Animal PET Imaging Research Articles

Position Sensitive APDs for Small Animal PET Imaging

In this paper, investigation of position sensitive avalanche photodiodes (PSAPDs) as optical detectors for reading out segmented scintillation arrays of LSO in high resolution PET modules is reported. PSAPDs with 8/spl times/8 mm/sup 2/ and 14/spl times/14 mm/sup 2/ area have been characterized with single LSO crystals and arrays. Energy resolution of 19% (FWHM) for 511 keV /spl gamma/-rays and coincidence timing resolution of /spl sim/3 ns (FWHM) have been recorded with PSAPD coupled to 1/spl times/1/spl times/20 mm/sup 3/ LSO detectors. Flood histogram studies have been successfully conducted by coupling multi-element element LSO arrays (1 mm pixels, 20 mm tall) to the PSAPDs. Finally, depth of interaction (DOI) resolution of <4.5 mm (FWHM) has been measured by coupling two PSAPDs on opposite ends of a 20 mm long LSO crystal with a 1/spl times/1 mm/sup 2/ cross section. Based on these results, PSAPDs appear to be promising for high resolution PET. An important advantage of these PSAPDs is significant reduction in electronic readout requirements.

Po-topic III-05: Techniques for determining the blood input function in small animal pet imaging

Obtaining the blood input function of radiotracers is an important parameter in examining rodent disease models with small animal PET imaging. The conventional clinical technique involves withdrawing multiple blood samples from the subject. However, this can be particularly difficult when dealing with laboratory mice that have small total blood volumes. In our institution, we have used inhaled anesthetics, physiologic monitoring, microsurgical techniques, and special restraining devices for microPET imaging of diseased mice. However, accurate blood input function measurement remains a challenge. We have investigated three independent techniques (1) blood sampling, (2) image value extraction and (3) beta-detector probe, to measure the true blood input function in mice. The advantages and limitations of these individual methods are described here. (1) Blood sampling followed by gamma counting is the most accurate way of measuring the exact radiotracer activity concentration in blood. It requires sterile microsurgery for arterial and venous catheter placement in the subject. Micro-renathane tubing is placed into the left common carotid artery before PET imaging. Rapid arterial blood sampling is used to generate time activity curves, concurrent with arterial blood sampling for labeled metabolite correction. The disadvantages of this method include the loss of blood volume (limiting its use in the same animal for multiple-tracer studies), and the limited time resolution. Nevertheless, this method is considered the gold standard to which the results from other methods are compared. (2) Image value extraction is based on averaging the pixel value in the region of interest (ROI) drawn from dynamic microPET images around the blood pool in the mouse heart. This method requires the least amount of effort in animal handling and provides similar time resolution compared with the blood sampling. The drawback is its quantitative accuracy due to limited image resolution and motion artifact. Typically, the extracted blood input function is under-estimated at early frames and over-estimated in late frames due to the partial volume effect and the cross-contamination of blood pool and myocardium activities. Techniques such as partial volume correction and factor analysis provide limited corrections for these errors. (3) Beta Microprobe (Biospace, Paris, France), a plastic, scintillator-based detector, provides a direct measurement of the beta emitter concentration in real-time. We have acquired a complete system and evaluated five different types of probes with sizes ranging from 0.25 mm to 0.5 mm in diameter. The initial results indicated insufficient detector sensitivity for the level of activity concentration used in typical mouse imaging. Methods to improve the detector sensitivity were further investigated. For example, improved light collection efficiency of the plastic scintillator was found to improve the overall sensitivity 2–3 fold. We anticipate obtaining the blood input function with sufficient statistics using the Beta Microprobe and we will compare its results with the blood sampling method.

Czech Republic & Germany: Constab — plastics masterbatch

In this paper, we present the design and characteristics of a novel low-noise front-end readout application-specific integrated circuit dedicated to CdZnTe (CZT) detectors for a small animal PET imaging system. A low-noise readout method based on the charge integration and the delayed peak detection is proposed. An eight-channel front-end readout prototype chip is designed and implemented in a 0.35 μm CMOS process. The die size is 2.3 mm ×2.3 mm. The prototype chip is tested in different methods including electronic test, energy spectrum test and irradiation test. The input range of the ASIC is from 2000e− to 180,000e−, reflecting the energy of the gamma ray from 11.2 keV to 1 MeV. The gain of the readout channel is 65 mV/fC at the shaping time of 1 μs. The best test result of the equivalent noise charge (ENC) is 58.9 e− at zero farad plus 5.4 e− per picofarad. The nonlinearity and the crosstalk are less than 3% and less than 2%, respectively, at the room temperature. The static power dissipation is about 3 mW/channel.

Molecular imaging: the application of small animal positron emission tomography.

The extraordinary advances in genomic technologies over the last decade have led to the establishment of new animal models of disease. The use of molecular imaging techniques to examine these models, preferably with non-destructive imaging procedures, such as those offered by positron emission tomography (PET), are especially valuable for the timely advancement of research. With the use of small animal PET imaging it is possible to follow individual subjects of a sample population over an extended time period by using highly specific molecular probes and radiopharmaceuticals. In this Prospect small animal PET imaging will be described, specifically focusing on the current technologies, its applications in molecular imaging and the logistics of performing small animal PET.

The biological application of small animal PET imaging

The short history of small animal PET is reviewed in the context of its application in the laboratory. Early work has demonstrated a role for the technique in both drug development and in the in vivo monitoring of neuroreceptor function with time. As spatial resolution approaches 1 mm, challenges in quantification remain. However, the ability to carry out animal PET studies that are analogous to human PET will form an important bridge between laboratory and clinical sciences.

Performance characteristics of the 3-D OSEM algorithm in the reconstruction of small animal PET images. Ordered-subsets expectation-maximixation.

Rat brain images acquired with a small animal positron emission tomography (PET) camera and reconstructed with the three-dimensional (3-D) ordered-subsets expectation-maximization (OSEM) algorithm with resolution recovery have better quality when the brain is imaged by itself than when inside the head with surrounding background activity. The purpose of this study was to characterize the dependence of this effect on the level of background activity, attenuation, and scatter. Monte Carlo simulations of the imaging system were performed. The coefficient of variation from replicate images, full-width at half-maximum (FWHM) from point sources and image profile fitting, and image contrast and uniformity were used to evaluate algorithm performance. A rat head with the typical levels of five and ten times the brain activity in the surrounding background requires additional iterations to achieve the same resolution as the brain-only case at a cost of 24% and 64% additional noise, respectively. For the same phantoms, object scatter reduced contrast by 3%-5%. However, attenuation degraded resolution by 0.2 mm and was responsible for up to 12% nonuniformity in the brain images suggesting that attenuation correction is useful. Given the effects of emission and attenuation distribution on both resolution and noise, simulations or phantom studies should be used for each imaging situation to select the appropriate number of OSEM iterations to achieve the desired resolution-noise levels.

Development of a small animal PET imaging device with resolution approaching 1 mm

This paper describes progress in the design and construction of a single-plane PET tomograph having a spatial resolution approaching 1 mm. The system consists of a 12-cm diameter ring with 360 LSO (Lu/sub 2/SiO/sub 5/) detectors viewed by 30 photomultiplier tubes. Thin (5-mm) crystals and a low energy threshold are used. Crystals are identified using both position arithmetic and energy criteria. To date, the system construction has been completed, system tuning has been carried out and imaging studies have begun.

A simple technique to increase the linearity and field-of-view in position sensitive photomultiplier tubes

Crossed anode wire position sensitive photomultiplier tubes (PS-PMTs) detect the location of a light source and provide the X and Y axis coordinates of the event. These coordinates are typically generated using Anger logic, where a resistor chain divides the current flow into two signals for each coordinate (X/sup +/, X/sup -/ and Y/sup +/, Y/sup -/). In the standard readout, identical resistor values are used across the entire resistor chain. While this arrangement provides a linear readout in the central portion of the photomultiplier face, the readout is non-linear and sometimes even double valued near the edges of the PS-PMT due to the truncation of the charge beyond the last anode wire. To counter this effect, we have increased the value of the resistance near the ends of each resistor chain in order to compensate for the charge lost beyond the anode wires. Measurements were made using a Hamamatsu R-3941 PS-PMT coupled to a pixellated BGO matrix of cut crystals with a 2 mm pitch in each direction. After changing the end resistors, the usable field-of-view increased by 39%. This simple modification should enhance the operation of PS-PMTs in applications such as positron emission mammography, and small animal PET imaging.