title: Endoluminal ultrasound applicators for MR-guided thermal ablation of pancreatic tumors: Preliminary design and evaluation in a porcine pancreas model.
abstract: OBJECTIVE
  Endoluminal ultrasound may serve as a minimally invasive option for delivering thermal ablation to pancreatic tumors adjacent to the stomach or duodenum. The objective of this study was to explore the basic feasibility of this treatment strategy through the design, characterization, and evaluation of proof-of-concept endoluminal ultrasound applicators capable of placement in the gastrointestinal (GI) lumen for volumetric pancreas ablation under MR guidance.
  
  
  METHODS
  Two variants of the endoluminal applicator, each containing a distinct array of two independently powered transducers (10 × 10 mm 3.2 MHz planar; or 8 × 10 × 20 mm radius of curvature 3.3 MHz curvilinear geometries) at the distal end of a meter long flexible catheter assembly, were designed and fabricated. Transducers and circulatory water flow for acoustic coupling and luminal cooling were contained by a low-profile polyester balloon covering the transducer assembly fixture. Each applicator incorporated miniature spiral MR coils and mechanical features (guiding tips and hinges) to facilitate tracking and insertion through the GI tract under MRI guidance. Acoustic characterization of each device was performed using radiation force balance and hydrophone measurements. Device delivery into the upper GI tract, adjacent to the pancreas, and heating characteristics for treatment of pancreatic tissue were evaluated in MR-guided ex vivo and in vivo porcine experiments. MR guidance was utilized for anatomical target identification, tracking/positioning of the applicator, and MR temperature imaging (MRTI) for PRF-based multislice thermometry, implemented in the real-time RTHawk software environment.
  
  
  RESULTS
  Force balance and hydrophone measurements indicated efficiencies of 48.8% and 47.8% and -3 dB intensity beam-widths of 3.2 and 1.2 mm for the planar and curvilinear transducers, respectively. Ex vivo studies on whole-porcine carcasses revealed capabilities of producing ablative temperature rise (ΔT > 15 °C) contours in pancreatic tissue 4-40 mm long and 4-28 mm wide for the planar transducer applicator (1-13 min sonication duration, ∼4 W/cm(2) applied acoustic intensity). Curvilinear transducers produced more selective heating, with a narrower ΔT > 15 °C contour length and width of up to 1-24 mm and 2-7 mm, respectively (1-7 min sonication duration, ∼4 W/cm(2) applied acoustic intensity). Active tracking of the miniature spiral coils was achieved using a Hadamard encoding tracking sequence, enabling real-time determination of each coil's coordinates and automated prescription of imaging planes for thermometry. In vivo MRTI-guided heating trials in three pigs demonstrated capability of ∼20 °C temperature elevation in pancreatic tissue at 2 cm depths from the applicator, with 5-7 W/cm(2) applied intensity and 6-16 min sonication duration. Dimensions of thermal lesions in the pancreas ranged from 12 to 28 mm, 3 to 10 mm, and 5 to 10 mm in length, width, and depth, respectively, as verified through histological analysis of tissue sections. Multiple-baseline reconstruction and respiratory-gated acquisition were demonstrated to be effective strategies in suppressing motion artifacts for clear evolution of temperature profiles during MRTI in the in vivo studies.
  
  
  CONCLUSIONS
  This study demonstrates the technical feasibility of generating volumetric ablation in pancreatic tissue using endoluminal ultrasound applicators positioned in the stomach lumen. MR guidance facilitates target identification, device tracking/positioning, and treatment monitoring through real-time multislice PRF-based thermometry.
authors.affiliation: Thermal Therapy Research Group, University of California, San Francisco, 2340 Sutter Street, S341, San Francisco, California 94115 and The UC Berkeley - UCSF Graduate Program in Bioengineering, University of California, Berkeley, and University of California, San Francisco, California 94115.
authors.forename: Matthew S
authors.initials: MS
authors.lastname: Adams
authors.affiliation: Thermal Therapy Research Group, University of California, San Francisco, 2340 Sutter Street, S341, San Francisco, California 94115.
authors.forename: Vasant A
authors.initials: VA
authors.lastname: Salgaonkar
authors.affiliation: Department of Radiology, Stanford University, Stanford, California 94305.
authors.forename: Juan
authors.initials: J
authors.lastname: Plata-Camargo
authors.affiliation: Thermal Therapy Research Group, University of California, San Francisco, 2340 Sutter Street, S341, San Francisco, California 94115.
authors.forename: Peter D
authors.initials: PD
authors.lastname: Jones
authors.affiliation: Department of Comparative Medicine, Stanford University, Stanford, California 94305.
authors.forename: Aurea
authors.initials: A
authors.lastname: Pascal-Tenorio
authors.affiliation: The UC Berkeley - UCSF Graduate Program in Bioengineering, University of California, Berkeley, and University of California, San Francisco, California 94115.
authors.forename: Hsin-Yu
authors.initials: HY
authors.lastname: Chen
authors.affiliation: Department of Comparative Medicine, Stanford University, Stanford, California 94305.
authors.forename: Donna M
authors.initials: DM
authors.lastname: Bouley
authors.affiliation: Department of Radiology, Stanford University, Stanford, California 94305.
authors.forename: Graham
authors.initials: G
authors.lastname: Sommer
authors.affiliation: Department of Radiology, Stanford University, Stanford, California 94305.
authors.forename: Kim Butts
authors.initials: KB
authors.lastname: Pauly
authors.affiliation: Thermal Therapy Research Group, University of California, San Francisco, 2340 Sutter Street, S341, San Francisco, California 94115 and The UC Berkeley - UCSF Graduate Program in Bioengineering, University of California, Berkeley, and University of California, San Francisco, California 94115.
authors.forename: Chris J
authors.initials: CJ
authors.lastname: Diederich
mesh_terms: D000818:Animals; D057785:Catheters; D004867:Equipment Design; D005240:Feasibility Studies; D005260:Female; D041981:Gastrointestinal Tract / Q000000981:diagnostic imaging; D057086:High-Intensity Focused Ultrasound Ablation / Q000295:instrumentation* / Q000379:methods; D053783:Magnetic Resonance Imaging, Interventional / Q000379:methods; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging / Q000601:surgery*; D066330:Printing, Three-Dimensional; D012984:Software; D034421:Sus scrofa; D013817:Thermography / Q000379:methods