title: Pseudoenhancement within the local ablation zone of hepatic tumors due to a nonlinear artifact on contrast-enhanced ultrasound.
abstract: OBJECTIVE
  Pseudoenhancement of an avascular region on contrast-enhanced ultrasound often occurs within an echogenic region of a radiofrequency ablation zone due to nonlinear ultrasound propagation through intervening microbubble-perfused tissue. The purpose of this study was to describe the imaging features of this artifact.
  
  
  METHODS
  Twenty-six patients with no tumor recurrence within ablation zones were included. Two radiologists assessed contrast-enhanced ultrasound pseudoenhancement in the arterial (< 30 seconds), portal (30-90 seconds), and late (> 90 seconds) phases. If pseudoenhancement was present, the following information was recorded: the degree, time to first appearance, progression over time, and location. The corresponding gray-scale echogenicity (hypo-, iso-, or hyperechoic) and lesion depth were also noted.
  
  
  RESULTS
  Fourteen lesions (14/26, 54%) showed pseudoenhancement on contrast-enhanced ultrasound. Fourteen (100%) corresponded to the hyperechoic area within the ablation zone on gray-scale ultrasound and were nonmarginal in location. Pseudoenhancement occurred more frequently in deep lesions (> or = 5 cm) than in superficial lesions (< 5 cm) (p = 0.002). Pseudoenhancement was initiated most frequently in the portal phase (9/14, 64%), followed by the arterial phase (4/14, 29%) and late phase (1/14, 7%). Progression in the degree of pseudoenhancement was shown in most cases (12/14, 86%) and no washout was seen.
  
  
  CONCLUSIONS
  Pseudoenhancement is frequently seen within ablation zones on contrast-enhanced ultrasound, particularly in deep echogenic lesions. However, pseudoenhancement follows enhancement of the parenchyma between the transducer and target. This observation is consistent with nonlinear propagation of the ultrasound beam, which increases with bubble concentration. Pseudoenhancement shows relatively late initiation, progression over time, and nonmarginal location; these findings are different from those seen in typical tumor recurrence, which shows early enhancement and washout at the margin of the ablation zone.
authors.affiliation: Department of Medical Imaging, University of Toronto, Toronto General Hospital, 585 University Ave., Toronto, ON M5G 2N2, Canada.
authors.forename: Hojun
authors.initials: H
authors.lastname: Yu
authors.forename: Hyun-Jung
authors.initials: HJ
authors.lastname: Jang
authors.forename: Tae Kyoung
authors.initials: TK
authors.lastname: Kim
authors.forename: Korosh
authors.initials: K
authors.lastname: Khalili
authors.forename: Ross
authors.initials: R
authors.lastname: Williams
authors.forename: Gord
authors.initials: G
authors.lastname: Lueck
authors.forename: John
authors.initials: J
authors.lastname: Hudson
authors.forename: Peter N
authors.initials: PN
authors.lastname: Burns
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D016477:Artifacts; D017115:Catheter Ablation; D003287:Contrast Media / Q000008:administration & dosage; D005260:Female; D005466:Fluorocarbons / Q000008:administration & dosage; D006801:Humans; D007090:Image Interpretation, Computer-Assisted; D008113:Liver Neoplasms / Q000000981:diagnostic imaging* / Q000601:surgery*; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D012307:Risk Factors; D014057:Tomography, X-Ray Computed; D014463:Ultrasonography
chemical_list: D003287:Contrast Media; D005466:Fluorocarbons; C042852:perflutren