Case 24 -  Introduction to Pyelonephritis

The kidneys are in the cortical phase of enhancement; the cortex is brighter than the medulla.  The images are not windowed optimally but there remains clear differentiation of the cortex from the medulla.  In the right kidney there are linear areas of hypo-enhancement which radiate from the renal pelvis and extend into the cortex.  This pattern is called a "striated nephrogram".  It is important to notice that the areas of decreased enhancement extend to the cortex.  There is no rim of enhancing tissue peripheral to the striations.

The striated nephrogram is the classic CT appearance of pyelonephritis.  It is caused by edema or pus backed up into the nephrons.  Sometimes there is also perinephric stranding.  Sometimes the abnormal kidney is enlarged (but not in this case).  On a nonconrast scan, an enlarged kidney with surrounding stranding should raise the concern for pyelonephritis.  Notice that there is no hydronephrosis.  Pyelonephritis with hydornephrisis and obstruction is a serious urological matter because the infection cannot drain into the bladder.  This is called pyonephrosis and usually requires the placement of a percutaneous nephrostomy tube.

Striated nephrograms can also be seen in acute obstruction, renal vein thrombosis, ATN, and renal infarct, but is most commonly caused by pyelonephritis.  Renal infarcts also look like radial regions of decreased enhancement.  However, they are usually wedge shaped.  Their lateral margins are linear and there is frequently a thin rim of enhancement along the renal capsule peripheral to the infarct.

Pyelonephritis usually represnts an ascending urinary tract infection.  It is more frequently unilateral than bilateral but can be bilateral.  There are often WBC in the urine and the patient should be systemically ill i.e. fever, white count, etc . . .  Flank pain may not be present.

Also of note in this patient is the liver.  There is a heterogenous, or mottled appearance to the liver. I increased the contrast to make this more apparent.  The pattern of enhancement is non anatomic.  It does not correspond to normal portal branching patterns. Diffusely heterogenous liver enhancement seen in patients with hepatitis, right heart failure, and hepatic vein thrombosis.  The hepatic veins are patent.

There is also periportal edema.  Notice the fluid attenuation along the portal triads.  These lucencies are not dilated bile ducts because they occur on both sides of the portal veins.  Bile ducts are only on one side of the potal vein.  Periportal edema is seen in patients with hepatitis, traumatic liver injury (edema and blood tracks along the portal triads), hepatic vein occlusion, low protein states and right heart failure.

Lastly, there is pericholecystic fluid.    The differential diagnosis for pericholecystic fluid is third spacing from low protein states or portal hypertension, volume overload, cholectystitis, and hepatitis.

I do not know for sure but I think this patient also has hepatitis.  Her presenting complaints was for the pyelonephritis. 

One last note on this case.  Look at the uterus.  This is one of the many ways a normal uterus can look.  The arcuate arteries (arrows) are nicely seen on this scan.

Take home point

• Striated nephrograms are the classic CT appeareance of  pyelonephritis.  Striated nephrograms extend through the cortex. Infarcts are wedged shaped with straight edges.
• Pyelonephritis and renal obstruction requires surgical attention.
• The differential diagnosis for periportal edema is volume recussication, hepatitis, traumatic liver injury (edema and blood tracks along the portal triads), hepatic vein occlusion, low protein states and right heart failure.
• The differential diagnosis for mottled liver enhancement is hepatitis, right heart failure, and hepatic vein thrombosis.