Radio-frequency-induced coagulation necrosis in rabbits: immediate detection at US with a synthetic microsphere contrast agent.
OBJECTIVE
To determine whether a synthetic ultrasonographic (US) contrast agent can be used to differentiate coagulation necrosis from untreated tumor immediately after radio-frequency ablative therapy.


METHODS
VX2 (adenocarcinoma) tumors (0.8-1.5-cm diameter) were implanted into 12 rabbits. Gray-scale and color Doppler US were performed with or without intravenous injection of a US contrast agent composed of poly-lactide-co-glycolic acid polymeric (PLGA) microspheres (2-micron diameter) filled with perfluorocarbon gas. Radio frequency was applied to each nodule for 6 minutes at 127 mA +/- 33 (mean +/- SD) (tip temperature, 92 degrees C +/- 2). Repeat US with a second dose of the contrast agent was performed immediately after ablation. In four animals, a third dose was administered 30-120 minutes after ablation. Radiologic-histopathologic correlation was performed and included in vivo staining and studies of mitochondrial function.


RESULTS
Intense contrast agent enhancement was seen throughout the tumor prior to ablation. At gray-scale US, ablation produced hyperechoic foci, which were within 1 mm of the foci identified at histopathologic examination in seven of 12 animals (58%). After the administration of contrast material, foci devoid of previously visualized enhancement, which measured 7.3-15.0 mm, were identified. These were within 1 mm of the size of the foci identified at histopathologic examination in 11 of 12 animals (92%, P < .01). In two animals, enhancement depicted viable tumor, which appeared hyperechoic, on nonenhanced images. On delayed images, hyperechoic areas decreased in size, whereas the nonenhanced region remained unchanged.


CONCLUSIONS
A PLGA microspherical US contrast agent enabled the immediate detection of coagulation necrosis as a region devoid of contrast enhancement after radio-frequency ablation in rabbit hepatic tumors. Therefore, this agent could provide real-time guidance during complex ablative procedures and may provide an efficient technique for postprocedural assessment.
Goldberg
S N
SN
Department of Radiology, Massachusetts General Hospital, Boston, USA. sgoldber@caregroup.harvard.edu
Walovitch
R C
RC
Straub
J A
JA
Shore
M T
MT
Gazelle
G S
GS
D000230:Adenocarcinoma / Q000000981:diagnostic imaging* / Q000473:pathology* / Q000601:surgery; D000818:Animals; D003287:Contrast Media; D004564:Electrocoagulation; D008113:Liver Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology* / Q000601:surgery; D008863:Microspheres; D009336:Necrosis; D011817:Rabbits; D011846:Radio Waves; D014463:Ultrasonography
D003287:Contrast Media