Magnetisation transfer based MRI Fat saturation

A recently reported technique relies on magnetisation transfer (MT). A brief recap of MT follows.
As you know, radiofrequency (RF) excitation pulses have to be at the Larmor frequency of the hydrogen atom (1H): on resonance. What you may not know is that in MRI we use 1H in free water molecules; other water molecules are around, such as those attached to macromolecules and membranes (we call these 1H bound or restricted). These other 1H have a very large range of Larmor frequencies and have such a short T2 relaxation time (less than 1ms, due to their restricted mobility) that they are not visible in MR images. We can excite or saturate some of the bound water protons by applying an RF pulse off-resonance (i.e. not on the resonant frequency of free water). Then the magnetisation of these bound protons is transferred to the free 1H protons and the free 1H behave as if they have received some of the off-resonance RF pulse directly. This magnetisation exchange is called magnetisation transfer (MT). MT is usually used to provide another contrast mechanism because the effect of MT varies between tissues; if we saturate the bound/restricted 1H , varying amounts of saturation occurs in the free 1H of tissues.

--> So back to the fat suppression method. It’s a simple image subtraction of an image with presaturation of both tissue protein and membrane phospholipid protons, from an image without presaturation. In the with-saturation image, efficient MT between water and tissue protein and membrane phospholipid means water gets saturated too, yielding an almost fat-only image. Subtract this from a regular image and you get a water-only image. The nice thing about this method is that fat signals are removed irrespective of their chemical shift (of which there is a range in vivo). It’s also not affected by B0 or B1 inhomogeneity. There is a small reduction of signal from water because not all the water gets saturated via MT, though in most tissues the eventual water signal-loss is small. A downside of the technique is a twofold increase in scan time, and possible misregistration between the two images before subtraction.

Source:http://www.revisemri.com/