Urine is produced in the kidneys by filtering out waste products from the bloodstream. In these circumstances the muscular wall is much thicker but the shape and volume of the bladder can be surprisingly variable. For other treatments, such as to the bladder, voiding is encouraged before treatment or imaging. For some treatments such as prostate, it is common for the bladder to be maintained at full capacity and in these cases the pressure can give the bladder a more consistent shape, although the paler muscular wall stretches thin. Thus it is not uncommon to find two or three separate ‘bulges’ of bladder appearing on CT slices around other structures. Since the bladder is essentially an elastic-walled bag of urine, it will deform readily around more solid anatomy adjacent to it. The bladder is the main organ of interest with regard to radiotherapy in the urinary system. The final stage of the ureters’ journey takes them around the posterior edge of the bladder towards the inferior ‘trigone’ area. Recall that the right kidney is the lower of the two due to the volume of liver above it. Each kidney is accompanied by an adrenal gland which is located superiorly and medially. The twin starting points of the urinary system are the kidneys, which are located at the rear of the abdomen, slightly anterior to the psoas muscles and lateral to the vertebral bodies. The shafts of the femora (5) lie laterally to this and the cancellous bony centres are clearly visible, surrounded by the higher density cortex. Fat within each ischiorectal and ischioanal fossa can be seen laterally to each ischium (9), separated centrally by the natal cleft. Central to Figure 2.1.14, the levator ani complex (11) intimately surrounds the anus. This can be improved by supporting the axial CT sections with coronal section imaging using high resolution T2 MR. These muscles make soft tissue differentiation in the inferior pelvis particularly challenging in both males and females. The urogenital diaphragm, formed by a thin sheet of striated muscle, is located anterior to the levator ani muscle group. The levator ani muscle group (11) forms the base of the funnel shaped pelvic diaphragm closing the inferior pelvic aperture.
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