Samir D. Sharma

Quantitative susceptibility mapping in the abdomen as an imaging biomarker of hepatic iron overload

Purpose: The purpose of this work was to develop and demonstrate feasibility and initial clinical validation of quantitative susceptibility mapping (QSM) in the abdomen as an imaging biomarker of hepatic iron overload. Theory and Methods: In general, QSM is faced with the challenges of background field removal and dipole inversion. Respiratory motion, the presence of fat, and severe iron overload further complicate QSM in the abdomen. We propose a technique for QSM in the abdomen that addresses these challenges. Data were acquired from 10 subjects without hepatic iron overload and 33 subjects with known or suspected iron overload. The proposed technique was used to estimate the susceptibility map in the abdomen, from which hepatic iron overload was measured. As a reference, spin‐echo data were acquired for R2‐based LIC estimation. Liver R2* was measured for correlation with liver susceptibility estimates. Results: Correlation between susceptibility and R2‐based LIC estimation was R2 = 0.76 at 1.5 Tesla (T) and R2 = 0.83 at 3T. Furthermore, high correlation between liver susceptibility and liver R2* (R2 = 0.94 at 1.5T; R2 = 0.93 at 3T) was observed. Conclusion: We have developed and demonstrated initial validation of QSM in the abdomen as an imaging biomarker of hepatic iron overload. Magn Reson Med 74:673–683, 2015.

Quantitative magnetic resonance imaging of hepatic steatosis: Validation in ex vivo human livers.

Emerging magnetic resonance imaging (MRI) biomarkers of hepatic steatosis have demonstrated tremendous promise for accurate quantification of hepatic triglyceride concentration. These methods quantify the proton density fat‐fraction (PDFF), which reflects the concentration of triglycerides in tissue. Previous in vivo studies have compared MRI‐PDFF with histologic steatosis grading for assessment of hepatic steatosis. However, the correlation of MRI‐PDFF with the underlying hepatic triglyceride content remained unknown. The aim of this ex vivo study was to validate the accuracy of MRI‐PDFF as an imaging biomarker of hepatic steatosis. Using ex vivo human livers, we compared MRI‐PDFF with magnetic resonance spectroscopy‐PDFF (MRS‐PDFF), biochemical triglyceride extraction, and histology as three independent reference standards. A secondary aim was to compare the precision of MRI‐PDFF relative to biopsy for the quantification of hepatic steatosis. MRI‐PDFF was prospectively performed at 1.5 Tesla in 13 explanted human livers. We performed colocalized paired evaluation of liver fat content in all nine Couinaud segments using single‐voxel MRS‐PDFF (n = 117) and tissue wedges for biochemical triglyceride extraction (n = 117), and five core biopsies performed in each segment for histologic grading (n = 585). Accuracy of MRI‐PDFF was assessed through linear regression with MRS‐PDFF, triglyceride extraction, and histology. Intraobserver agreement, interobserver agreement, and repeatability of MRI‐PDFF and histologic grading were assessed through Bland‐Altman analyses. MRI‐PDFF showed an excellent correlation with MRS‐PDFF (r = 0.984, confidence interval 0.978‐0.989) and strong correlation with histology (r = 0.850, confidence interval 0.791‐0.894) and triglyceride extraction (r = 0.871, confidence interval 0.818‐0.909). Intraobserver agreement, interobserver agreement, and repeatability showed a significantly smaller variance for MRI‐PDFF than for histologic steatosis grading (all P < 0.001). Conclusion: MRI‐PDFF is an accurate, precise, and reader‐independent noninvasive imaging biomarker of liver triglyceride content, capable of steatosis quantification over the entire liver. (Hepatology 2015;62:1444–1455)

Multisite, multivendor validation of the accuracy and reproducibility of proton-density fat-fraction quantification at 1.5T and 3T using a fat–water phantom

To evaluate the accuracy and reproducibility of quantitative chemical shift‐encoded (CSE) MRI to quantify proton‐density fat‐fraction (PDFF) in a fat–water phantom across sites, vendors, field strengths, and protocols.

Improving chemical shift encoded water–fat separation using object-based information of the magnetic field inhomogeneity

The purpose of this work was to improve the robustness of existing chemical shift encoded water–fat separation methods by incorporating object‐based information of the B0 field inhomogeneity.

MRI-based quantitative susceptibility mapping (QSM) and R2* mapping of liver iron overload: Comparison with SQUID-based biomagnetic liver susceptometry.

We aimed to determine the agreement between quantitative susceptibility mapping (QSM)‐based biomagnetic liver susceptometry (BLS) and confounder‐corrected R2* mapping with superconducting quantum interference device (SQUID)‐based biomagnetic liver susceptometry in patients with liver iron overload.

The effects of concomitant gradients on chemical shift encoded MRI

The purpose of this work was to characterize the effects of concomitant gradients (CGs) on chemical shift encoded (CSE)‐based estimation of B0 field map, proton density fat fraction (PDFF), and R2* .

Quantitative MR Imaging of Hepatic Steatosis: Validation in Ex Vivo Human Livers

Emerging magnetic resonance imaging (MRI) biomarkers of hepatic steatosis have demonstrated tremendous promise for accurate quantification of hepatic triglyceride concentration. These methods quantify the proton density fat‐fraction (PDFF), which reflects the concentration of triglycerides in tissue. Previous in vivo studies have compared MRI‐PDFF with histologic steatosis grading for assessment of hepatic steatosis. However, the correlation of MRI‐PDFF with the underlying hepatic triglyceride content remained unknown. The aim of this ex vivo study was to validate the accuracy of MRI‐PDFF as an imaging biomarker of hepatic steatosis. Using ex vivo human livers, we compared MRI‐PDFF with magnetic resonance spectroscopy‐PDFF (MRS‐PDFF), biochemical triglyceride extraction, and histology as three independent reference standards. A secondary aim was to compare the precision of MRI‐PDFF relative to biopsy for the quantification of hepatic steatosis. MRI‐PDFF was prospectively performed at 1.5 Tesla in 13 explanted human livers. We performed colocalized paired evaluation of liver fat content in all nine Couinaud segments using single‐voxel MRS‐PDFF (n = 117) and tissue wedges for biochemical triglyceride extraction (n = 117), and five core biopsies performed in each segment for histologic grading (n = 585). Accuracy of MRI‐PDFF was assessed through linear regression with MRS‐PDFF, triglyceride extraction, and histology. Intraobserver agreement, interobserver agreement, and repeatability of MRI‐PDFF and histologic grading were assessed through Bland‐Altman analyses. MRI‐PDFF showed an excellent correlation with MRS‐PDFF (r = 0.984, confidence interval 0.978‐0.989) and strong correlation with histology (r = 0.850, confidence interval 0.791‐0.894) and triglyceride extraction (r = 0.871, confidence interval 0.818‐0.909). Intraobserver agreement, interobserver agreement, and repeatability showed a significantly smaller variance for MRI‐PDFF than for histologic steatosis grading (all P < 0.001). Conclusion: MRI‐PDFF is an accurate, precise, and reader‐independent noninvasive imaging biomarker of liver triglyceride content, capable of steatosis quantification over the entire liver. (Hepatology 2015;62:1444–1455)

Quantitative magnetic resonance imaging phantoms: A review and the need for a system phantom

The MRI community is using quantitative mapping techniques to complement qualitative imaging. For quantitative imaging to reach its full potential, it is necessary to analyze measurements across systems and longitudinally. Clinical use of quantitative imaging can be facilitated through adoption and use of a standard system phantom, a calibration/standard reference object, to assess the performance of an MRI machine. The International Society of Magnetic Resonance in Medicine AdHoc Committee on Standards for Quantitative Magnetic Resonance was established in February 2007 to facilitate the expansion of MRI as a mainstream modality for multi‐institutional measurements, including, among other things, multicenter trials. The goal of the Standards for Quantitative Magnetic Resonance committee was to provide a framework to ensure that quantitative measures derived from MR data are comparable over time, between subjects, between sites, and between vendors. This paper, written by members of the Standards for Quantitative Magnetic Resonance committee, reviews standardization attempts and then details the need, requirements, and implementation plan for a standard system phantom for quantitative MRI. In addition, application‐specific phantoms and implementation of quantitative MRI are reviewed. Magn Reson Med 79:48–61, 2018.

Thrombus-mimicking artifacts in two-point Dixon MRI: Prevalence, appearance, and severity.

To evaluate the incidence and severity of potentially thrombus mimicking, flow‐induced misallocation artifacts in a clinical setting. Two‐point “Dixon” fat–water separation methods, with bipolar readout gradients, may suffer from flow‐induced fat–water misallocation artifacts. If these artifacts occur within blood vessels, they may mimic thrombus.

Crossover comparison of ferumoxytol and gadobenate dimeglumine for abdominal MR-angiography at 3.0 tesla: Effects of contrast bolus length and flip angle.

Ferumoxytol (FE) has gained interest as an alternative to gadolinium‐based contrast agents (GBCAs). The purpose of this study was to evaluate and optimize ferumoxytol dose and T1 weighting, in comparison to a conventional GBCA.