We have recently completed a large-area, codedaperture, gamma-ray imager for use in searching for radiation sources. The instrument was constructed to verify that weak point sources can be detected at considerable distances if one uses imaging to overcome fluctuations in the natural background. The instrument uses a rank-19, one-dimensional coded aperture to cast shadow patterns onto a 0.57 m 2 NaI(Tl) detector composed of 57 individual cubes each 10 cm on a side. These are arranged in a 19 x 3 array. The mask is composed of four-centimeter thick, onemeter high, 10-cm wide lead blocks. The instrument is mounted in the back of a small truck from which images are obtained as one drives through a region. Results of first measurements obtained with the system are presented.

Standard nuclear medicine imaging uses photon collimation and thus suffer from very low sensitivity, especially if high energy (>511 KeV) isotopes are to be imaged. Coded aperture techniques use a coded pattern mask instead of a collimator to encode the photon source distribution, thus every photon source contributes to the signal in the whole detector area. It significantly improves the system sensitivity while retaining the spatial resolution of the reconstructed images. We have developed coded aperture arrays which are specifically designed for near field imaging, rather than the far field imaging appropriate to X-ray astronomy and have placed an emphasis on reducing sidelobe response in order to increase utility for images with background.We have used a cyclic difference set uniformly redundant array as the coded aperture pattern; have conducted imaging experiments for point sources, 2-D sources (sources in a plane with arbitrary distribution), and 3-D sources (sources in 3-D with arbitrary distribution) of 511 KeV through phantoms; and have compared the experimental results with those from collimator systems. The coded aperture experiments have been conducted using a Siemens E.CAM gammdSPECT camera. Results from our experiments show significantly improved sensitivity for a coded aperture imaging system over collimator systems, while retaining reasonable resolution. We have demonstrated the possibility of using coded apertures and a conventional gamma camera to image gamma rays of high energy, such as 5 11 KeV, in nuclear medicine.

We introduce and demonstrate the utility of coded aperture (CA) nuclear scintigraphy for imaging small animals. CA imaging uses multiple pinholes in a carefully designed mask pattern, mounted on a conventional gamma camera. System performance was assessed using point sources and phantoms, while several animal experiments were performed to test the usefulness of the imaging system in vivo, with commonly used radiopharmaceuticals. The sensitivity of the CA system for 99mTc was 4.2 × 103 cps/Bq (9400 cpm/μCi), compared to 4.4 × 104 cps/Bq (990 cpm/μCi) for a conventional collimator system. The system resolution was 1.7 mm, as compared to 4–6 mm for the conventional imaging system (using a high-sensitivity low-energy collimator). Animal imaging demonstrated artifact-free imaging with superior resolution and image quality compared to conventional collimator images in several mouse and rat models. We conclude that: (a) CA imaging is a useful nuclear imaging technique for small animal imaging. The advantage in signal-to-noise can be traded to achieve higher resolution, decreased dose or reduced imaging time. (b) CA imaging works best for images where activity is concentrated in small volumes; a low count outline may be better demonstrated using conventional collimator imaging. Thus, CA imaging should be viewed as a technique to complement rather than replace traditional nuclear imaging methods. (c) CA hardware and software can be readily adapted to existing gamma cameras, making their implementation a relatively inexpensive retrofit to most systems.

No abstract

No abstract