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I am working on a skin disease classification problem where I have successfully created a classifier ( TensorFlow + Keras ) which can classify images of two skin diseases.

The sample image needs to be classified in this manner :

1. Whether the sample is an image of the skin.


2. Does the skin have any two of the diseases ( Melanoma or Psoriasis )


3. If a disease is found, to which class does it belong (CLASS1: Melanoma or CLASS2: Psoriasis

How can create Classifiers which could carry out the following tasks?
Do I need the image localization or CNNs or something like YoLo?
What steps should I implement?

I have created a classifier ( with an accuracy of 96% ) which can classify an image of the two diseases efficiently. But it can't detect the presence of the disease ( Step 2 in the above task list ).

The other answers are correct, I just want to expand since you seem to wonder where step 1. fits in.

I think you should add yet another class called Unknown. This class will be able to tell that it is not human skin, but preferably it should be even more precise. It also should be able to tell if a picture is a good input for disease detection. You only want to make classifications on data that is "similar enough" to data you trained on.

Examples of pictures to train as Unknown could be:

• Random pictures of whatever


• Picture containing human skin but is too far away


• Picture containing human skin but with a bad resolution / blurry


• Picture containing human skin but with bad lighting


• etc

When you got the negative data for the Unknown class your can use it in one of two ways. Either you train a general model that does everything in one or you train two specialist models.

General model

Build a general model for all your problems by making every distinction into classes. You can do this by adding another class of Unknown to the list @thanatoz wrote:

Healthy: 0
Melanoma: 1
Psoriasis: 2
Unknown: 3

Specialist models

Another approach is to built two models. One for skin detection and then one for disease detection, separating step 1 and step 2 in your list.

Skin detector - The first model will not know anything about diseases, it will only tell if a picture contains human skin or not. So the classes would be:

Unknown: 0
Skin: 1

Disease detector - This model is only activated if the skin detector has verified that you have a sample of representative human skin. This would then work the same as @thanaztoz answer with the classes being:

Healthy: 0
Melanoma: 1
Psoriasis: 2

You have to have sample tissues from skins with Melanoma, Psoriasis and Healthy Skins.

As is understood in the question, you developed a solution which can distinguish with 96% accuracy between melanoma and psoriasis. You are missing from the possibility of distinguish between sick and healthy skins.

The model you are looking for is a multiclass classification. Where CLASS3: Healthy skin.

You are doing ok so far, you just need to include this category and images of healthy skins in your model.

I suggest that you look up one-class classification.

When we say classification, we by default talk about so-called discriminative classifiers. I.e., models where we assume each sample to belong to exactly one of the pre-defined classes.

This doesn't work well with your application:

• as you already say, what if an image is not of skin at all?
  
  This "no-skin" class is ill-defined: besides not being skin, it could be anything. Such ill-defined classes are extremely difficult to train with a "normal" (discriminative) classifier because is is so hard to positively describe this class.


• Psoriasis does not prevent melanoma (actually, a quick internet search suggests that psoriasis patients have a higher cancer risk, including certain skin tumors): i.e. your disease classes are not mutually exclusive.

One-class classification relaxes this assumption: it models each class independently of all other classes, thus:

• it doesn't rely on having all classes that could appear in the samples pre-defined.
  
  In your case: what about other skin diseases? They certainly exist - and unless you're looking at a differential diagnosis (I'm not a medical doctor - but I do have the impression that melanoma vs. psoriasis


• a sample may be predicted to belong to none of the known classes.


• a sample may be predicted to belong to more than one class (e.g. "normal skin" and "psoriasis" or even all three: the image may contain some normal and some diseased skin.)


• In general, one-class methods deal well with "not-classes", i.e. where we have a well-defined class and the rest is not well defined ("not psoriasis" - could be anything from imitation leather to some kind of dermatitis)


• Consider the difference between skin (as in: not artificial leather, not t-shirt, no kitten etc.) vs. normal skin (as in: no skin disease). You may want to set up 2 different classes for that.

A quick search got me to 2 papers on arXiv that may be a good starting point for deep learning one-class classifiers: Learning Deep Features for One-Class Classification and Anomaly Detection using One-Class Neural Networks

However, these advantages of one-class classifiers are not avalable for "free": comparing one-class and discriminative classifiers for the same situation, you usually need more training examples to get the same predictive performance for the one-class classifier.

Generally speaking, medical diagnosis distinguishes between "diagnosis" and "differential diagnosis". Differential diagnosis means that there is information already that excludes many things, and the remaining question is to decide between a known list of possible diseases.

While differential diagnosis can be handled with discriminative classifiers, other diagnoses usually call for one-class architecture.

One class methods rely exclusively on examples of the "in-class" for training. For verification/validation, however, you need to carefully chose the out-of-class test samples. The recommendation is to find samples of the out-class which are expected to be hardest to get right.

In your case, talking to the medical doctors about differential diagnoses of psoriasis and melanoma may reveal which other skin lesions look similar to your target classes.

In addition, as you say, it will be good to make sure your classifier works on a number of BS samples (non-human skin, leather, artificial leather).

Another consideration that is separate from the one-class vs. discriminative classification:

• Do you want to detect the actual lesions, or


• Do you want to classify a patient of having psoriasis (or melanoma) even if the image you have does not contain a lesion ("If you have skin like this, you probably have a lesion somewhere")
  
  For both diseases this is thinkable: psoriasis is a systemic disease, so there could be specific characteristics of the skin even outside the lesion areas. Sun-exposure increases the melanoma risk - other parts of the skin may have characteristic changes of that photo-damage that are not yet cancer.

This is a simple problem of Multi-class classification having 2 classes:

'Healthy':0,
 'Melanoma':1,
 'Psoriasis':2

You can build a custom classifier to detect these images or fine-tune a pre-trained model to detect the following diseases. In the last layer of the model, the model could be given a threshold value of confidence above which the prediction id to be kept or not.

Apart from these, as these disease are going to exist even if there is a small patch somewhere, what could be done is using TTA (Test Time Augmentation) to find the diseases. You can read more about TTA here but will need to figure out some way of implementing the same in Keras.