Radiation Dose Optimization Techniques in MDCT Era: From Basics To Practice Chang Hyun Lee, MD JM Goo, MD, HJ Lee, MD EJ Chun, MD, CM Park, MD JG Im, MD Seoul National University Hospital Purpose 1. To discuss the importance of radiation dose modulation in MDCT 2. To review the characteristics of radiation dose modulation techniques in different MDCT scanners 3. To explain how to use and apply this techniques to the patients during MDCT scan Contents 1. Basic concepts in CT dose index - Fundamental dose parameters - CT dose index - Parameters affecting CTDI 2. Radiation dose increase in MDCT scanner - Radiation dose in MDCT - Radiation risk 3. Effective dose in various CT examination - Effective dose - Calculation and estimates of effective dose - Radiation dose summary 4. Radiation dose modulation techniques in different MDCT scanners – Reference mAs – Reference noise index – it’s advantages and disadvantages 5. Practical tips for optimizing radiation dose in MDCT Fundamental Dose Parameters BACK TO CONTENTS Fundamental Dose Parameters 10 mSv = 1 rem Fundamental Dose Parameters • Exposure – Roentgens (C/kg) or air kerma (J/kg – mGy) – Ionization in air per unit mass or amount of energy imparted per unit mass – Related to intensity of radiation at point of measurement – Irradiated area, penetrating power, tissue sensitivity에 대 한 고려가 없음. Fundamental Dose Parameters • Absorbed dose – Joules/kg or mGray (mGy) – Energy absorbed by material per unit mass – Depends on radiation type, energy & material irradiated – Quoted locally or averaged over area e.g. organ • Absorbed dose – amount of radiation energy deposited in the patient‟s body as a result of exposure • Radiation exposure: radiation source-related term • Radiation dose: body-related term D D ~ 3D Fundamental Dose Parameters • Effective dose – mSv – Measure of radiation risk to patient – Attempts to reflect equivalent whole body dose that results in same stochastic risk – Applies organ sensitivity factors – Enables risk comparison between different procedures and modalities Fundamental Dose Parameters NOTES • Absorbed dose, however, does not account for differing sensitivities of the organs to radiation damage. • Equivalent dose in a tissue is a product of the tissue type and the radiation weighting factor • Equivalent dose has the same numerical value as absorbed dose and is measured in sievert or rem. • Effective dose is computed by summing the absorbed doses in the organs weighted by their radiation sensitivity. • Effective dose estimates the whole-body dose that would be required to produce the same risk as partial-body dose delivered by a localized radiological procedure - useful in evaluating potential biological risk of a specific radiologic examination. CT Dose Index BACK TO CONTENTS CT dose index • CTDI was developed originally by Shope et al. back in 1981. • The basic radiation dose parameter in CT is the CT dose index (CTDI). • Represents absorbed radiation dose in a CT dose phantom, measured in the gray or rad. CT dose index: CTDI • CTDI has been defined for use with single-detector row CT scanners. • CTDI is the total energy absorbed within a dose profile deposited within one nominal collimation. CT dose index • Three derivatives of the CTDI • CTDI100: radiation exposure measured by means of an ionization chamber with a length of 100 mm. • CTDI100w: weighted radiation dose in axial (nonhelical) CT scan • CTDIvol: volume CTDI = CTDIw/pitch CT dose index: CTDI (Multiple-scan average dose) Measurement equipment: CTDI • Ionization chamber • Thermoluminescent Dosimeters (TLD) • X-ray film Ionization chamber: CTDI Ionization chamber: CTDI CTDI100 Average dose in scan plane: CTDIw • Weighted average CTDI represents the average dose in scan plane of Perspex phantom CTDIw = [2/3 CTDI100 (periphery) + 1/3 CTDI100 (center)] x 33.7 Average dose in scanned volume: CTDIvol • Axial: CTDIvol=CTDIw x (slice width/couch inc.) • Helical: CTDIvol = CTDIw / Pitch Noncontiguous exposure along z-axis CTDI • Advantages of CTDI as a dose descriptor • Disadvantages of CTDI Parameters affecting CTDI BACK TO CONTENTS Effect of scan parameters on CTDIvol • mA and scan time (mAs per rotation) • CTDIvol increase linearly with mA and scan time • E.g 2 x mAs = 2 x CTDIvol Variation of CTDIvol with kVp • CTDIvol increases with kVp • Approx ∝kVp2 ImPact 2005 CTDI and slice width • CTDI increases if irradiated width does not match nominal width CTDI = Area / nT Z-axis T1 T2 T3 Variation of CTDIvol with no. of slices • Number of slices • CTDIvol is independent of number of slices – Absorbed dose: energy absorbed per unit mass Effect of pitch on CTDIvol • CTDIvol is inversely proportional to pitch • E.g. doubling pitch halves the CTDIvol • … but only if mA remains constant • On some systems mA automatically adjusted for pitch so CTDIvol is constant Effect of patient size • For same scan parameters (mAs, kV) the dose increases as phantom/patient size decreases • For pediatrics CTDI can underestimate dose by ~ x 2 if measured in standard sized phantoms Radiation dose in MDCT BACK TO CONTENTS Radiation dose in MDCT • ~ 60 million CTs per year – doubled in 5 years • > 60,000 CT examinations at University of Alabama at Birmingham, USA • ~ 67% total radiation exposure is from CT Radiation Dose in MDCT Y Imanishi et al. Eur Radiol (2005) 15:41-46 CT examination is increasing in Japan 100,000 年スキャン数(x10,000) No. of image (x10,000) 80,000 年検査件数(x1,000) No. of exam/year (x1,000) CT装置数 No. of CT 60,000 40,000 20,000 0 1979 1989 2000 Nishizawa, Acta Radiol Jap 2004; 64:151-158 Scan area is also increasing Nishizawa, Acta Radiol Jap 2004; 64:151-158 Does multi-slice CT impart more or less radiation dose? • An increase by 10-30% may occur with multi-slice detector array ICRP (International Commission on Radiological Protection) Publication 87 Doses from Chest CT Helical CT MDCT (4) LDCT (single) kV 120 120 120 mA100-210 300-350 25 Rotation time (s/r) 1-1.5 0.5 2 Table feed (mm/s) 10 7-11 20/2s/r FOV (cm) 27 35 30 Organ dose (mGy) Bone marrow 5.91 7.19 2.51 Lung 20.94 19.59 3.09 Stomach 8.59 19.83 1.41 Breast 18.24 20.20 2.41 Liver 0.43 19.62 1.64 Esophagus 18.12 18.16 2.90 Thyroid 8.23 23.70 2.41 Effective dose (mSv) 7.62 11.0 1.40 Nishizawa, Acta Radiol Jap 2004; 64:suppl Radiation Risk BACK TO CONTENTS Audience Response Question • What is the conventional estimate of the long term risk of death from cancer for 10 mSv whole body exposure for an average person? A. Negligible B. 1:20,000 C. 1:2,000 D. 1:200 Audience Response Question • What is the conventional estimate of the long term risk of death from cancer for 10 mSv whole body exposure for an average person? A. Negligible B. 1:20,000 C. 1:2,000 D. 1:200 Audience Response Question • The radiation dose from a chest radiograph is approximately what fraction of the dose from yearly natural background radiation? A. 1:100 B. 1:10 C. 1:1 D. 10:1 Audience Response Question • The radiation dose from a chest radiograph is approximately what fraction of the dose from yearly natural background radiation? A. 1:100 B. 1:10 C. 1:1 D. 10:1 Audience Response Question • The radiation dose for One CT scan versus One chest radiograph? A. CT < CR B. CT > CR, CT < 10 CR C. CT > CR, 10 CR < CT < 100 CR D. CT > CR, CT = 100 ~ 250 CR E. CT > CR, CT > 500 CR Audience Response Question • The radiation dose for One CT scan versus One chest radiograph? A. CT < CR B. CT > CR, CT < 10 CR C. CT > CR, 10 CR < CT < 100 CR D. CT > CR, CT = 100 ~ 250 CR E. CT > CR, CT > 500 CR Relative Dose Beliefs Diagnostic CT Scans: Assessment of Patient, Physician, and Radiologist Awareness of Radiation Dose and Possible Risks. Lee CL et al. Radiology 2004 Cancer Risk Beliefs Diagnostic CT Scans: Assessment of Patient, Physician, and Radiologist Awareness of Radiation Dose and Possible Risks. Lee CL et al. Radiology 2004 Radiation Cancer Risk Conventional Theory • 5% excess cancer deaths per Sv (100 rem) • 1:2,000 (0.05%) excess cancer deaths per 10 mSv (1 rem) Brenner et al. Estimated Radiation Risks Potentially Associated with Full-Body CT Screening. Radiology 2004 Age-Dependent Cancer Risk Brenner et al. Estimated Radiation Risks Potentially Associated with Full-Body CT Screening. Radiology 2004 Radiation Risk in Context • Baseline risk of cancer in life time 20- 25% • Younger patients at higher risk • Late middle aged adult getting average CT has life time risk of cancer increased from ~20% to ~20.01% – Not measurable Radiation – Common Doses and Risks • Chest radiograph (adult): 0.02 mSv 0.0001 – 0.000002% deaths • UGI/BE: 1-7 mSv 0.005-0.035% deaths • Mammography 2.6 mGy, but – Effective Dose 0.13 mSv (so cancer risk much lower) • Natural background: 3 mSv/yr • Medical population average 0.3-0.6 mSv/yr Comparable Non-Radiation Risks • Assume 10 mSv (1 rem) CT scan – Smoking 140 cigarettes in a lifetime (lung cancer) – Spending 7 months in New York City (air pollution – lung cancer) – Driving 4,000 miles in a car (accident) – Flying 250,000 miles in a jet (accident) Audience Response Question • A single CT scan has a lifetime risk of death from cancer similar to: A. Smoking 7 cigarettes B. Driving 40,000 miles in a car C. Having a standard IVU D. Flying into low earth orbit Audience Response Question • A single CT scan has a lifetime risk of death from cancer similar to: A. Smoking 7 cigarettes B. Driving 40,000 miles in a car C. Having a standard IVU D. Flying into low earth orbit Effective dose BACK TO CONTENTS Effective dose • Effective dose – Estimate of stochastic radiation risk • Dose Length Product (DLP) – Related to stochastic radiation risk Random Carcinogenesis Genetic mutation Calculation of Effective dose • Direct approach impractical • Usual approach – Mathematical anthropomorphic phantom – Computer simulated irradiation using Monte Carlo techniques • Statistical calculations of photon interactions Estimates of Effective Dose • Effective dose = DLP x Conversion factor (mSv) CTDI radiation risk Diagram shows algorithm for the estimation of radiation exposure risk from CT European DRLs for CT Radiation dose - Summary KT Bae. JMRI 2004 Radiation dose - Summary • CTDI100 is the fundamental CT dose parameter • CTDIw characterises a CT scanner in terms of dose • CTDIvol is used to represent average absorbed dose to irradiated area • DLP for a given examination type is roughly proportional to stochastic radiation risk • Effective dose is used for more accurate risk estimates How ? ALARA (as low as possible reasonably achievable) Radiation Dose Modulation Techniques BACK TO CONTENTS Automatic Exposure Control (AEC) • Automatic exposure control systems are now available in all MDCT scanners • Each system operates on different basis, using a range of control methods • Tube current (mA) adjusted relative to patient attenuation Automatic Exposure Control (AEC) • Patients size AEC • Z-axis AEC • Rotational AEC • Combination Automatic Exposure Control (AEC) • Patient size AEC: adjust the tube current based upon the overall size of the patient. The same mA is used for an entire examination or scan. • Z-axis AEC: tube current is adjusted for each rotation of the x-ray tube. The mA would be low through the thorax and higher through the abdomen. • Rotational AEC: The tube current is decreased and increased rapidly (modulated) during the course of each rotation. The amplitude of mA modulation during rotational AEC reflect the patient asymmetry. Automatic Exposure Control (AEC) • Three levels of automatic exposure control. A) patient size AEC: higher mA is used for larger patient, b) z-axis AEC: higher mA used at more attenuating z-axis positions, c) rotational AEC: the degree of modulation depends on asymmetry at each z-axis position, d) combined effects of using all three levels of AEC. Benefits of AEC • Consistent image quality – Depending on its capabilities, there is less image noise variation between patients and within a single scan series. With rotational AEC, there is also a slight reduction in the variation of noise across the field of view. • Reduction in photon starvation artifact - Tube current is varied during the course of rotation, it can be increased for the most attenuating scan angles (e.g. laterally through the shoulders) Benefits of AEC • Potential for dose reduction through exposure optimization – AEC systems by themselves will not automatically lead to a reduction in patient dose. However, when used correctly, their introduction should generally tend to result in reduced dose. Benefits of AEC • Reduced tube loading (extended scan runs) – MDCT generally have fewer problems with tube cooling, but in general, if the radiation exposure to the patient is reduced by lowering tube currents, the heating of the x-ray tube is also reduced. Methods for AEC • Standard deviation (SD) based AEC - by specifying image quality in terms of SD of pixel values. - High SD values noisy image - Low SD values low noise image - set the tube current to achieve the requested SD on an image by image basis Methods for AEC • Standard deviation (SD) based AEC – Advantage: image quality resulting from protocols form different scanners can be compared more easily. – Disadvantage: easily to enter an SD which is lower than would be needed, resulting in higher patient doses than the ones achieved without AEC – Image noise is inversely proportional to the square of the tube current, so halving the SD results in an increase in the mA, and therefore the patient dose, by a factor of 4. Methods for AEC • Reference mAs AEC control – Uses the familiar concept of setting an mA (or mAs) related value – Assesses the size of the patient cross- section beig scanned, and adjusts the tube current relative to the reference value. Methods for AEC • Reference mAs AEC control – Advantages: permits more flexible adjustment of tube current according to patient size than with SD AEC control. With SD based systems, the AEC response to different patient sizes is pre-defined. – Can vary their response depending on the image quality requirements. – Users are familiar with typical mAs values for their scanners Methods for AEC • Reference Image AEC control – Uses reference image that has previously been scanned and judged to be of appropriate quality, and then attempts to adjusts the tube current to match the noise in the reference image. – Advantage: the required image quality is expressed using an existing clinical image rather than an abstract value of SD – Disadvantage: temptation to pick a „pretty‟ image. This lead to use higher doses than are necessary. It is also difficult to compare scan protocols, as there is no value associated with the image quality in the reference image. AEC systems on MDCT scanners Impact 2005 AEC systems on MDCT scanners • GE • AutomA: set a desired image quality by entering a “Noise Index” (NI). • Aims to achieve the same level of noise in each image • SmartmA: varies the tube current sinusoidally during the course of rotation AEC systems on MDCT scanners • Philips • DoseRight ACS (automatic current selector) provides patients based AEC, by use of a reference image • DoseRight DOM (Dose Modulation) – Z-Dom: Z-axis AEC – D-Dom: Tube current is set so that 90% of images will have equal or lower noise than the reference image, with remaining 10% of images in a series having equal or higher noise than the reference image. • Use feedback from the previous rotation to asses the amplitude of mA modulation used for rotational AEC AEC systems on MDCT scanners • Siemens • CARE Dose 4D use “image quality reference mAs”. • Adjust the tube current, setting a value that is higher or lower than the reference mAs depending upon the patient attenuation, relative to Siemens‟ reference patient size. • The degree to which the tube current is adjusted for patient size can be selected, using „weak‟, „average‟, or „strong (high degree of mA adjustment)‟, compensation settings. AEC systems on MDCT scanners • Toshiba • SureExposure in helical scanning only • Operated by selecting a target image SD form a drop down list. • AEC setup allows tube current to be limited by max. and min. values. • Variable length reference position selector to highlight a particular region. The AEC uses the mean attenuation within this scan region to calculate the tube current through this region. AEC systems on MDCT scanners Automatic Exposure Control Report 06013, 32 to 64 slice CT scanner comparison report version 1. Feb 2006, www.pasa.nhs.uk/cep. Clinical Use of AEC system • Clinical use of AEC system requires careful consideration • Generally lead to reduced patient doses • However, it is possible to increase doses operating an AEC system • Dose reduction generally accompanied by reduction of image quality • Goal is to achieve diagnostic image quality without an impact upon clinical usefulness Protocol optimization • The key is to use an appropriate image noise level, reference mAs or reference image in the AEC setup. • This process is not a straightforward • One approach is to focus image quality assessment upon the European Guidelines on Quality Criteria for Computed Tomography The Effect of Protocol Modification • There are significant differences from one system to another • Changing kVp will not affect the tube current on Siemens CARE Dose, but it will do on other AECs. • Changing the reconstruction kernel will alter the tube current Toshiba SureExposure, but not by others • It is important that users are aware of the behavior of their system, and the effect that varying scan and reconstruction parameters has upon the AEC. Monitoring patient dose • Use CTDIvol and DLP for monitoring • Before and after the introduction AEC, radiation dose can be assessed. • AEC exposure levels can be modified, although the effect of changing any other parameters such as beam collimation or kV should also be accounted for. AEC Future • Introduction of the AEC into clinical practice should be approached carefully • There is a need for education of users by the manufacturer • Scanner need to have default AEC protocols that draw a sensible compromise between the demand of image quality and radiation dose. • Common method for operation of the system would be of great help and it leads to the protocol uniformity and radiation dose optimization. Practical Tips BACK TO CONTENTS Clinical Issues in Radiation with MDCT • Pediatric exposure • Breast radiation for chest MDCT • Cumulative dose from follow-ups in some applications, e.g. renal stones • Scanning in pregnancy • Screening • Automatic Exposure Control • Low kVp to reduce dose and contrast volume Pediatric Exposure • Effective Doses in children may be 50% higher with common techniques • Strategy for dose reduction; – Be selective – Alternative diagnostic strategies – Reduce technique Breast Radiation with Chest MDCT • Breast radiation for chest CT • Especially scans for pulmonary embolism in low-risk young women • Dose to breasts may be up to 19 times that from a mammogram • Strategy for dose reduction: be selective Cumulative Dose • Rescanning (e.g. renal stones) • Often in young patients without serious conditions • Indication “creep” (lower threshold) • Up to 18 follow-ups recorded in one case! • Strategy: low dose, educate clinicians, be alert and recommend alternatives (e.g., radiography or ultrasound) Scanning in Pregnancy • 2nd-15th week gestation exposure – No concern < 50 mGy – Moderate concern 50-150 mGy – High concern > 150 mGy • Risk overestimated by physicians – 5% of obstetricians, 6% of family physicians recommend abortion after CT Scanning in Pregnancy • Principles of pregnancy policy Identify probability of pregnancy Minimize dose, consider alternative Document dose Low exposure – perform exam without delay High exposure – informed decision, awareness of risk Questionnaire, consent form Screening MDCT • Whole-body screening may increase cancer risk about 1% • Limit screening CT to proven applications • Use low-dose technique for chest, colon, coronary calcification scans Automatic Exposure Control • System adapts to changes in patient thickness: x-y and z-axis • Can reduce dose by 20-40% • Implementation differs for each vendor • Not yet perfected – may cause unsatisfactory increase in noise in some areas and patients • Should use with caution in patients with prosthesis Low kVp • Low kVp increases image contrast for IV contrast • Allows lower IV contrast doses • Low kVp increases noise at constant mAs • Probably satisfactory for vascular studies and small patients (increase mAs) • Not yet adequately clinically validated for many applications Strategies For Dose Reduction • Be selective, consider risk (young, sensitive) • Minimize technique ( mAs, pitch with 4 or fewer slice scanner) • Use AEC, low kVp when possible • Limit follow-up scans • Alternative diagnostic strategies, esp. Children and pregnancy • Be able to counsel for risk Actions for physician & radiologist… • Justification: Ensure that patients are not irradi ated unjustifiably. • Consider whether the required information be o btained by MRI, ultrasonography • Consider value of contrast enhancement or omitting pre-contrast scan • CT scanning in pregnancy may not be contrain dicated, particularly in emergency situations, al though examinations of the abdomen or pelvis should be carefully justified ICRP Publication 87 Actions for physician & radiologist (cont’d) • CT examination should not be repeated without clinical justification and should be limited to the area of interest • Clinician has the responsibility to communicate to the radiologist about previous CT examination of the patient • CT examination for research purpose that do not have clinical justification (immediate benefit to the person) should be subject to critical eval uation by an ethics committee ICRP Publication 87 Actions for physician & radiologist (cont’d) • CT examination of chest in young girls and young females needs to be justified in view of high breast dose • Once the examination has been justified, radiologist has the primary responsibility for ensuring that the examination is carried out with good technique References • ImPACT, www.impactscan.org • National conference on dose reduction in CT, with an emphasis on pediatric patients. AJR Am J Roentgenol. 2003 Aug;181(2):321-9. Linton OW, Mettler FA Jr; National Council on Radiation Protection and Measurements. • A new pregnancy policy for a new era. AJR Am J Roentgenol. 2003 Aug;181(2):335-40. El-Khoury GY, Madsen MT, Blake ME, Yankowitz J. • Physicians' perceptions of teratogenic risk associated with radiography and CT during early pregnancy. AJR Am J Roentgenol. 2004 May;182(5):1107-9. Pole M, Einarson A, Pairaudeau N, Einarson T, Koren G. • Dose reduction in pediatric CT: a rational approach. Boone JM, Geraghty EM, Seibert JA, Wootton-Gorges SL. • Estimated radiation risks potentially associated with full-body CT screening. Radiology. 2004 Sep;232(3):735-8. Brenner DJ, Elliston CD. • ICRP, Publication 87, 2000 • JRS, Guidelines for pediatric CT, 2005 • Nishizawa K, Acta Radiologica Jap, 2004National Research Counsil, 20 05 • Risk versus Benefit • For patients • Responsibility Thank you !