Separation of true fat and water images by correcting magnetic field inhomogeneity in situ.

Yeung, Hubert; Kormos, Donald W.

doi:10.1148/radiology.159.3.3704157

Cited by 90 publications

(54 citation statements)

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“…Note that because of the absolute value operation, the solution given by Equations [6] and [7] is not affected by the error phase 0 , even though 0 is in general spatially variant. When is not zero, Equations [6] and [7] do not provide a clean water and fat separation and the wateronly image or the fat-only image will in general contain admixture of both water and fat. To remedy the problem, Dixon proposed to form two magnitude images of ͉S 0 ͉ and ͉S 1 ͉ before they are summed or subtracted as in Equations [6] and [7].…”

Section: The Original Two-point Dixon Techniquementioning

confidence: 99%

“…When is not zero, Equations [6] and [7] do not provide a clean water and fat separation and the wateronly image or the fat-only image will in general contain admixture of both water and fat. To remedy the problem, Dixon proposed to form two magnitude images of ͉S 0 ͉ and ͉S 1 ͉ before they are summed or subtracted as in Equations [6] and [7]. However, it is easy to see that the water-only image thus obtained is really just an image where every pixel contains the dominant signal of the corresponding pixel, which can be either water or fat, rather than only water.…”

Section: The Original Two-point Dixon Techniquementioning

confidence: 99%

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Dixon techniques for water and fat imaging

2008

Magnetic Resonance Imaging

506

442

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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...

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Section: The Original Two-point Dixon Techniquementioning

confidence: 99%

Section: The Original Two-point Dixon Techniquementioning

confidence: 99%

Dixon techniques for water and fat imaging

2008

Magnetic Resonance Imaging

506

442

View full text Add to dashboard Cite

show abstract

“…water ambiguity problem (2)(3)(4). Ambiguity is intimately related to B0 field homogeneity, which in turn is related to the problem of multidimensional phase unwrapping (5).…”

Section: Introductionmentioning

confidence: 99%

“…Of particular clinical interest is the fat fraction, defined by Eq 3. [3] When the conditions leading to Eq 1 and 2 are satisfied, the fat fraction can be estimated by rearrangement of Eq 1 and 2.…”

Section: Introductionmentioning

confidence: 99%

Relaxation effects in the quantification of fat using gradient echo imaging

Bydder

Yokoo

Hamilton

et al. 2008

Magnetic Resonance Imaging

368

579

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Quantification of fat has been investigated using images acquired from multiple gradient echos. The evolution of the signal with echo time and flip angle was measured in phantoms of known fat and water composition and in 21 research subjects with fatty liver. Data were compared to different models of the signal equation, in which each model makes different assumptions about the T1 and/ or T2* relaxation effects. A range of T1, T2*, fat fraction and number of echos was investigated to cover situations of relevance to clinical imaging. Results indicate that quantification is most accurate at low flip angles (to minimize T1 effects) with a small number of echos (to minimize spectral broadening effects). At short echo times the spectral broadening effects manifest as a short apparent T2 for the fat component.

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“…Clearly, W and F can be easily solved from Eqs. [7] and [8] if P can be found. Although this phasor P is not completely known, it must be restricted to one of the following two possibilities, P u and P v , since the correct assignments of the (B, S) solutions must be either (B, S) ϭ (W, F) or (B, S) ϭ (F, W): [10] where P u and P v are the two possible error phasors constructed from the (B, S) solution pair.…”

mentioning

confidence: 99%