Fabian Springer

Non-invasive assessment and quantification of liver steatosis by ultrasound, computed tomography and magnetic resonance

Hepatic steatosis is the most prevalent liver disorder in the developed world. It is closely associated with features of metabolic syndrome, especially insulin resistance and obesity. The two most common conditions associated with fatty liver are alcoholic liver disease (ALD) and non-alcoholic fatty liver disease (NAFLD). Liver biopsy is considered the gold standard for the assessment of liver fat, but there is a need for less invasive diagnostic techniques. New imaging modalities are emerging, which could provide more detailed information about hepatic tissue or even replace biopsy. In the present review, available imaging modalities (ultrasound, computed tomography, magnetic resonance imaging and proton magnetic resonance spectroscopy) are presented which are employed to detect or even quantify the fat content of the liver. The advantages and disadvantages of the above-mentioned imaging modalities are discussed. Although none of these techniques is able to differentiate between microvesicular and macrovesicular steatosis and to reveal all features visible using histology, the proposed diagnostic modalities offer a wide range of additional information such as anatomical and morphological information non-invasively. In particular, magnetic resonance imaging and proton magnetic resonance spectroscopy are able to quantify the hepatic fat content hence avoiding exposure to radiation. Except for proton magnetic resonance spectroscopy, all modalities offer additional information about regional fat distribution within the liver. MR elastography, which can estimate the amount of fibrosis, also appears promising in the differentiation between simple steatosis and steatohepatitis.

Diffusion tensor imaging of the human calf muscle: distinct changes in fractional anisotropy and mean diffusion due to passive muscle shortening and stretching

The influence of passive shortening and stretching of the calf muscles on diffusion characteristics was investigated. The diffusion tensor was measured in transverse slices through the lower leg of eight healthy volunteers (29u2009±u20097u2009years) on a 3u2009T whole‐body MR unit in three different positions of the foot (40° plantarflexion, neutral ankle position (0°), and −10° dorsiflexion in the ankle). Maps of the mean diffusivity, the three eigenvalues of the tensor and fractional anisotropy (FA) were calculated. Results revealed a distinct dependence of the mean diffusivity and FA on the foot position and the related shortening and stretching of the muscle groups. The tibialis anterior muscle showed a significant increase of 19% in FA with increasing dorsiflexion, while the FA of the antagonists significantly decreased (∼20%). Regarding the mean diffusivity of the diffusion tensor, the muscle groups showed an opposed response to muscle elongation and shortening. Regarding the eigenvalues of the diffusion tensor, λ2 and λ3 showed significant changes in relation to muscle length. In contrast, no change in λ1 could be found. This work reveals significant changes in diffusional characteristics induced by passive muscle shortening and stretching. Copyright

Quantitative Analysis of Adipose Tissue in Single Transverse Slices for Estimation of Volumes of Relevant Fat Tissue Compartments A Study in a Large Cohort of Subjects at Risk for Type 2 Diabetes by MRI With Comparison to Anthropometric Data

Objectives:Aim of the study was to assess the potential value of cross-sectional adipose tissue evaluation in well defined single transverse slices for estimation of volumes of metabolically important adipose tissue compartments as visceral adipose tissue (VAT), nonvisceral adipose tissue (NVAT), and total body adipose tissue (TAT). In addition, validity of easily accessible anthropometric indices (waist-to-hip-ratio [WHR] and body mass index [BMI]) for prediction of volumes of those adipose tissue compartments were validated and compared. Materials and Methods:True volumes of main adipose tissue compartments VAT, TAT, and NVAT were carefully assessed based on whole-body magnetic resonance (MR) examinations in 367 volunteers (227 females, 140 males, age 18–69 years) at increased risk for type 2 diabetes. Volumes of VAT, NVAT, and TAT were compared with cross-sectional areas of adipose tissue in single transverse slices recorded on the level of (1) umbilicus (u), (2) head of humerus (h), (3) head of femur (f), and anthropometric data such as BMI and WHR. Separate analyses were performed in males and females. Results:In both genders, strong correlations were found between TAT and the cross-sectional areas: subcutaneous adipose tissue (u) with r = 0.88 in females and r = 0.92 in males, TAT(h) with r = 0.80 in females and r = 0.82 in males, and TAT(f) with r = 0.90 in females and r = 0.90 in males. VAT assessed by MR single slice evaluation at the umbilical level (VAT(u)) showed a strong correlation with total VAT in both genders (r = 0.93 in females, r = 0.87 in males). Suitable algorithms for predicting VAT in liter from fat planimetry in a slice recorded at the umbilical level were derived: VAT = 0.16 × (VAT(u) × body height) + 0.3 in women and VAT = 0.15 × (VAT(u) × body height) + 1.2 in men. Disregarding the body height, the best equations were as follows: VAT = 0.03 × VAT(u) + 0.3 in women and VAT = 0.02 × VAT(u) + 1.4 in men. In contrast, BMI versus VAT showed a lower correlation in both genders: r = 0.71 in females, r = 0.56 in males, and WHR versus VAT was only weakly correlated in females and in males (r = 0.49 and r = 0.55, respectively). For WHR versus TAT, significant results were only found in males (r = 0.44). Conclusions:Axial MRI at the umbilical level allowed for a fast and reliable estimation, especially for VAT and TAT in a collective at risk for type 2 diabetes. WHR and BMI were found clearly worse in prediction of VAT volumes compared with single slice evaluation at the umbilical level.

Three-dimensional ultrashort echo time imaging of solid polymers on a 3-Tesla whole-body MRI scanner.

ObjectiveWith the introduction of ultrashort echo time (UTE) sequences solid polymeric materials might become visible on clinical whole-body magnetic resonance (MR) scanners. The aim of this study was to characterize solid polymeric materials typically used for instruments in magnetic resonance guided interventions and implants. Relaxation behavior and signal yield were evaluated on a 3-Tesla whole-body MR unit. Materials and Methods:Nine different commonly used solid polymeric materials were investigated by means of a 3-dimensional (3D) UTE sequence with radial k-space sampling. The investigated polymeric samples with cylindrical shape (length, 150 mm; diameter, 30 mm) were placed in a commercial 8-channel knee coil. For assessment of transverse signal decay (T2*) images with variable echo times (TE) ranging from 0.07 milliseconds to 4.87 milliseconds were recorded. Spin-lattice relaxation time (T1) was calculated for all MR visible polymers with transverse relaxation times higher than T2* = 300 &mgr;s using an adapted method applying variable flip angles. Signal-to-noise ratio (SNR) was calculated at the shortest achievable echo time (TE = 0.07 milliseconds) for standardized sequence parameters. All relaxation times and SNR data are given as arithmetic mean values with standard deviations derived from 5 axially oriented slices placed around the isocenter of the coil and magnet. Results:Six of the 9 investigated solid polymers were visible at TE = 0.07 milliseconds. Visible solid polymers showed markedly different SNR values, ie, polyethylene SNR = 1146 ± 41, polypropylene SNR = 60 ± 6. Nearly mono-exponential echo time dependent signal decay was observed: Transverse relaxation times differed from T2*=36 ± 5 &mgr;s for polycarbonate to T2*=792 ± 7 &mgr;s for polyvinylchloride (PVC). Two of the investigated solid polymers were applicable to T1 relaxation time calculation. Polyurethane had a spin-lattice relaxation time of T1 = 172 ± 1 milliseconds, whereas PVC had T1 = 262 ± 7 milliseconds, respectively. Conclusions:A variety of solid polymers can be visualized by means of clinical whole-body MR scanners and 3D ultrashort echo time (UTE) sequences. The investigated polymers differ substantially in signal yield, signal-decay, and spin-lattice relaxation time. The knowledge of the signal behavior of solid polymers on whole-body clinical MR scanners may help to select suitable polymeric materials for instruments and implants which are visible using UTE sequences.

Magnetization transfer contrast imaging in bovine and human cortical bone applying an ultrashort echo time sequence at 3 Tesla

Magnetization transfer (MT) contrast imaging reveals interactions between free water molecules and macromolecules in a variety of tissues. The introduction of ultrashort echo time (UTE) sequences to clinical whole‐body MR scanners expands the possibility of MT imaging to tissues with extremely fast signal decay such as cortical bone. The aim of this study was to investigate the MT effect of bovine cortical bone in vitro on a 3 Tesla whole‐body MR unit. A 3D‐UTE sequence with a rectangular‐shaped on‐resonant excitation pulse and a Gaussian‐shaped off‐resonant saturation pulse for MT preparation was applied. The flip angle and off‐resonance frequency of the MT pulse was systematically varied. Measurements on various samples of bovine cortical bone, agar gel, aqueous manganese chloride solutions, and solid polymeric materials (polyurethane) were performed, followed by preliminary applications on human tibial bone in vivo. Direct on‐resonant saturation effects of the MT prepulses were calculated numerically by means of Blochs equations. Corrected for direct saturation effects dry and fresh bovine cortical bone showed “true” MTR values of 0.26 and 0.21, respectively. In vivo data were obtained from three healthy subjects and showed MTR values of 0.30 ± 0.08. In vivo studies into MT of cortical bone might have the potential to give new insights in musculoskeletal pathologies. Magn Reson Med, 2009.

Effects of in-pulse transverse relaxation in 3D ultrashort echo time sequences: analytical derivation, comparison to numerical simulation and experimental application at 3T.

The introduction of ultrashort-echo-time-(UTE)-sequences to clinical whole-body MR scanners has opened up the field of MR characterization of materials or tissues with extremely fast signal decay. If the transverse relaxation time is in the range of the RF-pulse duration, approximation of the RF-pulse by an instantaneous rotation applied at the middle of the RF-pulse and immediately followed by free relaxation will lead to a distinctly underestimated echo signal. Thus, the regular Ernst equation is not adequate to correctly describe steady state signal under those conditions. The paper presents an analytically derived modified Ernst equation, which correctly describes in-pulse relaxation of transverse magnetization under typical conditions: The equation is valid for rectangular excitation pulses, usually applied in 3D UTE sequences. Longitudinal relaxation time of the specimen must be clearly longer than RF-pulse duration, which is fulfilled for tendons and bony structures as well as many solid materials. Under these conditions, the proposed modified Ernst equation enables adequate and relatively simple calculation of the magnetization of materials or tissues. Analytically derived data are compared to numerical results obtained by using an established Runge-Kutta-algorithm based on the Bloch equations. Validity of the new approach was also tested by systematical measurements of a solid polymeric material on a 3T whole-body MR scanner. Thus, the presented modified Ernst equation provides a suitable basis for T1 measurements, even in tissues with T2 values as short as the RF-pulse duration: independent of RF-pulse duration, the variable flip angle method led to consistent results of longitudinal relaxation time T1, if the T2 relaxation time of the material of interest is known as well.

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 (∼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 rs = 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.

Quantitative Assessment of Visceral Fat in Morbidly Obese Patients by Means of Wide-Bore MRI and its Relation to Lower Esophageal Sphincter Pressure and Signs of Gastroesophageal Reflux

BackgroundThe aim of this study was to quantitatively assess visceral adipose tissue (VAT) by means of a wide-bore MR scanner in a cohort of morbidly obese patients referred for bariatric surgery. Furthermore, it was investigated whether gastroesophageal reflux disease (GERD) and lower esophageal sphincter pressure (LESP) are related to the volume of visceral fat masses.MethodsTwenty-five morbidly obese patients (nine male, 16 female) were prospectively enrolled. In addition to common anthropometric measures of obesity, VAT was determined quantitatively by multi-slice MRI. Symptoms of GERD were evaluated using a standardized questionnaire, while endoscopy of the upper gastrointestinal tract was performed to reveal pathologies of the gastroesophageal junction. LESP was evaluated by esophageal manometry.ResultsStudy population showed a body mass index (BMI) between 35.2 and 59.1xa0kg/m2. Waist-to-hip ratio and VAT were significantly higher (pu2009<u20090.0001; pu2009=u20090.0021) in males (1.05u2009±u20090.05; 8.89u2009±u20092.33xa0l) than in females (0.86u2009±u20090.07; 6.04u2009±u20091.28xa0l). VAT was not correlated to BMI. LESP values and GERD-related symptoms were neither dependent on anthropometric measures nor on VAT in our cohort.ConclusionsVAT did not show a positive correlation with BMI in our cohort of extremely obese subjects, indicating a pronounced fat deposition in subcutaneous tissue compartment. Moreover, this indicates that VAT is limited to a gender-dependent maximum volume for each individual and seems to be no further increasing in extremely obese subjects. This might be the reason that neither symptoms nor endoscopic findings of GERD nor LESP were significantly influenced by the stage of morbid obesity.

In Vitro Assessment of Needle Artifacts with an Interactive Three-dimensional MR Fluoroscopy System

PURPOSEnTo perform in vitro assessment of needle artifacts with an interactive three-dimensional (3D) near-real-time magnetic resonance (MR) fluoroscopy system for musculoskeletal interventions in a 1.5-T open-bore magnet.nnnMATERIALS AND METHODSnOne MR-compatible titanium needle, one MR-compatible Inconel (nickel-chromium superalloy) needle, and one MR-compatible carbon fiber needle were imaged with an interactive 3D MR sequence. Slice orientations, measurement parameters (fast low-angle shot, repetition time/echo time of 1,358/5 msec, flip angle of 15 degrees , bandwidth of 250 Hz/pixel; and true fast imaging with steady-state precession, repetition time/echo time of 770/2.2 msec, flip angle of 50 degrees , bandwidth of 558 Hz/pixel), phase encoding directions, and orientations to the main magnetic field (B(0)) were systematically varied. Artifact sizes, normalized artifact contrasts, and tip location errors were assessed for all variations of acquisition parameters and needles and compared with t tests.nnnRESULTSnMean artifact sizes, normalized artifact contrast, and tip location errors were 22.9 mm, 96%, and 5.4 mm, respectively, for the Inconel needle; 6.1 mm, 70%, and 0.3 mm, respectively, for the titanium needle; and 2.8 mm, 38%, and -1.9 mm, respectively, for the carbon fiber needle. Artifact widths depended on needle materials and needle orientation to B(0), with significant differences on ttests. Artifact contrast did not depend on measurement parameters. No significant influence on artifact character was found for changes in phase encoding direction and slice orientation.nnnCONCLUSIONSnBecause of its robustness in depicting needle artifacts, the interactive 3D MR fluoroscopy system appears to be suitable for MR-guided interventions. The titanium needle showed optimal artifacts with all combinations of measurement parameters. Artifacts with the other needles were too large (Inconel) or too small (carbon fiber).

Current problems and future opportunities of abdominal magnetic resonance imaging at higher field strengths.

Introduction of high-field-strength whole-body MR scanners to clinical routine made abdominal magnetic resonance (MR) imaging widely available. Higher field strength provides improved signal yield, but other issues such as shorter wavelength and increased power deposition of radiofrequency in tissue must also be taken into account. This review describes current problems and future opportunities of abdominal MR imaging at 3.0 T under special consideration of relevant physical properties and technical challenges: impact of higher field strength on signal-to-noise ratio, Larmor frequency, and chemical shift effects are elucidated in detail. Furthermore, changes in longitudinal and transverse relaxation times as well as increased susceptibility effects at 3.0 T are reported. General safety issues and limitations in radiofrequency power deposition are discussed. Subsequently, implications of the previously mentioned changed MR properties at 3.0 T on clinical abdominal examinations applying different sequence types are described.