*Quiz No. 5

What is the effect of pixel size on image quality?

An increased pixel size decreases the spatial resolution and thus detail of an image. Generally, the smallest object to be imaged must be two times the size of the smallest pixel.

What is the effect of pixel number on image quality?

An increased pixel number increases the contrast of an image, i.e. shades of grey.

*Describe the process of computed radiography.

Computed radiography uses a photostimulable phosphor plate instead of film, which is housed in an imaging cassette made of Gd₂O₂S. The phosphor plate is made of europium activated barium fluoride halide compounds, and this is where a latent image is formed when exposed to x-rays. The reason europium activated barium fluoride halide compounds are used is because the latent image does not fade for many hours and the output range is 300-500nm, which the used photomultiplier tubes are sensitive to. The also separate the stimulated and emitted spectrums well.

When exposed to radiation, electrons in the phosphor plate are excited to a higher energy level. They move into traps and the number of traps filled is proportional to the radiation intensity at that spot.

The cassette is then moved to an image reader and the imaging cassette is removed. A He-Ne laser beam then scans the phosphor plate in a raster pattern, causing the electrons to be released from the traps and emit a blue-green light. This light is directed using an optic fibre guide to a PM tube, which converts the visible light into an electrical signal and amplifies it. The electrical signal is converted to and stored as a pixel value.

To ready the plate for later use, it is exposed to a bright white light. This erases any residual trapped energy and the phosphor plate is returned to the imaging cassette.

What is dose creep or exposure creep?

Exposure creep occurs due to the wide dynamic range of computed radiography. This means that for changes in exposure, the optical density is not affected. Thus, we can overexpose a patient without producing a bad image.

What are advantages and disadvantages of computed radiography compared to screen film radiography?

Advantages:

• Similar to SF in workflow
• Large dynamic range
• Smoothing and edge enhancement

Disadvantages:

• Dose creep
• Poor spatial resolution
• Requires equal or higher dose to achieve same spatial resolution
• Lower absorption efficiency of phosphor plate
• Phosphor plate is more sensitive to scatter radiation

*What is CCD based digital radiography and what are its disadvantages?

A CCD, or charge-coupled device, is a light sensitive integrated circuit that consists of millions of cells that absorb light and produce a certain amount of charge, which corresponds to the amount of radiation received locally. Intensifying screens convert x-rays to light, which is detected by the CCDs and an electric signal is read out. However:

• CCDs are small
• Light must be demagnified
• Light is scattered before reaching the CCDs
• Geometric distortion occurs
• Decrease spatial resolution

*Describe DR using indirect flat panels and its disadvantages.

Indirect flat panels involve a 2 step process like film screen. A fluorescent material captures x-ray energy and converts it into light, which is then detected by an array of pixels consisting of photodetectors. The light is then converted into an electrical signal. Finally, the charge pattern is read out by a TFT.

• Light conversion is poor by the fluorescent material
• Light scattering occurs
• Image is degraded
• Decreased spatial resolution
• Increased image noise (speckled background)

What is the fill factor?

The fill factor is the percentage of the pixel area which is sensitive to image signal. It is equal to the light sensitive area divided by the detector element area.

*Describe DR using direct flat panels.

Direct flat panels do not have an intermediate step and instead use a photoconductive material to absorb the x-ray energy into an electrical signal. The photoconductive material (amorphous selenium) is coupled to a TFT, which produces electric charge and thus forms a signal directly. A high electric field is applied across the amorphous selenium to minimise the lateral spread of electrons, resulting in high spatial resolution.

What are the advantages of DR?

• Superior contrast resolution due to wide dynamic range
• Decrease patient dose
• Decrease retakes
• Fast acquisition
• Increased operational efficiency
• Images can be altered digitally to enhance or minimise features