Image Contrast, Noise, Resolution
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This document discusses the effects of kVp and mAs on various properties of x-ray images. It explains that kVp determines the highest x-ray energy and quality, while mAs determines the quantity of photons and exposure time. Higher kVp and mAs increase spatial resolution, contrast, and signal-to-noise ratio, but also increase radiation dose. The document covers these parameters for screen-film radiography as well as computed tomography, and how they impact visible properties like image noise, contrast, and resolution.
Image Contrast, Noise, Resolution
- 1. Effects of kVp and mAs on image spa4al resolu4on, contrast, dose, and noise Vibha Chaswal, Ph.D.
- 2. Peak Voltage (KVp) of an X-‐ray tube • Highest X-‐ray energy is determined by peak voltage applied across the x-‐ray tube. • With filtra4on: Eave = (1/2-‐1/3)Emax • KVp and filtra,on = Quality of the X-‐ray beam Ref: Bushberg
- 3. Milli Ampere Second (mAs) tube current • Tube current is the rate of electron flow from the cathode to anode, measured in milliamperes (mA) • 1 mA = 6.24 x 1015 • S = exposure 4me, dura4on of x-‐ray produc4on • Indicates quan4ty: Number of photons is propor4onal to mAs
- 4. Rules of Thumb • • • • • kVp and exposure: Exposure α (kVp)2 For a fixed exposure technique: (kVp1/kVp2)5 = mAs2/mAs1 kVp determines quan0ty, quality, and transmission through the object whereas mAs determines quan0ty
- 5. Image Contrast • Medical Imaging is the Process of Conver4ng Tissue Characteris4cs into a Visual Image • Contrast: Difference in the image gray-‐scale between closely adjacent regions on the image. Contrast sensitivity: imaging system's ability to translate physical object contrast into image contrast Increasing Contrast Sensi4vity Increases Image Contrast and the Visibility of Objects in the Body
- 6. Different defini4ons of contrast • Subject contrast: difference in some aspect of the signal prior to its being recorded (x-‐ray operators use different kVp and mAs to control subject contrast) • Displayed contrast: digital imaging (CT (x-‐ray tomography) imaging uses mAs for increasing contrast resolu4on (contrast to noise ra4o) kVp dependence:Once kVp is set, out of sight out of mind
- 7. Dose and contrast versus kVp For screen film radiography Ref: Bushberg
- 8. Computed Tomography: 3 steps Scan: produces image data Reconstruc,on: produces digital image => a matrix of pixels with CT numbers Digital to analog conversion: produces visible analog image represented by different shades of gray
- 9. Hounsfield Unit X-ray attenuation depends on both the density and atomic number (Z) of materials and the energy of the x-ray photons. For CT imaging a high KV (like 120-140) and heavy beam filtration is used. This minimizes the photoelectric interactions that are influenced by the Z of a material. Therefore, CT numbers are determined by the density of the tissues or materials.
- 10. Displayed Contrast: CT • Defined by difference in gray scale values of closely lying adjacent structures • Gray scale values assigned to pixels during DI to analog conversion of DI • Visible contrast can be controlled by window, level and zoom or post-‐processing techniques • CT imaging uses a high KV (like 120-‐140 kVp) and heavy beam filtra@on => minimizes the photoelectric interac@ons
- 11. Digital Radiography • Enhance contrast digitally using Window and Level and Ref: Bushberg
- 12. Digital Radiography • Post-‐ processing using the Edge enhancement filter Ref: Bushberg
- 13. Spa4al Resolu4on • Ability of an image system to dis4nctly depict two objects as they become smaller and closer together • Directly related to mAs = quan,ty of photons making the image • kVp set for a technique • LOTS of other factors that affect spa4al resolu4on • Quan4fied using MTF (cycles/mm)
- 14. Spa4al resolu4on of different imaging systems -‐ MTF Ref: Bushberg
- 15. Noise • Local varia4ons in contrast due to a background texture called noise that does not represent the ahenua4on in pa4ent • Random: e.g., caused by random varia4ons in x-‐ ray photons interac4ng in the 4ssue • Screen-‐film radiography: Visual percep,on of noise is reduced when the detected x-‐ray photons increase. • Increasing mAs and kVp decrease noise as the # of detected photons increase, so does pa,ent dose.
- 16. Signal-‐to-‐Noise Ra4o • • • • If N = photons/pixel then SNR = √N Noise and Dose: to increase the SNR by 2 the dose to the pa4ent (N) has to be increased by 4
- 17. Thank You!
Editor's Notes
- #7: Subject contrast can be consequence of a difference in intensity, energy fluence, x-ray energy, phase, radionuclide activity, relaxation characteristic etc.