Comparison of biphasic and reordered fat suppression for dynamic breast MRI

PURPOSE: To optimize a reordered k-space acquisition that applies intermittent fat saturation (FS) pulses to allow for a time-efficient reduction of fat signal in breast MR images, and compare it with an elliptic-centric biphasic FS method in terms of the degree of fat suppression and speed. MATERIALS AND METHODS: The behavior of the fat and water signals under the influence of the reordered sequence was characterized. This allowed us to optimize the flip angle and visualize the expected artifacts by deriving the point spread function (PSF) of the fat signal. We compared the two sequences by acquiring images with a varying number of FS pulses, with a corresponding difference in scan time. The quality of the images was assessed by comparison with images obtained with full fat suppression as measured by a root-mean-square (RMS) error metric. RESULTS: The reordered sequence allowed for an approximately twofold reduction in error compared to the biphasic sequence for the same scan time. With the reordered sequence and optimized scan parameters, we were able to reduce the time spent on fat suppression by as much as 99% with no discernible reduction in image quality. CONCLUSION: This method will allow robust fat suppression with virtually no extension in imaging time for dynamic contrast-enhanced (DCE)-MRI.     

Novel rapid fat suppression strategy with spectrally selective pulses.

Short repetition time gradient echo sequences are gaining popularity in clinical applications such as dynamic contrast enhancement imaging, cardiac imaging, and MR angiography. Performing fat suppression in these sequences is usually time consuming and often somewhat ineffective, due to the relatively short T(1) and long T(2) of fat. A novel rapid fat suppression strategy using spectrally selective pulses is introduced and compared with clinically popular sequences such as fat presaturated fast field echo (FFE) and turbo field echo (TFE) and binomial water-selective spatial-spectral excitation (SSE, or SPSP excitation) FFE. The new strategy combines fat presaturation with low-order binomial water-selective SSE pulses in a TFE sequence. This enables the use of a long echo train length to decrease exam time, but without creation of excess fat signal contamination of the resultant images. The fat nullification is also more reliable as fat signals in central k-space data are suppressed twice. An implementation of this strategy is compared with traditional methods in both phantom and human studies, confirming that the new technique provides strong fat suppression with few artifacts despite the short scan duration.     

Radial alternating TE sequence for faster fat suppression.

This study describes a steady-state sequence that uses a radial k-space trajectory and alternating echo times (TEs) between even and odd k-space views. The sequence generated a single data set that was used to reconstruct images with inherent fat suppression. This fat suppression results from the fat phase variation in alternate echoes giving rise to cancellation in the central portion of k-space. This new fat-suppression method provides inherent fat suppression in half the acquisition time relative to the radial two-point Dixon method. The improvement in k-space sampling efficiency is demonstrated in phantom and clinical images, and through measured point-spread functions (PSFs). As a result, the radial alternating TE sequence offers improved temporal resolution over a radial version of the two-point Dixon sequence by requiring fewer total projections to obtain the same effective resolution in water-based tissues.     

Optimization of chemical shift selective suppression of fat

Strategies to optimize flip angles for chemical shift selective fat suppression are discussed. Mathematical models for fat suppression in spoiled gradient recalled acquisition, spin echo, and RARE, which incorporate steady state conditions and multiple spectral components of fat, are developed. The optimal suppression flip angle is found to be larger than that determined with a single fat component model by more than 10° due to contributions from unflipped components such as olefinic and glycerol protons that lie outside the suppression band.     

Rapid fat suppression in MRI of the breast with short binomial pulses

PURPOSE: To develop a faster method of fat suppression for use in dynamic contrast enhanced MRI of the breast. MATERIALS AND METHODS: A method of fast fat suppression is presented using spatially nonselective rapid binomial pulses. In contrast to conventional binomial frequency-selective pulses, these short pulses are applied without interpulse delay, allowing for very rapid spectrally selective excitation. RESULTS: Effective water excitation and fat suppression were achieved in breast MRI at 3.0 Tesla with total excitation time as low as 160 microsec, which is several times shorter than the excitation time of currently used fat suppression techniques. Rapid fat suppression comes at the expense of increased specific absorption rate (SAR) and mildly degraded quality of suppression. A flexible tradeoff of short imaging time vs. SAR can be made to optimize imaging speed for fat-suppressed breast MRI. CONCLUSION: Rapid binomial pulses can be used for dynamic contrast enhanced breast MRI with excitation times significantly shorter than currently used fat suppression pulses. Shorter excitation time allows more rapid imaging, allowing greater temporal and spatial resolution for characterization of breast lesions.     

Three‐point dixon method enables whole‐body water and fat imaging of obese subjects

Dixon imaging techniques derive chemical shift‐separated water and fat images, enabling the quantification of fat content and forming an alternative to fat suppression. Whole‐body Dixon imaging is of interest in studies of obesity and the metabolic syndrome, and possibly in oncology. A three‐point Dixon method is proposed where two solutions are found analytically in each voxel. The true solution is identified by a multiseed three‐dimensional region‐growing scheme with a dynamic path, allowing confident regions to be solved before unconfident regions, such as background noise. 2π‐Phase unwrapping is not required. Whole‐body datasets (256 × 184 × 252 voxels) were collected from 39 subjects (body mass index 19.8‐45.4 kg/m²), in a mean scan time of 5 min 15 sec. Water and fat images were reconstructed offline, using the proposed method and two reference methods. The resulting images were subjectively graded on a four‐grade scale by two radiologists, blinded to the method used. The proposed method was found superior to the reference methods. It exclusively received the two highest grades, implying that only mild reconstruction failures were found. The computation time for a whole‐body dataset was 1 min 51.5 sec ± 3.0 sec. It was concluded that whole‐body water and fat imaging is feasible even for obese subjects, using the proposed method. Magn Reson Med 63:1659–1668, 2010. © 2010 Wiley‐Liss, Inc.     

Efficient fat suppression by slice‐selection gradient reversal in twice‐refocused diffusion encoding

Most diffusion imaging sequences rely on single‐shot echo‐planar imaging (EPI) for spatial encoding since it is the fastest acquisition available. However, it is sensitive to chemical‐shift artifacts due to the low bandwidth in the phase‐encoding direction, making fat suppression necessary. Often, spectral‐selective RF pulses followed by gradient spoiling are used to selectively saturate the fat signal. This lengthens the acquisition time and increases the specific absorption rate (SAR). However, in pulse sequences that contain two slice‐selective 180° refocusing pulses, the slice‐selection gradient reversal (SSGR) method of fat suppression can be implemented; i.e., using slice‐selection gradients of opposing polarity for the two refocusing pulses. We combined this method with the twice‐refocused spin‐echo sequence for diffusion encoding and tested its performance in both phantoms and in vivo. Unwanted fat signal was entirely suppressed with this method without affecting the water signal intensity or the slice profile. Magn Reson Med 60:1256–1260, 2008. © 2008 Wiley‐Liss, Inc.     

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.     

T2-weighted spine imaging with a fast three-point dixon technique: comparison with chemical shift selective fat suppression

PURPOSE: To develop a phased-array coil-compatible, fast three-point Dixon (TPD) technique, and compare its performance in T2-weighted spine imaging with that of the standard chemical shift selective (CHESS) fat suppression technique. MATERIALS AND METHODS: We acquired T2-weighted spine images of 27 patients using essentially identical scanning parameters with the fast TPD technique and standard fast spin echo (FSE) with CHESS fat suppression. A phased-array coil-compatible image reconstruction algorithm was developed to generate separate water and fat images from the data acquired with the fast TPD technique. Three neuroradiologists independently scored the images from the two different techniques for uniformity of fat suppression and lesion conspicuity using a four-point system (1 = poor, 2 = fair, 3 = good, 4 = best). RESULTS: The reviewers' mean scores were 3.2 and 2.1 for the uniformity of fat suppression, and 3.0 and 2.0 for the lesion conspicuity for the fast TPD and the CHESS fat suppression techniques, respectively. The fast TPD technique was statistically superior to the CHESS technique at P < 0.0005. CONCLUSION: The fast TPD technique provides superior fat suppression and lesion conspicuity, and potentially can be used as an alternative to T2-weighted imaging of the spine.     

Fast three‐dimensional dual echo dixon technique improves fat suppression in breast MRI

Purpose:

To compare qualitative and quantitative measures of the contrast‐enhanced dual‐echo Dixon technique with the commonly used standard three‐dimensional (3D) gradient echo (spectrally selective fat suppression) technique (SS‐FS) in breast MRI exams (bMRI).

Materials and Methods:

A total of 19 women, with prescheduled bMRI exam, were recruited to our study between 2006 and 2008. Dixon and standard SS‐SF techniques were used on both breasts of each patient. Image quality was rated in five categories: fat suppression quality, fat suppression uniformity, lesion margin clarity, lesion visibility, and axillary visibility. For quantitative assessment, we calculated the signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) of lesion to breast, SNR efficiency, and CNR efficiency.

Results:

Of 19 patients evaluated, 13 had a primary breast malignancy and 6 had benign lesions or negative exams. Dixon images were rated higher in four of five qualitative categories (P < 0.0001) and required a shorter scan time. Dixon images yielded significantly higher SNR (43.8) and CNR (40.1) values than did 3DGRE images (SNR = 34.8, CNR = 25.3; P < 0.05). SNR efficiency (36.30) and CNR efficiency (33.79) values for Dixon images were also higher than were 3DGRE images (SNR efficiency =25.7, CNR efficiency = 19.1; P < 0.05).

Conclusion:

Dixon images were superior to the standard SS‐SF images in both qualitative and quantitative assessment of 19 bMRI exams. The Dixon technique could replace standard SS‐SF technique in bMRI exam, after our findings have been confirmed in future studies with a larger sample size. J. Magn. Reson. Imaging 2010;31:889–894. ©2010 Wiley‐Liss, Inc.     

Consistent fat suppression with compensated spectral-spatial pulses

Reliable fat suppression is especially important with fast imaging techniques such as echo-planar (EPI), spiral, and fast spin-echo (FSE) T₂-weighted imaging. Spectral-spatial excitation has a number of advantages over spectrally selective presaturation techniques, including better resilience to B₀ and B₁, inhomogeneity. In this paper, a FSE sequence using a spectral-spatial excitation pulse for superior fat suppression is presented. Previous problems maintaining the CPMG condition are solved using simple methods to accurately program radio-frequency (RF) phase. Next an analysis shows how B₀ eddy currents can reduce fat suppression effectiveness for spectral-spatial pulses designed for conventional gradient systems. Three methods to compensate for the degradation are provided. Both the causes of the degradation and the compensation techniques apply equally to gradient-recalled applications using these pulses. These problems do not apply to pulses designed for high-speed gradient systems. The spectral-spatial FSE sequence delivers clinically lower fat signal with better uniformity than spectrally selective pre-saturation techniques.     

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.     

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.     

Chemical shift encoding‐based water–fat separation methods

The suppression of signal from fat constitutes a basic requirement in many applications of magnetic resonance imaging. To date, this is predominantly achieved during data acquisition, using fat saturation, inversion recovery, or water excitation methods. Postponing the separation of signal from water and fat until image reconstruction holds the promise of resolving some of the problems associated with these methods, such as failure in the presence of field inhomogeneities or contrast agents. In this article, methods are reviewed that rely on the difference in chemical shift between the hydrogen atoms in water and fat to perform such a retrospective separation. The basic principle underlying these so‐called Dixon methods is introduced, and some fundamental implementations of the required chemical shift encoding in the acquisition and the subsequent water–fat separation in the reconstruction are described. Practical issues, such as the selection of key parameters and the appearance of typical artifacts, are illustrated, and a broad range of applications is demonstrated, including abdominal, cardiovascular, and musculoskeletal imaging. Finally, advantages and disadvantages of these Dixon methods are summarized, and emerging opportunities arising from the availability of information on the amount and distribution of fat are discussed. J. Magn. Reson. Imaging 2014;40:251–268 . © 2014 Wiley Periodicals, Inc .     

The utility of chemical shift imaging and related fat suppression as standalone technique in cryptorchidism using low field MRI

Cryptorchidism is failure of one or both testes to descend completely into the scrotum. The testis can be located anywhere along its journey of descent, between the lower pole of the kidney and the inguinal canal. Objective: The aim of this study was to assess the effectiveness of chemical shift imaging and Dixon based fat suppression using low field MRI in non-palpable undescended testicle. Patients and methods: From July 2017 through February 2018, Twenty eight boys, presented by either unilateral or bilateral cryptorchidism, with total number of forty testicles, underwent MRI study using low field machine in T1-weighted dual gradient-echo in-phase and opposed-phase sequence with Dixon based fat suppression. Results: Based on the laparoscopic/operative data, twenty one testes were located at the inguinal region, whereas fifteen testes were pelvi-abdominal and four were absent. The whole image sets of CSI and Dixon fat suppression had the highest specificity and positive predictive value (100%) and the highest overall accuracy (95%) for detection of undescended testes. Conclusion: chemical shift imaging combined with Dixon based fat suppression is reliable imaging tool as a standalone technique for evaluating cryptorchidism, providing high specificity and diagnostic accuracy.     

4D radial coronary artery imaging within a single breath-hold: cine angiography with phase-sensitive fat suppression (CAPS).

Coronary artery data acquisition with steady-state free precession (SSFP) is typically performed in a single frame in mid-diastole with a spectrally selective pulse to suppress epicardial fat signal. Data are acquired while the signal approaches steady state, which may lead to artifacts from the SSFP transient response. To avoid sensitivity to cardiac motion, an accurate trigger delay and data acquisition window must be determined. Cine data acquisition is an alternative approach for resolving these limitations. However, it is challenging to use conventional fat saturation with cine imaging because it interrupts the steady-state condition. The purpose of this study was to develop a 4D coronary artery imaging technique, termed "cine angiography with phase-sensitive fat suppression" (CAPS), that would result in high temporal and spatial resolution simultaneously. A 3D radial stacked k-space was acquired over the entire cardiac cycle and then interleaved with a sliding window. Sensitivity-encoded (SENSE) reconstruction with rescaling was developed to reduce streak artifact and noise. Phase-sensitive SSFP was employed for fat suppression using phase detection. Experimental studies were performed on volunteers. The proposed technique provides high-resolution coronary artery imaging for all cardiac phases, and allows multiple images at mid-diastole to be averaged, thus enhancing the signal-to-noise ratio (SNR) and vessel delineation.