Iterative model reconstruction: Improved image quality of low-tube-voltage prospective ECG-gated coronary CT angiography images at 256-slice CT

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Abstract

Objectives

To investigate the effects of a new model-based type of iterative reconstruction (M-IR) technique, the iterative model reconstruction, on image quality of prospectively gated coronary CT angiography (CTA) acquired at low-tube-voltage.

Methods

Thirty patients (16 men, 14 women; mean age 52.2 ± 13.2 years) underwent coronary CTA at 100-kVp on a 256-slice CT. Paired image sets were created using 3 types of reconstruction, i.e. filtered back projection (FBP), a hybrid type of iterative reconstruction (H-IR), and M-IR. Quantitative parameters including CT-attenuation, image noise, and contrast-to-noise ratio (CNR) were measured. The visual image quality, i.e. graininess, beam-hardening, vessel sharpness, and overall image quality, was scored on a 5-point scale. Lastly, coronary artery segments were evaluated using a 4-point scale to investigate the assessability of each segment.

Results

There was no significant difference in coronary arterial CT attenuation among the 3 reconstruction methods. The mean image noise of FBP, H-IR, and M-IR images was 29.3 ± 9.6, 19.3 ± 6.9, and 12.9 ± 3.3 HU, respectively, there were significant differences for all comparison combinations among the 3 methods (p < 0.01). The CNR of M-IR was significantly better than of FBP and H-IR images (13.5 ± 5.0 [FBP], 20.9 ± 8.9 [H-IR] and 39.3 ± 13.9 [M-IR]; p < 0.01). The visual scores were significantly higher for M-IR than the other images (p < 0.01), and 95.3% of the coronary segments imaged with M-IR were of assessable quality compared with 76.7% of FBP- and 86.9% of H-IR images.

Conclusions

M-IR can provide significantly improved qualitative and quantitative image quality in prospectively gated coronary CTA using a low-tube-voltage.

Introduction

Coronary computed tomography angiography (CTA) using 64-slice or newer generation multidetector CT (MDCT) has become the non-invasive modality of choice for the detection and rule-out of clinically significant coronary artery disease (CAD) in stable patients with suspected or known CAD [1], [2]. However, risks from the radiation dose at coronary CTA are of concern [3]. According to Hausleiter et al. the mean effective dose at coronary CTA is 12 mSv (range 5–30 mSv) [4]. To comply with the as-low-as-reasonably-achievable (ALARA) principle, techniques such as imaging at reduced tube voltage [5], electrocardiography (ECG)-dependent tube current modulation [6], prospective electrocardiogram (ECG)-gating [7], high-pitch helical scanning on dual-source CT [8], the application of noise reduction filters [9], and scan length optimization [10] have been developed to reduce the radiation exposure. Low-tube-voltage techniques yield higher contrast enhancement than the standard 120-kVp tube voltage because the X-ray output energy at low tube voltages is closer to the iodine k edge of 33 keV [11]. However, increased image noise, a byproduct of low-tube-voltage settings, is a serious problem [12].

An iterative reconstruction algorithm for CT was introduced to help reduce the quantum noise associated with standard convolution-filtered back-projection (FBP) reconstruction algorithms [13]. Earlier investigations that evaluated the image quality of a hybrid type of iterative reconstruction (H-IR) in coronary CTA indicated that a 50–63% radiation dose reduction was possible without compromising the image quality [14], [15]. More complex iterative reconstruction approaches have modeled the shape of the X-ray beam as the focal spot and as it emerges from the anode, interaction of the X-ray beam within the voxel in the patient, etc. [16]. Being computationally intensive, these model-based approaches required reconstruction times that are significantly longer than FBP. Further advancements in computing hardware have addressed the historical limitations of clinically impractical reconstruction times, thereby enabling the implementation of newer generation of model-based iterative reconstructions that could also be used in coronary CT angiography. One such example is the iterative model reconstruction algorithm that represents the latest advance in the field of reconstruction techniques This is achieved by incorporating reconstruction as part of an optimization process, wherein, by imposing constraints on a penalty-based cost function (that has knowledge of the characteristics of a given CT system), image smoothness can be enforced to effectively control the level of noise reduction [17]. Thus by iteratively penalizing the noise and minimizing the cost function, images with significantly reduced noise can be obtained.

We hypothesized that a combination of a low-tube-voltage technique and this new model-based type of iterative reconstruction (M-IR) can provide diagnostically acceptable image quality at low radiation dose. The purpose of this study was to investigate the effect of M-IR on the quantitative and qualitative evaluation of low-tube-voltage prospectively ECG-gated coronary CTA images by comparing them with images reconstructed with FBP and H-IR.

Section snippets

Materials and methods

This retrospective study was approved by the institutional review board; informed consent was waived.

CT radiation dose

The mean calculated CTDIvol was 11.2 ± 3.4 mGy (range 4.9–14.7 mGy) and mean DLP was 145.2 ± 46.5 mGy cm (range, 64.3–185.7 mGy cm). The mean effective dose for 100-kVp coronary CTA was 2.0 ± 0.7 mSv (range 0.9–2.6 mSv).

Qualitative image quality

The results of our qualitative image quality assessment are shown in Table 2. With respect to all image quality parameters (image noise, beam-hardening artifact, vessel sharpness, and overall image quality) the visual scores were significantly higher for M-IR than for H-IR and FBP (p < 0.01).

Discussion

To our knowledge, this study is the first comparative evaluation in a clinical setting of the knowledge-based iterative reconstruction, compared to FBP and H-IR algorithm in coronary CTA, although we found a publication by Scheffel et al. comparing image quality of coronary artery plaque visualization at coronary CTA in an ex vivo setting by use of human hearts comparing FBP, a H-IR and a model-based iterative reconstruction algorithm [21].

Unlike the prior-generation iterative reconstruction

Conclusions

The M-IR algorithm can provide significantly improved qualitative and quantitative image quality at low-tube-voltage prospective ECG-gated coronary CTA using a low radiation dose. This may benefit non-obese patients with low and stable heart rates by radiation exposure saving.

Conflict of interest

One author (Mani Vembar) is an employee of Philips Ltd., and two authors (Seitaro Oda and Wm. Guy Weigold) had control of inclusion of all data and information for this study. The other authors (Seitaro Oda, Gaby Weissman, and Wm. Guy Weigold) have no conflict of interest.

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