Myocardial Local Frequency Shift Mapping

Junmin Liu^{1}, James W Goldfarb^{2}, and Maria Drangova^{1,3}

* Data
acquisition:* Data were acquired at 1.5 T with a dark blood
double inversion recovery gradient echo sequence (1
slice per breath-hold, repetition time 20 ms; 12 echo times, 2.4 – 15.5 ms (1.2
ms spacing); flip angle 20°; bandwidth 1860 Hz/pixel; in-plane resolution 2.3 ×1.7 mm

** The B0CS-LFS
approach** comprises three steps. First, a field (Δf

The second step is to remove the unwanted constant, B0, and CS phase components. To remove the constant phase term, the Hermitian products between echoes were calculated as follows:

$I_{j,hp} =I_{j}×I_{2}*, even, $ [1a]

$I_{j,hp} =I_{j}×I_{3}*, odd, $ [1b]

where I_{j}
is the complex image at the jth echo and * denotes the complex
conjugate. The LFS at each individual
echo (f_{j}) were estimated by removing the B0- and CS-related
phase terms:

$f_{j }=angle(I_{j,hp}×exp(-φ_{b0,j})×exp(-φ_{CS,j}))/(2π×ΔTE_{j}) $ [2]

where φ_{b0,j} is equal to 2π×Δf_{b0}×ΔTE_{j}; φ_{CS,j} is
calculated using the six-peak fat model and FF map determined from step 1; ΔTE_{j} is equal to (TE_{j} – TE_{2})
and (TE_{j} – TE_{3}) for the even and odd echoes,
respectively.

The third step is to calculate the final LFS map, defined as the mean over all included echoes on a pixel-by-pixel basis.

While no reference standard exists for myocardial LFS mapping, we compared the B0CS-LFS results to a high-pass filtered approach (HPF-LFS) adapted from the cardiac SWI method described in (5), where filtered phase was scaled to frequency and averaged over the last nine echoes.

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[7]. Liu J, Drangova M. Method for B0 off-resonance mapping by non-iterative correction of phase-errors (B0-NICE). Magn Reson Med 2015;74(4):1177-1188.

B0-NICE results in the VLA plane: (a), (b) and
(c) are the T2*, even-echo B0 and even-echo FF maps, respectively. The ellipse
in (a) indicates an acute hemorhagic infarct region. The odd-echo results (not
shown) are similar.

The LFS map: (a) generated using the HPF-LFS
method (64-point Hanning); (b) and (c) are obtained using the B0CS-LFS method with
the Hanning and 17×17 moving-average
filters, respectively. (b) and (c) are very similar. The ellipse is the same as
in the T2* image in Figure 1.

Magnitude (a), HPF-LFS (b) and B0CS-LFS (c); the
line profiles along the line-segment in (a) are plotted in (d). An epi- to
endo-myocardium gradient is observed in both LFS maps; the B0CS method facilitates
delineation of the tissue boundary (myocardium shaded in grey).

The LFS maps in the SA ((a) and (b)) and HLA
((c) and (d)) planes by using the HPF-LFS and the B0CS-LFS methods.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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