CT Imaging of Kidneys, Ureters, and Bladder: Comprehensive Overview
CT KUB (Non-Contrast)
CT KUB is the gold standard for detecting urinary stones, with thin-slice acquisition from T10 vertebral body to pubic symphysis to minimize radiation while capturing the entire urinary tract 1, 2.
Technique
- Scan range: Upper border of T10 to pubic symphysis 2
- Patient positioning: Supine (prone only if specifically requested) 3
- Dose parameters:
- Slice thickness: Thin-slice acquisition (typically 1-3mm)
Reconstructions
- Axial, coronal, and sagittal multiplanar reformations (MPR)
- Maximum intensity projections (MIP) for stone visualization 1
- 3D volume rendering when needed 1
HU Values & Windows
- Stones: >100-200 HU (calcium-based stones typically 400-1000+ HU)
- Soft tissue window: W:350-400, L:40-50
- Bone window: W:2000-3000, L:300-500 for stone detection
CT Urography (CTU)
CTU requires unenhanced images followed by IV contrast-enhanced nephrographic and excretory phases (≥5 minutes post-injection), with split-bolus protocols preferred when appropriate to reduce radiation exposure 5, 1, 3.
Standard Multi-Phase Protocol
Phase 1: Unenhanced
- Same as CT KUB above
- Detects stones and baseline attenuation
- CTDIvol 2-3 mGy 4
Phase 2: Nephrographic Phase (70-90 seconds post-injection)
- Contrast: 100-120 mL at 2.5-3 mL/s
- Timing: 70-90 seconds after injection
- Purpose: Optimal renal parenchymal enhancement, mass characterization
- Expected HU:
- Renal cortex: 120-180 HU
- Renal medulla: 80-120 HU
- Renal vessels: 250-350 HU
Phase 3: Excretory Phase (7-10 minutes post-injection)
- Timing: Fixed at 7 minutes post-injection (no need for test slices) 3
- Furosemide: 20 mg IV before contrast is mandatory to improve ureteral opacification 3
- Purpose: Visualize collecting system, ureters, bladder
- Expected HU: Opacified urine 100-300 HU
Split-Bolus Protocol (Radiation Reduction)
Split-bolus combines nephrographic and excretory phases into a single acquisition, reducing radiation dose from ~15.6 mGy to ~6.7 mGy 6.
Technique 3, 6
- Pre-hydration: 800 mL oral water 20 minutes before scan
- First bolus: 50 mL contrast at 2.5 mL/s
- Wait: 6 minutes (360 seconds)
- Second bolus: 70 mL contrast at 2.5 mL/s
- Scan: 60 seconds after second bolus (420 seconds total)
Advantages
- Single contrast-enhanced acquisition
- 57% radiation dose reduction 6
- Virtual unenhanced images can replace true unenhanced (87.5% sensitivity for stones) 6
Limitations
- Suboptimal for arterial phase evaluation
- May have incomplete bladder opacification (only 26.2% complete) 6
- Not ideal when vascular assessment critical
Triple-Bolus Protocol (For UPJO/Vascular Assessment)
For ureteropelvic junction obstruction requiring vascular evaluation, triple-bolus provides superior arteriovenous differentiation 7.
- Adds arterial phase for crossing vessel assessment
- Higher HU values in all phases
- Better arteriovenous differentiation (κ = 0.77) 7
Protocol Selection Algorithm
Use CT KUB (non-contrast only) when:
- Acute flank pain/suspected stone disease 1
- Follow-up of known stones
- No concern for mass or urothelial malignancy
Use Standard Multi-Phase CTU when:
- Gross hematuria with risk factors for malignancy 5, 4
- Staging urothelial carcinoma 5
- Characterizing renal masses
- Need for arterial phase imaging
Use Split-Bolus CTU when:
- Age <40 years or lower-risk patients 5
- Microscopic hematuria evaluation
- Radiation dose reduction priority 3
- No need for separate arterial assessment
Use Triple-Bolus CTU when:
- UPJO with suspected crossing vessels 7
- Vascular anatomy critical for surgical planning
Reconstructions for CTU
Essential Reconstructions 1, 4
- Axial images: 1-3mm slice thickness
- Coronal/sagittal MPR: For anatomic overview
- MIP: 5-10mm thick slabs for stone/collecting system visualization
- 3D volume rendering: For surgical planning, vascular anatomy
Advanced Techniques
- Dual-energy CT: Stone composition analysis, virtual unenhanced images 1, 6
- Curved planar reformations: Following ureter course
Window Settings Summary
| Structure | Window Width | Window Level |
|---|---|---|
| Soft tissue | 350-400 | 40-50 |
| Kidney parenchyma | 300-350 | 50-70 |
| Bone/Stone | 2000-3000 | 300-500 |
| Lung | 1500 | -600 |
Key HU Values Reference
| Structure/Phase | Expected HU |
|---|---|
| Unenhanced kidney | 30-50 |
| Nephrographic cortex | 120-180 |
| Nephrographic medulla | 80-120 |
| Renal artery (arterial) | 300-400 |
| Renal vein (venous) | 100-150 |
| Opacified urine | 100-300 |
| Calcium stones | 400-1000+ |
| Uric acid stones | 200-500 |
Critical Technical Points
Common pitfalls to avoid:
- Overscanning beyond T10 superiorly (occurs in 69% of cases) 2
- Performing test slices for excretory timing (unnecessary—use fixed 7-minute delay) 3
- Omitting furosemide (significantly impairs ureteral opacification) 3
- Using split-bolus when arterial phase needed for vascular assessment 7
Dose optimization: