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Ultrasonography of the urinary system

6,570 bytes removed, 09:56, 2 August 2019
In cases of decreased renal function, the most relevant assessments in a renal ultrasound are renal sizes, [[echogenicity]] and any signs of [[hydronephrosis]]. Renal enlargement usually indicates [[diabetic nephropathy]], [[focal segmental glomerular sclerosis]] or [[myeloma]]. Renal atrophy suggests longstanding chronic renal disease.<ref name="thorp2004">{{cite journal|title=An Approach To The Evaluation Of An Elevated Serum Creatinine|url=http://ispub.com/IJIM/5/2/10525|author=M Thorp|journal=The Internet Journal of Internal Medicine|year=2004|volume=5|issue=2}}</ref>
 
==Basic screening==
*'''Scan''' through the renal volumes in two projections, looking for focal changes or hydronephrosis.
[[File:Measures of the kidney.jpg|thumb|130px|Figure 1. Measures of the kidney. L = length. P = parenchymal thickness. C = cortical thickness.<ref name=Hansen2015/>]]
*Measure '''renal size''': 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" />
*Measure '''cortical thickness''': 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 1). 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" />
*Evaluate the '''echogenicity''' of the kidneys, comparing to the liver on the right side, and to the spleen on the left side if possible. A hyperechoic renal cortex indicates chronic kidney disease ([[#Chronic kidney disease|see section below]]).
*Have at least a quick glance at the '''bladder''' for excessive distension or obvious pathology.
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File:Ultrasonography of keyhole sign of lower urinary tract obstruction.jpg|Ultrasound of the urinary bladder, showing the "keyhole sign" indicating lower urinary tract obstruction. The main "hole" corresponds to a distended urinary bladder (white arrow) and the slit (black arrow) is a distended proximal urethra.
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==Findings in the normal kidney==
In the longitudinal scan plane, the kidney has the characteristic oval bean-shape. The right kidney is often found more caudally and is slimmer than the left kidney, which may have a so-called dromedary hump due to its proximity to the spleen. The kidney is surrounded by a capsule separating the kidney from the echogenic perirenal fat, which is seen as a thin linear structure.<ref name="Hansen2015" />
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:Doppler ultrasound of systolic velocity (Vs), diastolic velocity (Vd), acceleration time (AoAT), systolic acceleration (Ao Accel) and resistive index (RI) of normal kidney.jpg|thumb|350px|Figure 4. Doppler ultrasound (US) of a normal adult kidney with the estimation of the systolic velocity (Vs), the diastolic velocity (Vd), acceleration time (AoAT), systolic acceleration (Ao Accel) and resistive index (RI). Red and blue colors in the color box represent flow towards and away from the transducer, respectively. The specrogram below the B-mode image shows flow velocity (m/s) against time (s) obtained within the range gate. The small flash icons on the spectrogram represent initiation of the flow measurement.<ref name="Hansen2015" />]]
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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/>
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==Solid renal masses==
[[File:Cortical solid mass of renal cell carcinoma.jpg|thumb|Figure 8. Cortical solid mass, which later was shown to be renal cell carcinoma. Measurement of the solid mass on the US image is illustrated by ‘+’ and a dashed line.<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>}}
==Hydronephrosis==
One of the primary indications for referral to US evaluation of the kidneys is evaluation of the urinary collecting system. Enlargement of the urinary collecting system is usually related to urinary obstruction and can include the pelvis, the calyces and the ureter. [[Hydronephrosis]] is seen as an anechoic fluid-filled interconnected space with enhancement within the renal sinus, and normally, the dilated pelvis can be differentiated from the dilated calyces.<ref name="Hansen2015" /gallery[[File:Ultrasonography of hydronephrosis due to ureteropelvic junction obstruction.jpg|thumb|Figure 13. Hydronephrosis due to ureteropelvic junction obstruction in a pediatric patient.<ref namelink="Hansen2015" />]]Several conditions can result in urinary obstruction. In both adults and children, masses, such as abscesses and tumors, can compress the ureter. In children, hydronephrosis can be caused by ureteropelvic junction obstruction, ectopic inserted ureter, primary megaureter and posterior urethral valve (Figure 13). In the latter, both kidneys will be affected. In adults, hydronephrosis can be caused by urolithiasis, obstructing the outlet of the renal pelvis or the ureter, and compression of the ureter from, e.g., pregnancy and retroperitoneal fibrosis. Urolithiasis is the most common cause Ultrasonography of hydronephrosis in the adult patient and has a prevalence of 10%–15%.<ref name="Hansen2015" /> |[[File:Ultrasonography of dilatation of the ureter due to vesicoureteric reflux.jpg|thumb|Figure 14. Bilateral dilatation of the ureters due to vesicoureteric reflux in a pediatric patient.<ref name="Hansen2015" />]]Under normal conditions, the ureter is not seen with US. However, in, e.g., urinary obstruction and vesicoureteric reflux with dilation of the ureter, the proximal part in continuation with the renal pelvis, as well as the distal part near the ostium can be evaluated (Figure 14).<ref name="Hansen2015" /> The hydronephrosis is typically graded visually and can be divided into five categories going from a slight expansion of the renal pelvis to end-stage hydronephrosis with cortical thinning (Figure 15). The evaluation of hydronephrosis can also include measures of calyces at the level of the neck in the longitudinal scan plane, of the dilated renal pelvis in the transverse scan plane and the cortical thickness, as explained previously (Figure 16 and Figure 17).<ref name="Hansen2015" /> If the fluid in the dilated collecting system has echoes, pyonephrosis should be excluded by clinical exam, blood analysis and, in special cases, puncture or drainage. Hydronephrosis can also be caused by non-obstructive conditions, such as brisk diuresis in patients treated with diuretics, in pregnant women and in children with vesicoureteral reflux.<ref name="Hansen2015" /> <gallery widths="200" heights="200">File:Ultrasonography of end-stage hydronephrosis with cortical thinning.jpg|Figure 15. End-stage hydronephrosis with cortical thinning. Measurement of pelvic dilatation on the US image is illustrated by ‘+’ and a dashed line.<ref name=Hansen2015/>File:Ultrasonography of '''hydronephrosis with dilated anechoic pelvis and calyces, along with cortical atrophy.jpg|Figure 16. Hydronephrosis with dilated anechoic pelvis and calyces, along with cortical atrophy. The width of a calyx is measured on the US image in the longitudinal scan plane, and illustrated by ‘+’ and a dashed line.<ref name=Hansen2015/>File:Ultrasonography of hydronephrosis with dilated anechoic pelvis and calyces, along with cortical atrophy - measurement.jpg|Figure 17. Same patient as in Figure 16 with measurement of the pelvis dilation in the transverse scan plane illustrated on the US image with ‘+’ and a dashed line.<ref name=Hansen2015/>''']]
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Image fusion of ultrasound with a previously recorded dataset of CT or other modalities is rarely used in renal US. Reports on image fusion using CEUS or US combined with CT or MRI in the examination of renal lesions and in difficult US-guided renal interventions have been published (Figure 31). However, no recommendations have been published so far.<ref name="Hansen2015" />
 
[[Elastography]] is a US method to visualize the elasticity of tissue. Preliminary reports on US elastography used on transplanted kidneys to evaluate cortical fibrosis have been published showing promising results (Figure 32).<ref name="Hansen2015" />
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