Urinary bladder is a muscular elastic organ that stores urine from the kidneys via the ureters, and empties via the urethra. The bladder wall is 3-5 mm thick, but thinner when the bladder is extended. The inner wall has thick mucosal folds with surface glycocalyx, allowing the distension, and protecting the bladder from infections and toxic substances in the urine. Bladder and ureter walls contain numerous transporters that may help to maintain the high concentration gradients between urine and plasma; during bladder infections the bladder wall becomes more permeable to solutes (Lavelle et al., 2002).
Blood flow to the bladder is usually supplied by at least two arteries to different parts of the organ. Also the veins and lymph flow can be varied between individuals.
Human bladder can normally hold 3-5 dL of urine before the need to empty. After voiding, less then 50 mL urine should stay in the bladder, and >200 mL post-void residual volume (PVR) indicates inadequate emptying (Kolman et al., 1999).
Bladder cancer (urothelial carcinoma) is a common malignancy in men, and occurs also in women. If diagnosed early, bladder cancers have not penetrated the muscle layer (non-muscle invasive bladder cancer, NMIBC), but are confined to the epithelium or urothelium. Tumour tissue is usually removed by transurethral resection. If cancer is spread into urothelium, BCG immunotherapy is applied instead; bacillus Calmette-Guérin (BCG), which is an attenuated strain of Mycobacterium bovis (bovine tuberculosis causing bacterium), is put into the bladder via catheter, inducing local immune response that kills tumour cells. Also other immunotherapies, including radioimmunotherapy, and chemotherapy, can be applied to avoid surgical removal of the bladder (cystectomy).
Most bladder cancers overexpress EGFR, which can be targeted in diagnostic imaging and therapy.
PET radioligands and their radioactive metabolites are usually excreted into urine, and therefore bladder is often the critical organ in PET dosimetry. High urine activity can prevent detection of primary tumour in the bladder wall (Ahlström et al., 1996). Therefore [18F]FDG (Kosuda et al., 1997) and other tracers with high urine activity are usually considered of limited value to detect bladder cancer. High bladder activity may lead to severe scatter artifacts (Lawhn-Heath et al., 2018; Lindemann et al., 2019). The activity in the urine can be reduced by a diuretic and oral hydration and delaying the PET imaging (Anjos et al., 2007; Derlin et al., 2016). Oral hydration, on the other hand, may increase the fraction of [18F]FDG excreted in urine (Moran et al., 1999). In small animal studies, continuous flushing of the bladder can markedly improve the image quality and quantification (Deleye et al., 2014).
[1-11C]acetate and [11C]choline (and [18F]choline) are not excreted in urine, and can therefore provide good-quality images of bladder cancer and lymph node metastases; however, inflammation and infection can lead to false positives, and routine use is not yet recommended (Schröder et al., 2012; Salminen et al., 2016). In a prospective study, [1-11C]acetate PET/MRI was shown to be feasible for staging of bladder cancer (Salminen et al., 2018).
In PET imaging the anatomical CT or MR image usually cannot be used to delineate the bladder or bladder wall, because filling of the bladder during the scan changes the size and position of the organ (Bretin et al., 2017).
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Updated at: 2020-01-12
Created at: 2017-11-29
Written by: Vesa Oikonen