Janaka Wansapura

Matthew R Myers

Reza Dibaji

Rupak K Banerjee

Seyed Ahmad

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In Vivo Monitoring of HIFU
Induced Temperature Rise in
Porcine Liver Using Magnetic
Resonance Thermometry1
Seyed Ahmad Reza Dibaji
Department of Mechanical and Materials Engineering,
College of Engineering and Applied Science,
University of Cincinnati,
Cincinnati, OH 45221
Janaka Wansapura
Imaging Research Center,
Cincinnati Children’s Hospital Medical Center,
Cincinnati, OH 45221
Matthew R. Myers
Division of Solid and Fluid Mechanics,
Center for Devices and Radiological Health,
U.S. Food and Drug Administration,
Silver Spring, MD 20993
Rupak K. Banerjee
Department of Mechanical and Materials Engineering,
College of Engineering and Applied Science,
University of Cincinnati,
Cincinnati, OH 45221
1 Background
High intensity focused ultrasound (HIFU) is a noninvasive medi-
cal procedure during which a large amount of energy is deposited
in a short duration, which causes sudden localized rise in tissue
temperature, and ultimately, cell necrosis. In the preclinical charac-
terization of thermal fields generated by HIFU systems, the temper-
ature rise in an ex vivo or an in vivo tissue must be accurately
measured. The temperature rise can be measured using thin wire
thermocouples or magnetic resonance (MR) thermometry.
Among the two methods, thermocouples can be embedded
invasively in the animal tissue, and HIFU induced temperature
rise can be measured by focusing the beam on the thermocouple
junction for the desired sonication time. However, the temperature
rise measured using thermocouples is subject to several significant
sources of error. One source of error associated with direct
HIFU sonication of thermocouples is viscous-heating artifact [1].
Positioning errors represent another challenge in measuring tem-
perature by locating beam atop a thermocouple [2]. Consequently,
it has been difficult to accept that the temperature recorded by the
thermocouple is the actual temperature at the focus of the HIFU
beam. Therefore, there is a need for a method that can address
these limitations.
Unlike the measurement of temperature using thermocouples,
the MR thermometry is a noninvasive method that does not suffer
from the problems of positioning error and thermal artifacts. The
magnetic resonance imaging (MRI) scanner is capable of assess-
ing the transient temperature rise across the treatment volume, as
well as measuring the volume of the thermal lesion [3]. Although
the MR temperature monitoring was used to evaluate the feasibil-
ity of MR-guided HIFU ablation in the liver and kidney [4], the
effect of higher acoustic powers on initiation of cavitation and
possible maximum temperature rise in these organs were not
assessed using MR thermometry. In this study, MR thermometry
was used to monitor HIFU ablations performed on in vivo porcine
livers, at elevated acoustic powers of 10 W, 30 W, and 40 W to
assess the temperature field where initiation of cavitation is known
to occur. Temperature rise during the heating phase as well as the
temperature decay during the cooling phase was acquired. The
transient temperature profiles measured during the heating and
cooling phases for the three powers were compared, in order to
check the HIFU induced maximum temperature rise.
2 Methods
A set of HIFU ablations on in vivo porcine livers (n¼ 3) was
conducted in the MR bore. Based on the MR bore dimension, an
MR compatible positioning system was constructed for accurately
locating the HIFU transducer vertically on the porcine liver
(Fig. 1). The transducer, H102 (Sonic Concepts Inc., Bothell,
WA), having a focal length of 6.26 cm, outer radius of 3.2 cm,
inner radius of 1.1 cm, and frequency of 1.1 MHz, was oriented
vertically and coupled to the porcine liver via a plexiglass cou-
pling cone that was filled with degassed water (Fig. 1). The tip of
the cone was placed at selected locations atop of the exposed liver
surface and the lesion was formed below the tip. In order to pre-
vent the infiltration of noise signals in the MRI room, a low-pass
filter was incorporated in the electronic system [5]. Each porcine
liver was ablated in three different zones using acoustic powers of
10 W, 30 W, and 40 W, keeping the sonication time same (30 s).
Thus, there were a total of nine sonication zones (3 pigs 3
sonications/pig).
During the HIFU ablation procedure, MR imaging was per-
formed using a 3 Tesla whole body scanner (Achieva, Philips
Healthcare, Best, The Netherlands). MRI-derived temperature
rises were generated based on the proton resonance frequency
(PRF) shift thermometry [6]. The estimate in relative temperature
change DT, is given by
Fig. 1 Schematic of HIFU experimental setup
Fig. 2 Highest temperature map for the acoustic powers of
(a) 30 W and (b) 40 W
1Accepted and presented at the Design of Medical Devices Conference
(DMD2014), Minneapolis, MN, April 7–10, 2014.
DOI: 10.1115/1.4027082
Manuscript received February 21, 2014; final manuscript received March 3, 2014;
published online July 21, 2014. Editor: Arthur G. Erdman.
Journal of Medical Devices SEPTEMBER 2014, Vol. 8 / 030937-1Copyright VC 2014 by ASME
Downloaded From: https://medicaldevices.asmedigitalcollection.asme.org on 07/02/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use
DT ¼ /ðTÞ  /ðT0Þ
caB0TE
(1)
where /(T) is the phase in the current image, c is the
gyromagnetic ratio, B0 is the magnetic field strength, a is the PRF
change coefficient, TE is the echo time, and /(T0) is the phase
angle of a pixel outside the heated area, which corresponded to
the base temperature. Imaging parameters for the current study
were: c¼ 2.675 108 rad/s/T, a¼ 0.01 106 ppm/C, B0¼ 3.0 T,
TE¼ 0.01 s, number of slices¼ 3, and slice thickness¼ 3 mm.
The imaging temporal resolution was 3.15 s for the acoustic
powers of 30 W and 40 W while it was 2.58 s for the acoustic
power of 10 W.
3 Results
The sonication process was initiated at a time of 10 s for all
three powers. The presonication liver tissue temperature was
37 C. The highest temperatures measured by MR thermometry
during HIFU ablation procedure for the powers of 30 W and 40 W
are shown in Figs. 2(a) and 2(b), respectively. It can be seen in
Figs. 2(a) and 2(b) that the maximum temperature rise and ther-
mal lesion volume increase with the increase in power.
Figures 3(a)–3(c) show the HIFU induced temperature rise
derived from MR temperature maps, for the acoustic powers of
10 W, 30 W, and 40 W, respectively. The maximum temperature
rise, which has been averaged over three trials, for each acoustic
power of 10 W, 30 W, and 40 W was 30.2 6 8.6 C, 45.7 6 5.4 C,
and 56.6 6 9.8 C, respectively. The statistical averaging of three
sets of temperature data for different lesions shows some variabili-
ty particularly during the heating time period (between 10 s and
40 s). For lower powers (10 W and 30 W) the peak ablation tem-
perature was observed to be close to the end of sonication time
(40 s). However, at the highest power (40 W) some anomalies in
the temperature trace, e.g., plateau without a distinct peak value
between 30 s and 40 s were observed (Fig. 3(c)). This indicates
possible bubble cloud formation and the initiation of cavitation.
Such a phenomenon needs further assessment. Temperature
increases were also not proportional to power increases, due to the
cooling effects of blood flow in the highly perfused liver.
4 Interpretation
This study shows the feasibility of MR thermometry to acquire
the maximum temperature rise as well as localized cavitation dur-
ing an HIFU in vivo ablation procedure. One of the advantages of
using MR thermometry is that the maximum temperature rise at
the HIFU beam focus can be measured accurately without any
thermal artifact, which has been the major problem with the tem-
perature measurement using thermocouples. In order to improve
the accuracy of the temperature measurement, the spatial and tem-
poral resolution of MR thermometry can be increased. Despite the
limitation in increasing the spatial resolution, the location of max-
imum temperature rise (beam focus) can be found more accurately
in comparison with the thermocouple method, which is subjected
to positioning errors. For thermal dose calculation multiple ther-
mocouple arrangement is needed whereas for MR thermometry
such determination can easily be obtained without artifact and
noninvasively using the temperature field from MR thermometry.
Acknowledgment
This project was funded by NSF#1041508 and NSF#1137166.
References
[1] Morris, H., Rivens, I., Shaw, A., and Haar, G. T., 2008, “Investigation of the Vis-
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[3] Dasgupta, S., Wansapura, J., Hariharan, P., Pratt, R., Witte, D., Myers, M. R.,
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[6] Rieke, V., Vigen, K. K., Sommer, G., Daniel, B. L., Pauly, J. M., and Butts, K.,
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Fig. 3 HIFU induced temperature rise in porcine liver with
sonication period of 30 s using acoustic powers of (a) 10 W,
(b) 30 W, and (c) 40 W
030937-2 / Vol. 8, SEPTEMBER 2014 Transactions of the ASME
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In Vivo Monitoring of HIFU Induced Temperature Rise in Porcine Liver Using Magnetic Resonance Thermometry

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In Vivo Monitoring of HIFU Induced Temperature Rise in Porcine Liver Using Magnetic Resonance Thermometry

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