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卷积神经网络被大规模的应用在分类任务中,输出的结果是整个图像的类标签。但是UNet是像素级分类,输出的则是每个像素点的类别,且不同类别的像素会显示不同颜色,UNet常常用在生物医学图像上,而该任务中图片数据往往较少。所以,Ciresan等人训练了一个卷积神经网络,用滑动窗口提供像素的周围区域(patch)作为输入来预测每个像素的类标签。这个网络有两个优点:(1)输出结果可以定位出目标类别的位置;(2)由于输入的训练数据是patches,这样就相当于进行了数据增强,从而解决了生物医学图像数量少的问题。
(1)UNet采用全卷积神经网络。 (2)左边网络为特征提取网络:使用conv和pooling (3)右边网络为特征融合网络:使用上采样产生的特征图与左侧特征图进行concatenate操作。(pooling层会丢失图像信息和降低图像分辨率且是永久性的,对于图像分割任务有一些影响,对图像分类任务的影响不大,为什么要做上采样呢?上采样可以让包含高级抽象特征低分辨率图片在保留高级抽象特征的同时变为高分辨率,然后再与左边低级表层特征高分辨率图片进行concatenate操作) (4)最后再经过两次卷积操作,生成特征图,再用两个卷积核大小为1*1的卷积做分类得到最后的两张heatmap,例如第一张表示第一类的得分,第二张表示第二类的得分heatmap,然后作为softmax函数的输入,算出概率比较大的softmax,然后再进行loss,反向传播计算。
def get_unet():
inputs = Input((img_rows, img_cols, 1))
conv1 = Conv2D(32, (3, 3), activation='relu', padding='same')(inputs)
conv1 = Conv2D(32, (3, 3), activation='relu', padding='same')(conv1)
pool1 = MaxPooling2D(pool_size=(2, 2))(conv1)
# pool1 = Dropout(0.25)(pool1)
# pool1 = BatchNormalization()(pool1)
conv2 = Conv2D(64, (3, 3), activation='relu', padding='same')(pool1)
conv2 = Conv2D(64, (3, 3), activation='relu', padding='same')(conv2)
pool2 = MaxPooling2D(pool_size=(2, 2))(conv2)
# pool2 = Dropout(0.5)(pool2)
# pool2 = BatchNormalization()(pool2)
conv3 = Conv2D(128, (3, 3), activation='relu', padding='same')(pool2)
conv3 = Conv2D(128, (3, 3), activation='relu', padding='same')(conv3)
pool3 = MaxPooling2D(pool_size=(2, 2))(conv3)
# pool3 = Dropout(0.5)(pool3)
# pool3 = BatchNormalization()(pool3)
conv4 = Conv2D(256, (3, 3), activation='relu', padding='same')(pool3)
conv4 = Conv2D(256, (3, 3), activation='relu', padding='same')(conv4)
pool4 = MaxPooling2D(pool_size=(2, 2))(conv4)
# pool4 = Dropout(0.5)(pool4)
# pool4 = BatchNormalization()(pool4)
conv5 = Conv2D(512, (3, 3), activation='relu', padding='same')(pool4)
conv5 = Conv2D(512, (3, 3), activation='relu', padding='same')(conv5)
up6 = concatenate([Conv2DTranspose(256, (2, 2), strides=(
2, 2), padding='same')(conv5), conv4], axis=3)
# up6 = Dropout(0.5)(up6)
# up6 = BatchNormalization()(up6)
conv6 = Conv2D(256, (3, 3), activation='relu', padding='same')(up6)
conv6 = Conv2D(256, (3, 3), activation='relu', padding='same')(conv6)
up7 = concatenate([Conv2DTranspose(128, (2, 2), strides=(
2, 2), padding='same')(conv6), conv3], axis=3)
# up7 = Dropout(0.5)(up7)
# up7 = BatchNormalization()(up7)
conv7 = Conv2D(128, (3, 3), activation='relu', padding='same')(up7)
conv7 = Conv2D(128, (3, 3), activation='relu', padding='same')(conv7)
up8 = concatenate([Conv2DTranspose(64, (2, 2), strides=(
2, 2), padding='same')(conv7), conv2], axis=3)
# up8 = Dropout(0.5)(up8)
# up8 = BatchNormalization()(up8)
conv8 = Conv2D(64, (3, 3), activation='relu', padding='same')(up8)
conv8 = Conv2D(64, (3, 3), activation='relu', padding='same')(conv8)
up9 = concatenate([Conv2DTranspose(32, (2, 2), strides=(
2, 2), padding='same')(conv8), conv1], axis=3)
# up9 = Dropout(0.5)(up9)
# up9 = BatchNormalization()(up9)
conv9 = Conv2D(32, (3, 3), activation='relu', padding='same')(up9)
conv9 = Conv2D(32, (3, 3), activation='relu', padding='same')(conv9)
# conv9 = Dropout(0.5)(conv9)
conv10 = Conv2D(1, (1, 1), activation='sigmoid')(conv9)
model = Model(inputs=[inputs], outputs=[conv10])
model.compile(optimizer=Adam(lr=1e-5),
loss=dice_coef_loss, metrics=[dice_coef])
return model
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