CR image receptor
image readout
Exposed IPs are sent to the reader unit where analog data is received from this IP which is sent to the computer system to be converted into digital data. Reader unit is available in single or multiple plate configuration.
A typical CR reader unit has the following components -
- Drive mechanism - to move the IP through the scanning process
- Optical system - includes laser, beam shaping optics, collecting optics and optical filters.
- Photo detector - e.g. Photomultiplier tube (PMT)
- Analog to digital converter (ADC).
The difference in the machine of the manufacturers is due to the difference in the cr reader unit. In some devices the IP system moves and in some the optical components move.
Digitization of CR latent image consists of three important steps -
Scanning
The purpose of scanning is to convert the latent image into an electrical signal (voltage), which is then digitized and displayed as a digital image. The CR reader is first extracted from the cassette and fitted into the drive mechanism. The drive mechanism makes the cassette move at a uniform speed on the Y Axis, which is called slow scan mode.
A rotating and multifaceted mirror reflects the red light from a laser light source (He - Ne laser, 633 nm). This red light deflects in horizontal back and forth direction on the phosphor plate and is visible blue green light (390nm) release. This is called fast scan mode. Slow and fast scan mode is controlled by CR computer. This released light is collected by the PMT or CCD through a fiber optic light guide. PMT collects, amplifies and converts this visible light into a rical signal. Through PMT, the output signal is digitized by an ADC (analog to digital converter), which creates a digital image.
Sampling
The digital signal coming from the PMT is sampled before digitizing. ADC has an important performance characteristic sampling frequency. Sampling refers to the number of times an analog signal is converted into a digitized form. Increasing the sampling frequency of the analog signal increases the pixel density of the digital data, thereby increasing the spatial resolution of the digital image.
The smaller the sampling pitch, the smaller the sampling pitch, the closer the sample is to each other. That is, the distance between the sampling points is less. Increasing the sampling frequency decreases the sampling pitch, which also decreases the pixel size. The difference in CR equipment of the manufacturers is due to the sampling process of IP.
Some manufacturers fix the sampling frequency to maintain the spatial frequency and some make changes in the sampling frequency to keep the matrix size fix.
If the spatial resolution is set, the image matrix size is proportional to the IP size. Large matrix sizes are taken to maintain spatial resolution in large IPs.
If the IP size is changed by keeping the matrix size fixed, the spatial resolution of the digital image is affected.
Quantification or pixel bit depth
It controls the gray shade or contrast resolution of the image. During the quantification process, each pixel displays a brightness value that has a numerical value. Quantification represents the precision of the numerical value of each recorded point. Pixel size and pitch determine spatial resolution and pixel bit depth determines the system's ability to display gray shades. Pixel bit depth is determined by the type of ADC and by the CR System manufacturer. The higher the pixel bit depth, the higher is the contrast resolution of the digital image. e.g. 16 bit depth system has 216 pixel and 65536 gray shades.
