Introduction
Renal ultrasonography is the most appropriate and beneficial tool in diagnosing renal conditions. A patient with the renal disease will be advised for ultrasonography as the initial part of their investigations. Earlier, the relative reflectivity of the kidneys was compared to that of the liver and spleen to know if it was normal. It was believed that the renal parenchyma had a lesser reflectivity than the adjacent organs. It is evaluated qualitatively and quantitatively, though the qualitative method is the more commonly used method.
How Does an Ultrasonography Work?
To interpret an ultrasonography image, it is essential to know how an ultrasonographic image is captured. Sound waves of high frequency are generated and received by the ultrasound transducer placed on the skin in diagnostic ultrasonography. The sound wave returns are received and recorded on the computer. When the ultrasound machine works in bright mode or B-mode, a grayscale image is produced. The sound waves that return are recorded as bright dots. The brightness of the dots is proportional to the intensity of the sound waves reflected. Hence, echogenicity refers to how dark or how bright the image appears on the grayscale. The brighter the image is, the organ is more echogenic. When the kidney is considered, the echogenicity is compared to the organs near it, the liver or the spleen.
How Does a Normal Kidney Look Like in an Ultrasonography?
The normal parenchyma of the renal cortex is hypoechoic(less bright) or isoechoic ( similar brightness) as the adjacent organs, the liver or the spleen. Usually, an increase in echogenicity can indicate chronic kidney disease. It also shows an association with other kidney conditions like interstitial fibrosis, tubular atrophy, and glomerulosclerosis. An increase in echogenicity can also indicate acute kidney disease.
In normal conditions, structures that reflect the soundest waves appear to be the brightest, or white on a grayscale. The fat and fibrous structures reflect most sound waves and hence are seen brightest on the grayscale. In the case of the kidney, the renal capsule is a thin fibrous tissue and lies close to a layer of fat. This is seen as a bright rim in ultrasound with the adjacent areas appearing darker as they reflect lesser sound waves. The renal parenchyma is isoechoic or has similar echogenicity when compared to the normal spleen or a normal liver, which has intermediate echogenicity. Therefore, the liver should have normal echogenicity to compare the renal parenchyma with it. A fatty liver can be more echogenic, which makes the comparison difficult. The medulla of the kidney may appear a little bit darker but is similar to the cortex in most cases. The echogenicity of both kidneys is usually similar in normal cases. The center of the kidney, which is called the renal sinus, contains a mixture of fibrous and fatty tissue. This makes it appear relatively bright on an ultrasound. Bland fluid, urine, and blood do not transmit sound waves and are hence anechoic. They appear black on the ultrasound. This is the reason why the renal pyramids (which contain urine in the parallel tubules) appear darker.
What Does Increase in Echogenicity of the Kidney Parenchyma Signify?
An increase in the echogenicity of the renal parenchyma is probably due to an increase in the tissues or structures that can reflect the sound waves more, thereby increasing its brightness on an ultrasound image.
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Chronic kidney disease appears more bright in an ultrasonography because fibrous tissue is more echogenic.
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Conditions like acute interstitial nephritis( usually caused by side effects of certain drugs, characterized by an inflammatory infiltrate in the interstitium and can reduce renal functions) and glomerulonephritis (inflammation and damage to the glomeruli, which helps in filtration) can appear bright or hyperechoic due to the presence of inflammatory infiltrates.
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Acute tubular necrosis(ATN) is a condition in which the lack of oxygen or blood supply causes damage to the renal tubules and appears hyperechoic due to the presence of proteinaceous casts.
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In medullary nephrocalcinosis, the medulla appears bright on the ultrasonography due to the calcium deposits within the parenchyma of the renal medulla. ( Calcium deposits are highly echogenic in nature).
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Other diseases that can show hyperechoic renal medulla on ultrasound include sickle cell disease and gout.
Studies show that an increase in echogenicity of the renal parenchyma in ultrasound has 96% specificity for the presence of renal parenchymal disease.
Can an Increase in Echogenicity Help to Distinguish Chronic Kidney Disease and Acute Kidney Injury?
An increase in echogenicity may be present in an acute kidney injury or a chronic kidney disease. But this cannot be used as a reliable standard to diagnose the conditions. However, usually, the echogenicity of the renal parenchyma in chronic kidney diseases is less than that of the liver, in other terms, the renal parenchyma is usually hypoechoic when compared to the normal liver. Another fact to be noted is that diabetic kidney disease,(which can be a cause of chronic kidney disease) shows normal renal echogenicity.
Does Increased Echogenicity Help to Distinguish the Cause of Acute Kidney Injury?
Many studies have been done to check whether the increase in echogenicity can differentiate the various causes of acute kidney injury. Early studies show that ischemic acute tubular necrosis was characterized by a reduced or normal echogenicity, while nephrotoxic acute tubular necrosis was hyperechoic. But later studies disagree with this conclusion and showed that increased echogenicity was evident in some cases of ischemic acute tubular necrosis as well.
Increased echogenicity was observed in many cases of acute renal injury like acute interstitial nephritis, glomerulonephritis, multiple myeloma, nephropathy associated with HIV, and obstruction. A reduced echogenicity may be associated with renal infarcts and renal lymphoma. To conclude, the difference in the ultrasound alone will not be able to help in finding the cause of renal disease.
Conclusion
Ultrasonography is a helpful tool in diagnosing conditions that cause a change in the renal parenchyma. It can be used in adult as well as pediatric patients to assess obstruction, issues with the renal parenchyma, and to an extent to differentiate between acute and chronic renal conditions. Along with a clear idea of the patient's history and pathogenesis, an ultrasonogram can be a very helpful diagnostic tool.
