Unraveling the complexity of bladder-centric chronic pain by intravesical contrast enhanced MRI

Abstract

Bladder pain, a common symptom associated with various urological conditions, poses a diagnostic challenge as existing imaging modalities fail to pinpoint the bladder as the definitive source of pain. While bladder pain is often linked to localized inflammation resulting from urinary tract infections (UTIs) or Interstitial Cystitis/Bladder Pain Syndrome (IC/BPS), which can be exacerbated by emotional stress, current urine-based markers cannot precisely identify the specific site of inflammation within the urinary tract, spanning from the kidneys to the urethral meatus. Cystoscopy, currently considered the gold standard, is recommended by the American Urological Association (AUA) prior to aggressive treatment for IC/BPS patients, as it confirms the presence of bladder inflammation, particularly in Hunner lesions, and predicts a higher response rate to anti-inflammatory medication like cyclosporine. Nonetheless, the invasiveness of cystoscopy, which relies on investigator expertise, coupled with the significant variability in detecting Hunner lesions, underscores its limitations for inflammatory phenotyping. These factors contribute to the reluctance in choosing cystoscopy as a preferred diagnostic method and may also contribute to the lack of success observed in clinical trials assessing the efficacy of novel anti-inflammatory drugs. Given that immune cell infiltration into inflammatory sites relies on tight junction dilatation, the paracellular entry of injected or instilled paramagnetic dyes, mimicking the extravasation of colored dyes such as Evans blue dye could be a robust index of vascular or urothelial hyperpermeability-a characteristic sign of inflammation. This article aims to delve into the pathophysiology of bladder-centric chronic pain within the context of the challenging diagnosis of IC/BPS and explore the pivotal role of Stokesian and Fickian diffusion in the evolution of intravesical contrast-enhanced MRI (ICE-MRI) as a phenotyping tool for bladder pain, transitioning from laboratory research to practical clinical application.

Keywords: Bladder; Inflammation; Interstitial cystitis; MRI; Permeability.

Fig. 1.

Static and dynamic component of bladder barrier function- Polygonal umbrella cells with asymmetrical unit membrane on apical surface cover 70% of bladder lining and remaining 30% of bladder lining is erected by tight junctions. Both distension and inflammation increase the percentage of bladder surface area covered by tight junctions which permits accelerated extensive entry of instilled Evans blue/Gadobutrol—indicated by thicker blue line filling the widened gap between umbrella cells. ICE-MRI replaces transurethral instillation of Evans blue dye with 0.3 mL instillation of Gadobutrol and Ferumoxytol mixture to leverage the size dependent slower Stokesian diffusion of Ferumoxytol (r = 150 Å) for enhanced image contrast as its luminal retention darkens the lumen whereas paracellular diffusion into capillary perfused bladder mucosa of smaller sized (r = 4 Å) Gadobutrol (T1 shortening agent) brightens vesical veins (V) from bladder base carrying deoxyhemoglobin, thrown into sharp relief by dark, internal iliac artery (IA) and common iliac artery (CIA) delivering oxyhemoglobin (diamagnetic) to perfused abdominal organs. Adequate urothelial perfusion not only prevents the vicious feed forward loop of ischemia slowing the healing of injured urothelium but also prevents a direct entry of diffused Gadobutrol from mucosa to muscle. Turbo spin echo image in coronal orientation without fat saturation was acquired at repetition time (TR) 650 milliseconds (ms) and echo time (TE) of 36 ms, field of view (FOV) of 6 * 6 cm², matrix of 256 * 256, slice thickness of 2 mm, number of signal averages (NSA) = 2 at field strength of 7T.

Fig. 2.

Three-dimensional (3D) T1 weighted (Fast low angle shot) FLASH in axial orientation of female cat anaesthetized with ketamine/xylazine without any contrast enhancement (panel A–B) and post-instillation of 20 mL mixture of Gadobutrol and Ferumoxytol for ICE-MRI in vivo (panel C) and ex vivo (panel D–E). For improved contrast of bladder wall in panel C and E, pixel values were inverted whereas panel A–B displays serial raw images acquired 3 min apart and the oozing of oxygenated blood (red* increasing in size in panel B) displays the delivery of oxyhemoglobin (diamagnetic signal) that decreases signal in mucosa, thrown into sharp relief by the bright signal of lumen (L) containing 20 mL 0.25% acetic acid. Mucosal blood flow (marked by red*) in vivo not only precludes the direct entry of acetic acid (panel A–B)/formalin into detrusor muscle (#) but also explains why mucosa (white *) of live cat is less brighter than mucosa of isolated bladder ex vivo (panel E) as the blood flow rapidly washes away diffused Gadobutrol from * in vivo. Meanwhile, the absence of blood flow ex vivo explains the slow and steady diffusion of Gadobutrol into muscle (#) and outside of serosa ($) with the highest Gadobutrol concentration in * as indexed by the adjacent gray bar. The downhill concentration gradient from *to $ at 4 °C of free Gadobutrol-unfettered from the signal decay induced by luminal retention of Ferumoxytol in gelatin encased bladder (Panel D–E). Since signal intensity in T1 weighted MRI is determined by T1 relaxation time, perivesical fat of live animal appears brighter than muscle (#) and the imaging parameters at 3T for in vivo and ex vivo ICE-MRI were identical with gradient echo pulse sequence TR/TE of 5/2 ms, voxel of 0.683 * 0.683 * 2 mm³, FOV of 17.5 * 17.5 cm², matrix 256 * 256 and NSA = 1..

Fig. 3.

Panel A- Distension validates paracellular diffusion- While accumulation of Trypan Blue on luminal surface of bladder is distension dependent, smaller sized Fluorescein and Gadobutrol (1/3rd r of Trypan blue) diffuses at a faster rate without distension at instilled volume of 0.3 mL in rat and 50 mL for human subjects. Distension accelerates the paracellular Stokesian diffusion by reducing the diffusion length x and by increasing the human bladder blood flow rate >1 mL/s to elevate the concentration gradient φ for accelerated Fickian diffusion. Panel B- Impact of distension on pharmacokinetic parameters- In accordance with Fickian diffusion, simulated serum plots from published clinical studies on instilled fluorescein, methylene blue, thiotepa and lidocaine, depict that accelerated diffusion flux J evoked by distension generates a proportional rise in the absorption rate constant k_a (a greater rise in y-axis magnitude of the blue curve for the distended bladder wall at comparable time interval of x-axis for the orange curve of un-distended bladder. Peak represents C_max of both curves, dependent on φ and sensitive to urinary dilution and dwell time..

Fig. 4.

Capillary perfused urothelium significantly impacts the Fickian diffusion in ICE-MRI and in intravesical drug delivery as bladder vasculature explains why absence of blood flow ex vivo fails to predict significant water, urea, Na+ permeability in vivo. Erythrocytes engorged blood vessels (yellow *) just below the apical layer of umbrella cells are clearly visible in H&E-stained inflamed rat bladder cryosection than in histopathology of resected granuloma specimen from human bladder. Lumen is marked by L and estimated diffusion length for instilled drug/dyes to the nearest capillary is marked by x and urothelium is marked by U. Schematic illustration of mammalian bladder wall illustrates that superficial capillary plexus overlaid on mucosal plexus reduces the Fickian diffusion length x in intravesical delivery and ICE-MRI shorter than the histological thickness of urothelium..

Fig. 5.

Anticipated diagnostic chart with ICE-MRI to phenotype a large population of NHIC without any cystoscopic abnormality into bladder-centric and Extra-bladder phenotype.