Artifacts, Pitfalls, and Normal Variants

  • Outline

  • An Unfortunate but Common Pitfall, 1093

  • Technical Considerations, 1095

    • Mirror Image Artifact, 1095

  • Pelvis, Gravid and Nongravid Uterus, 1098

  • The First Trimester, 1114

  • The Fetal Head and Neck, 1122

  • The Fetal Thorax, 1132

  • The Fetal Abdomen, 1135

  • Amniotic Fluid and Membranes, 1143

  • The Fetal Skeleton, 1146

  • Multiple Gestations, 1151

When I began my involvement with diagnostic sonography nearly 4 decades ago, this chapter would have been considered ludicrous. Virtually all of sonography was considered to be either an artifact or a pitfall. Clinicians did not take this modality seriously, and few critical decisions were ever based on the results of the ultrasound examination alone. With time, improvements in both technology and our understanding of normal and abnormal findings made this a useful clinical diagnostic modality. It did not take long for important clinical decisions to be based solely on the results of the ultrasound examination, including surgery, early delivery of the fetus, or even termination of the pregnancy. This evolution, although welcomed by many, has placed a large responsibility on the sonologist. The phrase “primum non nocere”—first do no harm—has never been more true.

This chapter is not an attempt to explain the physical principles of sonography or artifact production. It is also unlikely that such a chapter could ever be all-inclusive. I have attempted to find examples of pitfalls, the diagnostic dilemmas that have the potential to lead us to the wrong diagnosis. I have tried to cover both basic potential pitfalls and some of the more esoteric normal variants. Undoubtedly, some readers will find examples here that are so basic as to seem almost insulting. I apologize in advance and will only respond that this chapter is meant to appeal to a wide audience that includes beginners and seasoned experts. I also do not attempt to give an overly detailed explanation for each example but try to offer what is theorized currently.

If I am able to avoid one false positive diagnosis and prevent unnecessary surgery, termination of a pregnancy, or even 20 weeks of an emotional roller coaster for expectant parents, I have fulfilled my goal.

An Unfortunate But Common Pitfall

I begin this chapter as the subheading suggests, with a common but unfortunate pitfall.

A 34-year-old woman presented to the emergency department complaining of mild pelvic pain with a known history of positive pregnancy test. A quantitative serum β-hCG (human chorionic gonadotropin) revealed a value of 3870 mIU/mL. (This value is nearly double most centers’ “discriminatory zone.”) A diagnostic ultrasound examination revealed no evidence of an intrauterine pregnancy (IUP) or adnexal masses. As a result, the patient was presumed to have an ectopic pregnancy (EP) and was treated with intramuscular methotrexate. A repeat sonogram obtained weeks later revealed a living IUP. The patient elected to terminate the pregnancy.

FIG 38-1

A, Transvaginal sonogram obtained at approximately 4.5 weeks demonstrating no evidence of an intrauterine pregnancy. Additional scans revealed normal ovaries without evidence of an adnexal mass. B, Transabdominal sonogram obtained 1.5 weeks later revealed a living intrauterine pregnancy.
When a woman of child-bearing age presents to her clinician or emergency room with pelvic pain or bleeding and a positive pregnancy test, clearly the first thought in the diagnostic tree is the possibility of an EP. Most clinicians are aware that the risk of a heterotopic pregnancy (in which both an extrauterine [ectopic] and IUP occur simultaneously) is quite low (1 in 15,000 to 1 in 30,000), although this phenomenon is more common in patients following assisted reproduction. Thus, the goal of sonography is to identify an IUP, making an EP unlikely. Two factors have helped achieve this goal:

  • 1.

    In the early 1980s, Kadar and others attempted to determine the maternal serum hCG level above which a gestational sac should be consistently seen during sonograms in a patient with a normal IUP. The value was termed the “discriminatory zone.” It should be noted that the values initially reported were significantly higher prior to the advent of transvaginal sonography in use today. Although this upper limit value today is variable at differing institutions, it tends to be between 1500 and 2000 mIU/mL (first IUP).

  • 2.

    With the advent of high-resolution transvaginal sonography, early pregnancies can now often be identified within the uterus when gestational sacs are as small as 2 to 3 mm.

A noninvasive method of treating EPs with methotrexate became available in the early 1980s. As a result, some clinicians, then and now, treat the patient with methotrexate when both an “empty uterus” at the time of sonography and an elevated β-hCG level (above the discriminatory zone) are present, in an attempt to treat the EP. Unfortunately, some of these patients subsequently have been found to have IUPs, not EPs, on follow-up. It is quite clear that even marked elevations of β-hCG above the discriminatory zone are not always reliable in excluding an intrauterine pregnancy when a gestational sac is not seen at sonography. In these cases, follow-up, rather than immediate treatment, is indicated. Likewise, even in the best of hands and with excellent ultrasound equipment, at 4 to 5 weeks’ gestation an IUP may not be identified sonographically, though the patient may in fact have an IUP. The report should ideally read “An ectopic pregnancy, or a pregnancy too early to be seen within the uterus, cannot be excluded.”

In October 2012, the Society of Radiologists in Ultrasound convened a multispecialty consensus conference to establish criteria on early first trimester diagnosis of miscarriage and exclusion of a viable IUP when seen in the uterus, as well as to discuss diagnosing and ruling out a viable IUP in a woman with a pregnancy of unknown location. These criteria and an editorial discussion were published in the New England Journal of Medicine ( ; ). The takeaway points from this panel regarding this pitfall were as follows:

  • A single hCG assessment, regardless of level, does not reliably determine a pregnancy’s location or viability (this is because hCG levels in women with nonviable IUPs, viable IUPs, and EP overlap substantially).

  • A single hCG level less than 3000 mIU/mL should not elicit treatment for presumed EP because there is a substantial risk that administering methotrexate will damage a normal IUP.

  • A single hCG level 3000 mIU/mL or higher in the absence of a sonographically visualized IUP indicates that a viable IUP is possible but unlikely. The most likely diagnosis is failed IUP, so if methotrexate is administered, it is more likely being given unnecessarily to a woman with a failed IUP than to one with an EP. At least one additional hCG level should be measured before initiating treatment for EP.

Technical Considerations

Mirror Image Artifact

A mirror image artifact is created when the ultrasound beam is reflected from a strong reflector that is often obliquely oriented and redirects the echoes off a secondary reflector back to the transducer. The ultrasound computer assumes the sound traveled in a straight line. The mirrored image results in the copy appearing on the same image, though deeper because of the additional time for the mirrored image to return to the transducer. In conventional gray-scale imaging, as well as color Doppler imaging, the mirrored artifact will always be the deeper structure.

FIG 38-2

Schematic representation of the mirror image artifact. Ultrasound (US) signals are normally reflected by the structure (fetal head) and by the reflective surface (posterior uterine wall and bowel) arriving on time to the transducer. Some ultrasound signals bounce back and forth between the head and the reflective surface, finally returning to the transducer. Because they arrive later than the original signals, they are represented as another structure behind/deep to the reflective surface.

(From Ahn H, Hernandez-Andrade E, Romero R, Ptwardhan M, et al: Mirror artifacts in obstetric ultrasound: case presentation of a ghost twin during the second-trimester ultrasound scan. Fetal Diagn Ther 34:248-252, 2013, used with permission.)

FIG 38-3

A, Longitudinal transvaginal sonography demonstrating an intrauterine pregnancy on the left and the mirror image artifact of the image on the right. B, Longitudinal transvaginal ultrasound image demonstrating an intrauterine pregnancy on the left and mirror image artifact on the right.

(From Malhotra R, Bramante RM, Radomski M, Nelson M: Mirror image artifact mimicking heterotopic pregnancy on transvaginal ultrasound: case series. West J Emerg Med 15(6):712-714, 2014, used with permission.)

FIG 38-4

Sonogram over the inguinal area in a gravid patient at 34 weeks with round ligament varices. Color Doppler imaging demonstrates prominent vessels with venous flow ( arrows ). A mirror image (M) of the varices is seen deep to the vessels.

FIG 38-5

A and C, Two cases of artifact ( arrows ) appear in the fetal stomach and duodenum. B and D, If one changes the scanning plane, these echoes will disappear from the area of concern.

FIG 38-6

Side lobe (gradient) artifact can occur with even the most sophisticated ultrasound scanning equipment. In this case, a side lobe artifact is seen overlying the fetal head ( arrows ). The artifactual nature of these echoes can be confirmed by noting that these echoes are not confined to the head but extend beyond its confines. In addition, changing the scan plane orientation will show the region to be normal.

FIG 38-7

The differentiation of real from artifactual echoes is often problematic during sonographic scanning. A common misconception is that changing to a lower frequency transducer and decreasing the overall system gain or just reducing the system gain will only cause the artifactual echoes alone to disappear. Although it is true that artifactual echoes will not be displayed with these maneuvers, it should be remembered that the real echoes will not be displayed either. A, In this patient, numerous low-level echoes ( arrows ) were seen swirling in the amniotic fluid during real-time scanning. Although the origin of the echoes is not known, they may represent shed fetal epithelial cells. B, When the system gain was decreased, these real echoes virtually disappeared. Although the hard copy images do not resolve this dilemma, the echoes were clearly seen to be moving and real when the sonologist was performing the scan.

FIG 38-8

Bowel within the pelvis can masquerade as ovarian cysts. A, In this patient, bowel gas and its shadowing create the appearance of a mass ( asterisk ). Bl, bladder. B, The strong reflection adjacent to the urinary bladder (Bl) or the “squared” appearance ( arrows ) to the “cyst” should make one suspicious that bowel gas artifact is causing this appearance. C, In a different patient, a true ovarian cyst ( cursors ) has borders on nearly every side and has enhanced through-sound transmission and internal echoes. Bl, bladder. D, In another patient, two different planes of section demonstrate what appears to be a large pelvic cyst (?C). In fact, this was due to bowel gas artifact. UB, bladder. E, Because this appearance was virtually indistinguishable from a pelvic cyst, a computed tomographic scan was performed immediately after the sonogram, confirming the artifactual nature of this “cyst.” The scans were normal without evidence of a pelvic mass.

Pelvis, Gravid and Nongravid Uterus

FIG 38-9

A, What appears to be a large pelvic cyst ( arrow ) is seen in this patient. Bl, bladder. B, Although the connection to the bladder was not seen in A, this bladder diverticulum (D) is clearly seen in another plane of section.

FIG 38-10

Dilated fallopian tubes simulating ovarian cysts. A, A transverse sonogram in this patient demonstrates what appear to be two ovarian cysts ( arrows ) posterior to the uterus ( cursors ). B, A longitudinal sonogram displays the elongated tubular nature of this fluid collection ( arrow ), which is more compatible with a dilated fallopian tube. U, uterus. C, In another patient, two large, rounded fluid collections are seen ( arrows ) simulating ovarian cysts. D, A longitudinal plane of section through one of these collections demonstrates the tubular (T) retort nature of these dilated fallopian tubes.

FIG 38-11

A 19-year-old woman presented with severe pelvic pain and negative pregnancy test. She had been in an emergency room 1 month earlier with the same symptoms. A, A pelvic sonogram demonstrates an enlarged ovary with peripheral small cysts. B, Doppler sonogram of the ovary demonstrates arterial and venous blood flow. Three hours later an avascular necrotic ovary from ovarian torsion was removed at surgery. Don’t be falsely reassured by an apparently normal Doppler sonogram when ovarian torsion is suspected.

FIG 38-12

Bowel simulating an ovarian cystic teratoma (dermoid). A, A large shadowing mass ( asterisk ) is seen adjacent to or possibly emanating from the ovary (Ov). B, Minutes later the normal ovary and adnexal structures are well seen. The bowel, which was the cause of the shadowing, was not seen.

FIG 38-13

Two round and oval structures are seen in the right adnexa. Although they might be mistaken for abnormal ovaries or paraovarian masses, they represent normal prominent bowel. Typical hypoechoic muscularis ( arrows ) is seen.

FIG 38-14

Pregnancy with coexistent ovarian carcinoma. Unfortunately, just because a patient is pregnant does not mean that she cannot have a concomitant ovarian cancer. A multilocular cystic mass ( arrows ) with solid components is seen adjacent to the gravid uterus.

FIG 38-15

A, Transverse axial sonogram of the pelvis in a nongravid patient. There appears to be either a duplication of the uterus or the uterus and an adjacent mass (C, F). Rt, right. B, A sagittal scan demonstrates that what was seen in the transverse scan was the cervix (C) and fundus (F) of an anteverted uterus. The dashed line represents the plane of section in A. H, head.

FIG 38-16

Longitudinal transvaginal sonogram through the cervix of a nongravid uterus. A and B, Multiple nabothian cysts ( arrows ) are seen in the cervix (Cx). At times these cysts can be quite large ( asterisk ). These retention cysts are quite common and should not be confused with low-implanted gestational sacs or other abnormalities. C and D, Two coronal T2-weighted magnetic resonance images through the cervix demonstrate multiple, round, T2-weighted hyperintense structures compatible with nabothian cysts ( arrows ).

FIG 38-17

A, Transverse scan through the gravid uterus demonstrates a uterine synechia ( arrow ) crossing the uterine cavity from the anterior to the posterior wall. The singleton fetus moved freely around the synechiae. This should not be confused with an amniotic band. B, In another patient a uterine synechia ( arrow ) is seen adjacent to the fetal head. C, Three-dimensional rendered image of the fetus in B demonstrating the synechia ( arrows ).

FIG 38-18

Sonogram of a gravid uterus in the second trimester demonstrating a circumvallate placenta (Pl). The infolding of the edge of the placenta ( arrows ) should not be confused with a uterine synechia.

FIG 38-19

A, Transverse scan of the pelvis demonstrating a left adnexal solid mass ( asterisk ). Although the anatomic position might suggest an ovarian lesion, the solid nature of the mass should suggest the possibility of a broad ligament myoma, which was ultimately demonstrated on magnetic resonance imaging. U, uterus, R, right. B, Coronal transvaginal scan demonstrating a broad ligament myoma (FIB) clearly separate from the uterus (UT) and ovary (RT OV).

FIG 38-20

A, Significant overdistention of the urinary bladder results in the appearance of a markedly elongated cervix ( double-headed arrow ) measuring 6.1 cm. One is visualizing not only the cervix but the apposed anterior and posterior walls of the lower uterine segment. B, Once the patient has emptied her urinary bladder, the true cervical length ( between calipers ) can be accurately measured at 3.09 cm.

FIG 38-21

A distended urinary bladder has caused the anterior and posterior walls of the lower uterine segment ( arrows ) to come near one another. The trapped amniotic fluid gives the appearance of an incompetent cervix. The cervix was normal in this case.

FIG 38-22

A myometrial contraction (M) is common in first and second trimester sonograms. This should not be confused with a myoma. Two features help make this distinction: First, in general, myometrial contractions tend to bulge inwardly without affecting the outer contour of the uterus. Uterine myomas tend to bulge inward and outward. Second, myometrial contractions may resolve during the time of scanning.

FIG 38-23

A myometrial contraction (M) in this second trimester pregnancy simulates a myoma. This should not be confused with a succenturiate lobe of the placenta. P, placenta.

FIG 38-24

Myometrial contractions of the lower uterine segment are common and may often touch one another. A, In this case the contractions ( asterisks ) of the lower uterine segment simulate an open and incompetent cervix. B, The normal-appearing cervix, in fact, is well seen ( calipers ) caudal to this contraction.

FIG 38-25

Myometrial contractions of the lower uterine segment may touch one another ( asterisks ). These opposed contractions have been referred to as “kissing contractions.” In this case, it gives the impression that the placenta (Pl) overlies the internal cervical os.

FIG 38-26

A, “Kissing contractions” of the lower uterine segment ( asterisks ). These contractions should not be confused with the cervix ( arrow ). B, With time there was partial resolution of the contractions ( asterisks ), so that the true cervical length (3.51 cm) could be measured ( arrow ).

FIG 38-27

Artifactual short cervix. A, Transperineal scan demonstrates a short cervix ( between calipers ) measuring 1.8 cm. Shadowing ( arrows ) from the rectum obscures the remainder of the cervix. B, A transvaginal scan in the same patient demonstrates a normal length to the cervix ( between calipers ) of 3.43 cm.

FIG 38-28

Dynamic changes of cervix. Three transvaginal scans of the cervix ( A to C ), taken minutes apart, demonstrate progressive dilatation of the cervix with fluid entering the endocervical canal. This finding should be reported to the referring obstetrician and the residual shortened closed length of the cervix should be reported, as well as the total cervical length.

FIG 38-29

A, A slightly oblique longitudinal scan through the uterus. A small area of placental tissue ( arrow ) appears separate from the major portion of the posterior placenta (Pl). This appears to be a succenturiate lobe of the placenta. B, When the scan plane is aligned slightly more midline the anterior placental tissue (Pl) in A can be seen to be continuous with the remainder of the placenta. C, A transverse axial plane of section demonstrates continuity between the anterior and posterior placental tissue (Pl).

FIG 38-30

In some patients, the surface area over which the placenta implants may be small. This short insertion site may create the false appearance of a thickened hydropic placenta (P).

FIG 38-31

Changing placenta previa. A, A transvaginal scan in a patient at 17 weeks demonstrates a complete placenta previa. The leading edge ( arrow ) of the posterior placenta (P) extends across the internal cervical os. B, Six weeks later at 23 weeks, the leading edge of the placenta extends to but not beyond the internal cervical os.

FIG 38-32

A, Transabdominal longitudinal sonogram demonstrating what appears to be complete placenta previa. The placenta (Pl ) appears to completely cover the internal cervical os ( arrow ). B, Transvaginal sonogram from the same patient demonstrates the placenta (Pl) extends to but does not cover the internal cervical os ( arrow ).

FIG 38-33

A, Veins present in the decidua basalis and myometrium contribute to a hypoechoic region beneath the placenta ( arrows ). This should not be misinterpreted as an abruption. B, Doppler interrogation of this region will often confirm the venous nature of this area. C, In this case, the veins ( arrows ) deep to the placenta (P) are clearly seen.

FIG 38-34

A hypoechoic area ( asterisk ) behind the placenta (Pl ) simulates placental abruption. Rather, this represents a focal myometrial contraction that resolved during the examination.

FIG 38-35

Prominent vessels ( arrows ) at the periphery of the uterus are common in the uterus and should not be mistaken as a precursor to an abruption or as trophoblastic disease. A, In this patient, the arcuate vessels ( arrows ) at the periphery of the uterus are quite prominent. B, With color Doppler flow imaging, the vascular nature of these structures is well seen.

FIG 38-36

Transverse scan of the gravid uterus demonstrates the fetal head (H). A calcified structure ( arrows ) is seen adjacent to the fetal head. At first glance, this appears to be another fetus but in fact it represents a calcified myoma.

FIG 38-37

Incarcerated uterus. A persistently retroflexed or retroverted uterus may become “trapped” within the sacral hollow. A, Longitudinal scan demonstrating an incarcerated uterus in the early second trimester. What appears to be the lower uterine segement is in fact the fundus (F) that is stuck within the sacral hollow. The cervix (C) is drawn anteriorly and superiorly. The urethra often becomes obstructed, and these patients are often seen in the emergency room with urinary obstruction. P, placenta. B, After manual reduction, the cervix (C) and uterus now have a more normal appearance. The placenta (P), which appeared to be posterior in the prereduction scan, is now seen anteriorly.

FIG 38-38

A, A transvaginal sonogram demonstrates an embryo and yolk sac ( arrow ) centrally positioned in what appears to be the uterus. B, A transabdominal sonogram of the same patient demonstrates that the pregnancy was in the fallopian tube (TUBE). A small fluid collection representing fluid within the endometrial cavity (decidual cast) of the uterus (UT) is seen. The patient had an ectopic pregnancy.

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Feb 16, 2019 | Posted by in ULTRASONOGRAPHY | Comments Off on Artifacts, Pitfalls, and Normal Variants
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