Background: Evaluation of left ventricular non-compaction (LVNC) is an increasingly common indication for cardiac magnetic resonance imaging (MRI). Fractal dimension (FD) is a unitless measure of geometrical complexity which can be used to quantify LV trabeculation. FD is increased in LVNC, but there have been few studies on FD in normal subjects. The aim of the study was to establish reference ranges for FD in a healthy population, and identify covariates which are associated with FD.

Methods: MRI was performed in 1,913 volunteers without hypertension, diabetes, or heart disease (1055 female, 858 male; median age 40, range 19-82). FD was derived from LV short-axis images, using a custom MATLAB box-counting algorithm. The maximal FD in the apical half of the LV was used for all analyses, as previously described.

Results: Normal ranges (2.5-97.5th percentile) for female and male subjects were 1.154 - 1.367 and 1.179 - 1.392, respectively. FD was significantly correlated with age, gender, ethnicity, body surface area (BSA), activity score, and systolic blood pressure. In multivariable analysis, FD was independently correlated with increased age (β 0.11, p<0.001), male gender (β 0.09, p<0.001), African/Afro-Caribbean ethnicity (β 0.18, p<0.001), increased BSA (β 0.27, p<0.001), and increased activity score (β 0.07, p=0.002). Since ethnicity was found to significantly affect FD, normal ranges were calculated for each subgroup (see table).

Conclusions: This is the largest study on FD in healthy subjects, and the first to present gender- and race-specific normal ranges. The association between FD and age suggests that LV trabeculation is a dynamic phenotype which may change with age.