When the party continues: Impulsivity and the effect of employment on young adults' post-college alcohol use.

BACKGROUND: The transition from college to work is both an exciting and potentially high risk time for young adults. As students transition from academic settings to full-time employment, they must navigate new social demands, work demands, and adjust their drinking behaviors accordingly. Research has shown that there are both protective factors and risk factors associated with starting a new job when it comes to alcohol use, and individual differences can moderate these factors. METHOD: 1361 students were recruited from 4 geographically diverse universities and followed 1month pre- and 1month post-graduation. Drinking frequency, quantity, consequences, and impulsivity were assessed. RESULTS: Full-time employment was related to increased drinking quantity but not related to changes in other drinking outcomes. However, impulsivity moderated the relationship between employment and drinking. For those reporting higher levels of impulsivity at baseline, full-time employment was associated with an increase in drinking variables (quantity and frequency), whereas drinking was unaffected by full-time employment status among those reporting lower levels of impulsivity. Implications for future research are discussed.

Noise reduction in oncology FDG PET images by iterative reconstruction: a quantitative assessment.

UNLABELLED: Tumor detection depends on the contrast between tumor activity and background activity and on the image noise in these 2 regions. The lower the image noise, the easier the tumor detection. Tumor activity contrast is determined by physiology. Noise, however, is affected by many factors, including the choice of reconstruction algorithm. Previous simulation and phantom measurements indicated that the ordered-subset expectation maximization (OSEM) algorithm may produce less noisy images than does the usual filtered backprojection (FBP) method, at equivalent resolution. To see if this prediction would hold in actual clinical situations, we quantified noise in clinical images reconstructed with both OSEM and FBP. METHODS: Three patients (2 with colon cancer, 1 with breast cancer) were imaged with FDG PET using a "gated replicate" technique that permitted accurate measurement of noise at each pixel. Each static image was acquired as a gated image sequence, using a pulse generator with a 1-s period, yielding 40 replicate images over the 10- to 15-min imaging time. The images were or were not precorrected for attenuation and were reconstructed with both FBP and OSEM at comparable resolution. From these data, images of pixel mean, SD, and signal-to-noise ratio (S/N) could be produced, reflecting only noise caused by the statistical fluctuations in the emission process. RESULTS: Noise did not vary greatly over each FBP image, even when image intensity varied greatly from one region to the next, causing S/N to be worse in low-activity regions than in high-activity regions. In contrast, OSEM had high noise in hot regions and low noise in cold regions. OSEM had a much better S/N than did FBP in cold regions of the image, such as the lungs (in the attenuation-corrected images), where improvements in S/N averaged 160%. Improvements with OSEM were less dramatic in hotter areas such as the liver (averaging 25% improvement in the attenuation-corrected images). In very hot tumors, FBP actually produced higher S/Ns than did OSEM. CONCLUSION: We conclude that OSEM reconstruction can significantly reduce image noise, especially in relatively low-count regions. OSEM reconstruction failed to improve S/N in very hot tumors, in which S/N may already be adequate for tumor detection.

Relation of thallium uptake to morphologic features of chronic ischemic heart disease: evidence for myocardial remodeling in noninfarcted myocardium.

OBJECTIVES: The aim of this study was to investigate the disparity between the extent of myocardial injury as assessed by thallium and the severity of left ventricular (LV) dysfunction in chronic ischemic heart disease. BACKGROUND: Although it is believed that thallium differentiates between viable and nonviable myocardium, in some patients with chronic ischemic heart disease, viable regions by thallium may fail to improve function after revascularization. METHODS: Thirteen transplant candidates with chronic ischemic heart disease (LV ejection fraction = 14 +/- 6% at rest) were studied prospectively with stress-redistribution-reinjection thallium single-photon emission computed tomography. We examined pretransplantation quantitative thallium uptake and post-transplantation extent and the histological distribution of collagen replacement in infarcted and noninfarcted myocardium and in 13 age-matched control hearts. RESULTS: The volume fraction of collagen varied inversely with wall thickness (r = -0.70, p < 0.001) and was higher in irreversible (30.9 +/- 15.8%) compared with reversible (20.2 +/- 12.6%, p < 0.001) or normal thallium segments (15.0 +/- 8.7%, p < 0.001). The irreversible thallium segments had lower wall thickness and more severe coronary artery narrowing (9.7 +/- 2.8 mm and 95 +/- 8%) compared with reversible (11.7 +/- 2.7 mm and 87 +/- 13%, p < 0.001) and normal thallium segments (12.8 +/- 2.6 mm and 80 +/- 14%, p < 0.001). Mean volume fraction of collagen was significantly lower in noninfarcted than it was in infarcted segments (13 +/- 6% vs. 36 +/- 13%, p < 0.001) but exceeded that in the control hearts (4 +/- 2%, p < 0.001). Noninfarcted segments had predominantly interstitial fibrosis with either microscopic or patchy areas of replacement fibrosis. CONCLUSIONS: In chronic ischemic heart disease with severe LV dysfunction, patterns of normal, reversible and irreversible thallium uptake correlated with the magnitude of collagen replacement, segmental wall thickness and severity of coronary artery narrowing. The finding of scattered areas of replacement fibrosis in noninfarcted myocardium may explain the observed disparity between LV contractile dysfunction and the extent of myocardial injury assessed by thallium.

Noninvasive estimation of the aorta input function for measurement of tumor blood flow with.

Quantitative measurement of tumor blood flow with [15O]water can be used to evaluate the effects of tumor treatment over time. Since quantitative flow measurements require an input function, we developed the profile fitting method (PFM) to measure the input function from positron emission tomography images of the aorta. First, a [11C]CO scan was acquired and the aorta region was analyzed. The aorta diameter was determined by fitting the image data with a model that includes scanner resolution, the measured venous blood radioactivity concentration, and the spillover of counts from the background. The diameter was used in subsequent fitting of [15O]water dynamic images to estimate the aorta and background radioactivity concentrations. Phantom experiments were performed to test the model. Image quantification biases (up to 15%) were found for small objects, particularly for those in a large elliptical phantom. However, the bias in the PFM concentration estimates was much smaller (2%-6%). A simulation study showed that PFM had less bias and/or variability in flow parameter estimates than an ROI method. PFM was applied to human [11C]CO and [15O]water dynamic studies with left ventricle input functions used as the gold standard. PFM parameter estimates had higher variability than found in the simulation but with minimal bias. These studies suggest that PFM is a promising technique for the noninvasive measurement of the aorta [15O]water input function.

Device-dependent activity estimation and decay correction of radionuclide mixtures with application to Tc-94m PET studies.

Multi-instrument activity estimation and decay correction techniques were developed for radionuclide mixtures, motivated by the desire for accurate quantitation of Tc-94m positron emission tomography (PET) studies. Tc-94m and byproduct Tc isotopes were produced by proton irradiation of enriched Mo-94 and natural Mo targets. Mixture activities at the end of bombardment were determined with a calibrated high purity germanium detector. The activity fractions of the greatest mixture impurities relative to 100% for Tc-94m averaged 10.0% (Tc-94g) and 3.3% (Tc-93) for enriched targets and 10.1% (Tc-94g), 11.0% (Tc-95), 255.8% (Tc-96m), and 7.2% (Tc-99m) for natural targets. These radioisotopes have different half-lives (e.g., 52.5 min for Tc-94m, 293 min for Tc-94g), positron branching ratios (e.g., 0.72 for Tc-94m, 0.11 for Tc-94g) and gamma ray emissions for themselves and their short-lived, excited Mo daughters. This complicates estimation of injected activity with a dose calibrator, in vivo activity with PET and blood sample activity with a gamma counter. Decay correction using only the Tc-94m half-life overestimates activity and is inadequate. For this reason analytic formulas for activity estimation and decay correction of radionuclide mixtures were developed. Isotope-dependent sensitivity factors for a PET scanner, dose calibrator, and gamma counter were determined using theoretical sensitivity models and fits of experimental decay curves to sums of exponentials with fixed decay rates. For up to 8 h after the end of bombardment with activity from enriched and natural Mo targets, decay-corrected activities were within 3% of the mean for three PET studies of a uniform cylinder, within 3% of the mean for six dose calibrator decay studies, and within 6% of the mean for four gamma counter decay studies. Activity estimation and decay correction for Tc-94m mixtures enable routine use of Tc-94m in quantitative PET, as illustrated by application to a canine Tc-94m sestamibi study.

A prospective study of 2-[18F] fluoro-2-deoxy-D-glucose/positron emission tomography scan, 99mTc-labeled arcitumomab (CEA-scan), and blind second-look laparotomy for detecting colon cancer recurrence in patients with increasing carcinoembryonic antigen levels.

BACKGROUND: An increasing carcinoembryonic antigen (CEA) level in the absence of disease on imaging studies can present a diagnostic challenge. We evaluated 2-[18F] fluoro-2-deoxy-D-glucose and positron emission tomography (FDG-PET) scan and CEA scan before second-look laparotomy as a means of localizing recurrent colorectal cancer. METHODS: Patients underwent computed tomography scan, bone scan, colonoscopy, and magnetic resonance imaging, and those without evidence of disease or resectable disease in the abdomen had FDG-PET and CEA scans. At second-look laparotomy, a surgeon blinded to the results of the FDG-PET and CEA scans performed an exploration and mapped findings. A second surgeon, with knowledge of the FDG-PET and CEA scans, then explored the patient; all lesions were biopsied or resected for pathology. RESULTS: In 28 patients explored, disease was found at operation in 26 (94%). Ten had unresectable disease. FDG-PET scans predicted unresectable disease in 90% of patients. CEA scans failed to predict unresectable disease in any patient. In 16 patients found to have resectable disease or disease that could be treated with regional therapy, FDG-PET scan predicted this in 81% and CEA scan in 13%. CONCLUSIONS: FDG-PET scan can predict those patients who would likely benefit from a laparotomy. If the FDG-PET scan indicates resectable disease, laparotomy can be considered. However, if the findings predict unresectable disease or the absence of disease, the patient should pursue systemic therapy or continued observation.

Measuring tumor blood flow with H(2)(15)O: practical considerations.

The ability to measure blood flow to tumors non-invasively may be of importance in monitoring tumor therapies, assessing drug delivery, and understanding tumor physiology. Of all the radiotracer methods that have been proposed to measure tumor blood flow, the method based on labeled water-H(2)(15)O-may be the most applicable to tumors. It is highly diffusible, does not participate significantly in metabolic processes during the short times involved in the study, and its uptake and clearance can be easily modeled. We present here an analysis of the bolus injection water methodology and how it might best be used to monitor tumor blood flow. Several different formulations of the basic methodology, based on previous applications in the heart and brain, are discussed. Potential problems of adapting these previous methodologies to tumor blood flow are presented.

Parametric images of blood flow in oncology PET studies using [15O]water.

UNLABELLED: PET was used to measure tumor blood flow, which is potentially valuable for diagnosis and assessing the effects of therapy. To help visualize regional differences in blood flow and to improve the accuracy of region-of-interest placement, a parametric imaging approach was developed and compared with the standard region-of-interest method. METHODS: Five patients with renal cell metastases in the thorax were studied using [15O]water and dynamic PET. To assess the reproducibility of the blood flow measurements, multiple water studies were performed on each patient. Model fitting was done on a pixel-by-pixel basis using several different formulations of the standard single-compartment model. RESULTS: The tumors studied spanned a wide range of blood flows, varying from 0.4 to 4.2 mL/min/g. These values were generally high compared with those of most other tissues, which meant that the tumors could be readily identified in parametric images of flow. The different model formulations produced images with different characteristics, and no model was entirely valid throughout the field of view. Although tumor blood flow measured from the parametric images was largely unbiased with respect to a standard regional method, large errors were observed with certain models in regions of low flow. The most robust model throughout the field of view had only 1 free parameter and, compared with a regional method, gave rise to a flow bias of 0.3%+/-3.1% for tumor and 16%+/-11% for low-flow soft tissue (muscle plus fat). With this model, tumor blood flow was measured with an SD of 7.6%+/-4.0%. CONCLUSION: Parametric imaging provides a convenient way of visualizing regional changes in blood flow, which may be valuable in studies of tumor blood flow.

Registration of planar emission images with reprojected CT data.

UNLABELLED: Planar gamma-camera imaging is still widely used clinically. Alignment of planar images with images from tomographic modalities, such as CT, or with other planar images would be desirable. Here, we present and evaluate a method for such an alignment, using planar transmission images acquired with the emission images and reprojection of the 3-dimensional CT data. This method permits determination of which CT slice corresponds to a particular row of pixels in the gamma-camera image and which column of pixels in that CT slice corresponds to a particular pixel in the emission data. METHODS: A method based on maximization of the correlation coefficient, previously used for 3-dimensional datasets, was modified to permit 2-dimensional registrations. Planar transmission measurements were obtained using a collimated 99mTc flood source in conjunction with planar emission studies. The CT data were first reprojected to permit the 2-dimensional registration. The registration method was evaluated for its accuracy and reproducibility. RESULTS: For phantom data, the registration errors were -0.1 +/- 1.0 mm for x-translations, 1.0 +/- 1.3 mm for y-translations, and -0.2 +/- 0.3 degrees for rotations. For patient data, the errors were 1.6 +/- 0.8 mm for x-translations, 1.3 +/- 1.0 mm for y-translations, and 0.5 +/- 0.5 degrees for rotations. An examination of the need for rescaling of the attenuation data (to compensate for the different photon energies used in the respective attenuation measurements) showed no significant impact on registration error. When 5 different regions of interest were used for the correlation coefficient calculation, the mean errors attributable to region-of-interest choice alone were 1.0 mm for x-translations, 2.0 mm for y-translations, and 1.2 degrees for rotations. CONCLUSION: In almost all instances, translational registration errors were kept to subpixel levels (pixel size, 2.6 mm) and rotational errors to 1 degrees or less. The 1 exception was in the easily avoidable case of "pitch" rotations of the patient of 2 degrees or more. The modified registration method provides a simple yet reliable way to provide cross-modality evaluation of planar emission data.

8:45-9:00. Using PET 18F-FDG, 11CO, and 15O-water for Monitoring Prostate Cancer During a Phase II Anti-angiogenic Drug Trial with Thalidomide.

Assessing prostate metastases is difficult with conventional radiographic modalities as few patients have soft tissue involvement and most have only bone lesions. Even with FDG PET, problems due to decreased avidity compared to other tumor types can occur. We assessed PET's ability to monitor changes in such tumors during an anti-angiogenic therapy. We measured changes in tumor blood flow (15O), blood volume (11CO), 18F-FDG uptake and "metabolic volume" before and during thalidomide treatment, to see if these changes correlated with changes in PSA values.Six patients with androgen-independent prostate cancer were imaged with 18F-FDG, 11CO, and 15O water before and during (mean interval 63 days, range 55-76 days) thalidomide therapy (200-1200mg/day). Lesions were visually identified on FDG images (9 bone, 5 soft tissue lesions). VOI's were generated by 3D region growing, with a 50% maximum pixel threshold. These VOI's were registered with, and applied to, the 11CO and water studies. Correlations with PSA values were done using the Spearman rank test.The change in maximum (r = 0.77, p = 0.06) and mean FDG value (r = 0.83, p = 0.03), functional FDG volume (r = 0.66, p = 0.14), and 11-CO blood volume (r = 0.77, p = 0.06) all correlated with the change in PSA. Changes in blood flow values were smaller than the variance of the method for repeated measures, likely due to low flow values in bone.Changes in blood volume measured by 11CO, and the mean and peak activity and functional volume measured by 18F-FDG, correlate with changes in PSA and may be useful in monitoring anti-angiogenic therapy in prostate cancer.

Segmentation of gated Tl-SPECT images and computation of ejection fraction: a different approach.

BACKGROUND: We describe a set of image processing algorithms and mathematical models that can be advantageously used in schemes for the segmentation of thallium-201-single photon emission computed tomography (SPECT) images and for computation of left ventricular ejection fraction (EF). METHODS: The system consists of two independent blocs for image segmentation and computation of function. The former is based on a multiresolution elliptical coordinate transformation and dynamic contour tracking. Computation of EF is formulated on the basis of both the endocardial and epicardial contours, and we compare this formulation with that using only the endocardial border for images with low signal-to-noise ratios. The accuracy of border detection was validated against manual border tracing on FDG-PET images, simulated Tl-201-SPECT images where the true underlying borders were known, and actual Tl-201-SPECT images. Finally, we compared EFs computed for FDG-PET, technetium-99m-SPECT and Tl-201-SPECT with those obtained from planar gated blood pool imaging. RESULTS: The automatically obtained results always were within the manual uncertainty range. Agreement between myocardial volumes from positron emission tomography and automatically obtained values from the simulated Tl-201-SPECT images was excellent (r = 0.95, n = 32). Agreement between EFs from planar gated blood pool imaging and the other image modalities was good (FDG-PET: y = 5.89 + 1.21x, r = 0.92, see = 6.24, n = 19, Tc-99m-SPECT: y = -3.86 + 1.06x, r = 0.88, see = 7.78, n = 9, Tl-201-SPECT: y = 17.8 + 0.81x, r = 0.77, see = 7.44, n = 26). For noisy input data the combined use of information from epicardial and endocardial contours gives more accurate EF values than the traditional formula on the basis of the endocardial contour only. CONCLUSIONS: Alternate approaches for segmentation and computation of function have been presented and validated. They might also be advantageously incorporated into other existing techniques.

Quantitative assessment of myositis in thigh muscles using magnetic resonance imaging.

Magnetic resonance (MR) imaging has been suggested as a technique for diagnosing and monitoring myositis, an inflammatory muscle disease. To date, the assessment of disease from MR images has been by subjective visual analysis. We describe here an objective, semi-automatic, computer-based method for quantifying the degree of disease from MR images, without the need for a radiologist or physician trained in the visual assessment of the MR images. The method is based on analysis of the histogram of intensity values produced from the MR images. The analysis yielded measures of the intensity and extent of disease. These two measures were combined to produce a calculated myositis index (CMI) which described the degree of disease evident from the MR images. This index was compared with a clinical assessment of the patient's condition, based on currently accepted, invasive and non-invasive, non-imaging criteria. Receiver operating characteristic (ROC) curve analysis showed that calculated myositis index agreed at least as well with clinical assessment as did visual analysis (receiver operating characteristic area = 0.93 and 0.94, p = not significant (NS), respectively, for separating remission from disease). Even using only two central MR slices for each patient, the receiver operating characteristic area for calculated myositis index was 0.92, implying that very short acquisition times are possible. We conclude that quantitative histogram analysis of MR images can be successfully performed with minimal operator input and using few MR slices. Agreement with more invasive clinical assessment is good and the method has the advantages of repeatability, objectivity, and decreased scan and analysis time.

13N-ammonia myocardial blood flow and uptake: relation to functional outcome of asynergic regions after revascularization.

OBJECTIVES: In this study we determined whether 13N-ammonia uptake measured late after injection provides additional insight into myocardial viability beyond its value as a myocardial blood flow tracer. BACKGROUND: Myocardial accumulation of 13N-ammonia is dependent on both regional blood flow and metabolic trapping. METHODS: Twenty-six patients with chronic coronary artery disease and left ventricular dysfunction underwent prerevascularization 13N-ammonia and 18F-deoxyglucose (FDG) positron emission tomography, and thallium single-photon emission computed tomography. Pre- and postrevascularization wall-motion abnormalities were assessed using gated cardiac magnetic resonance imaging or gated radionuclide angiography. RESULTS: Wall motion improved in 61 of 107 (57%) initially asynergic regions and remained abnormal in 46 after revascularization. Mean absolute myocardial blood flow was significantly higher in regions that improved compared to regions that did not improve after revascularization (0.63+/-0.27 vs. 0.52+/-0.25 ml/min/g, p < 0.04). Similarly, the magnitude of late 13N-ammonia uptake and FDG uptake was significantly higher in regions that improved (90+/-20% and 94+/-25%, respectively) compared to regions that did not improve after revascularization (67+/-24% and 71+/-25%, p < 0.001 for both, respectively). However, late 13N-ammonia uptake was a significantly better predictor of functional improvement after revascularization (area under the receiver operating characteristic [ROC] curve = 0.79) when compared to absolute blood flow (area under the ROC curve = 0.63, p < 0.05). In addition, there was a linear relationship between late 13N-ammonia uptake and FDG uptake (r = 0.68, p < 0.001) as well as thallium uptake (r = 0.76, p < 0.001) in all asynergic regions. CONCLUSIONS: These data suggest that beyond its value as a perfusion tracer, late 13N-ammonia uptake provides useful information regarding functional recovery after revascularization. The parallel relationship among 13N-ammonia, FDG, and thallium uptake supports the concept that uptake of 13N-ammonia as measured from the late images may provide important insight regarding cell membrane integrity and myocardial viability.

Thallium scintigraphy compared with 18F-fluorodeoxyglucose positron emission tomography for assessing myocardial viability in patients with moderate versus severe left ventricular dysfunction.

Thallium-201 reinjection imaging and positron emission tomography provide concordant information regarding myocardial viability in many patients with coronary artery disease and left ventricular (LV) dysfunction. It is unclear whether this concordance applies to patients with severe, as well as those with moderate, LV dysfunction. We studied 44 patients with chronic coronary artery disease and LV dysfunction, subgrouped on the basis of severity of dysfunction: 23 patients had moderate and 21 had severe dysfunction (ejection fractions 34 +/- 6% and 19 +/- 6%). Patients underwent exercise thallium single-photon emission computed tomography (SPECT) with 3- to 4-hour redistribution and reinjection imaging, as well as positron emission tomography (PET) imaging with 18fluorodeoxyglucose and 15O-water. Data were analyzed quantitatively in aligned transaxial PET and SPECT tomograms. A myocardial region was considered nonviable by PET if 18fluorodeoxyglucose activity was <50% of that in a normal region, associated with proportional reduction in blood flow. Similarly, regions were considered nonviable by thallium if activity was <50% of activity in normal regions on redistribution and reinjection studies. Thallium SPECT and PET data were concordant regarding viability in 98% and 93% of myocardial regions, respectively, in patients with moderate and with severe LV dysfunction. Lower concordance was observed only when regions with severe irreversible thallium perfusion defects on redistribution images were considered in both groups: 86% and 78%, respectively (p <0.01). Thus, thallium SPECT with reinjection yields information regarding regional myocardial viability that is similar to that provided by PET in patients with severe as well as moderate LV dysfunction. However, there is discordance in >20% of regions manifesting severe irreversible thallium defects in patients with severely reduced LV function.

Stress-induced reversible and mild-to-moderate irreversible thallium defects: are they equally accurate for predicting recovery of regional left ventricular function after revascularization?

BACKGROUND: In patients with coronary artery disease, stress-redistribution-reinjection thallium scintigraphy provides important information regarding myocardial ischemia and viability. Although both reversible and mild-to-moderate irreversible thallium defects retain metabolically active, viable myocardium, we hypothesized that stress-induced reversible thallium defects may better differentiate reversible from irreversible regional left ventricular dysfunction after revascularization. METHODS AND RESULTS: Twenty-four patients with chronic coronary artery disease underwent prerevascularization and postrevascularization exercise-redistribution-reinjection thallium single photon emission CT, gated MRI, and radionuclide angiography. After revascularization, mean left ventricular ejection fraction increased from 30+/-9% to 37+/-13% at rest (P<0.001). Before revascularization, abnormal contraction at rest was observed in 56 of 110 reversible and 20 of 37 mild-to-moderate irreversible thallium defects (51% and 54%, respectively). After revascularization, regional contraction improved in 44 of 56 reversible compared with 6 of 20 mild-to-moderate irreversible thallium defects (79% and 30%, respectively; P<0.001). The final thallium content (maximum tracer uptake on redistribution-reinjection images) was significantly higher in regions with reversible defects that improved than in those that did not improve after revascularization (86+/-16% versus 66+/-9%, P<0.001). In contrast, final thallium content was similar in regions with mild-to-moderate irreversible defects that improved and in those that did not improve after revascularization (69+/-9% versus 65+/-10%, P=NS). Furthermore, when asynergic regions were grouped according to the final thallium content, at 60% threshold value, functional recovery was observed in 83% of regions with reversible defects compared with 33% of regions with mild-to-moderate irreversible defects (P<0.001). CONCLUSIONS: These findings suggest that although both reversible and mild-to-moderate irreversible thallium defects after stress retain viable myocardium, the identification of reversible thallium defect on stress in an asynergic region more accurately predicts recovery of function after revascularization. Even at a similar mass of viable myocardial tissue (as reflected by the final thallium content), the presence of inducible ischemia is associated with an increased likelihood of functional recovery.

Noise characteristics of 3-D and 2-D PET images.

We analyzed the noise characteristics of two-dimensional (2-D) and three-dimensional (3-D) images obtained from the GE Advance positron emission tomography (PET) scanner. Three phantoms were used: a uniform 20-cm phantom, a 3-D Hoffman brain phantom, and a chest phantom with heart and lung inserts. Using gated acquisition, we acquired 20 statistically equivalent scans of each phantom in 2-D and 3-D modes at several activity levels. From these data, we calculated pixel normalized standard deviations (NSD's), scaled to phantom mean, across the replicate scans, which allowed us to characterize the radial and axial distributions of pixel noise. We also performed sequential measurements of the phantoms in 2-D and 3-D modes to measure noise (from interpixel standard deviations) as a function of activity. To compensate for the difference in axial slice width between 2-D and 3-D images (due to the septa and reconstruction effects), we developed a smoothing kernel to apply to the 2-D data. After matching the resolution, the ratio of image-derived NSD values (NSD2D/NSD3D)2 averaged throughout the uniform phantom was in good agreement with the noise equivalent count (NEC) ratio (NEC3D/NEC2D). By comparing different phantoms, we showed that the attenuation and emission distributions influence the spatial noise distribution. The estimates of pixel noise for 2-D and 3-D images produced here can be applied in the weighting of PET kinetic data and may be useful in the design of optimal dose and scanning requirements for PET studies. The accuracy of these phantom-based noise formulas should be validated for any given imaging situation, particularly in 3-D, if there is significant activity outside the scanner field of view.