Photoacoustic Tomography Appearance of Fat Necrosis: A First-in-Human Demonstration of Biochemical Signatures along with Histological Correlation

A 50-year-old woman with no past medical history presented with a left anterior chest wall mass that was clinically soft, mobile, and non-tender. A targeted ultrasound (US) showed findings suggestive of a lipoma. However, focal “mass-like” nodules seen within the inferior portion suggested malignant transformation of a lipomatous lesion called for cross sectional imaging, such as MRI or invasive biopsy or excision for histological confirmation. A T1-weighted image demonstrated a large lipoma that has a central fat-containing region surrounded by an irregular hypointense rim in the inferior portion, confirming the benignity of the lipoma. An ultrasound-guided photoacoustic imaging (PA) of the excised specimen to derive the biochemical distribution demonstrated the “mass-like” hypoechoic regions on US as fat-containing, suggestive of benignity of lesion, rather than fat-replacing suggestive of malignancy. The case showed the potential of PA as an adjunct to US in improving the diagnostic confidence in lesion characterization.

Fat necrosis, or cell death of adipose tissue, is a common benign condition that occurs from the lack of oxygen supply to adipose tissue [1]. As common causes include trauma or post-surgical changes [2], fat necrosis often presents as a palpable soft tissue mass at superficial regions [3].
Ultrasound (US) is the first-line imaging tool for these superficial lesions, but imaging appearances are extremely varied [4] due to the age of the lesion, which manifests as varying degrees of hardening, fibrosis, and degeneration. This often results in a diagnostic dilemma, which necessitates further cross-sectional imaging or invasive procedures, such as biopsy or excision for histological confirmation (Figures 1 and 2). There is, hence, an unmet clinical need for an adjunct imaging modality to US to improve diagnostic capability. Figure 1. A 50-year-old woman with no past medical history presented with a left anterior ches wall mass. Clinically, the mass was soft, mobile, and non-tender. (A): US image of the left anterio chest wall mass shows a wider than tall mass with well-circumscribed margins (bold arrows). contains internal fat echogenicity and is in keeping with a fat-containing lesion (i.e., lipoma). Withi the inferior portions (dotted arow), there are focal "mass-like" hypoechoic nodules seen, which ar surrounded by a hyperechoic capsule. (B): Doppler US of the focal "mass-like" nodules performed demonstrated mild increased peripheral vascularity around the hyperechoic capsule but no in creased vascularity within the hypoechoic "mass-like" nodules. Findings were indeterminate fo malignant change of a lipomatous lesion. Photoacoustic (PA) tomography, a hybrid optical imaging modality, is based on th light-induced ultrasound waves providing the contrast of optical imaging combined wit the high spatial resolution of ultrasound [5]. Its ability to provide the distribution of en dogenous chromophores, such as blood oxygenation [6], water [7], lipid [7][8][9][10] and re cently collagen [11,12], makes it attractive as a potential adjunct tool to various aspects o   Photoacoustic (PA) tomography, a hybrid optical imaging modality, is based on the light-induced ultrasound waves providing the contrast of optical imaging combined with the high spatial resolution of ultrasound [5]. Its ability to provide the distribution of endogenous chromophores, such as blood oxygenation [6], water [7], lipid [7][8][9][10] and recently collagen [11,12], makes it attractive as a potential adjunct tool to various aspects of Photoacoustic (PA) tomography, a hybrid optical imaging modality, is based on the light-induced ultrasound waves providing the contrast of optical imaging combined with the high spatial resolution of ultrasound [5]. Its ability to provide the distribution of endogenous chromophores, such as blood oxygenation [6], water [7], lipid [7][8][9][10] and recently collagen [11,12], makes it attractive as a potential adjunct tool to various aspects of ultrasound. This is particularly useful in regions abundant with these chromophores, such as superficial soft tissues and the breast, or in fat-containing and fibrotic/necrotic conditions, such as fat necrosis. However, these theoretical advantages and usages of chromophore differentiations have not been demonstrated in daily clinical usage. Herein, we present interesting images that demonstrate for the first time the biochemical signatures of fat necrosis derived by PA and its agreement with histopathology ( Figure 3). A detailed description of PA imaging protocol and image reconstruction is included in Appendix A. This work showcases the potential of PA as an adjunct for US to improve the diagnostic confidence for fat necrosis. ultrasound. This is particularly useful in regions abundant with these chromophores, such as superficial soft tissues and the breast, or in fat-containing and fibrotic/necrotic conditions, such as fat necrosis. However, these theoretical advantages and usages of chromophore differentiations have not been demonstrated in daily clinical usage. Herein, we present interesting images that demonstrate for the first time the biochemical signatures of fat necrosis derived by PA and its agreement with histopathology ( Figure 3). A detailed description of PA imaging protocol and image reconstruction is included in Appendix A. This work showcases the potential of PA as an adjunct for US to improve the diagnostic confidence for fat necrosis.  In this case, the authors have successfully demonstrated biochemical features of fat necrosis on PA as a first in-human demonstration with histopathological correlation. PA was able to identify the "mass-like" hypoechoic regions on US as fat-containing, rather than fat-replacing. On pathology, the lipid signals on PA correspond to liquefied necrotic fat from cystic degeneration, while the collagen signals on PA correspond to the fibrosis around the cavity. Hence, the biochemical capability of fat and collagen characterization could help to resolve ambiguous findings on US and improve diagnostic confidence for fat necrosis.
As it is crucial to obtain pathological correlation with US-PA images, excised tissues with no active blood signals had to be obtained. The authors believe the incorporation of blood signals in in vivo imaging would further intensify our understanding of the pathophysiology and respective imaging correlations for fat necrosis. Although the results from a single case may seem promising, more work must be done to validate these findings. In particular, more work must be done to validate findings of benign lipomas, fat necrosis, and malignant lipomatous tumors to investigate their biochemical differences. With more data, PA could potentially translate downstream into clinical imaging workflows for better characterization of superficial/breast lumps where fat-containing lesions are common. However, for widespread clinical adoption, there needs to be vast improvements in both hardware and software for PA imaging to improve its imaging depth (to at least 3-4 cm), its field of view, as well as spectral coloring.  Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Appendix A Appendix A.1. Materials and Methods
The excised lipoma was collected fresh from the operating theatre and subjected to US-PA imaging using the commercially available MSOT inVision 512-echo system (iThera Medical GmbH, Munich, Germany), fitted to a customized 2D handheld probe (specifications: 2D array of 256 detector elements (arranged along a 125 • arc on a spherical surface: radius 40 mm and 5 MHz centre frequency)). The detector was placed on the inferior portions of the lipoma to image the "mass-like" nodules.
PA images were acquired at near infrared wavelengths-700, 730, 760, 800, 850, 920, 930, 970, 1000, 1050, 1064, and 1100 nm-to allow for deep-tissue imaging. Big differences in absorption of light at these wavelengths by chromophores in breast tissue, such as blood, lipid, and collagen, have aided the unmixing of these chromophores. Data were processed using ViewMSOT 3.8 software (Release 3.8, Munich, Germany) and reconstructed using the backprojection algorithm, after applying a bandpass filter with cut-off frequencies of 50 kHz and 6.5 MHz. The distribution of collagen and lipid were visualized through spectral unmixing of the reconstructed data.