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CT Urography (CTU)

Definition

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Indications

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Technique / Protocol

Patient Preparation

• Oral hydration: 2-3 glasses of water before the scan (aids ureteric distension)
• IV hydration optional
• IV furosemide (10-20 mg) at time of contrast injection - promotes diuresis and ureteric opacification
• Empty bladder prior to scan

Equipment

• Multidetector CT (MDCT) with thin slices (2.5-5 mm)
• Reconstructions: axial, coronal, sagittal; MIP reformats for the collecting system

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Phases of Acquisition (Triphasic Protocol)

Phase 1: Non-contrast Phase

• Coverage: Top of kidneys to bladder base
• Purpose:
  • Detect calculi (high attenuation without contrast)
  • Identify fat-containing lesions (angiomyolipoma)
  • Detect parenchymal calcifications
  • Provide baseline attenuation for enhancement assessment
• Slice thickness: 2-2.5 mm

Phase 2: Nephrographic Phase

• Timing: 90-100 seconds after IV contrast injection (100-150 mL of non-ionic iodinated contrast at 2-4 mL/sec)
• Coverage: Abdomen and pelvis (kidneys to bladder)
• Purpose:
  • Homogeneous cortex + medulla enhancement - best phase for detecting small renal masses
  • Enhancement confirmed by comparison with non-contrast images (>20 HU increase = enhancement)
  • Evaluation of renal vasculature
• Key feature: Most sensitive phase for renal cell carcinoma detection

Phase 3: Excretory (Pyelographic) Phase

• Timing: 12-15 minutes after contrast injection
• Coverage: Full abdomen and pelvis
• Purpose:
  • Opacification and distension of renal calyces, pelvis, ureters, and bladder
  • Best phase for urothelial tumors (TCC/UTUC)
  • Evaluation of collecting system anatomy and filling defects
• Limitation: Ureteric peristalsis may cause non-opacification of segments

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Split-Bolus Technique (Dose Reduction Alternative)

• Contrast given in two boluses before a single post-contrast acquisition
• 1st bolus (smaller, e.g., 50 mL) → 7-9 minute delay → 2nd bolus (larger, e.g., 75-100 mL) → 120 seconds → single scan
• Cumulative effect: early bolus provides excretory information; later bolus gives nephrographic information
• Advantage: Eliminates one acquisition - reduces radiation dose significantly
• Disadvantage: Less reliable ureteric opacification compared to triphasic

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Techniques to Improve Ureteric Distension

1. IV furosemide - most effective; promotes diuresis
2. IV/oral hydration - increases urine flow
3. Prone positioning - uses gravity to fill dependent ureters with contrast
4. Compression belt - mechanical ureteric compression at the pelvic brim
5. Log-rolling - patient rolled prior to excretory acquisition to redistribute contrast

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Radiation Dose

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Findings / Diagnostic Value

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CT Urography vs. IVU

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Summary

• Calculi (non-contrast phase)
• Renal masses (nephrographic phase)
• Urothelial tumors and collecting system anatomy (excretory phase)

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Complete Phases of CT Urography (Four-Phase Protocol)

Phase 1: Non-contrast Phase

• Timing: Before contrast
• Purpose: Calculi, fat lesions, baseline attenuation

Phase 2: Corticomedullary Phase (CMP)

• Timing: 25-40 seconds after contrast injection (arterial/early venous)
• Purpose:
  • Renal cortex enhances brightly; medulla has not yet enhanced (clear corticomedullary differentiation)
  • Renal arteries and veins are well opacified - ideal for vascular anatomy, renal artery stenosis, arteriovenous malformations
  • Detection of hypervascular tumors (some RCCs, angiomyolipomas)
  • Pre-surgical planning (donor kidneys, partial nephrectomy)
• Limitation: Small renal masses can be missed because the medulla is relatively unenhanced, creating a heterogeneous background that obscures isodense lesions

Phase 3: Nephrographic Phase

• Timing: 90-100 seconds
• Cortex + medulla enhance uniformly - best for renal mass detection

Phase 4: Excretory Phase

• Timing: 12-15 minutes
• Collecting system, ureters, bladder opacified - best for urothelial lesions

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Practical Note

• Vascular assessment is needed (renal donor evaluation, suspected vascular lesion)
• Characterization of a known hypervascular mass
• Pre-operative planning

• Triphasic: Non-contrast + Nephrographic + Excretory (most common)
• Four-phase: Non-contrast + Corticomedullary + Nephrographic + Excretory (higher dose, ~25-35 mSv, used when vascular detail is required)

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Retroperitoneal Fibrosis - Radiology

Definition

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Imaging Modalities

1. Plain X-Ray / IVU (Intravenous Urography)

• Bilateral hydronephrosis and hydroureter (bilateral in 68%, unilateral in 20%)
• Medial deviation of the mid-ureters - the most characteristic sign
  • Normally ureters course laterally; in RPF they are pulled medially toward the midline
• "Maiden waist sign" - smooth, gradual tapering of the ureter at the level of obstruction (L4-L5)
• "Pipestem ureter" - long segment of ureter is narrowed and aperistaltic due to fibrotic encasement
• Site of obstruction: usually at the level of L4-L5, where the plaque is thickest
• Proximal dilation with distal narrowing - gives the classic IVU appearance

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2. Ultrasound

• Bilateral hydronephrosis - initial finding, prompts further workup
• May show a hypoechoic periaortic soft tissue mass
• Useful for monitoring treatment response - serial assessment of hydronephrosis
• Cannot characterize the fibrotic plaque well; does not differentiate benign from malignant
• Doppler: Can detect ureteric obstruction and assess renal resistive indices

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3. CT Scan (Modality of Choice for Diagnosis)

• Active/inflammatory RPF: enhances on arterial and venous phase
• Inactive/fibrotic RPF: little to no enhancement
• Decreasing enhancement on serial scans = treatment response

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4. MRI (Superior for Follow-Up and Tissue Characterization)

• Neither CT nor MRI reliably differentiates benign from malignant RPF
• Features suggesting malignant RPF:
  • Anterior elevation of the aorta from the spine (suggests lymphoma pushing from behind)
  • Lateral displacement of ureters (rather than medial)
  • Localized bone destruction
  • Infiltration of surrounding structures
  • Irregular, lobulated margins
• Features suggesting benign RPF:
  • Aorta hugging the spine (not elevated)
  • Medial deviation of ureters
  • Smooth, confluent plaque

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5. Retrograde / Antegrade Pyelography

• Performed when CT/MRI is contraindicated (renal impairment, contrast allergy) or when endoscopic intervention is planned
• Shows:
  • Long segment of ureteric narrowing at L4-L5
  • Smooth tapering (not abrupt cutoff - helps distinguish from malignant stricture)
  • Medial deviation of mid-ureter
  • Proximal hydronephrosis

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6. Nuclear Medicine / PET-CT

• FDG-PET/CT: Most sensitive imaging study for disease activity
  • Active RPF shows increased FDG uptake in the fibrotic plaque
  • Can differentiate active from inactive disease better than CT/MRI alone
  • Distinguishes RPF from malignant mimics (metastatic lymphoma, etc.)
  • May obviate the need for biopsy in typical cases
  • Useful for monitoring treatment response
• MAG3/DTPA renography (differential scintigraphy): Assesses individual renal function - important for surgical planning (ureterolysis)

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Differential Diagnosis on Imaging

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Summary of Modality Use

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Congenital Anomalies of the Kidney - Classification and Imaging Features of Fusion Anomalies

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PART A: CLASSIFICATION OF CONGENITAL ANOMALIES OF THE KIDNEY

I. Anomalies of Number

II. Anomalies of Volume / Size

• Renal hypoplasia - small kidney, reduced nephrons, normal architecture
• Renal dysplasia - abnormal differentiation; cystic dysplastic kidney

III. Anomalies of Position (Ectopia)

IV. Anomalies of Rotation (Malrotation)

• Normal: renal pelvis rotates 90° ventromedially during ascent
• Malrotation types (Weyrauch classification): Ventral, Ventromedial, Lateral, Dorsal positions
• Pelvis may face ventrally (non-rotation - most common), laterally, or dorsally

V. Anomalies of Fusion (Main focus - see Part B)

VI. Anomalies of the Collecting System

• Duplex collecting system (most common renal anomaly - 2% of population)
• Calyceal diverticulum
• Congenital megacalyces
• Pelvi-ureteric junction (PUJ) obstruction
• Duplex ureter / ectopic ureter / ureterocele

VII. Cystic Anomalies (separate classification)

• Autosomal dominant/recessive polycystic kidney disease
• Multicystic dysplastic kidney (MCDK)
• Medullary sponge kidney
• Simple renal cysts

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PART B: FUSION ANOMALIES OF THE KIDNEY - IMAGING FEATURES

Embryological Basis

• More common in males (2:1)
• Partial fusion - Horseshoe kidney (90% of all fusion anomalies), Crossed fused ectopia
• Complete fusion - Cake/Disc/Pancake kidney (rare, 2% of fused kidneys)

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1. HORSESHOE KIDNEY

Incidence

• Most common renal fusion anomaly - 1 in 400-500 individuals (Campbell-Walsh)
• Male:Female = 2:1
• Frequently associated with other congenital anomalies - Turner syndrome, trisomy 18, VACTERL

Anatomy

• Two distinct renal masses, one on each side of midline, connected at their lower poles (95%) by an isthmus
• Upper pole fusion: rare (5%)
• Isthmus lies at L3-L4 level, anterior to the aorta and IVC, just below the origin of the inferior mesenteric artery (IMA) - which prevents full cranial ascent
• Isthmus composition: functional renal parenchyma (85-95%) or fibrous tissue (5-15%)
• Renal pelvis: anteriorly placed (faces ventrally due to malrotation)
• Calyces: point posteriorly (fail to rotate); lowermost calyces may overlie the vertebral column

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Imaging Features

A. Plain X-Ray / IVU

• Vertical or obliquely lateral axis of each renal moiety (vs. normal oblique medially inclined axis)
• Lower pole medial orientation of collecting systems - medial tilt of lower poles toward midline
• Lowered position of kidneys (below L3)
• IVU shows "inverted lily" or "flower vase" appearance - collecting systems open anteriorly/medially
• Ureters course anteriorly over the isthmus - characteristic ureteric course with a high lateral insertion into renal pelvis
• Isthmus may be visible as a soft tissue density overlying the lumbar spine
• Complications: hydronephrosis (UPJ obstruction in up to 1/3 of patients), calculi, recurrent UTI

B. Ultrasound

• Two renal masses with medially oriented lower poles
• Isthmus visible anterior to aorta and IVC - hypo/isoechoic band of tissue crossing the midline
• Renal pelvis faces anteriorly (not medially as in normal kidney)
• Doppler: variable vascular supply - multiple aberrant vessels from aorta, IMA, iliac arteries

C. CT Scan (Investigation of Choice)

D. MRI

• MRI urography (MRU) gives equivalent anatomical information to CT
• Isthmus appears as a band of normal renal signal intensity tissue
• T2-weighted coronal images: best for showing collecting system orientation and ureteral course
• Preferred in children and pregnant women (no radiation)
• Gadolinium-enhanced: shows parenchymal enhancement of the isthmus, confirming functional tissue

E. Nuclear Medicine (MAG3/DTPA renogram)

• Assesses differential renal function of each moiety
• Evaluates drainage - identifies UPJ obstruction
• Post-operative follow-up after ureterolysis or pyeloplasty

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Complications of Horseshoe Kidney (Examinable)

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2. CROSSED FUSED RENAL ECTOPIA (CFRE)

Incidence

• Second most common fusion anomaly - 1 in 1300-7500
• Male predominance

Anatomy

• One kidney crosses the midline to the opposite side, and its parenchyma fuses with the other kidney
• The ureter of the crossed kidney crosses back to drain into the bladder on its own (original) side - key distinguishing feature
• Left kidney is most commonly the ectopic one (crosses to the right)
• Most common fusion pattern: upper pole of the inferior/crossed ectopic kidney fuses with the lower pole of the normally positioned kidney - produces an L-shape

McDonald and McClellan Classification of Crossed Ectopia:

Six Variants of Crossed Fused Ectopia (in decreasing order of frequency):

1. Type 1 - Inferior crossed fused ectopia (most common) - crossed kidney lies inferior, fused at its upper pole to lower pole of native kidney; L-shaped configuration
2. Type 2 - Sigmoid / S-shaped kidney - both kidneys on the same side, fused back-to-back with opposite pole orientation
3. Type 3 - Unilateral lump kidney - both fused with multiple contact areas; globular mass
4. Type 4 - Unilateral disc/shield kidney - flat, disc-shaped fusion
5. Type 5 - L-shaped kidney
6. Type 6 - Superior crossed fused ectopia (rarest) - crossed kidney lies superior

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Imaging Features of CFRE

A. Ultrasound

• Absence of kidney in the contralateral renal fossa (empty fossa on one side)
• Large unilateral mass of renal tissue in the other fossa
• Two separate renal sinuses may be identified within the single mass
• Doppler: separate blood supplies to each moiety
• May mimic a renal tumor or retroperitoneal mass

B. IVU / Excretory Urography

• Shows two collecting systems on one side of the abdomen
• One ureter crosses the midline back to drain on its original side
• Characteristic non-opacification of contralateral renal fossa

C. CT Scan (Best modality)

• Parenchymal mass on one side containing two fused renal units
• Two distinct collecting systems visible within the fused mass
• Each renal moiety enhances normally
• One ureter crosses the midline to drain into the ipsilateral bladder
• CT angiography delineates the dual separate blood supplies
• Coronal and axial views: identify the fusion plane and configuration type

D. Nuclear Medicine (DMSA / MAG3)

• DMSA scan: shows fused uptake of both functioning renal units on one side with absent contralateral uptake - pathognomonic appearance
• MAG3 renogram: assesses drainage and identifies PUJ obstruction (common due to malrotation)
• Widely used in children as first-line functional imaging

E. MRI / MRU

• T2 coronal: clearly shows fused renal mass, collecting system anatomy
• MRU: traces both ureters - shows the crossed ureter returning to its side
• No radiation - preferred in children

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3. COMPLETE FUSION ANOMALIES (Cake / Disc / Pancake Kidney)

Cake / Lump Kidney

• Complete fusion of both kidneys into a single midline mass (usually pelvic)
• 2% of all renal fusion anomalies
• No recognizable reniform shape; multiple collecting systems
• Blood supply: highly aberrant - multiple vessels from distal aorta and iliac vessels
• Imaging: pelvic soft tissue mass with renal parenchymal enhancement; bilateral ureteral drainage from single mass
• MRI best for characterization; CT angio for pre-surgical vascular mapping

Disc / Shield / Pancake Kidney

• Flat, disc-shaped bilateral fusion; both kidneys fused medially
• Located in the pelvis
• Multiple collecting systems and ureters drain normally

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SUMMARY TABLE: Fusion Anomalies at a Glance

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HIGH-YIELD EXAM POINTS (RGUHS)

1. Horseshoe kidney = most common fusion anomaly (1:400); lower pole fusion 95%; isthmus lies anterior to aorta BELOW the IMA (IMA prevents ascent)
2. IVU sign: vertical axis of renal pelves + medial tilt of lower poles = "inverted lily" appearance
3. UPJ obstruction in 33% of horseshoe kidneys - due to aberrant vessels or abnormal ureteral course over isthmus
4. Wilms tumor risk is doubled in horseshoe kidney
5. Crossed fused ectopia: left kidney crosses to right (most common); ureter crosses BACK to drain on correct side
6. DMSA scan = investigation of choice in children for CFRE - shows unilateral uptake of fused mass
7. Cake kidney = complete fusion; pelvic; 2% of fusion anomalies
8. MRI/MRU = preferred in children and pregnant women (no radiation)
9. CT angiography = mandatory before any surgery on horseshoe kidney (aberrant vasculature)
10. McDonald and McClellan classification of crossed ectopia - Type 1 (inferior, L-shaped) is most common (85%)

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Calcifications on KUB Skiagram and Various Modalities of Evaluation

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KUB Skiagram - Introduction

• Sensitivity for urinary calculi: ~45-80% (variable, depending on stone composition)
• Specificity: ~77%
• Limitation: Cannot detect radiolucent stones, cannot assess obstruction, many non-urinary calcifications mimic urinary stones

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CLASSIFICATION OF CALCIFICATIONS SEEN ON KUB

A. Normal Structures That Calcify (Physiological/Incidental)

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B. Pathological Calcifications on KUB

I. RENAL CALCIFICATIONS

1. Nephrolithiasis (Renal Calculi / Urinary Stones)

• Staghorn calculus - fills entire pelvicalyceal system; branching configuration; struvite (infection stones)
• Jackstone calculus - spiculated, calcium oxalate monohydrate
• Laminated - alternating rings; calcium phosphate / mixed
• Smooth rounded - uric acid (but radiolucent)

• Renal pelvis - overlying T12-L2 region
• PUJ - at L2-L3 level, just medial to lateral border of psoas
• Pelvic brim - at sacroiliac joint level
• VUJ (Vesico-ureteric junction) - at level of ischial spine; most common site of impaction
• Ureters run along the line of transverse processes of lumbar vertebrae → key anatomical landmark to identify ureteric stones

2. Nephrocalcinosis (Renal Parenchymal Calcification)

• Calcification in the peripheral cortex and septa of Bertin
• KUB appearance: Thin peripheral lines ("tramlines"), diffusely dense renal shadow, or punctate calcifications
• Causes:
  • Chronic cortical necrosis (bilateral, dense; "tram-track sign")
  • Chronic glomerulonephritis
  • Rejected transplant kidney
  • Oxalosis (primary hyperoxaluria)

• Calcification in the medullary pyramids
• KUB appearance: Dense bilateral aggregates in the papillary/medullary regions; triangular clusters
• Causes (mnemonic: "HARD RTA"):
  • Hyperparathyroidism (most common cause)
  • Renal tubular acidosis (Type 1, distal)
  • Medullary sponge kidney
  • Hypervitaminosis D
  • Sarcoidosis
  • Milk-alkali syndrome

3. Dystrophic Calcification in Renal Lesions

• Renal cell carcinoma - irregular, "eggshell" or central calcification; ~10% of RCCs
• Renal cyst - thin peripheral "eggshell" calcification of cyst wall (Bosnian category)
• Wilms tumor - stippled or faint calcification in children
• Angiomyolipoma - calcification rare
• Renal TB - "putty kidney" (dystrophic calcification of entire kidney); lobar or diffuse dense calcification; associated ureteric calcification ("pipestem ureter")
• Renal abscess / Xanthogranulomatous pyelonephritis - irregular calcification within mass

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II. URETERIC CALCIFICATIONS

• Calcified stones along the ureteric course
• Anatomical landmarks for tracing ureters on KUB:
  • Along the tips of transverse processes L1-L5
  • Across the sacroiliac joints at the pelvic brim
  • Down to the ischial spine level (VUJ)

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III. BLADDER CALCIFICATIONS

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IV. EXTRAURINARY CALCIFICATIONS VISIBLE ON KUB (Differential Diagnosis)

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IMAGING MODALITIES FOR EVALUATING KUB CALCIFICATIONS

1. Plain Radiograph (KUB Skiagram)

• First-line, widely available, low cost, low radiation
• Detects ~80% of urinary calculi (all radiopaque stones)
• Misses: uric acid stones, small stones, stones overlying bone/bowel gas
• Uses:
  • Initial assessment of urinary calculi
  • Follow-up of known stone size/burden
  • Post-ESWL or surgical follow-up (stone clearance)
• Limitation: Cannot assess function or obstruction; cannot differentiate ureteric stone from phlebolith with certainty

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2. Intravenous Urography / IVP (IVU)

• Historically the "gold standard" for urinary tract evaluation; now largely replaced by CT
• NECT KUB (control film) → IV contrast → serial films at 5, 15, 30 min
• Findings in urolithiasis:
  • "Standing column" sign - column of contrast in ureter proximal to obstruction (dilated, non-peristaltic ureter)
  • Delayed nephrogram and dense persistent nephrogram on affected side
  • Crescent sign - contrast in compressed calyces around obstructing stone
  • Identifies site and degree of obstruction
  • Differentiates ureteric stone from phlebolith (stone seen within opacified ureter)
• Advantage over plain KUB: Confirms whether opacity is within the urinary tract
• Contraindications: Contrast allergy, renal failure, metformin use (relative), pregnancy

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3. Ultrasound (USG)

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4. Non-Contrast CT (NCCT / CT KUB) - Gold Standard

• Detects ALL stones regardless of composition (including uric acid, indinavir)
• Soft tissue rim sign - halo of edematous ureteric wall around stone = confirms ureteric stone (vs. phlebolith)
• Comet tail sign - distinguishes phleboliths
• Identifies complications: hydronephrosis, hydroureter, perinephric fat stranding, urinoma, forniceal rupture
• Detects non-urological causes of pain (appendicitis, aortic aneurysm)
• HU (Hounsfield unit) attenuation predicts stone composition:
  • Uric acid: ~200-400 HU
  • Calcium oxalate: ~1000-1700 HU
  • Struvite: ~600-900 HU
  • Cystine: ~600-1100 HU

• Central lucency of phlebolith seen better on KUB than CT
• Stone size measurement slightly larger on KUB than CT (CT underestimates by ~12%)

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5. CT Urography (CTU)

• Triphasic: NECT + Nephrographic (90s) + Excretory (12-15 min)
• Combines stone detection, mass characterization, and urothelial evaluation
• For complex cases: hematuria + suspected stones + suspected tumor

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6. MRI / MR Urography (MRU)

• No ionizing radiation - preferred in pregnancy and children
• Does NOT visualize calcifications directly (calcium = signal void on MRI)
• MRU (heavily T2-weighted or Gd-enhanced): evaluates obstruction, collecting system anatomy
• Can identify the obstruction site but cannot characterize stone composition
• Role: Pregnant patients with suspected ureteric colic; children with congenital anomalies

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7. Radionuclide Imaging (Nuclear Medicine)

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8. Retrograde Pyelography

• Invasive; endoscopic opacification of ureter/pelvis from below
• Used when: CT/IVU contraindicated, pre-stenting planning, precise anatomical mapping
• Shows site of ureteric obstruction precisely

9. Antegrade Pyelography / Nephrostomy

• Percutaneous access via dilated pelvicalyceal system
• Used for decompression + diagnostic imaging when retrograde not feasible

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SUMMARY: MODALITY COMPARISON FOR KUB CALCIFICATIONS

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HIGH-YIELD EXAM POINTS

1. ~85% of urinary calculi are radiopaque on KUB; uric acid and indinavir stones are radiolucent
2. Staghorn calculus = struvite (infection stone); fills pelvicalyceal system; most opaque
3. Phlebolith vs ureteric stone: central lucency = phlebolith; soft tissue rim sign on CT = ureteric stone
4. Medullary nephrocalcinosis: causes = hyperparathyroidism (most common), RTA type 1, medullary sponge kidney
5. Cortical nephrocalcinosis: tramlines on KUB; causes = cortical necrosis, oxalosis
6. "Putty kidney" = TB; complete dystrophic calcification of kidney
7. Schistosomiasis = eggshell bladder wall calcification on KUB; S. haematobium
8. NCCT = gold standard for urinary calculi (sensitivity 96-99%); detects ALL stone types
9. MRU = choice in pregnancy (no radiation); does NOT directly show stones
10. KUB + USG = low-dose protocol for initial evaluation in select patients; NCCT for definitive diagnosis