The aim of this work was to investigate the effects that collim ators have on gamma camera images. The collimator is generally considered to be the weak link of the gamma cam era imaging system. This study has highlighted the following key areas in which changes could be made: improvements to the manufacturing accuracy of collimators by reducing hole misalignments, the design of laminated collimators to reduce collimator mass and to provide a more robust structure, and the use o f alternative collim ation materials for improved absorption, thus allowing the use of narrower septa to produce an improved efficiency while maintaining spatial resolution.
For the investigation a Monte Carlo simulation o f photon transport through a multihole collim ator was written which provided the average point source response function o f a collim ator in terms of the interactions undergone by the photons in the collim ator. The code enabled collim ator dimensions such as the hole size, septal thickness and length to be altered so that different collimators could be modelled. The collim ator simulation also allowed collimator defects such as hole misalignment to be simulated. A m inor modification to the code enabled the simulation of multi-layered collim ator structures. To consider the effects of collimators on images a M onte Carlo simulation of a line source phantom was written, which modelled the transport of photons from their origin to a plane external to the phantom. Another program combined the photon distribution on this plane with the collimator response function for that particular collimator to form the image in the crystal. A digital convolution of this image with the intrinsic cam era resolution produced the final system image. To quantify the various effects of collimators on images, a fitting routine was written which obtained the spatial resolutions and linearity of the images.
The collimator simulation was validated experimentally by obtaining single hole response functions for a range of source heights. Good agreement was seen for isotopes of two different energies at all source heights. The resolutions of the geometric response functions were then compared with theoretical expectations and good agreement found for a range of collimators and source heights. The effects caused by physical properties of perfect conventional lead collimators were investigated using the collimator simulation. Factors such as penetration and scatter, were considered and penetration was determined to cause a greater degradation to images than scatter. While penetrated photons caused an increase in the resolution of 1.27% photons scattered in the collim ator (but not in the object) only increased the width by a further 0.25%. Although the hole diameter caused
the greatest degradation to an image because o f the relatively large angular range of photons permitted, it could not be greatly reduced due to efficiency considerations.
In addition to considering perfect collim ators the collim ator sim ulation also enabled collimator defects to be modelled. The effects of hole misalignment on images were considered assuming a maximum hole angulation of 1°. This is a commonly quoted manufacturing accuracy. Collimator response functions were obtained for a range of collim ators each with a given hole angulation of up to 1°. Images o f the line source phantom from collimators with random angulations were then obtained by randomly selecting a given angulation for a particular location on the collimator. The angulations within each collimator were sampled from a Gaussian distribution. To simulate different degrees o f collimator angulation, the width of the Gaussian distribution was changed. Only angulations up to 1° were simulated. The major effect of hole angulation seemed to be in the linearity of the images; in collimators where over 55% of the holes had angulations of less than 0.25° the degradation in linearity was noticeable. This could lead to false hot or cold regions on the image. The resolution was affected m uch less noticeably than the linearity, however it did worsen with increase in angulation. Although these effects would have greater significance in tomographic imaging, improvements in accuracy of manufacture would be beneficial to planar imaging.
Conventional lead colhmators are heavy, due to the density of lead, and are prone to damage, because of the malleable nature of lead. In order to reduce the m ass of a collim ator and to provide a structure which is more robust, the Monte Carlo collimator code was modified to simulate layered collimator structures. From the response functions obtained from several different structures, a laminated collimator constructed from lead and aluminium was selected, with all layers of equal thickness. A layer thickness of 0.01 cm gave good results for the response function and was also a suitable thickness for m achining. The im ages obtained with the lam inated collim ators com pared very favourably with the conventional lead collimator, producing identical resolutions within statistical limits. The layer thickness selected produced a collimator with approximately 49% lead by height, resulting in a considerable reduction in mass. For a low energy general purpose collimator of the dimensions modelled throughout the thesis, this would produce a laminated collimator of approximately 6 kg compared with a lead collimator of approxim ately 10 kg. W hile this might not be significant for a low energy general purpose collimator, it would be of importance to higher energy collim ators and high resolution collimators. For low energy general purpose collimators however, the main advantage would be in the strength and robustness of the laminated collimator over the conventional lead collimator.
Although lead has excellent attenuation properties making it a good material for photon collimation, other materials do exist with better attenuation properties. Other materials are also less malleable than lead which would result in collimators which would be less likely to be damaged. A disadvantage of these other m aterials is that they are
generally m ore expensive than lead. Any alternative collim ator m aterial for use in com m ercial collimators would need to have better attenuation properties than lead, be widely available, cheap and easily machineable. An investigation into possible materials suggested tungsten and uranium as alternatives to lead, uranium having the best attenuation properties. Depleted uranium, a form of uranium in which there is a lower concentration of the U235 isotope than in naturally occurring uranium is a by-product of the nuclear industry and has high strength. Images were produced using the Monte Carlo code for a depleted uranium collimator with the same dimensions as the conventional lead collimator. The improvement in the spatial resolution for the uranium collim ator over the lead collimator was negligible when the same collimator dimensions were used. The advantage of using uranium, however, is that a narrower septa collim ator could be constructed than with lead without producing an increase in the penetrated photons. The use of narrower septa, while not altering the spatial resolution, will result in an increased efficiency, which would either improve image quality, enable shorter measurement times, or possibly allow a reduction in the activity adm inistered to the patient. W hen the response functions of several different septal thickness uranium collim ators were obtained from the numerical model, it was found that septal thicknesses of less than half the conventional septal thickness produced similar absorption properties. W hen images were formed using a uranium collimator with narrow septa, the final spatial resolution values obtained were at least as good as those of the conventional lead collimator, and the efficiency was approximately 20% higher for the narrow septa uranium collimator. In addition to providing a collimator of greater strength than lead, the narrow septa uranium collim ator would be lighter in weight at under 5 kg, which is less than half the mass of the conventional lead collimator.
6.1 Future work
Several extensions to the work performed for this thesis are suggested for future work. These suggestions are discussed briefly in this section.
In addition to the defect of collim ator hole m isalignm ent which has been presented in this thesis, other types o f collim ator defect exist. One, th at o f the m isalignm ent of collimator sides, has already been incorporated into the main Monte Carlo collimator simulation and is ready for use in image production. The code enables one or more sides of the collimator hole to be rotated by a predefined amount about the centre of the side. Average response functions would be obtained for a collimator with all holes having the same side misalignments. For production of the images, the same method will be used as for the hole misalignment simulation where the location of the photon on the collimator face is calculated, the appropriate response function obtained for that location, and the final location of the photon in the crystal calculated together with
the type o f interactions undergone in the collimator. In this way an image using a collimator with hole edge misalignment may be obtained.
The simulations of the laminated collim ator structure and the narrow septa uranium collim ator show that both are viable alternatives to the conventional lead collim ator. Both require experim ental confirm ation. A prototype o f the lam inated collimator could be constructed with round holes, as this would be quicker to obtain than a hexagonal hole prototype. Before a uranium collim ator is constructed it will be necessary to obtain a lump of depleted uranium of suitable dimensions to investigate the count rate obtained from the uranium on a gamma camera. If favourable results are obtained, a prototype narrow septa uranium collimator could be constructed. A laminated uranium/aluminium structure could also be investigated.
The eventual aim of the work is the improvement of gamma camera images, both by a reduction in the detection of unwanted photons, and by collim ator designs which exhibit few er manufacturing errors and are less prone to damage due to the materials from which they are constructed.