++
Transabdominal and transvaginal sonography are complementary imaging techniques and should be used together. In general, transvaginal imaging should not be performed without also performing a transabdominal scan, but this may not be practical in a busy clinical setting. Transvaginal imaging allows the probe tip to be placed very close to the organ of interest so that high-frequency probes can be used to generate high-resolution images. However, transvaginal probes have a limited field of view and objects more than a few centimeters away from the probe tip may not be seen. Transabdominal sonography uses lower frequency probes so the field of view is much larger and a better overview of pelvic structures can be obtained. The main drawback of transabdominal scanning is that the resolution is lower, so details of small pelvic structures are not as discernible, particularly ovaries and early pregnancies.
+++
Normal Nonpregnant Pelvis
+++
Transabdominal Scanning
++
Transabdominal scanning is usually accomplished using a 3.5–5 MHz ultrasound probe. The bladder is used as a window in transabdominal scanning, so it should be full to obtain optimal images. In the emergency setting, transabdominal scanning may be performed without a full bladder because it is not practical to have patients drink fluid and wait for an hour while their bladders fill. Intravenous fluid administration will typically lead to rapid bladder filling. Quality images are usually obtained without bladder filling in thin women and those with an anteflexed uterus. Gentle pressure on the probe can also be used to produce good-quality transabdominal images without filling the bladder.
++
The best transabdominal view for evaluating the uterus and its contents is the standard midline sagittal view (Figure 12-2). To obtain this view, the probe is placed on the abdominal wall in the midline just above the pubic bone, with the probe indicator pointing cephalad (Figure 12-3). By convention, the indicator on the probe should correlate with the left side of the monitor so that in sagittal images cephalad structures are on the left side. This view provides a longitudinal image of the uterus and the entire midline stripe should be visible. The cervix is seen just posterior to the bladder angle with the body of the uterus to the left of the angle and the vaginal stripe to the right. The ovaries can be seen by sliding the probe laterally, with the probe indicator still pointing cephalad, and aiming the beam toward the contralateral adnexa, using the bladder as a window. Sometimes when the bladder is very full or a large pelvic mass is present, better images can be obtained by placing the probe directly over the adnexa.
++
++
++
In some cases it may be easier to visualize the ovaries and other adnexal structures with the standard transabdominal transverse view (Figure 12-4). This view is obtained by placing the probe in the midline of the abdominal wall just above the pubic bone, with the probe indicator pointing to the patient's right side. This view provides a transverse image of the uterus and allows the midline of the uterus and the adjacent adnexa to be seen in the same image if the anatomy cooperates. In transverse images, anatomic structures have the same orientation as on a CT scan: right-sided structures are on the left side of the monitor and left-sided structures are on the right. To examine the entire pelvis in transverse planes, the probe should be kept in the midline suprapubic region with the probe indicator pointed to the patient's right and the beam should be aimed caudad and cephalad (Figure 12-5). This motion will allow the uterus to be viewed in transverse sections from the cervix to the fundus respectively.
++
++
++
The ovaries are most commonly found between the body of the uterus and the pelvic sidewall. In their normal location, the ovaries are bound posteriorly by the internal iliac artery and superiorly by the external iliac vein. These structures can be identified and used to help locate the ovaries. Normal ovaries appear as small discrete hypoechoic structures. Individual ovarian follicles are usually not visible with transabdominal imaging. Normal ovaries are not always seen with transabdominal sonography because they are relatively small and may be camouflaged by bowel or other surrounding structures with similar echogenicity. However, adnexal masses are frequently larger and very easy to identify with transabdominal imaging (Figure 12-4D).
+++
Transvaginal Sonography
++
Transvaginal scanning is different from other ultrasound techniques because the ultrasound probe is placed inside the vagina and very close to the organs of interest. Transvaginal sonography is accomplished using a specialized probe with a 5–7.5 MHz transducer. The probe has an indicator, similar to other ultrasound probes, which should correlate to the left side of the monitor screen. The sound beams may emanate straight out from the tip of the probe (end-fire) or at an angle from the tip of the probe (offset). End-fire probes are more versatile and make imaging planes easier to understand. This discussion of scanning planes will assume that an end-fire probe is being used.
++
Before the transvaginal probe is used, it must be thoroughly cleansed and covered by a rubber or vinyl sheath. Conducting gel should be placed inside the sheath before the probe is covered for appropriate sound transmission. Most sonologists use specially made latex condoms as probe covers while some prefer to use vinyl gloves as probe covers.115 Water-based lubricant, not conducting gel, should be used to lubricate the outside of the sheath before insertion into the vagina. Ultrasound conducting gel may be irritating to the vaginal mucosa. Patients should empty their bladder before transvaginal scanning is performed. A full bladder will straighten the angle between the uterus and vagina and move the body of the uterus away from the probe. Patient positioning is important in obtaining good transvaginal scans. The operator must be able to aim the probe anterior enough to see the fundus of an anteverted uterus. Scanning is best accomplished while the patient is in lithotomy stirrups or by elevating her pelvis on a pillow while she is in a frog-leg position. Many clinicians prefer to use lithotomy stirrups and perform transvaginal sonography as part of their pelvic examination, after the speculum and bimanual examinations.
++
Before inserting the transvaginal probe, the procedure should be explained to the patient. It is usually best to explain that transvaginal sonography is similar to the bimanual pelvic examination but visual rather than tactile information is obtained. Transvaginal sonography should not be painful and is usually very well tolerated by patients. Anxious patients may be given the option of inserting the probe into the vagina themselves.
++
The probe is initially inserted with the probe indicator pointed toward the ceiling (Figures 12-6 and 12-7). The uterus is easily recognized upon insertion of the probe. This is the standard transvaginal sagittal view; it produces a longitudinal image of the uterus similar to the transabdominal sagittal view but rotated 90 degree counterclockwise (Figure 12-8). The entire uterine midline stripe should be seen in this view. If the uterus is not seen immediately, then it may be extremely anteverted and the probe should be aimed upward toward the anterior abdominal wall, keeping the indicator pointed toward the ceiling. Lateral movement of the probe can be used to scan from side to side through the entire pelvis (Figure 12-6). The uterus appears as a relatively hypoechoic structure with thick walls and a well-defined border. The endometrial midline stripe is thin during the proliferative phase and thick during the secretory phase of the menstrual cycle (Figures 12-9 and 12-10). The cervix can be seen by pulling the probe back a few centimeters and aiming the probe tip downward toward the patient's back (Figure 12-11). In this view, the posterior cul-de-sac should be inspected for any evidence of free fluid.
++
++
++
++
++
++
++
After the uterus is identified, the ovaries can be found by their position relative to the uterus. They are usually found just lateral and posterior to the body of the uterus, between the uterus and the lateral pelvic wall. The sonographic appearance of the ovaries is distinct. They are relatively hypoechoic structures containing multiple anechoic follicles (Figures 12-12, 12-13, and 12-14). To find the ovaries in sagittal oblique planes, the probe is aimed laterally, with the probe indicator still toward the ceiling (Figure 12-6). The internal iliac artery and vein can often be identified and used as a guide because the normal position of the ovary is adjacent to these structures. Sometimes the ovaries cannot be identified with transvaginal sonography.116
++
++
++
++
The standard transvaginal coronal view may be better for surveying the entire pelvis. This view is obtained by turning the probe indicator toward the patient's right side (Figures 12-15 and 12-16). The coronal view gives a transverse image of the uterus and allows the uterus and ovaries to be seen in the same plane. The entire pelvis can be explored with oblique coronal planes by aiming the probe up toward the anterior abdominal wall and down toward the patient's back, keeping the probe indicator pointing toward the patient's right side (Figure 12-17).
++
++
++
++
Transvaginal sonography is a dynamic imaging technique. To visualize structures, they need to be very close to the tip of the probe. When structures are not readily visualized, operators should use their free hand to palpate the patient's anterior abdominal wall, similar to performing a bimanual pelvic examination.30, 69, 115 Pressure on the anterior abdominal wall will often bring an ovary or a mass into the field of view. Also, the abdominal hand and the transvaginal probe can be used together to manipulate pelvic contents and observe how the organs move in relation to one another. An ovary may be easier to identify if it is seen as a discrete structure moving independently from adjacent loops of bowel. Also, structures that appear as complex masses may be comprosed of multiple smaller structures that move independently of each other. Holding the transvaginal probe very still and observing for bowel peristalsis is a good method for differentiating bowel from other pelvic structures. Finally, the tip of the transvaginal probe can be used to try to localize pelvic pain. This may help the physician narrow the differential diagnosis when a mass or other abnormality is visualized.
++
Although several standard imaging planes have been described, the pelvis can often be scanned without concern for specific planes. Once an organ or a mass is identified, the probe can be turned in any direction that helps the operator obtain better images. Also, as long as the entire pelvis is imaged in a systematic organized manner, the use of specific planes is probably not crucial.117
+++
Normal Early Pregnancy
++
Both transvaginal and transabdominal sonography can be used to detect an early intrauterine pregnancy. Transvaginal ultrasound can identify an intrauterine pregnancy at about 5 weeks' gestational age (3 weeks postconception), about 7–10 days earlier than transabdominal ultrasound. The convention when referring to the age of a pregnancy is gestational age, which is the date from conception plus 2 weeks. An approximate correlation can be made between gestational age, β-hCG level, and pelvic ultrasound findings (Table 12-1).4, 30, 53, 69, 118–124
++
++
Transvaginal sonography is now the standard modality for evaluating early pregnancy. The following descriptions pertain to transvaginal sonography, except where specifically noted. The transvaginal technique is referred to as “endovaginal” sonography by some authors. The first sonographic sign of early pregnancy, the intradecidual sign, can be seen at 4–5 weeks (Figure 12-18). The intradecidual sign is a small sac, only a few millimeters in diameter, which is completely embedded within the endometrium on one side of the uterine midline, not deforming the midline stripe.30, 124, 125 There is a focal echogenic thickening of endometrium surrounding the sac. The intradecidual sign can be seen only by using a high-resolution technique (5 MHz or higher) and is not an accurate indicator of intrauterine pregnancy.125
++
++
A gestational sac can be clearly identified at about 5 weeks. With transvaginal sonography, a gestational sac can be seen in most patients with β-hCG levels of 1,000–2,000 mIU/mL and in all patients with levels above 2,000 mIU/mL.123 A gestational sac is characterized by a sonolucent center (chorionic sac) surrounded by a thick symmetric echogenic ring, known as the chorionic rim. This finding is seen in most intrauterine pregnancies but can also be seen surrounding a pseudogestational sac associated with an ectopic pregnancy.69 Doppler ultrasound can be used to measure peritrophoblastic flow in order to distinguish a true gestational sac from a pseudogestational sac.126 However, since this is outside the realm of bedside emergency sonography, identification of a simple gestational sac should not be used as definitive evidence of an intrauterine pregnancy.
++
Many authors consider a clear double decidual sign as the first definitive evidence of an intrauterine pregnancy.30, 69, 127 The double decidual sign is two concentric echogenic rings surrounding a gestational sac (Figure 12-19). The inner ring is the same structure as the chorionic ring and is called the decidua capsularis. The outer ring is called the decidua vera, derived from the stimulated endometrium of the uterus, while the thin hypoechoic layer between them is the endometrial canal.30, 127, 128 A gestational sac with a vague or an absent double decidual sign is not diagnostic of an intrauterine pregnancy and may be a pseudogestational sac. If two clear rings are seen, an intrauterine pregnancy is very likely. Unfortunately, the double decidual sign is present in only about half of all intrauterine pregnancies and is not 100% accurate.129
++
++
The yolk sac is the first structure that can be seen inside the gestational sac (Figure 12-20). Some authors consider the yolk sac the first definitive evidence of intrauterine pregnancy.53, 124 It is probably prudent for inexperienced sonologists to visualize the yolk sac before making a diagnosis of an intrauterine pregnancy, avoiding misinterpretation of more subtle findings like the double decidual sign. The yolk sac is a symmetric circular echogenic structure at the edge of the gestational sac. The yolk sac has a role in the transfer of nutrients to the embryo during the first trimester and early hematopoiesis takes place there. The yolk sac can first be seen by transvaginal sonography at about 5–6 weeks and then shrinks and disappears by about 12 weeks.130
++
++
The embryo appears as a thickening or small mass that is seen at the margin of the yolk sac between 5 and 6 weeks Figure 12-21A). The normal embryo will grow rapidly, about 1 mm per day. The embryo can first be seen when it is only 2–3 mm and cardiac activity may not be detectable initially. By 6 weeks, the embryo is a distinct structure separate from the yolk sac (Figure 12-21B). Also, the tiny vitelline duct, which connects the yolk sac to the base of the cord, can sometimes be seen between the yolk sac and the embryo.
++
++
Cardiac activity should be detected within the embryo at about 6 weeks. Any embryo measuring greater than 5 mm should have cardiac activity when transvaginal sonography is used. At 7 weeks, the embryo will be about 12 mm and the head of the embryo will be clearly distinguished. At this age, the embryo's head contains a single large cerebral ventricle and has an appearance similar to the yolk sac.69 At 8 weeks, the head of the embryo is about the same size as the yolk sac and limb buds begin to appear (Figure 12-22). Also, the physiologic midgut herniation can be visualized as an echogenic mass anterior to the trunk of the embryo. The bowel becomes intra-abdominal and the hernia disappears by 12 weeks. At 8 weeks and beyond, a thin echogenic line, the amnionic sac, may be seen surrounding the embryo.
++
++
At 10 weeks, organogenesis is complete and the embryo is now referred to as a fetus (Figure 12-23). Between 10 weeks and the end of the first trimester, the contours of the fetus become much more obvious. The fingers and toes can be identified and counted. Limb movements can be observed and bones and joints can be recognized. In the head, the falx cerebri becomes very distinct and the prominent choroid plexus can be seen in each of the lateral ventricles. The kidneys and bladder can be evaluated at 12 weeks. The heart and the stomach can also be identified inside the trunk and a four-chamber heart can be recognized by the end of the first trimester. Finally, the face and palate can be easily recognized late in the first trimester.
++
++
Routine screening for fetal abnormalities is not typically performed during the first trimester; the optimal time for this is at 18–20 weeks. However, some obvious abnormalities may be identified and it is important to know which structures are usually seen during the first trimester. There is some utility to evaluating nuchal thickness with transvaginal sonography, between 11 and 14 weeks, as a screening test for exomphalos and trisomies 18 and 13, but this is outside the realm of emergency bedside sonography.
++
Measurements of both the gestational sac and the embryo are accurate in the first trimester. Tables and formulas are available for calculating the gestational age using these measurements.4, 131 However, modern ultrasound software automatically calculates gestational age when calipers are placed on the structures of interest and appropriate presets are used.
++
The earliest measurement that can be used for pregnancy dating is mean sac diameter (MSD) of the gestational sac. MSD is the average of three orthogonal measurements of the gestational sac: (length + width + depth)/3. Pregnancy dating using MSD is only useful at 5–6 weeks, when the gestational sac is present but an embryo is not yet seen.
++
When an embryo is visible, at about 6 weeks, measurement of the crown-rump length (CRL) of the embryo should be used to date the pregnancy.53 When measuring CRL, it is important to measure the maximal embryo length, excluding the yolk sac (Figure 12-24). Errors can occur when the calipers are not carefully placed at the margins of the embryo. Also, the embryo can flex and extend slightly, changing the measurement. Nevertheless, gestational age determination by CRL is accurate to within 5–7 days.30
++
++
Measurement of the biparietal diameter (BPD) of the fetal skull is used for pregnancy dating at the end of the first trimester and during the second trimester. The BPD is a transverse measurement of the diameter of the skull at the level of the thalamus. The calipers should be positioned from the leading edge of the skull (outer table) on the near side to the leading edge of the skull (inner table) on the far side (Figure 12-25). Errors can be made by measuring the wrong part of the skull or if the image plane is not a true transaxial section through the fetal head. Pregnancy dating by BPD is also very accurate, especially prior to 20 weeks.69
++
++
Documentation of the chorionicity and amnionicity of a multiple pregnancy is important early in the pregnancy because it may be hard to determine later in pregnancy. There are several sonographic criteria that can be used to determine chorionicity and amnionicity in the first trimester. Two clear gestational (chorionic) sacs may be seen as early as 6 weeks; this is good evidence of dichorionic twins (Figure 12-26). Later in the first trimester, dichorionicity can be established by finding a thick septum separating the two chorionic sacs (Figure 12-27). If the septum separating the two pregnancies is thin, then it may be difficult to determine whether it is the wall of a chorionic sac or an amnionic membrane. When the septum is thin, identification of a chorionic peak can confirm a dichorionic twin pregnancy. A chorionic peak is a triangular projection of tissue, of the same echogenicity as the placenta, emanating from the placenta and tapering to a point in the intertwin membrane.30
++
++
++
The amnionicity of a monochorionic pregnancy can also be determined by first trimester sonography. Counting the number of yolk sacs is the easiest way to determine amnionicity; if there are two yolk sacs, then there must be two amnions.69 After about 8 weeks, amnionic membranes should be visible and diamnionic pregnancies should have a separate amnion surrounding each twin.