KEY CT PARAMETERS - WHAT ARE THEY

unkindnesskindUrban and Civil

Nov 15, 2013 (3 years and 4 months ago)

77 views

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


KEY CT PARAMETERS
-

WHAT ARE THEY
CALLED AND WHAT DO THEY MEAN?

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





0
500
1000
1500
2000
2500
3000
3500
4000
0
100
200
300
400
500
600
table position
attenuation I_0 / I
a.p. (measured)
lateral (calculated)
CT LOCALIZER RADIOGRAPH

Generic

Termzs

GE

Philips

Siemens

Toshiba

Hitachi

Neusoft

Neuroligica

CT localizer
Radiogragh

Scout

Surview

Topogram

Scanogram

Scanogram

Surview

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.
Planning View

2.
CT localizer radiograph

3.
CT survey projection

4.
Localizer Scan

5.
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.
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 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

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

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:

17%
30%
23%
20%
10%
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.

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 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
s

Toshiba

Hitachi

Neusoft

Neuroligica

Detector

Config

Det

Conf

Collimation
N x T (mm)

Det

Conf or
Acq

Det

Conf

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

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

TECH. PARAMETERS: TUBE CURRENT MODULATION


CARE Dose 4D

Topogram Evaluation: a.p. and lat.

0
500
1000
1500
2000
2500
3000
3500
4000
0
100
200
300
400
500
600
table position
attenuation I_0 / I
a.p. (measured)
lateral (calculated)
0
100
200
300
400
500
600
0
50
100
150
200
250
300
Table Position (mm)
Tube Current (mA)
90 degrees
(AP)

Shoulder
Region

Lung
Region

Abdomen

180 degrees
(LAT
)

Breast
Tissue

Long Axis Modulation

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

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.
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

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 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