Fat-suppressed three-dimensional dual echo Dixon technique for contrast agent enhanced MRI

PURPOSE: To develop a fast T1-weighted, fat-suppressed three-dimensional dual echo Dixon technique and to demonstrate its use in contrast agent enhanced MRI. MATERIALS AND METHODS: A product fast three-dimensional gradient echo pulse sequence was modified to acquire dual echoes after each RF excitation with water and fat signals in-phase (IP) and opposed-phase (OP), respectively. An on-line reconstruction algorithm was implemented to automatically generate separate water and fat images. The signal to noise ratio (SNR) of the new technique was compared to that of the product technique in phantom. In vivo abdomen and breast images of cancer patients were acquired at 1.5 Tesla using both techniques before and after intravenous administration of gadolinium contrast agent. RESULTS: In phantom, the new technique yields a close to the theoretically predicted 41% increase in SNR in comparison to the product technique without fat suppression (FS). In vivo images of the new technique show noticeably improved FS and image quality in comparison to the images acquired of the same patients using the product technique with FS. CONCLUSION: The three-dimensional dual echo Dixon technique provides excellent image quality and can be used for T1-weighted, fat-suppressed imaging with contrast agent injection.     

Dixon techniques for water and fat imaging

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 degrees out-of-phase. Dixon showed that from these two images, a water-only image and a fat-only image can be generated. The water-only 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 image-based 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.     

Late bone marrow changes in Hodgkin's disease patients: a characterization with proton chemical shift imaging

Our aim was to measure, by quantitative chemical shift imaging (CSI), the late therapy-induced changes in bone marrow (BM) of Hodgkin's disease (HD) patients. Fifteen HD patients treated with radiotherapy alone and radiochemotherapy (age at treatment between 11 and 50 years; post-treatment interval between 11 and 50 years; post-treatment interval between 15 and 127 months; applied dose 25.5 to 50 Gy), were studied with a 1.5 T MR imager. For the fat-water separation in-phase and opposed-phase (SE 1200/22) images were generated according to the Dixon method, with a modified post-processing. Long-term fatty replacement was seen in the irradiated BM only. The radiation fields were visualized as areas of high signal intensity in the T1-weighted images. There was a marked increase of the relative fat signal fraction in quantitative CSI without time, dose and age dependent recovery within the investigated ranges.Fatty replacement of the irradiated BM is a long-term effect in HD patients, probably induced by an obliteration of the microvasculature with consecutive fatty metaplasia.     

Quantitative proton chemical-shift imaging

Recently W. T. Dixon (Radiology 153, 189 (1984))introduced a simple method of proton chemical-shift imaging which requires only two images, a conventional (in-phase) image and an image in which fat and water protons are 180 degrees out of phase during signal acquisition, to separate the signals from fat and water protons. We have tested the application of this method to the quantitative determination of fat content and fat and water longitudinal relaxation times, and analyzed the effects of random and systematic errors. Ten phantoms were constructed with a range of fat contents (0-50% by weight) and water T1's (300-800 ms). Fat and water T1's were measured with a 0.6-T clinical imaging system in two ways: using the system as a spectrometer with all gradients off, and from least-squares fits to in-phase and out-of-phase image data made with six values of TR. The image-derived values of water T1 agreed well with spectrometer-derived values (r = 0.97) and the image derived fat fraction correlated strongly with the fat fraction by weight (r = 0.995). The effects of random and systematic errors were analyzed for a minimum data set of four images: in-phase and out-of-phase images at two values of TR. The pair of TR values which minimize the variance in water T1 were calculated, and for these pulse sequences the effects of two potential systematic errors were calculated: inhomogeneities in the main field, which will reduce the intensity in out-of-phase images compared to in-phase images even for pure water samples, and an incorrect shift of the 180 degrees pulse in the out-of-phase pulse sequence, corresponding to an inaccurate assumed chemical shift. With careful attention to such systematic effects the Dixon method is capable of producing reliable quantitative measurements.     

MRI化学位移同、反相位成像的体外实验模型研究

目的 通过建立同、反相位的体外实验模型，探讨与同相位相比，反相位上信号强度变化与脂肪含量的相关性，并从中确定反相位上信号强度明显下降时的确切脂肪含量的标准。材料与方法模型建立方法：将1％琼脂糖水溶液与色拉油先后倒入一长方形水槽内，在水-脂交界处采用层厚10mm倾斜面进行同、反相位检查。测出同、反相位图像各部感兴趣区(ROI)的信号强度(SI)，计算信号强度下降指数(SI指数)值，并且绘制脂肪含量与SI指数关系的曲线图。结果 最初脂肪比例较低时，随脂肪含量增加，反相位的信号强度呈逐渐下降表现；而当脂肪比例较高时，其信号强度随着脂肪含量的增加而升高，其反转点的脂肪含量为23％。结论 脂肪含量在23％左右时，反相位上的信号强度有较为明显的下降，而且脂肪含量愈接近该值，信号强度下降的程度就愈明显。     

Fat and water separation in balanced steady-state free precession using the Dixon method.

In this work the feasibility of separating fat and water signals using the balanced steady-state free precession (SSFP) technique is demonstrated. The technique is based on the observation (Scheffler and Hennig, Magnetic Resonance in Medicine 2003;49:395-397) that at the nominal values of TE = TR/2 in SSFP imaging, phase coherence can be achieved at essentially only two orientations (0 degrees and 180 degrees ) relative to the RF pulses in the rotating frame, under the assumption of TR < T2, and independently of the SSFP angle. This property allows in-phase and out-of-phase SSFP images to be obtained by proper choices of the center frequency offset, and thus allows the Dixon subtraction method to be utilized for effective fat-water separation. The TR and frequency offset for optimal fat-water separation are derived from theories. Experimental results from healthy subjects, using a 3.0 Tesla system, show that nearly complete fat suppression can be accomplished.     

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.     

An extended two-point dixon algorithm for calculating separate water,fat, and B0 images

A new algorithm is presented that provides separate water, fat, and B₀ images utilizing the in-phase and opposed-phase acquisitions of the two-point Dixon (2PD) method. The accuracy of the extended method (E2PD) compares favorably with the three-point Dixon (3PD) method, and the acquisition requires 2/3 the 3PD scan time. Slightly increased mismapping may occur in pixels containing an admixture of water and fat due to reduced SNR in the B₀ field map compared with the 3PD method.     

Two-point Dixon technique for water-fat signal decomposition with B0 inhomogeneity correction

To separate water and lipid resonance signals by phase-sensitive MRI, a two-point Dixon (2PD) reconstruction is presented in which phase-unwrapping is used to obtain an inhomogeneity map based on only in-phase and out-of-phase image data. Two relaxation-weighted images, a “water image” and a “fat image,” representing a two-resonance peak model of proton density, are output. The method is designed for T₁- or density-weighted spin-echo imaging; a double-echo scheme is more appropriate for T₂-weighted spin-echo imaging. The technique is more time-efficient for clinical fat-water imaging than 3PD schemes, while still correcting for field inhomogeneity.     

Articular cartilage of the knee: rapid three-dimensional MR imaging at 3.0 T with IDEAL balanced steady-state free precession--initial experience

Institutional review board approval and informed consent were obtained for this HIPAA-compliant study. In this study, iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) balanced steady-state free precession (bSSFP), fat-suppressed bSSFP, and fat-suppressed spoiled gradient-echo (GRE) sequences for 3.0-T magnetic resonance (MR) imaging of articular knee cartilage were prospectively compared in five healthy volunteers. Cartilage and fluid signal-to-noise ratio (SNR), cartilage-fluid contrast-to-noise ratio (CNR), SNR efficiency, CNR efficiency, image quality, and fat suppression were compared. Fat-suppressed bSSFP and IDEAL bSSFP had higher SNR efficiency of cartilage (P < .01) than did GRE. IDEAL bSSFP had higher cartilage-fluid CNR efficiency than did fat-suppressed bSSFP or GRE (P < .01). Fat-suppressed bSSFP and IDEAL bSSFP had higher image quality than did GRE (P < .01). GRE and IDEAL bSSFP had significantly better fat-water separation or fat saturation than did fat-suppressed bSSFP (P < .05). IDEAL bSSFP is a promising method for imaging articular knee cartilage.     

Multicoil Dixon chemical species separation with an iterative least-squares estimation method.

This work describes a new approach to multipoint Dixon fat-water separation that is amenable to pulse sequences that require short echo time (TE) increments, such as steady-state free precession (SSFP) and fast spin-echo (FSE) imaging. Using an iterative linear least-squares method that decomposes water and fat images from source images acquired at short TE increments, images with a high signal-to-noise ratio (SNR) and uniform separation of water and fat are obtained. This algorithm extends to multicoil reconstruction with minimal additional complexity. Examples of single- and multicoil fat-water decompositions are shown from source images acquired at both 1.5T and 3.0T. Examples in the knee, ankle, pelvis, abdomen, and heart are shown, using FSE, SSFP, and spoiled gradient-echo (SPGR) pulse sequences. The algorithm was applied to systems with multiple chemical species, and an example of water-fat-silicone separation is shown. An analysis of the noise performance of this method is described, and methods to improve noise performance through multicoil acquisition and field map smoothing are discussed.     

Magnetization-prepared IDEAL bSSFP: a flow-independent technique for noncontrast-enhanced peripheral angiography

Purpose:

To propose a new noncontrast‐enhanced flow‐independent angiography sequence based on balanced steady‐state free precession (bSSFP) that produces reliable vessel contrast despite the reduced blood flow in the extremities.

Materials and Methods:

The proposed technique addresses a variety of factors that can compromise the exam success including insufficient background suppression, field inhomogeneity, and large volumetric coverage requirements. A bSSFP sequence yields reduced signal from venous blood when long repetition times are used. Complex‐sum bSSFP acquisitions decrease the sensitivity to field inhomogeneity but retain phase information, so that data can be processed with the Iterative Decomposition of Water and Fat with Echo Asymmetry and Least‐Squares Estimation (IDEAL) method for robust fat suppression. Meanwhile, frequent magnetization preparation coupled with parallel imaging reduces the muscle and long‐T₁ fluid signals without compromising scan efficiency.

Results:

In vivo flow‐independent peripheral angiograms with reliable background suppression and high spatial resolution are produced. Comparisons with phase‐sensitive bSSFP angiograms (that yield out‐of‐phase fat and water signals, and exploit this phase difference to suppress fat) demonstrate enhanced vessel depiction with the proposed technique due to reduced partial‐volume effects and improved venous suppression.

Conclusion:

Magnetization‐prepared complex‐sum bSSFP with IDEAL fat/water separation can create reliable flow‐independent angiographic contrast in the lower extremities. J. Magn. Reson. Imaging 2011;33:931–939. © 2011 Wiley‐Liss, Inc.     

MR imaging of renal cell carcinoma: its role in determining cell type

Chemical shift gradient-echo MR imaging (CSI) can detect a small amount of fat as signal loss on opposed-phase images as compared with in-phase images. Cytoplasmic fat in clear cell renal cell carcinoma (RCC) or interstitial histiocytic fat in papillary cell RCC can be successfully demonstrated by this technique. T2*-weighted gradient-echo or echo-planar MR imaging can detect local susceptibility, for example, due to cytoplasmic or interstitial histiocytic hemosiderin deposition in papillary cell RCC. CSI can also show this focal susceptibility as excessive signal loss on in-phase images as compared with opposed-phase images. MR imaging can thus help predict the cell types (clear cell and papillary cell) of RCC. These findings may be important in the decision-making process in the management of patients with suspected RCC, particularly those who are not indicated for radical surgery.     

Fast spin-echo triple-echo dixon (fTED) technique for efficient T2-weighted water and fat imaging.

Previously published fast spin-echo (FSE) implementations of a Dixon method for water and fat separation all require multiple scans and thus a relatively long scan time. Further, the minimum echo spacing (esp), a time critical for FSE image quality and scan efficiency, often needs to be increased in order to bring about the required phase shift between the water and fat signals. This work proposes and implements a novel FSE triple-echo Dixon (fTED) technique that can address these limitations. In the new technique, three raw images are acquired in a single FSE scan by replacing each frequency-encoding gradient in a conventional FSE with three consecutive gradients of alternating polarity. The timing of the three gradients is adjusted by selecting an appropriate receiver bandwidth (RBW) so that the water and fat signals for the three corresponding echoes have a relative phase shift of -180 degrees , 0 degrees , and 180 degrees , respectively. A fully automated postprocessing algorithm is then used to generate separate water-only and fat-only images for each slice. The technique was implemented with and without parallel imaging. We demonstrate that the new fTED technique enables both uniform water/fat separation and fast scanning with uncompromised scan parameters, including applications such as T(2)-weighted separate water and fat imaging of the abdomen during breath-holding.     

Keyhole Dixon method for faster,perceptually equivalent fat suppression

PURPOSE: To reduce the acquisition time associated with the two-point Dixon fat suppression technique by combining a keyhole in-phase (Water + Fat) k-space data set with a full out-of-phase (Water - Fat) k-space data set and optimizing the keyhole size with a perceptual difference model. MATERIALS AND METHODS: A set of keyhole Dixon images was created by varying the number of lines in the keyhole data set. Off-resonance correction was incorporated into the image reconstruction process to improve the homogeneity of the fat suppression. A perceptual difference model (PDM) was validated with human observer experiments and used to compare the keyhole images to images from a full two-point Dixon acquisition. The PDM was used to determine the smallest keyhole width required to obtain perceptual equivalence to images obtained from the full two-point Dixon method. RESULTS: In experimental phantom studies, the keyhole Dixon image reconstructed from 96 of 192 Water + Fat k-space lines and 192 Water - Fat k-space lines was perceptually equivalent to the full (192 + 192) two-point Dixon images, resulting in a 25% reduction in scan time. Clinical images of a volunteer's knee, orbits, and abdomen created from the smallest, perceptually equivalent keyhole width resulted in a 27%-38% reduction in total scan time. CONCLUSION: This method improves the temporal efficiency of the conventional two-point Dixon technique and may prove especially useful for high-field systems where specific absorption rate (SAR) limits will constrain radiofrequency (RF)-based fat suppression techniques.     

Non‐contrast‐enhanced flow‐independent peripheral MR angiography with balanced SSFP

Flow‐independent angiography is a non‐contrast‐enhanced technique that can generate vessel contrast even with reduced blood flow in the lower extremities. A method is presented for producing these angiograms with magnetization‐prepared balanced steady‐state free precession (bSSFP). Because bSSFP yields bright fat signal, robust fat suppression is essential for detailed depiction of the vasculature. Therefore, several strategies have been investigated to improve the reliability of fat suppression within short scan times. Phase‐sensitive SSFP can efficiently suppress fat; however, partial volume effects due to fat and water occupying the same voxel can lead to the loss of blood signal. In contrast, alternating repetition time (ATR) SSFP minimizes this loss; however, the level of suppression is compromised by field inhomogeneity. Finally, a new double‐acquisition ATR‐SSFP technique reduces this sensitivity to off‐resonance. In vivo results indicate that the two ATR‐based techniques provide more reliable contrast when partial volume effects are significant. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Chemical shift-based true water and fat images: regional phase correction of modified spin-echo MR images

The process of separating water and fat signal in magnetic resonance images with the Dixon pulse sequence is hindered by phase errors in the water-fat opposed image. These errors arise from static field inhomogeneities, varying magnetic susceptibilities of different body tissues, and other causes. Phase correction performed with data from phantom imaging can compensate for static field inhomogeneities but not for the other effects. A regional phase-correction algorithm is presented that removes non-chemical-shift phase effects and produces true water and fat images. The technique has been applied with good results to 21 patients and healthy volunteers. The images include ones of the abdomen, knees, hips, spine, and head. This method of regional phase correction is an efficacious way of producing separate water and fat images with the Dixon pulse sequence.     

T1-weighted spoiled gradient-echo MR imaging of focal hepatic lesion: comparison of in-phase vs opposed-phase pulse sequence

The goal of our prospective study was to compare quantitatively and qualitatively in-phase and opposed-phase T1-weighted breath-hold spoiled gradient-recalled-echo (GRE) MR imaging technique for imaging focal hepatic lesion. Thirty-eight patients with 53 focal hepatic lesions had in-phase (TR = 12.3 ms, TE = 4.2 ms) and opposed-phase (TR = 10.1 ms, TE = 1.9 ms) GRE (flip angle = 30°, bandwidth ± 32 kHz, matrix size 256 × 128, one signal average) MR imaging at 1.5 T. Images were analyzed quantitatively by measuring the lesion-to-liver contrast and for lesion detection. In addition, images were reviewed qualitatively for lesion conspicuity. Quantitatively, lesion-to-liver contrast obtained with in-phase (3.22 ± 1.86) and opposed-phase pulse sequence (3.72 ± 2.32) were not statistically different (Student's t-test). No difference in sensitivity was found between in-phase and opposed-phase pulse sequence (31 of 53, sensitivity 58 % vs 30 of 53, sensitivity 57 %, respectively). Two lesions not seen with opposed-phase imaging were detected with in-phase imaging. Conversely, one lesion not seen on in-phase imaging was detected on opposed-phase imaging so that the combination of in-phase and opposed-phase imaging yielded detection of 32 of 53 lesions (sensitivity 60 %). Qualitatively, lesion conspicuity was similar with both techniques. However, in-phase images showed better lesion conspicuity than opposed-phase images in 9 cases, and opposed-phase images showed better lesion conspicuity than in-phase images in 7 cases. No definite advantage (at a significant level) emerged between in-phase and opposed-phase spoiled GRE imaging. Because differences in lesion conspicuity and lesion detection may be observed with the two techniques in individual cases, MR evaluation of patients with focal hepatic lesion should include both in-phase and opposed-phase spoiled GRE imaging. Received 30 October 1996; Revision received 6 January 1997; Accepted 8 January 1997     

Detection of hepatocellular carcinoma using double-echo FLASH sequence during the hepatic arterial phase.

PURPOSE: The purpose of this study was to compare the performance of in-phase and opposed-phase gradient-recalled echo (GRE) pulse sequences in paramagnetic contrast-enhanced magnetic resonance (MR) imaging of hepatocellular carcinomas (HCCs) during the hepatic arterial phase. MATERIAL AND METHODS: Thirty-four patients with 84 lesions with known or suspected HCCs, nine of whom had a fatty liver, were examined with double-echo GRE techniques under 1.5T before and 30 s after injection of gadopentenate dimeglumine at a dose of 0.1 mmol/kg. Echo times were 2.4 ms (opposed phase) and 5.0 ms (in phase). Contrast enhancement of the HCC detected in both in-phase and opposed-phase images was evaluated. The liver signal-to-noise ratio (SNR), lesion-liver contrast-to-noise ratio (CNR), and enhancement ratio (ER) were calculated for the largest lesion of each patient. RESULTS: In dynamic gadolinium-enhanced images of the 84 HCCs, 81 (96.4%) were detected in both in-phase and opposed-phase images, two (2.4%) were detected in only in-phase images, and one (1.2%) was detected only in opposed-phase images. The liver SNR, CNR, and ER were 46.7+/-16.1, 15.2+/-10.3, and 0.637+/-0.268 for in-phase images, and 48.9+/-16.9, 16.3+/-11.8, and 0.647+/-0.309 for opposed-phase images, respectively. In patients with a fatty liver, the SNR, CNR, and ER were 46.0+/-18.1, 21.7+/-17.9, and 0.525+/-0.231 for in-phase images, and 44.3+/-18.7, 26.0+/-21.3, and 0.793+/-0.124 for opposed-phase images, respectively. No significant statistical differences were found between the in-phase and opposed-phase images. CONCLUSION: Opposed-phase GRE imaging is equivalent to in-phase GRE sequences in patients with or without fatty liver for detection of HCC in dynamic gadolinium-enhanced images.     

Liver fat: effect of hepatic iron deposition on evaluation with opposed-phase MR imaging

PURPOSE: To retrospectively determine the effect of liver iron deposition on the evaluation of liver fat by using opposed-phase magnetic resonance (MR) imaging. MATERIALS AND METHODS: Committee on Human Research approval was obtained, and compliance with HIPAA regulations was observed. Patient consent was waived by the committee. Thirty-eight patients with cirrhosis (30 men, eight women; mean age, 58 years; range, 34-76 years) who underwent abdominal MR imaging and had contemporaneous liver biopsy were retrospectively identified. Two radiologists independently quantified liver fat according to the relative loss of signal intensity and compared this loss on opposed-phase and in-phase T1-weighted gradient-echo images. Liver fat percentage and presence of iron deposition were independently recorded by a pathologist. Generalized linear models, which included a mixed-random effects model, were used to determine the effect of iron deposition on the Spearman correlation coefficient for relative signal intensity loss versus histopathologically determined fat percentage. RESULTS: Liver iron deposition was found in 25 of 38 patients. Liver fat percentage (mean, 3%; range, 0%-25%) was identified histopathologically in 14 of 38 patients and in nine of 25 patients with iron deposition. For both readers, relative signal intensity loss at opposed-phase imaging was closely and significantly correlated (P < .05) with histopathologically determined liver fat percentage in patients without iron deposition (r = 0.7 for reader 1, r = 0.6 for reader 2), but no such correlation was found in patients with iron deposition (r = 0.1 for reader 1, r = -0.31 for reader 2; P > .05). CONCLUSION: Signal intensity loss on in-phase images caused by the presence of liver iron is a potential pitfall in the determination of liver fat percentage at opposed-phase MR imaging in chronic liver disease.