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Feasibility of CT angiography acquired with low tube voltage
and low iodine retrospective evaluation of radiation dose,
image quality and iodine load
Poster No.:
C-1347
Congress:
ECR 2015
Type:
Scientific Exhibit
Authors:
R. Bavaharan , A. Bherwani , R. Anand Kumar ; Trichy/IN,
1
2
2
3 1
3
Mumbai/IN, Erode/IN
Keywords:
Arteries / Aorta, Cardiovascular system, Contrast agents, CTAngiography, Image manipulation / Reconstruction, CT-High
Resolution, Comparative studies, Contrast agent-intravenous,
Statistics, Dosimetric comparison, Quality assurance, Patterns of
Care
DOI:
10.1594/ecr2015/C-1347
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Aims and objectives
With increasing usage of computed tomography (CT) procedures in recent years, there
is a growing awareness and concern among society, government, referring physicians
and patients towards the potential risk of cancer related to excessive radiation exposure.
Patients with chronic diseases who have to undergo frequent radiological examinations
for continuous monitoring, pediatric age group patients and young females are even more
prone to effects of radiation (See Fig 1). [1- 4]
Fig. 1 on page 4
In order to reduce the risk of radiation exposure, it is important to keep the radiation dose
as low as possible while maintaining an adequate image quality sufficient for making the
diagnosis. [3]
Contrast-induced renal failure is one of the most widespread complications of iodinated
contrast media (CM). It accounts for over 12% of all cases of the acute renal failure and
is associated with a high morbidity and mortality rate [5, 6].
Although the underlying mechanism of contrast-induced nephropathy (CIN) is not
fully understood, studies suggest that reduced renal blood flow induced by the
CM may subsequently contribute to its nephrotoxic potential. Figure 2 demonstrates
changes in the renal resistance index (RRI) after intra-arterial administration of
iopamidol (low-osmolar contrast medium; LOCM) compared with iodixanol (iso-osmolar
contrast medium; IOCM). Statistically significant increase in the RRI is suggestive
of renal vasoconstriction, is seen post administration of LOCM compared to minimal
vasoconstriction with IOCM [7]
Fig. 2 on page 5
Other characteristics of CM, such as osmolality [7], chemical composition [8] and viscosity
may also contribute to the risk of CIN. The nephrotoxic effects in high-risk patients
undergoing Angiography (NEPHRIC) study documented that IOCM may have better renal
tolerance than LOCM in high-risk patients [9].
The feasibility of CT angiography performed at low tube voltage with reduced radiation
dose and effective iodine content has been tested in several studies [10]. The basic
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rationale behind this concept is that the mean photon energy of 80 kVp in the X-ray
beam moves closer to the k-absorption edge of iodine (33.2 kVp). By taking advantage
of the properties of iodine, there is a potential to reduce both the radiation and the iodine
dose required to achieve the same degree of enhancement as with 120 kVp (see Fig
3). A higher vascular enhancement can be satisfactorily acquired with increased iodine
absorption at a reduced radiation dose level. [11, 12]
Fig. 3 on page 6
It is documented in many studies that an iterative reconstruction algorithm, adaptive
statistical iterative reconstruction (ASIR) produces images with increased contrast
resolution and reduced noise even at lower radiation doses.[13, 14].
In view of these hypothesis, a retrospective study was conducted to assess the
effectiveness and feasibility of CT angiography (coronary, pulmonary and abdominal)
performed at low tube voltage (100 kVp) using low iodine concentration contrast
medium (270mg/mL) in comparison with higher tube voltage (120 kVp) using high iodine
concentration contrast medium (350mg/mL).
Images for this section:
Page 4 of 33
Fig. 1: Effects of radiation on individuals of different age groups
Page 5 of 33
Fig. 2: Changes in the renal resistance index after intra-arterial administration of
iopamidol and iodixanol
Page 6 of 33
Fig. 3: Mass attenuation coefficient of idinated contrast material and tissue encountered
in diagnostic X-ray imaging
Page 7 of 33
Methods and materials
Study design
The study involved a retrospective analysis of 120 cases of CT angiography with
various medical indications during the period of June 2014 to September 2014. In all
these cases, the angiography was performed using a GE optima 660-128 slice scanner
using uniform scan settings except the tube voltage and the contrast medium. Iterative
reconstruction software namely ASIR 3D was used to reconstruct the images. The
Advantage Workstation AW 4.6 was used to process the images.
These cases were segregated into two groups:
Group A (n=60) which included cases performed using iodixanol (Visipaque™ 270,
General Electric Healthcare) 270 mg/mL on 100 KVp. Group B (n=60), which included
cases performed using iohexol (Omnipaque™ 350, General Electric Healthcare) 350mg/
mL on 120 kVp tube voltage.
The demographic characteristics of patients, contrast volume in the two groups have
been given in Table 1. Difference in Iodine load can be seen in Figure 4.
Table 1: Demographic characteristics of the patients.
Variables
Group A
Group B
p-Value
Male N (%)
31(51.7)
29(48.3)
Female N (%)
42(70.0)
18(30.0)
Age (in years) (M
±SD)
48.07 ± 15.0
49.47 ± 15.4
0.616
Body weight (in kg)
(M±SD)
64.56 ± 9.32
65.73 ± 9.61
0.503
Contrast Volume(in 64.75 ± 6.26
69.86 ± 10.63
0.002
mL) (M±SD)
NOTE: Table above suggests that there was no statistically significant difference between
two groups (Age, p value - 0.616 and bodyweight, p value - 0.503)
Fig. 4 on page 10
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Assessment of Study Parameters
Image Quality
Image quality was rated on axial, curved planar reconstructions and volume renderings
by six independent radiologists. These radiologists had more than 5 years of experience
in assessing body imaging and were blinded to all scanning and processing conditions.
Scores were allotted based on a four point scale on the presence of graininess, vessel
sharpness, streak artifact and the overall image quality. The scale used was as follows:
4-Excellent; 3-Good; 2-Acceptable; 1-Unacceptable,
Intravascular Attenuation
The CT attenuation value (Cv) (in Hounsfield units [HU]) was measured at circular region
of interest (ROI), placed in the center of the following vessels at uniform levels (see Fig
5-7).
•
•
•
Coronary angiography: Left main coronary artery (see Fig 5)
Abdominal angiography: Abdominal aorta at the level of renal origin (see Fig
6)
Pulmonary angiography: Main pulmonary artery (see Fig 7)
On the three levels, additional ROI was placed at the following regions and tissue value
(Ct) was measured. The values are shown as mean ± standard deviation (see Fig 5-7).
•
•
•
Coronary angiography: Paraspinal muscles (see Fig 5)
Abdominal angiography: Paraspinal muscles (see Fig 6)
Pulmonary angiography: Pectoralis major muscle (see Fig 7)
Fig. 5 on page 11
Fig. 6 on page 12
Fig. 7 on page 13
Determination of Contrast
The contrast value (C) was calculated using the CT attenuation value (Cv) of the vessel
and tissue value (Ct): C = [Cv - Ct]. (see Fig 5-7)
Calculation of Contrast-to-Noise Ratio
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The image noise (N) was defined as the mean standard deviation of the vessel
attenuations computed from the above mentioned positions. The contrast-to-noise ratio
(CNR) was determined by the equation CNR = C /N. (see Fig 5-7)
Radiation dose assessment
The dose-length product (DLP) and computed tomography dose index (CTDI) displayed
on the CT system were used to measure the radiation dose as shown in Fig 8.
Fig. 8 on page 14
Statistical Analysis
Statistical analyses were performed using SPSS software version 20.0. For all statistical
analyses, p<0.05 was considered significant.
t-Test was performed on background factors (patient age, body weight) to find out the
difference between populations in two groups. No statistically significant difference was
found in two groups (see Table 1), suggesting the groups to be comparable.
t-Test was further used to compare the following parameters between two groups: image
quality, intravascular attenuation, contrast to noise ratio (CNR), effective iodine load and
radiation dose (dose length product [DLP] and CT dose index [CTDI]).
Images for this section:
Page 10 of 33
Fig. 4: Iodine load between the groups
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Fig. 5: CT Coronary angiography (axial image): Left main coronary artery with ROI to
measure HU value (Cv) and another ROI in paraspinal muscles to measure the tissue
attenuation value (Ct)
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Fig. 6: CT Abdominal angiography (Axial image): Abdominal Aorta with ROI to measure
HU value (Cv) and another ROI in Paraspinal muscles to measure the tissue attenuation
value (Ct)
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Fig. 7: CT Pulmonary angiography (Axial image): Main pulmonary artery with ROI to
measure HU value (Cv) and another ROI in Pectoralis muscles to measure the tissue
attenuation value (Ct)
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Fig. 8: Screen shot taken from the workstation showing the radiation exposure doses
[CTDI (mGy), DLP (mGy-cm)]
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Results
Radiation Dose
Radiation dose measured in terms of CTDI was numerically lower in group A compared
to group B (p=0.228). The DLP was significantly lower in group A compared to group B
(p=0.038). Mean percentage difference between groups A and B for CTDI and DLP was
9.7% and 7.4% respectively (see fig 9 and 10).
Fig. 9 on page 18
Fig. 10 on page 18
Contrast-to-Noise Ratio
The contrast-to-noise ratio for the three positions were higher for Group A as compared
to Group B (24.22 vs. 21.94; p=0.24) with a percentage mean difference of 10.4% (see
Fig 11).
Fig. 11 on page 19
Intravascular Attenuation
The mean CT attenuation values for the left main coronary artery, main pulmonary artery
and abdominal aorta at the level of renal origin were higher for group A as compared
to group B. The overall CT attenuation values was also significantly higher for group
A compared to group B for the left main coronary artery, main pulmonary artery and
abdominal aorta at the level of renal origin (see Fig 12).
Fig. 12 on page 20
Qualitative Image Assessment
All the images were found to have sufficient quality for rendering clinical diagnosis, with
significantly better image quality assesment scores of all the parameters in group A, as
shown in Table 2 and overall image quality between two groups as shown in Fig 13.
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TABLE 2: Qualitative image assessment
Assessment parameters
Group A
Group B
Graininess (mean±SD)
3.33±0.59
3.14±0.56
Vessel sharpness(mean
±SD)
3.28±0.61
2.98±0.67
Streak artifact(mean±SD)
3.28±0.66
3.19±0.66
Overall image
3.28±0.51
3.07±0.51
quality(mean±SD)
(Assessment was done by 6 experienced radiologists blinded to all scanning and
processing conditions)
Fig. 13 on page 21
Overall Image Quality between Two Groups
Images in group A were found to be of better quality with finer details and better visibility.
Fig 14 demonstrates CT coronary angiography images in group A obtained from
volumetric 3D reconstruction (Fig 14 A, B, and C) and maximum intensity projection (Fig
14 D).
Fig. 14 on page 22
Fig 15 demonstrates CT whole body angiography obtained from volumetric 3D
reconstruction and maximum intensity projection in group A.
Fig. 15 on page 23
Comparison of image quality and finer details between two groups can be seen in figure
16-18.
Fig. 16 on page 24
Fig. 17 on page 25
Fig. 18 on page 26
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Images for this section:
Fig. 9: Difference in Radiation dose (CTDI) between two groups
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Fig. 10: Difference in Radiation dose (DLP) between two groups
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Fig. 11: Difference in Contrast Noise Ratio (CNR) between two groups
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Fig. 12: CT Attenuation values for group A and B in different vessels and overall
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Fig. 13: Image quality between two groups
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Fig. 14: CT coronary angiography images in group A obtained from volumetric 3D
reconstruction (A, B and C) and maximum intensity projection (D). (A) demonstrates
finer details of coronary structures. (B and C) demonstrates the 3 coronary vessels. (D)
demonstrates the 3 coronary arterial tree.
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Fig. 15: Whole body CT angiography obtained from volumetric 3D reconstruction (VRT)
and maximum intensity projection (MIP) in group A.
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Fig. 16: Comparison of image quality and finer details between two groups. (i, ii, iii and
iv) CT Coronary angiography obtained from volumetric 3D reconstruction.
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Fig. 17: Comparison of image quality and finer details between two groups. (i and ii) CT
Coronary angiography obtained from maximum intensity projection showing Left Anterior
Descending Artery.
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Fig. 18: Comparison of image quality and finer details between two groups. CT Renal
angiography obtained from volumetric 3D reconstruction, demonstrates the renal vessels
from origin to hilum
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Conclusion
Discussion
The findings in our present study have demonstrated a significant reduction in the
radiation dose and iodine load, maintaining higher CT attenuation values for all the
vessels at 100 kVp using low iodine concentration CM. Also the contrast to noise ratio and
diagnostic image quality obtained in group A is significantly better in spite of low KVp and
low iodine load. These findings were consistent with those reported by other studies. [15].
In our study we have used a combination of CT angiography of coronary, pulmonary, and
abdominal vessels with low tube voltage and low iodine CM. This is a unique study, as
no other study to our knowledge has focused on these three systems in a single study
protocol.
Using the attenuation properties of iodine discussed above and iterative reconstruction
technique namely ASIR 3D significantly reduced the image noise and improved the image
quality using a low iodine contrast medium.
In the current study, the automatic tube current modulation program was also used with
low tube voltage to reduce the image noise and minimize radiation dose exposure.
Fig. 19 on page 29
As shown in the Fig 19, this technology automatically changes the tube current by
monitoring of tissue attenuation along z-axis (Organ Dose Modulation - Smart mA & Ultra
Kernel) [16-18].
These advanced techniques thus reduce the radiation dose, image noise and drastically
improve the image quality [17]. The combination of low tube voltage and automatic
tube current modulation has been proved to be very efficient to reduce the dose as
demonstrated in several similar studies [18].
As documented in many studies, the key prevention to CIN, lies with reduction of injected
iodine load, however the practical scenario has remained unchanged due to the belief that
reducing the iodine concentration may adversely affect the enhancement / attenuation of
vessels and subsequently affect the diagnostic efficacy. [19, 20]
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As discussed, in our study we have taken advantage of the basic attenuation properties
of iodine along with the latest iterative reconstruction technology (ASIR) and automated
current modulation to achieve a better vascular enhancement and less image noise, using
low iodine concentration, lower osmolality and lower radiation for patient benefit.
In our study, despite a 7.3% reduction of volume of contrast, meaning 28.5% lesser iodine
load and 7.4% lower radiation, the degree of contrast enhancement (8.4%), contrast to
noise ratio (10.4%), overall image quality (7%) was significantly greater.
Conclusion
With this retrospective study of multi-system CT angiography we conclude that, using
a low iodine concentration contrast medium namely IODIXANOL (Visipaque 270) a
significantly better diagnostic image quality can be obtained with a low tube voltage (100
kVp) and iterative image reconstruction technique, thereby reducing the iodine load and
radiation exposure significantly.
The results of this study may be further extended to involve various other organ system
CT scan examinations in view of patient care.
Images for this section:
Page 29 of 33
Fig. 19: Measuring the attenuation in z-axis. Disclaimer: The image reproduced herein
for illustrative, educational and informational purposes. No copyright infringement is
intended and all copyrights belong to respective authors and publishers.
Page 30 of 33
Personal information
Thankful to GE Healthcare for supporting the study.
Authors:
Bavharan Rajalingam, Consultant Radiologist, Magnum Imaging & Diagnostics,
Tiruchirapalli, Tamil Nadu, India. [email protected]
Anand A. Bherwani, Medical Affairs Advisor, GE Healthcare, Core imaging division,
Mumbai, India. [email protected]
Anand Kumar, Consultant Radiologist, Rainbow scans, Tamil Nadu, India.
[email protected]
We would also like to thank:
Radiologists who were blinded to do the image quality assessment.
Dr. B.R Ramdas, Dr. M. Kamalnathan, Dr. S. Ramnivas, Dr. S. Vidyasagar, Dr. Siddique,
and Dr. Ilayraja.
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