Jan S. Iwanczyk

Vision 20/20: Single photon counting x-ray detectors in medical imaging.

Photon counting detectors (PCDs) with energy discrimination capabilities have been developed for medical x-ray computed tomography (CT) and x-ray (XR) imaging. Using detection mechanisms that are completely different from the current energy integrating detectors and measuring the material information of the object to be imaged, these PCDs have the potential not only to improve the current CT and XR images, such as dose reduction, but also to open revolutionary novel applications such as molecular CT and XR imaging. The performance of PCDs is not flawless, however, and it seems extremely challenging to develop PCDs with close to ideal characteristics. In this paper, the authors offer our vision for the future of PCD-CT and PCD-XR with the review of the current status and the prediction of (1) detector technologies, (2) imaging technologies, (3) system technologies, and (4) potential clinical benefits with PCDs.

Photon Counting Energy Dispersive Detector Arrays for X-ray Imaging

The development of an innovative detector technology for photon-counting in X-ray imaging is reported. This new generation of detectors, based on pixellated cadmium telluride (CdTe) and cadmium zinc telluride (CZT) detector arrays electrically connected to application specific integrated circuits (ASICs) for readout, will produce fast and highly efficient photon-counting and energy-dispersive X-ray imaging. There are a number of applications that can greatly benefit from these novel imagers including mammography, planar radiography, and computed tomography (CT). Systems based on this new detector technology can provide compositional analysis of tissue through spectroscopic X-ray imaging, significantly improve overall image quality, and may significantly reduce X-ray dose to the patient. A very high X-ray flux is utilized in many of these applications. For example, CT scanners can produce ~ 100 Mphotons/mm2 /s in the unattenuated beam. High flux is required in order to collect sufficient photon statistics in the measurement of the transmitted flux (attenuated beam) during the very short time frame of a CT scan. This high count rate combined with a need for high detection efficiency requires the development of detector structures that can provide a response signal much faster than the transit time of carriers over the whole detector thickness. We have developed CdTe and CZT detector array structures which are 3 mm thick with 16 times 16 pixels and a 1 mm pixel pitch. These structures, in the two different implementations presented here, utilize either a small pixel effect or a drift phenomenon. An energy resolution of 4.75% at 122 keV has been obtained with a 30 ns peaking time using discrete electronics and a 57Co source. An output rate of 6 times 106 counts per second per individual pixel has been obtained with our ASIC readout electronics and a clinical CT X-ray tube. Additionally, the first clinical CT images, taken with several of our prototype photon-counting and energy-dispersive detector modules, are shown.

Performance evaluation of A-SPECT: a high resolution desktop pinhole SPECT system for imaging small animals

Pinhole collimation of gamma rays to image distributions of radiolabeled tracers is considered promising for use in small animal imaging. The recent availability of transgenic mice, coupled with the development of /sup 125/I and /sup 99m/Tc labeled tracers, has allowed the study of a range of human disease models while creating demand for ultrahigh resolution imaging devices. We have developed a compact gamma camera that, in combination with pinhole collimation, allows for accessible, ultrahigh resolution in vivo single photon emission computed tomography (SPECT) imaging of small animals. The system is based on a pixilated array of NaI(Tl) crystals coupled to an array of position sensitive photomultiplier tubes. Interchangeable tungsten pinholes with diameters ranging from 0.5 to 3 mm are available, allowing the camera to be optimized for a variety of imaging situations. We use a three dimensional maximum likelihood expectation maximization algorithm to reconstruct the images. Our evaluation indicates that high quality, submillimeter spatial resolution images can be achieved in living mice. Reconstructed axial spatial resolution was measured to be 0.53, 0.74, and 0.96 mm full width at half maximum (FWHM) for rotation radii of 1, 2, and 3 cm, respectively, using the 0.5-mm pinhole. In this configuration, sensitivity is comparable to that of a high-resolution parallel hole collimator. SPECT images of hot- and cold-rod phantoms and a highly structured monkey brain phantom illustrate that high quality images can be obtained with the system. Images of living mice demonstrate the ability of the system to obtain high-resolution images in vivo. The effect of object size on the quantitative assessment of isotope distributions in an image was also studied.

An analytical model of the effects of pulse pileup on the energy spectrum recorded by energy resolved photon counting x-ray detectors.

Purpose: Recently, novel CdTe photon counting x-ray detectors (PCXDs) with energy discrimination capabilities have been developed. When such detectors are operated under a high x-ray flux, however, coincident pulses distort the recorded energy spectrum. These distortions are called pulse pileup effects. It is essential to compensate for these effects on the recorded energy spectrum in order to take full advantage of spectral information PCXDs provide. Such compensation can be achieved by incorporating a pileup model into the image reconstruction process for computed tomography, that is, as a part of the forward imaging process, and iteratively estimating either the imaged object or the line integrals using, e.g., a maximum likelihood approach. The aim of this study was to develop a new analytical pulse pileup model for both peak and tail pileup effects for nonparalyzable detectors. Methods: The model takes into account the following factors: The bipolar shape of the pulse, the distribution function of time intervals between random events, and the input probability density function of photon energies. The authors used Monte Carlo simulations to evaluate the model. Results: The recorded spectra estimated by the model were in an excellent agreement with those obtained by Monte Carlo simulations for various levels of pulse pileup effects. The coefficients of variation (i.e., the root mean square difference divided by the mean of measurements) were 5.3%–10.0% for deadtime losses of 1%–50% with a polychromatic incident x-ray spectrum. Conclusions: The proposed pulse pileup model can predict recorded spectrum with relatively good accuracy.

Intraoperative probes and imaging probes

Abstract. Intraoperative probes have been employed to assist in the detection and removal of tumors for more than 50 years. For a period of about 40 years, essentially every detector type that could be miniaturized had been tested or at least suggested for use as an intraoperative probe. These detectors included basic Geiger-Müller (GM) tubes, scintillation detectors, and even state-of-the-art solid state detectors. The radiopharmaceuticals have progressed from 32PO4- injections for brain tumors to sophisticated monoclonal antibodies labeled with iodine-125 for colorectal cancers. The early work was mostly anecdotal, primarily interdisciplinary collaborations between surgeons and physical scientists. These collaborations produced a few publications, but never seemed to result in an ongoing clinical practice. In the mid 1980s, several companies offered basic gamma-detecting intraoperative probes as products. This led to the rapid development of radioimmunoguided surgery (RIGS) and sentinel node detection as regularly practiced procedures to assist in the diagnosis and treatment of cancer. In recent years intraoperative imaging probes have been developed. These devices add the ability to see the details of the detected activity, giving the potential of using the technique in a low-contrast environment. Intraoperative probes are now established as clinical devices, they have a commercial infrastructure to support their continued use, and there is ongoing research, both commercial and academic, that would seem to ensure continued progress and renewed interest in this slowly developing field.

Pinhole SPECT of mice using the LumaGEM gamma camera

LumaGEM is a newly developed gamma camera for dedicated, small field of view, high spatial resolution imaging. The system consists of an array of 2/spl times/2/spl times/6 mm/sup 3/ NaI(Tl) pixels coupled to an array of position-sensitive photomultiplier tubes. It has a 125/spl times/125 mm/sup 2/ field of view. A pinhole collimator was used on LumaGEM to acquire SPECT images of mice that had transgenic modifications so as to model various diseases. Pinhole apertures of 1, 2 and 3 mm are interchangeable on the collimator and were used to acquire images. An iterative MLEM algorithm for pinhole SPECT was used to reconstruct the 128 projection images that covered 360/spl deg/ rotation. The reconstruction algorithm is based on a projector and backprojector pair implemented using a ray-tracing algorithm. The crucial reconstruction input parameters are the radius of rotation, center of rotation, and pinhole focal length. Ideal pinhole geometry is assumed, and no correction for attenuation has been made. The preliminary images presented here show detailed uptake in the mice subjects and are a convincing sign that animal SPECT can reach submillimeter spatial resolution and be a valuable tool in the study of diseases and the development of pharmaceuticals in animal models.

Quantitative fluorescence tomography using a combined tri-modality FT/DOT/XCT system

In this work, a first-of-its-kind fully integrated tri-modality system that combines fluorescence, diffuse optical and x-ray tomography (FT/DOT/XCT) into the same setting is presented. The purpose of this system is to perform quantitative fluorescence tomography using multi-modality imaging approach. XCT anatomical information is used as structural priori while optical background heterogeneity information obtained by DOT measurements is used as functional priori. The performance of the hybrid system is evaluated using multi-modality phantoms. In particular, we show that a 2.4 mm diameter fluorescence inclusion located in a heterogeneous medium can be localized accurately with the functional a priori information, although the fluorophore concentration is recovered with 70% error. On the other hand, the fluorophore concentration can be accurately recovered within 8% error only when both DOT optical background functional and XCT structural a priori information are utilized to guide and constrain the FT reconstruction algorithm.

Characterization of a novel photon counting detector for clinical CT: count rate, energy resolution, and noise performance

We report on a characterization study of a multi-row direct-conversion x-ray detector used to generate the first photon counting clinical x-ray computed tomography (CT) patent images. In order to provide the photon counting detector with adequate performance for low-dose CT applications, we have designed and fabricated a fast application specific integrated circuit (ASIC) for data readout from the pixellated CdTe detectors that comprise the photon counting detector. The cadmium telluride (CdTe) detector has 512 pixels with a 1 mm pitch and is vertically integrated with the ASIC readout so it can be tiled in two dimensions similar to those that are tiled in an arc found in 32-row multi-slice CT systems. We have measured several important detector parameters including the maximum output count rate, energy resolution, and noise performance. Additionally the relationship between the output and input rate has been found to fit a non-paralyzable detector model with a dead time of 160 nsec. A maximum output rate of 6 × 106 counts per second per pixel has been obtained with a low output x-ray tube for CT operated between 0.01 mA and 6 mA at 140 keV and different source-to-detector distances. All detector noise counts are less that 20 keV which is sufficiently low for clinical CT. The energy resolution measured with the 60 keV photons from a 241Am source is ~12%. In conclusion, our results demonstrate the potential for the application of the CdTe based photon counting detector to clinical CT systems. Our future plans include further performance improvement by incorporating drift structures to each detector pixel.

A 3D gantry single photon emission tomograph with hemispherical coverage for dedicated breast imaging

Abstract A novel tomographic gantry was designed, built and initially evaluated for single photon emission imaging of metabolically active lesions in the pendant breast and near chest wall. Initial emission imaging measurements with breast lesions of various uptake ratios are presented. Methods: A prototype tomograph was constructed utilizing a compact gamma camera having a field-of-view of Results: As iteration number increased for the tomographically measured data at all polar angles, contrasts increased while signal-to-noise ratios (SNRs) decreased in the expected way with OSEM reconstruction. The rollover between contrast improvement and SNR degradation of the lesion occurred at two to three iterations. The reconstructed tomographic data yielded SNRs with or without scatter correction that were >9 times better than the planar scans. There was up to a factor of ∼2.5 increase in total primary and scatter contamination in the photopeak window with increasing tilt angle from 15° to 45°, consistent with more direct line-of-sight of myocardial and liver activity with increased camera polar angle. Conclusion: This new, ultra-compact, dedicated tomographic imaging system has the potential of providing valuable, fully 3D functional information about small, otherwise indeterminate breast lesions as an adjunct to diagnostic mammography.

Mercuric iodide (HgI2) platelets for x-ray spectroscopy produced by polymer controlled growth

Abstract The low temperature red form of mercuric iodide has been grown by a new chemical transport method which introduces organic monomers or polymers during the crystal growth process. Resulting crystals are in the form of platelets which are more directly useful in radiation detector device application. Platelets near one centimeter in width and 200 μm in thickness have been grown in periods of a few days using only 99.9% (unpurified) starting material. Measurements of X-ray and gamma ray energy resolution from detectors fabricated from platelets have yielded a 1.15 keV (FWHM) value for the 59.5 keV Am-241 line and 400 ev (FWHM) for the 5.9 keV Fe-55 line. These are again among the highest resolution values ever measured for HgI2. Electrical carrier transport property values of HgI2 so grown equal the best values previously measured from vapor grown material.