IMAGE FORMATION  Introduction  The Invisible and Visible Image  Image Characteristics

2 By the end of this Lecture the student will be able to:  Define the terms, image and identify its types and methods of viewing  Identify the stages of image formation  Differentiate between the invisible (latent) and visible image  List and define the basic image characteristics  Differentiate between density and contrast and state their relationship Learning objectives

3 References John Ball& Tony Price; Chesney's Radiographic Imaging Websites IMAGE FORMATION

4 What is an Image?  The term image describe a recognizable pattern carrying information Other meaning  An optical appearance  A mental representation  A form or semblance  An idea or conception Visual images are of two types  Real images : Those have real existence such as photographic or radiographic images - Static ( RADIOGRAPH OR PHOTOGRAPH - Dynamic ( On a television screen)  Mental images: Pictures those generated within our minds IMAGE FORMATION

Medical imaging is essentially the extraction of anatomical and physiological information from the patient and the interpretation of these information for the purpose of diagnosing diseases The flow of the information from the patient to the observer is through three stages:  The formation of the invisible image  The conversion of the invisible image into a visible light image  Interpretation of the visible image IMAGE FORMATION

6 Scatter Stage.1 The formation of the invisible image  During radiographic examination X-ray passes through the patient  As x-rays penetrate through the body tissues it become modified each part of the beam is attenuated in a degree which depend on  The tissue type  The intensity of the beam  Thickness of the tissue TransmitAbsorb IMAGE FORMATION

Primary Radiation – The beam of photons, interacts with the pt ’ s body. Remnant Radiation – The resulting beam that is able to exit from the patient. Scatter Radiation – Radiation that interacts with matter & only continues in a different direction – not useful for image production. Attenuation – Primary radiation that is changed (partially absorbed) as it travels through the pt. IMAGE FORMATION

Path & Attenuation of X-ray Beam

9 Stage.1 The formation of the invisible image  During radiographic examination X-ray passes through the patient  As x-rays penetrate through the body tissues it become modified each part of the beam is attenuated in a degree which depend on  The tissue type  The intensity of the beam  Thickness of the tissue  Film is the simplest image receptor, but it is usually coupled with intensifying screens, which help reduce the dose to the patient.  X-ray film responds to a range of wavelengths and is in fact far more sensitive to light than it is to x-rays.  After exposure to x-rays an invisible image known as the latent image is formed. IMAGE FORMATION

10 Stage.2 The conversion of the invisible image into visible  Using the photographic effect of x-rays on a sensitive sheet ( x-ray film)  Using xeroradiographic effect process through exposing an electrically charged imaging plate  Using the fluorescent effect of x-ray on a sensitive screen where the x-rays is converted into light when touches these screens  Using the process of photon stimulated luminescence where a phosphor coated imaging plate is exposed to x-ray, the absorbed energy is stored in the phosphor layer until stimulated by laser photons to form the visible image IMAGE FORMATION Types of Conversion

Why you see what you see … The images have different levels of density – different shades of gray X-rays show different features of the body in various shades of gray. The gray is darkest in those areas that do not absorb X-rays well – and allow it to pass through The images are lighter in dense areas (like bones) that absorb more of the X-rays. IMAGE FORMATION

12 Stage.3 (Viewing) Interpretation of the visible image  The final image whether it be on a film sheet or on a television screen should be viewed under specific conditioned to evaluate its quality and then passes for diagnosis IMAGE FORMATION

13 Stage.3 (Viewing) Interpretation of the visible image IMAGE FORMATION Viewing by reflected light from a surface - Examples( texts, illustrations in books etc) Viewing by transmitted light - Through a semitransparent layer (e.g. Radiographs) Viewing by emitted light from fluorescent layer. - Examples( fluoroscopic image on the TV screen) Types of viewing

14 IMAGE FORMATION Image characteristics Noise Contrast Sharpness Resolution

15 IMAGE FORMATION Image characteristics Noise Real images consist of 2 components - Meaningful pattern (Signal) - Chaotic pattern ( Noise) The details of the structures is affected by noise +Noise = - Details

16 IMAGE FORMATION Image characteristics Signal to Noise Ratio (SNR) Under optimal condition the magnitude of signal is greater than the magnitude of the noise - SNR is Saied to be high ( More details) High SNR When the magnitude of nearest the magnitude of the noise - SNR is Saied to be Low ( Less details) Low SNR

17 IMAGE FORMATION Image characteristics Density and Contrast Density The amount of blackening “ darkness ” on the radiograph Contrast The differences between the blacks to the whites

18 HighOptimalLow Contrast Changes IMAGE FORMATION Image characteristics

19 IMAGE FORMATION Image characteristics Sharpness Describe the blackening changes at the boundaries between adjacent parts Element of Image Sharpness Recorded detail : the sharpness of the lines of the image. Distortion : the misrepresentation of the true size or shape of the image compared to the object.

20 True Shape Elongated Foreshortened lengthwidth IMAGE FORMATION Image characteristics Sharpness ( SHAPE DISTORTION – TERMS)

21 IMAGE FORMATION Image characteristics Resolution The resolution of a system Is its ability to demonstrate closely spaced structures in the subject as separate entities in the image

22 Visibility Sharpness density contrast detail distortion size shape Image characteristics IMAGE FORMATION Summary

Unit III23