Endovaginal coil combined with pelvic array coil magnetic resonance imaging for the preoperative staging of cervical cancer

Introduction

Cervical cancer is the most common malignant tumor of the female reproductive system in China and the second most common cause of cancer-related death among women (1). Due to the establishment of worldwide screening programs facilitating early detection of disease, a growing portion of newly detected cases are at the early stage, and the prognosis and 5-year survival rate of patients with cervical cancer have greatly improved; however, deficits in treatment compared to developed countries persist. As early identification and treatment of cervical cancer can result in a very high cure and survival rate, early screening of cervical lesions is critical. Preoperative staging of cervical cancer is critical to determining the need for surgical treatment and for informing prognosis (2). In the past, the staging of early-stage cervical cancer was based on clinical physical examination, but the accuracy was low due to subjective factors and technical conditions. Although conventional magnetic resonance imaging (MRI) using pelvic array coils can identify the tissue and determine the specific location of the tumor through multiangle and multiparameter imaging, it is located far away from cervix and thus the region of interest and has low signal-to-noise ratio (SNR). Endovaginal MRI was originally described in 1992 (3) and was shown to be valuable for preoperative staging at 0.5 T and 1.0 T (4), detecting low-volume disease, and assessing parametrial extension (5). Another study found that 3.0-T MRI was valuable in assessing patients suitable for fertility-sparing surgery (6). A direct comparison of the endovaginal technique with an external array technique indicated the superiority of using the endovaginal coil; however, this comparison did not include the combination of the endovaginal with the external array (7). With reference to previous research based on the placement of conventional pelvic coils, we placed an endovaginal coil in the vagina and thus nearer the cervix. According to our basic research, this could improve the SNR (8) and provide high-resolution images of the cervical area. We thus surmised that when combined with a pelvic array coil, it could provide superior visualization of small tumors (7), distinguish tumor boundaries, and reveal the infiltration of surrounding tissues. The purpose of the current study was to evaluate the ability of endovaginal coil in preoperatively staging cervical cancer and to examine its correlation with histopathological findings. We present this article in accordance with the STARD reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-24-998/rc).

Methods

Materials

A total of 49 patients with cervical cancer who underwent MRI from January 2023 to March 2024 at The First Affiliated Hospital of Chongqing Medical University were included in this study. The inclusion criteria were as follows: (I) cervical cancer confirmed by cervical biopsy; (II) no cervical physical therapy performed within 3 months before examination; and (III) completion of pelvic array coil and combined coil MRI scans before operation, with informed consent. Meanwhile, the exclusion criteria were as follows: (I) incomplete pelvic array coil and combined coil images, with artifacts greatly affecting the diagnosis; (II) a history of cervical resection, cervical neoplasia, or pelvic radiation treatment; (III) contraindications to MRI, cardiac pacemaker, a metal foreign body, claustrophobia, contrast agent allergy, or renal failure; and (IV) incomplete clinical data. This study was conducted in accordance with the Declaration of Helsinki (as revised in 2013) and was approved by the Ethics Committee of The First Affiliated Hospital of Chongqing Medical University (No. 2023-400). Informed consent was obtained from all participants.

MRI and endovaginal coil protocol

The first MRI scan was performed with a 3.0-T MAGNETOM Skyra device (Siemens Healthineers, Erlangen, Germany). The patient was placed in the supine position with an advanced head position. First, a 32-channel phased array coil (pelvic array coil) was placed to perform routine scans. Subsequently, an 8-channel phased endovaginal coil was placed tightly around the cervix with the conventional coil being retained (combined coils).

Endovaginal images were also obtained via the 3.0-T MAGNETOM Skyra under a 35-mm ring design solenoidal receiver coil. The endovaginal coil used in this study was designed by our research team and was inserted following digital vaginal examination and positioned around the cervix. This coil detected tissue signals in the magnetic field emitted after excitation by the MRI system through the inductance and capacitance resonance loop. The coil was made of environmentally friendly polycarbonate, had no proton signals, and met the safety standards. The scanning parameters for the endovaginal coil were as follows: repetition time (TR) =2,000 ms, echo time (TE) =88 ms, field of view (FOV) =180 mm, matrix =384×284, slice thickness =2 mm, acquisition time =150 s, and voxel size =0.44 (0.47×0.47×2) mm³. Meanwhile, the scanning parameters for the pelvic coil were as follows: TR =3,200 ms, TE =88 ms, FOV =200 mm, matrix =256×256, slice thickness =3 mm, acquisition time =104 s, voxel size values =1.83 (0.78×0.78×3) mm³.

Image analysis

Two or more radiologists read and analyzed the MRI images of the pelvic array coil and combined coils of all patients and then staged all cases according to the imaging findings in double-blinded manner according to the 2018 International Federation of Gynecology and Obstetrics (FIGO) staging system for cervical cancer (9). In case of discrepancies in staging, the two physicians reached an agreement via discussion.

Statistical analysis

All statistical analyses were performed with SPSS v. 28.0 (IBM Corp., Armonk, NY, USA). P<0.05 indicated statistical significance, and the Cohen kappa coefficient (κ) was used to determine intrarater reliability. Positive (PPV) and negative predictive values (NPV), sensitivity, specificity, and accuracy rate were calculated. Receiver operating characteristic (ROC) curve were plotted and the curve (AUC) was calculated. The closer the AUC value is to 1, the more diagnostic it is, and if the AUC value is 0.5, the diagnostic is almost meaningless.

Results

General information

This prospective study initially examined 49 patients: 2 patients were unwilling to cooperate during the examination, 1 case had a large lesion making coil placement difficult, one case had too many artifacts between the lesion and the coil, and 5 cases did not undergo surgical treatment and were lost to follow-up. Ultimately, 40 cases were included in this study. The patients’ age ranged from 26 to 70 years, with an average age of 52 years. Among the included patients, 8 (20.0%) were aged <40 years, and 32 (80.0%) were aged ≥40 years. Regarding the pregnancy status, 5 (12.5%) cases had been pregnant once, 17 (42.5%) had been pregnant twice (42.5%), and 18 (45.0%) had been pregnant three times. Moreover, 27 (67.5%) cases were human papillomavirus (HPV) positive, and 13 cases (32.5%) were HPV negative. The first clinical symptoms were vaginal bleeding after intercourse in 23 (57.5%) cases, irregular postmenopausal bleeding in 7 (17.5%) cases, vaginal discharge in 1 (2.5%) case, cervical lesions in 6 (15.0%) cases, and increased leucorrhea in 1 (2.5%) case.

Operation

The surgical modalities included laparoscopic and robot-assisted extensive total hysterectomy, bilateral adnexectomy, and pelvic lymph node dissection, as well as extensive abdominal hysterectomy with bilateral ovarian salpingectomy and pelvic lymph node dissection or cervical conization. The lesions had an average volume of 6.32±2.12 cm³ (range, 0.12–23.26 cm³). The postoperative pathological results were as follows: 29 (72.5%) cases with squamous cell carcinoma, 9 (22.5%) with adenocarcinoma, and 2 (8.23%) with small-cell neuroendocrine carcinoma. The pathological differentiation degree was as follows: 31 (87.34%) cases with moderately high differentiation and 9 (5.0%) with low differentiation. The pathological staging was as follows: 2 cases with stage IA (including IA2), 16 with stage IB (including IB1, IB2, and IB3), 14 cases with stage IIA (including IIA1 and IIA2), 3 with stage IIB3, and 5 with stage IIIC. Stage IB and stage IIA were more common.

Characteristics

For the cervical cancer signal characteristics, cervical cancer generally showed low and high signal intensities on T1 weighted imaging (T1WI) and T2 weighted imaging (T2WI), respectively. This is because cervical cancer is usually composed of dense cellular tissue, resulting in a low signal intensity on T1WI, whereas increased water and interstitial tissue result in a high signal intensity on T2WI. Cervical cancer myometrial infiltration can lead to a blurred and thickened cervical myometrium on imaging, which may be caused by tumor cell infiltration and tissue destruction. In severe cases, cervical cancer may penetrate the cervical myometrium and spread to the adjacent tissues or organs, such as parauterine invasion, with the following imaging findings: masses or abnormal signals in the parauterine region, usually with a high signal intensity on T2WI, and with faint, disappearing, thickened strips of abnormal signals in the parauterine space.

Correlation of staging with the pelvic array coil and pathology

The comparison between pelvic array coil and pathology in terms of staging is provided in Table 1, which also shows the number of cases of each stage that were under- or overstaged by pelvic array MRI. Two cases of stage IA disease were not detected by pelvic array imaging, and twelve cases were correctly staged as IB on pelvic array imaging. The accuracy for stage IB was 75.0% (12/16), and four cases were overstaged when compared with histopathology (two cases of overestimated stage IIA, one case of overestimated stage IIB, and one case of overestimated stage IB3). Eleven cases were correctly staged as IIA on pelvic array imaging, and the accuracy for stage IIA was 78.6% (11/14), with two cases underestimated as stage IB2 and one case overestimated as stage IIB. Two cases were correctly staged as IIB on pelvic array imaging, and the accuracy of stage IIB was 66.7% (2/3), with one case underestimated as stage IIA2. Four cases were correctly staged as IIIC, and the accuracy of stage IIIC was 80.0% (4/5). In this study, the accuracy of preoperative staging with pelvic array coil was 72.5% (29/40), and the κ value was 0.722 (κ<0.4 indicates poor consistency, κ between 0.4 and 0.75 indicates general consistency, and κ>0.75 indicates high consistency), showing that preoperative MRI with pelvic array coil staging of cervical cancer was consistent with postoperative pathology.

Correlation of staging with the combined coil and pathology

The comparison between the combined coil array and pathological staging is provided in Table 2, which also shows the number of cases of each stage that were under- or overstaged by the combined coil technique. One case of stage IA disease went undetected on combined coil imaging, and one case was correctly staged as IA. Fourteen cases were correctly staged as IB, and the accuracy for stage IB was 88.9% (14/16), with one case overestimated as stage IIA1 and one case overestimated as stage IB2. Twelve cases were correctly staged as IIA, and the accuracy for stage IIA was 85.7% (12/14), with one case underestimated as stage IIA1 and one case underestimated as stage IB3. Three cases were correctly as staged IIB, and the accuracy for stage IIB was 100% (3/3). Four cases were correctly staged as IIIC, and the accuracy for stage IIIC was 80% (4/5). In this study, the accuracy of preoperative staging with the combined coil was 85.0% (34/40), and a κ value of 0.813 indicated good agreement with postoperative pathology.

Comparison of the parametrial and vaginal invasion in cervical cancer between the two imaging examination methods

In our cohort of 40 patients, 8 had parametrial invasion (stage IIB or above), and 22 had vaginal invasion (stage IIA or above). Table 3 details the sensitivity, specificity, PPV, and NPV for detecting parametrial or vaginal invasion with the pelvic array coil and with the endovaginal plus pelvic array coil combination. The AUC of the pelvic array coil in diagnosing parametrial invasion was 0.812 [95% confidence interval (CI): 0.603–1.000], while that for the combined coil was 0.913 (95% CI: 0.756–1.000). Meanwhile, the AUC of the pelvic array coil in diagnosing vaginal invasion was 0.876 (95% CI: 0.757–0.996), while that for the combined could was 0.949 (95% CI: 0.869–0.999) (Figures 1,2). In the comparison between combined and pelvic array coil, the combined coil showed demonstrated greater consistency with the postoperative pathological findings in the diagnosis of parametrial and vaginal invasion (Figures 3,4). Regarding pelvic lymph node metastasis, the results of the two methods were similar, and the number of metastatic lymph nodes was 4 (pathology was 5).

Figure 1 Diagnosis of parametrial invasion by the two methods. AUC, area under the curve.

Figure 2 Diagnosis of vaginal invasion by the two methods. AUC, area under the curve.

Figure 3 Use of the combined coil in axial and sagittal cervical images. (A) In the axial scan the combined coil image showed that the lesion was located on the right side of the cervix uteri, lesion size 43 mm × 29 mm (white arrow), with no parametrial invasion. (B) The sagittal scan of the combined coil image (white arrow) showed the invasion of the posterior fornix (red arrow) involving the upper one-third of the vagina and indicating preoperative stage IIA. The postoperative pathology stage was IIA.

Figure 4 The combined coil image. A small lump was visible on the anterior cervix uteri, measuring 13 mm × 7 mm in size (green arrow), endovaginal coil (white arrow). The preoperative staging was IB1. The histopathology diagnosis was squamous cell carcinoma located on the anterior cervix uteri. The pelvic lymph nodes were not enlarged, and postoperative pathological staging was IB1.

Discussion

Our data indicated that staging performance of MRI for cervical cancer may be improved by using an endovaginal coil in conjunction with a pelvic array coil (Figure 5) at 3.0-T. Since the inclusion of imaging technology in the 2018 FIGO classification system, MRI has been routinely used staging of cervical cancer, but restricted to pelvic array imaging alone (10,11). More precise staging is needed for procedures that preserve fertility in patients with early cervical cancer (12). In our study, the preoperative staging accuracy for pelvic array coils was 72.5%, and that for the combined coil was 85.0%. Confirmation via endovaginal coil imaging improved the accuracy of preoperative staging. Multiple studies (5,13) have also reported that endovaginal coils, which are in close proximity to the cervix (the area of interest), provide a more accurate acquisition of smaller cervical tumors, with the location, scope, and depth of cervical cancer lesions being well correlated with the histopathological results. Endovaginal coil imaging is a new noninvasive imaging technique, and the images obtained by this coil are characterized by clear images and high resolution.

Figure 5 Diagram of the endovaginal coil. The measurements presented are in millimeters.

Parametrial invasion and vaginal invasion

Our study showed more consistent results than previous studies in the detection of parametrial and vaginal invasion. This is important because it affects management: those without parametrial invasion are eligible for primary surgery, whereas those with parametrial invasion are managed primarily with chemoradiotherapy. The tissue surrounding the uterus, the parauterine tissue, involves connective fibrous tissue extending from the uterus to the lateral wall of the pelvis; in T1WI and T2WI on MRI, these areas are often predominantly hyperintense in fat, whereas the ligaments appear hypotactic and as trailing vascular networks in the pelvis (14-16). Although conventional MRI has good soft-tissue resolution, the misdiagnosis rate is still approximately 50% (17). Mao et al. (18) reported that the high-resolution reduced field-of-view diffusion-weighted MRI had good performance in diagnosing cervical cancer with parametrial involvement. The use of an endovaginal coil offers addition resolution advantages due to the increased SNR available. In our study, the combined coil group showed more consistent results (95%) than pelvic array coil group in the diagnosis of parametrial invasion.

A number of new techniques have been developed to improve the staging of cervical cancer. These include the measurement of apparent diffusion coefficient, intravoxel incoherent motion diffusion-weighted imaging (DWI), and external phased array receiver (19-21). However, the problem of MRI displaying low accuracy in the assessment of parametrial invasion has not been addressed. The endovaginal coil acts as a spacer, filling and dilating the vagina and clearly demonstrating tumor infiltration of the vagina. This was evident from the accuracy for detecting vaginal invasion demonstrated in our patient cohort.

This study involved certain limitations which should be discussed. First, the number of cases included in this study was small, and not all of were appropriately staged. Therefore, there is a certain one-sidedness in the evaluation of MRI staging of cervical cancer. Second, endovaginal coil imaging was only performed with the T2WI sequence. The other imaging sequences, such as DWI, can be added in subsequent research to differentiate the changes after biopsy (such as the presence of edema, granuloma tissue, and fibrosis) from tumors to obtain better imaging data. Finally, artifacts may appear if the lesion is too large, the coil is placed in a shallow position, or if there was too much air between the coil and the lesion, and thus further optimization is needed. In the future, all cases of cervical cancer confirmed by biopsy and loop electrosurgical excision procedure (LEEP), should be included to determine total resection, as well as the normal length of cervical canals.

Conclusions

This study demonstrated that in the assessment of cervical cancer, MRI examination using endovaginal coil has a high sensitivity, high specificity, and good agreement among experienced and inexperienced observers in the staging of cervical cancer.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STARD reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-24-998/rc

Funding: This work was supported by the State Key Laboratory of Ultrasound in Medicine and Engineering (grant No. 2022KFKT005 to K.Z.).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-24-998/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki (as revised in 2013) and was approved by the Ethics Committee of The First Affiliated Hospital of Chongqing Medical University (No. 2023-400). Informed consent was obtained from all participants.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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