Kidney ultrasound – what is important for a urologist?

Abstract

Kidney ultrasound is one of the basic procedures in the practice of a urologist. Apart from the location and the size, description of renal morphology should contain the thickness of the anterior lip parenchyma in a transverse section and the location of possible narrowings. Uneven outline of the kidney is a sign of past inflammatory conditions. In the case of the pelvicalyceal system dilation, it is advised to specify the dimensions of the pelvis and calyces. Convex shape of the calyces proves elevated pressure within the pelvicalyceal system. Hydronephrosis is present when urinary retention has led to thinning the renal parenchyma. In each case, one should identify the reason for urinary retention in the upper urinary tract. Urinary retention on both sides requires one to exclude urinary bladder tumor, it may also be caused by a benign prostatic hyperplasia. Ultrasound examination is a sensitive method of renal stones detection, regardless of their chemical composition. Cyst description in an ultrasound image should cover its morphological features, differentiating between the so-called simple or complex cysts. In the case of a solid lesion, ultrasound makes it possible to detect parenchymal lesions usually starting with the size of 2–2.5 cm. It enables one to particularly diagnose angiomyolipomas. As regards the remaining parenchymal lesions, differentiation of the lesion nature is impossible. In some cases of angiomyolipoma, when it contains bleeding areas present and when it is deficient in adipose tissue, it resembles adenocarcinoma. It is necessary that the description includes the exact location, especially the dimensions and relation of the tumor to the renal sinus. In the case of larger lesions, respiratory motion of the kidney, the condition of the adrenal gland and the presence of enlarged lymph nodes should be controlled. Additionally, one should evaluate the renal vein and inferior vena cava in terms of a neoplastic plug presence.

Introduction

Ultrasound examination has a specific place in urology. In the majority of cases it constitutes first-line image examination and is frequently carried out by urologists when they are on duty. Therefore, it is a requirement during urology major, as a necessary skill. Ultrasound imaging is used not only for diagnostic needs. It also enables ultrasound guided prostate biopsy and percutaneous nephrostomy.

Ultrasound examination performed by a urologist and a radiologist should meet the same standards, although experience and knowledge gained in the field of the basic discipline differ. It has some impact on examination interpretation being the derivative of those elements.

The key issue is the reason for ultrasound examination. One should consider two basic situations. The first one – in the case of a patient without any ailments as regards the urinary tract, when it constitutes a screening test. The other one – in the case of a patient with particular ailments as regards the urinary tract, e.g. renal colic symptoms, then the exam has a specific diagnostic goal. In each case one should take into account the most significant principle that one pathology does not exclude other pathologies, therefore it is necessary to thoroughly analyze the whole urinary system available upon examination. The paper below has been written from the point of view of a urologist being interested in image diagnostics, especially ultrasound. While preparing this article, we were doing our best, obviously subjectively, to include the most important information, which is significant to make therapeutic decisions.

Discussion

Ultrasound of the kidneys usually enables one to make a preliminary diagnosis and only in the case of severe obesity, high location of the kidney or very narrow space between the costal arch and the iliac ala it may be hindered. Description of the location and the size of the kidney should be supplemented with the thickness of the parenchyma, determined as a standard in the transverse section within the anterior lip and other locations – if any narrowing is visible. Uneven outline of the kidney is a sign of past inflammatory conditions, which may be of clinical importance. Increased motion of the kidney, which may be the cause of pain usually present in the case of very slim women (80–90% of the cases), typically on the right, may be observed even in the course of the examination when positioned on the side, as the so-called increased respiratory motion. In order to diagnose the pathology, it is necessary to perform the examination when lying, then standing and sometimes even after some jumping. Lowering the kidney by over 5 cm makes it possible to diagnose such an abnormality⁽¹⁾.

From among anatomical defects, the most frequently observed one is a horseshoe kidney, which requires surgical treatment only in the case of additional pathologies, such as nephrolithiasis or dilation of the pelvicalyceal system secondary to an upper urinary tract obstruction⁽²⁾. In such a situation, it is necessary to carry out excretory phase enhanced CT. Initial ultrasound should take into account the condition of the septum (isthmus connecting both the kidneys), its thickness and possibly its vascularization in Doppler exam. When the examination shows no kidney in its typical location in the first row, it should be sought within the pelvis, as this is the most common ectopic location of the kidney⁽³⁾. It is accompanied by malrotation of the kidney, which may be erroneously described as narrowing the renal parenchyma. A much more difficult issue is the differentiation between agenesis and aplasia or hypoplasia of the kidney, as a small kidney may be hard to be observed and ultrasound is not the best method of looking for the kidney. It is worth bearing in mind that in the case of agenesis there is no ureteric bud created, so the bladder has no ureteral orifice, nor any ureteric fold on that side. In the case of aplasia, the ureter is residual, which may be proven by a cystoscopic examination. Ultrasound, on the other hand, is not capable of differentiating between agenesis and aplasia⁽⁴⁾. In the case of observing the pelvicalyceal system dilation, one should provide the dimensions of the pelvis and the calyces, taking into account the description of the calyceal fornix. Convex shape of the pelvicalyceal system proves elevated pressure within the pelvicalyceal system. In the case of excessive dilation of the renal calyces in the course of the upper urinary tract obstruction, the renal pelvis is also enlarged in a balloon-like manner, preserving its even outline. In the case of absence of such a relation, the isolated collecting system dilation (usually on one side) may prove an anatomical defect in the form of megacalycosis, which imitates pelvicalyceal system dilation, but does not entail problems with urinary outflow from the kidney and does not require any procedure to be performed. Ultrasound shows then visible multiple dilated renal calyces (over 20–25 pieces) and a correct renal pelvis⁽⁵⁾.

A high degree of urinary retention is often mistaken for hydronephrosis. Hydronephrosis is present when urinary retention has led to thinning the renal parenchyma. For the needs of differentiation from hydronephrosis, it is necessary to measure the thickness of the parenchyma and – in the case of thinning – determine whether the process is general, or applies only to particular sections of the parenchyma. At the same time, it is worth drawing the attention to the fact whether the other kidney is enlarged compensatory, which would prove a long-term disease. In the case of hydronephrosis, especially bilateral, it is necessary to assess the urinary bladder in terms of urinary retention after micturition in the course of benign prostatic hyperplasia as well as the presence of a urinary bladder tumor. It may sometimes happen that the dilation of the pelvicalyceal system may be the result of urinary bladder being full. In such a case, following micturition, the dilation should subside (sometimes paralysis of the detrusor muscle due to overfilling the bladder may require several times micturition to void the bladder). While looking for the causes, one should take into account both the age and the sex of the patient. In elderly men the most common reason for urinary retention in the upper urinary tract will be a benign prostatic hyperplasia (BPH), while in young women and men pelviureteric junction obstruction or nephrolithiasis. In every case, identify the cause of upper urinary tract obstruction – if possible. It should be borne in mind that the infiltrating urinary bladder transitional cell carcinoma often results in upper urinary tract obstruction. In the case of a bilateral obstruction, the risk of an infiltrating nature of the lesion in the bladder amounts to 90% (on one side the risk amounts to 50%)⁽⁶⁾. This aggressive nature of the carcinoma requires rapid action, i.e. radical cystectomy, and in the case of additional hydronephrosis – usually puncture nephrostomy is mandatory. In the case of slim people, there will be a dilated, broad ureter visible, because the orifice of ureter into the bladder is infiltrated.

As regards nephrolithiasis, ultrasound is a highly sensitive method of deposits detection, yet of limited possibilities to assess their size. It usually slightly increases the size of the deposits. The main advantage of this examination is that it makes it possible to thoroughly assess the location, i.e. the location of the calculus in a given anterior calyx, posterior calyx or in the middle and in a specific calyceal group: upper, central or lower. It is a piece of significant information supporting the planned procedure, e.g. percutaneous nephrolithotripsy (PCNL) or ureterorenoscopy for renal stones (RIRS). In the case of very small deposits at the threshold of the method’s capabilities, e.g. in the case of lack of acoustic shadow confirming the presence of a deposit, such s doubt should be taken into account in the description (example: “deposit suspected”). Differentiation between such calculi and echo from the arcuate arteries is very difficult and requires some experience. Although ultrasound detects all the kinds of deposits, regardless of their chemical composition (including deposits from uric acid or protein matrix), it is not a good method to assess the burden of a calculus – a very important parameter for planning procedures. Another significant parameter in the case of lithiasis of the lower pole is the length of the renal calyx and pelvicalyceal angle, which may be assessed solely in the case of urinary retention of the pelvicalyceal system.

Renal cysts are often described during an ultrasound examination. In the majority of cases, they are of no clinical importance, on condition that these are simple cysts. Therefore, the description of examination showing a thin-walled cyst with no internal echo should contain a conclusion: “simple cyst”. It is worth emphasizing the fact that the dimension of the cyst reaching even 10 cm does not constitute any indication for a surgery, in the case it is a simple and asymptomatic cyst. Cysts not meeting the criteria of a simple cyst should be additionally diagnosed with computed tomography with the use of the Bosniak classification⁽⁷⁾. It is common to come across an ultrasound examination description with the use of that scale. However, it was created on the basis of computed tomography image analysis and the basic part of information resulting from the nature of that examination may not be clearly translated into an ultrasound examination (e.g. contrast medium caught by the wall or septum of the cyst)⁽⁷⁾. A cyst description in ultrasound should cover its location within the kidney, size, wall outline, presence of calcifications, septa, type of content, e.g. dense. A significant supplement to the examination in grayscale may be the Doppler examination in order to assess any possible flows in the cyst wall or septa, which will prove the possible potential in terms of oncology. It seems that imaging using the power Doppler method owing to the ability to catch low-speed flows is more accurate in such a situation⁽⁸⁾. Every cyst should be analyzed separately and described, since sometimes a one-time presence of benign and malignant lesions may happen. Such an image is typical of the von Hippel-Lindau syndrome with simultaneous presence of cysts, adenomas and clear cell carcinomas, and the lesions are sometimes abundant and bilateral⁽⁹⁾. Another indication to have a cyst surgery may be pressure on the pelvicalyceal system, which usually applies to central cysts, namely the ones located in the renal sinus. In the case of the presence of bilateral abundant cysts, deforming the renal image, which are usually significantly enlarged, one should suspect autosomal dominant polycystic kidney disease (ADPKD). It is a genetic defect with a mild course at the beginning, which later may lead to renal failure. In the case of complications, such as the development of a nephrolithiasis or pressure on the pelvicalyceal system, treatment consists in a surgery. Owing to a more and more common presence of renal tumors, it is necessary to monitor these patients on a regular basis⁽¹⁰⁾. Ultrasound image is usually typical and does not raise any diagnostic doubts.

The availability of the ultrasound image and its low cost resulted in increase in renal tumors diagnosis. Over 60% of them are detected accidentally in the course of diagnosing abdominal pain, arterial hypertension or periodical tests, usually carried out to the request of the patient⁽¹¹⁾. Ultrasound examination enables one to detect parenchymal lesions usually starting from the size of 2–2.5 cm⁽¹²⁾, though some specialists claim that with good examination conditions it is possible to see 1 cm tumors. It often requires the doctor to be engaged as regards the time and performance of kidney imaging at different sections, while inhaling and exhaling, while changing the position of the patient. Difficulty as regards the examination increases along with the patient’s obesity and bad preparation for examination.

Quite frequently, kidney imaging for 5 minutes fails to provide an opportunity to find the lesion. It happens a lot of times in the practice of a urologist to find renal tumors with the diameter exceeding 5–6 cm not found upon ultrasound examination in patients performing them regularly once a year. An average tumor growth amounts to 0.3–1.1 cm a year; the bigger it gets, the faster its growth⁽¹³⁾. Delay in diagnosis may result in the need to remove the whole kidney. It exposes the patient to the risk of developing a chronic renal failure and shortening their lives⁽¹⁴⁾. Early detection increases the chance to perform renal tumor resection (nefron sparing surgery, NSS) and sparing the organ.

Ultrasound enables one to particularly diagnose angiomyolipoma type tumors (AML). In the case of lesions up to 4 cm, when the risk of self-rupture is minimum and the image of a hyperechogenic tumor is typical, it may be observed with the use of ultrasound. As regards the remaining parenchymal lesions, differentiation of the lesion nature between benign, e.g. adenoma or oncocytoma, and malignant nature is impossible. Also in some cases of AML when the tumor is deficient in adipose tissue, it starts to grow, bleed and take the form of a lesion of nonhomogeneous echostructure, the ultrasound image ceases to be typical. The tumor gets similar to adenocarcinoma and it is necessary to perform additional image diagnostics with the use of contrast enhanced computed tomography. The description should contain the exact location of the tumor, the size and relation to the renal sinus. It may also prove helpful to have the information concerning possible lack of bringing out of the renal outline, since in the course of a surgery – including laparoscopic one – it may prove necessary to apply intraoperative ultrasound in order to identify a lesion which is not palpable and not visible (not modeling the renal capsule). In the case of larger lesions, one should control the respiratory motion of the kidney, the condition of the adrenal gland and the presence of enlarged lymph nodes. Additionally, it is necessary to assess the renal vein and inferior vena cava in terms of the presence of a neoplastic plug. Its presence is typical of a clear cell carcinoma. The range of the neoplastic plug is an important piece of information. Especially as regards slim people, it is possible to thoroughly track the location of the neoplastic plug towards the diaphragm and hepatic vessels while inhaling and exhaling. To that end, it is also possible to be supported with transesophageal ultrasound, which makes it possible to select the appropriate surgery type. A reference method in the diagnostics of such lesions is Magnetic Resonance Imaging (MRI)⁽¹⁵⁾. Information on the presence and the size of a neoplastic plug should be highlighted in the description. In the case of a longer delay between the examination and the surgery, one may expect the plug to grow. The final diagnosis should be every time confirmed on the basis of computed tomography or MRI.

It is worth mentioning at this point the ultrasound examination following renal tumor resection. Postoperative healing process resulting from the operative technique may persist for up to 6 months. In the case of using hemostatic agents in the form of a sponge or cloth, one should expect an image resembling an abscess or recrudescence. In the early postoperative period (up to 3 months following the operation), the examination proves thus more helpful in the case of suspecting postoperative bleeding or monitoring the resorption of hematomata rather than to assess cancer recurrence.

There may also be lesions imitating renal tumors, such as the Bertin’s column hypertrophy, bulging left kidney or sinus fat deposit. In the case of any doubts, it is necessary to carry out CT – being a reference method – possibly static scintigraphy.

In conclusion, ultrasound examination is very important in the practice of a urologist. On the basis of renal examinations, their assessment in ultrasound proves useful when the doctor performing the examination understands the needs of a clinician and specialists know the possibilities and limitations of the method.

Conflict of interest

Authors do not report any financial or personal connections with other persons or organizations, which might negatively affect the contents of this publication and/or claim authorship rights to this publication.