Background Magnetization transfer saturation (MTsat) derives a semiquantitative index of magnetization transfer in faster acquisition times than quantitative magnetization transfer; the potential of MTsat for muscle imaging has not yet been explored. Purpose To evaluate the potential of MTsat to identify regional and sex differences in calf muscle. Study Type Prospective cohort study. Phantom/Subjects Vials with different agar and nickel nitrate concentrations providing a range of macromolecular fraction and T1. Seven male subjects (25 ± 7 years) and seven female subjects (28 ± 14 years); three subjects were scanned in three separate sessions to assess reproducibility. Field Strength/Sequence 3T, 3D fast low angle shot (FLASH) sequence with and without a magnetization transfer pulse; acquisition time of 4.12 minutes. Assessment The effectiveness of two methods of fat suppression was evaluated using the fat unsuppressed sequence as the reference and MTsat maps derived with and without transmit field inhomogeneity corrections were compared. Statistical evaluation of MTsat differences between calf muscles and between male and female cohorts was made. Statistical Tests Bland–Altman plots were used to assess fat suppression and B1+ correction. The coefficient of variation (CV) and the repeatability coefficient (RC) were calculated from the repeat sessions. Sex and regional differences were assessed using two‐way factorial analyses of variance (ANOVAs) with Bonferroni‐adjusted independent sample t‐tests for post‐hoc analyses. Results In phantoms, MTsat increased linearly with agar concentration and MTsat was independent of T1 (P = 0.229) evaluated in phantoms with two T1s. The CV and RC of MTsat ranged between 2.65 to 5.03% and 0.13 to 0.38, respectively, in the different calf muscles. MTsat of the tibialis anterior was significantly higher than other muscles (P < 0.05). MTsat in male subjects was significantly higher than in female subjects (P = 0.009). Data Conclusion MTsat maps of calf muscle acquired under 5 minutes may have the potential to detect regional and sex differences. Level of Evidence: 2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:1227–1237.
X-ray luminescence computed tomography (XLCT) has the potential to image the biodistribution of nanoparticles inside deep tissues. In XLCT, X-ray excitable nanophosphors emit optical photons for tomographic imaging. The lifetime of the nanophosphor signal, rather than its intensity, could be used to extract biological microenvironment information such as oxygenation in deep tumors. In this study, we propose the design, the forward model, and the reconstruction algorithm of a time domain XLCT for lifetime imaging with high spatial resolution. We have investigated the feasibility of the proposed design with numerical simulations. We found that the reconstructed lifetime images are robust to noise levels up to 5% and to unknown optical properties up to 4 times of absorption and scattering coefficients.
X-ray fluorescence computed tomography (XFCT) is a molecular imaging technique that can be used to sense different elements or nanoparticle (NP) agents inside deep samples or tissues. However, XFCT has not been a popular molecular imaging tool because it has limited molecular sensitivity and spatial resolution. We present a benchtop XFCT imaging system in which a superfine pencil-beam X-ray source and a ring of X-ray spectrometers were simulated using GATE (Geant4 Application for Tomographic Emission) Monte Carlo software. An accelerated majorization minimization (MM) algorithm with an L1 regularization scheme was used to reconstruct the XFCT image of molybdenum (Mo) NP targets. Good target localization was achieved with a DICE coefficient of 88.737%. The reconstructed signal of the targets was found to be proportional to the target concentrations if detector number, detector placement, and angular projection number are optimized. The MM algorithm performance was compared with the maximum likelihood expectation maximization (ML-EM) and filtered back projection (FBP) algorithms. Our results indicate that the MM algorithm is superior to the ML-EM and FBP algorithms. We found that the MM algorithm was able to reconstruct XFCT targets as small as 0.25 mm in diameter. We also found that measurements with three angular projections and a 20-detector ring are enough to reconstruct the XFCT images.
BACKGROUND: X-ray image quality relies heavily on the emitted X-ray photon number depending on X-ray tube current and exposure time. To accurately estimate the absorbed dose in an imaging protocol, it is better to simulate the X-ray imaging with a Monte Carlo platform such as GATE (Geant4 Application for Tomographic Emission). Although input of GATE is the X-ray photon number of the simulated X-ray tube, it lacks a good way to setup the photon number for a desired X-ray tube current setting. OBJECTIVE: To provide a method to correlate the experimental X-ray tube current exposure time and the X-ray photon number in GATE. METHODS: The accumulated radiation dose of a micro-computed tomography (CT) X-ray tube at different current exposure times was recorded with a general-purpose ion chamber. GATE was used to model the experimental microCT imaging system and record the total absorbed dose (cGy) in the sensitive volume of the ion chamber with different X-ray photon numbers. Linear regression models are used to establish a correlation between the estimated X-ray photon number and the X-ray tube settings. At first, one model establishes the relationship between the experimentally measured dose and the X-ray tube setting. Then, another model establishes a relationship between the simulated dose and the X-ray number in GATE. At last, by correlating these two models, a regression model to estimate the X-ray output number from an experimental X-ray tube setting (mAs) is obtained. RESULTS: For a typical micro-CT scan, the X-ray tube is operated at 50 kVp and 0.5 mA for a 500 ms exposure time per projection (0.25 mAs). For these X-ray imaging parameters, the X-ray number per projection is estimated to be 3.918×106 with 1.0 mm Al filter. CONCLUSION: The findings of this work provide an approach to correlate the experimental X-ray tube current exposure time to the X-ray photon number in the GATE simulation of the X-ray tube to more accurately determine radiation dose for an imaging protocol.
BACKGROUND: The time of flight (TOF) cone beam computed tomography (CBCT) was recently shown to reduce the X-ray scattering effects by 95%and improve the image CNR by 110%for large volume objects. The advancements in X-ray sources like in compact Free Electron Lasers (FEL) and advancements in detector technology show potential for the TOF method to be feasible in CBCT when imaging large objects. OBJECTIVE: To investigate feasibility and efficacy of TOF CBCT in imaging smaller objects with different targets such as bones and tumors embedded inside the background. METHODS: The TOF method used in this work was verified using a 24cm phantom. Then, the GATE software was used to simulate the CBCT imaging of an 8 cm diameter cylindrical water phantom with two bone targets using a modeled 20 keV quasi-energetic FEL source and various TOF resolutions ranging from 1 to 1000 ps. An inhomogeneous breast phantom of similar size with tumor targets was also imaged using the same system setup. RESULTS: The same results were obtained in the 24cm phantom, which validated the applied CBCT simulation approach. For the case of 8cm cylindrical phantom and bone target, a TOF resolution of 10 ps improved the image contrast-to-noise ratio (CNR) by 57%and reduced the scatter-to-primary ratio (SPR) by 8.63. For the case of breast phantom and tumor target, image CNR was enhanced by 12%and SPR was reduced by 1.35 at 5 ps temporal resolution. CONCLUSIONS: This study indicates that a TOF resolution below 10 ps is required to observe notable enhancements in the image quality and scatter reduction for small objects around 8cm in diameter. The strong scattering targets such as bone can result in substantial improvements by using TOF CBCT.
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