We present the tomographic images and performance measurements of the LBNL positron emission mammography (PEM) camera, a specially designed positron emission tomography (PET) camera that utilizes PET detector modules with depth of interaction measurement capability to achieve both high sensitivity and high resolution for breast cancer detection. The camera currently consists of 24 detector modules positioned as four detector banks to cover a rectangular patient port that is 8.2 6 cm 2 with a 5 cm axial extent. Each LBNL PEM detector module consists of 64 3 3 30 mm 3 LSO crystals coupled to a single photomultiplier tube (PMT) and an 8 8 silicon photodiode array (PD). The PMT provides accurate timing, the PD identifies the crystal of interaction, the sum of the PD and PMT signals (PD+PMT) provides the total energy, and the PD/(PD+PMT) ratio determines the depth of interaction. The performance of the camera has been evaluated by imaging various phantoms. The full-width-at-half-maximum (FWHM) spatial resolution changes slightly from 1.9 mm to 2.1 mm when measured at the center and corner of the field of the view, respectively, using a 6 ns coincidence timing window and a 300-750 keV energy window. With the same setup, the peak sensitivity of the camera is 1.83 kcps/ Ci. Index Terms-Biomedical imaging, depth of interaction (DOI), positron emission tomography (PET).

Abstract-We present an in situ calibration technique for the LBNL Positron Emission Mammography (PEM) detector module that is capable of measuring depth of interaction (DOI). The detector module consists of 64 LSO crystals coupled on one end to a single photomultiplier tube (PMT) and on the opposite end to a 64 pixel array of silicon photodiodes (PD). The PMT provides an accurate timing pulse, the PDs identify the crystal of interaction, the sum provides a total energy signal and the Γ=PD/(PD+PMT) ratio determines the depth of interaction. We calibrate using the 176 Lu natural background radiation of the LSO crystals. We determine the relative gain (K) of the PMT and PD by minimizing the asymmetry of the Γ distribution. We determine the depth dependence from the width of the Γ distribution with optimal K. The performance of calibrated detector modules is evaluated by averaging results from 12 modules. The energy resolution is a function of depth ranging from 24% fwhm at the PD end to 51% fwhm at the PMT end, and the DOI resolution ranges from 6 mm fwhm at the PD end to 11 mm fwhm at the PMT end. Index Terms-Biomedical imaging, calibration, detectors, depth of interaction (DOI), positron emission tomography (PET).I. INTRODUCTION N the design of PET detectors, there is a tradeoff between sensitivity and spatial resolution. Increasing crystal length to achieve higher sensitivity leads to deterioration of spatial resolution due to the increased probability of detecting a gamma photon that has penetrated through more than one crystal. By measuring the depth of interaction within a crystal, it is theoretically possible to overcome the need to compromise either of these detector parameters. Depth of interaction can be measured using light sharing by placing a photon detector at both ends of a continuous scintillator crystal [1][2][3]. In the calibration of detectors that utilize light sharing, gain balancing between detectors is an essential step. The most obvious way of calibrating depth in a detector utilizing light sharing is to use a collimated gamma beam to excite the scintillator crystal at known depths. However, this is not feasible for a fully assembled multicrystal detector, because the inner crystals are shielded from the excitation beam by their neighbors. One solution is to precalibrate each crystal before assembling the crystal array. However, in situ calibration is the only practical solution for ongoing calibration of the installed detectors. Such calibration is necessary to accommodate the effects of drift in photomultiplier tube gains, temperature sensitivity, and electronic aging.The LBNL PEM detector consists of 64 3 mm × 3 mm × 30 mm LSO crystals coupled to a single photomultiplier tube and an 8×8 silicon photodiode array [4]. The photomultiplier tube provides accurate timing. A custom PET readout IC (PETRIC) [5] amplifies the photodiode pulse signals and identifies the crystal of interaction by winner-take-all circuitry that chooses the crystal with the highest signal. The total energy is the sum o...