Key CT Parameters

Report
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
[email protected]
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
•
4000
a.p. (measured)
3000
2500
2000
1500
1000
500
0
600
500
400
300
table position
200
100
0
attenuation I_0 / I
3500
lateral (calculated)
CT LOCALIZER RADIOGRAPH
Generic
Termzs
GE
Philips
Siemens
CT localizer Scout Surview Topogram
Radiogragh
Toshiba
Hitachi
Neusoft
Scanogram Scanogram Surview
Neuroligica
Scout
CT LOCALIZER RADIOGRAPH
• Importance of centering
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.
2.
3.
4.
5.
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:
1.
2.
3.
4.
5.
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
kV
kV
kVp
kV
kV
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
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
mA
mA
Philips
Siemens
mAs
mAs
(axial)
Eff. mAs =
mAs/pitch
mAs/slice Eff. mAs
(helical)
(helical)
mAs
(axial)
Toshiba
Hitachi
mA
mA
Neusoft
Neuroligica
mA
mAs
mAs
mAs
Eff. mAs
(helical)
mAs/slice
mAs
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:
10%
20%
23%
1.
2.
3.
30%
4.
17%
5.
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:
1.
2.
3.
4.
5.
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
Siemen Toshiba
s
Hitachi
Neusoft
Neuroligica
Detector
Config
Det
Conf
Collimation
N x T (mm)
Det
Det
Conf or Conf
Acq
Det
Conf
Collimation
N x T (mm)
Det
Conf
TECH. PARAMETERS: COLLIMATION
• Changing Collimation has some influence on dose
• Wider Collimation settings are usually more efficient
TECH. PARAMETERS: COLLIMATION
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.
4000
a.p. (measured)
3000
2500
2000
1500
1000
500
0
600
500
400
300
table position
200
100
0
attenuation I_0 / I
3500
lateral (calculated)
Long Axis Modulation
Shoulder
Region
600
Breast
Tissue
Lung
Region
Abdomen
Tube Current (mA)
500
400
180 degrees
(LAT)
300
200
90 degrees
(AP)
100
0
0
50
100
150
Table Position (mm)
200
250
300
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
25%
25%
25%
25%
1.
2.
3.
4.
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
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.
2.
3.
4.
5.
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

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