Manganese Enhanced MRI (MEMRI) of Rat Endometrial Cysts

. Introduction Endometriosis, the presence of a viable endometrium outside the uterine cavity, is an estrogen-dependent condition that affects 5 million American women. Our laboratory has established a rat model of endometriosis in which the growth of the tissue is monitored non-invasively using MRI. In this model, the endometrial cyst has a fluid-filled interior which is comprised of various proteins including adhesion molecules (1), cytokines (2), and superoxide dismutase (3). Previously, manganese ion (Mn) has been used as an intracellular contrast agent that enters viable cells via voltage gated calcium channels (4). Proteins such as Mn-superoxide dismutase have been implicated in the recruitment of Mn from elsewhere (5). Therefore, we examined the potential for using manganese as a molecular contrast agent for endometrial cyst MRI signal enhancement. Methods Eight-week-old female Sprague-Dawley rats were pre-screened for at least 3 estrous cycles. Only consistent 4-day cyclers were used for uterine auto-transplantation. Surgery was performed on estrus confirmed by vaginal lavage. Using aseptic technique, a 5x5 mm patch prepared from the distal end of the right uterine horn was sewn with the endometrial side against the right peritoneal wall across a visible blood vessel. Before the three layer closure, 5 mls of sterile saline was used to generously hydrate the abdominal tissues. Manganese-Enhanced MRI (MEMRI) was performed at various time points (1 day, 1, 2 weeks post-Mn infusion) once the endometrial cyst reached a steady-state of growth (4.5 months post-surgery). There were two Mn infusion concentrations used, 8.14±0.92 and 4.05±0.55 nmoles/min/g BW. The animals were anaesthetized using a mixture of oxygen, and isoflurane (~1.5-2%). Respiratory signal was monitored and a T1weighted Fast Low Angle SHot (FLASH) sequence was used. All MRI images were acquired on a 4.7T Bruker BioSpec MRI spectrometer (Billerica, MA) using a half-birdcage surface coil. Short-axis images were acquired using a FLASH sequence. A pilot coronal image of the rat abdomen was obtained. This pilot coronal image provided a clear view of the endometrial cyst location. The short axis slices covered the entire endometrial cyst. The imaging parameters were as follows: matrix dimensions, 256x256; TE/TR, 4.5/400 ms; slice thickness, 1.0 mm; FOV, 4.5 cm; 4 averages; 20 slices. Fat-suppression was used to minimize the lipid chemical shift artifact and enhance the quality of endometrial cyst images. Image analysis was performed using ANALYZE software (AnalyzeDirect, KS); the ROI tools were used to select the areas of interest (cyst volume) and signal intensity values were recorded and analyzed. Results The fat-suppressed MRI pulse sequence provided high quality images for the observation of the endometrial cysts (Figure 1). In addition, the infusion of Mn in the endometriosis rat model clearly delineated and enhanced the cyst fluid cavity. These data suggest that the endometrial cyst has the potential to trap and retain Mn over a period of time (Figure 2). The time course of signal intensity enhancement suggests Mn was retained for ~ 2 weeks. Table 1 shows the normalized signal intensity enhancement over 2 weeks time. There is a small window for signal intensity enhancement between 8.14±0.92 and 4.05±0.55 nmoles/min/g BW infusion. This suggests there is room for improvement for both the infusion protocol and MRI pulse sequence (i.e. inversion recovery to optimizing T1-weighted images). Further analysis of cyst fluid content is warranted. Conclusions This study demonstrates that T1-weighted MRI enhancement in the endometriosis rat model in the presence of Mn 2+ provides clear signal enhancement which may be beneficial for analyzing the cyst volume and content. There was a relative narrow concentration versus contrast enhancement window suggesting further improvement of imaging optimization could be accomplished. In conclusion, MEMRI may be used as a possible method for enhancing detection of smaller endometrial lesions in a rat model and studying possible kinetics with content.