Dose Delivery Time Reduction Through Optimization of Focal Zone Path, Power and Size
Joshua Coon1, Urvi Vyas2, Allison Payne Ph. D. 4,5, Doug Christensen Ph. D. 2,4, Dennis Parker, Ph. D.3,5, Bob Roemer Ph. D. 2,4
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shorter when they use a combination of a focal zone path
that starts in the middle of the tumor and “stacks”
treatment points in the z-direction, the highest transducer
power that does not violate safety constraints, and a small
and tightly focused focal zone.
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Dose Delivery Time vs. Power Density
Path = Drill MBF; Perfusion = 0.5 kg m-3 s-1
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Introduction: A significant remaining obstacle to
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Figure 2: (left) An X-Z profile p-color graph of the most highly
focused transducer possible, corresponding to the “small” focal
zone size. This focal zone was translated rapidly in the X-Y plane
(right) to create larger effective focal zones with the same total
transducer power.
Results: Figure 3 shows that the Drill MBF and Drill MFB
paths have the shortest treatment times. The largest gains come
from two effects: a) The use of a “stack” in the z-direction which
reduces the amount of time the normal tissue is heated
maximizes the amount of temperature superposition in the
tumor and b) the order of the stack where the path starts in the
middle of the tumor, which preheats the next two focal zones in
each stack more efficiently than the other paths studied. This
experiment was repeated, and the same order of ranking of the
patterns was found for all four focal zone sizes (results not
shown). For each scan pattern, the smallest focal zone was
always the fastest, the second smallest focal zone was the next
fastest, etc.
Figure 4 demonstrates that the Drill MBF and MFB paths
continue to be faster than all other treatment paths studied
across all power densities studied. Also, the ordinal ranking (i.e.
which path was fastest, second fastest, etc.) of the paths
remained constant across all powers studied. Finally, the study,,
confirmed the results of figure 3 (which used the larger tumor)
for the treatment of a smaller tumor
Figure 5 shows that the small focal zone size produces a shorter
treatment time than all other focal zone sizes studied. Also, the
ordinal ranking of the focal zones is the same as was observed in
figure 3, with the smallest being the fastest, second smallest the
second fastest, etc. An additional result is that there is a
monotonic decrease in treatment time for all focal zone sizes
with increasing power. The treatment times are faster for the
small focal zones because the nonlinear gains from faster heating
and higher focal zone temperatures override the gains from the
larger area per pulse gain of large focal zones.
Figure 6 and figure 7 demonstrate that the Drill MBF and MFB
path and small focal zone are faster across all levels of perfusion
examined. The heat transfer losses due to perfusion in the larger
focal zones (due to larger heating area and longer heating times)
produces a larger percent increase in treatment time with
increasing perfusion than in the small focal zones.
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2+ Pulses Required
NT Cooling Required
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Small Focal Zone Power Density x (10 W m )
Figure 5: Dose delivery time vs. small FZ power
density for 4 FZ sizes and the large tumor. The
arrows label the start of powers for which 2+ pulses
(for the medium and large focal zones) and cooling
time (for the two smaller focal zones) is required.
Dose Delivery Time vs. Perfusion
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Power Density =0.66 W/m ; Size= Small FZ
X-Z Raster
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Total Dose Delivery Time (sec)
the widespread clinical use of HIFU is long treatment times
for large tumors. To minimize dose delivery time, several
user controlled parameters, including the size of the focal
zone and the path it follows, can be optimized. Most clinical
HIFU treatments to date have used a focal zone path that
treats the tumor in a back-to-front raster fashion because
several researchers (Zederic 2003), (Keshavarzi 2001),
(Damianou 1997), (Clarke 2000), (Gertner 1997),
(Techavipoo 2003), (Worthington 2001) have measured
increases in tissue attenuation after ablation in ex vivo tissue
and have determined that how much the tissue properties
change after treatment is dependant on the parameters
used during the treatment. For the time optimized
treatments of the current study, using the results from
(Damianou 1997) and (Worthington 2002) for <60 C max
temperature, 1 Mhz beam frequency, 1395 CEM/min, and
300 CEM, an attenuation change of less than than 25%
seems likely – a relatively small change that warrants
investigating if other FZ paths could be used to significantly
reduce treatment times.
Methods: HIFU treatments were simulated in a region
of tissue 4x4x3 cm with a central treatment (i.e. tumor)
volume of 1cm^3 (108 positions) or a subsection of this tumor
of 0.4cm^3 (45 positions). All physical tissue properties and
perfusion values were constant and homogeneous. A 43C
normal tissue temperature limit and 37C boundary condition
were used.
The SAR distribution was modeled using the HAS method
(Vyas 2008) based on a 256 random element phased array
transducer (13cm radius of curvature, 1Mhz frequency,
2x13mm focal spot at tightest focus) currently used by our
research group. Tissue heating inside the region was modeled
using the Pennes (Pennes 1948) model and tissue damage
modeled using thermal dose (Dewey and Sapareto 1984).
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Dose Delivery Time (sec)
Abstract: HIFU treatments can be made significantly
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Perfusion kg/(m s)
Figure 6: Dose delivery time versus homogenous
perfusion level for 4 paths for the small tumor.
Dose Delivery Time vs. Perfusion
Path = Drill
Path = Drill MBF; Power = 0.6 x 107 W m-3
MBF; Power = 0.6 x 107 W m-3
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Dose Delivery Time vs. Scan Path
Focal Zone Size= Small; Power = 0.66 x 107 W m-3; Perfusion = 0.5 kg m-3 s-1
Heating and Cooling Time Optimization: The
heating and cooling times at each FZ position in the
treatment path were optimized using a previously
developed (Payne 2008) routine to deliver at least 240 CEM
at each location in the minimum possible time such that
the temperature of the normal tissue always remained
below 43C. Treatments were delivered using one optimized
heating pulse at each FZ location and optimized cooling
times between pulses. Past dose from previous focal zone
heating was accounted for when heating a FZ position;
future dose was not.
NT Cooling Required
Domino
Dose Delivery Time (sec)
Figure 1: A schematic of the treatment region projected
onto the x-z plane. All distances are measured from the
transducer face.
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Figure 7: Dose delivery time vs. homogeneous
perfusion level for three FZ sizes. For the large tumor.
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MBF
Drill
MFB
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FBM
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BMF
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Focal Zone Path
Figure 3: Dose delivery time vs. FZ scanning path for
the small FZ and large tumor. Percentages are the
percentage of dose delivery time spent cooling the
normal tissue. Treatments with larger focal zone sizes
(results not shown) were also performed for all
cases, and the treatment times were always longer
than for the small focal zone.
Path: The large tumor was treated by ablating 36 points
References:
Focal Zone Size = Small; Perfusion = 0.5 (kg m-3 s-1)
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Future simulations will focus on
confirming the principles discovered in this initial study for
more complicated and patient specific tissue models with
inhomogeneous/changing physical tissue properties.
Future experimental verification will focus on verifying
these results in phantoms and in vivo.
generously supported by NIH Grant 1R01CA134599-01, the
Ben and Iris Margolis Foundation and the University of
Utah Center for High Performance Computing (CHPC)
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Future Work:
Acknowledgements: This work was
Dose Delivery Time vs. Power Density
Dose Delivery Time (sec)
in each of three treatment planes for a total of 108
treatment points. The small tumor, used for the treatments
in figures 4 & 6, consisted of a subsection of the large
tumor where 15 points in each of the three treatment
planes were ablated.
Paths fall into three general categories: a) Drill focal zones
treated in a “stack” in the z-direction (raster in x-y)
following 1 of 6 possible orderings for the three treatment
planes; b) Raster focal zones treated in one x-y plane in a
“typewriter” fashion before moving to the next plane; c)
Heuristic Focal Zones treated by using “knight jumps” or
“longest distance” between focal zones. Unless otherwise
noted, a raster pattern was used in the X-Y direction for all
of these paths.
Power: The power density of the hottest voxel in the
tumor was varied between values of approximately 0.31 to
1.18 x10^7 W/m^3 .
Size: Four different focal zone sizes in the x-y plane were
created by rapidly switching the tightly focused (small) focal
zone (figure 2) between adjacent treatment positions
(total power from the transducer kept constant) to create a
larger effective focal zone.
Perfusion: All focal zone sizes and paths were simulated
for homogeneous, constant perfusion values ranging from
0.5 to 10 kg/(m^3 s).
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Power Density x 10 (W m )
Figure 4: Dose delivery time versus small FZ power
density for four FZ paths and the small tumor. Other
paths were also studied, but were excluded for
clarity of presentation.
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1University
of Utah, Department of Physics
of Utah, Department of Bioengineering
3University of Utah, Department of Radiology
4University of Utah, Department of Mechanical Engineering
5Utah Center for Advanced Imaging Research (UCAIR)
2University