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Biomedical subjects

Fritz Schick

Publications and source records attributed to Fritz Schick.

At least 55 records · Page 3Linked to original sources
PubMed2005

Magnetic resonance imaging of the body trunk using a single-slab, 3-dimensional, T2-weighted turbo-spin-echo sequence with high sampling efficiency (SPACE) for high spatial resolution imaging: initial clinical experiences.

PURPOSE: The authors conducted a clinical evaluation of single-slab, 3-dimensional, T2-weighted turbo-spin-echo (TSE) with high sampling efficiency (SPACE) for high isotropic body imaging with large field-of-view (FoV). MATERIALS AND METHODS: Fifty patients were examined in clinical routine with SPACE (regions of interest: pelvis n=30, lower spine n=12, upper spine n=6, extremities n=4) at 1.5 T. For achieving a high sampling efficiency, parallel imaging, high turbofactor, and magnetization restore pulses were used. In contrast to a conventional TSE imaging technique with constant flip angle refocusing, the refocusing pulse train of the SPACE sequence consists of variable flip angle radiofrequency pulses along the echo train. RESULTS: Signal-to-noise ratio and contrast-to-noise ratio of SPACE images were of sufficient diagnostic value. The possibility of image reconstruction in multiple planes was of clinical relevance in all cases and simplified data analysis. CONCLUSION: The achievement of 3-dimensional, T2-weighted TSE magnetic resonance imaging with isotropic and high spatial resolution and interactive 3-dimensional visualization essentially improve the diagnostic potential of magnetic resonance imaging.

Abdominal Neoplasms↗
PubMed2005

Compensation of magnetic field distortions from paramagnetic instruments by added diamagnetic material: measurements and numerical simulations.

In minimally invasive procedures guided by magnetic resonance (MR) imaging instruments usually are made of titanium or titanium alloys (e.g., nitinol), because other more MR-compatible materials often cannot provide sufficient mechanical properties. Artifacts depending on susceptibility arise in MR images due to incorrect spatial encoding and intravoxel dephasing and thereby hamper the surgeon's view onto the region of interest. To overcome the artifact problem, compensation of the paramagnetic properties by diamagnetic coating or filling of the instruments has been proposed in the literature. We used a numerical modeling procedure to estimate the effect of compensation. Modeling of the perturbation of the static magnetic field close to the instruments reflects the underlying problem and is much faster and cost efficient than manufacturing prototypes and measuring artifact behavior of these prototypes in the MR scanner. A numerical model based on the decomposition of the susceptibility distribution in elementary dipoles was developed by us. The program code was written object oriented to allow for both maximum computational speed and minimum random access memory. We used System International units throughout the modeling for the magnetic field, allowing absolute quantification of the magnetic field disturbance. The field outside a simulated needlelike instrument, modeled by a paramagnetic cylinder (out of titan, chi =181.1) of length 8.0 mm and of diameter 1.0 mm, coated with a diamagnetic layer (out of bismuth, chi=-165.0) of thickness 0, 0.1, 0.2, 0.3, and 0.4 mm, was found to be best compensated if the cross-sectional area of the cylinder, multiplied by the absolute susceptibility value of the cylinder material, is equal to the cross-sectional area of the coating, multiplied by the absolute susceptibility value of the coating material. At the extremity of the coated cylinder an uncompensated field distortion was found to remain. We studied various tip shapes and geometries using our computational model: Suitable diamagnetic coating or filling of paramagnetic instruments clearly reduced tip artifacts and diminished the dependency of artifact size on orientation of the instrument with respect to B0 in the numerical studies. We verified the results of the simulations by measuring coated and uncoated titanium wires in a 1.5 T MR scanner.

Computer Simulation↗
PubMed2005

rf enhancement and shielding in MRI caused by conductive implants: dependence on electrical parameters for a tube model.

Radio frequency (rf) eddy-currents induced in implants made of conductive material might cause significant image artifacts in magnetic resonance imaging (MRI) such as shielding of the lumen of vascular stents. rf alteration near metal parts was assessed theoretically in the approximation of alternating current electrodynamics: The implant was modeled as tube with diameter d(o), resistance R, and reactance Y, constituting the secondary winding of a transformer. The transmitter coil of the scanner acted as primary winding and generated the linearly polarized rf field B1,app. Tube axis was assumed parallel to B1,app. The results of the calculations were as follows: Ninety percent of the applied rf-field amplitude is reached in the lumen at a ratio chi=R/Y approximately 2. A rapid drop occurs with the reduction of chi, whereas a further increase of chi causes only a small effect. With chi approximately 1/d(o)(Y approximately d2o,R approximately d(o)), conditions for rf alteration clearly depend on the diameter of the tube. Inside tubes with smaller diameter, rf shielding is less pronounced. rf alteration increases in good approximation with the square root of the strength of the static field B0. The following experiments were carried out: Tubes of similar diameter (d(o) approximately 8 mm) made of material of different conductivity (Cu, Nitinol, carbon fiber reinforced plastic with three different fiber structures) were examined at B0=0.2 and 1.5 T in water phantoms. Tube axis was aligned perpendicular to B0 and spin-echo technique was applied. Local rf enhancement near the outer surface of the metal tubes was detected applying manual reduction of the transmitter amplitude. Shielding inside a carbon fiber tube with d(o) approximately 8 mm and inside a smaller tube with d(o)=3.3 mm was compared. Both tubes showed the same wall structure and thickness (d(w)=0.4 mm). All measurements confirmed the theoretical results. Consequences for the construction of vascular stents are discussed, as well as problems with image artifacts due to rf enhancement near solid conductive implants.

Animals↗
PubMed2005

[Positive contrast in the detection of magnetically labeled cells by MRI--in vitro experiments].

PURPOSE: Magnetically labeled cells (MLC) cause local field inhomogenities within the single voxels as well as on a macroscopic scale. The related Larmor frequency shift near MLC was exploited to obtain bright visualization applying spectral selective saturation (SSS). METHODS: SK-Mel28 cells were labeled with the superparamagnetic iron oxide contrast agent SHU 555A. Low cell concentrations (0, 5, 20, 30, 50, 75, and 150 MLC/microl) and high cell concentrations (10 x 10(3), 30 x 10(3), 60 x 10(3), and 100 x 10(3) MLC/10 microl) were examined at 3 Tesla. Shimming and frequency adjustment to spectrometer reference frequency v0 was performed with the built in routine of the scanner. A 2D spin echo sequence with broadband excitation and refocusing pulses was used (BWex = 1.000 Hz). Prior to each TR, a non-selective saturation sinc pulse centred at v0 was applied. Bandwidth (BWsat) of this pulse was varied from 100 Hz to 800 Hz in logarithmic steps. RESULTS: Without SSS the highest value of Crel (i.e. relative MR contrast between labeled to unlabeled samples) was found for 150 MLC/microl and was given by 10%. Applying SSS led to positive contrast of the complete labeled volumes and to remarkable improvements in Crel. With increasing cell concentrations Crel raised to maximum, that was given by 52% (BWsat=100 Hz) and 28% (BWsat = 200 Hz) found for 75 MLC/microl. For 150 MLC/microl Crel decreased. A contrast clarification could also be detected near cell aggregations despite saturation. CONCLUSION: Using SSS positive contrast can be achieved for voxels containing MLC and voxels close to cell clusters. Under in vitro conditions positive contrast improved the sensitivity to detect MLC as compared to negative contrast imaging techniques. It seems reasonable, that positive contrast approaches can be applied in vivo as the underlying physical mechanism are comparable.

Cell Line, Tumor↗
PubMed2005

Thermal damage of the genitofemoral nerve due to radiofrequency ablation of renal cell carcinoma: a potentially avoidable complication.

OBJECTIVE: A technique is presented that may prevent damage to the genitofemoral nerve during percutaneous radiofrequency ablation of renal cell carcinomas. The genitofemoral nerve originates from the upper part of the lumbar plexus and descends laterally along the psoas major muscle, which is anatomically close to the kidney. During radiofrequency ablation, nearby healthy nerve tissue could be damaged by heat conductance. In the described technique, the radiofrequency applicator within the renal tumor is used as a lever to displace the kidney away from the psoas muscle. CONCLUSION: Three clinical cases are presented in this study. In one case, the technique was not applied, leading to coagulation of the genitofemoral nerve. In the other two cases, hazard to the genitofemoral nerve was averted by using the mentioned technique.

Aged↗
PubMed2004

3D proton MR spectroscopic imaging of prostate cancer using a standard spine coil at 1.5 T in clinical routine: a feasibility study.

The objective of this study was to demonstrate the feasibility of 3D proton MR spectroscopic imaging (MRSI) of the prostate using a standard spine instead of a dedicated endorectal coil at 1.5 T. Twenty-eight patients (25 with biopsy proven prostate cancers and three patients with a benign prostate hyperplasia) were examined. MRI and MRSI were conducted with commercial array surface coils at 1.5 T. Ratios of choline (Cho), creatine (Cr) and citrate (Ci) were calculated for tumour, central and peripheral zone retrospectively, based on axial T2 weighed MR images and histology reports. Prostate cancer was characterized by significantly elevated (Cho+Cr)/Ci ratio compared with non-tumourous prostate tissue. The quality of all proton MR spectra was considered to be good or acceptable in 17/28 patients (61%) and poor in 11/28 (39%) examinations. In 20/25 patients with proven malignancy (80%), MRSI was considered to be helpful for the detection of prostate cancer. In 4/25 patients with proven malignancy (16%) who underwent seed implantation, radiotherapy or hormone deprivation before MR examination spectroscopy was of poor and non-diagnostic quality. MRSI of the prostate is feasible within clinical routine using the spine array surface coil at 1.5 T. It can consequently be applied to patients even with contraindications for endorectal coils. However, spectral quality and signal-to-noise ratio is clearly inferior to 3D MRSI examinations with endorectal coils.

Adult↗
PubMed2004

The -913 G/A glutamine:fructose-6-phosphate aminotransferase gene polymorphism is associated with measures of obesity and intramyocellular lipid content in nondiabetic subjects.

Increases in glutamine:fructose-6-phosphate aminotransferase (GFAT) protein levels directly activate flux through the hexosamine biosynthetic pathway. This pathway has been involved as a fuel sensor in energy metabolism and development of insulin resistance. We screened the 5'-flanking region of the human GFAT gene for polymorphisms and subsequently genotyped 412 nondiabetic, metabolically characterized Caucasians for the two single-nucleotide polymorphisms (SNP) at positions -913 (G/A) and -1412 (C/G) with rare allele frequencies of 42% and 16%, respectively. The -913 G SNP was associated with significantly higher body mass index and percent body fat in men (P = 0.02 and 0.004, respectively), but not in women (P = 0.47 and 0.26, respectively). In the subgroup of individuals (n = 193) who underwent hyperinsulinemic-euglycemic clamp, an association of the -913 G SNP with insulin sensitivity independent of body mass index was not detected. Moreover, the -913 G allele in a group of 71 individuals who had undergone magnetic resonance spectroscopy was associated with higher intramyocellular lipid content (IMCL) in tibialis anterior muscle (4.21 +/- 0.31 vs. 3.36 +/- 0.35; P = 0.04) independent of percent body fat and maximal aerobic power. The -1412 SNP had no effect on percent body fat, insulin sensitivity, or IMCL. In conclusion, we identified two polymorphisms in the 5'-flanking region of GFAT, of which the -913 SNP seems to alter the risk for obesity and IMCL accumulation in male subjects.

Adult↗
PubMed2004

Solitary pulmonary nodules: dynamic contrast-enhanced MR imaging--perfusion differences in malignant and benign lesions.

PURPOSE: To determine whether dynamic contrast material-enhanced magnetic resonance (MR) imaging with use of kinetic and morphologic parameters reveals statistically significant differences between malignant and benign solitary pulmonary nodules. MATERIALS AND METHODS: Fifty-eight patients met the inclusion criteria of a solitary 5-40-mm pulmonary nodule without calcification or fat at computed tomography. Fifty-one patients were examined successfully; 46 received a histologic diagnosis, and five received a diagnosis by means of observation over 2 years. Dynamic MR images were acquired every 10 seconds for a total of 4 minutes. Diagnostic characteristics for differentiation were examined by using threshold values for maximum peak enhancement, slope of enhancement, and washout. Receiver operating characteristic curves were calculated to test the usefulness of these parameters. The diagnostic performance of a combination of curve profiles and morphologic contrast material distribution were tested by using a decision tree. RESULTS: Frequency of malignancy was 53% (27 of 51 nodules). Malignant nodules showed stronger enhancement with a higher maximum peak and a faster slope (P <.001). Significant washout (>0.1% increase in signal intensity per second) was found only in malignant lesions (14 of 27 lesions). Sensitivity, specificity, and accuracy were 96%, 88%, and 92%, respectively, for maximum peak; 96%, 75%, and 86% for slope; and 52%, 100%, and 75% for washout. When curve profiles and morphologic enhancement patterns were combined, sensitivity increased to 100%. CONCLUSION: Dynamic MR imaging delineates significant kinetic and morphologic differences in vascularity and perfusion between malignant and benign solitary pulmonary nodules. Washout seems to be highly specific for malignancy.

Adult↗
PubMed2004

FAIR true-FISP perfusion imaging of the kidneys.

Most arterial spin labeling (ASL) techniques apply echoplanar imaging (EPI) because this strategy provides relatively high SNR in short measuring times. Unfortunately, those techniques are very susceptible to static magnetic field inhomogeneities and perfusion signals from organs with fast transverse relaxation might decrease due to the exchange of water molecules in capillaries and organ tissue combined with relatively long echo times of EPI sequences. To overcome these problems a novel imaging technique, FAIR True-FISP, was developed. It combines a FAIR (flow-sensitive alternating inversion recovery) perfusion preparation and a true fast imaging with steady precession (True-FISP) data acquisition strategy. True-FISP was chosen since this sequence type does not show the mentioned disadvantages of EPI, but provides a similar SNR per measuring time. An important problem of this approach is that True-FISP sequences usually work in a steady state which is independent of a previous preparation of magnetization. For this reason a sequence structure had to be developed which keeps the advantages of True-FISP and makes the signal intensity sensitive to the FAIR preparation. Breathhold and nonbreathhold examinations of kidneys are presented and possible strategies to quantitative flow measurements are reported. It is shown that correction of spatially inhomogeneous receiver coil characteristics is easily feasible and leads to clinically valuable perfusion examinations of kidneys without application of potentially nephrotoxic contrast media.

Adult↗
PubMed2004

Sodium 3-D MRI of the human torso using a volume coil.

Sodium MR imaging is considered to provide clinically important information about the human body that is not achievable by hydrogen-based approaches. However, due to the low natural abundance in biological tissues, sodium signals usually lead to low spatial resolution, low SNR, and long acquisition times compared to conventional 1H imaging, even using well-adapted surface coils. For our study, a volume coil was designed with nearly homogeneous excitation/receive characteristics and a suitable geometry fitting the human torso. A sufficient penetration throughout the entire thorax, abdomen, or pelvis is provided allowing for sodium imaging of the kidneys, the liver with gall bladder, or the myocardium. All measurements were performed on a 1.5 T whole body scanner using a spoiled 3-D gradient echo sequence. Imaging parameters TE, TR, and readout bandwidth were optimized for sensitive recording of the sodium component with slow transverse relaxation. Nonselective RF excitation pulses with a duration of 2.5 ms and rectangular shape were applied to avoid SAR problems. Narrow receiver bandwidth and excitation near the Ernst angle provided clinically practicable examinations with measuring times of less than 15 min at a spatial resolution of 8 x 8 x 8 mm3. Under these conditions, SNR of 11 for the kidneys and vertebral disks, 9 for the spinal canal, and 6 for the liver was achieved. A special 3-D spin echo sequence was used to determine T2, times which resulted to 15.3 +/- 1.1 ms for liver, 27.7 +/- 7.2 ms for kidneys, and 24.0 +/- 4.7 ms for the content of the spinal canal.

Abdomen↗
PubMed2004

Region and volume dependencies in spectral line width assessed by 1H 2D MR chemical shift imaging in the monkey brain at 7 T.

High magnetic fields increase the sensitivity and spectral dispersion in magnetic resonance spectroscopy (MRS). In contrast, spectral peaks are broadened in vivo at higher field strength due to stronger susceptibility-induced effects. Strategies to minimize the spectral line width are therefore of critical importance. In the present study, 1H 2D chemical shift imaging at short echo times was performed in the macaque monkey brain at 7 T. Large brain coverage was obtained at high spatial resolution with voxel sizes down to 50 microl being able to quantify up to nine metabolites in vivo with good reliability. Measured line widths of metabolites decreased from 14.2 to 7.6 Hz with voxel volumes of 3.14 ml to 50 microl (at increased spatial resolution). The line width distribution of the metabolites (7.6+/-1.6 Hz, ranging from 5.5 to 10 Hz) was considerably smaller compared to that of water (10.6+/-2.4 Hz) and was also smaller than reported in 1H MRS at 7 T in the human brain. Our study showed that even in well-shimmed areas assumed to have minimal macroscopic susceptibility variations, spectral line widths are tissue-specific exhibiting considerable regional variation. Therefore, an overall improvement of a gross spectral line width--directly correlated with improved spectral quality--can only be achieved when voxel volumes are significantly reduced. Our line width optimization was sufficient to permit clear glutamate (Glu)-glutamine separation, yielding distinct Glu maps for brain areas including regions of greatly different Glu concentration (e.g., ventricles vs. surrounding tissue).

Animals↗
PubMed2004

Magnetic resonance imaging characteristics of six radiofrequency electrodes in a phantom study.

PURPOSE: To evaluate and compare visibility and artifacts in magnetic resonance (MR) compatible radiofrequency (RF) electrodes for MR-guided RF ablation. MATERIAL AND METHODS: Six different MR compatible electrodes for RF ablation including two internally cooled single needles, one internally cooled cluster needle, two expandable needles and one perfused needle were tested in a phantom study at 0.2 Tesla and at 1.5 Tesla field strength. Fluoroscopic, T1- and T2-weighted fast spin echo (FSE) and gradient echo (GE) sequences, which are usually used for MR-guided interventions, were evaluated. Qualitative and quantitative evaluations were performed. Length, width, noise, tip artifacts, global artifacts and global visualization of the RF electrodes that showed all sequences at different angles. RESULTS: Qualitative analysis showed that electrodes were well visualized at all angles and sequences and on both MR imagers. Quantitative analysis showed that artifact-induced widening of the shaft was increased in all electrodes by: a). use of fluoroscopic sequences, GE sequences, and fat saturation, b). increasing the angle between the needle and main magnetic field, and c). high field strength (1.5 T). Expandable needles produced fewer tip artifacts but broader signal voids along the shaft compared to nonexpandable needles. Cluster electrodes produced less widening than the other electrodes. CONCLUSION: Visibility and artifacts in all six MR compatible RF electrodes are satisfactory and these electrodes could be used for MR-guided radiofrequency ablation procedures.

Artifacts↗
PubMed2004

Intramyocellular lipids and insulin resistance.

Lipids are stored not only in adipocytes but also 'ectopically' in tissues such as muscle, liver, beta cells and others. From a metabolic perspective, intramyocellular lipids (IMCLs) have recently become a focus of interest. This review summarizes history, measurement techniques and interpretation of muscle lipid data. Problems in biopsies with the separation of those metabolically active lipid droplets in the cytoplasm of myocytes from further lipids in adipocytes are discussed as well as considerations important for analysis of correlations between IMCL content and insulin sensitivity under various circumstances. The relatively new approach to non-invasive assessment of the IMCL content by magnetic resonance spectroscopy (MRS) is described in detail and exemplary spectra from different skeletal muscle types in humans are presented. The MRS technique allows human examinations of large cohorts for a detailed assessment of the interactions among metabolic parameters such as age, measures of adiposity, hormonal and ethnic factors and insulin resistance. IMCLs are generally positively correlated with measures of obesity and negatively with insulin sensitivity. Paradoxically, physical fitness (maximal aerobic capacity) increases both IMCL content and insulin sensitivity and therefore has to be taken into account as a confounding factor. Intervention studies with MRS further allowed to elucidate the regulation of IMCL. Molecular mechanisms and potential genetic factors on IMCL regulation are discussed as well as possible mechanisms of current treatment strategies for improving insulin sensitivity.

Animals↗
PubMed2004

Numerical modeling of needle tip artifacts in MR gradient echo imaging.

Exact determination of needle tip position is obsolete for interventional procedures under control of magnetic resonance imaging (MRI). Exact needle tip navigation is complicated by the paramagnetism of microsurgical instruments: Local magnetic field inhomogeneities are induced resulting in position encoding artifacts and in signal voids in the surrounding of instruments and especially near their tips. The artifacts generated by the susceptibility of the material are not only dependent on the material properties themselves and on the applied MRI sequences and parameters, but also on the geometric shape of the instruments and on the orientation to the static magnetic field in the MR unit. A numerical model based on superposition of induced elementary dipole fields was developed for studying the field distortions near paramagnetic needle tips. The model was validated by comparison with experimental data using field mapping MRI techniques. Comparison between experimental data and numerical simulations revealed good correspondence for the induced field inhomogeneities. Further systematic numerical studies of the field distribution were performed for variable types of concentric and asymmetric tip shapes, for different ratios between tip length and needle diameter, and for different orientations of the needle axis in the external static magnetic field. Based on the computed local inhomogeneities of the magnetic field in the surroundings of the needle tips, signal voids in usual gradient echo images were simulated for a prediction of the artifacts. The practically relevant spatial relation between those artifacts and the hidden tip of the needle was calculated for the different tip shapes and orientations in the external field. As needle tip determination is crucial in interventional procedures, e.g., in taking biopsies, the present model can help to instruct the physician prior to surgical interventions in better estimating the needle tip position for different orientations and needle tip shapes as they appear in interventional procedures. As manufacturing prototypes with subsequent measurements of artifacts in MRI are a costly procedure the presented model may also help to optimize shapes of needle tips and of other parts of MR-compatible instruments and implants with low expense prior to production if some shape parameters can be chosen freely.

Artifacts↗
PubMed2003

Lipid content in the musculature of the lower leg assessed by fat selective MRI: intra- and interindividual differences and correlation with anthropometric and metabolic data.

PURPOSE: To assess the muscular lipid content (LC) in different muscle groups of the lower leg by a magnetic resonance imaging technique working with chemical shift selective excitation, and comparison with anthropometric and metabolic data. MATERIALS AND METHODS: Examinations were performed in 67 volunteers (54 male/13 female, age 29 +/- seven years) on a 1.5 T whole body imager, applying a highly selective spectral-spatial technique for fat selective MRI. LC was measured in six calf muscles and correlated with body mass index (BMI), percent body fat (PFAT), and insulin sensitivity (IS) of the subjects. RESULTS: Mean muscular LC of all subjects was between 2.0% (Tibialis posterior [TP]) and 3.8% (Peroneus muscles) with female subjects showing a significantly higher LC in all muscle groups (P < 0.05 each). LCs correlated moderately with BMI (R between 0.39 [TP] and 0.53 [GM]) and with PFAT (R between 0.38 [TP] and 0.62 [GM]). Insulin-resistant subjects showed slightly but not significantly increased LC compared to insulin-sensitive subjects in BMI-matched subgroups. CONCLUSION: The fat-selective MRI technique allows a reliable non-invasive measure of muscular lipids - even in muscle groups with inherent low LC - within a relatively short measurement time of about three minutes. The presented data reveal interesting interrelationships between LC and anthropometric and metabolic data, and therefore provide new insight into muscular fat metabolism.

Adult↗
PubMed2003

Relationship of serum adiponectin and leptin concentrations with body fat distribution in humans.

OBJECTIVE: We investigated whether serum concentrations of adiponectin are determined by body fat distribution and compared the findings with leptin. RESEARCH METHODS AND PROCEDURES: Serum concentrations of adiponectin and leptin were measured by radioimmunoassay (n = 394) and analyzed for correlation with sex, age, and body fat distribution, i.e., waist-to-hip ratio, waist and hip circumference, and subcutaneous adipose tissue area of the lower leg as assessed by magnetic resonance imaging. RESULTS: After adjusting for sex and percentage of body fat, adiponectin was negatively (r = -0.17, p < 0.001) and leptin was positively (r = 0.22, p < 0.001) correlated with waist-to-hip ratio. Leptin, but not adiponectin, correlated with both waist (r = 0.49, p < 0.001) and hip circumference (r = 0.46, p < 0.001). Furthermore, leptin, but not adiponectin, correlated with the proportion of subcutaneous fat of the lower leg cross-sectional area (r = 0.37, p < 0.001). DISCUSSION: These data suggest that both adipocytokines are associated with central body fat distribution, and serum adiponectin concentrations are determined predominantly by the visceral fat compartment.

Adiponectin↗