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

2. Disclosures Institutional research agreement, Siemens AG
Recipient research support Siemens AG
Consultant, Flaherty Sensabaugh Bonasso 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

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

10. Ct localizer radiograph
Importance of centering

11. 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

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

15. 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

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. 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

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. 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 /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
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

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)
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)

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

27. 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

28. Tech. parameters: Tube current modulation

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

30. 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

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

33. 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

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

35. 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)

36. 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

37. 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

38. 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 /20/2012
 Kanal et al. AJR 2007 Jul;189(1): and Kanal et al. AJR 2011 Aug;197(2):437-41

39. 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