Dershan Luo

Attenuation correction of PET cardiac data with low-dose average CT in PET/CT

We proposed a low-dose average computer tomography (ACT) for attenuation correction (AC) of the PET cardiac data in PET/CT. The ACT was obtained from a cine CT scan of over one breath cycle per couch position while the patient was free breathing. We applied this technique on four patients who underwent tumor imaging with F18-FDG in PET/CT, whose PET data showed high uptake of F18-FDG in the heart and whose CT and PET data had misregistration. All four patients did not have known myocardiac infarction or ischemia. The patients were injected with 555-740MBq of F18-FDG and scanned 1h after injection. The helical CT (HCT) data were acquired in 16s for the coverage of 100cm. The PET acquisition was 3min per bed of 15cm. The duration of cine CT acquisition per 2cm was 5.9s. We used a fast gantry rotation cycle time of 0.5s to minimize motion induced reconstruction artifacts in the cine CT images, which were averaged to become the ACT images for AC of the PET data. The radiation dose was about 5mGy for 5.9s cine duration. The selection of 5.9s was based on our analysis of the respiratory signals of 600 patients; 87% of the patients had average breath cycles of less than 6s and 90% had standard deviations of less than 1s in the period of breath cycle. In all four patient studies, registrations between the CT and the PET data were improved. An increase of average uptake in the anterior and the lateral walls up to 48% and a decrease of average uptake in the septal and the inferior walls up to 16% with ACT were observed. We also compared ACT and conventional slow scan CT (SSCT) of 4s duration in one patient study and found ACT was better than SSCT in depicting average respiratory motion and the SSCT images showed motion-induced reconstruction artifacts. In conclusion, low-dose ACT improved registration of the CT and the PET data in the heart region in our study of four patients. ACT was superior than SSCT for depicting average respiration motion in a patient study.

Improvement of the cine‐CT based 4D‐CT imaging

An improved 4D-CT utility has been developed on the GE LightSpeed multislice CT (MSCT) and Discovery PET/CT scanners, which have the cine CT scan capability. Two new features have been added in this 4D-CT over the commercial Advantage 4D-CT from GE. One feature was a new tool for disabling parts of the respiratory signal with irregular respiration and improving the accuracy of phase determination for the respiratory signal from the Varian real-time positioning and monitoring (RPM) system before sorting of the cine CT images into the 4D-CT images. The second feature was to allow generation of the maximum-intensity-projection (MIP), average (AVG) and minimum-intensity-projection (mip) CT images from the cine CT images without a respiratory signal. The implementation enables the assessment of tumor motion in treatment planning with the MIP, AVG, and mip CT images on the GE MSCT and PET/CT scanners without the RPM and the Advantage 4D-CT with a GE Advantage windows workstation. Several clinical examples are included to illustrate this new application.

Relation of external surface to internal tumor motion studied with cine CT

The accuracy of delivering gated-radiation therapy to lung tumors using an external respiratory surrogate relies on not only interfractional and intrafractional reproducibility, but also a strong correlation between external motion and internal tumor motion. The purpose of this work was to use the cine images acquired by four-dimensional computed tomography acquisition protocol to study the relation between external surface motion and internal tumor motion. The respiratory phase information of tumor motion and chest wall motion was measured on the cine images using a proposed region-of-interest (ROI) method and compared to measurement of an external respiratory monitoring device. On eight lung patient data sets, the phase shifts were measured between (1) the signal of a real-time positioning-management (RPM) respiratory monitoring device placed in the abdominal region and four surface locations on the chest wall, (2) the RPM signal in the abdominal region and tumor motions, and (3) chest wall surface motions and tumor motions. Respiratory waveforms measured at different surface locations during the same respiratory cycle often varied and had significant phase shifts. Seven of the 8 patients showed the abdominal motion leading chest wall motion. The best correlation (smallest phase shift) was found between the abdominal motion and the superior-inferior (S-I) tumor motion. A wide range of phase shifts was observed between external surface motion and tumor anterior-posterior (A-P)/lateral motion. The result supported the placement of the RPM block in the abdominal region and suggested that during a gated therapy utilizing the RPM system, it is necessary to place the RPM block at the same location as it is during treatment simulation in order to reduce potential errors introduced by the position of the RPM block. Correlations between external motions and lateral/A-P tumor motions were inconclusive due to a combination of patient selection and the limitation of the ROI method.

Predictors of survival in contemporary practice after initial radiosurgery for brain metastases

PURPOSE The number of brain metastases (BM) is a major consideration in determining patient eligibility for stereotactic radiosurgery (SRS), but the evidence for this popular practice is equivocal. The purpose of this study was to determine whether, following multivariate adjustment, the number and volume of BM held prognostic significance in a cohort of patients initially treated with SRS alone. METHODS AND MATERIALS A total of 251 patients with primary malignancies, including non-small cell lung cancer (34%), melanoma (30%), and breast carcinoma (16%), underwent SRS for initial treatment of BM. SRS was used as the sole management (62% of patients) or was combined with salvage treatment with SRS (22%), whole-brain radiation therapy (WBRT; 13%), or resection (3%). Median follow-up time was 9.4 months. Survival was determined using the Kaplan-Meier method. Cox regression was used to assess the effects of patient factors on distant brain failure (DBF), local control (LC), and overall survival (OS). RESULTS LC at 1 year was 94.6%, and median time to DBF was 10 months. Median OS was 11.1 months. On multivariate analysis, statistically significant predictors of OS were presence of extracranial disease (hazard ratio [HR], 4.2, P<.001), total tumor volume greater than 2 cm(3) (HR, 1.98; P<.001), age ≥60 years (HR, 1.67; P=.002), and diagnosis-specific graded prognostic assessment (HR, 0.71; P<.001). The presence of extracranial disease was a statistically significant predictor of DBF (HR, 2.15), and tumor volume was predictive of LC (HR, 4.56 for total volume >2 cm(3)). The number of BM was not predictive of DBF, LC, or OS. CONCLUSIONS The number of BM is not a strong predictor for clinical outcomes following initial SRS for newly diagnosed BM. Other factors including total treatment volume and systemic disease status are better determinants of outcome and may facilitate appropriate use of SRS or WBRT.

Radiation Pneumonitis: Correlation of Toxicity With Pulmonary Metabolic Radiation Response

PURPOSE To characterize the relationship between radiation pneumonitis (RP) clinical symptoms and pulmonary metabolic activity on post-treatment [(18)F]-fluorodeoxyglucose positron emission tomography (FDG-PET). PATIENTS AND METHODS We retrospectively studied 101 esophageal cancer patients who underwent restaging FDG-PET/computed tomography imaging 3-12 weeks after completing thoracic radiotherapy. The National Institutes of Health Common Toxicity Criteria, version 3, was used to score the RP clinical symptoms. Linear regression was applied to the FDG-PET/computed tomography images to determine the normalized FDG uptake vs. radiation dose. The pulmonary metabolic radiation response (PMRR) was quantified as this slope. Modeling was performed to determine the interaction of PMRR, mean lung dose (MLD), and the percentage of lung receiving >20 Gy with RP outcomes. RESULTS Of the 101 patients, 25 had Grade 0, 10 had Grade 1, 60 had Grade 2, 5 had Grade 3, and 1 had Grade 5 RP symptoms. Logistic regression analysis demonstrated that increased values of both MLD and PMRR were associated with a greater probability of RP clinical symptoms (p = 0.032 and p = 0.033, respectively). Spearmans rank correlation found no association between the PMRR and the dosimetric parameters (planning target volume, MLD, percentage of lung receiving >5-30 Gy). Twofold cross-validation demonstrated that the combination of MLD and PMRR was superior to either alone for assessing the development of clinical RP symptoms. The combined MLD (or percentage of lung receiving >20 Gy) and PMRR had a greater sensitivity and accuracy (53.3% and 62.5%, respectively) than either alone. CONCLUSION The results of this study have demonstrated a significant correlation between RP clinical symptoms and the PMRR measured by FDG-PET/computed tomography after thoracic radiotherapy.

Design of respiration averaged CT for attenuation correction of the PET data from PET/CT

Our previous patient studies have shown that the use of respiration averaged computed tomography (ACT) for attenuation correction of the positron emission tomography (PET) data from PET/CT reduces the potential misalignment in the thorax region by matching the temporal resolution of the CT to that of the PET. In the present work, we investigated other approaches of acquiring ACT in order to reduce the CT dose and to improve the ease of clinical implementation. Four-dimensional CT (4DCT) data sets for ten patients (17 lung/esophageal tumors) were acquired in the thoracic region immediately after the routine PET/CT scan. For each patient, multiple sets of ACTs were generated based on both phase image averaging (phase approach) and fixed cine duration image averaging (cine approach). In the phase approach, the ACTs were calculated from CT images corresponding to the significant phases of the respiratory cycle: ACT(050phs) from end-inspiration (0%) and end-expiration (50%), ACT(2070phs) from mid-inspiration (20%) and mid-expiration (70%), ACT(4phs) from 0%, 20%, 50% and 70%, and ACT(10phs) from all ten phases, which was the original approach. In the cine approach, which does not require 4DCT, the ACTs were calculated based on the cine images from cine durations of 1 to 6 s at 1 s increments. PET emission data for each patient were attenuation corrected with each of the above mentioned ACTs and the tumor maximum standard uptake value (SUVmax), average SUV (SUVavg), and tumor volume measurements were compared. Percent differences were calculated between PET data corrected with various ACTs and that corrected with ACT(10phs). In the phase approach, the ACT(10phs) can be approximated by the ACT(4phs) to within a mean percent difference of 2% in SUV and tumor volume measurements. In cine approach, ACT(10phs) can be approximated to within a mean percent difference of 3% by ACTs computed from cine durations > or =3 s. Acquiring CT images only at the four significant phases for the ACT can reduce radiation dose to 1/3 of the current 4DCT dose; however, the implementation of this approach requires additional hardware that is not standard equipment on PET/CT scanners. In the cine approach, we recommend a duration of 6 +/- 1 s in order to include variations of respiratory patterns in a larger population. This approach can be easily implemented because cine acquisition mode is available on all GE PET/CT scanners. The CT dose in the cine approach can be reduced to approximately 5 mGy by using the lowest mA setting (10 mA), while still maintaining good quality CT data for PET attenuation correction. In our scanning protocol, the ACT is only acquired if respiration-induced misregistration is observed (determined before the PET scan is completed), and therefore patients do not receive unnecessary CT radiation dose.

The clinical impact of the couch top and rails on IMRT and arc therapy

The clinical impact of the Varian Exact Couch on dose, volume coverage to targets and critical structures, and tumor control probability (TCP) has not been described. Thus, we examined their effects on IMRT and arc therapy. Five clinical prostate patients were planned with both 6 MV eight-field IMRT and 6 MV two-arc RapidArc techniques using the Eclipse treatment planning system. These plans neglected treatment couch attenuation, as is a common clinical practice. Dose distributions were then recalculated in Eclipse with the inclusion of the Varian Exact Couch (imaging couch top) and the rails in varying configurations. The changes in dose and coverage were evaluated using the dose-volume histograms from each plan iteration. We used a TCP model to calculate losses in tumor control resulting from not accounting for the couch top and rails. We also verified dose measurements in a phantom. Failure to account for the treatment couch and rails resulted in clinically unacceptable dose and volume coverage losses to the targets for both IMRT and RapidArc. The couch caused average prescription dose losses (relative to plans that ignored the couch) to the prostate of 4.2% and 2.0% for IMRT with the rails out and in, respectively, and 3.2% and 2.9% for RapidArc with the rails out and in, respectively. On average, the percentage of the target covered by the prescribed dose dropped to 35% and 84% for IMRT (rails out and in, respectively) and to 18% and 17% for RapidArc (rails out and in, respectively). The TCP was also reduced by as much as 10.5% (6.3% on average). Dose and volume coverage losses for IMRT plans were primarily due to the rails, while the imaging couch top contributed most to losses for RapidArc. Both the couch top and rails contribute to dose and coverage losses that can render plans clinically unacceptable. A follow-up study we performed found that the less attenuating unipanel mesh couch top available with the Varian Exact couch does not cause a clinically impactful loss of dose or coverage for IMRT but still causes an unacceptable loss for RapidArc. Therefore, both the imaging or mesh couch top and the rails should be accounted for in arc therapy. The imaging couch top should be accounted for in IMRT treatment planning or the mesh top can be used, which would not need to be accounted for, and the rails should be moved to avoid the beams during treatment.

Quantitative assessment of four‐dimensional computed tomography image acquisition quality

The purpose of the present work was to describe the development and validation of a series of tests to assess the quality of four‐dimensional (4D) computed‐tomography (CT) imaging as it is applied to radiation treatment planning. Using a commercial respiratory motion phantom and a programmable moving platform with a CT phantom, we acquired 4D CT datasets on two commercial multislice helical CT scanners that use different approaches to 4D CT image reconstruction. Datasets were obtained as the platform moved in various patterns designed to simulate breathing. Known inserts in the phantom were contoured, and statistics were generated to evaluate properties important to radiation therapy—namely, accuracy of phase‐binning, shape, volume, and CT number. Phase‐binning accuracy varied by as much as 5% for a 4D procedure in which images were reconstructed and then binned, but exhibited no variation for a 4D procedure in which projections were binned before reconstruction. The magnitude of geometric distortion was found to be small for both approaches, as was the magnitude of volume error. Partial‐volume effects in the direction perpendicular to the transverse planes of reconstruction affected volume accuracy, however. Computed tomography numbers were reproduced accurately, but 4D images exhibited more variation in CT number than static CT images did. Characterization of such properties can be used to better understand and optimize the various parameters that affect 4D CT image quality. PACS numbers: 87.53.‐j, 87.59.Fm

Effects of Respiration-Averaged Computed Tomography on Positron Emission Tomography/Computed Tomography Quantification and its Potential Impact on Gross Tumor Volume Delineation

PURPOSE Patient respiratory motion can cause image artifacts in positron emission tomography (PET) from PET/computed tomography (CT) and change the quantification of PET for thoracic patients. In this study, respiration-averaged CT (ACT) was used to remove the artifacts, and the changes in standardized uptake value (SUV) and gross tumor volume (GTV) were quantified. METHODS AND MATERIALS We incorporated the ACT acquisition in a PET/CT session for 216 lung patients, generating two PET/CT data sets for each patient. The first data set (PET(HCT)/HCT) contained the clinical PET/CT in which PET was attenuation corrected with a helical CT (HCT). The second data set (PET(ACT)/ACT) contained the PET/CT in which PET was corrected with ACT. We quantified the differences between the two datasets in image alignment, maximum SUV (SUV(max)), and GTV contours. RESULTS Of the patients, 68% demonstrated respiratory artifacts in the PET(HCT), and for all patients the artifact was removed or reduced in the corresponding PET(ACT). The impact of respiration artifact was the worst for lesions less than 50 cm(3) and located below the dome of the diaphragm. For lesions in this group, the mean SUV(max) difference, GTV volume change, shift in GTV centroid location, and concordance index were 21%, 154%, 2.4 mm, and 0.61, respectively. CONCLUSION This study benchmarked the differences between the PET data with and without artifacts. It is important to pay attention to the potential existence of these artifacts during GTV contouring, as such artifacts may increase the uncertainties in the lesion volume and the centroid location.

Reducing the effect of non-stationary resolution on activity quantitation with the frequency distance relationship in SPECT

The determination of quantitative parameters such as the maximum and total counts in a volume are influenced by the partial volume effect. The magnitude of this effect varies with the non-stationary spatial resolution inherent in SPECT imaging compared to the size and shape of the object, and the relative concentration of the object to its background. The objective of this investigation was to determine if the FDR (Frequency Distance Relationship) restoration filtering can reduce the impact of distance dependent spatial resolution on the quantitation of activity. An analytical projector which incorporates attenuation and distance dependent blurring was used to simulate small hot spherical objects in an cylindrical attenuator as imaged with a LEUHR collimator. FDR restoration filtering regularized using different Gaussian and parametric Wiener filters, was employed after attenuation correction with Bellinis method. Projections were also processed using Bellinis attenuation correction method followed by filtered backprojection and 3D Butterworth filtering with different cut-off frequencies. CCRs (Center Count Ratios) and TCRs (Total Count Ratios) were determined as the observed counts over true counts. Results show that after FDR restoration the CCR and TCR become approximately position invariant. However, when regularizing the FDR inverse filter with a Gaussian function, CCRs become highly unstable as the standard deviation (/spl sigma/) decreased below that at the center of rotation. The use of the Gaussian and parametric Wiener filters to regularize FDR filtering introduce noise in the CCRs but improve recovery of TCRs over that of the center of rotation.