Rebuilding IVUS images from raw data of the RF signal exported by IVUS equipment - Emerging Trends in Image Processing, Computer Vision, and Pattern Recognition - page 75

FIGURE 4 (a) RF envelope is shown as a black line over the wave profile of RF signal gray
line (data) and (b) DDP transformation.
2.5 Log-Compression
The next stage in the procedure described in Figure 2 , is to normalize the RF signal providing
values between 0 and 1 in order to permit work with a homogeneous range for all IVUS im-
ages. After this, the RF signal undergoes a transformation whose purpose is to map a narrow
range of grayscale values in an input image to a wide range of output levels [ 15 ] . This trans-
formation is defined as
where I nor represents the RF signal normalized and t is a constant empirically obtained to im-
prove the log-compression.
2.6 Digital Development Process
The DDP consist of a nonlinear intensity transformation, used frequently in photography, to
emphasize dark regions in images with a large dynamic range, without saturation or overex-
posure of brighter areas.
This way, in order to emphasize the edges borders and improve contrast gain, a DDP was
applied to the image [ 15 ] .
Each pixel value of an image was modified by the following equation to produce an image
with beter CNR: