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CT KUB - Complete Urology Residency Teaching Guide

FROM BASICS TO EXAMINER-LEVEL MASTERY

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PART 1: WHAT IS CT KUB AND WHY IT REPLACED EVERYTHING

"CT has a reported sensitivity of 96% for stone detection with a specificity and positive predictive value of 100%."

• Campbell-Walsh Wein Urology

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PART 2: TECHNICAL PARAMETERS (Examiner Favourite)

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PART 3: ANATOMY ON CT KUB - LABEL EVERYTHING

3A - The Kidney

• Cortex: 30-60 HU (soft tissue density)
• Medulla: Slightly lower than cortex
• Renal sinus fat: Negative HU (black on CT)
• Renal pelvis: Fluid density, 0-10 HU
• Size: 9-12 cm length, 3 kidneys per vertebral body height

1. Size - enlarged = obstruction, infection, infiltration
2. Shape - smooth vs. lobulated vs. scarred
3. Cortex thickness - thinning = chronic obstruction or CKD
4. Collecting system - dilated? (hydronephrosis)
5. Perinephric fat - clean/stranding/collection?
6. Any calcification inside?

3B - The Three Physiological Narrowings of the Ureter

KIDNEY → URETER STARTS AT UPJ
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    1. UPJ (Ureteropelvic Junction)
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    Abdominal ureter (runs on psoas)
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    2. Iliac Crossing (pelvic brim where ureter crosses iliac vessels)
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    Pelvic ureter
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    3. UVJ (Ureterovesical Junction) ← most common site of impaction
         ↓
         BLADDER

• UPJ: Transition from wide renal pelvis to narrow ureter
• Iliac crossing: Ureter angulates over common iliac vessels
• UVJ: Narrowest point of entire urinary tract (2-3 mm lumen)

3C - Ureteral Course on CT

• Abdominal ureter: Runs retroperitoneally, anterior to transverse processes of vertebral bodies, medial to psoas muscle
• At the pelvic brim: Crosses anterior to the common iliac artery bifurcation
• Pelvic ureter: Courses laterally and posteriorly
• UVJ: Enters the posterior-inferior bladder wall obliquely (this oblique tunnel is the anti-reflux mechanism)

3D - The Bladder

• Smooth-walled, thin wall (<3 mm when distended, <5 mm when collapsed)
• Urine inside = black (0 HU fluid)
• Check for: intraluminal stones, wall thickening, masses, diverticula

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PART 4: HOUNSFIELD UNITS - THE LANGUAGE OF CT (Critical for Exam)

• HU > 900: Stone is hard - poor SWL response, prefer URS or PCNL
• HU 200-400: Likely uric acid - try oral alkalinization first (potassium citrate to urine pH 6.5-7.0) before intervention
• Dual-energy CT can specifically distinguish uric acid from calcium stones based on material decomposition

"Certain predictions regarding stone composition can be made based on CT scan data, with increasing resistance to fragmentation associated with higher Hounsfield unit (HU) measurements."

• Campbell-Walsh Wein Urology

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PART 5: PRIMARY STONE SIGNS ON CT KUB

Sign 1: The Stone Itself

• High-density focus (>200 HU) in the expected ureteral course
• Measure in 2 planes (axial + coronal) - report the LARGEST dimension
• Specify location: upper ureter / mid ureter / lower ureter / UVJ

Sign 2: The Tissue Rim Sign

• A thin ring of soft tissue (ureteral wall edema) around the stone
• Distinguishes ureteral stone from a phlebolith (pelvic vein calcification)
• Phleboliths do NOT have a tissue rim
• Positive predictive value >90% for acute ureteral obstruction
• Also called the "halo sign"

Sign 3: Comet Tail Sign (for phleboliths)

• Soft tissue "tail" extending from a calcified pelvic focus
• Represents the pelvic vein itself
• Presence = it IS a phlebolith, NOT a stone

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PART 6: SECONDARY SIGNS OF OBSTRUCTION

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PART 7: THE PHLEBOLITH PROBLEM - Most Common Interpretive Pitfall

1. Look for tissue rim sign around the calcification
2. Look for comet tail sign (phlebolith)
3. Look for secondary signs - hydronephrosis, ipsilateral perinephric stranding
4. Trace the ureter on serial axial slices - is the calcification IN the ureter?
5. Check coronal reconstruction - very helpful for UVJ stones
6. HU value (phleboliths tend to be lower, 150-300 HU)

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PART 8: STONE COMPOSITION - TYPES AND CT CHARACTERISTICS

• Calcium oxalate (alone or with calcium phosphate): 75-80% - most common
• Struvite (magnesium ammonium phosphate): 10-20% - infection stones, women with recurrent UTI
• Uric acid: 5% - pure uric acid, radiolucent on plain film
• Cystine: <1-3% - genetic (autosomal recessive cystinuria)
• Calcium phosphate: 5%

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PART 9: STAGHORN CALCULI

• Large branching calcification filling the renal collecting system
• May show cortical thinning if chronic (renal damage)
• Look for signs of underlying obstruction or abscess (xanthogranulomatous pyelonephritis - XGP)

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PART 10: STONE SIZE AND MANAGEMENT DECISION (Examiner Must-Know)

• Ureteral stones: URS (ureteroscopy) now preferred for most sizes. SWL for proximal ureter <10mm.
• Renal stones <10 mm: SWL or flexible URS (equivalent, both first-line per EAU/AUA)
• Renal stones 10-20 mm: SWL or URS. PCNL if in lower pole or if SWL/URS fails.
• Renal stones >20 mm: PCNL first-line
• High HU (>900): Avoid SWL - poor fragmentation. Prefer URS/PCNL.

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PART 11: NON-STONE DIAGNOSES ON CT KUB

Renal Masses

• Renal cell carcinoma: Solid or complex cystic mass, evaluate using Bosniak classification on contrast CT
• Angiomyolipoma (AML): Fat-containing mass - fat density (-40 to -120 HU) within a renal lesion is pathognomonic
• Oncocytoma: Solid enhancing mass, may have central scar (needs contrast CT to differentiate)

Renal Infection

• Acute pyelonephritis: Wedge-shaped areas of reduced parenchymal attenuation (best on contrast CT - "striated nephrogram")
• Renal abscess: Fluid collection with thick enhancing wall
• Xanthogranulomatous pyelonephritis (XGP): Large non-functioning kidney, staghorn stone, fat-containing inflammatory masses replacing renal parenchyma - "Bear's paw sign" on CT

Vascular

• Renal infarction: Wedge-shaped cortical defect with absent perfusion. Subtle hypoattenuation on non-contrast CT. Can mimic renal colic.
• Abdominal aortic aneurysm (AAA): Life-threatening mimic of renal colic. ALWAYS scroll to look at the aorta on CT KUB.

Other

• Appendicitis: Look in the right iliac fossa on every scan
• Ovarian pathology: Ovarian cyst, torsion, ectopic pregnancy (right iliac fossa pain in women)
• Adrenal incidentaloma: Check adrenal glands routinely

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PART 12: CT KUB REPORTING - WHAT TO SAY (Systematic Approach)

• Size, shape, cortical thickness
• Any calculi? Size, HU, location in collecting system
• Hydronephrosis? (grade it: mild/moderate/severe)
• Perinephric stranding/collection?
• Parenchymal abnormalities?

• Trace entire course
• Any calculi? Measure size in 2 planes, record HU, state location relative to 3 narrowings
• Tissue rim sign present?
• Hydroureter?

• Wall thickness
• Any intraluminal calculi or masses?
• Post-void residual (if available)

• Aorta, liver, spleen, adrenals, bowel, lymph nodes, spine

"There is a [size] mm calcification (HU = X) in the [right/left] [location - proximal/mid/distal] ureter at the [UPJ / crossing iliac vessels / UVJ]. A tissue rim sign is [present/absent]. There is associated [mild/moderate/severe] ipsilateral hydronephrosis and hydroureter to the level of the calculus. Perinephric fat stranding is [present/absent]. No forniceal rupture. The contralateral kidney is normal. The bladder is unremarkable."

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PART 13: DUAL-ENERGY CT (Advanced Concept)

• Can specifically identify uric acid stones (low atomic number materials are differentiated)
• Can distinguish between calcium and non-calcium stones
• Colour maps: uric acid stones = red, calcium stones = blue (on most platforms)
• This matters because uric acid stones are the ONLY type amenable to medical dissolution therapy (oral alkalinization)

"Single-source dual-energy CT has been shown to accurately predict stone composition in stone formers with uric acid stones, cystine stones, and calcium oxalate stones."

• Campbell-Walsh Wein Urology

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PART 14: SPECIAL SCENARIOS IN EXAM

Pregnancy

• CT is avoided (radiation)
• First-line: Ultrasound (with Doppler)
• If inconclusive: MRI (no radiation)
• Plain KUB only if absolutely necessary with fetal shielding

Paediatric patients

• Children are more radiosensitive - cumulative dose of 50 mGy to brain increases brain cancer risk 2.8x
• Use low-dose CT or preferably ultrasound first
• MRI if needed

Radiolucent stone on plain KUB but visible on CT

• Think: uric acid stone
• Check serum uric acid, urine pH (typically acidic < 5.5 in uric acid stone formers)
• Treatment: Alkalinize urine to pH 6.5-7.0 with potassium citrate - stone may dissolve

Stone INVISIBLE on CT (0 HU)

• Think: drug stone (indinavir, other protease inhibitors) or matrix stone
• Indirect signs only - hydronephrosis, hydroureter
• Diagnosis confirmed with retrograde pyelogram or ureteroscopy

Unilateral non-obstructing stone vs. incidental finding

• Stones <3 mm in kidney with no hydronephrosis and no symptoms = likely incidental
• Still document size and HU for follow-up comparison
• Risk of passage/symptoms if stone moves to ureter

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PART 15: COMMON EXAMINER QUESTIONS WITH MODEL ANSWERS

6 mm: low passage rate, consider ureteroscopy (URS) especially for symptomatic patients or those with solitary kidney, bilateral obstruction, renal impairment, or infection. 10 mm renal: SWL or URS; > 20 mm: PCNL.

1. Non-contrast phase - detects calcifications, hemorrhage
2. Nephrographic phase (corticomedullary, 100-120 sec after contrast) - evaluates parenchyma, vascular supply, renal masses
3. Excretory phase (delayed, 5-15 min) - evaluates the opacified collecting system, ureters, urothelium - used for TCC/urothelial tumor detection

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QUICK REFERENCE SUMMARY CARD

CT KUB - THE ESSENTIALS
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MODALITY: Non-contrast helical CT
COVERAGE: Top of kidneys → Bladder base
SENSITIVITY: 97-98% | SPECIFICITY: 97-100%
DOSE: ~8.5-10 mSv (low-dose: <3-4 mSv)

3 SITES OF IMPACTION (proximal→distal):
  1. UPJ (Ureteropelvic Junction)
  2. Iliac Crossing (pelvic brim)
  3. UVJ (Ureterovesical Junction) ← MOST COMMON

STONE HU VALUES:
  Ca oxalate/phosphate : >400 (most common 75-80%)
  Struvite             : 400-800 (10-20%)
  Uric acid            : 200-400 (5%) ← RADIOLUCENT on KUB
  Cystine              : 200-400 (<3%)
  Indinavir/Matrix     : ~0 → INVISIBLE on CT

PRIMARY SIGN: Hyperdense focus in ureteral course
  + Tissue rim sign = confirms ureteral stone

SECONDARY SIGNS (confirms obstruction):
  • Hydronephrosis
  • Hydroureter
  • Perinephric stranding
  • Forniceal rupture (20-45 HU peri-renal fluid)

STONE vs PHLEBOLITH:
  Stone      → TISSUE RIM SIGN present
  Phlebolith → COMET TAIL SIGN present, no hydronephrosis

TREATMENT BY SIZE:
  <4mm:    MET + observe (80% pass spontaneously)
  4-6mm:   MET (60% pass)
  6-10mm:  Consider URS
  >10mm:   URS/SWL (location-dependent)
  >20mm:   PCNL

HIGH HU (>900) → Hard stone → SWL fails → Use URS/PCNL
LOW HU (200-400) + Radiolucent on KUB → Uric acid → Alkalinize

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• Campbell-Walsh Wein Urology, Chapters on Imaging and Urolithiasis (primary reference)
• Comprehensive Clinical Nephrology 7e, Chapter on CT Density (Table 6.4)
• EAU Guidelines on Urolithiasis 2023
• NICE NG118 - Renal and ureteric stones 2024