Optimization of SPECT-CT Hybrid Imaging Using Iterative Image Reconstruction for Low-Dose CT: A Phantom Study

Abstract

Background: Hybrid imaging combines nuclear medicine imaging such as single photon emission computed tomography (SPECT) or positron emission tomography (PET) with computed tomography (CT). Through this hybrid design, scanned patients accumulate radiation exposure from both applications. Imaging modalities have been the subject of long-term optimization efforts, focusing on diagnostic applications. It was the aim of this study to investigate the influence of an iterative CT image reconstruction algorithm (ASIR) on the image quality of the low-dose CT images.

Methodology/principal findings: Examinations were performed with a SPECT-CT scanner with standardized CT and SPECT-phantom geometries and CT protocols with systematically reduced X-ray tube currents. Analyses included image quality with respect to photon flux. Results were compared to the standard FBP reconstructed images. The general impact of the CT-based attenuation maps used during SPECT reconstruction was examined for two SPECT phantoms. Using ASIR for image reconstructions, image noise was reduced compared to FBP reconstructions for the same X-ray tube current. The Hounsfield unit (HU) values reconstructed by ASIR were correlated to the FBP HU values(R2 ≥ 0.88) and the contrast-to-noise ratio (CNR) was improved by ASIR. However, for a phantom with increased attenuation, the HU values shifted for low X-ray tube currents I ≤ 60 mA (p ≤ 0.04). In addition, the shift of the HU values was observed within the attenuation corrected SPECT images for very low X-ray tube currents (I ≤ 20 mA, p ≤ 0.001).

Conclusion/significance: In general, the decrease in X-ray tube current up to 30 mA in combination with ASIR led to a reduction of CT-related radiation exposure without a significant decrease in image quality.

Fig 1. Phantom geometries.

(A) Central transaxial slice across the uniformity module of the Catphan^® 500 phantom (outer diameter 200 mm) with 5 ROIs, (B) Catphan^® 500 phantom with 4 different labeled sensitometry samples and the positions of the background ROIs in the outer part of the phantom section. SPECT examinations were performed with (C) an elliptic phantom (semi-axes: 310 x 230 mm) and (D) the same elliptic phantom extended by additional bottles. CT phantom geometry (A, B) and SPECT phantom geometries (C, D) were not represented using the same scale.

Fig 2. Reconstructed detail of the uniformity module of the Catphan^® 500 phantom.

The phantom was scanned with a constant tube voltage (U = 120 kVp) and with two different X-ray tube currents (left: I = 20 mA; right: I = 40 mA). The images were reconstructed by FBP, ASIR50% and ASIR100% (top row: FBP, middle row: ASIR50%, bottom row: ASIR100%). All images were windowed with the same window level and width. The scale is in centimeters.

Fig 3. Image noise of FBP and iterative (ASIR) reconstructed CT images.

(A, B) Image noise in the uniformity module of the Catphan^® 500 phantom reconstructed by the FBP, ASIR50% and ASIR100% versus X-ray tube current: (A) head geometry (Catphan^® phantom), (B) body geometry (Catphan^® phantom with additional annulus). (C, D) Scatterplots of image noise in ASIR (ASIR50% and ASIR100%) and FBP reconstructed images for equal X-ray tube currents: (C) head geometry, (D) body geometry. The line of identity for both methods (FBP and ASIR) is represented by the dotted line.

Fig 4. HU values of different sensitometry samples of the Catphan^® phantom.

Mean HU versus X-ray tube current for FBP and ASIR reconstructions within three different sensitometry samples: (A) Teflon, (B) water equivalent and (C) air. All measurements were performed with a phantom for the head geometry (standard Catphan^® phantom) and for the body geometry (Catphan^® phantom with additional annulus). The tube voltage was always U = 120 kVp.

Fig 5. CNR of different sensitometry samples of the Catphan^® phantom.

CNR of four different sensitometry samples of head geometry (Catphan^® phantom, top row A-D) and of the body geometry (Catphan^® phantom with additional annulus, bottom row, E-H) for FBP and ASIR reconstructions. The CNRs were examined for Teflon (A, E), Acrylic (B, F), LDPE (C, G), and air (D, H). Due to the definition of the CNR, the slopes of LDPE and air are negative.

Fig 6. Bias in SPECT-CT examinations.

(A) Reconstructed HU values of CT scans reconstructed with FBP and ASIR, (B) calculated attenuation coefficients μ and (C) reconstructed SPECT counts (C) versus X-ray tube current for both SPECT phantoms (standard and extended) and for a fixed tube voltage of U = 120 kVp. The reconstructed SPECT counts (C) of the extended phantom for I = 120 mA are scaled to match with the mean values of the standard phantom for the identic X-ray tube current.