Photoacoustic imaging of tumour vasculature

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Vascular imaging is indispensable to visualise cancer. Since solid cancers require neovascularisation (the process by which new vascular structures assemble) for their growth and progression, this imaging technique is useful for the diagnosis of cancer. Understanding of the blood supply to the tumour results in precise diagnosis and favourable treatment outcomes. Dr Masakazu Toi and Dr Yoshiaki Matsumoto at the Department of Breast Surgery, Kyoto University Graduate School of Medicine, harness the high-resolution ability of photoacoustic imaging, detecting light-induced ultrasound mostly from haemoglobin, to visualise tumour-associated vessels in the human breast in vivo.

The human heart pumps approximately five litres of blood around the body each day through our circulatory system, the network of blood vessels that deliver oxygenated blood to all organs and tissues, and remove blood containing waste products.

However, many diseases are caused by changes in the circulatory system, or by unwanted changes to blood vessel growth and formation. For example, vascular dementia is a common type of dementia caused by reduced blood flow to the brain. Heart attacks, some cancers and diseases that induce defects in collagen, a protein that makes many of the body’s tissues, can also be caused by abnormal blood supply. Angiogenesis, or the formation of new blood vessels from pre-existing vessels, is particularly relevant in cancer as solid tumours require a good blood supply to survive and grow.

Schematic illustration showing the PAI-04 system configuration and the alternating irradiation sequence (pulse-to-pulse wavelength switching) adopted in this study. The PA controller controls the laser oscillation and PA wave reception. In this apparatus, laser beams of two different wavelengths are used for alternating irradiation. The generated PA wave is received by the hemispherical probe array (HDA) and sent to the data acquisition system (DAS). In addition to the PA controller, another unit controls ultrasonic (US) transmission and reception. Data received by the US transducer are sent to the US unit. The US transducer and the HDA are integrated as a transducer module, and the entire system is configured to move simultaneously during scanning. The space between the holding cup and the transducer module is filled with circulating water. Water for acoustic matching with the test object is poured into the holding cup.
Photo Credit: https://creativecommons.org/licenses/by/4.0/; https://www.nature.com/articles/s41598-018-33255-8/figures/1

Diagnostic imaging techniques are employed to characterise and monitor circulatory diseases. In particular, angiography is a technique that uses X-rays to check blood vessels. Angiographies involve injecting a special dye into the blood, as blood vessels do not show up well on normal X-rays. Other techniques such as CT and MRI scans are also used. One disadvantage of these types of imaging techniques is that they involve a contrast agent, such as the dye used in angiographies, and this is not well tolerated by some patient groups. Also, it can be dangerous for children to be exposed to high levels of the radiation often used in these imaging methods.

Due to the above challenges, there is a need for imaging techniques for blood vessels which do not require invasive approaches. One such technique is photoacoustic imaging (PAI). This is a novel imaging approach that does not require contrast agents or expose patients to radiation.

Photoacoustic imaging

PAI works by using a phenomenon called the photoacoustic effect. In essence, this is the formation of sound waves following the absorption of light by a particular material. The material absorbs the light which in turn raises the temperature of the material and causes it to expand. This expansion generates sound. The sound signals emitted can then be detected by a computer and analysed to determine the 3D position of the subject – in this case, the blood vessels. PAI has the dual advantages of high tissue specificity and contrast due to the optical measurements, and high spatial resolution due to the ultrasonic wave measurements. This means that PAI is a non-invasive approach that can be used to image small vascular structures.

“The blood vessels around a breast tumour have different S-factors, suggesting that there are both veins leading away from the tumour, and arteries leading into it.”

In addition, PAI can also be used for optical imaging and it is possible to image the haemoglobin oxygen saturation of the blood. This is calculated from measurements collected at two different wavelengths and was termed the ‘S-factor’. The oxygen saturation can also be translated to colour, resulting in a clear visual representation of the varying oxygen saturation levels of different blood vessels. Therefore, this imaging approach has the ability to provide detailed information about both vascular structure and function.

Examples of palm and thigh PA images obtained from healthy subjects in vivo. (a–c) Examples of palm PA images obtained using an alternate irradiation sequence. No body motion correction was performed. (a) The maximum intensity projection (MIP) image of the whole palm. (b) An image after deletion of the subcutaneous veins from the whole palm image. (c) An enlarged image of the region of (b) indicated by the white dashed line shows the common palmar digital arteries. In Fig. 2c, the blood vessels anatomically determined to be the common palmar digital arteries are designated A1–3, and the veins accompanying them are V1–3. The assignment of these arteries and veins corresponded to the blood vessel colour of the S-factor image (i.e., the magnitude relationship of the S-factor). (d) An example PAT image obtained in an anterolateral thigh. A stem portion of perforator (P) vessels and a bundle representing an artery (A) and a vein (V) were observed. (e) Schematic illustration of the measured tissue in the body of a subject.
Photo Credit: https://creativecommons.org/licenses/by/4.0/; https://www.nature.com/articles/s41598-018-33255-8/figures/3

Vascular imaging in breast cancer

Breast cancer is the most common cancer in women worldwide. Angiogenesis is key to tumours metastasizing and spreading to other parts of the body, and also to supply oxygen and nutrition for tumour growth. In breast cancer, angiogenesis is also associated with a poor prognosis and transformation from mammary hyperplasia (increased cell division) to malignancy.

Dr Masakazu Toi and Dr Yoshiaki Matsumoto at the Department of Breast Surgery, Kyoto University Graduate School of Medicine, have used PAI to visualise tumour-associated blood vessels in the human breast.

The PAI method involved a holding cup, in which the breast was placed. Water circulates underneath the holding cup, providing the material through which sound waves travel. Lasers, producing light waves at several different frequencies, were then used to irradiate the breast. A photoacoustic wave is produced, which is received by a probe and sent to the data acquisition unit, which processes the data.

3D images of vasculatures around the patient’s breast tumour were acquired, as well as areas of the body in human subjects with and without breast cancer. The team were able to scan regions of the hands, legs and breast in an imaging region with 140mm diameter. Using additional information from ultrasound imaging, Dr Toi and Dr Matsumoto were able to build an accurate 3D picture of the smallest blood vessels, including arterioles (small blood vessels branching out from an artery) and venules (small blood vessels that unite to form a vein), within this area. The results also showed that the blood vessels around a breast tumour have different S-factors. This suggests that there are both veins leading away from the tumour, and arteries leading into it. The images also showed some areas of hypoxia, or oxygen deprivation. Dr Toi and Dr Matsumoto hypothesise that this may represent areas of bleeding within the tumour itself or may be due to necrosis and fibrosis secondary to a lack of blood flow.

Examples of images obtained from a breast cancer patient in vivo. (a) A fusion image of the PA image taken at 797 nm and 3D-US images. Hypoechoic portions of the US were extracted based on the US signal intensity and reconstructed into a 3D volume by stacking the continuous B-mode images. The cubed US data were coloured red. Several clustered fine blood vessels were observed in the vicinity of the tumour. (b) A US B-mode image of the dotted-line portion indicated by 1–1 in (a). A hypoechoic region is visible and is indicated by the yellow arrowhead. (c) A fusion image of the S-factor and 3D-US images (red colour). The nipple at the centre of this image is coloured light blue because the absorption spectrum of melanin in the nipple and the spectrum of low oxygen saturation are almost the same. (d) An S-factor image of the same region with enlargement of the tumour vicinity but without fusion with the US tumour image. (e) The entire MIP image in the axial direction of the S-factor around the lesion. (f) The slab MIP image at the centre of the lesion corresponding to (e).
Photo Credit: https://creativecommons.org/licenses/by/4.0/; https://www.nature.com/articles/s41598-018-33255-8/figures/5

The 3D ultrasound image was effective in identifying the tumour position, meaning that these images could be overlapped with the PAI vessel imaging to better visualise the vascularisation of the tumour and surrounding area.

The researchers were also able to determine the haemoglobin oxygen saturation levels of the vessels and use this to identify whether the vessel was an artery or a vein. Finally, they were able to image tumour-associated blood vessels in high resolution.

“The work done by Dr Toi and Dr Matsumoto represents one of the first studies to report high-resolution imaging of blood vessels in live human cancer tissue.”

Future studies

In breast cancer imaging, it was possible to visualise small, tumour-related vessels using PAI. The work done by Dr Toi and Dr Matsumoto represents one of the first studies to report high-resolution imaging of blood vessels in live human cancer tissue.

The vessels that were imaged in this case were less than 10mm underneath the skin. In other clinical cases, they were able to image vessels at the depth of around 30mm from the skin. However, in future studies, the researchers aim to extend their imaging approach to reach blood vessels situated even deeper under the skin. They aim to do this by optimising the laser irradiation method and sensor sensitivity.

PAI can also be applied to other fields, such as plastic surgery, cancer diagnosis and dermatological disease treatment. One example highlighted by Dr Toi and Dr Matsumoto is that of plastic surgery, as better understanding of where blood vessels lie will aid safer and more accurate reconstructive surgery. In imaging areas of the thigh, the researchers hope to improve surgical approaches, as knowing precisely where blood vessels lie will enable surgeons to use the most effective approach. For example, when removing a skin flap from the thigh for reconstructive surgery elsewhere in the body, this knowledge is particularily useful since a good blood supply is crucial to the success of the transplanted flap of tissue.

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Another example of how this approach could be used is in the evaluation of drug treatments, as it would be possible to evaluate the impact of treatment on blood vessels as well as monitor the effectiveness of treatment, thereby encouraging a more personalised approach to treatment that could be tailored according to the reaction of an individual to a particular treatment. Novel drugs which target angiogenesis could be useful in interrupting disease progression. Furthermore, the good blood supply to tumours provides the opportunity for effective drug delivery to combat tumour growth.

Future studies done by the group at Kyoto University will aim to identify unusual characteristics of oxygen saturation patterns which are consistently associated with tumour-related blood vessels. They propose the use of artificial intelligence to achieve this, as well as the option to use a combination of PAI and other imaging techniques. Using a high-resolution imaging modality, such as PAI, will ultimately help elucidate underlying mechanisms of cancers and other vascular-associated conditions.


In addition to breast cancer, do you have plans to explore the use of PAI for other diseases?

Future studies will examine vascular disorders and damages caused by lifestyle-related diseases or infectious diseases.

 

References

  • Matsumoto, Y., Asao, Y., Sekiguchi, H., Yoshikawa, A., Ishii, T., Nagae, K. I., Kobayashi, S., Tsuge, I., Saito, S., Takada, M., Ishida, Y., Kataoka, M., Sakurai, T., Yagi, T., Kabashima, K., Suzuki, S., Togashi, K., Shiina, T., & Toi, M. (2018). Visualising peripheral arterioles and venules through high-resolution and large-area photoacoustic imaging. Scientific reports, 8(1), 14930. Available at: https://doi.org/10.1038/s41598-018-33255-8

Research Objectives

Dr Toi and Dr Matsumoto utilise photoacoustic imaging to visualise tumour-associated vessels in the human breast in vivo.

Funding

  • ImPACT programme, Cabinet Office, Japan

Collaborators

  • Luxonus Inc.
  • Canon Inc.

Bio

Masakazu Toi, MD, PhD, is a professor at the Department of Breast Surgery, Kyoto University Graduate School of Medicine. Dr Toi has been widely involved in the diagnosis, treatment and research of breast cancer, participating in national and international clinical trials and industry-academic collaborations. He has extensive expertise in tumour angiogenesis and is a leading researcher on the photoacoustic imaging in breast cancer and other vascular diseases.

Masakazu Toi

Yoshiaki Matsumoto, MD, PhD is a programme-specific assistant professor at the Department of Breast Surgery, Kyoto University Graduate School of Medicine. Among his research contributions are the studies on cancer metabolism and the clinical application of the photoacoustic imaging technique. He has been working on the application of artificial intelligence to photoacoustic images of tumour vessels in breast cancer to study the normalisation of them induced by drug treatment.

Yoshiaki Matsumoto

Contact
Kyoto University
54 Shogoin
Kawahara-cho, Sakyo-ku
Kyoto 606-8507, Japan

E: toi@kuhp.kyoto-u.ac.jp
E: yoshiaki@kuhp.kyoto-u.ac.jp
T: +81 75 751 3660
W: https://www.kuhp.kyoto-u.ac.jp/department/departments/breast-surgery.html
W: http://www.brca.jp/

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