Image Receptor Unsharpness By Professor Stelmark

Analog Systems Films Intensifying screens

Screen-Film As was previously stated, screen-film is the most widely used IR in radiology. Several characteristics must be considered when one is selecting screen-film: contrast, speed, spectral matching, anticrossover/antihalation dyes, and requirement for a safelight.

For direct-exposure film, speed is principally a function of the concentration and the total number of silver halide crystals. For screen-film, silver halide grain size and shape are the principal determinants of film speed.

Crossover Until recently, silver halide crystals were usually fat and three dimensional. Most emulsions now contain tabular grains, which are flat silver halide crystals, and provide a large surface area/volume ratio. The result is improved covering power and significantly lower crossover.

When light is emitted by a radiographic intensifying screen, it not only exposes the adjacent emulsion, it can also expose the emulsion on the other side of the base. When light crosses over the base, it causes increased blurring of the image.

The addition of a light-absorbing dye in a crossover control layer reduces crossover to near zero. The crossover control layer has three critical characteristics: (1)It absorbs most of the crossover light (2)It does not diffuse into the emulsion but remains as a separate layer (3)It is completely removed during processing

SCREENS Use of film to detect x-rays and to image anatomical structures is inefficient. In fact, less than 1% of the x-rays incident on radiographic film interact with the film and contribute to the latent image. Most radiographs are made with the film in contact with a radiographic intensifying screen because the use of film alone requires a high patient dose. A radiographic intensifying screen is a device that converts the energy of the x-ray beam into visible light. This visible light then interacts with the radiographic film, forming the latent image.

Usually, the radiographic film is sandwiched between two screens. The film used is called double-emulsion film because it has an emulsion coating on both sides of the base.

High-speed screens have low spatial resolution, and fine-detail screens have high spatial resolution. Spatial resolution improves with smaller phosphor crystals and thinner phosphor layers.

Reflective Layer Between the phosphor and the base is a reflective layer. When x-rays interact with the phosphor, light is emitted isotropically. Less than half of this light is emitted in the direction of the film. The reflective layer intercepts light headed in other directions and redirects it to the film. The reflective layer enhances the efficiency of the radiographic intensifying screen, nearly doubling the number of light photons that reach the film.

In mammography, the screen is positioned in contact with the emulsion on the side of the film away from the x-ray source, to reduce screen blur and improve spatial resolution.

A wire mesh test for screen/film contact should be performed at least annually (more often with larger cassette sizes because they are more prone to develop poor contact).

Screen FactorScreen SpeedRecorded DetailPatient Dose Thicker phosphor layer ↑↓↓ Larger phosphor crystal size ↑↓↓ Reflective layer↑↓↓ Absorbing layer↓↑↑ Dye in phosphor layer ↓↑↑

Computed Radiography CR IRs can be portable or fixed in a table or upright x-ray unit. The CR IR includes a cassette that houses the imaging plate (IP). The radiation exiting the patient interacts with the IP, where the photon intensities are absorbed by the phosphor. Although some of the absorbed energy is released as visible light (luminescence), a sufficient amount of energy is stored in the phosphor to produce a latent image. Luminescence is the emission of light when stimulated by radiation

The IP primarily consists of a support layer, phosphor layer, and protective layer. The phosphor layer is composed of barium fluorohalide crystals doped with europium, referred to as the photostimulable phosphor (PSP). This type of phosphor emits visible light when stimulated by a high-intensity laser beam, a phenomenon termed photostimulable luminescence.

A PMT collects, amplifies, and converts the visible light to an electrical signal proportional to the range of energies stored in the IP. The signal output from the PMT is digitized by an ADC in order to produce the digital image. To digitize the analog signal from the PMT, it must first be sampled. An important performance characteristic of an ADC is the sampling frequency, which determines how often the analog signal is reproduced in its discrete digitized form. Increasing the sampling frequency of the analog signal increases the pixel density of the digital data and improves the spatial resolution of the digital image

PSP crystal size affects spatial resolution: powder vs needle shaped crystals

PSP phosphor layer thickness

The closer the samples are to each other (increased sampling frequency), the smaller the sampling pitch, or distance between the sampling points. Increased sampling frequency decreases the sampling pitch and results in smaller-sized pixels. The distance between the midpoint of one pixel to the midpoint of an adjacent pixel describes the pixel pitch. Spatial resolution is improved with an increased number of smaller pixels resulting in a more faithful digital representation of the acquired analog image.

For a fixed matrix size CR system, using a smaller IP for a given field of view (FOV) results in improved spatial resolution of the digital image. Increasing the size of the IP for a given FOV results in decreased spatial resolution.

Fixed matrix size. A fixed matrix size will vary the sampling frequency for a different IP size. A larger IP size will result in a larger pixel size and decrease spatial resolution.

Fixed sampling frequency. A fixed sampling frequency will maintain a fixed spatial resolution. A larger IP size will have a larger matrix to maintain the same pixel size

Detector Size Detector size is critical. Detectors must be large enough to cover the entire area to be imaged and small enough to be practical. For chest x-rays, the detector field needs to be at least 17 × 17 inches so that both lengthwise and crosswise examinations are possible. Special examinations such as leg length and scoliosis series may require dedicated detectors.

Detector size or field of view (FOV)—The detector size and FOV describe the useful image acquisition area of an imaging device. Cassette-less digital systems have a fixed FOV which makes some projections difficult, while cassette-based CR systems have flexible FOVs like screen/film. Detector element (DEL)—The detector element is the smallest resolvable area in a TFT- or CCD-based digital imaging device

Depending on the detector’s physical characteristics, spatial resolution can vary a great deal. Spatial resolution of a-Se for direct detectors and CsI for indirect detectors is higher than CR detectors but lower than film/screen radiography. Excessive image processing, in an effort to alter image sharpness, can lead to excessive noise. Digital images can be processed to alter apparent image sharpness; however, excessive processing can lead to an increase in perceived noise. The best resolution will be achieved by using the appropriate technical factors and materials.