Abstract
The authors evaluate the energy dependent noise and bias properties of monoenergetic images synthesized from dual-energy CT (DECT) acquisitions. These monoenergetic images can be used to estimate attenuation coefficients at energies suitable for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging. This is becoming more relevant with the increased use of quantitative imaging by PET/CT and SPECT/CT scanners. There are, however, potential variations in the noise and bias of synthesized monoenergetic images as a function of energy. The authors used analytic approximations and simulations to estimate the noise and bias of synthesized monoenergetic images of water-filled cylinders with different shapes and the NURBS-based cardiac-torso (NCAT) phantom from 40 to 520 keV, the range of SPECT and PET energies. The dual-kVp spectra were based on the GE Lightspeed VCT scanner at 80 and 140 kVp with added filtration of 0.5 mm Cu. The authors evaluated strategies of noise suppression with sinogram smoothing and dose minimization with reduction of tube currents at the two kVp settings. The authors compared the impact of DECT-based attenuation correction with single-kVp CT-based attenuation correction on PET quantitation for the NCAT phantom for soft tissue and high-Z materials of bone and iodine contrast enhancement. Both analytic calculations and simulations displayed the expected minimum noise value for a synthesized monoenergetic image at an energy between the mean energies of the two spectra. In addition the authors found that the normalized coefficient of variation in the synthesized attenuation map increased with energy but reached a plateau near 160 keV, and then remained constant with increasing energy up to 511 keV and beyond. The bias was minimal, as the linear attenuation coefficients of the synthesized monoenergetic images were within 2.4% of the known true values across the entire energy range. Compared with no sinogram smoothing, sinogram smoothing can dramatically reduce noise in the DECT-derived attenuation map. Through appropriate selection of tube currents for high and low kVp scans, DECT can deliver roughly the same amount of radiation dose as that of a single kVp CT scan, but could be used for PET attenuation correction with reduced bias in contrast agent regions by a factor of ≈ 2.6 and slightly reduced RMSE for the total image. When DECT is used for attenuation correction at higher energies, there is a noise amplification that is dependent on the energy of the synthesized monoenergetic image of linear attenuation coefficients. Sinogram smoothing reduces the noise amplification in DECT-derived attenuation maps without increasing bias. With an appropriate selection of CT techniques, a DECT scan with the same radiation dose as a single CT scan can result in a PET image with improved quantitative accuracy.
Highlights
Quantitative imaging by positron emission tomography (PET) and single-photon emission computed tomography (SPECT) is receiving increased attention for clinical applications.[1–5] In quantitative PET and SPECT, correction for the effect of photon attenuation is of paramount importance.[6]
A list of energies of interest are given in Table I, and Fig. 1 plots the mass attenuation coefficients of common materials over the energy ranges relevant to PET/CT and SPECT/CT imaging
We addressed the energy dependent noise and bias properties for dual-energy CT (DECT) derived attenuation correction for SPECT
Summary
Quantitative imaging by positron emission tomography (PET) and single-photon emission computed tomography (SPECT) is receiving increased attention for clinical applications.[1–5] In quantitative PET and SPECT, correction for the effect of photon attenuation is of paramount importance.[6]. Quantitative imaging by positron emission tomography (PET) and single-photon emission computed tomography (SPECT) is receiving increased attention for clinical applications.[1–5]. In quantitative PET and SPECT, correction for the effect of photon attenuation is of paramount importance.[6]. The xray computed tomography (CT) component in a PET/CT or SPECT/CT system provides precise anatomical localization of regions identified on the tracer uptake images, but is used for attenuation correction of the PET or SPECT emission data.[7–10]. At the energies of x-ray CT, attenuation is due to Compton scatter and photoelectric absorption, while at SPECT energies, and PET energies, Compton scatter is the dominant process for biological materials. A list of energies of interest are given, and Fig. 1 plots the mass attenuation coefficients of common materials over the energy ranges relevant to PET/CT and SPECT/CT imaging.
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