Technique for selective fat saturation in MR imaging

In a system for MR imaging wherein a volume within an imaging subject positioned with respect to said system contains water molecules and fat tissue proximate to one another, a method for exciting a slice of interest in said volume and for saturating bands of water molecules and fat tissue in said volume so that said bands are selectively positioned with respect to said slice and to each other, said method comprising the steps of:

• specifying a first gradient magnetic field and a first RF carrier frequency for prospective use in generating said saturated bands, said first gradient magnetic field having a specified polarity with respect to an axis, said first RF frequency comprising the sum of a center frequency and a first offset frequency having a magnitude and polarity determined by said first gradient magnetic field, said first gradient magnetic field and said first offset frequency comprising the parameters of a first parameter set;
• specifying a second gradient magnetic field and a second RF carrier frequency for prospective use in generating said slice, said second gradient magnetic field having a specified polarity with respect to an axis, said second RF frequency comprising the sum of a center frequency and a second offset frequency having a magnitude and polarity determined by said second gradient magnetic field, said second gradient magnetic field and said second offset frequency comprising the parameters of a second parameter set;
• computing first and second terms, respectively comprising the width of said water molecule band divided by the frequency bandwidth thereof, and the width of said slice divided by the frequency bandwidth thereof;
• determining whether the sum of said first and second terms is positive or negative;
• and
• in the event said sum is positive, reversing the polarities of the parameters of at least one of said parameter sets, and using said parameters of reversed polarity in applying respective MR pulse sequences to said volume to excite said slice and to saturate said bands so that said band of saturated fat tissue is closer to said slice than said saturated band of water molecules.

In an MR imaging system, wherein a volume within an imaging subject positioned with respect to an axis defined by said system contains water molecules and fat tissue proximate to one another, apparatus for selectively saturating bands of water molecules and fat tissue and exciting a slice of interest in said volume comprising:

• gradient means for generating gradient pulses of variable polarity proximate to said volume, one polarity of said gradient pulses comprising a standard polarity for said system;
• excitation means for applying an RF excitation pulse of a first RF frequency to said volume in association with a first one of said gradient pulses having a first polarity, to saturate water molecules in a first band and fat tissue in a second band, and for applying an RF excitation pulse of a second RF frequency to said volume in association with a second one of said gradient pulses having a second polarity to excite said slice, said first RF frequency comprising the sum of a center frequency and a first offset frequency having a polarity corresponding to said first polarity of said first gradient pulse, said second RF frequency comprising the sum of a center frequency and a second offset frequency having a polarity corresponding to said second polarity of said second gradient pulse;
• and
• means for receiving input data representing one or more specified geometry factors, and for selecting the respective polarities of said first and second gradient pulses, and the polarities of said first and second offset frequencies respectively corresponding thereto, in response to said input data to position said second band of saturated fat tissue within a specified position relative to said slice, the polarity of at least one of said first and second gradient pulses being reversed from said standard polarity.