Ultrasonography is a routine diagnostic procedure for the detection of focal splenic lesions. However, differentiation between benign and malignant lesions is difficult in many cases using conventional ultrasonography, and clear discrimination criteria are lacking.1 Therefore, histologic or cytologic examination is necessary to confirm the diagnosis of splenic lesions.
Contrast-enhanced ultrasonography is a new diagnostic imaging procedure to investigate tissue vascularity and perfusion dynamics. A major advance of contrast-enhanced ultrasonography is the development of contrast-specific software allowing pulse subtraction imaging, and the development of second-generation contrast agents, which allow low-mechanical index (MI) ultrasonography. First-generation contrast agents have contrast effect only at high MI leading to microbubble destruction.2 Therefore, continuous or high-frame rate observation cannot be performed with first-generation contrast agents. On the other hand, second-generation contrast agents are of diagnostic value even with a very low MI because of their physical behavior during insonation. Second-generation contrast agent perflubutane microbubblesa produce stable contrast effect at low MI without destruction and allow continuous real-time imaging.3 Previous studies showed that contrast-enhanced ultrasonography with second-generation contrast agents could evaluate the tumor perfusion dynamics and improve the differentiation between benign and malignant focal splenic lesions in humans4 and dogs.5–7
In addition to vascular imaging, the second-generation contrast agent perflubutane microbubbles is phagocytized by Kupffer cells,8,9 allowing for long-lasting parenchymal contrast enhancement of the liver.10 Parenchymal imaging improves the diagnostic accuracy to differentiate benign and malignant focal liver lesions in humans3,11,12 and dogs,13,14 given that malignant tumors have little or no reticuloendothelial system and appear as hypoechoic defects.
Perflubutane microbubbles allow for parenchymal imaging of the normal spleen in dogs.15 The purpose of this study was to determine whether analysis of the enhancement pattern after injection of perflubutane microbubbles would allow one to differentiate benign from malignant focal splenic lesions.
Materials and Methods
Twenty-nine focal splenic lesions in 29 dogs were examined. The study population was recruited from dogs with single or multiple focal splenic lesions detected by conventional ultrasonography in the Veterinary Teaching Hospital of the Graduate School of Veterinary Medicine, Hokkaido University. The final diagnosis was confirmed by histology or cytology.
Contrast-enhanced ultrasonography was performed with a ultrasound machineb with a 5–11 MHz broadband linear probec or a 3.75 MHz convex probe.d A single focal zone was placed at the deepest part of the lesion. The MI was set at 0.1–0.2 MI to minimize microbubble destruction. The gain was set so that few signals from the underlying splenic parenchyma were present. The cranial abdomen was shaved and the dogs were restrained in dorsal recumbency. Scan planes were chosen to show both a splenic lesion and normal parenchyma in 1 image. Perflubutane microbubbles (0.12 μL microbubbles/kg) was injected via an intravenous catheter in the cephalic vein. Catheters were flushed with saline (0.9% NaCl) solution immediately after the injection. Real-time imaging was performed from preinjection to 1 minute after injection of perflubutane microbubbles for the vascular phases. Images for parenchymal phase were obtained 7–10 minutes after injection according to our previous study.15 All images were recorded on a hard disk for off-line analysis.
Qualitative assessment of the vessel appearance was performed immediately after injection of perflubutane microbubbles. Vessel appearance in lesion was divided into 3 groups in comparison with the vessel in surrounding normal parenchyma: (1) similar, (2) different, and (3) invisible.
Qualitative assessment of the enhancement pattern was performed in the early vascular phase (5–10 seconds after injection), late vascular phase (25–30 seconds after injection), and parenchymal phase (7–10 minutes after injection). The timing of all 3 phases was defined based on the results of our previous study.15 The contrast enhancement in the surrounding parenchyma was used as an in vivo reference. Enhancement pattern were defined subjectively based on echogenicity of lesion parenchyma in comparison with the surrounding normal parenchyma: (1) hypoechoic, (2) isoechoic, and (3) heteroechoic.
Diagnosis was confirmed in all 29 dogs, by histology in 24 dogs and ultrasound-guided aspiration cytology in 5 dogs. It was ensured by size and location that the lesions imaged were the ones sampled.
The statistical significance of differences between benign and malignant lesions was calculated with the G-test and 2-tailed Fisher's exact test. In addition, the sensitivity and specificity were calculated with 95% confidence intervals (CI). P < .05 was considered significant. Statistical analysis was performed with a standard computer software program.e
Twenty-nine dogs were included in this study. Of the 29 dogs, 13 dogs had benign nodules and 16 dogs had malignant tumors (Table 1).
Clinical data of 29 dogs with splenic lesions.