T06 Distortion (1) PDF - Radiography Distortion

TOPIC 6 - DISTORTION 1  Objectives ▪ To define magnification (size distortion). ▪ To draw and label a geometric representation of magnification in conventional radiography ▪ To demonstrate the effects of SOD, OID & SID on size distorti...

TOPIC 6 - DISTORTION 1  Objectives ▪ To define magnification (size distortion). ▪ To draw and label a geometric representation of magnification in conventional radiography ▪ To demonstrate the effects of SOD, OID & SID on size distortion using geometry ▪ To describe factors affecting shape distortion (shortening and elongation) using geometry ▪ To define shape distortion (true distortion) ▪ To draw the size and position of various projected structures on IR ▪ To perform MF calculation problems 2  Distortion It is a misrepresentation of the true size and shape of the object Factors Affecting Distortion: o Object thickness o Object shape o Object position and alignment Types of Distortion: o Magnification or size distortion o True distortion or shape distortion 3  Distortion S: SID o Source of radiation SOD FS: OID GEOMETRY OF IMAGE FORMATION o Focal spot - Small FS S Source CR: (Focal Spot) or Large FS o Central Ray SOD I: MF: o Image receptor (IR) SID A O B o Magnification factor Object OS: Ug: OID o Object Size Image o Geometric C I D Receptor IS: Unsharpness o Image Size 4  THE X-RAY BEAM DIVERGES FROM THE FS – (POINT SOURCE) Penumbra = Ug Amount of blurriness due to magnification and beam divergence 5 6  Distortion A minimum OID is needed to generate magnification If OID is very small, SID & SOD will not have a great effect on Magnification If OID is large, magnification can be decreased by increasing the SOD or SID 7 Types of distortion Magnification or size distortion True distortion or shape distortion 8  Magnification or size distortion Magnification occurs when IS > OS Magnification occurs because the XR beam diverges from the FS 9 Geometry of Magnification Triangles “SAB” and “SCD” are similar because they have the same shape and angles, but they are different in size According to the law of geometry we can say that: 10 Problem: Calculate the IS: OS = 12 cm OID = 20 cm SOD = 80 cm SID = SOD + OID Solution: SID = SOD + OID = 80 cm + 20 cm = 100 cm IS = (SID/SOD) X OS IS = (100 cm / 80 cm) X 12 cm IS = 15 cm 11  Magnification Factor (MF) IS or SID MF = ------ MF = ------- OS SOD MF > 1 12  % Magnification (% MF): (IS - OS) or (SID - SOD) % MF = -------------- X 100 % MF = ------------------ X 100 OS SOD MF & % MF: % MF = (MF -1) X 100 MF = % MF + 1 100 MF 1.0 1.2 1.5 2.0 2.5 3.0 %MF 0% 20% 50% 100% 150% 200% 13  Factors Affecting Magnification Magnification α OID Magnification 1/α SID Magnification 1/α SOD GEOMETRIC UNSHARPNESS (UG) IS CAUSED BY MAGNIFICATION AND BY FOCAL SPOT SIZE 14 15  Thickness & Location of object PA Orbits AP Orbits ↓ OID, ↓ size distortion ↑ OID, ↑ size distortion, orbits are magnified Image result for ap facial bones xray Image result for ap vs pa orbits x ray image skull xray caldwell method 16  http://www.stritch.luc.edu/lumen/MedEd/MEDICINE/PULMONAR/CXR/atlas/images/328b3.jpg cardiomegaly 17  Which image was done PA, why? Assuming that both images were done PA, which one was done with shorter SID, WHY? Assuming that both images were done PA & same SID, which one was done with shorter OID, WHY? Assuming that both images were done PA, same SID & OID, which one was done with shorter SOD, WHY? 18  Magnification Problems Which OID would produce a magnification factor of 2.5, if the SID is 100 cm? SID SID 100cm Solution: a) MF = -------, SOD = ------- = ---------- = 40cm SID = 100cm SOD MF 2.5 MF = 2.5 SOD = ? b) OID = SID - SOD = 100 cm - 40cm OID = ? OID = 60cm 19 An image measuring 18 cm long represents a % Magnification of 20% of the actual size of the object. What is the length of the object? Solution: 1) IS = 18 cm MF = (%MF / 100) +1 %MF = 20% MF = (20 / 100) +1 OS = ? MF = 1.2 2) OS = IS / MF OS = 18 cm /1.2 OS = 15 cm 20 An object measuring 24 cm long projects a 20% magnification on the image. What is the image size? Solution: 1) OS = 24 cm MF = (%MF/100) +1 %MF = 20% MF = (20/100) +1 IS = ? MF = 1.2 2) IS = MF X OS IS = 1.2 X 24 cm IS = 28.8 cm 21 The patient's wrist is placed at 10" from the image receptor and exposed at 40" SID. A measurement of the actual size of the wrist reveals that it is 2.5" wide. What will be the Magnification Factor on the radiograph? Solution: 1) OID = 10” SOD = SID - OID SID = 40” SOD = 40" - 10" OS = 2.5” SOD = 30" MF = ? 2) MF = SID / SOD MF = 40" / 30" MF = 1.33 22 The size of the screw (pin) on the image is 20 cm long, and the actual size is 15 cm long. i) What is the magnification factor? Solution: IS = 20 cm OS = 15 cm SID = 100 cm i) MF = ? ii) OID = ? i) MF = IS / OS MF = 20 cm / 15 cm MF = 1.33 23  True Distortion (Shape Distortion) Change in the shape of the image due to unequal magnification (foreshortening & elongation) Occurs when ROI is not parallel to IR: o ∆ OID & ∆ SOD (e.g., #2 & #3 in diagram) Results in ∆ Ug 24 25  Elongation and Foreshortening Elongation Elongation o IR not parallel to Object o CR ≈ perpendicular to object Foreshortening o Object not parallel to IR o CR ≈ perpendicular to image S Prevention of Distortion: Foreshortening o Keep object and image (IR) B parallel to each other Objec t Object o Keep object in line with the primary beam A C F IR Film D ' I 26  Marginal Rays Rule: o Long anatomical parts require more diverging rays ✓ examples: femur, leg, humerus, T/L spines o Distortion of distal and proximal anatomy is greater than the central anatomy o To reduce marginal rays rule distortion: ↑ SID and/or do additional smaller views on the ROI 27  Distortion of intervertebral disc spaces LUMBAR REGION IR IR 28 A P N O T S E T R E I R O I R O R IR 29 Superimposition, Displacement & Magnification of Structures To decrease magnification, reduce OID, increase SID & SOD How to prevent superimposition and or displacement of structures: o Positioning of patient: ▪ Supine, prone, decubitus, upright, Trendelenburg, LPO/RPO, LAO/RAO, internal/external rotation, flexion/extension). o Tube angulation: ▪ Cephalic, caudal, proximal, distal, medial, lateral. o Compression o Breathing phase 30  MAGNIFICATION Vs. IMAGE SIZE & DISPLACEMENT Source SOD SID Object OID Image A B C 31 IR IR 32 33 Other factors that affect magnification and distortion: Mattress Cast, bandages, etc Table-bucky distance Isocentric x-ray units maintain the CR perpendicular to the IR Examples: C-arm radioscopy units, panorex 34 DISTANCE BETWEEN THE BUCKY TRAY AND TABLETOP: GE UNITS → 9 CM (3.5”) PHILIPS UNIT → 8.5 CM (3.3”) 35  MAGNIFICATION FACTOR USED FOR TEMPLATING The magnification factor is used to determine the size of the prosthesis needed before surgery to replace joints such as the hip and knee joints 36 L 37 The Zimmer ball and pelvis king mark tools are used to calculate magnification factor (MF) on IMAGES SIZES: 1.5mm, 2.0mm, 2.5mm 38  R ZIMMER BALL 39 40 ZIMMER BALL 41 pelvis king mark tool 42 43 44