Kidneys and Adrenal Glands
IMBUS of the kidneys is important in the hospital and clinic. Adrenal gland abnormalities can be seen incidentally when the kidneys are imaged.
IMBUS of the kidneys is most frequently needed in clinic for the question of hydronephrosis. However, it is important to identify some additional kidney diseases. The adrenal glands are small and near the kidneys, so large lesions in the adrenal need to be recognized during a kidney exam. Ultrasound is not sensitive for evaluating adrenal glands so we do not need to worry about increasing the diagnosis of small incidentalomas. However, spotting a larger adrenal mass can be an important finding for some patients.
The kidneys lie in the retroperitoneum, behind all other abdominal organs, and really are shaped like “beans”. However, these beans are not identically shaped and are rotated medially. The left kidney may lie more cephalad than the right kidney. The vessels in this region were covered in a previous chapter but are labeled again in this frontal anatomy diagram.
The cephalad portion of the kidney is tilted posteriorly. Notice in the following image, shown from a lateral view, that the most caudal intercostal space on each side usually gives a long axis, coronal view of the kidneys. Ribs cover the upper third of each kidney.
The liver is cephalad and anterior to the right kidney and the spleen is cephalad to the left kidney as shown in the next image.
Medial to the kidneys are the psoas muscles and directly posterior are the quadratus lumborum muscles, along with adipose tissue. The kidneys move up and down on these muscles during respiration. Ultrasound can usually penetrate the posterior tissue and give acceptable views of the kidneys from the back. This posterior long axis view is mostly parasagittal, which means the kidney hilum is always at the medial border. Here is the posterior anatomy.
KIDNEY MICRO ANATOMY:
A section through a normal kidney shows three microscopic areas. The central portion, usually called the renal sinus, consists of the calyces, fatty tissue, and the renal pelvis. The next zone out is the renal medulla, which is made up of the medullary pyramids, separated by renal columns. Most external is the renal cortex. The renal columns are portions of the cortex that anchor to the renal pelvis so the tissue is identical to cortical tissue.
With IMBUS, the renal sinus is a single hyperechoic area, except for whatever anechoic urine may be present. The medulla and renal cortex together form a peripheral more hypoechoic area called the parenchyma. The medullary pyramids, when accumulating urine, are seen as small, anechoic areas in the inner parenchyma. Here is a still image of a normal kidney with the areas labeled P for parenchyma, S for renal sinus, and MP for medullary pyramid.
IMAGING THE KIDNEYS:
There are several different approaches to each kidney and an IMBUS-Advanced physician uses what works best for a patient. The standing patient position may be better for many patients, but supine or lateral decubitus positions are traditional. The standing position also works well for a posterior approach to the kidney. Don’t accept poor views of the kidney until several approaches have been tried.
The kidneys move a good deal with respiration; make sure to have patients halt their respiration for a handful of seconds whenever the best view of a kidney is found. Subtle kidney abnormalities may be difficult to distinguish if kidneys are constantly in motion.
Have the patient stand or sit with her back toward you. Find the medial rib margin with your fingers and apply gel medial to this rib. The curvilinear probe, indicator obliquely cephalad, is placed as in the following image on a patient’s right side. Move around medial to the rib in this region to find the kidney and optimize the long axis view.
Here is a clip of a kidney in the posterior window. There are no interfering ribs. The hyperechoic sinus and hypoechoic parenchyma are seen, along with a few medullary pyramids. The hilum is not in view.
This view of the kidney is parasagittal. Finding the hilum in this view requires fanning/sliding to the medial edge of the kidney. Blood vessels will be seen mostly cross-section with Color while the ureter will usually be a long axis, non-vascular tube if it is dilated.
Lateral intercostal kidney window:
Use fingers to find the most caudad intercostal space at the anterior axillary line and put the probe so the non-indicator edge is at the costal margin. Here is the hand/probe position for a left lateral intercostal view of the kidney.
Rotate the probe slightly to get the best long axis view. Sometimes the rib shadows prevent a clear view of the whole kidney. The hilum is usually a little easier to find with this lateral intercostal view because it is a coronal cut and the hilum is always at the kidney edge away from the probe. Here is a normal right kidney with a breath hold.
The renal sinus should be hyperechoic and the parenchyma hypoechoic but not black. When the long axis view is optimized, freeze and measure the longitudinal dimension of the kidney as in the following image.
Normal adult kidneys vary with patient height but in general are between 9-12 cm in length.
The other occasionally worthwhile measurement of the kidney is the width of the renal parenchyma. Measure from the hyperechoic outer capsule to the beginning of the hyperechoic renal sinus in the middle of a kidney where the parenchyma seems thickest. This means measuring past any medullary pyramids as in the following example.
Normal renal parenchyma width is between 1.4-2.4 cm and varies with the patient’s size. There is gradual shrinkage of the parenchyma and kidney length with age and chronic renal disease. Parenchyma can also become more hyperechoic in some renal diseases.
After measurement of the kidney in the long axis, fan/slide through the kidney in this plane, making sure to see the hilum on the medial side where the ureter and vessels enter and leave. Don’t miss a dilated ureter, which is the first sign of hydronephrosis. As a practical rule, an IMBUS exam should not be able to visualize a normal ureter, so in this view, any “vessel” medial and posterior to the kidney that does not have Color/Power Doppler flow should be considered an enlarged ureter. If there is concern about a kidney, rotate the probe to a transverse view with the indicator physician left and fan/slide through the length of the kidney. Repeat the process on the left kidney.
ABNORMAL KIDNEY FINDINGS
LARGE KIDNEYS: Kidneys can be large in acute nephritis and occasionally in early diabetic nephropathy and amyloid renal disease. The parenchyma is often more hyperechoic in these conditions as well, but be sure the gain is set correctly using surrounding tissue. Here is an image from a patient with acute SLE nephritis showing increased echogenicity of the parenchyma. The kidney measured greater than 12 cm in length.
SMALL KIDNEYS: Small kidneys usually indicate chronic intrinsic renal disease, chronic infection, or the result of renal artery stenosis, particularly if only one kidney is small. If a small kidney is congenital, the contralateral kidney is usually hypertrophied. It is mostly a loss of parenchyma that reduces the size of the kidney. The following is a patient with late stage diabetic nephropathy with an 8 cm kidney and a parenchymal width < 1 cm.
HORSESHOE KIDNEYS:This is a congenital malformation in which the lower poles of both kidneys are joined together by a parenchymal bridge that crosses up and over the aorta as a hypoechoic structure. The lower poles of each kidney don’t taper normally and they look indistinct in the standard views. Here is a clinic patient with the hypoechoic parenchymal bridge crossing the aorta and connecting to the left kidney.
A horseshoe kidney by itself causes no kidney trouble, but it can be associated with other genitourinary abnormalities so it is worth noting.
KIDNEY STONES: Stones in the kidney appear in the renal pelvis or calyces and can be difficult to see because this area is already hyperechoic, so the stone does not stand out. Only an acoustic shadow from the stone may distinguish it. To accentuate a shadow, reduce the gain somewhat. Here is a kidney with two stones. The posterior shadows were subtle, and best seen when viewing the clip frame by frame.
Color/Power Doppler will cause some kidney stones to light up and “twinkle”, even when there is no shadow, but we do not have a clip of this to show in the kidney. Twinkling of a stone at the end of a ureter is shown in the Bladder/Pelvis chapter. Reports indicate that 80% of renal stones exhibit twinkling artifact. Machine settings determine the amount of twinkling and having the focus position posterior to most of the renal sinus can enhance the twinkling. However, twinkling is not specific to kidney stones as demonstrated in the following clip with dramatic twinkling of a calcified lateral hip mass that turned out to be heterotopic ossification. Notice the dense posterior shadow. Twinkling probably indicates a rough surfaced calcification, which can be seen in various organs from various causes.
HYDRONEPHROSIS: Basic hydronephrosis was covered during IMBUS-Core, but here is a little more detail about some aspects. The following diagram depicts the progressive severity of hydronephrosis.
In mild hydronephrosis, the ureter and renal pelvis first dilate with urine. The calyces and medullary pyramids are still normal in size. The ureter comes medially out of the hilum and runs caudad toward the bladder. That means the hilum of each kidney must be identified carefully with Color/Power Doppler. If a dilated ureter cannot be found, be skeptical of diagnosing hydronephrosis. Here is a clip of a patient with mild to moderate hydronephrosis. This study should have been performed with less depth and with halted patient breathing. The dilated ureter comes into best view in a different plane from the best view of the renal pelvis dilation.
The dilated ureter is important because normal adults can show some urine in the renal pelvis when they are actively forming urine, but the ureter is never dilated. Here is a still image from a normal adult with good urine in the renal pelvis and medullary pyramids but there was no ureteral dilation.
Ureteral dilation is also important for differentiating a congenital condition called “ampullary pelvis” in which the renal pelvis is dilated but the calyces and ureter are normal. Here is an example.
This would be a difficult call, but seeing that the medullary pyramids were normal, along with no dilated ureter should create hesitation in diagnosing hydronephrosis. A decision must be made with each patient about whether it is better to over-call or under-call mild hydronephrosis. Uncertainty about early hydronephrosis might lead to confirmatory CT or formal ultrasound in some patients, but sometimes a patient can drink 250-500 mL of fluid over a fairly short period of time and come back in several hours to be re-examined. The patient could also come back the following day to be sure that hydronephrosis was not progressing. In the next chapter, ureteral bladder jets will be discussed as an aid for decision making with possible early hydronephrosis.
As hydronephrosis increases, the major calyces, the minor calyces, and the medullary pyramids eventually dilate and finally begin to compress the renal parenchyma. Moderate hydronephrosis becomes an easier diagnosis but make sure the ureter is dilated. Here is moderate hydronephrosis with an easily seen dilated ureter.
Severe hydronephrosis is usually easy to identify, as in the following clip, but never forget the dilated ureter (not well seen in this clip) to be sure that this isn’t severe polycystic kidney disease (polycystic disease should be bilateral) or multiple and large parapelvic cysts (see below).
A warning about left sided hydronephrosis: rarely, a large aortic aneurysm can cause obstruction of the left ureter, so in middle aged or elderly patients the aorta should be imaged in patients with left sided symptoms suggesting renal colic when any degree of hydroureter or hydronephrosis is seen.
Summary IMBUS clinic approach to possible kidney stones: Kidney stones < 5 mm in size cause symptoms but rarely obstruct a ureter, don’t seem to benefit from tamsulosin therapy, and don’t need instrumentation. Thus, a patient with symptoms consistent with a kidney stone but normal kidneys and ureters by IMBUS can be treated with simple pain medication and hydration at home while observing for stone passage and symptom relief. These patients could have early clinic followup and would usually not need a CT scan. Conversely, a patient with symptoms suggesting ureteral stone and mild or greater hydroureter and hydronephrosis should proceed immediately to CT because the location and size of the stone dictate how quickly urology might need to be involved. Obviously, the CT would also confirm the stone diagnosis and allow treatment with tamsulosin to begin.
RENAL CYSTS: Pure, simple cysts are always benign and usually clinically silent, although hemorrhage and acute pain can rarely occur in these cysts. However, make sure there are no complex features. Cysts that have complex features and are over 2 cm should have CT imaging. Simple cysts can be multiple and are completely anechoic with no internal echoes if the gain is correct. There should be posterior acoustic enhancement and often lateral wall shadows. They should be round or oval in shape and have a thin, sharp posterior wall. This next clinic patient had several simple cysts in the left kidney, but on the right side, there was a dominant cyst that was not quite round, was distorting the kidney a bit, and seemed to have some solid elements to it. The physicians appropriately obtained CT imaging and a renal tumor extending into the renal vein was found. It was resected for possible cure and identified as a renal cell carcinoma.
Simple cysts can bulge off of a kidney (pararenal), be in the cortex (cortical), or be in the renal medullary area (parapelvic). Here is a single, simple pararenal cyst off of the lower pole of a kidney. The cyst is round with no internal echoes, slight posterior acoustic enhancement, and a lateral wall shadow.
A large, hypoechoic medullary pyramid could cause confusion with a parapelvic cyst, but look for a cyst to be anechoic and to have posterior acoustic enhancement. Here is a clip of a parapelvic cyst extending out into the cortex. There is strong posterior acoustic enhancement.
Cysts can be very large, but as they become multiple and bilateral the worry is about polycystic kidney disease and confirmatory imaging is needed (either contrast enhanced CT or MR). The following clinic patient had multiple large cysts in both kidneys and had a mother who had been diagnosed with polycystic kidney disease.
RENAL MASSES: This is a difficult topic. There is no information about screening for renal tumors with ultrasound. Most renal tumors are considered dangerous and finding them early is seen as a benefit. In asymptomatic patients, such tumors are found incidentally. There is a normal renal variant with a hypertrophied renal column (columns of Bertin) that make physicians worry about cancer, although the tissue is completely isoechoic with the rest of the renal parenchyma, which would rarely be the case with a renal neoplasm.
Consistent with American College of Radiology guidelines, solid renal masses less than 1 cm in greatest diameter are not biopsied and even when found by CT are followed at 3-6 month intervals by CT/MR until they are over 1 cm. Thus, an IMBUS-identified renal mass that is clearly less than 1 cm can be followed with IMBUS at 3-month intervals for size change. Any mass that becomes greater than or equal to 1 cm should get an MR, which will help decide biopsy or direct resection.
Hypernephroma/renal cell carcinoma is the greatest fear. Such tumors can be hyperechoic or hypoechoic, are inhomogeneous, can cause a bulge in the renal outline, and are often scalloped along the border. Here is a left kidney in which a hypoechoic bulge greater than 1 cm was identified and was diagnosed as a clear cell renal carcinoma.
Unfortunately, the left kidney may infrequently have a benign malformation called a “dromedary hump” which is a little bulge opposite the border of the spleen, as in the following image. This is normal kidney tissue so it should be isoechoic with the rest of the parenchyma. The decision to get confirmatory CT or MR can be difficult.
Here is a clip showing a more ominous, larger lesion that is bulging and with a mixture of hyperechoic and hypoechoic elements. This was also a renal cell carcinoma.
Angiomyolipomas are hyperechoic parenchymal lesions reminiscent of the “snow ball” angiomas in the liver and spleen. These can be distinctive enough to be diagnosed as benign and not pursued, but confirmatory US would be obtained when a lesion was not distinctly hyperechoic.
ADRENAL GLAND MACRO ANATOMY
Imaging normal adrenal glands may be difficult. The reason to cover this topic briefly is for the infrequent times when a fairly large adrenal mass would be seen during a kidney exam.
Normal adrenal glands are 3-7 cm in length and lie cephalad and medial to the kidneys, as in the following view from the back of a patient.
The right adrenal lies posterior to the liver and just lateral to the IVC. The left adrenal is posterior to the spleen and lateral to the aorta.
IMAGING ADRENAL GLANDS
The important thing is to realize where the adrenal glands lie when we are doing kidney, liver, and spleen exams. With the kidneys in long axis, the adrenal glands will be just cephalad of the kidney and a bit medial. The right adrenal gland will abut the liver and be lateral to the IVC. The left adrenal gland will be against the spleen and lateral to the aorta.
The best view of the adrenal glands will be intercostal through the liver and through the spleen, getting the kidneys long axis. From these views, a very slight fan medial should bring in the adrenal glands. Here is a normal right adrenal gland showing as a triangular shaped, slightly hyperechoic structure between the superior pole of the kidney and the liver.
Next is a normal left adrenal region in the patient above with the parapelvic cyst.
ADRENAL GLAND PATHOLOGY
An IMBUS exam can probably only visualize an adrenal mass larger than 1 cm and at this point, malignancy and hormones are the issues. The malignancy pathway is determined by a patient’s history of cancer and the CT/MR size and appearance. Thus, the IMBUS rule must be that lesions greater than 1 cm would go to CT or MR (or PET/CT in a patient with a history of cancer).
Adrenal hemorrhage is the acute medical condition that could be visualized with IMBUS. Unilateral hemorrhage is often not symptomatic, but bilateral hemorrhage can lead to adrenal crisis. An early hematoma is solid with diffuse or heterogenous echogenicity. Later, the mass becomes mixed cystic and solid. Calcification can occur as early as several weeks.
Patient symptoms might clearly suggest hormone disturbances, but catecholamines, cortisol, and mineralocorticoids are the worries. A patient could have a 24-hour ABPM to establish normal blood pressure and pheochromocytoma would then be highly unlikely and catecholamine measurements would not be needed immediately. A normal serum potassium argues strongly against pursuing mineralocorticoid excess. A 1 mg overnight dexamethasone suppression test is probably needed to eliminate the possibility of cortisol excess.
Adrenal cyst: Adrenal cysts can be of any size. Larger cysts may reduce function in that adrenal. Here is a large adrenal cyst pushing up against a kidney.
Adenomas: Solid lesions large enough for IMBUS viewing could be benign or malignant and may be functional or not. Surprisingly, a large registry found that even “non-functioning” adrenal adenomas over the long term had a higher incidence of diabetes and cardiovascular events, suggesting that some of these adenomas may have low-level function causing long-term harm despite lab results within the normal range. Perhaps we should be suspicious of adenomas with high normal hormonal levels. Here is a fairly large, non-functioning right adrenal adenoma.
Pheochromocytoma: Pheochromocytomas can be small to large. IMBUS would not detect small ones, but this is an image of a large pheochromocytoma with cystic and solid parts.
Metastasis: Adrenal metastasis is classic in small cell lung cancer but can also occur in other tumors. IMBUS can’t differentiate a metastasis from other benign or malignant lesions. Here is a left adrenal metastasis from small cell lung cancer that was pushing up into the spleen.
Primary carcinoma: Adrenal carcinoma is unusual and may or may not secrete hormones. Patients might just have weight loss and anorexia. Here is an example.