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The African Review of Physics (2016) 11:0011
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Evaluation of Image Quality of Head Computed Tomography at the University of
Maiduguri Teaching Hospital, Nigeria
K. L. Ibrahim1,*, A. Ahidjo 2, I. C. Nwobi3 and S. M. Abdullahi1
Department of Physics, University of Maiduguri, Maiduguri, Nigeria
2
University of Maiduguri Teaching Hospital, Maiduguri, Borno State, Nigeria
3
Department of Medical Radiography, University of Maiduguri, Nigeria
1
Assessment of image quality of head computed tomography (CT) at University of Maiduguri Teaching Hospital
(UMTH) is crucial for diagnosis and treatment of patients with head disease or injury. Therefore, it is an essential practice to
evaluate the image quality in order to determine if they meet the established standard criteria for clinical diagnostic task.
This paper determined the factors that affect image quality of head CT at UMTH by subjective assessment including image
noise and artifacts, it also try to establish a baseline on image quality criteria for the locality. A total of 78 head CT images
were retrospectively selected using Philips Brilliance 16-slice 3rd generation CT scanner. Image quality criteria for brain
general CT were evaluated based on recommendations by the European guidelines on quality criteria for CT. Image noise
and the overall image quality of head CT were graded on a 5-point scale, while artifacts on head CT images were graded on
a 3-point scale. The results obtained show that brain CT examinations has a high mean score value of 84% with no
significance difference when compared with a published work. Assessment of image noise shows that 87% of head CT
images meet acceptable standard for clinical diagnostic task, while that of overall image quality shows that nearly 81% meet
acceptable standard for clinical diagnostic task. Artifacts assessment shows that 47% of head CT images obtained have
artifacts; about 6% of the CT images were affected while 35% of the CT images were not affected for the clinical diagnostic
task. In general, it has been shown that head CT images obtained at UMTH Maiduguri, Nigeria achieved optimum
performance of diagnostic quality.
1.
Introduction
The advent of computed tomography (CT) has
revolutionized diagnostic medicine as it provides a
non-invasive way to allow visualization of internal
structure within the human body. CT is a technique
that obtains cross-sectional images of an object
using multiple x-ray measurements taking at
different angle around the object [1]. Due to its
accuracy, higher sensitivity, and wider availability,
CT has become the diagnostic modality of choice
for head trauma and other head diseases. For
clarity, the human head comprises of the brain and
skull; where the skull is the bony skeleton of the
head and the most complex osseous structure in the
body shielding the brain [2], the brain on the other
hand is an organ of soft nervous tissue that controls
thought, memory, emotion, touch, motor skills,
vision, respirations, temperature, hunger and every
process that regulates our body. The human brain
contains over 100 billion neurons [3], it is protected
and supported by the bones of the skull, cranial
________________
*
[email protected]
meninges, cerebrospinal fluid, and blood brain
barrier.
Indications for CT of the head are numerous,
which include acute head trauma, suspected
intracranial hemorrhage, calcification, vascular
occlusive disease, shunt malfunction, mental status
change, headache, cranial nerves dysfunction,
seizures etc. Head CT plays a critical role in the
evaluation of intracranial abnormalities (such as
trauma, stroke, and hemorrhage) [4]. CT is a
proven useful tool in the diagnosis of the bony
skull abnormalities; it provides accurate
visualization and objective information of the bony
structures and cranial structures [5,6]. Due to its
high sensitivity, CT allows rapid assessment of the
extent and type of brain pathology, which ensures
patients who require urgent surgical intervention
receive such care at the earliest opportunity [7].
However, the clinical quality that makes head CT
useful for medical diagnostic task lies in its ability
to produce quality images with superior
resolutions, uniformity, low noise, and absence of
artifacts. At University of Maiduguri Teaching
Hospital (UMTH), a lot of head CT images are
being produced but the factors that affect the image
The African Review of Physics (2016) 11:0011
quality were not assessed. The consequence of this
is that there may be production of poor images,
which may lead to an inappropriate and missed
diagnosis of the patient or increase in radiation
dose to the patient as a result of repeat.
There are many tasks which involve evaluation
of image quality, these different tasks are best
performed by different assessment methods and the
outcome is often referred to as technical (or
physical) image quality or clinical image quality
depending on the method used [8]. Methods to
assess image quality often focus on the technical
(or physical) aspects of the image according to
international commission on radiation units and
measurements [9], which are more straightforward
and objective. This is a limitation for many
hospitals in developing countries including UMTH
because they lack the phantoms and quality control
kits needed to assess the physical image quality. To
address these challenges, we focused on the clinical
aspect of image quality by subjective assessment
using visual grading methods of diagnostic quality.
This assessment method is expensive, time
consuming and there is inconsistency in observer
perception. However, it is considered the most
important evaluation of image quality according to
the International Atomic Energy Agency (IAEA)
since it occurs in the clinical setting using images
of clinical patients. In addition, the benefit of visual
grading is not limited to specific diagnosis.
2.
Materials and Methods
Patients
All head CT examinations in this study were
retrospective. Approval for this study was obtained
from our local research ethic committee of the
Radiology Department, University of Maiduguri
Teaching hospital (UMTH). Inclusion criteria were
patients (male and female) up to age 1 and above
with indications for head CT. A total of 78 head CT
images were randomly selected at the hospital’s CT
scan machine from January 2012 to October 2013.
CT Imaging Technique
The instrument used for collecting data is Philips
Brilliance 16-Slice 3rd Generation CT scanner
(axial/helical)
located
at
the
Radiology
Department, UMTH Maiduguri, Nigeria. The CT
scanner is made up of three primary systems,
including the gantry (with table or couch), the
computer, and the operating console. During the
CT scan, the patient position is supine, head first
into the gantry with the head in the head-holder
whenever possible. The table height is centered
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such that the external auditory meatus (EAM) is at
the center of the gantry. To reduce or avoid ocular
lens exposure, the gantry is tilted to an angle only
for axial CT scan. Helical CT scan does not require
gantry tilt because of the fast scan time. Contrast is
administered to patients if indicated by the
radiologist.
Table 1. Head CT protocols.
Scan type
Axial
Helical
Rotation time(s)
0.5
1.5
Tube voltage (kV)
120
120
Tube current (mAs)
120-400
120-350/slice
FOV(mm)
250
250
Gantry tilt
±30;
0.5 increment
-
Slice thickness(mm)
6
5
Imaging protocols for head CT (Table 1) were in
conformity with the requirements of the American
Association of Physicist in Medicine [10] and the
European Guidelines on head CT examinations.
Criteria for Image Quality Evaluation
All head CT images were viewed and assessed on
the Philips Brilliance work station on a DICOM
monitor. The criteria for image quality evaluation
were subjective using visual grading methods of
diagnostic quality. Images were assessed with the
help of an experienced radiologist and a
radiographer. Fig. 1 shows a typical head CT image
of patients (a) normal and (b) abnormal. The
criteria for grading image quality of anatomical
structures were recommended by the European
guidelines on image quality criteria for computed
tomography [11].
The African Review of Physics (2016) 11:0011
(a)
69
Image criteria scoring were based on the
perception of YES for fulfilling a criterion or NO
for not fulfilling a criterion. Percentage criteria
fulfillment was recorded and the result of image
criteria of the scanner in this locality A was
compared with a reference scanner B using the
method of Calzado et al. [12].
Subjective evaluation of image noise and
overall image quality were both graded on a 5point scale; 1= unacceptable 2= below average 3=
average 4= above average 5= excellent. A score
greater than or equal to 3 is considered to meet the
acceptable standard for the clinical diagnostic task.
All artifacts were graded on a 3-point scale; 1=
not present 2= present and affecting diagnosis 3=
present and not affecting diagnosis. The grading
method for this study was adopted from
Namasivayam et al. [13] and Udayasankar et al.
[14]
Statistical Analysis
Simple descriptive statistical tools like mean and
percentages were used to analyze the data with the
aid of Microsoft Excel 2007. Two-sample t-test
was used in our study to compare percentages.
3. Results
(b)
Fig.1: Head CT images (a) normal (b) abnormal
The criteria for brain general CT examinations
were listed as follows:
Visualization:
(i) Whole cerebrum
(ii) Whole cerebellum
(iii) Whole skull base
Critical Reproduction:
(iv) Visually sharp reproduction of the border
between white and grey matter
(v) Visually sharp reproduction of the basal
ganglia
(vi) Visually sharp reproduction of the ventricular
system
(vii) Visually sharp reproduction of the
cerebrospinal fluid space around the
mesescephalon
(viii) Visually sharp reproduction of the
cerebrospinal fluid space over the brain
Image Criteria of Brain CT Examination
The results on image quality criteria evaluation for
brain general CT examination at UMTH (A) and
that of reference scanner (B) [12] are summarized
in Table 2. The figures in Table 2 represent the
percentage of examinations for which the criteria
were fulfilled.
The African Review of Physics (2016) 11:0011
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Table 2: Image criteria scoring for brain general CT examination
Criteria
% criteria fulfillment
(A)
% criteria fulfillment
(B)
(i) visualization of whole cerebrum
92
90
(ii) visualization of whole cerebellum
75
100
(iii) visualization of whole skull base
95
100
(iv) visually sharp reproduction of the border between
80
30
(v) visually sharp reproduction of the basal ganglia
68
20
(vi) visually sharp reproduction of the ventricular
96
100
78
90
white and grey matter
system
(vii) visually sharp reproduction of the cerebrospinal
fluid space around the mesescephalon
(viii) visually sharp reproduction of the cerebrospinal
90
89
fluid space over the brain
Mean image quality score (%)
It can be seen that individual criterion in the index
study were fulfilled to a larger extent with
percentage fulfillment of each greater than 50%
and a high mean score value of 84% was obtained.
Subjective Assessment of Image Noise
The results of subjective image noise are
summarized in Table 3
From Table 3, subjective assessment of image
noise as adopted shows that scores 3, 4, and 5 have
a total of 68(87.18%), which meets the acceptable
standard for clinical diagnostic task. Test scores 1
and 2 with total of 10(12.82%) did not meet the
acceptable standard.
84
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Table 3: Subjective assessment of image noise of
head CT
Scores
Frequency
Percentage
1
7
8.97
2
3
3.85
3
26
33.33
4
20
25.64
5
22
28.21
Total
78
100
(%)
The African Review of Physics (2016) 11:0011
Assessment of Overall Image Quality
The results of overall image quality by subjective
assessment are summarized in Table 4.
Table 4: Assessment of overall image quality of
head CT
Scores
Frequency
Percentage (%)
1
8
10.26
2
7
8.97
3
24
30.77
4
16
20.51
5
23
29.49
Total
78
100
From the results in Table 4, it can be shown that
scores 3, 4, and 5 have a total of 63(80.77%),
which meets the acceptable standard for clinical
diagnostic task. Scores 1 and 2 with a total of
15(19.23%) did not meet the acceptable standard.
Artifacts Assessment
The results of assessed image artifacts are
summarized in Table 5.
Table 5: Assessment of artifacts in head CT images
Scores
Frequency
Percentage (%)
1
46
58.97
2
5
6.41
3
27
34.62
Total
78
100
Artifact result shows that score 1 has 46
(58.97%), which represents images without any
form of artifacts. Score 2 has 5 (6.41%), which
represent images with artifacts degrading image
quality for the diagnostic task. Test score 3 has 27
(34.62%) representing images with artifacts
without interfering diagnosis.
4.
Discussion
Assessment of image quality is crucial to assure
that the CT scanner has achieved optimum
performance of diagnostic quality. Methods to
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access the image quality often focus on the
physical and technical aspect of the image
according to International Commission on
Radiation Units [9], which is straight forward and
objective. However, objective assessment of the
physical image quality could not be conducted
because there are no phantoms and quality control
kits at the time of the study and has limited our
study only to subjective assessment using visual
grading based on human perceptions.
From the results obtained, the image quality
evaluation for brain general CT examination in our
study was compared with that of Calzado et al,[12]
and the result has not shown any significant
difference t(169) = 0.991, (P =0.32). When
comparing individual criterion, it can be shown that
all image criteria in this locality has no percentage
score of less than 50%, unlike criteria (iv) and (v)
in the published work with percentage score of
30% and 20%, respectively. This is a very good
indication that image quality criteria in our locality
achieved optimum performance of diagnostic
quality.
The subjective assessment of image noise and
overall image quality were both graded on a 5point scale and artifacts were graded on a 3-point
scale as adopted in [13,14]. The Image noise has
total percentage of 87.18% of scores greater than or
equal to 3 representing an acceptable level for
clinical diagnostic task, only 12.82% were below
acceptable level. In this case, acceptable level of
image noise means that the grainy appearance on
head CT images is moderate, low, or even absent to
allow visualization of low and high contrast objects
as well as any disease or abnormality present on the
images. When the grainy appearance is high, that
is, when the noise level is high, it is obvious that
low contrast and high contrast objects are not
discernible as well as any disease or abnormal
structure that may be present on the images. Thus,
these images are considered unacceptable and
discarded for clinical diagnostic task.
The overall image quality has a total of 80.77%
of scores greater than or equal to 3 that represents
the acceptable level for clinical diagnostic and
19.23% below the acceptable level. The overall
image quality evaluation tends to find a balance of
all the image quality factors affecting anatomical
structures, including image noise and artifacts.
Both the assessment of image noise and overall
image quality has modal score of 3, which is
comparable to that of Namisavayam et al. [13] for
neck CT. Although the result of their study is that
all CT images were acceptable for the diagnostic
task at standard protocol, some of the images in our
The African Review of Physics (2016) 11:0011
study were below acceptable limit at standard
protocol.
Artifacts were found to be present in 32
(41.03%) out of the total 78 head CT images
assessed. The result shows that 5 (6.41%) of head
CT images with artifacts affected the image quality
for clinical diagnostic task while 27 (34.62%) of
head CT images with artifacts did not affect the
image quality for the clinical diagnostic task.
Udayasankar et al, [14] graded only streak artifacts
using the same adopted grading method. Our study
on the other hand graded stair-step artifact, streak
artifact, and motion artifact. These were the
artifacts found in head CT in our study. Stair step
artifacts were present in 26 (81.25%) out of the 32
images with artifacts, and these were mostly
viewed around the edges of sagittal and coronal
reformatted images and have reduced the image
quality in these formats. Smaller collimation and
overlapping reconstruction were used to reduce
these artifacts, only one image was degraded and
not useful for the diagnostic task. There were 5
(15.63%) images with streak artifact, three of these
images were badly affected and discarded as a
result of beam hardening and scatter due to metal
and bone. Only 1 image was presented with motion
artifact, which is rarely common, which was the
case of a 1year old baby. The image quality was
highly degraded and not useful for clinical
diagnostic task.
Under clinical conditions, the subjective image
quality of head CT scans and the capability of CT
to assess different features are influenced by a
number of variables, including device, FOV, voxel
size, tube voltage and current, and other technical
factors [15]. This might be responsible for the poor
images produced. Clinical assessment of different
features might also be affected by the observer
performance and viewing conditions, as well as by
hardware and software specifications and the
presence of artifacts. In general, the result indicate
very good image quality performance from head
CT images obtained at the hospital’s CT scan
machine with about 90% head CT protocols
meeting the standard for good imaging.
5.
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when compared to that of a published work. Each
individual criterion in this study fulfilled a
percentage score greater than 50% and the mean
image quality score was rated high (84%). Factors
affecting image quality like image noise and
artifacts were also evaluated using adopted method
and the results indicate very good image quality
performance from CT images obtained at the
hospital’s CT scan machine. The major
shortcoming is that there is dearth of information
on image quality data in our locality and
underscores the importance of this study as base
line for further investigation.
Acknowledgements
The authors acknowledge the Management and
Staff of Radiology Department, University of
Maiduguri Teaching Hospital, Nigeria who
permitted the study to be conducted at the CT scan
unit.
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A generally accepted principle is that image quality
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Clinical performance by subjective evaluation was
used in this study. It has been shown that head CT
images obtained from UMTH Maiduguri, Nigeria
reveal generally a good image quality performance
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Received: 9 May, 2016
Accepted: 15 July, 2016
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