Radiology -Imaging congenital renal abnormalities and lesions 1.embryology 2. Classification 3. Imaging protocol ct and MRI 4. Renal agenesis and associated syndromes 5. Renal hypoplasia,suprapneumary kidney, cross fused ectopia it's types 6. Abnormal positioned kidney ,renal ectopia 7. Fusion abnormalities 8. Duplex pelvic calyces system 8. Normal mimicer 9. All cystic renal dysplasia and it's association

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CT MRI protocol congenital renal anomalies imaging radiology 2024

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Radiology - Imaging of Congenital Renal Abnormalities

1. Embryology

The kidney develops through three sequential phases:

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

Key events:

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

Key molecular regulators:

Pathway	Role	Clinical relevance
GDNF/RET	GDNF (from mesenchyme) activates RET on ureteric bud, drives branching	Loss → agenesis
EYA1/SIX1	Metanephric mesenchyme transcription; also expressed in ear/branchial tissue	Mutations → BOR syndrome
SHH/GLI3	Hedgehog signaling regulates ureteric branching	Loss → agenesis (Smith-Lemli-Opitz, Pallister-Hall)

2. Classification

Renal abnormalities are sub-classified by six categories:

Category	Examples
Number	Agenesis, supernumerary kidney
Size/Parenchyma	Hypoplasia, dysplasia
Position	Pelvic/iliac/thoracic ectopia, malrotation
Fusion	Horseshoe kidney, crossed fused ectopia, pancake kidney
Collecting system	Duplex system, ureterocele, UPJ obstruction, bifid pelvis
Cystic	ADPKD, ARPKD, MCDK, obstructive dysplasia, simple cysts

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

Phase	Timing	Purpose
Non-contrast	Baseline	Calculi, gross anatomy
Arterial/corticomedullary	25-30 s	Vascular supply, accessory arteries (pre-op)
Nephrographic	70-100 s	Parenchymal assessment, fusion anomalies, isthmus
Excretory/urographic	5-10 min	Collecting system: duplication, ureterocele, ectopic insertion, UPJ

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

4. Renal Agenesis and Associated Syndromes

Unilateral renal agenesis (1 in 500-1,000 births): failure of ureteric bud to join metanephric blastema; absent/rudimentary ipsilateral ureter in most cases.

Population	Associated anomalies
Males (up to 70%)	Absent epididymis, vas deferens, seminal vesicle; seminal vesicle cyst
Females (70%)	Müllerian anomalies: vaginal/uterine agenesis, unicornuate uterus = MRKH syndrome
All (10%)	Absent ipsilateral adrenal gland
All	VACTERL; Turner syndrome; trisomy 21/22; cardiac, skeletal anomalies

Imaging clue on plain film: Left renal agenesis - splenic flexure lies medially to the lesser curve of the stomach (bowel occupies renal fossa)

Bilateral renal agenesis (1 in 4,000 births): Potter sequence - oligohydramnios → pulmonary hypoplasia + facial anomalies (low-set ears, broad flat nose, prominent infraorbital folds)

MRI abdomen showing absent right kidney with ureteric remnant (white arrows) and ectopic testicular tissue in inguinal canal (asterisk)

MRI abdomen: absent right kidney and testis; underdeveloped ureteric remnant (arrows) with ectopic insertion into the prostatic urethra

Associated syndromes:

Syndrome	Gene	Renal phenotype
BOR (Branchio-Oto-Renal)	EYA1, SIX1	Agenesis, hypodysplasia, VUR, duplication
Fraser syndrome	FRAS1, FREM2	Bilateral agenesis/hypoplasia
Pallister-Hall	GLI3	Agenesis, dysplasia, hydronephrosis
Smith-Lemli-Opitz	DHCR7	Agenesis, hypodysplasia
Okihiro (radial ray)	SALL4	Unilateral agenesis, VUR, cross-fused ectopia

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

Types of crossed renal ectopia

Type	Description
Crossed fused (most common)	Kidney migrates across midline, fuses to the lower pole of the contralateral kidney
Simple crossed (nonfused)	Kidney crosses midline, does not fuse; lies at the pelvic rim
Bilateral ectopia	Both kidneys cross the midline; native ureters still insert normally into the bladder

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

Imaging:

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

Neonatal US (left) and DMSA (right) showing crossed fused ectopia with fused kidneys to the right of midline

Crossed fused ectopia: (A) US - parenchymal band; (B) DMSA - fused kidneys right of midline

6. Abnormally Positioned Kidney / Renal Ectopia

Normal ascent: pelvis → L1-2 retroperitoneum, with 90° medial rotation. Failure at any point = ectopia.

Position	Notes
Pelvic kidney	Most common ectopic type; true pelvis; prone to UPJ obstruction, trauma
Iliac kidney	Iliac fossa
Abdominal/lumbar	Between pelvis and normal position
Thoracic kidney	Rare; through Bochdalek diaphragmatic defect; diagnosed on chest X-ray/CT
Crossed ectopia	Crosses midline (see above)

Axial CECT showing right pelvic kidney with duplex configuration

Right-sided pelvic kidney with duplex configuration on CECT

Complications: Decreased function, UPJ obstruction, VUR, nephrolithiasis, increased trauma risk.

Malrotation: Abnormal rotation around the long axis; UPJ faces anteriorly (most common); may cause extrinsic obstruction.

Imaging approach: If kidney absent in retroperitoneal fossa, systematically scan pelvis, iliac fossa, and mediastinum. CT/MRI define position, vasculature (multiple anomalous vessels common), and collecting system. DMSA confirms function.

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)

Horseshoe kidney: axial and coronal CECT

Sagittal CT showing inferior mesenteric artery draped over horseshoe kidney isthmus (arrow)

IMA (arrow) draped over horseshoe kidney isthmus - key CT finding

Associations:

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

8. Duplex Pelvicalyceal System (Ureteral Duplication)

Most common congenital urinary tract anomaly: incidence 0.8-5%

Arises when two separate ureteric buds develop from a single Wolffian duct.

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 moiety ureter inserts ectopically - below and medial to the normal orifice, sometimes extravesical. Lower moiety ureter inserts normally (above and lateral).

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

Imaging approach:

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

9. Normal Mimickers

Mimicker	What It Simulates	Key Distinguishing Features
Persistent fetal lobulation	Renal mass or cortical scar	Regular scalloping along inter-lobar septa; no parenchymal loss; isoechoic/isodense to cortex
Column of Bertin (prominent septum)	Cortical mass / pseudotumor	Isoechoic to cortex on US; isodense on all CT phases; located between pyramids; no mass effect; normal enhancement on contrast CT
Dromedary hump	Left renal mass	Left lateral cortical bulge from splenic impression; isodense to rest of cortex on all CT phases
Junctional parenchymal defect	Cortical scar or angiomyolipoma	Triangular hyperechoic/fat-containing defect at upper pole anteriorly (junction of lobar units); contains fat on CT
Extrarenal pelvis	Hydronephrosis	Pelvis extends outside sinus; calyces normal caliber; no obstruction on renogram
Corticomedullary differentiation in neonates	Hydronephrosis/dilated calyces	Normal US appearance; hypoechoic medullary pyramids; resolves with age
Accessory renal arteries	Pathological vascular structure	Normal variant (~30% unilateral); supplies lower pole; critical to identify pre-operatively (can cross UPJ causing obstruction)
Retro-aortic / circum-aortic left renal vein	Venous obstruction	Normal variant; no associated renal deformity; gonadal/adrenal veins drain differently

10. Cystic Renal Dysplasia - Full Spectrum and Associations

Renal Dysplasia (General Definition)

Abnormality of tissue patterning: decreased nephron number, loss of corticomedullary differentiation, metaplastic transformation of mesenchyme to cartilage/bone, disorganized stroma.

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

Extra-renal associations:

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

Condition	Inheritance	Gene	Cyst Location	Key Associations	US Appearance
ARPKD	AR	PKHD1 (chr 6)	Collecting ducts (<2mm microcysts)	Hepatic fibrosis, Caroli, portal HTN	Enlarged echogenic kidneys
ADPKD	AD	PKD1/PKD2	All nephron segments	Berry aneurysms, liver/pancreas cysts	Bilateral enlarged, innumerable cysts
MCDK	Sporadic	--	Large non-communicating cysts	Contralateral UPJ obstruction, VUR	Cysts without pelvis, involutes
Obstructive dysplasia	Acquired (obstruction)	--	Parenchymal + dilated system	PUV, ureterocele	Echogenic cortex, dilated system
Nephronophthisis	AR	NPHP1-20	Corticomedullary interface	Senior-Løken, Joubert syndrome	Small kidneys, interface cysts
Medullary sponge	Sporadic/AD	--	Collecting tubules (medullary)	Nephrocalcinosis, stones	Echogenic medullary pyramids
Simple cysts	Acquired	--	Cortex/medulla	--	Anechoic, posterior acoustic enhancement

References:

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)

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