Radiologic assessment of kidney disease

INTRODUCTION — A large number of radiologic studies are used to evaluate patients with kidney disease. These tests are performed alone or in combination for the detection, diagnosis, and/or the evaluation of multiple conditions. They are most often used to assess urinary tract obstruction, kidney stones, kidney cyst or mass, kidney size, disorders with characteristic radiographic findings, renal vascular diseases, and vesicoureteral reflux (VUR).

The more commonly used imaging studies include:

●Ultrasonography

●Computed tomography (CT)

●Magnetic resonance imaging (MRI)

●Plain film of the abdomen

●Renal arteriography

●Renal venography

●Voiding cystourethrography (VCUG)

●Radionuclide studies

●Retrograde or anterograde pyelography

Because of safety, ease of use, and the information provided, the most commonly used radiographic technique in patients presenting with kidney disease is kidney ultrasonography. Since obstruction is a readily reversible disorder, all patients presenting with kidney failure of unknown etiology should undergo ultrasonography, the modality of choice to detect possible obstructive disease. (See "Clinical manifestations and diagnosis of urinary tract obstruction (UTO) and hydronephrosis".)

The following is a brief introduction to the use of radiologic studies in the evaluation of patients with a variety of suspected or confirmed kidney disorders. Detailed discussions concerning the findings and optimal use of these modalities, as well as other techniques, are presented separately in topics reviewing individual diseases for which these modalities are routinely employed.

RADIOLOGIC STUDIES

B-mode ultrasonography — B-mode ultrasonography is the most appropriate imaging modality for evaluation of chronic kidney disease and should be part of the initial workup. (See "Chronic kidney disease (newly identified): Clinical presentation and diagnostic approach in adults".)

It can detect advanced irreversible parenchymal disease, cystic diseases, and chronic urinary obstruction. It is also the initial imaging study of choice in evaluating unexplained acute kidney injury, specifically to exclude urinary tract obstruction since it allows the patient to avoid the potential allergic and toxic complications of radiocontrast media (image 1 and image 2) [1]. Ultrasound can, in the majority of affected patients, diagnose hydronephrosis and often establish its cause. Since urinary tract obstruction is easily diagnosed and reversible when treated early, ultrasonography should be performed in all patients presenting with kidney failure of unknown etiology. (See "Clinical manifestations and diagnosis of urinary tract obstruction (UTO) and hydronephrosis".)

While ultrasound is useful for detecting proximal obstruction of the ureter (image 3), it is less sensitive for showing the level and cause of obstruction when obstruction is in the lower abdomen or pelvis because the ureter is usually obscured by overlying bowel. One exception is when a stone is impacted at the ureterovesical junction; when this is suspected, a pelvic ultrasound examination may be useful. The bladder should be examined in all cases of hydronephrosis, whether or not the proximal ureter is dilated.

Although less sensitive than computed tomography (CT) in detecting kidney masses, ultrasonography can be useful in differentiating cystic from solid masses that can be indeterminant on CT [2,3]. (See "Simple and complex kidney cysts in adults".)

It is also commonly used to screen for and diagnose polycystic kidney disease and to assess prognosis through measurement of kidney length [4]. (See "Autosomal dominant polycystic kidney disease (ADPKD) in adults: Epidemiology, clinical presentation, and diagnosis", section on 'Diagnosis'.)

Ultrasonography should also be considered in patients with pyelonephritis in certain situations (males, recurrent infections, and incomplete response to antimicrobial therapy) to rule out the presence of obstruction, kidney or perinephric abscesses, and other complications of pyelonephritis (image 4). However, CT is more sensitive and accurate in identifying abscesses. (See "Acute complicated urinary tract infection (including pyelonephritis) in adults and adolescents", section on 'Imaging'.)

Findings on ultrasonography can provide evidence of irreversible kidney disease, based upon kidney size and cortical thickness. Increased echogenicity is a nonspecific finding seen with many diffuse kidney diseases that does not necessarily indicate irreversible disease. In contrast, the combination of increased echogenicity and kidney length <10 cm almost always indicates untreatable disease [5].

In addition, ultrasound is also useful in detecting tubular interstitial disorders that can present as increased medullary echogenicity, nephrocalcinosis, and loss of papillae [6,7]. While it is a reasonable initial imaging study for kidney stones, ultrasonography can miss small stones, and CT should be the initial exam in patients with acute renal colic since the stone is likely to be in the ureter and not visible by ultrasound.

Doppler ultrasonography — Doppler ultrasonography can be used to evaluate renal vascular flow in patients with possible renal artery stenosis, renal vein thrombosis, or kidney infarction. There are two components of this examination: direct interrogation of the renal artery and vein, and indirect assessment through measurement of velocity waveforms in intrarenal arteries (spectral analysis). Spectral analysis is far easier to perform; the tardus-parvus pattern is most specific (96 percent) for renal artery stenosis but has a sensitivity of only 43 percent [8]. CT and magnetic resonance (MR) are more sensitive and are usually required for confirmation. Conversely, direct Doppler of the renal artery is technically challenging [9] and is only reliable in high-volume centers when performed and interpreted by experienced radiologists. The sensitivity and specificity range from 65 to 75 percent and 80 to 90 percent, respectively, with a negative predictive value of 70 to 90 percent [10]. (See "Establishing the diagnosis of renovascular hypertension", section on 'Duplex Doppler ultrasonography'.)

The resistive index is calculated by the following:

A high renal resistive index (>0.7) can be observed in a wide variety of disorders but is dependent primarily upon extrarenal hemodynamics rather than intrarenal factors [11,12]. As a result, the renal resistive index is of no utility in the diagnosis of kidney parenchymal disease. Resistive index is commonly measured in transplanted kidneys but is an insensitive and nonspecific indicator of rejection [13-15]. While the resistive index has prognostic value, this is probably related to systemic factors rather than kidney abnormalities [13].

Computed tomography — CT often provides complementary information to that obtained with kidney ultrasonography. In particular, CT with contrast is used to evaluate complex kidney cysts and possible masses detected by ultrasonography [16] and to visualize the ureters. (See "Simple and complex kidney cysts in adults".)

There are several indications for CT in patients with kidney or urinary tract disorders:

●Noncontrast, low-radiation dose CT scan is the gold standard for the radiologic diagnosis of kidney stone disease (image 5A-B), including the detection of small stones or ureteral stones not detectable by ultrasound or stones not visualized on plain films of the abdomen, and is the appropriate initial imaging test for suspected renal colic. (See "Kidney stones in adults: Diagnosis and acute management of suspected nephrolithiasis".)

●CT (or magnetic resonance imaging [MRI]) is used to evaluate kidney masses that are detected by ultrasonography. This usually requires administration of intravenous contrast, but some masses, specifically angiomyolipomas, can be diagnosed without contrast. (See "Clinical manifestations, evaluation, and staging of renal cell carcinoma", section on 'CT or ultrasonography'.)

●CT is used to confirm, localize, and identify the cause of ureteral obstruction that is suspected, but not visible, by ultrasonography. This is usually performed with intravenous contrast for optimal visualization of the ureters, but the ureters can often be visualized without intravenous contrast.

●Autosomal dominant polycystic kidney disease in the earliest stages can be diagnosed by CT with a higher sensitivity than with kidney ultrasonography, particularly in younger patients. However, ultrasonography is usually the preferred initial screening test, and CT scanning is rarely required. (See "Autosomal dominant polycystic kidney disease (ADPKD) in adults: Epidemiology, clinical presentation, and diagnosis", section on 'Establishing the diagnosis of ADPKD'.)

●CT angiography is the best imaging modality to evaluate the renal vasculature, specifically to diagnose renal artery stenosis or renal vein thrombosis (accuracy greater than 90 percent for renal artery stenosis) [17], thereby obviating the need for invasive angiography. (See "Establishing the diagnosis of renovascular hypertension" and "Renal vein thrombosis in adults", section on 'Evaluation and diagnosis'.)

Consultation with the radiologist is recommended prior to ordering contrast-enhanced CT scans since this can prevent the unnecessary use of radiocontrast agents.

Magnetic resonance imaging — MRI of the kidneys is performed in a variety of clinical settings:

●MR angiography has a role in evaluating patients with suspected renovascular hypertension or renal vein thrombosis (in those patients who cannot undergo CT angiography). While it has lower spatial resolution than CT, the sensitivity for detecting renal artery stenosis is 90 percent or greater and does not necessarily require administration of gadolinium as a contrast agent [18]. Administration of gadolinium during MRI was strongly linked to an often severe disease called nephrogenic systemic fibrosis (NSF) among patients with reduced estimated glomerular filtration rate (eGFR), particularly those requiring dialysis. As a result, the US Food and Drug Administration recommended that gadolinium-based imaging be avoided, if possible, in patients with an eGFR <30 mL/min/1.73 m². However, the risk of NSF varies considerably among the different gadolinium compounds and may be minimal with some, so prior consultation with the radiologist is advised. The use of gadolinium in patients with reduced kidney function is discussed at length elsewhere. (See "Nephrogenic systemic fibrosis/nephrogenic fibrosing dermopathy in advanced kidney disease".)

●Contrast-enhanced MRI angiography has equivalent sensitivities and specificities to CT angiography for the diagnosis of renal vein thrombosis [19]. (See "Renal vein thrombosis in adults", section on 'Evaluation and diagnosis'.)

●MRI is used in the evaluation of kidney masses, including suspected or confirmed kidney malignancy. MRI is especially useful for distinguishing and characterizing complex solid and cystic masses (see "Simple and complex kidney cysts in adults", section on 'Complex kidney cysts'). It may be a useful adjunct when ultrasonography and CT scanning are nondiagnostic and/or radiocontrast media cannot be administered due to allergy or to reduced kidney function. (See "Clinical manifestations, evaluation, and staging of renal cell carcinoma", section on 'MRI' and "Prevention of contrast-associated acute kidney injury related to angiography" and "Prevention of contrast-associated acute kidney injury related to angiography", section on 'Epidemiology'.)

●MRI is also used to measure total kidney volume in patients with autosomal dominant polycystic kidney disease, which is important in determining prognosis and following progression [20].

Abdominal radiography — Abdominal radiography (ie, plain film of the abdomen) has little or no role in patients with suspected kidney disease. Among patients presenting with symptoms suggestive of nephrolithiasis, a plain film of the abdomen can identify calcium-containing, struvite, and cystine stones (image 6) but will miss radiolucent uric acid stones and may miss small radiopaque stones or stones overlying bony structures. As previously mentioned, the diagnostic test of choice when nephrolithiasis is suspected is a noncontrast low-dose CT scan. (See 'Computed tomography' above.)

Renal arteriography — Renal arteriography is used less frequently because of the availability of noninvasive tests such as CT and MR angiography. However, arteriography has the advantage that angioplasty can be performed at the same time. Arteriography remains useful in certain settings, such as the patient with suspected polyarteritis nodosa [21]. Arteriography is often diagnostic in this disorder, demonstrating multiple aneurysms and irregular constrictions in the larger vessels, with occlusion of smaller penetrating arteries (image 7). (See "Clinical manifestations and diagnosis of polyarteritis nodosa in adults", section on 'Arteriography and cross-sectional imaging'.)

Voiding cystourethrography — Voiding cystourethrography (VCUG) is primarily used to establish the presence and severity of vesicoureteral reflux (VUR) (image 8 and image 9). (See "Urinary tract infections in infants older than one month and children less than two years: Acute management, imaging, and prognosis", section on 'Voiding cystourethrogram' and "Clinical presentation, diagnosis, and course of primary vesicoureteral reflux".)

VCUG is also used to diagnosis posterior urethral valves (image 10 and image 11) and to provide information on bladder shape and function in children with bladder dysfunction. (See "Clinical presentation and diagnosis of posterior urethral valves", section on 'Diagnosis' and "Evaluation and diagnosis of bladder dysfunction in children", section on 'Voiding cystourethrogram'.)

Radionuclide studies — Radionuclide studies include renal scans and radionuclide cystography.

Renal scans — Renal scans can provide both functional and anatomic information. Although CT can provide similar information (see 'Computed tomography' above), renal scans are the preferred imaging modality in children and infants because of the reduced radiation exposure compared with CT.

Renal scans using the radioisotope technetium Tc-99m mertiatide (Tc-99mMAG3) assess renal excretory function. It is the study of choice in differentiating between obstructive and nonobstructive hydronephrosis in infants and children and can also identify a difference in function between the two kidneys. This test may be used in adults but is rarely required. (See "Postnatal evaluation and management of hydronephrosis", section on 'Diuretic renography'.)

Static renal scans using the radioisotope Tc-99m succimer (DMSA) provide better visualization than Tc99mMAG3 scans for focal kidney parenchymal abnormalities and for assessment of a difference in kidney function between the two kidneys (image 12). DMSA scans can also be used in children with a febrile urinary tract infection to detect acute pyelonephritis or as a follow-up test to detect focal kidney scarring. (See "Urinary tract infections in infants older than one month and children less than two years: Acute management, imaging, and prognosis", section on 'Kidney scintigraphy' and "Evaluation of congenital anomalies of the kidney and urinary tract (CAKUT)", section on 'Static renal scan'.)

Radionuclide cystogram — Radionuclide cystograms (RNCs) are also used to detect VUR (picture 1). Although VCUG provides greater anatomic detail, there is increased radiation exposure with VCUG compared with RNC. As a result, RNC is often used preferentially for follow-up imaging in patients with VUR. (See "Clinical presentation, diagnosis, and course of primary vesicoureteral reflux", section on 'Diagnosis'.)

Retrograde or anterograde pyelography — Antegrade pyelography has been largely supplanted by ultrasonography and CT and is generally performed only during therapeutic nephrostomy or ureteral stent placement. However, retrograde pyelography may be indicated when findings on CT are uncertain or after trauma to the lower urinary tract. Retrograde studies can also be useful for localizing the site of obstruction when there is insufficient kidney function to excrete intravenous contrast. Advantages of these procedures are avoidance of intravenous contrast and the ability to perform therapeutic procedures.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Chronic kidney disease in adults".)

SUMMARY

●Kidney ultrasonography is the initial test of choice to exclude urinary tract obstruction, thereby avoiding the potential allergic and toxic complications of radiocontrast media. Although less sensitive than computed tomography (CT) for initially detecting a kidney mass, ultrasonography can also be useful in differentiating a simple benign cyst from a more complex cyst or a solid tumor and in detecting complications of pyelonephritis in a patient who has an incomplete response to antimicrobial therapy. In addition, ultrasonography can identify cortical thinning and decreased kidney size, indicative of irreversible kidney disease. (See 'B-mode ultrasonography' above.)

●Doppler kidney ultrasonography can be used to evaluate renal vascular flow in multiple disorders, including renal vein thrombosis, kidney infarction, and renal artery stenosis, but lacks sensitivity and requires a high level of expertise. (See 'Doppler ultrasonography' above.)

●Noncontrast, low-dose CT scanning is the gold standard for the radiologic diagnosis of kidney stone disease, including the detection of stones not visualized on plain films or sonography. In addition, CT is also used to confirm and localize ureteral obstruction and to evaluate and stage kidney tumors. CT angiography is the best method to diagnose renovascular disorders such as renal artery stenosis and renal vein thrombosis. (See 'Computed tomography' above.)

●Magnetic resonance imaging (MRI) of the kidneys is performed in a variety of clinical settings, including suspected renovascular hypertension, renal vein thrombosis, and in the evaluation of kidney masses, including suspected or confirmed kidney malignancy. While administration of gadolinium during MRI has been linked to nephrogenic systemic fibrosis (NSF) among patients with reduced kidney function, this does not appear to be an issue with newer gadolinium compounds. (See 'Magnetic resonance imaging' above.)

●Renal arteriography is used less frequently because of the availability of noninvasive tests such as CT scanning and MRI. However, it remains useful in certain settings and allows for therapeutic procedures such as angioplasty and stent placement. (See 'Renal arteriography' above.)

●Radionuclide scanning is used primarily in children to minimize radiation exposure. It also provides functional data for each kidney. Scanning using the radioisotope Tc-99m succimer (DMSA) is more sensitive than intravenous urography in detecting early vesicoureteral reflux (VUR) and kidney scars and is the preferred test for many pediatric nephrologists and radiologists. (See 'Radionuclide studies' above and 'Voiding cystourethrography' above.)