Week 7 Lecture on Plain Film Radiography PDF

Preview

Summary

Week 7 Lecture on Plain Film RadiographyWeek 7 Lecture on Plain Film RadiographyWeek 7 Lecture on Plain Film RadiographyWeek 7 Lecture on Plain Film Radiography...

Description

Diagnostic Imaging 1 - CHIR916 Week 7 Lecture: Basic theory in Plain Film Radiography s

Anthony J. Buxton MIR. ARMIT. MHEd Casual Lecturer in Diagnostic Radiography MRP0001718773

DISCLAIMER: VERY IMPORTANT TO NOTE ABOUT THE LECTURE DELIVERY: It is essential for you to understand that there is considerably more information that you need to know that IS NOT included on each pdf slide. The slides, as given to you, as pdf files before the lecture are in essence the “bones” of the lecture material. If you only review the pdf slides you will miss a considerable amount of valuable information that is essential for you to be aware of at assessment time. I am offering you three (3) methods of obtaining this additional information: 1. Attending the lecture, 2. Viewing and listening to the entire lecture on iLearn and 3. Reading and reviewing the material in the recommended text(s). As with all PG level material, and to a large extend UG, it is expected that students “read wider” on the topic and can expect a few questions in the final examination that has not been specifically addressed in the lecture series but certainly relevant and alluded to.

Specific Learning Outcomes of today’s Imaging lecture

Plain film Radiography • Be aware of what radiographic film is and how it is processed. • A basic understanding of sensitometry and densitometry in film/screen imaging • Understand how the film characteristic curve affects imaging quality • Understanding of Intensifying Screens, their construction, spectral matching, and speed • Understanding of “quantum mottle” in film screen radiography.

Image Formation • Exit (remnant) beam – Radiation remaining after interacting with the patient – Image forming x-rays and scatter radiation • Image forming x-rays must be transformed to a form intelligible to the radiologist • Media used to convert x-rays into visible image = Image Receptor: – Radiographic film (type of photographic film) – Fluoroscopic image intensifier – Television monitor – Laser imaging system – Solid-state detectors

Image production

Radiographic Film • Constructed of: – Polyester plastic base – Emulsion applied to both sides of base by a thin layer of transparent adhesive – Cover of overcoat of hard gelatin

PROTECTIVE OVERCOAT EMULSION PLASTIC BASE EMULSION PROTECTIVE OVERCOAT

ADHESIVE

Overcoat Layer • The outer or supercoat layer is made of a hard transparent gelatin that protects the film from damage during storage and handling • It softens during processing to allow the developing chemicals to reach the crystals in the emulsions

Image Receptor for Plain Film Radiography • • • • • • •

•

Films are held inside a film cassette Usually constructed from aluminium Opens like a book Various sizes An intensifying screen is present on each side of the cassette Screen-film compatability is essential for optimum image quality (Spectral Matching) Poor screen-film contact destroys detail and spatial resolution because light from the screen diffuses before it reaches the film Areas of poor film screen contact appear as blurred and unsharp areas on the image

18 x 24

24 x30 cm

18 x 43

35 x 43 cm

Intensifying Screens • Part of the image receptor – Includes the cassette, film and intensifying screen • Increase the efficiency of x-ray absorption and decrease the patient dose • Converts a single x-ray into thousands of lower-energy light photons, which expose the film • Only about 1% of the film’s optical density is produced by x-rays; the other 99% results from intensifying screen light • Enables a lower patient dose as less x-ray radiation is used to produce a diagnostic image • Increases blur, but minimised with modern screens

Intensifying Screens X-rays

Absorption by intensifying screen phosphors

Emission of many photons of visible light

Exposure of film

Intensifying Screen • •

•

•

SCREEN BASE – a 1-mm-thick plastic screen base provides support for the phosphor layer; strong and flexible REFLECTIVE LAYER – between the base and phosphor layer: redirects the scattered light from the phosphor back toward the film. Increases the efficiency of the screen, doubles the number of light photons reaching the film PHOSPHOR LAYER – active layer of the screen made up of crystals embedded in a clear plastic support medium. Emits light during stimulation by x-rays PROTECTIVE LAYER – a thin plastic layer about 0.01mm thick that protects the phosphor layer from abrasion when the film is inserted into and removed from the cassette

INTENSIFYING SCREEN CONSTRUCTION NB: This is a single screen second not shown. X-rays

Base Reflective layer

Phosphor Protective layer

X-ray film

Luminescence • A material that emits light due to outside stimulus is luminescent or a phosphor • X-ray hits the the electron returns to the atom it releases a photon of light • The phosphor crystals convert x-rays into visible light by means of two mechanisms: – FLUORESCENCE – is – PHOSPHORESCENCE – is the after the stimulation ceases – In intensifying screens most of the light output is due to fluorescence – Phosphorescence or afterglow and is undesirable

Phosphor Materials • Intensifying screens were developed by Thomas Edison in the early 1900s • These used calcium tungstate (CaWO4) crystals as a phosphor • Calcium tungstate crystals and are efficient in converting x-ray energy into light • These were used until the mid to late1970s when rare earth screens were introduced

Phosphor Materials • RARE EARTH SCREENS use elements from the rare earth section (Z=57 to 70) of the periodic table • Rare earth elements used in intensifying screens include: – Gadolinium – Lanthanum – Yttrium

• Absorb 5x more x-rays than calcium tungstate screens and emit 4x more light for each x-ray absorbed • Emit green light

Phosphor Layer • The SPEED and RESOLUTION of intensifying screens is determined by: – – – –

Phosphor material Size of phosphor Distribution of crystals Thickness of phosphor layer

• There is a trade-off between speed, patient dose, and resolution • Thicker screens have higher speed and require a lower patient dose but have poorer spatial resolution and higher contrast

Intensification Factor • The efficiency of the screen is described by the intensification factor. • It is the ratio of the mAs values required to produce the same optical density without and with a screen – IF = Exposure required without screen/Exposure required with screen

• – Therefore, the lower the exposure when using the screen

• Typical intensification factors range from 25 to several hundred (most common 100 – 600) • High-speed screens have higher intensification factors than slow-speed

Spectral Matching • Refers to • Film sensitivity: – Silver halide crystals are designed to be sensitive to different wave-lengths or colours of light (spectral response) – Conventional blue-sensitive film is sensitive to blue wavelengths of light – Green-sensitive, or orthochromatic, film is sensitive to green wavelengths of light

• These films are matched to the spectrum of light emitted from the intensifying screens • If not matched correctly then efficiency decreases and patient dose increases

Spectral Matching

Fosbinder RA, Kelsey CA. Essentials of Radiologic Science. McGraw-Hill 2002 pp 226

Safelights • To avoid exposure of film from light prior to processing • Lights with a colour filter – Colour filter allows only light to be emitted that has wavelengths longer than the spectral response of the film – Therefore, the light will not expose the film but will allow enough light to illuminate the darkroom • Blue-sensitive film can use an amber filter • Blue and green-sensitive film can use a red filter

Film Processing

What is Densitometry and Sensitometry • Densitometer – A device . – Must be calibrated before use – An x-ray film is passed under the point light source and the Optical Density (OD) of the region of the film is displayed • Sensitometer – A device that exposes a piece of x-ray film with a step wedge of KNOWN optical densities – Must be used in the dark room – Once exposed the film must be immediately processed

Explanation of Optical Density Qu: The OD of a region of a lung field is 2.5. What percentage of visible light is transmitted through the region of the image?

Ans: From the Table (10.1) we can see that an OD of 2.5 is equal to 2 of every 625 light photons that are being transmitted

Creating a characteristic Curve for a specific Film/Screen combination

Characteristic Curve including Base Fog

Characteristic Curve • Reflects the three most important characteristics of radiographic image receptor – Speed – Contrast – Latitude

• A plot of the optical density as a function of the logarithm of the relative exposure is called the characteristic curve

Characteristic Curve •

•

The optical density in the toe region ranges from Large changes in exposure in this region result in small changes in OD The straight-line portion of the curve is the range used in radiology – Small changes in exposure result in large changes in OD

•

In the shoulder region of the curve, most of the silver halide crystals have been exposed, and any further exposure does not produce much additional blackening (Called D Max)

Base and Fog Density • Describes the initial film density • Base density: – Tint of polyester support base (approx 0.1) • Fog density – Inadvertent development of silver grains with no useful information – Exposure to background radiation – Heat and chemicals during storage • The optical density in the base plus fog region should be less than 0.2

Image Receptor Contrast • Is the difference in optical density between two areas in the image • High contrast film: produces a very black and white image • Low contrast film: produces a greyer image • The film contrast is fixed by the manufacturer • Film contrast controlled by size and distribution of silver halide crystals – High contrast: smaller grains, uniform sizes – Low contrast: larger grains, wider range of grain sizes

• Film contrast is described by the slope of the straight-line portion of characteristic curve • Films with steeper straight-line portions have higher contrast • Inversely proportional to the exposure latitude

Film-Screen Latitude • Describes the range of exposures that produce an acceptable radiograph – Can be considered to be the margin of error possible to obtain an OD within the straight line portion of the curve • Films can have wide or narrow latitude – Wide lattitude means that the mAs can vary more and still result in a diagnostically useful image • • Narrow-latitude films have • Generally, higher-speed films have high-contrast and narrower latitude; lower-speed film have lower contrast and wider latitude

Image Receptor Contrast and Latitude Latitude Film A

Useful density range

Latitude Film B

Fosbinder RA, Kelsey CA. Essentials of Radiologic Science. McGraw-Hill 2002 pp 193

Image Receptor Speed •

•

• • • •

Sensitivity of the image receptor to x-rays and light – Describes how efficiently an image is obtained for a given exposure – Measures the ability of an image receptor to respond to low x-ray exposure – Describes the exposure required to produce an optical density of 1.0 above base fog – A faster film requires less exposure and a lower mAs setting to produce the same optical density – Therefore the characteristic curve of a faster film will be closer to the y-axis than the curve of a slower film Determined by silver halide grain size and shape – Large grain emulsions more sensitive than small grain emulsions – Speed limited by emulsion thickness: the thicker the emulsion the less will be exposed Double emulsion film optimises speed – Allows use of 2 intensifying screens – double the light output for the same exposure The image receptor is identified as fast or slow according to its sensitivity If an image receptor is twice as fast it therefore needs half the mAs to produce the same OD A faster image receptor is associated with higher film contrast, increased radiographic noise and decreased spatial and contrast resolution

Image Receptor Speed

OD = 1 + base fog

Fast Film

Slow Film

Fosbinder RA, Kelsey CA. Essentials of Radiologic Science. McGraw-Hill 2002 pp 192

Image Receptor Speed • Relative values set around a standard speed of an average calcium tungstate screen = 100 • Rare earth film/screen systems range in speed from 50 to 1200 • Faster screens, lower patient dose, less thermal stress on the x-ray tube, reduced shielding for x-ray rooms • • There are 3 types of screens: – Detail (slow) screens – Medium-speed screens – High-speed screens

Image Receptor Speed IMAGE RECEPTOR TYPE

SPEED

Detail

50

Medium-speed

100

High-Speed

400 - 1200

Image Receptor Speed • The mAs must be changed to compensate for a change in image receptor speed • The amount of change is given by the ratio of the screen speeds – (old screen speed / new screen speed) • Changing from a high-speed to a detail screen requires an increase in mAs to maintain the same optical density – ie. Changing from a 100 screen speed to a 50 screen speed would require a doubling of mAs to maintain OD

Spatial Resolution • Is the minimum separation between two objects at which they can be recognized as two separate objects. • Spatial resolution depends on the thickness of the screen and the phosphor size • Thicker high-speed screens have poorer spatial resolution

Fosbinder RA, Kelsey CA. Essentials of Radiologic Science. McGraw-Hill 2002 pp 230

Film Noise • • • •

Random fluctuations of optical density of the image Largely inherent in the imaging system Lower noise gives improved contrast resolution 4 components: – Film graininess • Distribution in size and space of silver halide crystals • Inherent in film, negligible – Structure mottle • Distribution of phosphor in intensifying screen • Inherent in screen, negligible – Quantum mottle • Increased with decreased number of x-rays forming the image • Improved with high mAs, low kVp and slower image receptors – Scatter radiation • and increased patient thickness • Reduced using beam-restricting devices and grids

Any Questions? SUBHEAD

Thank you for your attention....