Is data['image'] returning a numpy array?
If so, you could just slice it using e.g. data['image'][:4], if you want to get the first 4 images.
You could try to write your custom Dataset, load all data in __init__, and get a single sample in __getitem__. Here is some dummy example:
class MyDataset(Dataset):
    def __init__(self, mat_path):
        data = io.loadmat(mat_path)
        self.images = torch.from_numpy(data['images'])
        self.targets = torch.from_numpy(data['tumorMask'])

    def __getitem__(self, index):
        x = self.images[index]
        y = self.targets[index]
        return x, y

    def __len__(self):
        return len(self.images)

Also, have a look at the Data loading tutorial 103 to see how the Dataset and DataLoader play together.