Ca2+ transient detection and segmentation with the Astronomically motivated algorithm for Background Estimation And Transient Segmentation (Astro-BEATS)

Bolin Fan; Anthony Bilodeau; Frederic Beaupre; Theresa Wiesner; Christian Gagne; Flavie Lavoie-Cardinal; Renee Hlozek

q-bio.NCMar 19, 2026

Ca2+ transient detection and segmentation with the Astronomically motivated algorithm for Background Estimation And Transient Segmentation (Astro-BEATS)

Authors: Bolin Fan, Anthony Bilodeau, Frederic Beaupre, Theresa Wiesner, Christian Gagne, Flavie Lavoie-Cardinal, Renee Hlozek

Organizations: Dunlap Institute for Astronomy and Astrophysics, University of Toronto, Toronto, Canada · David A. Dunlap Department for Astronomy and Astrophysics, University of Toronto, Toronto, Canada · CERVO Brain Research Center, Québec, Canada · Institute Intelligence and Data, Université Laval, Québec, Canada ·

Abstract

Fluorescence-based Ca $^{2+}$ -imaging is a powerful tool for studying localized neuronal activity, including miniature Synaptic Calcium Transients, providing real-time insights into synaptic activity. These transients induce only subtle changes in the fluorescence signal, often barely above baseline, which poses a significant challenge for automated synaptic transient detection and segmentation. Detecting astronomical transients similarly requires efficient algorithms that will remain robust over a large field of view with varying noise properties. We leverage techniques used in astronomical transient detection for miniature Synaptic Calcium Transient detection in fluorescence microscopy. We present Astro-BEATS, an automatic miniature Synaptic Calcium Transient segmentation algorithm that incorporates image estimation and source-finding techniques used in astronomy and designed for Ca $^{2+}$ -imaging videos. Astro-BEATS outperforms current threshold-based approaches for synaptic Ca $^{2+}$ transient detection and segmentation. The produced segmentation masks can be used to train a supervised deep learning algorithm for improved synaptic Ca $^{2+}$ transient detection in Ca $^{2+}$ -imaging data. The speed of Astro-BEATS and its applicability to previously unseen datasets without re-optimization makes it particularly useful for generating training datasets for deep learning-based approaches.

Figures & tables

Panel of Figure 1 — Figure 1: Overview of Astro-BEATS in the context of mSCTs analysis. Raw Ca 2+ -imaging videos are processed using Astro-BEATS , an unsupervised source-finding algorithm used to generate segmentation masks of transients in these videos. The RHT [ 10 ] is used to generate a dendritic foreground mask, a mask of pixels that contain the foreground GCaMP fluorescence signal (top row, described in Supplementary Material A ). A dynamic estimate of the GCaMP6f fluorescence signal in the dendrites is used to estimate the average foreground in each frame. We take the difference image between this weighted foreground and our raw Ca 2+ video to isolate mSCTs from the foreground (middle row, described in Supplementary Material B ). Finally, density-based spatial clustering of applications with noise (DBSCAN) source-finding is used on pixels in the difference image to identify and segment transients (bottom row, described in Section 2.2.2 ).

Panel of Figure 2 — Figure 2: A) Precision, Recall and F 1 Score for mSCT detection by Astro-BEATS , IDT and AQuA (N=9 videos). The center line represents the median value while, the boxes represents the 2 nd and 3 rd quartile range. Whiskers represent the maximum and minimum value and raw data points are plotted beside each box plot. B) Comparison of transients detected by Astro-BEATS compared to manual ground truth annotations (True positive (TP); gray, FN; red, FP; blue). Volume ( $V$ ) is expressed in units of voxels (1 voxel = 0.00256 $\mu$ m 2 ms) and maximum brightness ( $B_{\mathrm{max}}$ ) is obtained from the transient signal map ( $\sigma$ ). See Supplementary Table 2 for descriptions of these variables. C) Comparison of transients detected by Astro-BEATS and IDT (detected by both algorithms: green, detected by Astro-BEATS only: grey, detected by IDT only: black cross, missed by both algorithms: red). D) Example images of out-of-focus, synaptic and dendritic transients during their temporal maximum. We also show the image 1s before the temporal peak of each transient. E) The $V$ and $B_{\mathrm{max}}$ of each transient types. F) The proportion of manually annotated Ca 2+ transients recovered by Astro-BEATS and IDT , for various types of Ca 2+ transients.

Panel of Figure 3 — Figure 3: A) Segmentation DiCE scores calculated over manually segmented transients, using expert segmentation masks as ground truth. Since there is variation between expert annotations, we use inter-expert agreement DiCE scores as a baseline of comparison for segmentation performance ( Section 3.1.1 ). When comparing segmentation DiCE score across transient types described in Section 2.1.1 , we find that Astro-BEATS has comparable DiCE scores to IDT for synaptic transients, while having improved DiCE scores for dendritic and out-of-focus events. C) Example Annotations generated by Astro-BEATS compared to expert annotations. We chose example transients with Astro-BEATS DiCE segmentation scores closest to the median.

Parameter	Astro-BEATS	IDT	AQuA
Computation time	$\sim 0.6$ s/frame	$\sim 30$ s/transient	$\sim 30$ min/video
	( $\sim 5$ min/video)	( $\sim 3$ hr/video)
Annotation Precision	0.92 $\pm$ 0.06	0.94 $\pm$ 0.10	0.56 $\pm$ 0.34
Annotation Recall	0.75 $\pm$ 0.14	0.38 $\pm$ 0.18	0.34 $\pm$ 0.22
Annotation F 1 score	0.81 $\pm$ 0.11	0.50 $\pm$ 0.17	0.39 $\pm$ 0.26
Segmentation DiCE score	0.66 $\pm$ 0.18	0.58 $\pm$ 0.21	0.41 $\pm$ 0.24

Table 1: Comparison of performance of mSCT segmentation and detection methods. In addition to improvements in annotation and comparable performance in segmentation, Astro-BEATS is significantly faster than other approaches.

Panel of Figure 4 — Figure 4: A) Precision, Recall, and F 1 score for the optimal performing (see Section 2.5 ) 3D U-Net method trained using Astro-BEATS and IDT annotations as ground truth. Despite similar mean values, the variance in both Precision and Recall is reduced for the 3D U-Net AB compared to the 3D U-Net IDT and Astro-BEATS only. With the exception of a single video, 3D U-Net AB annotations yielded scores of $\mathrm{F}_{1}>0.8$ . B) DiCE score distribution for Astro-BEATS , the 3D U-Net AB , and the 3D U-Net IDT . A cutoff of DiCE $>$ 0.45 is assigned to shown the fraction of segmentations with shapes vastly different from the expert segmentation. Some examples of these non-representative segmentation generated using Astro-BEATS and improved using 3D U-Net AB are shown in C).

Appendix figures & tables8 assets

Supplementary material from the paper’s appendix.

Appendix

Supplementary Fig 1: Large-scale fluctuation in the brightness of GCaMP6f on the dendritic shaft. The dendrite appears brighter at $t=0~\mathrm{s}$ compared to $t=30~\mathrm{s}$ . These brightness fluctuations are accounted for in Astro-BEATS by taking a separate time-dependent background estimate for each frame.

Supplementary Fig 2: A) Optimization of the DBSCAN parameters. The range of DBSCAN parameters that yield good F 1 scores are shown in Table 1 . We find that for a range of $400<\mathrm{Min}_{\mathrm{pts}}<700$ and $5\sigma<\mu<9\sigma$ , we obtain F ${}_{1}>0.7$ , which we consider to be acceptable. B) Precision, Recall and F 1 scores obtained by finding the DBSCAN parameters that achieve the highest F 1 score for one video, and applied over the remaining 9 test videos.

Supplementary Fig 3: A) Comparison of the performance of AQuA , IDT , and Astro-BEATS before and after manual expert reverification. B) We see that, prior to manual expert reverification, many TP detections made by Astro-BEATS are falsely labeled as FP or FN, despite their high brightness and volume due to misalignment error. C) Reverification allows us to better quantify the true number of FN and FP detections.

Supplementary Fig 4: A) TP detections made by Astro-BEATS that were classified as FP or FN due to misalignment error. B) Small and dim transients detected using Astro-BEATS but missed through manual annotation of the original testing dataset.

Supplementary Fig 5: A) Precision and Recall of Astro-BEATS on the out-of-distribution dataset described in Ref. [ 5 ] . Astro-BEATS parameters were not re-optimized for this new dataset and parameters listed in Supplementary Table 1 were used. This represents a hypothetical use case for applying Astro-BEATS to a new dataset without further fine tuning. B) Example raw data and transient segmentation maps taken from a Ca 2+ -imaging videos of this out-of-distribution dataset. Red circles in the raw video indicate detected transients in the segmentation map.

Parameter	Description	Range Tested	Value used here
$\mu$	Initial brightness threshold	$[3,11]~\sigma$	$7\sigma$
$\mathrm{Min}_{\mathrm{pts}}$	Minimum points per cluster	$[100,1000]$	500
$\epsilon$	Maximum spatial separation	$[3,11]$ pixels	7 pixels
$\epsilon_{t}$	Maximum time separation	$[1,3]$ frames	2 Frames

Appendix

Supplementary Table 1: Parameters of the DBSCAN algorithm. Values used in this work are obtained using the procedure outlined in Supplementary Material C (see Supplementary Figure 2 ).

Parameter	Description
$A$	Area of 2D projection of detection within aperture
	in number of square pixels
	(1px 2 = 0.0256 $\mu$ m 2 ).
	Note that detections with footprint area larger
	than our aperture ( Section 2.2.3 ) will be
	recorded as filling our entire aperture, having

Appendix

Supplementary Table 2: Properties of mSCTs calculated for each segmented event in the Ca 2+ -imaging video.

Parameter	Range Tested	Optimal Value Found
Smoothing	$[0.5,2]\sigma$	0.5 $~\sigma$
Minimum size of groups of
connected tentative active voxels	$[5,15]$ voxels	10 voxels

Appendix

Supplementary Table 3: Range of AQuA parameters used to optimize algorithm performance.

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