Density

Density may also be referred to as optical or radiographic density. Density in radiography is a measurable quantity: in its simplest sense it is the degree of ‘blackening’ seen on the image. For film/screen systems, when thought of in this way density is easy to evaluate and correct: is the film too dark (decrease exposure) or too light (increase exposure)?

In the case of a radiographic film the density we can measure is the transmitted density (D); this is defined as the base 10 logarithm of the ratio of the light incident upon the film (I_o) to the light transmitted through the film (I_t):

The use of a logarithmic measure is appropriate as the response of the eye to visual stimulation is itself logarithmic.^1,2

In order to be useful, the range of densities demonstrated on the image needs to be within the range for visual perception and differentiation, usually considered to be approximately D = 0.25 → 2.50 (Figure 3.1).³ Although a density of >2.5 may not be immediately differentiated by eye, densities of up to 4 may be recorded on film.⁴ In effect, too much information has been recorded; it is sometimes (but not always) possible to use and view this information by use of increased illumination (‘bright light’) or photographic reduction.

Figure 3.1 Characteristic curve.

Users of digital radiography systems need to be aware of the impact of over- and underexposure on the image. Underexposure of a digital radiographic image will not result in an image that has low density. In fact, the image will generally be manipulated by the system to be displayed with an adequate optical density of approximately 1.2 no matter how much or how little radiation the system receives.

Underexposure instead causes problems with the signal-to-noise ratio, and with underexposure the image will appear grainy as a result of quantum mottle. The image must be closely examined to recognise this appearance, as from a distance the image may appear diagnostic. In most cases where fine detail is required for diagnosis, low signal-to-noise ratio in the image will result in the image being repeated.

Overexposure will also not result in an image of high densities. Again, the optical density of the overexposed image will be approximately 1.2, but in this case overexposures (high patient doses) result in high signal-to-noise ratios and image quality will be increased. The temptation, especially when using digital techniques, is to overexpose, as the safety net of image manipulation will prevent the need for a repeat examination, but this practice leads to each individual exposure being higher than necessary for the individual patient. Clearly this is a temptation to be avoided, and professional standards in the application of the ‘as low as reasonably achievable’ (ALARA) principle need to be maintained: give the right exposure for the individual patient.

Variation of applied mAs is often given as the controlling factor for density,⁵ although the effect of variation of kVp on intensity, and therefore density, must also be considered. However, in general it is considered better to use a fixed kVp for each examination, using variations of mAs to control required changes in density.⁶

Contrast

Image contrast is a combination of subject contrast, which is the contrast produced due to the anatomical area under examination, and the receptor (radiographic) contrast, which is the contrast produced as a result of the image receptor being employed; and may be influenced by subjective contrast, which is the effect on contrast perception due to the observer or observing conditions.

The image itself is produced by means of differences in the attenuation of the X-ray beam within the patient. The differences thus produced in the transmitted beam are due to anatomical variations within the patient part under examination, in turn producing visible differences in density and contrast in the resultant image.

The contrast formed on the image in this way is termed ‘subject contrast’, due to the inherent ‘contrast’ which is the result of varying tissue types and densities of the body part under examination. Subject contrast can be influenced and manipulated by use of positive and negative contrast media, and the application of varying kVp techniques as described below.

Contrast can be shown to be inversely proportional to the applied kVp, hence in general at lower kVp values greater subject contrast is obtained. This is because, in the diagnostic range, the main interaction processes responsible for attenuation are photoelectric absorption and Compton scatter. Photoelectric absorption for a given beam energy is proportional to the cube of the atomic number and directly proportional to the density of the structure imaged, hence using the exposure ranges where photoelectric absorption is the dominant process (lower kVp) will maximise subject contrast. As digital systems manipulate the acquired image to produce a fixed image contrast (as described in Chapter 1), the direct relationship between kVp and subject contrast can be lost.

There is again a dose trade-off, as use of low kVp may increase skin dose. Several studies support the use of high kVp as a means of dose reduction. Guidelines for paediatric radiography recommend the use of 55–60 kVp, even for extremity work,⁷ but the increase in kVp will reduce subject contrast and hence image definition.⁸ Commonly forgotten in departments that have adapted this technique for adult use is the requirement for additional copper filtration to optimise the useful spectrum. Failure to use this additional filtration results in a reduction in image quality without the full benefit of the dose reduction intended.

kVp is the exposure factor by which contrast can be manipulated. If an image has adequate density but lacks contrast, even after digital manipulation, then kVp should be reduced; however, as kVp reduction will also reduce the number of photons reaching the image receptor, and hence density, an appropriate increase in mAs is required to maintain the final image density.

For intrinsically high-contrast examinations such as the chest, the use of high kVp enables better visualisation of lung structures despite a reduction in overall image contrast. This is because at low energies the high subject contrast of the thorax, together with high radiographic contrast produced, makes the overall image contrast such that all structures cannot be demonstrated within the useful density range.

High contrast, lower kVp, can be referred to as ‘short scale’,⁵ i.e. fewer shades of grey are represented within the image; consequently, fewer are available to represent the structures to be demonstrated. Use of a high kVp (120+)⁹ reduces the radiographic contrast but enables all structures to be visualised within the useful density range. Low contrast produces a ‘long scale’ image,⁵ with more shades of grey available for image depiction; the result is a ‘flatter’ image but with greater detail, particularly of lung parenchyma.

The ‘flat’ or grey appearance of such images does not suit all subjective tastes, and as such the technique is not universally accepted; however, this subjectivity is difficult to reconcile with accepted best practice, in terms of both image quality and dosimetry. Film readers need to educate themselves to accept these changes and embrace best practice,⁷ the evidence for which is now long established.⁹

Subject contrast will be affected both by pathological processes, which may change the appearance from the expected ‘norm’, and the effects of scatter, which are discussed below.

As mentioned above, subjectivity in image viewing can be an important factor when considering image contrast, and ‘subjective contrast’ requires some consideration.

Not to be confused with subject contrast as described above, subjective contrast is due to the observer rather than inherent in the image,¹⁰ but is nonetheless important to consider. The observer needs to be considered: eye strain and fatigue can have an effect on perception and several short viewing (or reporting) sessions are preferable to a single extended session; aids to visual acuity should be used as required (e.g. spectacles should be worn if they are needed!).

Viewing conditions need to be optimal. A dim viewing box in high ambient lighting, or holding a radiograph up to a window, are not ideal viewing conditions and will not enable accurate appreciation of either the radiographic density or the contrast demonstrated on the image. Viewing boxes should be matched for brightness and colour of illumination, checked on a regular basis, and used in appropriate conditions, i.e. in low ambient lighting.¹¹ Digital viewing stations should be of appropriate resolution and correctly adjusted.

As already stated, the amount of scatter reaching the image receptor will also affect image contrast. An increase in scatter reduces radiographic contrast by contributing a general increase in the overall image density, without any positive contribution to image definition.

Related posts:

Film/screen imaging Paranasal sinuses Pelvis and hips Forearm, elbow and humerus Investigations of the genitourinary tract Dental radiography

Stay updated, free articles. Join our Telegram channel

Join