Abstract:Macromolecular proton fraction (MPF) is a quantitative MRI parameter describing the magnetization transfer (MT) effect and defined as a relative amount of protons bound to biological macromolecules with restricted molecular motion, which participate in magnetic cross-relaxation with water protons. MPF attracted significant interest during past decade as a biomarker of myelin. The purpose of this mini review is to provide a brief but comprehensive summary of MPF mapping methods, histological validation studies,… Show more
“…Our approach is based on the classical theory of dipole-dipole relaxation, 33,34 where the relaxation parameters R 1,2 are determined by the spectral densities J (m) (𝜔), which are the Fourier transforms of the correlation functions G (m) (𝜏), Equations ( 7) and (8). In the case of 3D diffusion, the latter decrease with 𝜏 as G (m) (𝜏) ∼ 𝜏 −3∕2 at 𝜏 >> 𝜏 d (𝜏 d is the characteristic diffusion time), 33,34 and the integrals defining the Fourier transforms converge at any 𝜔.…”
Deciphering salient features of biological tissue cellular microstructure in health and diseases is an ultimate goal of MRI. While most MRI approaches are based on studying MR properties of tissue "free" water indirectly affected by tissue microstructure, other approaches, such as magnetization transfer (MT), directly target signals from tissue-forming macromolecules. However, despite three-decades of successful applications, relationships between MT measurements and tissue microstructure remain elusive, hampering interpretation of experimental results. The goal of this paper is to develop microscopic theory connecting the structure of cellular and myelin membranes to their MR properties.Theory and Methods: Herein we introduce a lateral diffusion model (LDM) that explains the T 2 (spin-spin) and T 1 (spin-lattice) MRI relaxation properties of the macromolecular-bound protons by their dipole-dipole interaction modulated by the lateral diffusion of long lipid molecules forming cellular and myelin membranes. Results: LDM predicts anisotropic T 1 and T 2 relaxation of membrane-bound protons. Moreover, their T 2 relaxation cannot be described in terms of a standard R 2 = 1/T 2 relaxation rate parameter, but rather by a relaxation rate function R 2 (t) that depends on time t after RF excitation, having, in the main approximation, a logarithmic behavior: R 2 (t) ∼ lnt. This anisotropic non-linear relaxation leads to an absorption lineshape that is different from Super-Lorentzian traditionally used in interpreting MT experiments.
Conclusion: LDM-derived analytical equations connect the membrane-boundprotons T 1 and T 2 relaxation with dynamic distances between protons in neighboring membrane-forming lipid molecules and their lateral diffusion. This sheds new light on relationships between MT parameters and microstructure of cellular and myelin membranes.
“…Our approach is based on the classical theory of dipole-dipole relaxation, 33,34 where the relaxation parameters R 1,2 are determined by the spectral densities J (m) (𝜔), which are the Fourier transforms of the correlation functions G (m) (𝜏), Equations ( 7) and (8). In the case of 3D diffusion, the latter decrease with 𝜏 as G (m) (𝜏) ∼ 𝜏 −3∕2 at 𝜏 >> 𝜏 d (𝜏 d is the characteristic diffusion time), 33,34 and the integrals defining the Fourier transforms converge at any 𝜔.…”
Deciphering salient features of biological tissue cellular microstructure in health and diseases is an ultimate goal of MRI. While most MRI approaches are based on studying MR properties of tissue "free" water indirectly affected by tissue microstructure, other approaches, such as magnetization transfer (MT), directly target signals from tissue-forming macromolecules. However, despite three-decades of successful applications, relationships between MT measurements and tissue microstructure remain elusive, hampering interpretation of experimental results. The goal of this paper is to develop microscopic theory connecting the structure of cellular and myelin membranes to their MR properties.Theory and Methods: Herein we introduce a lateral diffusion model (LDM) that explains the T 2 (spin-spin) and T 1 (spin-lattice) MRI relaxation properties of the macromolecular-bound protons by their dipole-dipole interaction modulated by the lateral diffusion of long lipid molecules forming cellular and myelin membranes. Results: LDM predicts anisotropic T 1 and T 2 relaxation of membrane-bound protons. Moreover, their T 2 relaxation cannot be described in terms of a standard R 2 = 1/T 2 relaxation rate parameter, but rather by a relaxation rate function R 2 (t) that depends on time t after RF excitation, having, in the main approximation, a logarithmic behavior: R 2 (t) ∼ lnt. This anisotropic non-linear relaxation leads to an absorption lineshape that is different from Super-Lorentzian traditionally used in interpreting MT experiments.
Conclusion: LDM-derived analytical equations connect the membrane-boundprotons T 1 and T 2 relaxation with dynamic distances between protons in neighboring membrane-forming lipid molecules and their lateral diffusion. This sheds new light on relationships between MT parameters and microstructure of cellular and myelin membranes.
“…Среди известных методов количественной МРТ уникальное положение занимает недавно разработанный метод быстрого картирования макромолекулярной протонной фракции (МПФ) [11,12]. В основе метода картирования МПФ лежит эффект переноса намагниченно-сти -явления, вызванного некогерентным обменом магнитной энергией между водой и макромолекулами в биологических объектах [13]. Принципиальной особенностью МПФ как биофизического параметра является его чувствительность и специфичность к содержанию миелина в нервной ткани [13].…”
Section: Abstract: аNomaly Of the Corpus Callosum Myelination Fetal M...unclassified
“…В основе метода картирования МПФ лежит эффект переноса намагниченно-сти -явления, вызванного некогерентным обменом магнитной энергией между водой и макромолекулами в биологических объектах [13]. Принципиальной особенностью МПФ как биофизического параметра является его чувствительность и специфичность к содержанию миелина в нервной ткани [13]. Количественная оценка миелинизации мозга в течение пренатального периода и в раннем детстве может открыть путь для выявления невропатологических причин задержки развития и дальнейшего прогноза интеллектуальных способностей и будущей социальной продуктивности ребенка при различных аномалиях головного мозга.…”
Section: Abstract: аNomaly Of the Corpus Callosum Myelination Fetal M...unclassified
“…White intermittent arrows indicate the CC hypoplasia and interhemispheric cyst secondary to CC hypoplasia (б). Black arrows show the communication of the third ventricle with interhemispheric cyst secondary to the CC agenesis (в)ния миелинизации методом МПФ в пренатальном периоде[12,13] и в раннем детском возрасте[14], позже опубликованы работы по исследованию траекторий поздней стадии миелинизации в подростковом возрасте[7].Многочисленные данные свидетельствуют о нейропластическом ремоделировании связей мозга при АМТ, которые начинаются еще во внутриутробном периоде. Знания о взаимосвязях аномалии МТ с внутриутробным структурным развитием мозга на уровне количественного определения маркеров нейрогенеза ограничены.…”
INTRODUCTION: There is evidence, indicate early compensatory axonal remodeling connections in the prenatal period, providing a favorable neurological outcome in isolated anomalies of the corpus callosum (CC). Mapping of the macromolecular proton fraction (MPF) is proven method of quantitative determination of myelin, which has been adapted for prenatal studies.OBJECTIVE: To investigate the relationship between CC anomalies and prenatal myelination of the brain using the fast macromolecular proton fraction (MPF) mapping.MATERIALS AND METHODS: Fetal MR imaging were performed on a 1.5 scanner (Achieva, Philips) using a 16-channel body coil. Of 66 fetal brains MRI, 12 studies were selected with MT abnormalities (22.8±2.8, 19–28.5 WG) and 21 without brain pathology (23.1±2.3, 20–29.5 WG). The images were analyzed according to structural MRI data (T2-Ssh and T1-GE, EPI, DWI, MYUR, T2-BFE-DYN) by two experienced radiologists. Fast-3D-MPF scan protocol with the MPF maps reconstruction was carried out according to a specialized protocol (open-source software: https://www.macroatomicmri.org /). Quantitative data were obtained by choosing the region of interest (ROI) in numerous brain structures bilateral (bridge, medulla oblongata, thalamus, cerebellum, and cerebral hemispheres). Statistics: distinctions between the groups and structures were assessed using repeated-measures analysis of covariance (ANCOVA), Pearson correlation analysis.RESULTS: MPF was significantly increased in the CC anomalies group as compared to controls in the medulla (3.26±0.63% vs. 2.75±0.59%, р=0.001) and cerebellum (2.02±0.55% vs. 1.76±0.34%, р=0.006). In hemispheres significant correlation with GA was observed in CC anomalies group (r=0.81, р=0.002), but was absent in controls (r=0.32, р=0.16).CONCLUSION: Primary observed MPF increase in the medulla and cerebellum as well as the dependence of the large hemispheres myelination on gestational age indicates that fetal cerebral matter undergoes early compensatory axonal remodeling in the cases of the interhemispheric connections’ reduction.
“…11 However, traditional MTR depends on MRI pulse sequence parameters and does not provide the true macromolecule content. Quantitative magnetization transfer (qMT) can measure the fraction of bound protons versus total protons, the so called Macromolecule Proton Fraction (MPF) [12][13][14][15][16][17][18][19][20] which reflects myelin content. 21,22 While initially qMT has been limited by low resolution, high radio frequency energy deposition (especially, at magnetic fields 3 T and higher), and long acquisition times, new approaches have been proposed to overcome some of these constraints.…”
Objective
Multiple sclerosis (MS) is a common demyelinating central nervous system disease. MRI methods that can quantify myelin loss are needed for trials of putative remyelinating agents. Quantitative magnetization transfer MRI introduced the macromolecule proton fraction (MPF), which correlates with myelin concentration. We developed an alternative approach, Simultaneous‐Multi‐Angular‐Relaxometry‐of‐Tissue (SMART) MRI, to generate MPF. Our objective was to test SMART‐derived MPF metric as a potential imaging biomarker of demyelination.
Methods
Twenty healthy control (HC), 11 relapsing–remitting MS (RRMS), 22 progressive MS (PMS), and one subject with a biopsied tumefactive demyelinating lesion were scanned at 3T using SMART MRI. SMART‐derived MPF metric was determined in normal‐appearing cortical gray matter (NAGM), normal‐appearing subcortical white matter (NAWM), and demyelinating lesions. MPF metric was evaluated for correlations with physical and cognitive test scores. Comparisons were made between HC and MS and between MS subtypes. Furthermore, correlations were determined between MPF and neuropathology in the biopsied person.
Results
SMART‐derived MPF in NAGM and NAWM were lower in MS than HC (p < 0.001). MPF in NAGM, NAWM and lesions differentiated RRMS from PMS (p < 0.01, p < 0.001, p < 0.001, respectively), whereas lesion volumes did not. MPF in NAGM, NAWM and lesions correlated with the Expanded Disability Status Scale (p < 0.01, p < 0.001, p < 0.001, respectively) and nine‐hole peg test (p < 0.001, p < 0.001, p < 0.01, respectively). MPF was lower in the histopathologically confirmed inflammatory demyelinating lesion than the contralateral NAWM and increased in the biopsied lesion over time, mirroring improved clinical performance.
Interpretation
SMART‐derived MPF metric holds potential as a quantitative imaging biomarker of demyelination and remyelination.
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