Urinary System and Kidneys

Urinary System Anatomy

• Kidney

  • Nephron: Functional unit of the kidney; each kidney contains over a million nephrons.

• Ureter

• Bladder

• Urethra

Urinary System Function

• Kidney Functions

  • Produces urine and removes waste.

  • Influences blood pressure, volume, and intake of salt and water.

    • Renin-angiotensin system.

  • Regulates serum electrolytes.

  • Regulates acid-base balance.

• Ureter Function

  • Tubal ducts leading from the kidney to the bladder.

  • Transports urine to the bladder.

• Bladder Function

  • Collects and stores urine.

• Urethra Function

  • Carries urine from the bladder to the outside of the body.

Kidney Anatomy

• Paired, bean-shaped structures in a sagittal and oblique plane in the retroperitoneal cavity.

• Located between the 1st and 3rd lumbar vertebrae.

  • Superior poles lie more posterior and medial.

  • Inferior poles lie more anterior and lateral.

  • Left kidney is typically more superior than the right kidney.

• Anterior to the psoas & quadratus lumborum muscles.

• Medial to the transverse abdominus muscle, liver, and spleen.

• Renal cortex is surrounded by a fascia capsule of connective tissue (Gerota fascia).

Kidney Structure

• Renal cortex

  • Outer portion of the kidney, bound by the renal capsule.

  • Contains glomerular capsules and convoluted tubules.

• Medulla

  • Inner portion of the renal parenchyma.

  • Contains renal pyramids, tubules, and the loops of Henle.

• Column of Bertin

  • Inward extension of the renal cortex between the renal pyramids.

• Renal sinus

  • Central portion of the kidney.

  • Contains major and minor calyces, peripelvic fat, fibrous tissues, arteries, veins, lymphatics, and part of the renal pelvis.

• Renal hilum

  • Contains renal artery, vein, and ureter.

Arterial Supply of the Kidney

• Renal artery

  • Main renal artery arises from the lateral aspect of the aorta.

  • Multiple or ipsilateral variations can occur.

  • Courses posterior to the renal vein.

  • Supplies the kidney, ureter, and adrenal glands.

• Segmental artery

  • Bifurcations after entering the renal hilum (4-5 segmental arteries).

• Interlobar artery

  • Branches off the lobar arteries and courses alongside the renal pyramids.

• Arcuate Artery

  • Branches of the interlobar arteries between the cortex and medulla.

• Interlobular arteries

  • Branches of the arcuate arteries that enter the renal glomeruli.

Indications for Kidney Examination

• Abnormal lab values

• UTI (Urinary Tract Infection)

• Flank pain

• Hematuria (blood in urine)

• Hypertension (high blood pressure)

• Decreased urine output

• Trauma

Lab Values Related to Kidney Function

• Creatinine

  • Normal range: 0.6-1.2 mg/dL

  • Increased in renal failure, chronic nephritis, or urinary obstruction.

  • Waste product from meat protein and body wear and tear.

  • More specific for renal function than BUN (Blood Urea Nitrogen).

• Blood Urea Nitrogen (BUN)

  • Normal range: 11-23 mg/dL

  • Elevated in urinary obstruction, renal dysfunction, or dehydration.

  • Decreased in over hydration, pregnancy, liver failure, smoking, and inadequate protein consumption.

  • Produced from the breakdown of food proteins.

• Hematuria

  • Visible or microscopic red blood cells in the urine.

  • Associated with early renal disease.

• Proteinuria

  • Abnormal amount of proteins in the urine.

  • Associated with nephritis, nephrolithiasis, carcinoma, polycystic disease, hypertension, and diabetes mellitus (DM).

  • Increased risk for progressive renal dysfunction.

• Concentration-Dilution Urinalysis

  • Detects chronic renal disease.

Sonographic Evaluation of the Kidney

• Patient positioning

  • Supine

  • Left posterior oblique (LPO)

  • Left lateral decubitus

  • Right posterior oblique (RPO)

  • Right lateral decubitus

  • Prone

  • Infants and small children.

Kidney Size (Sonographic Evaluation)

• Kidney size (adults)

  • 9-12 cm long

  • 4-5 cm wide

  • Minimum of 1 cm in cortical thickness

• Kidney size (children)

  • Based on age

    • >1 year: 7-8 cm

    • \<1 year: 5-6 cm

Sonographic Appearance of Kidney Structures

• Renal capsule

  • Well-defined echogenic line around the kidney.

• Renal cortex

  • Fine, moderate-low level echogenicity.

  • Less echogenic compared to the liver parenchyma.

  • Cortical thickness > 1cm.

• Medulla (aka Renal Pyramid)

  • Hypoechoic (can appear anechoic).

• Renal sinus

  • Hyperechoic, echogenic.

Color Doppler Evaluation of the Kidney

• Used to assess blood flow within the kidney.

PW Doppler Evaluation of the Kidney

• Renal Arterial Doppler

  • Low resistive.

  • Accurate in interlobar arteries.

  • Sample superior and inferior poles.

  • Low resistive waveform.

Anatomical Variations of the Kidney

• Dromedary hump

• Extra renal pelvis

• Fetal lobulation

• Hypertrophied column of Bertin

• Junctional parenchymal defect

Dromedary Hump

• Cortical bulge on the lateral aspect of the kidney, most often on the left kidney.

• Asymptomatic; incidental finding.

• Sonographic findings

  • Bulge on lateral (left kidney cortex).

  • Echogenicity is equal to the cortex.

Extra Renal Pelvis

• Renal pelvis extrudes from the renal hilum.

• Asymptomatic; incidental finding.

• Sonographic appearance

  • Anechoic oval-shaped structure medial to the renal hilum.

  • Pelvis appears as a central cystic area that is either partially or entirely beyond the confines of the renal substance.

Fetal Lobulation

• Immature renal development (\<5 years old), but can persist into adulthood (51%).

• Lobulations in the renal contour.

• Asymptomatic; incidental finding.

• Sonographic appearance

  • The surface of the kidney is indented in between the calyces.

Hypertrophied Column of Bertin

• Enlarged column of Bertin that extends from the cortex to the renal pelvis.

• Asymptomatic; incidental finding.

• Sonographic appearance

  • Cortex echogenicity extending into the medulla.

Junctional Parenchymal Defect

• Embryonic remnant of the fusion site between the upper and lower portions of the kidney.

• Asymptomatic; incidental finding.

• Sonographic appearance

  • Triangular echogenic area in the anterior aspect of the kidney.

Congenital Anomalies of the Kidney

• Agenesis

• Cake kidney

• Crossed fused ectopia

• Duplication

• Horseshoe kidney

• Pelvic kidney

• Sigmoid kidney

Cake Kidney

• Fusion of the entire medial aspect of both kidneys.

• Variant of a horseshoe kidney.

• Found in the pelvis.

• Pelvic mass on exam.

• Asymptomatic.

Crossed Fused Ectopia

• Both kidneys are fused in the same quadrant.

• Two separate collecting systems.

• Two normally located adrenal glands.

• Asymptomatic.

• Abdominal mass.

Duplication (Kidney)

• Two distinct collecting systems.

• Involves kidney, ureter, and/or renal pelvis.

• Partial or complete.

• Increased renal length.

• Two distinct collecting systems.

• Can be mistaken for a hypertrophied column of Bertin.

Horseshoe Kidney

• Fusion of kidneys at the inferior poles.

• Connected by an isthmus.

• Functioning parenchymal or non-functioning fibrotic tissue.

• Most common form of renal fusion.

• Asymptomatic.

• Pulsatile abdominal mass.

Pelvic Kidney

• Failure for kidney to ascend during development.

• Associated with a short ureter.

• Renal artery and vein are located more inferiorly.

• Asymptotic or pelvic pain.

• Lies in an oblique plane.

Sigmoid Kidney

• Variant of a horseshoe kidney.

• Superior pole of one kidney is fused to the inferior pole of the contralateral kidney.

• S-shaped.

• Asymptomatic.

• Abdominal mass.

Cystic Pathology of the Kidney

• Simple cyst

• Complex cyst

• Parapelvic cyst

• Peripelvic cyst

• Polycystic kidney disease

• Multicystic dysplasia

Simple Cyst

• Cystic mass with smooth, thin, well-defined border.

• Round or oval shape.

• Sharp interface between the cyst and renal parenchyma.

• No internal echoes (anechoic).

• Increased posterior acoustic enhancement.

• Acquired lesions, probably from obstructed ducts or tubules.

• Estimated incidence: 50% of the population older than 50 years of age.

• Asymptomatic; incidental finding.

• Solitary or multiple.

• One or both kidney involvement.

Complex Cyst

• May contain septations, thick walls, calcifications, internal echoes, and mural nodularity.

• Internal echoes are often the result of protein content, hemorrhage, and/or infection.

• Any irregularity at the base of the cyst should be considered a malignant growth.

• If septa are thicker than 1 mm with vascularity on color or power Doppler, the lesion is presumed malignant.

Parapelvic Cyst

• Small cysts that originate from the renal sinus, most likely lymphatic in origin.

• Do not communicate with the collecting system.

• Largely asymptomatic; may occasionally cause pain, hematuria, hypertension, or obstruction.

Polycystic Kidney Disease

• Inherited disorder.

  • Autosomal-recessive (ARPKD): childhood form (rare).

  • Autosomal-dominant (ADPKD): adult form (common).

• Normal renal parenchyma is replaced with cysts (bilateral).

• Symptoms: Palpable abdominal mass, hypertension, hematuria, colicky pain, elevated BUN and creatinine.

Multicystic Dysplasia

• Non-inherited disorder due to urinary obstruction in early embryologic development.

• Multiple cysts of varying shape and size.

• Uteropelvic junction obstruction and malrotation.

• Unilateral.

• Symptoms: Palpable abdominal mass, flank pain, hypertension.

Inflammatory Conditions of the Kidney

• Renal abscess

• Acute tubular necrosis (ATN)

• Chronic renal failure

• Glomerulonephritis

• Pyelonephritis

Acute Tubular Necrosis (ATN)

• Most common renal disease to produce acute renal failure; reversible.

• Etiology: Toxic drug exposure, hypotension, trauma, surgery of the heart or aorta, jaundice, sepsis.

• Asymptomatic or renal failure.

• Sonographic appearance: Bilateral enlarged kidneys with hyperechoic renal pyramids.

Chronic Renal Failure

• Loss of renal function due to diseases such as glomerulonephritis, chronic pyelonephritis, renal vascular disease, diabetes.

• Symptoms: Elevated BUN and creatinine, proteinuria, polyuria, headaches, fatigue, weakness, anemia.

• Sonographic appearance: Renal atrophy, hyperechoic parenchyma, thin renal cortex, difficult distinguishing kidney from surrounding structures.

Glomerulonephritis

• Necrosis and/or proliferation of cellular elements occurring in the glomeruli.

• Enlarged, poorly functioning kidney.

• Etiology: Immune diseases, infection, strep throat, lupus, chronic Hep C, vasculitis.

• Symptoms: Asymptomatic, proteinuria, decreased urine output, hypertension, hematuria, fatigue, and edema.

• Sonographic appearance: Hyperechoic renal cortex and enlarged kidneys.

Pyelonephritis

• Inflammatory condition in which bacteria ascends from the bladder.

• Symptoms: Flank pain, fever, chills, dysuria, pyuria, and leukocytosis.

• Sonographic appearance: Focal swelling of the kidney(s), well-defined renal pyramids.

Pyonephrosis

• Severe complication from pyelonephritis; infected kidney becomes obstructed and pus accumulates.

Obstruction of the Kidney

• Hydronephrosis

• Medullary sponge kidney

• Nephrolithiasis

Hydronephrosis

• Obstruction to the urinary tract.

• Grades I-III

  • Grade one (I): mild dilation of the renal calyces.

  • Severe grade three (III): massive dilation of the renal calyces, loss of renal parenchyma, resistive index (RI) of >.7.

• Symptoms: Flank pain, hematuria, fever, leukocytosis.

Medullary Sponge Kidney (MSK)

• Developmental abnormality within the medullary pyramids.

• Dilation of the distal collecting system (Ducts of Bellini) leads to urine stasis and stone formation.

• Asymptomatic; benign congenital condition.

• Sonographic appearance: Hyperechoic foci in the renal papillae.

Nephrolithiasis

• A stone formed within the kidney due to urinary stasis.

• Hyperechoic focus with posterior acoustic shadowing within the corticomedullary junction.

• Symptoms: Asymptomatic, renal colic, flank pain, hematuria.

Benign Pathology of the Kidney

• Adenoma

• Angiomyolipoma

• Lipoma

• Renal sinus lipomatosis

Adenoma

• Glandular epithelium; most common cortical tumor.

• Symptoms: Asymptomatic, hematuria.

• Sonographic appearance: Well-defined hypoechoic or isoechoic mass.

Angiomyolipoma

• Most common benign renal tumor; composed of fat, muscle, and blood vessels (hemorrhage risk).

• Found in 80% of patients with Tuberous Sclerosis (TS).

• Symptoms: Asymptomatic, flank pain, hematuria.

• Sonographic appearance: Well-defined hyperechoic mass, can distort renal architecture.

Lipoma

• Tumor consisting of fat cells.

• Asymptomatic.

• Occurs most often in women.

• Sonographic appearance: Hyperechoic well-defined mass.

Renal Sinus Lipomatosis

• Deposits of fat in the renal sinus with parenchymal atrophy.

• Increased echogenicity of the renal sinus with thinning of the renal cortex with normal renal contour.

• Etiology: Obesity, previous urinary obstruction, chronic renal infection, and steroid therapy.

• Asymptomatic or elevated creatinine.

Malignant Pathology of the Kidney

• Renal cell carcinoma

  • Adenocarcinoma (85%)

• Transitional cell carcinoma

• Wilms tumor (nephroblastoma)

Renal Cell Carcinoma

• Irregular mass with a range of echogenicity, highly vascular with indistinct borders.

• 4 stages:

  • 1. Confined to the kidney.

  • 2. Spread to the periphrenic fat.

  • 3. Extension to the renal vein, IVC, or lymph nodes.

  • 4. Extension to near or distant structures.

• Symptoms: Painless hematuria, uncontrolled hypertension.

Wilms Tumor

• AKA: Nephroblastoma; most common abdominal malignancy in children.

• Typical age: 2.5-3 years of age.

• Risk factors: \<5 years old, male, Beckwith-Widemann syndrome, and omphalocele.

• Symptoms: Palpable mass, abdominal pain, nausea/vomiting, hematuria, hypertension.

• Sonographic appearance: Solid, well-defined mass with variable echogenicity, echogenic rim, and calcifications.

Vascular Disorders of the Kidney

• Renal artery stenosis

• Renal artery aneurysm

• Renal vein thrombosis

• Renal vein tumor

Renal Artery Stenosis (RAS)

• Blockage in the renal artery causing obstruction or narrowing.

• Etiology: Atherosclerosis or fibromuscular dysplasia (FMD) - "String of pearls" mid-distal RA.

• Hypertension and renal insufficiency.

• Sonographic appearance: PSV of >180cm/s with spectral broadening at stenosis site- “tardus parvus” signal distally; RAR >3.5; kidney atrophy or infarct.

Renal Vein Thrombosis

• Obstruction within the renal vein.

• Enlarged renal vein with continuous or absent flow with an enlarged kidney.

• Symptoms: Flank pain or hematuria.

Renal Dialysis

• A process of diffusing blood across a membrane to remove substances a normal kidney would eliminate.

• For end-stage renal disease.

Renal Transplant

• Three types: Deceased-donor, Living-donor, Preemptive.

• Transplanted kidney is usually placed in the anterior iliac fossa.

• Renal artery anastomosed to the ipsilateral internal iliac artery.

• Renal vein to the ipsilateral external iliac vein.

• Ureter is placed in the superior portion of the urinary bladder.

Nephrocalcinosis

• Can be seen in rejected renal transplants as well as chronic glomerulonephritis, chronic hypercalcemia, and sickle cell disease.

• Sonographic findings:

  • Can affect the cortex: increased cortical echogenicity with spared pyramids.

  • Or medulla: medullary nephrocalcinosis pyramids become more echogenic than the adjacent cortex.

The Urinary Bladder Anatomy

• Apex: Superior portion of the bladder.

• Neck: Inferior portion of the bladder continuous with the urethra.

• Trigone: Region between the apex and neck.

• Ureters: Enter the bladder wall at an oblique angle ~5cm above the bladder outlet.

• Normal bladder values

  • Wall thickness when distended= 3cm

  • Wall thickness when empty=5cm

  • Wall thickness is greater in infants

  • Postvoid residual should not exceed 20mL

Congenital Abnormalities of the Urinary Bladder

• Bladder diverticulum

• Bladder ureterocele

• Urachal sinus

Bladder Diverticulum

• Weakness in the bladder walls causes an outpouching.

• Anechoic pedunctulation of the urinary bladder

• Small neck.

• May enlarge when bladder contracts.

Bladder Ureterocele

• Congenital obstruction of the ureteric orifice.

• Hyperechoic septation seen within the bladder at the ureter insertion.

• Demonstrated when urine enters the bladder.

• Cystlike enlargement of the lower end of the ureter.

Urachal Sinus

• A ligament that extends from the belly button to the bladder as part of normal fetal development.

• A urachal sinus when there is an opening allowing a connection between the umbilicus and the bladder.

• Symptoms: Asymptomatic or fluid draining from the umbilicus.

Pathology of the Urinary Bladder

• Bladder sludge

• Bladder calculus

• Bladder polyp

• Cystitis

• Bladder malignancy

Bladder Sludge

• Stagnant urine that becomes thick.

• Homogenous low-level echoes that are mobile with position change

• Debris in the bladder.

• Asymptomatic.

Bladder Calculus

• Can develop in the bladder or migrate from the kidneys.

• Hyperechoic focus within the bladder with posterior acoustic shadowing.

• Mobile with patient positioning.

• Asymptomatic or hematuria.

Bladder Polyp

• Echogenic intraluminal mass with smooth margins and immobile with position change.

• Symptoms: Asymptomatic or frequent urination.

Cystitis

• Infection in the bladder.

• Increase in bladder wall thickness with mobile internal echoes.

• Symptoms: Dysuria, urinary frequency, leukocytosis.

Bladder Tumors (Malignant)

• 95% are transitional cell carcinomas.

• Echogenic mass with irregular margins, immobile with vascularity.

• Symptoms: Painless hematuria, frequent urination, Dysuria.