Chapter 6: Anatomical Variants and Ultrasound Artifacts

INTRODUCTION

Anatomical variants are those variations in normal anatomy that could be misinterpreted as pathological conditions. Many anatomical variants occur as remnants of embryological development and fetal circulation, commonly visualized in the atria. Anatomical variants can be differentiated from artifacts (or errors in interpretation) as they persist despite sonographic changes in transducer frequency, gain, compression, or depth; and are seen in multiple image planes. Ultrasound artifacts usually occur due to a violation of the assumptions inherent to all ultrasound systems. Fundamentally, ultrasound imaging assumes that sound travels in a straight line, travels directly back from a reflector, and travels at exactly 1540 m/s through soft tissue. Additionally, it is assumed that the ultrasound beam is very thin, reflections are entirely from structures within the main axis of the beam, and the intensity of reflections is related only to the tissue characteristics of the reflector.¹ Artifacts can be distinguished from anatomical variants as they tend to cross known anatomical planes and boundaries and usually disappear with alternative imaging planes or sensitivity changes such as changes to the Doppler baseline or the pulse repetition frequency. Thus it is vital to be knowledgeable about the common anatomical variations and ultrasound imaging artifacts to ensure accurate echocardiographic interpretation and to avoid unnecessary interventions.²

THE EMBRYOLOGY OF ANATOMICAL VARIANTS³

In the fourth week of gestation, the atria and the sinus venosus evolve and merge with the embryological heart. Initially, the sinus venosus receives venous blood from left and right sinus horns (Fig. 6-1A and B). Soon thereafter, the veins to the left sinus horn are obliterated and the remnants become the coronary sinus. The right sinus horn enlarges to create the smooth-walled part of the right atrium (RA), which displaces the trabeculated tissue of the primitive RA into the periphery and into the right atrial appendage (RAA), resulting in the prominent pectinate muscles that are characteristic of the atrial appendages. Right and left venous valves mark the junction of the original right sinus horn and the primitive RA. The left venous valve disappears as it fuses with the developing atrial septum. The right venous valve of the right sinus venosus horn develops inferiorly into (1) the valve of the inferior vena cava (IVC), or the eustachian valve, which directs fetal blood flow from the IVC across the foramen ovale; and (2) the valve to the coronary sinus, or the thebesian valve (Fig. 6-2). Superiorly, the convergence of the smooth and trabeculated tissue of the RA results in the crista terminalis. Concurrently, the atrial septum forms with migration of the septum primum toward the endocardial cushion. The septum secundum forms through invagination of the atrial walls and migrates to cover the fenestrations formed in the septum primum. Eventually septum primum and septum secundum fuse, leaving the foramen ovale as the only residual interatrial communication. Incomplete coverage of the septum primum fenestrations leads to ...