascular permeability and blood flow) were displayed as color-coded images and analyzed as a function of time of therapy and radiation dose. All meningeomas showed a high exchange rate constant k21 (median, 5.7 min-1; range, 1.9-23.0 min-1) and a high amplitude A (median, 1.5 arbitrary units; range, 1.1-2.7) prior to X-ray treatment. During radiotherapy we found a dose related significant (p < .01) increase of k21 accompanied by an increase of the amplitude A as compared to the pretreatment values. Analysis of tumor volume 6, 18, and 50 wk after X-ray treatment revealed two different groups. In the responder group (n = 13) the median of the tumor volume decreased from 10.0 to 7.5 cm3. For this group, we found a significant drop (p < .01) of the median of the amplitude A and a decrease of the exchange rate constant k21. In the nonresponder group (n = 4) the median of the tumor volume increased after radiation from 3.5 to 4.5 cm3. The pharmacokinetic analysis revealed a decrease of the amplitude A-and an increase of the exchange rate constant k21. The response of meningeomas to radiotherapy is influenced by the effect of X-rays on tumor microcirculation. This effect on tumor microcirculation can be derived by analysis of pharmacokinetic maps obtained from dynamic MR images. Furthermore, these pharmacokinetic maps can possibly be used to differentiate groups of patients who respond or do not respond to radiotherapy and, thus, could benefit from another treatment modality.

Contrast Media↗
PubMed1997

Functional MR imaging of the prefrontal cortex: specific activation in a working memory task.

Functional magnetic resonance imaging was used to identify cortical regions activated by a working memory task involving letter detection. Twenty four normal subjects were scanned with a conventional 1.5-T magnet while performing one of two tasks: In the activation task, subjects responded by pressing a button whenever any presented letter was the same as the second last in the sequence. In the control condition, subjects had to respond to a single predefined letter without memory update requirements. The activation task and the control condition were identical with regard to perceptual input and motor output. They were different only regarding the task demand. Movement artifacts were minimized in a two way strategy and eight subjects were excluded from further analysis. Functional MR data from the remaining 16 subjects were analyzed on the basis of anatomical regions-of-interest which were manually defined in each subject. The engagement of working memory produced significant activation in the dorsolateral prefrontal cortex (Brodmann's areas 9, 10, 46, and 47) in both hemispheres. Results demonstrate the applicability of the paradigm within a clinical MRI setup and corroborate previous findings of non-lateralized dorsolateral prefrontal activation during continuous context updating and active maintenance.

Adult↗
PubMed1997

Serial MR imaging of intracranial metastases after radiosurgery.

PURPOSE: To evaluate the spatiotemporal evolution of radiosurgical induced changes both in metastases and in normal brain tissue adjacent to the lesions by serial magnetic resonance (MR) imaging. METHODS AND MATERIALS: Thirty-five intracranial metastases of different primaries were treated in 25 patients by single high-dose radiosurgery. MR images acquired before radiosurgery were available in all patients. Sixty-three follow-up MR studies were performed in these patients including T2- and contrast-enhanced T1-weighted MR images. The average follow-up time was 9 +/- 5 months (mean +/- standard deviation [SD]). Based on contrast-enhanced T1-weighted MR images, tumor response was radiologically classified in the following four groups: stable disease was assumed if the average tumor diameter after treatment did not show a tumor shrinkage of more than 50% and an increase of more than 25%, partial remission as a shrinkage of tumor size of more than 50%, a disappearance of contrast-enhancing tumor as a complete remission, and an increase of tumor diameter of more than 25% as tumor progress. Moreover, we analysed signal changes on T2-weighted images in brain parenchyma adjacent to the enhancing metastases. RESULTS: The overall mean survival time was 10.5 +/- 7 months, with a 1-year actuarial survival rate of 40%. Stable disease, partial or complete remission of the metastatic tumor was observed in 22 patients (88%). Central or homogeneous loss of contrast enhancement appeared to be a good prognostic sign for stable disease or partial remission. This association was statistically significant (p < 0.05). Three patients (12%) suffered from tumor progression. In eight patients (32%) with stable disease or partial remission, signal changes on T2-weighted images were observed in tissue adjacent to the contrast enhancing lesions. A progression of the high signal on T2-weighted images was seen in seven of the eight patients between 3 and 6 months after therapy, followed by a signal regression 6-18 months after irradiation. CONCLUSION: MR imaging is a sensitive imaging tool to evaluate tumor response as well as the presence or absence of adjacent parenchymal changes following radiosurgery. Loss of homogeneous or central contrast enhancement on Gd-enhanced MR images appeared to be a good prognostic sign for tumor response. Tumor shrinkage seems not to be dependent on time. In addition, most cases of radiation induced changes in normal brain parenchyma observed on T2-weighted images seem to be self limited.

Brain↗
PubMed1997

Iterative reconstruction of PET images using a high-overrelaxation single-projection algorithm.

An iterative image reconstruction procedure is described which is able to calculate high-precision images within eight iterative steps. High initial overrelaxation parameters are used which drop down towards about one during continuing iteration. The parameters are determined pragmatically, postulating-maximum gain of image quality during a sequence of iterative steps. Results with simulated and measured data show that parameters derived from one data set may be widely used for other data sets. The acceleration of iterative reconstruction achieved by the estimation of optimum overrelaxation parameters is important for large data sets, especially for fully 3D reconstruction.

Algorithms↗
PubMed1997

PET 2-fluoro-2-deoxyglucose uptake in rat prostate adenocarcinoma during chemotherapy with gemcitabine.

UNLABELLED: This study was performed to investigate the effect of the new chemotherapeutic agent gemcitabine on glucose transport and metabolism in prostate carcinoma in vitro and in vivo. METHODS: After transplantation of rat prostate adenocarcinoma cells, dynamic PET measurements with fluorine-18-labeled 2-fluoro-2-deoxy-D-glucose (18FDG) were performed in 15 animals before and 1 day after therapy with 90 mg/kg of body weight (n = 8) and 180 mg/kg of body weight (n = 7) gemcitabine. In the second examination, the animals received a simultaneous injection of 18FDG and [3H]thymidine. Quantitative evaluation of the PET data was done using the standardized uptake value (SUV) as well as a three-compartment pharmacokinetic model. Furthermore, the incorporation of [3H]thymidine into the DNA was determined. In vitro measurements of the FDG, 3-O-methylglucose and thymidine uptake were performed immediately and 4 hr after a 24-hr incubation period with different doses of gemcitabine. RESULTS: FDG-SUV and the metabolic rate of FD 3 utilization did not change significantly after therapy. However, the values for the transport rate constants K1 and K2 increased significantly. The incorporation of thymidine into the DNA of treated tumors showed an 80% decline as compared with a control group. In the cell culture experiments, a dose-dependent increase of FDG (up to 178%) and 3-O-methylglucose uptake (up to 305%) was demonstrated. The thymidine uptake showed a 96% decline in the nucleic acid fraction and an increase of up to 337% in the cytoplasmic fraction. CONCLUSION: The more global measures of FDG metabolism as SUV and metabolic rate of FDG utilization were unchanged after therapy, while DNA synthesis and cell viability declined. However, in vitro and in vivo evidence of an enhancement of glucose transport is presented, indicating that quantification by modelling may be superior for the evaluation of metabolic effects during chemotherapy.

Adenocarcinoma↗
PubMed1997

Performance evaluation of a whole-body PET scanner using the NEMA protocol. National Electrical Manufacturers Association.

UNLABELLED: This study evaluates the performance of the newly developed high-resolution whole-body PET scanner ECAT EXACT HR+. METHODS: The scanner consists of four rings of 72 bismuth germanate block detectors each, covering an axial field of view of 15.5 cm with a patient port of 56.2 cm. A single block detector is divided into an 8 x 8 matrix, giving a total of 32 rings with 576 detectors each. The dimensions of a single detector element are 4.39 x 4.05 x 30 mm3. The scanner is equipped with extendable tungsten septa for two-dimensional two-dimensional measurements, as well as with three 68Ge line sources for transmission scans and daily quality control. The spatial resolution, scatter fraction, count rate, sensitivity, uniformity and accuracy of the implemented correction algorithms were evaluated after the National Electrical Manufacturers Association protocol using the standard acquisition parameters. RESULTS: The transaxial resolution in the two-dimensional mode is 4.3 mm (4.4 mm) in the center and increases to 4.7 mm (4.8 mm) tangential and to 8.3 mm (8.0 mm) radial at a distance of r = 20 cm from the center. The axial slice width measured in the two-dimensional mode varies between 4.2 and 6.6 mm FWHM over the transaxial field of view. In the three-dimensional mode the average axial resolution varies between 4.1 mm FWHM in the center and 7.8 mm at r = 20 cm. The scatter fraction is 17.1% (32.5%) for a lower energy discriminator level of 350 keV. The maximum true event count rate of 263 (345) kcps was measured at an activity concentration of 142 (26.9) kBq/ml. The total system sensitivity for true events is 5.7 (27.7) cps/Bq/ml. From the uniformity measurements, we obtained a volume variance of 3.9% (5.0%) and a system variance of 1.6% (1.7%). The implemented three-dimensional scatter correction algorithm reveals very favorable properties, whereas the three-dimensional attenuation correction yields slightly inaccurate results in low- and high-density regions. CONCLUSION: The ECAT EXACT HR+ has an excellent, nearly isotropic spatial resolution, which is advantageous for brain and small animal studies. While the relatively low slice sensitivity may hamper the capability for performing fast dynamic two-dimensional studies, the scanner offers a sufficient sensitivity and count rate capacity for fully three-dimensional whole-body imaging.

Adenocarcinoma, Follicular↗
PubMed1996

Improved visualization of breast lesions with gadolinium-enhanced magnetization transfer MR imaging.

A pulse sequence with magnetization transfer as the main contrast mechanism (MT-FLASH) was developed for improved imaging of breast lesions that requires neither fat suppression nor postprocessing. After optimization of the sequence in phantom and volunteer studies, a clinical pilot study with 14 patients was performed. In carcinomas the relative signal increase after Gd-DTPA administration was on average 34% in MT-FLASH images compared with 169% in conventional T1-weighted (T1W) three-dimensional FLASH images. In MT-FLASH images, all lesions demonstrated a signal intensity higher than that of fat; in T1W images, all lesions have a lower signal intensity. The average postcontrast carcinoma-to-fat contrast-to-noise ratios were +11.6 and -14.2, respectively. The conspicuity of 12 of 13 carcinomas was improved in postcontrast MT-FLASH imaging enables excellent visualization of Gd-DTPA-enhancing breast lesions.

Adult↗
PubMed1996

Age dependency of the regional cerebral blood volume (rCBV) measured with dynamic susceptibility contrast MR imaging (DSC).

The changes of the regional cerebral blood volume (rCBV) with age were studied using dynamic susceptibility contrast MRI (DSC). We examined an unselected, random sample of 71 consecutive patients referred for work-up of suspected intracranial tumors (35 normal examinations, 36 tumors) with a standard 1.5 T clinical MR system. Determination of the rCBV was performed with a T2*-weighted simultaneous dual (SD) FLASH sequence (TR/TE1/TE2/alpha = 32/25/16/10 degrees, 55 images) after bolus injection of Gd-DTPA. Absolute quantification of the rCBV was achieved by normalizing the measured tissue concentration-time curves with the integrated arterial input function (AIF), which was simultaneously measured in the brain feeding arteries. The rCBV (mean +/- SD) was 8.4 +/- 2.9 ml/100 g and 4.2 +/- 1.7 ml/100 g in gray and white matter, respectively, with a decline of about 3% and 6% per decade for white and gray matter, respectively. We conclude that DSC using a SD FLASH sequence allows the simultaneous measurement of the AIF and the tissue concentration-time curve and thus an absolute quantification of the rCBV, which is the basis for interperson comparisons and follow-up studies.

Adult↗
PubMed1996

Macroscopic tumor volume of malignant glioma determined by contrast-enhanced magnetic resonance imaging with and without magnetization transfer contrast.

The purposes of this study were to compare the conspicuity and lesion volume of contrast-enhancing macroscopic malignant glioma determined by postcontrast magnetic resonance (MR) imaging with and without magnetization transfer (MT) saturation, and to discuss possible implications for radiotherapy planning. Nineteen patients (age 24-60 years) with histologically proven malignant glioma were prospectively examined by MR imaging. After the administration of gadolinium dimeglumine (0.1 mmol/kg body weight), the lesions were imaged with an MT-weighted FLASH (fast, low-angle shot) pulse sequence and with a conventional T1-weighted spin-echo (SE) sequence without MT saturation. The mean tumor volumes of gliomas measured on MT-weighted FLASH images were significantly (p < .01) larger than those obtained from T1-weighted SE images (45 +/- 15 cm3 vs. 33 +/- 10 cm3). The mean contrast-to-noise ratio of enhancing lesions on MT-weighted FLASH was 48 +/- 14 compared with 30 +/- 14 on SE images, representing a significant (p < .01) improvement. We conclude that the volume of contrast enhancement of malignant glioma identified on MT-weighted FLASH images represents the area of disrupted blood-brain barrier. If this volume of subtle contrast enhancement is caused by tumor infiltration and represents the boost target volume for stereotactic radiosurgery or brachytherapy, MT-weighted FLASH images would be better than T1-weighted SE images to define these volumes. These improved delineation of areas at highest risk for recurrence following radiation therapy should enhance the efficacy of treatment planning for high-boost therapy.

Adult↗
PubMed1996

Functional MR imaging of semantic information processing and learning-related effects using psychometrically controlled stimulation paradigms.

Functional magnetic resonance imaging (fMRI), in conjunction with carefully designed, psychometrically optimized stimulation procedures, was used to investigate the relation between brain activation and the processing of word associations. A semantic discrimination task of word-pair similarity was performed by normal subjects (n = 17) within a clinical 1.5-Tesla whole-body MRI system. A color similarity task of psychometrically equivalent difficulty, as indicated by behavioral data acquired online during fMRI, served as active control condition. Comparisons between tasks dramatically improved results compared to comparisons between task and resting condition. The language paradigm selectively activated left frontal and left fronto-temporal areas. Cortical activation during the semantic task decreased significantly over three runs of the same word list and was paralleled by decreased reaction times. No such changes were observed in the active control condition indicating selective learning of the language task only. When combined with psychological activation schemes and the acquisition of behavioral data, fMRI represents a powerful tool for the study of brain-behavior interaction.

Adult↗
PubMed1996

Assessment of the biodistribution and metabolism of 5-fluorouracil as monitored by 18F PET and 19F MRI: a comparative animal study.

The effective clinical use of the anticancer drug 5-fluorouracil (5-FU) requires the non-invasive assessment of its transport and metabolism, particularly in the tumor and the liver, where the drug is catabolized to alpha-fluoro-beta-alanine (FBAL). In this study, the potentials and limitations of dynamic 18F PET and metabolic 19F MRI examinations for noninvasive 5-FU monitoring were investigated in ACI and Buffalo rats with transplanted MH3924A and TC5123 Morris hepatomas, respectively. Selective 5-[19F]FU and [19F]FBAL MR images were acquired 5 and 70 min after 5-FU injection using a CHESS MRI sequence. After administration of 5-[18F]FU, the kinetics of the regional 5-[18F]FU uptake were measured by dynamic PET scanning over 120 min. To allow a comparison between PET and MRI data, standardized uptake values (SUV) were computed at the same points in time. The TC5123 hepatoma showed a significantly (p < 0.002) higher mean SUV at 5 and 70 min post-5-FU injection than the MH3924A cell lines, whereas there were no significant differences between the mean SUV measured in the liver of both animal populations. In contrast to the PET data, no significant differences in the mean 5-[19F]FU and [19F]FBAL MR signal values in the tumor of both models were observed. The MR images, however, yielded the additional information that 5-FU is converted to FBAL only in the liver and not in the hepatomas.

Animals↗
PubMed1996

[Functional magnetic resonance tomography during the activation of working memory].

AIMS: Functional magnetic resonance imaging was used in conjunction with a letter detection task for the study of working memory in 16 normal subjects. Because of movement artifacts, data from only 9 subjects were analysed. METHODS: In the activation task, subjects responded by pressing a button whenever any presented letter was the same as the second last in the sequence. In the control condition, the subjects had to respond to a fixed letter. Hence, the activation condition and the control condition differed only subjectively, i.e., regarding the task demand, whereas the stimuli and the type and frequency of response were identical. RESULTS: The activation condition produced significant activation in the dorsolateral prefrontal cortex (Brodmann's areas 10, 46, and 9). CONCLUSIONS: In contrast to experimental tasks previously used rather extensively to study the prefrontal cortex, the present paradigm is characterized by its simplicity, interpretability, and its ties to known neurophysiology of the frontal cortex.

Adult↗
PubMed1996

Texture analysis of differently reconstructed PET images.

The insufficient number of projections and the statistics of radioactive decay are supposed to lead to different textures in the images reconstructed by the diverse algorithms used in positron emission tomography (PET). In our study, four reconstruction methods were examined, including the filtered backprojection and three algebraic approaches. Grey-level morphological operators were used to extract the texture. To quantify the observations, we used the texture parameters directionality, fractal dimension, and lacunarity. By increasing the number of iteration steps, each of the algebraic reconstruction methods evolve the texture from coarse to fine granularities, but surprisingly, during the iteration process all methods follow a quite similar 'trail' of possible textures within the active zone. The texture of the termination points of the movement have been compared with the result that the 'hils' and 'valleys' of the activity distribution are located in the same positions. The texture derived from the filtered backprojection is included in the set of textures produced by the algebraic methods. Artefacts are restricted to zones without activity. We found that the texture is independent of the number of counts when more than ten counts are registered in each pixel within the examined region of interest. A high reliability in the reconstructed images is given. All methods allow texture based tissue classification.

Algorithms↗