arXiv:2610.40705v1 Announce Type: new Abstract: Despite their scale, large medical imaging datasets may underrepresent clinically relevant pathological presentations. Synthetic data can expand coverage, but findings must remain compatible with surrounding anatomy. This is challenging in ultrasound, where findings resemble surrounding tissue and must be positioned relative to anatomical landmarks. We propose DiffUltra, a task-guided latent diffusion pipeline that models pathological structure and spatial context, and screens synthetic images for downstream utility. DiffUltra conditions generation on structural masks decomposing findings into anechoic, hypoechoic, and hyperechoic subregions. Two placement strategies insert findings into pathology-absent backgrounds or replace existing findings with structurally matched pathology from other patients. A filter trained exclusively on real data retains images whose predictions agree spatially with target boxes. We evaluate DiffUltra for lung ultrasound detection of pleural effusion and consolidation, using over 700,000 expert-annotated frames from 598 patients at 12 sites. Averaged across three real-data scales, synthetic insertion significantly improved video-level AUROC for both pleural effusion (p = 0.017) and consolidation (p < 0.01). For pleural effusion, frame-level average precision improved by 4.5 percentage points on average and by 6.1 points using only 10% of real data (p < 0.01 for both). Synthetic insertion also yielded larger average precision gains than an existing diffusion-based lesion-insertion method at every scale. At full scale, targeted synthesis improved AUROC for both pathologies by 1.1-3.9 points in an underrepresented anatomical zone and on a held-out acquisition system. These results suggest that anatomically constrained, task-guided synthesis can help address coverage gaps in ultrasound training data.