A new concept for improved quantitative analysis of reversible transverse relaxation in tissues with variable microscopic field distribution.

PURPOSE: The intravoxel distribution of the magnetic field strongly influences signal dephasing after RF excitation and the resulting signal decay in gradient echo-based MRI. In this work, several different field distribution models were applied and tested for analysis of microscopic field characteristics within pixels. THEORY: A flexible model for improved pixel-wise characterization of the underlying field distribution is introduced. The proposed symmetric alpha-stable (SαS) distribution covers Lorentzian, Gaussian, and intermediate field distributions in a continuous way using a two-parametric (width and shape) function. METHODS: The new model was applied on human brain, potatoes (homogeneous isotropic tissue), and stems of pineapple (anisotropic fibrous tissue). Effects of microscopic structure and background gradients on the shape and the widths of the microscopic field distribution were analyzed using gradient echo sampling of the spin echo and multigradient-echo sequences. Effects of non-Lorentzian shapes of microscopic field distributions on the results of common T2∗ measurements with mono-exponential fitting of signal values were tested. RESULTS: Many pixels of the examined objects showed field characteristics in between Lorentzian and Gaussian shapes. Microscopic field inhomogeneities caused by microscopic susceptibility effects and background gradients sometimes led to rather Gaussian than Lorentzian field distribution. In cases with nearly Gaussian field distribution, mono-exponential fitting of the signal decay resulted in different T2∗ values, depending on the sampling points. CONCLUSIONS: Using the concept of more flexible distributions for characterization of microscopic susceptibility effects in tissue provides better fitting of data and nearly sampling point-independent results than common T2∗ measurements with mono-exponential fitting.

Spontaneous mechanical and electrical activities of human calf musculature at rest assessed by repetitive single-shot diffusion-weighted MRI and simultaneous surface electromyography.

PURPOSE: Assessment of temporal and spatial relations between spontaneous mechanical activities in musculature (SMAM) at rest as revealed by diffusion-weighted imaging (DWI) and electrical muscular activities in surface EMG (sEMG). Potential influences of static and radiofrequency magnetic fields on muscular activity on sEMG measurements at rest were examined systematically. METHODS: Series of diffusion-weighted stimulated echo planar imaging were recorded with concurrent sEMG measurements. Electrical activities in sEMG were analyzed by non-parametric Friedman and two-sample Kolmogorov-Smirnov test. Direct correlation of both modalities was investigated by temporal mapping of electrical activity in sEMG to DWI repetition interval. RESULTS: Electrical activities in sEMG and number of visible SMAMs in DWI showed a strong correlation (ρ = 0.9718). High accordance between sEMG activities and visible SMAMs in DWI in a near-surface region around sEMG electrodes was achieved. Characteristics of sEMG activities were almost similar under varying magnetic field conditions. CONCLUSION: Visible SMAMs in DWI have shown a close and direct relation to concurrent signals recorded by sEMG. MR-related magnetic fields had no significant effects on findings in sEMG. Hence, appearance of SMAMs in DWI should not be considered as imaging artifact or as effects originating from the special conditions of MR examinations. Spatial and temporal distributions of SMAMs indicate characteristics of spontaneous (microscopic) mechanical muscular action at rest. Therefore, DWI techniques should be considered as non-invasive tools for studying physiology and pathophysiology of spontaneous activities in resting muscle. Magn Reson Med 79:2784-2794, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Volumetric assessment of spontaneous mechanical activities by simultaneous multi-slice MRI techniques with correlation to muscle fiber orientation.

The purpose of this work was assessment of volumetric characteristics of spontaneous mechanical activities in musculature (SMAMs) by diffusion-weighted simultaneous multi-slice (DW-SMS) imaging and spatial correlation to anatomical structure, as revealed by fusion to fiber tractographic information derived from diffusion-tensor imaging (DTI). The feasibility of using DW-SMS to image spontaneous events in human musculature was assessed by phantom measurements. Series of DW-SMS images and DTI datasets were recorded from the resting calf of three human subjects. Simultaneously recorded SMAMs in multiple slices were analyzed regarding spatial extension by the Kolmogorov-Smirnov test. Direct correlation of spatial distribution of SMAMs and fiber orientation was investigated by mapping of muscle fibers to multi-slice SMAM datasets. The DW-SMS strategy allows simultaneous assessment of SMAMs in several slices of resting skeletal musculature, since 73.9% of SMAM-affected volumes have shown SMAMs in multiple DW-SMS slices. Spatial extension of SMAMs was highly correlated over different simultaneously recorded DW-SMS slices, and affected areas followed the orientation of muscle fibers with a connectivity ratio up to 57.18 ± 14.80% based on event count and connectivity count maps. In 89.2% of all SMAM-affected datasets muscle fiber connectivity was shown in at least two adjacent slices. Direct correlation between SMAMs in human lower leg musculature and underlying anatomical structure was revealed by high muscle fiber connectivity (89.2%). SMAMs have shown a wide distribution along the longitudinal muscle direction (73.9% in multiple DW-SMS slices) with direct involvement of muscle fibers. Correlation between SMAMs in multiple DW-SMS slices and crossing muscular fiber tracts provides evidence that SMAMs result from physiological processes in musculature. Fusion of DW-SMS with DTI facilitates non-invasive studies of muscle fiber involvement in SMAMs in resting muscle.

A fully automatic reference deconvolution strategy to increase the accuracy of in vivo lipid signal quantification.

PURPOSE: Lipid signals measured by (1)H MR spectroscopy cannot be adequately quantified by common fitting routines like VARPRO or AMARES, if lipid spectra are distorted by irregular spatial and temporal inhomogeneities of the static magnetic field during readout. A fully automatic reference deconvolution algorithm is presented that eliminates these distortions before application of fitting routines. METHODS: The measured signal of the dominant methyl resonance is isolated with aid of a spectral estimator (estimation of parameters via rotational invariance techniques) and used as reference signal for estimation of distortions. A Wiener filter is applied to deconvolve those distortions in the lipid spectrum. Performance of the algorithm is assessed for different bandwidths and shapes of distortions, using artificially distorted as well as measured data. RESULTS: Application of the fully automatic reference deconvolution algorithm on simulated spectra yields a distinct increase in quantification accuracy. Deconvolved in vivo spectra of subcutaneous fat indicate reduced spectral overlap after application of the proposed strategy. CONCLUSION: The proposed method is helpful for in vivo magnetic resonance spectroscopy of adipose tissue to correct for effects of field inhomogeneities within the voxel and for inevitable eddy current effects. Quantification accuracy is improved by eliminating distortions before application of fitting routines.

Whole-Body Diffusion Tensor Imaging: A Feasibility Study.

OBJECTIVE: The aim of this study was to demonstrate the feasibility of whole-body diffusion tensor imaging (DTI) as a promising tool for research applications, for instance, for investigation of systemic muscle diseases. MATERIALS AND METHODS: Twelve healthy volunteers (mean age, 26.6 years; range, 20-39 years) underwent whole-body magnetic resonance imaging at 3 T using an echo planar imaging sequence (b value, 400 s/mm) with 6 different spatial encoding directions. Coronal maps of DTI parameters including mean diffusivity, fractional anisotropy, and diffusion tensor eigenvalues (λ1-3) were generated using in-house MATLAB routines. Diffusion tensor imaging parameters were evaluated by region-of-interest analysis in skeletal muscle, cerebral gray and white matter, the kidneys, and the liver. RESULTS: The acquisition time was 79 minutes 12 seconds. The different organs could be clearly depicted on the parametrical maps. Exemplary values in skeletal muscle were mean diffusivity, 1.67 ± 0.16 × 10(-3) mm2/s; fractional anisotropy, 0.26 ± 0.03; λ1, 2.17 ± 0.20 × 10(-3) mm2/s; λ2, 1.64 ± 0.17 × 10(-3) mm2/s; and λ3, 1.22 ± 0.12 × 10(-3) mm2/s. CONCLUSION: Whole-body DTI is technically feasible. Further refinements are required to achieve a higher signal-to-noise ratio and improved spatial resolution. A possible clinical application could be the assessment of systemic myopathies.

Addressing spontaneous signal voids in repetitive single-shot DWI of musculature: spatial and temporal patterns in the calves of healthy volunteers and consideration of unintended muscle activities as underlying mechanism.

Single-shot diffusion-weighted MRI sensitive to different types of incoherent motion inside tissue shows sporadic signal voids with a considerable size (>1 cm) in calf musculature at rest. Spatial and temporal patterns of these signal voids and their dependence on measurement conditions were tested systematically in order to obtain more insight into the underlying mechanism. Lower leg muscles of 10 healthy subjects were examined by recording series of 1000 echo-planar single-shot scans with repetition time 500 ms and b-value 100 s/mm(2) . Effects of strength and orientation of motion sensitization gradients and of repetition times were analysed. Potential influences of arterial blood pulsations and positioning of the subject were studied. Comparison of calf muscle groups showed more frequent signal voids in gastrocnemius and soleus muscle compared with tibialis muscles. Large inter-individual variance in the total number of signal voids visible in a transverse slice of the lower leg was observed (minimum 40/1000 scans; maximum >550/1000 scans). Typical sizes of the affected muscular areas ranged from 1.5 to 2.5 cm in the transverse and from 1.5 to 7 cm in the head-feet direction. Signal voids occurred nearly independent of the cardiac phase and with similar frequencies for supine and prone positions. Resting calf muscles show spontaneous signal voids in single-shot DWI at low b-values with an irregular temporal and spatial pattern. Values of mean diffusivity, diffusion tensor parameters, and IVIM-derived perfusion are expected to be clearly distorted by such signal voids if no rejection of affected data is applied. Several potential causes for the signal voids are discussed. The most probable explanation for the phenomenon is seen in the occurrence of spontaneous incoherent mechanical activity in musculature based on weak muscle fibre contractions. If this is the case it opens up a new field for studies on the physiological role and regulation of these unintended muscle activities.

Quick water-selective excitation of fast relaxing tissues with 3D UTE sequences.

PURPOSE: The aim of this study was to implement a time effective 1-1 double pulse water-selective excitation (WE) into a three-dimensional ultrashort echo time (UTE) sequence (WE-UTE) for visualization of short-T2 tissues with positive contrast and sufficient suppression of surrounding fat. METHODS: First, an analytical description of magnetization components in the steady state applying WE-UTE was derived and results were compared with numerical simulations based on Bloch's equations. Parameters were optimized for best positive contrast between short-T2 tissues and fat under consideration of variable relaxation properties over a broad range. Maximal signal yield and signal efficiency of on-resonant protons were compared with UTE sequences with and without off-resonance fat saturation (FatSat). WE-UTE was exemplarily applied for in-vivo musculoskeletal imaging on a 3T whole-body MR unit. RESULTS: Steady state magnetization of WE-UTE could be described analytically and showed excellent accordance with numerical simulations. Even for tissues with T2 = 1 ms WE-UTE resulted in 79% of maximal signal yield of UTE without FatSat and was more efficient regarding signal yield if compared with UTE with FatSat. Using WE-UTE in-vivo tendons and ligaments could be well delineated with positive contrast to surrounding fat. CONCLUSION: WE-UTE provides a quick method for visualizing short-T2 tissues with positive contrast.

Utilizing echo-shifts in k-space for generation of positive contrast in areas with marked susceptibility alterations.

A technique for generation of positive contrast near susceptibility alterations utilizing echo-shifts in k-space is introduced, based on altered Larmor-frequencies and resulting phase-shifts accumulating during the echo-time at the site of local magnetic field gradients. 3D gradient-echo raw-data is acquired and weighted with an inverse Hanning filter. The filter partly suppresses central raw-data points, while maintaining outer areas. Reconstruction of the filtered raw-data results in images where pixels with apparent magnetic field gradients are highlighted against homogeneous pixels. Further processing steps are introduced to remove remaining intensities in the homogeneous parts of the filtered image. Feasibility is shown by an agar phantom containing magnetically labeled cells, with concentrations of 25, 50, 100, and 250 cells/µL, and by images of the human head. The technique allows detection of echo-shifted pixels with automatic suppression of magnetically homogeneous parts while keeping post-processing time short. Fewer than four labeled cells per pixel were clearly displayed with positive contrast. Application to the human head shows bright veins and complete suppression of homogeneous regions. The presented technique has high potential for specific detection of low concentrations of labeled cells or susceptibility altered regions in vivo with positive contrast, whereas areas with low spin density are not highlighted.

Positive contrast imaging of iron oxide nanoparticles with susceptibility-weighted imaging.

Superparamagnetic iron oxide particles can be utilized to label cells for immune cell and stem cell therapy. The labeled cells cause significant field distortions induced in their vicinity, which can be detected with magnetic resonance imaging (MRI). In conventional imaging, the signal voids arising from the field distortions lead to negative contrast, which is not desirable, as detection of the cells can be masked by native low signal tissue. In this work, a new method for visualizing magnetically labeled cells with positive contrast is proposed and described. The technique presented is based on the susceptibility-weighted imaging (SWI) post-processing algorithm. Phase images from gradient-echo sequences are evaluated pixel by pixel, and a mask is created with values ranging from 0 to 1, depending on the phase value of the pixel. The magnitude image is then multiplied by the mask. With an appropriate mask function, positive contrast in the vicinity of the labeled cells is created. The feasibility of this technique is proved using an agar phantom containing superparamagnetic iron oxide particles-labeled cells and an ex vivo bovine liver. The results show high potential for detecting even small labeled cell concentrations in structurally inhomogeneous tissue types.

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 (29 +/- 7 years) on a 3 T whole-body MR unit in three different positions of the foot (40 degrees plantarflexion, neutral ankle position (0 degrees ), and -10 degrees 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 ( approximately 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, lambda(2) and lambda(3) showed significant changes in relation to muscle length. In contrast, no change in lambda(1) could be found. This work reveals significant changes in diffusional characteristics induced by passive muscle shortening and stretching.

Influence of steady background gradients on the accuracy of molecular diffusion anisotropy measurements.

Spatial susceptibility variations of body components lead to local gradients of the static magnetic field. Effects of such background gradients on fractional diffusion anisotropy (FA) measurements on whole-body magnetic resonance units operating at 1.5, 3.0 and 7.0 T were analyzed theoretically and experimentally. Analytical expressions were derived for the cases of diffusion occurring in isotropic media and in tissues with cylindrical symmetry (e.g., white matter tracts or skeletal musculature). Typical magnitudes of background gradient strengths were estimated from in vivo and in vitro measurements with B0 field mapping sequences. Additionally, numerical simulations of magnetic field distributions and resulting field gradients were performed considering tissue-air interfaces in simplified geometrical arrangements. For media with isotropic diffusion, both measurements and analytical calculations showed increasing FA inaccuracy with stronger coupling between diffusion-encoding and background gradients. For cylindrical symmetry, FA values were estimated for a standard diffusion tensor imaging protocol in a realistic scenario. At 1 mm distance from a water-air interface, susceptibility-related background gradients amount to approximately 9 mT/m at 7 T and lead to a relative error of the measured FA of up to 48%. The error in the anisotropy assessment rises considerably with increasing field strength and must be taken into account especially for experimental and clinical studies on modern high-field systems.

An Efficient Modelling-Simulation-Analysis Workflow to Investigate Stump-Socket Interaction Using Patient-Specific, Three-Dimensional, Continuum-Mechanical, Finite Element Residual Limb Models.

The lack of an efficient modelling-simulation-analysis workflow for creating and utilising detailed subject-specific computational models is one of the key reasons why simulation-based approaches for analysing socket-stump interaction have not yet been successfully established. Herein, we propose a novel and efficient modelling-simulation-analysis workflow that uses commercial software for generating a detailed subject-specific, three-dimensional finite element model of an entire residual limb from Diffusion Tensor MRI images in <20 min. Moreover, to complete the modelling-simulation-analysis workflow, the generated subject-specific residual limb model is used within an implicit dynamic FE simulation of bipedal stance to predict the potential sites of deep tissue injury. For this purpose, a nonlinear hyperelastic, transversely isotropic skeletal muscle constitutive law containing a deep tissue injury model was implemented in LS-DYNA. To demonstrate the feasibility of the entire modelling-simulation-analysis workflow and the fact that detailed, anatomically realistic, multi-muscle models are superior to state-of-the-art, fused-muscle models, an implicit dynamic FE analysis of 2-h bipedal stance is carried out. By analysing the potential volume of damaged muscle tissue after donning an optimally-fitted and a misfitted socket, i.e., a socket whose volume was isotropically shrunk by 10%, we were able to highlight the differences between the detailed individual- and fused-muscle models. The results of the bipedal stance simulation showed that peak stresses in the fused-muscle model were four times lower when compared to the multi-muscle model. The peak interface stress in the individual-muscle model, at the end of bipedal stance analysis, was 2.63 times lower than that in the deep tissues of the stump. At the end of the bipedal stance analysis using the misfitted socket, the fused-muscle model predicted that 7.65% of the residual limb volume was injured, while the detailed-model predicted 16.03%. The proposed approach is not only limited to modelling residual limbs but also has applications in predicting the impact of plastic surgery, for detailed forward-dynamics simulations of normal musculoskeletal systems.

Tumor detection by diffusion-weighted MRI and ADC-mapping--initial clinical experiences in comparison to PET-CT.

OBJECTIVE: To evaluate the clinical potential of diffusion-weighted-imaging (DWI) with apparent diffusion coefficient (ADC)-mapping for tumor detection. MATERIALS AND METHODS: A single-shot echo-planar-imaging DWI sequence with fat suppression and ability for navigator-based respiratory triggering was implemented. Nineteen patients (11 melanoma, 4 prostate cancer, 1 non-Hodgkin lymphoma, and 3 lung cancer) were examined by positron emission tomography (PET) with an integrated computed tomography scanner (PET-CT) and DWI. Images at b = 0, 400, and 1000 s/mm2 were acquired and ADC maps were generated. PET examinations were used as a reference for tumor detection. Four hundred twenty-four regions of interest were used for DWI and 73 for PET data evaluation. RESULTS: DWI and ADC maps were of diagnostic quality. Metastases with increased tracer uptake were clearly visualized at b = 1000 s/mm2 with the exception of mediastinal lymph node metastases in cases of lung cancer. ADC mapping did not improve detection rates. CONCLUSIONS: DWI is a feasible clinical technique, improving the assessment of metastatic spread in routine magnetic resonance imaging examinations.

Effects on MRI due to altered rf polarization near conductive implants or instruments.

In magnetic resonance imaging near metal parts variations in radio frequency (rf)-amplitude and of receive sensitivity must be considered. For loop structures, e.g., vascular stents, B1 produces rf eddy currents in accordance to Faraday's law; the B1-related electrical rf field E1 injects directly to elongated structures (e.g., wires). Locally, the rf magnetic field Bl,ind (induced B1) is superimposed onto the rf field from the transmitter coil, which near the metal can dominate spin excitation. Geometry and arrangement of the parts determine the polarization of B(1,ind). Components parallel to B0 are of special interest. A copper sheet (100 mm x 15 mm, 3 mm thick) and a 27 cm long copper wire were examined in a water phantom using the spin-echo (SE) technique. In addition to rf-amplitude amplification, rf-phase shift due to z components of B(1,ind) could be detected near the metallic objects. Periodic rf-amplitude instabilities had an amplified effect for phase-shifted regions. Phase-encoding artifacts occurred as distinct ghosts (TR=200 ms) or band-like smearing (TR=201 ms) from affected spin ensembles. SE phase imaging can potentially be used in interventional magnetic resonance imaging for background-free localization of metallic markers.

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.

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.

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.

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.

Rapid assessment of longitudinal relaxation time in materials and tissues with extremely fast signal decay using UTE sequences and the variable flip angle method.

OBJECTIVES: : To develop suitable strategies for quantification of longitudinal relaxation time (T1) by means of ultrashort echo time (UTE) sequences and the variable flip-angle approach in materials and tissues with extremely fast signal decay. MATERIALS AND METHODS: : A recently published modified Ernst equation, which correctly accounts for in-pulse relaxation of transverse magnetization, was used to numerically determine optimal flip angles for reliable assessment of T1 in case of extremely short effective transverse relaxation time (T2*). Various ratios of repetition time (TR) to T1 and radiofrequency (RF) pulse duration (TRF) to T2* were evaluated. Theoretical considerations were applied to solid polymeric material (T2* = 0.295 milliseconds), and T1 quantification was performed using various optimized flip-angle approaches at different RF pulse durations (TRF = 0.1-0.4 milliseconds). Furthermore, in vivo measurement of T1 in cortical bone was exemplarily performed in 3 healthy volunteers to test the applicability of the proposed method in vivo. For in vitro and in vivo studies, MR imaging was performed on a 3 T whole-body MR system using a 3D UTE sequence with a rectangular excitation pulse and centric radial readout. RESULTS: : Optimal flip angles were shown to be strongly dependent on TR/T1 and TRF/T2* ratios. Exemplarily, longitudinal relaxation time of the investigated solid polymeric material was determined to T1 = 223.1 ± 3.1 milliseconds with RF pulse duration of TRF = 0.2 milliseconds, and 12 acquired flip angles ranging from 5 to 60 degrees. Using only 2 optimized flip angles (8 degrees, 44 degrees), T1 of the same material was determined to T1 = 223.8 ± 4.2 milliseconds in a markedly less acquisition time. In vivo evaluation of cortical bone was feasible and showed T1 values of 80.4 ± 25.1 milliseconds, exemplarily. CONCLUSIONS: : Using the modified Ernst equation, it seems possible to rapidly evaluate 3D distribution of longitudinal relaxation time in materials and tissues with extremely fast signal decay by means of UTE sequences and only 2 measurements with optimized flip angles.

[Relaxation times T1, T2, and T2* of apples, pears, citrus fruits, and potatoes with a comparison to human tissues]. / T1-, T2- und T2*-Relaxationswerte von Äpfeln, Birnen, Zitrusfrüchten und Kartoffeln im Vergleich zu menschlichen Geweben.

The aim of the project was a systematic assessment of relaxation times of different fruits and vegetables and a comparison to values of human tissues. Results provide an improved basis for selection of plant phantoms for development of new MR techniques and sequences. Vessels filled with agar gel are mostly used for this purpose, preparation of which is effortful and time-consuming. In the presented study apples, (malus, 8 species), pears, (pyrus, 2 species), citrus fruits (citrus, 5 species) and uncooked potatoes (solanum tuberosum, 8 species) from the supermarket were examined which are easily available nearly all-the-year. T1, T2 and T2* relaxation times of these nature products were measured on a 1.5 Tesla MR system with adapted examination protocols and mono-exponential fitting, and compared to literature data of human parenchyma tissues, fatty tissue and body fluid (cerebrospinal fluid). Resulting values were as follows: apples: T1: 1486-1874 ms, T2: 163-281 ms, T2*: 2.3-3.2 ms; pears: T1: 1631-1969 ms, T2: 119-133 ms, T2* : 10.1-10.6 ms, citrus fruits (pulp) T1: 2055-2632 ms, T2: 497-998 ms, T2* : 151-182 ms; citrus fruits (skin) T1: 561-1669 ms, T2: 93-119 ms; potatoes: T1: 1011-1459 ms, T2: 166 - 210 ms, T2* : 20 - 30 ms. All T1-values of the examined objects (except for potatoes and skins of citrus fruits) were longer than T1 values of human tissues. Also T2 values (except for pears and skins of citrus fruits) of the fruits and the potatoes tended to be longer. T2* values of apples, pears and potatoes were shorter than in healthy human tissue. Results show relaxation values of many fruits to be not exactly fitting to human tissue, but with suitable selection of the fruits and optionally with an adaption of measurement parameters one can achieve suitable contrast and signal characteristics for some purposes.