Kidneys, Ureters, and Bladder Imaging
- Author: Paul G Schmitz, MD, DPharm, FACP, CSH; Chief Editor: Caroline R Taylor, MD more...
Various imaging studies are available to evaluate patients with suspected renal or urinary tract disease, including the following:
Plain films of the abdomen
Magnetic resonance imaging
Common uses and limitations of each of these imaging modalities are discussed below.
Plain Films of the Abdomen
Plain films of the abdomen are now rarely used to evaluate kidney and urinary tract disease. If obtained, plain films may reveal radiopaque kidney stones (usually calcium-containing stones but also struvite and cystine). An advantage of the plain film is that it can be performed in pregnant and pediatric patients, since the amount of radiation exposure is limited.
The sensitivity and specificity of plain abdominal films in detecting a stone is low in patients with renal colic and no history of kidney stones. However, plain films can be used for follow-up of stone clearance, growth, or recurrence after operative or conservative treatment of stones.
It is difficult to distinguish vascular calcifications from ureteral calcifications with plain radiography. Owing to its higher sensitivity, CT imaging has replaced plain films for the diagnosis of urolithiasis and nephrolithiasis.
Plain films are not sensitive enough to exclude tumors of the kidney or urothelial tract. This imaging technique does provide general information regarding kidney size and shape.
Plain abdominal films are indicated for the evaluation of radiopaque kidney stones (calcium-containing stones, struvite, cystine).
Renal ultrasonography is invaluable as a screening test for urinary tract dilatation (hydronephrosis), a hallmark of urinary tract obstruction. However, dilation of the urinary tract may also be observed in polyuria and normal pregnancy (uterine enlargement causes partial urinary tract obstruction). Urinary tract dilation may persist indefinitely, even after relief of urinary tract obstruction. Parapelvic cysts may also be mistaken for pelvocaliectasis.
Ultrasonography remains the procedure of choice for evaluation of acquired or hereditary polycystic kidney disease. Renal masses are also readily identified with ultrasonography. Advanced kidney disease is usually accompanied by scarring and thinning of the renal cortex with small kidneys (< 9 cm in longitudinal length). These features are easily characterized with renal ultrasonography.[4, 5]
Although renal ultrasonography was once routinely used to identify kidney stones, noncontrast helical computed tomography has supplanted ultrasonography in the diagnosis of nephrolithiasis.
Color-Doppler ultrasonography is used to measure flow or velocity of blood in the main renal artery. It is primarily used to detect renal vascular occlusive disease. Color-Doppler flow studies in the renal artery are highly operator-dependent.
Indications of renal ultrasonography are as follows:
Evaluation of cystic kidney disease
Diagnosis of hydronephrosis
Measurement of kidney size and echogenicity as part of an evaluation of chronic kidney disease
Detection of renal artery occlusive disease via Doppler images
No exposure to radiation or contrast in pregnancy
Limitations of renal ultrasonography are as follows:
Interpretation is operator-dependent
Large body habitus renders the interpretation difficult
Intravenous pyelography (IVP) was the earliest imaging technique to define the anatomy of the renal and urinary tract using iodinated contrast injection, which is excreted by the kidneys into the collecting system. IVP can be used to detect kidney stones and delineate the level of obstruction in patients with urinary tract obstruction. The acquisition of data is slower than other forms of imaging (eg, CT scanning). In pregnant patients, IVP with limited contrast can be performed if ultrasonography is unrevealing. IVP is an excellent modality to diagnose medullary sponge kidney and papillary necrosis.
Indications of IVP are as follows:
Delineation of the gross anatomy of the renal and urinary tract
Evaluation of medullary sponge kidney and papillary necrosis
Avoid IVP in patients with allergy to iodine contrast dye and in patients with impaired renal function (generally a serum creatinine level >2 mg/dL).
Computed tomography (CT) provides similar information as renal ultrasonography but with additional detail due to high spatial resolution. CT scan is an excellent tool to evaluate masses, traumatic injury to the kidney, stones, and pyelonephritis.
Noncontrast helical CT scanning is the procedure of choice to evaluate kidney stones. CT scanning is also used to differentiate malignant from nonmalignant renal masses. Moreover, CT scanning is essential to evaluate the local spread of a renal malignancy. High-resolution CT angiography is excellent in defining the anatomy of the renal arteries and veins (eg, renal vein thrombosis).
CT scanning is superior to ultrasonography in identifying renal cysts, since it is capable of detecting small cysts (2-3 mm in diameter).
Because of safety and cost, renal ultrasonography is still used to screen for polycystic kidney disease.
With the advent of multidetector CT scanning, CT urography is a feasible option to replace intravenous urography.
Multiphase CT urography has a higher diagnostic yield in evaluating the etiology of hematuria and identifying urothelial tumors than intravenous urography. Some investigators believe it is comparable to cystoscopy and provides complementary data by simultaneously delineating extraurinary disease.
Indications of CT scanning are as follows:
Criterion standard for diagnosing nephrolithiasis
Evaluating kidney masses and staging renal tumors
Evaluating polycystic kidney disease
The primary limitation of CT scanning is the risk of radiation and administration of contrast.
In the precontrast phase, a scan is obtained for baseline calcifications, stones, and space-occupying lesions in the kidney and urinary tract.
Within 70 seconds after injection of contrast, the renal vasculature is identified and renal cell carcinoma can be accurately staged.
In the nephrographic phase (ie, up to 180 seconds after contrast injection), renal masses can be differentiated from simple cysts, as malignant masses will enhance with contrast.
In the excretory phase (5 minutes after contrast injection), the ureter, bladder and pelvicaliceal system is imaged.
Limited CT urography (with an excretory phase only) can be performed to minimize radiation exposure.
Contrast should be avoided if the patient is allergic, has renal failure, or is pregnant.
A multidetector CT scan with 4-64 sections is used.
No preparation is required, but prehydration is typically performed, and the bladder should be empty before the procedure is begun. Intravenous saline administration or administration of 10 mg of intravenous furosemide is often used to increase opacification and distension of the collecting system.
Position of patient
The supine position seems to be satisfactory in most patients, but the prone position improves opacification of the distal urinary tract in some patients. Turning the patient several times prior to the excretory phase is necessary to prevent layering of contrast.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) provides a useful alternative to CT scanning in individuals at risk for toxicity from intravenous contrast. It may also offer an advantage in the evaluation of small renal masses. Magnetic resonance angiography has proven useful in the evaluation of stenosis in the mid and proximal renal arteries.
Recently, progressive systemic fibrosis (nephrogenic systemic fibrosis [NSF]) in patients with kidney failure has been recognized. This disorder has been reported only in patients receiving gadolinium, a contrast agent used to enhance the standard MRI. Although rare, these cases invariably progressed to death. To date, all of these cases have occurred in patients with advanced renal disease. Therefore, MRI with gadolinium contrast is typically avoided in patients with a serum creatinine level exceeding 2 mg/dL (estimated glomerular filtration rate [eGFR] < 30 mL/min), unless deemed urgent, and postexamination dialysis may be indicated in selected cases. Newer contrast agents at very low doses are under investigation as an alternate approach.
Newer modalities, such as magnetic resonance renography, have shown promising results in assessing morphology and function of the kidneys; however, the risk of gadolinium contrast remains a significant concern in patients with renal insufficiency. Several recent studies of non–contrast-enhanced magnetic resonance angiography have revealed excellent sensitivity in detecting renal artery stenosis; however, larger studies are needed before this approach can be recommended routinely in the evaluation of renal artery disease. Magnetic resonance urography is commonly used in children and pregnant women to avoid the risk of ionizing radiation.
Indications of MRI are as follows:
Detailed assessment of the kidney anatomy
Noninvasive assessment of kidney function
Estimation of GFR 
Magnetic resonance renography 
Assessment of congenital anomalies of the kidney, bladder, and urinary tract
T2-weighted magnetic resonance techniques rely on high signal intensity of urine for image contrast. Images can be obtained quickly and in any image plane. The images are appealing when compared to intravenous urography.
The signal-to-noise ratio (SNR) is increased with phased-array surface coils to achieve maximal interpretable resolution. A further increase in SNR is achieved by imaging with higher field strength; however, this also increases susceptibility artifacts from gas-filled bowel, and this technique therefore needs further investigation.
Magnetic resonance urography can be complemented with T2 weighting and excretory images after administration of intravenous gadolinium. Multiple acquisitions of static fluid MRIs can ensure adequate visualization of the entire ureter and assess fixed narrowing or obstruction.
When contrast is used for magnetic resonance urography, T1-weighted images are used to examine the kidney and vasculature. Intravenous furosemide is used to augment visualization of the excretory system. T1-weighted images are obtained to visualize the bladder for tumors before gadolinium reaches the bladder, as masses can be obscured because of heterogeneous enhancement.
The image quality is less robust with an undistended urinary system. Several interventions such as intravenous fluids, diuretics, compression devices, and gadolinium chelate aid in improving the resolution of MRI. Respiratory and ureteral peristaltic movements may interfere with signal acquisition; however, forced breath-holding may improve the image.
A higher degree of patient cooperation and radiologist supervision is required.
MRI is not very sensitive for detecting calcifications, although renal calculi can be inferred from filling defects or ureteral dilatation.
The sensitivity of MRI in detecting urothelial and kidney malignancies is less well known than CT imaging.
Radionuclide scanning has been successfully used to evaluate renal perfusion in various settings, including renal artery stenosis and thrombosis. Although a radionuclide study can provide an assessment of renal tubular function, it is nonspecific and therefore cannot establish a definitive renal diagnosis. Radionuclide cystography is widely used by pediatric nephrologists to detect early vesicoureteral reflux in children.
Differential renal function can be estimated from the uptake and clearance of tracer by each kidney over a specified period; 99mTc dimercaptosuccinic acid [DMSA] is traditionally used.
Blood samples must be obtained frequently after the tracer injection, and, if the GFR is less than 30 mL/min, an additional sample should be obtained at 24 hours. Renal blood flow can be estimated as a fraction of cardiac output depending on the amount of radioactivity in the kidney.
Urinary obstruction can also be identified based on the relative tracer excretion via each kidney.
Renal angiography is the criterion standard for direct visualization of the renal vasculature. It is invaluable in the diagnosis and prognosis of renal artery stenosis and renal vein thrombosis. Renal arteriography may also provide complementary information in the evaluation of a renal mass, especially for mapping before surgery.
Retrograde pyelography is an essential tool for localizing the site of urinary tract obstruction. It may also prove therapeutic (eg, ureteral stents can be placed to relieve an obstruction). It has been supplanted by ultrasonography or CT scanning in most settings. However, it is helpful in patients with a known pelvic malignancy when hydronephrosis is absent owing to ureteral encasement.
Diuretic renography is widely used to discriminate functional versus anatomical obstruction after identification of a dilated upper urinary tract (usually with ultrasonography or CT scanning). Two important functional aspects of kidney function can be assessed: (1) clearance of each kidney and (2) the flow of urine through the urinary tract.
Furosemide is administered with a radiopharmaceutical (usually MAG 3, technetium-99m-mercaptoacetyl triglycine). However, a single-kidney GFR of less than 15 mL/min significantly limits the usefulness of diuretic renography because of diuretic resistance.
Diuretic renography is primarily used to determine whether a dilated urinary tract is secondary to obstructive lesions (eg, tumors) or nonobstructive causes (eg, persistent dilation after relief of a previous obstruction).
The patient should be adequately hydrated to produce 1-3 mL/min of urine; 500 mL of oral hydration is given 30 minutes before the procedure. In some cases, the urine specific gravity is measured to ensure adequate hydration (ideally, the specific gravity should be less than 1.015). The bladder should be emptied before the test.
The dosage should be reduced in children based on body surface area. In infants, the washout interpretation is difficult owing to variable GFR, sodium absorption, renal blood flow, and urine-concentrating ability. Nonetheless, ruling out of obstruction is vital in this setting.
Patients with a single-kidney GFR of less than 15 mL/min will not respond to diuretic administration.
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