CT Pulmonary Angiography at Reduced Radiation Exposure and Contrast Material Volume Using Iterative Model Reconstruction and iDose4 Technique in Comparison to FBP
CT Pulmonary Angiography at Reduced Radiation Exposure and Contrast Material Volume Using Iterative Model Reconstruction and iDose4 Technique in Comparison to FBP
Azien Laqmani 0 1
Maximillian Kurfürst 0 1
Sebastian Butscheidt 0 1
Susanne Sehner 1
Jakob Schmidt-Holtz 0 1
Cyrus Behzadi 0 1
Hans Dieter Nagel 1
Gerhard Adam 0 1
Marc Regier 0 1
0 Department for Interventional and Diagnostic Radiology and Nuclear Medicine, University Medical Center Hamburg-Eppendorf , Hamburg, Germany , 2 Department of Medical Biometry and Epidemiology, University Medical Center Hamburg-Eppendorf , Hamburg, Germany , 3 Science & Technology for Radiology , Buchholz , Germany
1 Editor: Gayle E. Woloschak, Northwestern University Feinberg School of Medicine , UNITED STATES
To assess image quality of CT pulmonary angiography (CTPA) at reduced radiation exposure (RD-CTPA) and contrast medium (CM) volume using two different iterative reconstruction (IR) algorithms (iDose4 and iterative model reconstruction (IMR)) in comparison to filtered back projection (FBP). 52 patients (body weight < 100 kg, mean BMI: 23.9) with suspected pulmonary embolism (PE) underwent RD-CTPA (tube voltage: 80 kV; mean CTDIvol: 1.9 mGy) using 40 ml CM. Data were reconstructed using FBP and two different IR algorithms (iDose4 and IMR). Subjective and objective image quality and conspicuity of PE were assessed in central, segmental, and subsegmental arteries.
Data Availability Statement; All relevant data are within the paper
Competing Interests: The authors have declared
that no competing interests exist.
Noise reduction of 55% was achieved with iDose4 and of 85% with IMR compared to FBP.
Contrast-to-noise ratio significantly increased with iDose4 and IMR compared to FBP
(p<0.05). Subjective image quality was rated significantly higher at IMR reconstructions in
comparison to iDose4 and FBP. Conspicuity of central and segmental PE significantly
improved with the use of IMR. In subsegmental arteries, iDose4 was superior to IMR.
CTPA at reduced radiation exposure and contrast medium volume is feasible with the use
of IMR, which provides improved image quality and conspicuity of pulmonary embolism in
central and segmental arteries.
With advances in CT technology offering high temporal and spatial resolution, the diagnostic
accuracy of CT pulmonary angiography (CTPA) has increased with reported sensitivities and
specificities of 83–100% and 89–97%, respectively [
]. However, the utilization of CTPA
increased dramatically, raising concerns with respect to radiation exposure [
]. As the high
diagnostic value of CTPA has been outlined in numerous studies, the focus has been shifted to
the optimization of CTPA protocols in order to comply to “as low as reasonably achievable”
(ALARA) principles. Moreover, the use of iodinated contrast medium (CM) for CTPA
examinations also remains a concern since it may contribute to contrast-induced nephropathy
(CIN), especially in patients with pre-existing renal impairment [
]. The risk of developing
CIN is increased with higher volumes of iodinated CM . Therefore an optimized CTPA
protocol with a combination of both reduced radiation dose and reduced CM volume would be
desirable. Low tube voltage CTPA protocols have shown the ability to reduce radiation dose
and, by shifting the average x-ray photon energy closer to the k-absorption edge of iodine, to
reduce CM volume in non-obese patients [
]. A drawback of low tube voltage is the
increased image noise, which can impair image quality . The increased image noise can be
compensated by iterative reconstruction (IR). Previous studies have shown the feasibility of
low KV CTPA examinations combined with IR, facilitating good image quality [
iterative model reconstruction (IMR) has been introduced, which offers a higher image noise
reduction strength, thus a greater increase in CNR can be achieved, which can be potentially
advantageous for reduction of CM volume. A combination of both, low kilo voltage CTPA
protocol and IR may facilitate the reduction of both, radiation dose and CM volume, while
maintaining image quality. Therefore, the aim of this study was to assess image quality of CTPA at
reduced radiation exposure (RD-CTPA) and CM volume using the two different IR algorithms
iDose4 and IMR in comparison to filtered back projection (FBP).
Materials and Methods
Between November 2014 and January 2015, a total of 52 patients with a body weight less than
100 kg who were referred to our emergency department for a clinically indicated CTPA for
suspected PE were included. Demographic patient data including sex, age, height, and weight were
noticed. Body-Mass-Index (BMI) was calculated using the formula (patient weight (kg)/
[patient height (m)]2).
The retrospective study was approved by the local Clinical Institutional Review Board
(Ethic committee of the medical chamber of Hamburg) with a waiver of informed consent.
CT pulmonary angiography acquisition protocol
All CTPA examinations were performed on a 256-slice MSCT scanner (Brilliance iCT, Philips,
Best, The Netherlands) using the following parameters: tube voltage: 80 kV; detector
collimation: 128 x 0.625 mm; pitch 0.915; tube rotation time: 0.4 s. The automatic exposure control
system (automatic current selection (ACS)) combined with z-axis dose-modulation (Z-DOM))
were used with a quality reference tube charge setting of 130 effective mAs, resulting in a
CTDIvol setting of 2.6 mGy (without considering the additional dose saving effect of Z-DOM).
The CTPA scan was performed in a craniocaudal direction from the top of the aortic arch
to the level of the bottom of the heart within a single breath hold at mid inspiration in order to
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avoid valsalva maneuvers. In cases of additionally suspected pneumonia, the scan area was
The injection protocol consisted of intravenous administration of 40 mL contrast medium
with an iodine concentration of 300 mg/ml (Imeron 300, Bracco Altana Pharma, Milan, Italy)
at a flow rate of 4 mL/sec followed by a 20-ml saline flush at the same flow rate via a
commercially available injector (Medrad1, Stellant1 D, Bayer HealthCare, USA). The CT acquisition
was triggered using a bolus tracking technique (Bolus Pro Ultra, Philips Healthcare) with the
region of interest (ROI) placed in the pulmonary trunk. Data acquisition started with a delay of
4 seconds after exceeding a threshold level of 130 Hounsfield units (HU).
iDose4 and IMR technique
iDose4 (Philips Healthcare, Cleveland, OH, US) represents a hybrid IR algorithm with 2
denoising components: an iterative maximum likelihood-type sinogram restoration based on
Poisson noise distribution and a local structure model fitting on image data that iteratively
decreases the noise [
]. The iDose4 levels (1–7) define the increasing strength of noise
IMR is the latest IR approach introduced by Philips Healthcare and is an advanced, model
and knowledge based IR technique that states to overcome the trade-off between image noise
and spatial resolution that is typical for traditional FBP [
]. Noise reduction by up to factor 10
(17), which is even larger compared to other hybrid algorithms (factor up to 3 (10, 11, 13, 21)),
provides virtually noise free images. But IMR aims at accounting for not only the noise
behavior of the image but also the data statistics, image statistics, and system models during its
iterative cycle. Hence, different aspects of image quality can be targeted, such as image smoothness,
spatial resolution and artifacts. The vendor offers different “IMR definitions” (i.e. filter
settings) for reconstruction of the images, whereby “BodyRoutine” (BR) is the recommended
setting for CT angiography examinations. The IMR levels (1–3) define the increasing strength of
noise reduction [
All raw data were reconstructed with FBP, the HIR iDose4 and with a prototype
implementation of the new knowledge-based IR algorithm IMR (Philips Healthcare, Cleveland, OH, US).
Axial source images were reconstructed with a slice thickness of 1 mm and an increment of 0.5
mm using a standard body filter (B) for FBP and iDose4. For IMR, the recommended setting
“BodyRoutine” (BR) was used. In order to compare different reconstruction levels of the two
IR algorithms, two increasing iDose4 levels 4 (iDose4 L4) and 6 (iDose4 L6) and three
increasing IMR levels 1 (IMR-BR1), 2 (IMR-BR2), and 3 (IMR-BR3) were used. The choice of iDose4
levels applied was based on a previous investigation [
]. Thus, six image series were
reconstructed for each of the 52 patients, resulting in an overall number of 312 data sets.
Quantitative image analysis
As described in detail previously [
], CT numbers (CT-N) of pulmonary arteries (PA),
defined as the attenuation in Hounsfield units (HU), was measured in axial sections by placing
circular regions of interest (ROIs) at nine different levels: the pulmonary trunk, right
pulmonary artery, left pulmonary artery, right upper lobe artery, right middle lobe artery, right lower
lobe artery, left upper lobe artery, left lingular artery and the left lower lobe artery, respectively.
To minimize any bias from a single measurement, each selected artery was measured at three
adjacent 1-mm slices, and the results were averaged for further calculations. Based on the
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CT-N of the nine different levels, an average pulmonary vessel CT-N (CT-Nvessel) was
Objective image noise (OIN) was defined as the mean of the standard deviations (SD) of
CT-N. For measuring mean background CT-N (CT-Nbackground), circular ROIs were placed
within the pectoral muscles on both sides, and the attenuation was averaged. Contrast-to-noise
ratio (CNR) was calculated by use of the following formula: CNR = (CT-N vessel−CT-N
background)/ OIN. To ensure measurement accuracy, the initial ROI was placed on the image
reconstructed with FBP and was then copied and pasted onto the corresponding five data sets of
iDose4 and IMR reconstructions.
Qualitative image analysis
Two radiologists with 7 and 11 years of experience in CTPA, who were blinded to the
reconstruction technique applied, independently evaluated each data set by using axial sections and
standard CTPA window settings (window width, 450 HU; window center, 50 HU). To
standardize subjective evaluation, images obtained from five patients not included in the study
were read prior to assessment in consensus. For image analysis, all 312 reconstructed image
series were randomized and rendered anonymous. The readers were allowed to zoom in and
change the window level and width for assessing structures ad libitum.
All RD-CTPA reconstructions were rated according to a 5-point scale (1 indicating worst
through to 5 indicating best) for subjective image quality, subjective image noise, and blotchy
image appearance using evaluation criteria being published previously [
]. The obtained
fivepoint scoring system is described in Table 1.
Conspicuity of pulmonary embolism
Assessment of conspicuity of PE followed the method previously described by Laqmani et al.
] using a three-point scale as follows: 1, subtle, may be an artifact; 2, sufficient, filling defect
definable; and 3, excellent, filling defect clearly definable. The anatomic location of any filling
defects and the number of the affected PA were recorded. The location of PE was classified as
either central/lobar (pulmonary trunk, right/ left PA and lobar PA), segmental or
subsegmental. To identify PA, the nomenclature outlined by Remy-Jardin et al [
] was used, which was
adapted by Ghaye et al [
Subsequently, a consensus readout session was performed for all data sets in a side-by-side
comparison in order to ensure the assessment of all filling defects. These results were used as
the reference standard.
Radiation dose estimation
The volume weighted CT dose index (CTDIvol), the effective tube charge and the dose-length
product (DLP) of each examination were recorded as provided by the scanner. The estimates
for the calculation of the effective dose (E) were based on the European Guidelines on quality
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criteria in CT using the formula E = EDLP DLP, where E is the effective dose in mSv and
EDLP = 0.014 mSv / mGy cm (dose length coefficient for chest) [
Sample characteristics are given as absolute and relative frequencies or mean +/- standard
deviation, whichever is appropriate. To account for the repeated measurement structure of the
data, defined by three 1 mm adjacent image slices and nine different vessels per patient and
reconstruction level, multilevel mixed-effects linear regression was calculated for the
quantitative parameters. In this model, the reconstruction level was included as predictor for the
For the qualitative Likert scale an analogous model was used while the repeated
measurements were defined by the ratings of the two readers. To evaluate the subjective image quality,
the interaction between reconstruction level and each subjective image quality characteristic
was included as predictor. For the evaluation of conspicuity of PE, the interaction between
reconstruction level and the three different vessel groups was included as predictor. All models
were controlled adjusted for BMI, age, gender, tube charge and effective dose. The results were
estimated as marginal means, which are represented in graphs with 95% confidence intervals
The quantitative parameter CT-Nvessel as well as the qualitative scores for filling defect
conspicuity were analysed in an analogous manner with the interaction between vessel, respective
vessel groups (central/lobar, segmental, and subsegmental), and reconstruction technique and
level as fixed effect. If this interaction was insignificant, only the main effects were included.
Posthoc tests for comparison of marginal means were calculated with contrast tests, using
Wald tests with correction for multiplicity by Bonferroni. P-values <0.05, two sided were
considered significant. All analyses were computed using Stata 14.1 (STATA Corporation, College
Station, Texas, USA).
The study population consisted of 23 women and 29 men. The mean age of the 52 patients
was 65.3 ± 17.5 years (range: 20 to 94), the mean body weight and height were 71.7 ± 11.3 kg
(range: 49 to 95 kg) and 173 ± 7.5 cm (range: 155 to 188 cm), respectively. The calculated mean
BMI was 23.9 ± 3 kg/m2 (range: 16.9 to 30.4).
The mean exposure was 97 ± 33 effective mAs (range: 45 to 191), the mean CTDIvol and DLP
were 1.9 ± 0.6 mGy (range: 0.9 to 3.7) and 66 ± 27 mGy cm (range: 22 to 145), respectively.
These resulted in a mean effective dose of 0.9 ± 0.4 mSv (range: 0.3 to 2.0). The average net (i.e.
imaged) scan length was 24.9 ± 6.1 cm (range: 15.1 to 35.9).
Quantitative results (±95%-CI) are graphically displayed in Fig 1. None of the confounder
showed a significant effect on the quantitative parameters. The mean CT-Nvessel in RD-CTPA
studies reconstructed with FBP was slightly but not significantly higher than that in
reconstructions with iDose4 and IMR (CT-Nvessel: FBP: 406.5 HU; iDoseL4 401.5 HU; iDoseL6: 400 HU;
IMR-BR1: 399 HU; IMR-BR2: 399.3 HU; IMR-BR3: 399.4).
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Fig 1. Quantitative analysis of RD-CTPA being reconstructed with FBP, iDose4 and IMR. Error bars represent the 95% confidence interval.
OIN was reduced up to 41% to 55% with iDose4 and up to 74% to 85% with IMR compared
to FBP (p<0.001) (Figs 1 and 2). CNR calculations showed a statistically significant progressive
Fig 2. Objective image noise reduction with application of iDose4 and IMR compared with FBP. Objective image noise of this RD-CTPA significantly
decreased from FBP (97 HU) to iDoseL4 (70 HU), to iDoseL6 (54 HU), to IMR-BR1 (38 HU), to IMR-BR2 (32) and IMR-BR3 (23 HU).
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increase between FBP, the different iDose4 and IMR reconstructions. FBP had the lowest CNR
value with 4.4 and IMR-BR3 showed the highest CNR value with 32.6 (p < 0.001).
Mean scores (95%-CIs) of qualitative image analysis are graphically displayed in Fig 3. None of the
confounder showed a significant interaction with reconstruction technique/level or image quality.
Subjective image quality improved significantly with the application of iDose4 and IMR
compared with FBP (p<0.001) (Figs 3 and 4). While CTPA images reconstructed with FBP
(mean score 2.9; [2.8–3.1]) were graded as having the lowest image quality, the IMR level 1
(mean score 4.8; [4.7–4.9]) and the IMR level 2 (mean score: 4.9; [4.8–5]) were rated as
providing the best image quality with no significant differences between these two IMR levels.
The perception of subjective image noise (Figs 3 and 4) significantly improved with
application of IMR in comparison with FBP and to a lower level also in comparison with iDose4
(p<0.001). The middle and the highest IMR levels (IMR-BR1 and IMR-BR2) were graded to
have no perceived image noise. A minor blotchy image appearance was noticed with the use of
low and middle IMR levels, which did not affect diagnostic confidence. This blotchy smoothed
image appearance was further enhanced with application of the highest IMR level 3 (p< 0.001),
mildly limiting diagnostic confidence of the readers (mean score: 3.5; [3.4–3.6]) (Figs 3 and 4).
There was an excellent interobserver agreement for the evaluation of image quality between
both readers (ICC: 0.001; reliability 99%).
Pulmonary embolism conspicuity results
Ten patients with PE were identified, whereas filling defects were identified in 1 main pulmonary
artery, 33 central/lobar PA (8 right and left pulmonary arteries, 25 lobar arteries), 61 segmental
arteries and in 36 subsegmental arteries. The final side-by-side readout of corresponding FBP,
iDose4 and IMR data sets did not disclose any additional filling defects in any of the different
Fig 3. Qualitative analysis of RD-CTPA being reconstructed with FBP, iDose4 and IMR. The plots show the mean scores for subjective image quality,
subjective image noise and blotchy image appearance. Error bars represent the 95% confidence interval. Subjective 5-point grading scale (1 indicating
worst through 5 indicating best).
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Fig 4. Image quality, image noise and image appearance of RD-CTPA being reconstructed with FBP, iDose4 and IMR. Transverse
RD-CTPA image reconstructed with FBP, iDoseL4, iDoseL6, IMR-BR1, IMR-BR2 and IMR-BR3. Subjective image quality and image noise
improved with the application of iDose4 and IMR compared with FBP. Simultaneously, blotchy appearance increases moderately with
application of iterative reconstruction.
Mean scores (95%-CIs) of qualitative results of conspicuity of PE are graphically displayed
in Fig 5. In central/lobar (Fig 6) and segmental PA (Fig 7), the conspicuity of PE improved
with use of IMR compared with FBP and to a lower level also compared with iDose4 (p< 0.05).
IMR-BR1 was rated to be the best reconstruction setting for PE conspicuity in central/lobar
and also in segmental PA.
For the assessment of PE in subsegmental PA, iDose4 level L6 (mean score: 2.4; [2.2–2.6])
was rated to have the best image quality. iDoseL6 was superior to FBP (p< 0.05) and to
IMR-BR3 (p: 0.004) (Fig 8).
There was an excellent interobserver agreement for the conspicuity of PE between both
readers (ICC: 0.015; reliability 98.5%).
In this study we evaluated a dose-saving protocol with 80 kVp as well as a reduced amount of
CM in combination with IR. Our results showed that such a combined RD-CTPA protocol and
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Fig 5. Qualitative analysis of conspicuity of pulmonary embolism in central/lobar, segmental and subsegmental pulmonary arteries. The plots
show the mean subjective image scores (error bars represent the 95% confidence interval). Subjective 3-point grading scale: 1, subtle, may be an artifact; 2,
sufficient, filling defect definable; and 3, excellent, filling defect clearly definable.
Fig 6. Conspicuity of lobar pulmonary embolism in RD-CTPA being reconstructed FBP, iDose4 and IMR. Transverse RD-CTPA image
reconstructed with FBP, iDoseL4, iDoseL6, IMR-BR1, IMR-BR2 and IMR -BR3 demonstrating a right-sided pulmonary embolism with filling-defect in the
right lower lobe artery being obscured at FBP. With application of the iterative reconstruction algorithms iDose4 and IMR a significant decrease of image
noise and streak artifacts was achieved, enabling a better conspicuity of the filling defect with IMR and to lower extent also with iDose4 compared to FBP.
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Fig 7. Conspicuity of segmental pulmonary embolism in RD-CTPA being reconstructed FBP, iDose4 and IMR. Transverse RD-CTPA image
reconstructed with FBP, iDoseL4, iDoseL6, IMR-BR1, IMR-BR2 and IMR -BR3 demonstrating a right-sided segmental pulmonary embolism. Conspicuity of
the filling defect improved with application of the iterative reconstruction algorithms iDose4 and IMR.
reduced CM amount provide good to excellent image quality in non-obese patients by the
additional application of IR algorithms. The knowledge-based IR algorithm IMR yielded the
best objective and subjective image quality for the assessment of pulmonary arteries and for the
depiction of central, lobar and segmental emboli at a reduced effective radiation dose of
0.92 ± 0.3 mSv.
Optimized radiation dose and contrast medium CTPA protocols are of significant
importance, since the use of CTPA is steadily increasing within the last years [
]. A study from
one large academic institution reported a fivefold increase in CTPA examinations from 2001 to
2007 . A multi-centric study estimated that one in 330 females undergoing CTPA at the age
of twenty years will develop radiation-induced cancer [
]. Furthermore, the contrast medium
administration required for CTPA bears the potential risk of CIN. Although the risk of CIN
may be overestimated [
], it’s incidence and severity is directly related to the contrast dose
]. The most suitable approach to reduce both the radiation dose and the CM volume during
CT is to lower the tube voltage [
]. Thus, an increase in attenuation values of iodinated
CM and CNR can be achieved, which can facilitate both the improvement of image quality and
the reduction of CM volume [
]. Despite the increase of CNR due to increase in attenuation
values, the simultaneously increase in image noise is one disadvantage of low tube voltage and
can limit a further dose reduction [
]. Previous studies have reported high image noise
values for 80 kVp CTPA protocols [
] if the effective dose was reduced [
]. To address
image noise, recent studies have combined low kilo voltage with IR in CTPA protocols. Lu
et al. [
] reported sufficient image quality in a high-pitch 80 kVp CTPA protocol with an
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Fig 8. Conspicuity of subsegmental pulmonary embolism in RD-CTPA being reconstructed FBP, iDose4 and IMR. Detailed enlargement of a
subsegmental pulmonary artery with embolus. RD-CTPA image reconstructed with FBP, iDoseL4, iDoseL6, IMR-BR1, IMR-BR2 and IMR -BR3.
Whereas the filling defect is well definable in FBP images, it’s conspicuity slightly decreases with iDose4 and markedly decreases with IMR.
effective radiation dose of 0.9 ± 0.2 mSv by application of the sinogram affirmed iterative
reconstruction algorithm. Pontana et al. [
] demonstrated that raw data-based iterative
reconstruction provided improved OIN levels in low-voltage half-dose CT angiograms when
compared with standard-dose FBP images with a reduced effective radiation dose of 1.03 mSv. Our
results demonstrate that a reduced radiation dose to less than 1 mSv can be achieved using an
80 kVp RD-CTPA protocol and that by application of IMR both objective and subjective
image quality can be effectively improved at this reduced dose setting. Quantitative
measurements demonstrated a substantial noise reduction capability of IMR, which was even superior
to the hybrid IR iDose4. IMR provided a noise reduction of up to 85% and 66% compared to
FBP and to iDose4, respectively. CNR calculations demonstrated that IMR provided the highest
values, which were significantly higher than FBP and iDose4. According to the literature, a
minimum CNR of five is required for a reliable detection of PE [
]. In our study, FBP
provided only a CNR of 4.1 whereas IMR provided a CNR of 26.8 [
]. This significant
improvement in CNR is based on the IMR induced noise reduction. Two recent studies on low kilo
voltage coronary CT angiography using IMR supported our results by reporting significant
noise reductions and improvement of CNR and image quality in angiographic studies [
IMR is a knowledge-based IR approach in which reconstruction becomes a process of
optimization that takes into account the data statistics, image statistics, as well as system models [
It generates optimal images by iteratively minimizing the difference between acquired data and
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their ideal form, providing noise and artifact free images [
]. The consecutive
improvement of CNR offers the opportunity to reduce the amount of CM. Zhang et al.  reported
the feasibility of CM volume reduction in combination with a low kilo voltage coronary CT
angiography protocol by the use of IMR. The CM volume commonly applied in CTPA of
nonobese patients varies between 60 and 100 ml [
29, 33, 34
]. According to the literature, a
minimum attenuation value of 211 to 250 HU in PA permits reliable exclusion of acute and chronic
]. The mean attenuation of about 400 HU inside the PA in our study
using only 40ml of CM markedly exceeded these values. Our results show that an excellent
enhancement of the PA can be achieved using the low CM amount of 40 ml, thus reducing the
risk of CIN. A recent study by Lu et al.  suggested that even a further reduction of CM
amount could be achieved in high-pitch CTPA at 80 kVp when being combined with IR by
offering sufficient image noise reduction. However, previous studies reported that excessive
noise reduction could result in an over-smoothing and blurring of the images [
]. In our
study, overall subjective image quality improved with application of both IR algorithms iDose4
and IMR compared to FBP, whereas the best overall image quality was rated at the low and
middle IMR strength levels. However, the high iDose4 level 6 provided a distinct blotchy
appearance, which was further enhanced applying the highest IMR level 3. The use of low and
middle IMR levels 1 and 2 resulted in a minor blotchy image appearance, which did not affect
iDose4 can overcome the change in image appearance to some degree due to its hybrid
iterative algorithm. A previous phantom study showed that iDose4 reduces the noise power
spectrum uniformly over the entire spatial frequency ranges, thus providing an image appearance
which is comparable to what radiologists have been used to over the past three decades [
Knowledge-based IR such as IMR is non-linear, and the spatial resolution can vary with the
object contrast and noise level [
]. However, in a phantom study obtained by Mehta et al.,
IMR provided both improvement of low and high-contrast spatial resolution, respectively [
These experimental results are not fully consistent with our clinical study, as the conspicuity of
small subsegmental emboli degraded with IMR compared to FBP and iDose4. The ability to
distinguish between noise and structures obviously diminishes with increasing IR strength
when structures and differences in density become small. This has already been reported in a
number of other studies, assessing iterative reconstruction algorithms of various vendors [
]. As a consequence the edges of the filling defects become diffuse with IMR, making it
difficult to resolve small structures as small subsegmental emboli, even though the overall image
noise is remarkably decreased by IMR. This presumed decrease of structure edges with IMR
did not compromise the conspicuity of filling defects in central/ lobar and segmental PA, rather
the application of IMR resulted in higher conspicuity of filling defects in comparison to FBP
and iDose4. Here, IMR benefits from its superior strength in noise and streak artifact
Streak artifacts caused by mediastinal structures, contrast within the cardiac chambers,
inflow of highly concentrated CM into the brachiocephalic vein and superior vena cava during
CTPA are a well-reported concern and can obscure the pathways of the central and segmental
]. These artifacts are markedly reduced by IMR, thus enabling a better conspicuity of
the underlying structures such as filling defects in central and segmental PA.
Our study has several limitations. Firstly, the IMR prototype was not officially cleared for
clinical use; hence it was not possible to reduce the dose beyond the level that is appropriate for
iDose4. As a consequence we cannot state if a further radiation dose reduction would also
provide sufficient image quality when being combined with IMR. Secondly, we limited patient
enrollment to individuals with a maximum body weight of 100 kg. Hence, further
investigations have to be obtained to evaluate the influence of IR on reduced dose settings in CTPA
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protocols in obese patients exceeding 100 kg. Thirdly, patients had not undergone the gold
standard of lung scintigraphy. Thus, false negative findings cannot definitely be excluded.
Fourthly, even though the subjective image evaluation was obtained in a blinded manner,
complete blinding to the image reconstruction techniques was not feasible because of the inherent
image texture differences among the three reconstruction methods.
In conclusion, CTPA at reduced radiation exposure and contrast medium volume is feasible
with application of IMR, which provides improved image quality and conspicuity of
pulmonary embolism in central and segmental arteries.
Conceived and designed the experiments: AL HDN GA MR.
Performed the experiments: AL MK SB JS MR.
Analyzed the data: AL SS GA MR.
Contributed reagents/materials/analysis tools: AL MK SB JS CB SS HDN GA MR.
Wrote the paper: AL SS GA MR.
Provided technical support for the implantation of the IMR-prototype: HDN.
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