Image Contrast & Contrast Resolution PDF

Topic 5: Image Contrast & Contrast Resolution 1  Objectives To define image contrast (IC), Image To describe how scatter affects SC Receptor Contrast (IRC) & SC and IC To state the re...

Topic 5: Image Contrast & Contrast Resolution 1  Objectives To define image contrast (IC), Image To describe how scatter affects SC Receptor Contrast (IRC) & SC and IC To state the relationship between IC, To compare the various types of SC and IRC beam limiting devices To describe the relationship between To calculate the contrast scale of SC, IC & Exposure latitude improvement factor (K) (EL) To justify the choice of kVp for To compare the advantages and examinations involving CM disadvantages of short & long SoC To differentiate between objective images and subjective contrast To describe the factors affecting To list the factors affecting distinctness subjective and objective image To state the relationship between contrast kVp, SC, EL and SoC To define Contrast resolution and To list and describe the factors the factors that affect it affecting SC 2  VIDEOS Subject Contrast: https://www.youtube.com/watch?v=_EcKx0SY3cg&t=850s Contrast Resolution (second half of video): https://www.youtube.com/watch?v=vhsNkWds7eI Contrast Resolution (second half of video): https://www.youtube.com/watch?v=Jq88t-MNUAg Scatter Control Devices: https://slideplayer.com/slide/4156368/ kVp and contrast kVp and Contrast – YouTube kVp & radiographic contrast explained || Ask The Rad Tech – YouTube kVp and x-ray Contrast (Guide for Radiologic Technologists) - YouTube 3 Diagnostic Value requires ❑ IC ❑ Sharpness ❑ adequate scale of contrast ❑ Minimum noise 4  Image Contrast (IC) IC is the difference between brighter and darker areas on an image IC is a function of Subject Contrast and IR Contrast o minimum Subject Contrast and IR Contrast are required to generate IC 5  Subject Contrast SC is the ratio or variation in X-ray intensity transmitted through one part of the subject compared to another IC α SC 6 7 FACTORS THAT AFFECT SUBJECT CONTRAST 8  HOW DOES kVp AFFECT SC? SC 1/α kVp IC 1/α kVp o As kVp ↑ Primary beam to scatter ratio in exit beam ↓ o As kVp ↓Primary beam to scatter ratio in exit beam ↑ 9  40% of 15% change in kVp is required to see a small change in IC This is due to the inherent wide dynamic range of digital systems, bit depth, and smart image processing algorithms 10 HOW DOES FILTRATION AFFECT SC? Effect of Inversely proportional filtration on: proportional Ave. beam energy ☺ SC ☺ IC ☺ SoC ☺ 11 HOW DOES CONTRAST MEDIA AFFECT SC? CM increases SC CM may ↑ or ↓ Z of tissue depending on: o Concentration of CM o Composition of surrounding tissues 12  HOW DOES CONTRAST MEDIA AFFECT SC? Dilution of CM: o Barium studies: (100 – 120 kVp range) ▪ Thick Ba or diluted barium is used on examinations of the digestive system (oesophagus, stomach, large & small intestines o Iodine studies: (80 kVp) ▪ Concentrated iodine: angiograms ▪ Diluted Iodine: urinary system imaging 13 HOW DOES EFFECTIVE ATOMIC NUMBER AFFECT SC? ∆ Z α ∆ transmission α Subject Contrast EFFECTIVE ATOMIC NUMBER (Zeff): calculates the average Z for a compound or mixture of materials TISSUES CONTRAST MEDIA Bones: Iodine: 13.8 53 soft tissues: Barium: 7.4 56 Fat: Air: 5.9 7.6 14  body part thickness Part Tickness Vs. Subject Contrast (SC) Incident (i) Exposure (Ei) SC = E2/E1 B B (Soft Tissues) A (Soft Tissues) A (Soft Tissues) (Soft Tissues) Transmitted Exposure (Et) E2 E1 E2 E1 (ii) Tissue Density Vs. SCSubject 1 < SC2 Contrast (SC) Incident 15  Exposure (Et) E2 E1 E2 E1 HOW DOES TISSUE DENSITY AFFECT SC? (ii) Tissue Density Vs. Subject Contrast (SC) Incident Exposure (Ei) A B A B (Air) (Bone) SC = E2/E1 (Fat) (Muscle) Transmitted Exposure (Et) E2 E1 E2 E1 SC1 > SC2 (iii) Compression Vs. Subject Contrast (SC) 16  A B A B HOW DOES COMPRESSION AFFECT SC? (Air) (Bone) SC = E2/E1 (Fat) (Muscle) Transmitted Exposure (Et) E2 E1 SC increases E E with compression 2 1 (iii) Compression Vs. Subject Contrast (SC) Incident compression Exposure (Ei) A A B (Bone) SC = E2/E1 B (Bone) (Soft Tissues) (Soft Tissue) Transmitted Exposure (Et) E2 E1 E2 E2 E1 E2 SC may be ↓ because of gas shadow being pushed away and replaced with ST; this would ↓ SC 17  HOW DOES SCATTERED RADIATION AFFECT SC? Scatter ↓ SC The amount of scatter in the remnant beam ↑ when: o kVp ↑ - even though increasing kVp lowers the scatter produced, the % of scatter in the forward direction increases o part thickness ↑ o Amount/Size of tissue irradiated ↑ o tissue density ↑ o OID ↓ SC α primary beam to scatter ratio 18  HOW DOES SCATTERED RADIATION AFFECT SC? How to Reduce scatter o Prevent scatter formation: ▪ ↓ kVp ▪ ↓ field size ▪ ↑ compression o Prevent scatter from reaching the IR by: ▪ Grids ▪ air gaps (increasing OID) ▪ back of IR made of absorbing material (↓ back scatter) ▪ more absorbing IR fronts and table 19  Grids improve IC Contrast Improvement Factor (K) The ability of the grid to improve image contrast is specified by the contrast improvement factor (k). A contrast improvement factor of 1 indicates no improvement. IC with grid K = ---------------------- IC without grid Most grids have contrast improvement factors between 1.5 and 2.5. In other words, the IC is approximately doubled when grids are used. The highervthe grid ratio the more effective the grid is to remove scatter 20  COLLIMATION IMPROVES IC & LOWERS PD Collimation is the most Efficient way to ↓ scatter, ↑ IC & ↓ PD o collimation and compression are most effective/useful when: ▪ Thicker body parts are x-rayed ▪ Non-grid (or low grid ratio) ▪ kVp is ↑ 21  Image Receptor Contrast - IRC IRC - A measurement of the ability of the IR to record the SC into IC IRC may amplify or reduce SC IRC depends on the characteristics of the IR o Dynamic range o Bit Depth o Image Processing Algorithms 22  CONTRAST RESOLUTION Contrast Resolution is the ability of the IR to distinguish between structures of similar tissue, e.g., liver and pancreas Contrast resolution depends on: o kVp o Subject composition (thickness, tissue density, etc.…) o Dynamic range o Bit depth o Grids o Collimation (PB to scatter ratio) o SNR The contrast resolution of digital Image Receptors is superior to film 23 Image Contrast – Contrast Resolution up to 1% difference in tissue attenuations are detected by latest DR systems Film systems up to 10% 24 CONTRAST RESOLUTION 26  Long SoC vs. Short SoC If SoC is too long, then IC is lower causing for detail visibility to decrease, however it will display more structures in the ROI If SoC is too short, then the IC is higher causing for detail visibility to increase, however it will display less structures in the region of interest as they may be hidden in the high dark areas or the white areas in the ROI. As we saw in the PACS and GE equipment, post processing image software ca be applied to the image to manipulate the contrast but to a limit. 27  HOW DO THE VIEWING CONDITIONS AFFECT IC the viewer’s ability to assess: o subjective contrast o subjective definition or distinctness Intensity of light from monitor o If brightness ↑: ▪ subjective IC in dark areas will ↑ ▪ subjective IC in light areas will ↓ o If brightness ↓: ▪ subjective IC in dark areas will ↓ ▪ subjective IC in light areas will ↑ 28  HOW DO THE VIEWING CONDITIONS AFFECT IC If brightness Intensity of surrounding lights increases (room lights), then: o subjective IC in dark areas will ↓ o subjective IC in bright areas will ↑ Uniformity of light: o Non uniformity of light would result in local variation in IC as described above 29  HOW DO THE VIEWING CONDITIONS AFFECT IC when viewing images, the observer should: be able to adapt to extreme changes in brightness Have good visual acuity Avoid vision fatigue Use adequate viewing distance Use a magnifying glass for small ROI reduce glare (or dazzle) by eliminating “parasite” lights: o eliminate ceiling/window lights – dimmed lights o apply electronic masking outside the collimated field 30  AUTOMATIC ELECTRONIC COLLIMATION Also called automatic electronic masking black mask around the ROI improves subjective IC: ↓ glare 31 Electronic masking reduces Dazzle and glare 32  Es-tu doué en perception de couleurs ?! Alors jette un coup d’oeil à la figure suivante... 33 34 La couleur des carrés “A” et “B” est-elle identique!? 35 36  Non?! Allez, vérifie encore une fois... 37 38 The brain forces itself as much as possible to image the squares as they should be... One obscure and the other white... It doesn't matter that they're the same color. Since the contiguous squares are supposed to be a different color... Your brain makes you see them that way... Le cerveau se force un maximum pour imager les carrés tels qu’ils devraient être... L’un obscure et l’autre blanc... Peu lui importe qu’ils soient de la même couleur. Les carrés contigus étant supposés être de couleur différente... Ton cerveau fait en sorte que tu les voies ainsi... 40