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Dose from Computed Tomography CT Patient Dose Tube rotates around patient during study Dose distribution different than radiography Patient CT Dose depends on X-Ray Beam kVp Image Quality & Processing Selections noise mA detector time slice thickness filtration collimation Direct efficiency matrix resolution reconstruction algorithm Indirect CT Patient Dose In theory only image plane is exposed In reality adjacent slices get exposed because X-ray Tube beam diverges interslice scatter Patient Detector CT Dose Phantom Comes in 2 sizes “Head” 16 cm diameter “Body” 32 cm diameter Measuring CT Dose “Pencil” ion chamber used Pencil pointed in “Z” direction Measurement Procedure Standardize protocol kVp mAs scan time slice thickness table movement field of view Select appropriate phantom head body Measuring Dose “Pencil” ion chamber used Pencil pointed in “Z” direction Single thin slice irradiated Long chamber captures scatter Dose Phantom Chamber Hole for Chamber Active Chamber Area (exposed area of chamber) Z Slice Z Field Measurement of CTDI Pencil ion chamber length must >> slice thickness chamber receives radiation from all parts of dose distribution including scatter Dose Chamber Partial Irradiation Correction Only part of chamber is irradiated Chamber calculates exposure as: charge from ionization ----------------------------------mass of air in entire chamber BUT Because only part of chamber irradiated, displays incorrect exposure Meter assumes full chamber irradiated Partial chamber irradiation correction required Dose CT Dose Conventions CTDI (Computed Tomography Dose Index) CTDI100 CTDIW CTDIVOL DLP (Dose Length Product) Effective Dose CTDI100 Measurement made with 100 mm chamber Includes scatter “tails” Dose Phantom Chamber 100 mm Beam Z CTDIW Weighted average of center & peripheral measurements Represents “average” dose in scan plane CTDIW = 1/3 Center Dose + 2/3 Avg Periphery Dose CTDIC CTDiP 16 cm phantom: Measurements do not vary much 32 cm phantom: Peripheral measurement about 2X center Beam Pitch & Dose! Table motion in one rotation Beam Pitch = --------------------------------------Beam width Beam Pitch > 1 Beam Pitch = 1 Beam Pitch & CT Dose It’s easy. Higher pitch means table moves patient through beam faster table motion during one rotation Beam Pitch = -------------------------------------------Beam width Dose inversely proportional to pitch Smaller pitch Higher dose Higher pitch Lower dose Definition of CTDI vol CTDIVOL corrects dose for beam pitch CTDIvol = CTDIw / Pitch Table motion in one rotation Beam Pitch = --------------------------------------Beam width Dose Length Product DLP DLP = CTDIvol* length of scan DLP units mGy*cm Displayed on report page Effective Dose Represents single dose to entire body that gives same cancer risk Allows comparison of different non-uniform exposures Units: Rem = 1 rad for x-rays Sievert = 1 Gray for x-rays Measure dose to various organs and multiply by these weighting factors Tissue / Organ Gonads Weighting Factor .20 Bone marrow .12 Colon .12 Lung .12 Stomach .12 Bladder .05 Tissue / Organ Liver Weighting Factor .05 Esophagus .05 Thyroid .05 Skin .01 Bone .01 Surface Remainder .05 Effective Dose Example (2 mSv X 0.12) + 1 mSv X 0.05) = 0.29 mSv 1 mSv (thyroid) = 2 mSv (lung) 0.29 mSv (whole body) Effective Doses for Various Studies Eff. Dose PET F-18 FDG Cardiac scan Tc-99m pertechnetate Bone scan Tc-99m MDP Brain scan Tc-99m HMPAO Mammogram Coronary angiogram (high) Coronary angiogram (low) CT body CT head Barium enema (10 images, 137 sec fluoro) Barium swallow (24 images 106 sec fluoro) IVP (6 films) 0 200 400 600 800 1000 1200 1400 1600 1800 mrem What affects CT Dose? Multi-slice’s wide beam diverges more Is less parallel Multi-slice can expose ends which are not imaged Some machines prevent this CT Dose Tradeoff More dose required to improve spatial resolution for same noise smaller pixels maintaining same dose / pixel More dose required to improve noise for same spatial resolution Noise Resolution Dose CT Dose Reduction Reduce mAs Increases image noise Noise inversely proportional to square root of dose Proper technique: maximum tolerable noise as determined by radiologist CT Noise Reduction Increase pixel size BUT Larger pixels obscure visualization of small objects poorer spatial resolution CT Noise Reduction: Increase Slice Width more photons reach detectors BUT More partial volume effect more different tissue types in each voxel CT Noise Reduction Increase overall detector efficiency More photons detected for same technique CT Phototiming Allow operator to specify image quality GE nomenclature: “Noise Index” Modulate mA as tube rotates around patient Patient moves through gantry Goal: keep photon flux to detector constant during study Rotational Beam Modulation Operator specifies maximum mA mA reduced as tube rotates around patient to provide only as radiation as needed Z-axis Beam Modulation Scanner determines changes in attenuation along zaxis from scout study mA changed as patient moves through gantry CT Beam Modulation Note more variation in mA during tube rotation in thorax than in abdomen. CT Dose Considerations http://library.thinkquest.org/ NCRP Reports Ionizing Radiation Exposure of the Population of the United States # 93 Published in 1987 Data from early 1980’s Extensive data on medical exposures # 160 Re-write in 2008 National Council on Radiation Protection and Measurements CT Usage Annual growth U.S. Population: <1% CT Procedures: >10% ~ 67,000,000 procedures in 2006 about 10% pediatric CT Computed Tomography — An Increasing Source of Radiation Exposure David J. Brenner, Ph.D., D.Sc., and Eric J. Hall, D.Phil., D.Sc. New England Journal of Medicine, 2007 U.S. per capita Exposure 1982 Total Exposure (mSv) 3.6 (360 mrem) 0.53 2006 6.0 3.0 ~500% Medical Exposure (mSv) medical exposure increase 24 year exposure increase • Total: ~2.5 mSv • CT: ~ 1.5 mSv CT Usage 16% of imaging procedures 23% of total per capita exposure 49% of medical exposure Multi-slice CT Doses Other considerations Other considerations Tendency to cover more volume (anatomy) Better availability of equipment Other Reasons for High CT Doses Repeat Exams No adjustment of technique factors for different size patients No adjustment for different areas of body CT Usage “On the basis of …data on CT use from 1991 through 1996, it has been estimated that about 0.4% of all cancers in the United States may be attributable to the radiation from CT studies…By adjusting this estimate for current CT use this estimate might now be in the range of 1.5 to 2.0%.” Computed Tomography — An Increasing Source of Radiation Exposure David J. Brenner, Ph.D., D.Sc., and Eric J. Hall, D.Phil., D.Sc. New England Journal of Medicine, 2007 CT Dose Risk Natural incidence of cancer: 1 in 5 Increase in risk from typical CT study << natural incidence This is public health concern because of large large increase in volume for CT. FDA Website: http://www.fda.gov/RadiationEmittingProducts/RadiationEmittingProductsandProcedures/Medi calImaging/MedicalX-Rays/ucm115329.htm Dose reduction mechanisms Customize settings to patient size mA kVp Reduce z-axis coverage Reduce field of view Consider alternate procedures http://www.acr.org/SecondaryMainMenuCategories/quality_safety/app_criteria.aspx Big Brother California SB 1237 would require hospitals & clinics to record CT doses Accreditation Reporting of “overdosing” Concerns Siemens automatically increases mA when pitch is increased GE protocols can be Changed Over-written CTDI (dose index) Radiation measured in phantom Useful in comparing units Of limited use in calculating accurate dose to specific patient Concerns CT dose report often not convenient to view No dose database on CT scanners Review of dose information in DICOM headers is cumbersome manual process