Female Pelvis - Trans abdominal view (1)

Image #1- A transverse view of the female pelvic organs through a full urinary bladder.

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Observations

In this image we are using a full urinary bladder as a sonographic window for visualizing the uterus (in the center of the image) and both ovaries(which are located on either side of the uterine fundus). We recall that sonography of the abdomen requires no special sonographic windows since most of the organs are close to the abdominal wall without intervening bowel loops. However, evaluation of the female pelvis is performed either transabdominally through a full urinary bladder (which functions to displace the overlying small bowel loops which would otherwise impede the ultrasound beam) or transvaginally. In the latter instance a probe is introduced into the vagina with its tip in a vaginal fornix. The transducer's proximity to the pelvic organs avoids intervening bowel loops and also allows us to use a higher frequency probe (often 5 or 7 megaHertz), which in turn improves image resolution.

In this transverse image the understood conventions are the same as for the images of the abdomen, namely, that the patient's right side is to the left of the image, and we are looking at the patient as though they were lying supine on a table with the observer positioned towards the patient's feet. Note that in a transverse projection the contour of the bladder is symmetric with relatively straight lateral borders. A second clue that we are in a transverse projection is the shape of the uterus, which is the ovoid structure directly posterior to the center of the bladder. The uterus has relatively high level echoes in its wall and a central thin echogenic line representing the endometrium, which we call the endometrial stripe. We will contrast this appearance of the uterus to its shape in the longitudinal projection in the next image. The ovaries are the ovoid, slightly heterogeneous structures located to either side of the uterine fundus. Note that we cannot rely upon ovarian shape as an indicator of the imaging plane (transverse versus longitudinal) since they have a similar appearance in both planes. We can recognize that they are ovaries by two characteristics: (1) sonographic echotexture, and (2) position. The normal echotexture is heterogeneous since the ovary is composed of stromal tissue (solid) and developing follicles (tiny cysts). Small ovarian follicles are difficult to resolve as individual structures in this image due to the relatively low frequency of the transducer (3.5 MegaHertz) and the far distance of the ovary from the ultrasound transducer (approximately 8 cm). (Both of these factors can be eliminated through the use of a transvaginal probe as we will subsequently see). A small follicle, however, is just barely visible along the extreme lateral aspect of the left ovary. The second feature that helps us identify these structures as ovaries is their para-midline location, on either side of the uterine fundus. In this particular patient both ovaries are present at the same level in relationship to the uterine fundus, which is not the usual case In most instances only one ovary can be identified in any one sonographic imaging plane, but each is still generally positioned lateral to the midline.

Returning to a consideration of the uterus, note that its sonographic echotexture is quite regular, in contradistinction to the appearance of the ovary. There is an outer, thicker, uniformly echogenic layer of myometrium; a thinner hypoechoic inner compact layer of myometrium, and finally the central endometrial stripe. The appearance of the endometrial stripe varies depending on the phase of the menstrual cycle. In this instance, thethin linear echogenic endometrial stripe (which actually represents the two apposed layers of endometrium compressed against each other), indicates that the patient is in the early proliferative phase of the cycle.

The hyperechoic areas surrounding the uterus represent a combination of collapsed bowel and/or mesenteric fat. The intensely hyperechoic focus interposed between the lateral aspect of the uterus and the left ovary which demonstrates a posterior acoustic shadow is gas within a bowel loop. Finally the alternating linear echogenic structures along the top aspect of the bladder are ultrasound artifacts which are spurious echoes produced by the ultrasound transducer at a region of a strong acoustic impedance mismatch (namely between the rectus muscle and the top of the urinary bladder) Much of the correct interpretation of ultrasound images relies upon recognition of such artifacts.

Female Pelvis - Trans abdominal view (2)

Image #2- Female pelvis - Transabdominal view

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Observations

This longitudinal (sagittal) midline image of the uterus (same patient as Image 1) is obtained through a full urinary bladder. The patient's head (superior, or cephalad) is towards the left and the feet (inferior, or caudal) are towards the right. The uterus is comprised of an uppermost portion called the fundus, a mid-to-lower portion called the corpus, and a lowermost portion, the uterine cervix. The demarcation between these three regions is gradual and not strictly defined sonographically. However, the approximate junction between cervix and corpus can be inferred by a slight concavity best seen along the posterior aspect of the lower uterus. The sonographic key to the recognition of the uterus is the identification of the endometrial stripe, which represents two closely apposed layers of endometrium. In the presence of endometrial fluid (some examples: in early pregnancy, at time of menses, or with endometritis), these layers can be separately identifed, since fluid is anechoic. Themyometrium comprises the bulk of the uterus and consists of a thin inner hypoechoic compact layer and an outer more echogenic zone. Thevagina is the thin hypoechoic band extending obliquely from the lower cervix (external os), just posterior to the back wall of the bladder. The angle that the uterus makes with the vagina in this particular case is greater than ninety degrees (90°) and hence this uterine position is retroverted. Note also that here the uterine fundus is in the same plane as the corpus. Occasionally, the fundus may be separately tipped, either posteriorly (retroflexion) or anteriorly (anteflexion). Note the size relationship between the cervix and fundus+corpus: the combined length of fundus and corpus is roughly twice the length of the cervix. This is the typical relationship for a menstruating patient. In premenarchal girls and postmenopausal women the fundus and corpus shrink, so that their combined length is approximately the same as that of the cervix. Posterior and superior to the uterine fundus is a highly echogenic interface with absence of echoes posterior to it. This represents the sacrum and, similar to the vertebral bodies that we encountered in the examination of the upper abdomen, they allow no sound wave transmission. Directly posterior to the uterus is a cluster of high level echoes likely reflecting collapsed bowel and/or mesenteric fat. The junction of the vagina with the posterior lip of the cervix denotes a potential space called the posterior vaginal fornix. During transvaginal ultrasound the tip of the ultrasound transducer typically is placed into this space.

Uterus (sagittal)

Image #3 Longitudinal View of the Uterus.

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Observations

This transabdominal sagittal image of the uterus (in a different patient) was obtained with a 5 megaHertz (MHz) transducer. The use of this higher frequency transducer (images 1 and 2 utilize a 3 MHz transducer) coupled with a shorter distance between the ultrasound transducer and the uterus (due to a less filled urinary bladder) both account for the improved clarity of the image of the uterus compared to the prior examples. The size of the uterus in sagittal and A/P planes has been measured (here being 9.7 x 4 cm). The size of the uterus in a menstruating female is variable and tends to increase with increasing parity. But more important for determining normalcy than size is the evaluation of the echotexture of the myometrium and endometrium. As shown here, the myometrium should be uniformly echogenic. Any focal hypoechoic area would indicate the presence of a fibroid (leiomyoma). In this particular example the endometrial stripe is nearly invisible (indicating that the patient has just undergone menstruation). Note that the stroma of the cervix is slightly more echogenic than the myometrium of the uterine corpus and fundus. This is likely artefactual, due to enhanced through transmission of sound waves behind the deepest portion of the bladder.

In addition to the elongated contour of the uterus, a second clue that we are imaging the pelvis in a longitudinal plane is the appearance of the bladder. Unlike the prior transverse image (Image 1) the bladder here has a more elongated pear-shape, being narrow at the top and wider near the base (or trigone area) of the bladder. The vagina is not included on this image.

Uterus (transverse)

Image #4 Transverse Uterus (click here to restore original image).

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Observations

This image is obtained through the upper portion of the uterus (same patient as Image 3). The transverse imaging plane can be inferred by the squared-off or trapezoidal shape of the bladder as well as the oval shape of the uterine corpus. As noted previously, the endometrial stripe is not visible, indicating that this patient has just undergone menstruation. During the first half of the menstrual cycle (at which time estrogen secretion by the developing ovarian follicles is steadily increasing) there is a gradual change in the appearance of the endometrium. What appears first is a thin echogenic line which gradually thickens to form a multilayered configuration. This reflects growth of the endometrial glands under the influence of estrogen synthesis by the ovarian follicles. This multilayered or proliferative (transvaginal, example 1, transvaginal, example 2) endometrium is preparing for possible implantation by a fertilized egg. Following mid-cycle ovulation, in the second half of the menstrual cycle, the endometrium is under the influence of progesterone secretion by the corpus luteum. The endometrial glands fill up with mucus, changing the endometrium into a thick echogenic configuration (secretory phase) (example 3). The endometrium usually obtains it thickest dimensions at this point (< 14mm). As luteinizing hormone levels decrease, the level of progesterone secretion by the corpus luteum falls, signaling the onset of menstruation. Thus, the sonographic appearance of the endometrium (invisible, thin echogenic, multi-layered, thick echogenic) is frequently an excellent indicator of the phase of the menstrual cycle. Additionally, sonography is a useful tool for the evaluation of dysfunctional uterine bleeding. An abnormally thickened endometrium may indicated hyperplasia, polyps, or endometrial carcinoma, while a normal appearing but displaced endometrial stipe would indicate a submucosal fibroid.

Following menopause the endometrium gradually atrophies and should be no thicker than 4 mm in A/P dimension, making ultrasound a useful tool for the evaluation of post-menopausal bleeding. Since the commonest cause of post-menopausal bleeding is endometrial atrophy, the sonogram in such cases should show a barely perceptible endometrial stripe, no thicker than 4 mm. If a thickened endometrial stripe is seen, further investigation (including aspiration and biopsy) should be performed.

On this transverse image note the echogenic interface where the sound waves are reflected off the sacrum leading to an acoustic shadow behind. By contrast, the two hyperechoic areas (one directly posterior to the uterus and the other to the left side of the uterine fundus) show multiple reverberation artifacts due to bowel gas within the sigmoid colon.

Left Ovary (transverse)

Image #5 Transverse View of the Left Ovary.

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Observations

This transverse view of the left ovary in a different patient is obtained through a full urinary bladder. Towards the left side of the image (ie toward the patient's right), a portion of the uterine fundus can be seen. Note the normal sonographic texture of the ovary, consisting of multiple anechoic folliclesalternating with echogenic ovarian stromal tissue. The size of the normal ovary is less variable than that of the uterus, but to account for variations in ovarian shape, we can measure ovarian volume (which is obtained by multiplying the length, width, and heighth of the ovary times one-half). It should normally be less than or equal to 14 cc. Two such dimensions can be obtained in any one single image. (In this image we can obtain an A/P andtransverse dimension and the third or longitudinal dimension would require a different plane of section - see next image for illustration). Selected examples of abnormalities that would cause an increase in ovarian size include: polycystic ovaries, ovarian hyperstimulation syndrome (such as would occur following assisted reproduction techniques utilizing Clomid or Pergonal), ovarian torsion, or a focal mass (such as a dermoid tumor) or cyst (hemorrhagic cyst or endometrioma as two common examples).

Note the normal appearance of the small follicles in this ovary. They typically are less than 10 mm with the exception of the follicle seen in the lower right portion of the ovary. This is the dominant follicle for this particular menstrual cycle, and indicates the follicle that will likely undergo subsequent ovulation and formation of the corpus luteum in the second half of the cycle. This dominant follicle (currently measuring over 12mm) will subsequently reach its maximal size as it forms the corpus luteum, typically 20-25 mm. Hence the presence of an ovarian cyst on a pelvic ultrasound should not be misconstrued as pathologic, since a corpus luteum or dominant follicle will be found in most menstruating patients.

Since the ovary is an ovoid structure, its appearance is similar in transverse and sagittal planes and hence we need to rely upon analysis of the bladder shape (or imaging labels, usually found in the upper left hand corner of the image) for determination of the imaging plane.

Left Ovary (sagittal)

Image #6 Longitudinal View of the Left Ovary.

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Observations

In this longitudinal view (same patient as image 5) note the pear-shaped bladder (similar to the shape in longitudinal image 3). Again note the appearance of the ovary with the peripheral small follicles and the central more echogenic stroma. The dominant follicle is not in this plane or section. Follicles are generally seen in most premenopausal patients undergoing ultrasound evaluation. If no follicles are identified in a pre-menopausal patient, this may represent primary pituitary or ovarian failure, or it may reflect the fact that the patient is undergoing hormonal suppression (such as occurs during oral contraceptive therapy or during intensive chemotherapy). In a post-menopausal patient follicles should not be found. Any cystic structure seen in this patient population should be viewed with some clinical suspicion and at least warrants a follow-up examination.

Note the strong acoustic shadows occurring just superior to the ovary (due to the sacrum), as well as the areas of the hyperechogencity, indicating enhanced through transmission posterior to the bladder (seen in the lower right hand aspect of the image).

Uterus & Left Ovary (transverse)

Image #7 Transverse View of the Uterus and Left Ovary.

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Observations

In this different patient, note the similarity in size of the uterine fundus and of the left ovary but also their different echotextures: namely, the multiple follicles seen within the normal left ovary compared to the more uniform echotexture of the uterus. These ovarian follicles are developing under the influence of FSH secretion from the pituitary, but in this particular patient no dominant follicle is seen (because it is occurring in the contra-lateral ovary, see next image).

Right Ovary (sagittal)

Image #8 Longitudinal View of the Right Ovary.

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Observations

Note the larger cystic structure seen in this sagittal projection of the right ovary (same patient as image 7). This is the dominant follicle in this patient who is at mid-cycle. Following a luteinizing hormone surge from the pituitary, ovulation occurs, which results in the release of the egg from within this follicle as well as a small amount of fluid. The fluid collects in the retrouterine peritoneal space (pouch of Douglas) and may account for the pain that is typically felt at this point in the menstrual cycle (see next two images). Under the influence of high levels of luteinizing hormone, the ruptured follicle becomes the corpus luteum, the source of progesterone secretion in the second half or secretory phase of the cycle. This corpus luteum can persist as a cystic structure but more typically "fills in" (becomes echogenic) due to the presence of internal hemorrhage as well as growth and hyperplasia of the remaining luteinized, hormone-producing cells. While the corpus luteum typically involutes around the start of the next menstrual cycle, occasionally this involution can be delayed over several cycles such that at any one time there may be more than one dominant-appearing cystic structure within the ovary.

Normal Ovarian Follicles (transverse)

Image #9 Transvaginal Normal Ovarian Follicles (click here to restore original image.)

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Image #10

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Observations

 

Uterus & Ovaries (transverse)

Image #11 Longitudinal View of the Uterus.

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Observations

On this longitudinal view of the uterus note the presence of anechoic areas just posterior to the uterine corpus, fundus, and cervix. This indicatesfree fluid in the cul-de-sac (or pouch of Douglas), a potential retro-uterine peritoneal space). This amount of fluid can be normal at mid-cycle and is typically quickly resorbed. The pouch of Douglas is one of two dependent spaces in the peritoneum (the other is the pouch of Morison in the right upper quadrant between the right kidney and liver). Ascites of any etiology (cardiac, liver failure, peritonitis, malignancy) will accumulate in these areas first. Simple or serous fluid typically is anechoic (such as in this case), but complex fluid (due to presence of hemorrhage or infection) may show fine low level echoes in the fluid. Ultrasound is a powerful tool for detecting small amounts of fluid within the peritoneal cavity and can also useful in guiding paracentesis.