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Imaging of Congenital Renal Abnormalities

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1. Embryology

• Pronephros (weeks 3-4): rudimentary, non-functional, regresses completely
• Mesonephros (weeks 4-8): temporarily functional, gives rise to the Wolffian (mesonephric) duct
• Metanephros (week 5 onward): the permanent kidney - arises from two elements:
  • Ureteric bud: an outgrowth of the Wolffian duct that induces the metanephric blastema and eventually forms the ureter, renal pelvis, calyces, and collecting ducts
  • Metanephric blastema (mesenchyme): forms nephrons, glomeruli, and tubules

• The ureteric bud must contact the metanephric mesenchyme at precisely the right time and location - early or late budding leads to renal malformations, dysplasia, hypoplasia, or agenesis
• From weeks 6-9, the kidneys ascend from the pelvis to the retroperitoneum (L1-L2), rotating 90° medially so the hilum faces anteromedially
• Arterial supply shifts progressively cranially from lateral sacral branches to final supply from the aorta at L2
• Failure of ascent or abnormal budding underlies most anomalies

• GDNF/RET signaling: GDNF from metanephric mesenchyme activates RET receptor on ureteric bud, driving branching morphogenesis
• EYA1/SIX1: expressed in metanephric mesenchyme; mutations cause Branchio-Oto-Renal (BOR) syndrome
• Sonic Hedgehog (SHH)/GLI3: Hedgehog loss-of-function produces renal agenesis/hypodysplasia (seen in Smith-Lemli-Opitz syndrome, Pallister-Hall syndrome)

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2. Classification of Congenital Renal Anomalies

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3. Imaging Protocols - CT and MRI

Ultrasound (first-line)

• Antenatal US: most anomalies first detected here; assesses kidney size, echogenicity, presence, and collecting system dilatation
• Normal kidney: slightly less echogenic than liver, difference increases with age
• Corticomedullary differentiation can mimic hydronephrosis in neonates (normal variant)
• Fetal lobulation visible up to age 5

CT Protocol

• Non-contrast phase: baseline attenuation, calculi, gross anatomical survey
• Arterial/corticomedullary phase (25-30 s): renal vasculature, accessory arteries - important before surgery on anomalous kidneys
• Nephrographic phase (70-100 s): parenchymal assessment, masses, fusion anomalies, isthmus of horseshoe kidney
• Excretory/urographic phase (5-10 min): collecting system anatomy - identifies duplication, ureterocele, UPJ obstruction, ectopic insertion
• CTU (CT Urography): combines above phases; gold standard for complex collecting system anomalies, ectopia, and duplex systems in adults
• Multiplanar reconstructions (coronal, sagittal) are essential for fully characterizing positional and fusion anomalies

MRI Protocol

• Preferred in children, pregnant patients, and radiation-sensitive situations
• T2 HASTE/SSFSE coronal: excellent for collecting system (bright urine), anomalies of position and fusion
• T1 pre/post gadolinium: parenchymal function, enhancement patterns
• MR Urography (MRU): static fluid MRU (heavily T2-weighted, no contrast) and excretory MRU (with gadolinium and furosemide) for functional assessment
• DWI: adjunct for cystic lesion characterization and function in dysplastic kidneys
• Allows excellent soft-tissue delineation without radiation - ideal for pediatric duplex systems, ectopic ureters, and cystic diseases

Nuclear Medicine

• DMSA scan: cortical function, scarring, split function - used for horseshoe kidney isthmus function (Fig. 27.14), unilateral agenesis confirmation
• MAG3/DTPA diuretic renogram: UPJ obstruction, drainage curves, split function
• VCUG (Micturating Cystourethrogram): vesicoureteric reflux, posterior urethral valves, ureterocele

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4. Renal Agenesis and Associated Syndromes

• Results from failure of the ureteric bud to join the metanephric blastema
• Often asymptomatic; contralateral kidney undergoes compensatory hypertrophy
• Ipsilateral absent/rudimentary ureter in most cases

• Males (up to 70%): absent epididymis, vas deferens, seminal vesicle; seminal vesicle cyst
• Females (70%): Müllerian anomalies - vaginal/uterine agenesis, unicornuate uterus = Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome
• Absent ipsilateral adrenal gland: 10%
• VACTERL association (vertebral, anorectal, cardiac, tracheo-esophageal, renal, limb)
• Turner syndrome, trisomy 21/22, cardiac anomalies, skeletal (clubfeet)
• Ipsilateral seminal vesicle cysts (associated with ejaculatory duct obstruction)

• Potter sequence: oligohydramnios → pulmonary hypoplasia + facial anomalies (low-set ears, broad flat nose, prominent infraorbital folds, limb deformities)
• Previously uniformly fatal; rare survivors described with aggressive NICU support

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5. Renal Hypoplasia, Supernumerary Kidney, and Cross-Fused Ectopia

Renal Hypoplasia

• Small kidney with reduced number of nephrons - developmental arrest in first trimester
• Distinct from dysplasia (which involves abnormal tissue patterning)
• Imaging: decreased kidney size; may show hyperechoic cortex or small cystic foci on US
• CT/MRI: small smooth kidney, preserved but diminished parenchyma, normal architecture
• Differential: acquired atrophy (scarred, irregular surface) vs hypoplasia (smooth surface, normal internal structure)

Supernumerary (Suprapneumary) Kidney

• Rare: presence of an additional kidney, distinct from the normal two
• Usually occurs on the left (probable ureteric bud duplication)
• Located caudal to the normal kidney
• Drainage: either separate ureter or branch of a bifid ureter
• Imaging: CT/MRU shows separate renal parenchymal unit with its own collecting system and blood supply

Cross-Fused Renal Ectopia - Types

• More common in males; left kidney is more commonly the ectopic one
• The ureter from the ectopic lower kidney crosses the midline and usually inserts normally into the bladder on its native side
• Abnormalities of vasculature are common when crossed fused ectopia exists
• Complications: UPJ obstruction, nephrolithiasis, VUR, infection

• US: parenchymal band conjoining both kidneys, abnormal location
• DMSA scan: functional tissue on the ectopic side confirms renal tissue
• CT/MRU: defines parenchymal fusion, vascular supply, collecting system

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6. Abnormally Positioned Kidney / Renal Ectopia

• Decreased function
• UPJ obstruction (aberrant vessel crossing)
• Vesicoureteric reflux
• Nephrolithiasis
• Increased susceptibility to trauma (no bony protection)
• Higher incidence of anomalies in the contralateral kidney as well

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7. Fusion Abnormalities

Horseshoe Kidney

• Most common renal fusion anomaly: 1 in 400-1,000 births; 2:1 male predominance
• Results from contact and fusion of both kidneys (usually lower poles, 90% of cases) during ascent
• The isthmus lies anterior to the IVC and aorta but posterior to the inferior mesenteric artery (IMA) - the IMA prevents full ascent
• Isthmus: fibrous band or functioning parenchyma (85-95%)
• Renal axes: lower poles face medially (opposite of normal)

• Turner syndrome (up to 7% have horseshoe kidney)
• Trisomy 18, trisomy 21
• Neural tube defects
• UPJ obstruction (high incidence due to high ureteric insertion), VUR, nephrolithiasis
• Increased risk of Wilms tumor, carcinoid tumors, transitional cell carcinoma, and lymphoma

Pancake Kidney

• Bilateral pelvic kidneys that fuse - produces a single flat pelvic renal mass
• Both collecting systems present within fused mass
• Diagnosed with CT/MRU

L-Shaped / Sigmoid / Lump Kidney

• Other configurations of crossed fused ectopia in which the orientation of the fused unit differs

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8. Duplex Pelvicalyceal System (Ureteral Duplication)

Types

• Partial (bifid) duplication: two pelves join proximal to the UVJ; may show Y-shaped ureter
  • Bifid pelvis: two pelvices joined proximal to UPJ (upper calyces → upper pelvis; mid/lower → lower pelvis)
  • Y-ureter: ureters join distal to UPJ - can cause yo-yo (ureteroureteral) reflux
• Complete duplication: two entirely separate ureters inserting separately into the bladder (or ectopically)

Weigert-Meyer Rule

• Upper pole moiety: tends to obstruct (ectopic insertion, ureterocele)
• Lower pole moiety: tends to reflux (laterally displaced orifice)

Ureterocele

• Cystic dilatation of the intravesical distal ureter
• Orthotropic (simple) ureterocele: at normal position, typically adults
• Ectopic ureterocele: in duplex system, upper moiety, inserts below normal (females >> males)
• Cobra-head/spring-onion sign on IVU/CTU
• US: thin-walled cystic structure at bladder base

• US: echogenic band ("columnar of Bertin") between upper and lower moieties; dilated upper pole in obstruction
• CT/MRU: defines both moieties, ureteric courses, and site of insertion
• VCUG: demonstrates VUR into lower moiety; may opacify ureterocele
• MRU preferred in children for complete anatomical mapping without radiation

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9. Normal Mimickers

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10. Cystic Renal Dysplasia - Full Spectrum and Associations

A. Renal Dysplasia (General)

• Abnormality of tissue patterning: decreased nephron number, loss of corticomedullary differentiation, metaplastic transformation of mesenchyme to cartilage/bone, abnormal stroma
• Typically small for age; polymorphic cysts may or may not be present
• Histology: paucity of glomeruli/tubules, disorganized elements, thickened/dilated renal tubules

B. Multicystic Dysplastic Kidney (MCDK)

• Most severe form of renal dysplasia
• Kidney replaced almost entirely by large polymorphic cysts with no discernible functioning parenchyma
• Incidence: ~1 in 4,000; more common in males; usually unilateral
• Contralateral kidney undergoes compensatory hypertrophy
• Natural history: spontaneous involution over time (size decreases; may completely involute)
• Detected on antenatal US: large peripheral cystic areas, no identifiable renal parenchyma, cysts do not communicate
• CT/MRI: multiple non-communicating cysts of varying size, no renal pelvis identifiable, absent normal parenchyma
• DMSA: absent/minimal function
• Associations: contralateral UPJ obstruction (~5-10%), VUR, contralateral hypoplasia

C. Obstructive Cystic Renal Dysplasia (ORD)

• Results from prolonged urinary tract obstruction during gestation
• Secondary to: posterior urethral valves, ureterocele, urethral agenesis
• US: hyperechoic thinned cortex, loss of corticomedullary differentiation, dilated collecting system, parenchymal cysts

D. Autosomal Recessive Polycystic Kidney Disease (ARPKD)

• PKHD1 gene mutation (chromosome 6); 1 in 20,000 live births
• Affects kidneys and liver (congenital hepatic fibrosis)
• Kidneys: cystic dilatation of collecting ducts → multiple microcysts usually <2 mm
• US: large bilaterally echogenic kidneys in neonate (microcysts too small to resolve individually); oligohydramnios on antenatal US
• With age: cysts may become large enough to be visible
• Liver: Caroli disease (intrahepatic biliary ectasia), congenital hepatic fibrosis, portal hypertension

E. Autosomal Dominant Polycystic Kidney Disease (ADPKD)

• PKD1 gene (chromosome 16) ~85%; PKD2 gene (chromosome 4) ~15%
• 1 in 400-1,000 live births - most common inherited cause of ESRD
• PKD1 mutations: earlier and more severe disease
• Symptoms: hematuria, flank pain, hypertension, proteinuria, renal insufficiency

• Intracranial (berry) aneurysms (~10%) - major cause of morbidity
• Hypertension
• Colonic diverticulosis
• Mitral and aortic valve dysfunction
• Aortic dissection
• Multiple biliary hamartomas (von Meyenberg complexes)
• Cysts in liver, spleen, pancreas

F. Simple Renal Cysts

• Common with increasing age; not congenital per se but commonly encountered
• Classified by Bosniak criteria (I-IV) for malignancy risk stratification
• Parapelvic cysts arise from renal parenchyma adjacent to the sinus
• Peripelvic cysts arise from lymphatics within the renal sinus

G. Medullary Sponge Kidney (MSK)

• Cystic dilatation of medullary collecting tubules
• Associated with nephrocalcinosis and recurrent stones
• IVU/CTU: "paintbrush" or "bouquet of flowers" appearance - contrast fills dilated tubules
• Not a true dysplasia but included in cystic renal spectrum

H. Nephronophthisis / Juvenile Nephronophthisis

• Autosomal recessive; commonest genetic cause of end-stage renal failure in children
• Small/normal-sized kidneys; corticomedullary cysts on imaging; loss of corticomedullary differentiation
• Associated with retinal dystrophy (Senior-Løken syndrome), Joubert syndrome

I. Summary Table - Cystic Diseases

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• Grainger & Allison's Diagnostic Radiology, Chapter 27-28 (Congenital Abnormalities of the Urinary Tract, Congenital Cystic Renal Disease)
• Brenner & Rector's The Kidney, Chapter 72 (Spectrum of Kidney and Urinary Tract Disorders in Children)
• Campbell-Walsh Wein Urology (Radiologic Anatomy of the Renal Parenchyma; CAKUT genetics)Here is a complete, organized reference covering all the topics you requested.

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Radiology - Imaging of Congenital Renal Abnormalities

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1. Embryology

• Pronephros (weeks 3-4): rudimentary, non-functional, completely regresses
• Mesonephros (weeks 4-8): temporarily functional, gives rise to the Wolffian (mesonephric) duct
• Metanephros (week 5+): the permanent kidney, arising from two elements:
  • Ureteric bud: outgrowth of the Wolffian duct - forms ureter, renal pelvis, calyces, and collecting ducts
  • Metanephric blastema (mesenchyme): forms nephrons, glomeruli, and tubules

• Ureteric bud must contact the metanephric mesenchyme at precisely the right time - early or late budding leads to renal malformations, dysplasia, hypoplasia, or agenesis
• Weeks 6-9: kidneys ascend from pelvis to L1-L2 retroperitoneum with 90° medial rotation (hilum faces anteromedially)
• Arterial supply shifts progressively cranially; failure of earlier arteries to involute = accessory renal arteries (~30% unilateral, 10% bilateral)
• Failure of ascent = ectopic kidney; failure of fusion arrest at midline = horseshoe/fused kidney

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2. Classification

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3. Imaging Protocol - CT and MRI

First-line: Ultrasound

• Antenatal US: most anomalies first detected here
• Normal kidney: slightly less echogenic than liver (difference increases with age)
• Neonatal corticomedullary differentiation can mimic hydronephrosis (normal)
• Fetal lobulation visible up to age 5

CT Urography (CTU) Protocol

• Multiplanar reconstructions (coronal, sagittal) are mandatory for positional and fusion anomalies
• CTU is the gold standard for complex collecting system anomalies in adults

MRI / MR Urography (MRU)

• Preferred in children and pregnancy (no radiation)
• T2 HASTE/SSFSE coronal: collecting system, fusion, position (bright urine)
• T1 pre/post gadolinium: parenchymal enhancement, function
• Static MRU: heavy T2-weighted, no contrast - outlines dilated collecting systems
• Excretory MRU: gadolinium + furosemide - functional and anatomical mapping
• DWI: adjunct for cystic lesion characterization and dysplastic kidney assessment

Nuclear Medicine

• DMSA scan: cortical scarring, split function, confirms ectopic/fused renal tissue
• MAG3/DTPA diuretic renogram: UPJ obstruction assessment
• VCUG: VUR, posterior urethral valves, ureterocele

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4. Renal Agenesis and Associated Syndromes

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5. Renal Hypoplasia, Supernumerary Kidney, and Cross-Fused Ectopia

Renal Hypoplasia

• Small kidney with reduced nephron number - developmental arrest in first trimester
• Distinct from dysplasia (abnormal tissue patterning) and acquired atrophy (irregular scarred surface)
• Imaging: Small, smooth kidney; preserved but diminished parenchyma; may show hyperechoic cortex or small cysts on US

Supernumerary Kidney

• Extra kidney (beyond the normal pair); rare
• Usually on the left (probable ureteric bud duplication)
• Located caudal to the normal kidney
• Drained by a separate ureter or a branch of a bifid ureter
• CT/MRU: separate renal unit with its own parenchyma, collecting system, and vascular supply

Cross-Fused Renal Ectopia

• More common in males; the left kidney is more commonly the ectopic one
• The ectopic kidney's ureter crosses the midline and inserts ipsilaterally into the bladder
• Complications: UPJ obstruction, stones, VUR, infection

• US: parenchymal band between kidneys; abnormal position
• DMSA: confirms functional renal tissue in the ectopic location
• CT/MRU: defines fusion, vascular supply, collecting system

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6. Abnormally Positioned Kidney / Renal Ectopia

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7. Fusion Abnormalities

Horseshoe Kidney

• Most common fusion anomaly: 1 in 400-1,000 births; 2:1 male predominance
• Both kidneys (usually lower poles, 90%) fuse across the midline during ascent
• The isthmus lies anterior to the aorta and IVC, posterior to the inferior mesenteric artery (IMA) - which halts further ascent
• Isthmus composition: fibrous or functioning parenchyma (85-95%)
• Lower poles point medially (reversed renal axes)

• Turner syndrome (7%), trisomy 18, trisomy 21, neural tube defects
• UPJ obstruction (high ureteric insertion), VUR, nephrolithiasis
• Increased risk: Wilms tumor, carcinoid, transitional cell carcinoma, lymphoma

Pancake (Disc) Kidney

• Bilateral pelvic kidneys that fuse anteriorly, producing a single flat pelvic renal mass with two collecting systems

Other Fusion Types (variants of crossed fused ectopia)

• Sigmoid/S-shaped kidney: the two kidneys fuse at opposite poles
• L-shaped kidney: perpendicular configuration
• Lump/cake kidney: irregular complete fusion

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8. Duplex Pelvicalyceal System (Ureteral Duplication)

Types

• Partial (bifid) duplex:
  • Bifid pelvis: two pelvices joined proximal to UPJ
  • Y-ureter (bifid ureter): ureters join distal to UPJ; can cause yo-yo (ureteroureteral) reflux if one segment disproportionately enlarged
• Complete duplex: two entirely separate ureters from kidney to bladder (or ectopically)

Weigert-Meyer Rule

• Upper pole: tends to obstruct (ectopic/ureterocele)
• Lower pole: tends to reflux (laterally displaced orifice, shorter submucosal tunnel)

Ureterocele

• Cystic dilatation of the intravesical distal ureter
• Orthotropic (simple): at normal position; typically in adults; usually asymptomatic
• Ectopic: in duplex system, upper moiety, inserts below trigone; more common in females; may prolapse into urethra
• Cobra-head / spring-onion sign on IVU or CTU

• US: echogenic band (column of Bertin) between moieties; dilated upper pole in obstruction; ureterocele as thin-walled cyst at bladder base
• CTU/MRU: full anatomical map of both moieties, ureteric courses, insertion sites
• VCUG: VUR into lower moiety; may opacify ureterocele
• Approximately 1/3 of patients with complete duplication have an additional congenital anomaly

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9. Normal Mimickers

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10. Cystic Renal Dysplasia - Full Spectrum and Associations

Renal Dysplasia (General Definition)

Multicystic Dysplastic Kidney (MCDK)

• Extreme form: kidney almost entirely replaced by large polymorphic non-communicating cysts, no identifiable renal pelvis, no functioning parenchyma
• Incidence ~1 in 4,000; more common in males; usually unilateral
• Natural history: gradual spontaneous involution
• Antenatal US: large peripheral cysts, no parenchyma
• DMSA: absent/minimal function
• Associations: contralateral UPJ obstruction (~5-10%), VUR, contralateral hypoplasia

Obstructive Cystic Renal Dysplasia

• Due to prolonged gestation urinary obstruction (posterior urethral valves, ureterocele, urethral agenesis)
• US: hyperechoic thinned cortex, loss of corticomedullary differentiation, dilated collecting system, parenchymal cysts

Autosomal Recessive PKD (ARPKD)

• PKHD1 gene, chromosome 6; 1 in 20,000 births
• Kidneys: cystic dilatation of collecting ducts → microcysts <2 mm
• Liver: congenital hepatic fibrosis, Caroli disease, portal hypertension
• Early US: bilaterally enlarged echogenic kidneys (microcysts too small to resolve); oligohydramnios antenatally
• With age: cysts become large enough to see individually

Autosomal Dominant PKD (ADPKD)

• PKD1 (chr 16, ~85%) and PKD2 (chr 4, ~15%); 1 in 400-1,000 births
• Most common inherited cause of ESRD
• Imaging: bilaterally enlarged kidneys, innumerable cysts of variable size

• Intracranial (berry) aneurysms (~10%)
• Colonic diverticulosis
• Mitral and aortic valve dysfunction
• Aortic dissection
• Von Meyenberg complexes (biliary hamartomas)
• Cysts in liver, spleen, pancreas

Medullary Sponge Kidney (MSK)

• Cystic dilatation of medullary collecting tubules
• Associated with nephrocalcinosis and recurrent stones
• CTU/IVU: "paintbrush" or "bouquet of flowers" - contrast fills dilated tubules

Nephronophthisis

• Autosomal recessive; most common genetic cause of ESRD in children
• Small/normal-sized kidneys; corticomedullary cysts; loss of differentiation
• Associated with: retinal dystrophy (Senior-Løken syndrome), Joubert syndrome (cerebellar vermis hypoplasia)

Summary Table - Cystic Renal Diseases

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• Grainger & Allison's Diagnostic Radiology, Chapters 27-28 (Congenital Abnormalities of the Urinary Tract; Congenital Cystic Renal Disease)
• Brenner & Rector's The Kidney, Chapter 72 (Spectrum of Kidney and Urinary Tract Disorders in Children)
• Campbell-Walsh Wein Urology (CAKUT genetics, Radiologic Anatomy of the Renal Parenchyma)