Flat Multi-Condition Head

Overview

When predicting severity grades (Normal/Moderate/Severe) for multiple conditions simultaneously (e.g., 5 spinal conditions × 5 vertebral levels = 25 labels, each with 3 classes), a flat 75-dimensional output head avoids the complexity of multiple classification heads. During training, use cross-entropy on all 75 logits. At inference, slice the output into 25 groups of 3 and apply softmax per group. This is simpler to implement, faster to train, and often matches multi-head performance.

Quick Start

import torch
import torch.nn as nn
import timm

N_LABELS = 25       # conditions × levels
N_CLASSES = 3       # normal, moderate, severe
N_OUTPUT = N_LABELS * N_CLASSES  # 75

class MultiConditionModel(nn.Module):
    def __init__(self, model_name, in_chans=30):
        super().__init__()
        self.backbone = timm.create_model(
            model_name, pretrained=True,
            in_chans=in_chans, num_classes=N_OUTPUT, global_pool='avg'
        )

    def forward(self, x):
        return self.backbone(x)  # (B, 75)

# Training: reshape for per-label CE loss
logits = model(images)  # (B, 75)
logits = logits.view(-1, N_LABELS, N_CLASSES)  # (B, 25, 3)
loss = nn.CrossEntropyLoss()(logits.view(-1, N_CLASSES), labels.view(-1))

# Inference: per-condition softmax
logits = model(images)[0]  # (75,)
for i in range(N_LABELS):
    probs = logits[i*3:(i+1)*3].float().softmax(0).cpu().numpy()
    predictions.append(probs)

Workflow

1. Set num_classes = N_labels × N_classes in the backbone
2. Train with CE loss on reshaped (B × N_labels, N_classes) logits
3. At inference, slice flat output into per-label chunks
4. Apply softmax per chunk to get per-condition probability distributions

Key Decisions

• Flat vs multi-head: Flat is simpler; multi-head allows per-condition learning rates
• Loss weighting: Weight rare conditions higher with class weights in CE loss
• Shared backbone: All conditions share features — works well when inputs overlap
• Label ordering: Keep consistent ordering between training labels and output slicing

References

• RSNA2024 LSDC DenseNet Submission
• RSNA2024 LSDC Submission Baseline