Floris Jansen

CZT gamma camera with pinhole collimator: Spectral measurements

To date, CZT has been investigated for a variety of nuclear imaging applications using mostly parallel hole collimators. The image quality realized has evolved to make these configurations competitive with the mature conventional configurations using scintillators such as NaI. However the full potential of CZT detectors is still evolving. In our investigations we have found that there are significant additional advantages of the combination of monolithic CZT detectors with pinhole collimators over the known advantages of CZT detectors by themselves. In particular we show here the improved energy response spectrum for a selection of common nuclear medicine isotopes. The improved energy resolution is important to quality control, image fidelity and stability, and new applications such as dual isotope imaging.

Effect of detector energy response on image quality of myocardial perfusion SPECT

The goal of this study was to evaluate the effect of the detector energy response on the quality of 99mTc myocardial perfusion SPECT images. A Data Spectrum torso phantom was prepared to model the low-dose rest portion of a standard one-day myocardial perfusion protocol. Projection data were acquired with a recently developed Ultra-Fast Cardiac SPECT System (UFC, GE Healthcare). UFC utilizes an array of CZT detector modules and pinhole collimators. A point source in air was used to measure the 99mTc spectrum in CZT. In addition to acquiring emission data, the phantom was scanned with high resolution CT and converted into a 3D model for the SimSET Monte Carlo simulation package, which was then used to generate photon history files. We developed a collimator-detector response module that operates on the SimSET photon history files. This module performs multi-pinhole collimation followed by a stochastic energy blurring operation and generates projection data. The simulated CZT detector response was derived from a measured spectrum, and ideal energy response served as reference. Simulation results were compared to actual torso phantom acquisitions. Components of the resulting projection data (amount of primary and scattered photons) and reconstructed slices were compared. For fixed energy acceptance windows, the asymmetric CZT energy response shape leads to a 30% reduction of the scatter component in measured data and contributes to superior reconstructed image quality.

Image quality improvements with time-of-flight positron emission tomography for molecular imaging

The renewed interest in time of flight (TOF) PET is being driven by the development of detector technology and the growth of new molecular imaging applications. In this study we performed Monte Carlo simulations to quantify image quality improvements as a function of timing resolution for imaging scenarios that mimic whole body and brain imaging. We implemented an iterative image reconstruction algorithm that incorporated time-of-flight information. Image quality, as measured through contrast and noise metrics, consistently improved with improved timing resolution. We simulated and compared a conventional PET scanner with a TOF-PET scanner with 200 ps timing resolution. For lesions embedded in 40 cm and 20 cm cylindrical objects, lesion contrast improved by a factor of 3.4 and 2.3, respectively. TOF-PET holds the potential for significantly improved imaging over conventional PET scanners of equivalent sensitivity.

A theory for the ultrasonic modulation of incoherent light in turbid medium

A diffusion approximation to the radiative transfer in a medium with varying refractive index has been proposed as a theoretical model for the ultrasonic tagging of fluorescence or FluoroSound, in a scattering medium. It has been found that the diffuse modulation is a defocusing effect. Defocusing is related to scatter - more the scatter, more the defocusing and there exists a component of the defocusing effect of scatter at the ultrasonic frequency. This is in contrast to the modulation for ballistic photons that originates in the focusing effect of the acoustic lens created by the ultrasonic wave. Simulations with circular phantoms of 1.5 and 2.0cm radius have shown that defocusing is minimum when the acoustic lens is midway between the source and the detector. These results are consistent with physics and demonstrate the capability of the model to function as a predictive tool for FluoroSound instrument design. Both ballistic and diffuse FluoroSound signatures can help in the simultaneous localization of the anomaly and determination of its optical properties. As an adjunct, optimally designed ultrasound beams can be also used to enhance diffuse photon modulation signal through acoustic guidance. Optical properties provide a way to discriminate between normal and diseased tissue. FluoroSound could therefore potentially achieve a fusion of anatomical and functional information non-invasively in a single measurement. The additional information made available by this method will improve the speed and accuracy of optical imaging as a tool in the identification and validation of targets.

Revolving multipinhole SPECT for small animal imaging

Multi-pinhole collimators are often used to obtain the detection sensitivity necessary for small animal SPECT with submillimeter spatial resolution. However, in comparison to pinhole collimators with single aperture, multipinhole acquisitions run the risk of yielding overlapping projection data, which can result in reconstructed images that exhibit multiplexing artifacts. In this work, we study the multiplexing artifacts produced by a nine-pinhole collimator in three different acquisition geometries: (1) circular multipinhole (CMP) in which the nine-pinhole collimator is rotated around the object with a simple circular gantry rotation, (2) helical multipinhole (HMP) that uses a circular gantry rotation combined with a table translation to produce a helical acquisition, and (3) revolving multipinhole (RMP) produced with a circular orbit of the gantry combined with a concurrent revolution of the nine-pinhole collimator. The pinhole configurations were simulated for a SPECT scanner with a 40 40 cm2 detector. The reconstructed images obtained using the different imaging geometries were compared by calculating the lesion detectability index, contrast-ratio, normalized standard deviation and percentage error using a series of simulation experiments. The improved sampling obtained with the RMP imaging geometry produced a faithful reconstruction of a larger FOV compared to the single pinhole (SP) case. For example, the contrast ratio for equivalent noise levels obtained from a Defrise phantom were approximately 98%, 92%, 75% and 35% for RMP, HMP, CMP and SP respectively. Results show that the improved angular sampling of the RMP imaging geometry diminishes the effects of multiplexing patterns thereby facilitating the accurate convergence of the resulting SPECT image.

Uniformity correction using non-uniform floods

Calibration and quality control (QC) of gamma cameras with fixed pinhole collimators presents unique challenges since the usual assumption of uniform flood intensity at the surface of the detector is violated. For the GE Discovery NM 530c∗, a fast cardiac SPECT system with multiple pinholes and CZT detectors focused on the heart, the problem is further complicated by the fact that the plane of the pinhole may not be parallel to either the detector or the flood source. To address this, we derived an expression for the geometrical response to an ideal uniform flood source, then added further terms to approximate the attenuation and scatter behavior of a real flood source. The model was validated with Monte Carlo simulations for a range of angles for both flood and pinhole. Accurate knowledge of the flood source orientation is essential for good uniformity correction; we developed a jig that permits repeatable flood positioning for rapid daily QC. Alternatively we have shown that the angle of the flood can be calculated from observed systematic flood non-uniformity. Experimental measurements show that variations in pinhole penetration as a function of angle can be detected in the residual error of the floods; the magnitude of the effect agrees very well with predictions from a simple model of the knife edge collimator. Uniformity effects attributable to the finite stopping power of CZT were also observed. The methods described in this paper have been implemented in the GE Discovery NM 530c and Discovery NM/CT 570c imaging systems, and portions of the technology are patent pending.

Design of a Modular and Efficient CAMAC/Lab VIEW-Based Data Acquisition System for a Time of Flight PET Test-Bed

A high-speed data acquisition (DAQ) system has been designed for a time of flight PET test-bed. The requirements of the system were flexibility, data throughput and data integrity. The software is modular so that modifications and additions can be integrated easily into the existing software architecture. The program operation is driven by commands read from a script file, simplifying implementation of complex acquisition sequences. The heart of the program is the DAQ module, which efficiently transfers data from CAMAC to file. Another software module offers online or offline analysis capabilities. The software, written in LabVIEW, communicates with a novel high-speed USB2 CAMAC controller (CCUSB). The CCUSB offers significant improvements over its GPIB predecessor, supporting FIFO buffered DAQ and a variety of data readout modes. Four readout modes have been evaluated in order to maximize the DAQ rate for this particular system. A highest sustained data rate of 15.7 k events/s was achieved for approximately 60 input channels using a 22Na flood phantom. Flexibility in the software design accommodates both current and future hardware configurations without the need to edit the LabVIEW code.

Efficient algorithm for modeling keel-edge pinhole response

We describe an efficient method for computing the system response (sensitivity and point spread function) of a conventional gamma camera with multiple keel-edge pinhole collimators, taking into account geometric acceptance effects at the collimator as well as detector response. An efficient way of storing the resulting system matrix permits us to achieve very fast iterative reconstruction with reasonable memory requirements.

A virtual study of shape-based optical reconstruction

We demonstrate the use of a priori knowledge of lesion shape from ultrasound to improve quantitative accuracy of 2D optical tomography. Mean absorption in the lesion can be estimated within 20% of actual value.

Evaluation of different multi-pinhole imaging geometries for SPECT imaging of Parkinsonian disorders

Focusing multi-pinhole (MP) collimators are increasingly being used for small animal as well as targeted VOI imaging. We use focused MP collimators to improve the resolution and sensitivity of photons detected from the striatal region of the brain. Simulations were based on activity distributions derived from clinical SPECT images of normal and Parkinson’s Disease patients injected with 99mTc-Trodat. Radioactive counts extracted from the clinical images were mapped onto regions of the Zubal brain phantom for input into the SPECT simulator. Simulated images were then generated modeling single pinhole (SP), nine pinhole (9PH) and 21 pinhole (21PH) collimators attached to one of the heads of a clinical SPECT scanner. The images were reconstructed using OSEM and evaluated after every iteration. The resulting image quality was evaluated using the ideal VOIs from the Zubal phantom for metrics such as contrast to noise ratio (CNR), bias, mean Uptake Ratio and standard error of the mean. In addition, for cross validation an automated feature detection and analysis tool was used for the detection and stratification of the simulated PD images. The CNR for the 9PH and 21PH was observed to increase by 66% and 81% while the corresponding noise levels dropped by 71% and 84%. Similarly the absolute bias was 64%, 28% and 22% for the SP, 9PH and 21PH respectively. Our results showed an improved performance of the MP collimators over the SP collimator configuration. The 21PH case performed well in terms of CNR and absolute bias, while the 9PH case resulted in the most accurate estimate of the true Uptake Ratio. The MP configurations were consistently observed to be superior to the single pinhole. In conclusion, focusing MP collimators were found to give improved quantification and better resolution compared to traditional SPECT acquisitions. The improved CNR enables more refined 3D visualization of the striatum, which could translate to better stratification of Parkinsonain disorders.