Human Chorionic Gonadotropin

hCG from the blastocyst is first detectable in maternal serum 6 to 8 days following fertilization. Most current urine pregnancy test kits yield positive results 3 to 4 days after implantation. By 7 days after implantation, which is the time of expected menses, 98% of urine pregnancy tests will be positive. Current pregnancy tests are immunoassays of hCG based on monoclonal antibodies to the β-subunit of hCG and are highly sensitive and rapid, with virtually no cross-reactions. If the urine test is negative, suspicion for EP is lowered, but not erased, because a urine pregnancy test will not detect levels of hCG below 25 mIU/mL. Importantly, a quantitative serum hCG test can detect levels approaching zero. If the serum hCG is negative, then an EP is almost certainly excluded, although chronic EP is still possible and rupture of an EP has been reported after the hCG dropped to zero. However, both of these conditions are extremely rare.

There has been much discussion in the literature regarding discriminatory serum hCG levels above which an IUP should be definitively visible by TVS. Levels of 1000 to 2000 mIU/mL (1st International Reference Preparation [IRP]) were previously used as discriminatory levels for expected visualization of an IUP on TVS. However, most researchers and practitioners now agree that no single serum hCG level should be used to differentiate an IUP from an EP. Rather, analysis of serial hCG levels and follow-up TVS are used to reach the correct diagnosis. That said, if the serum hCG level is above 3000 mIU/mL (1st IRP) and the uterus is empty, a normal IUP is unlikely, although there have been anecdotal reports of delivery of a normal pregnancy following an initial TVS demonstrating an empty endometrial cavity with hCG levels as high as 4336 mIU/mL. There are several possible explanations for this shift in discriminatory threshold levels. First, the incidence of multiple gestations has increased, mainly as a result of ART, and hCG levels for a given gestational age would thus be higher in a woman carrying a multiple gestation, compared to a singleton pregnancy, upon which earlier discriminatory numbers were derived. Second, data upon which earlier discriminatory numbers were based came from single institution studies with small patient populations. Third, clinical management and treatment have changed. Now, MTX is often given to treat EPs. If an erroneous, false positive diagnosis of EP is made and an IUP is not recognized, treatment with MTX can result in serious harm to the developing embryo. Hence, the current goal is to achieve 100% specificity with no false positive diagnoses of EP.

In general, the serum hCG level is lower for a given gestational age in the setting of an EP than with an IUP. Hence, when clinical findings are suspicious for possible EP, TVS is indicated even if the serum hCG is very low. Approximately 70% of women with EPs have a slow rise in serum hCG, slower than expected for a normal IUP, or a slow fall in hCG, slower than typical for spontaneous miscarriage. Hence, a suboptimal increase (i.e., <53% in 48 hours) or plateau of serial hCG values suggests an EP. Of note, serum hCG levels in some women with EPs may be similar to expected hCG levels in women with normal IUPs. One study revealed that up to 27% of women diagnosed with EP had hCG curves resembling normal IUPs. Others have found that 15% to 20% of EPs have hCG doubling times similar to those for IUPs. In addition, studies have shown that serum hCG levels may increase by only 35% to 66% over 48 hours for some normal IUPs, rather than doubling in that time frame, as has been previously expected. Furthermore, in 8% of spontaneously resolving EPs, the serum hCG level will fall in a pattern mimicking spontaneous miscarriage. As a result, the use of serum hCG levels alone for follow-up is unreliable for the diagnosis and management of EP.

Using sonography alone for follow-up is also not reliable because some EPs are too small to visualize. Additionally, 6% of apparent miscarriages on ultrasound examination may actually have an underlying EP. A presumed completed miscarriage based on TVS findings of an empty uterus should be treated as a pregnancy of unknown location (PUL) until confirmed as a miscarriage based on serial hCG levels and follow-up TVS. There are reports of patients discharged to home with presumed miscarriage subsequently returning with pain, bleeding, and ruptured EP.

PUL is a descriptive term referring to a pregnancy that is detected biochemically (positive pregnancy test), but not yet visible sonographically or laparoscopically. This term, a temporary descriptor, is used as a holding pattern until the location of the pregnancy is established. The incidence of PULs is dependent not only upon the expertise of the examiners and the adequacy of the sonographic evaluation but also upon gestational age. As women present earlier in pregnancy, more PULs are reported. In 1999 Banerjee and colleagues reported an 8% incidence (135 cases) of PULs in their series of first trimester sonograms. On follow-up, 14% of these turned out to be EPs; 27% developed into normal IUPs; 9% resulted in miscarriages; and 50% resolved spontaneously. Kirk and Bourne reported that in 8% to 31% of early pregnancies, the gestation will not be visualized on the initial TVS. The vast majority of PULs are eventually found to be IUPs or failed IUPs, and only approximately 7% to 20% are EPs. However, follow-up is very important. There is no reliable method by which to predict which PULs are IUPs and will either progress normally or miscarry and which PULs are EPs and, of these, which EPs will spontaneously resolve and which are likely to rupture. Unfortunately, rupture of EPs has been reported with declining or very low hCG levels, including some rare cases with negative hCG values. It is therefore incorrect to assume that because the hCG level is low, an EP is unlikely or stable or to assume that an EP is too small to visualize and thus not perform a sonogram (see Fig. 33-3 ).

Molecular Biomarkers

Studies are ongoing in attempts to identify diagnostic molecular biomarkers that can reliably distinguish EPs from both normal and abnormal IUPs at an early stage, when the location of the pregnancy is not yet known. These data would be extremely useful in the early diagnosis of EP, obviating the need for multiple follow-up blood tests, ultrasound examinations, and clinic visits, while at the same time decreasing the risk of EP rupture. One of the main challenges in these investigations is that there is no suitable animal model for tubal EP, so research must be performed directly in women. Biomarkers of trophoblastic function, corpus luteal function, angiogenesis, endometrial function, inflammation, muscle damage, and impaired tubal transport are currently being investigated. Although a variety of serum, plasma, and urine biomarkers have been evaluated both independently and in combination, most do not have adequate discriminatory capability to be clinically useful. One controlled study of patients with EPs and viable IUPs with hCG levels lower than 1500 mIU/mL demonstrated that a four-marker test, including progesterone, vascular endothelial growth factor (VEGF), inhibin A, and activin A, could predict EP with 100% accuracy. However, additional investigations that include patients with abnormal IUPs are necessary to further assess the utility and reliability of this panel of markers.

Transvaginal Sonography

In a patient suspected of having an EP, quantitative serum hCG levels and TVS are used in conjunction for diagnosis. It is reasonable to proceed with TVS while the quantitative hCG is being measured or independent of hCG level. Justification for performing TVS prior to obtaining the serum hCG results includes reports that up to 50% of ruptured EPs have extremely low hCG levels (<1000 mIU/mL) and that hCG levels do not necessarily correlate with risk of rupture.

Although TVS is much more sensitive than transabdominal sonography, most experts recommend that scanning begin with transabdominal pelvic ultrasound, albeit without intentional filling of the bladder. The aim of this limited transabdominal pelvic scan is to obtain an overview of the pelvis; establish the size, location, and position of the uterus; locate an ectopic sac in a high location ( Fig. 33-4 ); and assess for free intraperitoneal fluid, including in the Morison pouch, between the liver and right kidney. This is an excellent method for identifying a large hemoperitoneum that may be missed by TVS. In a supine patient, blood may accumulate in the upper quadrants with only a minimal residual amount of fluid seen in the pelvis. Subsequently, a detailed TVS is performed to provide higher resolution evaluation of the uterus and adnexa.

Value of a transabdominal scan in finding an ectopic pregnancy. A, Transabdominal longitudinal scan of the pelvis showing the echogenic tubal ring of a small ectopic pregnancy ( arrow ) situated above the uterine fundus (not shown). B, Transabdominal midline longitudinal scan of the uterus (U) shows a small amount of anechoic fluid in the endometrial cavity ( asterisks ), indicating a small amount of intrauterine blood. Transvaginal sonography could not image the ectopic sac because of its high location.

The yield and reliability of TVS depend upon transducer frequency, patient body habitus, uterine position, presence of leiomyomas, and operator expertise. Optimal transducer frequency for TVS is usually between 5 and 10 MHz. Depth is set such that there is visualization of pelvic structures 2 to 3 cm beyond the margin of the uterus or ovary, so as not to miss an adjacent or exophytic mass. Overall gain and time gain compensation (TGC) curves are set to optimize the image within the field of view. The object of interest is placed within the focal zone, and the use of multiple focal zones improves resolution. Harmonic imaging and spatial compounding will also improve resolution. Magnification of the area of interest within the uterus or adnexa may improve visualization. Gentle bimanual palpation exerted with one hand on the transvaginal transducer and the other hand pressing on the anterior abdominal wall can be helpful and may serve to identify the area of the patient’s pain, where disease is most likely to be found. This maneuver may move structures of interest toward the focal zone such that they are easier to see and will compress bowel, thereby eliminating artifact from air within the bowel lumen. In addition, exerting compression on an adnexal mass and ovary can help differentiate an exophytic corpus luteum, attached to the ovary, from an adjacent EP, which typically move apart, separate from the ovary. Three-dimensional (3D) sonography may be helpful, particularly in evaluation of EPs in unusual locations.

The first structure imaged by ultrasound is the cervix; the endocervical canal should be perpendicular to the ultrasound beam and the walls of the cervix in continuity with the myometrium of the uterine corpus. This positioning allows for evaluation of a possible cervical EP or miscarriage in progress and provides optimal visualization of the cul-de-sac, posterior to the cervix. The cul-de-sac is the most dependent portion of the female pelvis and thus is a common location for EPs or hemoperitoneum. Subsequently, the endometrium, myometrium, and adnexa are thoroughly evaluated. Any fluid within the endometrial cavity is assessed for location, shape, rim, and presence of internal echoes to determine the possibility of an IUP. If an ectopic gestational sac is not identified between the uterus and the ovary, one must scan as far lateral as possible in the pelvis, recognizing that this may result in some discomfort for the patient. Potential “blind spots,” where EPs can be located and are difficult to detect include the pelvic side walls, deep cul-de-sac, above the uterine fundus, and anteriorly between the uterus and bladder. The examination must include targeted evaluation in the region of the patient’s pain. Finally, if the hepatorenal space was not checked at the beginning of the examination, one should keep the patient supine and scan, looking for fluid in the Morison pouch.

Color, power, or spectral Doppler sonography may be used to demonstrate blood flow to an EP and to differentiate between vascularized trophoblastic tissue and avascular hematoma. However, Doppler sonography of the endometrium is not advised in order to avoid insonating a possible early IUP that is not yet visible. Color Doppler ultrasound parameters should be optimized for the detection of slow flow. This includes decreasing the scale or pulse repetition frequency (PRF), increasing gain and color saturation, decreasing the wall filter, setting higher color priority, and using a small color box. Spectral Doppler tracings should be obtained to confirm blood flow demonstrated on color Doppler images, as color Doppler sonography is sensitive to motion, which may create color artifact mimicking blood flow.

Uterine Findings

In the setting of EP, sonographic findings in the uterus are nonspecific and include absence of an intrauterine gestational sac and fluid in the endometrial canal. If there is no visible intrauterine gestational sac, diagnostic possibilities include EP, very early IUP, failed IUP, or heterotopic pregnancies. At this point, the pregnancy is deemed to be a PUL, and, in current practice, the diagnosis of EP should be based upon positive findings rather than on the absence of an intrauterine gestational sac with serum hCG above a specific discriminatory level. Hence, the search continues. Although the presence of an IUP makes the probability of an EP much less likely, heterotopic pregnancies have been reported in 1 out of 4000 pregnancies in the general population, and the incidence is as high as 1 in 100 to 300 pregnancies in women who have undergone ART.

Endometrial thickness has not been shown to reliably predict the presence of an EP or the outcome of a PUL. Likewise, decidual cysts have not been proved to be predictive of EPs, despite initial reports that a thin-walled endometrial cyst was associated with a high risk for EP ( Fig. 33-5 ). Although Laing and coworkers reported that a thin-walled decidual cyst can be differentiated from an early intrauterine gestational sac with its thicker echogenic wall (intradecidual sac sign) ( Fig. 33-5B ), most authors rely on the identification of a yolk sac within an endometrial fluid collection to definitively diagnose an intrauterine gestational sac.

Decidual cysts. A, Transvaginal sagittal scan of a retroverted uterus demonstrates two small anechoic cysts ( arrows ) within the decidua, neither of which has a thick rim. Note small amount of free fluid ( asterisk ) adjacent to the uterus. This patient had a tubal ectopic pregnancy (not shown). B, Transvaginal transverse scan of another patient shows a small decidual cyst ( arrow ) and a very early intrauterine gestational sac (intradecidual sac sign) ( arrowhead ). Note the thicker echogenic rim around the early intrauterine pregnancy.

In the past, it was thought that intracavitary blood or secretions were relatively common findings in patients with EPs. This fluid, often containing low level echoes, located centrally within the uterine cavity and surrounded by a single echogenic decidual layer, was termed a “pseudogestational sac” or “pseudosac” ( Fig. 33-6 ). Thus, it could be differentiated from a true intrauterine gestational sac, which is either eccentric and embedded in a single layer of the endometrium (intradecidual sign) or surrounded at least partially by two echogenic rims of tissue with an interleaving hypoechoic layer (the double decidual sac sign). However, at present, fluid in the endometrial canal is observed in only 10% to 20% of women with EPs, possibly because of sonographic evaluation at earlier gestational ages. Benson and associates report that in current practice any round or oval well-defined fluid collection in the mid to upper endometrial cavity is statistically more likely to represent an IUP (99.98%) rather than a “pseudosac” associated with an EP (0.02%). Note that an endometrial fluid collection with a tapered or pointed edge is much more likely to be associated with an EP (see Fig. 33-6 ). In conclusion, the terms “pseudosac” and “pseudogestational sac” should probably be abandoned as they are confusing and of limited use. In current practice, endometrial fluid is infrequently observed in patients with EP.

Intrauterine fluid associated with EP, referred to as pseudogestational sacs. A, Transvaginal sagittal scan of the uterus demonstrating an oblong intrauterine fluid collection with pointed margins ( arrowheads ) surrounded by a single decidual lining ( arrow ). There was an adnexal tubal ectopic pregnancy (not shown). B, Transvaginal scan of another patient with fluid in the endometrial canal with pointed edge ( arrowhead ). Intraluminal echoes ( asterisk ) represent blood, surrounded by single thin decidual lining ( arrow ). The patient had an ectopic pregnancy (not shown). C, Transvaginal color Doppler scan of a third patient demonstrating a right adnexal ectopic pregnancy adjacent to the uterus surrounded by a large hematoma ( long arrow ). The ectopic pregnancy demonstrates no blood flow on color Doppler interrogation. Fluid-containing low level echoes ( asterisk ) consistent with hemorrhage is noted centrally within the endometrial cavity. The fluid is surrounded by a single echogenic rim of decidual tissue ( short arrow ). This appearance has been referred to as a pseudogestational sac . However, most authors now prefer to avoid this potentially confusing terminology and use of this phrase is discouraged.

Adnexal Findings

Visualization of an extrauterine gestational sac containing a yolk sac or an embryo with or without a heartbeat is diagnostic of an EP with 100% positive predictive value (PPV) ( Fig. 33-7 ). The next most reliable sonographic finding is an adnexal tubal ring representing an empty gestational sac that appears as a round anechoic fluid collection with a thick echogenic rim. This has also been described as the bagel, donut, or sugar donut sign ( Figs. 33-2B, 33-3A, and 33-8 ). The adnexal tubal ring sign has a 95% PPV for EP and is seen in approximately 50% of cases. The PPV increases if the tubal ring is clearly separate from the ipsilateral ovary ( Fig. 33-8B ). Color Doppler sonographic imaging may increase the conspicuity of a small tubal ring ( Fig. 33-8C ). Although some EPs will demonstrate circumferential vascularity, most EPs will demonstrate only segmental or focal vascularity, and some may demonstrate no flow at all on color Doppler ultrasound imaging ( Figs. 33-2B and 33-6C ). EPs may also appear as a nonspecific adnexal mass representing hemorrhage with blood clot. These masses can be homogeneous, heterogeneous, or complex; may appear either predominantly cystic or solid; and have variable echogenicity ranging from hypo- to hyperechoic, depending upon the time course since hemorrhage occurred ( Fig. 33-9 ). Hemorrhage contained within the fallopian tube (hematosalpinx) will appear tubular or ovoid in shape ( Fig. 33-10 ). Internal vascularity increases the likelihood that the mass represents an EP rather than bland hemorrhage from a ruptured corpus luteum. In the setting of a positive pregnancy test and an empty uterus, any adnexal mass that is clearly not either an intraovarian corpus luteum, paraovarian cyst, or pedunculated myoma has a 92% PPV for EP. EPs are most commonly found situated between the ovary and uterus or in the cul-de-sac, which is the most dependent portion of the female pelvis. Although rare, it is possible to occasionally see twinning in ectopic locations. Possibilities include twin sacs in one fallopian tube, one twin in each tube, monochorionic twins, and conjoined twins ( Fig. 33-11 ).

Definitive ultrasound findings of ectopic pregnancy. A, Transvaginal scan of an ectopic gestational sac showing a thick hyperechoic rim and a yolk sac ( arrow ) within the anechoic center, but no embryo is seen. B, Transvaginal scan of an ectopic gestational sac with a thick hyperechoic rim, situated between the uterus (U) and ovary (O). There is a yolk sac ( arrow ) and an adjacent tiny embryo ( arrowhead ) with no cardiac activity. C, Transvaginal scan of an ectopic gestational sac shows a very early embryo with M-mode (tracing of cardiac motion not shown). D, Transvaginal scan of the adnexa in a patient with an ectopic gestational sac containing a larger embryo. Cardiac activity was noted on real-time imaging. Note the rhombencephalon ( arrow ) in the posterior fossa. There is a large perigestational hematoma ( asterisks ) that is less echogenic than the trophoblastic tissue at the periphery of the gestational sac ( arrowhead ).

Ectopic pregnancies presenting as tubal rings in three different patients. A, Transvaginal scan shows a typical adnexal tubal ring ( between calipers ), also called a “sugar donut” or “bagel” sign, representing an empty ectopic gestational sac with a round anechoic fluid-filled center surrounded by a thick echogenic rim. Note a small amount of adjacent free pelvic fluid. B, Transvaginal scan shows the small tubal ring of an ectopic pregnancy ( arrow ) separate from and medial to the right ovary (ROV). C, Transvaginal transverse color Doppler scan shows the tubal ring of an ectopic pregnancy located between the uterus (U) and left ovary (O). Note the incomplete color Doppler “ring of fire.”

Ectopic pregnancies presenting as nonspecific adnexal masses: varying appearances in five different patients. A, Transvaginal scan shows a complex, heterogeneous, predominantly solid-appearing mass. B, This ectopic pregnancy is well marginated with a relatively homogeneous, echogenic, solid appearance ( arrow ), likely due to blood clot around the ectopic sac within the fallopian tube. C, Transvaginal scan shows a complex predominantly solid mass with hyperechoic components ( arrow ) surrounded by more hypoechoic hematoma, representing an ectopic pregnancy situated between the uterus (U) and right ovary (O). D, Transvaginal scan shows a large heterogeneous, predominantly cystic mass in the left adnexa. E, Transvaginal scan shows a large heterogeneous pelvic mass.

Tubular adnexal masses representing hematosalpinges from ectopic pregnancies. A, Transvaginal longitudinal scan demonstrates a distended fallopian tube with intraluminal complex fluid representing blood. Pathologic specimen revealed a tubal ectopic pregnancy. B, Transvaginal scan in another patient with an ectopic pregnancy demonstrates a well-marginated, tubular heterogeneous structure ( calipers ) consistent with a fallopian tube filled with blood. The gestational sac was not recognizable. A fallopian tube filled with blood (hematosalpinx) will appear oblong on long axis and round on short axis. Smooth well-defined margins and tubular shape help differentiate a tube filled with blood from an adnexal hematoma.

(B from Bryan-Rest LL, Scoutt LM: Ectopic pregnancy. In Fielding JR, Brown DL, Thurmond AS (eds): Gynecologic Imaging. Philadelphia, Elsevier, 2011, p 335, Fig. 22-4B, used with permission.)

Twin ectopic pregnancies. A, Transvaginal color Doppler image of dichorionic diamniotic twin ectopic gestational sacs ( arrows ), the most common form of twinning resulting from embryo transfer techniques. Note that blood flow shown in the tubal rings is discontinuous, segmental, and focal, rather than a complete 360 degree ring of vascularity, which is more commonly observed with the corpus luteum. B, Transvaginal color Doppler scan shows monochorionic twin embryos with central color pixels ( arrows ) representing cardiac motion in an ectopic tubal pregnancy.

Cul-de-Sac Findings

Echogenic free fluid indicative of hemoperitoneum may be the only initial finding in up to 25% of patients with EPs ( Fig. 33-12 ). Hemoperitoneum typically collects in the cul-de-sac as this is the most dependent portion of the female pelvis in the upright position. A pregnant woman with an empty uterus has a 90% probability of EP if a moderate to large amount of echogenic free peritoneal fluid is found. However, the volume of free fluid has low sensitivity, specificity, and PPV for determining tubal rupture. The amount of free fluid is a good indicator of the risk of hemodynamic instability. Therefore, the upper abdomen should always be assessed as the presence of free fluid in the Morison pouch suggests that there is a relatively large volume and would increase concern for potential hemodynamic instability. When evaluating free fluid, careful attention should be paid to technique as high gain settings can create artifactual echoes. Comparison of cul-de-sac fluid with known anechoic fluid such as urine in the bladder or fluid in an ovarian cyst/follicle at similar depth will help reveal whether echoes are real or artifactual ( Fig. 33-12B ). One can also look for layering or movement of low-level echoes. It should be remembered that on occasion acute hemoperitoneum may appear as anechoic fluid. Occasionally, hemoperitoneum in the Morison pouch may appear so echogenic that it approximates the echogenicity of adjacent hepatic parenchyma and may be mistaken for liver because of the silhouette effect ( Fig. 33-13 ). Also, clotted blood may appear echogenic and mass-like, and thus may be difficult to differentiate from loops of bowel ( Fig. 33-12D ). After the initial clotting stage, fibrinolysis begins and hemoperitoneum will appear heterogeneous, making it more difficult to visualize a small ectopic gestational sac amid amorphous material in the cul-de-sac (see Figs. 33-3 and 33-12B ).

Variable sonographic appearance of hemoperitoneum due to rupture of ectopic pregnancy. A, Transvaginal transverse scan shows homogeneous, low-level echoes representing blood ( arrow ) in the cul-de-sac, posterior to the uterus (U). B, Echogenic hemoperitoneum ( arrow ) outlines the ovary (O), which contains several small anechoic follicles, and an ectopic gestational sac ( asterisk ) with an echogenic rim. C, Hemoperitoneum, shown posterior to the uterus (U), is nearly anechoic in this patient with a small area of low-level echoes ( arrow ). Note a small amount of fluid in the endometrial cavity. D, Transvaginal longitudinal pelvic scan shows complex heterogeneous clotted blood ( arrow ) posterior to the uterus (U) mimicking bowel loops. E, Transvaginal transverse power Doppler scan through the cul-de-sac shows complex heterogeneous material with no demonstrable internal blood flow, consistent with clotted blood. Note that there is a small amount of adjacent anechoic free fluid.

Hemoperitoneum from ruptured ectopic pregnancy may mimic liver parenchyma. A, In this patient with hypotension and ruptured ectopic pregnancy (not shown), more than a liter of intraperitoneal blood ( arrow ) accumulated in the Morison pouch, between the right lobe of the liver (L) and the right kidney (RK). This was initially overlooked, mistaken for liver parenchyma owing to similar echogenicity. B, Hemoperitoneum ( arrow ) under the liver (L) was eventually correctly recognized when an anechoic stripe was noted, separating liver (L) from hemoperitoneum ( arrow ). RK, right kidney.

(Images courtesy of Mindy Horrow, MD, Albert Einstein Medical Center, Philadelphia, PA.)

Pitfalls

General limitations in the sonographic diagnosis of EP include lack of operator experience, training, skill, persistence, or lack of attention to details. Patient obesity, leiomyomas, uterine anomalies, retroflexion of the uterus, or hydrosalpinx may also limit sonographic evaluation. Careful and complete patient history should address risk factors such as prior EP, ART, PID, or tubal surgery. If the patient presents at an early gestational age, the EP may be too small to visualize and may fall below the limits of resolution of the ultrasound equipment.

Potential pitfalls in image interpretation include misconstruing fragments of decidua or blood clot within an intrauterine fluid collection as either a yolk sac or embryo, thus leading to a false positive diagnosis of IUP ( Fig. 33-14 ). It can also be very difficult to differentiate an exophytic corpus luteum from an EP ( Fig. 33-15 ), particularly because 80% of EPs will be located ipsilateral to the ovarian corpus luteum ( Fig. 33-16 ). There are several useful points of distinction ( Fig. 33-17 ). A corpus luteum arises from the ovary, and intraovarian EPs are extremely rare (see later). Although a corpus luteum can be exophytic, a peripheral crescent of ovarian parenchyma, termed the “claw sign,” will typically be observed surrounding at least part of the corpus luteum. However, if a mass adjacent to the ovary represents an EP, it will be separate from the ovary with an acute intervening angle between the structures and no “claw sign” (see Fig. 33-17 ). When prodded with the transvaginal probe or with palpation on the anterior abdominal wall, the EP will typically slide away from the ovary, whereas a corpus luteum will remain attached to and move with the ovary. The tubal ring of an EP is typically thinner and more echogenic than the homogeneous thick wall of the corpus luteum ( Figs. 33-15A, 33-15C, 33-16A, and 33-17C ). However, there is considerable overlap in the echogenicity and thickness of the walls of a corpus luteum and an EP. As a corpus luteum involutes, the inner margin may become crenated or stellate in appearance ( Fig. 33-15A and B ). Internal echoes are relatively common as a result of hemorrhage and may mimic a yolk sac or embryo (see Fig. 33-15B ). Magnified views using harmonic imaging or spatial compounding may be helpful given that definitive identification of a yolk sac or embryo in an adnexal ring-like structure confirms the diagnosis of EP. Both a corpus luteum and an EP can demonstrate marked peripheral vascularity on color Doppler imaging, termed the “ring of fire,” with low-resistance spectral Doppler waveforms with increased diastolic flow. Neither the degree of mural vascularity depicted on color Doppler ultrasound nor the characteristics of flow on spectral Doppler waveforms is helpful in differentiating an EP from a corpus luteum. Although the “ring of fire” sign was initially described as a pattern of color Doppler flow suggestive of EP, subsequent experience has shown that a complete circumferential (360-degree) “ring of fire” is actually more commonly seen with corpora lutea ( Figs. 33-15B, 33-15D , and 33-16B ). Blood flow in the tubal ring of an EP is more likely to be focal and segmental ( Figs. 33-8C and 33-11A ).

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