T1‐weighted 3D dynamic contrast‐enhanced MRI of the breast using a dual‐echo dixon technique at 3 T

Purpose:

To evaluate a single‐pass fast spoiled gradient echo (FSPGR) two‐point Dixon sequence and a gradient echo sequence with spectral fat suppression in their performance at 3 T for fat suppressed contrast‐enhanced bilateral breast imaging.

Materials and Methods:

Twenty patients were prospectively enrolled in an imaging protocol that included axial Dixon and 3D FSPGR with spectrally selective fat saturation sequences as part of patient care in this study. Qualitative analysis was performed retrospectively by two readers who scored the images for homogeneity and degree of fat saturation, severity of artifacts, and quality of normal anatomical structures. Enhancing lesions were scored according to the confidence with which American College of Radiology (ACR) BI‐RADS magnetic resonance imaging (MRI) features were identified.

Results:

The Dixon sequence showed superior fat saturation homogeneity, quality of posterior anatomical structures, and decreased artifact severity that were statistically significant (P < 0.0001). The degree of fat saturation was scored higher in the Dixon sequence, although the difference did not reach statistical significance. There were no significant differences between the 3D T1‐weighted FSPGR and Dixon groups for assessing lesion features.

Conclusion:

Our findings suggest that the Dixon technique is an effective fat suppression method for contrast‐enhanced breast MRI. The Dixon technique also seemed to provide better anatomical definition of posterior structures and improvement in severity of artifacts. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

T1‐corrected fat quantification using chemical shift‐based water/fat separation: Application to skeletal muscle

Chemical shift‐based water/fat separation, like iterative decomposition of water and fat with echo asymmetry and least‐squares estimation, has been proposed for quantifying intermuscular adipose tissue. An important confounding factor in iterative decomposition of water and fat with echo asymmetry and least‐squares estimation‐based intermuscular adipose tissue quantification is the large difference in T₁ between muscle and fat, which can cause significant overestimation in the fat fraction. This T₁ bias effect is usually reduced by using small flip angles. T₁‐correction can be performed by using at least two different flip angles and fitting for T₁ of water and fat. In this work, a novel approach for the water/fat separation problem in a dual flip angle experiment is introduced and a new approach for the selection of the two flip angles, labeled as the unequal small flip angle approach, is developed, aiming to improve the noise efficiency of the T₁‐correction step relative to existing approaches. It is shown that the use of flip angles, selected such the muscle water signal is assumed to be T₁‐independent for the first flip angle and the fat signal is assumed to be T₁‐independent for the second flip angle, has superior noise performance to the use of equal small flip angles (no T₁ estimation required) and the use of large flip angles (T₁ estimation required). Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Robust fat suppression at 3T in high‐resolution diffusion‐weighted single‐shot echo‐planar imaging of human brain

Single‐shot echo‐planar imaging is the most common acquisition technique for whole‐brain diffusion tensor imaging (DTI) studies in vivo. Higher field MRI systems are readily available and advantageous for acquiring DTI due to increased signal. One of the practical issues for DTI with single‐shot echo‐planar imaging at high‐field is incomplete fat suppression resulting in a chemically shifted fat artifact within the brain image. Unsuppressed fat is especially detrimental in DTI because the diffusion coefficient of fat is two orders of magnitude lower than that of parenchyma, producing brighter appearing fat artifacts with greater diffusion weighting. In this work, several fat suppression techniques were tested alone and in combination with the goal of finding a method that provides robust fat suppression and can be used in high‐resolution single‐shot echo‐planar imaging DTI studies. Combination of chemical shift saturation with slice‐select gradient reversal within a dual‐spin‐echo diffusion preparation period was found to provide robust fat suppression at 3 T. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Optimization of alternating TR‐SSFP for fat‐suppression in abdominal images at 3T

Magnetic resonance imaging is widely used in the work‐up and monitoring of patients with Crohn's disease. Balanced steady‐state free precession sequences are an important part of the imaging protocol and until now primarily 1.5T scanners have been used in daily clinical practice. This is largely because running balanced steady‐state free precession sequences in 3T magnets has technical problems related to increased B₀ inhomogeneity and specific absorption rate (SAR) deposition. A modified form of alternating repetition time steady‐state free precession sequence is presented to acquire 3D‐isotropic abdominal images with fat‐suppression at 3T within a breath‐hold. The modifications include an adjusted radiofrequency pulse shape, suitable phase‐cycling scheme and TR₁/TR₂ ratio. Results show that the proposed sequence is successful in obtaining high contrast 3D‐isotropic abdominal images within a breath‐hold. Furthermore, the proposed methodology is easy to implement in a clinical setting and does not require any postprocessing steps. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

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.     

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.     

Automatic intra‐subject registration‐based segmentation of abdominal fat from water–fat MRI

Purpose:

To develop an automatic registration‐based segmentation algorithm for measuring abdominal adipose tissue depot volumes and organ fat fraction content from three‐dimensional (3D) water–fat MRI data, and to evaluate its performance against manual segmentation.

Materials and Methods:

Data were obtained from 11 subjects at two time points with intermediate repositioning, and from four subjects before and after a meal with repositioning. Imaging was performed on a 3 Tesla MRI, using the IDEAL chemical‐shift water–fat pulse sequence. Adipose tissue (subcutaneous—SAT, visceral—VAT) and organs (liver, pancreas) were manually segmented twice for each scan by a single trained observer. Automated segmentations of each subject's second scan were generated using a nonrigid volume registration algorithm for water–fat MRI images that used a b‐spline basis for deformation and minimized image dissimilarity after the deformation. Manual and automated segmentations were compared using Dice coefficients and linear regression of SAT and VAT volumes, organ volumes, and hepatic and pancreatic fat fractions (HFF, PFF).

Results:

Manual segmentations from the 11 repositioned subjects exhibited strong repeatability and set performance benchmarks. The average Dice coefficients were 0.9747 (SAT), 0.9424 (VAT), 0.9404 (liver), and 0.8205 (pancreas); the linear correlation coefficients were 0.9994 (SAT volume), 0.9974 (VAT volume), 0.9885 (liver volume), 0.9782 (pancreas volume), 0.9996 (HFF), and 0.9660 (PFF). When comparing manual and automated segmentations, the average Dice coefficients were 0.9043 (SAT volume), 0.8235 (VAT), 0.8942 (liver), and 0.7168 (pancreas); the linear correlation coefficients were 0.9493 (SAT volume), 0.9982 (VAT volume), 0.9326 (liver volume), 0.8876 (pancreas volume), 0.9972 (HFF), and 0.8617 (PFF). In the four pre‐ and post‐prandial subjects, the Dice coefficients were 0.9024 (SAT), 0.7781 (VAT), 0.8799 (liver), and 0.5179 (pancreas); the linear correlation coefficients were 0.9889, 0.9902 (SAT, and VAT volume), 0.9523 (liver volume), 0.8760 (pancreas volume), 0.9991 (HFF), and 0.6338 (PFF).

Conclusion:

Automated intra‐subject registration‐based segmentation is potentially suitable for the quantification of abdominal and organ fat and achieves comparable quantitative endpoints with respect to manual segmentation. J. Magn. Reson. Imaging 2013;37:423–430. © 2012 Wiley Periodicals, Inc.     

Simultaneous T2 and lipid quantitation using IDEAL‐CPMG

Muscle damage, edema, and fat infiltration are hallmarks of a range of neuromuscular diseases. The T₂ of water, T_2,w, in muscle lengthens with both myocellular damage and inflammation and is typically measured using multiple spin‐echo or Carr–Purcell–Meiboom–Gill acquisitions. However, microscopic fat infiltration in neuromuscular diseases prevents accurate T_2,w quantitation as the longer T₂ of fat, T_2,f, masks underlying changes in the water component. Fat saturation can be inconsistent across the imaging volume and removes valuable physiological fat information. A new method is presented that combines iterative decomposition of water and fat with echo asymmetry and least squares estimation with a Carr–Purcell–Meiboom–Gill–sequence. The sequence results in water and fat separated images at each echo time for use in T_2,w and T_2,f quantification. With knowledge of the T_2,w and T_2,f, a T₂‐corrected fat fraction map can also be calculated. Monte‐Carlo simulations and measurements in phantoms, volunteers, and a patient with inclusion body myositis are demonstrated. In healthy volunteers, uniform T_2,w and T₂‐corrected fat fraction maps are present within all muscle groups. However, muscle‐specific patterns of fat infiltration and edema are evident in inclusion body myositis, which demonstrates the power of separating and quantifying the fat and water components. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Combination of complex‐based and magnitude‐based multiecho water‐fat separation for accurate quantification of fat‐fraction

Multipoint water–fat separation techniques rely on different water–fat phase shifts generated at multiple echo times to decompose water and fat. Therefore, these methods require complex source images and allow unambiguous separation of water and fat signals. However, complex‐based water–fat separation methods are sensitive to phase errors in the source images, which may lead to clinically important errors. An alternative approach to quantify fat is through “magnitude‐based” methods that acquire multiecho magnitude images. Magnitude‐based methods are insensitive to phase errors, but cannot estimate fat‐fraction greater than 50%. In this work, we introduce a water–fat separation approach that combines the strengths of both complex and magnitude reconstruction algorithms. A magnitude‐based reconstruction is applied after complex‐based water–fat separation to removes the effect of phase errors. The results from the two reconstructions are then combined. We demonstrate that using this hybrid method, 0–100% fat‐fraction can be estimated with improved accuracy at low fat‐fractions. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.