Awake humans in supine posture produce urine at a faster rate with postural shift in body fluids elevating the renal perfusion, which raises the glomerular filtration rate. While a rapid rise in ultrasound measurement of bladder volume during initial stages of sleep (ref.1) is consistent with rapid urine production ≥ 2mL/min but the lack of a steady rise in bladder volume during deep sleep implies water absorption from bladder (ref.2) can offset urine production of ≤ 1mL/min by kidney during sleep.  Because the signal of magnetized water protons is intrinsically sensitive to urine osmolality, we hypothesized that water reabsorption from urine stored in bladder (ref.2) will raise the signal of stored urine higher than the fresh urine during 90seconds MRI scan without water loading required for prolonged cine MR urography or cystoscopy.

Eight female subjects aged 26-77 years consented for prospective MRI without any fluid consumption for >60min prior to Prisma 3T scan with 4-channel flexible receiver coil. Bladder was first rapidly localized by HASTE scan, and then 90s T2-weighted Turbo spin echo scans were acquired for 30-45 axial and sagittal slices of 3mm slice thickness in 16cm field of view, which were volumetrically rendered by Image J to display grey and color coded signal differences between ureter jets and stored urine.

Scanned subjects could be stratified by ~2cm or ~6cm long spiral jets from ureteral orifice on posterior wall, indexing spot urine production rate of ~2mL/min and >10mL/min, respectively. The downhill gradient of greyscale and color-coded signal intensity (SI) from urine stored in lumen, yellowish red >742 artificial units (a.u.) to greenish yellow, <700 a.u. for fresh urine of ureteral jets indexes an uphill osmolality gradient resulting from free water reabsorption from urine stored in bladder (ref.2). Peristaltic contractions of ureter results in even spacing of expanded and contracted ureteral segments in the abdomen and ureteral jets resemble funnel cloud on axial slice and resulting vortex mixes fresh urine of lower osmolality with the higher osmolality of stored urine. While short greenish yellow jets against red lumen produced lower urine volume of 120-150mL (n=6), higher bladder volume of 250-450mL resulted from long green jets reaching the anterior wall against yellowish red lumen (n=2).

MRI scan non-invasively corroborated past reports of bladder raising urine osmolality (ref.1-3) and awake adults without water-loading producing urine at twice the rate estimated from awake voided volume, ≥2mL/min. While long ureteral jets of lower signal (yellow-green) in supine posture symbolizes dilute urine produced during initial stages of sleep (ref.1), dramatically higher signal of stored urine is consistent with homeostatic reabsorption of free water via passive paracellular diffusion resulting in higher osmolality of stored urine (ref.2). Therefore, intravesical absorption rate of tritiated water (≥1mL/min) mirrors the water reabsorption from stored urine for homeostasis of plasma osmolality which slows the  steady rise in bladder volume during deep sleep (ref.1) by raising the osmolality of morning urine. In fact, homeostatic water reabsorption from adult bladder reduces the awake voided urine volume to 1.2±0.25L, which is ≥30% lower than the voided volume estimated from urine production rate of ≥2mL/min (ref.2) of awake adults having  "water-tight" bladder.

Therefore, constant water reabsorption from bladder results in higher osmolality and higher signal of stored urine than of fresh urine (ref.2). Bladder reduces voided urine volume by ≥ 30%  and erects false-positive match between average urine production rate of adult kidney and that of 40 weeks old fetus.  MRI is superior to ultrasound in displaying ureteral jets and urine reabsorption by bladder for objective phenotyping of storage symptoms and establishing the bladder as a site in pathophysiology and treatment of nocturia and OAB.

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