Interpretation
Vasa previa is traditionally classified into TypeⅠand Type Ⅱ, whereas
more recent papers have described Type Ⅲ. TypeⅠvasa previa is with
velamentous cord insertion (VCI), Type II shows interconnecting vessels
between two lobes of placenta in a bipartite placenta or connecting
vessel with a succenturiate lobe of the
placenta[10], and Type Ⅲ refers to boomerang
vessels beyond the edges without placental mass protection[11].
The American College of Obstetricians and Gynecologists recommends the
usage of color Doppler in patients who are at a tremendous risk for vasa
previa[12,13]. TypeⅠ vasa previa will be more
easier to be detected by ultrasound, because it just forms at the
velamentous part of the cord. Trials have shown Ultrasound performed
best for the diagnosis of VCI with a sensitivity, specificity and
positive predictive value of 100, 99.9 and 85.7%, respectively, with
routine use of color Doppler[14]. TypeⅡvasa previa
is more difficult to be diagnosed by ultrasound, because the vasa previa
forms between two lobes of placenta with normal umbilical
insertion[15].
No large studies have been performed evaluating the effectiveness of MRI
in detecting vasa previa[16]. There has been a
case study by Kikuchi et al, in which MRI was used as a problem solving
measure in identifying the placenta and expediting
management when ultrasonography proved to be
inconclusive[17]. But studies have demonstrated
MRI may aid in the assessment of placental structures, number of lobes
and position, and can play an associated role when diagnosis by
ultrasound evaluation is equivocal, especially in case of vasa previa
associated with two lobed placentas[18].
In the 12 patients from the study group, MRI demonstrated vasa previa in
11 patients. The rest one was excluded of vasa previa and reached term
delivery. In all the 51 patients from the control group, MRI had
excluded vasa previa. Pathology examination after delivery had confirmed
these MRI diagnoses. The study had shown excellent sensitivity for MRI
in diagnosing potential vasa previa when no conclusive ultrasonic result
returned, and specificity as well.
The reason we considered that ultrasonic examination had not detected
the vasa previa may include the following, but not limited to these
reasons:
Firstly, fetal vessels may run at an unfavorable insonation angle to the
relatively
fixed transducer. The most common sections for pelvic ultrasonography
are standard horizontal, sagittal, coronal, and oblique sections
originate from the standard ones[19,20].
Thereupon, if the previa vessel long axis was not just in the insonation
plane, it would not form a curve image but only a tiny cross section. In
this circumstance, the color Doppler could only catch a round red or
blue signal in small diameter. Addionally, because of the patients’
low-lying placentas like the 4th, 5th, 7th and 8th paitents, or even
with really tiny velamentous parts like 2nd and 3rd patients (Figure 2
& Figure 3), the ultrasound could not distinguish the blood flow of
vasa previa from that of the placenta.
Secondly, visualization of vasa previa may be difficult with
transvaginal sonography alone. Even if the transvaginal probe actually
rotated and remained at the previa vessel long axis direction, the
ultrasonic definition of transvaginal gray scale might not be high
enough to distinguish the vessel[21]. In this
circumstance, the color Doppler could be beneficial, however, the color
Doppler scanning should be turned on before the transvaginal probe
rotation. Still, due to the low lying placenta making the lower uterine
segment not formed well, the previa vessel was not just over the
cervical internal os, but posterior to the relatively thick cervical
tissue in transvaginal scanning, bringing in further obstacle to
identify it, like the 2nd, 4th, 5th, 7th and 8th paitents. Similarly,
ultrasound could not clearly exclude vasa previa that with velamentous
vessel moving laterally away from cervix as the lower uterine segment
extended during the third trimester, like the 12th patient (Figure 12).
Thirdly, in some cases, the vasa previa just carried only a small
portion of blood flow, as in TypeⅡand Ⅲ. And recognized from
ultrasonologists’ feedback, in other cases, the fetal heads may be so
low that compress the previa vessels, making them really difficult be
seen. If that happens in TypeⅠvasa previa only with three umbilical
vessels in the velamentous part, two umbilical arteries and one
umbilical vein, the continuous fetal heart rate monitoring should show
fetal distress. But if that is in TypeⅠwith mangrove or even parachute
morphology like 4th patient (Figure 4), and merely a tiny branch
compressed, or in TypeⅡand Ⅲ, no fetal distress would display. Thus, the
blood flow in that previa vessel might be very slow, or not be
continuous but intermittent. If there was too low velocity signal that
the ultrasonic Doppler sampling gate could not fit well to distinguish
it, or even no flow at the time gap during ultrasonic scanning, the vasa
previa could still not be identified[22], like 5th
and 6th patients (Figure 5 & Figure 6).
Addionally, from this study, it shows that when the vasa previa is close
to lower uterine lateral wall, the vessel signal might disguise itself
among the uterine blood flow signals. That causes some TypeⅡand Ⅲ cases
more difficult to be identified, like 7th to 11th patients.
MRI can be an alternative in identifying vasa previa, if the ultrasound
scanning results are inconclusive, owing to some advantages it possesses
in this field[23].
Firstly, MRI is independent of the placental location, the maternal
obesity, the maternal bladder, the operator, and the directions of
velamentous vessels, unlike ultrasound[24]. It may
be most useful as an adjunct in difficult cases where ultrasonography is
equivocal, the patient is difficult to scan, or if the placenta is
implanted in the posterior uterus[25].
Secondly, MRI is tomography, so the image definition for spatial
structures and soft tissues will be able to detect absolutely thin
vessels[26].
Thirdly, MRI can reconstruct images in three
dimensions[27]. A single ultrasonic image only
shows one certain plane. While in MRI, the coronal, horizontal and
sagittal aspects can be exhibited as 3 simultaneous images within a
single display. That assists to determine the vessels positions, within
amnion or intramuscular, regardless of different vascular directions.
Therefore, any direction of the previa vessels path can be confirmed,
not like the ultrasonography that needs the previa vessel’s long axis
within the certain scanning plane to see the vessel curve.
Traditional conceptions claimed MRI with several disadvantages, include
cost, time, patient monitoring, and the general lack of expertise in
interpreting the studies[28]. However, these
disadvantages diminished in recent years.
Firstly, MRI costs higher than ultrasound. As the 3-D and 4-D techniques
have been widely applied in ultrasonic equipment and elevated the
individual scan cost, the financial gap between MRI and ultrasonography
is less obvious nowadays[29].
Secondly, MRI is relatively unfit to emergent cases comparing with
ultrasound. In real clinical work, if there’s suspected vasa previa with
emergent issues such as vaginal bleeding, the most imminent management
is cesarean section, not either ultrasound or MRI
scanning[30].
Thirdly, MRI is not so available as ultrasound in underdeveloped areas
as our country. But accurate ultrasonography diagnosis also needs well
experienced and skilled ultrasonologists, which would not be so
available in remote hospitals. As more MRI devices widely deployed in
recent years, MR scanning might be possible there. Specialists at
tertiary centers can analyse the images uploaded from hundreds of miles
away. Advanced than ultrasound, MRI digital evidence is fully kept, so
any colleagues can review and discuss the previous scanning results at
any time when preferred and reach more accurate
diagnoses[31].