Category: 4. Renal Ultrasound Pathology

It is important to realize that not all focal hyperechoic lesions within the kidney are consistent with urolithiasis. A stone should have posterior acoustic shadowing. If a hyperechoic lesion with no acoustic shadow is seen within the kidney, you must consider a renal mass such as renal cell carcinoma (RCC) or angiomyolipoma (AML) (Forman 1993).

Renal masses can be detected incidentally during renal point of care ultrasound (POCUS). Any suspicious renal mass or abnormal kidney shape warrants referral to an expert and further workup (Sweeney, 1996).

Renal Cell Carcinoma (RCC)

• Most common Renal Malignancy in Adults.
• Highly variable with regards to size, shape, and location.
• Variable echogenicity: isoechoic, hypoechoic, or echoic

Angiomyolipoma

• Most common Benign Renal Tumor. (Tublin, 2011)
• Clearly circumscribed (i.e. angiomyolipoma) or irregular in shape (i.e. renal cell carcinoma).
• Often unilateral, but if bilateral, higher chance of malignancy.
• In some circumstances, renal cell carcinoma and angiomyolipoma can look very similar

Renal cysts are non-malignant, fluid-filled anechoic structures that are usually located in the parenchyma of the kidneys penetrating the cortex and medullary pyramids. They are common incidental POCUS findings and are usually benign and asymptomatic. (Bates, 2010)

Renal Cyst Ultrasound Findings

• Thin-walled and smooth, without septations or any internal elements.
• The anechoic areas of cysts are localized and do not connect to the calyces.
• Round or oval in shape
• Are located in the cortex or by the pelvis.

Editor’s Note: If these criteria are not fulfilled, you should consider a complex renal cyst, renal abscess, or malignancy that requires further workup. Also, make sure to differentiate kidney cysts from liver cysts by acquiring multiple ultrasound views.

PCKD and ARCD

If you see multiple renal cysts, this could indicate Polycystic Kidney Disease (PCKD) or acquired renal cystic disease (ARCD).

• Polycystic Kidney Disease (PCKD) is an inherited disorder that presents with an abundance of irregular renal cysts of varying size that distort the kidney’s shape bilaterally. Symptoms include flank pain, hematuria, hypertension, and renal failure.
• Acquired Renal Cystic Disease (ARCD) likewise presents with multiple kidney cysts but it is an acquired disorder and is seen in patients with end-stage renal disease on hemodialysis. It is associated with a higher risk of renal malignancy than PCKD.

Kidney stones (also known as nephrolithiasis, urolithiasis, ureteral stone, renal stone, or renal calculi) result because of stony secretions in the bladder or urinary tract.

According to the National Kidney and Urologic Diseases Information Clearing House, more than half a million people visit emergency rooms, and nearly three million people visit their health care provider, for problems with kidney stones.

These patients classically present with:

• Severe pain in their abdomen, groin, genitals, or flank
• Fever and chills
• Severe nausea or vomiting
• Hematuria

Kidney Stone Size

The cutoff size for a significant kidney stone size is around 5mm. Stones <5mm have a significantly higher chance (81-98%) of spontaneously passing while stones >5mm will have an exponentially lower chance (9-65%) of spontaneously passing (Jendeberg 2010). See the table below for spontaneous passage rates for specific ureteral stone sizes.

Ureteral Stone Size	Spontaneous Passage %
0-2mm	98%
3mm	98%
4mm	81%
5mm	65%
6mm	33%
>6.5mm	9%

Ureteral Stone Size and Spontaneous Passage Rate

Kidney Stone Location

Kidney stones may be located within the renal parenchyma, at the ureteropelvic junction (between the ureter and the renal pelvis), within the ureter, or at the ureterovesical junction (between the ureter and the bladder).

Ultrasound Findings for Kidney Stones:

There are multiple ways to look for the presence of kidney stones on ultrasound. The ultrasound findings you see will depend on the size/location of the kidney stone and the amount of obstruction.

Here are some of the more commonly described ultrasound findings for kidney stones:

1. Direct Visualization of Stone
2. Hydronephrosis of affected kidney
3. Absence of Ureteral Jets
4. Twinkling Artifact

1. Direct Ultrasound Visualization of Kidney Stone

The most definitive way to see a kidney stone on ultrasound is by direct visualization. Unfortunately, this is a rare occurrence since most kidney stones lie in the ureter or the ureterovesical junction. With surrounding bowel gas it is difficult to directly visualize the bladder.

Sometimes renal stones can be seen in the renal cortex, pelvis, or the ureteropelvic junction. Look for a hyperechoic structure with Acoustic Shadowing.

2. Hydronephrosis of Affected Kidney

An indirect sign of obstructive uropathy from a kidney stone is to look for hydronephrosis on renal ultrasound. (Go to the previous section on how to use renal ultrasound to diagnose hydronephrosis)

A misconception is that the absence of hydronephrosis on renal ultrasound rules out a kidney stone. This isn’t true. If a kidney stone is small, it may still cause symptoms and renal colic without dilating the kidney.

While not 100% specific, there is evidence that patients with none or mild hydronephrosis on ultrasound are significantly more likely to have ureteral stones <5 mm and will likely pass them without the need for surgical intervention. 

3. Absence of Ureteral Jets

Urine flows from the kidney, through the ureters, and finally enters the bladder at the trigone. Ultrasound “Ureteral Jets” are a color Doppler finding during bladder ultrasound that detects the flow of urine into the bladder at the level of the trigone.

To evaluate for ureteral jets properly make sure you are in the transverse view of the bladder and decrease your color Doppler scale to about 10-20cm/s or you can use power Doppler. Urine is not continuously released into the bladder, and actually occurs at regular intervals. Therefore, you may need to wait up to 5-10 minutes to see if there are any ureteral jets.

If a suspected kidney stone is lodged within the ureter or ureterovesical junction, you may find an absence of ureteral jets on the obstructed side when scanning the bladder.

Here are examples of ultrasound ureteral jets from the right and left kidneys:

4. Twinkling Artifact

The Twinkling Artifact is a sonographic artifact located behind calcifications of ureteral calculi when color Doppler is applied. It will appear as a multicolored high-intensity signal, similar to signals produced by turbulent flow with aliasing.

This artifact may also be associated with a color comet-tail artifact, which, similar to a greyscale comet-tail artifact, occurs when an ultrasound beam encounters 2 strong parallel reflectors. This leads to the “tail” like appearance from numerous reverberation signals that the transducer is receiving.

The twinkling artifact has a sensitivity and specificity of 90% and 100%, respectively, for kidney stone at the ureterovesical junction.

Ultrasound vs CT Scans for Kidney Stone Workup

A landmark study published in 2014 by the New England Journal of Medicine conducted at 15 various emergency departments evaluated the accuracy of Point of Care Ultrasound (POCUS) versus computed tomography (CT) scan as the initial imaging method for patients with suspected kidney stones.

The authors found that there was NO significant difference between POCUS and CT scans with regards to the following:

• Diagnostic accuracy
• Return emergency department visits or hospitalizations
• Incidence of high-risk diagnoses with complications
• Serious adverse events
• Average Pain score

The POCUS group however had a significantly lower amount of cumulative radiation exposure compared to the CT group.

This NEJM study concluded that Point of Care Ultrasound should be the first screening tool for patients with suspected kidney stones.

Editors Note: This is not to say that patients should only undergo ultrasound imaging, but rather the researchers advocate that it should be used as the initial diagnostic test. From there, it should be at the doctor’s discretion to order further imaging studies based on their clinical judgment.

Hydronephrosis Mimics can present as false positives for true hydronephrosis.

Here are some of the most common causes of Hydronephrosis Mimics:

• Extrarenal Pelvis
• Parapelvic Cysts
• Vascular Malformation

Extrarenal Pelvis

An extrarenal pelvis is a normal and often benign anatomical variant that lies predominantly outside the renal sinus. Regarding extrarenal pelvis vs hydronephrosis, an extrarenal pelvis on ultrasound will appear as an anechoic structure adjacent to the renal sinus without any pelviectesis, caliectasis, or cortical thinning. You can also place color Doppler on it to make sure there is no arterial/venous flow present.

Parapelvic Cyst

A parapelvic cyst is a cyst that forms near the renal pelvis. They are normally painless unless they grow too large and compress the kidney. Though they are associated with a history of urinary obstruction, the cyst itself rarely causes any obstruction.

Prominent Renal Vasculature and Vascular Malformation

Prominent renal arteries/veins and Renal Vascular Malformations (Abnormal communication between renal arteries and veins) can also cause false positives for hydronephrosis. Color Doppler indicates blood flow and can distinguish renal prominent vessels or vascular malformations from hydronephrosis (Bates, 2010).

Hydronephrosis, kidney stones, renal cysts, and masses are all detectable with ultrasound. Here, we will explain the key findings and clinical presentations of these common conditions.

Hydronephrosis Ultrasound Grading

Hydronephrosis is defined as abnormal enlargement and distension of a kidney caused by obstructive uropathy (blockage of normal urine flow into the ureter). Patients may present with varying symptoms depending on the causes of hydronephrosis.

Here are some of the most common causes of hydronephrosis and obstructive uropathy:

• Kidney Stones
• Bladder Stones
• Benign Prostatic Hyperplasia (BPH)
• Cancer (bladder, renal, uterine, colon, cervical)
• Dysfunctional Bladder (urinary retention)

Evaluating for Hydronephrosis with renal ultrasound is one of the most common Point of Care Ultrasound (POCUS) applications. Thankfully POCUS is easy to perform/interpret while being very sensitive and specific for diagnosing hydronephrosis.

Initially, it may seem confusing or hard to memorize all of the terms to grade hydronephrosis using ultrasound. However, hydronephrosis grading is actually very simple!

Tip: Don’t memorize the hydronephrosis grading system, just think that the structures closest to the obstruction will be the first to be dilated. So the order of hydronephrosis will be Renal Pelvis -> Major Calyces -> Minor Calyces -> Renal Cortex. Using this method, you can derive the entire hydronephrosis grading system.

Now there are few different ways to grade hydronephrosis. The simplest way is to grade hydronephrosis as Mild, Moderate, or Severe. The other way, which you may see in radiology reports, is to use a 1-4 hydronephrosis grading system with descriptions of the hydronephrosis findings (i.e. peviectasis, caliectasis, cortical thinning). The figure below sums up the most commonly used hydronephrosis grading systems.

Grade 0 Hydronephrosis – Normal

Sometimes you may see the term “Grade 0” Hydronephrosis. Don’t worry there isn’t any actual hydronephrosis, that just means that the kidney appears normal!

Grade 1 Hydronephrosis (Mild)

Grade 1 Hydronephrosis or “Mild” Hydronephrosis occurs when there is dilatation of the renal pelvis (Pelviectasis) without dilatation of the calyces. The renal cortex (parenchyma) is preserved and does not show any atrophy.

Note: Grade 1 Hydronephrosis (Pelviectasis) can be found in healthy/normal patients that have not voided for a prolonged amount of time. Make sure to have the patient void prior to the renal ultrasound exam to accurately access for hydronephrosis.

Grade 2 Hydronephrosis (Mild-Moderate)

Grade 2 Hydronephrosis or “Mild-Moderate” Hydronephrosis occurs when you have dilatation of the renal pelvis (Pelviectasis) and dilatation of the Calcyces (Caliectasis), specifically the MAJOR Calyces. The renal cortex (parenchyma) is preserved and does not show any atrophy.

Hydronephrosis Grade 3 (Moderate)

Grade 3 Hydronephrosis or “Moderate” Hydronephrosis occurs when you have dilatation of the renal pelvis (Pelviectasis) and dilatation of the Calcyces (Caliectasis), specifically the MAJOR and MINOR Calyces. Mild cortical thinning may be seen.

Hydronephrosis Grade 4 (Severe)

Grade 4 Hydronephrosis or “Severe” Hydronephrosis occurs when there is significant/gross dilatation of the renal pelvis (Pelviectasis) and Calcyces (Caliectasis) resulting in renal cortical thinning. There will also be renal atrophy and loss of the borders between the renal pelvis and calyces. Grade 4 hydronephrosis is also sometimes called the “Bear Claw” Sign since the anechoic areas resemble a paw print.