Robert A. Mintzer

Design and Performance of a New SPECT Detector for Multimodality Small Animal Imaging Platforms

A new detector for single photon emission computed tomography (SPECT) has been developed for the Siemens microCATreg II and Inveon Multimodality preclinical imaging systems. The detector provides an active imaging area of 15 cm times 15 cm. We review the design of this new SPECT detector and present some key performance characteristics. Integral and differential uniformity were 3.7% and 3.0%, respectively. Mean energy resolution for 99mTc (140 keV) was 12.5%. Sensitivity as high as 1400 cps/MBq was measured for 99mTc on a dual-detector system, and a spatial resolution of 0.7 mm (FWHM) was obtained using 0.5 mm single pinhole collimators. Additionally, we present data from representative preclinical SPECT studies acquired with single and multi-pinhole collimators and multiple isotopes. Reconstructed images demonstrate that this detector is capable of high-resolution SPECT for multimodality small animal imaging.

Modeling of the Point Spread Function by Numerical Calculations in Single-Pinhole and Multipinhole SPECT Reconstruction

In conventional reconstruction of single photon emission computed tomography (SPECT) data acquired with a single-pinhole or multipinhole system, the point spread function (PSF) may be either approximated by some analytical equations or substituted by the sensitivity function, which is the integral of the PSF. We have developed a method to numerically calculate the PSF for a pinhole system in order to improve image resolution over a sensitivity-function-based method. The method calculates the probability of photon penetration through the pinhole edges using a ray-tracing approach. To calculate the transmission by the collimator plate along each ray, we trace the ray through the collimator by analytical calculations. The PSF is calculated for only one detector angle, and a Gaussian rotator is used to rotate the image grid for other detector angles in the iterative reconstruction. To evaluate our method, we measured the sensitivities of four keel-edged single-pinhole plates and scanned an ultramicro Derenzo phantom on a single-pinhole system and a five-pinhole system and performed two mouse bone scans on the five-pinhole system using the 140 keV photons of Tc-99m. The numerical calculations of sensitivities for the single-pinhole plates agreed well with the measurements. Results for both types of data scans showed that modeling of the PSF improved image resolution. In conclusion, we found that modeling of the PSF by numerical calculations increases the resolution of reconstruction for single-pinhole and multipinhole SPECT imaging.

Design and performance of a new pixelated- LSO/PSPMT gamma-ray detector for high resolution PET imaging

The gamma-ray detector developed for the Siemens Inveontrade small animal PET systems affords improved light collection and reduced number of photodetectors, while providing 67% greater axial field of view over the previous Focustrade system design. This is achieved using a tapered, multiple-element lightguide to couple the detectors 1.6 mm pitch, 20 times 20 LSO crystal array (32 mm square) to the 23.5 mm square photosensitive area of a Hamamatsu R8900 C12 PSPMT. The response of 900 production detectors to 511 keV photons has been analyzed. The average peak-to-valley ratio of crystal rows and columns derived from 2-d event position histograms was 4.08 plusmn 0.26 (plusmn1 SD; range of 2.96 to 5.60), and the average detector energy resolution derived from individual crystal spectra was 12.3 plusmn 0.5% FWHM (plusmn1 SD; range of 11.2% to 13.9%). Application of this compact detector in conjunction with the Quicksilvertrade electronics architecture enables flexible system design for high performance PET imaging.

Intraoperative gamma imaging of axillary sentinel lymph nodes in breast cancer patients

Sentinel lymph node (SLN) biopsy is now standard practice in the management of many breast cancer patients. Localization protocols vary in complexity and rates of success. The least complex involve only intraoperative gamma counting of radiotracer uptake or intraoperative visualization of blue-dye uptake; the most complex involve preoperative gamma imaging, intraoperative counting and intraoperative dye visualization. Intraoperative gamma imaging may improve some protocols. This study was conducted to obtain preliminary experience and information regarding intraoperative imaging. Sixteen patients were enrolled: 8 in a protocol that included intraoperative counting and dye visualization (probe/dye), 8 in a protocol that involved intraoperative imaging, counting and dye visualization (camera/probe/dye). Preoperative imaging of all 16 patients was performed using a GE 500 gamma camera with a LEAP collimator (300 cpm/muCi). The results of this imaging were not, however, given to the surgeon until the surgeon had completed the procedures required for the study. A Care Wise C-Trak probe was used for intraoperative counting. A Gamma Medica Inc. GammaCAM/OR (12.5 x 12.5 cm FOV) with a LEHR collimator (135 cpm/muCi) was used for intraoperative imaging. Times from start of surgery to external detection of a radioactive focus and to completion of excision of SLNs were recorded. Foci were detected preoperatively via imaging in 16/16 patients. Intraoperative external detection using the probe was accomplished in less than 4 min (mean = 1.5 min) in 15/16 patients, and via intraoperative imaging in 6/8 patients. The average time for completion of excision of nodes was 19 min for probe/dye and 28 min for camera/probe/dye. In one probe/dye case, review of the preoperative images prompted the surgeon to resume axillary dissection and remove one additional SLN.

High-resolution hand-held gamma camera

A high resolution, hand-held scintillation camera has been designed and built for specific Nuclear Medicine applications. Primary intended applications are pre-surgical and intra-operative lymphoscintigraphy. The detector head is highly compact with a 1-inch by 1-inch physical field of view. A variety of easily interchangeable collimators including parallel hole, diverging hole, and pinhole allow several choices of image parameters including variable spatial resolution, sensitivity and field of view. The camera can be operated in imaging mode or as a probe in a non-imaging mode. Surgeons performing sentinel node surgeries have the option of using the device asa standard audio-guided counting probe or as an imaging device to improve surgical management. The 20 mm FOV camera has 1 mm intrinsic spatial resolution. System FWHM in air is 2.1 mm and 2.6 mm at 0 cm from a high-resolution parallel hole collimator, respectively. FWHM of 3.8 mm is measured 2 cm from a 3 mm pinhole. Pinhole sensitivity is 600 cps/MBq above a 125 cps/MBq background for a 1 cm lesion 1 cm below a water surface. Nodes are identified in images even when overall count rate is not above the background from a nearby injection site.

Continuously sampled digital pulse processing for inveon small animal PET scanner

The quicksilver event processing module (EPM) is a key component of a high performance data acquisition platform from Siemens Molecular Imaging (Knoxville, TN) for use in the Inveontrade line of multimodal PET and SPECT preclinical imaging systems. The cards main purpose is to condition, digitize and process incoming analog pulses from PMT or APD based PET or SPECT detectors. Analog pulses from a detector are digitized using a 100 MHz continuous sampling ADC and read into a Xilinx Virtex II Pro FPGA for processing. The FPGA performs digital integration, baseline offset correction and pileup rejection. Because these functions are done in the digital domain, different algorithms can be quickly re-implemented and tested. The EPM has the ability to capture raw event ADC samples, allowing for the quick development and comparison of new algorithms in software on actual event samples. The Inveontrade small animal PET scanner uses a larger LSO block detector and new analog front end than previous generation scanners which increases the likelihood of pileup events. The digital pulse processing methods presented here have been evaluated to obtain the best energy and positioning performance from the high pixel count Inveontrade detectors while maintaining high stability across countrates.

Gamma cameras for intraoperative localization of sentinel nodes: technical requirements identified through operating room experience

Various nuclear medicine techniques are used for localization of sentinel lymph nodes (SLNs). Procedures that include high-quality preoperative imaging and skilled intraoperative use of a gamma counting probe are almost always successful. Those that involve only the intraoperative use of a gamma probe are generally less successful. For a variety of reasons, high-quality preoperative imaging is not possible at many institutions and thus many institutions use procedures that involve only intraoperative use of a gamma probe. It has been proposed that procedures involving intraoperative imaging be developed and evaluated. To better identify technical requirements for an intraoperative imaging system and protocol, five breast cancer patients were imaged intraoperatively, as well as preoperatively. The intraoperative imaging was performed using a small (127 mm /spl times/ 127 mm) field-of-view (FOV) gamma camera mounted on an articulating arm (Gamma Medica GammaCAM/OR). Intraoperative imaging was performed following administration of anesthesia and following preparation of a sterile surgical field about the involved breast. The camera and arm were draped in a sterile sheath, and the operators of the camera were attired in sterile surgical wear. Intraoperative images were acquired pre-incision and post-excision. Images were acquired for 2 to 3 minutes each. Members of the surgical/nuclear medicine team observed and assessed the ease or difficulty of the acquisitions of images. Conclusions included that a camera for SLN localization should exhibit low noise, should have very good shielding from all non-imaging directions, should have very low collimator penetration, and should have very good sensitivity at 140 keV. The system should have tools for flexible display windowing, convenient region-of-interest definition, and rapid image analysis. These features should be readily available and be easily controlled by the individual positioning the camera. The FOV should be at least 127 mm /spl times/ 127 mm but probably no larger than 200 mm /spl times/ 200 mm. A system should also have a means by which its camera can be easily repeatably positioned.

Implementations of Maximum-Likelihood Position Estimation in a Four-PMT Scintillation Detector

Maximum-likelihood (ML) position estimation in small scintillation detectors often involves the use of a look-up table (LUT) to map event characterization vectors to the associated position estimates. As the number of possible characterization vectors determines the size of the LUT, the number of these vectors needs to be manageable. This implies that a critical component of an implementation of ML position estimation is the mapping of detector output signals to characterization vectors. The use of different photomultiplier tube (PMT) output mappings in ML estimation in a small camera with four round PMTs and a single 5 x 100 x 100 mm NaI(T1) crystal was investigated. The employed mappings of the four detector signals resulted in 20 (5 bits/PMT), 22 (8 bits each for Anger x and y; 6 bits for the sum signal), 24 (6 bits/PMT), and 28 (7 bits/PMT) bit event characterizations. Using present implementations of ML estimation, significant improvement in image quality results if a 24-bit characterization is used rather than 20 or 22. Improvement in image quality is not as marked when 28-bit characterization is used instead of 24, but measurable improvement is obtained.

A data acquisition and Event Processing Module for small animal SPECT imaging

The Quicksilver event processing module (EPM) designed for the Siemens Inveon Dedicated PET (Positron Emission Tomography) preclinical scanner, has been leveraged in the development of a new SPECT (Single Photon Emission Computed Tomography) EPM module. This new module is used in Inveon SPECT systems and provides 16 channels of high speed data acquisition and event processing functions. Fifteen of these channels are used for processing 7 X signals, 7 Y signals and a SUM signal all received from the SPECT detector electronics. Custom mixed-signal CMOS ASICs and high speed ADCs are utilized to provide the front-end analog portion of the data acquisition. A high performance FPGA provides the digital portion of the data acquisition running at 100 MHz and the subsequent event processing. This FPGA also provides multiple high speed serial data channels for external interconnection. These interconnections allow the module to be replicated as needed in a distributed parallel processing architecture for flexible, high performance SPECT imaging. The module also provides controllable high voltage needed to bias the SPECT detectors. A four head Nal(Tl) based SPECT system is currently being built using one SPECT EPM per SPECT head.

CHAPTER 7 – Single-Photon Emission Computed Tomography

This chapter explains the fundamentals of imaging systems, gamma cameras, data acquisition in Single-photon emission computed tomography (SPECT) systems, and their common clinical applications. There are two common forms of single-photon emission imaging namely, planar and tomographic. A planar image depicts a single view of a radiotracer distribution in a patient while a tomographic image is a slice or volume image of the radiotracer distribution computed from multiple images acquired from multiple camera positions. Both imaging methods are used routinely in nuclear medicine clinics, and both use a gamma camera to collect the data. To illustrate basic uses of gamma cameras and provide examples of planar images, this chapter discusses three of the most commonly performed planar imaging studies. This includes thyroid studies, ventilation/perfusion studies, and whole-body bone studies. Though gamma camera and SPECT-system emission technologies are relatively mature, and many standard clinical SPECT protocols exist for the management of patients with various diseases and ailments, advancement of emission detection and imaging technology continues.