Yu-Ching Ni

A three-dimensional registration method for automated fusion of micro PET-CT-SPECT whole-body images

Micro positron emission tomography (PET) and micro single-photon emission computed tomography (SPECT), used for imaging small animals, have become essential tools in developing new pharmaceuticals and can be used, among other things, to test new therapeutic approaches in animal models of human disease, as well as to image gene expression. These imaging techniques can be used noninvasively in both detection and quantification. However, functional images provide little information on the structure of tissues and organs, which makes the localization of lesions difficult. Image fusion techniques can be exploited to map the functional images to structural images, such as X-ray computed tomography (CT), to support target identification and to facilitate the interpretation of PET or SPECT studies. Furthermore, the mapping of two functional images of SPECT and PET on a structural CT image can be beneficial for those in vivo studies that require two biological processes to be monitored simultaneously. This paper proposes an automated method for registering PET, CT, and SPECT images for small animals. A calibration phantom and a holder were used to determine the relationship among three-dimensional fields of view of various modalities. The holder was arranged in fixed positions on the couches of the scanners, and the spatial transformation matrix between the modalities was held unchanged. As long as objects were scanned together with the holder, the predetermined matrix could register the acquired tomograms from different modalities, independently of the imaged objects. In this work, the PET scan was performed by Concordes microPET R4 scanner, and the SPECT and CT data were obtained using the Gamma Medicas X-SPECT/CT system. Fusion studies on phantoms and animals have been successfully performed using this method. For microPET-CT fusion, the maximum registration errors were 0.21 mm /spl plusmn/ 0.14 mm, 0.26 mm /spl plusmn/ 0.14 mm, and 0.45 mm /spl plusmn/ 0.34 mm in the X (right-left), Y (upper lower), and Z (rostral-caudal) directions, respectively; for the microPET-SPECT fusion, they were 0.24 mm /spl plusmn/ 0.14 mm, 0.28 mm /spl plusmn/ 0.15 mm, and 0.54 mm /spl plusmn/ 0.35 mm in the X, Y, and Z directions, respectively. The results indicate that this simple method can be used in routine fusion studies.

Feasibility study of using PEImager scanner for positron emission mammography

The purpose of this work is to study the feasibility of using PEImager scanner for positron emission mammography (PEM). PEM can be performed by using two opposite detectors. The two-detector positron projection imaging has less depth information, because of the limited number of line of responses (LORs). In this work, an iterative back projection algorithm is employed for reconstruction of projection data. Although the number of LORs are limited, the locations and sizes of hot spots in breast phantom still can be determined from the reconstructed images.

Improvements on a patient-specific dose estimation system in nuclear medicine examination

The purpose of this paper is to develop a patient-specific dose estimation system in nuclear medicine examination. A dose deposition routine to store the deposited energy of the photons during their flights was embedded in the widely used SimSET Monte Carlo code and a user-friendly interface for reading PET and CT images was developed. Dose calculated on ORNL phantom was used to validate the accuracy of this system. The ratios of S value for (99m)Tc, (18)F and (131)I computed by this system to those obtained with OLINDA for various organs were ranged from 0.93 to 1.18, which were comparable to that obtained from MCNPX2.6 code (0.88-1.22). Our system developed provides opportunity for tumor dose estimation which cannot be known from the MIRD. The radiation dose can provide useful information in the amount of radioisotopes to be administered in radioimmunotherapy.

Noninvasive measurement of radiopharmaceutical time–activity data using external thermoluminescent dosimeters (TLDs)

In this study, we present a new method for estimating the time-activity data using serial timely measurements of thermoluminescent dosimeters (TLDs). The approach is based on the combination of the measurement of surface dose using TLD and Monte Carlo (MC) simulation to estimate the radiopharmaceutical time-activity data. It involves four steps: (1) identify the source organs and outline their contours in computed tomography images; (2) compute the S values on the body surface for each source organ using a MC code; (3) obtain a serial measurement of the dose with numerous TLDs placed on the body surface; (4) solve the dose-activity equation to generate organ cumulative activity for each period of measurement. The activity of each organ at the time of measurement is simply the cumulative activity divided by the timespan between measurements. The usefulness of this method was studied using a MC simulation based on an Oak Ridge National Laboratory mathematical phantom with 18F-FDG filled in six source organs. Numerous TLDs were placed on different locations of the surface and were repeatedly read and replaced. The time-activity curves (TACs) of all organs were successfully reconstructed. Experiments on a physical phantom were also performed. Preliminary results indicate that it is an effective, robust, and simple method for assessing the TAC. The proposed method holds great potential for a range of applications in areas such as targeted radionuclide therapy, pharmaceutical research, and patient-specific dose estimation.

Scatter fraction performance tests for positron imaging system with dual plane geometry

NEMA NU 2 and IEC 61675–1 standards are widely used for characterizing the performance of PET scanners. These documents specify procedures for acquiring and analyzing data by using standard phantoms and radio-sources. However, some dedicated PET systems, such as breast and prostate PET systems, have non-ring geometric design may not meet these standards. In this work, we proposed certain modifications to the NEMA NU 2 for analyzing the scatter fraction (SF) of a dual-plane positron imaging system reasonably. A projection-map with geometric compensation was performed instead of sinogram to get analyzed profile. Three methods to determine analyzed window (4 × FWHM, 40 mm width, and 14 mm width) were used to estimate SF. The effects of diameter and location of line source were also considered. Three diameters (0.6, 2, 3.2 mm) and nine locations of line source were set for GATE simulation. The equivalent SF of the dual-plane system was calculated as the area-weighted average of nine-position SF values. The results reveal that the presented method here can estimate SF value better, compared with the methods suggested by the standards.

Detection-ability evaluation of the PEImager for positron emission mammography applications

This work is a pilot study of using a dual-head scanner in positron emission mammograph (PEM). A positron emission imager (PEImager) developed at our laboratory was used as a PEM prototype to obtain data. Dual-head projection imaging mode was used in the PEM study. An iterative algebraic reconstruction was employed to reconstruct projection data to obtain tomograms. A cylindrizal phantom filled with water was applied to simulate a breast and five hollow spheres (2 mm-10 mm diameters) filled with F-18 fluoride simulated tumors in the breast phantom. Preliminary data revealed that the locations and sizes of the hot spots in the breast phantom were determined from the reconstructed images. The ability to detect the tumor embedded in the radioactive water was evaluated. At a tumor-to-normal tissue ratio 20:1, a 3 mm tumor was detected; 5 mm and 10 mm tumors could be detected at the ratios of 10:1 and 5:1, respectively.

Application of the Intraoperative Dual Photon Emission Computed Tomography System in Sentinel Lymph Node Detection: A Simulation Study

The sentinel lymph node (SLN) hypothesis is applied as part of the standard procedure for identifying early-stage breast cancer. Thus, an imaging system that can locate SLNs in operating rooms is desired. Many 2-D probe imaging systems and a freehand single-photon emission-computed tomography (fhSPECT) system have been proposed. However, 2-D probe imaging systems are affected by shine-through and shadowing effects. Here, we propose an alternative to 3-D imaging systems, i.e., a dual-photon emission computed tomography (DuPECT) system, which integrates both preoperative and intraoperative information to locate SLNs using cascade isotopes such as Se-75. The system consists of a LaBr3-based probe and planar head, a collimation system, and a coincidence circuit. For each disintegration, the slat and parallel-hole collimator define a plane and a line, respectively, which represent the possible flight paths of each photon. SLNs can be located using the line-plane intersection. Here, the performance is evaluated using Monte Carlo software developed in our laboratory, integrated with SimSET and GATE software. A measurement study indicates that the randoms rate increases with increased initial activities, while the scatter rate is lower than 1.2 count/s for various activities. In a simulated imaging study, four injection sites and two LNs placed at various depths are minimally distinguishable. However, the LNs are clearly identifiable in the absence of injection sites. Our results indicate that the proposed three-dimensional imaging system has the potential to identify injection sites and various SLNs. However, difficulties with low sensitivity for LN detection, especially in the presence of activity from injection sites, and the choice of appropriate radioisotope must be overcome for its clinical usage.

Scatter feature of a positron imager with dual plane geometry

Dedicated positron imagers with dual plane detectors for breast get high scatter fraction (SF) due to 3D data acquisition. In this study, we investigate the contribution of scatter from each breast and body outside of FOV, and its effects on lesion visibility. Beam stoppers (BSs) were designed to estimate the scatter distribution. All the data in this study were simulated using GATE. The FOV of this system is 196.8 mm×98.4 mm and the crystal element is 1.64×1.64×10 mm3 LYSO. A XCAT phantom with F-18-FDG was simulated to obtain the body-scatter contribution to the left/right breast imaging individually. Besides, the breast phantom without body was simulated to obtain the scatter contribution from each breast. To mimic breast, three lesions with 3, 5, 8 mm diameters were placed inside a 100×100×80 mm3 box phantom. Various T/B ratios were simulated for lesion visibility. Three BSs made of lead were placed between the upper detector and the box phantom. The phantom was scanned twice (w/wo BS). The BS blocks partially the recording of the primary events without affecting the scatter and random. From the counts of two scans, we can estimate the scatter distribution. The results of XCAT phantom show that the SF of left (right) breast imaging was 33.3% (33.1%) and 30.2% (29.8%), respectively, with and without the inclusion of body. The SF of box phantom was 36.4% which was higher than XCAT phantom due to the larger volume. At the T/B ratio 3:1 condition, the 3 mm lesion was faintly visible suffered from scatter. The scatter distribution estimated by BS was shown. The results indicate that left and right breast imaging are suffered almost the same scatter effect. The scatter from out of FOV could be ignored. The results of SF values and lesion visibility study show the scatter effect of positron imager with dual plane geometry could not be ignored. The BSs were designed to estimate the scatter distribution, and could be used in scatter correction in the future.

Measurement and analysis of geometric effects on partial iso-centric x-ray tomosynthesis system

X-ray digital tomosynthesis based on plurality sets of projection data, within a restricted angular range, is capable of forming a cross-sectional image of 3D objects and providing acceptable resolution image but with lower radiation dose than computer tomography (CT). We are developing a tomosynthesis system based on an object rotating micro-CT system. This study is to evaluate the changes and to model the behaviors caused by projection angle variation of tomosynthesis geometry. The result proved that source-to-pixel distance plays an important role on image uniformity. Further studies to accomplish a well optimized tomosynthesis acquisition and system models to improve reconstructed image quality are on-going.