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By Dr. Purushottama Sagireddy, DaVita Nephrologist
Plain radiographs, or “x-rays” as they are commonly known, have been in existence for over 100 years, since William K. Roentgen discovered this technology in 1895. Even though there has been an explosion of technology in the imaging modalities, the age-old radiographs still play a role in the diagnosis of chronic kidney disease (CKD). The different tools available include plain radiographs (x-rays), ultrasound, computerized tomography (CT), magnetic resonance imaging (MRI) and angiography.
Plain Radiographs (X-rays)
Intravenous Urography (IVU)
IVU is used to measure kidney size and shape and in the evaluation of the pelvis and ureters (the tubes leading from the kidneys to the bladder). The major drawback for this test is the use of contrast dyes, which can have serious side effects including renal failure. Although the risk is less with the newer non-ionic contrasts, the risk still exists.
This technique uses contrast dye like the IVU but can provide more information about the blood vessels. The angiogram helps to assess for renal artery stenosis—when the lining of the main artery supplying blood to the kidney narrows or is blocked—like is used in diagnosing coronary heart disease.
Ultrasound is used to evaluate a person’s native kidneys (the ones you were born with) as well as transplanted kidneys. It can measure the size and appearance of the kidneys and detect tumors, congenital anomalies, swelling and blockage of urine flow. A newer technique called Color Doppler is used to assess clots, narrowing, pseudo-aneurysms—in the arteries and veins—of the original and transplanted kidneys. This is the least invasive of all the procedures. It is portable and, above all, does not require radiation. To our knowledge so far, there have been no reports of side effects from the procedure itself.
Computed tomography (CT scan or Cat scan)
This is a digital x-ray modality. A computer is used to reconstruct multiple x-ray data samples. It is best used to detect kidney stones or tumors. It can assess most of the details similar to ultrasound, but this has the risk of radiation exposure and, sometimes, intravenous (IV) contrast dye is used, which itself can cause kidney damage.
Magnetic resonance imaging (MRI)
MRI involves a large and powerful magnet. Hydrogen ions in the body are used to obtain pictures of the body parts. But in regard to the kidney, an MRI gives the same information as a CT scan. In the past it was thought that the advantage was that the contrast material called gadolinium, used in an MRI, had no risk of kidney damage. However, gadolinium has now been associated with nephrogenic systemic fibrosis (NSF), a potentially fatal skin disease in people with decreased kidney function. And, in a small fraction of patients, separate from NSF, gandolinium may decrease glomerular filtration rate (GFR), similar to other contrast dyes. Talk to your physician to find out if an angiogram or CT scan would be a better choice than an MRI.
Now with this knowledge of the types of tests that are available, I would like to discuss some clinical situations and describe which test is better for what particular kidney disease.
Acute renal failure (ARF)
Obstruction of urine flow is one of the causes of acute renal failure. It accounts for about 5 percent of the cases of acute renal failure. But, if the obstruction is identified, then it is very easy to correct. Ultrasound is the best way to assess obstruction as an initial screening method. Although ultrasound is the principal test for evaluating this, a CT scan is advantageous when ultrasound has technical difficulties, like being unable to visualize the kidneys because of a lack of preparation or body build. The role of MRI is very limited, as it does not add any more information than the ultrasound and CT scan.
Chronic kidney disease (CKD)
Ultrasound allows assessment of kidney size, the thickness of the cortex area of the kidney and evaluation of cortical echogenicity (how the sound waves echo off the kidney). A connection has been found between cortical echogenicity and the prevalence of global sclerosis (hardening of the kidney), focal tubular atrophy (wasting away of the tubules in the kidney) and the number of hyaline casts per glomerulus (when proteins in the tubules form impressions of the tubules called “casts” in the filter that cleans the blood). It is also helpful in identifying the cysts (including polycystic kidney disease) in the kidney. The role of CT scan is very limited except it can help to identify the cystic diseases much better than ultrasound. MRI has the same limitations as the CT scan.
Renal vein thrombosis
Contrast (dye) CT and MRI are the best ways of identifying renal vein thrombosis. Angiography is the test of choice, but it is more invasive.
Angiography is the test of choice for the renal hypertension or high blood pressure caused by narrowing of the renal arteries that carry blood to the kidneys. Initial evaluation could be done by the use of ultrasound. The size of the kidneys and, in expert hands, the Doppler sonography of the renal arteries can help in evaluating narrowing of the arteries (stenosis). CT angiography can also help identify the stenosis, but it is limited in use because of the contrast (dye) agent and the risk of kidney damage with it. Magnetic Resonance Angiography (MRA) is fast evolving as a pre-screen for angiography. As the technology develops the MRA might be the way to go for renal artery stenosis as this has no risk of contrast nephropathy and it is non-invasive.
CT scan is the preferred test to detect renal infections as it can help in identifying gas, stones, calcifications (stone-like calcium deposits) within the kidney, bleeding, abscesses and obstruction.
CT scan is the best way to assess for stone disease. Even though IVU may be utilized sometimes, it is not as sensitive as the CT scan.
Ultrasound and CT scan are complimentary in evaluating a renal mass of any kind. These two modalities can be used effectively to identify a cystic lesion, mass or renal cell carcinoma (cancer). So most of the time when there is doubt about a mass by one test the other test is helpful. MRI better identifies the renal vein thrombosis, which occurs as a complication of kidney cancer.
Ultrasound is the most commonly used test to identify most of the complications that are seen with kidney transplant. Sometimes a nuclear medicine scan is used to assess the function of the transplanted kidney.
Dr. Purushottama Sagireddy is a DaVita nephrologist in Stockton, California. He graduated from the Guntur Medical College in Guntur, Andhra Pradesh, India and is board certified in Internal Medicine and Nephrology. Dr. Sagireddy performed his residency in Internal Medicine at the University of Illinois, College of Medicine in Chicago and his fellowship in Nephrology at the University of Texas Health Science Center in San Antonio.
Dr. Sagireddy is the Chief of Staff at Dameron Hospital in Stockton, California and is chairman of the fistula first initiative at the St. Joseph Medical Center. He is a member of the American Society of Nephrology, Renal Physicians Association, San Joaquin County Medical Society and Indian Medical Council.
Dr. Sagireddy is married and has three children. He enjoys tennis, basketball, gardening and current affairs.
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