Sub-voxel position identification in Cadmium Zinc Telluride detectors for Low Dose Molecular Breast Imaging

在英国，乳腺癌是最常见的一种疾病，直接影响到八分之一的女性。癌症治疗在过去几十年里有了长足的发展，以至于目前有令人难以置信的尖端技术和技术可用，癌症患者的成功率不断上升。然而，此类治疗成功的一个主要限制因素是癌症被诊断的阶段。这种疾病诊断得越早，根除这种疾病的治疗效果就越好。因此，还需要开发复杂的诊断技术。大约一半的筛查年龄女性的乳房被认为是“乳房X光检查致密”。虽然这是一个正常和常见的特征，但它限制了可用于检测癌症的筛查方法的诊断能力。所有常见的诊断技术对乳房的诊断能力都是有限的，这些乳房被认为是乳房造影致密的。乳房X光检查致密的女性患乳腺癌的风险更高，被检测到的几率更低。因此，在筛查乳腺癌时，需要其他方法来提高诊断性能。分子乳腺成像是诊断核医学的一个分支，它利用放射性同位素Tech-99m来获得功能图像并识别乳房内的病变。放射性同位素被注射到患者体内，在那里它移动到乳房区域，并被癌细胞吸收。半衰期为6小时，同位素在患者体内衰变，同时释放出特有的141keV伽马射线。腐烂产物离开身体，在这样做的过程中与其路径上的材料相互作用。位于患者外部的伽马相机检测发射的伽马射线，以处理事件并重建乳房内放射性核素摄取的图像，这是癌症组织的指示。MBI的主要优势是该技术能够在早期发现乳腺癌。然而，与任何成像技术一样，尽管MBI对于乳房致密的女性来说具有其他方式无法比拟的优势，但它也不是没有缺点。与MBI相关的主要缺点是由于给药源给患者提供的剂量明显高于传统乳房X光检查中的典型剂量。最近的研究试图开发这种方法学，以降低MBI中使用的源的活度，进而减少给患者的剂量，而不影响系统的敏感性或特异性。(1)碲锌镉是一种可用于MBI的探测器材料，主要是因为它具有良好的位置分辨率。这一特性使探测器对141keV伽马射线高度敏感，因此可以在不影响图像质量的情况下向患者提供较低活度的同位素。最终项目目标是开发一种针对MBI应用而优化的CZT探测器系统。该项目寻求实现探测器系统的最佳位置分辨率，例如可以向患者提供低活度源，从而减少提供的剂量，进而减少与成像方式相关的辐射风险。首先，有必要量化CZT探测器中的电荷共享效应，以便了解问题的严重性并最终得到解决。基于Geant4的仿真研究将有助于开发旨在最大化系统可获得的位置分辨率的算法。通过本论文，将相应地调查和分析不同的像素几何和准直器设计如何影响系统的位置分辨率，以确定最终探测器系统的最佳性能。最终的系统将是

Breast cancer is the most common form of the disease in the UK, directly affecting 1 in 8women. Cancer treatment has developed significantly over the last few decades such that thereare currently incredibly sophisticated techniques and technologies available with an evergrowing success rate for cancer sufferers. However, a major limiting factor to the success ofsuch treatments is the stage at which the cancer is diagnosed. The earlier the disease isdiagnosed, the greater the probability of treatments working to eradicate the disease. Hence,developments of sophisticated diagnostic techniques are also required.Approximately half of women of screening age have what are considered 'mammographicallydense breasts'. Although this is a normal and common characteristic, it limits the diagnosticabilities of the screening methods available which are used to detect cancer. The abilities of allcommon diagnostic techniques are limited when conducted on breasts that are considered to bemammographically dense. Women with mammographically dense breasts have a higher risk ofdeveloping breast cancer and a lower chance of having it detected. Hence, alternative methodsare required to improve the diagnostic performance when screening for breast cancer.Molecular Breast Imaging is a branch of diagnostic Nuclear Medicine which utilises theradioisotope Tech-99m to obtain functional images and identify lesions within the breast. Theradioisotope is injected into the patient where it moves to the breast region and is preferentiallyup-taken by cancer cells. With a half-life of 6 hours, the isotope decays within the patient,emitting a characteristic 141 keV gamma ray as it does so. The decay products leave the body,interacting with the material in their path as they do so. A gamma camera situated outside thepatient detects the emitted gammas in order to process the events and reconstruct the image ofradionuclide uptake in the breast, which is indicative of cancerous tissue.The main advantage of MBI is the ability of the technique to detect breast cancer at an earlystage. However, as with any imaging technique, although MBI offers good advantages overother modalities for women with dense breasts, it is not without its downfalls. The maindisadvantage associated with MBI is the dose delivered to the patient as a result of the sourceadministered, which is significantly higher than the typical dose delivered in conventionalmammography. More recent studies have sought to develop the methodology in order to reducethe activity of the source used in MBI, in turn reducing the dose delivered to the patient, withoutcompromising the sensitivity or specificity of the system. (1) Cadmium Zinc Telluride is adesirable detector material for use in MBI primarily due to its good position resolution. Thisproperty makes the detector highly sensitive to 141 keV gamma rays and therefore allows for anisotope of lower activity to be administered to patients without compromising the image quality.The ultimate project aim is to develop a CZT detector system optimised for application in MBI.The project seeks to achieve the best possible position resolution of the detector system suchthat a low activity source can be administered to the patient, hence reducing the dose deliveredand, in turn, the radiation risks associated with the imaging modality. Firstly, it is necessary toquantify charge sharing effects in CZT detectors to enable the magnitude of the problem to beunderstood and, ultimately, solved. A Geant4 based simulation study will aid the development ofalgorithms which will aim to maximise the obtainable position resolution of the system.Throughout the thesis, how different pixel geometries and collimator designs affect the positionresolution of the system will be investigated and analysed accordingly in order to determine theoptimal properties of the final detector system. The final system will b