Transabdominal Technique

The transabdominal technique has long been used to evaluate first trimester pregnant patients and is best done with a curvilinear probe. Before the ultrasound examination, a Foley catheter can be inserted to fill the bladder. The practice of filling the bladder accomplishes two things. First, a full bladder displaces bowel out of the anterior cul-de-sac and acts as an acoustic window to the pelvic organs. Second, a full bladder generally aligns the uterus such that its long axis parallels the abdominal wall. This allows the transmitted sound waves to strike the uterus at a nearly perpendicular angle, which produces better reflections when the returning echoes are received by the transducer and plotted on the monitor. Although this optimizes image quality, adequate images can often be achieved without this maneuver.

In the sagittal view, in which the uterus is seen in its long axis, the probe is placed just superior to the pubic symphysis with the indicator pointed toward the patient’s head. The anterior-most organ on screen is the fluid-filled, triangular-shaped bladder. Just posterior to the bladder is the pear-shaped uterus. The uterus more commonly lies in an anteverted position, with the fundus pointed toward the anterior abdominal wall, but it may also be seen in a retroverted lie with the fundus pointing posteriorly toward the spine. The endometrial stripe serves as the landmark for identifying the longitudinal uterus in the midline. The endometrial stripe is a result of the endometrial mucosal lining coming together to form a hyperechoic, curvilinear line that continues as the cervical stripe more inferiorly (Fig. 120.1, A and B). After the midline of the longitudinal uterus is identified, the probe is panned from side to side to evaluate the entire width of the uterus. Much of the hyperechoic area surrounding the uterus is the bowel and rectum, which are poorly defined because of their solid and gas contents.

Fig. 120.1 A and B, Transabdominal views of the sagittal plane of the uterus revealing the typical landmarks of the uterus, endometrial stripe, cervix, bladder, and vaginal stripe.

In the transverse view, the probe is rotated 90 degrees counterclockwise so that the indicator is pointed toward the patient’s right. With the probe angled inferiorly to visualize into the pelvis, the anterior-most organ is the fluid-filled, rectangular-shaped bladder. Posterior to the bladder is an ovular-shaped cut of the uterus in the transverse view with the hyperechoic endometrial stripe in the center. The rectum and bowel are hyperechoic areas surrounding the bladder and uterus posteriorly and laterally. The uterus should be evaluated from the top of the fundus inferiorly through to the cervix. Though not always visualized, the ovaries may be seen lateral to the fundus of the uterus in either the sagittal or transverse view.

Advantages of the transabdominal technique include (1) an overall view of the true pelvis and (2) a faster examination. The lower-frequency curvilinear probe will penetrate deeper into the pelvis, and although the images might be a little fuzzy, a “full” view of the pelvis is provided. Second, in patients who are farther along in the first trimester or whose bladder is not completely empty, adequate transabdominal views can be obtained without filling the bladder. Less preparation time is needed to scan these patients, and enlisting the assistance of a coworker to chaperone is unnecessary.

The main disadvantage of the transabdominal technique results from the use of a lower-frequency probe. The lower frequency (2- to 5-MHz curvilinear probe) means less resolution and thus the pregnancy must be farther along (usually 7 to 8 weeks’ gestational age) to be visualized. The transvaginal probe is a higher-frequency probe (5 to 9 MHz) with increased resolution, which means that pregnancies as early as 5 to 6 weeks can be visualized.

Endovaginal Technique

The arrival of endovaginal transducers in the 1980s significantly improved the quality of ultrasound imaging in female patients. Simplified, the engineering design placed an ultrasound transducer on the end of a stick that could be inserted into the vaginal vault. This permits the transducer scanning head to be in close proximity to the pelvic organs, which has several important implications in ultrasound imaging. First, there is a clearer path to transmit and receive echoes, and second, the shorter distance between the transducer and the pelvic organs allows higher transducer frequencies to be used.⁶ In contrast to the transabdominal approach, the quality of images from an endovaginal approach is enhanced with an empty bladder because of less distortion of the pelvic anatomy.

The high-frequency endocavitary probe is prepared by directly placing ultrasound gel on the transducer head, sheathing the probe with a sterile cover, and then adding more sterile gel over the sheathed transducer head. If it is not possible to place the patient in the lithotomy position on a gynecology examination table, it is important to adequately prop up the patient’s pelvis for any modified positioning. It is often necessary to manipulate the handle of the probe below the level of the pelvis for better visualization. The covered probe is then inserted into the vaginal vault to begin scanning. Some patients and practitioners have found it more comfortable for patients to insert the probe.

For the sagittal view, the transducer is inserted so that the indicator is straight up and down, with the body sliced into left and right halves. Even after voiding, the bladder usually retains a small amount of fluid, which enables this anechoic structure to be used as a landmark in endovaginal imaging. After initially placing the probe into the vaginal vault, the bladder or uterus (or both) may not initially be visible. To look for these structures, the handle of the probe should be brought down and the footprint of the probe tilted more anteriorly and angled up to get the uterus in its full, longitudinal (sagittal) plane. In this plane, the anterior-most structure is a sliver of bladder, the pear-shaped uterus is posterior to the bladder, and the more hyperechoic bowel and rectum surround the uterus (Fig. 120.2, A to C). As in transabdominal imaging, the endometrial stripe serves as the landmark for identifying the longitudinal uterus in its midline. Once it is identified, the uterus should be interrogated along its entire width by fanning from side to side.

Fig. 120.2 A and B, Transvaginal views of the sagittal plane of the uterus revealing the typical landmarks of the uterus and endometrial stripe. C, Illustration depicting positioning of the probe.

For the coronal view the probe is rotated 90 degrees counterclockwise, with the body cut into anterior and posterior components. As in the sagittal view, the probe is tilted anteriorly by moving the handle of the probe downward to obtain transverse cuts of the uterus. Depending on the depth of probe insertion, some part of the bladder may be seen. Deep to the bladder lies the ovular uterus, which is surrounded by the rectum and bowel.

Advantages of the endovaginal technique include (1) superior resolution and (2) a wider field of view. The shorter pathway from the transducer to the pelvic structures permits the use of higher frequencies. Sound attenuation is limited, and waves reflected from nearby objects are maximized. These higher-frequency probes, with their inherent enhanced resolution, provide the EP with more clinical confidence when interpreting images. In addition, most endovaginal transducers incorporate a beam angle that provides a 120- to 180-degree field of view.

Disadvantages of the endovaginal technique include (1) increased invasiveness, (2) more time to prepare and perform the scan, and (3) decreased depth of view. As the endovaginal probe is inserted into vaginal vault, there is a small risk of transmitting disease through an improperly sanitized probe. Second, the taboo of having an unknown physician insert a probe into the vaginal vault may require greater explanation of the procedure. Optimal examination is performed with a gynecology table, which may be a limited resource. More time commitment is involved in preparation for the endovaginal study and obtaining a chaperone. Third, the higher-frequency probes used for endovaginal scanning limit the range of the transmitted echo. Generally, a full view of the pelvis is not achieved with transvaginal scanning.

Intrauterine Pregnancy

To define a pregnancy as intrauterine, it is imperative to have a clear understanding of the criteria necessary to support the diagnosis by ultrasound (Box 120.3). The first component of the criteria is to visualize a gestational sac with a mean sac diameter (height, width, and length) larger than 5 mm and an echogenic ring. Second, this gestational sac must be clearly visualized within the endometrial lining of the uterus and many times may demonstrate a “double decidual sac sign.” Finally, this gestational sac, which lies within the endometrial echo of the uterus, must contain a yolk sac or a fetal pole. Although this may be excessive for a radiologist who uses the double decidual sac sign as the earliest criteria for an IUP, it is a conservative safety measure that has served EPs well to avoid confusing the pseudogestational sac of an ectopic pregnancy with a true IUP. Because ectopic pregnancies secrete hormones that stimulate the endometrial lining, occasionally there may be changes in the endometrial lining that can approximate the appearance of a gestational sac. This pseudosac will never have an inner yolk sac or fetal pole. Use of these two additional findings for defining an IUP helps ensure that the EP will not be confused. Whether scanning from a transabdominal or an endovaginal approach, all these criteria should be visualized and documented on every sonogram. The safest way to ensure that these criteria are present is a systematic approach to documenting these landmarks. In the endovaginal approach, documentation is best achieved by obtaining a single sagittal view of the gestational and yolk sacs clearly within the endometrial echo of the uterus, along with the inferiorly positioned endometrial stripe as it passes through the corpus and cervical region of the uterus (Fig. 120.3, A and B). In the transabdominal approach, documentation is best achieved by obtaining a single sagittal view of the gestational and yolk sacs clearly within the endometrial echo of the uterus, along with the anteriorly positioned bladder and the inferiorly positioned vaginal stripe (Fig. 120.4, A and B). By identifying pregnancies as intrauterine, ectopic pregnancy has effectively been excluded with reasonable certainty. There is a theoretic risk for a heterotopic pregnancy of approximately 1 in 30,000 pregnancies.⁷ When assisted reproduction is involved, the heterotopic rate is 1 in 7000 overall and as high as 1 in 900 with induction of ovulation.⁸ Even when a live IUP is identified in an assisted reproduction patient, close follow-up with the fertility specialist is recommended.

Fig. 120.3 A and B, Endovaginal views of the sagittal plane showing an intrauterine pregnancy. The gestational sac is located within the endometrial echo of the uterus and contains a yolk sac.

Fig. 120.4 A and B, Transabdominal view of the sagittal plane showing an intrauterine pregnancy. The gestational sac is located within the endometrial echo of the uterus and contains a yolk sac. The landmarks of the vaginal stripe and bladder help confirm the proper location.

Live Intrauterine Pregnancy

Once the pregnancy is determined to be intrauterine, the next designation is viability. This involves real-time documentation of the cardiac activity normally present at 6 weeks’ gestational age when using the endovaginal technique and at 7 to 8 weeks’ gestation when using the transabdominal technique. The criteria for documentation of a live IUP is detailed below (see Box 120.3). Simply having a fetal pole within the endometrial echo of the uterus will not suffice (Fig. 120.5, A and B). For our purposes, an IUP is not live until cardiac activity is documented. The flickering of cardiac activity is an unmistakable sign of life that can be identified by the novice sonographer and the patient as well. Documentation can be confirmed by video in B mode or by identifying the fetal heart rate by M mode (Fig. 120.6). Pulsed wave Doppler should never be used to document fetal heart rates because of the theoretic risk to the fetus of the increased heat generated by this form of ultrasound. Novice sonographers will occasionally use the zoom feature and record or print an image of a pregnancy without any definite landmarks confirming the pregnancy as intrauterine (Fig. 120.7). The problem is that ectopic pregnancies can be indistinguishable with this particular example of a cone-down view, which does not allow visualization of the landmarks that support the criteria for an IUP.

Fig. 120.5 A and B, Endovaginal views of the sagittal plane showing an intrauterine pregnancy. The gestational sac is located within the endometrial echo of the uterus and contains a fetal pole.

Fig. 120.6 M-mode documentation of the fetal heart rate confirming that an intrauterine pregnancy is a live intrauterine pregnancy.

Fig. 120.7 Transabdominal technique in an unknown plane of a live pregnancy.

This image demonstrates that with the zoom function, proper identification of landmarks (bladder, vaginal stripe, uterine fundus) is lost such that proper documentation of intrauterine location is not possible.

Abnormal Intrauterine Pregnancy

A pregnancy can be categorized as abnormal when the gestational sac is disproportionate to its contents. For example, by the time that a gestational sac measures 13 mm in diameter, a yolk sac should clearly be visualized. Likewise, by 18 mm, all intrauterine gestational sacs containing live embryos should have cardiac activity. When using conservative criteria, it is safe to conclude that any sac larger than 13 mm without a yolk sac or larger than 18 mm without evidence of cardiac activity is termed abnormal (Fig. 120.8, A and B). It has been shown in the obstetric ultrasound literature that gestational sacs reaching 13 mm in size with no yolk sac or fetal pole have virtually no viability and ultimately result in miscarriage. In most cases in which viability is equivocal, there is no rush to make a specific diagnosis (IUP versus abnormal IUP). Because these pregnancies have only a gestational sac visualized on ultrasound, obstetric consultation and close follow-up in 48 hours, repeated ultrasound, and quantitative measurement of β-human chorionic gonadotropin will be required.

Fig. 120.8 A and B, Endovaginal coronal plane views of an abnormal intrauterine pregnancy. The gestational sac is located within the endometrial echo of the uterus but is larger than 10 mm and does not have a fetal pole.

Extrauterine Pregnancy

Ectopic pregnancies represent approximately 2% of reported pregnancies, and ectopic pregnancy–related deaths account for 9% of all pregnancy-related deaths.⁹ The most common implantation site is within the fallopian tube (95.5%). Sites of tubal implantation in descending order of frequency are ampullary (73.3%), isthmus (12.5%), fimbrial (11.6%), and interstitial (2.6%). The remaining sites are ovarian (3.2%) and abdominal (1.3%).¹⁰ Visualization of an ectopic pregnancy on ultrasound, even for experienced sonographers, is frequently difficult. As opposed to an IUP, for which ultrasound confirms the diagnosis, an extrauterine pregnancy is often not substantiated by ultrasound and is just presumed because of the absence of an IUP or other findings of a normal pregnancy. The criteria for diagnosis of an extrauterine gestation is a gestational sac outside the endometrial echo of the uterus with evidence of a yolk sac or fetal pole (Fig. 120.9, A and B). In patients who do not have this definitive finding, the diagnosis can be made either intraoperatively or by serial ultrasound in patients who are being monitored very closely. A “ruptured” ectopic pregnancy is frequently suspected on clinical grounds. Supportive sonographic findings of an intraperitoneal fluid collection or an adnexal mass (or both) further strengthen this suspicion.¹¹ Recent literature suggests that a transabdominal approach in which a large intraperitoneal fluid collection is revealed will decrease the time to diagnosis and treatment.¹⁴ A pregnant female with free fluid visualized in the right upper quadrant should have these findings communicated immediately to obstetric consultants to help facilitate a shorter time to the operating room.

Fig. 120.9 A and B, Endovaginal coronal plane views of an extrauterine gestation. The gestational sac is located outside the endometrial echo of the uterus and has a yolk sac.

No Definitive Pregnancy

The diagnosis of no definitive pregnancy is established when a technically adept emergency ultrasound examination fails to diagnose an intrauterine or extrauterine gestation (Fig. 120.10, A and B). When this is encountered, three possible diagnoses exist. First, it is possible that an early IUP is present but no definitive signs are visualized within the uterus by ultrasound. Second, the products of conception my have been aborted and the empty uterus is a result of a miscarriage. Finally, a concealed ectopic pregnancy is not identified by emergency ultrasound.

Fig. 120.10 A and B, Endovaginal sagittal plane views of a uterus with no definitive pregnancy identified. There is no gestational sac in the uterus. In addition, free fluid is seen in the pouch of Douglas.

Absence of proof is not proof of absence. It should be borne in mind that patients in whom no definitive pregnancy is diagnosed are at high risk for an ectopic pregnancy. The presence of free fluid in the pouch of Douglas or elsewhere in the pelvis should increase suspicion for an ectopic pregnancy substantially. The literature varies widely in defining ectopic pregnancy rates in patients with indeterminate or no definitive pregnancy. This may be due to variation in the specific criteria used to define this category. Many researchers have added subgroups such as probable ectopic pregnancy and probable IUP.^11,12 The take-home message is that all patients in whom no definitive IUP is diagnosed should be managed closely with our obstetric colleagues regardless of the β-human chorionic gonadotropin level. An interdepartmental policy addressing this issue would clearly be advantageous to each specialty and the patient.