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Key CT Parameters - What Are They Called and What Do They Mean?

Understand the important technical parameters in CT scanning that affect radiation dose and image quality, including CT localizer radiograph, kV, tube current modulation, and pitch.

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Key CT Parameters - What Are They Called and What Do They Mean?

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  1. Key CT Parameters - What Are They Called and What Do They Mean? Michael McNitt-Gray, PhD, DABR, FAAPM Professor, Department of Radiology Director, Biomedical Physics Graduate Program David Geffen School of Medicine at UCLA mmcnittgray@mednet.ucla.edu

  2. Disclosures • Institutional research agreement, Siemens AG • Recipient research support Siemens AG • Consultant, Flaherty SensabaughBonasso PLLC • Consultant, Fulbright and Jaworski, LLC

  3. Purpose • Introduce some of the important tech. parameters in CT scanning that affect both radiation dose and image quality • Describe the terms used by the major manufacturers • Discuss similarities and differences between them.

  4. Important Reference • AAPM Website (www.aapm.org) • CT Protocols Link • Lexicon Tab • Excel document • http://www.aapm.org/pubs/CTProtocols/documents/CTTerminologyLexicon.pdf

  5. AAPM Lexicon • from Working Group on nomenclature and CT protocols

  6. Technical parameters • CT localizer radiograph • kV • mA, mAs, effective mAs (aka mAs/slice) • Pitch • Tube Current Modulation (TCM) Systems • One form of Automatic Exposure Control (AEC) Systems

  7. Tech. parameters: CT Localizer Radiograph • The scanned projection radiograph, often acquired by the CT system to allow the user to prescribe the start and end locations of the scan range • Used for Planning CT Scan Start and End Locations • ALSO – All Automatic Exposure Control systems use this to plan adjustments based on patient size/attenuation

  8. Technical parameters

  9. CT Localizer Radiograph

  10. Ct localizer radiograph • Importance of centering

  11. Each manufacturer has a different name for the projectional image that is used for planning a CT exam, including Scout, Surview, Topogram, and Scanogram, but the generic name is actually the: • Planning View • CT localizer radiograph • CT survey projection • Localizer Scan • Monitoring Scan 6

  12. Planning View CT localizer radiograph CT survey projection Localizer Scan Monitoring Scan Each manufacturer has a different name for the projectional image that is used for planning a CT exam, including Scout, Surview, Topogram, and Scanogram, but the generic name is actually the: Answer: 2, CT localizer radiograph Ref: AAPM CT Lexicon version 1.3 04/20/2012

  13. Tech. parameters: kV • Tube potential • The electric potential applied across an x-ray tube to accelerate electrons towards a target material, expressed in units of kilovolts (kV) • Often reduced in peds/smaller patients • kV selection methods part of AEC • NOTE: In CT, all scans are constant kV; There is no kV modulation or varying of kV within the scan

  14. Tube potential

  15. Tech. parameters: kV • Contrast in image • Lower kV can give more contrast, especially with iodinated contrast agents (exploit k-edge) • Tube output (mR/mAs) • Lower kV yields lower tube output –> noise increase • So, reducing kV often involves increasing mAs to offset noise increase

  16. Tech. parameters: kV • Dose • CTDIvol (kV)2.5 • So, reducing kV from 120 to 80 • (80/120) 2.5 = .36 (64% reduction) • IF mAs is held constant

  17. Tech. parameters: kV • Dose • CTDIvol (kV)2.5 • So, reducing kV from 120 to 80 • (80/120) 2.5 = .36 (64% reduction) • IF mAs is held constant

  18. Tech. parameters: Tube current, etc. • Tube current (in mA) • Tube Current time product (in mAs) • Effective Tube Current Time Product • Effective mAs • mAs/Slice • = mAs/pitch

  19. Tube current, etc.

  20. Manufacturers use different terms for the tube current, tube current time product or the effective tube current time product. The definition of the effective tube current time product is: • The number of electrons accelerated across an x-ray tube per unit time, expressed in units of milliampere (mA) • The product of tube current and exposure time per rotation, expressed in units of milliampere • seconds (mAs). • In helical scan mode, the product of tube current and rotation time (expressed in mAs) ÷ pitch • In axial mode, this is equal to tube current × (scan angle ÷ 360) × rotation time. • In helical mode, this is equal to tube current × rotation time. 5

  21. The number of electrons accelerated across an x-ray tube per unit time, expressed in units of milliampere (mA) The product of tube current and exposure time per rotation, expressed in units of milliampere • seconds (mAs). In helical scan mode, the product of tube current and rotation time (expressed in mAs) ÷ pitch In axial mode, this is equal to tube current × (scan angle ÷ 360) × rotation time. In helical mode, this is equal to tube current × rotation time. Manufacturers use different terms for the tube current, tube current time product or the effective tube current time product. The definition of the effective tube current time product is: Answer: (3) mAs ÷ pitch; this is also known as mAs/Slice in some systems.   Ref: AAPM CT Lexicon version 1.3 04/20/2012

  22. Tech. parameters: Pitch • Pitch = Table feed per rotation/nominal collimation • Pitch = I/NT • Influences: • Total scan time (e.g. breathold) • Dose (?) • Effective width of reconstructed image thickness • minor effect in most MDCT)

  23. Tech. parameters: Pitch • ONLY influences dose if everything else is constant • GE, Toshiba – use mA and Pitch independently • If Pitch  , CTDIvol and patient dose • Philips, Siemens – use effective mAs or mAs/slice • EffmAs = mAs/pitch • System AUTOMATICALLY adjusts mAs with changes in pitch to provide a constant effmAs • If Pitch  then mAs  and no net change in CTDIvol

  24. Tech. parameters: Collimation • Detector Configuration • Nominal Collimation - NxT • N = Number of Detector Channels • T = Width of each Detector Channel • Example: 64 x 0.625mm • N= 64, T=0.625mm, NT = 40mm

  25. Detector configuration (DET CONF)

  26. Tech. parameters: Collimation • Changing Collimation has some influence on dose • Wider Collimation settings are usually more efficient

  27. Tech. parameters: Collimation

  28. Tech. parameters: Tube current modulation

  29. CARE Dose 4D Topogram Evaluation: a.p. and lat.

  30. Long Axis Modulation Lung Region Breast Tissue Shoulder Region Abdomen 180 degrees (LAT) 90 degrees (AP)

  31. CTDIvol in Context of AEC • When Tube current modulation is used: • CTDIvol reported is based on the average mA used throughout the scan

  32. Scan where Tube Current Modulation was used Blue Curve Represents actual instantaneous mA Red Curve Represents avgmA for each image Yellow Curve Represents avgmA over entire scan Overall avg is used for CTDIvol reported in Dose Report

  33. Tube Current Modulation • LOTS of Different Names • Siemens: CareDose4D • GE: Smart Scan, Auto mA, Smart mA • Philips: DOM, Z-DOM • Toshiba: SureExposure, SureExposure3D

  34. Tube Current Modulation • Siemens: CareDose4D • User sets a “Quality Reference mAs” • System uses online modulation (180 degree lag) • The mAs (or effective mAs, if helical scan) that would be used on a “standard sized” patient • Quality Reference mAs is NOT the max or min • ACTUAL mAs (eff. mAs) can be larger than this (should be for large patients) • ACTUAL mAs (eff. mAs) can be less than this (should be for smaller patients)

  35. Tube Current Modulation • GE SmartmA • User sets: Max mA, min mA and Noise Index (NI) • NI is approximately the standard deviation in a 20 cm water phantom scanned under these conditions • The higher the NI, the lower the mA • The lower the NI, the higher the mA • Scanner output is influenced by recon. image thickness (Kanal AJR 2007) • Attempts to keep noise constant across patient size/anatomy

  36. While all tube current modulation systems base their calculations from the CT localizer radiograph, the image quality reference parameters vary from system to system. Which of the following will result in an increase in dose for a patient of a given size where the scan is being performed with AEC • Decreasing the Noise Index (NI) on a GE Scanner • Decreasing the Quality Reference mAs on a Siemens Scanner • Increasing the Standard Deviation on a Toshiba Scanner • Increasing the Standard Deviation (% ) a Hitachi Scanner 6

  37. Decreasing the Noise Index (NI) on a GE Scanner Decreasing the Quality Reference mAs on a Siemens Scanner Increasing the Standard Deviation on a Toshiba Scanner Increasing the Noise Index (NI) on a GE Scanner Increasing the Standard Deviation (% ) a Hitachi Scanner While all tube current modulation systems base their calculations from the CT localizer radiograph, the image quality reference parameters vary from system to system. Which of the following will result in an increase in dose for a patient of a given size where the scan is being performed with AEC  Answer: 1, Decreasing the Noise Index on a GE Scanner  Ref: AAPM CT Lexicon version 1.3 04/20/2012 Kanal et al. AJR 2007 Jul;189(1):219-25 and Kanal et al. AJR 2011 Aug;197(2):437-41

  38. Summary • Introduce some of the important tech. parameters that affect both radiation dose and image quality • CT localizer radiograph, kV, mA/mAs/effective mAs, pitch and TCM • Describe the terms used by the major manufacturers • Discuss similarities and differences between them. • Important Resources – AAPM CT Protocols Lexicon

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