The effect of activity outside the field of view on image quality for a 3D LSO-based whole body PET/CT scanner

Whole-body biodistribution and the influence of body activity on brain kinetic analysis of the 11C-PiB PET scan

Dynamic ¹¹C-PiB PET imaging with kinetic analysis has been performed for accurate quantification of amyloid binding in patients with Alzheimer's disease (AD). In this study, we measured the whole-body biodistribution of ¹¹C-PiB in nine subjects. We then evaluated the effect of body activity on quantitative accuracy of brain ¹¹C-PiB three-dimensional (3D) dynamic PET. Based on clinical biodistribution data, we conducted phantom experiments to estimate the effect of body activity on quantification of the brain 3D dynamic ¹¹C-PiB PET data and the error introduced by body activity using six different PET camera models. One of the PET cameras was used to acquire ¹¹C-PiB brain 3D dynamic PET data on a patient with AD. We calculated the distribution volume ratio (DVR) in two kinetic methods using both the original human time-activity-curve (TAC) data and the TAC corrected for the error caused by body activity. In the early phase, both healthy subjects and patients with AD showed a biodistribution of ¹¹C-PiB that reflected regional blood flow. In the simulated early phase of the phantom experiments, activity outside the field of view led to a maximum 6.0% overestimation of brain activity in the vertex region. Conversely, the effect of body activity on the DVR estimate was small (≤1.2%), probably because the tested kinetic methods did not rely heavily on early phase data. These results indicate that the effect of body activity on brain ¹¹C-PiB PET quantification is generally small and that it depends on the method of kinetic analysis, the region of interest, and the PET camera model used.

Effects of a novel tungsten-impregnated rubber neck shield on the quality of cerebral images acquired using 15O-labeled gas

The present study aimed to validate the effects of a novel tungsten-impregnated rubber neck shield on the quality of phantom and clinical ¹⁵O-labeled gas positron emission tomography (PET) images. Images were acquired in the presence or absence of a neck shield from a cylindrical phantom containing [¹⁵O]H₂O (phantom study) and from three individuals using [¹⁵O]CO₂, [¹⁵O]O₂ and [¹⁵O]CO gas (clinical study). Data were acquired in three-dimensional (3D) mode using a Discovery PET/CT 710. Values for cerebral blood flow, cerebral blood volume, oxygen extraction fraction, and cerebral metabolic rate of oxygen with and without the neck shield were calculated from ¹⁵O-labeled gas images. Arterial radioactivity and count characteristics were evaluated in the phantom and clinical studies. The coefficient of variance (CV) for the phantom study and the standard deviation (SD) for functional images were also analyzed. The neck shield decreased the random count rates by 25-59% in the phantom and clinical studies. The noise equivalent count rate (NECR) increased by 44-66% in the phantom and clinical studies. Random count rates and NECR in [¹⁵O]CO₂ images significantly differed with and without the neck shield. The improvement in visual and physical image quality with the neck shield was not observed in the phantom and clinical studies. The novel neck shield reduced random count rate and improved NECR in a 3D PET study using ¹⁵O-labeled gas. The image quality with the neck shield was similar to that without the neck shield.

Evaluation of PET quantification accuracy in vivo. Comparison of measured FDG concentration in the bladder with urine samples

Quantitative positron emission tomography (PET) requires accurate scanner calibration, which is commonly performed using phantoms. It is not clear to what extent this procedure ensures quantitatively correct results in vivo, since certain conditions differ between phantom and patient scans.

AIM

We, therefore, have evaluated the actual quantification accuracy in vivo of PET under clinical routine conditions.

PATIENTS, METHODS

We determined the activity concentration in the bladder in patients undergoing routine [18F]FDG whole body investigations with three different PET scanners (Siemens ECAT EXACT HR+ PET: n = 21; Siemens Biograph 16 PET/CT: n = 16; Philips Gemini-TF PET/CT: n = 19). Urine samples were collected immediately after scan. Activity concentration in the samples was determined in well counters cross-calibrated against the respective scanner. The PET (bladder) to well counter (urine sample) activity concentration ratio was determined.

RESULTS

Activity concentration in the bladder (PET) was systematically lower than in the urine samples (well counter). The patient-averaged PET to well counter ratios for the investigated scanners are (mean ± SEM): 0.881 ± 0.015 (ECAT HR+), 0.898 ± 0.024 (Biograph 16), 0.932 ± 0.024 (Gemini-TF). These values correspond to underestimates by PET of 11.9%, 10.2%, and 6.8%, respectively.

CONCLUSIONS

The investigated PET systems consistently underestimate activity concentration in the bladder. The comparison of urine samples with PET scans of the bladder is a straightforward means for in vivo evaluation of the expectable quantification accuracy. The method might be interesting for multi-center trials, for additional quality assurance in PET and for investigation of PET/MR systems for which clear proof of sufficient quantitative accuracy in vivo is still missing.

Small lesions detectability with the Biograph 16 Hi-Rez PET/CT scanner and fast imaging protocols: performance evaluation using an anthropomorphic thoracic phantom and ROC analyses

OBJECTIVES

The purpose of this study was to evaluate the impact on lesion detectability of fast imaging protocols using 18F-FDG and a 3-dimensional LSO-based PET/CT scanner.

METHODS

An anthropomorphic thoracic phantom was used simulating the anatomical structures of radioactivity distribution for the upper torso of an underweight patient. Irregularly shaped targets of small dimensions, the zeolites, were located inside the phantom in an unpredictable position for the observers. Target-to background ratios and target dimensions were selected in order to sample the range of detectability. Repeated imaging was performed to acquire PET images with varying emission scan duration (ESD) of 1, 2, 3 and 4 min/bed and background activity concentrations of 10, 5 and 3 kBq/mL in the torso cavity. Three observers ranked the targets and a receiver operating characteristic analysis was performed for each acquisition protocol.

RESULTS

Detection performances improved when passing from a short (ESD = 1 min) protocol to longer (ESD C 2 min) protocols. This improvement was established with adequate statistical significance.

CONCLUSIONS

Short image acquisition times of 1 min/bed using 18F-FDG and the specific scanner model considered in the study lead to reduced lesion detectability and should be avoided also in underweight patients.

The effect of activity outside the field-of-view on image signal-to-noise ratio for 3D PET with (15)O

Activity outside the field-of-view (FOV) degrades the count rate performance of 3D PET and consequently reduces signal-to-noise ratios (SNRs) of reconstructed images. The aim of this study was to evaluate a neck-shield installed in a 3D PET scanner for reducing the effect of the outside FOV activity. Specifically, we compared brain PET scans ((15)O(2) and H(2)(15)O) with and without the use of the neck-shield. Image SNRs were directly estimated by a sinogram bootstrap method. The bootstrap analysis showed that the use of the neck-shield improved the SNR by 8% and 19% for H(2)(15)O and (15)O(2), respectively. The SNR improvements were predominantly due to the reduction of the random count rates. Noise equivalent count rate (NECR) analysis provided SNR estimates that were very similar with the bootstrap-based results for H(2)(15)O, but not for (15)O(2). This discrepancy may be due to the fundamental difference between the two methods: the bootstrap method directly calculates the local SNR of reconstructed images, whereas the NECR calculation is based on the whole-gantry count rates, indicating a limitation of the conventional NECR-based method as a tool for assessing the image SNR. Although quantitative parameters, e.g. cerebral blood flow, did not differ when examined with and without the neck-shield, the use of the shield for brain (15)O study is recommended in terms of the image SNR.

Influence of reconstruction settings on the performance of adaptive thresholding algorithms for FDG-PET image segmentation in radiotherapy planning

The purpose of this study was to analyze the behavior of a contouring algorithm for PET images based on adaptive thresholding depending on lesions size and target‐to‐background (TB) ratio under different conditions of image reconstruction parameters. Based on this analysis, the image reconstruction scheme able to maximize the goodness of fit of the thresholding algorithm has been selected. A phantom study employing spherical targets was designed to determine slice‐specific threshold (TS) levels which produce accurate cross‐sectional areas. A wide range of TB ratio was investigated. Multiple regression methods were used to fit the data and to construct algorithms depending both on target cross‐sectional area and TB ratio, using various reconstruction schemes employing a wide range of iteration number and amount of postfiltering Gaussian smoothing. Analysis of covariance was used to test the influence of iteration number and smoothing on threshold determination.

The degree of convergence of ordered‐subset expectation maximization (OSEM) algorithms does not influence TS determination. Among these approaches, the OSEM at two iterations and eight subsets with a 6–8 mm post‐reconstruction Gaussian three‐dimensional filter provided the best fit with a coefficient of determination R2=0.90 for cross‐sectional areas ≤ 133 mm² and R2=0.95 for cross‐sectional areas > 133 mm². The amount of post‐reconstruction smoothing has been directly incorporated in the adaptive thresholding algorithms. The feasibility of the method was tested in two patients with lymph node FDG accumulation and in five patients using the bladder to mimic an anatomical structure of large size and uniform uptake, with satisfactory results.

Slice‐specific adaptive thresholding algorithms look promising as a reproducible method for delineating PET target volumes with good accuracy.

PACS numbers: 87.57.nm, 87.55.D‐, 87.57.uk