title: High intensity focused ultrasound with large aperture transducers: a MRI based focal point correction for tissue heterogeneity.
abstract: OBJECTIVE
  The risk of undesired tissue damage to thoracic cage, heart, and lung during MR guided HIFU ablations of breast cancer can be greatly reduced if a phased array transducer design with a lateral beam direction is used in combination with a large aperture. The disadvantage is an increased sensitivity to focus aberrations due to tissue heterogeneity. Here, the authors propose to restore the focal coherence by using a matched aperture phase correction, which is based on a noninvasively obtained tissue model.
  
  
  METHODS
  The method combines high resolution MRI with ultrasound wave measurements of different tissue types to determine a phase correction, which compensates focal point aberrations caused by tissue heterogeneity. 3D segmentation of tissue is used to quantify the relative proportion of each tissue type along a line running from the center of each element of the phased array to the target focal point.
  
  
  RESULTS
  For tissue types with a celerity difference of 3%, the proposed method allows to quantify the phase aberration with an accuracy of 6° ± 20° and a correlation factor R(2) = 0.95. Using the refocusing method for a complex heterogeneous phantom resulted in 95% of the maximal pressure, whereas only 70% of the maximal pressure is obtained in absence of any phase correction.
  
  
  CONCLUSIONS
  Since the proposed refocusing algorithm is compatible with a standard interventional preplanning and requires only a minimal amount of processing, it presents a promising approach to compensate for aberration in heterogeneous tissues such as the human breast.
authors.affiliation: Philips Healthcare, Toronto L6C 2S3, Canada. charles.mougenot@philips.com
authors.forename: Charles
authors.initials: C
authors.lastname: Mougenot
authors.forename: Matti
authors.initials: M
authors.lastname: Tillander
authors.forename: Julius
authors.initials: J
authors.lastname: Koskela
authors.forename: Max O
authors.initials: MO
authors.lastname: Köhler
authors.forename: Chrit
authors.initials: C
authors.lastname: Moonen
authors.forename: Mario
authors.initials: M
authors.lastname: Ries
mesh_terms: D000465:Algorithms; D001943:Breast Neoplasms / Q000473:pathology* / Q000601:surgery*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005260:Female; D057086:High-Intensity Focused Ultrasound Ablation / Q000295:instrumentation* / Q000379:methods*; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014159:Transducers