P1120992

1.2 Ovarian structures in the marę

1.2.1    Follicles

1.2.1.1    Sonographlc images of follicles

The ultrasonic image of follicles exhibits features characteristic for fluid filled vesicles.

Some of the components of the image can be related to the presence of real morphological structures of the follide (Kahn and Lejdl 1987 b), while some ultrasonic image pattems typical for follicles are induced by phys-ical phenomena when uitrasound waves impact onto vesides and their fluid contents. They are thus seen as principal features which are referred to as artifacts. These image components which result from the inter-action between uitrasound waves and fluid filled ves-ides freąuentfy do not reflect the presence of actual tissue components and should be viewed as artificial products. An understanding of their origin helps to avoid misinterpretations and permits proper conclu-sions about the naturę of a vesicle.

When uitrasound waves impact vertically onto a smooth reflection surface, some waves are reflected back to the transducer and are depicted as an intense echo on the screen (Fig 1.8). The echogenicity is strong-est where the uitrasound waves fali perpendicularly onto the front and back walis of the vesicle (Fig 1.9) which result in specular reflections. In the area where the uitrasound waves fali obliąuely onto the follicle wali a smaller amount of the energy is reflected, the remain-der being deflected away from the transducer and not received. At these sites the follicular wali is less echoic and thus darker. If the sound waves were to fali ob-licjueły onto a perfectly level border surface they would all be dcflectcd and no signals would be rcceived (Hassler 1984). Bordering layers in the body are, how-ever, irregular and a varying number of sound waves are always reflected back to the transducer. Where the sound waves hit the follicular walls at a tangent, they are largely deflected (Rg 1.8 and 1.9) with hardly any echo signals reaching the probe from such sites. Veiy few sound waves continue to progress in a straight linę from these sites into deeper tissues. The vast bulk of the uitrasound energy is deflected lateraUy from here. The result is that narrow, sometimes slightly widening, echo shadows form below such areas of tangential sound wave impact.

Amplified echoes or brighter images are seen behind larger fluid filled vesicles (Rg 1.8 and 1.9). Uitrasound is much less attenuated by fluid than by other body tissues. When sound waves penetrate larger fluid bodies they reach the deeper lying tissues with morę energy and cast a much morę intense echo than neighboring waves that followed a longer path through layers of tis-sue with morę wave absorbency. Also on its path back towards the transducer the reflected uitrasound penet-rating through fluid looses less energy and is absoibed to a lesser extent. This creates the impression of an echogenicity behind a fluid filled vesicle, the so called enhancement artifact. The width of the enhancement area is determined by the diameter of the fluid filled body. LateraUy, the field of echo enhancement is usually demarcated by the narrow ultrasonic shadows which originate from where the sound waves impact tangen-tially onto the lateral walls of the fluid body.