Comparison of optimised endovaginal vs external array coil T2-weighted and diffusion-weighted imaging techniques for detecting suspected early stage (IA/IB1) uterine cervical cancer

A 37-mm ring-design solenoidal receiver coil similar to one previously described for use at 1.5-T [18] was used as part of a coil array with an externally placed flex-L coil (Philips Medical Systems, Best, The Netherlands). The endovaginal coil was designed and built in-house. It was covered in a disposable sheath and required simple manual insertion, without visual or imaging guidance, and manipulated so that the ring encompassed the cervix. The coil was immobilised in an external clamp as previously described. Vaginal air introduced during insertion was aspirated using a Ryles tube to minimise image distortion.

A receive-only SENSE XL Torso coil 16-element phased array was used (Philips Medical Systems). This is a flexible volume coil consisting of anterior and posterior sections, each of eight elements – four upper and four lower. Coil dimensions were 74 mm × 558 mm × 600 mm for each half (upper and lower). Each element had a Z coverage of 22.5 cm.

We initially established the most appropriate imaging protocols for both the endovaginal technique and the external array technique. As high a resolution as possible with the T2-W sequences without compromising image quality was deemed to be a minimum requirement. We matched the voxel size of the external array imaging to the endovaginal DW imaging (Table 1). From our experience in reporting these images, planes sagittal and coronal to the cervix were selected. The endovaginal images had a spatial resolution of 0.35 mm³ (0.42 × 0.42 × 2 mm), while the torso array had a spatial resolution of 3 mm³ acquired (1 × 1 × 3 mm). In order to minimise acquisition time, DWI for comparison was done in the coronal plane only as parametrial extension is best demonstrated in this plane. For the endovaginal technique we used the ZOOM sequence [19] which substantially reduces image distortions that arise due to susceptibility artefacts from the endovaginal coil. This allows a spatial resolution on the DW images of 3.1 mm³ (1.25 × 1.25 × 2 mm). Because of the lower image SNR without the internal coil, it was not possible to derive any meaningful data with the ZOOM-DWI. A conventional echo-planar imaging (EPI) sequence with a pixel resolution equivalent to that of the ZOOM-DWI image was therefore selected (1.25 × 1.25 mm) but a slice thickness of 3 mm was selected to match the slice thickness of the T2-W images and retain SNR, giving a voxel size of 4.7 mm³. Details of the endovaginal coil and external array coil protocols are given in Table 1. SNR measurements were made in a test-object with each coil combination using the optimised sequences and corresponding reconstruction filters, multichannel reconstruction and parallel imaging. A 2.5-cm diameter gel test object (Eurospin, T1 = 500 ms, T2 = 50 ms) surrounded by mineral oil test objects to mimic a body load was imaged with the endovaginal-flex-L coil combination followed by the external array coil, repeating each sequence three times. Mean signal intensity measured from a 200 mm² region of interest on the central slice within the gel test object from the three repetitions was divided by the mean of the standard deviation of background noise. There was near equivalence of the SNR on T2-W images with each coil combination. There was a 62 % reduction in SNR on the b = 800 s/mm² ZOOM-DWI images from the endovaginal coil compared to the corresponding T2-W images, but a seven times reduction in SNR on the b = 800 s/mm² DWI from the external array coil compared to the endovaginal DWI images (Table 1).

The study was approved by the local Institutional Review Board. All patients gave informed, written consent. Over a 15-month period (May 2013–Aug 2014), a further follow-on cohort from our previous study [15, 20] of 51 patients were referred for MRI following an abnormal smear test and subsequent histological evidence of invasive cervical cancer. Twenty-two complained of at least one episode of abnormal bleeding unrelated to any preceding surgical procedure. All were successfully prospectively recruited for assessment with endovaginal T2-W MRI followed by high-resolution external phased array MRI at 3.0-T. Three patients were staged as Federation Internationale Gynecology and Obstetrics FIGO 2B at examination under anaesthesia, so that 48 consecutive patients with Stage 1 cervical cancer aged 24–53 years (mean 34.4 ± 8.2 years) were included in this study (Fig. 1). Forty-two patients had a cone or LLETZ biopsy at a median of 4.0 weeks (range 2–12 weeks) prior to MRI in which some or the entire tumour may have been excised; at histology, 26 were squamous cell carcinoma and 16 adenocarcinoma. The other five had punch biopsies only (three squamous cell carcinoma, two adenocarcinomas); of these three had undergone cone or LLETZ biopsies more than 9 months previously.

Images were obtained at 3.0-T (Philips Achieva). Hyoscine butyl bromide (Buscopan) 20 mg intramuscularly (IM) was administered to reduce artefact from bowel peristalsis.

Endovaginal imaging was performed first. T2-W images were acquired in three orthogonal planes to the cervix and covered it entirely. DWI slice-matched images were acquired in free-breathing coronal to the cervix as parametrial extension is best assessed in this plane. Sequence parameters and acquisition time are given in Table 1. Isotropic apparent diffusion coefficient (ADC) maps based on the monoexponential rate of signal decay with increasing diffusion weighting were generated with the system software (Philips Medical Systems) using all b-values.

Following the endovaginal imaging, the coil was removed and, following a 5-minute off-bed break, external array coil imaging was done. T2-W images were acquired in planes sagittal and coronal to the cervix. DW-slice-matched images were obtained coronal to the cervix. As with the endovaginally acquired data, isotropic ADC maps based on the monoexponential rate of signal decay with increasing diffusion weighting were generated with the system software (Philips Medical Systems) using all b-values.

At the time of clinical image reading, the reporting radiologist (20 years’ experience of pelvic MRI), with access to clinical information and all imaging data sets noted the absence or presence of tumour and the absence or presence of tumour invasion into the right and left parametrium. Where tumour was noted, a tumour volume was measured by drawing a region-of-interest around the tumour on every slice on which it appeared and multiplying the total measured tumour area by the slice thickness plus interslice gap.

On completion of the patient accrual, an independent radiologist (5 years’ experience of pelvic MRI) without access to the clinical data separately scored first the anonymised coronal and sagittal T2-W external array images as positive or negative for tumour within the cervix; the absence or presence of parametrial extension was also noted as absent or present. This was then repeated with inclusion of the corresponding coronal ADC maps. At a subsequent occasion 2 weeks later to avoid memorisation, the same observer scored the randomized, anonymised coronal and sagittal T2-W endovaginal images without and then with the addition of the corresponding coronal ADC maps. The absence or presence of right and left parametrial extension was also noted.

Tissue was acquired at radical hysterectomy (n = 10), radical vaginal trachelectomy (RVT) (n = 13), cold knife cone (n = 11) or LLETZ procedure (n = 4) in 38 patients. Of the remainder, four went to chemoradiotherapy and were known to have prior positive biopsies and six were deemed to have negative MRI scans with clear margins on the initial cone or LLETZ biopsy so no further intervention at that point was deemed necessary. These latter six cases were therefore excluded from the analysis because of a lack of post-MRI confirmation of the absence of tumour. All specimens were fixed in formalin. For hysterectomy and RVT specimens, a shave of the proximal endocervical resection margin (at or near the internal os) was sliced longitudinally (sagittal plane centrally, coronal plane laterally). The cervix together with the parametrium was sliced into transverse sections up to approximately 1 cm from the ectocervix. The remaining 1 cm of distal cervix together with vaginal cuff was sectioned longitudinally (sagittal plane centrally, coronal plane laterally or sagittally throughout). All slices (each 3–4 mm thick) were processed and embedded in paraffin; 2–3 micron sections were deparaffinised and stained with haematoxylin and eosin. According to the thickness of the slice obtained at macroscopy, each histology slide was separated from the next by 3–4 mm.

Sensitivity, specificity, and positive and negative predictive values for detecting the presence of tumour by the reading radiologist were obtained. For the independent radiologist blinded to the clinical information sensitivity, specificity, and positive and negative predictive values of T2-W without and with the addition of DWI were established for detecting the presence of tumour. Data were obtained for the endovaginal technique and separately for the external array-only technique. Similar data were derived for right and left parametrial extension. A Cohen’s kappa to assess agreement between observers for interpreting endovaginal T2-W + DW imaging together was calculated between reading and independent radiologists and between the internal and external coil readings of the independent radiologist on T2-W and separately T2-W + DW imaging.