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High School Poster

HS Poster

Poster 111 - Minimizing Motion Artifacts During Photoacoustic Microscopy Using ECG Gating

Saturday, October 26, 2024
10:00 AM - 12:00 PM EST
Location: Exhibit Hall E, F & G

Introduction: Cardiovascular diseases impact 523 million people globally, with heart disease accounting for 85% of related deaths. Studying the cardiovascular system, particularly the myocardium and vascular oxygen saturation, is vital for early disease detection, timely intervention, and improved patient outcomes.
Photoacoustic microscopy (PAM) is an advanced imaging technique that offers unique advantages in studying vascular properties. First, PAM is label-free because it uses endogenous optical absorption. Second, PAM can reach a micrometer resolution, being able to resolve single red blood cells. Third, PAM can provide functional microvascular parameters, such as saturation of hemoglobin and blood flow speed. These advantages make PAM a good approach to studying the heart vessels.
In PAM imaging, a sample irradiated by a short pulsed light thermally expands and instantaneously generates ultrasound (US) waves, which are detected by a US transducer and used to reconstruct images. However, PAM cannot be used directly to image a beating heart because of cardiac motion artifacts. In a preclinical mouse model, the heart moves a few millimeters regularly, making capillaries, typically < 10 micrometers wide, blurred or distorted. PAM’s imaging quality is thus significantly affected. Because even motions in the range of 10 µm can result in significant blurring and distortion of vascular structures, minimizing motion artifacts is crucial for accurate imaging. In this work, we reviewed reported methods for reducing motion artifacts in in vivo heart imaging. We also proposed a novel ECG gating-aided PAM system to minimize the motion artifact in heart imaging with PAM.

Materials and

Methods: ECG gating: We used PowerLab and LabChart to obtain ECG signals from the mouse. After obtaining the first ECG cycle and recording the amplitude values for the peak (Ap) and trigger point (At) we identify the motionless period (tML) in subsequent ECG cycles by calculating a trigger point (t0, At) and the delay time (Δt). All these timings are clearly indicated in Fig.1 and processing steps 1-7 are shown in Fig.2. To allow enough time for signal transmission and processing, we use two neighboring ECGs for gating. PAM imaging will be performed during the motionless period (tML).
PAM imaging of beating mouse heart: In the PAM system, we used a nanosecond pulsed laser as the light source. The light is focused on the imaging sample, i.e., a beating mouse heart, which is mounted on a glue holder for global motion removal. The illuminated sample absorbs the photons, heats up, and expands, generating acoustic waves, which are detected by a US transducer. We used Galvo mirrors for 2D imaging of a field of view (FOV) of 40 x 40 µm2. in one imaging period. By capturing multiple such FOVs, we can stitch them together to form a larger image. The resolution can reach 3 µm, allowing us to resolve 6-8 µm wide capillaries.
Motion artifact quantification: Different frames were aligned, and the lateral shifts of the same spot were measured as the motion artifact.

Results, Conclusions, and Discussions:

Results: We reviewed three reported methods for reducing motion artifacts in mouse heart imaging: suction stabilizer, glue stabilizer, and glue stabilizer combined with ECG gating.
When there is no motion suppression method applied, the movement of the heart is typically in the order of several millimeters because of respiration and cardiac motion.2 To limit the global motion artifact, mouse hearts were mounted to holders: the suction holder in Ref [2] reduced the motion artifact to < 2-5 μm in endoscopic fluorescence imaging, and the glue holder in Ref [3] reduced it to about 4 μm in a laser scanning microscope. Adding ECG gating to the glue holder setup can further reduce the motion artifact to 2 μm, facilitating studying the 6-8 μm wide capillaries that are important to study heart physiology.
Our experimental results using PAM show motion artifacts of 6 μm when only using the stabilizer [Fig.3]. Because our future goal is to achieve ~0.5 μm motion artifacts in PAM imaging, we propose introducing ECG gating into PAM imaging. [Fig.4] The detailed workflow of the system can be seen in Fig.2. Specifically, in steps 1-7, an imaging window of ~80 ms with minimal motion artifact is identified with ECG gating, and in steps 8-13, PAM imaging of a 40 x 40 μm2 field of view (FOV) is performed. For a larger region of interest, we can divide it into small FOVs and perform imaging multiple times and reconstruct the whole image later.
Conclusion and

Discussion: With ECG gating, the quality of PAM imaging on the beating heart can be significantly improved. The capillaries will be much less blurry or distorted. These images can help understand the physiology of heart vessels and thus heart diseases to provide insight into diagnosis and treatment.

Acknowledgements (Optional):

References (Optional): 1. Hu S, Wang LV. J Biomed Opt. 2010;15:011101.
2. Jung K, Kim P, Leuschner F, Gorbatov R, Kim JK, Ueno T, Nahrendorf M, Yun SH. Circ Res. 2013;112
3. Lee S, Vinegoni C, Feruglio PF, Fexon L, Gorbatov R, Pivoravov M, Sbarbati A, Nahrendorf M, Weissleder R. 2012;3:1054.
4. Sudhir R, Rajni D, Qingyou D, Sukhodub A, Jovanović S, Jovanovic A. Pflugers Arch. 2020;472:1-9.