Ultrasound technique

The kidneys are best evaluated with ultrasound by first obtaining an LS of the kidney, including the upper and lower poles, with the kidney roughly perpendicular to the beam. This may necessitate using various angles and patient positions, and it may be necessary to scan from the posterior aspect. Finding the maximum length at the start of the procedure, by twisting the probe to include both poles, enables the operator to establish the lie of the kidney and provides a good starting point from which to fully and carefully evaluate the organ. In this way the operator is less likely to miss pathology or underestimate renal length.

The right kidney is readily demonstrated through the right lobe of the liver. Generally a subcostal approach displays the (more anterior) lower pole to best effect, while an intercostal approach is best for demonstrating the upper pole (Fig. 7.1). The left kidney (LK) is not usually demonstrable in a true sagittal plane because it lies posterior to the stomach and splenic flexure. The spleen can be used as an acoustic window to the upper pole by scanning coronally, from the patient’s left side, with the patient supine or decubitus (left side raised) but, unless the spleen is enlarged, the lower pole must usually be imaged from the left side posteriorly. Coronal sections of both kidneys are particularly useful as they display the renal pelvicalyceal system and its relationship to the renal hilum (Fig. 7.1C). This section demonstrates the main blood vessels and ureter (if dilated). As with any other organ, the kidneys must be examined in both longitudinal and transverse (axial) planes and the operator must be flexible in his/her approach to obtain the necessary results.

Fig. 7.1 • (A) Sagittal section through the normal RK, using the liver as an acoustic window. The central echoes from the renal sinus are hyperechoic due to the fat content. The hypoechoic, triangular, medullary pyramids are demonstrated in a regular arrangement around the sinus. The cortex is of similar echogenicity to the liver.

(B) Transverse section through the hilum of the RK. Colour Doppler shows flow in the main renal vein and artery.

(C) LK in coronal section. The renal hilum is seen furthest from the transducer. (Compare this with the sagittal section of the RK in which cortex is seen all the way around the PCS.)

(D) Pyramids (p) are arranged regularly around the pelvicalyceal system. The renal cortical thickness is represented by the white line.

The bladder should be filled and examined to complete the renal tract scan. An excessively full bladder may cause mild dilatation of the pelvicalyceal system, which will return to normal following micturition.

Normal ultrasound appearances of the kidneys

The cortex of the normal kidney is slightly hypoechoic when compared to the adjacent liver parenchyma, although this is age dependent. In young people it may be of similar echogenicity and in the elderly it is not unusual for it to be comparatively hyperechoic and thinner. The medullary pyramids are seen as regularly spaced, hypoechoic (not echo-free) triangular structures between the cortex and the renal sinus (Fig. 7.1). The tiny reflective structures often seen at the margins of the pyramids are echoes from the arcuate arteries which branch around the pyramids.

The renal sinus containing the pelvicalyceal system is hyperechoic due to sinus fat which surrounds the vessels. The main artery and vein can be readily demonstrated at the renal hilum and should not be confused with a mild degree of pelvicalyceal dilatation. Colour Doppler can help differentiate.

The kidney develops in the fetus from a number of lobes that fuse together. Occasionally the traces of these lobes can be seen on the surface of the kidney, forming fetal lobulations (Fig. 7.2A); these may persist into adulthood. The issue for the sonographer is being able to recognise these as normal variations, as distinct from a renal mass, or renal scarring.

Fig. 7.2 • (A) RK demonstrating fetal lobulations (arrows).

(B) TS through the base of the bladder demonstrating a left ureteric jet.

(C) An enlarged prostate indents the bladder. This may be associated with incomplete emptying and/or retention. A post-micturition scan is useful in estimating residual volume.

Normal ultrasound appearances of the lower renal tract

When the bladder is distended with urine, the walls are thin, regular and hyperechoic. The walls may appear thickened or trabeculated if the bladder is insufficiently distended, making it difficult to exclude a bladder lesion. The ureteric orifices can be demonstrated in a transverse section at the bladder base. Ureteric jets can easily be demonstrated with colour Doppler at this point and normally occur between 1.5 and 12.4 times per minute (a mean of 5.4 jets per minute) from each side (Fig. 7.2B).¹

It is useful to examine the pelvis for other masses, e.g. related to the uterus or ovaries, which could exert pressure on the ureters causing proximal dilatation. The prostate is demonstrated transabdominally by angling caudally through the full bladder (Fig 7.2C). The investigation of choice for the prostate is transrectal ultrasound, however, an approximate idea of its size can be gained from transabdominal scanning.

When prostatic hypertrophy is suspected, it is useful to perform a post-micturition bladder volume measurement to determine the residual volume of urine (see next section).

Measurements

The normal adult kidney measures between 9 cm and 12 cm in length. A renal length outside the normal range may be an indication of a pathological process and measurements should therefore form part of the protocol of renal scanning. Technique is extremely important if the renal length is not to be underestimated (see above.) Obtaining the maximum renal length may involve an intercostal scan with rib shadowing over the central portion of the kidney, or may necessitate scanning from the posterior of the patient. A subcostal section, which foreshortens the kidney, often underestimates the length and it is more accurate to measure a coronal or posterior longitudinal section with the beam perpendicular to the renal axis.

The cortical thickness of the kidney is generally taken as the distance between the capsule and the margin of the medullary pyramid (Fig. 7.1D). This varies between individuals and within individual kidneys, and tends to decrease with age.

The bladder volume can be estimated for most purposes by taking the product of three perpendicular measurements and multiplying by 0.56:

Haemodynamics

The vascular tree of the kidney can be effectively demonstrated with colour Doppler (Fig. 7.3). By manipulating the system sensitivity and using a low PRF, small vessels can be demonstrated at the periphery of the kidney.

Fig. 7.3 • (A) Colour Doppler of the RK demonstrating normal intrarenal perfusion throughout the kidney.

(B) Power Doppler demonstrating renal perfusion.

(C) The waveform from the main renal artery at the hilum of the kidney is of low resistance with good EDF. The spectrum from the adjacent vein can be seen below the baseline.

(Di) Coronal section through the aorta taken from the patient’s right side showing the origin of the left renal artery (LRA).

(Dii) The LRA appears in blue as it is flowing away from the transducer.

Demonstration of the extrarenal main artery and vein with colour Doppler is most successful in the coronal or axial section by identifying the renal hilum and tracing the artery back to the aorta or the vein to the IVC. The best Doppler signals – i.e. the highest Doppler shift frequencies – are obtained when the direction of the vessel is parallel to the beam, and taken on suspended respiration. The LRV is readily demonstrated between the SMA and aorta by scanning just below the body of the pancreas in transverse section. The origins of the renal arteries may be seen arising from the aorta in a coronal section (Fig. 7.3D).

The normal adult renal vasculature is of low resistance with a fast, almost vertical systolic upstroke and continuous forward end diastolic flow. Resistance generally increases with age.² The more peripheral arteries are of lower velocity with weaker Doppler signals, and are less pulsatile than the main vessel.

Radioisotope scans

Although the ultrasound scan is invaluable in assessing the morphology of the kidneys, it is not able to assess function. The administration of a radioactive tracer, however, reveals valuable information regarding renal function and an isotope scan may often be performed in addition to ultrasound.

A DTPA (diethylene triaminepenta-acetic acid) scan, in which the isotope is intravenously injected as a bolus, can assess renal perfusion, with further data reflecting renal uptake, excretion and drainage during later images. A DMSA (dimercaptosuccinic acid) scan shows uptake of isotope which is proportional to functioning renal tissue. Relative renal function can be determined between kidneys and localized areas of poor or absent function, such as scars, are clearly demonstrated.

Duplex kidney

This term is used to describe a spectrum of possible appearances, from two separate kidneys with separate collecting systems and duplex ureters, to a more simple division of the pelvicalyceal system at the renal hilum (Fig. 7.4A). The latter is more difficult to recognize on ultrasound, but the two moieties of the pelvicalyceal system are separated by a zone of normal renal cortex which invaginates the kidney – a hypertrophied column of Bertin (see below).

Fig. 7.4 • (A) Duplex kidney showing two separate intrarenal collecting systems (arrows). These drained into a single ureter on intravenous urethrogram (IVU).

(B) TS through the abdomen demonstrating the fused lower poles of the horseshoe kidney (arrow) anterior to the spine.

(C) Coronal section through a horseshoe kidney with the isthmus of the kidney (i) anterior to the aorta and IVC.

(D) MAG3 scan demonstrating a horseshoe kidney with a poorly functioning LK and isthmus. Differential function is 86% on the right and 14% on the left.

(E) TS through the RK demonstrating a baggy extrarenal pelvis. The pelvicalyceal system remains undilated and this should not be confused with hydronephrosis.

(F) Hypertrophied column of Bertin.

(G) Colour Doppler of the hypertrophied column reveals regular, normal interlobar vessels (as opposed to the irregularly distributed, increased flow through a renal cell carcinoma [RCC]).

Duplex kidney is the most common congenital renal abnormality. It may be associated with other anomalies such as reflux, ectopic ureteric orifice or ureterocele, and may predispose the patient to infection or obstruction of the upper moiety or, rarely, the lower moiety.³ The main issue for the sonographer here is that one moiety may be mistaken on ultrasound for the entire kidney, especially if bowel gas overlies part of the kidney, and the operator must ensure that both renal poles are properly demonstrated. A chronically obstructed moiety in an adult patient may masquerade as a renal cyst or as fluid-filled bowel.

The main renal artery and vein may also be duplicated, which can occasionally be identified using colour or power Doppler.

Ectopic kidneys

The kidney normally ascends from the pelvis into the renal fossa during its course of development. During this ‘migration’ it rotates inwards so that the renal hilum faces medially. A failure of this mechanism causes the kidney to fall short of its normal position, remaining in the pelvis, i.e. a pelvic kidney. Usually it lies on the correct side, however, occasionally it can cross to the other side, lying inferior to its normally placed partner – crossed renal ectopia. Frequently it may fuse with the lower pole of the other kidney – crossed fused renal ectopia – resulting in what appears to be a very long, unilateral organ.

Horseshoe kidneys

In the horseshoe kidney, the kidneys lie one on each side of the abdomen but their lower poles are fused by a connecting band of renal tissue, or isthmus, which lies anterior to the aorta and IVC (Fig. 7.4B–D). The kidneys tend to be rotated and lie with their lower poles medially.

It may be difficult to visualize the isthmus due to bowel gas anterior to it, but a horseshoe kidney should always be suspected when the operator is unable to confidently identify the lower poles of the kidneys, or when the lower pole seems unusually anterior and medial.

When the isthmus can be seen, it is important not to confuse it with other abdominal masses, such as lymphadenopathy. CT is occasionally performed because of this and normally clarifies the findings.

Extrarenal pelvis

Not infrequently, the renal pelvis projects outside the kidney, medial to the renal sinus. This is best seen in a transverse section through the renal hilum. It is frequently ‘baggy’ containing anechoic urine, which is prominently demonstrated on the ultrasound scan (Fig. 7.4E). The importance of recognizing the extra-renal pelvis lies in not confusing it with dilatation of the PCS, or with a para-pelvic cyst or collection.

Hypertrophied column of Bertin

The septum of Bertin is an invagination of renal cortex down to the renal sinus. It occurs at the junctions of original fetal lobulations and is present in Duplex systems, (see above) dividing the two moieties. Particularly prominent, hypertrophied columns of Bertin may mimic a renal tumour. It is usually possible to distinguish between the two as the column of Bertin does not affect the renal outline and has the same acoustic characteristics as the adjacent cortex (Fig. 7.4F, G).

Colour or power Doppler are helpful in revealing the normal, regular vascular pattern (as opposed to the chaotic and increased blood flow pattern of malignant renal tumours). If doubt persists, particularly in a symptomatic patient, CEUS reliably differentiates a prominent column of Bertin from tumour.⁴ An isotope scan can also be helpful demonstrating normally functioning renal tissue.

Renal humps

These are areas of renal cortex, which form a bulge in the renal outline. Like the hypertrophied column of Bertin, a hump may mimic a renal mass. Careful scanning can usually solve the dilemma as the cortex remains constant in thickness. The most usual manifestation is the splenic hump on the left kidney, which is a flattening of the upper pole with a lateral prominence just below the margin of the spleen.

Humps are basically a variation in the shape of the kidney rather than an area of hypertrophied tissue.

Ultrasound appearances

Like cysts in any other organ, they must display three basic characteristics – anechoic, a thin, well-defined capsule and exhibit posterior enhancement – if they are to be legitimately called ‘simple’. It can be difficult to appreciate the posterior enhancement if the hyperechoic perirenal fat lies distal to the cyst; scanning from a different angle (Fig. 7.5) is helpful.

Fig. 7.5 • (A) 3 Simple renal cysts with posterior enhancement.

(B) Small renal cyst, Bosniak 2, containing calcification following episodes of infection. This remained stable on follow-up.

(C) Example of a Bosniak 2F cyst with a thin septum and nodule of calcification on the wall. No contrast enhancement was demonstrated and the cyst remained stable.

Haemorrhage or infection can give rise to low-level echoes within a cyst and in some cases the capsule may display calcification.

Whilst a solitary, simple cyst can almost certainly be ignored, cysts with more complex acoustic characteristics may require further investigation, e.g. CT. A calcified wall may be associated with malignancy. Increasingly small renal cysts are incidentally discovered on ultrasound due to improved technology and they are by no means always simple. In 1989 Bosniak⁵ proposed a classification of cysts to be used with CT to differentiate benign from malignant lesions (Fig. 7.5B, C; Table 7.1). This has been used over the years in conjunction with ultrasound findings in order to highlight possible malignancy. Whilst it broadly works, it is by no means a definitive test⁶ and complex renal cysts should normally be monitored or undergo CT to evaluate further.

Table 7.1 Bosniak renal cyst classification for CT⁵

These lesions can now be successfully characterized into the Bosniak classification using CEUS⁷ (see also Fig. 7.10.) CEUS is able to differentiate the vascularized solid components of complex renal masses at least as well as CT, and can also be used to monitor lesions, thereby reducing the radiation dose from CT.

Ultrasound appearances

The disease is always bilateral, causing progressively enlarging kidneys with multiple cysts of various sizes, many having irregular margins (Fig. 7.6). There may be little or no demonstrable normal renal tissue and the kidneys may become so large that they visibly distend the abdomen.

Fig. 7.6 • Autosomal dominant (‘adult’) polycystic disease. Numerous cysts of varying size are seen within in the renal bed. No discernable renal architecture is apparent. A cyst containing debris i.e. haemorrhage (calipers) is present.

APKD predisposes the patient to urinary tract infections, stones and cyst haemorrhage. The liver, spleen and pancreas should also be examined on ultrasound for associated cysts. Although there have been reports of an increased incidence of RCC in APKD, there is no substantial evidence for this and it is generally considered that the risk of RCC is the same as the general population.^8,9

Angiomyolipoma

These are usually solitary, asymptomatic lesions, found incidentally on the scan. They tend to be well-defined and highly reflective, often rounded lesions containing blood vessels, muscle tissue and fat as the name suggests. They are usually asymptomatic, although the larger lesions can haemorrhage, causing haematuria and pain. AMLs are also associated with tuberose sclerosis, when they are often multiple and bilateral (Fig. 7.8). Because the contrast between the hypoechoic renal parenchyma and the hyperechoic AML is so great, very small lesions in the order of a few millimetres can be recognized on ultrasound.

Fig. 7.8 • (A) AML in the RK (calipers).

(B) Multiple, small AMLs in a patient with tuberose sclerosis.

An AML may cause a diagnostic dilemma, particularly in patients presenting with haematuria. They tend to be smaller and more echogenic than RCCs, and sometimes demonstrate shadowing, which is not normally seen in small carcinomas.¹³ When doubt persists, CT is usually able to differentiate in these cases by identifying the fat content of the lesion. However this is not infallible as a small number of AMLs (<5%) do not contain fat and a number of other tumours, such as lipoma, liposarcoma and some RCCs, also may contain fat.¹⁴

Adenoma

The renal adenoma is usually a small, well-defined hyperechoic lesion, similar in appearance to the AML. It is felt that adenomas are frequently early manifestations of renal carcinoma as distinct from a benign lesion^14,15 and the two may be histologically indistinguishable.

Renal adenomas are often found in association with an RCC in the same or contralateral kidney,¹⁶ although these are radiologically indistinguishable from metastases. Because of the controversy surrounding the distinction between adenomas and small RCCs, the management of patients with these masses is uncertain. Most incidentally discovered, small (less than 3 cm) parenchymal renal masses are slow growing and may safely be monitored with CT or ultrasound, particularly in the elderly.¹⁷

There are a number of other benign renal tumours including leiomyoma, haemangioma, fibroma, oncocytoma and lymphangioma. Ultrasound is usually unable to characterize these, and CT may be helpful in evaluating the kidney further.¹⁴

Ultrasound appearances

The RCC is a heterogeneous mass that often enlarges and deforms the shape of the kidney (Fig. 7.9). The mass may contain areas of cystic degeneration and/or calcification. It has a predilection to spread into the ipsilateral renal vein and IVC (see also Chapter 8). The increasing use of ultrasound, and its improved quality has led to an increase in the detection of small tumours, often in asymptomatic patients. Around 50% of all RCCs diagnosed fall into this category.²⁰

Fig. 7.9 • (A) The LK is almost completely replaced by a large renal carcinoma. The ureter also contains tumour thrombus.

(B) CT of the case in (A).

(C) A small RCC (calipers) discovered incidentally during abdominal US.

(D) Colour Doppler of an exophytic RCC reveals disorganized, multidirectional blood flow.

(E) Large RCC completely replacing the right kidney.

Colour Doppler reveals a disorganized and increased blood flow pattern in larger masses with high velocities from the arteriovenous (AV) shunts within the carcinoma. CEUS may demonstrate a variety of contrast uptake patterns, with heterogeneous uptake, or hyper-enhancement in the sinusoidal phase.^4–721 CEUS is also helpful in identifying residual tissue after tumours have been ablated (Fig. 7.10).

Fig. 7.10 • CEUS:

(A) Small RCC (calipers) pre-contrast.

(B) The lesion in (A) is hypervascular and takes up contrast throughout.

(C) Another small RCC post-ablation (arrow) before contrast.

(D) The lesion in (C) is avascular following successful ablation – note the normal contrast uptake in the background kidney.

Smaller renal cell carcinomas can be hyperechoic and may be confused with benign angiomyolipoma. The latter has well-defined borders whilst a renal cell carcinoma is ill-defined: differentiation may not be possible on all occasions – biopsy or interval scan may be required.

Clearly smaller masses have a better prognosis and are likely to be early in stage with no metastases.²⁰ With larger masses, liver, adrenal and lymph node metastases may be demonstrated on ultrasound. CT is used to stage the disease and will also demonstrate if metastases are present in the lungs.