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Flashcards in Kidneys Deck (30)
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How can you tell between hydronephrosis and renal cysts?

  • On imaging, renal sinus cysts will show a single or multiple discrete cystic lesions that do not communicate with each other.
  • In true hydronephrosis, all the dilated fluid-filled spaces are contiguous.


After diagnosing a renal or ureteric calculus, one should always evaluate the presence of _______.



Give examples of hydronephrosis without obstruction

  • Vesicoureteral reflux or pregnancy may cause a dilated ureter without obstruction. Pregnancy preferentially affects the right side.


Give examples of GU obstruction without hydronephrosis

Obstruction without hydronephrosis may also be seen in:

  • Very acute obstruction.
  • Obstruction with dehydration, where there is insufficient urine production to create a pressure back up.
  • Obstruction with ruptured fornix. Increased pressure from obstruction may cause a fornix to rupture, which would decompress the renal pelvis and spill fluid into the perinephric space.


What can happen to Resistive Index of the kidney during acute obstruction?

  • The renal resistive index (RI) may be elevated in acute obstruction, thought to be due to cytokine-mediated renal artery vasoconstriction.
  • The resistive index is calculated with pulse-width doppler of the renal segmental or arcuate arteries.

RI = (PSV-EdV)/PSV (peak systolic velocity and end diastolic velocity

Higher resistive indices correlate with higher resistance. With no diastolic flow, RI = PSv/PSv = 1. Reversal of diastolic flow technically causes RI >1, although in such cases RI is not measured.

  • An RI of >0.7 on the affected side, or a difference of >0.1 between kidneys, suggests acute obstruction.
  • Bilateral elevated RIs (>0.7) are nonspecific and can be due to any number of medical renal processes.
  • The resistive index is not used to diagnose chronic obstruction.


What is an angiomyolipoma?

Large AMLs have an increased risk of what?

US appearance?



  • An angiomyolipoma is a benign hamartoma made up of blood vessels (angio), smooth muscle (myo), and fat (lipoma).
  • Although benign, there is an increased risk of hemorrhage if >4 cm in size. The hemorrhage may be caused by microaneurysm rupture within the vascular elements of the AmL.
  • On ultrasound, AML is echogenic due to the fat component. There is considerable overlap between the ultrasound appearance of AML and renal cell carcinoma
  • About one-third of AML demonstrate shadowing, which is a specific finding for AML.
  • Multiple AMLs are seen in tuberous sclerosis.


What is a renal oncocytoma? 

US appearance?


  • Oncocytoma is a benign renal tumor arising from tubular cells.
  • on ultrasound, oncocytoma is indistinguishable from renal cell carcinoma (RCC). It may be hypoechoic, isoechoic, or hyperechoic. A spoke-wheel vascular pattern is sometimes seen on color Doppler.
  • Due to imaging overlap with RCC, oncocytomas are treated surgically, even if the typical stellate or spoke-wheel vessels are seen.


Renal Cell Carcinoma

Staging system?

US appearance?

What else must be evaluated if there is an RCC?


  • Renal cell carcinoma (RCC) is the most common solid renal mass.
  • The staging of RCC uses the Robson system, which is discussed in the genitourinary section.
  • RCC is most often isoechoic to renal cortex, but can occasionally be hypoechoic or even hyperechoic (mimicking AML). A hypoechoic rim and intratumoral cystic changes are typically seen only in RCC, which may help to distinguish it from AML.
  • In the presence of a renal mass, the renal veins must be carefully evaluated as RCC has a propensity for venous invasion. venous invasion is Robson stage IIIA, and the presence of venous invasion has important implications for surgical approach.
  • Color and spectral Doppler are helpful in differentiating bland renal vein thrombus (which would not be stage IIIA) from tumor thrombus. Tumor thrombus will have color Doppler flow with an arterial waveform.


What are the solid renal masses?

  • AML
  • Oncocytoma
  • RCC
  • Renal Lymphoma


Name the renal infections

  • Acute diffuse pyelonephritis
  • Focal pyelonephritis
  • Renal abscess
  • Emphysematous pyelonephritis
  • Tuberculous pyelonephrits
  • Xanthogranulomatous pyelo
  • Pyonephritis
  • HIV assosiated nephropathy


What is the most common US appearance of acute diffuse pyelonephritis?

  • Normal kidney
  • Occasionally generalized renal edema and engorgement can be seen



What is focal pyelonephritis and what is the classic US appearance?

  • Focal pyelonephritis is a focal or multifocal infection of the renal parenchyma.
  • The classic ultrasound appearance is a hypoechoic mass (or masses) with low-amplitude echoes that disrupts the corticomedullary junction. A distinct wall is lacking.


What is a renal abscess?

How often are UAs negative in the setting of a renal abscess?


US appearance?

  • A renal abscess is a focal necrotic parenchymal infection with a defined wall.
  • Urinalysis may be negative up to 30% of the time if the infection does not involve the collecting system.
  • Small abscesses (<3cm) often undergo a trial of conservative medical therapy, while larger abscesses are typically drained.
  • Ultrasound shows a fluid-filled renal mass with a distinct wall, which may be multiloculated.


What is emphysematous pyelonephritis?

What population gets it? Management? Mortality rate?

US appearance? 

  • A complication of acute pyelonephritis characterized by replacement of renal parenchyma by gas. It is caused by gas-forming organisms, most commonly E. coli.
  • It is almost exclusively seen in diabetic or immunocompromised individuals.
  • It is a surgical emergency requiring broad-spectrum antibiotics and emergent nephrectomy. mortality can reach 40%.
  • Ultrasound shows high-amplitude echoes in the renal

    parenchyma representing gas locules with posterior dirty acoustic shadowing.


?What characterizes tuberculous pyelonephritis?

What is a putty kidney ?

  • Tuberculous pyelonephritis, caused by hematogenous spread of M. tuberculosis, is characterized by focal cavitary renal lesions with calcification.
  • A putty kidney is an atrophic, calcified kidney seen in end-stage renal tuberculosis.


What is xanthogranulomatous pyelonephritis?

US appearance?

  • Xanthogranulomatous pyelonephritis results from repeated cycles of chronic lowgrade infection caused by an obstructing calculus that leads to fibrofatty replacement of renal parenchyma.
  • On ultrasound, the kidneys are enlarged with areas of mixed echogenicity. A central stone is nearly universally present, which may be staghorn in morphology.

MNEMONIC = Xanthogranuloma"STONE" pyelonefibrofatty replacement


What is pyonephrosis?


US appearance?


  • Pyonephrosis is an infection of an obstructed collecting system and is a surgical emergency. Treatment is emergent relief of obstruction, either with percutaneous nephrostomy or ureteral stent.
  • Ultrasound features echoes within a dilated collecting system. A fluid level may be present.


What is HIV associated nephropathy?

What does it most commonly result in?

How does it present?

US appearance?

  • The HIV virus may directly infect the renal parenchyma to produce HIV nephropathy, most commonly resulting in focal segmental glomerulosclerosis (FSGS).
  • HIV nephropathy clinically presents with nephritic renal failure.
  • The kidneys are characteristically echogenic. Enlarged echogenic kidneys are specific for HIV nephropathy, although the kidneys are enlarged only about 20% of the time.


What is autosomal dominant polycystic disease?


Natural history of disease?

Increased risk of malignant transformation?

US appearance?



  • ADPKD is the most common cause of multiple renal cysts in adults and is associated with cysts in the liver and other visceral organs. (MNEMONIC - cysts in kidney, liver, head (aneurysms), butt (diverticulosis)
  • 15% of patients have saccular cerebral aneurysms.
  • The natural history of ADPKD is renal failure by middle age.
  • Does not confer an increased risk of renal cell carcinoma; however, complex cysts with internal hemorrhage are difficult to distinguish from renal cell carcinoma.
  • Imaging of ADPKD shows markedly enlarged kidneys with innumerable cysts of varying size and echogenicity.


What is autosomal recessive polycystic kidney disease?

Prognosis? Presentation? US findings?

What eventually develops in these patients?

  • ARPKD is a diagnosis of infancy.
  • Prognosis is poor. If the child survives infancy, hepatic fibrosis usually develops.
  • ARPKD presents in utero as enlarged echogenic kidneys since the cysts are too small to be individually resolved by ultrasound.


What is acquired renal cystic disease?

Name an important difference between this and ADPKD

  • Patients on long-term dialysis often develop many small renal cysts superimposed upon atrophic kidneys.
  • The acquired cystic disease does confer an increased risk of renal cell carcinoma, in contrast to ADPKD.


What is the goal of US evaluation of renal transplants?

Where are transplanted kidneys implanted?

A lower or higher RI suggest renal dysfunction?

  • The goal of ultrasound evaluation after renal transplant is to determine whether there is a treatable surgical or vascular complication.
  • Ultrasound is not useful for differentiating among the various kinds of parenchymal rejection.
  • The transplanted kidney is implanted in the right or left iliac fossa (right more commonly), and is often very well imaged due to its superficial location.
  • An elevated RI (>0.7) suggests renal dysfunction, but this finding is nonspecific.


What are the surgical complications following renal transplant?

  • Ureteral obstruction is apparent on ultrasound as hydronephrosis.
  • Fluid collection (blood, pus, urine) is highly dependent on timing:
    • Immediately postoperative: Hematoma.
    • 1 - 2 weeks postoperative: Urinoma.
    • 3 - 4 weeks postoperative: Abscess.
    • 2nd month and beyond: Lymphocele.
      • MNEMONIC: BloodPeePusLy


What are the vascular complications following renal transplant?

What does doppler show in these?

  • Renal vein thrombosis: The renal artery Doppler may show reversal of diastolic flow.
  • Renal artery stenosis: Elevated flow velocities are seen at the site of stenosis, with a parvus et tardus waveform distal to the stenosis. Usually takes several weeks to months to develop.
  • Pseudoaneurysm is usually due to renal biopsy.


What are the medical complications following renal transplant?

  • Medical complications generally cannot be differentiated on ultrasound. A biopsy is necessary for diagnosis, although the time elapsed since the transplant may be a helpful clue.
  • Hyperacute rejection: occurs in first few hours after transplant.
    • Hyperacute rejection is very rare and is due to ABO blood type incompatibility.
  • Acute tubular necrosis (ATN): occurs in the immediate few postoperative days.
    • ATN is usually a sequela of pre-implantation ischemia.
  • Acute rejection: occurs within three months of transplant.
  • Chronic rejection: occurs after three months of transplant.
  • Drug toxicity may be caused by cyclosporine, which is nephrotoxic.


What could occur s/p kidney transplant that is associated with EBV?

US appearance? 

  • Post-transplant lymphoproliferative disorder (PTLD) is a type of lymphoma that is thought to be due to immune suppression and Epstein-Barr virus proliferation.
  • It can arise anywhere in the body. Any new mass in any organ in a transplant patient should raise concern for potential PTLD.
  • Ultrasound of renal PTLD will show an amorphous hypoechoic mass that may simulate a fluid collection on grayscale images. Unlike fluid, PTLD will demonstrate doppler flow.


Differential of medullary nephrocalcinosis

US appearance

  • Hyperparathyroidism is the most common cause of medullary nephrocalcinosis.
  • Renal tubular acidosis (distal type).
  • Medullary sponge kidney is caused by ectatic tubules in the medullary pyramids leading to stasis and stone formation.
  • Papillary necrosis.
  • In a child, treatment with furosemide can lead to medullary nephrocalcinosis.


Differential of cortical nephrocalcinosis

Overall much more rare compared to medullary nephrocalcinosis. 

  • Acute cortical necrosis.
  • Hyperoxaluria (rare).
  • Alport syndrome.
  • Autosomal recessive polycystic kidney disease.


The differential for echogenic kidneys

  • Echogenic kidneys are most commonly due to medical renal disease, such as diabetic nephropathy, glomerulosclerosis, acute tubular necrosis, etc.
  • HIV nephropathy causes enlarged and echogenic kidneys.


Differential of echogenic renal mass

  •  Angiomyolipoma (AML). A shadowing echogenic renal mass is relatively specific for AML.
  • Malignant neoplasm (atypical appearance).
  • Renal calculus.
  • Intrarenal gas.
  • Milk of calcium, caused by crystals precipitating out of supersaturated solution.
  • Sloughed papilla, secondary to papillary necrosis, may appear as an echogenic mass in the collecting system.