Virtual measurements of paracellular permeability and chronic inflammation via color coded pixel-wise T₁ mapping

Abstract

To assess whether quantitative T1 relaxometry can measure permeability, assess chronic inflammation and mural thickening of mouse bladder wall. Adult female C57BL6 mice unexposed to radiation (controls) or 40 weeks post-irradiation10Gy were scanned at 9.4T before and after instillation (0.1mL) of aqueous, novel contrast mixture containing 4mM Gadobutrol and 5mM Ferumoxytol. Rapid Acquisition with Refocused Echo (RARE) sequence with variable Repetition Times (TR) generated pixel wise map of T1 relaxation times for segmented bladder wall layers from voxel-wise, nonlinear least square data fitting of TR dependent signal intensity acquired with TR array of 0.4-10s followed by the histology of harvested bladder. Significant differences between pre-contrast and post contrast T1 (DT1) noted in urothelium and lamina propria of both groups but only in detrusor of irradiated group (p<0.001; Two-way ANOVA). Nearly two-fold higher Gadobutrol permeability (550 ± 73μM vs 294 ± 160μM; p<0.01) derived as per 1/DT1 = r1. [C] in urothelium of irradiated group (0.75± 0.04mm vs 0.44± 0.08mm; p<0.001). Inflammation and bladder wall thickening predicted by MRI was subsequently confirmed by histology and altered expression of CD45 and zonula occludens-1 (ZO-1) relative to controls. Novel contrast mixture enhanced MRI relies on the retention of large molecular weight Ferumoxytol in lumen for negative contrast, while permeation of the non-ionic, small molecular weight Gadobutrol through ZO-1 generates positive contrast in bladder wall for virtual measurement of paracellular permeability and assessment of chronic inflammation in thin and distensible bladder wall, which is also defined by its variable shape and location within pelvis.

Functional and histologic imaging of urinary bladder wall after exposure to psychological stress and protamine sulfate

“…The shortening of T 1 relaxation time can help us derive Gadobutrol permeability in any given region of interest (ROI) of bladder wall. Because the longitudinal relaxation rate of water (R1 = 1/T 1 ) is linearly related to the Gadobutrol concentration 21 , 23 , 29 , Gadobutrol permeability can be computed from the pre-contrast and post-contrast T 1 maps using the Eq. ( 1 ) where [Gd] is the unknown Gadobutrol concentration, r 1 is the relaxivity of Gadobutrol (measured 3.58 mM −1 s −1 at 9.4 T, Fig.…”

“…To bring a paradigm shift in the permeability assay of bladder mucosa, we report on the measurement of bladder wall T 1 relaxivity for indexing the innate host response to chronic psychological stress and PS exposure. Since bladder wall T 1 relaxivity is linearly related to the paracellular entry of [Gd] into the extracellular space of bladder wall either across the tight junction of capillaries after intravenous injection of Gadobutrol or its analog) 24 or across the tight junction complex of umbrella cells after instillation (as in irradiated mouse bladder 29 ), we hypothesized that the quantitative T 1 mapping of rat bladder can serve as a robust index for compromised bladder permeability of rat exposed to WAS 13 . A minimally invasive MRI based permeability assay can provide global information of bladder permeability and avoid the site-selection bias of biopsy 2 , 4 , 5 , 12 or the complexity and errors intrinsic to biochemical assays 10 , 16 .…”

“…The extensive plasma uptake 16 , 27 of the instilled radiolabeled urea in IC/BPS patients 10 through a first order process validated the distributed model for intravesical pharmacokinetics. The venous clearance of instilled Gadobutrol is argued by the venous clearance of the eight times larger inulin when instilled into rat bladder 44 and therefore the venous clearance of diffused Gadobutrol not only engenders a logarithmic concentration decline across the bladder wall thickness 29 but also prevent any additional rise in the mucosal concentration with the repeat instillation.…”

“…The passive, concentration dependent, paracellular diffusion of instilled Gadobutrol 29 (604.71 Daltons), cations 10 and of large molecular weight permeability probes such Evans blue dye (961 Daltons) 17 , 45 into bladder mucosa led us to infer that the compromised mucosal barrier increases the penetration of irritants from urine for inciting the underlying afferent nerves and drive the pain and LUTS of IC/BPS patients. Thus, a significant rise in [Gd] ingress after PS exposure in the WAS group can potentially replicate the flare up of BPS/IC symptoms 46 in a non-infective rodent model without relying on bacterial products.…”

“…As described recently for mouse bladder 29 , MRI was performed in a 9.4 T/30 cm Bruker AVANCE III HD scanner (Bruker BioSpin, Billerica, MA, USA) running ParaVision 6.0.1 equipped with a 12 cm BGA-12SHP gradient set, an 86 mm transmit coil and 35 mm 2 × 2 receiver array over the bladder. Rats were maintained under isoflurane anesthesia via a nose cone (2–3% in 1:1 Oxygen: Air).…”