The accuracy of iodine quantification was assessed in a phantom study. The capacity of iodine-density images to show perfusion defects has been investigated in a patient survey.
All acquisition scans have been performed with the SOMATOM Definition Edge CT (Siemens Healthineers, Forchheim, Germany) with a single source and a split-filter (TwinBeam Dual-Energy), which consists of a layer of gold (0.05 mm) and a layer of tin (0.06 mm) placed in front of the tube output, resulting in the separation of the photon energy spectra (Fig. 1). The gold filtration leads to a low-energy spectrum and the tin to a high-energy spectrum, with a mean photon energy of 68 keV and 86 keV, respectively [2]. The tin filtration eliminates low-energy photons and shifts the x-ray spectrum to higher energies.
The Siemens Syngo.via application “DE Lung PBV” was used to obtain and analyze iodine maps. Iodine maps are generated by a post-processing algorithm applied to the low-energy image and the high-energy image generated by the DECT. It is based on three-material decomposition principles applied to main elements of pulmonary parenchyma: soft tissue, air and iodine. The iodine map quantifies the iodine concentration in each CT voxel: the software provides a colour coded multiplanar view of the lung parenchyma, to be superimposed to the anatomic image.
32 and 16 cm diameter PMMA phantoms (CT Body and Head Dose phantoms) and a 30 cm diameter home-made phantom with an external thickness of PMMA and the interior of low-density material were scanned; five vials with different iodine concentration (40, 20, 10, 5, 2.5 mg/ml) were prepared with contrast medium (Iopamiro® 370 mg/ml, Bracco) and placed in different positions.
In the first phase of the study, we varied the main acquisition/reconstruction parameters in order to identify those responsible for a possible variation in the measured iodine values: mAs (100 – 200 – 300 – 400), reconstruction kernel filter (Qr40 – Br38 – Qr61); we also checked the most involved parameters in the split filter acquisition that are acquisition direction (caudo-cranial or cranio-caudal) and pitch (0.25 – 0.3 – 0.35 – 0.4); at the end, we tested how the iodine ratio in the Syngo.via application (1.36 – 1.46 – 1.56 – 1.66) influences iodine maps. The information obtained from the phantom study was used to develop an optimized acquisition protocol for clinical practice.
The patient scans were performed using the following parameters: tube voltage AuSn/120kVp, 64x0.6 collimation, 200 reference mAs, 0.33s rotation time, 0.35 pitch, caudo-cranial direction, reconstruction kernel Qr40. The study population included 20 patients (mean age 64 ± 17 years) with suspected pulmonary embolism.
The radiation dose assessment was performed with the radiation dose index monitoring system NEXO[DOSE]® (Bracco Injeneering S.A., Lausanne, Switzerland), integrated with the PACS of our hospital [3]. Relevant data are extracted from different sources: Digital Imaging and COmmunications in Medicine (DICOM) header, patient protocol and Radiation Dose Structured Report (RDSR). NEXO[DOSE]® tracks patient information and exposure data, e.g. the CTDIvol and the dose length product (DLP).
