Through-slice cardiac motion varying the myocardial slice-profile of 2D bSSFP conventional short-axis cines during the cardiac cycle

Background: In standard 2D cine bSSFP cine imaging, the myocardial signal intensity (mcSI) varies due to through-plane motion (1,2), an effect largely absent in central slices of 3D cines covering the left ventricle (3). However, it is still unclear how exactly this periodic contraction and relaxation, occurring on a time-scale similar to native myocardial T1, affects the 2D cine slice profile. To address this gap we aimed, first, to calculate the cine bSSFP slice-profile of moving myocardium during the heart-cycle, and second, to verify the slice-profile calculations in-vivo, for the first time.

Methods: The conventional bSSFP mcSI during periodic mid-ventricular through-slice motion (as in healthy subjects (4)), was calculated for finely subdivided “subslices” (2,5) within the 8mm slice-thickness (SLT) (FA 60°, TR 3.0ms, native T1 and T2). The transverse magnetization (Mxy) in each subslice enabled derivation of the changing slice-profile during the 2D cine (Fig. 1). To confirm the slice-profile in-vivo, thin-slab 3D mid SAX bSSFP cine images were acquired in a healthy volunteer at 1.5T (Philips, Ingenia): SLT 8mm or 20mm with 7x1.3mm or 10x2mm subslices, TR 3.8 or 4.2ms, TE 1.9 or 2.1ms, FA 60°, 25 phases. For comparison, a standard 2D bSSFP cine was acquired (SLT 8mm, TR 3.3ms, TE 1.7ms, FA 60°, 30 phases). Myocardial contours were obtained (Neosoft SuiteHeart) and mcSI derived by in-house scripts (Matlab R2022).

Results: With myocardial motion, the slice profile varies with cardiac phase as partially recovered myocardium enters one side of the slice (edge-slice myocardial-enhancement (ESME); Fig. 1). Simulations show that, during contraction, the magnetization from myocardium at the basal side dominates the total slice magnitude (Fig. 1 and 2). Similarly, the apical subslices experience ESME during early and late filling of the LV (Fig. 1 and 2). The simulated ESME was confirmed in-vivo (Fig. 3a,c), and is, as predicted (2), reduced for thicker SLT (Fig. 3b,d). The mcSI from a standard 2D bSSFP cine is confirmed as arising from the ESME effects summed over the subslices (Fig. 3e).
The in-vivo thin-slab 3D cines prove that the ESME effects occur in clinically applied SAX 2D bSSFP cines. Contrast-injection or disease-altering T1 may interact with the varying cine slice profile (1,6).

Conclusion: Theoretical calculations with in-vivo confirmation, show that the myocardial brightness variations during the cardiac cycle in conventional 2D bSSFP cines include stark changes in the cine slice-profile as influenced by incoming edge-slice myocardial enhancement.

Figure 1: Illustration of the edge-slice myocardium-enhancement (ESME) effect and simulations of the myocardial signal intensity as it varies during the heart cycle and through-slice: The myocardium in conventional SAX cines (c) is brightened by through-slice motion (yellow arrows (a)). Simulations show that the myocardial SAX cine slice profile (b, red) varies significantly with motion during the cardiac cycle (see also Fig.2B). The simulated “subslices” (d) show that the signal intensity of the SAX cine myocardium (e) is weighted towards incoming myocardium during the cycle. The vertical axes on (d,e) indicate the typical strength of motion-induced variations in mid-SAX cine magnitude.
Figure 2: (a) Simulation results plotted for each subslice, showing the myocardial signal intensity (mcSI) as it varies during the heart cycle resulting from through-plane motion in normal heart function (as in [4]). (b) The simulated mcSI as it varies through-slice, plotted for each cine frame. Dashed lines mark cine frames of minimal mcSI; full thick lines for frames at which mcSI peaks. The dotted black line corresponds to the more saturated myocardial slice profile in absence of motion. The nominally defined slice thickness (SLT) corresponds, for this 8mm SLT simulation, to the seven central subslices (8 to 14); the “out-of-slice” subslices 1 to 7 and 15 to 21 are not shown in (a) for clarity.
Figure 3: In-vivo verification of the ESME effect: (a) the myocardial cine signal intensity (mcSI) of the subslices of a 3D short-axis bSSFP cine with 8mm slice thickness (SLT) with 7x1.3mm subslices, and (b) 20mm SLT with 10x2mm subslices; (c) the ESME effect plotted along the through-slice direction for 8 mm SLT and (d) 20mm SLT. During systolic contraction, the magnetization from myocardium at the basal side (left of plot) contributes more to the slice than do the subslices at the apical side (right of plot). Similarly, the apical-subslices are enhanced by ESME during early- and late-diastole. Dashed lines show cine frames of minimal mcSI; full thick lines for frames at which mcSI peaks. Note also that the in-vivo plots of panel c and d only show the subslices within the prescribed SLT (i.e. corresponding to simulated mcSI in subslices 8-14 in Fig 2b.) (e) The summations over the subslices of the 3D cines show similar 3-peak patterns during the entire cardiac cycle as typically observed for a conventional 2D cine.