Measuring liver fat fraction with complex-based chemical shift MRI: the effect of simplified sampling protocols on accuracy

Application and research progress of magnetic resonance proton density fat fraction in metabolic dysfunction-associated steatotic liver disease: a comprehensive review

Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD), formerly known as Non-Alcoholic Fatty Liver Disease (NAFLD), is a chronic liver disorder associated with disturbances in lipid metabolism. The disease is prevalent worldwide, particularly closely linked with metabolic syndromes such as obesity and diabetes. Magnetic Resonance Proton Density Fat Fraction (MRI-PDFF), serving as a non-invasive and highly quantitative imaging assessment tool, holds promising applications in the diagnosis and research of MASLD. This paper aims to comprehensively review and summarize the applications and research progress of MRI-PDFF technology in MASLD, analyze its strengths and challenges, and anticipate its future developments in clinical practice.

Non-Invasive Imaging Methods to Evaluate Non-Alcoholic Fatty Liver Disease with Fat Quantification: A Review

Hepatic steatosis without specific causes (e.g., viral infection, alcohol abuse, etc.) is called non-alcoholic fatty liver disease (NAFLD), which ranges from non-alcoholic fatty liver (NAFL) to non-alcoholic steatohepatitis (NASH), fibrosis, and NASH-related cirrhosis. Despite the usefulness of the standard grading system, liver biopsy has several limitations. In addition, patient acceptability and intra- and inter-observer reproducibility are also concerns. Due to the prevalence of NAFLD and limitations of liver biopsies, non-invasive imaging methods such as ultrasonography (US), computed tomography (CT), and magnetic resonance imaging (MRI) that can reliably diagnose hepatic steatosis have developed rapidly. US is widely available and radiation-free but cannot examine the entire liver. CT is readily available and helpful for detection and risk classification, significantly when analyzed using artificial intelligence; however, it exposes users to radiation. Although expensive and time-consuming, MRI can measure liver fat percentage with magnetic resonance imaging proton density fat fraction (MRI-PDFF). Specifically, chemical shift-encoded (CSE)-MRI is the best imaging indicator for early liver fat detection. The purpose of this review is to provide an overview of each imaging modality with an emphasis on the recent progress and current status of liver fat quantification.

A Standardized Approach for MRI-PDFF is Necessary in the Assessment of Diagnostic Performances of the Ultrasound-Based Hepatic Fat Quantification Tools

The recently developed ultrasound-based hepatic fat quantification tools have the potential to be implemented in daily practice with wide acceptance due to inherited advantages of ultrasound technology. Researchers intensively focused on this topic and the accumulated evidences that support clinical usefulness of these tools. However, differences in the researcher-dependent factors of the utilized MRI-PDFF technique, the recommended reference standard, may hinder the better understanding of the diagnostic performances of these tools. Therefore, a standardized approach for MRI-PDFF technique, which is established with international consensus may be considered as important.

Free-breathing, non-ECG, simultaneous myocardial T1 , T2 , T2 *, and fat-fraction mapping with motion-resolved cardiovascular MR multitasking

PURPOSE

To develop a free-breathing, non-electrocardiogram technique for simultaneous myocardial T1 , T2 , T2 *, and fat-fraction (FF) mapping in a single scan.

METHODS

The MR Multitasking framework is adapted to quantify T1 , T2 , T2 *, and FF simultaneously. A variable TR scheme is developed to preserve temporal resolution and imaging efficiency. The underlying high-dimensional image is modeled as a low-rank tensor, which allows accelerated acquisition and efficient reconstruction. The accuracy and/or repeatability of the technique were evaluated on static and motion phantoms, 12 healthy volunteers, and 3 patients by comparing to the reference techniques.

RESULTS

In static and motion phantoms, T1 /T2 /T2 */FF measurements showed substantial consistency (R > 0.98) and excellent agreement (intraclass correlation coefficient > 0.93) with reference measurements. In human subjects, the proposed technique yielded repeatable T1 , T2 , T2 *, and FF measurements that agreed with those from references.

CONCLUSIONS

The proposed free-breathing, non-electrocardiogram, motion-resolved Multitasking technique allows simultaneous quantification of myocardial T1 , T2 , T2 *, and FF in a single 2.5-min scan.

Relationship between controlled attenuated parameter and magnetic resonance imaging-proton density fat fraction for evaluating hepatic steatosis in patients with NAFLD

We used cross-sectional and longitudinal studies to comprehensively compare hepatic steatosis measurements obtained with magnetic resonance imaging-proton density fat fraction (MRI-PDFF) and controlled attenuated parameter (CAP) in hepatic steatosis in adults with nonalcoholic fatty liver disease (NAFLD). A total of 185 participants with NAFLD and 12 non-NAFLD controls were recruited. CAP and MRI-PDFF data were collected at baseline from all participants and from 95 patients included in the longitudinal study after 24 weeks of drug or placebo intervention. Pearson correlation, linear regression, and piecewise linear regression analyses were used to evaluate the relationship between the two modalities. Linear analysis suggested a positive correlation between CAP and MRI-PDFF (r = 0.577, p < 0.0001); however, piecewise linear regression showed no correlation when CAP was ≥331 dB/m (p = 0.535). In the longitudinal study, both the absolute and relative change measurements were correlated between the two modalities; however, the correlation was stronger for the relative change (relative r = 0.598, absolute r = 0.492; p < 0.0001). Piecewise linear regression analysis revealed no correlation when CAP was reduced by more than 53 dB/m (p = 0.193). Conclusions: We found a correlation between CAP and MRI-PDFF measurements for grading hepatic steatosis when CAP was <331 dB/m. While the measured absolute change and relative change were correlated, it was stronger for the relative change. These findings have implications for the clinical utility of CAP or MRI-PDFF in the clinical diagnosis and assessment of NAFLD.

Comparison of reader agreement, correlation with liver biopsy, and time-burden sampling strategies for liver proton density fat fraction measured using magnetic resonance imaging in patients with obesity: a secondary cross-sectional study

Background

The magnetic resonance imaging (MRI)-based proton density fat fraction (PDFF) has become popular for quantifying liver fat content. However, the variability of the region-of-interest (ROI) sampling strategy may result in a lack of standardisation of this technology. In an effort to establish an accurate and effective PDFF measurement scheme, this study assessed the pathological correlation, the reader agreement, and time-burden of different sampling strategies with variable ROI size, location, and number.

Methods

Six-echo spoiled gradient-recalled-echo magnitude-based fat quantification was performed for 50 patients with obesity, using a 3.0-T MRI scanner. Two readers used different ROI sampling strategies to measure liver PDFF, three times. Intra-reader and inter-reader agreement was evaluated using intra-class correlation coefficients and Bland‒Altman analysis. Pearson correlations were used to assess the correlation between PDFFs and liver biopsy. Time-burden was recorded.

Results

For pathological correlations, the correlations for the strategy of using three large ROIs in Couinaud segment 3 (S3 3L-ROI) were significantly greater than those for all sampling strategies at the whole-liver level (P < 0.05). For inter-reader agreement, the sampling strategies at the segmental level for S3 3L-ROI and using three large ROIs in Couinaud segment 6 (S6 3L-ROI) and the sampling strategies at the whole-liver level for three small ROIs per Couinaud segment (27S-ROI), one large ROI per Couinaud segment (9L-ROI), and three large ROIs per Couinaud segment (27S-ROI) had limits of agreement (LOA) < 1.5%. For intra-reader agreement, the sampling strategies at the whole-liver level for 27S-ROI, 9L-ROI, and 27L-ROI had both intraclass coefficients > 0.995 and LOAs < 1.5%. The change in the time-burden was the largest (100.80 s) when 9L-ROI was changed to 27L-ROI.

Conclusions

For hepatic PDFF measurement without liver puncture biopsy as the gold standard, and for general hepatic PDFF assessment, 9L-ROI sampling strategy at the whole-liver level should be used preferentially. For hepatic PDFF with liver puncture biopsy as the gold standard, 3L-ROI sampling strategy at the puncture site segment is recommended.

[Evaluation of the Validity of ROI Setting in CEI Used for the Assessment of Liver]

PURPOSE

The enhancement effect ratio using ethoxybenzyl (EOB) contrast is useful in the assessment of liver fibrosis. Since the enhancement effect ratio is calculated by setting a region of interest (ROI) in the liver, the ROI setting method may affect the enhancement effect ratio. One of the methods of setting the ROI in liver fibrosis evaluation is by placing the ROI in each Quinault segment, but this method requires considerable time. Therefore, it is necessary to consider a reproducible ROI setting method in contrast to the method of placing ROIs in each Quinault segment.

METHOD

In contrast to the method of placing one ROI in each Quinault segment, we examined the method of setting four ROIs (two in the right lobe and two in the left lobe) and two ROIs (one in the right lobe and one in the left lobe). The size of the ROI was set to 1 cm², 4 cm², and the maximum area that fits within each placement area.

CONCLUSION

In the ROI setting method for CEI calculation, reproducibility can be maintained by setting the number of ROIs in four locations and by setting ROIs of 4 cm² or more.

Automated Whole-Liver MRI Segmentation to Assess Steatosis and Iron Quantification in Chronic Liver Disease

Background Standardized manual region of interest (ROI) sampling strategies for hepatic MRI steatosis and iron quantification are time consuming, with variable results. Purpose To evaluate the performance of automatic MRI whole-liver segmentation (WLS) for proton density fat fraction (PDFF) and iron estimation (transverse relaxometry [R2*]) versus manual ROI, with liver biopsy as the reference standard. Materials and Methods This prospective, cross-sectional, multicenter study recruited participants with chronic liver disease who underwent liver biopsy and chemical shift-encoded 3.0-T MRI between January 2017 and January 2021. Biopsy evaluation included histologic grading and digital pathology. MRI liver sampling strategies included manual ROI (two observers) and automatic whole-liver (deep learning algorithm) segmentation for PDFF- and R2*-derived measurements. Agreements between segmentation methods were measured using intraclass correlation coefficients (ICCs), and biases were evaluated using Bland-Altman analyses. Linear regression analyses were performed to determine the correlation between measurements and digital pathology. Results A total of 165 participants were included (mean age ± standard deviation, 55 years ± 12; 96 women; 101 of 165 participants [61%] with nonalcoholic fatty liver disease). Agreements between mean measurements were excellent, with ICCs of 0.98 for both PDFF and R2*. The median bias was 0.5% (interquartile range, -0.4% to 1.2%) for PDFF and 2.7 sec^-1 (interquartile range, 0.2-5.3 sec^-1) for R2* (P < .001 for both). Margins of error were lower for WLS than ROI-derived parameters (-0.03% for PDFF and -0.3 sec^-1 for R2*). ROI and WLS showed similar performance for steatosis (ROI AUC, 0.96; WLS AUC, 0.97; P = .53) and iron overload (ROI AUC, 0.85; WLS AUC, 0.83; P = .09). Correlations with digital pathology were high (P < .001) between the fat ratio and PDFF (ROI r = 0.89; WLS r = 0.90) and moderate (P < .001) between the iron ratio and R2* (ROI r = 0.65; WLS r = 0.64). Conclusion Proton density fat fraction and transverse relaxometry measurements derived from MRI automatic whole-liver segmentation (WLS) were accurate for steatosis and iron grading in chronic liver disease and correlated with digital pathology. Automated WLS estimations were higher, with a lower margin of error than manual region of interest estimations. © RSNA, 2021 Online supplemental material is available for this article. See also the editorial by Moura Cunha and Fowler in this issue.

Comparison of the Effects of Hepatic Steatosis on Monoexponential DWI, Intravoxel Incoherent Motion Diffusion-weighted Imaging and Diffusion Kurtosis Imaging

RATIONALE AND OBJECTIVES

Diffusion-weighted imaging (DWI) techniques have drawn attention for their capability of staging hepatic fibrosis. However, the diagnostic performance of DWI for hepatic fibrosis might be affected by hepatic steatosis because hepatic steatosis and fibrosis may have a similar effect on diffusion/perfusion parameters. Therefore, the purpose of our study is to investigate the effect of hepatic steatosis on DWI parameters.

MATERIALS AND METHODS

51 patients with MR elastography liver stiffness values below 3.45kPa underwent DWI with multiple b-values and a multi-echo Dixon sequence for fat quantification. Correlation analysis was conducted between fat fraction and DWI parameters, and DWI parameters were compared between steatosis and non-steatosis groups.

RESULTS

Significant negative correlation was observed between fat fraction and apparent diffusion coefficient (ADC) (r = -0.62, p <0.001), pure molecular diffusion (D) (r = -0.62, p <0.001), corrected ADC (D_app) (r = -0.36, p = 0.01) and a positive correlation with mean kurtosis (K_app) (r = 0.53, p <0.001). The results of the comparison of DWI parameters were that ADC, D and D_app were statistically lower in the steatosis group (p < 0.001, p < 0.001 and p = 0.026, respectively) and K_app was significantly higher in the steatosis group (p <0.001) compared to the non-steatosis group. However, perfusion-related parameters (D* and f) did not show any statistical significance.

CONCLUSION

DWI parameters except for perfusion-related parameters (D* and f) are affected by changes in hepatic steatosis. Thus, hepatic steatosis may be considered as a possible confounding factor in DWI-based assessment of liver fibrosis.

Does chemical shift imaging offer a biomarker for the diagnosis and assessment of disease severity in multiple myeloma?: A cross-sectional study

To investigate whether chemical shift imaging (CSI) is useful for differentiating myelomatous infiltration from hematopoietic bone marrow (BM) and for quantitatively assessing disease severity.In this retrospective study, spinal MRI, including a sagittal iterative decomposition of water and fat with echo asymmetry and least-squares estimation T2 fast spin-echo sequence, was performed on 76 myeloma patients (45 men, 67.0 ± 11.4 years; 31 women, 66.5 ± 11.0 years) and 30 control subjects (20 men, 67.0 ± 8.4 years; 10 women, 67.0 ± 9.2 years). The fat-signal fraction (FF) and mean signal dropout ratio (DR) were calculated from lumbar BM that contained no focal lesions. The BM plasma cell percentage (BMPC%) and serological data were obtained. As DR is highest when FF = 50%, the patients were divided into 2 groups: a water-dominant group (FF < 50%) and a fat-dominant group (FF > 50%).Serum monoclonal protein (M protein), β2-microglobulin, and BMPC% were significantly higher in the water-dominant group than in the fat-dominant group. In the water-dominant group, DR correlated significantly with BMPC% and M protein, whereas in the control group, DR showed a weak correlation with age but no correlation with other clinical factors. No significant differences in any clinical data were seen between high and low DR.CSI proved ineffective for differentiating myelomatous infiltration from hematopoietic BM. For myeloma patients with relatively high BM cellularity, a small signal drop on opposed-phase images indicated a higher tumor burden. For BM with relatively low cellularity, disease severity was not reflected by CSI.

Sample strategies for quantification of hepatic fat fraction mean MRI in healthy cats during body weight gain

Hepatic fat fraction (HFF) can be non-invasively estimated with magnetic resonance imaging (MRI) multiple echo gradient recalled echo (GRE) sequence. The aim of this study was to test different methods of sampling strategies to quantify the HFF in healthy cats during body weight gain. Twelve healthy adult male cats were examined in a 3 Tesla MRI unit. Sequences included morphological images, and multiple echo GRE sequence. Cats were scanned at the beginning of the study and twice, each 20 weeks apart during body weight gain. HFF was calculated with 5 different methods of sampling on the multiple echo GRE sequence with different number, size and position of regions of interest (ROIs) and by 2 operators. Results indicated that HFF increased with increasing body weight, and the increase was appreciated with all the 5 methods. There was overall excellent agreement (interclass correlation coefficient = 0.820 (95% confidence interval:0.775–0.856)) between the 2 operators. HFF in the left lateral hepatic lobe was lower than in the other analyzed lobes. HFF measured on large free-hand drawn ROIs was higher than HFF measured with smaller ROIs size. This study proves that different sampling methods for quantification of HFF on multiple echo GRE sequence have overall excellent repeatability and ability to appreciate increased HFF.

Magnetic Resonance Elastography of the Liver in Children and Adolescents: Assessment of Regional Variations in Stiffness

RATIONALE AND OBJECTIVES

We describe our experience in measuring parenchyma stiffness across the liver Couinaud segments in lieu of the conventional practice of using a single slice-wise "global" region-of-interest. We hypothesize that the heterogeneous nature of fibrosis can lead to regional stiffness within the organ, and that it can be reflected by Couinaud segment-based magnetic resonance elastography measurements.

MATERIALS AND METHODS

This retrospective study involved from 173 patients (116 males, 57 females, 1.0-22.5 years, 14.7 ± 3.5 years) who underwent exams between June 2017 and September 2018. Liver stiffness across the eight Couinaud segments was measured in addition to a single-slice global measurement by two analysts. Inter- and intrarater analysis was performed in a subset of 20 cases. Individual segment stiffness values, the average across the segments, and the coefficients of variation (CoV) were compared to global single-slice-derived values using linear and Lin's concordance correlation coefficients. Linear correlations between stiffness values versus age, gender, and body-mass-index (BMI) were also evaluated.

RESULTS

We observed CoVs ranging from 3.1%-79.2%, 17.2 ± 7.2%. The CoV was not correlated with age or BMI (r² < 0.01, p = 0.99 for both). The CoV did not differ between males (17.1 ± 5.6%) and females (17.3 ± 9.8%) (p = 0.88). There were no correlations between global stiffness versus age (r² = 0.02, p = 0.84) or BMI (r² = 0.03, p = 0.68). A range of 0.58-0.86 was observed for Lin's concordance correlation coefficient between segmental stiffness, the average stiffness across segments, and global stiffness. Segments II and VII had the highest frequency of being the stiffest Couinaud segment. The average stiffness across the segments correlated strongly with the single-slice global measurement (r² = 0.88, p< 0.01).

CONCLUSION

There exists potential variations in parenchyma stiffness across the liver Couinaud segments, which may reflect the heterogeneous nature of fibrosis. This variation can potentially provide additional diagnostic and clinical information.