The urinary system begins its development along with the reproductive system as they develop from the same origin, often described as the urogenital ridge. This is why anomalies in one system may have associated anomalies in the other system. The urogenital ridge will form during the fourth week of gestation and divide into the nephrogenic cord, to form the urinary tract, and the gonadal ridge, to form the reproductive tract. The correct development of the urinary tract is through a complex interaction between embryonic tissues, the mesonephric duct, the ureteric duct, and the metanephric blastema. A disruption of this intricate process through either a genetic or environmental cause can result in congenital anomalies of the urinary system, including renal agenesis, multicystic dysplastic kidney (MCDK), or infantile polycystic kidney disease (PKD). What follows is very basic information on the embryology of the kidneys, and the reader is encouraged to read more detailed information about the formation of the urinary system.

The kidneys will develop in three morphologic stages called the pronephros, the mesonephros, and the metanephros. The pronephros is the earliest nephric stage and is a transitory, nonfunctional structure that disappears by the fourth week of gestation. Pronephros is derived from Greek and means before kidney. Its purpose is not well understood, but it is thought that it might provide a structure for the mesonephros. The mesonephros forms just caudal to the pronephros and develops into the mesonephric tubules and the mesonephric duct, also known as the Wolffian duct. This structure provides partial function, whereas the kidney continues to develop and completely regresses by the 12th week of gestation. The Wolffian duct will regress in females and become Gartner duct that can form a cyst called a Gartner duct cyst, which is usually located along the anterolateral wall of the proximal third of the vagina. They can sometimes be identified on an endovaginal examination. In males, the Wolffian duct develops into parts of the male reproductive system, including the epididymis, vas deferens, seminal vesicles, and the ejaculatory duct. In females, the mesonephric duct develops into the paramesonephric duct, or Müllerian duct, which develops into the female reproductive system, including the uterus and the cervix. In males, the Müllerian duct will regress and become the appendix testes. The ureteric bud comes off the mesonephric duct and develops into the different parts of the collecting system, such as the calyces, collecting ducts, renal pelvis, and ureter. The ureteropelvic junction (UPJ) is the last to canalize and is the most common site of obstruction in congenital hydronephrosis. During the fifth week of gestation, the metanephros phase begins, and the metanephric mesoderm will form the excretory parts of the kidneys that form the nephrons, glomerulus, Bowman capsule, and the convoluted tubules (Pathology Box 12-1). Note that the collecting system and the parenchyma develop from two different sources. The kidneys become functional at about 12 weeks of gestational age. Nephrogenesis, development of the kidney, will continue through 32 to 36 weeks of gestational age.

The kidneys initially develop in the pelvic region where they lie close together in the sacral region. As the fetus grows and the abdomen enlarges, the kidneys are drawn apart and ascend to their final position in the abdomen around T12-L3. During this ascent, the kidneys will rotate 90 degrees, causing the hilum, which initially faces ventrally, to face medially. This ascent process occurs between 6 and 9 weeks of gestational age. While in the pelvis, the kidneys will receive their blood supply from the common iliac arteries. As the kidneys ascend, they are vascularized by a succession of transient, higher arteries off the abdominal aorta that corresponds to the level of the kidneys, until the kidneys reach their final position and the main renal arteries are formed. These lower vessels should regress as they become unnecessary, but if they persist, they become accessory renal arteries.

The normal urinary system consists of two kidneys, two ureters, the urinary bladder, and the urethra. The kidneys are in the retroperitoneum on either side of the vertebrae. The ureters take the urine from the kidney to the urinary bladder, where it is stored temporarily until it is excreted from the body through the urethra. The left kidney is located at about the level of T12-L3, whereas the right kidney is slightly lower in the abdomen owing to the larger size of the liver (Fig. 12-1A). The left kidney will be slightly higher, superior, in the abdomen and more toward the back, posterior, then the right kidney. The kidneys lie on the posterior abdominal wall on the quadratus lumborum muscle on either side of the spine in the perineal space of the retroperitoneum (Fig. 12-1B, C). The long axis of the kidney is in an oblique plane parallel to the lateral border of the psoas muscle. It is not in a sagittal plane. The upper pole of the kidney is more medial and lies more posterior than the lower pole. This is why the lower pole of the kidney tilted up toward the skin surface (Fig. 12-2).

Renal length gives an approximation of renal function; therefore, it is important for the sonographer to measure the kidneys carefully. Kidney size has been correlated with the patient’s height, age, and body mass index (BMI). The size

of the kidneys is roughly the size of the patient’s fist. In adults, the normal kidney is approximately 10 to 14 cm long in males and 9 to 13 cm long in females. A general number used is renal length is between 9 and 12 cm. The kidneys measure about 5 to 7 cm in width, 3 to 5 cm in thickness, and weigh approximately 120 to 170 g (Fig. 12-3A). The width and thickness measurements are measured at the level of the renal hilum in a transverse plane that is perpendicular to the long axis of the kidney. The two measurements should be at a right angle to one another (Fig. 12-3B). The kidneys are slightly larger in men, and the left kidney is usually slightly longer than the right. The difference between the two normal kidneys should be within 15 mm if the right kidney is smaller and 10 mm if the left kidney is smaller. The rule of thumb that is commonly used in sonography is that the kidneys should be within 2 cm of each other in length. If one kidney is more than 2 cm or shorter by at least 2 cm in length, when compared with the other kidney, the sonographer needs to try and determine the cause. The potential reasons for this discrepancy in size are discussed later in this chapter. When a kidney is removed, the remaining kidney will start to grow again and increase in size, called hypertrophy, to compensate for assuming the function of the other kidney, measuring greater than 12 to 13 cm in length. As people age, their kidneys tend to shrink. Therefore, a 9.5-cm kidney in an 80-year-old woman may be normal, whereas in a 26-year-old, it would be small.

The cortical thickness is measured from the base of the pyramid to the capsule and is normally between 6 and 10 mm. If the pyramids are difficult to see, the parenchymal thickness can be measured, instead of measuring from the edge of the sinus echo to the capsule and should be between 15 and 20 mm (Fig. 12-4). Cortical thickness appears to be closely related to estimated glomerular filtration rate (eGFR), and studies have shown that in patients with a low eGFR, their cortical thickness is reduced in size. Some nephrologists use the cortical thickness over the renal length measurement to determine the severity of renal failure as well as to follow the patient for progression of the disease.

Another sonographic method to evaluate the kidney for renal failure is by obtaining a renal volume. Renal volume can be used to help assess renal function, especially in patients with CKD, because renal volume will decrease with a decrease in renal function. Renal volume is a more sensitive indicator of declining renal function as opposed to a single linear measurement. It is calculated by measuring the three axes of the kidney and using the formula for an ellipsoid:

Volume = length × width × thickness × π/6

An easier version to use for calculation is volume = length × width × thickness × 0.523 (where π/6 = 3.14/6 = 0.523). Some ultrasound (US) units can calculate the renal volume from the three measurements when the volume tool is activated. Because the kidney is not a true ellipse, renal volumes are underestimated using this method. A normal renal volume is between 110 and 190 mL in men and between 90 and 150 mL in women (Pathology Box 12-2). Renal volumes calculated using a 3D transducer and 3D software have been shown to be more accurate when compared with renal volumes obtained using MRI.

Obtaining accurate renal length measurements is very important for patient care as medical decisions are made based on these measurements. It is important when measuring the length of the kidney to ensure that the full length of the kidney is being accurately measured, as it is easy to oblique the transducer and foreshorten the length, creating a false measurement. Look at the cortex as it should be about the same thickness as it surrounds the sinus echoes. Check the individual measurements of each kidney and compare the measurements between the kidneys to make sure that they make sense. The difference in size between the two kidneys should be less than 2 cm, and the measurements should be repeated if the difference is greater than 2 cm and a reason for the discrepancy cannot be determined. Sometimes, the lower pole may be hard to define due to a rib shadow or overlying bowel gas. A kidney that measures less than 9 cm is usually abnormal, so if a measurement is less than 9 cm, it is important to reevaluate the image, especially if the other kidney is a normal size. If the sinus echoes touch or are near the border of a pole, then the measurement will be inaccurate as this image has been captured in an oblique plane. If seeing the lower pole border is difficult due to a rib shadow, try using a sector transducer as it can get between the ribs and show both the borders of the kidney. It is acceptable for the rib shadow to go through the middle of the kidney when obtaining a length measurement. If the issue is bowel gas, try placing the transducer closer to the scanning table and angling anteriorly toward the kidney. If that does not work, turn the patient into an oblique or decubitus position. A last resort is to turn the patient into a prone position. Sometimes, pathology can interfere or mask the poles of the kidney, and the sonographer will need to make their best judgment on how to measure the length of the kidney. This will involve using the border of the mass to represent the pole of the kidney (Fig. 12-5A-H). There will be a slight variation in measurements between sonographers, called intervariability, and between repeated measurements from the same sonographer, called intravariability. If you realize that the sonographer on the prior study measured incorrectly, do not try to make your measurements match theirs, as this is very poor patient care. The radiologist, supervisor, or manager should directly discuss scanning errors with the employee that measured incorrectly. It is important for sonographers to remain professional and resist discussing errors or mistakes of colleagues among staff because this leads to a hostile working environment.

The kidney is bean shaped with a convex lateral border, a concave medial border, an upper pole, and a lower pole. The kidney is divided into the renal parenchyma, which consists of the renal cortex and the medulla, and the renal sinus (Fig. 12-6A, B). The renal cortex lies under the capsule and contains Bowman capsule, the glomerulus, and the proximal and distal convoluted tubules of the nephron. It is in the renal cortex where filtration occurs. The medulla is the inner portion of the renal parenchyma and contains the loop of Henle, which is where reabsorption takes place. The renal medulla consists of 7 to 14 renal pyramids, which are separated from each other by an inward extension of the renal cortex called a renal column or column of Bertin, through which the blood vessels pass. On average, there are eight renal pyramids in each kidney. The renal pyramids along with the surrounding cortical tissue create one lobe of the kidney. The arcuate arteries, which are located at the base of the pyramids, separate the medulla from the cortex and are termed the corticomedullary junction. The apex of the pyramid, called the renal papilla, fits into the cup-like
cavity of a minor calyx to collect the urine. There are between 7 and 14 minor calyces, which converge to form the two to three major calyces. Urine flows from a minor calyx into a major calyx and then into the funnel-shaped renal pelvis, which narrows to form the ureter at the UPJ. The renal sinus is a cavity located within the medial aspect of the kidney and contains blood vessels, the renal pelvis, and fat. Renal parenchyma surrounds the centrally located, hyperechoic renal sinus, which abuts the columns of Bertin without a connective tissue interface (Fig. 12-7). On the medial border of the kidney is an indentation called the renal hilum, which is where the renal arteries, nerves, lymphatics, and
fat enter the kidney and the renal veins and ureter leave the kidney. The renal veins are anterior to the renal arteries, and the ureter is posterior to the renal vessels (Fig. 12-8A, B). The kidneys are surrounded by an inner fibrous capsule, a middle layer of perinephric fat, an outer layer of fibrous connective tissue that also surrounds the adrenal gland, and, finally, an outer layer of pararenal fat. The innermost layer is the fibrous renal capsule, which covers the surface of the kidney and goes between the kidney and the adrenal gland to provide support to the renal tissue and protection from infection. There is not any adipose tissue found between the renal parenchyma and the renal capsule. The renal capsule is covered by a layer of perirenal fat, called the adipose capsule or renal fat pad, which helps support the kidney and is a protective layer acting as a shock absorber (Fig. 12-9A-C). The next layer anchors the kidneys to the surrounding structures and is the perirenal fascia, which is a collagen-filled, fibrous connective tissue that envelops the kidney, the adrenal gland, and the perirenal fat to form the perirenal space. The perirenal fascia divides into two layers that pass in front of and behind the kidney. The anterior and posterior fascial leaflets fuse, closing off the perirenal space from the rest of the retroperitoneum. The anterior perirenal fascia is called Gerota fascia, which separates the perinephric fat from the paranephric fat (Pathology Box 12-3). It is named after Dr. Dimitrie Gerota, an anatomist and the first Romanian radiologist and later urologist after losing his hand to radiodermatitis. The posterior renal fascia is a fibrous sheath covering the posterior side of the kidney and the perinephric space and is called Zuckerkandl fascia, named after the Hungarian Zuckerkandl brothers who worked together at The University of Vienna. Emil was an anatomist and surgeon, and Otto was a urologist. Emil described the posterior fascia in 1883 in a paper but did not recognize the anterior fascia. In 1895, Dr. Gerota described the anterior fascia and named the posterior fascia after Emil Zuckerkandl. The posterior pararenal space of the retroperitoneum is posterior to the Zuckerkandl fascia. Over time, the Zuckerkandl name has been forgotten and most people refer to both fasciae as Gerota fascia. Finally, there is another layer of fat, called the pararenal fat. The fatty layers are important in protecting and holding the kidneys in their normal position. When the amount of fatty tissue around the kidney rapidly decreases owing to rapid weight loss, it may cause a ureter to become kinked, causing hydronephrosis.

The kidneys receive their blood supply from the renal arteries, which arise from the lateral borders of the aorta, at around the level of L1-2, just distal to the superior mesenteric artery (SMA). Approximately 20% to 30% of the cardiac output enters the renal arteries. They are the first lateral branches off the aorta, and their long axis is best seen in a transverse plane. The renal arteries are approximately 4 to 6 cm in length and 5 to 6 mm in diameter and are located posterior to the renal veins (Fig. 12-10). Because the aorta lies to the left of the midline, the right renal artery (RRA) is longer than the left and travels posterior to the inferior vena cava (IVC) on its way to the kidney (Fig. 12-11A, B). The left renal artery (LRA) is more superior in location and courses posterior to the left renal vein (LRV) (Fig. 12-12). The main renal artery supplies branches to the adrenal gland, ureter, perinephric tissue, and the renal capsule as it approaches the renal hilum. The renal artery branches are an example of anatomic end arteries, which means that there is no communication between the arteries so that if one arterial branch becomes thrombosed or damaged, the renal parenchyma supplied by that vessel will become necrotic. The renal artery supplies blood to the five segments of the kidney: superior or apical, anterior superior, anterior inferior, inferior, and posterior. As the renal artery approaches the renal hilum, it divides into anterior and posterior segmental arteries, with the anterior branch receiving approximately 75% and the posterior branch approximately 25% of the blood flow. The first division of the renal artery is the posterior branch that passes behind the renal pelvis to become the posterior segmental artery. This branch supplies blood to a large portion of the posterior kidney. The main renal artery continues as the anterior branch, and at the renal hilum, it divides into the four anterior branches: the apical, upper, middle, and lower segmental branches. The apical segmental artery will supply blood to the anterior and posterior surfaces of the upper pole. The lower anterior segmental artery will supply blood to the anterior and posterior surfaces of the lower pole. The upper and middle segmental arteries supply blood to the remainder of the anterior surface. The segmental arteries then course through the renal sinus and branch into the lobar arteries. As the lobar arteries reach the minor calyces, they further divide into the interlobar arteries, which run between the pyramids, supplying them with blood. At the base of the pyramids, the interlobar arteries bend to form the incomplete arches of the arcuate arteries, indicating the true corticomedullary junction. The arcuate arteries travel across the top of the renal pyramids and give rise to the cortical radiate arteries, which were formerly known as interlobular arteries. The cortical radiate arteries come off the side of the arcuate artery at right angles and are arranged radially over the basal surface of the pyramids, perpendicular to the renal surface (Fig. 12-13). The cortical radiate arteries pass through the cortex as they divide to form the afferent arterioles. The afferent arterioles send the blood into a high-pressure capillary bed called the glomerulus where filtration takes place. The blood exits the glomerulus through the efferent arterioles. The efferent arterioles form a second capillary network around the tubule, which is called the peritubular capillaries, which empty their blood into a small network of venules that converge to become the cortical radiate veins. The blood next travels to the arcuate veins and then the interlobar veins, which form the main renal vein. Note that there are no segmental veins. The renal veins emerge from the renal hilum to enter the side wall of the IVC at about the level of L2 (Pathology Box 12-4). The right renal vein is typically about 3 to 4 cm in length as it is right next to the IVC, whereas the LRV is about 6 to 7 cm in length. The renal veins have a diameter of 10 to 12 mm. The LRV courses between the SMA and the aorta to reach the IVC (Fig. 12-14A-C). The LRV receives venous blood from the left adrenal vein, left gonadal (ovarian, testicular) vein, and the inferior phrenic vein. The right renal vein does not receive venous blood from other veins. The
nutcracker syndrome is when the LRV, the nut, is compressed between the SMA and the aorta, the nutcracker. Some patients may be asymptomatic, whereas others have hematuria, flank, or abdominal pain. Two common variants of the LRV are a retroaortic LRV, where the vein courses posterior to the aorta, and a circumaortic LRV, where both a retroaortic LRV and a normal renal vein pass anterior to the aorta (Fig. 12-15).

The renal capsule appears sonographically as a bright reflector surrounding the kidney. It is a specular reflector and is only seen when imaged perpendicular to the sound beam (Fig. 12-18).

The renal parenchyma is best appreciated on a longitudinal view of the kidney and will demonstrate the cortex and the medullary pyramids. The renal cortex extends from the renal capsule to the bases of the pyramids and into the spaces between them, the columns of Bertin. The normal adult renal cortex is homogeneous with the echogenicity of the cortex less than, hypoechoic, or equal to, isoechoic, to that of the adjacent liver. Studies have shown that the cortex may appear isoechoic to the liver in adults with normal renal function. As the spleen is more echogenic than the liver, a normal left kidney will always be less echogenic than the spleen (Fig. 12-19A). In order to correctly compare the right kidney against the liver, it must be a normal echogenicity. Fatty livers will be markedly increased in echogenicity, and there will be a big difference in echogenicity between the two organs (Fig. 12-19B). Livers with hepatitis can cause the kidney to appear echogenic because of the decreased echogenicity of the liver (Fig. 12-19C, D). When the renal cortex is more echogenic than the liver, this is abnormal and is discussed later in this chapter.

Posterior to the cortex is the inner portion of the renal parenchyma, which is called the medulla and contains the renal pyramids. The renal pyramids are uniform in distribution and in size and shape. The broader base of each pyramid faces the cortical area, and its apex or papilla points toward the renal pelvis. Sonographically, the medullary pyramids are cone or triangle shaped and are hypoechoic relative to the renal cortex and can be seen in about 50% of normal adults. The pyramids are more visible when the kidney cortex is echogenic (Fig. 12-20).

The renal sinus is the central portion of the kidney and contains the major and minor calyces, the renal pelvis, renal vessels, fat, nerves, and lymphatics. The fat in the sinus
is an extension of the perirenal fat. The renal sinus will appear as an oval highly echogenic structure in the center of the kidney. This bright echogenicity is not because the renal sinus is a dense structure but is caused by the multiple interfaces from fat, the calyces, renal pelvis, vessels, and lymphatics. The normal renal sinus should be more echogenic than both the normal renal cortex and the normal liver. The renal cortex is the least echogenic structure in the upper abdomen, whereas the renal sinus is the most echogenic structure (Pathology Box 12-5).

The normal renal pelvis is not seen with US unless it is dilated. If the patient is well hydrated, the renal pelvis may sometimes be seen as a lighter gray echo pattern inside the echogenic renal sinus.

From the renal pelvis, urine flows into the ureters, which are approximately 25- to 30-cm long and 3 to 4 mm in diameter. The smooth muscle wall of the calyces, pelvis, and ureters contracts rhythmically and propels urine by peristalsis to the bladder where it is stored. The ureter is not seen sonographically unless it is dilated. When dilated, the proximal ureter can be seen, leaving the kidney. There are three areas along the course of the ureter where it narrows, allowing a stone to become stuck: at the UPJ, as the ureter enters the pelvis and crosses over the common iliac artery bifurcation, and at the ureterovesical junction (UVJ), which is the most common place (Pathology Box 12-6).

No patient preparation is needed for a renal sonogram. If the bladder needs to be imaged, then it must contain urine and the patient should drink 16 to 24 ounces of fluid about 30 minutes before the examination. If an abdominal study is also ordered, then the patient will need to be NPO (nothing by mouth) for 6 to 8 hours.

The most common transducer to use is a curved linear-array transducer of the appropriate frequency for the size of the patient. Common frequencies used are in the 1 to 6 MHz range. To scan between the ribs, a sector transducer may be utilized. Scanning is usually performed with harmonics turned on, but this can cause penetration issues, especially on patients with fatty livers. If the echoes in the far field are absent or washed out, owing to the overall gain being increased, turn harmonics off and evaluate the image. It may be a more diagnostic image without using harmonics. Sometimes, the sonographer may need to take two images, one with and one without harmonics, to have images with the best resolution and an image with proper penetration (Fig. 12-21A, B). Compound imaging is another control that can be used to help reduce artifacts in the image. Sometimes, it may interfere with the acoustic shadow or enhancement artifacts. Always try scanning with these controls on and off to see which gives the better image.

The native kidneys are scanned in a supine position to start, using the liver and spleen as acoustic windows. Usually, the transducer is placed subcostal on the side of the patient aiming toward the midline, making the imaging plane is a variation of a coronal plane. Images may also be obtained from an intercostal approach. Usually, both approaches are needed. If the kidneys are not adequately seen from a supine position, then a posterior oblique position can be tried. If one or the other kidney is still not very well seen, then scanning the kidney from a right or left lateral decubitus view may allow better visualization of the kidney. If the kidneys are still not optimally seen, scanning them with the patient in a prone position can often visualize them as there will be no issue with the ribs or bowel gas. When scanning in a prone position, a pillow or a rolled sheet can be placed under the abdomen at the level of the kidneys to help increase the space between the iliac crest and the ribs. This is a good position to scan newborns. Because turning a patient into a prone position can be difficult, one work around is while the patient is in a decubitus position, place the transducer on the back of the patient as if they were in a prone position (Fig. 12-22). It may take scanning the patient in a variety of positions to properly evaluate the entire kidney.

The long axis of the kidney is in an oblique plane, so the transducer will also need to be in an oblique plane. When the patient is supine, the kidneys are closer to the stretcher as they are retroperitoneal organs and located in the back of the patient. Having the transducer in the middle of the side of the patient is typically not a good spot to see the kidney. The transducer should be near the stretcher and angled toward the abdominal wall, until the kidney is seen. (Sometimes, the sonographer’s hand is resting on the stretcher to see the kidney.) If the upper pole is not well seen owing to the angle of reflection issues, moving the transducer up toward the head and scanning between the ribs will give a better angle and the upper pole can be better appreciated (Fig. 12-23A, B). In transverse images, the upper pole can be obtained between the ribs, the mid from either an intercostal approach, especially the left kidney, or a subcostal approach and the lower poles from a subcostal approach (Fig. 12-24A-C).

When the patient is scanned from a subcostal approach, a deep inspiration will bring the kidneys down from under the ribs. When scanning from an intercostal approach, the goal is to get the kidney in a good position between the ribs. This can take trying a variety of breathing styles, including having the patient in quiet breathing, which is letting the patient breath normally or in a full or partial expiration. On some patients, a combination of breathing styles and approaches may be needed. For example, to see the upper pole of the kidney, an intercostal approach is used with the patient is in full expiration, whereas the lower pole is best seen from a subcostal approach with the patient in a deep inspiration.

This section discusses when to include the bladder as part of the examination. The anatomy and pathology of the bladder is discussed in Chapter 13.

In some departments, a renal sonogram always includes longitudinal and transverse images of the bladder, whereas in other departments, it is only scanned when clinically indicated. To evaluate the bladder, there needs to be some urine inside of it.

When there is bilateral hydronephrosis, the bladder should be scanned to see if it is too full and causing the hydronephrosis. If the patient has a full bladder, ask them to take their time and empty their bladder fully. (Some patients will quickly empty their bladder but not fully because they do not want to hold up the room.) After the bladder is emptied, the kidneys should then be rescanned to evaluate if the hydronephrosis stayed the same or went away. If there is still some degree of hydronephrosis present, recheck the bladder to evaluate how full it is.

When there is hematuria clinically, images of the bladder should be obtained to evaluate if the bladder is the source of the bleeding. Kidney stones, blood clots, tumors, or thickened bladder walls caused by cystitis can all be the causes of the hematuria.

If there is a concern for kidney stones, color Doppler should be used to evaluate the bladder for ureteral jets. The speed of the urine leaving the ureter is fast enough to produce a Doppler shift, and the jet can be appreciated with color Doppler as a sudden burst of color in the bladder toward the transducer (Fig. 12-25). Normally, ureteral jets should occur twice or more per minute and should be symmetric with color Doppler in a healthy individual. The jets are directed upward and toward the contralateral side crossing the midline. A nonobstructive stone is suspected with a weak jet or when color constantly “dribbles” out of the ureteral orifice. If a color jet is not visualized, then obstruction should be suspected, especially if multiple jets have been seen on the contralateral side.

This section discusses when to add color and spectral Doppler to the examination. Renal arterial and venous Doppler examinations are discussed in Chapter 6.

Color and spectral Doppler may need to be added to a routine renal US for various reasons, such as patients with pyelonephritis, medical renal disease, and evaluation of a renal mass. The renal preset should be used over a general or abdominal one. A renal preset optimizes the US machine, especially the Doppler settings, for the kidneys, whereas the general or abdominal presets are optimized for the liver (Fig. 12-26A, B). The sonographer should adjust the color velocity scale and color gain to appropriate settings to allow for good visualization of flow inside the kidney and in the main renal vessels.

The following paragraphs of this section discuss scenarios when color and spectral Doppler would be appropriate to include with a routine renal examination. It should be noted that adding color and spectral Doppler to an examination does not automatically allow the sonographer to upcharge to a more expensive renal Doppler study that was not originally ordered. The billing or coding department should be consulted as up-charging inappropriately puts the department at risk for being fined for fraudulent coding.

Some departments might require a color Doppler image with or without a spectral Doppler waveform of the main renal artery and vein at the hilum of the kidney to assess inflow and outflow in all studies. Some clinicians just want to know that there is flow to the kidney and flow leaving the kidney. In some patients, the renal vein may be prominent or dilated, which could mimic hydronephrosis. Color Doppler should be used to investigate this area of dilatation to show if it is a vein (Fig. 12-27A-E).

The resistive index (RI) is an arterial measurement that is obtained from the interlobar artery and relates to renal vascular resistance and is influenced by the amount of diastolic flow. This measurement reflects the condition of the vascular bed where blood flow is traveling.

The RI is determined by the following equation:

RI = peak systole − end diastole/peak systole

The RI is calculated by the US unit and is determined by the sonographer placing the calipers at peak systole and end diastole. An RI does not require the use of angle correction. The normal kidney should have an RI between 0.6 and 0.7. A RI greater than 0.7 is suggestive of increased vascular pressure, resistance, or compliance inside the kidney. It is important to properly measure the signal as it is easy to mistake noise or mirror artifact for diastolic flow (Fig. 12-28A-E). Observing the color Doppler pattern will help determine when end diastole occurs. If the color is flashing, that is, the arterial flow completely disappears in diastole, then there is no flow in end diastole and the RI is 1. If the arterial flow just about disappears and end-diastolic flow is minimal, then the RI will be greater than 0.8. If the arterial flow is seen very well during diastole, then the RI will be normal, less than 0.7. End-diastolic flow is correctly measured right before the next systolic upstroke as this represents the end of the diastolic cycle. End-diastolic flow should not be measured where diastolic flow stops if it is before the systolic upstroke. Some common examples when an RI should be measured in a kidney include patients with medical renal disease, those with AKI, those with CKD, and patients with hypertension caused and not caused by renal artery stenosis.

A color or power Doppler image of the entire kidney should be obtained to show perfusion in patients with AKI or CKD to evaluate for flow inside the kidney (Fig. 12-29). Patients who have pyelonephritis need a perfusion image to look for areas of flow voids that might be caused by early abscess formation. A common reason is to show perfusion in a “pseudotumor” on a patient with a dromedary hump or fetal lobulations to show that these areas have normal perfusion (Fig. 12-30A-C). A renal mass will not demonstrate perfusion but flow on the border of the mass and sometimes specs of color inside the mass.

An uncommon reason is when one kidney is unexpectedly found to be very small, less than 6 cm, to document for renal artery occlusion by showing a lack of flow in the kidney (Fig. 12-31).

Color Doppler can be helpful in identifying small stones as it produces what is called the twinkling artifact that is caused by the calcium in the stone. Any bright echoes in the kidney should be investigated with color Doppler. The twinkling artifact does not occur with every stone nor does seeing the artifact always mean that it is caused by a stone (Fig. 12-32A-C).

A long-axis image of the middle of the kidney measuring the longest length of the kidney and the renal cortical thickness. One to two longitudinal images of the kidney documenting from midline to its lateral border. One to two longitudinal images of the kidney documenting from midline to its medial border. An image demonstrating the liver and right kidney together and the spleen and left kidney together for parenchymal comparison (Fig. 12-33A-E).

A transverse image at the mid pole of the kidney, with the transducer angled to be perpendicular to the kidney. Note that the current AIUM/ACR/SRU/SPR protocol does not require a transverse or an anteroposterior (AP) (or height) measurement, but some departments still may require these measurements. The department can add to a protocol, but they cannot delete any required images. Next one to two transverse images of the kidney documenting from the upper pole to the mid kidney and one to two transverse images of the kidney documenting from the mid kidney to the lower pole (Fig. 12-34A-E).

Some departments might require a color Doppler and spectral Doppler waveform of the main renal artery and vein at the hilum (Fig. 12-35A-C). Some protocols may require images of the bladder and images of bladder jets on all patients. Images to determine postvoid residual volume are not routinely documented unless these images are requesting by the physician (Fig. 12-36) (Pathology Box 12-8).

The dromedary hump is a common variation of kidney shape and is caused by the splenic impression onto the superolateral mid pole of the left kidney. It is called a dromedary hump because it resembles the hump of a dromedary camel. Sonographically, it appears as a focal bulge in the mid-pole region, giving the kidney a more triangular shape. This area will have the same echo texture as the surrounding kidney tissue (Fig. 12-37). A dromedary hump is considered a renal pseudotumor as it may mimic the appearance of a renal mass. To differentiate between the two, interrogating the area with color or power Doppler will demonstrate normal parenchymal flow patterns, whereas a renal mass will show peripheral flow around the mass with possibly scattered flow inside (Fig. 12-38A-C).

Embryologically, the kidneys originate as distinct lobules that will fuse together as they develop. When these lobules do not fuse and there is incomplete fusion of the developing renal lobules, this is termed persistent fetal lobulations. This gives the kidney a bumpy contour or outline, as opposed to a smooth, flat, and continuous one. Persistent fetal lobulations have been described as indentations or a scalloped appearance of the renal outline between the renal pyramids (Fig. 12-39). Like the dromedary hump, fetal lobulations are considered pseudotumors and can be worrisome for a renal mass. Color or power Doppler should be used to document normal parenchymal flow underneath these “bumps” (Fig. 12-40A, B).

Columns of Bertin are a normal variant where there are double layers of renal cortex between the renal pyramids, thus widening the space between them. They are named after Exupere Joseph Bertin, a French anatomist, who first
described them in 1744. Columns of Bertin are not hypertrophic tissue but occur because of an incomplete fusion of the fetal lobes, causing two adjacent septa to become a large column with double the thickness. Columns of Bertin are present in approximately 50% of the population, with 20% being bilateral.

Sonographic criteria include indentation of the renal sinus, continuous with the adjacent renal cortex, and have the same echogenicity as that of the cortex. They can present with varying depths within the medullary substance of the kidneys and are usually located at the junction of the upper and middle thirds of the kidney. They have been described


as a renal pseudotumor; however, the echogenicity of the column is isoechoic and continuous with the renal cortex (Fig. 12-41). Columns of Bertin may indent deep into the renal sinus and mimic a duplicated collecting system; however, a transverse scan through the column will demonstrate the echogenic sinus at the bottom (Fig. 12-42).

A junctional parenchymal defect is a fusion defect usually seen in the upper pole of the right kidney, although there are reports in the literature of it being seen in the lower pole and in the left kidney. The defect is due to a partial fusion at the junction of two embryonic parenchymatous masses called ranunculi, which is where the term junctional parenchymal defect originates. On US, it is seen as a triangular or linear echogenic structure, which extends from the capsule to the renal sinus, near the junction of the upper and middle poles. The extension of perirenal fat into the defect causes it to be echogenic. A junctional parenchymal defect has no clinical significance and is discovered incidentally during an abdominal US or CT (Fig. 12-43).

An extrarenal pelvis is an anatomic variant where part of the renal pelvis is located outside the renal hilum. It appears dilated as opposed to the intrarenal pelvis, which is surrounded by sinus fat that helps to keep it compressed. It is found in approximately 10% of the population. Patients will be asymptomatic, and it is usually an incidental finding on US or CT. An extrarenal pelvis is best appreciated in a mid-pole transverse image as a midline longitudinal image will show no evidence of hydronephrosis. When the transducer is aimed at the medial aspect of the kidney, the fluid will be seen extending outside the renal border (Fig. 12-44A-D). Sonography will demonstrate a cystic area lying partially or entirely outside the renal pelvis on a transverse mid-pole image. Transverse images of the upper and lower poles will be normal. Care should be taken not to mistake an extrarenal pelvis for hydronephrosis by noting lack of dilated calyces
or a hydroureter. As there will be a normal midline image of the kidney that does not show any hydronephrosis, it should be easy not to confuse an extrarenal pelvis for hydronephrosis.

Renal sinus lipomatosis refers to a condition when there is an accumulation of excessive fat within the renal sinus. It is usually found in patients in their sixth or seventh decade, who are obese, or have had exposure to steroids. Renal sinus lipomatosis has no clinical significance, and patients are asymptomatic with normal urine output and renal function tests. It is usually bilateral and found incidentally. Sonographically, it is seen as an enlarged echogenic central sinus that is usually less echogenic than a normal renal sinus, and what appears to be cortical

thinning, due to the widening of the sinus echoes. These patients may also have a fatty pancreas, which appears as a bright, echogenic pancreas, that has no clinical significance (Fig. 12-45A, B).