Mammography Basics in Dose and Image Quality

Mammography Basics in Dose and Image Quality Why is it so important to have high quality in mammo? Why use Mo/Mo at low energies for screen-film mammo? What are the radiographic technique choices available? How does technique affect image quality and dose? Background Radiographic Technique & Dose Image Formation Image Quality Radiographic Technique & Dose Image Formation Image Quality When was this film taken? 1950s 1960s 1970s 1980s Stafford Warren c. 1939 Raul Leborgne c. 1951 1st dedicated mammo system CGR Senographe, 1969 Gold et al, RadioGraphics 1990 The Good: Reduces breast cancer mortality Widespread screening tool Low cost High resolution High speed (acquisition, diagnosis) Acceptable dose The Bad: False negatives 30% to 50% or more missed cancers overlapping dense tissue limited contrast / subtle findings False positives low incidence of cancer: 1 / 7 recalled, 1 / 80 biopsied, but only 1 / 250 cancer add expense and patient anxiety The Ugly: "Interpreting mammograms, even good ones, is considered the hardest task in radiology, requiring extensive training and specialization.” 1 Legal burden: #1 source of malpractice Average award for ≤5mo delay Dx of BR CA = $250K 2 1 NY Times 10/24/2002 2 Physician Insurers Assoc America. Breast cancer study, 3rd ed. 2002 Background Radiographic Technique & Dose Image Formation Image Quality X-ray enters body: No interaction, no dose X-ray flies straight through body, hits detector to form image Yes interaction, yes dose Photoelectric effect X-ray totally absorbed Lower energies, dep. on atomic # and density Compton scatter X-ray deflected, may or may not hit detector Higher energies, dep. on density only   PHOTOELECTRIC INTERACTIONS Attenuation Coefficient (per mm) Yes, Yes, Yes Maybe No Can't PHOTON ENERGY (keV) IODINE The K Edge 33 keV Photoelectric Attenuation in Soft Tissue Total Attenuation in Soft Tissue Photoelectric Compton Element Symbol Atomic Number (Z) K Edge (keV) Hydrogen H 1 .013 Beryllium Be 4 .11 Carbon C 6 .28 Nitrogen N 7 .40 Oxygen O 8 .53 Sodium Na 11 1.1 Aluminum Al 13 1.6 Silicon Si 14 1.8 Sulfur S 16 2.5 <20 keV, photoelectric effect dominates: Maximum contrast, maximum dose >25 keV, Compton scatter dominates Minimal contrast in soft tissue, still has dose Great for imaging other parts of body e.g. CXR! Lower energy also better for photoelectric absorption in screen So traditionally, most mammo done at 20-25 keV Target / filter Determines shape or quality of spectrum Broad or spikes? skewed to left or right? Tube potential energy Determines max energy of spectrum Also affects amount of x-rays Exposure NO effect on spectrum shape Affects amount of x-rays   X-RAY SPECTRUM Photons Photon Energy (keV) kV 100 ma 50 ma 25 kV 50 kV 100 Few low energy photons penetrate anyway do NOT affect image quality do contribute to dose Very thin sheet or foil just 1mm of light metal attenuates ~ same as 1cm of tissue for mammo, e.g.,25-50 µm of Mo or Rh Removes low energy photons before they enter Pt Shifts beam quality (effective energy) higher Same image quality, much lower dose! K-edge filters: Use heavy attenuation just above k-edge to kill undesired higher part of spectrum Mo/Mo passes up to 20 keV Mo/Rh adds 20-23 keV,­quality/energy (for thicker or denser breasts) 1mm Aluminum 1cm Muscle Penetration Photon Energy (keV) no filter 1mm filter 3mm filter # of Photons Moly/Moly Spectrum Molybednum Anode Characteristic Radiation 17.6 keV Molybednum Filter 20 keV 19.7 keV Moly/Rhodium Spectrum Molybdenum Anode Characteristic Radiation 17.6 keV 19.7 keV Rhodium Filter 23.22 keV Photon Energy (keV) Relative Exposure Relative Exposure Photon Energy (keV) Soft, blurry background Nearly uniform in mammo 20-50% Reduce contrast by same amount, 20-50% (could be worse, CXR is 65-95%) Reduced with grid (3:1 to 5:1 for mammo) Bucky Factor: ­exposure (~2x) due to atten of primary & scatter through grid   Background Radiographic Technique & Dose Image Formation Image Quality Why care about attenuation? Attenuation provides “subject contrast” aka “physical contrast,” which is captured by detector to form “image contrast” (i.e., x-ray image is formed by different attenuations of x-ray beam within patient's body) Objects with increased attenuation (fluid, FGT, calcs) produce shadows relative to decreased attenuation (fat) X-RAY IMAGE CONTRAST Object Penetration Low High Receptor Receptor Image Image Eternal conflict! Body: low contrast, low dose Lesion: high contrast, high dose Greater·­kVp, ­more photons penetrate, so can lower mAs to equalize, reduce dose But lower kVp means less photons penetrate lesion, ­increased contrast Body Penetration Reduces Dose Object Penetration Reduces Contrast Receptor BODY PENETRATION X-ray Tube X-ray Beam kV 100 kV 60 10% 2% OBJECT PENETRATION AND CONTRAST Latitude or dynamic range: ratio between lightest and darkest areas of a system ­Greater contrast, ­greater difference between dark and light, ­greater challenge in display with finite DR L: raw image “For processing” Very wide latitude R: processed “For presentation” Narrow latitude, high contrast Which has highest contrast? Background Radioographic Technique & Dose Image Formation Image Quality Anthropomorphic phantom, anatomy of real Pt QA phantom, like ACR but more stuff Contrast Distinguishes target from background fat vs glandular, lesion vs glandular kVp, scatter, image processing Noise Obscures targets in noise Exposure, detector, image processing Resolution Ability to perceive fine detail Detector, scatter, image processing Which image is better?  Why? 25 kVp 32 kVp 1 2 3 4 1 2 3 4 35 kVp 23 kVp 23 kVp 29 kVp 35 kVp 344 mAs - 1.8s - 1.4 mGy 84 mAs - 0.5s - 0.9 mGy 38 mAs - 0.5s - 0.6 mGy If pixels are 85x85 µm, what is the smallest object you can see? 165 µm 130 µm Better quality with poorer resolution? SFM FFDM SFM FFDM Contrast Detail Phantom: Going from bottom to top, greater contrast Going left to right, greater size No Grid Grid Increased Contrast Increased Noise No Grid - AEC 49 mAs - 0.5 mGy Grid - AEC 80 mAs - 0.84 mGy No Grid - Manual 80 mAs - 0.84 mGy Lower Dose Less Contrast Higher Dose Greater Contrast HIgher Quality Higher Dose Greater Contrast Less Noise Best Quality Mo/Rh ­higher energy, less dose for thick/dense breasts vs. Mo/Mo Raising kVP  lowers dose, lowers contrast (fixed by img proc) Important factors in mammo image quality: contrast, noise, and resolution Grid w/ AEC: less scatter, ­greater contrast, similar noise (due to AEC), higher quality, ­higher dose Why is it so important to have high quality in mammo? Why use Mo/Mo at low energies for screen-film mammo? What are the radiographic technique choices available? How does technique affect image quality and dose? Mammo quality is important because of challenging clinical tasks and physics complexity S/F mammo optimized for Mo at low energies for best compromise between contrast vs dose Mammo image formed by technique choices: target / filter, kVp, mAs, mag, grid, detector These affect dose and image quality:contrast, noise, resolution X-ray tube has high voltage potential between cathode and anode (28 kVp) Electrons accelerate from cathode to anode, picking up energy (28 keV) Electrons strike anode, generate x-rays(range or “spectrum” of 0–28 keV) Cathode (-) Anode (+) X-rays 3 contributions: Bremsstrahlung aka braking or general radiation Continuous spectrum up to max energy (0–28 keV) Characteristic radiation Sharp spikes unique to anode material k-edge energies (for Mo target, ~18 & 20 keV) Filtering to reduce dose Remove low-energy (won’t make it through breast) Remove high-energy above k-edge (not useful for contrast, for Mo filter, ~20 keV) Number of x-rays Initial Bremsstrahulung Characteristic x-ray peak Final Spectrum X-ray Energy Max kV No interaction X-ray flies straight through, hits detector Photoelectric effect X-ray totally absorbed Depends on atomic # and density Dominates at lower energies Compton scatter X-ray deflected, hits detector? Depends on density only Dominates at higher energies Penetrate Scatter Photoelectric Interaction Energy Compton Interaction Minimal difference between fat and FGT, hence low contrast of mammo. Almost no difference between tumor and FGT. Difference even smaller at higher energies. Breast Tumor Fibroglandular Tissue Adipose Tissue Energy (keV) µ (cm -1)