Bladder ultrasound: calculation of volume
Sonographic evaluation of the urinary bladder should be performed in any patient with dilated collecting system or frequent urinary tract infections, especially in a male. The examination is performed with the patient in supine position with suprapubic area exposed. The probe is placed longitudinally in the mid-line above the pubic symphysis with probe marker towards patient’s head to obtain sagittal view of the bladder. Then it should be angled laterally and ‘swept’ to left and right to examine the lateral borders. The probe is then rotated 90 degrees counter clockwise to obtain the transverse view and swept superior to inferior to image the bladder completely. The volume of the bladder is estimated by orthogonal measurements, assuming it to be an ellipsoid (= 0.52 × the three orthogonal dimensions). Both pre and post-void measurements should be taken to diagnose urinary retention. Because bladder is not a true ellipsoid, alternative formulae exist such as 0.75 x 3 dimensions and 0.66 x 3 dimensions. Practically, we just want to know if the patient is retaining urine and knowing the ‘precise’ volume rarely matters.