Estimation of metabolite <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript> relaxation times using tissue specific analysis,signal averaging and bootstrapping from magnetic resonance spectroscopic imaging data

Object A novel method of estimating metabolite T ₁ relaxation times using MR spectroscopic imaging (MRSI) is proposed. As opposed to conventional single-voxel metabolite T ₁ estimation methods, this method investigates regional and gray matter (GM)/white matter (WM) differences in metabolite T ₁ by taking advantage of the spatial distribution information provided by MRSI. Material and methods The method, validated by Monte Carlo studies, involves a voxel averaging to preserve the GM/WM distribution, a non-linear least squares fit of the metabolite T ₁ and an estimation of its standard error by bootstrapping. It was applied in vivo to estimate the T ₁ of N-acetyl compounds (NAA), choline, creatine and myo-inositol in eight normal volunteers, at 1.5 T, using a short echo time 2D-MRSI slice located above the ventricles. Results WM-T _1,NAA was significantly (P < 0.05) longer in anterior regions compared to posterior regions of the brain. The anterior region showed a trend of a longer WM T ₁ compared to GM for NAA, creatine and myo-Inositol. Lastly, accounting for the bootstrapped standard error estimate in a group mean T ₁ calculation yielded a more accurate T ₁ estimation. Conclusion The method successfully measured in vivo metabolite T ₁ using MRSI and can now be applied to diseased brain.     

Hyperpolarized <Superscript>3</Superscript>He lung ventilation imaging with <Emphasis Type="Italic">B</Emphasis><Subscript>1</Subscript>-inhomogeneity correction in a single breath-hold scan

The signal-to-noise ratio (SNR) of hyperpolarized noble gas MR images is sensitive to the flip angle used. Variations in flip angle due to B ₁-inhomogeneity of the RF coil cause intensity variation artifacts in lung ventilation images which may mask or mimic disease. We show these artifacts can be minimized by using the optimal flip angle and corrected if the local flip angle is known. Hyperpolarized ³He lung images were obtained in ten healthy subjects using both a conventional gradient-echo sequence and a new hybrid pulse sequence designed to simultaneously acquire lung ventilation images and corresponding flip-angle maps in comparable imaging time. Flip-angle maps and corrected images were calculated from the hybrid scan and compared with conventional images. The qualitative theoretical dependence of flip angle on SNR was verified. Ventilation images and flip-angle maps were successfully obtained with the hybrid sequence. Corrections to image intensity calculated from the flip-angle maps appeared reasonable for images acquired using an average flip angle near optimal. Use of the optimal flip angle is crucial to the quality of lung ventilation images. Artifactual intensity variations due to RF-coil inhomogeneity may be identified and potentially corrected using our hybrid sequence. Acknowledgement The authors thank John M. Christopher, RT(R)(MR) for valuable assistance performing the studies, and Jaime F. Mata, MS, Jing Cai, MS, and Andrew G. Reish, BS, for excellent operation of the ³He polarization system. We gratefully acknowledge support for this research from the Commonwealth of Virginia Technology Research Fund (Grant No. IN2002-01), Siemens Medical Solutions and Amersham Health.     

The Applicability of Sr-deficient <Emphasis Type="Italic">n</Emphasis>-type SrTiO<Subscript>3</Subscript> for SOFC Anodes

Here we discuss the effect of preparation conditions on structural stability and electrical properties of Sr-deficient n-type SrTiO₃. In particular, an explanation of a wide scatter of conductivity values in Y- and Nb-doped SrTiO₃. reported in the literature is proposed, based on the existing defect chemistry model of n-doped SrTiO₃. It was confirmed that when sintered in air, Sr-deficient SrTiO₃ doped with Nb and/or Y, remains single phase until the solubility limit (e.g., 30% for Nb or 4% for Y). However, when sintered at low po₂, the material transforms from a vacancy compensated to an electronically compensated compound with a strontium deficient second phase. Measured at 800°C in low po₂, the maximum conductivity of these multi-phase compounds was 340 S/cm and 100 S/cm for the Nb-doped and Y-doped sample, respectively. However, the conductivity dropped dramatically to less than 10 S/cm when samples of the same compositions were sintered in air, again measured in reducing atmosphere.     

A novel alternative to classify tissues from <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript> and <Emphasis Type="Italic">T</Emphasis><Subscript>2</Subscript> relaxation times for prostate MRI

Objective

To segment and classify the different attenuation regions from MRI at the pelvis level using the T ₁ and T ₂ relaxation times and anatomical knowledge as a first step towards the creation of PET/MR attenuation maps.

Materials and methods

Relaxation times were calculated by fitting the pixel-wise intensities of acquired T ₁- and T ₂-weighted images from eight men with inversion-recovery and multi-echo multi-slice spin-echo sequences. A decision binary tree based on relaxation times was implemented to segment and classify fat, muscle, prostate, and air (within the body). Connected component analysis and an anatomical knowledge-based procedure were implemented to localize the background and bone.

Results

Relaxation times at 3 T are reported for fat (T ₁ = 385 ms, T ₂ = 121 ms), muscle (T ₁ = 1295 ms, T ₂ = 40 ms), and prostate (T ₁ = 1700 ms, T ₂ = 80 ms). The relaxation times allowed the segmentation–classification of fat, prostate, muscle, and air, and combined with anatomical knowledge, they allowed classification of bone. The good segmentation–classification of prostate [mean Dice similarity score (mDSC) = 0.70] suggests a viable implementation in oncology and that of fat (mDSC = 0.99), muscle (mDSC = 0.99), and bone (mDSCs = 0.78) advocates for its implementation in PET/MR attenuation correction.

Conclusion

Our method allows the segmentation and classification of the attenuation-relevant structures required for the generation of the attenuation map of PET/MR systems in prostate imaging: air, background, bone, fat, muscle, and prostate.

     

Low Temperature Sintering of BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript>-Based Middle-<Emphasis Type="Italic">k</Emphasis> Dielectric Composition for LTCC Applications

Effect of glass addition on the low-temperature sintering and microwave dielectric properties of BaTi₄O₉-based ceramics were studied to develop the middle-k dielectric composition for the functional substrate of low-temperature co-fired ceramics. When 10 wt% of glass was added, sufficient densification was obtained and the relative density more than 98% was reached at the sintering temperature of 875C. The microwave dielectric properties were k = 32, Q × f = 9000 GHz, and t_cf = 10 ppm/C. As the added amount of glass frit with base dielectric composition, phase changes from BaTi₄O₉ to BaTi₅O₁₁ and Ba₄Ti₁₃O₃₀ was observed, which result in the modification of microwave dielectric properties.     

3D <Emphasis Type="Italic">T</Emphasis><Subscript>2</Subscript>-weighted imaging at 7T using dynamic k<Subscript>T</Subscript>-points on single-transmit MRI systems

Objectives

For turbo spin echo (TSE) sequences to be useful at ultra-high field, they should ideally employ an RF pulse train compensated for the B ₁ ⁺ inhomogeneity. Previously, it was shown that a single k_T-point pulse designed in the small tip-angle regime can replace all the pulses of the sequence (static k_T-points). This work demonstrates that the B ₁ ⁺ dependence of T ₂-weighted imaging can be further mitigated by designing a specific k_T-point pulse for each pulse of a 3D TSE sequence (dynamic k_T-points) even on single-channel transmit systems

Materials and methods

By combining the spatially resolved extended phase graph formalism (which calculates the echo signals throughout the sequence) with a gradient descent algorithm, dynamic k_T-points were optimized such that the difference between the simulated signal and a target was minimized at each echo. Dynamic k_T-points were inserted into the TSE sequence to acquire in vivo images at 7T.

Results

The improvement provided by the dynamic k_T-points over the static k_T-point design and conventional hard pulses was demonstrated via simulations. Images acquired with dynamic k_T-points showed systematic improvement of signal and contrast at 7T over regular TSE—especially in cerebellar and temporal lobe regions without the need of parallel transmission.

Conclusion

Designing dynamic k_T-points for a 3D TSE sequence allows the acquisition of T ₂-weighted brain images on a single-transmit system at ultra-high field with reduced dropout and only mild residual effects due to the B ₁ ⁺ inhomogeneity.

     

Acoustic noise reduction in <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript>- and proton-density-weighted turbo spin-echo imaging

Objective

To reduce acoustic noise levels in T ₁-weighted and proton-density-weighted turbo spin-echo (TSE) sequences, which typically reach acoustic noise levels up to 100 dB(A) in clinical practice.

Materials and methods

Five acoustic noise reduction strategies were combined: (1) gradient ramps and shapes were changed from trapezoidal to triangular, (2) variable-encoding-time imaging was implemented to relax the phase-encoding gradient timing, (3) RF pulses were adapted to avoid the need for reversing the polarity of the slice-rewinding gradient, (4) readout bandwidth was increased to provide more time for gradient activity on other axes, (5) the number of slices per TR was reduced to limit the total gradient activity per unit time. We evaluated the influence of each measure on the acoustic noise level, and conducted in vivo measurements on a healthy volunteer. Sound recordings were taken for comparison.

Results

An overall acoustic noise reduction of up to 16.8 dB(A) was obtained by the proposed strategies (1–4) and the acquisition of half the number of slices per TR only. Image quality in terms of SNR and CNR was found to be preserved.

Conclusions

The proposed measures in this study allowed a threefold reduction in the acoustic perception of T ₁-weighted and proton-density-weighted TSE sequences compared to a standard TSE-acquisition. This could be achieved without visible degradation of image quality, showing the potential to improve patient comfort and scan acceptability.

     

Torque ripple minimization with optimized <Emphasis Type="Italic">i</Emphasis><Subscript><Emphasis Type="Italic">sx</Emphasis></Subscript> estimation in vector control of current source inverter induction motor

In variable speed applications of high-power induction motors, current source inverters are usually used instead of voltage source inverters. Power switches delay and switching losses in high-power ranges, prevents inverters to operate properly in high frequencies. So the voltage and current waveforms of the motor supply consist of high harmonic content causing a high-torque ripple. One of the parameters that influences the torque ripple most is the stator flux reference value. Motors do not usually operate in their nominal load. Therefore, the optimum value of the flux is not equal to the nominal value. In this paper, an optimal i _sx reference for vector control system of current source inverter fed induction motor is produced by applying a simple estimator. Simulation results show that torque ripple has been reduced, especially in the loads less than their nominal power, as significantly as power factor has been improved. Also flux reduction causes to noticeable reduction in core and switching losses.     

The impact of fibre orientation on <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript>-relaxation and apparent tissue water content in white matter

Objective

Recent MRI studies have shown that the orientation of nerve fibres relative to the main magnetic field affects the R₂*(= 1/T₂*) relaxation rate in white matter (WM) structures. The underlying physical causes have been discussed in several studies but are still not completely understood. However, understanding these effects in detail is of great importance since this might serve as a basis for the development of new diagnostic tools and/or improve quantitative susceptibility mapping techniques. Therefore, in addition to the known angular dependence of R₂*, the current study investigates the relationship between fibre orientation and the longitudinal relaxation rate, R₁ (= 1/T₁), as well as the apparent water content.

Materials and methods

For a group of 16 healthy subjects, a series of gradient echo, echo-planar and diffusion weighted images were acquired at 3T from which the decay rates, the apparent water content and the diffusion direction were reconstructed. The diffusion weighted data were used to determine the angle between the principle fibre direction and the main magnetic field to examine the angular dependence of R₁ and apparent water content.

Results

The obtained results demonstrate that both parameters depend on the fibre orientation and exhibit a positive correlation with the angle between fibre direction and main magnetic field.

Conclusion

These observations could be helpful to improve and/or constrain existing biophysical models of brain microstructure by imposing additional constraints resulting from the observed angular dependence R₁ and apparent water content in white matter.

     

Intravascular contrast agent <Emphasis Type="Italic">T1</Emphasis> shortening: fast <Emphasis Type="Italic">T1</Emphasis> relaxometry in a carotid volunteer study

OBJECTIVE: To measure the T1 times in blood after the administration of the intravascular contrast agent gadofosveset trisodium in humans. MATERIALS AND METHODS: In a pilot study for parameter optimization, the T1-shortening induced by the injection of a single dose (0.03 mmol/kg body weight) of the MR contrast agent Vasovist (Bayer Schering Pharma AG) was measured at B (0) = 1.5 T as a function of time. In four sessions, T1 measurements were performed in the carotid vein of 9 volunteers up to 30 min after injection. T1 times were measured using a segmented saturation recovery turboFLASH (SSRTFL) pulse sequence with 7 different saturation recovery delay times in a total acquisition time of 20 s. RESULTS: The SSRTFL measurements showed T1 times of about 100 ms immediately after injection, which gradually increased to 175 ms at 30 min. The time curve of the R1 = 1/T1 averaged over all volunteers could be described with an exponential decay with a time constant T = 330 +/- 65 s and an amplitude DeltaR1 = 4.1 +/- 0.3 s(-1), and a constant offset of R1(0) = 5.7 +/- 0.2 s(-1). Mean relaxation values are in excellent agreement with theoretical predictions. CONCLUSION: An analytical expression for the initial T1-shortening of Vasovist was derived which can now be used for optimization of the pulse sequence parameters in clinical studies.     

Dielectric properties and defect mechanisms of (1-<Emphasis Type="Italic">x</Emphasis>)Ba(Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> -<Emphasis Type="Italic">x</Emphasis>BiYbO<Subscript>3</Subscript> ceramics

(1-x)Ba(Fe_0.5Nb_0.5)O₃ -xBiYbO₃ (BFN-xBY) ceramics were prepared by a conventional solid-state reaction method. The dielectric properties and relaxation behavior of BFN-xBY ceramics were analyzed according to dielectric and impedance spectroscopy. Dielectric permittivity of the ceramics increases with increasing temperature below 500 K then remains unchanged up to 700 K, while corresponding loss factor decreases with the increase of temperature below 500 K then increase slowly. Defect compensation mechanism of this system was analyzed in detail. The giant dielectric behavior of the ceramics arises from the internal barrier layer capacitor (IBLC) effect. Polarization effect at insulating grain boundaries between semiconducting grains accompanied by a strong Maxwell-Wagner (MW) relaxation mode. The characteristic of grain boundaries was revealed using impedance spectroscope and the universal dielectric response law.     

High cerebral <Emphasis Type="Italic">scyllo</Emphasis>-inositol: a new marker of brain metabolism disturbances induced by chronic alcoholism

Cerebral metabolic changes that concur to motor and/or cognitive disorders in actively drinking alcoholics are not well established. We tested the hypothesis that chronic alcoholics exhibit profound alterations in the cerebral metabolism of scyllo-inositol. Brain metabolism was explored in nine actively drinking and 11 recently detoxified chronic alcoholics by in vivo brain ¹H-MRS and in vitro ¹H-MRS of blood serum and cerebrospinal fluid. The cohort was composed of individuals with acute, subacute or chronic encephalopathy or without any clinical encephalopathy. Chronic alcoholism is associated with a hitherto unrecognized accumulation of brain scyllo-inositol. Our results suggest that scyllo-inositol is produced within the central nervous system and shows a diffuse but heterogenous distribution in brain where it can persist several weeks after detoxification. Its highest levels were observed in subjects with a clinically symptomatic alcohol-related encephalopathy. When detected, brain scyllo-inositol takes part in a metabolic encephalopathy since it is associated with reduced N-acetylaspartate and increased creatine. High levels of cerebral scyllo-inositol are correlated with altered glial and neuronal metabolism. Our findings suggest that the accumulation of scyllo-inositol may precede and take part in the development of symptomatic alcoholic metabolic encephalopathy.     

Self-consistent calculation for valence subband structure and hole mobility in <Emphasis Type="Italic">p</Emphasis>-channel inversion layers

Using six- and eight-band k⋅p models—with parameters calibrated against the bulk band structure obtained using non-local empirical pseudopotentials—we have employed a new hybrid self-consistent method to calculate the valence subband structure in p-channel inversion layers of InAs, InSb, GaAs, In_0.53Ga_0.47As, and GaSb. This method involves two separate stages: first, density-of-states (DOS) of the three lowest-energy subbands (heavy, light, and split-off holes) is calculated using the triangular-well approximation. Then, the self-consistent calculation is performed using the DOS previously obtained, but shifting each subband by the amount obtained from the self-consistent eigenvalues obtained during the self-consistent iteration. Finally, we present results regarding the hole mobility in Ge p-channel inversion layers. The results are compared to those obtained employing the subband structure computed with the triangular-well approximation and also with experimental data.     

Controlled<Emphasis Type="Bold"> E</Emphasis>-field gradient coils

Peripheral neural stimulation is a major problem in current gradient coil designs. Induced current problems in patients relate directly to gradient strength and modulation frequency. Current designs of gradient coil tend to limit ultra-high-speed imaging methods such as echo-planar imaging through the effect of induced currents which produce tingling sensations and involuntary muscle twitch. Neural stimulation could also trigger epileptic fits and/or cardiac fibrillation. For reduction of induced currents, an important aspect is the coil geometry. It is desirable to design the gradient coil in such a way as to prevent closed loop circulating currents within the body. Preliminary results using a four-sector gradient coil with rectangular geometry, operating in a low mutual coupling mode, indicate significant reduction in the E-field within the subject volume of the coil. Reduction in induced currents in the patient allows safer operation at higher magnetic field strengths together with faster scans currently prohibited through neural stimulation effects in standard coil geometries.     

Identification of myocardial diffuse fibrosis by 11 heartbeat MOLLI <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript> mapping: averaging to improve precision and correlation with collagen volume fraction

Objectives

Our objectives involved identifying whether repeated averaging in basal and mid left ventricular myocardial levels improves precision and correlation with collagen volume fraction for 11 heartbeat MOLLI T ₁ mapping versus assessment at a single ventricular level.

Materials and methods

For assessment of T ₁ mapping precision, a cohort of 15 healthy volunteers underwent two CMR scans on separate days using an 11 heartbeat MOLLI with a 5(3)3 beat scheme to measure native T ₁ and a 4(1)3(1)2 beat post-contrast scheme to measure post-contrast T ₁, allowing calculation of partition coefficient and ECV. To assess correlation of T ₁ mapping with collagen volume fraction, a separate cohort of ten aortic stenosis patients scheduled to undergo surgery underwent one CMR scan with this 11 heartbeat MOLLI scheme, followed by intraoperative tru-cut myocardial biopsy. Six models of myocardial diffuse fibrosis assessment were established with incremental inclusion of imaging by averaging of the basal and mid-myocardial left ventricular levels, and each model was assessed for precision and correlation with collagen volume fraction.

Results

A model using 11 heart beat MOLLI imaging of two basal and two mid ventricular level averaged T ₁ maps provided improved precision (Intraclass correlation 0.93 vs 0.84) and correlation with histology (R ² = 0.83 vs 0.36) for diffuse fibrosis compared to a single mid-ventricular level alone. ECV was more precise and correlated better than native T ₁ mapping.

Conclusion

T ₁ mapping sequences with repeated averaging could be considered for applications of 11 heartbeat MOLLI, especially when small changes in native T ₁/ECV might affect clinical management.

     

Towards accurate and precise <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript> and extracellular volume mapping in the myocardium: a guide to current pitfalls and their solutions

Mapping of the longitudinal relaxation time (T ₁) and extracellular volume (ECV) offers a means of identifying pathological changes in myocardial tissue, including diffuse changes that may be invisible to existing T ₁-weighted methods. This technique has recently shown strong clinical utility for pathologies such as Anderson-Fabry disease and amyloidosis and has generated clinical interest as a possible means of detecting small changes in diffuse fibrosis; however, scatter in T ₁ and ECV estimates offers challenges for detecting these changes, and bias limits comparisons between sites and vendors. There are several technical and physiological pitfalls that influence the accuracy (bias) and precision (repeatability) of T ₁ and ECV mapping methods. The goal of this review is to describe the most significant of these, and detail current solutions, in order to aid scientists and clinicians to maximise the utility of T ₁ mapping in their clinical or research setting. A detailed summary of technical and physiological factors, issues relating to contrast agents, and specific disease-related issues is provided, along with some considerations on the future directions of the field.     

Correction of <Emphasis Type="Italic">B</Emphasis><Subscript>0</Subscript>-induced geometric distortion variations in prospective motion correction for 7T MRI

Objective

Prospective motion correction can effectively fix the imaging volume of interest. For large motion, this can lead to relative motion of coil sensitivities, distortions associated with imaging gradients and B ₀ field variations. This work accounts for the B ₀ field change due to subject movement, and proposes a method for correcting tissue magnetic susceptibility-related distortion in prospective motion correction.

Materials and methods

The B ₀ field shifts at the different head orientations were characterized. A volunteer performed large motion with prospective motion correction enabled. The acquired data were divided into multiple groups according to the object positions. The correction of B ₀-related distortion was applied to each group of data individually via augmented sensitivity encoding with additionally integrated gradient nonlinearity correction.

Results

The relative motion of the gradients, B ₀ field and coil sensitivities in prospective motion correction results in residual spatial distortion, blurring, and coil artifacts. These errors can be mitigated by the proposed method. Moreover, iterative conjugate gradient optimization with regularization provided superior results with smaller RMSE in comparison to standard conjugate gradient.

Conclusion

The combined correction of B ₀-related distortion and gradient nonlinearity leads to a reduction of residual motion artifacts in prospective motion correction data.

     

On high suppression of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> and CO emissions in gas-turbine plants with combined gas-and-steam cycles

In this work are given results of analyzing processes of production of nitrogen oxides (NO _x ) and afterburning of CO when firing natural gas at combined-cycle gas-turbine plants. It is shown that for suppressing emissions of the said microcomponents it is necessary to lower temperature in hot local zones of the flame in which NO_x is formed, and, in so doing, to avoid chilling of cold flame zones that prevents afterburning of CO. The required lowering of the combustion temperature can be provided by combustion of mixtures of methane with steam, with high mixing uniformity that ensures the same and optimum fraction of the steam “ballast” in each microvolume of the flame. In addition to chilling, the steam ballast makes it possible to maintain a fairly high concentration of hydroxil radicals in the flame zone as well, and this provides high burning out of fuel and reduction in carbon monoxide emissions (active steam ballast). Due to this fact the fraction of steam when firing its mixtures with methane in a gas-turbine plant can be increased up to the weight ratio 4: 1. In this case, the concentrations of NO _x and CO in emissions can be reduced to ultra-low values (less than 3 ppm).     

Pulsed arterial spin labelling at ultra-high field with a <Emphasis Type="Italic">B</Emphasis><Stack><Subscript>1</Subscript><Superscript>+</Superscript></Stack>-optimised adiabatic labelling pulse

Objective

Arterial spin labelling (ASL) techniques benefit from the increased signal-to-noise ratio and the longer T ₁ relaxation times available at ultra-high field. Previous pulsed ASL studies at 7 T concentrated on the superior regions of the brain because of the larger transmit radiofrequency inhomogeneity experienced at ultra-high field that hinders an adequate inversion of the blood bolus when labelling in the neck. Recently, researchers have proposed to overcome this problem with either the use of dielectric pads, through dedicated transmit labelling coils, or special adiabatic inversion pulses.

Materials and methods

We investigate the performance of an optimised time-resampled frequency-offset corrected inversion (TR-FOCI) pulse designed to cause inversion at much lower peak B ₁ ⁺ . In combination with a PICORE labelling, the perfusion signal obtained with this pulse is compared against that obtained with a FOCI pulse, with and without dielectric pads.

Results

Mean grey matter perfusion with the TR-FOCI was 52.5 ± 10.3 mL/100 g/min, being significantly higher than the 34.6 ± 2.6 mL/100 g/min obtained with the FOCI pulse. No significant effect of the dielectric pads was observed.

Conclusion

The usage of the B ₁ ⁺ -optimised TR-FOCI pulse results in a significantly higher perfusion signal. PICORE–ASL is feasible at ultra-high field with no changes to operating conditions.

     

Volume measurements of individual muscles in human <Emphasis Type="Italic">quadriceps femoris</Emphasis> using atlas-based segmentation approaches

Objectives

Atlas-based segmentation is a powerful method for automatic structural segmentation of several sub-structures in many organs. However, such an approach has been very scarcely used in the context of muscle segmentation, and so far no study has assessed such a method for the automatic delineation of individual muscles of the quadriceps femoris (QF). In the present study, we have evaluated a fully automated multi-atlas method and a semi-automated single-atlas method for the segmentation and volume quantification of the four muscles of the QF and for the QF as a whole.

Subjects and methods

The study was conducted in 32 young healthy males, using high-resolution magnetic resonance images (MRI) of the thigh. The multi-atlas-based segmentation method was conducted in 25 subjects. Different non-linear registration approaches based on free-form deformable (FFD) and symmetric diffeomorphic normalization algorithms (SyN) were assessed. Optimal parameters of two fusion methods, i.e., STAPLE and STEPS, were determined on the basis of the highest Dice similarity index (DSI) considering manual segmentation (MSeg) as the ground truth. Validation and reproducibility of this pipeline were determined using another MRI dataset recorded in seven healthy male subjects on the basis of additional metrics such as the muscle volume similarity values, intraclass coefficient, and coefficient of variation. Both non-linear registration methods (FFD and SyN) were also evaluated as part of a single-atlas strategy in order to assess longitudinal muscle volume measurements. The multi- and the single-atlas approaches were compared for the segmentation and the volume quantification of the four muscles of the QF and for the QF as a whole.

Results

Considering each muscle of the QF, the DSI of the multi-atlas-based approach was high 0.87 ± 0.11 and the best results were obtained with the combination of two deformation fields resulting from the SyN registration method and the STEPS fusion algorithm. The optimal variables for FFD and SyN registration methods were four templates and a kernel standard deviation ranging between 5 and 8. The segmentation process using a single-atlas-based method was more robust with DSI values higher than 0.9. From the vantage of muscle volume measurements, the multi-atlas-based strategy provided acceptable results regarding the QF muscle as a whole but highly variable results regarding individual muscle. On the contrary, the performance of the single-atlas-based pipeline for individual muscles was highly comparable to the MSeg, thereby indicating that this method would be adequate for longitudinal tracking of muscle volume changes in healthy subjects.

Conclusion

In the present study, we demonstrated that both multi-atlas and single-atlas approaches were relevant for the segmentation of individual muscles of the QF in healthy subjects. Considering muscle volume measurements, the single-atlas method provided promising perspectives regarding longitudinal quantification of individual muscle volumes.