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Barbara K. Blok, M.D, Beatrice Hoffmann, M.D., Ph.D., RDMS

I.  Introduction and Indications

Acute flank pain and abdominal pain with hematuria are relatively common presenting complaints in the emergency department.  Although urinary obstruction is a likely diagnosis in such patients, the differential diagnosis includes life-threatening disease processes, most importantly an expanding or ruptured abdominal aortic aneurysm.  Emergency bedside sonography is a tool that can rapidly confirm the diagnosis of acute urinary obstruction and help exclude life-threatening processes.
It is important to know common medical terms used to describe the pathophysiology of urinary retention.  The structural impediment to the flow of urine is termed obstructive uropathy.  Unless this obstruction develops slowly it is typically painful, which is called renal colic.  The most common cause is a kidney stone dislodged into the ureter called ureterolithiasis.  Urine flow is blocked by the stone leading to back-up and dilation of the proximal ureter (hydroureter). As the obstruction progresses, more proximal structures like the renal collecting system (renal pelvis and calyces) becomes dilated, termed hydronephrosis.  If the hydronephrosis is severe, the renal parenchyma becomes compressed and if lasting long enough (about 2-4 weeks), can cause loss of renal function.
As described above, the most common cause of renal colic and hydronephrosis is ureterolithiasis.  But in general everything able to obstruct the inner lumen of the collecting system or causing extrinsic compression can block urinary flow and lead to renal colic.

Bedside renal sonography in the emergency department is also useful in the patient presenting with decreased urinary output or anuria, acute renal failure or pyelonephritis.  Similar to the renal colic patient it allows the examiner to narrow the differential diagnosis by evaluating the retroperitoneal anatomical structures for abnormalities but gives only limited clues for the functional status of the urinary system.

II.  Anatomy

The kidneys are retroperitoneal organs that are protected by the lower ribs posteriorly.  The right kidney lies just below the liver, while the left lies just below the spleen.  The superior pole of both kidneys is tilted slightly medially and posteriorly (oblique lie).  Each kidney has a fibrous outer cortex, a middle layer consisting of medulla (pyramids) with surrounding cortex (columns of Bertin) and an inner renal sinus that contains the calyces and renal pelvis with larger blood vessels, lymphatics and fatty tissue. 
The entrance to the renal sinus is on each medial aspect of the kidneys.  It is termed renal hilum and contains the major branches of renal vein, artery, ureter, lymphatics and connective tissue.
The whole renal complex including the kidney, adrenal gland, renal hilum and perinephric fat is surrounded by a fascial layer, called Gerota’s fascia.

Illustration 1:  Overview of kidney anatomy.

The ureter leaves the kidney through the renal hilum and is a tubular, muscular structure that travels retroperitoneal and anterior to the psoas muscle towards the bladder.  It courses medially then crosses over the iliac vessels and travels along the pelvic sidewalls before turning anteriorly and medially to enter the fundus of the bladder. 
There are three natural anatomic narrowing of the ureters: The ureteropelvic junction, as the renal pelvis narrows to become the ureter; the crossing of the iliac vessels, due to external bending and compression and the ureteral vesicular junction, due to intrinsic narrowing.  The bladder lies just posterior to the pubis and anterior to the rectum (males) or uterus (females).  It is a muscular structure that, as it distends, expands to fill the pelvis and, eventually, abuts the anterior abdominal wall.

III.  Scanning Technique, Normal Findings and Common Variants

A 3.5-5 MHz probe is typically used to scan the kidney.  For the right kidney, have the patient lie supine and place the probe in the right lower intercostal space in the midaxillary line.  Use the liver as your “acoustic window” and aim the probe slightly posteriorly (toward the kidney).  Gently rock the probe (up and down or side to side) to scan the entire kidney.  If needed, you can have the patient inspire or exhale, which allows for subtle movement of the kidney.  Obtain longitudinal (long axis) and transverse (short axis) views (Figure 1-3 and video clip1 and 2).

	Figure 1a		Figure 1b		Figure 2

Figure 1a and 1b:  Longitudinal images of normal right kidney. Figure 2:  Transverse image
of normal right kidney.  Note the position of the gallbladder (GB) next to the liver and inferior vena cava (IVC).

 

	Video clip 1		Video clip 2

Video clip 1:  Video showing longitudinal axis of normal right kidney. Video clip 2:  Video showing the transverse
axis of normal right kidney.

 

Figure 3:  Transverse view of renal blood supply of the right kidney. 
(Color Doppler shows hilum vessels in red and blue).

For the left kidney have the patient lie supine or in the right lateral decubitus position.  Place the probe in the lower intercostal space on the posterior axillary line.  The placement will be more cephalad and posterior than when visualizing the right kidney.  Again gently rock the probe to scan the entire kidney.  Obtain longitudinal and transverse views (Video clip 3 and 4).

	Video clip 3		Video clip 4

Video clip 3:  Video showing the normal left kidney in longitudinal axis. Video clip 4:  Video showing the normal
left kidney in transverse axis.

Depending on which axis you use to obtain you images, the sonographic shape of the kidney will change.  On longitudinal view, the kidney will appear football-shaped and will typically be 9-12 cm in length and 4-5 cm in width (normally within 2 cm of each other).  On transverse view, the kidney appears C-shaped.  The normal kidney will have a bright area surrounding it which is made up of Gerota’s fascia and perinephric fat.  The periphery of the kidney will appear grainy gray which is made up of the renal cortex and pyramids.  Sometimes you can see the individual pyramids, but this is not always the case.  The central area of the kidney, the renal sinus, will appear bright (echogenic) and consists of the calyces, renal pelvis and the renal sinus fat.  Always scan both kidneys for comparison and correlation to clinical picture.  The ureters are generally not well visualized by ultrasound, but, when distended may appear as a tubular structure extending inferiorly from the kidney (Figure 4).  The bladder, when distended with fluid, can be easily visualized in the lower pelvis as a rather thick walled, fluid-filled structure.

There are many normal variations in the anatomic structure of the kidneys.  Some common ones that you may identify include:  Double collecting system, where the renal sinus is divided by a hypertrophied column of Bertin; horseshoe kidney, where the left and right kidney are connected to each other, usually at the lower pole; Renal ectopia, where one or both kidneys are outside the normal renal fossa. (1-4)

IV.  Pathology

The diagnosis of acute renal colic has three major components: 
1.  The patient’s clinical presentation. 
2.  The presence of blood on urinalysis.  89% of patients with ureterolithiasis have > 0 RBC per high power field on urine microscopy.
3.  Diagnostic imaging, which may include intravenous pyelogram (IVP), CT scan or ultrasound.

The goal of bedside renal ultrasonography is to rapidly evaluate the patient presenting to the ED with flank pain, abdominal pain with hematuria or decreased urinary output to answer a few basic questions:

                        Is there hydronephrosis?
                        Unilateral or bilateral?
                        Is there fluid around the kidney?
                        Is the bladder distended?
                        Are stones seen?
                        Is the aorta normal (see abdominal aortic aneurysm)

The primary sonographic abnormality you will identify in the patient with suspected acute renal colic is hydronephrosis.  The degree of hydronephrosis relates to the degree and extent of obstruction (Illustration 2, figures 5 - 7).  The alternative imaging studies that are used to diagnose acute renal colic include IVP and spiral CT scan.  In IVP, intravenous contrast is injected and a series of plain abdominal radiographs are obtained.  The intravenous contrast agent is filtered by the kidney and appears bright white on the radiographs.  The radiographs are reviewed to evaluate for a delay in renal filtering of the intravenous contrast agent and for evidence of hydronephrosis as the contrast is filtered.  A delay in filtering of the intravenous contrast along with the presence of hydronephrosis indicates obstruction.  With high resolution spiral CT, images are obtained from the kidney to the bladder without the use of IV contrast.  This allows the reader to view serial cross sectional images of the kidney and ureter to identify hydronephrosis.  In comparison to ultrasound or IVP, CT scan can routinely identify ureteral stones and provide accurate measurements of the stone’s size.  Just remember, no study can identify hydronephrosis or ureteral calculi 100% of the time.

The advantages of emergency renal ultrasonography include:

                        Bedside
                        No radiation
                        No IV contrast
                        Repeatable
                        Fast

The disadvantages of emergency renal ultrasonography are that it does not assess renal function (as IVP does) and it cannot typically identify/size the ureteral stone.

Figure 4:  Dilated ureter seen below bladder (transverse view).

Illustration 2:  Overview of degrees of hydronephrosis.

 

Figure 5:  Longitudinal view of right kidney with mild hydronephrosis.

 

	Figure 6a		Figure 6b

Figure 6a:  Longitudinal axis of kidney with moderate hydronephrosis. Figure 6b:  Transverse view of same kidney with
moderate hydronephrosis.

Figure 7:  Kidney with severe hydronephrosis.  Note that the shape of the kidney is completely obliterated
by the severe hydronephrosis.

Dehydrated patients may not show hydronephrosis if the obstruction is early.  After complete obstruction to flow, there is an acute rise in intrarenal pressure.  The renal pelvis and calyces dilate first.  On ultrasound you will see echo-free areas distending the normal bright white (hyperechoic) central area of the kidney.  As obstruction continues, the renal parenchyma becomes compressed and you see thinning of the pyramids.   Always scan both kidneys for comparison and correlation to the patient’s clinical presentation.
Renal or ureteral stones may be seen on ultrasonography in the patient with acute renal colic.  Look for bright objects that cast a shadow within the kidney (Figures 8 – 12).  If you can, follow the dilated ureter down toward the bladder.  A bright object that casts a shadow within the ureter or at the junction of the bladder is consistent with ureteral nephrolithiasis.  It is often difficult to identify ureteral stones.

	Figure 8		Figure 9		Figure 10

	Figure 11		Figure 12

Figure 8 - 12:  Several examples of kidney stones are shown.  Kidney, kidney stone (blue) with the cast shadow are labeled in each figure.

In the evaluation of the patient with anuria, acute renal failure or decreased urinary output look for evidence of bilateral obstruction to flow.  You may find a distended, fluid-filled bladder (despite the patient’s best efforts to void), indicating bladder outlet obstruction, or a normal bladder with bilateral hydronephrosis, indicating a mass lesion compressing both ureters.  Other abnormal findings that you may identify when scanning the kidney include: 
Simple or complex renal cysts: peripheral, smooth, hypoechoic and with or without internal echoes; multiple in polycystic disease (Figure 13 to 19).

 

	Figure 13		Figure 14		Figure 15

Figure 13 - 15:  Several examples of renal cysts.  Kidneys with cyst (green) and posterior enhancement are shown.

 

	Figure 16		Figure 17

	Figure 18		Figure 19

Figure 16 -19:  Images of polycystic kidneys with multiple cysts.

 

Pyelonephritis:  sonographic appearance is most commonly normal, but you may find hypoechoic cortex and loss of demarcation between the outer cortex and middle pyramids and columns of Bertin.
Renal mass: may have any echotexture (hyperechoic, anechoic etc.) and appear anywhere within the kidney (Video Clip 5 and 6)
Transplant kidney: a normal echotexture kidney, typically in a pelvic location (Figure 20 and 21).

	Video clip 5		Video clip 6

Video clip 5 and 6:  Video showing a right kidney mass.

 

	Figure 20		Figure 21

Figure 20 and 21:  Renal transplant in longitudinal view located in the pelvic fossa.  Figure 21 shows a small intra-parenchymal bleed after a biopsy (blue area).

Chronic renal failure: Kidneys appear small and hyperechoic.
Ureteral stents:  have a characteristic appearance but can be difficult to visualize due to size and position (Figure 22 and 23).

	Figure 22		Figure 23

Figure 22 and 23:  Images showing a ureteral stent (green) in the proximal dilated ureter (blue).

Bladder:
Stones: bright object within the bladder that casts a posterior shadow.  Foreign bodies: Foley catheter in bladder. 
Bladder mass: hyperechoic or mixed, irregular shaped mass within the bladder (Figure 24-26).

	Figure 24a		Figure 24b

	Figure 25a		Figure 25b

Figure 24a, b and 25a, b:  Images 24a and 24b show the Foley in the bladder with the balloon deflated.  In images 25a and b the balloon is inflated.

 

Figure 26:  Fungating bladder mass.

V.  Pearls and Pitfalls Renal cysts can sometimes be mistaken for hydronephrosis.  Cysts are typically single and arise in the periphery of the kidney, but can be multiple as in polycystic kidney disease.  A collecting system located outside the kidney is termed extrarenal pelvis.  This can mimic early hydronephrosis but is a normal developmental variant.  The overhydrated patient may have mild hydronephrosis without obstruction.  In this case, both kidneys will show evidence of mild hydronephrosis The underhydrated patient may not have hydronephrosis on initial renal scanning, despite the presence of obstruction and renal colic  Mild hydronephrosis may be seen in the pregnant patient or one with a full bladder.  Again, both kidneys will appear similar Kidneys with chronic disease may not have normal sonographic appearance, making identification difficult In some cases an adjacent structure, such as a fluid-filled gallbladder, may be mistaken as renal (in this case mistaken as a renal cyst). Remember, the patient with multiple renal cysts may also have liver cysts.  Perform a quick ultrasound of the liver to screen for liver cysts in these patients.  Additionally, certain forms of polycystic kidney disease are associated with intracerebral aneurysms; followup is important in these patients. Be sure to scan the aorta for AAA in the patient who clinically appears to have acute renal colic, but in whom the renal scanning is normal.

 

VI.  References

1. Walsh J, ed., Campbell’s Urology. Philadelpia, PA, W.B. Saunders, 2002.
   
   
2. Ma, OJ and Mateer JR, eds.
   Emergency Ultrasound, New York, NY: McGrawHill; 2003.
   
   
3. Cosby, KS and Kendall JL, eds.
   Practical Guide to Emergency Ultrasound. Lippincott, Williams & Wilkins: Philadelphia, PA,2006.
   
   
4. Simon BC and Snoey ER, eds.Ultrasound in Emergency and Ambulatory Medicine. Mosby: St. Louis, MO,1997.