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A simple procedure to determine the focal length of a pinhole collimator is presented, which is important for planar and SPECT imaging. The procedure involves the use of a point source and three linear stages. The source is moved by carefully controlled relative shifts in two dimensions to a number of different positions within a plane nearly perpendicular to the detector's plane. For each position a projection is acquired. The focal length is calculated from the shift in the centroid of the projections via a maximum-likelihood fit. A useful byproduct of the procedure is calibration of the position of the source, which can then be used for planar imaging studies such as sensitivity and point-spread function. This calibration maps the coordinate system of the stages to the coordinate system of the aperture. The method was experimentally validated for three different focal lengths. [ABSTRACT FROM AUTHOR]

Pinhole SPECT is sensitive to alignment and calibration. For circular orbits, it is typically assumed that the radius of rotation (ROR) is constant. Uncompensated deviation in the ROR from a constant has the potential to worsen resolution. Herein, we describe a method for determining the ROR as a function of gantry angle. This method uses a laser to serve as a reference line and a laser target mounted to the pinhole collimator to measure position. The target rotates with the collimator and is read-out at multiple angles. The two-dimensional read-out values are used to calculate the ROR as a function of angle. Experimental radionuclide SPECT projection data were acquired. The projection data were reconstructed using the angular-dependent correction and using an average ROR. The results indicate that it is difficult to distinguish the effect of this correction when the amplitude is less than about 0.5 mm. However, the correction is very pronounced for an amplitude of about 1 mm. [ABSTRACT FROM AUTHOR]

Pinhole collimation yields high sensitivity when the distance from the object to the aperture is small, as in the case of imaging small animals. Fine-resolution images may be obtained when the magnification is large since this mitigates the effect of detector resolution. Large magnifications in pinhole single-photon emission computed tomography (SPECT) may be obtained by using a collimator whose focal length is many times the radius of rotation. This may be achieved without truncation if the gamma camera is large. We describe a commercially available clinical scanner mated with pinhole collimation and an external linear stage. The pinhole collimation gives high magnification. The linear stage allows for helical pinhole SPECT. We have used the system to image radiolabeled molecules in phantoms and small animals. [ABSTRACT FROM AUTHOR]