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Journal Cover   Magnetic Resonance Materials in Physics, Biology and Medicine
  [SJR: 0.928]   [H-I: 40]   [1 followers]  Follow
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0968-5243 - ISSN (Online) 1352-8661
   Published by Springer-Verlag Homepage  [2281 journals]
  • Active decoupling of RF coils using a transmit array system
    • Abstract: Objective Implementation of a decoupling method for isolation of transmit and receive radio frequency (RF) coils for concurrent excitation and acquisition (CEA) MRI in samples with ultra-short T2*. Materials and methods The new phase and amplitude (PA) decoupling method is implemented in a clinical 3T-MRI system equipped with a parallel transmit array system. For RF excitation, two transmit coils are used in combination with a single receive coil. The transmit coil is geometrically decoupled from the receive coil, and the remaining B 1-induced voltages in the receive coil during CEA are minimized by the second transmit coil using a careful adjustment of the phase and amplitude settings in this coil. Isolation of the decoupling scheme and transmit noise behavior are analyzed for different loading conditions, and a CEA MRI experiment is performed in a rubber phantom with sub-millisecond T2* and in an ex vivo animal. Results Geometrical (20 dB) and PA decoupling (50 dB) provided a total isolation of 70 dB between the transmit and receive coils. Integration of a low-noise RF amplifier was necessary to minimize RF transmit noise. CEA MR images could be reconstructed from a rubber phantom and an ex vivo animal. Conclusion CEA MRI can be implemented in clinical MRI systems using active decoupling with parallel transmit array capabilities with minor hardware modifications.
      PubDate: 2015-08-05
  • Effect of $$ T_{2}^{*} $$ T 2 ∗ correction on contrast kinetic model
           analysis using a reference tissue arterial input function at 7 T
    • Abstract: Objectives We aimed to investigate the effect of \( T_{2}^{*} \) correction on estimation of kinetic parameters from T 1-weighted dynamic contrast enhanced (DCE) MRI data when a reference-tissue arterial input function (AIF) is used. Materials and methods DCE-MRI data were acquired from seven mice with 4T1 mouse mammary tumors using a double gradient echo sequence at 7 T. The AIF was estimated from a region of interest in the muscle. The extended Tofts model was used to estimate pharmacokinetic parameters in the enhancing part of the tumor, with and without \( T_{2}^{*} \) correction of the lesion and AIF. The parameters estimated with \( T_{2}^{*} \) correction of both the AIF and lesion time-intensity curve were assumed to be the reference standard. Results For the whole population, there was significant difference (p < 0.05) in transfer constant (K trans) between \( T_{2}^{*} \) corrected and not corrected methods, but not in interstitial volume fraction (v e). Individually, no significant differences were found in K trans and v e of four and six tumors, respectively, between the \( T_{2}^{*} \) corrected and not corrected methods. In contrast, K trans was significantly underestimated, if the \( T_{2}^{*} \) correction was not used, in other tumors for which the median K trans was larger than 0.4 min−1. Conclusion \( T_{2}^{*} \) effect on tumors with high K trans may not be negligible in kinetic model analysis, even if AIF is estimated from reference tissue where the concentration of contrast agent is relatively low.
      PubDate: 2015-08-05
  • Phase-corrected bipolar gradients in multi-echo gradient-echo sequences
           for quantitative susceptibility mapping
    • Abstract: Objective Large echo spacing of unipolar readout gradients in current multi-echo gradient-echo (GRE) sequences for mapping fields in quantitative susceptibility mapping (QSM) can be reduced using bipolar readout gradients thereby improving acquisition efficiency. Materials and methods Phase discrepancies between odd and even echoes in the bipolar readout gradients caused by non-ideal gradient behaviors were measured, modeled as polynomials in space and corrected for accordingly in field mapping. The bipolar approach for multi-echo GRE field mapping was compared with the unipolar approach for QSM. Results The odd–even-echo phase discrepancies were approximately constant along the phase encoding direction and linear along the readout and slice-selection directions. A simple linear phase correction in all three spatial directions was shown to enable accurate QSM of the human brain using a bipolar multi-echo GRE sequence. Bipolar multi-echo acquisition provides QSM in good quantitative agreement with unipolar acquisition while also reducing noise. Conclusion With a linear phase correction between odd–even echoes, bipolar readout gradients can be used in multi-echo GRE sequences for QSM.
      PubDate: 2015-08-01
  • Automatic segmentation of subcutaneous mouse tumors by multiparametric MR
           analysis based on endogenous contrast
    • Abstract: Object Contrast-enhanced T1-weighted imaging is usually included in MRI procedures for automatic tumor segmentation. Use of an MR contrast agent may not be appropriate for some applications, however. We assessed the feasability of automatic tumor segmentation by multiparametric cluster analysis that uses intrinsic MRI contrast only. Materials and methods Multiparametric MRI consisting of quantitative T1, T2, and apparent diffusion coefficient (ADC) mapping was performed in mice bearing subcutaneous tumors (n = 21). k-means and fuzzy c-means clustering with all possible combinations of MRI parameters, i.e. feature vectors, and 2–7 clusters were performed on the multiparametric data. Clusters associated with tumor tissue were selected on the basis of the relative signal intensity of tumor tissue in T2-weighted images. The optimum segmentation method was determined by quantitative comparison of automatic segmentation with manual segmentation performed by three observers. In addition, the automatically segmented tumor volumes from seven separate tumor data sets were quantitatively compared with histology-derived tumor volumes. Results The highest similarity index between manual and automatic segmentation (SImanual,automatic = 0.82 ± 0.06) was observed for k-means clustering with feature vector {T2, ADC} and four clusters. A strong linear correlation between automatically and manually segmented tumor volumes (R 2 = 0.99) was observed for this segmentation method. Automatically segmented tumor volumes also correlated strongly with histology-derived tumor volumes (R 2 = 0.96). Conclusion Automatic segmentation of mouse subcutaneous tumors can be achieved on the basis of endogenous MR contrast only.
      PubDate: 2015-08-01
  • IVIM analysis of brain tumors: an investigation of the relaxation effects
           of CSF, blood, and tumor tissue on the estimated perfusion fraction
    • Abstract: Object We sought to investigate the dependence of intravoxel incoherent motion (IVIM)-related perfusion fraction (f) estimates on the transverse relaxation of brain tissue, blood, and cerebrospinal fluid (CSF), attempting to overcome the influence of CSF on conventional f maps. Materials and methods Eighteen patients with gliomas underwent DWI with 14 b-values (0–1,300 s/mm2) and two distinct echo times (TEs). Regions of interest representing tumour and normal brain tissue were analysed by calculating the f values for both TEs. A mask for pixels with relevant CSF partial volume was subsequently created. The f values were tested for significant differences. Results We found statistically significant differences between the two TEs in the f values for cortical and juxtacortical structures and non-enhancing areas of the tumour /oedema. Normal white matter and gadolinium-enhancing tumour tissue appeared insensitive to TE variation. In all tissue types examined, the masking of voxels with considerable CSF content was able to overcome issues of erroneous f estimation and calculation of f values insensitive to TE changes was feasible. Conclusion Due to the complex interaction in the relaxation rates of CSF, blood, and tumour tissue, the estimation of f values is affected by the choice of TE. Only f values in normal white matter and tumour tissue—which largely comprise blood voxels with minor CSF partial volume—may be clinically applicable in the present form of IVIM-based DWI analysis. Going a step further, and after removing voxels with heavily TE-susceptible f values, we were able to obtain accurate and TE-independent f values in contrast-enhancing tumour tissue, white matter, and grey matter, which were essentially consistent with those reported in the literature.
      PubDate: 2015-08-01
  • Fast PRF-based MR thermometry using double-echo EPI: in vivo comparison in
           a clinical hyperthermia setting
    • Abstract: Objective To develop and test in a clinical setting a double-echo segmented echo planar imaging (DEPI) pulse sequence for proton resonance frequency (PRF)-based temperature monitoring that is faster than conventional PRF thermometry pulse sequences and not affected by thermal changes in tissue conductivity. Materials and methods Four tumor patients underwent between one and nine magnetic resonance (MR)-guided regional hyperthermia treatments. During treatment, the DEPI sequence and a FLASH PRF sequence were run in an interleaved manner to compare the results from both sequences in the same patients and same settings. Temperature maps were calculated based on the phase data of both sequences. Temperature measurements of both techniques were compared using Passing and Bablok regression and the Bland–Altman method. Results The temperature results from the DEPI and FLASH sequences, on average, do not differ by more than ΔT = 1 °C. DEPI images showed typically more artifacts and approximately a twofold lower signal-to-noise ratio (SNR), but a sufficient temperature precision of 0.5°, which would theoretically allow for a fivefold higher frame rate. Conclusion The results indicate that DEPI can replace slower temperature measurement techniques for PRF-based temperature monitoring during thermal treatments. The higher acquisition speed can be exploited for hot spot localization during regional hyperthermia as well as for temperature monitoring during fast thermal therapies.
      PubDate: 2015-08-01
  • Improving the robustness of 3D turbo spin echo imaging to involuntary
    • Abstract: Objective 3D TSE imaging is very prone to motion artifacts, especially from uncooperative patients, because of the long scan duration. The need to repeat this time-consuming 3D acquisition in the event of large motion artifacts substantially reduces patient comfort and increases the workload of the scanner. Materials and methods A new sampling strategy enables homogenized collection of k-space data for 3D TSE imaging. It is combined with Frobenius norm-based motion-detection to enable freely stopped acquisition in 3D TSE imaging whenever excessive subject motion is detected. Results The feasibility and reliability of the proposed method were demonstrated and evaluated in in-vivo experiments. Conclusion It is shown that the additional overhead related to repeat scanning of the 3D TSE sequence as a result of patient motion can be substantially reduced by using the homogenized k-space sampling strategy with automatic scan completion as determined by Frobenius norm-based motion-detection.
      PubDate: 2015-08-01
  • Velocity mapping of the aortic flow at 9.4 T in healthy mice and mice
           with induced heart failure using time-resolved three-dimensional
           phase-contrast MRI (4D PC MRI)
    • Abstract: Objectives In this study, we established and validated a time-resolved three-dimensional phase-contrast magnetic resonance imaging method (4D PC MRI) on a 9.4 T small-animal MRI system. Herein we present the feasibility of 4D PC MRI in terms of qualitative and quantitative flow pattern analysis in mice with transverse aortic constriction (TAC). Materials and methods 4D PC FLASH images of a flow phantom and mouse heart were acquired at 9.4 T using a four-point phase-encoding scheme. The method was compared with slice-selective PC FLASH and ultrasound using Bland–Altman analysis. Advanced 3D streamlines were visualized utilizing Voreen volume-rendering software. Results In vitro, 4D PC MRI flow profiles showed the transition between laminar and turbulent flow with increasing velocities. In vivo, 4D PC MRI data of the ascending aorta and the pulmonary artery were confirmed by ultrasound, resulting in linear regressions of R 2 > 0.93. Magnitude- and direction-encoded streamlines differed substantially pre- and post-TAC surgery. Conclusions 4D PC MRI is a feasible tool for in vivo velocity measurements on high-field small-animal scanners. Similar to clinical measurement, this method provides a complete spatially and temporally resolved dataset of the murine cardiovascular blood flow and allows for three-dimensional flow pattern analysis.
      PubDate: 2015-08-01
  • Comparing velocity and fluid shear stress in a stenotic phantom with
           steady flow: phase-contrast MRI, particle image velocimetry and
           computational fluid dynamics
    • Abstract: Object This study aims to validate phase-contrast magnetic resonance imaging (PC-MRI) measurements of a steady flow through a severe stenotic phantom using particle image velocimetry (PIV) and computational fluid dynamics (CFD). Materials and methods The study was performed in an axisymmetric 87 % area stenosis model using an inlet Reynolds number (Re) of 160, corresponding to a jet Re of 444. Velocity patterns and estimated fluid shear stresses from three modalities were analyzed and compared qualitatively and quantitatively. Results Visual analysis via contour subtraction and Bland–Altman plots showed good agreement for flow velocities and less agreement for maximum shear stress (MSS). The Pearson’s coefficients of correlation between PC-MRI and PIV were 0.97 for the velocity field and 0.82 for the MSS. The corresponding parameters between PC-MRI and CFD were 0.96 and 0.84, respectively. Conclusion Findings indicate that PC-MRI can be implemented to estimate velocity flow fields and MSS; however, this method is not sufficiently accurate to quantify the MSS at regions of high shear rate.
      PubDate: 2015-08-01
  • The separation of Gln and Glu in STEAM: a comparison study using short and
           long TEs/TMs at 3 and 7 T
    • Abstract: Objectives This study aimed to determine the optimal echo time (TE) and mixing time (TM) for in vivo glutamine (Gln) and glutamate (Glu) separation in stimulated-echo acquisition mode at 3 and 7 T. We applied a short TE/TM (20/10 ms) for a high signal-to-noise-ratio and a field-specific long TE/TM (3 T: 72/6 ms; 7 T: 74/68 ms) for optimal Gln and Glu separation of the Carbon-4 proton resonances. Materials and methods Corresponding Gln and Glu spectra were simulated using VeSPA software, and measured in a phantom and human brains at 3 and 7 T. Results Higher spectral separation for Gln and Glu was achieved at 7 than 3 T. At 7 T, short TE/TM provided comparable spectral separation and in vitro Gln and Glu quantification compared to long TE/TM. Moreover, it showed greater reliability in in vivo Gln and Glu detection and separation than long TE/TM, with significantly lower Cramer–Rao lower bounds (Gln: 14.9 vs. 75.8; Glu: 3.8 vs. 6.5) and correlation between Gln and Glu (p = 0.004). Conclusion Based on the optimal separation for Gln and Glu, a short TE/TM at 7 T is proposed for future in vivo Gln and Glu acquisition.
      PubDate: 2015-08-01
  • Single-channel, box-shaped, monopole-type antenna for B1+ field
           manipulation in conjunction with the traveling-wave concept in 9.4 T
    • Abstract: Object We have developed a single-channel, box-shaped, monopole-type antenna which, if used in two different configurations, excites complementary B1+ field distributions in the traveling-wave setup. Materials and methods A new monopole-type, single-channel antenna for RF excitation in 9.4 T magnetic resonance imaging is proposed. The antenna is entirely made of copper without lumped elements. Two complementary B1+ field distributions of two different antenna configurations were measured and combined as a root sum of squares. B1+ field inhomogeneity of the combined maps was calculated and compared with published results. Results By combining B1+ field distributions generated by two antenna configurations, a “no voids” pattern was achieved for the entire upper brain. B1+ inhomogeneity of approximately 20 % was achieved for sagittal and transverse slices; it was <24 % for coronal slices. The results were comparable with those from CP, with “no voids” in slice B1+ inhomogeneity of multichannel loop arrays. The efficiency of the proposed antenna was lower than that of a multichannel array but comparable with that of a patch antenna. Conclusion The proposed single-channel antenna is a promising candidate for traveling-wave brain imaging. It can be combined with the time-interleaved acquisition of modes (TIAMO) concept if reconfigurability is obtained with a single-antenna element.
      PubDate: 2015-08-01
  • Measurement reproducibility of magnetic resonance imaging-based finite
           element analysis of proximal femur microarchitecture for in vivo
           assessment of bone strength
    • Abstract: Introduction Osteoporosis is a disease of weak bone. Our goal was to determine the measurement reproducibility of magnetic resonance assessment of proximal femur strength. Methods This study had institutional review board approval, and written informed consent was obtained from all subjects. We obtained images of proximal femur microarchitecture by scanning 12 subjects three times within 1 week at 3T using a high-resolution 3-D FLASH sequence. We applied finite element analysis to compute proximal femur stiffness and femoral neck elastic modulus. Results Within-day and between-day root-mean-square coefficients of variation and intraclass correlation coefficients ranged from 3.5 to 6.6 % and 0.96 to 0.98, respectively. Conclusion The measurement reproducibility of magnetic resonance assessment of proximal femur strength is suitable for clinical studies of disease progression or treatment response related to osteoporosis bone-strengthening interventions.
      PubDate: 2015-08-01
  • Repeatability of in vivo quantification of atherosclerotic carotid artery
           plaque components by supervised multispectral classification
    • Abstract: Objective To evaluate the agreement and scan–rescan repeatability of automated and manual plaque segmentation for the quantification of in vivo carotid artery plaque components from multi-contrast MRI. Materials and methods Twenty-three patients with 30–70 % stenosis underwent two 3T MR carotid vessel wall exams within a 1 month interval. T1w, T2w, PDw and TOF images were acquired around the region of maximum vessel narrowing. Manual delineation of the vessel wall and plaque components (lipid, calcification, loose matrix) by an experienced observer provided the reference standard for training and evaluation of an automated plaque classifier. Areas of different plaque components and fibrous tissue were quantified and compared between segmentation methods and scan sessions. Results In total, 304 slices from 23 patients were included in the segmentation experiment, in which 144 aligned slice pairs were available for repeatability analysis. The correlation between manual and automated segmented areas was 0.35 for lipid, 0.66 for calcification, 0.50 for loose matrix and 0.82 for fibrous tissue. For the comparison between scan sessions, the coefficient of repeatability of area measurement obtained by automated segmentation was lower than by manual delineation for lipid (9.9 vs. 17.1 mm2), loose matrix (13.8 vs. 21.2 mm2) and fibrous tissue (24.6 vs. 35.0 mm2), and was similar for calcification (20.0 vs. 17.6 mm2). Conclusion Application of an automated classifier for segmentation of carotid vessel wall plaque components from in vivo MRI results in improved scan–rescan repeatability compared to manual analysis.
      PubDate: 2015-07-11
  • The repeatability of T2 relaxation time measurement of human knee
           articular cartilage
    • Abstract: Objectives To assess short- and long-term repeatability of T2 relaxation time measurements of the knee articular cartilage. Materials and methods The right knees of nine asymptomatic volunteers (age 30–38 years, five male, four female) were imaged at 1.5 T in three sessions 1 and 2 weeks apart. To observe short-term repeatability, the measurements were repeated three times within one of the three imaging sessions for each volunteer. T2 relaxation time was mapped using a multi-slice multi-echo spin echo sequence in axial and sagittal planes. Cartilage was manually segmented and repeatability, as measured by root-mean-square coefficient of variation (CVRMS) was evaluated both for the entire bulk cartilage of each joint surface in the slice and separately for each region of interest (ROI) at different topographical locations and separately for the superficial and deep half of each ROI. Results For bulk T2, the long-term repeatability was 3.2, 5.4, and 3.7 %, and the short-term reproducibility was 3.9, 3.9, and 3.4 % for bulk femoral, tibial, and patellar cartilage, respectively. There were no significant differences between long-term and short-term repeatability in superficial or deep cartilage when comparing CVRMS values (p = 0.338 and 0.700, respectively). For individual ROIs, the repeatability varied between 2.5 and 22.2 % depending on the topographical location. Conclusion The current results show mostly good repeatability. However, there were remarkable variations of T2 between bulk cartilage and different ROIs, bulk cartilage showing better repeatability. With careful patient positioning T2 can be accurately determined for different cartilage surfaces.
      PubDate: 2015-07-11
  • Optical tracking with two markers for robust prospective motion correction
           for brain imaging
    • Abstract: Objective Prospective motion correction (PMC) during brain imaging using camera-based tracking of a skin-attached marker may suffer from problems including loss of marker visibility due to the coil and false correction due to non-rigid-body facial motion, such as frowning or squinting. A modified PMC system is introduced to mitigate these problems and increase the robustness of motion correction. Materials and methods The method relies on simultaneously tracking two markers, each providing six degrees of freedom, that are placed on the forehead. This allows us to track head motion when one marker is obscured and detect skin movements to prevent false corrections. Experiments were performed to compare the performance of the two-marker motion correction technique to the previous single-marker approach. Results Experiments validate the theory developed for adaptive marker tracking and skin movement detection, and demonstrate improved image quality during obstruction of the line-of-sight of one marker when subjects squint or when subjects squint and move simultaneously. Conclusion The proposed methods eliminate two common failure modes of PMC and substantially improve the robustness of PMC, and they can be applied to other optical tracking systems capable of tracking multiple markers. The methods presented can be adapted to the use of more than two markers.
      PubDate: 2015-06-30
  • Acoustic-noise-optimized diffusion-weighted imaging
    • Abstract: Objective This work was aimed at reducing acoustic noise in diffusion-weighted MR imaging (DWI) that might reach acoustic noise levels of over 100 dB(A) in clinical practice. Materials and methods A diffusion-weighted readout-segmented echo-planar imaging (EPI) sequence was optimized for acoustic noise by utilizing small readout segment widths to obtain low gradient slew rates and amplitudes instead of faster k-space coverage. In addition, all other gradients were optimized for low slew rates. Volunteer and patient imaging experiments were conducted to demonstrate the feasibility of the method. Acoustic noise measurements were performed and analyzed for four different DWI measurement protocols at 1.5T and 3T. Results An acoustic noise reduction of up to 20 dB(A) was achieved, which corresponds to a fourfold reduction in acoustic perception. The image quality was preserved at the level of a standard single-shot (ss)-EPI sequence, with a 27–54 % increase in scan time. Conclusions The diffusion-weighted imaging technique proposed in this study allowed a substantial reduction in the level of acoustic noise compared to standard single-shot diffusion-weighted EPI. This is expected to afford considerably more patient comfort, but a larger study would be necessary to fully characterize the subjective changes in patient experience.
      PubDate: 2015-06-20
  • Proton spectroscopic imaging of brain metabolites in basal ganglia of
           healthy older adults
    • Abstract: Object We sought to measure brain metabolite levels in healthy older people. Materials and methods Spectroscopic imaging at the level of the basal ganglia was applied in 40 participants aged 73–74 years. Levels of the metabolites N-acetyl aspartate (NAA), choline, and creatine were determined in "institutional units" (IU) corrected for T1 and T2 relaxation effects. Structural imaging enabled determination of grey matter (GM), white matter (WM), and cerebrospinal fluid content. ANOVA analysis was carried out for voxels satisfying quality criteria. Results Creatine levels were greater in GM than WM (57 vs. 44 IU, p < 0.001), whereas choline and NAA levels were greater in WM than GM [13 vs. 10 IU (p < 0.001) and 76 versus 70 IU (p = 0.03), respectively]. The ratio of NAA/cre was greater in WM than GM (2.1 vs. 1.4, p = 0.001) as was that of cho/cre (0.32 vs. 0.16, p < 0.001). A low voxel yield was due to brain atrophy and the difficulties of shimming over an extended region of brain. Conclusion This study addresses the current lack of information on brain metabolite levels in older adults. The normal features of ageing result in a substantial loss of reliable voxels and should be taken into account when planning studies. Improvements in shimming are also required before the methods can be applied more widely.
      PubDate: 2015-06-01
  • Multiparametric oxygen-enhanced functional lung imaging in 3D
    • Abstract: Objective To develope a self-gated free-breathing 3D sequence allowing for simultaneous T 1-weighted imaging and quantitative \(T_{2}^{ *}\) mapping in different breathing phases in order to assess the feasibility of oxygen-enhanced 3D functional lung imaging. Materials and methods A 3D sequence with ultrashort echo times and interleaved double readouts was implemented for oxygen-enhanced lung imaging at 1.5 T. Six healthy volunteers were examined while breathing room air as well as 100 % oxygen. Images from expiratory and inspiratory breathing phases were reconstructed and compared for the two breathing gases. Results The average \(T_{2}^{*}\) value measured for room air was 2.10 ms, with a 95 % confidence interval (CI) of 1.95–2.25 ms, and the average for pure oxygen was 1.89 ms, with a 95 % CI of 1.76–2.01 ms, resulting in a difference of 10.1 % (95 % CI 8.9–11.3 %). An 11.2 % increase in signal intensity (95 % CI 10.4–12.1 %) in the T 1-weighted images was detected when subjects were breathing pure oxygen compared to room air. Furthermore, a significant change in signal intensity (26.5 %, 95 % CI 18.8–34.3 %) from expiration to inspiration was observed. Conclusions This study demonstrated the feasibility of simultaneous \(T_{2}^{*}\) mapping and T 1-weighted 3D imaging of the lung. This method has the potential to provide information about ventilation, oxygen transfer, and lung expansion within one experiment. Future studies are needed to investigate the clinical applicability and diagnostic value of this approach in various pulmonary diseases.
      PubDate: 2015-06-01
  • Characterizing blood oxygen level-dependent (BOLD) response following
           in-magnet quadriceps exercise
    • Abstract: Object There have been no studies to investigate the effects of cycling exercise protocols, as well as repeated bouts of exercise, on the blood oxygen level-dependent (BOLD) response in the quadriceps muscles. This study characterized BOLD signal recovery following non-ischemic bouts of exercise in the quadriceps muscles of healthy adults in order to provide a basis for application of a protocol for clinical populations. Materials and methods Healthy male subjects (23.7 ± 2.0 years of age, n = 10) completed three cycles of one-minute exercise (65 % of maximum workload), with two  minutes of rest between each bout, on an MRI-compatible ergometer. The BOLD responses during recovery were fitted to a sigmoid model, and response kinetics (post-exercise intensity [S0]), response time (α), change in baseline BOLD signal (κ), and inflection point (β)] were measured. Results The sigmoid function fit well to the post-exercise BOLD data (r 2 = 0.95 ± 0.04). The mean response time was 10.5 ± 3.8 seconds, change in baseline BOLD intensity was 0.15 ± 0.068, and time to half-peak was 20.2 ± 8.6 seconds. Conclusion The proposed sigmoid model is a robust method for quantifying quadriceps BOLD response post-exercise without induced ischemia. Extension of this model to evaluate microvascular responses in patients with chronic disease could improve our understanding of exercise intolerance.
      PubDate: 2015-06-01
  • Free-breathing, zero-TE MR lung imaging
    • Abstract: Object The investigation of three-dimensional radial, zero-echo time (TE) imaging for high-resolution, free-breathing magnetic resonance (MR) lung imaging using prospective and retrospective motion correction. Materials and methods Zero-TE was implemented similarly to the rotating-ultra-fast-imaging-sequence, providing 3D, isotropic, radial imaging with proton density contrast. Respiratory motion was addressed using prospective triggering (PT), prospective gating (PG) and retrospective gating (RG) with physiological signals obtained from a respiratory belt and interleaved pencil beam and DC navigators. The methods were demonstrated on four healthy volunteers at 3T. Results 3D, radial zero-TE imaging with high imaging bandwidth and nominally zero echo-time enables efficient capture of short-lived signals from the lung parenchyma and the vessels. Compared to Cartesian encoding, unaccounted for free-breathing respiration resulted in only benign blurring artifacts confined to the origin of motion. Breath holding froze respiration but achieved only limited image resolution (~1.8 mm, 30 s). PT and PG obtained similar quality expiratory-phase images at 1.2 mm resolution in ~6 min scan time. RG allowed multi-phase imaging in ~15 min, derived from eight individually stored averages. Conclusion Zero-TE appears to be an attractive pulse sequence for 3D isotropic lung imaging. Prospective and retrospective approaches provide high-quality, free-breathing MR lung imaging within reasonable scan time.
      PubDate: 2015-06-01
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