A large number of radiologic tests are performed alone or in combination to assess urinary tract obstruction, kidney stones, renal cyst or mass, kidney size, disorders with characteristic radiographic findings, renal vascular diseases, and vesicoureteral reflux (VUR). This article is a brief introduction to the use of radiologic studies in the evaluation of patients with a variety of suspected or confirmed renal disorders.
The more commonly used imaging studies include:
- Computed tomography (CT)
- Magnetic resonance imaging (MRI)
- Plain film of the abdomen
- Intravenous urogram
- 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 renal disease is renal ultrasonography.
Since obstruction is a readily reversible disorder, all patients presenting with renal failure of unknown etiology should undergo ultrasonography, the modality of choice to detect possible obstructive disease.
The test of choice to exclude urinary tract obstruction since it allows one to avoid the potential allergic and toxic complications of radiocontrast media.
While ultrasound is useful for detecting proximal obstruction of the ureter, it is less sensitive for showing the level and cause of obstruction when obstruction is in the lower abdomen or pelvis, especially when the ureter is obscured by overlying bowel.
Useful in differentiating a simple, benign renal cyst from a more complex cyst or a solid tumor.
Used to screen for and diagnose polycystic kidney disease.
Ultrasonography or CT scanning should also be considered in patients with pyelonep
hritis who continue to have an incomplete response to antimicrobial therapy to rule out the presence of obstruction, renal or perinephric abscesses, and other complications of pyelonephritis.
To assess the presence of irreversible kidney disease, based upon kidney size and cortical thickness. Increased echogenicity is a nonspecific finding seen with many diffuse renal 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.
Useful in diagnosing nephrocalcinosis, which can be caused by various etiologies like medullary cystic disease, renal tubular acidosis, and hyperparathyroidism; however, it cannot be used to reliably exclude the presence of small, non-obstructing stones.
Doppler ultrasonography can be used to evaluate renal vascular flow in multiple disorders.
- Renal artery stenosis
- Renal vein thrombosis
- Renal infarction
However, CT and magnetic resonance (MR) are more sensitive for these conditions and are usually required for confirmation.
Used to obtain the renal resistive index, which is calculated from the following formula:
(Peak systolic velocity - end diastolic velocity) ÷ Peak systolic velocity, the normal renal resistive index is < 0.7.
A high renal resistive index can be observed in a wide variety of disorders and is dependent primarily on extrarenal hemodynamics rather than intrarenal factors, commonly measured in transplanted kidneys although it is an insensitive and nonspecific indicator of rejection.
While the resistive index has prognostic value, this is probably related to systemic factors rather than renal abnormalities.
CT often provides complementary information to that obtained with renal ultrasonography.
In particular, CT with contrast is used to evaluate complex renal cysts and possible masses detected by ultrasonography.
Conversely, renal ultrasonography can often clarify whether indeterminant masses on CT scanning are cystic.
There are several other indications for CT in patients with renal disease:
MAGNETIC RESONANCE IMAGING
- Non-contrast helical CT scan is the gold standard for the radiologic diagnosis of renal stone disease, 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.
- CT is used for confirmation and localization of ureteral obstruction that is suspected, but not visible, by ultrasonography.
- Autosomal dominant polycystic kidney disease can be diagnosed with CT scanning with a higher sensitivity than that obtained with renal ultrasonography, particularly in younger patients.
- CT is used to evaluate and stage renal tumors and to diagnose renal vein thrombosis.
Performed in a variety of clinical settings:
MR angiography has a role in evaluating patients with suspected renovascular hypertensionand, in many patients, has reduced the need for renal angiography.
However, the 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 USFDA 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.
MRI, along with renal venography and CT scanning, are considered gold standards for the diagnosis of renal vein thrombosis.
MRI is used in the evaluation of renal masses, including suspected or confirmed renal cell carcinoma. MRI is especially useful for distinguishing and characterizing complex solid and cystic masses.
PLAIN FILM OF THE ABDOMEN
A plain film of the abdomen is not commonly performed in patients with suspected renal disease.
Among patients presenting with symptoms suggestive of nephrolithiasis, a plain film of the abdomen can identify calcium-containing, struvite, and cystine stones but will miss radiolucent uric acid stones and may miss small radiopaque stones or stones overlying bony structures.
In the past, intravenous urogram (also called intravenous pyelogram [IVP]) was the principal radiologic technique used in evaluating the patient with possible renal disease.
It provides detailed information concerning caliceal anatomy and the size and shape of the kidney and is useful in detecting renal stones.
IVP has high sensitivity and specificity for the detection of renal stone disease and provides data on the degree of obstruction. However, noncontrast-enhanced helical CT scanning is the gold standard for the radiologic diagnosis of renal stone disease.
Renal arteriography is used less frequently because of the availability of noninvasive tests such as CT and MR angiography. However, it remains useful in certain settings, such as the patient with suspected polyarteritis nodosa.
Arteriography is often diagnostic in this disorder, demonstrating multiple aneurysms andirregular constrictions in the larger vessels, with occlusion of smaller penetrating arteries.
Primarily used to establish the presence and severity of vesicoureteral reflux
Diagnose posterior urethral valves and to provide information on bladder shape and function in children with bladder dysfunction.
Renal Scans -
Renal scans can provide both functional and anatomic information.
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.
Used in children with a febrile urinary tract infection to detect acute pyelonephritis or as a follow-up test to detect focal renal scarring.
Retrograde or anterograde pyelography -
Antegrade or retrograde pyelography has been used to diagnose urinary tract obstruction but has largely been supplanted by ultrasonography and CT scanning.
However, pyelography may be indicated when the history is highly suggestive of urinary tract obstruction (eg, unexplained acute renal failure [ARF] with a bland urine sediment in a patient with known pelvic malignancy) and hydronephrosis is absent on ultrasonography and CT scanning due to possible urinary encasement.
Retrograde studies can also be useful for localizing the obstruction when there is insufficient renal function to excrete intravenous contrast.
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