Renal and Urinary Ultrasound
Ultrasound is a versatile and inexpensive imaging modality that can be used to characterize acute and chronic renal pathologies. It is considered to be the first-line imaging modality for evaluating azotemic patients to diagnose urinary obstruction. It is effective in detecting renal calculi and vascular pathology. It is also valuable for distinguishing cystic lesions from renal masses. However its complex internal architecture and variable appearance make distinguishing medical causes of renal disease complex. Our goal in this chapter is to use renal POCUS (focused renal ultrasound) to distinguish causes of renal obstruction, calculi and infection.
Relevant Renal Anatomy
The kidneys are retroperitoneal organs with the left kidney situated between the level of the T12-L3 vertebra and usually up to 2cm higher than its right counterpart.
Anatomic structure of the kidney and relevant structures that can be seen on ultrasound. 1. Renal capsule; 2. Nephron within the cortex layer; 3. Renal pyramid; 4. Renal papilla; 5. Minor calix; 6. Major calix; 7. Renal sinus; 8. Renal pelvis; 9. Ureter. Image modified from P Jaworksi CC BY-SA 3.0
Equipment and Technique:
The curvilinear probe (2-5MHz) allows the sonographer to visualize deeper and is the optimal choice of probe to use to visualize the kidneys.
The exam is typically performed in the supine position but due to the kidney's posterior lie in the abdomen, lateral decubitus or even prone may be necessary to optimally visualize the organ. Scanning should occur in a longitudinal plane (or long axis ) and then on its short axis. Deep inspiration causes to diaphragm to displace these structures and move them away from the acoustic shadows cast by the rib cage. Subcostal views can also be used but will not be used as part of the focused renal exam.
Renal artery evaluation has a high failure rate (up to 30%) on expert hands. Intra-renal studies are easier but interpretation of the results are variable which is why Doppler ultrasound is not included as part of the focused renal ultrasound examination.
We will be using two windows to interrogate the kidney in B-mode ultrasound. Remember that the evaluation is both in long and short axis. Lets first take a look at the technique before going into the the structures that can be visualized on examination.
Have the patient lie supine with the probe positioned at the mid axillary line. Have the probe marker pointing towards the patient's head. This maneuver should generate a long axis view of the kidney. To improve visualization you might have to ask the patient to take a deep breath to displace the kidneys down to avoid the acoustic shadow generated by the ribs. Rotate the probe 90 degrees from here to get a short axis view of the kidney.
The examination starts at the mid axillary line to evaluate the long axis of the kidney. Probe indicator pointing up as the probe is moved posteriorly.
The left kidney is located more posterior and into the patients back.
The left kidney is examined more posteriorly than that of the right kidney. The lateral decubitus position may be better to visualize the left kidney.
Bladder: ureteral jets
Ureteric jets represent the visualization of normal physiological periodic efflux of urine from the distal end of each ureter into the bladder. As the urine reaches the vesicoureteric junction, it is forced into the bladder after a smooth muscle contraction of the ureter and under ideal conditions, contract twice or more per minute. These jets can be visualized with the use of Color Flow Doppler (CFD) and are seen as a sudden burst of color in the bladder lasting a few seconds .
Ureteral jets. These are seen as shorts burst of color of short duration at the vesicoureteral junction.
Closer look: sono anatomy of the kidney
The kidney is enveloped in an echogenic capsule which makes this structure easy to identify on ultrasound. The cortex appears isoechoic its echogenicity is very similar to that of the liver or spleen. The medullary pyramids are triangular hypoechoic structures that appear darker than the cortex and the pyramidal shape is not easily visualized on ultrasound. Between the pyramids, the cortex extends in the form of columns (of Bertin). The collecting system is hypoechoic not well visualized unless there is distention. Renal sinus fat is echogenic and occupies a significant portion of the inner kidney.
Anatomic structure of the kidney and its appearance with B mode ultrasound. 1, renal cortex; 2, pyramids, 3, collecting system (minor and major calices and renal pelvis). On ultrasound the cortex has the same echogenic appearance as the liver, the pyramids (renal medulla) is hypoechoic but not anechoic and the renal fat is hyperechoic. You can pause the clip on the right. The collecting system is not well visualized under normal conditions and in fact you cannot distinguish it on this clip. Image modified from P Jaworksi CC BY-SA 3.0
Renal Doppler imaging is via direct visualization of renal arteries/veins. A precise interrogation of the vascular structure of the kidney is beyond the scope of a focused kidney evaluation. However a simple scan can assist in determining if patient with features of hydronephrosis actually has prominent vascular arteries or veins.
Perfusion of the kidney. Depiction of the renal vasculature on the left and on the right a Color Flow Doppler (CFD) window w a low nyquist level to evaluate for both arterial and venous blood. CFD can determine if the ultrasound image seen corresponds to hydronephrosis vs prominent renal vasculature. Image modified from P Jaworksi CC BY-SA 3.0.
The following section displays a range of pathologies that can be seen with performing a focused renal ultrasound examination:
Ultrasound (US) is insensitive to acute pyelonephritis as an unremarkable renal ultrasound exam is visible in most cases. The American College or Radiology Appropriateness Criteria for acute pyelonephritis does not recommend imaging for uncomplicated cases. In complicated cases it can offer a low-risk rapid acquisition modality. Some features of abnormal US findings include particulate matter or debris in the collecting system, abnormal echogenicity with edematous (hypoechoic) or hyperechoic (hemorrhagic) areas, gas bubbles or reduced cortical vascularity with the use of Power Doppler.
We can use US to evaluate for complications such as:
Emphysematous pyelonephritis is a fulminant necrotizing infection of the renal parenchyma associated with the formation of gas and predominantly affects patients with diabetes. On US, we see pockets of pair with posterior acoustic shadowing de to the presence of gas.
Emphysematous pyelonephritis. This longitudinal scan of the kidney shows multiple large echogenic foci of air extending into the parenchyma. Notice the acoustic shadow cast by this infection. Image courtesy of SM Almoghazy,MD ( Radiopaedia.org, rID: 88419)
Pyonephrosis is the presence of pus in the kidney and its upper collecting system that can be associated with an obstruction. On US we see the presence of a dilated of the pelvicalyceal system coupled with:
Echogenic debris in the collecting system.
Fluid filled levels in the collecting system.
Incomplete shadows of gas in the collecting system.
Pyonephrosis. Air-fluid levels (arrows) and echogenic debris in the collecting system is seen. Image courtesy of A Dixon MD (Radiopaedia.org, rID: 10432 ) and MS Patel, MD (Radiopaedia.org, rID: 16173)
Renal abscess is the collection of fluid that is infected within the kidney parenchyma. This is typically secondary to complication from acute pyelonephritis. Severe vasospasm and inflammation can result in liquefactive necrosis and its formation.
On US, renal abscess presents as a well-defined hypoechoic area within the parenchyma of the kidney (cortex or coticomedullary region).
Renal abscess. There is a well-defined parenchymal collection with internal echoes (notice the upper marker on the image to the left) and no apparent blood flow through the structure on the image to the right. Images courtesy MS Patel, MD (Radiopaedia.org, rID: 48616)
Renal infarct appears as an absence of perfusion of the renal cortex using Color Flow Doppler or Power Doppler.
Renal infarct. Notice the absence of perfusion when with the use of Power Doppler. Image courtesy of G Fananapazir, MD (Abdominal Radiology 43(9):1-9)
Hydronephrosis is the abnormal enlargement and distention of the structures of the kidney caused by obstruction of flow. The kidney keeps producing urine but unable to empty creating a buildup of pressure. The most common causes of hydronephrosis include kidney or bladder stones, benign prostatic hyperplasia, cancer (of the kidneys and those related anatomic structures) and urinary retention.
This is the most common indication to use POCUS. Renal POCUS can detect Hydronephrosis with a sensitivity of 77-90% and a specificity of 71- 96% with a positive predictive value 91% compared to CT scans. On imaging we see predictible progressive dilation of structures that continue to balloon because of the buildup of pressure. The structures closest to the obstruction dilate first which implies that the renal pelvis dilates first, followed by the major calyces. Then come the minor calyces. Finally the renal cortex. These compromised structures are filled with pressurized urine and thus appear anechoic on ultrasound and so we focus on the dark areas of the images and its progression to the cortex.
Mild Hydronephrosis (Grade 1):
The first structures to be compromised are those closest to the obstruction, the renal calyces and pelvis (pelviectasis) appear dilated. There is preservation of the cortico-medulla differentiation. Ureters are also not typically seen unless they are distended (hydroureter). Note that normal patients that have not voided for a prolonged amount of time might have these findings.
Comparison side by side of normal appearing kidney on US vs mild hydronephrosis (image courtesy of thepocusatlas.com). Notice the dilated renal pelvis.
Moderate Hydronephrosis (Grade 2):
We now also have dilation of the calyces (caliectasis) on top of the existing pelviectasis. The renal cortex does not present with thinning. As noted by nephropocus.com imagine a black branching tree.