Effect of kilovoltage and quality reference mAs on CT-based attenuation correction in 177Lu SPECT/CT imaging: a phantom study

Abstract

Introduction: CT-based attenuation correction (CT-AC) plays a major role in accurate activity quantification by SPECT/CT imaging. However, the effect of kilovoltage peak (kVp) and quality-reference mAs (QRM) on the attenuation coefficient image (μ-map) and volume CT dose index (CTDI_vol) have not yet been systematically evaluated. Therefore, the aim of this study was to fill this gap and investigate the influence of kVp and QRM on CT-AC in ¹⁷⁷Lu SPECT/CT imaging.

Methods: Seventy low-dose CT acquisitions of an Electron Density Phantom (seventeen inserts of nine tissue-equivalent materials) were acquired using various kVp and QRM combinations on a Siemens Symbia Intevo Bold SPECT/CT system. Using manufacturer reconstruction software, ¹⁷⁷Lu μ-maps were generated for each CT image, and three low-dose CT related aspects were examined. First, the μ-map-based attenuation values (μ_measured) were compared with theoretical values (μ_theoretical). Second, changes in ¹⁷⁷Lu activity expected due to changes in the μ-map were calculated using a modified Chang method. Third, the noise in the μ-map was assessed by measuring the coefficient of variation in a volume of interest in the homogeneous section of the Electron Density Phantom. Lastly, two phantoms were designed to simulate attenuation in four tissue-equivalent materials for two different source geometries (1-mL and 10-mL syringes). ¹⁷⁷Lu SPECT/CT imaging was performed using three different reconstruction algorithms (xSPECT Quant, Flash3D, STIR), and the SPECT-based activities were compared against the nominal activities in the sources.

Results: The largest relative errors between μ_measured and μ_theoretical were observed in the lung inhale insert (range: 18%-36%), while it remained below 6% for all other inserts. The resulting changes in ¹⁷⁷Lu activity quantification were -3.5% in the lung inhale insert and less than -2.3% in all other inserts. Coefficient of variation and CTDI_vol ranged from 0.3% and 3.6 mGy (130 kVp, 35 mAs) to 0.4% and 0.9 mGy (80 kVp, 20 mAs), respectively. The SPECT-based activity quantification using xSPECT Quant reconstructions outperformed all other reconstruction algorithms.

Conclusion: This study shows that kVp and QRM values in low-dose CT imaging have a minimum effect on quantitative ¹⁷⁷Lu SPECT/CT imaging, while the selection of low values of kVp and QRM reduce the CTDI_vol.

Keywords: 177Lu SPECT/CT; Attenuation correction; CTDIvol; Quantitative SPECT/CT.

Fig. 1

Positioning of the cubic volume of interest of 3 × 3 × 8 voxels in the bone insert with 1250 mg/cm³ hydroxyapatite in 130 kVp and 35 mAs image. A whole phantom. Blue dotted line indicates position of inner inserts, orange dotted line indicates position of outer inserts. B Zoom over the bone insert and placing of the cubic volume of interest

Fig. 2

Digital phantom from the CIRS electron density phantom. A Segments from the high resolution CT image. B µ-map based on theoretical values from NIST database [17]. C Theoretical attenuation correction factor (ACF_theoretical) image. $I_0$ is the image with attenuation correction and $I$ is the image without attenuation correction

Fig. 3

Segmented VOIs for the noise analysis. Illustration performed on imageJ [28]

Fig. 4

Quantitative SPECT: A Frontal view of 10-mL syringe insert. B Lateral view of 10-mL syringe insert. C Segmentation boundaries using 10-mL syringe insert with sources inside of NEMA Phantom

Fig. 5

Mean measured attenuation coefficients (µ_measured). Error bars consider a coverage factor (k) of 2

Fig. 6

Relative error between the mean measured attenuation coefficients (µ_measured) and the theoretical attenuation coefficients (µ_theoretical). Error bars consider a coverage factor (k) of 2

Fig. 7

Relative error between the attenuation correction factor (ACF_measured) and the theoretical attenuation correction factor (ACF_theoretical) for: A Insert located in the inner section of the phantom. B Inserts located in the outer section of the phantom. Error bars consider a coverage factor (k) of 2

Fig. 8

Noise analysis. Measured noise coefficient of variation together with the mean CTDI_vol for each kVp and QRM combination

Fig. 9

Relative error matrix between the measured and nominal activity for the four tested reconstruction algorithm and attenuation material of: A 1-mL syringe sources and B 10-mL syringe sources. Materials: PS Polystyrene, PTFE Polytetrafluoroethylene, PA Polyamide, and PP Polypropylene