FIGURE 131.19 Full bladder measured in two different planes Bladder Ultrasound Bladder ultrasound is performed for a variety of reasons For the child who has not voided for a prolonged period, assessing bladder size can inform the practitioner if there is a problem with urinary retention More commonly, bladder ultrasound is done prior to bladder catheterization or suprapubic aspiration to assess volume of urine and thereby improve success rates It may be used dynamically as well to guide needle placement and improve success rates of suprapubic aspiration Technique The same approach as the pelvic view of the FAST examination is used when performing a bladder ultrasound A high-frequency linear probe or low-frequency abdominal probe may be used A full bladder will appear as a well-circumscribed, fluid-filled (anechoic) structure within the pelvis Once the bladder is identified, it should be measured in at least two planes (Fig 131.19 ) Although studies differ with respect to exact measurements, generally speaking if the wall-to-wall distance measures greater than cm in both planes, catheterization or suprapubic aspiration will likely be successful