Tetsuro Mizuta

Quantification of Cerebral Blood Flow and Oxygen Metabolism with 3-Dimensional PET and 15O: Validation by Comparison with 2-Dimensional PET

Quantitative PET with 15O provides absolute values for cerebral blood flow (CBF), cerebral blood volume (CBV), cerebral metabolic rate of oxygen (CMRO2), and oxygen extraction fraction (OEF), which are used for assessment of brain pathophysiology. Absolute quantification relies on physically accurate measurement, which, thus far, has been achieved by 2-dimensional PET (2D PET), the current gold standard for measurement of CBF and oxygen metabolism. We investigated whether quantitative 15O study with 3-dimensional PET (3D PET) shows the same degree of accuracy as 2D PET. Methods: 2D PET and 3D PET measurements were obtained on the same day on 8 healthy men (age, 21–24 y). 2D PET was performed using a PET scanner with bismuth germanate (BGO) detectors and a 150-mm axial field of view (FOV). For 3D PET, a 3D-only tomograph with gadolinium oxyorthosilicate (GSO) detectors and a 156-mm axial FOV was used. A hybrid scatter-correction method based on acquisition in the dual-energy window (hybrid dual-energy window [HDE] method) was applied in the 3D PET study. Each PET study included 3 sequential PET scans for C15O, 15O2, and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{H}_{2}^{15}\mathrm{O}\) \end{document} (3-step method). The inhaled (or injected) dose for 3D PET was approximately one fourth of that for 2D PET. Results: In the 2D PET study, average gray matter values (mean ± SD) of CBF, CBV, CMRO2, and OEF were 53 ± 12 (mL/100 mL/min), 3.6 ± 0.3 (mL/100 mL), 3.5 ± 0.5 (mL/100 mL/min), and 0.35 ± 0.06, respectively. In the 3D PET study, scatter correction strongly affected the results. Without scatter correction, average values were 44 ± 6 (mL/100 mL/min), 5.2 ± 0.6 (mL/100 mL), 3.3 ± 0.4 (mL/100 mL/min), and 0.39 ± 0.05, respectively. With the exception of OEF, values differed between 2D PET and 3D PET. However, average gray matter values of scatter-corrected 3D PET were comparable to those of 2D PET: 55 ± 11 (mL/100 mL/min), 3.7 ± 0.5 (mL/100 mL), 3.8 ± 0.7 (mL/100 mL/min), and 0.36 ± 0.06, respectively. Even though the 2 PET scanners with different crystal materials, data acquisition systems, spatial resolution, and attenuation-correction methods were used, the agreement of the results between 2D PET and scatter-corrected 3D PET was excellent. Conclusion: Scatter coincidence is a problem in 3D PET for quantitative 15O study. The combination of both the present PET/CT device and the HDE scatter correction permits quantitative 3D PET with the same degree of accuracy as 2D PET and with a lower radiation dose. The present scanner is also applicable to conventional steady-state 15O gas inhalation if inhaled doses are adjusted appropriately.

Performance evaluation of a high-sensitivity large-aperture small-animal PET scanner: ClairvivoPET.

ObjectiveIn this study, we evaluated the performance of a newly commercialized small-animal positron emission tomography (PET) scanner, ClairvivoPET, which provides significant advantages in spatial resolution, sensitivity, and quantitative accuracy.MethodsThis scanner consists of depth of interaction detector modules with a large axial extent of 151 mm and an external 137Cs source for attenuation correction. Physical performances, resolution, sensitivity, scatter fraction (SF), counting rate including noise equivalent count (NEC) rate, quantitative accuracy versus activity strength, and transmission accuracy, were measured and evaluated. Animal studies were also performed.ResultsTransaxial spatial resolution, measured with a capillary tube, was 1.54 mm at the center and 2.93 mm at a radial offset of 40 mm. The absolute sensitivity was 8.2% at the center, and SFs for mouse-and rat-sized phantoms were 10.7% and 24.2%, respectively. Peak NEC rates for mouse-and rat-sized uniform cylindrical phantoms were 328 kcps at 173 kBq/ml and 119 kcps at 49 kBq/ml, respectively. The quantitative stability of emission counts against activity strength was within 2% over 5 half-lives, ranging from 0.6 MBq to 30 MBq. Transmission measurement based on segmented attenuation correction allowed 6-min and 10-min scans for mouse-and rat-sized cylindrical phantoms, respectively. Rat imaging injected with 18F-NaF resulted in visibility of fine bone structures, and mouse imaging injected with 18F-D-fluoromethyl tyrosine demonstrated the feasibility of using this system to obtain simultaneous time activity curves from separate regions, such as for the heart and tumors.ConclusionsClairvivoPET is well suited to quantitative imaging even with short scan times, and will provide a number of advantages in new drug development and for kinetic measurement in molecular imaging.

Self normalization for continuous 3D whole body emission data in 3D PET

Continuous 3D scanning with a large-aperture PET scanner can provide high sensitivity over most of the axial FOV in whole-body studies. To minimum the artifact depended on the uniformity of the different lines-of-response (LORs) in sinograms, accurate normalizing algorithms will be needed. In this study, we propose self-normalization method in which full correction factors are derived from the emission data itself without using conventional normalize scan of cylinder phantom. In this method, transaxial block profile and crystal efficiency were calculated from the original emission data, and correction factors applied itself. We implemented proposed method to a 5 ring GSO PET scanner which has continuous 3D scan mode and evaluated. To accurate correction factor, components were calculated from dataset which summed in the direction of the bed movement. To investigate the performance, we compared the proposed method with conventional component based normalization using uniform cylinder phantom. And To evaluate clinical performance, we also /sup 18/FDG human studies were performed. In both phantom and human studies, the block profile and crystal efficiencies can be calculated correctly using proposed method. Evaluating percent standard deviation, self-normalization improved transaxial uniformity since it reduced ring artifacts. Especially, the accuracy of transaxial block profiles which influenced easily by some physical phenomena has unproved. Our self-normalize method was accurate enough for continuous 3D scanning with a large aperture PET scanner. The image quality using self-normalization was superior than conventional component normalization. This method contributes to the improvement of system operation , since regular acquisition of cylinder phantom for normalization is not necessary.

Development of a high resolution whole-body DOI PET system

High spatial resolution Positron Emission Tomography (PET) imaging provides functional data about a patients body and provides us with a range of new possibilities, including medicine development, new diagnostic methods, and others. The use of Depth of Interaction (DOI) detectors is one effective technique for the improvement of whole-body PET imaging, as is evident in the fact that most small animal PET images are taken using a DOI system. In this study, we developed a prototype whole-body DOI- equipped PET system and evaluated its performance. This scanners DOI detectors consist of dual-layer (front and rear) 9 x 10 array of GSO/GSO crystals, a light guide and two rectangular double-anode photo multiplier tubes (PMTs). The DOI layers are discriminated according to decay time differences, which are controlled by Ce concentrations. The DOI detectors are arranged in a circular detector ring, with a diameter of 664 mm. In order to avoid parallax errors, coincidental events, including rear layer detection, were addressed to the nearest neighbor LOR front-layer coincidence pair. To evaluate the spatial resolution of this PET system at various radial positions, 22Na point sources at 1, 10, 20, and 25 cm offset from the center were reconstructed, both with and without DOI information. Results showed that spatial resolution degradation at 25 cm improved from 195 % to 154 % as a result of using DOI data. During a visual evaluation using a Derenzo phantom, image reconstruction using DOI data clarified the image and corrected hot rod shapes. The results presented here show that DOI detectors were useful in creating an effective whole-body PET system.

Implementation of noise reduction for PET using hybrid nonlinear wavelet shrinkage method

Positron emission tomography (PET) imaging provides the functional information and precise physiological uptake of radioactivity in a patients body. But the photon level of PET system has fewer level, the shortcoming of PET is low SNR. The quantum noise in emission data can have a significant influence on the statistical uncertainty of PET measurements. The reduction of the quantum noise in images is an important issue. Conventional post-processing is not completely suitable, since it leads to loss of the spatial resolution while reducing noise. In this study, we present a method for denoising the quantum noise in images. Our proposed methods is to use a hybrid wavelet shrinkage processing in 3D sinogram(bin,angle,slice) domain. One is applied to axial(bin,angle) space to reduce detector dependence noise element, the other is applied to 3D space to reduce quantum noise. We optimized three parameters in each wavelet processing. The efficacy of the filtering technique was shown on a derenzo phantom. Finally, we also performed human FDG studies to evaluate clinical performance. As result, we have found that the optimized parameters and reconstructed image without post filtering in proposed study reduce COV while maintaining average and spatial resolution. We therefore conclude that wavelet de-noising technique is an effective tool to process PET images and thus to provide better image quality. We expect that our study can provide benefits to clinical applications of PET images.

Implementation of Histogram Based Soft-tissue Segmentation for Single Spiral Transmission Scanning in Whole Body PET

3D continuous emission and spiral transmission (CEST) scanning provides a high-throughput whole-body PET study by using two dedicated detectors for 3D emission and singles transmission, together with continuous bed movement. To supress the amount of transmission scatter components (TSC) in singles transmission data, the transmission detector was designed to have a short axial extent with a highly collimated 137Cs point source, and to remove the amount of emission contamination (EC) in post-injection scanning, real-time EC correction was implemented. However, transmission images can be still affected by residual EC and TSC, depending on patient size and injected dose. This produces slight variations in attenuation coefficients, depending on the patients axial and radial positions. In this study, we developed a new soft-tissue segmentation (STS) method based on histogram scaling at each axial position of the spiral transmission. Peaks, corresponding to soft-tissue in a histogram of attenuation coefficients, were found at each axial position and the transmission image was scaled using the ratio of soft-tissue histogram peaks to the theoretical water attenuation coefficient. In scaled transmission images, pixel values near soft-tissue peaks were replaced with their theoretical water attenuation coefficients. Quantitative evaluation of the transmission images obtained was performed under various acquisition conditions, both with and without the proposed STS method. Final imaging performance evaluations included quantification of emission images reconstructed using both STS attenuation correction and hybrid scatter correction. Results showed that the proposed STS method for spiral transmission scanning provided quantitative images that contained activity, independent of object size.

Retrospective Respiratory Motion Compensation Under Deep Breathing in Spiral Transmission Scanning of 3D PET

3D continuous emission and spiral transmission (CEST) scanning can minimize respiratory motion artifacts inherent in normal free breathing, due to the averaging of emission and transmission images over multiple breathing cycles during the stay time of axial detector rings. However, under deep breathing, which is synchronized approximately with the rotation speed of the transmission source, mushroom-shaped artifacts are produced occasionally where the diaphragm and lung intersect. In this work, we have developed a retrospective respiratory motion-compensation method applied to transmission data which was acquired using six detector rings under deep breathing. Our scheme uses a hybrid compensation method that is based on the filtering and summation of different time-stamp images (FDT) and the filtering of sharp edges (FSE) in the axial plane. FDT calculates the deviations from the average image produced using six time-frame images. FDT first identifies pixels in six time frames whose deviations are over the predetermined threshold, then replaces those pixel values with the minimum-deviation pixel value. Since the bed moves continuously through these detector rings, the six time-frame images can be obtained by reconstructing the six transmission sinograms acquired by each detector ring. After FDT compensation is applied, FSE detects residual sharp edges in the axial direction and applies median filtering, using widths calculated based on edge sharpness. We evaluated this method using patient data that displayed artifacts at the intersection of the diaphragm and lung. These results show that this proposed method provides a practical means to significantly reduce artifacts without using a respiratory gating device.

A scatter-compensated crystal interference factor in component-based normalization for high-resolution whole-body PET

On a positron emission tomography (PET) scanner consisting of block detectors, coincidence responses to scattered radiation may differ from those to true depending on the crystal pair position within a coincidence block pair. Furthermore, these differences are considered to vary according to the radial position of the coincidence block pair. These conditions create ringing artifacts in the reconstructed image due to the lack of scatter compensation in detector normalization. In component-based normalization, a scatter-compensated crystal interference factor is therefore required in addition to the scatter-compensated block profile and intrinsic crystal efficiencies. In this study, we propose a scatter-compensated component-based normalization scheme using an annulus phantom, which provides true and scattered radiations over a large transaxial field of view, and evaluates the quality of three different-sized phantom images with whole-body PET. The results showed that the proposed normalization method significantly reduces the ringing artifacts in reconstructed images with different scattered/true fractions. The proposed algorithm, which introduced the scatter-compensated crystal interference factor, worked well under different scattered/true ratio conditions and was considered to be a robust, practical normalization method in high-resolution whole-body PET.

Optimization of enhanced energy window on a whole-body DOI PET system

The use of depth of interaction (DOI) detectors is one effective technique to improve the spatial resolution of whole-body PET imaging. We have developed a whole-body PET system with DOI detectors that achieves less than 3 mm (FWHM) uniform spatial resolution, independent of the radial position in the transaxial field of view. On the other hand, improvements in sensitivity and noise equivalent count rate (NECR) were expected by implementing the DOI-dependent extended energy window (DEEW) method, employing a different energy window for each layer. Furthermore, the energy-based selective coincidence (ESC) method reduces multiple coincidences. In this study, we investigated sensitivity and NECR improvements using the DEEW method and the optimized enhanced energy window setting. The DOI detector consists of dual-layer GSO crystals, and the DOI detectors are arranged into a circular detector ring with a diameter of 664 mm. We evaluated the sensitivity and NECR performance based on the NEMA NU2-2001 standard. ESC and DEEW (1st layer: 412-624 keV, 2nd layer: 200-300 keV + 400-624 keV) resulted in better system performance than the conventional method.

[Accuracy of Resolution Recovery in PSF-based Fully-3D PET Image Reconstruction: Simulation and Phantom Study in Multicenter Trial].

PURPOSE Recently, the quality of positron emission tomography (PET) images has rapidly improved using resolution recovery algorithm with point spread function (PSF). The aim of this study was to investigate the accuracy of the resolution recovery algorithm using three different PET systems. METHODS Three PET scanner models, the GE Discovery 600 M (D600M), SIEMENS Biograph mCT (mCT), and SHIMADZU SET-3000GCT/X (3000GCT) were used in this study. The radial dependences of spatial resolution (full width at half maximum: FWHM) were obtained by point source measurements (0.9 mmφ). All PET data were acquired in three-dimensional (3D) mode and reconstructed using the filtered back projection (FBP) , 3D-ordered subsets expectation maximization (3D-OSEM or dynamic row-action maximum likelihood algorithm) , and 3D-OSEM+PSF (PSF) algorithms. Two indicators, aspect ratio (ASR) and resolution recovery ratio (RRR), were calculated from measured FWHMs and compared among the three PET scanners. RESULTS In D600 and 3000GCT, distortions of the radial direction were slightly increased at circumference of field of view (FOV). On the other hand, random distortions were occurred in both radial and tangential direction in mCT. ASRs calculated from 3D-OSEM images at circumference of FOV were 2.06, 1.22, and 2.04 on D600M, mCT, and 3000GCT, respectively. ASR improved with PSF in all PET scanners. On the other hand, RRR with PSF were calculated 57.6%, 61.4%, and 31.6%, respectively. CONCLUSION Our results suggest that the spatial resolutions of PET images could be improved with PSF algorithm in all PET systems; however, effect of PSF was different depending on PET systems. Furthermore, PSF algorithm could not completely improve spatial resolutions in circumference of FOV.