often ineffective and traumatic for the patient [6]. Furthermore, there is no practical and accurate imaging technique for monitoring therapy.
The limitations of traditional diagnostic methods have motivated the study of the applicability of other imaging modalities to breast imaging. There is ongoing work in the field using CT [25, 26], MRI [27, 28] and PET [29, 30, 31].
The functional information provided by PET is useful since cel-lular changes associated with cancer can occur well before structural changes such as calcifications appear [6]. As early as in 1989, whole-body FDG PET was studied in its application to breast imaging [32].
In this study an 82% sensitivity of whole-body PET to breast tumors of >5mm in size was reported. Subsequent studies have reported sen-sitivities ranging from 80% to 100% and specificities ranging from 75%
to 100%, with reports of highest sensitivities coming from studies that included patients with large tumors [33]. FDG has also been studied for use in the monitoring of breast cancer treatment [34]. However, several limitations have prevented PET from being incorporated into standard breast cancer evaluation. Limitations include low sensitiv-ity, inadequate spatial and contrast resolutions for small lesion, as well as high cost and long scan times. Dedicated positron emission mam-mography (PEM) systems have been developed to overcome some of these difficulties [33, 35, 31]. For improved spatial and contrast resolu-tion, PEM devices use high-resolution detectors in a geometry which increases sensitivity in a ROI by employing a small FOV. However, these are highly specialized systems with a very narrow range of ap-plicability in clinical practice. Furthermore, they would involve a high financial costs to implement on a large scale.
As a means of obtaining high resolution in a reduced FOV, the PET insert geometry offers a practical solution to the limitations of
using whole-body PET for breast imaging applications. The tech-nique makes use of a clinical scanner and its only requirement is that a limited number of high-resolution detectors to be connected with a, possibly already-existing, system. This application has been pro-posed using the ring insert concept, employing a partial ring of high-resolution scintillation detectors in coincidence with a whole-body scanner [36, 37]. A prototype of the system has shown improvements in spatial resolution and contrast recovery in a cylindrical phantom [16]. The application of a probe geometry to breast imaging using a more realistic breast phantom will be the subject of matter of chapter 8 of the present work.
2.6 Summary
The PET insert concept aims to enhance the spatial resolution of a conventional scanner by placing additional detectors in the FOV.
Ring and probe insert geometries have been envisioned. This chapter illustrates how placing the insert detectors closer to the subject and reducing the size of their individual elements creates high-resolution coincidence events. Further enhancement may be obtained by using insert scintillation detectors with one-to-one coupling or solid-state detectors. Various implementations of the PET insert concept are currently under investigation by several groups and have been dis-cussed in this chapter. Finally, a clinical application of the probe geometry has been proposed. A PET probe could provide functional information useful for breast imaging, providing higher sensitivity and resolution than whole-body PET, while offering an alternative to the highly specialized PEM devices.
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