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Michael McNitt-Gray, PhD, DABR, FAAPM Professor, Department of Radiology Director, Biomedical Physics Graduate Program David Geffen School of Medicine.

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Presentation on theme: "Michael McNitt-Gray, PhD, DABR, FAAPM Professor, Department of Radiology Director, Biomedical Physics Graduate Program David Geffen School of Medicine."— Presentation transcript:

1 Michael McNitt-Gray, PhD, DABR, FAAPM Professor, Department of Radiology Director, Biomedical Physics Graduate Program David Geffen School of Medicine at UCLA KEY CT PARAMETERS - WHAT ARE THEY CALLED AND WHAT DO THEY MEAN?

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 GEPhilipsSiemensToshibaHitachiNeusoftNeuroligica CT localizer Radiogragh ScoutSurviewTopogramScanogram SurviewScout

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: 1.Planning View 2.CT localizer radiograph 3.CT survey projection 4.Localizer Scan 5.Monitoring Scan

12 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: 1.Planning View 2.CT localizer radiograph 3.CT survey projection 4.Localizer Scan 5.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 GEPhilipsSiemensToshibaHitachiNeusoftNeuroligica Tube potential kVkVpkV kVpkV

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. Generic Terms GEPhilipsSiemensToshibaHitachiNeusoftNeuroligica mA mAs mAs (axial) mAs (axial) mAs Eff. mAs = mAs/pitch mAs/slice (helical) Eff. mAs (helical) Eff. mAs (helical) mAs/slicemAs

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: 1.The number of electrons accelerated across an x-ray tube per unit time, expressed in units of milliampere (mA) 2.The product of tube current and exposure time per rotation, expressed in units of milliampere seconds (mAs). 3.In helical scan mode, the product of tube current and rotation time (expressed in mAs) ÷ pitch 4.In axial mode, this is equal to tube current × (scan angle ÷ 360) × rotation time. 5.In helical mode, this is equal to tube current × rotation time.

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: 1.The number of electrons accelerated across an x-ray tube per unit time, expressed in units of milliampere (mA) 2.The product of tube current and exposure time per rotation, expressed in units of milliampere seconds (mAs). 3.In helical scan mode, the product of tube current and rotation time (expressed in mAs) ÷ pitch 4.In axial mode, this is equal to tube current × (scan angle ÷ 360) × rotation time. 5.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 GEPhilipsSiemen s ToshibaHitachiNeusoftNeuroligica Detector Config Det Conf Collimation N x T (mm) Det Conf or Acq Det Conf Det Conf Collimation N x T (mm) Det Conf

26 TECH. PARAMETERS: COLLIMATION Changing Collimation has some influence on dose Wider Collimation settings are usually more efficient

27 TECH. PARAMETERS: COLLIMATION CollimationCTDI w (mGy/100 mAs) 64x.625mm8.5 32x.625mm9.0 16x.625mm10.5 8x.625mm12.5 4x.625mm12.4 2x.625mm15.1

28 TECH. PARAMETERS: TUBE CURRENT MODULATION

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

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

31

32 CTDI VOL IN CONTEXT OF AEC When Tube current modulation is used: CTDI vol 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 1.Decreasing the Noise Index (NI) on a GE Scanner 2.Decreasing the Quality Reference mAs on a Siemens Scanner 3.Increasing the Standard Deviation on a Toshiba Scanner 4.Increasing the Standard Deviation (% ) a Hitachi Scanner

38 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 1.Decreasing the Noise Index (NI) on a GE Scanner 2.Decreasing the Quality Reference mAs on a Siemens Scanner 3.Increasing the Standard Deviation on a Toshiba Scanner 4.Increasing the Noise Index (NI) on a GE Scanner 5.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


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