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PreXionLED AcousticX

Photoacoustic Imaging Technology

PreXionLED AcousticX

AcousticX is Photoacoustic imaging system using LED array light source, which is compact, power-saving and reasonable price system to assist all of researchers who are contributing to expanding use of photoacoustic imaging clinically.

PreXionLED AcousticX

We have developed the photoacoustic imaging technology using LED array light source. Fig.2 shows the LED array light source. It can be integrated onto the ultrasound transducer (Fig.1) which is known as the device used by doctors and technologists to help diagnose and monitor various conditions in different parts of the body.
Photoacoustic imaging is different from ultrasound imaging, but both are detecting sound wave collected by the transducer. Therefore the photoacoustic imaging capabilities can be incorporated into the conventional ultrasound system, which gives the effective functionality by using a combination of both imaging modalities.

fig1-2

Fig.1                  Fig.2

Principle of photoacoustic imaging

The difference between ultrasound and photoacoustic imaging is shown in Fig.3.

● Ultrasound (US) vs. Photoacoustic Imaging (PAI) (Fig. 3)

Mechanism What can be done
US Transmit US pulse,
then receive echoPreXionLED AcousticX
>Localize boundaries
>Structural imaging
>Blood flowPreXionLED AcousticX
PAI Expose Near Infrared pulse, then receive
photoacoustic signal
PreXionLED AcousticX
>Localize absorber
>Detect distribution of blood
>Functional imaging
>Visualize Needle clearly

In ultrasound imaging, the transducer transmits ultrasonic waves, then the reflected waves come back from the body, which are detected by the transducer. The reflector location can be calculated by measuring the propagation time, and its signal intensity is converted to the brightness, then the two dimensional image is constructed. So, the ultrasound imaging is called structural imaging. Also, the blood flow is shown as red or blue color in Color Doppler mode.
On the other hand, in photoacoustic imaging, the transducer does not transmit the ultrasonic waves. Instead, a short pulse of near-infrared light is exposed into the body from the body surface. Then the adiabatic expansion occurs at the light absorber in the body, which generates the weak acoustic wave. By receiving this weak signal, the light absorber is localized and the distribution of the light absorber is analyzed. For example, photoacoustic imaging can detect the distribution of blood, which means that photoacoustic imaging can identify the functional activities of tissues by visualizing the presence of small blood vessels, the content of hemoglobin and its degree of oxygenation. By using multiple wave-length light, vessels between artery and vein can be distinguished. So, the photoacoustic imaging is called as functional imaging. Also, the clinical metal needle can be visualized clearly.

Wave-length of light

There may be a concern that the light cannot penetrate deeper in our body. But the light used in photoacoustic imaging is called as near-infrared light. The wave-length of near-infrared is approximately between 700nm and 2500nm (Japanese Industrial Standards definition). Near-infrared light has the maximum penetration depth in tissue, and it can non-invasively detect the information from our body, it is applied for some biological imaging devices.
However, the tissue has substances to absorb and scatter light, and in particular, hemoglobin and water has strong light absorption.
Fg.4 shows the strength of absorption dependency with the wave-length. The strength of absorption and scattering is on the vertical axis, the wave-length of light is on the horizontal axis.

PreXionLED AcousticX Fig.4

As shown here, the light wave-length below 700nm has a strong absorption of hemoglobin in blood, while the wave-length beyond 1300nm, the absorption by water is strong.
Since the absorbance of both hemoglobin and water is less around the wave-length range between 700nm and 1300nm, which means that this range has the maximum penetration depth, it is called as the optical window, and also known as the “biological window”, therefore it is suitable for photoacoustic imaging.
When the nanosecond pulsed light is exposed into the body, the light is absorbed by the absorber inside tissue, it induces instantaneous temperature rise, and then the volume containing absorber instantaneously expands and consequentially builds up a pressure, which leads to the emission of an elastic wave. The elastic wave is in a frequency range of the ultrasound, it can be detected by ultrasound transducer.
The sound wave propagates approximately at 1500m/s in the body, so the optical absorber can be localized by measuring the propagation time, and estimate the density of absorber from the signal strength.

Importance of clinical metal needle visibility in POC and the visualization of needle by photoacoustic imaging

POC (point-of-care) is a term that is used in the sense that examination, diagnosis and treatment that is carried out in the immediate vicinity of the patient. In POC field, ultrasound imaging is often used not only for diagnosis purpose but as the guide to perform following actions relating to something examination and treatment while viewing the image.
In such field where rapid diagnosis and treatment is required, compact, simple, and safe equipment is desired. There are clinical procedures that uses not only injection needle but also various type of metal needles. In particular, they are regional anesthesia, biopsy for sampling cells, and paracentesis procedure to remove the accumulated ascites, etc. In recent years, very small ultrasound diagnostic imaging equipment has become widely used, which assists physicians to do accurate and safe procedure while monitoring tip of the needle inserted. However, performing needle biopsy requires skill even for physicians. As an example, sometimes training exercise of the ultrasound image guided needle procedure event is held at the scientific congress or medical seminar. Thus, the visibility of needle on ultrasound equipment is very important.

Fig.5 shows an example image that chicken breast was scanned by conventional ultrasound. It was punctured by a 0.9mm diameter needle, where a tip of the needle was hardly visualized.

PreXionLED AcousticX Fig.5

Fig.6 shows a photoacoustic signal from the needle acquired by photoacoustic imaging at the same timing as the Fig.5 was captured. Here the needle was clearly shown.

PreXionLED AcousticX Fig.6

Fig.7 shows a combined image of both ultrasound and photoacoustic signal. The combined image gives the better visibility of needle while viewing tissue structures.

PreXionLED AcousticX Fig.7

Clinical application opportunities of photoacoustic imaging

In addition to clear visualization of needle, there are many clinical application opportunities which have been researched and introduced by photoacoustic researchers.
Photoacoustic imaging can be integrated to endoscopic ultrasound device. It can gives more information around suspicious lesion which can’t be acquired by ultrasound alone. It may also help guiding targeted biopsy procedure.(Washington Univ. in St. Louis) [1]
X-ray mammography is used for breast cancer exam. The radiation dose is relatively low, but it uses X-ray. Also it is said that dense breasts are common in Asian country populations. Such dense breast tissue can make it harder to find cancer on a mammogram. There is a research that photoacoustic reveals a clear contrast between blood-vessel dense tumor areas and normal vessel environments.(Univ. of Twente) [2]
Trans-rectal ultrasound (TRUS) transducer is used for prostate biopsy to collect tissue samples that may include cancer cells from many parts of prostate. TRUS transducer combining photoacoustic can help detection of neurovascular bundle during radical prostatectomy procedure. It may help guiding targeted prostate biopsy procedure.(National Defense Medical College) [3]
Carotid plaque is one of the reason that causes ischemic stroke. It is said that there is atheromatous plaque, which is at risk to rapture. Lipid deposition inside the arterial wall is a key indicator of plaque vulnerability. An intravascular photoacoustic (IVPA) catheter is considered a promising device for quantifying the amount of lipid inside the arterial wall.(Purdue Univ.) [4]
[References]
[1] Photoacoustic endoscopy. Yang JM, Maslov K, Yang HC, Zhou Q, Shung KK, Wang LV. Opt Lett. 2009 May 15;34(10):1591-3.
[2] M. Heijblom, D. Piras, W. Xia, J. van Hespen, J. Klaase, F. van den Engh, T. van Leeuwen, W. Steenbergen, and S. Manohar Visualizing breast cancer using the Twente photoacoustic mammoscope: What do we learn from twelve new patient measurements? Opt. Express 20, 11582-11597 (2012).
[3] Development of photoacoustic imaging technology overlaid on ultrasound imaging and its clinical application
Author(s): Miya Ishihara; Kazuhiro Tsujita; Akio Horiguchi; Kaku Irisawa; Tomohiro Komatsu; Makoto Ayaori; Takeshi Hirasawa; Tadashi Kasamatsu; Kazuhiro Hirota; Hitoshi Tsuda; Katsunori Ikewaki; Tomohiko Asano
[4] High-speed ‘label-free’ imaging could reveal dangerous plaques: November 4, 2014

Challenges to be solved to make photoacoustic imaging clinically available

As described above, there will be many opportunities to utilize photoacoustic imaging. However, it is not yet clinically available. What is the major factor to prevent expanding use of photoacoustic imaging? It is because the size of equipment is huge and expensive.
Currently the photoacoustic imaging system which is commonly used consists of the solid-state laser and the tunable wavelength device such as optical parametric oscillator.
As shown on Fig.8, the laser-based photoacoustic system needs the precise alignment of optical elements and also the heavy optical table and the robust chassis to avoid the beam-distortion caused by the unexpected vibration. So, the laser-based photoacoustic imaging system tends to become bigger and the drive power supply of the system is also big, it requires relatively large space to install than that for ultrasound equipment. Moreover the configuration of the system is complicated, the system price also tends to be expensive. So these are the factors that prevent the expanding use of photoacoustic imaging for clinical applications.

PreXionLED AcousticX Fig.8

In such situation, we have studied whether we can develop and realize the photoacoustic imaging prototype system using the high intensity LED array light source.
As the size comparison shown on Fig.9, the photoacoustic imaging system can be configured with the high intensity LED array light source, the system doesn’t need optical table and robust chassis, so the entire system becomes compact and portable to fit small place.

PreXionLED AcousticX Fig.9

Table 1 shows the comparison between LED light source system and Laser light source system. The size of laser system is about 15,000 times bigger than LED system. The power consumption of laser system is about 1,000 times higher than LED system. The system cost of laser system is over 10 times higher than LED system. Therefore the ultra-compact high-intensity pulsed LED light source that can be integrated onto ultrasound transducer has been developed.

PreXionLED AcousticX Table 1

Our competitiveness is that we have many fundamental pending patents related to LED based photoacoustic imaging system, which have been created through the previous experiences during the course of development. Therefore these patents block others to make the similar system.

We expect to gain momentum in the deployment of the photoacoustic imaging technology to medical field. Toward the practical use in the medical field, we hope to promote the development of applications and product development in cooperation with the multiple medical device manufacturers and the medical field professionals.

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