Changes in whole-body fat distribution, intrahepatic lipids, and insulin resistance of obese adolescents during a low-level lifestyle intervention.

The aim of this study was to analyze changes in adipose tissue (AT) distribution, intrahepatic lipids (IHL), and insulin resistance (IR) among a group of obese adolescents undergoing a 7-months low-level lifestyle intervention. Thirty-nine obese Caucasian adolescents (mean age 13.9 years, body mass index standard deviation score (BMI-SDSLMS) 2.14) were included. AT and IHL were determined by T1-weighted magnetic resonance (MR) imaging and single-voxel MR spectroscopy; IR was estimated using the homeostatic model assessment (HOMA-IR). The lifestyle intervention led to a reduction of both BMI-SDSLMS (boys 2.27 to 2.17; girls 2.00 to 1.82) and HOMA-IR (boys 6.1 to 4.4 (p = 0.008); girls 6.2 to 4.7 (p = 0.030)). IHL dropped in both genders (boys 7.5 to 4.3 %; girls 4.6 to 3.4 %) positively correlating with HOMA-IR (boys r = 0.52; girls r = 0.68), while in contrast visceral AT did not change significantly. CONCLUSIONS: Although the lifestyle intervention only slightly reduced BMI-SDSLMS, insulin sensitivity improved in both genders and came along with a marked reduction of IHL. This suggests that IHL might play the dominant role regarding insulin resistance in the youth, especially if compared to other AT compartments such as visceral AT. WHAT IS KNOWN: • MR imaging/spectroscopy can be used to evaluate body fat distribution and intrahepatic lipids in the youth. • The strength of associations between body fat compartments and insulin resistance is under scientific debate. WHAT IS NEW: • The study emphasizes that even a low-level lifestyle intervention has a beneficial effect. • The study suggests that intrahepatic lipids are an important factor in the development of insulin resistance.

Predicting volumes of metabolically important whole-body adipose tissue compartments in overweight and obese adolescents by different MRI approaches and anthropometry.

OBJECTIVES: To evaluate 5-slice stack/single-slice MRI approaches and anthropometric measures as predictors for metabolically relevant whole-body adipose tissue (AT) compartments in overweight/obese adolescents. METHODS: Forty adolescents (22 males, age 11.4-16.1 years) were included with a BMI above the 90th percentile. Volumes of whole-body AT compartments, i.e. total AT (TAT), subcutaneous AT (SCAT) and visceral AT (VAT), were determined using a breath-hold T1-weighted-FSE-MR-sequence and semi-automated segmentation serving as the gold standard. SCAT, VAT and TAT was estimated by either axially oriented single-slices or 5-slice-stacks centred at specific anatomic landmarks (umbilicus, head of femur and humerus). Furthermore, anthropometric measures were also evaluated as predictors of whole-body AT compartments. RESULTS: Strong correlations were found for both genders between TAT/SCAT and single-slice evaluation (e.g. whole-body SCAT-SCAT at umbilicus level: r = 0.91 (m), r = 0.92 (f)) or anthropometry (SCAT-BMI: r = 0.93 (m, f)). VAT was correlated to VAT at umbilicus (r = 0.71 (m), r = 0.94 (f)) but only weakly to anthropometry. CONCLUSIONS: Anthropometric measures and single-slice MRI can accurately predict TAT/SCAT which cannot be improved by evaluation of 5-slice stacks. Prediction of VAT by 5-slice stack/single-slice MRI protocols seems only to be accurate in females. Anthropometry cannot be reliably used for prediction of VAT in both genders. Thus, MRI seems to be necessary for quantification of VAT in overweight/obese adolescents of both genders.

RF tissue-heating near metallic implants during magnetic resonance examinations: an approach in the ac limit.

PURPOSE: State of the art to access radiofrequency (RF) heating near implants is computer modeling of the devices and solving Maxwell's equations for the specific setup. For a set of input parameters, a fixed result is obtained. This work presents a theoretical approach in the alternating current (ac) limit, which can potentially render closed formulas for the basic behavior of tissue heating near metallic structures. Dedicated experiments were performed to support the theory. METHODS: For the ac calculations, the implant was modeled as an RLC parallel circuit, with L being the secondary of a transformer and the RF transmission coil being its primary. Parameters influencing coupling, power matching, and specific absorption rate (SAR) were determined and formula relations were established. Experiments on a copper ring with a radial gap as capacitor for inductive coupling (at 1.5 T) and on needles for capacitive coupling (at 3 T) were carried out. The temperature rise in the embedding dielectric was observed as a function of its specific resistance using an infrared (IR) camera. RESULTS: Closed formulas containing the parameters of the setup were obtained for the frequency dependence of the transmitted power at fixed load resistance, for the calculation of the resistance for optimum power transfer, and for the calculation of the transmitted power in dependence of the load resistance. Good qualitative agreement was found between the course of the experimentally obtained heating curves and the theoretically determined power curves. Power matching revealed as critical parameter especially if the sample was resonant close to the Larmor frequency. CONCLUSIONS: The presented ac approach to RF heating near an implant, which mimics specific values for R, L, and C, allows for closed formulas to estimate the potential of RF energy transfer. A first reference point for worst-case determination in MR testing procedures can be obtained. Numerical approaches, necessary to determine spatially resolved heating maps, can be supported.

Optimized in-phase and opposed-phase MR imaging for accurate detection of small fat or water fractions: theoretical considerations and experimental application in emulsions.

OBJECT: To optimize strategies and measurement parameters for quantification of small fat and water fractions (<10%) in mixtures of both components by 4-point in-phase and opposed-phase gradient-echo imaging and to compare theoretical results with in-vitro experiments using emulsions. MATERIALS AND METHODS: Theoretical analysis was based on steady-state signal equations for spoiled GRE-sequences and on relaxation properties of water and fat components. For quantification, signals were corrected for T2*-decay, T1-decay, and signal contributions from double bonds. Theoretical results were exemplarily compared to measurements at 1.5 T on emulsions with either low water or fat fractions (0.5-10%) using spoiled 2D- and 3D-GRE-sequences. Excitation flip angle was varied in order to determine suitable values for sensitive detection of small fat/water fractions. RESULTS: Theoretical results and measurements correlated well, especially for 3D-sequences. Maximal sensitivity to a small signal fraction (S (fat) and S (water), respectively), was provided at the Ernst angle of the lower concentrated component. For 2D-sequences, the nominal flip angle had to be increased for compensation of slice profile effects and B(1) inhomogeneities. IP- and OP-echoes are recommended to be acquired in separate measurements with smallest possible receiver bandwidth to increase SNR/unit-time. Lowest detectable fat/water concentration in emulsions under typical conditions regarding spatial resolution and measuring time was approximately 1%. CONCLUSION: Using IP/OP-imaging with optimized parameters and post-processing, a sensitive and reliable detection of small fat/water fractions larger than 1% is possible in emulsions.

Quantification of direct current in electrically active implants using MRI methods.

The aim of this study was to evaluate a variety of phase- and magnitude-based MRI methods at 1.5 T and 3 T regarding their sensitivity and accuracy with respect to the quantification of electrical direct current via the induced magnetic field inhomogeneity. For this, a phantom was constructed which was specially designed to reduce RF effects and which provided a one-dimensional electrical direct current in a thin copper conductor perpendicular to the static magnetic field of the scanner. The current was varied between 4 mA and 472 mA. The analysis of FLASH phase images as well as trueFISP and MAGSUS images revealed that the accuracy of the MR current measurement depended on the method and the field strength: the mean of the absolute deviations of the measured current values from the adjusted current values varied between 9% and 21%. The phase measurement with a FLASH sequence was found to be more sensitive than the trueFISP and MAGSUS measurements. In FLASH magnitude images as well as in images of spin echo sequences with on- and off-resonant frequency selective saturation pulses the extension of the artifact increased with the electrical current. MRI methods for the quantification of electrical direct current might e.g. play a role in functional testing of electrically active devices in the human body in terms of measuring the present current. One-dimensional electrical direct current in a thin, straight conductor could also be applied to the visualization of instruments in interventional MRI procedures. Currents below 100 mA would be sufficient to create distinct artifacts, at least under simplified conditions (homogeneous background etc.).

Assessment of relevant hepatic steatosis in obese adolescents by rapid fat-selective GRE imaging with spatial-spectral excitation: a quantitative comparison with spectroscopic findings.

OBJECTIVE: To test the feasibility of fat-selective GRE imaging using a spectral-spatial excitation technique for determination of intrahepatic lipid content (IHL) in obese adolescents. METHODS: Fat-selective MR imaging (1.5 T) was applied to record a single axial slice through a representative liver region within a single breath-hold. The sequence uses six equidistant slice-selective excitation pulses with binomial amplitude ratios to achieve high selectivity for lipid signals after appropriate shimming. IHL(MRI) content was quantified using signal intensity of adjacent subcutaneous adipose tissue. As the gold standard for IHL quantification, single-voxel stimulated echo magnetic resonance spectroscopy (MRS) was applied. IHL(MRS) was quantified using the water peak as a reference. RESULTS: Forty-five MR examinations could be performed, and IHL(MRS) content ranged from 0.7% to 19.1%. Results from MRS and fat-selective imaging correlated well with Spearman coefficients between r = 0.78 and r = 0.86. There were no relevant regional differences in IHL within the liver parenchyma (p > 0.6359). Fat-selective imaging was able to reliably identify patients with IHL content above 5% with positive/negative likelihood ratio of 11.8 and 0.05, respectively. CONCLUSION: Fat-selective MR imaging provides both a reliable and a convenient method of rapidly quantifying IHL content in obese adolescents.

Quantitative assessment of intrahepatic lipids using fat-selective imaging with spectral-spatial excitation and in-/opposed-phase gradient echo imaging techniques within a study population of extremely obese patients: feasibility on a short, wide-bore MR scanner.

OBJECTIVES: The aim of this study was to evaluate the feasibility of 2 established magnetic resonance imaging based techniques to quantify intrahepatic lipids (IHL) within a study population of extremely obese patients by means of a short, wide-bore MR scanner. Fat-selective imaging using a spectral-spatial excitation technique and in-phase/opposed-phase (IN/OP) gradient echo imaging were applied and results were compared. Results for IN/OP technique were corrected for T1- and T2*- relaxation effects. Furthermore, image quality was assessed for both techniques. Differences in regional fat distribution were assessed using parameter maps of voxel-wise calculated IHL. MATERIALS AND METHODS: MR examinations of 20 extremely obese patients were included in the study (7 males, 13 females; mean age 40.4 +/- 12.6 years; mean body mass index 46.3 +/- 6.6 kg/m2). IHL, in terms of fat signal fractions, was calculated from simultaneously acquired IN/OP-images using a double-echo gradient echo technique. For correction of transverse relaxation effects an additional multiecho gradient echo sequence was applied in each subject, whereas correction of longitudinal relaxation was performed using literature values for T1 of water and lipid protons in the liver parenchyma. A highly selective spectral-spatial excitation technique with 6 binomial radiofrequency pulses was used for fat-selective imaging. In this case, signal intensity of adjacent subcutaneous adipose (approximately 100% fat) was used as an internal reference for IHL quantification. RESULTS: IN/OP-imaging provided sufficient image quality in all subjects, whereas fat-selective imaging was hampered by insufficient homogeneity of the static magnetic field in 1 of 20 subjects. Hepatic T2* values ranged from 20.1 milliseconds to 42.2 milliseconds. Results for IHL from both techniques were highly correlated with r(s) = 0.915 (P < 0.0001). Mean values for IHL were 16.5% +/- 9.2% and 10.6% +/- 7.3%, for IN/OP and spectral-spatial excitation technique, respectively, showing a slightly lower estimation of IHL by the spectral-spatial excitation method. In the examined cohort of extremely obese subjects a relatively high number of 4 out of 20 cases (20%) were found with uneven distribution of IHLs. CONCLUSIONS: The presented data confirm that both methods are reliable tools for quantification of IHL, if inherent drawbacks and limitations are taken into account. Inhomogeneity of the static magnetic field observed in examinations of extremely obese patients limits the use of spectral-spatial excitation, if performed without time-consuming shimming procedures. Necessity to correct for transverse and longitudinal relaxation effects using the IN/OP method requires additional measurements and postprocessing procedures, which might hamper the clinical applicability. Moreover, significant regional differences in IHL may exist in some patients especially if pronounced hepatic steatosis is present.