Diagnostic Utility of Sonography and Magnetic Resonance Imaging in Evaluating Placenta Previa and Its Variants Using Intraoperative Findings as the Gold Standard

By Izza Shahid¹, Nadeem Ibrahim¹, Sadaf Shafique², Zunaira Zulfiqar¹

Affiliations

1. Department of Radiology, Combined Military Hospital, Gujranwala, Pakistan
2. Department of Obstetrics and Gynaecology, Combined Military Hospital, Gujranwala, Pakistan

doi: 10.29271/jcpsp.2025.06.698

ABSTRACT
Objective: To evaluate the diagnostic utility of ultrasonography and magnetic resonance imaging (MRI) for the diagnosis of placenta previa (PP) with accurate grading, keeping peroperative findings as the gold standard.
Study Design: A cross-sectional study.
Place and Duration of the Study: Department of Radiology, Combined Military Hospital, Gujranwala, Pakistan, from March 2023 to April 2024.
Methodology: Pregnant women aged 18-45 years in their third trimester with suspected PP were evaluated using transabdominal ultrasound and non-contrast MRI (1.5T). Perioperative findings during caesarean section served as the gold standard. Diagnostic accuracy was calculated for ultrasonography and MRI in diagnosing the accurate type of PP in terms of sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), keeping peroperative findings as the gold standard.
Results: In a total of 110 cases, the mean and gestational ages were 30.56 ± 3.46 years and 33.25 ± 2.95 weeks, respectively. The sensitivity of ultrasound was 83.3%, while MRI demonstrated sensitivity at 95.0%. Ultrasound and MRI showed equally high specificity at 98.0%. The PPV was 98.0% for ultrasound and 98.3% for MRI. The NPV was 83.1% for ultrasound and 94.2% for MRI. The diagnostic accuracy of ultrasound was 90.0%, while MRI achieved a diagnostic accuracy of 96.4%. The diagnostic performance of USG and MRI did not reveal significant differences (p = 0.065).
Conclusion: Ultrasonography can be a useful, cost effective, and safest initial assessment tool for diagnosis and grading of PP in the late pregnancy. It showed overall comparable results of USG and MRI.

Key Words: Diagnostic accuracy, Gold standard, Transabdominal ultrasound, Placenta previa, Ultrasonography.

INTRODUCTION

Placenta previa (PP) is known to be a high-risk obstetric condition exhibiting abnormal implantation of the placenta near or directly over the internal cervical os, typically within 2 cm of this opening.¹ This condition is a major cause of bleeding during pregnancy and poses significant risks to both maternal and foetal health. PP can be classified into 4 types depending upon the placenta's relationship with the internal os (IO). In type I, the placenta is located in the lower uterine segment but does not reach the IO. Type II occurs when the edge of the placenta extends to the IO without covering it. In type III, the placenta partially obstructs the IO, while in type IV, it completely covers the IO. Understanding these classifications is essential for proper clinical assessment, risk evaluation, and management throughout pregnancy and delivery.^2,3

Various factors play important roles in the development of PP, including previous caesarean sections, multiple pregnancies, and pregnancy at advanced age.⁴  History of caesarean section is considered to be a major risk factor for the development of PP.⁵ There are several morbidities associated with abnormal placentation, which include severe blood loss during delivery with need for blood transfusions, sometimes leading to pulmonary embolism, ventilator use, reoperation, and ICU admission.⁶ Prompt and early diagnosis of PP is essential due to the high risk of antepartum haemorrhage (APH).

Ultrasonography (USG) is a crucial tool for the early detection and diagnosis of PP during routine antenatal scans. The placenta's position is usually evaluated during the anomaly scan, which is performed in the 2^nd trimester. If the placenta is found to be low- lying during this initial assessment, a follow-up USG in the 3^rd trimester becomes essential to confirm whether the placenta's position has persisted or shifted.⁷ This follow-up scan helps in accurately diagnosing PP, determining its types, and planning appropriate management strategies to ensure maternal and foetal well-being. USG can provide detailed information about the placenta's location and signs of abnormal placenta adherence.⁸ Magnetic resonance imaging (MRI) can be an important investigation tool for PP evaluation preoperatively. MRI can detect various features of abnormal placenta location or invasion and further guide in the proper management of patients.⁹

This study would be helpful in addressing the critical need for accurate prenatal diagnosis and grading of PP. Although USG is widely used as the initial imaging modality, it has limitations in accurately determining the type and extent of PP, especially in complex cases. MRI has emerged as a complementary tool with superior soft tissue resolution, potentially offering higher diagnostic accuracy. However, comparative data evaluating the performance of these two modalities against perioperative findings, the gold standard, remain limited. Assessing the diagnostic accuracy, sensitivity, specificity, and predictive values of USG and MRI may provide evidence to optimise imaging protocols and improve clinical decision-making for better maternal and foetal outcomes. This study was performed to evaluate the diagnostic utility of USG and MRI for the diagnosis of PP with accurate grading, keeping peroperative findings as the gold standard.

METHODOLOGY

This cross-sectional, validation study was conducted at the Department of Radiology, Combined Military Hospital, Gujranwala, Pakistan, from March 2023 to April 2024. Ethical approval was obtained from the Institutional Ethical Review Board (ERB No: 21-2023, Dated: 20-02-2023). Inclusion criteria were pregnant females aged between 18 and 45 years, in their 3^rd trimester with clinical suspicion of PP based on history, physical examination, or prior imaging. Exclusion criteria were women with a history of bleeding disorders, on anticoagulant therapy, who experienced APH before 24 weeks of gestation, or those with contraindications to MRI (e.g., metallic implants or claustrophobia). Considering the specificity of MRI in correctly diagnosing placental invasion as 92.3%,¹⁰ with 95% confidence level, and 5% margin of error, the sample size was calculated to be 110.

Eligible participants were enrolled consecutively after taking written informed consent. All participants underwent transabdominal USG using a high-resolution USG machine. All examinations were performed by a certified radiologist with at least 5-year post-fellowship experience. Non-contrast MRI was performed on all participants using a 1.5T MRI scanner. MRI findings were documented systematically. Imaging protocols were standardised to ensure consistency in acquisition and interpretation. During caesarean delivery, placental location and grading were confirmed by direct visualisation and manual examination, serving as the gold standard in collaboration with the obstetrics team. These findings included direct visualisation and manual examination of the placenta. Diagnostic accuracy of USG and MRI in detecting and grading PP, using perioperative findings as the reference standard, was evaluated.

Data analysis was done using IBM-SPSS version 26.0. Diagnostic accuracy was calculated for USG and MRI in diagnosing the accurate type of PP in terms of sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), keeping peroperative findings as the gold standard. Sensitivity measured the ability of each modality to correctly identify cases of PP and its variants, while specificity assessed their effectiveness in ruling out the negative cases. PPV indicated the probability, that a positive test result corresponded to an actual case, whereas NPV reflected the likelihood that a negative result accurately excluded the condition. Overall diagnostic accuracy was determined by calculating the proportion of correctly classified cases (true positives and true negatives) for each imaging modality. The McNemar test was applied to seek the agreement between USG and MRI findings, taking p <0.05 as statistically significant.

RESULTS

In a total of 110 cases, the mean and gestational ages were 30.56 ± 3.46 years and 33.25 ± 2.95 weeks, respectively. USG detected PP in 51 (46.4%) patients, whereas 58 (52.7%) patients were positive on MRI for PP. Figure 1 shows sagittal T2WI MRI image exhibiting that the placenta was predominantly posterio-inferior and was completely covering the os along the inferior aspect with partial extension along the anterior wall as well. There was thinning of myometrium along the inferio-lateral aspect, however, the interface with the bladder wall appeared normal, and no bladder wall invasion was noted. Intra-uterine pregnancy was seen within coephalic lie. Urinary bladder wall was smooth and showed normal wall thickness.

Figure 2 is showing that placenta is low lying and is predominantly anterior completely covering the internal os with partial extension along posterior wall (type IV).

Peroperative findings diagnosed PP in 60 (54.5%) patients. Table I is showing characteristics and diagnostic findings of patients studied.

According to peroperative findings, 60 (54.5%) patients were having PP. Among 60 (54.5%) with positive peroperative findings, PP was graded as type I, II, III, and IV in 12 (20.0%), 12 (20.0%), 24 (40.0%), and 2 (20.0%) patients, respectively (Figure 3).

Table   I:   Patient   characteristics   and   diagnostic   findings   (n = 110).

Characteristics		Frequency
Age (years)	18-30	54 (49.1%)
	31-45	56 (50.9%)
Ultrasonography findings of PP	Positive	51 (46.4%)
	Negative	59 (53.6%)
Magnetic resonance imaging findings for PP	Positive	58 (52.7%)
	Negative	42 (47.3%)
Peroperative for PP	Positive	60 (54.5%)
	Negative	50 (45.5%)

Table  II:  Diagnostic  accuracy  of  USG  and  MRI  for  placenta  previa  and  its  grading  keeping  peroperative  findings  as  the  gold  standard.

Parameters	Ultrasound findings	MRI findings
Sensitivity	83.3%	95.0%
Specificity	98.0%	98.0%
PPV	98.0%	98.3%
NPV	83.1%	94.2%
Diagnostic accuracy	90.0%	96.4%

Table III: Contingency table for diagnostic accuracy of USG and MRI compared to peroperative findings.

Ultrasonography findings	Peroperative findings
	Positive	Negative
Positive	50 (True positive)	1 (False positive)
Negative	10 (False negative)	49 (True negative)
Magnetic resonance imaging findings	Peroperative findings
Positive	57 (True positive)	1 (False positive)
Negative	3 (False negative)	49 (True negative)
Total	60	50

Figure 1: T2WI MRI image showing type 4 placenta previa.

Figure 2: USG findings showing type 4 placenta previa.

Figure 3: Placeta previa grading as peroperative findings (n = 60).

The sensitivity of USG was 83.3%, while MRI demonstrated sensitivity at 95.0%. Both USG and MRI showed equally high specificity at 98.0%. The PPV was 98.0% for USG and 98.3% for MRI. The NPV was 83.1% for USG and 94.2% for MRI. The diagnostic accuracy of USG was 90.0%, while MRI achieved a diagnostic accuracy of 96.4% (Table II). Table III is giving detailed breakdown of diagnostic elevation of USG and MRI in comparison to peroperative findings.

Among cases identified as positive by MRI, USG correctly iden- tified 84.5%, while 15.5% were missed. Among MRI-negative cases, USG correctly classified 96.2%, misidentifying 3.8% as positive. The McNemar test yielded a p-value of 0.065, indicating no statistically significant difference in the diagnostic performance of USG and MRI.

DISCUSSION

PP is the commonest cause of antepartum haemorrhage (APH), and its incidence is increasing with concomitant increase in rates of caesarean sections.¹¹ This study noted that the mean age of patients was 30.56 ± 3.46 years. The findings of this study are quite similar to Rosenberg et al.’s study where they reported a similar mean age of 30.4 years.¹² This study found that about 65% of PP cases had a history of previous uterine surgery. Marshall et al. reported that the incidence of PP escalates with the number of previous uterine surgeries.¹³ APH is a critical symptom of PP, affecting approximately 0.5% of all pregnancies and significantly contributing to perinatal mortality and maternal morbidity.¹⁴ Women with PP have 4-fold raised risk of 2^nd trimester vaginal bleeding, with APH occurring in 20-80% of cases.¹⁵

MRI is considered the most accurate investigation tool for PP diagnosis and grading. One study by Ding et al. showed that the MRI group had higher diagnostic accuracy (94%) and sensitivity (98%) as compared to the USG group (72% and 77%).¹⁶ This study reported similar diagnostic accuracy (96.4%) and higher than USG (90.0%). The high-resolution technology of MRI is beneficial for the proper visualisation of the placenta and cervix, resulting in a superior accuracy rate over USG. The USG findings can be limited by factors such as foetal position and placental changes. MRI is a useful tool for evaluation of morbidly adherent placenta (MAP) diagnosis, along with PP cases. One study reported sensitivity of 100% and specificity of 85.5% for accurate diagnosis of MAP.¹⁷

The ideal time for performing placental localisation USG in the diagnosis of PP varies between 32 and 36 weeks of gestation.¹⁸ USG has shown excellent overall performance with a sensitivity of 90.7% and a specificity of 96.9%.¹⁹ This study found the diagnostic accuracy of USG to be comparable to above-mentioned study, with sensitivity of 83.3% and specificity of 98.0%. A study conducted in Cameroon, the authors reported sensitivity and specificity of 82% and 99%, respectively. Some researchers have reported that endovaginal scans yield higher diagnostic accuracy for the accurate diagnosis of PP, rather than transabdominal scans, yielding sensitivity of 87.5%.²⁰ Hong et al. in a recently published systematic review and meta-analysis comparing the utility of USG, and MRI in terms of diagnostic accuracy in placenta accreta spectrum disorders, the pooled analysis revealed no statistically significant difference between USG and MRI. The sensitivity (p = 0.81) and specificity (p = 0.41) of the two modalities were comparable.²¹ These findings suggest that while both modalities are effective, their diagnostic performances remain statistically similar, emphasising the importance of clinical context and expertise when selecting the appropriate imaging tool. The researchers further recommended that routine MRI should not be employed for diagnosing the PP as it is highly expensive and not always available in resource-constraint settings.²⁰ A local study by Nawab et al.²² concluded that the diagnostic accuracy of USG is better than MRI in diagnosing placenta accreta, which further emphasises the role of USG in accurately diagnosing PP, such as the present study. Ultrasound is the safest, most accessible, and cost-effective tool for diagnosing PP, and it remains the first-line imaging modality. However, MRI was included in this study due to its superior soft tissue contrast and multiplanar capability, which are particularly valuable in cases with inconclusive ultrasound findings, suspected PAS, or posterior placenta. This study demonstrated higher sensitivity (95.0% vs. 83.3%) and overall diagnostic accuracy (96.4% vs. 90.0%) of MRI compared to ultrasound, supporting its adjunctive role in complex cases. Transvaginal ultrasound (TVUS), while more accurate than transabdominal scanning, was not used in this study due to institutional limitations and patient discomfort concerns in late pregnancy.

Based on the findings of this study, it is recommended that ultrasound should remain the first-line imaging modality for diagnosing PP due to its high specificity, accessibility, and cost-effectiveness. However, MRI should be considered in cases with inconclusive ultrasound findings or high clinical suspicion of complex placental pathology, as it has demonstrated superior sensitivity and overall diagnostic accuracy. Standardised imaging protocols should be implemented to enhance diagnostic consistency. While perioperative findings remain the gold standard, future studies with larger, grade-specific sample sizes are needed to validate diagnostic performance for each subtype of PP.

The limitations of this study include its single-centre design, which may limit the generalisability of the findings to other settings. Operator dependency in USG could introduce variability in diagnostic accuracy, as results are influenced by the skill and experience of the radiologist. The study also relied on non-contrast MRI, which, while avoiding foetal exposure to contrast agents, might have limited the detection of certain placental abnormalities. Potential selection bias may exist due to the inclusion criteria, as only patients with suspected PP in the 3^rd trimester were included, excluding earlier diagnoses and asymptomatic cases.

CONCLUSION

Ultrasonography can be a useful, cost-effective, and safest initial assessment tool for diagnosis and grading of PP in the late pregnancy. It showed overall comparable results of USG compared to MRI. Future prospective studies should evaluate the role of late pregnancy ultrasound in assessing types of PP and optimal delivery timing with appropriate surgical management involving larger population.

ETHICAL APPROVAL:
Approval was obtained from the Ethical Committee of Combined Military Hospital (ERB No: 21-2023, Dated: 20-02-2023).

PATIENTS’ CONSENT:
Informed consent was obtained from the patients to publish the data concerning this case.

COMPETING INTEREST:
The authors declared no conflict of interest.

AUTHORS’ CONTRIBUTION:
IS: Data collection, drafting, and critical revision.
NI: Design, conception, proofreading, and critical revision.
SS, ZZ: Data collection, data analysis, and critical revision.
All authors approved the final version of the manuscript to be published.

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