title: T1-weighted MR image contrast around a cryoablation iceball: a phantom study and initial comparison with in vivo findings.
abstract: OBJECTIVE
  To correlate T1-weighted (T1w) magnetic resonance (MR) image contrast around a cryoablation iceball with temperature in a phantom study and to compare this to its in vivo appearance during MR-guided focal cryoablation of prostate cancer.
  
  
  METHODS
  A MR-compatible cryoneedle was inserted into identical gel phantoms (n = 3) on a 1.5 T MR system. Two fiber-optic temperature sensors were placed parallel to the needle. A fast 3D T1w gradient echo (GRE) sequence (TR/TE/FA = 4.81/1.98/6°) was used to monitor iceball progression. Normalized signal intensity (SI) was correlated with temperature. The same T1w sequence was used during MR-guided prostate cryoablation in ten consecutive patients at the authors' institution. In vivo findings were quantitatively compared to the phantom data.
  
  
  RESULTS
  In the phantom study, the cryoablation iceball appeared in the T1w MR images as a sharply delineated signal void. A 2.2 ± 0.2 mm wide hyperintense rim directly surrounded the iceball at cooled but nonfreezing temperatures (<20 °C) in the gel. Normalized SI was maximum at 8.4 ± 2.4 °C, showing a 35.6%-43.0% (mean 40.5%) increase with respect to baseline before cooling. In the clinical procedures, the same image contrast was observed in vivo in all patients. In vivo, width of the hyperintense rim was 1.6 ± 0.6 mm. Normalized SI increases with respect to nontreated prostate ranged 28.4%-55.6% (mean 36.8%). On quantitative analysis, normalized SI changes along a linear region of interest from surrounding tissue onto the iceball center were similar between the patients and phantom setting (root mean square difference 0.06).
  
  
  CONCLUSIONS
  The hyperintense rim around the iceball in fast T1w GRE images corresponded to cooled but nonfreezing temperatures (<20 °C) proximal to the frozen zone. The same image contrast was observed both in a phantom study as well as in vivo in the human prostate during cryotherapy. Potentially, monitoring of this rim could be useful in order to maintain a safe margin from at-risk tissues during MR-guided prostate cryoablation procedures.
authors.affiliation: Department of Radiology and Nuclear Medicine, Radboud University Medical Centre, Nijmegen 6525GA, The Netherlands.
authors.forename: Christiaan G
authors.initials: CG
authors.lastname: Overduin
authors.affiliation: Department of Radiology and Nuclear Medicine, Radboud University Medical Centre, Nijmegen 6525GA, The Netherlands.
authors.forename: Joyce G R
authors.initials: JG
authors.lastname: Bomers
authors.affiliation: Department of Radiology and Nuclear Medicine, Radboud University Medical Centre, Nijmegen 6525GA, The Netherlands.
authors.forename: Sjoerd F M
authors.initials: SF
authors.lastname: Jenniskens
authors.affiliation: Department of Medical Physics, Medisch Spectrum Twente, Enschede 7500KA, The Netherlands.
authors.forename: Maarten F
authors.initials: MF
authors.lastname: Hoes
authors.affiliation: MIRA Institute for Biomedical Engineering and Technical Medicine, University of Twente, Enschede 7500AE, The Netherlands.
authors.forename: Bennie
authors.initials: B
authors.lastname: Ten Haken
authors.affiliation: Department of Radiology and Nuclear Medicine, Radboud University Medical Centre, Nijmegen 6525GA, The Netherlands.
authors.forename: Frank
authors.initials: F
authors.lastname: de Lange
authors.affiliation: Department of Radiology and Nuclear Medicine, Radboud University Medical Centre, Nijmegen 6525GA, The Netherlands and MIRA Institute for Biomedical Engineering and Technical Medicine, University of Twente, Enschede 7500AE, The Netherlands.
authors.forename: Jurgen J
authors.initials: JJ
authors.lastname: Fütterer
authors.affiliation: Department of Radiology and Nuclear Medicine, Radboud University Medical Centre, Nijmegen 6525GA, The Netherlands.
authors.forename: Tom W J
authors.initials: TW
authors.lastname: Scheenen
mesh_terms: D003287:Contrast Media / Q000737:chemistry; D003452:Cryosurgery / Q000295:instrumentation* / Q000379:methods; D017679:Cryotherapy / Q000295:instrumentation* / Q000379:methods; D005336:Fiber Optic Technology; D005782:Gels / Q000737:chemistry; D006801:Humans; D007053:Ice; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D009339:Needles; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D013696:Temperature
chemical_list: D003287:Contrast Media; D005782:Gels; D007053:Ice