Changes

The kidney is divided into parenchyma and renal sinus. The renal sinus is hyperechoic and is composed of calyces, the renal pelvis, fat and the major intrarenal vessels. In the normal kidney, the urinary collecting system in the renal sinus is not visible, but it creates a heteroechoic appearance with the interposed fat and vessels. The parenchyma is more hypoechoic and homogenous and is divided into the outermost cortex and the innermost and slightly less echogenic medullary pyramids. Between the pyramids are the cortical infoldings, called columns of Bertin (Figure 12). In the pediatric patient, it is easier to differentiate the hypoechoic medullar pyramids from the more echogenic peripheral zone of the cortex in the parenchyma rim, as well as the columns of Bertin (Figure 2).<ref name="Hansen2015" />[[File:Normal adult kidney.jpg|thumb|Figure 1. Normal adult kidney. Measurement of kidney length on the US image is illustrated by ‘+’ and a dashed line. * Column of Bertin; ** pyramid; *** cortex; **** sinus.<ref name="Hansen2015" />]] The length of the adult kidney is normally 10–12 cm, and the right kidney is often slightly longer than the left kidney. The adult kidney size is variable due to the correlation with body height and age; however, normograms for pediatric kidney size are available.<ref name="Hansen2015" /> Cortical thickness should be estimated from the base of the pyramid and is generally 7–10 mm. If the pyramids are difficult to differentiate, the parenchymal thickness can be measured instead and should be 15–20 mm (Figure 3). The echogenicity of the cortex decreases with age and is less echogenic than or equal to the liver and spleen at the same depth in individuals older than six months. In neonates and children up to six months of age, the cortex is more echogenic than the liver and spleen when compared at the same depth.<ref name="Hansen2015" />

File:Normal pediatric adult kidney.jpg|Figure 2. Normal pediatric adult kidney. Measurement of kidneylength on the US image is illustrated by ‘+’ and a dashed line. * Column of Bertin; ** pyramid; *** cortex; **** sinus.<ref name="Hansen2015" />File:Measures of the Normal pediatric kidney.jpg|Figure 3. Measures of the Normal pediatric kidney. L = length. P = parenchymal thickness. C = cortical thickness* Column of Bertin; ** pyramid; *** cortex; **** sinus.<ref name=Hansen2015/>

[[File:Renal cell carcinoma with both cystic and solid components.jpg{{Main|thumb|Figure 9. Renal cell carcinoma with both cystic and solid components located in the cortex. Measurement of tumor on the US image is illustrated by ‘+’ and a dashed line.<ref name="Hansen2015" />]] A [[solid renal mass]] appears in the US exam with internal echoes, without the well-defined, smooth walls seen in cysts, often with Doppler signal, and is frequently malignant or has a high malignant potential. The most common malignant renal parenchymal tumor is renal cell carcinoma (RCC), which accounts for 86% of the malignancies in the kidney. RCCs are typically isoechoic and peripherally located in the parenchyma, but can be both hypo- and hyper-echoic and are found centrally in medulla or sinus. The lesions can be multifocal and have cystic elements due to necrosis, calcifications and be multifocal (Figure 8 and Figure 9). RCC is associated with von Hippel–Lindau disease, and with tuberous sclerosis, and US has been recommended as a tool for assessment and follow-up of renal masses in these patients.<ref name="Hansen2015" /> However, US is not the primary modality for the evaluation Ultrasonography of solid tumors in the kidney, and CT is the first choice modality. Nevertheless, hemorrhagic cysts can resemble RCC on CT, but they are easily distinguished with Doppler ultrasonography. In RCCs, Doppler US often shows vessels with high velocities caused by neovascularization and arteriovenous shunting. Some RCCs are hypovascular and not distinguishable with Doppler US. Therefore, renal tumors without a Doppler signal, which are not obvious simple cysts on US and CT, should be further investigated with CEUS, as CEUS is more sensitive than both Doppler US and CT for the detection of hypovascular tumors.<ref name="Hansen2015" /> Other malignant tumors in the kidney are transitional cell carcinoma and squamous cell carcinoma, which arise from the urothelium and are found the renal sinus, as well as adenocarcinoma, lymphoma and metastases, which can be found anywhere in the kidney (Figure 10).<ref name="Hansen2015" /> Benign solid tumors of the kidney are oncocytoma and angiomyofibroma. Oncocytoma has a varying ultrasonic appearance, but may have a central scar or calcification as a hallmark. Angiomyofibroma are often found in patients with tuberous sclerosis. They are composed of fat, smooth muscle tissue and vascular elements. The echogenicity is governed by the composition of these elements, but the lesion is often hyperechoic (Figure 11 and Figure 12).<ref name="Hansen2015" /> Benign tumors are difficult to separate from malignant tumors using US. Thus, solid renal masses found on US are difficult to classify and should be further evaluated with CT. In special cases of cystic or solid renal masses, additional US guided biopsy or drainage is performed to identify the histologic tumor type before a decision on surgery is made.<ref name="Hansen2015" /> <gallery widths="200" heights="200">File:Ultrasonography of renal lymphoma.jpg|Figure 10. Solid tumor in the renal sinus seen as a hypoechoic mass, later found to be lymphoma. The ‘1’ and ‘2’ on the US image are reference points used for CT fusion (not shown).<ref name=Hansen2015/>File:Ultrasonography of angiomyolipoma.jpg|Figure 11. [[Angiomyolipoma]] seen as a hyperechoic mass in the upper pole of an adult kidney.<ref name=Hansen2015/>File:Ultrasonography of multiple angiomyolipomas in tuberous sclerosis.jpg|Figure 12. Patient with tuberous sclerosis and multiple angiomyolipomas in the kidney. Measurement of kidney length on the US image is illustrated by ‘+’ and a dashed line.<ref name=Hansen2015/></gallery>}}