Design, construction and performance of one-dimensional lead foil parallel plate and fan-beam collimators for gamma ray imaging

Abstract

Precise manufacturing of lead foil collimators for gamma-ray imaging is critical to obtain acceptable resolution, sensitivity and uniformity. Actual fabrication poses numerous technical challenges because of the mechanical characteristics of lead foil and the requirement to maintain 50 μm tolerances or better over large areas. This paper describes the materials and techniques used to construct precision one-dimensional parallel and converging slice collimators for a full-ring SPECT imaging system. The methods devised are generally applicable in the construction of similar gamma-ray or X-ray lead foil collimators. Materials used include lead foil with 2% antimony and 0.5% tin, a machinable rigid polymethacrylimide foam and thin film adhesive. A carefully controlled fabrication process was developed to maximize the uniformity of the collimator slices. Sheets of precise thickness foam, adhesive and lead foil were precut and bonded using a heated press. A specially designed vacuum chuck was used for cutting and shaping the slices. Tolerances on foam thickness could be held to 50 μm. Finished slices were stacked together on a surface plate with precut adhesive film placed between each slice. A completed stack of finished slices was heated to bond the entire collimator. Three collimators have been constructed to date. Two are parallel plate collimators, one designed for 140 keV 99mTc gamma rays and one designed for 511 keV annihilation radiation. The third is a converging collimator with 2 : 1 magnification designed for imaging 131I. The measured uniformity and resolution are in good agreement with the design objectives and reflect the variance in the material dimensions. The uniformity varied from 5% standard deviation for the 99mTc collimator to 14% standard deviation for the 131I collimator. The intrinsic resolution varied from 8.5 mm FWHM for the 99mTc parallel collimator to 1.9 mm FWHM for the 131I collimator. The sensitivity of the 99mTc collimator is comparable to current dual head SPECT cameras with LEAP collimators, 670 cpm/μCi. The sensitivity of the 131I converging and 511 keV parallel collimators are an order of magnitude less than that of the 99mTc collimator.