Elsevier

Clinical Positron Imaging

Volume 1, Issue 2, Spring 1998, Pages 135-144
Clinical Positron Imaging

Review
Quantitative Brain PET: Comparison of 2D and 3D Acquisitions on the GE Advance Scanner

https://doi.org/10.1016/S1095-0397(98)00009-0Get rights and content

Abstract

Purpose: Recent developments in the design of positron emission tomography (PET) scanners have made three-dimensional (3D) data acquisition attractive because of significantly higher sensitivity compared to the conventional 2D mode (with lead/tungsten septa extended). However, the increased count rate in 3D mode comes at the cost of increased scatter, randoms, and dead time. Several schemes to correct for these effects have been proposed and validated in phantom studies. In this study, we evaluated the overall improvement afforded by 3D imaging in quantitative human brain PET studies carried out at our institution.

Methods: Subjects were studied using sequential/interleaved 2D and 3D data acquisition with a GE Advance scanner. We calculated regional and global cerebral glucose metabolism with [18F]flourodeoxyglucose (FDG) and estimated rate constants for striatal [18F]fluorodopa (FDOPA) uptake.

Results: FDG: Global mean glucose metabolic rates were in almost complete agreement (within 1%) between the two modes whereas the regional differences ranged from −7.7% to +9% for all cortical structures. However, for small regions (<2 cm2) like caudate nuclei, the maximum difference was 14.7%. FDOPA: A significant improvement in image quality was evident in 3D mode and there was complete agreement between the estimated parameters in the two scanning modes for the same noise equivalent counts: Striatal-to-occipital ratio (SOR) and striatal FDOPA uptake (KiFD) had mean differences of less than 2% and 5%, respectively.

Conclusions: 3D FDG studies can be done with either half the injected dose or half the scan duration to a comparable 2D study. 3D PET imaging has distinct advantages over 2D in the quantitative fluorodopa studies.

Introduction

Dramatic increases in the sensitivity of positron emission tomography (PET) scanners have been achieved by the implementation of 3D data acquisition. On the hardware side, most new PET scanners now have a retractable lead/tungsten septa that can be moved in or out of the field-of-view (FOV).1, 2, 3, 4 On the software side, improved scatter corrections schemes like the dual energy window and model-based techniques have been introduced to reduce the increased scatter component in the 3D PET signal.5, 6, 7, 8

Our objective was to evaluate the quantitative recovery of clinical parameters of interest using the 3D capability of the GE Advance PET scanner to perform the following radiotracer studies in patients: (1) [18F]fluorodeoxyglucose (FDG) studies to estimate regional glucose metabolic rate (rCMRGlc) in normal subjects and patients with neurological disease; and (2) [18F]fluorodopa (FDOPA) studies to asses the integrity of the presynaptic dopaminergic system by estimating striatal radiotracer uptake in parkinsonism and related conditions. The 2D and 3D performance characteristics of the GE Advance scanner have been previously published.9, 10 Using the CTI/Siemens (Knoxville, TN) ECAT-831 Neuro PET scanner in 3D mode, Cherry et al. have demonstrated the improved detection of cerebral blood flow changes.3 Rakshi et al. have published the improvements afforded by 3D implementation for [18F]fluorodopa studies using the ECAT 953B (CTI/Siemens) scanner.11 Quantitative as well as qualitative evaluation of the GE Advance scanner in 3D mode for FDG studies have been recently presented in brief forms.12, 13, 14 We now present the comparative results of 2D and 3D data acquisition modes for our [18F]FDG and [18F]FDOPA studies performed on the GE Advance scanner using the commercially supplied scatter correction.5 Scatter correction in 3D is a very active area of research and new algorithms are being developed and refined. Besides scatter effects, differences arise in 3D and 2D modes due to axial slice width, randoms, dead time, and out-of-field activity. The focus of this paper is to evaluate whether an overall quantitative improvement in brain PET images can be obtained in 3D mode on the GE Advance PET scanner.

Section snippets

Scanner Performance

GE Advance scanner was used for these studies (General Electric, Milwaukee, WI). The characteristics of this tomograph relevant to 2D and 3D are presented in Table 1. Axial field-of-view (FOV): 15.2 cm; Number of detector (bismuth germanate, BGO) rings: 18; Coincidence window width: 12.5 nsec. The scanner includes shielding (32-mm lead) on both front and back of the detector units to prevent detection of events from outside the scan planes. Slice thickness = 4.2 mm; pixel size = 2.3 mm; a 6 mm

Results

3D and 2D images for FDOPA are illustrated in Figure 1. A significant improvement in image quality for the 3D acquisition is evident (3D and 2D data were acquired for equal scan duration).

Fdg

We could not discern any spatial dependency in the rCMRGlc errors. However, the maximum errors were associated with ROIs having the smallest areas like pons and caudate nuclei. These larger errors could arise from the difference in 2D and 3D axial resolution and partial volume effects on small structures (<4 cm3). For most of the gray matter regions, the difference between 2D and 3D rCMRGlc ranged between −5 to +5%. The excellent correlation between 2D and 3D data sets suggests that for FDG/PET

Conclusions

The advantage of 3D PET scanning is governed by the type of study and the clinical or research question that is being posed. For neurotransmitter studies like FDOPA where the injected activity is small and constrained by the radiation dosimetry, 3D can substantially improve the scan data. In case of autoradiographic FDG studies, the injected dose is usually not a limiting criterion except if multiple studies on the same subject are planned. Nonetheless, the total study duration can be shortened

Acknowledgements

Dr. Ken Kazumata is the recipient of Veola S. Kerr Fellowship from the Parkinson’s Disease Foundation.

Dr. David Eidelberg is a Cotzias Fellow of the American Parkinson’s Disease Association.

This work was supported by NIH R01-01001 and by the National Parkinson Foundation.

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