Pyonephrosis Ultrasound and Computed Tomography Features: A Pictorial Review

Abstract

Urinary tract infections (UTIs) are the most frequent community-acquired and healthcare-associated bacterial infections. UTIs are heterogeneous and range from rather benign, uncomplicated infections to complicated UTIs (cUTIs), pyelonephritis and severe urosepsis, depending mostly on the host response. Ultrasound and computed tomography represent the imaging processes of choice in the diagnosis and staging of the pathology in emergency settings. The aim of this study is to describe the common ultrasound (US) and computed tomography (CT) features of pyonephrosis. US can make the diagnosis, demonstrating echogenic debris, fluid/fluid levels, and air in the collecting system. Although the diagnosis appears to be easily made with US, CT is necessary in non-diagnostic US examinations to confirm the diagnosis, to demonstrate the cause and moreover to stage the pathology, defining extrarenal complications. In emergency settings, US and CT are differently used in the diagnosis and staging of pyonephrosis.

Importance of the hounsfield unit value measured by computed tomography in the differentiation of hydronephrosis and pyonephrosis

“…Patodia et al studied a group of 91 patients with pyonephrosis and 410 patients without pyonephrosis and revealed that there are risk factors that increase the incidence of pyonephrosis including the stone size, the severity of hydronephrosis, the time of presentation, and the function of the kidney [6]. Our results corroborate some of these findings (although not the same points of evaluation in our study), where we found that The two leading causes of pyonephrosis are obstruction and infection; hence any risk factor increasing the incidence of urinary tract infections (UTI) could also lead to pyonephrosis [7]. In our study, we found that patients with pyonephrosis had comorbid conditions (especially diabetes in 38% of cases), and they were more frequently of the female gender compared to those seen in the hydronephrosis group (61.5% vs 38.5%).…”

“…However, differentiation of simple hydronephrosis from pyonephrosis may be difficult on CT as it is mostly diagnosed by the presence of indirect signs. The imaging of obstruction can be performed with NCECT with an accuracy of 97% in the detection of stones within the ureters, although contrast-enhanced CT scans are more suitable to detect infection, in addition to any parenchymal or functional changes [7].…”

The “Black Pattern”, a Simplified Ultrasound Approach to Non-Traumatic Abdominal Emergencies

“…Abdominal fluid in non-traumatic patients may be the expression of inflammatory diseases and infection and can be either free or organized. Free fluid around parenchymas and viscera is the expression of a complicated pathology or an extra-visceral extension of the disease where free fluid can stand next to the affected organ or organize into infected collections [ 25 , 26 ] ( Figure 3 and Figure 5 ).…”

“…The diagnostic capability of US in characterizing fluid components is comparable to MR imaging. US can differentiate transudative, exudative, and complex fluid components such as debris and fluid/fluid levels [ 8 , 25 , 52 ]. The presence of heterogeneous fluids within the vessels, the urinary tract, or the biliary system can rapidly orient the diagnosis ( Table 2 ).…”

“…US findings include hydronephrosis with echogenic material in the collecting system, with a shifting urine debris. This finding has a 100% positive predictive value for suspected pyonephrosis and can thus ascertain the presence of urosepsis [ 25 , 57 , 58 ] ( Figure 10 ).…”