How to Order CT Abdomen for Ruling Out Bladder Obstruction
For suspected bladder obstruction, order a CT urography (CTU) without and with IV contrast, which includes unenhanced, nephrographic, and excretory phases—this provides comprehensive evaluation of the entire urinary tract including the bladder, ureters, and kidneys to identify both the presence and cause of obstruction. 1
Why CTU is the Optimal Study
CTU provides superior anatomic detail compared to standard CT abdomen/pelvis because the excretory phase (obtained at least 5 minutes post-contrast) specifically opacifies and distends the urinary tract for optimal visualization of the bladder and collecting systems. 1 Standard contrast-enhanced CT abdomen/pelvis without dedicated urographic phases does not adequately evaluate the bladder because it lacks this critical excretory phase imaging. 1
The multiphasic protocol includes three essential components:
- Unenhanced phase for detecting calculi (which may cause obstruction) 1
- Nephrographic phase for evaluating renal parenchyma 1
- Excretory phase (≥5 minutes post-contrast) for urothelial and bladder visualization 1
When Non-Contrast CT is Insufficient
Non-contrast CT abdomen/pelvis alone is primarily useful only for stone detection (97% sensitivity) but provides severely limited evaluation of bladder wall abnormalities, masses, or complex structural causes of obstruction. 2, 3 While it can identify hydronephrosis and assess renal size, it cannot adequately characterize the bladder itself or determine the underlying etiology of obstruction. 4
Clinical Context Matters
If Urolithiasis is the Primary Concern:
Order CT abdomen/pelvis without IV contrast (stone protocol) when obstructive urolithiasis is the primary suspected cause, as this maximizes stone visualization. 4 However, recognize that this approach is less comprehensive for evaluating other causes of bladder obstruction. 4
If Bladder Pathology is Suspected:
For suspected bladder tumors, fistulas, structural abnormalities, or bladder outlet obstruction from prostatic disease or pelvic masses, CTU with IV contrast is the preferred examination. 1 The addition of IV contrast is particularly helpful in assessing for pelvic masses as a cause of obstruction. 4
For Traumatic Bladder Injury:
CT cystography (with direct bladder contrast instillation via catheter) is specifically indicated for traumatic bladder injuries, bladder fistulas, and leaks. 1 This technique has been shown to be as accurate as conventional cystography for identifying bladder injury. 5
Technical Specifications to Include in Your Order
When ordering CTU, specify:
- No oral or rectal contrast required 1
- Thin-slice acquisition (1-1.5 mm) with multiplanar reformations 1
- Hydration protocol: oral or IV saline, consider low-dose furosemide to improve urinary tract distension 1
- Timing: excretory phase at minimum 5 minutes post-contrast (inadequate delay results in suboptimal bladder opacification) 1
When to Consider Alternative Imaging
If IV contrast is contraindicated (chronic kidney disease, iodine allergy), order MR urography (MRU) as the alternative, which provides functional information without nephrotoxic contrast. 1 MRU combines heavily T2-weighted sequences with contrast-enhanced T1-weighted imaging when gadolinium can be used safely. 1
Ultrasound with color Doppler can evaluate for hydronephrosis, bladder distension, postvoid residual volume, and prostatomegaly, but provides less comprehensive evaluation than CTU. 4 Ultrasound has 93% sensitivity and 100% specificity for hydronephrosis but only 50% sensitivity for detecting hydroureter. 3
Critical Pitfalls to Avoid
- Do not order standard "CT abdomen/pelvis with contrast" when you specifically need bladder evaluation—this lacks the excretory phase necessary for proper urothelial visualization. 1
- Do not assume bilateral hydronephrosis can wait for evaluation even with normal creatinine—both kidneys are at simultaneous risk without contralateral functional reserve. 6
- Do not order contrast-enhanced CT as first-line for suspected stones—it may reduce sensitivity for small calculi compared to non-contrast technique. 2
- Recognize that perirenal fat stranding on CT may indicate bladder outlet obstruction and warrants further urodynamic evaluation, particularly in older male patients. 7