Acta Med. Okayama, 2018
Vol. 72, No. 2, pp. 153‑164
CopyrightⒸ 2018 by Okayama University Medical School.

Original Article

http://escholarship.lib.okayama-u.ac.jp/amo/

Image Quality of the Coronary Angiography with Noise Reduction
Technology to Decrease the Radiation Dose
Masayuki Kumashiroa,b, Takahiro Kataokaa, Shinobu Yokotab, Shinobu Nakagawab,
Keisuke Otsukib, Nobuyuki Miyakeb, Shinji Osumib, and Kiyonori Yamaokaa＊aGraduate School of Health Sciences, Okayama University, Okayama 700-8558, Japan,
Department of Radiological Technology, Kurashiki Central Hospital, Kurashiki, Okayama 710-8602, Japanb

We examined the effects of a reduced exposure dose on the quality of images from an angiography device augmented with a noise reduction algorithm. Before its clinical application, we compared the diameter of the discrimination limit of the hole with that in the conventional method by a visual evaluation with a contrast-detail(C-D) phantom imaged using the target dose. Based on the results, a reducible dose was determined andapplied clinically. The sample population consisted of patients being followed up after percutaneous coronary
intervention (PCI) for coronary artery disease; we evaluated the effects of the exposure reduction on image
quality. A significant dose reduction was observed by the noise-reduction method compared to the conventional method; the radiation dose to the flat panel detector (FPD) could be reduced to 70 nGy per frame.
Clinically, a dose reduction of approx. 40% was obtained while maintaining image quality almost equal to thatof the conventional method.
Key words: image quality, radiation dose, noise reduction, percutaneous coronary intervention, contrast-detailphantomPercutaneous coronary intervention (PCI) in
patients with coronary artery disease has been an
important development, but the increasing radiation
exposure dose to both patients and operators is a problem [1-3]. Cases of skin disorders caused by an increasein the patient exposure dose due to prolonged fluoroscopy times and increased numbers of images wererecently reported [4-8]. The International Commission
on Radiological Protection (ICRP) and the U.S. Foodand Drug Administration (FDA) have reported theeffectiveness of the measurement and recording of
exposure doses for patients for the prevention of theoccurrence of skin disorders due to radiation [9-11].
According to the guidelines of the Radiation Safety
Received September 8, 2017 ; accepted November 24, 2017.
Corresponding author. Phone : +81-86-235-6852; Fax : +81-86-235-6852
E-mail : yamaoka@md.okayama-u.ac.jp (K. Yamaoka)＊

in the Practice of Cardiology Writing Group Members,a PCI increases the risk of skin disorders. It is thus necessary to identify the safe radiation dose for PCIs, andstaying below the threshold safe radiation dose isimportant [12].
In recent years, a system to reduce the radiation has
been applied in clinical practice. The new technology
uses an image noise reduction algorithm (Clarity IQ)
specifically designed for X-ray images which combinesa spatial filter and a temporal filter [13 , 14]. Both filters
include an analysis operation that reveals the predominant structures in the image and excludes them from thefilter phase. This approach is deployed at different spatial scales by using multiresolution image decomposition [15 , 16].
Conflict of Interest Disclosures: No potential conflict of interest relevantto this article was reported.

Kumashiro et al.

An angiography apparatus equipped with noise
reduction technology (NRT) based on the Clarity IQ
algorithm has been introduced and applied in clinicalpractice to reduce exposure during interventional
radiology (IVR) procedures [17-21], resulting in dose
reductions of > 50%. However, these studies differ
regarding the initial dose to be compared, the target
dose reduction, the fluoroscopic time, and the frame
number due to variations in physiques and in the procedure used, and thus comparisons of only the dosevalues versus reduction rates are inadequate.
The Clarity IQ noise reduction algorithm uses several functions to reduce the radiation dose. Noise
reduction consists of both temporal and spatial noise
reduction. Temporal noise reduction refers to processing performed over time and thus across subsequent
images. Spatial noise reduction refers to processing
performed over different regions within a single image.
Temporal noise is reduced by averaging several frames.
The Clarity IQ algorithm uses motion compensation byaligning moving structures before averaging. By utilizing this motion compensation, more frames can beused and more powerful temporal filtering can beapplied. As a result, the noise of the moving structure
is further reduced.
In the clinical application of these techniques, a significant reduction in the radiation dose causes a deterioration of the image quality due to X-ray quantum
mottle; the signal buried in the noise component ismore emphasized and the image quality deteriorates. It
is necessary to determine how much the dose can be
reduced without degrading the image quality.
The optimal ratio of image quality to radiation doseshould be determined in accordance with the As Low As
Reasonably Achievable (ALARA) principle. However,to the best of our knowledge there has been no reporton how much the dose can be reduced in clinical applications, or on the verification of blood vessel visualization when clinically adopting the lowest dose acceptablefor imaging diagnosis.
A previous study evaluated the image quality byswitching consecutively from the conventional method
to the Clarity IQ algorithm for the evaluation of thesame patient [18], and the method had drawbacks of anincrease in the radiation dose and contrast media for thepatients. In the present study we conducted a similar
comparison of the conventional method and the Clarity
IQ algorithm within the context of a normal follow-up

Acta Med. Okayama Vol. 72, No. 2

study that did not increase the patient burden. Our
study also differs from other reports in terms of how
much the radiation dose can be reduced from the target
dose in a phantom experiment and in the clinical applications performed based on the phantom experiment.
Our aim was to determine how much noise reduction technology can reduce the radiation dose without
degrading the image quality, by using a phantom examined with cine angiography equipment, and we applied
the results clinically to verify the technology’s usefulness.

Materials and Methods
Phantom study.
1. Contrast-detail phantom X-ray acquisition
For X-ray acquisition we used a commercially available 240 × 240 mm contrast-detail (C-D) phantom(Burger Phantom 41318-000, Kyoto Kagaku, Kyoto,
Japan). This C-D phantom consists of concave hollow
targets arranged in a matrix of 15 rows of 15 holes. The
diameters and depths of these holes ranged from
1.0 mm to 8.0 mm (0.5-mm increments). Of these, thesmallest identifiable hole diameter was defined as theidentification limit diameter (ILD). Holes with diameters and depths of 1.0 mm to 4.5 mm were used to measure the ILD at each hole depth (Fig. 1).
4.5
4
3.5

Depth (mm)

154

3
2.5
2
1.5
1
1

1.5

2

2.5

3

3.5

4

4.5

Diameter (mm)Fig. 1
Diameters and depths of the holes in the C-D phantom(concave type).

April 2018

The first reference images were collected using a
conventional flat panel detector (FPD) angiography
apparatus (Allura FD 10; Philips Healthcare, Best, The
Netherlands) with standard image processing and exposure system settings (the Xper 10; Philips Healthcare,Best, The Netherlands). Targeted study images werelater collected using the same device with the noise
reduction system (i.e., the Clarity IQ algorithm;
Philips Healthcare, Best, The Netherlands).
The experimental system consisted of a C-D phantom placed on 20-mm-thick acrylic resin and positioned so that the center of the phantom was an isocenter. The distance between the focus of the X-ray tube
and the FPD was 105 cm. To obtain the same tube
voltage and avoid differences due to X-ray energy, alarge focus (0.8 mmφ) was used for the Xper system,
and a small focus (0.5 mmφ) was used for the Clarity
system. Copper (0.4-mm) and aluminum (1-mm) filters were added to the X-ray tube.
A total of 11 dose groups with an incident dose perframe on the FPD of 44-140 nGy/frame (nGy/f) in both
systems were set based on the manufacturer’s instructions. The irradiation conditions for obtaining thesedoses included a tube voltage of 70-76 kVp and a current of 496-821 mA, provided by the automatic exposure control (AEC) in the radiation imaging system.
The actual incident dose (ID) per frame (frame rate: 15
frame/sec) was estimated from the dose area product(DAP) (mGy•cm2) by the built-in area dosimeter. The
DAP was obtained from the internal trans­mission ionization chambers (KermaX Plus; IBA Dosimetry,
Schwarzenbruck, Germany) configured in each device.
2. Visual evaluation
We compared the quality of the images obtained by
the Clarity method with the images obtained by theconventional Xper method and the ILD (in mm) of thecylinder of the C-D phantom. Five radiological technologists (M.K, K.O, S.N, N.M and S.Y) and 1 clinicalengineering technician (T.S > 10 years of experienceamong them) who routinely participate in quantitative
coronary analyses (QCAs) in our hospital’s angiography
department acted as observers.
For the review, the acquisition image obtained byeach method was displayed on a diagnostic quality image
review monitor (MML1942-PER; Philips Healthcare).
The images were presented to the reviewers in a random
order, and the reviewers were blinded to the image
characteristics. To maintain consistent observation

Reduction of Radiation Dose in Cine Angiography

155

conditions, the room lighting was constant, and theobservations were performed under conditions automatically displayed on the image review monitor.
Differences in each ILD between the two acquisition
methods were evaluated using a paired two-sided
Student’s t-test, and p-values < 0.05 were consideredsignificant.
Clinical study.
1. Patients
This clinical study was conducted with the approvalof the Ethics Committee of Kurashiki Central Hospital(no. 1563). The subjects were patients being followed
up after a PCI. Immediately after the PCI the patientswere examined with a conventional reference angio­
graphy device (Allura Xper FD 10 Biplane; Philips
Healthcare); the follow-up examinations were performed within 2 years with a new device (Allura Clarity
FD 10 Biplane; Philips Healthcare). Examinations withthe conventional system were conducted between May
2011 and February 2013; examinations with the new
system were conducted between January 2013 andMarch 2014.
Among these subjects, the 25 patients who did notexperience a significant change (≥ 50%) in the stenosis
rate in all 3 branches of the coronary artery were
included in both studies. All 25 patients were examinedwithin a routine protocol after giving informed consent.
The demographics of the 25 patients were as follows: 21
males, 4 females; age: 70 ± 10 years; BMI: 25.1 ±5.7kg/m 2 ; period between examinations : 11.4 ± 5.8
months. Among the final diagnoses, there were 9 casesof old myocardial infarction (OMI) and 16 cases ofangina pectoris (AP); 24 patients underwent stentplacement.
The examinations were performed by 16 cardiologists with > 7 years of experience; the operator was notalways the same before and after the examination of apatient. Contrast medium was injected using a5-French diagnostic catheter; volumes of 7 mL in theleft coronary artery (LCA) and 5 mL in the right coronary artery (RCA) were manually injected.
As a routine examination in all cases, radiographs
were obtained from 8 directions for the LCA, 4 directions for the RCA, and 2 directions for the left ventricle(LV) with the biplane system. The imaging directions
were as follows. For LCA: right anterior oblique(RAO), left anterior oblique (LAO) cranial, RAO caudal, LAO, anteroposterior (AP) cranial, LAO caudal,

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Kumashiro et al.

RAO cranial, and AP caudal; for RCA: AP cranial,
LAO, RAO caudal, and LAO cranial; and for
LV: RAO and LAO.
The group that underwent cine angiography using
the conventional angiography device was the “reference
study (RS)” group; the group examined with the systemequipped with the Clarity IQ algorithm was the “newstudy (NS)” group.
2. Clinical X-ray acquisition chain
In the reference study, the programmed exposure
conditions were as follows: the imaging dose per frame
was 120 nGy, the fluoroscopic dose was 40 nGy, andthe tube voltage and current were determined by theautomatic exposure unit on the largest field of view (8inches). In the new study, the programmed exposure
conditions were as follows: the imaging dose per frame
was 70 nGy, the fluoroscopic dose was 34 nGy, and thetube voltage and current were given by the automatic
exposure unit on the same field of view.
3. Visual evaluation
To determine the potential dose reduction, we compared each phantom image obtained by the device
equipped with the noise reduction processing algorithmwith the corresponding image obtained by the conventional method.
Images of the bilateral coronary arteries of 25
patients examined using both types of equipment were
evaluated in randomized, blinded, offline readings.
The images used for the visual evaluation were as follows: AP cranial 30° was used for the LCA and LAO
55° was used for the RCA. All images were displayed aspairs on the reference image monitor (MML 1942-PER,
Philips). The observations were performed under conditions automatically displayed on the image review
monitor; the images did not undergo post-processing.
The observers were not notified of the characteristics of
the image.
The images were evaluated in randomized, blinded,offline readings with cine images and still images
obtained at the same angle. The anonymized images
were displayed in pairs on 2 monitors (randomly on theleft or right side). Seven experienced radiological technologists (M.K, K.O, D.S, S.N, N.M, S.O and S.Y > 10
years of experience in angiography) graded the coronary arteries (RCA and LCA) according to the criteria
described herein. The evaluated image quality criteria
were given a score of 1 to 5 as follows. 1, very
poor: unsatisfactory for diagnosis; 2, mediocre: small

Acta Med. Okayama Vol. 72, No. 2

arteries not discernible at all and larger arteries not
sharply defined; 3, average: fair vessel visualizationthat is useful for diagnosis, with distal parts of small
arteries invisible; 4, good: good for visualization, withsmall vessels also visible; and 5, very good or excellent : superior visualization of the vasculature.
Differences in each score between the 2 acquisition
methods in the LCA and RCA were respectively evaluated using Wilcoxon rank sum test, and p-values < 0.05
were considered significant.
At the same time, the visibility of the peripheral
small coronary arteries was also scored by 7 same technologists by comparing the NS and RS images. Thisscore ranged from +2 to −2 as follows: +2, NS excellent; +1, NS good; 0, average; −1, RS good; −2, RS
excellent. Similar evaluations were conducted by threecardiologists (S.K, M.O, and T.M) with > 10 years ofexperience in PCI, and we assessed the validity of theevaluation by the 7 technologists by performing a
receiver operating characteristic (ROC) analysis anddetermining the area under the curve (AUC) based on
the evaluation by the cardiologists. When at least 2 ofthe 3 cardiologists scored the peripheral small coronary
arteries as +2, +1 or 0, we defined the signal as present(signal score 1). When a negative value was provided
by 2 or more of the cardiologists, we defined the signal
as absent (signal score 0). Differences in each score
between the technologists and the cardiologists and also
those between the LCA and RCA were evaluated usinga paired two-sided Student’s t-test; p-values < 0.05 wereconsidered significant.
Statistical analyses were performed with EZR(Saitama Medical Center, Jichi Medical University),
which is a graphical user interface for R (The R
Foundation for Statistical Computing, ver. 1.33) [22].
4. Dose evaluation
For all 25 cases, the fluoroscopy times, total number of frames, DAP, and total air kerma (AK) in free air
at each interventional reference point of the frontal andlateral devices were calculated on both types of equipment and compared. Each value was calculated using apaired two-sided Student’s t-test; p-values < 0.05 wereconsidered significant.
The conversion from DAP to AK (mGy) was madeusing a DAP calculation model [23]. The DAP is definedas the product of the area of the cross-section of the
X-ray beam and the average AK over that cross-section.
The implementation of the DAP calculation model can

Reduction of Radiation Dose in Cine Angiography

April 2018

be divided into 2 parts: the calibrated AK and theeffective area of the beam. Specifically, the DAP isdetermined by a combination of the calibrated AK as afunction of tube voltage and 4 pre-filters in the collimator, the heel effect of the X-ray tube, the shutter position, the wedges rotation and translation, the extra-focal radiation, and the collimator scatter contribution.
In both the Xper and Clarity methods, the calibrated
AK is measured as a function of the tube voltage withthe different pre-filters.
As mentioned above, the DAP and AK can be
determined by the built-in dosimeter, but we carriedout our own dosimetric measurements (Accu-Gold;
Radcal, Monrovia, CA, USA) in order to determine
the actual AK rate (mGy/min) of both units in the fluoroscopy mode (fluoroscopic dose) and radiographymode (imaging dose). The installation point of thedosimeter was set at the IVR reference point based on
Interna­tional Electrotechnical Commission (IEC) standard 60601-2-43. In this case, the ID of the reference
study was 130 nGy/f.
The correction coefficient (CC) between the measured and calculated values was determined, and thefinal dose reduction rates of DAP and AK were corrected based on this coefficient.

Results
Phantom study. Table 1 shows the estimatedincident dose per frame calculated with the built-in area
dosimeter based on the Xper dose of 120 nGy/f. The
error rate exceeded 5% at 70 nGy/f for Xper, 44 nGy/f
and 60 nGy/f for Clarity.
Table 1Xper

Clarity

157

Table 2 shows the average ILD for the depth of theC-D phantom at each dose of both systems. In thecomparison of Xper and Clarity, the visibility in images
taken using Clarity was excellent (p < 0.04) at all doses of
70 to 140 nGy/f at depths of 1 and 1.5 mm. A significant difference (p < 0.03) was observed at 70 nGy/f at a
depth of 2 mm, but no significant difference wasobserved at 85 nGy/f, or at any dose at depths of≥ 2.5 mm.
Table 3 shows the results of the comparison of ILDs
between the Xper dose of 120 nGy/f and each Clarity
dose. A significant difference was observed at 44 nGy/f
at a depth of ≤ 2 mm and at 60 nGy/f at a depth of
1.5 mm or less (p < 0.05). However, in the comparison
among the doses of 70, 79, and 85 nGy/f, there was no
significant difference at any depth (p > 0.17). The lowest
dose was estimated using the results of the visual evaluation; 70 nGy/f did not significantly affect the image
quality.
The results of the comparison by the 6 observers of
the ILD measurements at 70 and 120 nGy/f of both
systems are illustrated in Fig. 2. Compared to Xper
120 nGy/f, the average ILD of Xper 70 nGy/f wassignificantly increased when the depth was ≤ 2mm(p < 0.012). However, in comparison with Clarity
70 nGy/f, the average ILD from 1 mm to 3.5 mm wasequal or slightly increased; no significant difference
was observed (p > 0.17). Based on these results, we used
70 nGy/f as a clinical dose reduction target.
Clinical study.
1. Visual evaluation
Table 4 summarizes the evaluation of the images of
25 patients by the seven observers. All images acquired

The estimated incident dose (ID) per frame calculated with a built-in area dosimeter based on Xper 120 nGy/fID(nGy/f)Voltage(kV)

Current(mA)

DAP rate(mGy•cm2/f)

AK rate(mGy/f)Normalized
ID (nGy/f)

70
85
120
140

74
76
74
75

518
528
766
814

8.67
9.86
13.94
15.74

0.122
0.140
0.233
0.229

74.7
84.9
120.0
135.5

44
60
70
79
85
120
140

70
73
74
76
76
74
75

496
510
518
524
527
763
821

6.31
7.47
8.37
9.07
9.60
14.00
15.90

0.090
0.110
0.120
0.132
0.139
0.202
0.230

54.4
64.4
72.1
78.1
82.7
120.6
136.9

DAP, dose area product; AK, air kerma.

error
(%)
6.8

－0.2

0.0

－5.3

23.7
7.4
3.0
－1.2
－2.8
0.5
－2.3

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Kumashiro et al.

Table 2

Acta Med. Okayama Vol. 72, No. 2

The average values (standard deviations) of ILDs obtained by the six observers at 1.0-3.5 mm depth of the C-D phantom

ID (nGy/f)
Depth (mm)

1

1.5

2

ILDs (mm)

2.5

3

3.5

44

Clarity

3.08 (0.93)

2.17 (0.56)

1.67 (0.39)

1.33 (0.45)

1.17 (0.39)

1.00 (0.00)

60

Clarity

2.92 (0.76)

2.17 (0.56)

1.58 (0.56)

1.33 (0.45)

1.08 (0.39)

1.00 (0.00)

70

Xper
Clarity
P Value3.00 (0.57)
2.50 (0.61)＜ .01

2.50 (0.45)
1.91 (0.67)＜ .05

1.83 (0.48)
1.50 (0.45)＜ .05

1.42 (0.50)
1.17 (0.39)
NS

1.17 (0.39)
1.08 (0.39)
NS

1.00 (0.00)
1.00 (0.00)
NS79

Clarity

2.42 (0.69)

1.83 (0.56)

1.50 (0.45)

1.17 (0.39)

NS

1.00 (0.00)

85

Xper
Clarity
P Value

3.08 (0.53)
2.25 (0.64)＜ .01

2.42 (0.38)
1.67 (0.70)＜ .05

1.58 (0.56)
1.42 (0.50)
NS

1.25 (0.48)
1.17 (0.39)
NS

1.08 (0.39)
1.00 (0.00)
NS

1.00 (0.00)
1.00 (0.00)
NS

120

Xper
Clarity
P Value

2.50 (0.45)
2.17 (0.56)＜ .05

1.83 (0.38)
1.50 (0.61)＜ .05

1.33 (0.45)
1.25 (0.48)
NS

1.08 (0.39)
1.08 (0.39)
NS

1.00 (0.00)
1.00 (0.00)
NS

1.00 (0.00)
1.00 (0.00)
NS

140

Xper
Clarity
P Value

2.42 (0.38)
2.08 (0.45)＜ .05

1.75 (0.49)
1.41 (0.50)＜ .05

1.33 (0.45)
1.08 (0.39)
NS

1.08 (0.39)
1.00 (0.00)
NS

1.00 (0.00)
1.00 (0.00)
NS

1.00 (0.00)
1.00 (0.00)
NS

Values are given as mean (standard deviation) or number. ILD, identiﬁcation limit diameter.

Table 3Comparison of IDLs between Xper 120 nGy/f and each ID of clarity
ID (nGy/f)

Xper
Clarity
Clarity
Clarity
Clarity
Clarity
Clarity

Depth (mm)

1

1.5

120
44
60
70
79
85
120

2.50 (0.45)
3.08 (0.93)＊
2.92 (0.76)＊
2.50 (0.61)
2.42 (0.69)
2.25 (0.65)
2.17 (0.56)＊

1.83 (0.38)
2.17 (0.56)＊
2.17 (0.56)＊
1.91 (0.67)
1.83 (0.56)
1.67 (0.76)
1.50 (0.61)＊

ILDs (mm)22.51.33 (0.45)
1.67 (0.39)＊
1.58 (0.56)
1.50 (0.45)
1.50 (0.45)
1.42 (0.50)
1.25 (0.48)

1.08 (0.39)
1.33 (0.45)
1.33 (0.45)
1.17 (0.39)
1.17 (0.39)
1.17 (0.39)
1.08 (0.39)

Values are given as mean (standard deviation) or number. ＊p ＜ .05. ID, incident dose.

4.5
4

X-per

P < .001
P < .001

3.5

P < .05

3
ILD (mm)70nGy/f

Clarity 70nGy/f
X-per

120nGy/fClarity 120nGy/f

2.5
2
1.5
1
0.5
0

Fig. 2
The average identiﬁcation limit diameter(ILD) at each hole depth in the C-D phantom.
1

1.5

2
2.5
Depth (mm)3

3.5

Reduction of Radiation Dose in Cine Angiography

April 2018

in this study were of diagnostic quality and were used toassess clinically relevant information.
In the LCA evaluation, none of the 7 observers
observed any significant difference between the RS and
NS. The average of the NS (i.e., 3.70) was almost thesame as the average value of the RS (3.71). In the RCA
evaluation, one observer reported a significant decreasein the NS, but the others did not recognize a significant
difference between the RS and NS. Although the average value of the RS was 3.58, the average of NS was
reduced to 3.33, but the difference was not significant(p = 0.062). Typical LCA cases are shown in Fig. 3, andtypical RCA cases are shown in Fig. 4.
Panels A and B of Fig. 5 compare the evaluation ofimages by the 7 technologists between the LCA andRCA for the 25 patients. In the LCA, 62% of the new
study (NS) images were evaluated as excellent, good, or
equivalent (non-inferior) compared to the reference
study (RS) images (excellent: 7%; good: 29%; average: 26%). In the RCA, 47% of the NS images were
evaluated as excellent, good, or equivalent (non-inferior) compared to the RS images (excellent: 6%; good:
Table 4

159

23%; average: 18%).
Table 5 compares the evaluation scores of the LCA
and RCA given by the technologists and cardiologists.
The LCA score was −0.02 for the technologists and 0.12
for the cardiologists, and the RCA score was −0.29 forthe technologists and −0.15 for the cardiologists; thecardiologists’ scores were somewhat higher for both the
LCA and RCA, but significant differences were notobserved between the 2 groups (LCA: p = 0.255; andRCA: p = 0.250). The AUCs were 0.840 (95%CI 0.6161.000) in the LCA and 0.849 (95%CI 0.696-1.000) in the
RCA, which suggested that the results of the evaluationby the technologists were almost the same as those of
the evaluation by the cardiologists with high probability.
For the LCA, the sensitivity was 0.900 and the specificity was 0.800, with a threshold value of 2.571 (Fig. 6A).
For the RCA, the sensitivity was 0.769 and the specificity was 0.917, with a threshold value of 3.000 (Fig. 6B).
2. Dose evaluation
Table 6 shows the calculated and measured AK rates(mGy/min) of both systems for each fluoroscopic andimaging dose. Both the measured and calculated values

Diagnostic image quality obtained by the 7 technologists
Score
Observer

A

B

C

D

E

F

G

Average

LCA

ReferenceNew
P Value

3.88 (0.65)
3.64 (0.76)
NS

3.80 (0.74)
3.44 (0.69)
NS

3.44 (0.80)
3.68 (0.76)
NS

3.60 (0.71)
3.84 (0.67)
NS

3.88 (1.11)
4.00 (1.14)
NS

3.60 (0.71)
3.44 (0.69)
NS

3.80 (0.74)
3.88 (0.65)
NS

3.71 (0.62)
3.70 (0.61)
NS

RCA

ReferenceNew
P Value

3.64 (0.81)
3.40 (0.88)
NS

3.28 (0.80)
3.00 (0.69)
NS

3.52 (0.86)
3.24 (0.78)
NS

3.40 (0.79)
3.20 (0.91)
NS

4.04 (1.16)
3.48 (1.24)
NS

3.44 (0.85)
3.20 (0.77)
NS

3.72 (0.75)
3.24 (0.78)＜ 0.01

3.58 (0.71)
3.33 (0.71)
NS

Values are given as mean (standard deviation) or number.
abFig. 3A
The LCAs of the patient with
AP: a 66-year-old male, BMI ＝ 25.1,
plain old balloon angioplasty [POBA],
left anterior descending artery [LAD]#7).
Average grading score (a ＝ 3.57 ＜ b ＝
3.85). a ＝ reference study (RS), b ＝
new study (NS).

160
aKumashiro et al.

Acta Med. Okayama Vol. 72, No. 2
bFig. 3B
The LCAs of the patient with
AP: a 79-year-old male, BMI ＝ 22.4,
stent LAD#6 and left circumflex(LCX)#13, POBA LAD#9. Average
grading score: a [RS] ＝ 4.00 ＞ b [NS]＝ 3.85.
ab

Fig. 4A
The RCAs of the patientwith AP: a 66-year-old male, BMI ＝25.1. Average grading score (a [RS] ＝3.14 ＜ b [NS] ＝ 4.00).
ab

Fig. 4B
The RCAs of the patientwith AP: a 79-year-old male, BMI ＝22.4, POBA RCA#2. Average gradingscore, a [RS] ＝ 4.00 ＞b [NS] ＝ 3.14.

in the fluoroscopic dose reference system were consistent at 27 mGy/min. However, in the new system, themeasured value increased to 23 mGy/min compared tothe calculated value of 21 mGy/min (CC 1.095). In
addition, in the reference system of the imaging dose,the calculated value was approx. 174 mGy/min and themeasured value of 173 mGy/min was approximately thesame (CC 0.995). However, in the new system, themeasured value decreased to 91 mGy/min compared tothe calculated value of 95 mGy/min (CC 0.958). It was
thus necessary to correct the DAP value by a 9.5%
increase in the fluoroscopic dose but by a 4.2% decreaseReduction of Radiation Dose in Cine Angiography

April 2018

A

20

16

20

12
10
8
6

14
12
10
8
6
4

4

2

2
0

Reference Study Excellent
Reference Study Good
Average
New Study GoodNew Study Excellent

18
16

14

Number of subjects

Number of subjects

B

Reference Study Excellent
Reference Study Good
Average
New Study GoodNew Study Excellent

18

161

0

Ａ

Ｂ

Ｃ

Ｄ

Ｅ

Ｆ

Ａ

Ｇ

Ｂ

Ｃ

Ｄ

Ｅ

Ｆ

Ｇ

Observers

Observers

Fig. 5
The relative evaluation of images of the LCAs (left) and RCAs (right) of 25 patients by the 7 observers, comparing the reference
and new studies.

Table 5

Relative image quality obtained by the 7 technologists and 3 cardiologists
Technologists－0.02 (0.72)－0.29 (0.73)

LCA
RCA
P Value

Cardiologists

P Value

AUC

95%CI

0.12 (0.65)

NS
NS

0.840
0.849

0.616-1.000
0.696-1.000－0.15 (0.49)＜ 0.01NS

Values are given as mean (standard deviation) or number. AUC, area under the curve.

A

1.0

0.8

0.8

0.6

0.6

Sensitivity

Sensitivity

1.0

0.4

0.4

0.2

0.2

0.0

0.0
1.0

0.8

0.6

0.4

0.2

0.0

1.0

0.8

0.6

0.4

0.2

0.0

Speciﬁcity

Speciﬁcity

Fig. 6B

ROC curves in the LCA, (left) and the RCA (right).

in the imaging dose.
Table 7 shows the average values and standard deviations (SDs) of the fluoroscopic time (min), the number of imaging frames, the DAP rate (mGy•cm2/f), andthe front and lateral AK (mGy). The average fluoro-

scopic time was 6.1 min in the reference study and6.2 min in the new study; the difference was not significant (p = 0.426). The average number of frames in the
new study was 1,717, which was slightly higher than
the 1,661 of the reference study, but the difference was162

Kumashiro et al.

Table 6

Acta Med. Okayama Vol. 72, No. 2

The calculated and measured dose rates in the Xper and Clarity systems for each ﬂuoroscopic and imaging dose

Fluoroscopic Dose
Imaging Dose

Xper
Clarity
Xper
ClarityCalculated AK rate(mGy/min)Measured AK rate(mGy/min)Correction
Coeﬃcient

Error
(%)

27
21
174
95

27
23
173
91

1.000
1.095
0.995
0.958

0.0
9.5
－0.5
－4.2

Table 7
The average values and SDs of ﬂuoroscopic time (min), number of imaging frames, DAP per frame (mGy•cm2/f), and front andlateral AKs (mGy)

Fluoroscopic Time (min)Frame Number
DAP rate (mGy•cm2/f)
AK-F (mGy)
AK-L (mGy)

Complete Study (n ＝ 25)
BMI ＜ 25 kg/m2 (n ＝ 13)
BMI ＞ 25 kg/m2 (n ＝ 12)

Reference Study

New Study

P Value

6.1 (3.6)
1661 (294)24.1 (6.9)
20.6 (5.8)
27.9 (8.6)
306 (106)493 (174)

6.2 (4.2)
1717 (227)
14.8 (4.2)
12.5 (3.5)
17.4 (5.0)
216 (87)
326 (140)

NSNS＜ .001＜ .001＜ .001＜ .001＜ .001

Values are given as mean (standard deviation) or number. DAP, dose area product; AK-F, air kerma in frontal system; AK-L, air kerma in lateral system.

not significant (p = 0.140).
In the new complete study, the average DAP rate wassignificantly reduced by 39% (24.1 to 14.8 mGy•cm2/f,
p < 0.001), and the reduction rate estimated by the correction coefficient obtained from the measured value
was 40% (23.9 to 14.2 mGycm2/f), which agreed withthe 40% reduction rate (120 to 72.1 nGy/f) of the ID
corrected with the phantom experiment. Similarly, theaverage value of the AK decreased by 29% (306 to
216 mGy) in the frontal system and by 34% (493 to
326 mGy) in the lateral system. A total decrease of 32%
was confirmed, and the difference was significant(p < 0.001).

Discussion
The results obtained from the phantom experimentssuggested that the new cine angiography system using
noise reduction technology (NRT) can be used toreduce the dose without degrading image quality, even
if the dose is reduced by about 40%. Next, therefore,
we applied the new system in a clinical setting, and our
findings demonstrated that it is possible to reduce theimaging dose to 40% while maintaining a nearly equal
image quality. We observed that the incident dose to the
FPD could be reduced from 120 to 70 nGy per frame.
Deterioration of the image was observed at ≤ 60 nGy,

and we estimated that there was a clear branch pointcaused by degradation of the image quality due to X-ray
quantum mottle between 60 and 70 nGy.
Because there was no temporal fluctuation in thephantom experiment using the Clarity IQ device,attention should be focused only on the influence of
spatial noise. Since the spatial noise filter detects noise
in a single image and filters on a pixel-by-pixel basis,neighboring pixels should be considered. It is therefore
possible to enhance only the signal strength. We thus
speculate that the Clarity IQ algorithm can obtain thesame images as Xper but at a lower dose. However,
below a certain dose, the influence of quantum mottle
increases and the signal becomes buried in noise components. In this situation, we cannot separate the pixel
intensity of the signal and the noise; it seems that thesignal is averaged together with the noise and the rendering ability is reduced.
To reduce the exposure dose of the patients andoperators, it is necessary to optimize the dose and perform image quality evaluations using phantom experiments. Using the C-D phantom, we determined theminimum dose required for cine angiography that isnecessary to maintain the quality of the image by the
ILD. This constitutes another unique application of the
exposure reduction technology to clinical practice.
However, the disadvantage of this technique is that it

April 2018

only evaluates the ILD, not the discriminable minimum
contrast (discrimination contrast) in the contrast-detail
diagram (C-D curve) method [24]. It was impossible toevaluate the small size when the depth was ≥ 2.5 mm,
as the disk was up to 1 mm in diameter.
Although this method can easily perform relative
comparisons, such as the influence of various parameters on images, it depends on subjective judgment criteria, so it is likely to be affected by observation conditions. In the visual evaluation of the C-D phantom, thestandard deviation of the ILD was a maximum of 0.93
in the low-dose range and the error was large. This may
also be attributed to the fact that the window and level
could not be freely selected so that the observer could
easily evaluate the signal to maintain constant observation conditions. It seems necessary to compare theobjective visual evaluation methods such as ROC [25]and the physical evaluation.
In dose assessments, an AK value is displayed as thesum of the imaging and fluoroscopic doses, whereas the
DAP shows the individual photograph and the fluoroscopic doses. In our study, the fluoroscopic dose was
reduced by 15%, from 40 nGy/f to 34 nGy/f, to maintain the visibility of the guidewire during a PCI. The
reduction rate of the AK was thus lower than that of the
DAP by an average of 32% on the front and side faces.
Cate et al. [18] evaluated the area dose per frame; in
39 cases (average BMI = 26.4 kg/m2) the patient’s DAP
was reduced from 55 to 26 mGy•cm2/f. According toour present data, the DAP of 25 patients (average BMI =25.1 kg/m2) was reduced from 24 to 15 mGy•cm2/f. In
our study, the dose was further reduced by approx.
42%, which was due to differences in the physiques ofWestern and Japanese patients. It is also necessary toconsider that the reduction value used in this study wasa limited dose range. In fact, Cate et al. [18] reported
that overall, 85% of the image sets were considered to
be of better or equal (non-inferior) quality compared to
the reference cine. However, in our study the percentages of image sets better or equal to the reference cinewere 62% for LCA and 47% for RCA, or less than half
the sets in the latter case.
The relative score was −0.02 in the LCA and −0.29 in
the RCA, and the evaluation was slightly inferior in the
RCA. In our clinical assessment, we used the ROC
method to investigate the authenticity of the technologists’ evaluation. However, our ROC evaluation verified the reliability of the technologists’ evaluation by

Reduction of Radiation Dose in Cine Angiography

163

comparing the judgment criteria of the cardiologists as
the true signal and the signal in normal angiography,
and it did not directly compare the visibility between
Xper and Clarity. The more images which were highly
evaluated by the reviewer in both comparisons, we consider that the visualization capability is excellent systemwith a high probability. The finding that 62% (LCA)
and 47% (RCA) of the image sets were evaluated as
equivalent or better in our study did not exceed theresults by the conventional method. We thus consider
the 2 systems nearly equivalent.
The lower value of the RCA compared to the LCA
was shown by the Xper of the conventional method aswell as by the Clarity. The fundamental differences thatare common to both systems may be due to differencesin the amount of contrast agent, differences in theimaging angle, and the influence of the AEC. We useda large focus on Xper and a small focus on Clarity, andthus using a small focus on the X-ray tube, the voltage
was increased due to the action of the AEC in thepatients with a high BMI. We speculate that along withthe rise in the tube voltage, the deterioration of image
quality is due to a reduction of contrast and an increaseof scattered X-rays.
In the visual evaluation of the RCA by the 7 technologists, the score was ≤ 3 in 4 of the 25 cases; the average BMI of these 4 patients was 29.7 kg/m2, which ishigher than the average value of the 25 cases (25.1 kg/
m2). We suspect that the tube voltage was changing to
the high-voltage side in these 4 cases. As the target dose
was set to the lowest dose range, in the case of halation
within the field of view or an insufficient degree ofinspiration by the patient or an overlap of the vertebral
bodies and other organs, we speculated that therequired dose could not be obtained due to the automatic exposure function. In these cases, it seems necessary to consider the dose-range setting assumptions.
Although our study used the same cases, the examination was performed by 16 cardiologists who differed
between the 2 parts of the studies. Since influences due
to differences in fluoroscopic time among operators
have also been reported [20], it is also necessary to consider the influence of operator differences. In addition,
since there was an interval of approx. 11 months (max.
23 months) between examinations, the patients’ information also changed, and thus a complete comparison
is difficult.
To exclude these factors, Cate et al. [18] performed

164

Kumashiro et al.

Acta Med. Okayama Vol. 72, No. 2

contrast examinations at the same angle twice consecutively in the same patient. Although the reproducibilityof both examinations was maintained, there is concern
about an increase in the exposure dose and contrast
medium. A sufficient explanation for the patient andhis/her agreement are necessary, and it is difficult toimplement this without patient cooperation.
Our facility uses a biplane system; there are reportsthat biplane systems are expected to reduce the contrast
agent and other reports that the exposure dose will
increase [26 , 27]. However, targeted blood vessels andstenosis sites have been identified in followed-up
patients and others who are repeatedly examined. As
patient information also continues to be updated, it ismore likely that the benefit of radiation reduction willcompensate for the slight decrease in image quality.
In conclusion, we used a phantom to investigate
whether a decrease in the radiation dose could beachieved by using noise reduction technology in theangiography apparatus. Our results demonstrated that
it is possible to reduce the incident dose to the FPD doseto 70 nGy per frame. We applied the technique topatients after a follow-up PCI, and a dose reduction of
approx. 40% was achieved while image quality wasessentially maintained.
Acknowledgments. We thank all the cardiologists at Kurashiki
Central Hospital who participated in this study, Mr. Hitoshi Imao (Chuoh
College of Medical Technology, Tokyo) for the statistical analysis advice,and Mr. Yoshiji Kodama (Philips Japan, Tokyo) for providing documents.

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