Sweep Profile V.0.97 Free Download
Thomas Merino
124 consecutive patients with normal ejection fraction underwent both clinically indicated transthoracic echocardiography (TTE) and CMR within 2 months. Interpolated 3D reconstruction of the MA over time was performed with semi-automated atrioventricular junction (AVJ) tracking in long-axis cine-CMR images, producing an MA sweep volume over the cardiac cycle. CMR-based diastolic function was evaluated, using the following parameters: peak volume sweep rates in early diastole (PSRE) and atrial systole (PSRA), PSRE/PSRA ratio, deceleration time of sweep volume (DTSV), and 50% diastolic sweep volume recovery time (DSVRT50); these were compared with TTE diastolic measurements.
3D MA sweep volumes generated by semi-automated AVJ tracking in routinely acquired CMR images yielded diastolic parameters that were effective in identifying patients with diastolic dysfunction when correlated with TTE-based variables.
Mitral annular (MA) dynamics has been previously studied with various methods [16]-[18]; however, the relationship between CMR MA motion and diastolic function remains largely unexplored. In a prior study, a 2D manual tracking technique in assessing CMR MA motion had been reported [19]. In this paper, we aimed to expand on this prior technique and identify patients with diastolic dysfunction by using 3D MA sweep volumes calculated from routinely acquired long-axis cine-CMR images, comparing the results to TTE.
For each cardiac phase, two AVJ locations were tracked in each of the two-, three-, and four-chamber long-axis CMR views (Figure 2A), creating six independent spatial 3D coordinates within the AVJ that were tracked over the cardiac cycle. Note that 2D image coordinates were transformed into the corresponding 3D space coordinates, using spatial information about the image acquisition locations from DICOM headers. A 3D spline curve was then used to interpolate these 6 distinct 3D spatial coordinates sampled within the MA at each cardiac cycle phase, in order to create a reconstruction of the MA in 3D space (Figure 2B), using solid-modeling software Rhinoceros (McNeel, WA, USA). This is analogous to methods that have been used to reconstruct the 3D shape of the MA from ultrasound images [25]. 3D spline curves have been similarly used to reconstruct the 3D MA structure in prior CMR studies [18],[26]. A 3D MA incremental sweep volume (Vn) was then generated for each cardiac phase tn, using the MA areas at tn-1 and tn, and the 3D distance (positive or negative) through which the MA traversed (Figure 2C, D); the net sweep volume at a given cardiac phase was derived from the sum of the incremental volumes starting from end-diastole.
Interpolated 3D reconstruction of mitral annulus. (A) AVJ points were tracked in two-, three-, and four-chamber cine-CMR views to create six distinct spatial points (highlighted in red) sampled in the mitral annulus (MA) per cardiac phase. (B) 3D spline interpolation was applied to the 3D space locations of these points to create a 3D reconstruction of the mitral annulus (MA). (C, D) A 3D MA sweep volume (V, highlighted in green) was generated for each cardiac phase relative to the MA area at t1 (end diastole), by summing incremental volumes calculated from the MA area at that phase and the distance the MA traversed from the previous phase.
Mitral annulus sweep volume curve and associated diastolic parameters. (A) Representative 3D models of MA sweep volumes at different phases of the cardiac cycle. (B) Representative MA sweep volume (upper) and sweep rate (lower) profiles from a subject with normal diastolic function. Both curves were normalized to end-systolic sweep volume (ESSV). Cardiac intervals were identified based on the transitions in slope of the sweep volume curve, distinguishing systole, early diastole, mid-diastole, and atrial systole (AS). 50% diastolic sweep volume recovery time (DSVRT50) was measured as the time required in diastole for the MA to recover 50% of ESSV, and was adjusted for RR interval. Normalized peak sweep rates in early diastole (PSRE) and atrial systole (PSRA) are shown in the sweep rate curve. Sweep volume acceleration time (ATSV) was measured from onset of early diastole to the time of PSRE, and sweep volume deceleration time (DTSV) was measured by linear extrapolation of PSRE to baseline.
Three time-interval parameters were selected to characterize diastolic function from the sweep volumes: (1) acceleration time (ATSV), measured from ED onset to the time of PSRE; (2) deceleration time (DTSV), measured by linear extrapolation of PSRE to baseline; and (3) 50% diastolic sweep volume recovery (DSVRT50), defined as the time required in diastole for the MA to recover 50% of its end systolic sweep volume (adjusted for RR interval); this index had been introduced in a prior study to describe CMR LV volume filling [12].
Our CMR method successfully generated MA sweep volume curves for all subjects in the study. The approximate processing time for each case was less than 3 minutes. The processing included the initial user delineation of AVJ points, review and possible correction of semi-automated tracking results, MA reconstruction in 3D, and the identification of cardiac intervals within the sweep volume curve. Manual corrections of AVJ locations were primarily necessary in images with blurring artifacts affecting the regions of interest. Approximately 30% of cine image series required at least one manual correction. Cases that did not require manual correction were analyzed in under 1 minute.
This study demonstrates that CMR diastolic parameters derived from 3D MA sweep volumes were reproducible and could accurately differentiate between patients with normal diastolic function and diastolic dysfunction, as established by TDI. CMR-based measurements of peak sweep rates were also strongly correlated with analogous TDI velocity indices. Diastolic assessments using CMR 3D MA sweep volumes were validated in a diverse population of 124 subjects with normal systolic function measures, which revealed an increased prevalence of CMR based diastolic dysfunction in patients with HTN, CAD, and LVH. This suggests that MA sweep volume may contribute to the evaluation of LV diastolic function, potentially providing additional prognostic information and guidance that could be useful in management before frank heart failure occurs [28].
To facilitate the measurement of MA sweep volumes, AVJ tracking was performed semi-automatically, using a simple NCC feature-tracking algorithm. NCC is an image-processing technique that has been used for many applications in motion tracking. NCC provides a means to assess the degree of similarity ("correlation") between two images, as a function of pixel position [23]. Assuming that the immediate surroundings of a given point in an initial image provide the features of a template, the algorithm "slides" and centers this template at each pixel within a neighborhood of this point in a subsequent image, and a correlation coefficient is calculated between the template and the subsequent image for each such pixel position. This process produces a map of the correlation between the point (and its surroundings) in the initial image and the points within that corresponding neighborhood in the subsequent image; the location of the maximum correlation represents the likely location of the initial point (and its surrounding template) within the subsequent image. In this study, NCC enabled semi-automated AVJ tracking that reduced the long post-processing time previously required with manual AVJ tracking [19]. In addition, the use of reproducibility analyses here showed that the use of NCC limited variability in calculated MA sweep volume measurements between users.
MA sweep mechanics were reported here in terms of "percentage sweep volume recovery". This index showed that patients with normal diastolic function recovered nearly 70% of the end-systolic sweep volume by the end of early diastole, or 10.6 mL in absolute volume, but patients with diastolic dysfunction only recovered 54%, or 6.8 mL. These findings were in rough overall agreement with a prior study that reported an average MA excursion volume of 6 mL in nine healthy subjects [17]. However, this prior study involved subjects with a considerably lower average stroke volume (52 mL vs. 93 mL), which may explain the moderate difference in results. Overall, the CMR parameters discussed here were all consistent in demonstrating blunted early-diastolic MA kinetics in patients with abnormal TDI velocities.
We also investigated the time intervals of the MA sweep volume. The results showed that patients with TTE-based diastolic dysfunction had significantly longer DTSV (144 55 ms) compared to the normal diastolic function group (96 37 ms). ATSV was slightly longer in the diastolic dysfunction group, but the difference was not significant. Both findings were consistent with a published TDI study that examined acceleration and deceleration times pertaining to MA velocities (normal DT = 84 ms, diastolic dysfunction DT = 156 to 168 ms) [31].
DSVRT50 was an additional CMR diastolic function parameter, which measured the time needed for the MA to recover 50% of its end systolic sweep volume, adjusted for R-R interval. On average, longer DSVRT50 times were observed in patients with TTE-based diastolic dysfunction. This parameter was analogous to an index used in a prior study named "diastolic volume recovery" [12], which accounted for both heart rate and volume status when assessing diastolic dysfunction in CMR LV volumetric filling. Lastly, it is important to note that PSRE, PSRE/PSRA, and DSVRT50 each independently predicted TTE-based diastolic dysfunction after controlling for age, LVH, HTN, and CAD.
The CMR-derived MA sweep volume was found here to effectively characterize global diastolic function. Prior echocardiography studies have demonstrated that MA excursion volume plays an important role in LV function [17],[38]. In addition, reports have long recognized the relationship between LV long-axis function and MA displacement [39]. For example, the longitudinal MA excursion in diastole envelops and effectively transfers blood from the atrium to the ventricle, separately from the flow of blood across the location of the MA. This can occur even while the blood remains relatively stationary in relation to the apex. Similarly, in atrial systole, the MA is pulled away from the apex by the atrial pectinate muscles, "over" the blood, to facilitate further ventricular filling (and also increasing the ventricular pre-stretch, thus augmenting contractility). As a result, MA excursion is a central component of diastole, along with the transmitral pressure gradient.
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