FRCR: Physics Lectures Diagnostic Radiology

1. FRCR: Physics LecturesDiagnostic Radiology Lecture 4 Film-screen radiography Dr Tim Wood Clinical Scientist

2. Overview • Film-screen radiography • Processing • Intensifying screens and the film cassette • The characteristic curve and sensitivity • Image quality

3. The story so far…

4. What is ‘image quality’ • Image quality describes the overall appearance of the image and its fitness for purpose • Remember: There is always a play-off between image quality and patient dose • We only need images that are of diagnostic quality (fit for purpose) – not pretty pieces of art! • The main factors to consider are: • Contrast • Spatial resolution • Noise

5. Image contrast • The final contrast in the image will depend on a number of factors, such as; • Subject contrast – an inherent property of the patient being imaged that will depend on the attenuation coefficients of the tissues (or contrast media), the thickness of structures, the nature of any overlapping tissues and the incident X-ray spectrum (kVp, filtration, etc – discussed previously) • Detector properties – film and digital detectors each have different implications for the contrast in the final image (these will be discussed in the next lectures) • Scattered radiation – scatter can degrade image contrast if it reaches the detector as it conveys no information about where it came from. Scatter rejection techniques may be used to remove this.

6. Contrast and scatter • Primary radiation carries the information to be imaged • Scatter obscures this as it carries no information about where it came from • The amount of scatter (S) may be several times the amount of primary (P) in the same position • The ratio S/P depends on thickness of patient • For typical PA chest = 4:1 (20% of photons carry useful information) • For typical lateral pelvis = 9:1 (10% of photons…)

7. Contrast and scatter • Scatter is generally quite uniform across the image • Reduces contrast that would otherwise be produced by the primary by a factor of (1+S/P) – up to 10 times! Intensity S+P 10 % contrast reduced S 8 Scatter = 4 x Primary (PA Chest) S+P contrast = (10-9)/10 x 100 = 10% Primary contrast = (2-1)/2 x 100 = 50% P 2 Position

8. Scatter reduction • Scatter and patient dose may be reduced by; • Reducing the field area with collimation (reduce volume of tissue generating scatter) • Compress the tissue to minimise overlying structures (reduce volume of tissue generating scatter) • Scatter may be reduced at the expense of increased patient dose by; • Reducing kVp – less forward scatter is produced and is much less penetrating (less scatter produced) • Use an anti-scatter grid to remove scatter from the X-ray beam • Use an air-gap to reduce the intensity of scatter reaching the detector

9. Spatial Resolution • Spatial resolution describes the ability to see fine detail within an image • Fine detail is clearer when the contrast is high • e.g. microcalcifications • May be expressed as the smallest visible detail, but most common descriptor is the highest frequency of lines that can be resolved in a high-contrast bar pattern

10. Limiting factors • Geometric Unsharpness – due to finite focal spot size (discussed previously) • Movement Unsharpness – imaging moving structures may result in blur e.g. heart, lungs, etc • Minimise by immobilisation e.g. compression in mammography • Breath hold techniques • Use shortest possible exposure times (may be at the expense of high mA) • Absorption Unsharpness – gradual changes in absorption near tapered/rounded structures • Minimise by careful patient positioning • Detector resolution – will be discussed in following lectures

11. Noise • Noise is a random, usually unwanted, variation in brightness or colour information in a visible image • Noise is one of the most important limiting factors to contrast and spatial resolution • The most significant source is quantum noise (or mottle) due to the low levels of radiation used to form an image • Other sources include film grain and electronic noise in the image receptor

12. Noise • Image noise is most apparent in image regions with low signal level, such as shadow regions or underexposed images • Noise gives a grainy, mottled, textured or snowy appearance to an image • Noise can mask fine detail in a radiograph • Noise reduces visibility of parts of a radiographic image • Noise is particularly an issue with image details that are already of low contrast

13. Signal on detector Position on detector So what does this all really mean? Something with lots of contrast is easier to see than… Signal on detector … something with little contrast Position on detector

14. Signal on detector Position on detector So what does this all really mean? But there is some loss of sharpness due to various process, so the sharp edge… Signal on detector … actually becomes blurred, which may not matter for large structures… Position on detector

15. So what does this all really mean? Signal on detector … but if the structure is small… Position on detector Signal on detector … it may start to disappear into the background… Position on detector

16. Signal on detector Signal on detector Position on detector Position on detector So what does this all really mean? Up to now we haven’t considered the third component, noise… … which may not be a problem if there isn’t too much…

17. Signal on detector Position on detector So what does this all really mean? … but if there is lots of noise in the system, the detail may be obscured… Signal on detector … whilst the low level of noise may be problematic if contrast is low to start with… Position on detector

18. So what does this all really mean? Signal on detector … or resultion is poor Position on detector

22. Pause…

23. Film-screen radiography

24. Film-Screen Imaging • Traditionally, all X-ray image capture has been through X-ray film Emulsion Protective layer Adhesive layer Film base Emulsion

25. Film • Polyester film base gives mechanical strength to the film – does not react to X rays • Emulsion consists of silver halide grains (AgBr) • The image is formed by the reaction of AgBr grains to X-ray photons • The sensitivity of the film depends on number of grains • Must be evenly distribution • Typically each crystal is about 1 μm in size • larger grains = more sensitive (contrast), • smaller grain = better resolution • Adhesive layer ensures emulsion stays firmly attached to base • Protective layer prevents mechanical damage

26. Film • Film is actually much more sensitive to visible light and UV than it is to X-rays • Hence, use a fluorescent screen to convert X-ray photons to light photons • Enables lower patient dose! • A latent image is formed upon exposure, which cannot be seen unless the film undergoes chemical processing • Mobile silver ions are attracted to electrons liberated by light photons, forming a speck of silver metal on the surface

27. Processing • The invisible latent image is made visible by processing • There are three stages to this process; • Development • Fixing • Washing

28. Processing • First stage is development: • Film is immersed in an alkaline solution of a reducing agent (electron donor) • Reduces positive silver ions to metallic grain of silver (black specks) • Unexposed crystals are unaffected by the developer – bromide ions repel the electron donor molecules • However, given sufficient time, the developer will penetrate the unexposed crystals • The amount of background fog is dependent upon the time, strength and temperature of the developer

29. Processing • Second stage is fixing: • If the film is exposed to light after the first stage, the whole film becomes black • To ‘fix’ the film, unaffected grains are dissolved by an acid solution, leaving the X-ray image in the form of black silver specks • Final stage is washing: • The film is washed in water and dried with hot air • Inadequate washing would result in a brown/yellow film over time (from excess acid) and smell

30. Processing • Automatic processors use a roller system to transfer the film through the different solutions • Regular Quality Assurance of the processor is vital for producing good quality radiographs • Image is then viewed by transmission of light from a light box with uniform brightness • Dark = lots of X-rays • Light = relatively few X-rays e.g. through bone

31. Production of a Radiograph

32. Logarithms • A logarithm is an exponent – the exponent to which the base must be raised to produce a given number • 104 = 10x10x10x10 = 10,000 • = log1010000 = 4 • i.e., 4 is the logarithm of 10000 with base 10 • Seen in many applications • Richter earthquake scale • Sound level measurements (decibels = dB) • Optical Densities blackness on film (OD) • Written as log10x or if no base specified in physics texts as log x it is interpreted as the same

33. Properties of logs • log101 = 0 • log1010 = 1 • log10xy = log10x + log10y • log10x/y = log10x - log10y

34. Optical Density • Optical Density: the amount of blackening in the film • Defined as the log of the ratio of the intensities of the incident and transmitted light • log is used as the eyes response is logarithmic

35. Optical Density • Optical density can be measured with a densitometer • From the definition, if 1% of light is transmitted, D = 2.0 • If 10% is transmitted, D = 1.0 • The density of an area of interest on a properly exposure film should be about 1.0 • Lung field may be ~2.0 • Areas with D>3.0 too dark to see any detail on a standard light box

36. Contrast • Contrast is the difference in optical densities Contrast = OD1 – OD2 • High contrast - e.g. black and white • Low contrast – e.g. grey and grey!

37. Intensifying screens • Film is relatively insensitive to X-rays directly • Only about 2% of the X-rays would interact with the emulsion • Requires unacceptably high doses to give a diagnostic image • An intensifying screen is a phosphor sheet the same size as the film, which converts the X-rays to a pattern of light photons • The intensity of the light is proportional to the intensity of X-rays • The pattern of light is then captured by the film • One exception is intraoral dental radiography, where screens are not practical

38. Intensifying screens • Modern intensifying screens use rare earth materials, which emit light that is matched to the sensitivity of the film being used • Spectral match between the emission of the screen and the absorption in the film e.g. blue or green • K-edges clinically relevant (39-61 keV) • Rare earth screens used as they very efficient at converted absorbed X-ray energy into light • Results in a ‘faster’ (more sensitive) system • The sensitive emulsion of the film must be in close contact with the screen

39. Intensifying screens • General radiography film usually double coated with emulsion on each side of the base • The front screen absorbs ~1/3 of X-rays and ~1/2 light travels forward and is absorbed by front layer of emulsion • Rear screen absorbs ~1/2 of X-rays transmitted through the front and exposes the rear emulsion • ~2/3 of total X-ray fluence absorbed in screens • Mammography only uses a single screen to maximise spatial resolution (more on this later) • Screen materials chosen to have no phosphorescence (delayed fluorescence) to avoid ghost images

40. The film-cassette • Flat, light tight box with pressure pads to ensure film in good contact with the screen(s) mounted on the front (and back) • The tube side of the cassette is low atomic number material (Z~6) to minimise attenuation • Rear of cassette often lead backed to minimise back scatter (not in mammo)

41. The characteristic curve Optical density • Plotting OD against log exposure gives the Characteristic Curve of the X-ray film • Different types of film – subtle differences but all basically the same Saturation Linear region, gradient = gamma Solarisation Fog Log exposure

42. The characteristic curve • Depends on type of film, processing and storage • Fog: Background blackening due to manufacture and storage (undesirable) • Generally in the range 0.15-0.2 • Linear portion: useful part of the curve in which optical density (blackening) is proportional to the log of X-ray exposure • The gradient of the linear portion determines contrast in an image and patient exposures must lie within this region • Need to match this to the clinical task! • Hence, film suffers from a limited and fixed dynamic range

43. The characteristic curve Optical density • Gradient of linear region = Gamma,  = OD2 – OD1 log E2-log E1 • Gamma depends on • Emulsion • Size and distribution of grains • Film developing • Gamma ~ Contrast • Latitude = useful range of exposures Linear region Latitude Log exposure

44. The characteristic curve • Gamma and latitude are inversely related • High gamma = low latitude • Wide latitude (low gamma) for chests • High gamma (low latitude) for mammography • At doses above the shoulder region, the curve flattens off at D~3.5 • Saturation, whereby all silver bromide crystals have been converted to silver • At extremely high exposures density will begin to fall again due to solarization • Not relevant to radiography

45. Film Speed • Definition: 1 / ExposureB+F+1 • Reciprocal of Exposure to cause an OD of 1 above base plus fog • Speed of film = sensitivity = amount of radiationrequired to produce a radiograph of standard density • Speed shifts H-D curve left and right • Fast film requires less radiation (lower patient dose) • Speed is generally used as a relative term defined at a certain OD; one film may be faster than another at a certain point on the curve

46. Factors affecting speed • Size of grains – larger means faster • This is the main factor and conflicts with the need for small crystals to give good image sharpness. • Fast films are grainier but reduce patient dose • Thickness of emulsion • Double layers of emulsion give faster films • Radiosensitisers added • (X-ray energy)

47. Effect of developing conditions • Increasing developer temperature, concentration or time increases speed at the expense of fog • Developer conditions should be optimised for maximum gamma, and minimum fog • Automatic processor has temperature controls and time maintained by roller speed • Concentration is controlled by automatic replenishment of the chemicals

48. Film-screen sensitivity • Intensification factor • Each X-ray photon generates ~1000 light photons • Just under half of these will reach the film • ~100 light photons to create a latent image • Hence, more efficient process • Intensification factor is the ratio of air KERMA to produce D = 1 for film alone, to that with a screen • Intensification factor typically 30-100 • Speed class • Most common descriptor of sensitivity • Speed = 1000/K, where K is air KERMA (in μGy) to achieve D = 1 • Typically 400 speed (K = 2.5 μGy)

49. Image quality • Contrast • Contrast in film-screen radiography is due to both subject contrast, scatter and gamma • Remember, high gamma = high contrast = low latitude (and vice-versa) • Contrast is fixed for any given film and processing conditions • Image detail may be lost if contrast is too high as it may be lost in the saturated or fog regions • Hence, vital to match gamma to the clinical task • Ambient light conditions and viewing box uniformity may also impact on the subjective contrast presented to the Radiologist • Use a darkened room, mask off unused areas of lightbox, etc

50. Image quality • Screen-unsharpness • The film-screen system has inherent unsharpness additional to geometric, motion and absorption • Only partly due to finite size of the emulsion crystals • Most significant effect is due to spread of light from the point of X-ray absorption in the phosphor, to detection by the film • Depends on the point in the phosphor where the interaction occurs • Thicker phosphor layers more sensitive (absorb more X-rays), but result in more blurring – allow lower patient doses