Optimization of the Acquisition Time and Injected Dose of 18 F-Fluorodeoxyglucose Based on Patient Specifications for High-Sensitive Positron Emission Tomography/Computed Tomography Scanner.

Background This study was aimed to optimize the fluorodeoxyglucose (FDG)-administered dose and scan time based on patient specifications using a highly sensitive five-ring bismuth germanium oxide (BGO)-based positron emission tomography/computed tomography (PET/CT) scanner (Discovery IQ). Methods We retrospectively analyzed 101 whole-body 18 F-FDG PET/CT images. Patient data were reconstructed using ordered subset expectation maximization with resolution recovery algorithms (OSEM + SharpIR). Signal-to-noise ratio (SNR) was calculated for each patient, standardized to SNR norm , and plotted against three body index parameters (weight, body mass index, and lean body mass). Two professional physicians blindly examined image quality at different patient time per bed positions to determine the minimum acceptable quality. To select images of acceptable quality, the noise index parameter was also measured. A new dose-time product (DTP) was established for each patient, and a predicted injected dose was assumed. Results We found an almost linear association between patient weight and normalized SNR, and patient weight had the highest R 2 in the fitting. The redesigned DTP can reduce results by approximately 74 and 38% compared with ordinary DTP for 80- and 160-s scan durations. The new dose regimen formula was found to be DTP = c/t × m 1.24 , where m is the patient weight, t is the scan time per bed position, and c is 1.8 and 4.3 for acceptable and higher confidence states, respectively, in Discovery IQ PET/CT. Conclusion Patient weight is the best clinical parameter for the implementation of 18 F-FDG PET/CT image quality assessment. A new dose-time regimen based on body weight was proposed for use in highly sensitive five-ring BGO PET-CT scanners to significantly reduce the injection dose and scan times while maintaining sufficient image quality for diagnosis.

COMPARISON OF CBCT AND CT IN TERMS OF DOSE VALUE OF ORGANS AT RISK IN PARANASAL SINUS IMAGING.

Irradiated dose to the organs at risk surrounding the paranasal sinuses was compared in cone beam computed tomography (CBCT) and multi spiral computed tomography with respect to the organs' relative positions to the imaging field. A head and neck Alderson-Rando phantom equipped with thermoluminescence dosemeters pellets was irradiated according to three routine CBCT protocols and one protocol in multi spiral computed tomography. Dose value of organs outside the imaging field as well as those measured dose of organs inside the field were assessed. The highest measured doses were obtained from CT scan for most of the organs investigated in this study, whereas the lowest one was associated with the low-resolution mode of CBCT. Also, statistical analysis showed no significant differences between the dose values of out-of-field organs in all CBCT modes, whereas significant differences were observed between the radiation doses of CT and CBCT modes for all organs at risk inside and outside of the imaging field. CBCT is recommended on the basis of having a lower dose; however, the image qualities were the same in the two employed modalities, so the approach of lower dose can be made.

Analysis of induced error by susceptibility effect in low-density gel dosimeters.

PURPOSE: In low-density (LD) gel dosimeter, diffusive spin-spin relaxation rate (R2)-dispersion caused by susceptibility-induced internal gradient leads to a significant deviation in the measured R2 from the real value. In this study, the effect of induced internal gradient on R2 was visualized and quantified algebraically as an important cause of inaccuracy in LD gel dosimeters. MATERIALS AND METHODS: In this method, two sets of LD and unit-density (UD) gel dosimeters were prepared. The LD gel was made by mixing the UD gel with expanded polystyrene spheres. The R2 was used to determine the spatially resolved decay rates due to diffusion in internal magnetic field. The internal gradient was calculated for a multiple spin-echo sequence. RESULTS: It is shown that in a LD gel, the internal gradient leads to overestimation of mean R2 value (R2mean). Pixel-by-pixel R2 measurements inside a LD gel showed significant deviation from R2 mapping in UD gel. CONCLUSION: It appears that significant differences between R2mean in a selected region of interest and pixel-by-pixel R2 values are the main source of inaccuracy in dose mapping of a LD gel.

A novel quantification method for low-density gel dosimeter.

AIM: Low signal-to-noise ratio (SNR) images of lung-like (low-density [LD]) gel dosimeters, compared to unit-density (UD) gels, necessitate the use of different quantification methods. SETTING AND DESIGN: In this study, a new method is introduced based on noise correction and exponential (NCEXP) fitting. The feasibility of NCEXP method for quantifying dose absorption in LD gels is evaluated. MATERIALS AND METHODS: Sensitivity, dose resolution, detectable dynamic range, and correlation of the calibration curve for both UD and LD gel dosimeters are the parameters, which we analyze to investigate the consequences of new method. Results of NCEXP method are compared to maximum likelihood estimation of rician distribution (MLE-R) and variable echo number (VAREC) quantification methods. RESULTS: Dose response of LD gel dosimeter shows wider detectable dynamic range as compared to UD gel. Using NCEXP method for both LD and UD dosimeter gels, a more sensitive calibration curve with a superior dose resolution is obtained. The advantage of new quantification method is more significant for LD dosimeter gel analysis, where SNR decreases as a result of higher absorbed doses (≥10 Gy). Despite the inverse effect of the VAREC method on detectable dose range of UD gel, no specific changes are observed in dynamic dose range of LD gel dosimeter with different quantification methods. The correlations obtained with different methods were approximately of the same order for UD and LD gels. CONCLUSION: NCEXP method seems to be more effective than the MLE-R and VAREC methods for quantification of LD dosimeter gel, especially where high-dose absorption and steep-dose gradients exist such as those in intensity-modulated radiation therapy and stereotactic radiosurgery.

Influencing Factors on Reproducibility and Stability of MRI NIPAM Polymer Gel Dosimeter.

INTRODUCTION: At present, the polymer gel dosimeter is considered to be the best possible dosimeter for measuring 3-dimesional radiation dose distribution in radiotherapy. These gels are normally toxic; therefore, manufacturing, handling and discarding them require special attention. In order to find less toxic recipe, N-isopropyle acrylamide polymer gel (NIPAM) was introduced. In this study, the reproducibility and stability of NIPAM polymer gel dose response together with some influencing factors related to MR imaging were studied. METHODS: The NIPAM gel was prepared according to a method, described by senden et al in 2006. The gels were irradiated approximately 2 h after manufacturing and MR images of the gel were made 24 h after irradiation. The effects of different batches, post-irradiation time and the MRI room temperature on reproducibility and stability of polymer gel dose response were explored by analyzing the NMR response (R2) of the gel. RESULTS: : In a fixed temperature, the response of the gel was found to be stable 24 h after irradiation. The results showed that the dose response of the NIPAM polymer gel is highly reproducible in the same and different batches of chemical. No inhomogeneity was observed for magnetic fields in the specified position of measurements and 5°C fluctuation was recorded for MRI room temperature. CONCLUSION: Fluctuation in MRI room temperature necessitates that stringent attention to be paid to controlling the gel temperature at the time of imaging. The new formulation of polymer gel ensures stability of the gels' spatial resolution and makes it a suitable dosimeter for distant or remote measurements.