Relaxation effects in the quantification of fat using gradient echo imaging

Abstract

Quantification of fat has been investigated using images acquired from multiple gradient echoes. The evolution of the signal with echo time and flip angle was measured in phantoms of known fat and water composition and in 21 research subjects with fatty liver. Data were compared to different models of the signal equation, in which each model makes different assumptions about the T1 and/or T2* relaxation effects. A range of T1, T2*, fat fraction and number of echoes was investigated to cover situations of relevance to clinical imaging. Results indicate that quantification is most accurate at low flip angles (to minimize T1 effects) with a small number of echoes (to minimize spectral broadening effects). At short echo times, the spectral broadening effects manifest as a short apparent T2 for the fat component.

Figure 1

MR spectra of Intralipid (solid line) and fatty liver (dashed line) taken with TR 1500 ms and TE 20 ms. The water peak (near 4.8 ppm) in Intralipid is shifted relative to the fat peak (near 1.2 ppm) due to the temperature difference of approximately 20°C between the phantom and the in vivo samples.

Figure 2

Panel A shows a plot of data acquired from Intralipid over the TE range 2.3 to 11.5 ms. Best-fit curves to two models are shown, as described in the text: a broad fat peak containing two components (solid line) and a composite peak containing five components (dashed line). Panel B shows the same data set over a longer range of TE and best-fit curves to the two models. Panel C shows the 1%, 5%, 10% and 20% fat fractions estimated at different flip angles; overlaid are best-fit curves to the expected dependence due to T1 effects. Panel D shows the amplification factor A₁ (defined in Eq 9) and the best-fit curve to the data points.

Figure 3

Panels (A) to (F) show fat fraction maps estimated using models (i) to (vi). Columns were doped with gadolinium (left), no agent (middle) and ferumoxides (right). The known fat fractions are 20.0, 14.9, 9.8, 4.7, 2.5, 1.1 and 0 %. The images are windowed identically between 0 and 30% therefore the rows should appear iso-intense.

Figure 4

The median fat fractions in the center of each vial from Figure 3 are plotted against the known fat fraction for the three doping conditions, seven dilutions and six models. Panel (A) shows the fat fractions in Gd-doped vials. Panel (B) shows the fat fractions in undoped vials. Panel (C) shows the fat fraction in Fe-doped vials. Relaxation values from the different models are given in Table III.

Figure 5

Error (calculated by Eq 12) for the six models as a function of the number of data points (echos) used in the curve-fitting. Echos were added by including a later echo time acquisition therefore the Error is largely a measure of the breakdown in the assumption of two components at long echo times (e.g. see Figure 2B). The plots in Figure 4 correspond to the use of five echos in this plot.

Figure 6

In vivo fat fraction and T2* maps from a research subject who was scanned at five minute intervals during the infusion of a T2* contrast agent. Panels (A) and (B) show fat fraction maps obtained by model (i) with two echos (i.e. 2-point Dixon) at the start and end of the infusion (corresponding with the minimum and maximum contrast agent effect). There is a noticeable drop in the fat fraction (17.4 to 13.9 %). Panels (C) and (D) show the same data processed using model (iv), in which the fat fraction is relatively stable (24.6 to 25.7 %). Panels (E) and (F) show the T2* maps determined from model (iv) for the same data.

Figure 7

Plots of the signal intensity in the liver (taken from the ROI from Figure 6) at different times during the infusion of T2* contrast agent. Best-fit curves to model (iv) are shown and the fitted values are given in Table IV.

Figure 8

Fat fractions determined by models (iv) and (v) using GRE imaging (circle and square markers, respectively) plotted against the fat fraction determined by spectroscopy using the STEAM sequence. Panel A shows the results using a flip angle of 10° and panel B shows the results using a flip angle of 90°. Assuming spectroscopy represents the true fat fraction, the solid lines show expected trends from Eq 9 with TR 122 ms, T1(water) = 490 ms and T1(fat) = 260 ms (24). Note the non-linearity due to T1-weighting is more evident in the high flip angle data.