HHM-ZUNet: hybrid hierarchical model based on Z-Net and modified U-Net++ with variable multi-attention for brain tumor detection and classificationNarmada Kari, Sanjay Kumar Singh and Roshan M. BodileEur. Phys. J. Plus, 140 8 (2025) 805DOI: https://doi.org/10.1140/epjp/s13360-025-06736-9