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

Disclosures Institutional research agreement, Siemens AG Recipient research support Siemens AG Consultant, Flaherty Sensabaugh Bonasso PLLC Consultant, Fulbright and Jaworski, LLC

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.

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

AAPM Lexicon from Working Group on nomenclature and CT protocols

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

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

Technical parameters

CT Localizer Radiograph Generic Termzs GE Philips Siemens Toshiba Hitachi Neusoft Neuroligica CT localizer Radiogragh Scout Surview Topogram Scanogram

Ct localizer radiograph Importance of centering

CT localizer radiograph CT survey projection Localizer 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: Planning View CT localizer radiograph CT survey projection Localizer Scan Monitoring Scan 6

Answer: 2, CT localizer radiograph 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 Answer: 2, CT localizer radiograph Ref: AAPM CT Lexicon version 1.3 04/20/2012

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

Tube potential Generic Termzs GE Philips Siemens Toshiba Hitachi Neusoft Neuroligica Tube potential kV kVp

Tech. parameters: kV Contrast in image Tube output (mR/mAs) 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

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

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

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

Tube current, etc. Generic Terms GE Philips Siemens Toshiba Hitachi Neusoft Neuroligica mA mAs mAs (axial) (axial) Eff. mAs = mAs/pitch mAs/slice (helical) Eff. mAs (helical) mAs/slice

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

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. Answer: (3) mAs ÷ pitch; this is also known as mAs/Slice in some systems.   Ref: AAPM CT Lexicon version 1.3 04/20/2012

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)

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 Eff mAs = mAs/pitch System AUTOMATICALLY adjusts mAs with changes in pitch to provide a constant eff mAs If Pitch  then mAs  and no net change in CTDIvol

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

Detector configuration (DET CONF) Generic Terms GE Philips Siemens Toshiba Hitachi Neusoft Neuroligica Detector Config Det Conf Collimation N x T (mm) Conf or Acq Collimation N x T (mm)

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

Tech. parameters: Collimation CTDIw (mGy/100 mAs) 64x.625mm 8.5 32x.625mm 9.0 16x.625mm 10.5 8x.625mm 12.5 4x.625mm 12.4 2x.625mm 15.1

Tech. parameters: Tube current modulation

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

Long Axis Modulation Lung Region Shoulder Region Breast Tissue Abdomen 180 degrees (LAT) 90 degrees (AP) Topogram used to assign mAs variations along long-axis of patient

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

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

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

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)

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

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

Decreasing the Noise Index (NI) on a GE 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 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  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

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