Applications of the Dixon technique in the evaluation of the musculoskeletal system / Aplicações da técnica Dixon na prática radiológica do sistema musculoesquelético

Abstract The acquisition of images with suppression of the fat signal is very useful in clinical practice and can be achieved in a variety of sequences. The Dixon technique, unlike other fat suppression techniques, allows the signal of fat to be suppressed in the postprocessing rather than during acquisition, as well as allowing the visualization of maps showing the distribution of water and fat. This review of the Dixon technique aims to illustrate the basic physical principles, to compare the technique with other magnetic resonance imaging sequences for fat suppression or fat quantification, and to describe its applications in the study of diseases of the musculoskeletal system. Many variants of the Dixon technique have been developed, providing more consistent separation of the fat and water signals, as well as allowing correction for many confounding factors. It allows homogeneous fat suppression, being able to be acquired in combination with several other sequences, as well as with different weightings. The technique also makes it possible to obtain images with and without fat suppression from a single acquisition. In addition, the Dixon technique can be used as a quantitative method, allowing the proportion of tissue fat to be determined, and, in more updated versions, can quantify tissue iron.

Resumo A aquisição de imagens com supressão do sinal da gordura é um recurso de grande utilidade diagnóstica, existindo várias sequências capazes de realizá-la. A técnica Dixon, ao contrário de outras técnicas de supressão de gordura, permite suprimir a contribuição do sinal de gordura no pós-processamento e não durante a aquisição, além de permitir a visualização de mapas com a distribuição da água e da gordura. Esta revisão sobre a técnica Dixon almeja ilustrar os princípios físicos básicos, comparar a técnica com outras sequências de ressonância magnética para supressão ou quantificação de gordura, e descrever suas aplicações no estudo de doenças do sistema musculoesquelético. Muitas variantes da técnica Dixon foram desenvolvidas, proporcionando separação mais consistente dos sinais de gordura e água e permitindo correção de muitos fatores de confusão. Permite obter supressão homogênea de gordura, podendo ser adquirida de forma combinada com várias outras sequências, bem como com diferentes ponderações. Esta técnica possibilita também a obtenção de imagens com e sem supressão de gordura a partir de uma única aquisição. Adicionalmente, a técnica Dixon pode ser utilizada como recurso quantitativo, pois permite a mensuração do porcentual de gordura e, em versões mais atualizadas, consegue quantificar ferro tecidual.

Advances in the application of magnetic resonance imaging in hepatic fat quantification in patients with nonalcoholic fatty liver disease / 临床肝胆病杂志

Hepatic fat content is an important index for the early diagnosis, disease grading, and outcome evaluation of nonalcoholic fatty liver disease (NAFLD). Various magnetic resonance imaging (MRI) methods have been used to determine hepatic fat content in NAFLD, among which proton density fat fraction obtained by magnetic resonance spectroscopy and chemical-shift-encoded MRI can achieve precise quantification of hepatic fat and therefore, it is considered the imaging gold standard for the diagnosis of fatty liver disease and has been applied in clinical research. This article reviews the research advances in the MRI techniques for quantification of hepatic fat content, in order to provide a reference for clinical application and experiment.

Fat Quantification in the Vertebral Body: Comparison of Modified Dixon Technique with Single-Voxel Magnetic Resonance Spectroscopy

OBJECTIVE: To compare the lumbar vertebral bone marrow fat-signal fractions obtained from six-echo modified Dixon sequence (6-echo m-Dixon) with those from single-voxel magnetic resonance spectroscopy (MRS) in patients with low back pain. MATERIALS AND METHODS: Vertebral bone marrow fat-signal fractions were quantified by 6-echo m-Dixon (repetition time [TR] = 7.2 ms, echo time (TE) = 1.21 ms, echo spacing = 1.1 ms, total imaging time = 50 seconds) and single-voxel MRS measurements in 25 targets (23 normal bone marrows, two focal lesions) from 24 patients. The point-resolved spectroscopy sequence was used for localized single-voxel MRS (TR = 3000 ms, TE = 35 ms, total scan time = 1 minute 42 seconds). A 2 × 2 × 1.5 cm³ voxel was placed within the normal L2 or L3 vertebral body, or other lesions including a compression fracture or metastasis. The bone marrow fat spectrum was characterized on the basis of the magnitude of measurable fat peaks and a priori knowledge of the chemical structure of triglycerides. The imaging-based fat-signal fraction results were then compared to the MRS-based results. RESULTS: There was a strong correlation between m-Dixon and MRS-based fat-signal fractions (slope = 0.86, R² = 0.88, p 20%). CONCLUSION: Given its excellent agreement with single-voxel-MRS, 6-echo m-Dixon can be used for visual and quantitative evaluation of vertebral bone marrow fat in daily practice.

Evaluation of thyroid-associated ophthalmopathy activity by using magnetic resonance fat quantification technology / 中华实验眼科杂志

Objective@#To explore the value of magnetic resonance fat quantification technology (MR mDixon-quant) in evaluating thyroid-associated ophthalmopathy (TAO) activity.@*Methods@#Case control study was performed.Fifty patients (100 eyes) with TAO which met Mourits criterion were enrolled from September 2016 to March 2018 in Ningxia Hui Autonomous Region People's Hospital.The TAO patients were grouped by clinical activity score (CAS). Twenty nine patients (58 eyes) with CAS≥3 served as TAO active group, twenty one patients (42 eyes) with CAS<3 served as TAO non-active group.Twenty three healthy subjects (46 eyes) were collected as normal control group at the same time.All subjects underwent orbital MR mDixon-quant and MR imaging.Orbital fat content (FF values) and exophthalmus was calculated on MR work-station.FF values and degree of exophthalmus were analyzed by analysis of variance.The correlation of FF values and CAS in TAO patients was conducted by Spearman rank correlation analysis.The cut-off values of FF in predicting TAO activity was determined by receiver operating characteristics (ROC) curve.The Kappa consistency test was chosen to assess the consistency of CAS and FF values.Intraclass correlation coefficient (ICC) were used to analyze the intra-observer and inter-observer repeatability of the FF values and the degree of exophthalmus.This study was approved by the Ethics Committee of Ningxia Hui Autonomous Region People's Hospital (No.20160718). All the candidates signed informed consent.@*Results@#The FF values were (85.190±4.346)%, (88.715±5.686)% and (82.345±5.445)% in TAO active group, TAO inactive group and normal control group, respectively, with a significant difference among the 3 groups (F=17.072, P<0.001). The FF values of TAO active group and TAO inactive group were significantly higher than that of control group, and the FF value of TAO active group was significantly lower than that of TAO inactive group (all at P<0.01). The degrees of exophthalmos were (20.221±1.714), (20.855±2.103) and (15.363±1.667)mm in TAO active group, TAO inactive group and normal control group, respectively, with a significant difference among the three groups (F=126.298, P<0.01). The degrees of exophthalmos in TAO active group and TAO inactive group were significantly higher than that in normal control group (both at P<0.01), there was no statistical difference in degrees of exophthalmos between TAO active group and TAO inactive group (P<0.05). Spearman rank correlation revealed that FF value was negative correlated with CAS (rs=-0.443, P<0.01). The cut-off value of FF value was 87.180%, the sensitivity, specificity, positive predictive value, negative predictive value and accuracy were 74.14%(43/58), 66.67%(28/42), 75.44%(43/57), 65.12%(28/43) and 71.00%(71/100), respectively.There were consistencies of FF values with the CAS (Kappa=0.431, P<0.001). The intra-observe and inter-observer reproducibility were fairly good for FF value (ICC=0.953, 0.920) and degree of exophthalmus (ICC=0.935, 0.917).@*Conclusions@#MR mDixon-quant can quantitative measure orbital fat with good measurement repeatability.Fat quantification technology by MR is valuable in evaluating TAO activity.

Application of navigator IDEAL-IQ in liver fat quantification:a comparative study with MRS / 实用放射学杂志

Objective To quantitatively assess the liver fat content by Nav IDEAL-IQ and compare with conventional breath-hold IDEAL-IQ and magnetic resonance spectroscopy (MRS).Methods Twenty volunteers,twenty fatty liver patients and twenty patients with other liver diseases were enrolled in this study.IDEAL-IQ,Nav IDEAL-IQ and MRS imaging were performed and fat fraction (FF) were measured respectively.The results were compared by t-test to explore whether there were differences between these groups. Spearman correlation analysis was also used to test the correlation.Results No significant differences in the FF measured by IDEAL-IQ,Nav IDEAL-IQ and MRS in normal volunteers [(4.09±0.82)%,(3.94±0.83)%,(3.92±0.85)%],fatty liver [(15.5±6.1)%,(11.8±6.5)%,(1 2 .4 ± 7 .2 )% ] and other liver lesions patients [(4.20±0.84)%,(4.00±0.94)%,(3.97±0.96)%]were found between these groups (P>0.05). There was a significant positive correlation between Nav IDEAL-IQ and MRS,and the correlation coefficients of these groups were 0.959,0.786 and 0.945,respectively.Conclusion The results of Nav IDEAL-IQ of fat quantification are consistent with MRS results and it increases the success rate of fat quantitative testing.In a word,it is a noninvasive,convenient and accurate way to measure fat fraction.

T₂* Mapping from Multi-Echo Dixon Sequence on Gadoxetic Acid-Enhanced Magnetic Resonance Imaging for the Hepatic Fat Quantification: Can It Be Used for Hepatic Function Assessment?

OBJECTIVE: To evaluate the diagnostic value of T₂* mapping using 3D multi-echo Dixon gradient echo acquisition on gadoxetic acid-enhanced liver magnetic resonance imaging (MRI) as a tool to evaluate hepatic function. MATERIALS AND METHODS: This retrospective study was approved by the IRB and the requirement of informed consent was waived. 242 patients who underwent liver MRIs, including 3D multi-echo Dixon fast gradient-recalled echo (GRE) sequence at 3T, before and after administration of gadoxetic acid, were included. Based on clinico-laboratory manifestation, the patients were classified as having normal liver function (NLF, n = 50), mild liver damage (MLD, n = 143), or severe liver damage (SLD, n = 30). The 3D multi-echo Dixon GRE sequence was obtained before, and 10 minutes after, gadoxetic acid administration. Pre- and post-contrast T₂* values, as well as T₂* reduction rates, were measured from T₂* maps, and compared among the three groups. RESULTS: There was a significant difference in T₂* reduction rates between the NLF and SLD groups (−0.2 ± 4.9% vs. 5.0 ± 6.9%, p = 0.002), and between the MLD and SLD groups (3.2 ± 6.0% vs. 5.0 ± 6.9%, p = 0.003). However, there was no significant difference in both the pre- and post-contrast T₂* values among different liver function groups (p = 0.735 and 0.131, respectively). A receiver operating characteristic (ROC) curve analysis showed that the area under the ROC curve for using T₂* reduction rates to differentiate the SLD group from the NLF group was 0.74 (95% confidence interval: 0.63–0.83). CONCLUSION: Incorporation of T₂* mapping using 3D multi-echo Dixon GRE sequence in gadoxetic acid-enhanced liver MRI protocol may provide supplemental information for liver function deterioration in patients with SLD.

Comparison of breath-hold and respiratory-triggered proton MR spectroscopy in quantification of liver fat content / 实用医学杂志

Objective To compare the consistency and correlation of multiple breath-hold (BH) with respiratory-triggered (RT) 1H-MRS for quantification of hepatic lipid content. Methods Sixty subjects were underwent RT 1H-MRS of the liver (Couinaud segment VII) and BH 1H-MRS at 1.5 Tesla Magnetic Resonace Imaging (MRI). The peak areas of water and methylene obtained on RT and BH 1H-MRS were recorded respectively and the liver fat fraction was calculated. Pearson correlation coefficient , Bland-Altman 95% limit of agreement, and concordance correlation coefficient were calculated. Results Mean liver fat fraction measured in RT and BH 1H-MRS were (8.6 ± 8.7)% and (9.4 ± 9.3)% respectively. There was a strong correlation between RT and BH 1H-MRS(r = 0.973, P < 0.000 1, concordance correlation coefficient = 0.95). With the Bland-Altman method, 91.7% data points were within the 95% limits of agreement. Conclusion RT and BH 1H-MRS are alternative tools for intrahepatic lipid quantification. These two methods have a strong correlation and perfect consistency.

Whole-Body Muscle MRI in Patients with Hyperkalemic Periodic Paralysis Carrying the SCN4A Mutation T704M: Evidence for Chronic Progressive Myopathy with Selective Muscle Involvement

BACKGROUND AND PURPOSE: Hyperkalemic periodic paralysis (hyperKPP) is a muscle sodium-ion channelopathy characterized by recurrent paralytic attacks. A proportion of affected individuals develop fixed or chronic progressive weakness that results in significant disability. However, little is known about the pathology of hyperKPP-induced fixed weakness, including the pattern of muscle involvement. The aim of this study was to characterize the patterns of muscle involvement in hyperKPP by whole-body magnetic resonance imaging (MRI). METHODS: We performed whole-body muscle MRI in seven hyperKPP patients carrying the T704M mutation in the SCN4A skeletal sodium-channel gene. Muscle fat infiltration, suggestive of chronic progressive myopathy, was analyzed qualitatively using a grading system and was quantified by the two-point Dixon technique. RESULTS: Whole-body muscle MRI analysis revealed muscle atrophy and fatty infiltration in hyperKPP patients, especially in older individuals. Muscle involvement followed a selective pattern, primarily affecting the posterior compartment of the lower leg and anterior thigh muscles. The muscle fat fraction increased with patient age in the anterior thigh (r=0.669, p=0.009), in the deep posterior compartment of the lower leg (r=0.617, p=0.019), and in the superficial posterior compartment of the lower leg (r=0.777, p=0.001). CONCLUSIONS: Our whole-body muscle MRI findings provide evidence for chronic progressive myopathy in hyperKPP patients. The reported data suggest that a selective pattern of muscle involvement-affecting the posterior compartment of the lower leg and the anterior thigh-is characteristic of chronic progressive myopathy in hyperKPP.

Comparison of different MR fat quantification methods at 3.0 T in a phantom study / 中华放射学杂志

Objective To investigate the value of MR imaging methods for the quantification of fat content in a customized lipid phantom at 3.0 T.Methods Eleven homogeneous fat-water phantoms (50 ml)with fat volume percentages from 0 to 100％ were constructed with reference to Bernard's methods.Fat tractions of the lipid phantom were acquired using water selective saturation (WS),fat selective saturation (FS),in-and out-of-phase imaging (IOP),iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) gradient echo imaging and IDEAL Quant imaging methods on a 3.0 T MRI system.For statistical comparisons,paired-sample t test,Pearson correlation test,and Bland-Ahman maps were applied.Results Evaluated fat fractions acquired by WS,IDEAL Gradient echo imaging and IDEAL Quant were (49.6±28.8)％,(46.0±28.4)％,(51.0±32.0)％,the result has no significant difference with the true fat contents(t values were-0.186,-2.218,2.713;P values were 0.856,0.051,0.055).Evaluated fat fractions acquired by FS,corrected algorithm and IOP were (64.2±26.7)％,(58.9±31.9)％ and (45.3±32.3)％,these three kinds of methods have significant difference with the true fat contents (t values were 5.168,4.273,-6.441;P＜0.01).All the chemical shift imaging methods correlated with the true phantom model fat fractions,r values were 0.977(FS),0.978 (corrected algorithm),0.982 (WS),0.99 8(IOP),0.993 (IDEAL Gradient echo imaging),0.999 (IDEAL Quant) (all P＜0.01).Each method's 95％ confidence interval of the mean difference acquired by Bland-Altman map was WS (-14.7％ to 13.8％),FS (-3.6％ to 32.0％),corrected algorithm (-4.6％ to 22.5％),IOP(-9.4％ to 0.0％),IDEAL gradient echo imaging (-15.9％ to 7.8％),IDEAL Quant(-2.0％ to 4.0％).IDEAL Quant had the best correlation and confidence with the true fat fraction.Conclusions Chemical shift imaging methods (IOP,IDEAL Gradient echo imaging,IDEAL Quant) can acquire more accurate fat quantification results than chemical saturation imaging methods (FS,Corrected algorithm,WS) in a customized lipid phantom at 3.0 T.IDEAL Quant can acquire the best fat quantification result compared with the other imaging methods.

Background Gradient Correction using Excitation Pulse Profile for Fat and T2* Quantification in 2D Multi-Slice Liver Imaging

PURPOSE: The objective of this study was to develop background gradient correction method using excitation pulse profile compensation for accurate fat and T2* quantification in the liver. MATERIALS AND METHODS: In liver imaging using gradient echo, signal decay induced by linear background gradient is weighted by an excitation pulse profile and therefore hinders accurate quantification of T2* and fat. To correct this, a linear background gradient in the slice-selection direction was estimated from a B0 field map and signal decays were corrected using the excitation pulse profile. Improved estimation of fat fraction and T2* from the corrected data were demonstrated by phantom and in vivo experiments at 3 Tesla magnetic field. RESULTS: After correction, in the phantom experiments, the estimated T2* and fat fractions were changed close to that of a well-shimmed condition while, for in vivo experiments, the background gradients were estimated to be up to approximately 120 microT/m with increased homogeneity in T2* and fat fractions obtained. CONCLUSION: The background gradient correction method using excitation pulse profile can reduce the effect of macroscopic field inhomogeneity in signal decay and can be applied for simultaneous fat and iron quantification in 2D gradient echo liver imaging.