The MP-IDEAL reconstruction described above assumes that the fat spectrum is known a priori. One option to provide the required information is to assume that the frequencies and amplitudes of the fat peaks can be measured once and considered to be constant for a defined set of conditions (range of body parts, pulse sequence type and parameters). The fat spectrum can be measured by MR spectroscopy. The frequencies of the fat peaks (fp) can be accurately determined from this spectrum. The normalized areas of the peaks represent the relative amplitudes in Eq. 1 (αp), normally measured with spectroscopy analysis softwares (e.g. jMRUI (17 (link))). However, in our studies, it was found that some spectral peaks are close to each other and possess broad line widths, making it difficult to differentiate them.
Given the knowledge of the frequencies of the fat side peaks as determined by spectroscopy, another method to determine the relative amplitudes a priori is to use an over-sampled multi-echo acquisition and the multi-species IDEAL reconstruction. In practice, 16 echoes can be acquired using a multi-echo sequence to collect the echoes as rapidly as possible in a single repetition. It has been demonstrated previously that the IDEAL algorithm can be extended to fit for multiple chemical species (22 (link)). By treating each of the fat peaks as an independent chemical species with known frequency locations (fp), a 16-pt IDEAL reconstruction can fit for the 6 fat peaks plus the water peak independently, providing separate images for each fat peak as well as an image of the water peak. An ROI is then drawn in the fat area and the pixel intensities in the ROI are averaged to form the final, calibrated relative amplitude αp at each peak frequency.Figure 2b shows an example of using sixteen echoes to fit for six fat frequencies independently. The re-synthesized signals using the calibrated αp are plotted in solid green, which demonstrates an excellent fit to the acquired data. This spectrum pre-calibration needs to be performed only once and the calibrated spectrum can be used in all MP-IDEAL reconstructions that fit the predefined criteria for that pre-calibration.
Given the knowledge of the frequencies of the fat side peaks as determined by spectroscopy, another method to determine the relative amplitudes a priori is to use an over-sampled multi-echo acquisition and the multi-species IDEAL reconstruction. In practice, 16 echoes can be acquired using a multi-echo sequence to collect the echoes as rapidly as possible in a single repetition. It has been demonstrated previously that the IDEAL algorithm can be extended to fit for multiple chemical species (22 (link)). By treating each of the fat peaks as an independent chemical species with known frequency locations (fp), a 16-pt IDEAL reconstruction can fit for the 6 fat peaks plus the water peak independently, providing separate images for each fat peak as well as an image of the water peak. An ROI is then drawn in the fat area and the pixel intensities in the ROI are averaged to form the final, calibrated relative amplitude αp at each peak frequency.
