# Strategy 1: Drop — remove unreadable or empty images
def is_valid_image(path):
    try:
        img = cv2.imread(path, cv2.IMREAD_GRAYSCALE)
        if img is None:
            return False
        if img.mean() < 5:           # nearly black
            return False
        if img.shape[0] < 50 or img.shape[1] < 50:  # too small
            return False
        return True
    except Exception:
        return False

# Strategy 2: Impute — rare for images, but possible (e.g., in painting to fill in missing patches). Generally avoided for diagnostic data.

# Strategy 3: Flag — track which patients are missing which modalities,
#   and let the model condition on availability. Common in multi-modal healthcare ML.