Xin Liu, Cole Streiff, Meihua Zhu, Muhammad Ashraf, David J. Sahn METHODS Ten freshly harvested rabbit hearts were studies. A small latex balloon was mounted on vinyl tubing and fixed within each LV cavity. The proximal end of the tube was attached to a pulsatile pump apparatus. The pump was calibrated to deliver stroke volumes (SV) of 2 ml and 4 ml at stroke rates (SR) of 60 and 120 beats per minute (bpm). Fetal myocardial hypertrophy was simulated by fixing different amounts of myocardial tissue to the LV epicardium. Four D image loops of pulsatile motion were acquired on Philips IU 22 Ultrasound system with new Matrix Array X6-1 probe. Each of the ten hearts was passively pumped by a pulsatile pump interfaced with a small balloon fixed within the LV. The hearts were mounted in a water bath and imaged by a matrix array transducer. Reference values for LV mass were determined by immersing rabbit heart in a water filled graduated cylinder, and measuring displacement of fluid level. Image data was analyzed offline in Q-Lab. The operator defined the endocardial and epicardial apical boundaries of the basal myocardium. Manual traces of the LV endocardium and epicardium at ten different short axis views were used to create a model of the myocardial volume. The LV mass was determined by the difference between the epicardial volume and the endocardial volume multiplied by the estimated density of the fetal myocardium (1.050 g/cm³). RESULTS Linear regression analysis showed good correlation between the 4D echo-derived heart masses and r-masses (R2= 0.8870; Figure 2). Echo-derived measurements of LV mass also showed good correlation with reference values at all SRs and SVs. At 2ml & 60bpm: r = 0.95, at 2ml & 120bpm: r = 0.95; at 4ml & 60bpm: r = 0.93, and at 4ml & 120bpm: r = 0.95, with P<0.01 for all values.There was excellent interobserver and intraobserver agreement regarding the 4D data to measure mass (r = 0.983, a mean difference of -0.32g,-3.85% of the mean; r = 0.984, a mean difference of -0.28g, -3.38% of the mean; Figure 3). ABSTRACT Background: This study tested the feasibility of new 4D fetal echocardiography to evaluate LV mass in an experimental model. Methods: Ten fresh rabbit hearts were studied. Fetal myocardial hypertrophy was simulated by fixing different amounts of myocardial tissue to the LV epicardium. A small latex balloon was mounted on vinyl tubing and fixed within each LV cavity. The proximal end of the tube was attached to a pulsatile pump apparatus. The pump was calibrated to deliver stroke volumes (SV) of 2 ml and 4 ml at stroke rates (SR) of 60 and 120 beats per minute (bpm). 4D data was acquired and analyzed in QLab. Reference values for LV mass were determined by displacement method. Results: Linear regression analysis showed good correlation between the 4D echo-derived heart masses and r-masses(R2= 0.8870),Echo-derived measurements of LV mass aslo showed good correlation with reference values at all SRs and SVs. At 2ml & 60bpm: R2 = 0.9051, at 2ml & 120bpm: R2 = 0.8980; at 4ml & 60bpm: R2 = 0.8609, and at 4ml & 120bpm: R2 = 0.8982. There was also excellent interobserver (r = 0.98, a mean difference of -0.32g,-4.4% of the mean) and intraobserver (r = 0.98, a mean difference of -0.28g, -3.8% of the mean) agreement. Conclusions: In this controlled phantom study, high resolution 4D echocardiography is suggested feasible for evaluation of fetal myocardial hypertrophy. CONCLUSIONS Our rabbit model study suggests that 4D echocardiography could be an accurate and reproducible modality for assessing myocardial hypertrophy and fetal left ventricular mass. DISCLOSURES No relationships to disclose: Xin Liu Cole Streiff Meihua Zhu Muhammad Ashraf David J. Sahn Evaluation of Fetal Myocardial Hypertrophy by 4D Echocardiography: An in Vitro Study Xin Liu, Cole Streiff, Meihua Zhu, Muhammad Ashraf, David J. Sahn Oregon Health & Science University, Portland, OR  BACKGROUND Since the fetal heart is in a proliferative state of development, it appears to respond rapidly with physiological adaptive mechanisms. Many fetuses, especially those of diabetic mothers, are at high risk to develop myocardial hypertrophy. Accurate monitoring of fetal hypertrophy caused by maternal diabetes may guide treatment and influence the prognosis of fetuses at risk. Image based assessment of fetal ventricular volume, growth and functional maturation has largely been based on M-mode or 2-dimensional (2D) echo. M-mode echocardiography is highly dependent on the incident ultrasound beam and geometric assumptions about the shape of the ventricles. LV mass measured by 2D echocardiography using area-length calculation methods also have inherent limitation assuming that the LV has a prolate ellipsoid shape. It has limited accuracy and repeatability. A series of new techniques, such as spatiotemporal image correlation (STIC) and virtual organ computer-aided analysis (VOCAL), have been used to evaluate the fetal ventricular mass. However, these are indirect motion-gated offline scanning modes and may cause stitching artifacts due to misregistration from fetal movement and sudden change in fetal heart rate. Real time 3-dimensional (RT3D) or 4-dimensional (4D) echocardiography may allow more accurate and reproducible measurements of LV mass without geometric assumptions. This study aimed to evaluate the capability of 4D echo to assess cardiac mass in a hypertrophic fetal heart model at different stroke volumes (SV) and stroke rates (SR). Bland-Altman plot Four-D Echo derived computation of LV mass