Use of fat suppression in R₂ relaxometry with MRI for the quantification of tissue iron overload in beta-thalassemic patients

Pancreatic iron quantification with MR imaging: a practical guide

Accurate determination of pancreatic iron status is crucial for preventing impairment of the exocrine and endocrine function of the pancreas and for prospectively stratifying the cardiac iron risk. The following article should be a sort of practical guide for radiologists interested in quantifying pancreatic iron overload by Magnetic Resonance Imaging (MRI). After a brief background on iron-deposition diseases, we will describe basic principles and relative advantages and disadvantages of the more widely used and clinically feasible MRI-based techniques for pancreatic iron assessment. These methods can be classified into signal intensity ratio (SIR) and relaxometry methods. We will examine different technical aspects representing the key for accurate and precise relaxation time measurement.

Evaluation of a single-breath-hold radial turbo-spin-echo sequence for T2 mapping of the liver at 3T

OBJECTIVES

T2 mapping of the liver is a potential diagnostic tool, but conventional techniques are difficult to perform in clinical practice due to long scan time. We aimed to evaluate the accuracy of a prototype radial turbo-spin-echo (rTSE) sequence, optimized for multi-slice T2 mapping in the abdomen during one breath-hold at 3 T.

METHODS

A multi-sample (fat: 0-35%) agarose phantom doped with MnCl₂ and 80 subjects (73 patients undergoing abdomen MR examination and 7 healthy volunteers) were investigated. A radial turbo-spin-echo (rTSE) sequence with and without fat suppression, a Cartesian turbo-spin-echo (Cart-TSE) sequence, and a single-voxel multi-echo STEAM spectroscopy (HISTO) were performed in phantom, and fat-suppressed rTSE and HISTO sequences were performed in in vivo measurements. Two approaches were used to sample T2 values: manually selected circular ROIs and whole liver analysis with Gaussian mixture models (GMM).

RESULTS

The rTSE-T2s values exhibited a strong correlation with Cart-TSE-T2s (R² = 0.988) and with HISTO-T2s of water (R² = 0.972) in phantom with an offset between rTSE and Cart-TSE maps (mean difference = 3.17 ± 1.18 ms). The application of fat suppression decreased T2 values, and the effect was directly proportional to the amount of fat. Measurements in patients yielded a linear relationship between rTSE- and HISTO-T2s (R² = 0.546 and R² = 0.580 for ROI and GMM, respectively).

CONCLUSION

The fat-suppressed rTSE sequence allows for fast and accurate determination of T2 values of the liver, and appears to be suitable for further large cohort studies.

KEY POINTS

•Radial turbo-spin-echo T2 mapping performs comparably to Cartesian TSE-T2 mapping, but an offset in values is observed in phantom measurements. •Fat-suppressed radial turbo-spin-echo T2 mapping is consistent with T2 of water as assessed by MRS in phantom measurements. •Fat-suppressed radial turbo-spin-echo sequence allows fast T2 mapping of the liver in a single breath-hold and is correlated with MRS-based T2 of water.

Physiological variation of the vertebral bone marrow water T2 relaxation time

The aim of this study was to investigate physiological variations of the water T2 relaxation time in vertebral bone marrow with respect to age, body mass index (BMI), sex and proton density fat fraction (PDFF) based on single-voxel magnetic resonance spectroscopy (MRS) at 3 T. Multi-TE single-voxel STEAM MRS data of a single lumbar vertebra (L4 or L5) from 260 subjects (160/100 female/male, age: 0.7/37.1/77.7 years, BMI: 13.6/26.2/44.5 kg/m² [min./median/max.]) with no history of vertebral bone marrow pathologies were retrospectively included. All data were processed using a joint series T2-constrained time domain-based water-fat model. Water T2 and PDFF data were analyzed using (a) Pearson's correlation r and (b) multiple linear regression without interactions of the independent variables. Min./median/max. water T2 and PDFF were 11.2/21.1/42.5 ms and 4.0%/36.8%/82.0%, respectively. Pearson's correlation coefficients were significant (P < .05) for water T2 versus age (r = -0.429/-0.210 female/male) and for water T2 versus PDFF (r = -0.580/-0.546 female/male) for females and males, respectively. Females showed significant higher water T2 values compared with males (P < .001). Multiple linear regression for water T2 without interactions revealed a R² = 0.407 with PDFF (P < .001) and sex (P < .001) as significant predictors. The current study suggests that under physiological conditions vertebral bone marrow water T2 is negatively correlated with age and PDFF and shows significant differences between females and males. The observed systematic trends are of relevance for the evaluation of T2 values and T2-weighted bone marrow parameters. Further research on the exact mechanisms and drivers of the observed water T2 behavior is required.

Evaluation of liver iron overload with R2* relaxometry with versus without fat suppression: both are clinically accurate but there are differences

OBJECTIVES

To assess clinically relevant difference in hepatic iron quantification using R2* relaxometry with (FS) and without (non-FS) fat saturation for the evaluation of patients with suspected hepatic iron overload.

METHODS

We prospectively enrolled 134 patients who underwent 1.5-T MRI R2* relaxometry with FS and non-FS gradient echo sequences (12 echoes, initial TE = 0.99 ms). Proton density fat fraction for the quantification of steatosis was assessed. Linear regression analyses and Bland-Altman plots including Lin's concordance correlation coefficient were performed for correlation of FS R2* with non-FS R2*. Patients were grouped into 4 severity classes of iron overload (EASL based), and agreement was evaluated by contingency tables and the proportion of overall agreement.

RESULTS

A total of 41.8% of patients showed hepatic iron overload; 67.9% had concomitant steatosis; and 58.2% revealed no iron overload of whom 60.3% had steatosis. The mean R2* value for all FS data was 102.86 1/s, for non-FS 108.16 1/s. Linear regression resulted in an R-squared value of 0.99 (p < 0.001); Bland-Altman plot showed a mean R2* difference of 5.26 1/s (SD 17.82). The concordance correlation coefficient was only slightly lower for patients with steatosis compared with non-steatosis (0.988 vs. 0.993). The overall agreement between FS and non-FS R2* measurements was 94.8% using either method to classify patients according to severity of iron storage. No correlation between R2* and proton density fat fraction was found for both methods.

CONCLUSION

R2* relaxometry showed an excellent overall agreement between FS and non-FS acquisition. Both variants can therefore be used in daily routine. However, clinically relevant differences might result when switching between the two methods or during patient follow-up, when fat content changes over time. We therefore recommend choosing a method and keeping it straight in the context of follow-up examinations.

KEY POINTS

• Both variants of R2* relaxometry (FS and non-FS) may be used in daily routine. • Clinically relevant differences might result when switching between the two methods or during patient follow-up, when fat content changes over time. • It seems advisable choosing one method and keeping it straight in the context of follow-up examinations.

Evaluation of two-point Dixon water-fat separation for liver specific contrast-enhanced assessment of liver maximum capacity

Gadoxetic acid-enhanced magnetic resonance imaging has become a useful tool for quantitative evaluation of liver capacity. We report on the importance of intrahepatic fat on gadoxetic acid-supported T1 mapping for estimation of liver maximum capacity, assessed by the realtime ¹³C-methacetin breathing test (¹³C-MBT). For T1 relaxometry, we used a respective T1-weighted sequence with two-point Dixon water-fat separation and various flip angles. Both T1 maps of the in-phase component without fat separation (T1_in) and T1 maps merely based on the water component (T1_W) were generated, and respective reduction rates of the T1 relaxation time (rrT1) were evaluated. A steady considerable decline in rrT1 with progressive reduction of liver function could be observed for both T1_in and T1_W (p < 0.001). When patients were subdivided into 3 different categories of ¹³C-MBT readouts, the groups could be significantly differentiated by their rrT1_in and rrT1_W values (p < 0.005). In a simple correlation model of ¹³C-MBT values with T1_inpost (r = 0.556; p < 0.001), T1_Wpost (r = 0.557; p < 0.001), rrT1_in (r = 0.711; p < 0.001) and rrT1_W (r = 0.751; p < 0.001), a log-linear correlation has been shown. Liver maximum capacity measured with ¹³C-MBT can be determined more precisely from gadoxetic acid-supported T1 mapping when intrahepatic fat is taken into account. Here, T1_W maps are shown to be significantly superior to T1_in maps without separation of fat.

Estimation of pancreatic R2* for iron overload assessment in the presence of fat: a comparison of different approaches

OBJECTIVES

To propose a method for estimating pancreatic relaxation rate, R2*, from conventional multi-echo MRI, based on the nonlinear fitting of the acquired magnitude signal decay to MR signal models that take into account both the signal oscillations induced by fat and the different R2* values of pancreatic parenchyma and fat.

MATERIALS AND METHODS

Single-peak fat (SPF) and multi-peak fat (MPF) models were introduced. Single-R2* and dual-R2* assumptions were considered as well. Analyses were conducted on simulated data and 20 thalassemia major patients.

RESULTS

Simulations revealed the ability of the MPF model to correctly estimate the R2* value in a large range of fat fractions and R2* values. From the comparison between the results obtained with a single R2* value for water and fat and the dual-R2* approach, the latter is more accurate in both water R2* and fat fraction estimation. In patient's data analysis, a strong concordance was found between SPF and MPF estimated data with measurements done with manual signal correction and from fat-saturated images. The MPF method showed better reproducibility.

CONCLUSION

The MPF dual-R2* approach improves reproducibility and reduces image analysis time in the assessment of pancreatic R2* value in patients with iron overload.

Iron deposition quantification: Applications in the brain and liver

Iron has long been implicated in many neurological and other organ diseases. It is known that over and above the normal increases in iron with age, in certain diseases there is an excessive iron accumulation in the brain and liver. MRI is a noninvasive means by which to image the various structures in the brain in three dimensions and quantify iron over the volume of the object of interest. The quantification of iron can provide information about the severity of iron-related diseases as well as quantify changes in iron for patient follow-up and treatment monitoring. This article provides an overview of current MRI-based methods for iron quantification, specifically for the brain and liver, including: signal intensity ratio, R₂ , R2*, R2', phase, susceptibility weighted imaging and quantitative susceptibility mapping (QSM). Although there are numerous approaches to measuring iron, R₂ and R2* are currently preferred methods in imaging the liver and QSM has become the preferred approach for imaging iron in the brain.

LEVEL OF EVIDENCE

5 Technical Efficacy: Stage 5 J. Magn. Reson. Imaging 2018. J. MAGN. RESON. IMAGING 2018;48:301-317.

Inter-method reproducibility of biexponential R2 MR relaxometry for estimation of liver iron concentration

PURPOSE

To assess the reproducibility of biexponential R₂ -relaxometry MRI for estimation of liver iron concentration (LIC) between proprietary and nonproprietary analysis methods.

METHODS

This single-center retrospective study, approved by investigational review board and compliant with the Health Insurance Portability and Accountability Act, included 40 liver MRI exams in 38 subjects with suspected or known iron overload. From spin-echo images of the liver, acquired at 5 different echo times (TE = 6-18 ms), biexponential R₂ maps were calculated using 1 proprietary (FerriScan, Resonance Health Ltd., Claremont WA, Australia) and 3 nonproprietary (simulated annealing, nonlinear least squares, dictionary search) analysis methods. Each subject's average liver R₂ value was converted to LIC using a previously validated calibration curve. Inter-method reproducibility for liver R₂ and LIC were assessed for linearity using linear regression analysis and absolute agreement using intraclass correlation and Bland-Altman analysis. For point estimates, 95% confidence intervals were calculated; P values < 0.05 were considered statistically significant.

RESULTS

Linearity between the proprietary and nonproprietary methods was excellent across the observed range for R₂ (20-312 s^-1 ) and LIC (0.4-52.2 mg/g), with all coefficients of determination (R² ) ≥ 0.95. No statistically significant bias was found (slope estimates ∼ 1; intercept estimates ∼ 0; P values > 0.05). Agreement between the 4 methods was excellent for both liver R₂ and LIC (intraclass correlations ≥ 0.97). Bland-Altman 95% limits of agreement in % difference between the proprietary and nonproprietary methods were ≤ 9% and ≤ 16% for R₂ and LIC, respectively.

CONCLUSION

Biexponential R₂ -relaxometry MRI for LIC estimation is reproducible between proprietary and nonproprietary analysis methods.

The utility of susceptibility-weighted imaging to evaluate the extent of iron accumulation in the choroid plexus of patients with β-thalassaemia major

AIM

To assess iron accumulation in the choroid plexus of β-thalassaemia patients using fast spin echo (FSE) T2-weighted, gradient echo (GRE) T2*-weighted, susceptibility-weighted imaging (SWI) and compare the results.

MATERIALS AND METHODS

Eighteen patients with transfusion-dependent β-thalassaemia and the control group underwent magnetic resonance imaging (MRI) examinations. Signal intensities were separately evaluated using a "number of hypointensity in the choroid plexus" (NHICP) grading system on axial FSE T2-weighted, GRE T2*-weighted, and SWI images. The NHICP grading system scores were compared using the chi-squared test. Spearman's correlation analysis was used to explore relationships between the variables and NHICP grading system scores.

RESULTS

The sensitivity of each technique was calculated: FSE T2-weighted imaging=0.17, GRE T2*-weighted imaging=0.48, and SWI=0.81. Three-sample test for equality of proportions showed that chi-squared=74.85, df=2, p<0.0001. All of the FSE T2-weighted, GRE T2*-weighted, and SWI images differed significantly in terms of their capacity to reveal iron accumulation in the choroid plexus. Of the three methods, SWI was the most sensitive.

CONCLUSIONS

SWI is useful for revealing iron deposition in the brains of β-thalassaemia patients, especially those in the early stages of disease, and it can be used to predict disease prognosis. The present study contributes to an understanding of the important role played by the choroid plexus in brain iron metabolism.

Accurate estimate of pancreatic T2* values: how to deal with fat infiltration

PURPOSE

We examined different approaches aimed to deal with the signal fluctuation of pancreatic T2* values due to fat infiltration in order to obtain accurate estimates of iron overload.

METHODS

Pancreatic T2* values were assessed in 20 patients (13 females, 37.24 ± 9.12 years) enrolled in the Myocardial Iron Overload in Thalassemia network without and with the application of fat suppression-FS (T2*-NoFS and T2*-FS). T2* values were assessed in three different ways: (1) from the immediate fit (original T2*); (2) discarding the echoes until the achievement of a good visual concordance between the signal and the model (final_vis T2*); (3) eliminating the echoes until the achievement of a fitting error (known) <5% (final_thres T2*).

RESULTS

For the T2*-NoFS sequence the original T2* values were significantly higher than the final_vis T2* values (difference:4.8 ± 6.1 ms; P < 0.0001) and the final_thres T2* values (difference:4.3 ± 6.1 ms; P = 0.006). For the T2*-FS sequence the original T2* values were comparable to final_vis and final_thres T2* values. The original T2*-FS values were significantly different from the original T2*-NoFS values. The final_vis T2*-FS values were comparable to the final_vis T2*-NoFS values and the final_thresh T2*-FS values were comparable to the final_thresh T2*-NoFS values. For both T2*-FS and T2*-NoFS sequences, the final_thres T2* values were not significantly different from the final_vis T2* values and no bias was present.

CONCLUSIONS

In the clinical practice, an accurate pancreatic iron overload assessment should be done by applying FS and, when needed, by discarding the TEs until the fitting error goes below 5%.

Does fat suppression via chemically selective saturation affect R2*-MRI for transfusional iron overload assessment? A clinical evaluation at 1.5T and 3T

PURPOSE

Fat suppression (FS) via chemically selective saturation (CHESS) eliminates fat-water oscillations in multiecho gradient echo (mGRE) R2*-MRI. However, for increasing R2* values as seen with increasing liver iron content (LIC), the water signal spectrally overlaps with the CHESS band, which may alter R2*. We investigated the effect of CHESS on R2* and developed a heuristic correction for the observed CHESS-induced R2* changes.

METHODS

Eighty patients [female, n = 49; male, n = 31; mean age (± standard deviation), 18.3 ± 11.7 y] with iron overload were scanned with a non-FS and a CHESS-FS mGRE sequence at 1.5T and 3T. Mean liver R2* values were evaluated using three published fitting approaches. Measured and model-corrected R2* values were compared and statistically analyzed.

RESULTS

At 1.5T, CHESS led to a systematic R2* reduction (P < 0.001 for all fitting algorithms) especially toward higher R2*. Our model described the observed changes well and reduced the CHESS-induced R2* bias after correction (linear regression slopes: 1.032/0.927/0.981). No CHESS-induced R2* reductions were found at 3T.

CONCLUSION

The CHESS-induced R2* bias at 1.5T needs to be considered when applying R2*-LIC biopsy calibrations for clinical LIC assessment, which were established without FS at 1.5T. The proposed model corrects the R2* bias and could therefore improve clinical iron overload assessment based on linear R2*-LIC calibrations. Magn Reson Med 76:591-601, 2016. © 2015 Wiley Periodicals, Inc.

Effect of inversion recovery fat suppression on hepatic R2* quantitation in transfusional siderosis

OBJECTIVE. The purpose of this study is to evaluate whether the application of spectral pre-saturation inversion recovery (SPIR) fat suppression in standard multiecho gradient-echo sequences has a significant effect on hepatic R2* quantitation in patients with iron overload syndromes. MATERIALS AND METHODS. Eighty patients were scanned with a multiecho gradient-echo sequence without and with the application of SPIR. Six different postprocessing approaches were used to extract R2* values for maximum generality. RESULTS. SPIR fat suppression lowered R2* values by 3.9-7.0% (p < 0.0001 in all pairwise comparisons), independently of the postprocessing algorithm. Coefficients of variation for R2* ranged from 4.5% to 10.0%. Regardless of the size of the ROI (area of homogeneous tissue or entire liver profile in the slice), pixelwise approaches combined with an exponential-plus-constant fitting model yielded the lowest coefficients of variation (4.5% and 5.1%), whereas truncated exponential fits of the averaged signals produced the highest coefficients of variation (7.8% and 10%). For R2* values exceeding 200 Hz, a Bland-Altman analysis showed a bias that grew linearly for all postprocessing methods. CONCLUSION. SPIR fat suppression resulted in systematically lower hepatic R2* estimates. Because calibration curves were derived using images without fat suppression, these biases should be corrected when reporting liver iron concentrations estimated from fat-suppressed multiecho T2*-weighted images.

Measurement of liver fat fraction and iron with MRI and MR spectroscopy techniques

Diffuse liver disease is a widespread global healthcare burden, and the abnormal accumulation of lipid and/or iron is common to important disease processes. Developing the improved methods for detecting and quantifying liver lipid and iron is an important clinical need. The inherent risk, invasiveness, and sampling error of liver biopsy have prompted the development of noninvasive imaging methods for lipid and iron assessment. Ultrasonography and computed tomography have the ability to detect diffuse liver disease, but with limited accuracy. The purpose of this review is to describe the current state-of-the-art methods for quantifying liver lipid and iron using magnetic resonance imaging and spectroscopy, including their implementation, benefits, and potential pitfalls. Imaging- and spectroscopy-based methods are naturally suited for lipid and iron quantification. Lipid can be detected and decomposed from the inherent chemical shift between lipid and water signals, whereas iron imparts significant paramagnetic susceptibility to tissue, which accelerates proton relaxation. However, measurements of these biomarkers are confounded by technical and biological effects. Current methods must address these factors to allow a precise correlation between the lipid fraction and iron concentration. Although this correlation becomes increasingly challenging in the presence of combined lipid and iron accumulation, advanced techniques show promise for delineating these quantities through multi-lipid peak analysis, T2 water mapping, and fast single-voxel water-lipid spectroscopy.

Diagnostic accuracy of dual-echo (in- and opposed-phase) T1-weighted gradient recalled echo for detection and grading of hepatic iron using quantitative and visual assessment

OBJECTIVES

Detection and quantification of hepatic iron with dual-echo gradient recalled echo (GRE) has been proposed as a rapid alternative to other magnetic resonance imaging (MRI) techniques. Co-existing steatosis and T1 weighting are limitations. This study assesses the accuracy of routine dual-echo GRE.

METHODOLOGY

Between 2010 and 2013, 109 consecutive patients underwent multi-echo (ME) MRI and dual-echo GRE for quantification of hepatic iron. Liver iron concentration (LIC) was calculated from ME-MRI. Relative signal intensity (RSI) and fat signal fraction (FSF) were calculated from dual-echo GRE. Four radiologists subjectively evaluated dual-echo GRE (±subtraction). Diagnostic accuracy was compared between techniques and correlated with biopsy using Fisher's exact test, Spearman correlation and regression.

RESULTS

The sensitivity of visual detection of iron ranged from 48 to 55%. Subtraction did not increase sensitivity (p < 0.001). Inter-observer variability was substantial (κ = 0.72). The specificity of visual detection of iron approached 100% with false-positive diagnoses observed using subtraction. LIC showed a higher correlation with histopathological iron grade (r = 0.94, p < 0.001) compared with RSI (r = 0.65, p = 0.02). Univariate regression showed an association between RSI and LIC (B = 0.98, p < 0.001, CI 0.73-1.23); however, the association was not significant with multi-variate regression including FSF (p = 0.28).

CONCLUSIONS

Dual-echo GRE has low sensitivity for hepatic iron. Subtraction imaging can result in false-positive diagnoses.

KEY POINTS

• Routine liver MRI studies cannot effectively screen patients for iron overload. • Concomitant hepatic steatosis and iron limits diagnostic accuracy of routine liver MRI. • Dual-echo GRE subtraction imaging causes false-positive diagnoses of iron overload. • Dedicated MRI techniques should be used to diagnose and quantify iron overload.

Fat and iron quantification in the liver: past, present, and future

Liver fat, iron, and combined overload are common manifestations of diffuse liver disease and may cause lipotoxicity and iron toxicity via oxidative hepatocellular injury, leading to progressive fibrosis, cirrhosis, and eventually, liver failure. Intracellular fat and iron cause characteristic changes in the tissue magnetic properties in predictable dose-dependent manners. Using dedicated magnetic resonance pulse sequences and postprocessing algorithms, fat and iron can be objectively quantified on a continuous scale. In this article, we will describe the basic physical principles of magnetic resonance fat and iron quantification and review the imaging techniques of the "past, present, and future." Standardized radiological metrics of fat and iron are introduced for numerical reporting of overload severity, which can be used toward objective diagnosis, grading, and longitudinal disease monitoring. These noninvasive imaging techniques serve an alternative or complimentary role to invasive liver biopsy. Commercial solutions are increasingly available, and liver fat and iron quantitative imaging is now within reach for routine clinical use and may soon become standard of care.

Quantification of liver iron with MRI: state of the art and remaining challenges

Liver iron overload is the histological hallmark of hereditary hemochromatosis and transfusional hemosiderosis, and can also occur in chronic hepatopathies. Iron overload can result in liver damage, with the eventual development of cirrhosis, liver failure, and hepatocellular carcinoma. Assessment of liver iron levels is necessary for detection and quantitative staging of iron overload and monitoring of iron-reducing treatments. This article discusses the need for noninvasive assessment of liver iron and reviews qualitative and quantitative methods with a particular emphasis on magnetic resonance imaging (MRI). Specific MRI methods for liver iron quantification include signal intensity ratio as well as R2 and R2* relaxometry techniques. Methods that are in clinical use, as well as their limitations, are described. Remaining challenges, unsolved problems, and emerging techniques to provide improved characterization of liver iron deposition are discussed.