Point Response Function Research Articles

Two detector MWPC positron camera with honeycomb lead converters for medical imaging: Performance and developments

A MWPC camera for nuclear medicine imaging with positron emitting isotopes has been built. The two planar detectors are converter coupled MWPC with sensitive area 48×48 cm 2 placed 50 cm apart with clear space in between for the patient. The effective sensitivity of 5.8% for each detector with a spatial resolution of 7 mm fwhm on the image plane is achieved by use of honeycomb lead-on-plastic converters. Present performance in imaging test objects and in clinical applications is described, using various computer 3D reconstruction algorithms. Developments in progress include the use of small diameter lead oxide glass tube converters; techniques to eliminate multiple scatter events and improve the point response function; and electronics to provide accidental event subtraction.

Improved depth resolution in coded aperture γ-ray imaging systems

We derive a rigorous expression for the three-dimensional point response function associated with the combined encoding-decoding process of coded aperture imaging. From this general expression we derive the two-dimensional point response function appropriate to a coded aperture imaging system which images a two-dimensional slice through a, three-dimensional source distribution. This point response function is then calculated numerically for a number of different aperture coding functions and decoding functions. The graphed results suggest that simple modifications of the decoding function can lead to significant improvements in the depth resolving capability of a coded aperture imaging system. Suggestions for future research are given.

A computer simulation of the performance of a spark chamber gamma camera

The response of a spark chamber gamma camera to three-dimensional test objects has been simulated in a computer. The simulation was based on efficiency and point response function measurements for single parallel hole collimators using a Geiger counter as the detector. Measurements and simulations were also done using a NaI scintillation detector. A comparison of the two simulations shows that the product of activity in test objects and exposure time for the spark chamber system is of the order of 1000 times that for a NaI scintillation detector for the same visualization statistics. This results from the spark chamber's low detection efficiency and a loss of contrast caused by an inability to discriminate against scattered radiation.

On the residence-time distribution for a system with velocity profiles in its connections with the environment

A point-stimulus point-response function is defined that allows rigorous expressions to be written for the residence time distribution for a system where there are complicating factors such as inlet concentration profiles and inlet and outlet velocity profiles. This function is capable of being measured directly; the residence time distribution and other response functions may be calculated from it. It is not possible to calculate the residence time distribution from the results of experiments in which tracer is generated and/or measured in situ at locations were there are significant velocity profiles unless further information is available.