Scintillator and photodetector array optimization for functional breast imaging

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Abstract

Nuclear Medicine methods have been proposed as a means of imaging primary breast lesions and regional metastatic involvement based on tumor physiology. Recently, the positive predictive value of scintimammography using 99Tc labelled SestaMIBI has been reported to be as high as 81%, with an associated negative predictive value of 97%. Visualization of small (<1 cm) lesions using scintimammography may be complicated, however, by the effects of overlying and underlying background uptake of MIBI in the breast soft tissue and the deterioration of lesion contrast with distance from the gamma camera. For these reasons dedicated compact gamma cameras have been proposed and successfully used. Nevertheless, the detection of very small tumors (<5–10 mm) is still very difficult. Many parameters affect the breast small tumors detection. The degree of pixellation of both the scintillator and photodetector is critical for the intrinsic position resolution of the detector and for the overall imaging performance. In this paper, we examine the basic imaging properties of systems using arrays of scintillators and pixellated photodetectors. The influence of readout systems is also taken into account. Simulations as well as preliminary experimental results are presented.

Introduction

Breast cancer is the most commonly diagnosed malignant neoplasm and the second leading cause of cancer-related death among women. X-ray film-screen mammography plays a major role in both screening and post-surgical or therapy management of this disease but has the drawback of low positive predictive value. Breast cancer detection based on tissue function may prove beneficial and serve a complementary role to X-ray mammography. Nuclear medicine methods have been proposed as a means of imaging primary breast lesions and regional metastatic involvement based on tumor physiology. The positive predictive value of scintimammography using 99mTc labelled SestaMIBI has been reported to be as high as 81%, with an associated negative predictive value of 97%. Visualization of small (<1 cm) lesions using scintimammography may be complicated, however, by the effects of overlying and underlying background uptake of MIBI on the breast soft tissue and the spatial resolution limit when imaging with standard gamma cameras. Several clinical trials are currently in progress or have recently been completed. The aim was to evaluate 99mTc MIBI scintimmammography breast imaging effectiveness. The results show [1] that the Anger camera is not the right detector for early breast tumor detection. In fact, it does not fit the organ anatomy with consequent obvious limitation in overall spatial resolutions. Moreover, its intrinsic spatial resolution is not adequate in detecting small lesions. Also, a complete tomographic scan close to the breast cannot be performed. For these reasons recently there has been a growing interest in compact discrete dedicated gamma cameras [2], [3].

Section snippets

Dedicated high-resolution detectors

The dedicated high-resolution gamma detectors are essentially mini-Anger cameras that, fitting the organ, allow shorter imaging distances, thus improving collimator contribution to spatial resolution. Furthermore, arrays of scintillators and segmented photodetectors allow better intrinsic resolution. So the overall position resolution is greatly increased with respect to the traditional Anger cameras.

Different groups [2], [3] have developed compact dedicated gamma cameras. Measurements with

Simulations

Simulations have been performed in order to understand better the choice of scintillator and photodetector segmentation. The simulation software has been extensively described elsewhere [7]. Arrays of CsI(Tl) scintillators and photodetectors of different segmentation have been considered. Here we summarize the main results. A comparison of the performances shows that:

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    segmented scintillators have better performances with respect to the continuous ones;

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    high degree of scintillator pixellation is

Measurements

Previously reported measurements [8], [9] made with CsI(Tl) pixels of 4.2×4.2 mm2 and 2.5×2.5 mm2 cross-sections have confirmed the simulation results: increasing the degree of segmentation of the scintillator requires photodetector pixel size of approximately the same dimension at the anode level. For better understanding the limits of scintillator pixel size, measurements have been done with CsI(Tl) arrays of different degree of pixellation (1.5×1.5×3 and 2×2×3 mm3, pixels are isolated by 0.2 mm

Summary and outlook

The best practical detector for functional breast imaging will be obtained as a compromise between many and often conflicting requirements. An optimization process is needed to design a dedicated gamma camera able to detect small tumors (<1 cm, possibly <0.5 cm). Simulations and measurements have been performed in order to understand the role of segmentation of the scintillator and of the photodetector for improving the spatial resolution. The best solution at this stage seems to be a 1.5×1.5×3 mm3

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