Chapter 19 & 20 Image Quality & Techniques

1. Chapter 19 & 20 Image Quality & Techniques • There are three geometric factors that affect radiographic quality. • Magnification • Distortion • Focal Spot Blur • We have explored these factors in the laboratory.

2. Magnification • All objects on the radiograph are larger that their actual size. This is called magnification. • The magnification factor is the image size divided by the object size. • At 40” (100 cm) factor is 1.1 • At 72” (180 cm) factor is 1.05

3. Magnification • Usually we do not know the size of the object so we must determine the magnification factor another way. • Image size S0D • MF = ----------------= ------------ • Object Size SID

4. Minimizing Magnification • Large SID: Use the less divergent beam. • Chest X-rays are done at 72” SID to minimize magnification. • Lateral C-spine done at 72” • Small OID: Get patient as close to the film as possible. • Basic principle for positioning.

5. Distortion • Distortion is the misrepresentation of the true size and shape of the object being radiographed. • The amount of distortion depends upon the thickness, position and shape.

6. Thickness • Thick objects are more distorted than thin objects because of the greater change in Object Image Distance.

7. Thickness • The position of the object relative to the central axis will cause greater distortion with thick and/or irregular shaped objects.

8. Object Position • If the object plane and image plane are parallel the image will not be distorted. • If the object plane and image plane are not parallel, distortion will occur.

9. Spatial Distortion • When multiple objects at different OID’s occur, we get spatial distortion due to unequal magnification. • Two arrows appear as one. • When shifted laterally more distortion occurs

10. Object Shape Distortion • When the object plane is not parallel to the image plane as when inclined, shape distortion occurs. • This will result in foreshortening.

11. Focal Spot Blur • Focal spot blur is caused by the effective size of the focal spot, which is larger at the cathode side. • Focal spot blur is the most important factor in determining spatial resolution.

12. Focal Spot Blur • Focal spot blur is impacted by the Object to Image Distance.

13. Focal Spot Blur Heel Effect • There is more to the heel affect than just the attenuation of the beam by the anode. • The focal spot blur is smaller at the anode side and larger at the cathode side.

14. Taking advantage of the Anode Heel Effect

15. Did you see a problem ? • If the tube is mounted correctly for the AP Full spine, Chest and A-P Thoracic Spine, the patient must stand on his head for the lateral thoracic spine!!!! • For erect radiography, the use of the anode heel affect is limited.

16. Object Factors that Affect Quality • Subject Contrast • Patient or part thickness • Tissue mass density • Affective atomic number • Object shape • kVp

17. Radiographic Contrast • Radiographic Contrast is how the film looks. • It is the combination of receptor contrast and subject contrast. • Contrast occurs between structures of different densities.

18. Thickness • The thicker the body part or body section, the greater attenuation of the beam. Contrast is relative to the number of x-rays leaving the body. • Remember that x-rays are merely shadows of the anatomy based upon attentation of the beam.

19. Thickness • Radiographs of thin patients will have more contrast than those of large patients. • Thicker object absorb more rays and will appear lighter than thin objects.

20. Tissue Mass Density • Different sections of the body have may equal thickness yet different mass density. • Two wrist may be the same thickness but the one that is swollen will have greater mass density due to water in the tissues.

21. Effective Atomic Number • While Compton interactions with tissues are not impacted by the relative atomic number of tissues, the photoelectric effect vary with the cube of the atomic number. • When the effective atomic number of adjacent tissues is very different, subject contrast is very high.

22. Object shape • The object shape not only influences the geometry but also through its contribution to subject contrast. • A vessel on end has high contrast while other have lower contrast.

23. Varying tissue densities • Bone absorbs most x-rays leaving a white shadow. • Water absorbs less x-rays leaving a light gray shadow • Fat absorbs fewer x-rays leaving a dark gray shadow. • Air absorbs little x-rays and is black on the film.

24. Varying densities in the Body • Air, oil, water and metal (natural) absorb different degrees of the x-rays and produce contrast. • Heavy metals are used as contrast media to enhance contrast in the body in medical radiology. Principle ones are Barium and Iodine.

25. kVp • We have little control over the previous factors controlling subject contrast. • BUT!!!!! • The absolute magnitude of subject contrast is greatly influenced by the kVp of operation.

26. kVp • kVp also influences film contrast but not to the extend it controls subject contrast. • Low kVp = high contrast = short scale • High kVp = low contrast = broad scale

27. Motion Blur • If any element of the x-ray moves during exposure, we get motion blur. • Patient motion is the most common cause of motion blur. • Motion blur is more common in erect radiography.

28. Ways to Control Motion Blur • Use the shortest possible exposure time • Restrict patient motion by instructions or restraining device. • Use a large SID • Use a small OID

29. Tools to Improve Quality • Patient Positioning • Get the patient close to the film reduce OID. • Center the beam to the area of interest. • Get the area of interest parallel to the beam or film. • Restrain motion and communicate with the patient. • Use short exposure times.

30. Tools to Improve Quality • Image receptors • Use the correct film & screen combination for the examination. Extremity for wrist. Regular for spines. • Intensifying screens reduce patient exposure at least 20 times. • As the speed of the image receptor increases, radiographic noise and contrast resolution decreases.

31. Tools to Improve Quality • Image receptors • Low contrast imaging procedures have wider latitude and a larger margin of error in producing acceptable radiographs. • Use the highest speed system that will provide adequate contrast and density over the entire spectrum of examinations.

32. Tools to Improve Quality • Technique selection • We must select the optimum technical factors. • We must get the exposure time as low as possible so high frequency machine are important. • Contrast controlled by the kVp used • Density controlled by the mAs used.

33. Tools to Improve Quality • kVp has a greater influence than mAs. • Technique selection is a balancing act. Balancing contrast, density and exposure.

34. Chapter 20 Radiographic Technique • Several factors influence the selection of technical factors. The primary factors that impact exposure and image quality are: • SID • mAs • kVp

35. Patient Factors • The anatomic thickness and body composition greatly impact the technical factors. • The technique chart is based upon the Sthenic Body Type.

36. Patient Factors • Sthenic is strong & active • Hyposthenic is thin but healthy • Hyperstenic is obese • Astenic is small, frail sometime emaciated, and often elderly

37. Patient Thickness • The thickness of the patient should not be guessed. • It should be measured with calipers. • Patient thickness is measured in cm.

38. Body composition • The type of tissue in the area of exposure will impact the technical factors. • The tissue types in the chest are different from the abdomen. • Disease processes will also impact the exposure factors. Obtaining a good clinical history is important. History must be communicated to the radiographer.

39. Radiolucent (Destructive) Active TB Atrophy Bowel obstruction Cancer Degenerative arthritis Emphysema Osteoporosis Pneumothorax RadiopaqueConstructive) Aortic aneurysm Ascites Atelectasis Cirrosis Hypertrophy Metastasis Pleural Effusion Pneumonia Sclerosis Classifying Pathology

40. Image Quality Factors • Image quality factors include • OD • Contrast • Image Detail • Image Distortion • OD is the optical density or radiographic density. OD is controlled by the mAs and SID.

41. Optical Density • Numerically low OD is a low number like 0.25. • Dark is a high number like 2.20 to 4.0 • Light is underexposed • Dark is over exposed • If density is the only factor that needs to be changed, change the mAs.

42. Optical Density • A 30% change in mAs is needed to make a perceptible change in optical density. • Usually when a change in optical density is needed, the mAs is either doubled or halved. • kVp must be changed by 4% to produce the same change in optical density. • Changing kVp will also impact penetration and contrast.

43. 30 - 50 Rule • If the film is under exposed, double the mAs. • If the film is over exposed, cut the mAs in half. • If the film is slightly underexposed, increase the mAs 30%. • If the film is slightly overexposed, reduce the mAs 30%.

44. 30% Density Change • The lower image was the first image taken. It was dark but normally would be acceptable. • The top image was the mAs reduced 30%. The air fluid levels in the sinus is easier to see.

45. 15% Rule • The OD can be changed with kVp but it will also impact exposure and contrast also. • Increase of 15% in kVp is equal to cutting the mAs in half. • Decrease of 15% in kVp is equal to doubling the mAs. • If the film is underexposed, increase kVp 15%. • If the film is overexposed, decrease kVp 15%.

46. Contrast • The function of contrast is to make the anatomy more visible. • Contrast is the difference in density of adjacent structures. • The relative penetrability of the x-ray through different tissues determines the image contrast.

47. Contrast • Contrast can be measured as the Gray Scale of Contrast. It is the range of optical density from white to black on the image. • Contrast is controlled by kVp.

48. Adjusting Contrast with 15% Rule • An 15% increase in kVp and a reduction of mAs by 50% will produce the same OD but lower contrast. • Used to reduce exposure or reduce exposure time/ • An 15% decrease in kVp and doubling the mAs will produce the same OD but higher contrast.

49. Image Detail • The sharpness of image detail refers to the ability to see structural lines or borders of tissue in the image. • The visibility of image detail is best measured by the contrast resolution. • The geometric factors of focal spot selection, SID and OID will impact sharpness.

50. Image Detail • Visibility of image detail is impacted by factors such as image fog. • Scatter radiation reduces the ability to visualize lines of detail. • Light fog or processing can impact the visibility of structures. • Collimation, screen combination and the use of a grid are other factors that impact image detail.