Recent advances in imaging and understanding interstitial cystitis

Interstitial cystitis/bladder pain syndrome originating from the bladder

IC/BPS patients of this category^8,9 are considered to have an organic disease of the bladder wall. Pathological examination of biopsy specimens from the bladder wall of such IC/BPS patients typically finds a higher incidence of mucosal hemorrhages (glomerulations), submucosal inflammation¹³ and a high density of mast cells^14,15, which is associated with hyperexcitability¹⁶ of afferent nerve fibers¹⁷. Cystoscopic detection of glomerulations in the bladder mucosa of IC/BPS patients is the defining characteristic of the ESSIC type 2C classification, while the detection of Hunner’s lesion (a denudation of urothelium) is a sign of classic type or ESSIC type 3C, more commonly referred to as Hunner-type IC/BPS. Hunner-type IC/BPS is seen in 5–10% of cases and the analysis of their bladder wall biopsy is generally associated with high expression of T and B cell markers¹⁸, low expression of urothelial markers, high density of mast cells, focal lymphoid aggregates, overexpression of IL-6, IL-10, IL-17A, and inducible NOS (iNOS) mRNA¹⁴. Analysis of urine specimens found higher concentration of immunoglobulin and inflammatory mediators. Our current understanding is that most IC/BPS patients in the population do not present with Hunner’s lesions and therefore are classed as having non-Hunner-type IC/BPS¹⁹. To date, bladder wall biopsy is the only available method to detect mast cells²⁰ and molecular signatures of chronic inflammation in the bladder wall, but the method is limited by invasiveness and potential complications. Different fixation/staining techniques for mast cells²¹ also add to the confusion. Therefore, there is a need for a non-invasive imaging method to identify patients with organic disease of the bladder wall.

Contribution of bladder permeability and interstitial cystitis/bladder pain syndrome

Multiple lines of evidence now support a role for increased bladder permeability in IC/BPS patients with organic disease of the bladder wall^22–24. Denudation of the glycosaminoglycan (GAG) layer located at the luminal surface of the bladder^24,25 by protamine sulfate leads to increased bladder permeability, which can be ameliorated by long-term oral or intravesical administration of GAG substitutes. Past published studies used the serum uptake of intravesically instilled urea²⁴ or radionuclide²⁶ as an index of bladder permeability. However, serum uptake²⁶ of instilled substances is subject to considerable inter-individual variability in the distended bladder wall thickness²⁷ and the volume of systemic distribution. Large divergence in the published findings^24,26 can easily be explained by the differences in the molecular weight (60 versus 487 Da) of urea and the radionuclide used as a probe.

The contribution of bladder permeability to the pathophysiology of IC/BPS is also supported by the immunohistochemical and ultrastructural studies of biopsies taken from IC/BPS patients. Decreased expression of tight junction protein and adhesive junction proteins including E-cadherin and zonula occludens-1 (ZO-1) as well as the differentiation marker uroplakin^22,28 supports the role of bladder permeability in IC/BPS pathogenesis. Electron microscopy of the urothelium in the biopsies revealed the contribution of leaky tight junctions in IC/BPS²³. Umbrella cells are pleomorphic in the apical layer of the urothelium, and a decrease in the microplicae (ridges) in umbrella cell membranes was suggested to be pathognomonic for IC/BPS²⁹. Furthermore, upregulation of the purinoceptor P2X3 and an increased release of adenosine triphosphate from the urothelium is also reported in IC/BPS patients³⁰. However, biopsy-based studies are sensitive to site selection, which can lead to the apparent variability in the morphological differences between the urothelium of IC/BPS patients and controls^29,31,32.

Contribution of fibrosis in interstitial cystitis/bladder pain syndrome

Fibrosis, or tissue scarring, often results from the evolutionarily conserved process of wound healing to resolve chronic inflammation. Progressive fibrotic changes in the bladder wall of IC/BPS patients are characterized by excessive deposition of extracellular matrix within the lamina propria and smooth muscle (Figure 1), generation of contractile fibroblasts (myofibroblasts), and decreased capillary density^33,34. These histological changes are associated with the upregulation of collagen genes, collagen I, collagen III, fibronectin, and transforming growth factor-β1 (TGF-β1)³⁵ and the downregulation of sonic hedgehog, WNT gene family, WNT2B, WNT5A, WNT10A, and WNT11 in the biopsy of non-Hunner-type IC/BPS patients^33,36–40. Recent studies report the association of YKL-40 antigenic expression in detrusor mast cell granules and submucosal macrophages with detrusor fibrosis³⁴ and the activation of mammalian target of rapamycin (mTOR) with fibrosis in ketamine-induced⁴¹ IC/BPS.

Figure 1. Magnetic resonance imaging (MRI) is superior to computed tomography (CT) in bladder wall segmentation.

Contrast-enhanced T₁-weighted MRI (Panel C) is superior to CT (Panel A) and unenhanced T₁-weighted MRI (Panel B) for resolving the thickened bladder wall of the same female ulcerative interstitial cystitis/bladder pain syndrome (IC/BPS) patient as indicated by a red line drawn across the bladder wall. Representative T₁-weighted fast low angle shot (FLASH) images acquired at the flip angle of 14° in orthogonal orientation before (Panel B) and after novel contrast mixture (NCM) instillation (Panel C) demonstrate that NCM-enhanced MRI can non-invasively segment the bladder wall into a middle layer of bright signal intensity sandwiched between inner and outer layers of lower signal intensity. Fibrotic changes reported in the pathological results of the biopsy from the IC/BPS patient are visible as the bright middle layer from the diffusion of gadobutrol from NCM instilled in the lumen (hypointense circular region in Panel C). Fast acquisition of a single slice (5 mm thickness) over a single breath hold of approximately 17 seconds with imaging parameters of repetition time (TR)/echo time (TE) of 5.5/2 milliseconds, field of view of 180×180 mm², matrix of 256×256 and 10 averaging achieved in-plane resolution of 0.35 mm (after 2x interpolation) with minimal influence of motion and chemical shift artifacts (Panel B and C). Akin to CT (Panel A), T₁-weighted MRI (Panel B) with similar imaging parameters as in Panel C could not resolve the bladder wall from the lumen in the absence of NCM owing to the relatively long intrinsic T₁ relaxation time of >1 second for the bladder wall and of urine in the lumen. Although the bladder wall is demarcated clearly in pre-contrast T₂-weighted image (Panel D and E) acquired at the TR/TE of 6.9/3.1 milliseconds, there is underestimation of bladder wall thickness, especially of the inner layer from 3.62 mm in panel C to 1.21 mm in Panel E. Instillation of gadobutrol (GBCA) in the absence of ferumoxytol enhances the T₁ contrast of urine as well as the bladder wall, which precludes it from affording improved image contrast. This figure is an original image taken in our clinic for this publication.

Changes in collagen gene expression in biopsy specimens representing the mucosa and submucosa tissue layers of the bladder wall are consistent with the increased collagen staining in the intra- and inter-fascicular muscle tissue of IC/BPS patients¹⁵. Histological detection of fibrosis in the bladder wall is associated with increased passive tissue stiffness during stretch, reduced bladder compliance⁴², uninhibited detrusor contractions⁴³, higher urinary frequency, and lower bladder capacity⁴³. However, decreased bladder capacity⁷ is not a specific marker for IC/BPS patients, as both IC/BPS and overactive bladder patients⁴⁴ exhibit decreased bladder capacity. Fibrotic changes in non-Hunner-type IC and Hunner-type IC^15,33,34,45 are significantly associated with the failure of standard treatment and the need for aggressive treatment options of steroids or hydrodistension with deep bladder biopsies, pain clinic, or surgery (e.g. urinary diversion and cystectomy) in extreme cases. Therefore, earlier detection of fibrotic changes can guide the pre-emptive addition of anti-fibrotic therapy for improved treatment outcomes.

Planar imaging techniques

Cystoscopy. Cystoscopy is a planar imaging technique with a small field of view (FOV) for direct, real-time image guidance of the bladder wall. The ICS regards cystoscopy as the standard technique to identify IC/BPS patients with Hunner’s lesions⁴⁷. Two-dimensional (2D) images generated via cystoscope show Hunner’s lesions on the luminal surface of bladder wall as either singular or multiple erythematous mucosal patches, with small vessels radiating toward a discrete central pale scar with fibrin or coagulum. Fissure and glomerulations (petechial hemorrhages) are visible during bladder emptying after hydrodistention with cascade bleeding. The absence of Hunner’s lesions on cystoscopic examination classifies the patients as non-Hunner-type IC/BPS. The detection of Hunner’s lesions can guide appropriate surgical treatment of the lesion including transurethral cauterization or fulguration and consideration of hydrodistension under general anesthesia in the absence of Hunner’s lesions⁴⁸. However, small FOV of cystoscopy limits its sampling to small tissue volumes, and it can easily miss bladder wall changes that are morphologically indistinct from normal bladder wall⁴⁹. Moreover, this gross morphological discrimination is unable to exclude patients with an abnormality in their pelvic floor.

Ultrasound. Available ultrasound techniques to image the bladder wall produce widely variable outcomes depending on the anatomical approach (translabial/transperineal and suprapubic), frequency, and other factors⁴⁶. Detrusor smooth muscle is hypoechogenic (dark), whereas the mucosa and the outer layer of the adventitia are hyperechogenic (bright). Since bladder wall and detrusor become thinner as the bladder fills, values of the bladder wall thickness (BWT) drop by about 1 mm for each increase in bladder volume distension by 50 mL. Therefore, most measurements of BWT in the range of 3 mm to 6.5 mm are made with bladder filling volume of less than 50 mL using a 5–9 MHz probe. Since image resolution with ultrasound is only limited to 1 mm, a distended bladder wall may only be represented by around 3 image pixels. Ultrasound is generally used to detect renal involvement, pelvic floor muscle mobility⁵⁰, and thickening of the bladder wall in recurrent UTI^51,52 and in IC/BPS⁵³ patients.

Near-infrared imaging. The application of conventional fluorescent microscopy for imaging the bladder wall is limited owing to the issues pertaining to absorption by tissues, scattering, and auto-fluorescence of visible light⁵⁴. Recently, fluorochromes emitting in the near-infrared (NIR) band were shown to emit light with tissue penetration approaching 10–15 cm⁵⁵. Low background auto-fluorescence and minimal absorption by tissue components⁵⁴ in the NIR band make it suitable for deep-tissue imaging⁵⁶ (Figure 2). In a rodent study, the instillation of liposomes containing NIR dye allowed visualization of the instilled liposomes in anaesthetized mouse bladder wall⁵⁷. NIR spectroscopy (NIRS) is a non-invasive, functional, transcutaneous optical technique that uses NIR light to monitor changes in the concentration of oxyhemoglobin and deoxyhemoglobin in the bladder wall. Thus, optical monitoring of bladder wall oxygenation is feasible with NIRS⁵⁸ to potentially delineate the contribution of ischemia to IC/BPS symptoms.

Figure 2. Near infra-red imaging.

Representative images for the rat pelvic area in visible light and in near infra-red (NIR) light after instillation of liposomes containing a trace amount of NIR dye. Deep tissue imaging of the bladder wall is possible, as light emitted from the bladder lumen in the NIR band encounters low background auto-fluorescence and minimal absorption by tissue components. This figure is an original image taken in our clinic for this publication.

Tomographic imaging techniques

Tomographic imaging techniques are capable of three-dimensional (3D) display and acquisition of multiple slices across multiple planes. Such a large image volume is enormously rich in potential information, but the complex instrumentation and intensive image analysis preclude real-time feedback. The potential of tomographic image visualization and quantitative analysis in the diagnosis and treatment of IC/BPS remains unrealized.

X-ray computed tomography. Abdominal computed tomography (CT) is a tomographic imaging technique which was recently used to detect the treatment-associated decrease in bladder wall thickening of IC/BPS patients⁵⁹, identify hidden lesions under scanned area, and exclude malignancies. However, CT exposes patients to high doses of ionizing radiation (X-ray) and lacks the sensitivity to detect the onset and progression of fibrosis. Figure 1 illustrates the relative contrast resolution (degree of difference between brightest and darkest component of an image) of the bladder wall with CT and MRI.

Magnetic resonance imaging. MRI is a safe imaging technique that does not rely on ionizing radiation for tomographic imaging of visceral organs. MRI affords higher contrast and spatial resolution of soft tissue in multiplanar images⁶⁰, which makes it one of the preferred imaging modalities in urology. MRI is especially superior to CT in reliably producing slices more suited to detect lesions in the dome and base of the bladder wall. MRI also allows diffusion imaging and dynamic contrast enhancement in the bladder wall⁶⁰. However, there is no reference signal for MRI corresponding to the standard reference signal of 0 Hounsfield units for the radio density of water in CT. So, MR image quality in terms of both image contrast (Figure 1) and spatial resolution is described by contrast resolution or contrast-detail measured by contrast–noise ratio (CNR)⁶¹.

Conventional MRI

Exquisite soft tissue contrast in conventional T₁- and T₂-weighted MRI arises principally from the differences in the intrinsic tissue relaxation times T₁ (spin–lattice relaxation time)⁶² and T₂ (spin–spin relaxation time)^10,63,64. Variation in signal sensitivity (rise in CNR) to highlight the differences in T₁⁶² and T₂^63,64 for T₁-weighted and T₂-weighted images^63,65 is commonly achieved by manipulating the acquisition parameters (e.g. flip angle [FA], echo time [TE], repetition time [TR], inversion time, etc.). Unenhanced T₁-weighted MRI requires a TR of >4–5 times the T₁ relaxation time of the target organ. Hence, a relatively long intrinsic T₁ relaxation time of >1 second for bladder wall and of >6 seconds for urine at magnetic field strength of 3 Tesla (T)^62,66 requires a TR of >5 seconds, which prolongs the image acquisition (>5 minutes)⁶⁷ and the resulting image quality of the thin human bladder wall^68,69 becomes highly susceptible to artifacts from involuntary bowel and respiratory motion of the subject⁷⁰. Moreover, multiple signal averages⁶³ with prolonged image acquisition for T₁-weighted MRI of the bladder wall can only achieve in-plane resolution of >0.5 mm^71,72.

We recently showed that the use of a short TR of 5.5 milliseconds increases the image resolution in T₁-weighted MRI (Figure 1B), but the isointense signal in the bladder wall and urine⁶² makes it difficult to differentiate the thin human bladder wall from the urine. Though the human bladder wall is more distinguishable in T₂-weighted MRI (Figure 1D)^63,73, T₁-weighted MRI⁶⁷ is preferable for its higher signal-to-noise ratio (SNR) and accurate measurement of BWT^69,73–75 for characterizing bladder disorders⁵¹. As shown in Figure 1, the thickness of the inner layer (urothelium) of the bladder wall is underestimated in T₂-weighted images^75,76. The bladder wall appears as a thin layer of intermediate intensity on T₂-weighted images and is likely to correspond to the thickness indicated by the middle and outer layer on the contrast-enhanced T₁-weighted image (Figure 1C)⁶⁵. It appears that the highly intense signal of urine in unenhanced T₂-weighted images submerges the signal from a portion of the urothelium layer, leading to BWT underestimation⁷⁵. Since lesions in the bladder wall associated with IC/BPS are generally restricted to the urothelium, T₂-weighted MRI can easily miss the important details for pathological characterization of IC/BPS. T₂-weighted MRI is also not suitable for staging the bladder cancer before invasion of muscular layer⁶⁵.

Furthermore, the sandwich composition of the bladder wall’s histology can displace the respective MR signals in the readout direction to produce chemical shift mis-registration of the perivesical fat signal and exaggerate motion artifacts^66,77,78. Chemical shift artifact refers to the signal alterations that result from the 440 Hz difference in the resonant frequencies of fat and water protons at 3T^66,78. Therefore, variable shape, location, and histological composition of the bladder wall limits the utility of conventional MRI in the diagnosis and treatment of IC/BPS. Nevertheless, urinary bladder can be visualized by MRI in multiple cross-sectional orientations with parallel image series in cross-sectional imaging planes and intersectional at different orientations. However, the information in 2D image slices is non-continuous owing to slice gaps. Hence, improvements in bladder wall imaging call for enhancement of contrast, SNR and reduced susceptibility to artifacts⁷⁹ during image acquisition and 3D reconstruction.

Contrast-enhanced MRI

One of the traditional approaches to enhance T₁ contrast of the human bladder wall is the intravenous injection of gadolinium-based contrast agents (GBCAs). Entry of blood containing the injected GBCAs into the middle layer of the bladder wall can briefly differentiate it into three layers: a thin inner layer of low intensity, a middle layer of strong intensity, and a thick outer layer of intermediate intensity in post-contrast images⁶⁵. However, subsequent entry of the GBCA excreted into urine within 3 minutes of injection^80,81, and continuous accumulation of urine in bladder ultimately reduces the contrast between the lumen and bladder wall^69,73,82. Moreover, the T₂ relaxation effect of GBCAs^80,83 causes pseudolayering in the urine collected in the lumen⁷³. To overcome the drawbacks associated with the intravenous administration of GBCAs, several groups have now tried intravesical administration of GBCAs in bladder cancer⁸⁴, vesicoureteral reflux⁸⁵, and IC/BPS⁸⁶. But the enhancement of T₁ contrast in the urine by the instilled GBCA analog gadolinium–diethylenetriaminepentaacetic acid prevents it from offering a good image contrast in the bladder wall^69,84,86 and accurately measuring the BWT. Likewise, instillation of superparamagnetic iron oxide (SPIO) nanoparticles⁸⁷ has also been tried without much success to improve the contrast of the human bladder wall⁸⁸.

Our group recently reported that novel contrast mixture (NCM) can improve the image contrast of the bladder wall in T₁-weighted MRI of rat⁸⁸ and human bladder⁶¹ (Figure 1C, Figure 3, and Figure 4). NCM is a homogenous mixture of gadobutrol (GBCA) diluted 1:250 and ferumoxytol (SPIO) diluted 1:104 in sterile water for irrigation (Panel A). Gadobutrol (transparent liquid) with a molecular weight of 604.71 Da, whether injected intravenously or instilled into bladder, reaches the extracellular space in the lamina propria to produce T₁ contrast (signal increase). Meanwhile, the large molecular weight of 750 kDa restricts the bladder wall diffusion of ferumoxytol (vial with brown-colored liquid) to primarily exert T₂ contrast (signal decrease) in the lumen and produce a localized increase in proton dephasing of T₁-weighted FLASH (fast low angle shot) MRI at a TR/TE of 5.5/2 milliseconds. T₁-weighted FLASH images of the tubes containing gadobutrol or NCM in Figure 4 A–B illustrate that the presence of ferumoxytol (5 mM) in the NCM tube masks the gadobutrol-mediated signal enhancement, which is visible with an increase in FA from 6° (panel A) to 14° (panel B) in the right tube containing gadobutrol 4 mM alone. The concentration of gadobutrol (4 mM) is the same in both tubes identified as NCM and gadobutrol. Likewise, the separation of gadobutrol away from NCM instilled in the lumen dramatically increases the signal intensity in the bladder wall of ulcerative IC/BPS patients (Figure 4E–F) but not in the non-ulcerative IC/BPS patient (Figure 4C–D), which indicates that a greater physical separation of gadobutrol from ferumoxytol occurs from the diffusion of gadobutrol into the bladder wall away from the NCM instilled in the bladder lumen of ulcerative IC/BPS patients.

Figure 3. Contrast-enhanced magnetic resonance imaging (MRI) relies on the diffusion of gadolinium into the bladder wall.

Novel contrast mixture-enhanced MRI relies on the differences in the contrast mechanisms and molecular weight of two US Food and Drug Administration-approved agents (gadobutrol diluted 1:250 and ferumoxytol diluted 1:104) for increasing the contrast resolution of the bladder wall (Panel A). Gadobutrol, a transparent liquid, is a gadolinium-based contrast agent (GBCA) with a molecular weight of 604.71 Da that reaches the extracellular space in the lamina propria to shorten T₁ (positive contrast) or produce higher signal intensity (Panel A and B). Meanwhile, the large molecular weight of 750 kDa for ferumoxytol (vial with brown-colored liquid) restricts its bladder wall diffusion, and it produces a localized increase in proton dephasing, which decreases the signal intensity (negative contrast) in T₁-weighted images. Instillation of novel contrast mixture 50 mL can non-invasively segment the inner layer (urothelium) and the outer layer of adventitia (dark signal) from the bright signal in the middle layer composed of the lamina propria and detrusor smooth muscle (Panel C). Bladder wall histology in Panel C is shown for illustration and does not represent the pathological characterization of the interstitial cystitis/bladder pain syndrome (IC/BPS) patient shown in Panel B. This figure is an original image taken in our clinic for this publication.

Figure 4. Quantitative measurement of gadolinium diffusion.

T₁-weighted fast low angle shot (FLASH) images with constant repetition time (TR) of 5.5 milliseconds (ms) at flip angle (FA) of 6° (Panel A, C, and E) and 14° (Panel B, D, and E) demonstrate that gadobutrol-mediated signal enhancement (visible in the right tube containing gadobutrol 4 mM alone) is suppressed by the presence of ferumoxytol (5 mM) in the novel contrast mixture (NCM) tube, as the gadobutrol concentration of 4 mM is the same in both tubes (Panel A and B). T₁-weighted FLASH images demonstrates that greater separation of gadobutrol into the bladder wall away from the NCM instilled in the bladder lumen occurs in ulcerative IC/BPS patients (Panel E and F) than in non-ulcerative interstitial cystitis/bladder pain syndrome (IC/BPS) patient (Panel C and D), which is evident from the dramatic increase in signal intensity in Panel F relative to Panel D at FA of 14°. Catheter used for instillation is shown by C in panel C–H. Constant TR of 5.5 ms at different FAs achieves the stable steady-state conditions necessary for the differences in signal intensity of the same slice to become a function of T₁ relaxation time as indicated by the color panel in Panel H. Greater shortening of T₁ relaxation time (blue color) in ulcerative IC/BPS patients is consistent with higher diffusion of gadobutrol into the expanded extracellular matrix of the thickened bladder wall of IC/BPS patients. This figure is an original image taken in our clinic for this publication.

Post-contrast images of human bladder wall were acquired after 50 mL NCM instillation to achieve artifact-free visualization of the bladder wall with an in-plane resolution of <0.5 mm. The use of a short TR of 5.5 milliseconds with 10 averages shortened the image acquisition from 5 minutes to a single breath hold of 17 seconds and reduced the susceptibility to motion artifacts. A higher number of pixels representing the bladder wall and high readout bandwidth of 574 Hz/pixel ensure that the mis-mapping of the water and fat signals is limited to a pixel (i.e. 0.7 pixels = 440 Hz/574 Hz)^66,78. Contrast-enhanced MRI (CE-MRI) of the ulcerative IC/BPS patient after the instillation of NCM revealed segmentation into three layers of different signal intensity in post-contrast images taken at the FA of 14°, where the middle layer of bright signal intensity was sandwiched between the inner and outer layers of lower signal intensity. The constant TR of 5.5 milliseconds at different FAs achieves the stable steady-state conditions necessary for the signal intensity measurement of the same slice at different FA to become a function of the T₁ relaxation time (Figure 4G and 4H).

In a study published by another group, GBCA was instilled at the rate of 2–2.5 cc per second over a 5–10 minute period or a total volume of 600–800 mL⁸⁶. In comparison, a standard volume of 50 mL for NCM instillation is likely to be more tolerable for a subset of severe IC/BPS patients with low bladder capacity of 200–400 mL^7,89. Bladder wall thinning of approximately 0.41 mm measured after 50 mL instillation for NCM-enhanced T₁-weighted MRI⁶¹ is consistent with the ultrasound measurements of BWT distended to similar volumes⁴⁶. It is known that overdistention of the bladder wall can provoke motion artifacts in sensitive patients and the consequent thinning of the bladder wall can also hinder the differentiation of tissue layers and the luminal texture⁹⁰.

Pelvic magnetic resonance imaging as a tool to assess the contribution of bladder permeability in interstitial cystitis/bladder pain syndrome

Instead of serum uptake, T₁-weighted CE-MRI in IC/BPS patients relies on the penetration of an instilled GBCA into the bladder wall as a non-invasive measure of bladder permeability^86,91,92. Towner et al. reported that instillation of GBCA bestowed greater skewness and kurtosis in the probability distribution for the bladder wall signal intensity⁸⁶. It is worth stating here that the signal intensity measurement in MRI depends on several factors, including intrinsic properties of the tissue and the acquisition parameters, receiver coil geometry, sensitivity, and signal amplifier gains. These variables introduce a non-linearity into the signal intensity measurement and differences in hardware used at different places preclude any direct comparisons of intensity values across patients or imaging centers. Moreover, hypointense signal intensity in the inner layer of the bladder wall (Figure 1C and Figure 3) after NCM instillation indicates that quantitative measurement of the bladder wall signal intensity following GBCA instillation is especially vulnerable to the MRI artifacts^62,66,77,78.

Passive diffusion of gadobutrol into the bladder wall is presumed to create a downhill concentration gradient of GBCA with the highest concentration in the innermost layer. However, signal intensity in different tissue layers of the bladder wall does not show a similar downhill gradient owing to the T₂ relaxation effect of GBCA at higher concentrations^80,83, which reduces the relative signal intensity of the inner layer compared to the middle layer of the bladder wall. The T₂ relaxation effect of the GBCA is also known to cause pseudolayering in urine containing excreted GBCA accumulated in the bladder after intravenous injection of the GBCA⁸⁰. Also, a role for an increased proton dephasing from localized perturbation of the magnetic field by ferumoxytol cannot be ruled out in the lowering of the signal intensity in the inner layer. Since signal intensity measurement in CE-MRI can introduce errors in bladder wall permeability measurement, the interpretation of the imaging data can be simplified by direct calculation of the proton density and spin relaxation times. Hence, acquisition T₁ relaxation time can facilitate improved characterization of the bladder wall, enhance image tissue contrast, and provide a more direct link between the observed signal changes and pathological changes detected by bladder wall biopsy. Besides, quantitative relaxometry (quantitative measurement of T₁ relaxation time) can facilitate the standardization of bladder wall imaging for longitudinal study on the same patient and multicenter studies.

In a small pilot study of six subjects, we used the differences in signal intensity at different FAs to calculate the T₁ relaxation time, which is an intrinsic property of tissues. T₁ graphically represents the first order time constant required for the z-component of net magnetization to reach (1-1/e) or about 63% of its maximum signal intensity⁹³. A change in the longitudinal relaxation rate (1/T₁) of the tissue is directly proportional to the GBCA concentration in the tissue⁹⁴. However, the acquisition of artifact-free, high-resolution imaging of human bladder wall is a prerequisite for mapping the T₁ relaxation time and for deriving the bladder wall permeability data. Therefore, automatic acquisition of the pre-contrast and post-contrast pixel wise T₁ maps obtained after NCM (Figure 4G and 4H) can be a robust measure of bladder wall permeability. The variable FAs⁹⁵ method is a preferred alternative to the Look-Locker method for quantitative measurement of bladder wall T₁, as it does away with the inversion pulse and clinically infeasible times required to more fully characterize the recovery curves. The differences in signal intensity at different FAs in different layers of the bladder wall were transformed to a homogenous blue color indicating the shortening of the bladder wall T₁ following diffusion of gadobutrol from instilled NCM (Figure 4).

Studies on the heart and liver have demonstrated that fibrotic changes^96,97 increase the T₁ relaxation time and therefore quantitative T₁ measurement⁹⁸ of the bladder wall is proposed as an objective and reproducible parameter for the non-invasive detection of diffuse histological changes such as edema and fibrosis without resorting to bladder wall biopsy. Indeed, the bright middle layer in the bladder wall (Figure 1C, Figure 3B, and Figure 4F) is consistent with the evidence of fibrosis noted in the bladder biopsy and bladder wall thickening noted in CT of the same IC/BPS patient. Therefore, quantitative measurement of bladder wall T₁ can become a non-invasive biomarker of diffuse tissue changes leading to increased bladder wall permeability, which can help the clinician to discriminate IC/BPS from other pelvic floor defects. In recent years, the use of T₁ mapping techniques has been simplified and can be readily integrated into clinical MRI examination.

Pelvic magnetic resonance imaging as a tool to assess the contribution of pelvic floor hypertonicity in interstitial cystitis/bladder pain syndrome

The anatomies of pelvic structures^99,100 are critical for the diagnosis of pelvic floor hypertonicity. The contribution of pelvic floor hypertonicity to pain in 15 female IC/BPS patients and age-matched controls was investigated with T₂-weighted MRI without instilling or injecting any contrast agents. Increased pelvic floor hypertonicity in IC/BPS patients was linked to shortened levator muscles, wider posterior puborectalis angle, and decreased puborectal distances. While the total urethral length and M line were similar in two cohorts, the H line was shorter and the vaginal cuff and bladder neck distances to the H line were longer in patients with IC/BPS. These observations need to be considered in light of the known age dependence in the displacement of the bladder and vagina⁹⁹ and as-yet-unknown contribution of neuronal factors, decrease in muscle strength/mass, or fat deposition. Pelvic magnetic resonance 3D reconstructed images can reveal the anatomical relationships of pelvic organs with each other¹⁰¹.

Brain magnetic resonance imaging as a tool to assess the contribution of central pain processing

In a multicenter study, high-resolution T₁-weighted MRI¹¹ and functional MRI (fMRI)¹² of brain was used to detect alterations in central pain processing of well-phenotyped IC/BPS patients. Compared to healthy controls, the pain, mood (anxiety), and urological symptoms of 33 IC/BPS patients were associated with a notably elevated volume of gray matter in a number of different brain regions. A separate study examined the 10-minute resting brain fMRI of 85 IC/BPS patients and 85 female healthy controls¹² to detect blood oxygen level-dependent signal, which was then transformed to the frequency domain. Altered frequency distributions in viscerosensory (post insula), somatosensory (postcentral gyrus), and motor regions (anterior paracentral lobule and medial and ventral supplementary motor areas) were detected in IC/BPS patients relative to controls. IC/BPS patients also showed increased functional connectivity of the anterior paracentral lobule and medial and ventral supplementary motor areas to the midbrain (red nucleus) and cerebellum. Patients who experienced pain during bladder filling had the highest level of increased functional connectivity.