1IntroductionCT dosesCTDIEReducing CT dosesCT skin dose40 mGy4 radxray skin dose2 mGy02 radCataractsDeterministic effectCarcinogenesisStochastic effect2Dose distributions in CT measured in phantom ID: 866050
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1 1 Adult & pediatric Walter Huda PhDSUNY
1 Adult & pediatric Walter Huda PhDSUNY Upstate Medical University, Syracuse NY IntroductionCT doses(CTDI/E)Reducing CT doses CT skin dose40 mGy/4 radx-ray skin dose2 mGy/0.2 rad CataractsDeterministic effect CarcinogenesisStochastic effect 2 Dose distributions in CT measured in phantoms Computed Tomography Dose Index (CTD
2 I)CTDI D(z) dz Contiguous CT imaging 16
I)CTDI D(z) dz Contiguous CT imaging 16 cm and Acrylic phantoms= 1.19Z = 6.56 3 Helical scanning & dose Pitch of 1.0 has dose axial scanPitch of 0.75 has 33% more dosePitch of 1.5 has 66% less dose 1 CTDI is independentof section thickness T& number of sections NPatient risk depend on sectionthickness T & number of section
3 s N Dose (risk) in CTis best measured by
s N Dose (risk) in CTis best measured by effective dose (E) E = iwi Computing adult and pediatricdoses in head CT examinations Measurementsmade at level of the Dimensions& average HU127 patients undergoing head CT examinations Patient age (Yr) 020406080100 Mean HU 0100150200250300 Patient age (Yr) 05101520 Radius (mm)
4 506090100 Patient age (Yr) 20406080100 R
506090100 Patient age (Yr) 20406080100 Radius (mm) 60708090100 Heads modeled as water equivalentcylinders with a radius rInfants ~60 mmAdults ~90 mm 2 CTDI is equal to: Energy imparted/Section mass CTDI for any sized radius(Energy imparted/Section mass)Monte Carlo modeling203:417-422 kVp 708090100110120130140150 CTDI (mG
5 y) 020305060 340 mAs0 - 6 monthsAdults E
y) 020305060 340 mAs0 - 6 monthsAdults Energy imparted CTDIDirectly irradiated mass(i.e., kVp 708090100110120130140150 Energy imparted (mJ) 06080100140160 340 mAs0 - 6 monthsAdults For an anthropomorphic phantom& head CT scan, compute :Effective dose EEnergy imparted 3 Computeratio for different sized patients (i.e., n
6 ewborn to adults) Effective dose per uni
ewborn to adults) Effective dose per unit energy imparted for head x-ray examinations vs size Patient age (year) 110 Effective dose per unit energy imparted (E/ mSv J 708090100110120130140150 Effective dose (mSv) 01345 340 mAs0 - 6 monthsAdults Weight (kg) 405060708090 Effective dose (mSv) 23569 Chest CTr ² = 0.465 Patient
7 dose:is proportional to mAsincreases by
dose:is proportional to mAsincreases by factor of 5 when going from 80 to 140 kV(@ constant mAs) Doses from CT are a major concern 4 Frequency of CT examinationsGermany 1994 CT contribution to collective medical doseGermany 1994 1 Dose reduction strategiesTechnology (AEC)Dose reduction (ALARA)Optimization Tube current mod
8 ulation for non-circularcross-sections o
ulation for non-circularcross-sections of the body GE Smart mA (LightSpeed Ultra) High dose scanLow dose scan 120 kVp206 mAs 120 kVpA Metastasis in vertebral bodyB Subcutaneous gluteal metastatisC Small gas inclusionD ureter opacified c contrast 2 Optimizing CT with respect to dose Increasing the x-ray photon energy (kVp)
9 reduces image contrast Photon Energy (ke
reduces image contrast Photon Energy (keV) 50556065707580 Relative HU 0.00.51.0 MuscleIodine The mAs used to make a CT image important Contrast to noise ratio (CNR) defines CT image quality. Low High 3 image (120 kVp) mAs 406080200400100 Noise (HU) 100 16 cm32 cm CT CNR can be changed via mAs;CT is quantumnoise limited
10 X-ray tube voltage (kVp) 80100120140 Rel
X-ray tube voltage (kVp) 80100120140 Relative energy imparted 1234 IodineSoft tissueConstant CNR at each kVp Stochastic risks are important in CTDeterministic riskshould not occur Effective doses in CT1 2 mSv for head5 10 mSv for body CT doses dominatemedical exposuresALARA principleshould be used to minimize patient