Background and Indications for Examination
Pelvic ultrasound was first introduced into the acute care setting to answer simple yes or no questions such as “Is there an intrauterine pregnancy?” Pelvic ultrasound applications in the emergency setting have now expanded to include the nonpregnant patient. The improvement in skills now allows the clinician to evaluate for other pelvic pathology such as adnexal cysts or masses, ovarian torsion, fibroids, or tuboovarian abscesses.
The complaint of abdominal or pelvic pain in the young female patient comprises a significant number of the total emergency department visits throughout most institutions. Both transabdominal ultrasound (TAUS) and transvaginal ultrasound (TVUS) can help the physician diagnose the cause of these complaints and can guide clinical decisions and expedite patient care. While point-of-care pelvic ultrasound is a very useful tool in the acute care setting, it still remains focused and is not intended to examine all potential pathology.
Probe Selection and Technical Considerations
A wide footprint curvilinear probe is appropriate for transabdominal pelvic imaging. This probe has a lower frequency with better penetration, which is suitable for imaging the pelvic organs through the abdominal wall. Smaller footprint or phased array probes may be used for transabdominal pelvic imaging but are not optimal.
The endocavitary probe has a higher frequency, which produces an image with better resolution. It also has a wider field of view and provides more detail. It is used for all transvaginal pelvic scanning, especially when the structure of interest is not visualized well using transabdominal imaging alone, such as in an early intrauterine pregnancy (IUP) or in evaluation of the adnexa.
In TAUS the sonographer should start with a deeper field and survey the areas surrounding both the uterus and bladder. This is important in the evaluation for free fluid, which will appear in the most dependent areas first, posterior to the uterus. Also, a deeper field allows the sonographer to better evaluate the adnexa for pathology. Once the far field is completely evaluated, the depth can be decreased so that the uterus appears more superficial and intrauterine structures better visualized.
The zoom control allows for magnification of a particular portion of the image being scanned. This becomes important when attempting to visualize small structures, such as obtaining the fetal heart rate (FHR) of an early pregnancy. The zoom function can sometimes distort the image, therefore the depth should be optimized first before the image is magnified.
The total gain can be adjusted in order to increase the signal returning to the transducer and brighten the image. In cases where only the far field needs to be brighter, the time-gain compensation (TGC) can be increased and the near field left alone. The gain should not be increased too high or the image can appear “washed” out and white, which can obscure free fluid in dependent areas, especially behind the uterus.
Color-flow Doppler identifies blood flow and is useful in the evaluation of the ovaries for torsion. Ovarian torsion demonstrates a decrease in blood flow compared to the unaffected side. Color flow also identifies blood vessels, such as the iliac artery and vein, which serve as landmarks for the ovaries. In addition, this modality can be used to diagnose retained products of conception (RPOC) after a pregnancy or abortion. The area in the uterus that contains debris will display more color flow compared to the normal endometrium. Unless absolutely necessary, Doppler should be avoided in the patient with early pregnancy due to the increased mechanical and thermal energy involved. M-mode should be used for FHR, as described below, not Doppler.
M-mode detects motion over time and is used to calculate the FHR of a pregnancy. Once the pregnancy is identified, the M-mode cursor can be placed over the fetal heart. The heart rate is then calculated graphically on one side of the screen depicting the motion of the heart over time.
Gestational age estimations based on crown-rump-length (CRL), biparietal diameter (BPD), and other modalities are usually included in calculation packages supplied by the machine. In order to access these calculations, the sonographer must choose the obstetrical preset on the machine prior to scanning.
Normal Ultrasound Anatomy
The uterus is located midline in the true pelvis superior and posterior to the urinary bladder and anterior to the rectum. The normal uterus is approximately 7–9 cm long, 2–4 cm thick, and 4–6 cm wide. The myometrium is the largest component of the uterus and has a uniform low-level echogenicity and appears hypoechoic relative to the endometrium. The endometrial stripe is composed of a canal surrounded by two layers of endometrium. The brightness and thickness of this stripe varies depending on the patient’s menstrual cycle. The stripe is seen as a thin hyperechoic line, approximately 2–3 mm, at the end of the cycle (Fig. 12-1). At other points during the cycle, it will appear thicker and can be up to 15 mm during the proliferative phase.
Transvaginal view of normal pelvic anatomy. The bladder (B) is visualized as an anechoic structure sitting anterior and inferior to the uterus (U). The endometrial stripe (ES) is seen as a hyperechoic line in the center of the uterus. RV: retrovesicular space, between the bladder and uterus, PD: pouch of Douglas, between the uterus and rectum.
The vaginal stripe is seen as an echogenic stripe that lies within the vaginal canal in between the anterior and posterior walls. Its presence indicates the lack of debris or foreign body within the vaginal vault. It is only visualized on a transabdominal scan and is best seen in the sagittal plane (Fig. 12-2).
Sagittal transabdominal view of normal pelvic anatomy. The bladder (B) is visualized as an anechoic structure sitting anterior and inferior to the uterus (U). The vaginal stripe (VS) is seen as a bright white hyperechoic line within the vaginal canal. The pouch of Douglas (PD) lies between the uterus and the rectum, the most dependent area in the female pelvis, where free fluid collects first. BW: bowel, RV: retrovesicular space. The uterus is tipped forward toward the bladder and anterior abdominal wall (anteverted) and the fundus is tipped forward in relation to the cervix (anteflexed).
The position of the uterus is described in terms of version and flexion, with version describing the orientation of the overall uterus, and flexion describing any bending within the uterus. The most common position is anteverted where the entire uterus, including the cervix, is tilted forward toward the anterior abdominal wall overlying the bladder (see Fig. 12-2). The uterus can be pushed slightly backward into a horizontal position when the bladder is full and distended. A retroverted uterus is tilted backward away from the bladder toward the posterior abdominal wall and spine (Fig. 12-3). Version of the uterus is best seen in a sagittal plane, and orientation is particularly important when using TVUS, as a normal uterus may appear retroverted (or vice versa) if the probe is turned over. The other common uterine variant described is flexion, in which the uterus bends on itself. The uterus can have any combination of positions mentioned earlier: anteverted-anteflexed, anteverted-retroflexed, retroverted-anteflexed, or retroverted-retroflexed.
Sagittal transabdominal view of a retroverted uterus. The bladder (B) is visualized as an anechoic structure sitting anterior and inferior to the uterus. The vaginal stripe (VS) is seen as a bright white hyperechoic line within the vaginal canal. The endometrial stripe (ES) is seen as a hyperechoic line in the center of the uterus. The uterus is tipped backward away from the bladder toward the posterior abdominal wall (retroverted).
The posterior cul-de-sac or “pouch of Douglas” is the space between the posterior wall of the uterus and rectum (see Figs. 12-1 and 12-2). It is the most dependent area in the female pelvis, the site where free fluid will collect first. This space can have a variable appearance depending on its contents. It may appear completely anechoic when containing new blood or may have a heterogeneous appearance if clots are present. The anterior cul-de-sac (retrovesicular or uterovesicular space) is the space between the urinary bladder and the anterior wall of the uterus (see Figs. 12-1 and 12-2). Due to its anterior position, it is less gravity-dependent and does not collect free fluid as quickly as the posterior space. If fluid collects in the uterovesicular space, it indicates a significant amount of fluid in the pelvis.
The ovaries are usually found anterior and medial to the iliac blood vessels. These vessels are the primary landmarks for the ovaries, but they can be displaced according to the position of the uterus and can even be found in the cul-de-sac. Ovaries have a medium-level echogenicity with scattered anechoic follicles in the periphery, creating the “chocolate-chip cookie” appearance (Fig. 12-4). Ovarian size varies with age and with the menstrual cycle as the number and size of the follicles change. They are smallest at the time of menstruation and the largest at mid-cycle. The dominant follicle or corpus luteum can often measure up to 2 cm or larger in diameter. The ovaries are more readily identified in premenopausal women, as they tend to decrease in size with increasing age. In addition, they are usually more easily found using TVUS where the probe can be placed closer to the ovary.
Fallopian tubes, mesosalpinges, and broad ligaments may be difficult to identify sonographically unless pathology is present.
The urinary bladder lies anterior and inferior to the uterus. It appears as an anechoic structure with its sonographic size dependent on the urine volume present. It serves as a landmark for the uterus in TAUS scanning; therefore, it is preferably full during this exam (see Fig. 12-2). In TVUS, a full bladder can displace uterine and adnexal anatomy, thus an empty bladder is preferred (see Fig. 12-1).
Normal Ultrasound Anatomy in Early Pregnancy
The “intradecidual sign” is the earliest evidence of an IUP, although it is difficult to visualize by even expert sonographers. It consists of an embryo that is completely embedded within the uterine endometrium, but does not displace the endometrial stripe. On ultrasound, it appears as a simple anechoic area within the uterine cavity and can be seen as early as 41/2 weeks’ gestational age on a TVUS. It is not sufficient enough evidence to prove an IUP exists, therefore not routinely used by clinicians to make the diagnosis.
The “double decidual sac sign” is more evidence that a true IUP exists, but it should also be relied on with caution. This sign is seen as two rings, the inner being the decidua capsularis and the outer the decidua vera that surrounds an anechoic area within the uterine cavity (Fig. 12-5). This sign can be seen on TVUS at about 5 weeks and on TAUS at 6 weeks’ gestational age. Although its presence is good evidence that an IUP exists, the novice clinician can misinterpret this finding and it alone should not be used to make a secure diagnosis.
The gestational sac is an anechoic space surrounded by an echogenic rim that is visualized on TVUS at about 5 weeks and TAUS at 6 weeks’ gestational age (see Fig. 12-5). The sac is usually visualized when it reaches a diameter of 5–6 mm.
There may be an anechoic area within the uterus that is not a gestational sac, but possibly free fluid or a pseudo-gestational sac. Visualizing the “double decidual sac sign,” as described earlier, surrounding the anechoic area is good evidence that a gestational sac exists, but other criteria should be met to confidently make this conclusion. The presence of both a yolk sac with or without a fetal pole inside the anechoic area is more definitive proof that a true gestational sac exists. A normal gestational sac is located in the upper uterine body or fundus, and embedded in the middle of the uterine wall. In early pregnancy, the mean gestational sac diameter (MGSD) is used to estimate the gestational age.
The yolk sac is attached to the developing embryo and provides nourishment while the circulatory system is still developing. It usually disappears during early pregnancy as the circulatory system takes over. Its diameter in an early pregnancy is consistently 6-7 mm and is important in establishing a normal IUP. Its absence in the first trimester or an unusually large diameter may signify an abnormal pregnancy.
On ultrasound, the yolk sac is a round echogenic ring with an anechoic center that is often referred to as a “cheerio.” It can be seen on ultrasound within the gestational sac at approximately 6 weeks on TVUS and at 7 weeks on TAUS. It is usually found eccentrically attached to one of the walls of the lumen (Fig. 12-6). A yolk sac visualized within the gestational sac is definitive evidence that a true IUP exists.
The fetal pole is the developing embryo that is first seen attached to one end of the yolk sac (see Fig. 12-6) within the gestational sac. It is usually visualized at 6 weeks on TVUS and about 8 weeks on TAUS. The fetal pole is usually identified when the CRL is approximately 2–4 mm. The FHR can often be seen flickering within the fetal pole when the CRL reaches 5 mm. The FHR is often better visualized on TVUS at this early stage and often is too small to assess properly with TAUS. The presence of both the fetal pole and yolk sac is more definitive evidence that an IUP exists.
The yolk sac begins to disappear by the end of the first trimester and its remnants form the umbilical cord, which attaches the fetus to the placenta. The placenta grows throughout pregnancy, with the size of the placenta proportional to the age and health of the fetus. The thickness of the placenta can be used to approximate gestational age and is usually less than 3–4 cm at its maximum. In the early second trimester, the placenta should be easily visualized by TAUS as a homogenous structure that surrounds the gestational sac and fetus. A normal placenta will be visualized superiorly to the fetus close to the uterine fundus (Fig. 12-7). The different layers of the placenta will become more apparent in the third trimester. The amniotic fluid will begin to accumulate in larger amounts and will be seen as an anechoic area surrounding the fetus.
Imaging Tips and Protocol
Transabdominal ultrasound (TAUS) is the initial exam performed on any patient presenting with acute pelvic pain or bleeding (pregnant or nonpregnant). The curvilinear abdominal probe is used and the patient should be scanned in both the transverse (short-axis) and sagittal (long-axis) orientations.
The patient should be placed in the supine position, similar to other abdominal exams. In the sagittal view, the probe is placed midline on the patient’s lower abdomen, just below the pubic bone, with the indicator pointed cephalad. The vaginal stripe and urinary bladder serve as landmarks in TAUS imaging, and therefore should be identified first. The patient should ideally have a full bladder as this cystic structure is used as a sonographic window in pelvic scanning. The monitor will show the patient’s head on the left side of the screen, the feet on the right side, the anterior surface at the top of the screen, and the posterior surface at the bottom. The urinary bladder appears as a triangular or elongated structure with a thick wall around an anechoic area. The vaginal stripe appears as a bright white line within the canal. Once the landmarks are identified, the uterus will be seen superior and posterior to the bladder. The vaginal stripe can be followed up through the canal to the cervix and into the uterus (see Fig. 12-2).
Once an adequate sagittal image is obtained, the sonographer should interrogate the surrounding area by fanning the probe medially and laterally. In addition to surveying the organs for pathology, the anterior and posterior cul-de-sacs should be carefully examined looking for possible free fluid. This will appear as an anechoic or hypoechoic area outside the uterus, in between the bladder anteriorly, or rectum posteriorly.
While usually much better visualized with transvaginal imaging, imaging of the adnexa should be attempted to identify obvious pathology. When seen, the ovaries are anterior and medial to the iliac vessels (see Fig. 12-4).
In order to obtain a transverse view, the transducer is then rotated 90° counterclockwise with the indicator pointing toward the patient’s right side. This image depicts the bladder and uterus both in their short axis with the bladder visualized anterior to the uterus (Fig. 12-8). The monitor will show the patient’s right on the left side of the screen, the patient’s left on the right side of the screen, the anterior surface at the top of the screen, and the posterior surface at the bottom. In this orientation, the uterus should be scanned from the cervix up to the fundus by fanning the probe in both superior and inferior directions. Once again, both the anterior and posterior cul-de-sacs should be examined for free fluid. The probe can be moved to the left and right in order to visualize the adnexa.
Transverse transabdominal view of normal pelvic anatomy. The bladder (B) is seen as an anechoic structure anterior and inferior to the uterus. The endometrial stripe (ES) is visualized as a hyperechoic line in the center of the uterus. Both potential spaces, the “pouch of Douglas” (PD) and the “retrovesicular space” (RV) are seen on the transverse view.
Transvaginal ultrasound (TVUS) provides the sonographer with better resolution and image quality compared to abdominal scanning. It should be used in the evaluation of uterine and adnexal pathology when TAUS does not provide enough detail. The endocavitary probe, with its long shape and higher frequency, allows the sonographer to obtain a closer and more detailed view of the pelvic anatomy. In TVUS imaging, the patient should be scanned in two orientations, both sagittal and coronal.
The patient should be asked to empty their bladder before the exam because if it is full and distended, it can distort the anatomy, and interfere with visualization. The patient should be placed in a supine position with their legs abducted and bent at the knees at approximately a 45° angle. This is typically referred to as a “frog-legs position.” A pelvic bed is extremely helpful in this exam. If not available, the pelvis should be elevated by placing a bedpan or blanket under the lower back to allow the tail of the probe to go low enough. The exam should be fully explained to the patient before starting the ultrasound. A sterile probe cover and gel should be placed over the endocavitary probe before beginning.
The TVUS is usually started in a sagittal orientation by inserting the probe into the vaginal canal with the indicator pointing toward the ceiling. The images produced on the screen in TVUS are more difficult for the novice sonographer to interpret. A full understating of the orientation of the transducer indicator in relation to the patient’s body is essential before learning to use the endocavitary probe. In the sagittal view, the left side of the screen is the patient’s anterior body surface as the indicator is pointing toward the ceiling initially. The right side of the screen then represents the patient’s posterior body surface, the top of the screen is the patient’s feet (caudad), and the bottom of the screen is the patient’s head (cephalad).