Professor Department of Radiology Director Biomedical Physics Graduate Program David Geffen School of Medicine at UCLA mmcnittgraymednetuclaedu Key CT Parameters What Are They Called and What Do They Mean ID: 760644
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Slide1
Michael McNitt-Gray, PhD, DABR, FAAPMProfessor, Department of RadiologyDirector, Biomedical Physics Graduate ProgramDavid Geffen School of Medicine at UCLAmmcnittgray@mednet.ucla.edu
Key CT Parameters - What Are They Called and What Do They Mean?
Slide2Disclosures
Institutional research agreement, Siemens AG
Recipient research support Siemens AG
Consultant, Flaherty
Sensabaugh
Bonasso
PLLC
Consultant, Fulbright and
Jaworski
, LLC
Slide3Purpose
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.
Slide4Important Reference
AAPM Website (www.aapm.org)CT Protocols LinkLexicon TabExcel documenthttp://www.aapm.org/pubs/CTProtocols/documents/CTTerminologyLexicon.pdf
Slide5AAPM Lexicon
from Working Group on nomenclature and CT protocols
Slide6Technical 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
Slide7Tech. 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
Slide8Technical parameters
Slide9CT Localizer Radiograph
Generic
Termzs
GE
Philips
Siemens
Toshiba
Hitachi
Neusoft
Neuroligica
CT localizer
Radiogragh
Scout
Surview
Topogram
Scanogram
Scanogram
Surview
Scout
Slide10Ct localizer radiograph
Importance of centering
Slide11Each 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 projectionLocalizer Scan Monitoring Scan
6
Slide12Each 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 projectionLocalizer Scan Monitoring Scan
Answer: 2, CT localizer radiograph
Ref: AAPM CT Lexicon version 1.3 04/20/2012
Slide13Tech. 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
Slide14Tube potential
Generic
Termzs
GE
Philips
Siemens
Toshiba
Hitachi
Neusoft
Neuroligica
Tube potential
kV
kVp
kV
kV
kVp
kV
kV
Slide15Tech. 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
Slide16Tech. 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
Slide17Tech. 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
Slide18Tech. 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
Slide19Tube current, etc.
Generic
Terms
GE
Philips
Siemens
Toshiba
Hitachi
Neusoft
Neuroligica
mA
mA
mA
mA
mA
mAs
mAs
(axial)
mAs
(axial)
mAs
mAs
mAs
Eff.
mAs
=
mAs
/pitch
mAs
/slice (helical)
Eff.
mAs
(helical)
Eff.
mAs
(helical)
mAs
/slice
mAs
Slide20Manufacturers 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) ÷ pitchIn 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
Slide21Manufacturers 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) ÷ pitchIn 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
Slide22Tech. 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)
Slide23Tech. 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
Slide24Tech. 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
Slide25Detector configuration (DET CONF)
Generic TermsGEPhilipsSiemensToshibaHitachiNeusoftNeuroligicaDetectorConfigDetConfCollimation N x T (mm) DetConf or AcqDetConfDetConfCollimation N x T (mm) DetConf
Slide26Tech. parameters: Collimation
Changing Collimation has some influence on dose
Wider Collimation settings are usually more efficient
Slide27Tech. parameters: Collimation
Collimation
CTDI
w
(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
Slide28Tech. parameters: Tube current modulation
Slide29CARE Dose 4D
Topogram Evaluation: a.p. and lat.
Slide3090 degrees (AP)
Shoulder Region
Lung Region
Abdomen
180 degrees (LAT
)
Breast Tissue
Long Axis Modulation
Slide31Slide32CTDIvol in Context of AEC
When Tube current modulation is used:
CTDI
vol
reported is based on the
average
mA
used throughout the scan
Slide33Scan 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
Slide34Tube Current Modulation
LOTS of Different Names
Siemens: CareDose4D
GE: Smart Scan, Auto
mA
, Smart
mA
Philips: DOM, Z-DOM
Toshiba:
SureExposure
, SureExposure3D
Slide35Tube 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
)
Slide36Tube 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
Slide37While 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 ScannerIncreasing the Standard Deviation on a Toshiba ScannerIncreasing the Standard Deviation (% ) a Hitachi Scanner
6
Slide38While 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 ScannerIncreasing the Standard Deviation on a Toshiba ScannerIncreasing 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
Slide39Summary
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