Magnetism of materials: theory and practice in magnetic resonance imaging

All substances exert magnetic properties in some extent when placed in an external magnetic field. Magnetic susceptibility represents a measure of the magnitude of magnetization of a certain substance when the external magnetic field is applied. Depending on the tendency to be repelled or attracted by the magnetic field and in the latter case on the magnitude of this effect, materials can be classified as diamagnetic or paramagnetic, superparamagnetic and ferromagnetic, respectively. Knowledge of type and extent of susceptibility of common endogenous and exogenous substances and how their magnetic properties affect the conventional sequences used in magnetic resonance imaging (MRI) can help recognize them and exalt or minimize their presence in the acquired images, so as to improve diagnosis in a wide variety of benign and malignant diseases. Furthermore, in the context of diamagnetic susceptibility, chemical shift imaging enables to assess the intra-voxel ratio between water and fat content, analyzing the tissue composition of various organs and allowing a precise fat quantification. The following article reviews the fundamental physical principles of magnetic susceptibility and examines the magnetic properties of the principal endogenous and exogenous substances of interest in MRI, providing potential through representative cases for improved diagnosis in daily clinical routine.

Intraepineurial fat quantification and cross-sectional area analysis of the sciatic nerve using MRI in Charcot-Marie-Tooth disease type 1A patients

The objectives of this study were to assess the fat fraction (FF) and cross-sectional area (CSA) of the sciatic nerve in Charcot-Marie-Tooth disease type 1A (CMT1A) patients using Dixon-based proton density fat quantification MRI and to elucidate its potential association with clinical parameters. Thigh MRIs of 18 CMT1A patients and 18 age- and sex-matched volunteers enrolled for a previous study were reviewed. Analyses for FF and CSA of the sciatic nerve were performed at three levels (proximal to distal). CSA and FF were compared between the two groups and among the different levels within each group. The relationship between the MRI parameters and clinical data were assessed in the CMT1A patients. The CMT1A patients showed significantly higher FF at level 3 (p = 0.0217) and significantly larger CSA at all three levels compared with the control participants (p < 0.0001). Comparisons among levels showed significantly higher FF for levels 2 and 3 than for level 1 and significantly larger CSA for level 2 compared with level 1 in CMT1A patients. CSA at level 3 correlated positively with the CMT neuropathy score version 2 (CMTNSv2). In conclusion, the sciatic nerve FF of CMT1A patients was significantly higher on level 3 compared with both the controls and the measurements taken on more proximal levels, suggesting the possibility of increased intraepineurial fat within the sciatic nerves of CMT1A patients, with a possible distal tendency. Sciatic nerve CSA at level 3 correlated significantly and positively with CMTNSv2, suggesting its potential value as an imaging marker for clinical severity.

Optimization of breath-holding time for intrahepatic fat quantification using proton-magnetic resonance spectroscopy at 1.5-Tesla magnetic resonance imaging

Purpose We determined the minimum required breath-holding time (Modified 1H-MRS) and conducted a backward study on patients who underwent intrahepatic fat quantification using the point resolved spectroscopy method. We investigated the correlation between dual echo-MRI (DE-MRI) and liver attenuation index (LAI) measurements and evaluated the validity of the analysis results to determine whether breath-holding time (BHT) could be reduced.Methods In the MRS-fat fraction (MRS-FF) analysis, using only the reference datasets, the minimum required breath-holding time was set to 14 s. To evaluate the validity of the obtained MRS-FF, we compared the correlation using the dual echo-fat fraction (DE-FF) and LAI as a reference values, the correlation coefficient was investigated by obtaining the Spearman’s rank correlation coefficient. Next, we compared the correlation in two subgroups of the abovementioned case groups: (1) those with a BHT of 23 s and (2) those with a BHT between 14 to 22 s.Results The correlation coefficients between MRS-FF and DE-FF and MRS-FF and LAI were 0.841 (p = 0.00) and − 0.726 (p < 0.01), respectively. Next, in the subgroup analysis, the correlation coefficients of MRS-FF and DE-FF and MRS-FF and LAI were 0.849 (p = 0.00) and − 0.733 (p = 0.00), respectively, in group (1) and 0.886 (p = 0.00) and − 0.761 (p = 0.00), respectively, in group (2).Conclusion In the imaging using Modified 1H-MRS, the present method suggested that the reduction in breath-holding time from 14 to 22 s did not affect the MRS-FF analysis results.