Abstract : Left ventricular hypertrophy (LVH) is a crucial prognostic indicator in the course of cardiovascular pathologies [1]. The main objective of this study is to compare the diagnostic agreement between Transthoracic Echocardiography (ECHO), which is widely used in clinical practice, and Cardiac Magnetic Resonance (CMR) tomography, considered the gold standard for visualization, in measuring left ventricular mass (LVM) and left ventricular mass index (LVMI), as well as to mathematically and clinically evaluate the impact of geometric deformations (dilatation and asymmetry) on the bias and error rates between these two methods [2], [3]. Data from 94 patients who underwent both ECHO and CMR examinations at our clinic with an initial diagnosis or suspicion of LVH were retrospectively analyzed. In the ECHO examination, LVM was calculated using the Devereux formula based on M-mode parameters (IVSd, PWd, LVIDd) [4]. To preserve the reliability of patients with incomplete height and weight data in the archive, reverse mathematical modeling was applied using the absolute mass and mass index parameters from the CMR report to calculate the individual Body Surface Area (BSA), and ECHO parameters were indexed according to this area. Inter-method differences were evaluated using the T-test, Pearson correlation, and Bland-Altman analysis [5]. Of the patients in the study group, 84% were male (n = 79) and 16% were female (n = 15); the mean age was determined as 55.4 ± 13.5 years. The mean LVM calculated by ECHO (242.4 ± 95.1 g) was found to be statistically significantly higher than the mean LVM measured by CMR (224.2 ± 47.9 g) (p < 0.05). The net difference between the two methods was +18.2 g (overestimation). Subgroup analyses revealed that in cases of eccentric hypertrophy with enlarged left ventricular internal diameter (LVIDd > 65 mm) and in patients with asymmetrically enlarged septum (septum/posterior wall > 1.3), the mathematical deviation rate of ECHO increased sharply, overestimating the actual mass by more than 30% in some cases [6]. Although the ECHO method is highly accessible for initial screening and mass examination of LVH, its reliance on geometric assumptions leads to significant diagnostic inaccuracies, especially in asymmetric and dilated hearts [7]. For precise selection of treatment strategy, particularly in assessing the risk of sudden cardiac death, CMR examination must be applied as an absolute clinical reference in the management of patients with structural deformation [8].