1. Master in Medical Physics 2015 to 2016
Quality control in CT "Image quality and
dose measurement"
Francisco J.Hernández Flores∗
International Centre for Theoretical Physics
fhernand@ictp.it
August 27, 2015
Abstract
In this task report two practices about quality control in computed tomography, during the practices
in the first session we made measure about image quality (CT Number Linearity, Spatial Resolution,
Low contrast Resolution, Spatial Uniformity, Pixel dimension) and the second practical we make dose
measurements in CTDI and kerma in air. is very important consider routine of quality control can help
ensure equipment operates appropriately so that dose is optimized for the necessary image quality. Is
very useful the quality control of image quality must be related to dose, because is always possible to
increase the image quality increasing the amount of radiation in the patient. The result of the CTDI for
body standard 5.16% and head standard 3.26% of the diference we when compared the CTDI delivered
for the CT these values are lees than limit value is 20%, the result obtained for DLP Was 35.4 mGycm
and air kerma was 54.2 mGy using field of view for head and 177.1 mGy using field of view for body.
I. Introduction
Image quality can be expressed in terms of physical parameters such as CT Number Linear-
ity, Spatial Resolution, Low contrast Resolution, Spatial Uniformity, Noise, Pixel dimension,
depends on the technological characteristics of the scanner, the exposure factors used and the
conditions of image viewing The Computed tomography (CT) scanners is very useful because
create cross-sectional images of high radiographic contrast. This is particularly important for
diagnosis and radiotherapy treatment planning thus involving soft tissue (that is, organs not
including lung or bone) as the contrast available from CT images is vastly superior to that
gained from projection radiography.
In these practices we used three phantom one of them is called catphan 600 this was used
for the quality control image measurement (CT Number Linearity, Spatial Resolution, Low
contrast Resolution, Spatial Uniformity, Pixel dimension); the other two phantom are PMMA
material one of them is used for calculate CTDI in body standard and the other is used for
determine CTDI in head standard.
During the practice we evaluate CT dose index (CTDI) is a standardized measure of radiation
dose output of a CT scanner which allows the user to compare radiation output of different CT
scanners. In the past CTDI100 (measured over a 100 mm long ionization chamber) and CTDIw
(weighted average of dose across a single slice) were used; for helical scanners in current use,
for obtain this parameter in both phantom standard body and standard head, we made in
different hole of the fantom in the center and the periphery.
∗Exercise of Radiology 1
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2. Master in Medical Physics 2015 to 2016
II. Theory
I. Spatial resolution
Spatial resolution refers to the ability to see small details [2]. For determining the spatial
resolution there are two different possibilities, which are also described in the standards dealing
with CT [2]. The first one is calculating the Modulation Transfer Function (MTF) from the
measured edge of a cylinder, the second one is measuring the contrast of material to background
in dependence on the size of the structures of line-pairs.
II. Noise
Noise of a CT image is due to the variation of CT numbers in the image of a homogenous
object. It is defined as standard deviation of CT numbers in a ROI selected in the central area
the image of a homogenous object [1].
III. CT number uniformity
Mean value of CT numbers in the image of a homogenous object. Maximum variation of
mean value of CT number among a central ROI and 4 ROI at the peripheral of the image of a
homogenous object. [?]
IV. Computed tomography dose index CTDI
The computed tomography dose index, CTDI100,c, measured in center of phantom for a single
rotation of a CT scanner is the quotient of the integral of the dose along a line parallel to the
axis of rotation of the scanner over a length of 100 mm and the nominal slice thickness, T . The
integration range is positioned symmetrically about the volume scanned, thus: [4]
CTDI100,c =
1
T
+50
−50
D(z)dz (1)
The integral of the single scan radiation dose profile along the z axis, normalized to the thickness
of the imaged section (¸Sslice thicknessˇT).
Weighted CTDI(CTDIW) is approximation to average dose in X-Y plane for perspex phantom.
CTDIw =
1
3
CTDI100,c +
2
3
CTDI100,p (2)
CTDI100,c Pencil chamber inserted in the central hole; CTDI100,p Pencil chamber 10 mm under
the surface (peripheral holes )
Global dose can be considered using Dose length product (DLP)
DPL = CTDIw × scanlength (3)
scan length is equal at product of slice thickness × number of slice. [3]
III. Material and Method
The equipment used during these practices we used the CT, the catphan phantom, one phantom
standard head of PMMA material and phantom standard body of PMMA material, ionization
chamber of 10 cm and electrometer. During the first practice was image quality measurement
the first procedure was make the warmup of the CT after aline the phantom with the laser, set
zero , after scan the image in different part of the catphan phantom depending on the type of
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3. Master in Medical Physics 2015 to 2016
procedure to be performed.
In the second practice we realized the warmup of the CT in this case we used two different
size of phantom small for head and large for body from the same material PMMA, these was
alined and these have a insert of chamber in different position, in the center and four in the
periphery.
For the calculate the air kerma in air was used the chamber in the center aline wit the lase,
we obtained three reading using different field of view for head standard and body standard.
after we use the equation 4 for obtained the factor for different phantom relationship with CTDI
delivered for the CT.
CTDI = Kair ∗
Calib.f actor
thickness
(4)
IV. analysis and discussion
I. First Practice: Image quality measurement.
I.1 CT Number Linearity.
The figure 1a show the data obtained of the image 1b the numeric data can be seen in appendix
in table 3 this data from this image was analyzed for osiris 4 program for obtained the numeric
data.
(a) Number CT vs Linear attenuation coefficient (b) image hs1 catphan phantom
Figure 1: CT number linearity
In the figure 2 show the image
I.2 Spatial Resolution.
As alternative to determining the spatial resolution by calculating the MTF, how is show in the
figure 2.
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4. Master in Medical Physics 2015 to 2016
(a) Spatial resolution (b) MTF vs Line pair per milimeter (c) image hst3 catphan phantom
Figure 2: Spatial Resolution
I.3 Low contrast Resolution.
The low contrast resolution determines the size of detail that can be see when only a small
difference in specific density relative to the surrounding area, the graph 3a represent the number
of detail can be seen in the image during the practice.
(a) Low contrast (b) image hs5 catphan phantom
Figure 3: Low contrast Resolution
I.4 Spatial Uniformity.
The way as we calculate the spatial uniformity is the difference between periphery ROI and
central ROI. the result obtained were:up periphery 0.92 HU, right periphery 0.68 HU, down
periphery 0.18 HU and left periphery 0.28 HU.
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5. Master in Medical Physics 2015 to 2016
Figure 4: Spatial uniformity
I.5 Pixel dimension.
The distance of the holes is 50 mm distance measurement obtained was 49.7 mm getting a
discrepancy of 0.6%, we can be seen the distance between holes in image 5b
(a) pixel size (b) image hs1 catphan phantom
Figure 5: Pixel size
II. second Practice: Dose measurement
The table 1 represent the data used for Acquisition modality used in the obtained data for
calculate the CTDI. In this case was evaluated CTDIw usin the result of measure in both
Modality t[sec] kV mA
Thickness
(mm)
FOV
(mm) Matrix
CTDI
(mGy)
Head Std(S & S) 1 120 200 8 240(S) 512 44.3
Body Std(S & S) 0.5 120 250 32 400(L) 512 14.7
Table 1: Acquisition modality
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6. Master in Medical Physics 2015 to 2016
phantom in the center and periphery this data was evaluated using the equation 2, the table 2
show the results obtained for different modality, the percent difference of the data show in CT
with the value measure is below the tolerance limit; body standard 5.16% and head standard
3.26% the limit value is 20%.
Calculation
Center
(mean value) 1/3 C
Periphery
(mean value) 2/3 P
CTDIw
(mGy)
nCTDIw
(mGy/mAs)
Head Std(S & S) 32.2 10.7 35.4 23.6 42.9 0.214
Body Std(S & S) 26.63 8.88 61.2 40.8 15.5 0.062
Table 2: Acquisition modality
The results for air kerma was 54.2 mGy using field of view for head and 177.1 mGy using field
of view for body, the factor to the head standard was 0.66 and for body standard was 0.27. this
factors were found doing relationship with the CTDI delivered for the CT with the CTDI using
the air kerma.
V. Conclusion
• One way to protect patients is the periodic assessment of image quality and dose, to
determine the dose delivered to the patient and consider image quality generated by the
equipment
• We conclude that the measurement about the CTDI for the body standard and the head
standard during the practice, all the results are less than reference limit.
References
[1] Paola Bregant , Lecture Physics of Diagnostic with x-ray 2, ICTP Trieste Italy, 2015
[2] Jerrold T Bushberg the Essential Physics of Medical Imaging,second edition, Lippincott
Williams-Wilkins, 2012
[3] Mario Dedenaro Lecture notes in Exercise of Radiology 1, Master of Medical Physics ICTP-
Trieste university, 2015.
[4] Renato Padovani Lecture notes in Radiation Dosimetry, Master of Medical Physics ICTP-
Trieste university, 2015.
VI. Appendix
CT number linearity.
Material No CT µ[1/cm]
air -994 0.000
Polystryene -40 0.171
LDPE -100 0.174
air -996 0.000
Teflon 922 0.363
Delrin 314 0.245
Acrylic 120 0.215
Table 3: Measurement of CT number Linearity
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