Yusuf E. Erdi

Segmentation of lung lesion volume by adaptive positron emission tomography image thresholding.

It is common protocol in radionuclide therapies to administer a tracer dose of a radiopharmaceutical, determine its lesion uptake and biodistribution by gamma imaging, and then use this information to determine the most effective therapeutic dose. This treatment planning approach can be used to quantitate accurately the activity and volume of lesions and organs with positron emission tomography (PET). In this article, the authors focus on the specification of appropriate volumes of interest (VoI) using PET in association with computed tomography (CT).

Intensity of 18Fluorodeoxyglucose Uptake in Positron Emission Tomography Distinguishes Between Indolent and Aggressive Non-Hodgkin’s Lymphoma

PURPOSE (18)Fluorodeoxyglucose positron emission tomography (FDG PET) is widely used for the staging of lymphoma. We investigated whether the intensity of tumor FDG uptake could differentiate between indolent and aggressive disease. MATERIALS AND METHODS PET studies of 97 patients with non-Hodgkins lymphoma who were untreated or had relapsed and/or persistent disease and had not received treatment within the last 6 months were analyzed, and the highest standardized uptake value (SUV) per study was recorded. Correlations were made with histopathology. RESULTS FDG uptake was lower in indolent than in aggressive lymphoma for patients with new (SUV, 7.0 +/- 3.1 v 19.6 +/- 9.3; P < .01) and relapsed (SUV, 6.3 +/- 2.7 v 18.1 +/- 10.9; P = .04) disease. Despite overlap between indolent and aggressive disease in the low SUV range (indolent, 2.3 to 13.0; aggressive, 3.2 to 43.0), all cases of indolent lymphoma had an SUV <or= 13. A receiver operating characteristic (ROC) analysis demonstrated that the SUV distinguished reasonably well between aggressive and indolent disease (area under ROC curve, 84.7%), and an SUV > 10 excluded indolent lymphoma with a specificity of 81%. With a higher cutoff for the SUV, the specificity would have been higher. CONCLUSION FDG uptake is lower in indolent than in aggressive lymphoma. Patients with NHL and SUV > 10 have a high likelihood for aggressive disease. This information may be helpful if there is discordance between biopsy and clinical behavior.

Effect of respiratory gating on reducing lung motion artifacts in PET imaging of lung cancer

Positron emission tomography (PET) has shown an increase in both sensitivity and specificity over computed tomography (CT) in lung cancer. However, motion artifacts in the 18F fluorodioxydoglucose (FDG) PET images caused by respiration persists to be an important factor in degrading PET image quality and quantification. Motion artifacts lead to two major effects: First, it affects the accuracy of quantitation, producing a reduction of the measured standard uptake value (SUV). Second, the apparent lesion volume is overestimated. Both impact upon the usage of PET images for radiation treatment planning. The first affects the visibility, or contrast, of the lesion. The second results in an increase in the planning target volume, and consequently a greater radiation dose to the normal tissues. One way to compensate for this effect is by applying a multiple-frame capture technique. The PET data are then acquired in synchronization with the respiratory motion. Reduction in smearing due to gating was investigated in both phantoms and patient studies. Phantom studies showed a dependence of the reduction in smearing on the lesion size, the motion amplitude, and the number of bins used for data acquisition. These studies also showed an improvement in the target-to-background ratio, and a more accurate measurement of the SUV. When applied to one patient, respiratory gating showed a 28% reduction in the total lesion volume, and a 56.5% increase in the SUV. This study was conducted as a proof of principle that a gating technique can effectively reduce motion artifacts in PET image acquisition.

Respiratory Motion in Positron Emission Tomography/Computed Tomography: A Review

The development of positron emission tomography/computed tomography (PET/CT) scanners has allowed not only straightforward but also synergistic fusion of anatomical and functional information. Combined PET/CT imaging yields an increased sensitivity and specificity beyond that which either of the 2 modalities possesses separately and therefore provides improved diagnostic accuracy. Because attenuation correction in PET is performed with the use of CT images, with CT used in the localization of disease, accurate spatial registration of PET and CT image sets is required. Correcting for the spatial mismatch caused by respiratory motion represents a particular challenge for the requisite registration accuracy as a result of differences in temporal resolution between the 2 modalities. This review provides a brief summary of the materials, methods, and results involved in multiple investigations of the correction for respiratory motion in PET/CT imaging of the thorax, with the goal of improving image quality and quantitation. Although some schemes use respiratory-phase data selection to exclude motion artifacts, others have adopted sophisticated software techniques. The various image artifacts associated with breathing motion are also described.

Radiotherapy treatment planning for patients with non-small cell lung cancer using positron emission tomography (PET)

PURPOSE Many patients with non-small cell lung cancer (NSCLC) receive external beam radiation therapy as part of their treatment. Three-dimensional conformal radiation therapy (3DCRT) commonly uses computed tomography (CT) to accurately delineate the target lesion and normal tissues. Clinical studies, however, indicate that positron emission tomography (PET) has higher sensitivity than CT in detecting and staging of mediastinal metastases. Imaging with fluoro-2-deoxyglucose (FDG) PET in conjunction with CT, therefore, can improve the accuracy of lesion definition. In this pilot study, we investigated the potential benefits of incorporating PET data into the conventional treatment planning of NSCLC. Case-by-case, we prospectively analyzed planning target volume (PTV) and lung toxicity changes for a cohort of patients. MATERIALS AND METHODS We have included 11 patients in this study. They were immobilized in the treatment position and CT simulation was performed. Following CT simulation, PET scanning was performed in the treatment position using the same body cast that was produced for CT simulation and treatment. The PTV, along with the gross target volume (GTV) and normal organs, was first delineated using the CT data set. The CT and PET transmission images were then registered in the treatment planning system using either manual or automated methods, leading to consequent registration of the CT and emission images. The PTV was then modified using the registered PET emission images. The modified PTV is seen simultaneously on both CT and PET images, allowing the physician to define the PTV utilizing the information from both data sets. Dose-volume histograms (DVHs) for lesion and normal organs were generated using both CT-based and PET+CT-based treatment plans. RESULTS For all patients, there was a change in PTV outline based on CT images versus CT/PET fused images. In seven out of 11 cases, we found an increase in PTV volume (average increase of 19%) to incorporate distant nodal disease. Among these patients, the highest normal-tissue complication probability (NTCP) for lung was 22% with combined PET/CT plan and 21% with CT-only plan. In other four patients PTV was decreased an average of 18%. The reduction of PTV in two of these patients was due to excluding atelectasis and trimming the target volume to avoid delivering higher radiation doses to nearby spinal cord or heart. CONCLUSIONS The incorporation of PET data improves definition of the primary lesion by including positive lymph nodes into the PTV. Thus, the PET data reduces the likelihood of geographic misses and hopefully improves the chance of achieving local control.

Quantitation of respiratory motion during 4D-PET/CT acquisition

We report on the variability of the respiratory motion during 4D-PET/CT acquisition. The respiratory motion for five lung cancer patients was monitored by tracking external markers placed on the abdomen. CT data were acquired over an entire respiratory cycle at each couch position. The x-ray tube status was recorded by the tracking system, for retrospective sorting of the CT data as a function of respiration phase. Each respiratory cycle was sampled in ten equal bins. 4D-PET data were acquired in gated mode, where each breathing cycle was divided into ten 500 ms bins. For both CT and PET acquisition, patients received audio prompting to regularize breathing. The 4D-CT and 4D-PET data were then correlated according to their respiratory phases. The respiratory periods, and average amplitude within each phase bin, acquired in both modality sessions were then analyzed. The average respiratory motion period during 4D-CT was within 18% from that in the 4D-PET sessions. This would reflect up to 1.8% fluctuation in the duration of each 4D-CT bin. This small uncertainty enabled good correlation between CT and PET data, on a phase-to-phase basis. Comparison of the average-amplitude within the respiration trace, between 4D-CT and 4D- PET, on a bin-by-bin basis show a maximum deviation of approximately 15%. This study has proved the feasibility of performing 4D-PET/CT acquisition. Respiratory motion was in most cases consistent between PET and CT sessions, thereby improving both the attenuation correction of PET images, and co-registration of PET and CT images. On the other hand, in two patients, there was an increased partial irregularity in their breathing motion, which would prevent accurately correlating the corresponding PET and CT images.

PET/CT: a new imaging technology in nuclear medicine.

This review discusses the technical background of combined PET and CT and considers the clinical applications of PET/CT imaging. Questions addressed include: Is PET/CT superior to PET imaging alone? If so, in which patient populations and in what respect? Can PET/CT imaging affect patient management? Can PET/CT be practiced economically? While much work remains to be done, the available data clearly suggest that PET/CT decreases imaging time per patient and, even for the experienced reader, significantly reduces the number of equivocal PET interpretations. PET/CT also has the ability to improve accuracy of PET image interpretation and to affect clinical decision making, thereby improving patient management. The nuclear medicine community should approach this new technology with an open mind and focus on its clinical usefulness. The decision regarding whether PET/CT should be part of the equipment in a given nuclear medicine or radiology practice largely depends on the specific patient population. It is concluded that present skepticism concerning combined PET/CT will subside once critics of this new modality have had the opportunity to clearly see on images its many advantages compared with either PET alone or conventional image fusion approaches.

FDG-PET standardized uptake values in normal anatomical structures using iterative reconstruction segmented attenuation correction and filtered back-projection

Filtered back-projection (FBP) is the most commonly used reconstruction method for PET images, which are usually noisy. The iterative reconstruction segmented attenuation correction (IRSAC) algorithm improves image quality without reducing image resolution. The standardized uptake value (SUV) is the most clinically utilized quantitative parameter of [fluorine-18]fluoro-2-deoxy-D-glucose (FDG) accumulation. The objective of this study was to obtain a table of SUVs for several normal anatomical structures from both routinely used FBP and IRSAC reconstructed images and to compare the data obtained with both methods. Twenty whole-body PET scans performed in consecutive patients with proven or suspected non-small cell lung cancer were retrospectively analyzed. Images were processed using both IRSAC and FBP algorithms. Nonquantitative or gaussian filters were used to smooth the transmission scan when using FBP or IRSAC algorithms, respectively. A phantom study was performed to evaluate the effect of different filters on SUV. Maximum and average SUVs (SUVmax and SUVavg) were calculated in 28 normal anatomical structures and in one pathological site. The phantom study showed that the use of a nonquantitative smoothing filter in the transmission scan results in a less accurate quantification and in a 20% underestimation of the actual measurement. Most anatomical structures were identified in all patients using the IRSAC images. On average, SUVavg and SUVmax measured on IRSAC images using a gaussian filter in the transmission scan were respectively 20% and 8% higher than the SUVs calculated from conventional FBP images. Scatterplots of the data values showed an overall strong relationship between IRSAC and FBP SUVs. Individual scatterplots of each site demonstrated a weaker relationship for lower SUVs and for SUVmax than for higher SUVs and SUVavg. A set of reference values was obtained for SUVmax and SUVavg of normal anatomical structures, calculated with both IRSAC and FBP image reconstruction algorithms. The use of IRSAC and a gaussian filter for the transmission scan seems to give more accurate SUVs than are obtained from conventional FBP images using a nonquantitative filter for the transmission scan.

Use of PET to monitor the response of lung cancer to radiation treatment.

Abstract.Approximately 170,000 people arediagnosed with lung cancer in the United States each year. Manyof these patients receive external beam radiation for treatment. Fluorine-18 2-fluoro-2-deoxy-d-glucose positron emission tomography (FDG PET) is increasingly being used in evaluating non-small cell lung cancer and may be of clinical utility in assessing response to treatment. In this report, we present FDG PET images and data from two patients who were followed with a total of eight and seven serial FDG PET scans, respectively, through the entire course of their radiation therapy. Changes in several potential response parameters are shown versus time, including lesion volume (VFDG) by PET, SUVav, SUVmax, and total lesion glycolysis (TLG) during the course of radiotherapy. The response parameters for patient 1 demonstrated a progressive decrease; however, the response parameters for patient 2 showed an initial decrease followed by an increase. The data presented here may suggest that the outcome of radiation therapy can be predicted by PET imaging, but this observation requires a study of additional patients.

Validation of GATE Monte Carlo simulations of the GE Advance/Discovery LS PET scanners

The recently developed GATE (GEANT4 application for tomographic emission) Monte Carlo package, designed to simulate positron emission tomography (PET) and single photon emission computed tomography (SPECT) scanners, provides the ability to model and account for the effects of photon noncollinearity, off-axis detector penetration, detector size and response, positron range, photon scatter, and patient motion on the resolution and quality of PET images. The objective of this study is to validate a model within GATE of the General Electric (GE) Advance/Discovery Light Speed (LS) PET scanner. Our three-dimensional PET simulation model of the scanner consists of 12 096 detectors grouped into blocks, which are grouped into modules as per the vendors specifications. The GATE results are compared to experimental data obtained in accordance with the National Electrical Manufactures Association/Society of Nuclear Medicine (NEMA/SNM), NEMA NU 2-1994, and NEMA NU 2-2001 protocols. The respective phantoms are also accurately modeled thus allowing us to simulate the sensitivity, scatter fraction, count rate performance, and spatial resolution. In-house software was developed to produce and analyze sinograms from the simulated data. With our model of the GE Advance/Discovery LS PET scanner, the ratio of the sensitivities with sources radially offset 0 and 10 cm from the scanners main axis are reproduced to within 1% of measurements. Similarly, the simulated scatter fraction for the NEMA NU 2-2001 phantom agrees to within less than 3% of measured values (the measured scatter fractions are 44.8% and 40.9 +/- 1.4% and the simulated scatter fraction is 43.5 +/- 0.3%). The simulated count rate curves were made to match the experimental curves by using deadtimes as fit parameters. This resulted in deadtime values of 625 and 332 ns at the Block and Coincidence levels, respectively. The experimental peak true count rate of 139.0 kcps and the peak activity concentration of 21.5 kBq/cc were matched by the simulated results to within 0.5% and 0.1% respectively. The simulated count rate curves also resulted in a peak NECR of 35.2 kcps at 10.8 kBq/cc compared to 37.6 kcps at 10.0 kBq/cc from averaged experimental values. The spatial resolution of the simulated scanner matched the experimental results to within 0.2 mm.