1 Preface

Placenta previa means that the placenta is implanted in the lower segment of the uterus, partially or completely covering the internal opening of the cervix, and its severity can range from mild edge previa to severe complete previa. The position of the placenta plays a vital role in the development of the fetus. If placenta previa leads to the failure of the fetal head to pass through the cervix, it is very easy to cause serious complications such as fetal asphyxia, massive bleeding, and even endanger the life safety of pregnant women and fetuses. Therefore, timely and accurate diagnosis of placenta previa is very important. MRI technology is widely used in clinical medicine, especially in the diagnosis of gynecological and obstetric diseases. Compared with other examination methods, MRI technology can provide clearer and more accurate images, especially for soft tissue imaging. In the diagnosis of placenta previa, MRI technology can directly observe the location and size of the placenta and cervix, and can clearly reflect the distance between the placenta and the cervix, the coverage area and degree of the placenta, as well as the length and opening degree of the cervix [1]. In addition, MRI can also detect abnormal placental lesions, such as placental abruption, placental transposition, placental abruption, placental thickening and other lesions, which helps to take effective treatment measures in time to ensure the health of pregnant women and fetuses [2]. Based on this, this study selected 100 pregnant women with placenta previa to analyze the diagnostic value of MRI.

2 Data and Methods

2.1 General Information

The study selected 100 pregnant women with placenta previa who were admitted to our hospital from January 2017 to January 2020 as the study objects. The random number table was used to group 50 cases in Group A, aged from 24 to 35 years, with a mean of (30.24 ± 1.15) years; The gestational weeks ranged from 29 to 36 weeks, with a mean of (33.02 ± 1.15) weeks; The number of pregnancies was 1–4, with a mean of (2.15 ± 0.75); The body mass is 51–79 kg, with an average of (62.15 ± 2.33) kg; There were 48 singleton pregnancies and 2 twin pregnancies. One placenta of twin pregnancy is placenta previa, and the rest is normal placenta. The time of termination of pregnancy was 33 ~ 39 weeks, and 18 pregnant women had a history of cesarean section. More than 22 pregnant women were admitted to hospital with vaginal bleeding, and 28 pregnant women did not have vaginal bleeding. 50 cases in group B, aged from 22 to 34 years, mean (30.12 ± 1.20) years; The gestational weeks ranged from 28 to 36 weeks, with a mean of (33.07 ± 1.12) weeks; The number of pregnancies was 1–3, with a mean of (2.19 ± 0.74); Body mass is 50–77 kg, with an average of (62.11 ± 2.08) kg; There were 49 singleton pregnancies and 4 twin pregnancies. One placenta of twin pregnancy is placenta previa, and the rest is normal placenta. The time of termination of pregnancy was 34 ~ 39 weeks, and 20 pregnant women had a history of cesarean section. More than 25 pregnant women were admitted to hospital with vaginal bleeding, and 25 pregnant women did not have vaginal bleeding. After comparison, the data are recorded as P > 0.05.

  1. (1)

    Inclusion criteria

The time of MRI examination of pregnant women is in the middle and late pregnancy, 28 weeks and above; This pregnancy includes single pregnancy and twin pregnancy; There is no contraindication for MRI of pacemaker and steel plate implantation in pregnant women; Pregnant women and their families agreed to this examination.

  1. (2)

    Exclusion criteria

Pregnant women have unbalanced breathing, nervous tension and efficiency, and their cooperation with inspection activities is low; The fetus moves frequently, and the error rate is large when MRI is performed; Pregnant women with other pelvic diseases [3]; Pregnant women who finally stop treatment because they do not meet the inclusion criteria or have adverse events during the study.

2.2 Methods

2.2.1 Ultrasonic Inspection Method

The pregnant women in this study were examined with GEvolusionE8 Doppler ultrasound instrument under the frequency of 2 ~ 9 MHz. During the examination, the pregnant woman should keep the supine position, hold the urine as much as possible to keep the body bladder full, and then place the detection probe on the pregnant woman's abdomen. Scanning in multiple ways, such as cross cutting and straight cutting, should be adopted. During the examination, the growth and development of the fetus should be carefully observed to judge the relationship between the placental edge and the uterine orifice. Attention should be paid to constantly changing the direction during the examination, so that the pregnant women are in different positions. Observe, analyze and record the posterior placental space, surrounding tissues and hemodynamics, and fully understand the placental status of pregnant women. After the examination, the results will be diagnosed by senior doctors in a double-blind manner, and the diagnosis will be negotiated if the diagnosis results are inconsistent.

In the color Doppler ultrasound examination, the placental position was mainly observed. The placental position was lower than the fetal presentation and close to the lower segment of the uterus. The distance between the lower edge of the placenta and the inner mouth of the cervix is within 2 cm.

2.2.2 MRI Examination Method

During MRI examination, GE3.0TMRI scanner was used to examine the pelvic cavity of pregnant women. During the process, pregnant women first lay flat on the examination table, and the scanner was used to scan the pelvic cavity in sagittal, coronal and transverse axial positions. Pay attention to place the placenta of the pregnant woman at the core of the examination. The scanning layer is 5 mm thick, with a total of 30 layers. The specific scanning parameters are shown in Table 1.

Table 1 MRI scanning parameters
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Two doctors with the title of deputy director or above performed blind film reading. The MRI signs of placenta accreta met the following requirements: ① uneven signal in placenta; ② The uterine contour is locally convex; ③ Increased and thickened vascular shadows in placenta; ④ Tent like change or nodular process of bladder; ⑤ The boundary between uterine myometrium and placenta is unclear; ⑥ In T2WI sequence, intraplacental banding low signal shadow.

2.3 Observation Indicators

Observe the detection rate of disease types, including central type, marginal type, partial type, placental depression, adherent placenta, and placental implantation. Based on surgical pathology, the sensitivity formula is the true positive number divided by the false negative number; The formula of specificity is true negative number divided by (true negative number plus false positive number); The formula of missed diagnosis rate is the number of false negatives divided by (the number of true positives plus the number of false negatives); The formula of misdiagnosis rate is the number of false positives divided by the number of false positives plus the number of true negatives. To analyze the imaging manifestations of placenta accreta and non placenta accreta after MRI diagnosis.

2.4 Statistical Analysis

Using SPSS 280 statistical software for statistical processing. Measurement data are expressed in (x ± s), independent sample t-test is used for data comparison, counting data is expressed in [cases (%)], and data comparison is performed in χ2. The difference was statistically significant (P < 0.05) as shown in Table 2.

Table 2 Comparison of detection rates of disease types between the two groups [n/%]
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3 Results

3.1 Comparison of Detection Rate of Disease Types Between the Two Groups

The detection rate of central type, marginal type and partial type in group A was higher than that in group B, and there was a difference after data comparison (P < 0.05).

3.2 Analysis of Diagnosis Results of Two Groups

According to the gold standard of surgery and pathology, the number of true positive, true negative, false positive and false negative in group A is 42, 5, 2 and 1 respectively; Group B has 34 true positive, 2 true negative, 4 false positive and 10 false negative as shown in Table 3.

Table 3 Analysis of diagnosis results of two groups
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3.3 Comparison of Diagnostic Efficacy Between the Two Groups

With surgery and pathology as the gold standard, the diagnostic accuracy and sensitivity of group A were higher than that of group B, and the rate of missed diagnosis was lower than that of group B (P < 0.05) as shown in Table 4.

Table 4 Comparison of diagnostic efficacy between the two groups [n/%]
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3.4 MRI Diagnosis of Placenta Previa with Placenta Accreta

In group A, there were nine cases with placenta implantation and 41 cases without placenta implantation. There were differences in imaging manifestations of pregnant women, such as uneven intraplacental signal, increased/thickened intraplacental vascular shadows, and unclear boundaries between placenta and uterus (P < 0.05) as shown in Table 5.

Table 5 Comparison of images of pregnant women with placenta previa and placenta accreta diagnosed by MRI [n/%]
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4 Discussion

4.1 Overview of Placenta Previa

Placenta previa means that after 28 weeks of pregnancy, the lower edge of the placenta is significantly lower than the fetal presentation position and close to the cervical opening. Clinically, the distance from the lower edge of the placenta to the cervical orifice of the pregnant woman is taken as the determination standard of placenta previa, and it can be divided into marginal, partial, complete, etc. in different distance positions. Even if a pregnant woman has a low placental position after more than 28 weeks of pregnancy and does not meet the diagnostic requirements for placenta previa, attention should be paid to avoid placenta previa due to the increase of pregnancy cycle. In clinical practice, the main symptoms of placenta previa include painless repeated vaginal bleeding, which will increase the probability of anemia in pregnant women. In addition, if the amount of bleeding is too much at one time, it is necessary to carry out rescue in time to avoid death due to shock of pregnant women. In many types of placenta previa, the most bleeding is complete placenta previa, and the least bleeding is marginal placenta previa. At present, the etiology of placenta previa has not been clarified clinically, and it is generally believed to be related to factors such as the number of cesarean section. From the perspective of the development of placenta previa in pregnant women, the risk of placenta previa and placenta accreta in pregnant women with a history of cesarean section is high, especially in pregnant women with three or more cesarean sections, the risk of placenta accreta in pregnant women can reach 67%, while the risk of placenta accreta in pregnant women without a history of cesarean section is only 3.3% according to the research results at home and abroad [4].

The mechanism of placenta previa is that the decidua of the uterus of the pregnant woman will have a certain inhibitory effect on the growth of villi under the normal pregnancy activities of the pregnant woman. Therefore, the history of induced abortion of the pregnant woman, the elderly pregnant women and other conditions will have an impact on the decidua activity, so that the uterine decidua of the body cannot inhibit the growth of villi, and will continue to develop toward the uterine wall, and finally placenta implantation will occur. At present, in clinical diagnosis of placenta accreta, due to the lack of clinical symptoms and the defects of laboratory examination, it is impossible to make scientific judgments. For example, ultrasound examination can only provide reference for pregnant women with placenta accreta and placenta previa. Doctors often analyze the medical history and vaginal bleeding of pregnant women to provide the final diagnosis for pregnant women. However, the application of MRI technology can show the relationship between the placental structure of pregnant women and the myometrium of uterus by virtue of the high level of soft tissue resolution. In addition, it can also detect the placentas of pregnant women from multiple perspectives and open up horizons, and conduct scientific detection on the development of placental villi of pregnant women. The detection results will not be affected by the obesity of pregnant women and amniotic fluid [5]. It is effective and scientific.

4.2 Safety of Placental MRI

At present, ultrasound is the main examination method for placenta previa, but in the process of examination, misdiagnosis may occur due to factors such as maternal abdominal fat, intestinal gas accumulation, bladder filling, etc. At this time, the appearance of MRI technology can not only ensure that the examination activity has a wide field of vision and clear plane influence, and that the amniotic fluid, abdominal fat and other external factors of pregnant women have less interference with the examination activity, but also can quickly identify the soft tissues. The above advantages make MRI technology have advantages in the examination of anterior placentas. The placental examination of pregnant women with MRI technology is a non radiation, multi-faceted imaging combined with soft tissue resolution imaging examination, which is generally used in prenatal screening of pregnant women. In To verify the scientificity of this examination method, the US Food and Drug Administration, the British National Radiological Protection Council and other institutions have actively carried out practice to verify the safety and effectiveness of this method, and thus determined the white paper on MRI examination of pregnant women's placenta. China has also actively studied this examination method, but relevant domestic medical institutions have indicated that pregnant women within 90 days of pregnancy are not recommended to carry out this screening because some instruments, including MRI contrast media, contain metal components that will accompany drugs to enter the fetus via the placenta, which is not conducive to the growth and development of the fetus. Therefore, in the domestic medical field, it is generally not recommended that pregnant women undergo enhanced scanning.

Under placental MRI, fast spin echo sequence can help doctors quickly identify placental signals by virtue of high soft tissue resolution, show the situation of decidua, myometrium and serous layer, and clearly show the relationship between placenta and myometrium; In MRI technology, the balanced steady state autorotation and single shot fast automatic cyclotron wave are both included in the fast scanning sequence, which can minimize the impact on image quality during fetal examination. Even if the pregnant woman has irregular breathing during the examination, it will not interfere with the examination results. During MRI, diffusion weighted imaging can clearly show the diffusion of water molecules and determine the scope. Because placental tissue is presented in the form of high signal in diffusion weighted imaging, the placental condition can be clearly determined with equipment. However, gradient echo weighted imaging in MRI is generally a routine sequence for examination, which can evaluate the specific bleeding in the fetal membrane [6].

The normal placentas of pregnant women generally have three layers of structure, including the medial fetal chorion, the intermediate villous parenchyma, and the lateral basement membrane. With the application of MRI detection technology, the normal placenta of pregnant women shows that the myometrium of uterus is complete and continuous, and the placental signal is average. When pregnant women are detected in the third trimester, they can find placental aging, fibrosis, and signal discontinuity from image data. At present, the objects to be detected in the clinical examination of placental implantation using MRI technology include uterine myometrium, placental tissue signals, morphology, etc., which can intuitively show the abnormalities of the uterus, such as lack of basic tissue, thickening or thinning of myometrium, signal terminals, etc. From the weighted image, it can be seen that the slightly low signal in the muscular layer and the high signal in the line like branch. At this time, the difference in the thickness of the placenta will lead to different degrees of obstacles in signal reception. When the myometrium of the uterus of pregnant women is connected with the placenta, and the junction is uneven or fuzzy, MRI imaging can directly show the placental villi and blood vessels of pregnant women, and the uterine wall of pregnant women will appear circuitous thickening under T2WI. According to investigation and research, under the influence of bladder involvement, there will be “hump like” situation.

4.3 Analysis of the Results of This Study

4.3.1 Detection Value and Imaging Manifestations of MRI for Disease Types

In this study, the detection rate of central type preposition (28.00%), marginal type preposition (26.00%) and partial type preposition (12.00%) in group A was higher than that in group B (12.00, 10.00, and 2.00%), with significant difference in data comparison (P < 0.05). The reason is that MRI technology can obtain high-resolution images without using radiation, and has high accuracy and sensitivity for the detection of placenta previa. MRI technology can directly observe the location and size of the placenta and cervix, clearly reflect the distance between the placenta and the cervix, the coverage area and degree of the placenta, as well as the length and opening degree of the cervix and other information. In different types of placenta previa, MRI imaging will also be different. Central type of placenta previa means that the placenta completely covers the internal opening of the cervix, and the MRI image shows that the placenta is completely located in the center of the uterine cavity. On the MRI image, it can be seen that the edge of the placenta completely covers the internal opening of the cervix, and the upper edge of the internal opening of the cervix presents round or oval signs. Since the internal opening of the cervix is completely covered by the placenta, the head of the fetus cannot pass through the opening of the cervix. In this case, cesarean section and other surgical measures need to be taken in time. Marginal type of placenta previa refers to the part of the edge of the cervix that the placenta covers the inner mouth of the cervix. The MRI image shows that the placenta overlaps the inner mouth of the cervix. On MRI images, it can be seen that the placenta is located at the lower part of the cervix, and the edge is close to the inner mouth of the cervix, but it does not completely cover the inner mouth of the cervix. In this case, it is necessary to closely observe the situation of pregnant women. If the placenta fails to move to the upper part of the cervix at the later stage, cesarean section and other operations should be considered. Partial placenta previa means that the placenta partially covers the inner mouth of the cervix, and MRI image shows that the placenta and the inner mouth of the cervix are partially covered. On MRI images, it can be seen that the placenta is located at the lower part of the cervix, and some of its edges are in contact with the internal mouth of the cervix, but it does not completely cover the internal mouth of the cervix. In this case, it is necessary to closely observe the situation of pregnant women. If the placenta fails to move to the upper part of the cervix at the later stage, cesarean section and other operations should be considered.

4.3.2 Accuracy of MRI in Diagnosis of Placenta Previa

With surgical pathology as the gold standard, the diagnostic accuracy (94.00%) and sensitivity (97.67%) of group A were higher than those of group B (72.00, 77.27%), and the missed diagnosis rate (2.33%) was lower than that of group B (22.73%) (P < 0.05). The reason is that MRI technology has the advantage of high resolution, which can clearly show the location and size of the placenta and cervix. Compared with other imaging inspection methods, such as ultrasound, MRI images are clearer and more detailed, and are not easily affected by factors such as fetal occlusion and restriction during pregnancy, and changes in placental and cervical positions, so its detection accuracy is higher. MRI can be imaged on multiple planes, including transverse, sagittal and coronal, and can observe the relationship between placenta and cervix more comprehensively and stereoscopically. This can better judge whether the placenta covers the inner mouth of the cervix, the area and extent of placenta coverage and other important information, so as to improve the sensitivity and accuracy of diagnosis. This technology does not require the use of radiation, and is more safer for pregnant women and fetuses. Moreover, MRI imaging will not be affected by fetal movement or fetal position, and can be checked in different pregnancy periods, which is very helpful for the early diagnosis and follow-up of placenta previa [7]. In addition, MRI can also comprehensively evaluate multiple parameters, such as the thickness of placenta, the length of cervix, the distance between placenta and cervix, the size and shape of the internal orifice of cervix, etc., to judge the type and severity of placenta previa from multiple perspectives, and more accurately diagnose and evaluate [8].

4.3.3 Differential Effect of MRI on Placenta Accreta

Among the pregnant women in Group A, there were nine cases of placental implantation and 41 cases without placental implantation. There were differences in the imaging manifestations of the pregnant women, such as uneven intraplacental signal, increased/thickened intraplacental vascular shadows, and unclear boundaries between placenta and uterus (P < 0.05). The reason is that the magnetic properties and density of placenta accreta and non placenta accreta tissues are different. MRI can identify placenta accreta by detecting these differences. In MRI images, placental implantation usually appears as a low signal intensity area, because the magnetic properties and density of the tissue around placental implantation are different from those of placental tissue, resulting in the absorption of signals. On the contrary, non placental implanted tissues usually present high signal intensity areas, because their magnetic and density differences with surrounding tissues are small, and signals can be reflected back [9]. The MRI manifestations of placental implantation are: ① uneven signal in the placenta, increased/thickened vascular shadows in the placenta, and unclear boundary between the placenta and uterus; ② Low signal intensity area on T1 weighted imaging and high signal intensity area on T2 weighted imaging. ③ The uterine wall and myometrium around placental implantation can present high signal intensity areas, which is because these tissues have higher water content and more free water molecules. ④ Irregular inflammatory reactions, such as edema and local inflammation, may occur around placental implantation. The MRI findings without placental implantation were: ① high signal intensity areas on T1 weighted imaging and T2 weighted imaging; ② The tissues presenting high signal intensity areas are usually soft tissues with high water content, such as the cervix, endometrium and myometrium.

4.4 New Development of Placental MRI Imaging Technology

With the continuous progress of medical technology, MRI technology has been developed, and the imaging speed and scanning efficiency have been improved. Experts and scholars in the medical field are more actively exploring the development of placental MRI imaging sequences. However, no outstanding results have been obtained. Most of the sequence studies are in the initial stage of animal experiments, and most of them are 3D modeling, multimodal imaging technology, pop, etc. 3D modeling combined with computer technology has been applied in various stages. Professor He Hong studied the 3D model of placenta by combining the fast spin echo T2 weighted sequence and the digital medical 3D design system, which can be used as a guide for surgical activities [10]. The research also introduced 3D reconstruction technology and 3D printing technology, and analyzed their supervision effect on pregnant women's placentas, which can intuitively show the changes of placentas and improve the scientificity of placental examination. With the vigorous development of medical technology, more and more information technologies are integrated into MRI technology, such as multimodal MRI technology, which is to analyze the morphology and function of the examination site from multiple perspectives under the weighted mode. Tao Hui et al. used multimodal MRI technology in their research to analyze the evaluation effect of this technology on the placenta of pregnant women. The research will point out that multimodal MRI has a high sensitivity and specificity in placenta previa and placenta accreta of pregnant women, and the diagnostic efficiency in placenta accreta is as high as 75%. The study also carried out enhanced scanning for pregnant women with placenta accreta after delivery, and the results showed that there were enhancement signs in my residual placental tissue. This indicates that after the development of MRI technology towards multimodality in the future, the scanning technology will be strengthened, which can display the image signs more intuitively and clearly, and assist doctors in diagnosing placenta previa and placenta accreta.

Deep learning has a strong ability of automatic image extraction. Its application in the field of medical imaging can help image data processing, and combined with the computer aided detection and diagnosis system (CAD) of deep learning, it has brought changes to the field of artificial intelligence placenta previa imaging. At the same time, the clinical application of DBT is still in the stage of popularization, which also provides opportunities for in-depth learning and computer-aided detection and diagnosis systems to be integrated into the medical management process.

Deep learning is a mode based on multi-layer neural network as the organizational structure. Compared with the traditional computer-aided detection and diagnosis system, it can automatically extract learning features and penetrate into the information beyond human cognition in the image. The combination of deep learning and medical imaging can be divided into two levels: image and pixel. The image level is used to identify the presence of lesions and malignant and benign lesions, while the pixel level is used to segment lesions. Because it contains a large number of algorithms, targeted and representative algorithms should be used for in-depth learning in different fields. Taking convolutional neural network as an example, it is the most representative algorithm among them, and it is also a feedforward neural network. Under its organizational structure, there are several types of input, convolution, pooling, full connection, and output. The first is to import the original image through the input layer, extract its features through the convolution layer, simplify the pooling layer, fit and classify the full connection layer, and finally display the results by the output layer. Through deep learning combined with MRI examination, the following diagnostic values can be obtained: ① higher accuracy: the method based on deep learning can analyze and learn a large number of MRI images, thus improving the diagnostic accuracy and accuracy. ② Faster: Traditional diagnostic methods usually require manual judgment and interpretation of MRI images, which is time-consuming and has the risk of miscalculation. The method based on deep learning can automatically analyze MRI images and quickly screen and diagnose placenta previa. ③ Non invasive: MRI is a non-invasive examination method, which will not cause any harm or discomfort to the fetus and pregnant women. Joint in-depth learning can comprehensively evaluate MRI images, try to avoid other invasive examinations, and can effectively diagnose diseases. ③ Early detection: the method based on deep learning can screen early pregnant women and timely detect abnormal conditions such as placenta previa, which is helpful to take measures in time to reduce adverse effects on pregnant women and fetuses. In conclusion, MRI examination based on deep learning can improve the operation level and film reading technology of imaging personnel, find potential and hidden lesions, and have higher diagnostic accuracy for diseases such as placenta previa.

5 Conclusion

To sum up, under the application of MRI technology, doctors check the placentas and decidual villi of pregnant women. The efficiency of the examination is high, and the quality of the examination can be guaranteed. It is an important auxiliary examination way for clinical placenta previa and placental implantation symptoms. In this study, there were few placental implantation cases in the pregnant women included in the study, and at the same time, the examination of placental implantation ty** was not completed in a timely and effective manner. Therefore, the examination of normal placental conditions and placental implantation at a lower level in pregnant women is not scientific, and the examination effect needs to be improved. In the field of clinical medicine, a lot of practical exploration is still needed for the identification points of this examination activity.