IEEE 2002

1
Utilization of the High-Frequency Piezoelectric
Ceramic Hollow Spheres for Exposimetry and Tissue
Ablation
Osama M. Al-Bataineh1, Richard J. Meyer Jr.2,
Robert E. Newnham3 and Nadine Barrie
Smith1,4 1Department of Bioengineering, 2Applied
Research Laboratory, 3Material Research
Institute, 4Graduate Program in Acoustics The
Pennsylvania State University, University Park,
PA 16802, USA
Porcine kidney was used for in vitro
ultrasound thermal ablation experi-ments (Fig.
7). Each hollow sphere transducer was connected
to a signal generator with a contin-uous wave
(CW) signal at the resonance frequency and
amplified 40 dB in magnitude.
An automated exposimetry system (Fig. 4) was used
to calibrate and evaluate the hollow sphere
hydrophones. A high pressure focused acoustic
wave was produced using a hemisphere transducer
(10 cm diameter, 8 cm focal length) driven at 1.7
MHz sinusoidal signal amplified by a RF amplifier
with fixed 40 dB amplification.

• Introduction
• A novel miniature piezoelectric ceramic hollow
  sphere was investigated for potential use as a
  hydrophone to measure high pressure acoustic
  fields and as a minimally invasive interstitial
  ablation device. The hydrophone design using the
  hollow sphere has unique characteristics
  regarding its geometry, dimensions and
  sensitivity, which make it ideal for use in
  exposimetry of high pressure fields. Hollow
  spheres resist high intensity fields by
  redistributing the radial mechanical stresses
  into tangential directions based on the spherical
  geometry, and, at the same time, increase the
  sensitivity of the hydrophone by the radius to
  wall thickness ratio. As a therapy transducer
  device, the hollow sphere is considered suitable
  for interstitial thermal therapy due to its
  omni-directional transmitting characteristics,
  comparable density to human tissue, miniature
  size and high radial mechanical transmitting
  characteristics.

Figure 7 In vitro interstitial ablation device
experiment setup.
III. Results
A. Hollow sphere as a hydrophone
II. Materials and Methods
A. Hollow sphere construction
Fabrication

• Attach a platinum wire to a polystyrene core.
• Build the ceramic shell.
• Fire to vaporize the core.
• Connect the outer electrode.

Figure 4 The computerized exposimetry setup
showing a computer controlling the positioning
system and, at the same time, controlling an
oscilloscope connected via a GPIB interface
device. The underwater experiments were performed
inside the water tank (122 x 51 x 53 cm3) with a
transmitting transducer and the hollow sphere
hydrophone as the receiving transducer. The
hollow sphere hydrophone was connected to the
oscilloscope while the source transducer was
connected to the driving voltage source through a
RF amplifier.
Figure 8 End-of-cable sensitivity of selected
miniature hollow sphere hydrophones plotted as a
function of their driving voltage.

• Hollow sphere hydrophones could withstand four
  times the pressure of the needle type hydrophone.
• Nearly twice the sensitivity of the needle type
  hydro-phone is shown with no pre-amplification
  stages.

Figure 1. Schematic diagram showing the Co-firing
technique steps.
The relative calibration method was used to
calibrate the miniature hollow spheres to be used
as high pressure hydrophones. Experiments (Fig.
5) were performed to calibrate the hollow sphere
hydrophones and to determine the receiving
directionality pattern.
Characteristics

• An example of the omnidirectional receiving
  pattern.
• Repeated experimental results correlated well.

• Omni-directional
• High sensitivity
• Low overall density
• Miniature size
• Resist high pressures

• Calibrate the spherical transducer using a
  calibrated hydrophone (Precision Acoustics Ltd.,
  Dorchest, UK).
• Use the calibrated focused transducer to
  calibrate the hollow sphere hydrophone as a
  function of source pressure.
• Measure the receiving directionality pattern of
  the hollow spheres.

Figure 2. Picture showing different diameter
hollow spheres.
Figure 9 The receiving directionality pattern
plotted for the miniature hollow sphere
hydrophone 14-13 at 100 mV0p driving voltage.
The mean pressure values were plotted as a
function of rotation angle.
B. Hollow sphere as a hydrophone
B. Hollow sphere as an ablation device
For exposimetry perfor-mance tests, the hollow
sphere hydrophones with coaxial cables were
housed in cylindrical brass tubes which were used
to fix the hydrophones inside the water tank to
the automated positioning system. A poly-urethane
insulation layer was used to insulate the
hydro-phones to decrease water interaction with
the hydro-phones lead zirconate titanate (PZT)
ceramic material and to strengthen the hollow
sphere for underwater experiments.

• Necrosed tissue volume increased as a function of
  ultrasound exposure time.
• The localized contro-llable heating of the hollow
  sphere trans-ducer can achieve symmetrical damage
  to the targeted volume.

Figure 9 Tissue necrosis using the miniature
hollow sphere transducer 14-42 while increasing
the exposure time from 5 to 20 seconds.
Figure 5 Schematic diagram of the three
exposimetry experiments used to perform the (a)
calibration of the source transducer, (b)
calibration of the hollow sphere hydrophone using
the relative calibration method and (c)
measurement of the receiving directionality
pattern while rotating the hollow sphere
hydrophone around its axis.
Table 2 Lesion diameters as a function of
sonication time.
Figure 3. Schematic diagram (above) and photo
(below) showing the preparation of the miniature
hollow sphere as a hydrophone.
IV. Conclusions
C. Hollow sphere as an ablation device

• Hollow sphere hydrophones were found to withstand
  high-pressure focused ultrasound waves while
  maintaining linear sensitivity characteristics.
• An almost omni-directional receiving pattern was
  shown experimentally with high repeatability and
  accuracy for each tested hollow sphere
  hydrophone.
• The miniature hollow sphere transducer worked
  well as a minimally invasive ablation device for
  potential clinical applications.

Selected hollow spheres (Table 1) were used as
an interstitial ablation devices. Each transducer
was surrounded by a bolus of water (Fig. 6) to
serve as a coupling medium between the transducer
surface and the target tissue.
References
Table 1. Physical properties of the miniature
hollow spheres. The indicated () spheres were
used for hydrophone and ablation tests.
1 R. Newnham, J. Zhang, R. Meyer Jr., S. Alkoy,
J. Cochran Jr., and D. Markley, "Processing of
miniature hollow sphere transducers," Integrated
Ferroelectric, vol. 42, pp. 235-243, 2002. 2 C.
J. Diederich and K. Hynynen, Ultrasound
technology for hyperthermia, Ultrasound in Med.
Biol., vol. 25, no. 6, pp. 871-887, 1999. 3
O. M. Al-Bataineh, Biomedical applications of
the miniature hollow sphere transducer, M.S.
thesis, Department of Bioengineering, The
Pennsylvania State University, University Park,
PA, USA, 2002.
Figure 6 Hollow sphere transducer preparation
for tissue ablation experiments.