Automated method for accurate abdominal fat quantification on water-saturated magnetic resonance images

PURPOSE: To introduce and evaluate the performance of an automated fat quantification method for water-saturated magnetic resonance images. MATERIALS AND METHODS: A fat distribution model is proposed for fat quantification on water saturated magnetic resonance images. Fat from both full- and partial-volume voxels are accounted for in this model based on image intensity histogram analysis. An automated threshold method is therefore proposed to accurately quantify total fat. This method is compared to a traditional full-volume-fat-only method in phantom and human studies. In the phantom study, fat quantification was performed on MR images obtained from a human abdomen oil phantom and was compared with the true oil volumes. In the human study, results of the two fat quantification methods of six subjects were compared on abdominal images with different spatial resolutions. RESULTS: In the phantom study, the proposed method provided significantly more accurate estimations of true oil volumes compared to the reference method (P < 0.0001). In human studies, fat quantification using the proposed method gave much more consistent results on images with different spatial resolutions, and on regions with different degrees of partial volume averaging. CONCLUSION: The proposed automated method is simple, rapid, and accurate for fat quantification on water-saturated MR images.     

Liver fat content and T2*: simultaneous measurement by using breath-hold multiecho MR imaging at 3.0 T--feasibility

Research ethics committee approval was obtained for this study, and written informed consent was obtained from all participants. The purpose was to prospectively evaluate the feasibility of breath-hold multiecho in- and out-of-phase magnetic resonance (MR) imaging for simultaneous lipid quantification and T2* measurement. A spoiled gradient-echo sequence with seven echo times alternately in phase and out of phase was used at 3.0 T. Imaging was performed in a lipid phantom, in five healthy volunteers (all men; mean age, 37 years), and in five obese individuals with hyperlipidemia or diabetes (four men, one woman; mean age, 53 years). A biexponential curve-fitting model was used to derive the relative signal contributions from fat and water, and these results were compared with results of liver proton MR spectroscopy, the reference standard. There was a significant correlation between multiecho and spectroscopic measurements of hepatic lipid concentration (r(2) = 0.99, P < .001). In vivo, the T2* of water was consistently longer than that of fat and reliably enabled the signal components to be correctly assigned. In the lipid phantom, the multiecho method could be used to determine the fat-to-water ratio and the T2* values of fat and water throughout the entire range of fat concentrations. Multiecho imaging shows promise as a method of simultaneous fat and T2* quantification.     

Temperature‐induced tissue susceptibility changes lead to significant temperature errors in PRFS‐based MR thermometry during thermal interventions

Proton resonance frequency shift‐based MR thermometry (MRT) is hampered by temporal magnetic field changes. Temporal changes in the magnetic susceptibility distribution lead to nonlocal field changes and are, therefore, a possible source of errors. The magnetic volume susceptibility of tissue is temperature dependent. For water‐like tissues, this dependency is in the order of 0.002 ppm/°C. For fat, it is in the same order of magnitude as the temperature dependence of the proton electron screening constant of water (0.01 ppm/°C). For this reason, proton resonance frequency shift‐based MR thermometry in fatty tissues, like the human breast, is expected to be prone to errors. We aimed to quantify the influence of the temperature dependence of the susceptibility on proton resonance frequency shift‐based MR thermometry. Heating experiments were performed in a controlled phantom set‐up to show the impact of temperature‐induced susceptibility changes on actual proton resonance frequency shift‐based temperature maps. To study the implications for a clinical case, simulations were performed in a 3D breast model. Temperature errors were quantified by computation of magnetic field changes in the glandular tissue, resulting from susceptibility changes in a thermally heated region. The results of the experiments and simulations showed that the temperature‐induced susceptibility changes of water and fat lead to significant errors in proton resonance frequency shift‐based MR thermometry. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Human prostate: multisection proton MR spectroscopic imaging with a single spin-echo sequence--preliminary experience

The authors investigated the feasibility of a multisection proton magnetic resonance (MR) spectroscopic imaging technique for the acquisition of metabolic information in the human prostate. Multisection MR spectroscopic imaging was performed of a citrate phantom and of the prostates of eight adult volunteers. High-quality proton MR spectra and citrate metabolite maps of the prostate were obtained with this method.     

Fatty liver. Chemical shift phase-difference and suppression magnetic resonance imaging techniques in animals, phantoms, and humans.

In vitro animal and human models were used to evaluate the potential of chemical shift magnetic resonance imaging (MRI) for assessing fatty liver. Phantoms of varying fat content were created from mayonnaise-agar preparations. Fatty liver was induced in eight rats by feeding them ethanol for three to six weeks (36% of total calories), whereas eight control rats were fed a normal diet. T1-weighted in-phase and opposed-phase MR images were obtained of the phantoms animals, and 28 human subjects. Additional images obtained in animals included long TR images with in-phase and opposed-phase technique, and hybrid chemical shift water and fat suppression. The rats were killed and histologic status was graded blindly by a hepatopathologist as normal, mild, moderate, or severe fatty change, for correlation with MR grading. Quantitative analysis of MR images included fat signal fraction for animals, and relative signal decrease between in-phase and opposed-phase images for phantom and human data. Phantom in-phase signal increased linearly with respect to fat content, whereas opposed-phase signal decreased linearly. MRI and histologic grading of rat livers were highly correlated, especially when based on water suppression images (r = 0.91, P = .0001). Opposed-phase images were also highly correlated, while fat suppression images were less effective. There was no overlap between MR-derived fat fractions for control (2.6%-5.7%) versus ethanol-fed rats (7.7%-17.9%, P = .0002). Human liver considered to be fatty by visual inspection (n = 8) had higher relative signal decrease than nonfatty liver (n = 22) (P less than .001). Phantom, animal, and human data demonstrate that comparison of T1-weighted in-phase and opposed-phase images is both practical and sensitive in the detection and grading of fatty liver.     

Measurement of liver fat content using selective saturation at 3.0 T

PURPOSE: To validate an MRI technique for measuring liver fat content by calibrating MRI readings with liver phantoms and comparing MRI measurements in human subjects with estimates of liver fat content on liver biopsy specimens. MATERIALS AND METHODS: The MRI protocol consisted of fat and water imaging by selective saturation using a 3.0-T scanner. A water phantom and liver phantoms were scanned before each of 10 human subjects who underwent a liver biopsy to assess for nonalcoholic fatty liver disease (NAFLD). Liver fat content in human subjects was derived from a calibration curve generated by scanning the phantoms. Liver fat was also estimated by optical image analysis and pathologists' assessment of histologic sections. RESULTS: MRI measurements of the liver phantoms were highly reproducible. Measurements of liver fat content in human subjects made by MRI in two areas of the liver were strongly correlated (r=0.98, P<0.001). MRI measurements were highly associated with estimates of liver fat content made by optical image analysis (r=0.96, P<0.001) and with estimates made by the pathologists (r=0.93, P<0.001). CONCLUSION: We validated a technique for quantifying liver fat content based on selective fat and water imaging. The technique is accurate and reproducible and provides a noninvasive method to obtain serial measurements of liver fat content in human subjects.     

Chemical shift selective MR imaging using a whole-body magnet

We have applied a new method for separating water and fat resonances in proton magnetic resonance (MR) imaging to human studies using a whole-body MR imaging system at 2.0 T. Chemical shift selective (CHESS) MR imaging provides either a water or fat image in a single experimental run within the same time needed for a conventional composite image. Although the technique requires a spectral resolution of about 1 ppm over the entire imaging region, first images of the human head and hip indicated that CHESS MR imaging is extremely promising for use in clinical investigations. Moreover, CHESS MR imaging can be combined arbitrarily with other imaging modalities and is easy to implement in any high-field MR imaging system.     

Magnetic resonance imaging and spectroscopy for monitoring liver steatosis

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 in-phase/out-of-phase (IP/OP) and plus/minus fat saturation (+/-FS) 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) = 0.928, P < 0.0001) and MRI (IP/OP r(s) = 0.942, P < 0.0001; FS r(s) = 0.935, P < 0.0001). Following weight reduction, four of five patients with >5% weight loss had a decrease in IHCL of >or=50%. 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.     

Chemical shift-based water/fat separation in the presence of susceptibility-induced fat resonance shift

Chemical shift‐based water/fat separation methods have been emerging due to the growing clinical need for fat quantification in different body organs. Accurate quantification of proton‐density fat fraction requires the assessment of many confounding factors, including the need of modeling the presence of multiple peaks in the fat spectrum. Most recent quantitative chemical shift‐based water/fat separation approaches rely on a multipeak fat spectrum with precalibrated peak locations and precalibrated or self‐calibrated peak relative amplitudes. However, water/fat susceptibility differences can induce fat spectrum resonance shifts depending on the shape and orientation of the fatty inclusions. The effect is of particular interest in the skeletal muscle due to the anisotropic arrangement of extracellular lipids. In this work, the effect of susceptibility‐induced fat resonance shift on the fat fraction is characterized in a conventional complex‐based chemical shift‐based water/fat separation approach that does not model the susceptibility‐induced fat resonance shift. A novel algorithm is then proposed to quantify the resonance shift in a complex‐based chemical shift‐based water/fat separation approach that considers the fat resonance shift in the signal model, aiming to extract information about the orientation/geometry of lipids. The technique is validated in a phantom and preliminary in vivo results are shown in the calf musculature of healthy and diabetic subjects. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Extraneous lipid contamination in single-volume proton MR spectroscopy: phantom and human studies.

PURPOSETo determine the degree of extraneous lipid contamination in defined volumes of interest studied with single-volume proton MR spectroscopy.METHODSSingle-volume proton MR spectroscopy was performed on a fat/water phantom and in three volunteers using the stimulated-echo acquisition mode (STEAM) and point-resolved spectroscopy (PRESS) localization methods. Three different volumes of interest (8, 27, and 64 cm3) were examined at echo times of 20, 135, and 270 for the STEAM sequences and 135 and 270 for the PRESS acquisitions in both the phantom and the volunteers (volumes of interest were placed adjacent to but not encompassing fat-containing structures, such as the scalp and retroorbital fat). The degree of lipid contamination was then correlated with measurements of the section profiles.RESULTSThe PRESS method resulted in less extraneous lipid contamination in both phantom and volunteer studies. The STEAM method had the highest level of lipid contamination signal in phantom and human studies. In the volunteers, volumes of interest abutting fat-containing structures obtained with PRESS or STEAM sequences showed no lipid contamination. However, the STEAM sequences showed lipid signal in the volume of interest adjacent to orbital fat whereas the PRESS sequences did not. These observations are supported by the section profile studies, which showed that the actual volume excited by the STEAM sequence was 7% to 32% larger than that originally selected, while with PRESS the actual excited volume was 12% to 16% smaller than that originally selected.CONCLUSIONIn our MR unit, short-echo-time STEAM sequences (< or = 135 milliseconds) resulted in extraneous lipid contamination in phantom and human studies adjacent to the orbits. PRESS sequences showed no lipid contamination in volumes abutting fat structures in phantoms or humans. These results correlated closely with the configuration of the section profiles. Although these findings might be dependent on the MR unit used, our study could help determine extraneous lipid contamination for other MR units.     

3.0T ~1H-MRS联合梯度回波化学位移技术定量分析评估脂肪肝治疗效果

目的:探讨在3.0T MRI上联合运用氢质子波谱成像(1H-MRS)和梯度回波化学位移技术评估脂肪肝治疗效果的可行性。方法:搜集临床确诊的脂肪肝病例26例,于干预治疗前、干预治疗后3个月、6个月各行1次磁共振化学位移抑脂成像(梯度回波T1WI同/去相位双回波)和氢质子波谱成像(1 H-MRS),测得同/去相位序列的信号强度值(SIIP和SIOP),计算双回波脂变指数(FI)。测得1 H-MRS的水峰峰值(Pwater)和脂肪峰峰值(Plipid)、水峰峰下面积(Awater)、脂肪峰峰下面积(Alipid),计算肝细胞相对脂肪含量1(RLC1)及相对脂肪含量2(RLC2)。同期测量患者的血脂、谷氨酰转肽酶、腹围及身高体重指数(BMI),将其拟合成临床脂肪肝指数(FLI)。以FLI为参照标准,对不同时间点MRI所测得肝脏脂肪含量进行统计学分析。结果:FI、RLC1、RLC2与FLI进行秩相关性分析,呈正相关性(r>0,P<0.05)。干预治疗前后对照采用重复测量的方差分析,显示FI、RLC1、RLC2、FLI组间差异具有统计学意义(P=0.000),对时间(time)变化趋势的对比Polynomial检验显示time*type有统计学意义(P=0.000),提示FI、RLC1、RLC2在治疗前、治疗后3个月、治疗后6个月的变化是有差异的。可靠性分析显示,治疗前后组的FI和治疗前组的RLC1、RLC2的可重复性好,组内相关系数ICC≥0.75。结论:1H-MRS和梯度回波化学位移MRI可在一定程度上对脂肪肝进行定量测定,可作为脂肪肝动态监测、疗效评估和随访观察的有效手段。     

Non-invasive quantification of hepatic fat fraction by fast 1.0, 1.5 and 3.0 T MR imaging

INTRODUCTION: Even mild hepatic steatosis in a split liver donor may cause general liver failure and death in the donor. So far, CT density measurements or percutaneous biopsy is used to determine the presence of hepatic steatosis. Magnetic resonance imaging (MRI) may be an elegant method of non-invasive and non-radiation quantification of hepatic fat content. METHODS: Fast gradient echo (GRE) technique was used to discriminate between fat and water spins. Echo time (TE) was adjusted for field strength dependent in-phase and out-of-phase states at 1.0, 1.5 and 3.0 T. Continuous MR signal transition from 100% water to 100% fat was investigated using a wedge water-oil phantom, which was positioned in such a way, that no spatial resolution occurred, thereby combining water and fat in one slice. RESULTS: Using the phantom, a significant difference for a 5% difference in fat content was demonstrated in the range from 20 to 80% fat content (p<0.05) for all tested field strengths. In 25 patients MRI data were correlated with the percentage of fat determined by histologic evaluation of a CT-guided liver biopsy. Using the linear correlation calculated from the MRI phantom data at 1.0 T, we determined the liver fat from each patient's MRI measurements. Comparison of these data with the histologic quantified fat fraction of liver tissue showed a strong correlation (r(2)=0.93 for TE 6 ms and r(2)=0.91 for TE 10 ms). CONCLUSION: The described method can be used to determine the presence of hepatic steatosis of >10% with p<0.05.     

Assessment of the specific absorption rate and calibration of decoupling parameters for proton decoupled carbon-13 MR spectroscopy at 3.0 T

A strategy for proton decoupled carbon-13 MR spectroscopy ((1H)-13C MRS) with a strong static magnetic field (3.0 T) in vivo was investigated. The proton decoupling improves the signal-to-noise ratio, however, the effect of the decoupling power on the human body, especially in strong magnetic fields, should be considered. In order to establish a technique for monitoring the metabolism of glucose in the liver using (1H)-13C MRS at 3.0 T, two phantom experiments were performed. To assess whether the decoupling energy conformed to SAR limits defined by the IEC, temperature rises inside an agar gel phantom were monitored during a (1H)-13C MRS experiment. Then, the decoupling conditions of a glucose solution phantom were systematically optimized with combinations of decoupling bandwidth and power. The reliability of this procedure was discussed in conjunction with IEC guidelines.     

Determining disk hydration status with a MnCl2-based MR model

PURPOSE: An extensive body of literature demonstrates a strong correlation between intervertebral disk (IVD) hydration status (HS) and functional spinal integrity. However, to date, in vivo IVD HS assessment has relied largely on subjective and nonrepeatable measures. The aim of this study was to establish the consistency of signal homogeneity of a novel semisolid-state manganese chloride (MnCl2)-based phantom for HS correlation using conventional magnetic resonance (MR) imaging. MATERIALS AND METHODS: Sixteen MnCl2 phantoms, of increasing relative molar concentration (range 0.01 to 2.9 mM), underwent axial MR imaging. Phantom signal-to-noise ratio measures were recorded for each concentration on several sequence types. Coefficient of variance data were calculated to determine the degree of MR signal variation at each concentration. RESULTS: Analysis of variance testing suggested no significant difference in coefficient of variance data derived from phantom signal intensities using either T1- (P = .13) or T2-weighted sequence types (P = .96), suggesting a high degree of relative signal homogeneity. CONCLUSIONS: The findings of this study suggest that a MnCl2 phantom combined with a nonfield reactive, semirigid, gelatin suspension media can produce a predictable, concentration-related, homogeneous MR signal response. This may be an appropriate base material for a noninvasive model to allow accurate quantification of the hydration status of the in vivo human IVD.     

Fat quantification using three-point dixon technique: in vitro validation

RATIONALE AND OBJECTIVES: To test the repeatability, reproducibility and accuracy of the three-point Dixon (3PD) sequence for estimating true fat volume ratios using a fat/water phantom. MATERIALS AND METHODS: A phantom, constructed from test tubes of varying fat content, was imaged using the 3PD sequence on a 1.5T MRI scanner by two operators four times each. Fat volume ratios were calculated from these images and compared with true fat volumes. RESULTS: Measures of fat volume ratios calculated from the 3PD MR images correlated strongly with values for true fat volumes (r = 0.96). CONCLUSION: The 3PD technique was found to be highly reproducible and accurate, and may be useful for in vivo quantification of fat in lean tissues, such as the liver, pancreas or skeletal muscle.     

Quantification of pancreatic lipomatosis and liver steatosis by MRI: comparison of in/opposed-phase and spectral-spatial excitation techniques

OBJECTIVES: The goal of the present study was the assessment of pancreatic and hepatic fat content applying 2 established magnetic resonance (MR) imaging techniques: in-phase/opposed-phase gradient-echo MR imaging and fat-selective spectral-spatial gradient-echo imaging. Results of both approaches were compared, and influences of T1- and T2*-related corrections were assessed. The possibility of a correlation between pancreatic lipomatosis and liver steatosis was investigated. MATERIALS AND METHODS: Seventeen volunteers at risk for type 2 diabetes (6 male, 11 female; age, 26-70 years; body mass index, 19.4-41.3 kg/m2; mean, 31.7 kg/m2) were examined. Liver and pancreas fat content were quantified with 2 different gradient-echo techniques: one uses a spectral-spatial excitation technique with 6 binomial radio frequency pulses, which combines chemical shift selectivity with simultaneous slice-selective excitation. The other technique based on double-echo chemical shift gradient-echo MR provides in- and opposed-phase images simultaneously. Influences of T1 and individual T2* effects on results using in-phase/opposed-phase imaging were estimated and corrected for, based on additional T2* measurements. RESULTS: The fat content calculated from images recorded with the fat-selective spectral-spatial gradient-echo sequence correlated well with the fat fraction determined with in-phase/opposed-phase imaging and following correction for T1/T2* effects: pancreas r = 0.93 (P < 0.0001) and liver r = 0.96 (P < 0.0001). In-phase/opposed-phase imaging revealed a pancreatic fat content between 1.6% and 22.2% (mean, 8.8% +/- 5.7%) and a hepatic fat fraction between 0.6% and 33.3% (mean, 7.9% +/- 9.1%). The fat-selective spectral-spatial gradient-echo sequence revealed a pancreatic lipid content between 3.4% and 16.1% (mean, 9.8% +/- 4.0%) and a hepatic fat content between 0% and 28.5% (mean, 8.8% +/- 8.3%). With neither technique was a substantial correlation between pancreatic and hepatic fat content found. CONCLUSION: The presented results suggest that both methods are reliable tools for pancreatic and hepatic fat quantification. However, for reliable assessment of quantitative fat by the in-phase/opposed-phase technique, an additional measurement of T2* seems crucial.     

Spectrally selective pencil‐beam navigator for motion compensation of MR‐guided high‐intensity focused ultrasound therapy of abdominal organs

MR‐guided high‐intensity focused ultrasound (MR‐HIFU) is a noninvasive technique for depositing thermal energy in a controlled manner deep within the body. However, the MR‐HIFU treatment of mobile abdominal organs is problematic as motion‐related thermometry artifacts need to be corrected and the focal point position must be updated in order to follow the moving organ to avoid damaging healthy tissue. In this article, a fat‐selective pencil‐beam navigator is proposed for real‐time monitoring and compensation of through‐plane motion. As opposed to the conventional spectrally nonselective navigator, the fat‐selective navigator does not perturb the water–proton magnetization used for proton resonance frequency shift thermometry. This allows the proposed navigator to be placed directly on the target organ for improved motion estimation accuracy. The spectral and spatial selectivity of the proposed navigator pulse is evaluated through simulations and experiments, and the improved slice tracking performance is demonstrated in vivo by tracking experiments on a human kidney and on a human liver. The direct motion estimation provided by the fat‐selective navigator is also shown to enable accurate motion compensated MR‐HIFU therapy of in vivo porcine kidney, including motion compensation of thermometry and beam steering based on the observed three‐dimensional kidney motion. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Use of proton MR spectroscopy and MR imaging to assess obesity

We used ¹H MR spectroscopy and MR imaging at 9.4-T to quantify and localize fat and water in the abdominal regions of 12 lean, normal, and obese mice. The D₂O dilution method which measures also the equilibrium plasma D₂O concentration by ²H MR spectroscopy was used to quantify body water and fat. In obese mice, the intensity of the fat ¹H resonance was about 120% that of the water ¹H resonance, about threefold higher than its value (about 45%) in normal mice. In lean mice, the fat/water intensity ratio was about 1:4, about half that in normal mice. Total body water was similar in obese and normal mice (19.9 ± 1.5 and 18.7 ± 1.3 mL) despite their very different body weights (50.1 ± 3.1 g and 30.2 ± 3.1 g, respectively), but slightly lower in lean mice (14.8 ± 1.2 mL water; 22.1 g ± 2.0g weight). Selective methylene-proton images showed marked accumulation of fat in the abdomen and the retroperitoneal and subcutaneous spaces of obese mice. Selective water-proton images allowed clear resolution of the renal cortex, medulla, papilla, and urinary pelvis. The readily measurable resonance intensity ratio of abdominal fat to water is a sensitive index by which to characterize obesity.     

Echo combination to reduce proton resonance frequency (PRF) thermometry errors from fat

PURPOSE: To validate echo combination as a means to reduce errors caused by fat in temperature measurements with the proton resonance frequency (PRF) shift method. MATERIALS AND METHODS: Computer simulations were performed to study the behavior of temperature measurement errors introduced by fat as a function of echo time. Error reduction by combining temperature images acquired at different echo times was investigated. For experimental verification, three echoes were acquired in a refocused gradient echo acquisition. Temperature images were reconstructed with the PRF shift method for the three echoes and then combined in a weighted average. Temperature measurement errors in the combined image and the individual echoes were compared for pure water and different fractions of fat in a computer simulation and for a phantom containing a homogenous mixture with 20% fat in an MR experiment. RESULTS: In both simulation and MR measurement, the presence of fat caused severe temperature underestimation or overestimation in the individual echoes. The errors were substantially reduced after echo combination. Residual errors were about 0.3 degrees C for 10% fat and 1 degrees C for 20% fat. CONCLUSION: Echo combination substantially reduces temperature measurement errors caused by small fractions of fat. This technique then eliminates the need for fat suppression in tissues such as the liver.     

Applications of multiple quantum coherence to MR imaging

Another fundamental type of nuclear spin magnetization, multiple quantum coherence, is shown to provide a further means of differentiating materials detected in the magnetic resonance (MR) experiment. A sequence using pulsed magnetic field gradients and radiofrequency pulses is demonstrated to selectively image only lactic acid in a phantom consisting of bottles of water, concentrated lactic acid, and corn oil. In particular, since water dominates the signal in the body, the double quantum experiment also serves as a strong natural suppression technique. These types of experiments are predicted to be useful both in MR spectroscopy and imaging when the more dilute metabolites in tissue are to be examined.