We report a preliminary study of breast cancer imaging by microwave-induced thermoacoustic tomography. In this study, we built a prototype of breast cancer imager based on a circular scan mode. A 3-GHz 0.3~0.5-μs microwave is used as the excitation energy source. A 2.25-MHz ultrasound transducer scans the thermoacoustic signals. All the measured data is transferred to a personal computer for imaging based on our proposed back-projection reconstruction algorithms. We quantified the line spread function of the imaging system. It shows the spatial resolution of our experimental system reaches 0.5 mm. After phantom experiments demonstrated the principle of this technique, we moved the imaging system to the University of Texas MD Anderson Cancer Center to image the excised breast cancer specimens. After the surgery performed by the physicians at the Cancer Center, the excised breast specimen was placed in a plastic cylindrical container with a diameter of 10 cm; and it was then imaged by three imaging modalities: radiograph, ultrasound and thermoacoustic imaging. Four excised breast specimens have been tested. The tumor regions have been clearly located. This preliminary study demonstrated the potential of microwave-induced thermoacoustic tomography for applications in breast cancer imaging

Jin, Xing

Ku, Geng

Wang, Lihong V.

Xu, Minghua

Caltech Authors - Main

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SPIEDigitalLibrary.org/conference-proceedings-of-spie
Breast cancer imaging by microwave-
induced thermoacoustic tomography
Minghua  Xu, Geng  Ku, Xing  Jin, Lihong V. Wang, Bruno
D. Fornage, et al.
Minghua  Xu, Geng  Ku, Xing  Jin, Lihong V. Wang, Bruno D. Fornage, Kelly
K. Hunt, "Breast cancer imaging by microwave-induced thermoacoustic
tomography," Proc. SPIE 5697, Photons Plus Ultrasound: Imaging and
Sensing 2005: The Sixth Conference on Biomedical Thermoacoustics,
Optoacoustics, and Acousto-optics,  (25 April 2005); doi: 10.1117/12.589128
Event: SPIE BiOS, 2005, San Jose, CA, United States
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Breast cancer imaging by microwave-induced  
thermoacoustic tomography 
 
Minghua Xu, Geng Ku, Xing Jin and Lihong V. Wang∗ 
Optical Imaging Laboratory, Department of Biomedical Engineering 
Texas A&M University, 3120 TAMU, College Station, Texas 77843-3120 
 
Bruno D. Fornage and Kelly K. Hunt 
The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd, Houston, TX 77030  
 
 
ABSTRACT 
 
We report a preliminary study of breast cancer imaging by microwave-induced thermoacoustic tomography. In this 
study, we built a prototype of breast cancer imager based on a circular scan mode. A 3-GHz 0.3~0.5-µs microwave is 
used as the excitation energy source. A 2.25-MHz ultrasound transducer scans the thermoacoustic signals. All the 
measured data is transferred to a personal computer for imaging based on our proposed back-projection reconstruction 
algorithms. We quantified the line spread function of the imaging system. It shows the spatial resolution of our 
experimental system reaches 0.5 mm. After phantom experiments demonstrated the principle of this technique, we 
moved the imaging system to the University of Texas MD Anderson Cancer Center to image the excised breast cancer 
specimens. After the surgery performed by the physicians at the Cancer Center, the excised breast specimen was placed 
in a plastic cylindrical container with a diameter of 10 cm; and it was then imaged by three imaging modalities: 
radiograph, ultrasound and thermoacoustic imaging. Four excised breast specimens have been tested. The tumor regions 
have been clearly located. This preliminary study demonstrated the potential of microwave-induced thermoacoustic 
tomography for applications in breast cancer imaging. 
 
Keywords: Breast cancer, imaging, microwave, thermoacoustic tomography 
 
1. INTRODUCTION 
 
Breast cancer is the most common cancer among women, accounting for one out of every three cancer diagnoses. The 
incidence of breast cancer increases with age: approximately 3 out of 4 women with a new diagnosis of breast cancer 
each year are older than 50. Although breast cancer remains a leading cause of cancer deaths among women, the cure 
rate is much improved by early detection. X-ray mammography and ultrasonography are the current clinical tools for 
breast-cancer screening and detection. Mammography is the “gold standard”; however, it uses ionizing radiation and has 
difficulty in imaging pre-menopausal breasts, which are radiographically dense. Currently, ultrasonography is used only 
as an adjunct tool to x-ray mammography and tends to miss non-palpable tumors. 
 
The contrast in the microwave or radiofrequency (RF) regime is very sensitive to tissue physiological states—molecular 
constituents, ion concentration and mobility, free- and bound-water concentrations, and temperature.  Cancerous breast 
tissues absorb microwave or RF 2–5 times more strongly than surrounding normal breast tissues [1–3]. However, pure-
microwave or RF imaging is limited to poor resolution because of the long wavelengths utilized. Ultrasonic imaging has 
good resolution but has poor contrast for early-stage tumors. Microwave-induced thermoacoustic imaging combines the 
contrast advantage of pure-microwave imaging and the resolution advantage of pure-ultrasonic imaging, and therefore, 
has greater potential to detect early breast cancers and to assess and monitor treatments as well. 
 
In this paper, we present a preliminary study of breast cancer imaging by microwave-induced thermoacoustic 
tomography. The reconstruction is conducted by our proposed back-projection algorithm. We quantified the line spread 
                                                        
∗
 To whom all correspondence should be addressed. Telephone: 979-847-9040; fax: 979-845-4450-; electronic mail: 
LWang@tamu.edu; URL: http://oilab.tamu.edu 
Photons Plus Ultrasound: Imaging and Sensing 2005, edited by Alexander A. Oraevsky,
Lihong V. Wang, Proceedings of SPIE Vol. 5697 (SPIE, Bellingham, WA, 2005)
1605-7422/05/$15 · doi: 10.1117/12.589128
45
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function of the imaging system. Then, we moved the imaging system to the University of Texas MD Anderson Cancer 
Center and imaged four excised breast cancer specimens. 
 
2. METHOD 
 
In the initial study, we use 2-D circular measurement geometry. Figure 1 shows the experimental setup. A plexiglass 
container is filled with mineral oil. An unfocused transducer is immersed inside it and fixed on a rotation device. A step 
motor drives the rotation device and then moves the transducer scan around the sample on a horizontal x-y plane, where 
the transducer horizontally points to the rotation center. A sample is immersed inside the container and placed on a 
holder: it is made of a thin plastic material, which is transparent to microwaves. The transducer (V323, Panametrics) has 
a central frequency of 2.25 MHz and a diameter of 6mm.  
 
The microwave pulses transmitted from a 3-GHz microwave generator have a pulse energy of 10 mJ and a pulse width 
of 0.3 or 0.5 µs . A function generator (Protek, B-180) is used to trigger the microwave generator, control its pulse 
repetition frequency, and synchronize the oscilloscope sampling.  
 
In our experiments, the pulse repetition frequency is 50 Hz and the sampling frequency of the oscilloscope is 20 or 50 
MHz. Microwave energy is delivered to the sample by a rectangular waveguide with a cross section of 72 mm × 34 mm. 
A personal computer is used to control the steps. The signal from the transducer is first amplified through a pulse 
amplifier, then recorded and averaged 100~500 times by an oscilloscope (TDS640A, Tektronix), and finally transferred 
to a personal computer for imaging based on our proposed back-projection algorithms [4,5]. 
 
3. SPATIAL RESOLUTION 
 
We quantified the line spread function (LSF) of the imaging system with pulse duration. A metal wire with a diameter 
of 0.2 mm was buried in pork fat and then imaged by our imaging system with a scan radius of 75 mm. The 
photoacoustic image of the embedded wire is shown in Fig. 2(a).  Fig. 2(b) shows the profile of the LSF across the wire, 
where the ringing is caused primarily by the bandwidth of the imaging system. Its full width at half maximum (FWHM) 
is about 0.5 mm. We can, therefore, claim the spatial resolution of our experimental system reaches 0.5 mm, which 
agrees with the theoretical spatial-resolution limit for 3 MHz photoacoustic signals whose half wavelength is <0.5 mm 
in soft biological tissues.  It is worth noting that the claimed 0.5-mm resolution is a worst-case estimation of the true 
resolution because of the finite thickness of the wire (0.2 mm diameter). Of course, the detecting transducer has a finite 
physical size.  If it is close to the thermoacoustic sources, it cannot be approximated as a point detector.  Its size will 
blur the images and decrease the spatial resolution.  Therefore, in experiments, the transducer must be placed some 
distance away from the tissue samples. 
Microwave generator
Step motor
Sample
Transducer
Mineral oil Pulse amplifier
Computer
Function
generator
Oscilloscope
Driver
y
x
z
 
Figure 1: Diagram of experimental setup 
46     Proc. of SPIE Vol. 5697
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 -5 0 5 
5 
0 
-5 
X (mm) 
Y 
(m
m
) 
      
-4 -3 -2 -1 0 1 2
-0.5
0.0
0.5
1.0
 
0.5 mm
Am
pl
itu
de
 (a
.
u
.
)
X (mm)
  
                                                         (a)                                                                                      (b) 
Figure 2:  (a) Thermoacoustic image of a thin wire; (b) Profile across the wire. 
 
4. BREAST CANCER IMAGING 
 
We took the prototype of thermoacoustic imaging system to the University of Texas M.D. Anderson Cancer Center and 
conducted a preliminary study of breast imaging. In our initial study, we investigated tumors in breast mastectomy 
specimens. Four excised breast specimens were imaged. The tumor regions in these specimens can be located in the 
thermoacoustic images as expected.  
 
In experiments, we followed the following procedure: 
 
(a) First, a mammogram before the mastectomy of the breast was taken at the Cancer Center as a part of the clinical 
procedure. Fig. 3(a) shows the mammogram of the specimen in one of the four samples we investigated. A 
mammogram reflects the different attenuations of the x-ray beam within a patient's breast. Therefore, the image contrast 
produced by an object depends on its attenuation of the x-ray beam relative to the attenuation of the background tissue. 
To improve the contrast, the breast is often taken with standard compression. 
 
(b) After the mastectomy, the excised specimen was placed in a plastic cylindrical container with a diameter of 10 cm 
[Fig. 3(b)]. The nipple of the specimen faced the bottom of the container. For the specimen shown in Fig. 3, its 
thickness in the container was ~6 cm; but the thickness varied for different specimens. The container had a minimal 
effect on the transmission of microwave, ultrasound, and x-ray.  
 
(c) Then, a conventional B-mode gray-scale sonogram of the specimen [Fig. 3(c)] was taken using a real-time scanner 
(HDI 5000, Philips-ATL, Bothell, WA) equipped with a 5–12 MHz broadband linear array electronic transducer. The 
tumor region was marked by two lines. In a sonogram, the contrast is based on the tissues’ mechanical properties. 
 
(d) Next, the specimen was imaged using our thermoacoustic imaging system. A circular scan was carried out by a 
cylindrically focused ultrasound with a step size of 2-1/4 degrees. The scan radius was 7.5 cm. The reconstructed image 
[Fig. 3(d)] was computed by the back-projection method. We adjusted the image contrast and set a threshold level to 
suppress the background. The tumor region clearly shows up (marked by a circle), the location of which agrees with the 
original sample’s. Of course, the normal breast tissues also have certain microwave absorption. In principle, based on 
the pathological characteristics, the tumor region can be differentiated from the normal tissues. 
 
(e) Finally, another radiograph of the specimen was taken from the top of the cylindrical container [Fig. 3(e)]. In this 
case, the tumor region is not clearly imaged.  
 
 (f) After these imaging experiments, the specimen was rendered to the Department of Pathology for histopathological 
diagnosis. This lesion shown in Fig. 3 was diagnosed as invasive lobular carcinoma with a size of ~1.5 cm. 
 
 
 
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(a)                                                     (b)                                                    (c) 
 
              
-5.0 -2.5 0.0 2.5 5.0
5.0
2.5
0.0
-2.5
-5.0
Horizontal axis x (cm)
Ho
riz
on
ta
l a
xis
 
y 
(cm
)
0 
                                     
                                                               (d)                                                                    (e) 
 
Figure 3:  (a) Pre-operative mammogram showing suspicious density in the breast, which was taken with standard compression. (b) 
The mastectomy specimen placed in a plastic cylindrical container with a diameter of 10 cm (c) Sonogram of the specimen in the 
container. (d) Thermoacoustic image of the specimen in the container.  The rectangle marks the wave-guide aperture.  (e) Radiograph 
of the mastectomy specimen in the container. The tumor is marked by a circle in (a), (d) and (e) and by two white lines in (c). 
 
The rectangle in Fig. 3(d) marks the wave-guide aperture.  The wave-guide for this experiment was not large enough to 
cover the entire specimen.  Since then, we have upgraded our system with a larger wave-guide to overcome this 
problem. 
 
5. SUMMARY 
 
We built a prototype of microwave-induced thermoacoustic imaging system with circular measurement geometry. We 
also quantified the spatial resolution of the imaging system. Then, we conducted a preliminary study of breast cancer 
imaging. Four excised breast specimens have been tested. The tumor regions have been clearly located. This initial 
study shows that the microwave-induced thermoacoustic tomography has great potential in the application of breast 
cancer imaging. 
 
ACKNOWLEDGEMENTS 
 
This project was sponsored in part by the U.S. Army Medical Research and Materiel Command Grant No. DAMD17-
00-1-0455, the National Institutes of Health Grants No. R01 EB000712 and No. R01 NS46214, and Texas Higher 
Education Coordinating Board Grant No. ARP 000512-0063-2001. 
 
REFERENCES 
 
1. W. Joines, R. Jirtle, M. Rafal, and D. Schaeffer, Int. J. Radiat.Oncol. Biol. Phys. 6, 681 (1980). 
2. S. Chaudhary, R. Mishra, A. Swarup, and J. Thomas, Indian J. Biochem. Biophys. 21, 76 (1984). 
3. G. Ku and L.-H. Wang, Med. Phys. 28, 4 (2001). 
4. M.-H Xu and L.-H. Wang, IEEE Trans. Med. Imaging 21, 814 (2002). 
5. M.-H. Xu, Y. Xu, and L.-H. V. Wang, IEEE Trans. Bio-med Eng. 50, 1086 (2003). 
48     Proc. of SPIE Vol. 5697
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Breast cancer imaging by microwave-induced thermoacoustic tomography

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Breast cancer imaging by microwave-induced thermoacoustic tomography

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