Embryology and Anatomy
- The bladder is a hollow, distensible viscus with a strong muscular wall. Embryologically, it forms from the urogenital sinus, which is contiguous with the allantois, a hindgut diverticulum extending to the umbilicus. The allantois normally involutes by the 2nd month of gestation, forming the median umbilical ligament. Any persistent segments of the allantoic channel are called urachal remnants.
- The distal ureters enter the posterolateral bladder wall at the trigone. The urethral orifices form the apex of the bladder trigone.
- The bladder wall has 4 layers. The lumen is lined by uroepithelium with 3-7 layers of stratified flat cells, which are flexible and can change shape from cuboidal to flattened as the bladder distends, hence the term transitional epithelium. The 2nd layer is the lamina propria, containing connective tissue, lymphatics, and neurovasculature. The 3rd layer is the detrusor muscle (muscularis propria), a complex network of interlacing smooth muscle fibers. The inner and outer muscle fibers tend to be oriented longitudinally, but distinct layers are usually not discernible. Detrusor muscle fibers merge with the prostate capsule (or anterior vagina in females) and pelvic floor muscles. A 4th adventitial layer is formed by connective tissue. A serosal covering, formed by the peritoneum, is only at the bladder dome.
- The bladder is located within the extraperitoneal space and is surrounded by loose connective tissue and pelvic fat. The perivesical space contains the bladder and urachus. The prevesical space, also called the space of Retzius, extends ventrally to the pubic symphysis and communicates posteriorly with the presacral space. These spaces can expand to contain large amounts of fluid or hemorrhage, e.g., from an extraperitoneal bladder rupture or due to pelvic fractures.
Imaging Techniques and Indications
- Conventional cystography primarily evaluates for bladder leak, usually with abdominal trauma or iatrogenic injury, or for fistulae, although CT cystography is now the preferred modality. Cystography accuracy rate is 90-100% for detecting bladder injury, if performed properly.
- A scout film is performed before infusion of contrast. Adequate bladder distention with a minimum of 300 mL of contrast is imperative to exclude small leaks, which are easily missed with incomplete bladder distention. Ideally, oblique images should be obtained; however, in practice, these patients often have pelvic fractures and are not mobile. Diagnostic images can still be obtained in this situation, but it is imperative to obtain images of a maximally filled bladder and a postdrainage film. The importance of the postdrainage film cannot be overemphasized, because in ~ 10% of cases, the injury is only seen on the postdrainage film.
- Leaks may occur in the extraperitoneal space, appearing as focal collections of contrast, or in the intraperitoneal space, appearing as free-flowing contrast outlining bowel loops. Fistulae are also well demonstrated with contrast accumulating in the small bowel, colon, or vagina.
- CT cystography is equally sensitive and specific to conventional cystography for bladder leaks, with accuracy nearing 100%, and benefits from the detailed anatomic information obtained on cross-sectional imaging, hence it is now the preferred imaging modality. For fistulae, CT cystography is superior to conventional cystography, not only because of improved diagnosis by avoiding overlapping contrast obscuring the tract, but also due to detailed tract evaluation, which is crucial for clinical management and potential surgical planning.
- After routine CT, the bladder should be drained, and a minimum of 300 mL of dilute, water-soluble contrast should be instilled followed by a repeat scan through the bladder. Multiplanar reformations in the coronal and sagittal plane help define the anatomy and site of injury/fistulae. Unlike with conventional cystography, postdrainage imaging is not required. CT cystography can be performed immediately following the initial trauma CT, helping diagnose bladder injury quickly and accurately compared with conventional cystography. Note that filling the bladder antegrade with contrast, by performing delayed CT imaging, is inadequate for evaluating bladder injury and does not exclude injuries.
- Underdistention can erroneously simulate wall thickening, and overdistention can obscure small, sessile masses. CT does not accurately determine the depth of mural tumor invasion but evaluates for organ-confined vs. nonorgan-confined disease, lymphadenopathy, and distant metastasis. Increasingly, mpMR is being used for T staging to differentiate nonmuscle invasive (≤ T1) vs. detrusor muscle invasive (≥ T2) disease, which determines whether treatment is transurethral resection and possible intravesicular chemotherapy/phototherapy vs. radical cystectomy with radiation, chemotherapy, or both. Adding diffusion-weighted and dynamic contrast-enhanced imaging, MR is 90% sensitive and 88% specific for determining tumor invasion of the detrusor muscle.
- A full bladder is easily seen with transabdominal US, though the anterior wall is poorly evaluated secondary to reverberation artifact. Bladder stones, clots, and some masses are readily detected, and US benefits from being able to demonstrate mobility of a lesion. Color Doppler US is useful to evaluate the vascularity of masses and presence of ureteral jets. Postvoid residual is also easily calculated.
Approach to Bladder Masses
- Pathologic conditions of the bladder can manifest as either a focal bladder mass or diffuse wall thickening. Focal masses are most commonly neoplastic but may develop secondary to congenital, inflammatory, or infectious processes. Certain bladder abnormalities have pathognomonic appearances, such as ureteroceles. Secondary bladder inflammation, e.g., from Crohn disease or diverticulitis, may result in a focal bladder wall abnormality or fistulae. In many cases, the clinical, macroscopic, and radiologic findings for these masses may overlap; thus, histologic evaluation is often required.
- Bladder neoplasms can arise from any of the bladder layers. They are broadly classified as either epithelial or nonepithelial (mesenchymal) neoplasia. > 95% of bladder neoplasms are epithelial, predominately urothelial carcinoma (~ 90%). The spectrum of neoplasia ranges from benign papilloma, to carcinoma in situ, to invasive carcinoma. Other rarer primary epithelial tumors include squamous carcinoma and adenocarcinoma, the latter sometimes arising within an urachal remnant. Because epithelial masses derive from the most superficial layer of the bladder wall, they often appear as irregular, intraluminal filling defects.
- Neoplasms derived from mesenchymal tissue arise from muscle, nerve, cartilage, fat, fibrous tissue, and blood vessels. Benign tumors include leiomyoma, paraganglioma, fibroma, hemangioma, solitary fibrous tumor, neurofibroma, and lipoma. Malignant tumors include rhabdomyosarcoma, leiomyosarcoma, lymphoma, and osteosarcoma. Because mesenchymal tumors arise from the submucosal portion of the bladder wall, they often appear as smooth intramural lesions. If large, however, they can ulcerate and be confused with a mucosal mass.
- Some neoplasms can cause diffuse wall thickening (adenocarcinoma and lymphoma most commonly), but this is more typically seen in nonneoplastic disorders. Diffuse bladder wall thickening can develop secondary to many conditions, including infection with bacteria or viral infection, schistosomiasis, tuberculosis, and inflammatory conditions, such as cystitis cystica, cystitis glandularis, or eosinophilic cystitis. Exposure to chemotherapy (particularly with cyclophosphamide) or irradiation also cause diffuse wall thickening. The radiologic characteristics of these disorders tend to be nonspecific.
- Acute cystitis demonstrates bladder wall thickening and hyperemia and adjacent inflammatory changes. Bladder tuberculosis and schistosomiasis produce nonspecific bladder wall thickening and ulceration in the acute phase, suspected in immunocompromised patients or those from endemic regions. In the chronic phase, the bladder wall may calcify, a characteristic finding of chronic schistosomiasis. The diagnosis of chemotherapy or radiation cystitis should be clinically evident, but imaging may be used to determine severity and to assess complications.
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