الفهرس 
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Abstract
Several MR imaging–based techniques are currently in clinical use for the detection and quantification of fat-water admixtures. All of them depends on dividing the net MR signal to it’s water and fat components as summarized diagrammatically
Water Signal H2O
Observed MR Signal
Others
Fat Signal
CH
CH3
CH2
• Fat detection using the chemical shift imaging technique is done by comparing the signal in the IP and OP images (when it is higher in the IP images so steatosis is confirmed )while fat quantification is done by subtracting water and fat signals in the OP images as follows
CSI Water and Fat Singles add in the IP and Subtract in the OP echo
IP: Water + Fat Detect: IP > OP
OP: Water – Fat Quant: IP – OP
• Fat detection using the frequency selective imaging technique is done by comparing the signal in the NFS images and the FS images ( when it is higher in the NFS images so steatosis is confirmed )while fat quantification is done by subtracting the signal of the FS images from that of the NFS images as represented
FSI FS selectively Filters Out Fat Signals
NFS: Water + Fat Detect: NFS > FS
FS: Water Quant: Fat = NFS – FS
• Fat detection using the MR spectroscopy technique is done by detecting high fat peak at 1.3 ppm while fat quantification is done by adding the area under curve for each of the peaks representing the spectral components of the net fat signal as illustrated
MRS Non-localized MR signal represents an admixture of diverse chemical moieties
4.7 ppm: Water Detect: Signal at 1.3 ppm
1.3 ppm: Fat (CH2) Quant: Fat = CH2, CH3, CH=CH, etc
Out-of-phase/in-phase chemical shift imaging-1
It is now usually performed as a dual-echo GRE sequence, can help detect and quantify fat on the basis of the phase interference between fat and water signals (chemical shift effect of the second kind). This technique can be tailored for fat detection by using heavy T1 weighting, which amplifies the relative fat signal and provides high sensitivity for small amounts of fat in water-dominant admixtures.
Alternatively, it can be tailored for fat quantification by minimizing T1 weighting. The accuracy of fat quantification in short T2* tissues (e.g., liver with excess iron) can be improved by correcting for the T2* signal decay
Chemical shift imaging is robust in the presence of magnetic field inhomogeneity and can be performed during a single breath hold. Fat-water dominance ambiguity, a potential pitfall of this technique, can be resolved by repeating the acquisition with variable T1 weighting (achieved with the application of two flip angles) or by administering a T1-shortening agent (gadolinium).
Frequency selective imaging -2
Fat may also be detected and quantified with fat-saturated and non-fat-saturated GRE or spin-echo imaging. Unlike with magnitude out-of-phase and in-phase imaging, fat fraction estimates are unique for the entire range of fat content (0%–100%), and there is no ambiguity between fat-dominant and water-dominant tissues.
In principle, the accuracy of fat quantification may be improved by obtaining dual-echo echo-train spin-echo images at different echo times and using the estimated T2 relaxation rates to correct for T2 effects. The performance of fat saturation, which depends on the uniformity of the magnetic field, may be spatially variable and unpredictable. Caution should be exercised, since the success of fat saturation within the lesion or tissue of interest may be difficult to gauge.
3-Proton spectroscopy
Is the most specific MR imaging technique for fat detection and quantification, since it directly displays the signal intensity of different proton species within fatty acid molecules.
To measure the spectrum within a target lesion or tissue, a spatially localized MR spectroscopic sequence, such as PRESS or STEAM, is used to obtain data within a single voxel, typically larger than 1 × 1 × 1 cm. Multivoxel MR spectroscopy may be possible but is time consuming and may be impractical as a routine protocol.
The accuracy of fat quantification can be improved by obtaining spectra at multiple echo times and measuring and correcting for the T2 decay of individual frequency peaks. MR spectroscopic analyses are complex and require specialized off-line software packages and trained personnel.
In conclusion, noninvasive detection and quantification of fat is becoming more and more important clinically, largely due to the rapidly increasing prevalence of nonalcoholic fatty liver disease, and has in fact become part of routine clinical practice at many institutions to minimize the need for liver biopsy which is not appropriate at many clinical settings.
Many advanced methods of calculating an accurate fat fraction (corrected for T1 and T2* relaxation effects) have been proposed but remain largely in the realm of research and are not yet applicable in daily practice.
For imaging of fat in routine clinical practice, it is suggested to use a three-echo, dual-flip-angle approach. Fat detection will be facilitated with use of heavy T1 weighting (achieved with a higher flip angle), whereas a lower flip angle will facilitate fat quantification by minimizing T1 effects.
The use of two flip angles will also allow resolution of fat-water ambiguity. The use of three echoes will allow some correction for T2* effects, aiding in fat detection in a short T2* environment and allowing more accurate fat quantification than can be achieved with two echoes.
Finally, many techniques utilizing different imaging modalities have been adopted to detect and quantify hepatic steatosis, providing no solid results. But with the advent of MRI and it’s different recent applications, the process of hepatic steatosis detection and quantification has become easier and more accurate, minimizing the need for biopsy.
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