Ultrasound Diagnosis of Placenta Accreta: A Tutorial for Imagers - HD

Introduction

Hello, my name is Dr. Dolores Pretorius. I'm a professor of radiology at the University of California San Diego, and I'm also the director of fetal imaging at the UCSD Maternal Fetal Care and Genetics Center in La Jolla, California.

I'm going to talk about the ultrasound diagnosis of placenta accreta, a tutorial for imagers. And I did this presentation with the help of Allison Matti from UC San Diego.

Definition of Placenta Accreta

Placenta accreta is a clinical condition when part of the placenta or the entire placenta invades and is inseparable from the uterine wall.

So you can see this normal placenta right here. And then accreta here is when the placenta villi or in direct contact with the myometrium. Increta is when there is extension of the placenta into the myometrium, but not into the serosa. And percreta is when it, the placenta grows past the serosa layer here often into the bladder.

This is thought to be due to a defect in the decidua basalis but is most commonly seen when there's been prior uterine surgery, particularly cesarean sections. And the treatment of this is to do a cesarean hysterectomy because of massive hemorrhage.

Risk Factors

The risk factors for placenta accreta include prior uterine surgery, particularly C-section or a myomectomy or even instrumentation in the uterus, often from a DNC. Placenta previa, advanced maternal age, grand multiparity and conception by in vitro fertilization.

And you can see this area of percreta growing through the uterus in this specimen that was taken out from this hysterectomy. You can see in this cesarean section, any of these risk factors should prompt a sonographic search for placenta accreta.

Diagnosis

The diagnosis is typically by ultrasound in the second or third trimester, but this diagnosis can be made in the first trimester as it was in this patient at 12 weeks, which was done transabdominally. And this is a percreta case and you can see these multiple lucencies in this patient.

Here we have a patient that is at about 28 weeks and you can see the placenta growing through the serosal layer into the bladder. And here is a color picture showing this chaotic blood flow with vertical vessels going into this region of accreta.

Now the diagnosis of placenta accreta can also be made on MRI, and we mainly use MRI as an adjunct to ultrasound when we have ambiguous ultrasound findings. The findings on MRI is bulging of the placenta thinning of the myometrial wall here, placental lucencies or heterogeneous hypo intense T2 placental bands that come in vertically and obliteration of this normal tissue interface with the endometrium.

This white area here is the bladder, which is our landmark for us to be able to look for the accreta in that region.

Goals of the Module

So the goals of this module are first, unfortunately the diagnosis of placenta accreta can be easily missed even by experienced imagers. This tutorial is intended to educate medical providers on the sonographic appearance of placenta accreta with attention to criteria for an adequate study and common pitfalls.

In this module, we're going to show normal pictures of the placenta of gross pathology of posterior placenta and placenta previa. In addition, we're going to show images of the findings of placenta creta including low implantation of the gestational sac, placenta lakes, myometrial thinning interrupted serosa and a color color doppler abnormalities. We are also going to give you a quiz at the end to show you some of the pitfalls.

Normal Placenta Anatomy

So what is normal? Normal placenta anatomy as shown on this image with a fundal placenta? The placenta is this homogeneous texture sitting here in the top of this uterus with a hypoechoic myometrium. Notice that this is a shadowing from one of the limbs, whereas the fetus is right here. And then this white line is the serosa layer.

This is a gross pathology correlation showing normal placenta here and normal myometrium here. This patient had a focal percreta, but it was in another section of this pathologic specimen. Here we have normal myometrium and normal interface of the placenta.

This myometrium can be quite hypoechoic, but there's often lots of little vessels in that area. And this appearance of the myometrium varies from entirely echoes to shades of gray. It is always less echogenic than the placenta, however.

Now this is a picture with these overlaid colors of the same image showing you where this findings are. So the placenta is this blue area right back here. The bladder is this maroon color. The green line is the serosa and notice how bright it is there against the bladder. The orange area is the myometrium and then the cervix is right down here where they've measured it. This is the fetus.

Placenta Previa Without Accreta

So first we're going to look at placenta previa without accreta because the vast majority of accreta do occur in the setting of placenta previa.

So if we look at this picture first we see the placenta in blue, then we see the bladder appear in the maroon, the cervix in purple. And then when we look at this, we see that the serosa is nice and thin walled and white right here along the surface of the uterus that this yellow area is the myometrial clear space right here and this clear space is measuring.

If it's less than a millimeter then that has been shown to be predictive for placenta accreta. There should be a hypoechoic region between the retro placental vessels and their serosa or that clear space.

Now then we have this orange area. These are myometrial blood vessels. These are not placental lucencies and you can see them here as well as here. And there they are. Those are the blood vessels going along the edge of the myometrium.

We always need to have a picture of the bladder in the image to be able to assess whether the placenta could be growing into the bladder. Now this is a major pitfall if you don't tip the transducer to get the picture of the bladder with the placenta. And we need to look at this in two views, in order to look for disruption of the serosa, because this is an area that can be very tricky to recognize.

Signs of Placenta Accreta on Ultrasound

So the signs of accreta on ultrasound include one low implantation in the first trimester, two placental lakes, three myometrial thinning, four irregular placental interface, and five abnormal color doppler images.

Low Implantation

So first let's look at low implantation. This is a finding we see in the first trimester, because accreta can be suspected when you have a low implantation of the gestational sac.

So in this case we can see from the overlay the cervix is here and then the gestational sac is this blue area and then there's some hemorrhage above it and the fundus is over here. And this is a six weeks pregnancy.

Here we have a normal appearance of the cervix, the fundus and the sac much more superior in this patient. So when the sac is low, this should raise concern for placenta accreta.

Placental Lakes

The next finding we look for are placental lakes. It's often the first thing to catch your eye when you're looking at a pregnancy and you see that there's these lakes and it makes you say to yourself, oh, are there any risk factors? Does she have any prior surgical history like a C-section or a myomectomy to prompt us to look for the less obvious signs?

So here you can see these multiple lucencies where the yellow arrows are in this patient who was at 20 weeks gestation. Another image showing multiple lucencies. And that's different than this normal homogeneous placenta that we normally see.

If there's lots of lucencies, they're often called a moth eaten appearance to the placenta. Here's another case at 11 weeks where you can see these multiple lucencies and the placenta growing up here into this myometrium. And here at 35 weeks multiple moth eaten appearance to the placenta. And notice that's in difference to this normal very homogeneous placenta.

So one of the pitfalls is that the myometrial blood vessels that you see here can mimic placental lucencies. So you need to look for these and see if it's a contraction and your normal placenta is really on top versus whether or true lucencies this patient has lucencies scattered throughout this placenta. Whereas this is just the normal myometrium.

Myometrial Thinning

The next finding we look for is myometrial thinning. This may be the only sign of accreta in the initial scan, but it is frequently subtle and we suggest a low threshold for follow up.

So here we have a 12 week scan with normal myometrium below in the green area and a loss of the myometrium here in the yellow area. Say another patient at 11 weeks. And this is a percreta that goes up into the region of the serosa layer with multiple lucencies and thinning here, whereas here you can see the normal myometrium over there and over here and sometimes your normal myometrium can look very black in a clear space and you just turn on color like in this patient of 33 weeks gestational age to see that's just the normal myometrium.

Here we have myometrial thinning, so it's normal here and it's quite thin right along this region of the placenta also in this region of the placenta. Now this case you can see these multiple lucencies. It might have to make sure that if you pick up your transducer and have a light touch that to make sure that this is not just pressure of your transducer on the myometrium, but this turned out to be placenta accreta and this is a much more normal looking myometrium underneath that.

Interrupted Serosa

Okay, the fourth finding is interrupted serosa. It's most commonly seen in the bladder view, but it may be present anywhere along the placental interface.

So for example, in this case you can see the normal serosa here, but there is no serosa here or it is lost and that's a region of percreta. This was a 19 week transvaginal scan. Here is a 27 week with percreta and you can see the lack of the interface right here.

This patient, you have loss of the normal serosa right here and the normal serosa up here. Here again for comparison, previa only with a nice normal serosa. This patient had no visible serosa at the bladder interface right here and this had penetrated into the bladder at 18 weeks.

And this is a picture with the overlay showing the placenta and the bladder and the cervical area and then the normal myometrium back here behind here. And once again previa for comparison. This patient has a focal area of accreta and invasion here along the anterior abdominal wall at 27 weeks.

Few accreta patients will have this finding because the interrupted bladder serosa is much more common where you see the bladder interface. But this you can see is invading up here. It's near the bladder but we don't see the bladder here. Normal myometrium right here. This was a patient with percreta. And this is the path correlate of this patient with that percreta.

Now one of the pitfalls in the lower uterine segment is to view the bladder's curvature and see that there's a refractive artifact that comes off that curvature and mimics loss of the serosa. So here you have an absence of serosa and here you have normal echogenicity. So here it looks like it's absent and here we have the normal.

And this is a normal for comparison to show you this refractive shadowing that comes across the region of the bladder. So you really need to move your transducer in different positions to be able to decide whether it's real or whether it's artifactual.

Abnormal Color Doppler

The last finding we look for is abnormal color doppler. And you always have the rest of the placenta to look at for an internal comparison to see if it's a change from the normal to the areas of accreta of where the placenta is growing into the wall.

So for example, if we look at this, we can see this very chaotic flow with flow vertically going into the bladder. And notice that the bladder interface is right there. And this has lots of chaotic and vascular flow, but if the same patient you look at here is the chaotic flow and here's some nice normal flow off to the edge, in this patient at 35 weeks.

And often when I see multiple placenta lucencies, I'll have our team go and look at the interface along the entire placenta to see whether it's normal. Now one pitfall is that one image doesn't give you the answer. This looks like normal color flow and this looks like accreta and it's in the same patient. Okay? And that's because you're going to have some area of accreta right here, and some areas where it'll look more normal.

And that's because this is the area of focal abnormality. So always remember to move up and down in the same patient and make sure that you have some bladder as well as your placenta.

Quiz

Okay, now you're ready for the quiz. How do we know that this is an accreta? Can you point out the findings?

So this is the loss of the serosa. This is the irregular interface of the placenta bulging into the bladder. These are the lakes. This is irregular flow, vertical flow going into this myometrium in the same area of lac of ser. So that's accreta.

How about this 34 week transvaginal scan. We see the cervix right here. What do you think? Is it an accreta or not? This is an example of an inadequate image. The placenta accreta cannot be ruled out here because you don't have the bladder face in the image.

So right here we see the cervix and there's the bladder interface and we can see this a lot better that this is just placenta previa allowing us to rule out accreta. That's the normal myometrium, the normal serosa.

These pregnancies are both eight weeks and five days. Which one do you think might be accreta? Well the image on the left is accreta because we see a low sac. The top of the fundus is right up here, whereas this patient is normal, the sac is filling most of that uterus and extending up to the fundus. So low implantation.

Are these placental lucencies or normal myometrial vessels. How could we tell the difference? This is a transvaginal image at 19 weeks. Clearly the head is right here and there's your bladder interface.

Well the answer is that these are placental lucencies, but just one image doesn't definitely tell you that sometimes you need to go to other images because it can be quite tricky. Now in this case, I think when you go in and look at this transverse view, it really helps you because there's the normal serosa and here we have placenta growing through that serosa.

So this tells us that this is a placenta per accreta on these two pictures. Why do we suspect placenta accreta? Well, the region of the yellow is a worrisome part for thinning and loss of the myometrium. The normal hypoechoic myometrium is here. This looks like normal myometrium, but this looks abnormal and there's our normal for comparison.

Is this interrupted bladder serosa? Well the answer is we cannot say based on this image as there is refractive artifact obscuring the region of the serosa. So we need to come in from another angle, get rid of that shadowing and see that yes indeed there is a break in the serosa and this turned out to be placenta percreta.

A cleaner image shows that this patient did not have loss of serosa at that area. Okay, label each image based on color doppler. Do you think this is a case, these four different cases? Which ones are suspicious for accreta versus which ones are likely normal or no accreta?

So this is in accreta with the blood flow vertical going into the region of the myometrium. This is really just placenta previa with nice myometrial flow here. And the placenta over the cervix. This one's normal with the cervix here and placenta previa and this is abnormal with it turned out to be accreta with vertical vessels going in with lots of extra flow.

Conclusion

That's the end of our talk. We hope that these images will help you identify placenta accreta in the future.