Single region of interest versus multislice T2* MRI approach for the quantification of hepatic iron overload

Purpose

To evaluate the effectiveness of the single ROI approach for the detection of hepatic iron burden in thalassemia major (TM) patients in respect to a whole liver measurement.

Materials and Methods

Five transverse hepatic slices were acquired by a T2* gradient‐echo sequence in 101 TM patients and 20 healthy subjects. The T2* value was calculated in a single region of interest (ROI) defined in the medium‐hepatic slice. Moreover, the T2* value was extracted on each of the eight ROIs defined in the functionally independent segments. The mean hepatic T2* value was calculated.

Results

For patients, the mean T2* values over segments VII and VIII were significantly lower. This pattern was substantially preserved in the two groups identified considering the T2* normal cutoff. All segmental T2* values were correlated with the single ROI T2* value. After the application of a correction map based on T2* fluctuations in the healthy subjects, no significant differences were found in the segmental T2* values.

Conclusion

Hepatic T2* variations are low and due to artifacts and measurement variability. The single ROI approach can be adopted in the clinical arena, taking care to avoid the susceptibility artifacts, occurring mainly in segments VII and VIII. J. Magn. Reson. Imaging 2011;33:348–355. © 2011 Wiley‐Liss, Inc.     

MRI quantification of splenic iron concentration in mouse

Purpose:

To quantify hepatic and splenic iron load, which is a critical issue for iron overload disease diagnosis. MRI is useful to noninvasively determine liver iron concentration, but not proven to be adequate for robust evaluation of splenic iron load. We evaluated the usefulness of MRI‐derived parameters to determine splenic iron concentration in mice.

Materials and Methods:

A mouse model of experimental iron load was used. Multi‐echo spin‐echo images of liver and spleen were acquired at 4.7 Tesla. The parameters were tested at all echoes with and without an external reference. Splenic and hepatic iron concentrations were determined using biochemical assay as the gold standard.

Results:

Our results show that (i) use of an internal or external reference is essential; (ii) optimal echo times were TE = 19.5 ms and TE = 32.5 ms for the liver and spleen, respectively; (iii) in the liver, the relationship between biochemical and MRI iron concentration determinations is logarithmic; (iv) in the spleen, the best relationship is an inverse function.

Conclusion:

A single spin‐echo sequence allows robust estimation of hepatic and splenic iron content. Parameters classically used for hepatic iron concentration cannot be applied to splenic iron determination, which requires both the specific sequence and the adapted fitting function. J. Magn. Reson. Imaging 2010;32:639–646. © 2010 Wiley‐Liss, Inc.     

Independent estimation of T*2 for water and fat for improved accuracy of fat quantification

Noninvasive biomarkers of intracellular accumulation of fat within the liver (hepatic steatosis) are urgently needed for detection and quantitative grading of nonalcoholic fatty liver disease, the most common cause of chronic liver disease in the United States. Accurate quantification of fat with MRI is challenging due the presence of several confounding factors, including T*₂ decay. The specific purpose of this work is to quantify the impact of T*₂ decay and develop a multiexponential T*₂ correction method for improved accuracy of fat quantification, relaxing assumptions made by previous T*₂ correction methods. A modified Gauss‐Newton algorithm is used to estimate the T*₂ for water and fat independently. Improved quantification of fat is demonstrated, with independent estimation of T*₂ for water and fat using phantom experiments. The tradeoffs in algorithm stability and accuracy between multiexponential and single exponential techniques are discussed. Magn Reson Med 63:849–857, 2010. © 2010 Wiley‐Liss, Inc.     

Quantification of hepatic steatosis with MRI: The effects of accurate fat spectral modeling

Purpose

To develop a chemical‐shift–based imaging method for fat quantification that accounts for the complex spectrum of fat, and to compare this method with MR spectroscopy (MRS). Quantitative noninvasive biomarkers of hepatic steatosis are urgently needed for the diagnosis and management of nonalcoholic fatty liver disease (NAFLD).

Materials and Methods

Hepatic steatosis was measured with “fat‐fraction” images in 31 patients using a multiecho chemical‐shift–based water‐fat separation method at 1.5T. Fat‐fraction images were reconstructed using a conventional signal model that considers fat as a single peak at –210 Hz relative to water (“single peak” reconstruction). Fat‐fraction images were also reconstructed from the same source images using two methods that account for the complex spectrum of fat; precalibrated and self‐calibrated “multipeak” reconstruction. Single‐voxel MRS that was coregistered with imaging was performed for comparison.

Results

Imaging and MRS demonstrated excellent correlation with single peak reconstruction (r² = 0.91), precalibrated multipeak reconstruction (r² = 0.94), and self‐calibrated multipeak reconstruction (r² = 0.91). However, precalibrated multipeak reconstruction demonstrated the best agreement with MRS, with a slope statistically equivalent to 1 (0.96 ± 0.04; P = 0.4), compared to self‐calibrated multipeak reconstruction (0.83 ± 0.05, P = 0.001) and single‐peak reconstruction (0.67 ± 0.04, P < 0.001).

Conclusion

Accurate spectral modeling is necessary for accurate quantification of hepatic steatosis with MRI. J. Magn. Reson. Imaging 2009;29:1332–1339. © 2009 Wiley‐Liss, Inc.     

Assessment of hepatic and cardiac iron overload in thalassemia patients by magnetic resonance imaging: Our experience in Alexandria University

Aim of the study

The aim of this study was to assess the utility of MRI as a non-invasive technique for grading of the hepatic and cardiac iron overload in Thalassemia patients in a cohort of patients in Alexandria, Egypt.

T1 independent,T2* corrected MRI with accurate spectral modeling for quantification of fat: Validation in a fat‐water‐SPIO phantom

Purpose:

To validate a T₁‐independent, T₂*‐corrected fat quantification technique that uses accurate spectral modeling of fat using a homogeneous fat‐water‐SPIO phantom over physiologically expected ranges of fat percentage and T₂* decay in the presence of iron overload.

Materials and Methods:

A homogeneous gel phantom consisting of vials with known fat‐fractions and iron concentrations is described. Fat‐fraction imaging was performed using a multiecho chemical shift‐based fat‐water separation method (IDEAL), and various reconstructions were performed to determine the impact of T₂* correction and accurate spectral modeling. Conventional two‐point Dixon (in‐phase/out‐of‐phase) imaging and MR spectroscopy were performed for comparison with known fat‐fractions.

Results:

The best agreement with known fat‐fractions over the full range of iron concentrations was found when T₂* correction and accurate spectral modeling were used. Conventional two‐point Dixon imaging grossly underestimated fat‐fraction for all T₂* values, but particularly at higher iron concentrations.

Conclusion:

This work demonstrates the necessity of T₂* correction and accurate spectral modeling of fat to accurately quantify fat using MRI. J. Magn. Reson. Imaging 2009;30:1215–1222. © 2009 Wiley‐Liss, Inc.     

Fat quantification with IDEAL gradient echo imaging: correction of bias from T(1) and noise.

Quantification of hepatic steatosis is a significant unmet need for the diagnosis and treatment of patients with nonalcoholic fatty liver disease (NAFLD). MRI is capable of separating water and fat signals in order to quantify fatty infiltration of the liver (hepatic steatosis). Unfortunately, fat signal has confounding T(1) effects and the nonzero mean noise in low signal-to-noise ratio (SNR) magnitude images can lead to incorrect estimation of the true lipid percentage. In this study, the effects of bias from T(1) effects and image noise were investigated. An oil/water phantom with volume fat-fractions ranging linearly from 0% to 100% was designed and validated using a spoiled gradient echo (SPGR) sequence in combination with a chemical-shift based fat-water separation method known as iterative decomposition of water and fat with echo asymmetry and least squares estimation (IDEAL). We demonstrated two approaches to reduce the effects of T(1): small flip angle (flip angle) and dual flip angle methods. Both methods were shown to effectively minimize deviation of the measured fat-fraction from its true value. We also demonstrated two methods to reduce noise bias: magnitude discrimination and phase-constrained reconstruction. Both methods were shown to reduce this noise bias effectively from 15% to less than 1%.     

In vivo investigation of hepatic iron overload in rats using T2 maps: quantification at high intensity field (4.7-T)

In vivo quantitation of hepatic iron content is useful in diagnosis and staging of several iron related diseases. We used an experimental model of hepatic iron overload to determine the correlation between iron content and T2 relaxation time in rat liver. Experiments were carried out at 4.7T for high signal-to-noise ratio (SNR) using a spin-echo multiecho sequence with six echoes and minimum echo-time of 5.5 msec. The liver iron content was determined ex vivo by atomic absorption spectrophotometry (AAS). T2 maps were calculated in order to evaluate the space distribution of the iron content. We found good linear correlation between the in vivo liver transversal relaxation rate and the iron content within the range explored (106-4538 microg Fe/g liver wet wt.). T2 maps revealed that the decrease in T2 is not homogeneous through the liver parenchyma. This finding represents a physiological limitation to obtaining better correlation between T2 and iron content.     

Quantification of hepatic macrosteatosis in living,related liver donors using T1‐independent,T2*‐corrected chemical shift MRI

Purpose:

To evaluate the diagnostic implications of the iterative decomposition of water and fat using echo‐asymmetry and the least‐squares estimation (IDEAL) technique to detect hepatic steatosis (HS) in potential liver donors using histopathology as the reference standard.

Materials and Methods:

Forty‐nine potential liver donors (32 male, 17 female; mean age, 31.7 years) were included. All patients were imaged using the in‐ and out‐of‐phase (IOP) gradient‐echo (GRE) and IDEAL techniques on a 1.5 T MR scanner. To estimate the hepatic fat fraction (FF), two reviewers performed regions‐of‐interest measurement in 15 areas of the liver seen on the IOP images and on the IDEAL‐FF images. The magnetic resonance imaging (MRI) and pathology values of macrosteatosis were correlated using the Pearson correlation coefficient. We analyzed the diagnostic performance of IOP imaging and IDEAL for detecting HS.

Results:

The results of the hepatic‐FF estimated on IDEAL were well correlated with the histologic degree of macrosteatosis (γ = 0.902, P < 0.001). IDEAL showed 100% sensitivity and 91% specificity for detecting HS, and IOP imaging showed 87.5% sensitivity and 97% specificity, respectively.

Conclusion:

IDEAL is a useful tool for the preoperative diagnosis of HS in potential living liver donors; it can also help to avoid unnecessary biopsies in these patients. J. Magn. Reson. Imaging 2012;36:1124–1130. © 2012 Wiley Periodicals, Inc.     

菲立磁增强MRI在肝脏局灶性病变诊断中的价值

目的 评价菲立磁增强MRI在肝脏实性占位性病变诊断中的应用价值。材料与方法 对21例怀疑有肝脏局灶性占位病变患者行MR平行及菲立磁增强MRI检查。扫描序列包括频率选择脂肪抑制及非脂肪抑制ASTE T2WI、True FISP T2WI、频率选择脂肪抑制FLASH T1WI。比较增强前后T2WI及T2WI病灶及肝脏的信噪比（SNR）及对比噪声比（CNR）;观察增强前后病灶数量及形态;结合MR平扫及增强MRI表现进行定性诊断。结果 菲立磁增强T2WI及T2WI肝脏信号强度较平扫明显下降,病灶与肝脏的CNR较平扫明显提高,差异具有统计学意义。结论 菲立磁增强T2WI及T2WI可明显提高肝脏实性占位性病灶的检出率。菲立磁增强T1WI在脏局灶性病变的定性诊断中具有潜在价值,有待于进一步开发与研究。     

Estimation of liver T*2 in transfusion‐related iron overload in patients with weighted least squares T*2 IDEAL

MRI imaging of hepatic iron overload can be achieved by estimating T₂* values using multiple‐echo sequences. The purpose of this work is to develop and clinically evaluate a weighted least squares algorithm based on T₂* Iterative Decomposition of water and fat with Echo Asymmetry and Least‐squares estimation (IDEAL) technique for volumetric estimation of hepatic T₂* in the setting of iron overload. The weighted least squares T₂* IDEAL technique improves T₂* estimation by automatically decreasing the impact of later, noise‐dominated echoes. The technique was evaluated in 37 patients with iron overload. Each patient underwent (i) a standard 2D multiple‐echo gradient echo sequence for T₂* assessment with nonlinear exponential fitting, and (ii) a 3D T₂* IDEAL technique, with and without a weighted least squares fit. Regression and Bland–Altman analysis demonstrated strong correlation between conventional 2D and T₂* IDEAL estimation. In cases of severe iron overload, T₂* IDEAL without weighted least squares reconstruction resulted in a relative overestimation of T₂* compared with weighted least squares. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

菲立磁增强磁共振成像在肝脏疾病诊断中的价值

目的　评价菲立磁增强MRI在肝脏疾病诊断中的应用价值。方法　对 31例经CT或MRI检查确定或怀疑有肝脏病变者进一步行菲立磁增强MRI检查 ,分别测量增强前后肝脏、病变及背景噪声的T2 WI信号强度 ,计算增强前后肝脏及病变的信噪比(SNR)、对比噪声比 (CNR)。分析平扫及增强后扫描检测的病灶数目。结果　菲立磁增强后肝脏的SNR明显降低 (Ρ <0 .0 1) ;恶性病灶的SNR变化不明显 (Ρ >0 .0 5 ) ;病灶 -肝脏CNR明显增高 (Ρ <0 .0 1)。增强后病变的检出数量增加 ( χ2 =8.5 7,Ρ <0 .0 1)。结论　菲立磁能显著提高肝脏恶性肿瘤的检出率 ,而且对肝脏小病灶的鉴别诊断提供有利的依据     

Volumetric navigators for prospective motion correction and selective reacquisition in neuroanatomical MRI

We introduce a novel method of prospectively compensating for subject motion in neuroanatomical imaging. Short three-dimensional echo-planar imaging volumetric navigators are embedded in a long three-dimensional sequence, and the resulting image volumes are registered to provide an estimate of the subject's location in the scanner at a cost of less than 500 ms, ~ 1% change in contrast, and ~3% change in intensity. This time fits well into the existing gaps in sequences routinely used for neuroimaging, thus giving a motion-corrected sequence with no extra time required. We also demonstrate motion-driven selective reacquisition of k-space to further compensate for subject motion. We perform multiple validation experiments to evaluate accuracy, navigator impact on tissue intensity/contrast, and the improvement in final output. The complete system operates without adding additional hardware to the scanner and requires no external calibration, making it suitable for high-throughput environments.