Instrumentation optimization for positron emission mammography

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Abstract

The past several years have seen designs for PET cameras optimized to image the breast, commonly known as Positron Emission Mammography (PEM) cameras. The guiding principal behind PEM instrumentation is that a camera whose field of view is restricted to a single breast has higher performance and lower cost than a conventional PET camera. The most common geometry is a pair of parallel planes of detector modules, although geometries that encircle the breast have also been proposed. The ability of the detectors to measure the depth of interaction (DOI) is also a relevant feature. This paper finds that while both the additional solid angle coverage afforded by encircling the breast and the decreased blurring afforded by the DOI measurement improve performance, for small lesions the ability to measure DOI is more important than the ability to encircle the breast.

Introduction

An increasing number of PET cameras optimized to image the breast have been proposed or constructed [1], [2], [3], [4], [5], [6], [7], [8], [9]. These cameras, commonly known as Positron Emission Mammography (PEM) cameras, restrict the field of view to a single breast, and are expected to have higher performance and lower cost than a conventional PET camera. By placing the detectors close to the breast, the PEM geometry is able to subtend more solid angle around the breast than a conventional PET camera. In addition, gamma rays emitted in the breast have to pass through at most one attenuation length (∼10 cm) of tissue in the PEM geometry, but may have to travel through as much as four attenuation lengths of tissue in a conventional PET camera. These two factors significantly increase the sensitivity (the detected coincident event rate per unit activity in the field of view) in the PEM geometry.

There are two additional design features that have a large affect on the performance of PEM cameras—the geometric efficiency for detecting activity that is within the field of view and whether the detector module is capable of measuring the depth of interaction (DOI, described later). While having a high efficiency and the ability to measure DOI are both desirable, they come at some cost, and so we must estimate their benefits in order to maximize the cost/performance tradeoff. Therefore, this paper examines several PEM camera designs in order to identify which design tradeoffs have the greatest effect on the imaging performance.

Section snippets

Camera design

Four hypothetical camera designs are examined to identify the most important features. We examine the four combinations implied by two geometries (with different geometrical acceptance) and two DOI measurement ability options (with and without).

The Fisher information matrix

Comparing PEM camera designs poses several challenges. First, a task and a performance measure for this task must be defined. For example, we might compute the signal-to-noise ratio for a spherical “tumor” in a uniform background. Given the heterogeneity of tumors in patients, a single task is usually woefully inadequate to characterize performance. In the spherical tumor example, we should measure a variety of “tumors” with different diameters, signal to background ratios, positions within the

Non-instrumentation issues

Some of the critical limitations to PEM have nothing to do with camera design. For example, there is considerable interest in detecting small (3 mm diameter and below) tumors. However, small tumors will contain extremely low absolute amounts of activity, and so may be very difficult to observe above the background activity level. Assuming a 1 mCi injection into a 70 kg patient and a 3:1 tumor to normal tissue uptake ratio (a typical value for fluoro-deoxyglucose, which is the most commonly used

Conclusions

PEM offers significantly higher sensitivity for radiation sources in the breast than conventional PET cameras, mainly because of significantly increased solid angle coverage and reduced attenuation in the patient. There are several design features, notably increased solid angle coverage due to encircling the breast and incorporating detector modules that measure the DOI, which could be implemented in PEM cameras to improve their performance. Using the Fisher information matrix, we have explored

Acknowledgements

We would like to thank Dr. Ronald Huesman, Dr. Stephen Derenzo, Dr. Thomas Budinger, Dr. Chaincy Quo, and Dr. Jennifer Huber of Lawrence Berkeley National Laboratory for many useful discussions. This work was supported in part by the Director, Office of Science, Office of Biological and Environmental Research, Medical Science Division of the US Department of Energy under Contract No. DE-AC03-76SF00098 and in part by the National Institutes of Health, National Cancer Institute under Grant No.

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