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1. Direct Quantification of Intervertebral Disc Water Content Using MRI

   https://doi.org/10.1002/jmri.27171  

   Yang, Bo ; Wendland, Michael F. ; O'Connell, Grace D. ( April 2020 , Journal of Magnetic Resonance Imaging)

   Water content is a key parameter for simulating tissue swelling and nutrient diffusion. Accurately measuring water content throughout the intervertebral disc (NP = nucleus pulposus; AF = annulus fibrosus) is important for developing patient‐specific models. Water content is measured using destructive techniques, Quantitative MRI has been used to estimate water content and detect early degeneration, but it is dependent on scan parameters, concentration of free water molecules, and fiber architecture.

   To directly measure disc‐tissue water content using quantitative MRI and compare MRI‐based measurements with biochemical assays, and to quantify changes in disc geometry due to compression.

   Basic science, controlled.

   Twenty bone‐disc‐bone motion segments from skeletally mature bovines.

   7T/3D fast low angle shot (FLASH) pulse sequence and a T₂rapid imaging with refocused echoes (RARE) sequence.

   Disc volumes, NP and AF volumetric water content, and T₂relaxation times were measured through MRI; NP and AF tissue gravimetric water content, mass density, and glycosaminoglycan content were measured through a biochemical assay.

   Correlations between MRI‐based measurement and biochemical composition were evaluated using Pearson's linear regression.

   Mechanical dehydration resulted in a decrease in disc volume by up to 20% and a decrease in disc height by up to 35%. Direct water content measurements for the NP was achieved by normalizing MRI‐based spin density by NP mass density (1.10 ± 0.03 g/cm³). However, the same approach underestimated water content in the AF by ~10%, which may be due to a higher concentration of collagen fibers and bound water molecules.

   Spin density or spin density normalized by mass density to estimate NP and AF water content was more accurate than correlations between water content and relaxation times. Mechanical dehydration decreased disc volume and disc height, and increased maximum cross‐sectional area.

    

     J. Magn. Reson. Imaging 2020;52:1152–1162.

    

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2. Double diffusion encoding MRI for the clinic

   https://doi.org/10.1002/mrm.27043  

   Yang, Grant ; Tian, Qiyuan ; Leuze, Christoph ; Wintermark, Max ; McNab, Jennifer A. ( December 2017 , Magnetic Resonance in Medicine)

   The purpose of this study is to develop double diffusion encoding (DDE) MRI methods for clinical use. Microscopic diffusion anisotropy measurements from DDE promise greater specificity to changes in tissue microstructure compared with conventional diffusion tensor imaging, but implementation of DDE sequences on whole‐body MRI scanners is challenging because of the limited gradient strengths and lengthy acquisition times.

   A custom single‐refocused DDE sequence was implemented on a 3T whole‐body scanner. The DDE gradient orientation scheme and sequence parameters were optimized based on a Gaussian diffusion assumption. Using an optimized 5‐min DDE acquisition, microscopic fractional anisotropy (μFA) maps were acquired for the first time in multiple sclerosis patients.

   Based on simulations and in vivo human measurements, six parallel and six orthogonal diffusion gradient pairs were found to be the minimum number of diffusion gradient pairs necessary to produce a rotationally invariant measurement of μFA. Simulations showed that optimal precision and accuracy of μFA measurements were obtained using b‐values between 1500 and 3000 s/mm². The μFA maps showed improved delineation of multiple sclerosis lesions compared with conventional fractional anisotropy and distinct contrast from T₂‐weighted fluid attenuated inversion recovery and T₁‐weighted imaging.

   The μFA maps can be measured using DDE in a clinical setting and may provide new opportunities for characterizing multiple sclerosis lesions and other types of tissue degeneration. Magn Reson Med 80:507–520, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

    

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3. Subject‐loaded quadrifilar helical‐antenna RF coil with high B 1 + field uniformity and large FOV for 3‐T MRI

   https://doi.org/10.1002/cmr.b.21326  

   Athalye, Pranav S. ; Šekeljić, Nada J. ; Ilić, Milan M. ; Tonyushkin, Alexey A. ; Notaroš, Branislav M. ( September 2016 , Concepts in Magnetic Resonance Part B: Magnetic Resonance Engineering)

   A novel method for excitation of RFB₁field in high‐field (3‐T) magnetic resonance imaging (MRI) systems using a subject‐loaded quadrifilar helical antenna as an RF coil is proposed, evaluated, and demonstrated. Design, analysis, characterization, and evaluation of the novel coil when situated in a 3‐T MRI bore and loaded with different phantoms are performed and cross‐validated by extensive numerical simulations using multiple computational electromagnetics techniques. The results for the quadrifilar helical‐antenna RF body coil show (a) strong field penetration in the entire phantoms; (b) excellent right‐hand circular polarization (RCP); (c) high spatial uniformity of RCP RF magnetic field, B₁⁺, throughout the phantoms; (d) large field of view (FOV); (e) good transmit efficiency; and (f) low local specific absorption rate (SAR). The examples show that the new RF coil provides substantially better B₁⁺‐field uniformity and much larger FOV than any of the previously reported numerical and experimental results for the existing RF coil designs at 3 T in literature that enable comparison. In addition, helical RF body coils of different lengths can, for instance, easily provide an excellent RCP and highly uniform B₁⁺‐field within the MRI maximum FOV length of 50 cm, and even 100 cm. The proposed MRI RF coil yields a remarkable improvement in the field uniformity in the longitudinal direction, for various phantoms, with comparable efficiency and SAR levels.

    

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4. Independent Component and Graph Theory Analyses Reveal Normalized Brain Networks on Resting‐State Functional MRI After Working Memory Training in People With HIV

   https://doi.org/10.1002/jmri.28439  

   Jia, Chunying ; Long, Qunfang ; Ernst, Thomas ; Shang, Yuanqi ; Chang, Linda ; Adali, Tülay ( September 2022 , Journal of Magnetic Resonance Imaging)

   Cognitive training may partially reverse cognitive deficits in people with HIV (PWH). Previous functional MRI (fMRI) studies demonstrate that working memory training (WMT) alters brain activity during working memory tasks, but its effects on resting brain network organization remain unknown.

   To test whether WMT affects PWH brain functional connectivity in resting‐state fMRI (rsfMRI).

   Prospective.

   A total of 53 PWH (ages 50.7 ± 1.5 years, two women) and 53HIV‐seronegative controls (SN, ages 49.5 ± 1.6 years, six women).

   Axial single‐shot gradient‐echo echo‐planar imaging at 3.0 T was performed at baseline (TL1), at 1‐month (TL2), and at 6‐months (TL3), after WMT.

   All participants had rsfMRI and clinical assessments (including neuropsychological tests) at TL1 before randomization to Cogmed WMT (adaptive training,n = 58: 28 PWH, 30 SN; nonadaptive training,n = 48: 25 PWH, 23 SN), 25 sessions over 5–8 weeks. All assessments were repeated at TL2 and at TL3. The functional connectivity estimated by independent component analysis (ICA) or graph theory (GT) metrics (eigenvector centrality, etc.) for different link densities (LDs) were compared between PWH and SN groups at TL1 and TL2.

   Two‐way analyses of variance (ANOVA) on GT metrics and two‐samplet‐tests on FC or GT metrics were performed. Cognitive (eg memory) measures were correlated with eigenvector centrality (eCent) using Pearson's correlations. The significance level was set atP < 0.05 after false discovery rate correction.

   The ventral default mode network (vDMN) eCent differed between PWH and SN groups at TL1 but not at TL2 (P = 0.28). In PWH, vDMN eCent changes significantly correlated with changes in the memory ability in PWH (r = −0.62 at LD = 50%) and vDMN eCent before training significantly correlated with memory performance changes (r = 0.53 at LD = 50%).

   ICA and GT analyses showed that adaptive WMT normalized graph properties of the vDMN in PWH.

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5. An independent Monte Carlo–based IMRT QA tool for a 0.35 T MRI‐guided linear accelerator

   https://doi.org/10.1002/acm2.13820  

   Khan, Ahtesham Ullah ; Simiele, Eric A. ; Lotey, Rajiv ; DeWerd, Larry A. ; Yadav, Poonam ( November 2022 , Journal of Applied Clinical Medical Physics)

   To develop an independent log file–based intensity‐modulated radiation therapy (IMRT) quality assurance (QA) tool for the 0.35 T magnetic resonance‐linac (MR‐linac) and investigate the ability of various IMRT plan complexity metrics to predict the QA results. Complexity metrics related to tissue heterogeneity were also introduced.

   The tool for particle simulation (TOPAS) Monte Carlo code was utilized with a previously validated linac head model. A cohort of 29 treatment plans was selected for IMRT QA using the developed QA tool and the vendor‐supplied adaptive QA (AQA) tool. For 27 independent patient cases, various IMRT plan complexity metrics were calculated to assess the deliverability of these plans. A correlation between the gamma pass rates (GPRs) from the AQA results and calculated IMRT complexity metrics was determined using the Pearson correlation coefficients. Tissue heterogeneity complexity metrics were calculated based on the gradient of the Hounsfield units.

   The median and interquartile range for the TOPAS GPRs (3%/3 mm criteria) were 97.24% and 3.75%, respectively, and were 99.54% and 0.36% for the AQA tool, respectively. The computational time for TOPAS ranged from 4 to 8 h to achieve a statistical uncertainty of <1.5%, whereas the AQA tool had an average calculation time of a few minutes. Of the 23 calculated IMRT plan complexity metrics, the AQA GPRs had correlations with 7 out of 23 of the calculated metrics. Strong correlations (|r| > 0.7) were found between the GPRs and the heterogeneity complexity metrics introduced in this work.

   An independent MC and log file–based IMRT QA tool was successfully developed and can be clinically deployed for offline QA. The complexity metrics will supplement QA reports and provide information regarding plan complexity.

    

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