Image quality

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Chapter 14 Image quality

Ken Holmes, Marc Griffiths

KEY POINTS

Image quality affects the ease of extracting information from an image.

Good image quality will ensure the maximum amount of diagnostic information is gained from the image.

Sharpness is essential as blurring of an image will lower the image quality.

Contrast describes the difference in density between two adjacent structures.

Density is the degree of blackening on the film.

Fault finding is essential to ensure that image quality is optimal.

Understanding the cause of faults will ensure rectification of the cause and improved image quality.

INTRODUCTION

One of the fundamental roles of practitioners in radiography is to produce optimum radiographs for reporting. Image quality is difficult to define because it is subjective in its nature, but an optimum quality image enables the observer to extract information from the image and make an accurate diagnosis. Poor quality images have a poor signal-to-noise ratio and detract from the process of extracting information. However, there are characteristics of a radiograph, which may be evaluated, and enable the practitioner to distinguish the diagnostic quality of the image. These factors include:

• magnification and distortion

• sharpness

• radiographic density

• contrast

• latitude of the film or the resolution of the monitor (VDU).

These factors can be affected during the production and processing of the image (both analogue and digital processing) and once the image has been produced or displayed. It is essential that if a film is produced it is viewed under optimum viewing conditions and must not be held up to the nearest light bulb! Digital images must also be viewed on an appropriate monitor with diagnostic resolution.

SIGNAL-TO-NOISE RATIO

One concept used to define image quality is the signal-to-noise ratio. The signal is the useful information from the patient being imaged and the noise is anything that detracts from accessing the information. Useful information is derived from photoelectric interactions within the patient (absorption by body structures) and noise is derived from scattered photons, because the image receptor does not have the ability to determine their origin. In radiographic images if the signal level is high compared to the noise, structures within the body will be clearly seen, but if the signal level is similar to or less than the noise level then the structure will become obliterated.

Images produced in radiography are often a compromise between obtaining a perfect signal and reducing the noise. The production of images is constrained by a number of factors including the radiation dose level and each component of the imaging chain. For example, a faster film-screen system is utilised when imaging the abdomen than for the extremities and this enables the practitioner not only to produce a diagnostic image with minimal noise but to minimise the radiation dose to sensitive organs.

MAGNIFICATION

When producing a radiographic image all objects in the image are larger than the object being X-rayed. This magnification is due to the geometry of imaging. The ideal situation is to have the object being X-rayed as close as possible to the image receptor, parallel to the X-ray tube and the image receptor, with the radiation beam at right angles to the object. This minimises the magnification of the image but also, more importantly, the magnification of the unsharpness in the image (Figs 14.1 and 14.2).

Figure 14.1 Image with minimal magnification.

Figure 14.2 Image demonstrating magnification – if the body part is not in contact with the cassette, the result is magnification.

If the object being imaged is not parallel to the image receptor it will be magnified; however, each end will be magnified differently and this will produce distortion. This may be elongation or foreshortening of the image (Fig. 14.3).

Figure 14.3 Image demonstrating distortion – if the body part is not parallel with the cassette, the result is distortion. Distortion also occurs when the X-ray beam is angled across the object being X-rayed.

The focus to image receptor (SID; see p. 91) should be as long as possible and the distance between the patient and the image receptor (object–film distance, OFD) as short as possible. For practical reasons the SID is usually 110 cm for techniques on the X-ray table and 180 cm for chest and cervical spine erect film work. The OFD should be as close as possible and ideally the object is in contact with the image receptor. Using a Bucky assembly also causes magnification of the image but the grid is necessary to reduce scatter and improve the contrast of the image. Practically, the mechanism which moves the grid and houses the Bucky is kept as small as possible.

SHARPNESS

In order to determine the image quality the image must be sharp. Blurring will reduce the image quality and also reduce the diagnostic quality of the image. The factors affecting sharpness are:

• movement

• the resolution of the imaging system (film-screen combination or VDU)

• the geometry of the imaging system (relationship between the focus, patient and imaging system).

MOVEMENT UNSHARPNESS

Movement unsharpness should be kept to a minimum by careful radiographic technique. Making the patients comfortable, giving them precise instructions on breath holding and planning the procedures all help reduce this. Using short exposure times also helps reduce blur in the image. Exposure times as low as 0.003 seconds for a chest radiograph enable the practitioner to ensure minimum movement, even in paediatric patients.

RESOLUTION OF THE IMAGING SYSTEM

This is covered in more detail in other chapters of the book (see pp. 141, 154). However, it should be noted that the fastest film-screen combination or exposure index compatible with diagnostic quality should be used. When determining the quality required in an image the practitioner must be aware of what structures need to be defined. An image to determine the position of bones in a plaster cast following an orthopaedic reduction needs less resolution than the original image to diagnose the fracture. Ideally the smaller of the foci of the X-ray tube should be used (fine focus) but if this does not enable a short exposure time to be used on a patient likely to move then the practitioner may need to use a broad focus. This is another example of where the practitioner needs to make a decision that is a compromise between the ideal conditions and getting a diagnostic image.

GEOMETRY OF IMAGING

The positioning of the patient (geometry) to produce an image has a direct relationship to the quality of that image. The section on magnification (p. 162) outlined the ideal conditions needed to produce radiographic images and dealt with the requirements to reduce magnification (p. 163). It also emphasised that any increase in the size of the object also increases the unsharpness of the image and this will be explored in this section.

Figure 14.4 is a diagrammatic representation of image production and shows that a penumbra (unsharpness) is formed with any image that is produced from a finite source (focal spot). The diagram uses a large distance between the object and image receptor to illustrate the principle of the penumbra. In practice the amount of geometric unsharpness (U_g) in an image is small and may be much less than 1 mm.

Figure 14.4 Diagram to demonstrate the geometry of imaging.

Measurement of the penumbra (U_g) is a straightforward calculation using similar triangles. Figure 14.5 demonstrates the diagrammatic representation of similar triangles and the calculation to determine unsharpness in an image ofa finger due to geometric unsharpness. The values used are typical in radiography and the unsharpness is only 0.005 mm.