Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

tal hemodynamics is common, with one study finding blood flow restriction in at least one uterine artery in a majority of women (23). Women with an abnormal Doppler study before delivery have significantly higher blood pressures than those with a normal predelivery study; likewise, abnormal Doppler US is associated with an increased number of perinatal deaths and significantly higher fetal distress as compared to a normal Doppler study.

Placenta

Dynamic MR reveals early placental enhancement; it has heterogeneous enhancement during the second trimester and evolving into enhanced lobules during the third trimester; normal placental enhancement precedes myometrial en-hancement and it can be differentiated from myometrium.

A placental maturity classification was developed in 1979, based on US findings of placental texture (24); this classification, using a grading scale from 0 to 3, correlates with fetal pulmonic maturity as determined by the lecithin-sphingomyelin ratio. Those with premature placental aging are at increased risk of perinatal complications such as hypertension, oligohydramnios, and delayed intrauterine growth.

A relationship exists between smoking and placental calcifications.

Transabdominal US should detect most placenta previa, with only occasional transperineal or endovaginal US necessary. An MRI can provide substantially similar information, but at the expense of substituting a more complex study for a simpler one.

Placenta membranacea or placenta diffusa results when chorion fails to differentiate. It is associated with bleeding, abortion, and fetal death. Ultrasonography in placenta membranacea reveals total placenta previa covering the uterine wall and containing numerous lacuna.

With placenta increta, chorionic villi invade the myometrium while placenta percreta signifies that the villi penetrated through the myometrium. Both evolve in a setting of uterine scarring such as prior cesarean section, D&C, and other causes. An association exists with placenta previa. Placenta percreta can lead to uterine rupture and an acute abdomen.

ADVANCED IMAGING OF THE ABDOMEN

Ultrasonography suggests placenta accreta by detecting the absence of the typically visualized retroplacental sonolucent space. Doppler US in placenta percreta reveals placental extension into the myometrium and increased blood flow through the myometrium. Gadoliniumenhanced MRI also differentiates placenta accreta from placenta percreta.

The resultant bleeding with abnormal placentation can be massive and a total hysterectomy required for bleeding control. Hypogastric artery balloon occlusion in several women with abnormal placentation, performed after cesarean delivery but prior to hysterectomy and hypogastric artery ligation, was effective in decreasing blood loss (25).

Placental nontrophoblastic tumors include chorioangioma and teratoma. These tumors are benign and often overlooked, but multiple placental chorioangiomas lead to fetal cardiac failure or anemia. Their presence can be suggested by US.

Antepartum Bleeding

Discussed here are only some of the more unusual causes of pregnancy-related bleeding. Premature delivery and various placental abnormalities, discussed above, are common causes. Most first trimester bleeding is associated with either an ectopic pregnancy or partial placental separation from its myometrial implantation (separation occurring before the 20th week of pregnancy is called abortion and separation after the 20th week abruptio placentae). Common causes of bleeding during the last half of pregnancy are placenta previa (low placental implantation) and abruptio placentae.

Transabdominal US may not detect a cause for first trimester bleeding, and in these women endovaginal US, with its better resolution, is more helpful. If detected, a subchorionic hematoma has a crescent appearance elevating the chorionic membrane. Ultrasonography establishes whether a normal-appearing gestational sac is visible. A clot secondary to abruptio placentae often has a similar transabdominal US appearance to normal placenta and thus normal US does not exclude abruptio. At times only an abnormally thick placenta is detected.

FEMALE REPRODUCTIVE ORGANS

Most intraamniotic bleeding is secondary to trauma. It is common after amniocentesis. A rare cause of third trimester bleeding is cervical varices.

Postpartum Bleeding

Retained intrauterine placental tissue postpartum or postabortion is termed a placental polyp. It is associated with massive bleeding, at times months or even years after delivery. Ultrasonography after delivery has low sensitivity in detecting retained trophoblastic tissue; if detected, this tissue appears as a heterogenous, hyperechoic tumor. At times retained fluid is evident. Sonohysterography detects freefloating endometrial tissue, yet the role of endovaginal sonohysterography is not clear in this setting, although it is more sensitive than endovaginal US; it should decrease the number of curettages in women with bleeding.

Magnetic resonance imaging reveals most placental polyps as pedunculated intrauterine tumors. They are hyperintense on T2-weighted images, with blood often surrounding them. Postcontrast, these polyps enhance more than myometrium and appear as a high signal intensity uterine cavity tumor.

Bleeding postpartum is generally managed medically. At times hysterectomy is necessary for unresponsive major bleeding, but arterial embolization should be considered in intractable postpartum bleeding not controlled with vaginal packing and uterotonic drugs. In these patients angiography is useful not only to identify a bleeding site but also to act as a guide for selective embolization of the vessel involved. Embolization controls both immediate and delayed bleeding. Generally multiple arteries are embolized, including uterine, vaginal, ovarian arteries or even a division of the internal iliac arteries. Few complications are encountered with uterine artery embolization, and normal menses resumes in almost all women with obstetric hemorrhage who undergo selective uterine artery embolization (26).

It takes about 7 weeks for the postpartum uterus to involute to its baseline.

The differential diagnosis of a placental polyp includes a choriocarcinoma, although the latter is associated with elevated serum b-hCG levels. A leiomyoma has variable signal intensities.

Gestational trophoblastic disease and a uterine arteriovenous malformation are also in the differential.

A rare cause of late postpartum hemorrhage after cesarean section is an aneurysm bleeding into the uterus; angiography not only is diagnostic but also permits therapeutic embolization.

Gestational Trophoblastic Disease

Gestational trophoblastic diseases include a spectrum of disorders ranging from hydatidiform mole (molar pregnancy) to a choriocarcinoma and the rare placental site trophoblastic tumor. They originate from trophoblastic tissue and are believed to be due to abnormal fertilization. Most are located in the uterus, although an ectopic one originates in the ovary and other sites of an ectopic pregnancy.

Hydatidiform Mole

A hydatidiform mole is not a neoplasm, although an occasional one progresses to a persistent trophoblastic neoplasm (see below). A mole can be complete or partial.

A complete molar pregnancy develops when a normal sperm fertilizes an aberrant ovum containing abnormal maternal chromosomes. No viable fetus develops. The placenta contains abnormal chorionic villi and excessive trophoblastic proliferation, with the latter leading to elevated serum b-hCG levels. Following evacuation of a mole, b-hCG levels should gradually return to normal; otherwise continued trophoblastic disease should be suspected.

A partial mole contains a fetus, although usually an abnormal one. A hydatidiform mole can coexist with viable multiple pregnancies. The risk of molar pregnancy is increased at the extremes of reproductive life and in those with a prior mole.

Bleeding and a uterus larger than expected are the most common presentation. Preeclampsia is not uncommon. With current management nearly all of these patients can be cured.

A hydatidiform mole distends the uterine cavity but does not invade the myometrium. Computed tomography reveals a hypervascular tumor, at times containing cystic regions. Either

a diffusely enlarged uterus or a focal tumor is evident.A site of tumor origin needs to be established; at times a sonographically suspected mole turns out to be a mucoid ovarian cyst or even an endometrial carcinoma.

Among 74 women from the New England Trophoblastic Disease Center, US diagnosed a complete hydatid mole in only 10% of patients before the onset of symptoms (27). Ultrasonography of a hydatidiform mole identifies multiple anechoic regions, although this finding is seen only in about 80% of confirmed moles; a fetus is not detected unless a twin pregnancy is present. A partial mole is often not detected.

Magnetic resonance imaging reveals either diffuse uterine enlargement or a focal tumor, loss of normal zonal anatomy, and abnormal uterine vascularity.

Persistent Trophoblastic Neoplasia

Classified under persistent trophoblastic neoplasia complex are invasive mole, choriocarcinoma, and placental site trophoblastic neoplasm. The staging of gestational trophoblastic tumors is outlined in Table 12.3.

Invasive Mole

The presence of chorionic villi and trophoblastic proliferation in the myometrium constitutes an invasive mole. At times invasion is beyond the uterus and is embolic at distant sites. Some moles lead to severe hemorrhage.

Choriocarcinoma

A rare molar pregnancy evolves into a choriocarcinoma (nongestational ovarian choriocarcinoma is discussed later; see Malignant Germ Cell Tumor). Some choriocarcinomas are not preceded by a molar pregnancy but follow a term pregnancy. A choriocarcinoma developing after a full-term pregnancy often has a poor prognosis. A very rare choriocarcinoma develops independently of gestation; it consists of invasive trophoblastic proliferation and lack of formed villi. A gestational choriocarcinoma together with a viable pregnancy is even rarer. Rare infant–mother choriocarcinomas are reported.

Gestational choriocarcinomas invade the myometrium and spread hematogenously. Some first present as a metastasis, including to brain,

ADVANCED IMAGING OF THE ABDOMEN

Table 12.3. Tumor, node, metastasis (TNM) staging of gestational trophoblastic tumors

IT1 Tumor confined to uterus

Low risk: seven or fewer risk factors. High risk: eight or more risk factors.

FIGO, Federation Internationale de Gynecologie et Obstetrique (International Federation of Gynecology and Obstetrics); hCG, human chorionic gonadotropin.

Source: From the AJCC Cancer Staging Manual, 6th edition (2002), published by Springer-Verlag, New York, NY, used with permission of the American Joint Committee on Cancer (AJCC), Chicago, IL.