novel dual energy CT-based attenuation correction method in PET/CT systems: a phantom study

In present PET/CT scanners, PET attenuation correction is performed by relying on the information given by CT scan. In the CT-based attenuation correction methods, dual-energy technique [DECT] is the most accurate approach, which has been limited due to the increasing patient dose. In this feasibility study, we have introduced a new method that can implement dual-energy technique with only a single energy CT scan. The implementation was done by CT scans of RANDO phantom at tube voltages of 80 kV[P] and 140 kV[P]. The acquired data was used to obtain conversion curves [which scale CT numbers at different kV[P] to each other], in three regions including lung tissue [HU<-100], soft tissue [-100200] for the combination of 80 kV[P] /140 kV[P]. Therefore, with having the CT image in one energy, we generate the CT image at the second energy [from now we call it virtual dual-energy technique] using these kV[P] conversion curves. The attenuation map at 511 ke[V] was generated using bilinear [the most commonly used method in commercially available PET/CT scanners], real dual-energy and virtual dual-energy technique in a polyethylene phantom. In the phantom study, the created attenuation map using mentioned methods are compared to the theoretical values calculated using XCOM cross section library. The results in the phantom data show 10.1%, 4.2% and 4.3% errors for bilinear, dual-energy and virtual dual-energy techniques respectively. Further evaluation using a larger patient data is underway to evaluate the potential of the technique in a clinical setting

[Calculation of the scattered radiation profile in 64 slice CT scanners using experimental measurement]

One of the most important parameters in x-ray CT imaging is the noise induced by detected scattered radiation. The detected scattered radiation is completely dependent on the scanner geometry as well as size, shape and material of the scanned object. The magnitude and spatial distribution of the scattered radiation in x-ray CT should be quantified for development of robust scatter correction techniques. Empirical methods based on blocking the primary photons in a small region are not able to extract scatter in all elements of the detector array while the scatter profile is required for a scatter correction procedure. In this study, we measured scatter profiles in 64 slice CT scanners using a new experimental measurement. To measure the scatter profile, a lead block array was inserted under the collimator and the phantom was exposed at the isocenter. The raw data file, which contained detector array readouts, was transferred to a PC and was read using a dedicated GUI running under Mat Lab 7.5. The scatter profile was extracted by interpolating the shadowed area. The scatter and SPR profiles were measured. Increasing the tube voltage from 80 to 140 kVp resulted in an 80% fall off in SPR for a water phantom [d=210 mm] and 86% for a polypropylene phantom [d = 350 mm]. Increasing the air gap to 20.9 cm caused a 30% decrease in SPR. In this study; we presented a novel approach for measurement of scattered radiation distribution and SPR in a CT scanner with 64-slice capability using a lead block array. The method can also be used on other multi-slice CT scanners. The proposed technique can accurately estimate scatter profiles. It is relatively straightforward, easy to use, and can be used for any related measurement