HIFU Treatment Time Reduction Through Focal Zone Size and Spacing Selection
Joshua
Robert Roemer,
2,3,4
PhD
2Utah
3Department
of Physics,
Center for Advance Imaging Research (UCAIR),
of Mechanical Engineering,
4Department of Bioengineering, University of Utah, Salt Lake City, Utah USA
220
x
x
Axial Spacing
Treatment Time (sec)
Purpose: Reduce treatment times in HIFU treatments
through focal zone size and spacing selection.
Introduction: An obstacle to widespread clinical
implementation of HIFU for cancer treatment is the long
treatment times needed for large tumors. Previous
research has shown that treatment times can be greatly
reduced by strategically choosing the focal zone path1,
optimizing pulse heating and interpulse cooling times2,3
and applying higher power densities2,3. This research
expands previous work by looking at strategies for focal
zone size and spacing selection in a simulated tumor.
Simulation Setup: Treatments were simulated for a
1x1x16mm3 tumor in a 7x7x7cm3 tissue region with a
uniform perfusion of 0.5 kg/(m3 s). SAR patterns for the
treatment were obtained by using the Hybrid Angular
Spectrum method4,5 to simulate a 256 element phased
array (Imasonics, Inc.). The tumor center and geometric
focus (13 cm) for the transducer were both located at the
center of the tissue region; the long axis of the tumor was
parallel with the axis of the transducer. All acoustic and
thermal properties were those of water, except for
attenuation, which was that of muscle. SAR patterns
were electronically steered and transducer power was
fixed for all treatments. The focal zone size was
1x1x12mm3 (50% SAR line).
Treatment Time Optimization: Pulse heating times for
each treatment were optimized by using the fmincon
routine in Matlab 2011b., which minimized total heating
time subject to the constraint that all tumor voxels
reached 240 CEM.
Focal Zone Size: The tumor (Figure 1) was treated using
two different strategies: first using two discrete,
sequential pulses (Figure 2) at different axial positions, or
second a “rapid scanning” approach where two pulses
were time shared in a “duty cycle” between two locations
(Figure 3), which is equivalent to using a larger “effective”
focal zone.
Results: Figure 2 shows the treatment time versus focal
zone spacing for the two discrete pulse heating strategy.
The optimal spacing was 10mm, a separation which
provided a small amount of overlap for the 50% SAR lines
and good heating of the distal tumor.
Tumor
200
180
160
140
120
100
80
0
2
4
8
6
10
12
14
Distance between Focal Zones (mm)
Figure 2: Treatment Time vs. Focal Zone Spacing on the tumor
shown in Figure 1.
Transducer
Figure 1: Schematic (not to scale) showing the treatment
setup used in the optimal spacing simulations.
Figure 3 shows treatment time versus duty cycle for the
rapid scanning (larger effective focal zone) using the
optimal 10mm spacing. Rapid scanning always had longer
treatment times than discrete scanning.
Additional studies (results not shown) were also
performed for higher perfusions, lower transducer powers,
and a larger tumor. Optimal axial spacing was perfusion and
power independent for the range of variables studied.
Conclusions: Discrete scanning was always faster than rapid
scanning due to its higher power densities and better use
of nonlinear thermal dose. For discrete scanning, an
optimal axial spacing exists, corresponding to an optimal
overlap of the focal zones’ 50% SAR lines. For rapid
scanning, an optimal axial spacing and duty cycle (biased
towards the distal tumor) also exist.
Future Work: Additional studies will investigate optimal
transverse spacing and further compare rapid and discrete
scanning strategies.
Acknowledgments: This work was partially supported by
grants from NIH (R01-CA134599), Siemens Medical
Solutions, the Focused Ultrasound Foundation, a University
of Utah Synergy Grant, the Ben B. and Iris M. Margolis
Foundation, and the Center for High Performance
Computing at the University of Utah, Urvi Vyas, Allison
Payne, Doug Christensen, Martin Cuma , and Dennis Parker.
300
250
Treatment Time
1Department
1,2
Coon
10mm
200
150
100
Discrete Fastest Time
50
0
10
20
30
40
50
60
70
80
Percent Duty Cycle in Front Position
Figure 3: Treatment Time vs. Duty Cycle for rapid scanning.
Optimized duty cycles with spacings of 6, 8, 12, and 14mm gave
even longer rapid scanning treatment times .
References: 1. J. Coon, A. Payne, R. Roemer, “HIFU Treatment Time Reduction in
Superficial Tumors through Focal Zone Path Selection”, International Journal of
Hyperthermia, Accepted for publication, 2011.
2. Payne, Allison, Urvi Vyas, Adam Blankespoor, Douglas Christensen, and Robert
Roemer. "Minimisation of Hifu Pulse Heating and Interpulse Cooling Times."
International Journal of Hyperthermia 26, no. 2 (2010): 198-208.
3. Roemer, R., and A H Payne. "Minimization of Hifu Dose Delivery Time." In
International Society of Therapeutic Ultrasound. Seoul, Korea, 2007.
4. U. Vyas and D.Christensen. "Ultrasound Beam Propagation Using the Hybrid Angular
Spectrum Method." Paper presented at the Engineering in Medicine and Biology
Society, 2008. EMBS 2008. 30th Annual International Conference of the IEEE, 2008.
5. U. Vyas and D. Christensen. ‘”Ultrasound Beam Simulations in Inhomogeneous Tisse
Geometries Using The Hybrid Angular Spectrum Method”, IEEE Trans. Ultrason.
Ferroelectr. Freq. Control,Provisionally Accepted for Publication, 2011.