Jennifer S. Huber

An LSO scintillator array for a PET detector module with depth of interaction measurement

Presents construction methods and performance results for a production scintillator array of 64 optically isolated, 3/spl times/3/spl times/30 mm sized LSO crystals. This scintillator array has been developed for a PET detector module consisting of the 8/spl times/8 LSO array 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 and initial energy discrimination, the PD identifies the crystal of interaction, the sum provides a total energy signal, and the PD/(PD+PMT) ratio determines the depth of interaction (DOI). Unlike the previous LSO array prototypes, the authors now glue Lumirror reflector material directly onto 4 sides of each crystal to obtain an easily manufactured, mechanically rugged array with their desired depth dependence. With 511 keV excitation, the authors obtain a total energy signal of 3600 electrons, pulse-height resolution of 25% fwhm, and 6-15 mm fwhm DOI resolution.

Geometry and surface treatment dependence of the light collection from LSO crystals

Abstract We study the relative light collection efficiency for narrow LSO crystals as a function of surface finish and geometry. We explore both a specular and diffuse surface reflector finish, using LSO crystals that are either polished or etched. The crystals have a square cross-section with widths of 1.8, 2.2, 2.5, or 2.6xa0mm and lengths of 10, 20 or 30xa0mm. When optically coupled to a PMT on a square end and wrapped with Teflon on 5 sides, we excite them at 5 mm incremental depths with 511xa0keV photons and measure the photopeak position. The light collection is characterized by a maximum output (based on the photopeak position when excited at the PMT end) and a depth-dependent loss factor. Both the maximum output and loss factor are effectively independent of width, and the maximum output is only weakly dependent on length (6696±612, 5796±432, and 5328±288 photons for 10, 20, and 30xa0mm lengths). The loss factor is effectively independent of length, with a 0, 18, 27, and 22% reduction at excitation depths of 0, 10, 20, and 30xa0mm. The light collection is only weakly dependent on surface finish if it is polished or etched beyond a minimum time.

Characterization of a 64 channel PET detector using photodiodes for crystal identification

The authors present performance results for a prototype PET detector module consisting of 64 LSO scintillator crystals (3/spl times/3/spl times/20 mm) coupled on one end to a single photomultiplier tube and on the opposite end to a 64 pixel array of 3 mm square silicon photodiodes (typical pixel parameters are 5 pF capacitance, 300 pA dark current, and 73% quantum efficiency at 415 nm). The photomultiplier tube (PMT) provides an accurate timing pulse and energy threshold for all crystals in the module, the silicon photodiodes (PD) identify the crystal of interaction, the sum (PD+PMT) provides a total energy signal, and the PD/(PD+PMT) ratio determines the depth of interaction. With 32 of the channels instrumented, the detector module correctly identifies the crystal of interaction (where correct includes the adjacent 4 crystals) 79/spl plusmn/4% of the time with high detection efficiency. The timing resolution for a single LSO detector module is 750 ps fwhm, while its pulse height resolution at 511 keV is 24/spl plusmn/3% fwhm. The depth of interaction measurement resolution is 8/spl plusmn/1 mm fwhm.

Conceptual design of a high-sensitivity small animal PET camera with 4/spl pi/ coverage

We present a conceptual design of a high-sensitivity PET camera that completely encloses a small animal in a rectangular volume formed by 6 planar banks of detector modules. The 4/spl pi/ geometry and 3 attenuation-length fast scintillators provide significantly higher sensitivity than contemporary animal PET cameras, while the depth of interaction (DOI) measurement and small crystal width achieve isotropic, high spatial resolution. The absolute sensitivity is 24 kcps//spl mu/Ci, /spl sim/120 times higher than contemporary systems; the true event count rate is increased by covering 10 times the solid angle using 80% efficient detectors. For a 29 g mouse, the total scatter event rate is 11% of the total true event rate. A short (2 nsec) coincidence window and the absence of out of field activity implicit with whole animal coverage yield a small random fraction. Assuming a maximum system count rate of 10 Mcps (achievable with electronics under development), the noise equivalent count rate as a function of activity concentration has a maximum of 6.6 Mcps at 25 /spl mu/Ci/cc. 2D reconstruction algorithms indicate a spatial resolution of 2.3 mm fwhm through most of the field of view.

Characterization of the LBNL PEM camera

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/spl times/6 cm/sup 2/ with a 5 cm axial extent. Each LBNL PEM detector module consists of 64 3/spl times/3/spl times/30 mm/sup 3/ LSO crystals coupled to a single photomultiplier tube (PMT) and an 8/spl times/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//spl mu/Ci.

Effect of 176Lu background on singles transmission for LSO-based PET cameras

We explore how the radioactive background from naturally occurring 176Lu affects single photon transmission imaging for lutetium orthosilicate (LSO) scintillator-based PET cameras by estimating the transmission noise equivalent count rate (NECR) including this background. Assuming a typical PET camera geometry (80 cm detector ring diameter), we use a combination of measurement and analytic computation to estimate the counting rates due to transmission, scatter and background events as a function of singles transmission source strength. We then compute a NECR for singles transmission. We find that the presence of radiation from the naturally occurring 176Lu reduces the NECR by 60% or higher for source strengths less than 10 mCi, and that a 25% reduction of the NECR can occur even with a source strength of 40 mCi.

Calibration of a PEM detector with depth of interaction measurement

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 /spl Gamma/=PD/(PD+PMT) ratio determines the depth of interaction. We calibrate using the /sup 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 /spl Gamma/ distribution. We determine the depth dependence from the width of the /spl Gamma/ 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.

Conceptual design of a compact positron tomograph for prostate imaging

We present a conceptual design of a compact positron tomograph for prostate imaging using a pair of external curved detector banks, one placed above and one below the patient. The lower detector bank is fixed below the patient bed, and the top bank adjusts vertically for maximum sensitivity and patient access. Each bank is composed of 40 conventional block detectors, forming two arcs (44 cm minor, 60 cm major axis) that are tilted to minimize attenuation and positioned as close as possible to the patient to improve sensitivity. The individual detectors are angled to point toward the prostate to minimize resolution degradation in that region. Interplane septa extend 5 cm beyond the scintillator crystals to reduce random and scatter backgrounds. A patient is not fully encircled by detector rings in order to minimize cost, causing incomplete sampling due to the side gaps. Monte Carlo simulation (including randoms and scatter) demonstrates the feasibility of detecting and differentiating partial and whole prostate tumors with a tumor-to-background ratio of 2:1, utilizing the number of events that should be achievable with a 6-min scan after a 10 mCi injection (e.g., carbon-11 choline or fluorine-18 fluorocholine).

Longitudinal Evaluation of Left Ventricular Substrate Metabolism, Perfusion, and Dysfunction in the Spontaneously Hypertensive Rat Model of Hypertrophy Using Small-Animal PET/CT Imaging

Myocardial metabolic and perfusion imaging is a vital tool for understanding the physiologic consequences of heart failure. We used PET imaging to examine the longitudinal kinetics of 18F-FDG and 14(R,S)-18F-fluoro-6-thia-heptadecanoic acid (18F-FTHA) as analogs of glucose and fatty acid (FA) to quantify metabolic substrate shifts with the spontaneously hypertensive rat (SHR) as a model of left ventricular hypertrophy (LVH) and failure. Myocardial perfusion and left ventricular function were also investigated using a newly developed radiotracer 18F-fluorodihydrorotenol (18F-FDHROL). Methods: Longitudinal dynamic electrocardiogram-gated small-animal PET/CT studies were performed with 8 SHR and 8 normotensive Wistar-Kyoto (WKY) rats over their life cycle. We determined the myocardial influx rate constant for 18F-FDG and 18F-FTHA (KiFDG and KiFTHA, respectively) and the wash-in rate constant for 18F-FDHROL (K1FDHROL). 18F-FDHROL data were also used to quantify left ventricular ejection fraction (LVEF) and end-diastolic volume (EDV). Blood samples were drawn to independently measure plasma concentrations of glucose, insulin, and free fatty acids (FFAs). Results: KiFDG and KiFTHA were higher in SHRs than WKY rats (P < 3 × 10−8 and 0.005, respectively) independent of age. A decrease in KiFDG with age was evident when models were combined (P = 0.034). The SHR exhibited higher K1FDHROL (P < 5 × 10−6) than the control, with no age-dependent trends in either model (P = 0.058). Glucose plasma concentrations were lower in SHRs than controls (P < 6 × 10−12), with an age-dependent rise for WKY rats (P < 2 × 10−5). Insulin plasma concentrations were higher in SHRs than controls (P < 3 × 10−3), with an age-dependent decrease when models were combined (P = 0.046). FFA levels were similar between models (P = 0.374), but an increase with age was evident only in SHR (P < 7 × 10−6). Conclusion: The SHR exhibited alterations in myocardial substrate use at 8 mo characterized by increased glucose and FA utilizations. At 20 mo, the SHR had LVH characterized by decreased LVEF and increased EDV, while simultaneously sustaining higher glucose and similar FA utilizations (compared with WKY rats), which indicates maladaptation of energy substrates in the failing heart. Elevated K1FDHROL in the SHR may reflect elevated oxygen consumption and decreased capillary density in the hypertrophied heart. From our findings, metabolic changes appear to precede mechanical changes of LVH progression in the SHR model.

Multi-modality phantom development

Multi-modality imaging has an increasing role in the diagnosis and treatment of a large number of diseases, particularly if both functional and anatomical information are acquired and accurately co-registered. Although PET-CT has recently revolutionized the role of imaging for many kinds of cancer, ultrasound is the preferred imaging technology for many diseases such as prostate cancer. Transrectal ultrasound (TRUS) is an integral part of diagnosis and treatment for prostate cancer, so we are developing a dual imaging system that will acquire PET and TRUS data during the same patient imaging session and accurately co-register the images. In order to validate our methods prior to patient imaging, we will use a novel custom PET-TRUS prostate phantom. We present our initial PET- ultrasound phantom development, including PET and ultrasound images of a simple phantom, as well as discuss of our future phantom construction plans. We will use agar-gelatin tissue mimicking materials mixed with radioactive water solutions. Although we are currently focused on prostate imaging, this phantom development is applicable to all PET-ultrasound imaging applications. In addition, we discuss how to make a PET-ultrasound phantom also MRI and/or CT compatible.