Purpose: Contrast medium enhanced harmonic ultrasound is a recently developed ultrasound technique that improves the assessment of renal parenchymal vascularity. We performed a preliminary study to define the ability of contrast enhanced ultrasound with harmonic imaging to detect and localize renal scars in a porcine model of experimentally induced pyelonephritis.
Materials and methods: Vesicoureteral reflux was surgically induced in 10 pigs weighing 10 kg. with the contralateral kidney serving as a control. Approximately 4 weeks after the creation of vesicoureteral reflux cystography confirmed reflux. Infection was induced by placing a suspension of paraffin broth impregnated with Escherichia coli in the bladder of each pig. At 4 to 8 weeks (mean 6.5) after the induction of infection a radiologist blinded to the side of the previous procedure performed conventional gray scale ultrasound, color Doppler ultrasound and contrast enhanced ultrasound with harmonic power imaging. The animals were sacrificed; the kidneys were inspected and tissues from the upper, mid and lower poles were sent for culture and histological examination. Histology results were compared with the findings of the 3 ultrasound modalities.
Results: All 10 pigs survived the surgical procedures with no complications. Post-contrast harmonic ultrasound images were available in 9 pigs (18 renal units). Harmonic ultrasound revealed decreased blood flow in 11 renal units, including the upper pole in 7, lower pole in 3, and upper and lower poles in 2. Renal tissue cultures were positive in 11 renal units (6 refluxing units). Histological findings revealed fibrosis in 10 renal units (8 refluxing units). Harmonic ultrasound findings in these 10 renal units showed 9 with changes consistent with inflammation or scars. Using histological results as the gold standard for renal injury harmonic ultrasound was more sensitive than gray scale ultrasound for detecting renal injury with 90% sensitivity, 75% specificity, 81% positive predictive value and 85% negative predictive value.
Conclusions: Preliminary results indicate that contrast enhanced ultrasound with harmonic imaging is a sensitive method for determining renal scarring without the need for radiation exposure. It may be prove to be a useful adjunct for evaluating children who present with urinary tract infection.