In Ch. 3, we learned about the basic structure and function of four main types of digital image receptor systems, as well as some basic properties of the digital image, like matrix size, pixel size and bit depth. We discussed how the histogram is created from the image data and how it is rescaled to provide similar brightness and contrast to the reference image from the look-up table (LUT). In this chapter, we are going to dig a little deeper into the details of each image receptor system and focus on how these image receptor system features impact the image properties.

When comparing different digital image receptor systems, we need to focus on features of the system that maximize image quality, as well as those features that minimize patient dose. There are five performance characteristics than should be considered when considering the impact of the imaging system on image properties.

Key Performance Characteristics of Digital Image Receptor Systems

Sensitivity

Sensitivity is the term used to describe the ability of a detector system to recognize the x-rays that hit it. The sensitivity of the detector system controls the amount of radiation necessary to produce an optimum image. A detector with higher sensitivity will require less radiation to produce the same image than a detector with a lower level of sensitivity. This concept is sometimes discussed using the term speed or speed class. Speed is a term that persists from film-based imaging, where a “faster” system requires less radiation to produce the image than a “slower” system.  There are four factors that impact the sensitivity of the receptor system: absorption efficiency, conversion efficiency, emission efficiency, and fill factor. However, not all of these factors are relevant to every receptor system type.

Absorption Efficiency

Absorption efficiency is the primary factor related to image receptor sensitivity and is

Receptor System Considerations Affecting IR Exposure & Patient Dose

Fill Factor

Dynamic Range/Exposure Latitude

Tiling and Flat-Field Correction

Receptor System Considerations Affecting Brightness

Window Level

Receptor System Considerations Affecting Contrast

Window Width

Linear Response Rate = EXTREME Sensitivity to Scatter

Signal-to-Noise Ratio (SNR)

Receptor System Considerations Affecting Spatial Resolution

Scintillators and Light Spread

Matrix Size and Field of View

Pixel Density and Pixel Pitch

Sampling Frequency

Nyquist Frequency

Receptor System Considerations and Affecting Distortion