I’m using transfer learning to classify binary medical MRI brain images. I used 5-fold cross validation, SGD optimizer and scheduler with different learning rate with 1e-4 and 1e-5 to get the following results. The learning curve and the accuracy have weird behavior and I don’t know how to interpret and improve. Please help me indicate the problems and the things to try on.
The optimizer and scheduler settings I used:
params = [
            { 'params': parameters['base_parameters'], 'lr': sets.learning_rate }, 
            { 'params': parameters['new_parameters'], 'lr': sets.learning_rate }
            ]
        
optimizer = torch.optim.SGD(params=params, momentum=0.9, weight_decay=1e-4)
scheduler = optim.lr_scheduler.ExponentialLR(optimizer, gamma=0.95)

Averaged learning curve and accuracy results with lr = 1e-4(average over 5 folds):

Screen Shot 2021-04-29 at 12.28.57 PM909×280 29.4 KB

Averaged learning curve and accuracy results with lr = 1e-5:

Screen Shot 2021-04-29 at 12.29.09 PM917×283 28.8 KB

So I have a few questions regarding the results.

Will the result be different if I change from SGD to Adams with the same learning rate?
In lr = 1e-4 situation, the model predicts single class mostly. As I searched in the forum, people said with lower learning might save the issue and indeed with  lr = 1e-5, the model gives more diverse results (still poor accuracy). However, the train/validation accuracy stops improving and yet fluctuating after certain epochs. What might be the reason for this?
The loss  curve stops decreasing around 2.5, but in normal situations, isn’t the loss value lower than 1? Why is my loss value stops at 2.5?
The data augmentation method I used was randomly select the brain regions during training (i.e. some regions might be ignored). Will this cause the accuracy not improving as shown in the curve? Or should I use other data augmentation method (e.g. train on the whole brain with rotations)?

It might be complicated questions to answer, but i would be so appreciated if anyone could help me with this. Thank you!!