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Background
The sonographic appearance of the endometrium and endometrial cavity (EC) vary with the age of the patient, phase of the menstrual cycle, coexistent uterine pathology, and sonographic technique. The EC is normally a thin, hyperechoic line, positioned centrally within the endometrial echo. Identification of the endometrial cavity and its relationship to endometrial pathology or intrauterine fluid collections (in the case of early pregnancy) can be useful in achieving an accurate and early noninvasive diagnosis.

Educational Goals / Teaching Points
Demonstrate the importance of identifying the EC in distinguishing between early intrauterine gestational sacs and fluid collections within the EC. Show how displacement of the EC aids in identifying and characterizing focal endometrial masses and discriminating them from processes resulting in diffuse endometrial thickening. Describe sonographic techniques useful in identifying the EC.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Early sonographic demonstration of an intrauterine gestational sac (IUP) markedly lowers the probability of an ectopic gestation and helps to reassure the patient. An intrauterine gestational sac implants within the decidual reaction of the endometrium and is separated from the EC by an overlying layer of decidua (decidua capsularis). Demonstrating the EC as separate and distinct from the cystic area is predictive of an IUP. Pseudo sacs are nonspecific fluid collections within the uterine cavity, and are thus continuous with the sonographic EC. A gestation sac is usually round in all planes, whereas a pseudo sac is elongated or tear-drop shaped and may be inconsistent in shape or location. Overlap in the sonographic appearances of these entities is common, highlighting the importance of evaluating the EC. Descriptions of sonographic endometrial patterns has been described and standardized. Displacement of the EC, focal discontinuity or splitting of the EC (“bright edge”) are clues indicating focal endometrial masses, most commonly polyps. A thickened endometrial echo without EC displacement is characteristic of diffuse endometrial processes, such as hyperplasia or malignancy. Recognition of the EC position improves identification and characterization of endometrial pathology and provides earlier access to appropriate management. The EC is most conspicuous when the ultrasound beam reflects the EC at a perpendicular angle. When transvaginal sonographic fails to achieve optimal results, supplementation with transabdominal ultrasound using curved and linear array transducers can be highly effective in demonstrating the EC. The transabdominal approach is especially helpful in patients with enlarged or elongated uteri or in patients unable to undergo transvaginal sonography. The inclusion of video clips is essential as a retrospective aid in identifying the EC.

Conclusion
Sonographic identification and assessment of the EC enhances earlier recognition of intrauterine pregnancies and improves the diagnostic accuracy and detection of endometrial pathology.