Water–fat separation with IDEAL gradient‐echo imaging

177

142

Purpose:To compare noninvasive MRI and magnetic resonance spectroscopy (MRS) methods with liver biopsy to quantify liver fat content. Materials and Methods:Quantification of liver fat was compared by liver biopsy, proton MRS, and MRI using inphase/out-of-phase (IP/OP) and plus/minus fat saturation (ϮFS) techniques. The reproducibility of each MR measure was also determined. An additional group of overweight patients with steatosis underwent hepatic MRI and MRS before and after a six-month weight-loss program. Results:A close correlation was demonstrated between histological assessment of steatosis and measurement of intrahepatocellular lipid (IHCL) by MRS (r s ϭ 0.928, P Ͻ 0.0001) and MRI (IP/OP r s ϭ 0.942, P Ͻ 0.0001; FS r s ϭ 0.935, P Ͻ 0.0001). Following weight reduction, four of five patients with Ͼ5% weight loss had a decrease in IHCL of Ն50%. Conclusion:These findings suggest that standard MRI protocols provide a rapid, safe, and quantitative assessment of hepatic steatosis. This is important because MRS is not available on all clinical MRI systems. This will enable noninvasive monitoring of the effects of interventions such as weight loss or pharmacotherapy in patients with fatty liver diseases.

Section: Discussionmentioning

confidence: 99%

Magnetic resonance imaging and spectroscopy for monitoring liver steatosis

Cowin

Jonsson

Bauer

et al. 2008

177

142

“…In particular, NSA becomes zero for water-only and fat-only images when water and fat are equal. The SNR performance from this analysis forms the basis for the IDEAL (iterative decomposition of water and fat with echo asymmetry and least-squares estimation) technique (65,66). The essential component of the IDEAL technique is the combination of the iterative least-squares approach for water/fat separation (18) and the optimal sampling scheme under which NSA is independent of the water/fat ratios.…”

Section: The Data Acquisition Strategy and Other Considerationsmentioning

confidence: 99%

Dixon techniques for water and fat imaging

2008

513

442

In 1984, Dixon published a first paper on a simple spectroscopic imaging technique for water and fat separation. The technique acquires two separate images with a modified spin echo pulse sequence. One is a conventional spin echo image with water and fat signals in-phase and the other is acquired with the readout gradient slightly shifted so that the water and fat signals are 180°out-of-phase. Dixon showed that from these two images, a water-only image and a fat-only image can be generated. The wateronly image by the Dixon's technique can serve the purpose of fat suppression, an important and widely used imaging option for clinical MRI. Additionally, the availability of both the water-only and fat-only images allows direct imagebased water and fat quantitation. These applications, as well as the potential that the technique can be made highly insensitive to magnetic field inhomogeneity, have generated substantial research interests and efforts from many investigators. As a result, significant improvement to the original technique has been made in the last 2 decades. The following article reviews the underlying physical principles and describes some major technical aspects in the development of these Dixon techniques. MR IMAGES ACQUIRED IN VIVO usually contain signals from both water and fat. In many pulse sequences, fat appears hyperintense. However, the contribution from water is often of the primary interest for many practical applications. Without suppression, the bright fat signal may result in aggravated motion-related artifacts and, more importantly, reduce the underlying lesion conspicuity. Because of its chemical shift, fat is also responsible for the well-known spatial misregistration artifacts, which can appear both along the frequency encode direction and along the slice select direction. For a few other applications such as diagnosis of bone marrow diseases or hepatic steatosis, detection rather than suppression of the fat signal can also be valuable.Several approaches have been proposed and developed for achieving fat suppression. Perhaps the most popular is the chemical shift selective saturation (1) or its variants. In this approach, a frequency selective radiofrequency (RF) pulse and a spoiler gradient pulse are used in conjunction to first excite and then saturate the fat magnetization before water is excited for imaging. Alternatively, a frequency selective RF pulse can be used to directly excite only the water magnetization and leave the fat magnetization alone along the longitudinal axis. These techniques, particularly the selective saturation technique, are easy to implement and have been widely used with great success. However, both selective saturation pulses and selective excitation pulses increase the scan time. Another limitation for fat suppression by the selective saturation pulses is that it typically requires an accurate 90°flip angle, and as a result, its performance can be dependent on B1 homogeneity. Perhaps most importantly, fat suppression using the frequency selective approach...

“…Furthermore, effective water suppression can help improve differentiation between PV-fat and nonfat tissues, and therefore quantification methods that take PVE into consideration can be used to reduce the variability in VAT (11,17). Alternatively, more advanced multiecho techniques such as ''iterative decomposition of water and fat with echo asymmetry and least squares estimation'' (IDEAL) can be applied to generate fat-only MR images, which may help identify and therefore estimate PV fat amount due to greatly improved contrast (18,19). In addition to these methods, ultrafast imaging techniques can minimize potential motion-induced image blurring, and therefore improve PV fat quantification.…”

Section: Discussionmentioning

confidence: 99%

Impact of partial volume effects on visceral adipose tissue quantification using MRI

Zhou

Murillo

Peng

2011

Purpose: To quantitatively estimate the impact of partial volume effects on visceral adipose tissue (VAT) quantification using typical resolution magnetic resonance imaging (MRI). Materials and Methods:Nine normal or overweight subjects were scanned at central abdomen levels with a water-saturated, balanced steady-state free precession (b-SSFP) sequence. The water-saturation effectiveness was evaluated with region-of-interest analysis on fat, muscle, bowel, and noise areas. The number of full-volume (FV) and partial-volume (PV) fat pixels was estimated based on a gray-level histogram model of water-saturated images. Both FV and PV fat amounts were quantified.Results: High-quality, fat-only images were generated with the b-SSFP imaging method. Fat SNR was 77.7 6 25.6 and water-saturation was effective, with the average fat-to-water signal intensity ratio ¼ 20.7 6 3.8. The average ratio of partial-to full-volume fat amounts was 104.0%. The ratio was higher with lower body mass index (BMI) and PV fat amount only increased slightly when BMI increased.Conclusion: PV fat contributes a significant amount of fat to fat measurements on typical spatial resolution MRI on normal and overweight subjects. The relative PV fat contribution is markedly higher in slimmer patients. Inclusion of this portion of the adipose tissue will increase overall accuracy and decrease variability of VAT quantification using MRI. IMAGING OF HUMAN BODY FAT and its distribution have gained renewed attention because of the link between obesity and physical inactivity to multiple disease states. Excessive body fat, particularly visceral adipose tissue (VAT), correlates with adverse metabolic consequences especially those related to the metabolic syndrome such as diabetes type 2 and cardiovascular disease (1-3). Moreover, the global obesity epidemic has heightened the importance and magnitude of the problems (4). Thus, accurate and rapid methods for measuring VAT have been sought to better investigate its contribution to many disease states as well as the development of diagnostic biomarkers (5,6).VAT imaging and quantification have been challenging because of motion artifacts, the highly complicated nature of anatomic fat compartments, and the disseminated nature of visceral fat. Compared with computerized tomography (CT), magnetic resonance imaging (MRI) has no associated ionizing radiation and is more desirable for repeated studies for infants, children, and healthy subjects. MRI is therefore the method of choice for VAT quantification in such clinical research studies despite its known relatively poor spatial resolution due to signal-to-noise ratio (SNR) and scan duration limitation of a clinical scan. Human abdominal fat quantification by MRI has been historically performed using T1-weighted (T1W) turbo spin-echo (TSE) or gradient-echo imaging techniques. These techniques can generate moderate contrast between fat and nonfat tissues to enable classification of these pixels using postprocessing methods such as a manual contour drawing method (7)...