Bright Fat on FSE

Why is fat bright on fast spin echo images?  

When fast spin-echo (FSE) sequences first came into use in the 1990s, the most striking difference compared to conventional spin-echo (CSE) was "abnormally" bright fat seen on the FSE images. This phenomenon is pictured right.

Conventional spin-echo image (left) and fast spin-echo image (right), both with TR/TE_(eff) = 3000/80. Note brighter signal from scalp fat on FSE.

Although several factors may contribute to the FSE "bright fat" phenomenon, the dominant mechanism is thought to be T2 prolongation secondary to disruption of J-coupling interactions that take normally place between adjacent fat protons.

I know this statement requires some some further background and explanation, so please try to follow the discussion below!

Tissue fat exists primarily in the form of long-chain triglycerides. Each triglyceride molecule contains approximately 1 dozen types of ¹H nuclei in slightly different chemical environments. Neighboring nuclei in these long chains normally interact with each other through a distortion of their electron clouds in a process known as J-coupling. 

Structure of a typical triglyceride (fat) molucule

J-coupling interaction between neighboring protons is a quantum-derived effect mediated through distortions of electron clouds.

The strength of the J-coupling interaction is quantified by the factor J, a resonance frequency offset that for lipid protons has values in the range of 6 to 8 Hz. This small frequency shift means that various protons in the lipid will precess at slightly different frequencies. Their signals will interfere constructively and destructively in a cyclic fashion with each other. (This is similar to the more familiar water-fat chemical shift interference described in a separate Q&A, but involves only fat protons and is field independent). The first out-of-phase interference will occur at TE= 1/2J, or at approximately 60-80 ms. 

It should be noted that TE's of 60-80 ms are typically used to produce T2-weighed images in conventional spin-echo imaging. By choosing TE's in this range, the signal recorded from fat is actually lower due to J-coupling effects than it might have been should a shorter or longer TE have been chosen. 

In a sense, therefore, it might be said that fat on T2-weighted CSE imaging is abnormally dark, rather than the fat on FSE being abnormally bright.

In FSE imaging the multiple 180°-pulses occur at echo spacings on the order of 10 ms and break up this pattern of J-modulation dephasing, although the J-coupling offset itself is not refocused by the 180°-pulses.  A complete understanding of this process requires knowledge of quantum mechanics and is therefore well beyond the scope of this website.  Nevertheless, this process can be understood in the sense that multiple 180°-pulses, when applied at intervals shorter than 1/J, render all J-coupled spins chemically "equivalent."  The resultant signal is the no longer modulated by the coupling. Hence, the fat on FSE imaging appears brighter than that seen on CSE images.

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Advanced Discussion (show/hide)»

Although most authorities consider disruption of J-coupling to be the primary mechanism responsible for bright fat, additional factors may also be important. Henkelman et al have suggested the closely-spaced RF-pulses reduce diffusion-mediated dephasing as spins diffuse through regions with different local magnetic field suceptibilities (fat and water). Stimulated echoes from the multiple RF-pulses may also explain perhaps 10% of the bright fat effect. Mulkern et al have shown that the degree of relative fat signal increase has a field dependence, being relatively more prominent at 1.5T than at higher or lower fields. Since J-coupling is independent of field strength, this implies other mechanisms may be operational.

In any case I have wondered whether this question is still even relevant to the modern generation of MR users. FSE imaging has largely replaced CSE in nearly all aspects of MRI. Fat on FSE images no longer seems "abnormally" bright, since FSE has become the "new normal". Perhaps this discussion is now mostly of historical interest. In any case, the bright fat phenomenon on FSE does explain why we typically do fat-suppressed T2-weighted images routinely, whereas fat suppression was not commonly employed on CSE T2-weighted images in the "old days".

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References
     Henkelman RM, Hardy PA, Bishop JE, Poon CS, Plewes DB. Why fat is bright in RARE and fast spin-echo imaging. J Magn Reson Imaging 1992; 2: 533-540.
     Mulkern RV, Packard AB, Gambarota G. Field dependence of the bright fat effect in fast spin echo imaging: theory and experiment. Proc Intl Soc Mag Reson Med 2003; 11:1107.
    Stables LA, Kennan RP, Anderson AW, Gore JC. Density matrix simulations of the effects of J coupling in spin echo and fast spin echo imaging. J Magn Reson 1999;140:305-314.
     Stokes AM, Feng Y, Mitropoulos T, Warren WS. Enhanced refocusing of fat signals using optimized multipulse echo sequences. Magn Reson Med 2013;69;1044-55.

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Related Questions
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