CZT detectors: How important is energy resolution for nuclear breast imaging?

Semiconductor detectors, such as Cadmium Zinc Telluride (CZT), offer improved energy resolution (ER) and potentially improved tumor contrast in breast imaging. We evaluated the effect of changes in ER on tumor contrast. Studies were performed on a prototype CZT detector, a commercial CZT-based system, a multicrystal CsI system and a conventional NaI system. The same low energy high resolution collimator was used on each system and the same total counts (250 kcts) were acquired in each image. CZ-Tmodules in a detector can exhibit different ERs. We imaged a breast phantom containing 4 tumors (diameters 4.9, 7.2, 7.2, and 9.8 mm) over individual CZT modules with intrinsic ers of 5.8–17.5%. Tumor to background ratio was 6.2:1 and tumors were placed 1 and 4 cm deep in the 6-cm thick breast phantom. A standard ±10% energy window and narrower energy windows (−5%/+10% and −7%/ +10%) were used. Tumor contrast was determined by maximum tumor intensity divided by mean background intensity. Tumor contrast was not significantly affected by differences in ER between CZT modules. Tumor contrast improved by 2–10% with a narrow energy window. At the same ER, the CZT systems yielded slightly better contrast than the CsI system. All pixilated systems performed better than the NaI system. Scatter in the breast was found to be low and accounts for only ∼11% of counts in a ±10% energy window for a 6 cm- thick breast. No strong relationship was observed between ER and tumor detectability, which may be due to low scatter in the breast. Detector element size in pixilated systems appears to be more important than ER in determining tumor contrast.     

A Monte Carlo investigation of the spatial resolution performance of a small-animal PET scanner designed for mouse brain imaging studies

Our laboratory has developed PET detectors with depth-encoding accuracy of ∼2 mm based on finely pixelated crystals with a tapered geometry, readout at both ends with position-sensitive avalanche photodiodes (PSAPDs). These detectors are currently being used in our laboratory to build a one-ring high resolution PET scanner for mouse brain imaging studies. Due to the inactive areas around the PSAPDs, large gaps exist between the detector modules which can degrade the image spatial resolution obtained using analytical reconstruction with filtered backprojection (FBP). In this work, the Geant4-based GATE Monte Carlo package was used to assist in determining whether gantry rotation was necessary and to assess the expected spatial resolution of the system. The following factors were investigated: rotating vs. static gantry modes with and without compensation of missing data using the discrete cosine transform (DCT) method, two levels of depth-encoding, and positron annihilation effects for ¹⁸F. Our results indicate that while the static scanner produces poor quality FBP images with streak and ring artifacts, the image quality was greatly improved after compensation of missing data. The simulation indicates that the expected FWHM system spatial resolution is 0.70 ± 0.05 mm, which approaches the predicted limit of 0.5 mm FWHM due to positron range, photon non-colinearity and physical detector element size effects. We conclude that excellent reconstructed resolution without gantry rotation is possible even using FBP if the gaps are appropriately handled and that this design can approach the resolution limits set by positron annihilation physics.     

ClearPEM,un TEP dédié au sein

Conventional breast imaging (mammography, ultrasound, MRI) relies on the analysis of anatomical characteristics. Whole-body positron emission tomography (PET) allows for the acquisition of a metabolic image, yet is limited by its poor spatial resolution. The Crystal Clear Collaboration developed a PET dedicated for breast imaging, the ClearPEM, in order to offer a high-resolution nuclear imaging technique. The patient is installed in the prone position on the exam bed, with two detector plates rotating around to breast to acquire a 3-dimensional image. Two prototypes were built and installed in hospitals. We summarize the technical solutions necessary for the development of this system and present a summary of its performances as well as an outlook on preclinical and clinical tests.     

CdTe compact gamma camera for coded aperture imaging in radioguided surgery

The aim of this work was to assess the performance of a prototype compact gamma camera (MediPROBE) based on a CdTe semiconductor hybrid pixel detector, for coded aperture imaging. This probe can be adopted for various tasks in nuclear medicine such as preoperative sentinel lymph node localization, breast imaging with ^99mTc radiotracers and thyroid imaging, and in general in radioguided surgery tasks. The hybrid detector is an assembly of a 1-mm thick CdTe semiconductor detector bump-bonded to a photon-counting CMOS readout circuit of the Medipix2 series or energy-sensitive Timepix detector. MediPROBE was equipped with a set of two coded aperture masks with 0.07-mm or 0.08-mm diameter holes. We performed laboratory measurements of field of view, system spatial resolution, and signal-difference-to-noise ratio, by using gamma-emitting radioactive sources (¹⁰⁹Cd, ¹²⁵I, ²⁴¹Am, ^99mTc). The system spatial resolution in the lateral direction was 0.56 mm FWHM (coded aperture mask with holes of 0.08 mm and a 60 keV source) at a source-collimator distance of 50 mm and a field of view of 40 mm by side. Correspondingly, the longitudinal resolution in 3D source localization tasks was about 3 mm. MediPROBE showed a significant improvement in terms of spatial resolution when equipped with the high-resolution coded apertures, with respect to the performance previously reported with 1–2 mm pinhole apertures as well as with respect to adopting a 0.35 mm pinhole aperture.     

Innovations technologiques récentes en détection pour la Tomographie par Emission de Positons

Since the recognition of the clinical value of Positron Emission Tomography (PET) for the diagnosis and staging of several cancers, the PET systems have evolved to systems associating PET and Computed Tomography (CT). The main constraint for clinical imaging is to reduce the acquisition duration. As a consequence, PET detectors are faster and emit more light than the BGO crystal used previously. These detectors allow an improvement of the count rate performance of the PET systems, reducing the scattered and the random events while increasing the true events at high activity concentration. Among the new crystals, some allow measuring the time of flight of the annihilation photons. This measurement further improves the performance of the systems. The spatial resolution of clinical PET systems is still equal to 5 mm at best. High spatial resolution PET systems dedicated to small animal imaging have been developed. These systems use similar crystal materials as the clinical systems. However, in order to permit spatial resolution close to 1 mm, the crystal elements have much smaller transverse dimensions than that of clinical systems. The detectors are compact using position sensitive photomultipliers or photodiodes. In order to preserve the uniformity of the spatial resolution over the transverse field of view of the tomographs, solutions allowing the measurement of the depth of interaction of the photons in the crystal have been designed. New compact detectors based on semi conductors are currently investigated.     

A single-photon imaging system for the simultaneous quantitation of luminescent emissions from multiple samples

A combination of a two-dimensional photon detector (double-microchannel plate) with single-photon sensitivity and an optical projection system that allows space-resolved quantitation of luminescent emissions from spatially extended objects is described. A "luminescent image" of the object focused onto the detector is accumulated over a preset time and stored in a digital frame memory from which photon counts over areas of interest can be read. In this study, the object consisted of a microtiter plate containing luminescent samples which was placed below a projecting lens (2.0/21 mm, 36 X 24-mm format camera lens) at a distance of 38.5 cm. Although geometry substantially limited photon collection, the sensitivity achieved was only 10X less than that obtained with a dedicated photon-counting luminometer. A slightly diminished photon collection from peripheral wells was apparently caused by the projection system and could be corrected arithmetically. Both chemically generated luminescence (ATP bioluminescence) and cell-derived, superoxide-dependent luminescence (with lucigenin as chemilumigenic probe) were detected with excellent spatial resolution and linearity of response over a wide range.     

Five-year experience of quality control for a 3D LSO-based whole-body PET scanner: Results and considerations

PET scanners require routine monitoring and quality control (QC) to ensure proper scanner performance. QC helps to ensure that PET equipment performs as specified by the manufacturer and that there have not been significant changes in the system response since acceptance. In this work we describe the maintenance history and we report on the results obtained from the PET system QC testing program over 5 years at two centers, both utilizing a Siemens Biograph 16 HiRez PET/CT system. QC testing programs were based on international standards and included the manufacturer’s daily QC, monthly uniformity and sensitivity, quarterly cross-calibration and annual resolution and image quality.For the Winnipeg and Novara sites, two and one PET detector blocks have been replaced, respectively. Neither system has had other significant PET system related hardware replacements. The manufacturer’s suggested daily QC was sensitive to detecting problems in the function of PET detector elements. The same test was not sensitive for detecting long term drifts in the systems: the Novara system observed a significant deterioration over five years of testing in the sensitivity which exhibited a decrease of 16% as compared to its initial value measured at system installation. The measure of the energy spectrum, showed that the 511 keV photopeak had shifted to a position of 468 keV. This shift was corrected by having service personnel perform a complete system calibration and detector block setup.We recommend including tests of system energy response and of sensitivity as part of a QC program since they can provide useful information on the actual performance of the scanner. A modification of the daily QC test by the manufacturer is suggested to monitor the long term stability of the system. Image quality and spatial resolution tests have proven to be of limited value for monitoring the system over time.     

碲化镉（CdTe）探测器的原理及医学应用

本文介绍了一种新型的化合物半导体探测器——CdTe探测器,它具有灵敏度高,探测效率高、能量分辨率好、可以在室温下使用,以及对湿度不敏感和体积小等优点,并介绍了该探测器在医学应用方面的前景。     

An analysis of an alternative scintimammography system

A novel system for scintimammography has been developed, which has high spatial resolution, good sensitivity, better energy resolution, and lower cost than conventional systems. In addition, the system incorporates the potential to provide additional information and functionality beyond imaging, including x,y and z lesion location, integration with a stereotatic biopsy system and images both breasts at the same time. Methods: The system uses sheet NaI (Tl) crystals, PM Tubes, and slant hole collimators allowing complete separation of the breast from the chest wall while remaining in contact with the breast at all depths. The physical characteristics are presented, with images and compared with other systems. Results: The limit of lesion detectability in terms lesion size, depth and contrast has been investigated and compared with a current conventional system. Using accepted patient dose data, the anticipated radiopharmaceutical activity related to the lesion size is used for imaging tests and provides a measure of expected of lesion detectability. Conclusion: Large area, sheet NaI (Tl) detector systems with slant hole collimators are capable of imaging features of below 5mm size at both surface and at clinical depths with the amount of activity expected in lesions of this size with patient administered doses in the clinically acceptable range.     

Non-rigid registration of serial dedicated breast CT,longitudinal dedicated breast CT and PET/CT images using the diffeomorphic demons method

Rationale and objectivesDedicated breast CT and PET/CT scanners provide detailed 3D anatomical and functional imaging data sets and are currently being investigated for applications in breast cancer management such as diagnosis, monitoring response to therapy and radiation therapy planning. Our objective was to evaluate the performance of the diffeomorphic demons (DD) non-rigid image registration method to spatially align 3D serial (pre- and post-contrast) dedicated breast computed tomography (CT), and longitudinally-acquired dedicated 3D breast CT and positron emission tomography (PET)/CT images.MethodsThe algorithmic parameters of the DD method were optimized for the alignment of dedicated breast CT images using training data and fixed. The performance of the method for image alignment was quantitatively evaluated using three separate data sets; (1) serial breast CT pre- and post-contrast images of 20 women, (2) breast CT images of 20 women acquired before and after repositioning the subject on the scanner, and (3) dedicated breast PET/CT images of 7 women undergoing neo-adjuvant chemotherapy acquired pre-treatment and after 1 cycle of therapy.ResultsThe DD registration method outperformed no registration (p < 0.001) and conventional affine registration (p ≤ 0.002) for serial and longitudinal breast CT and PET/CT image alignment. In spite of the large size of the imaging data, the computational cost of the DD method was found to be reasonable (3–5 min).ConclusionsCo-registration of dedicated breast CT and PET/CT images can be performed rapidly and reliably using the DD method. This is the first study evaluating the DD registration method for the alignment of dedicated breast CT and PET/CT images.     

Development of a single detector ring micro crystal element scanner: QuickPET II

This article describes a single ring version of the micro crystal element scanner (MiCES) and investigation of its spatial resolution imaging characteristics for mouse positron emission tomography (PET) imaging. This single ring version of the MiCES system, referred to as QuickPET II, consists of 18 MiCE detector modules mounted as a single ring in a vertical gantry. The system has a 5.76-cm transverse field of view and a 1.98-cm axial field of view. In addition to the scanner and data acquisition system, we have developed an iterative reconstruction that includes a model of the system's detector response function. Evaluation images of line sources and mice have been acquired. Using filtered backprojection, the resolution for a reconstructed line source has been measured at 1.2 mm full width at half maximum. F-18-2-fluoro-2-deoxyglucose mouse PET images are provided. The result shows that QuickPET II has the imaging characteristics to support high-resolution, static mouse PET studies using 18-F labeled compounds.     

Mid-IR microspectroscopic imaging of breast tumor tissue sections

IR microspectroscopic imaging is a relatively new approach for the examination of tissue sections. In contrast to standard light microscopy based procedures, the IR approach requires neither sample staining nor fixation. The IR spectra of breast tumor tissue sections are obtained via a microscope equipped with a focal plane array detector. This enabled the simultaneous collection of individual mid-IR spectra from thousands of different sample positions with a spatial resolution near the diffraction limit. The analysis of the IR data reveals a high sensitivity of the IR approach toward changes in tissue biochemistry and variations in breast tissue architecture. Moreover, the data demonstrate the need for collecting spectra with high spatial resolution at the level of individual cells. This minimizes problems associated with tissue microheterogeneity and is an essential prerequisite for future studies aimed at developing IR microspectroscopic imaging as a complement to present diagnostic tools for breast cancer.     

High-resolution gamma-camera for molecular breast imaging: First clinical results

Scintimair mography is a molecular breast imaging technique using tumour-seeking radiopharmaceuticals; with standard gamma-cameras, is proved of value especially when mammography is indeterminate and in women with dense breasts; nevertheless, this technique shows a high sensitivity only for cancers >1 cm. The issue of detecting small cancers is critical for the future development and clinical usefulness of breast imaging with radiopharmaceuticals, because other modalities are increasingly employed for early identification of small abnormalities. The use of high-resolution dedicated cameras for breast imaging is the best option to improve small cancers' detection: they allow greater flexibility in patient positioning, and the availability of projections similar to those of mammography. Moreover, the detector can be placed directly against the breast, and a mild compression is possible, with the results of reducing breast thickness, increasing the target-to-background ratio and the sensitivity. Our first clinical findings using the dedicased camera Lumagem 3200S (Gamma Medica, Inc., Northridge, USA) are very satisfactory. Till now, 29 patients with BI-RADS category III and IV lesions ≤1 cm were prospectively evaluated using a conventional gamma-camera and the dedicated device. Four out nine (44%) of the malignant lesions were detected with the standard gamma-camera, whereas the high-resolution camera visualized all the breast cancers. The standard gamma-camera and the dedicated one showed the same specificity: 19 out of 20 (95%) benign lesions were negative. Our results indicate that molecular breast imaging with this dedicated camera is able to detect small cancers in patients with probably benign or low-suspicion to indeterminate mammographic findings.     

A task-based evaluation of PEM detector element size

Positron Emission Mammography (PEM) is a planar imaging method that utilizes arrays of discrete detector elements for the detection of radiotracer-avid breast cancer. In this investigation we have systematically studied, through computer simulations, the effect of detector element size (width and length) on breast lesion detection and localization tasks. The contrast-to-noise ratios of the spheres simulating breast lesions were calculated as a function of detector element dimension to gauge detectability. System resolution (fwhm) across the field-of-view was used as the metric for the localization task. For both tasks, individual detector elements of lyso with cross sectional dimensions of 2×2 mm (96×72 element arrays, step 2.1mm) and 3×3mm (65×49 element arrays, step 3.1 mm), and lengths of 10,15 and 20 mm were simulated. The results revealed that narrower pixel dimensions reduced the partial volume effect, while the thicker pixels increased pixel sensitivity, thus reducing noise per pixel and increasing the contrast-to-noise ratio.     

High-Resolution Ultrasound-Switchable Fluorescence Imaging in Centimeter-Deep Tissue Phantoms with High Signal-To-Noise Ratio and High Sensitivity via Novel Contrast Agents

For many years, investigators have sought after high-resolution fluorescence imaging in centimeter-deep tissue because many interesting in vivo phenomena—such as the presence of immune system cells, tumor angiogenesis, and metastasis—may be located deep in tissue. Previously, we developed a new imaging technique to achieve high spatial resolution in sub-centimeter deep tissue phantoms named continuous-wave ultrasound-switchable fluorescence (CW-USF). The principle is to use a focused ultrasound wave to externally and locally switch on and off the fluorophore emission from a small volume (close to ultrasound focal volume). By making improvements in three aspects of this technique: excellent near-infrared USF contrast agents, a sensitive frequency-domain USF imaging system, and an effective signal processing algorithm, for the first time this study has achieved high spatial resolution (~ 900 μm) in 3-centimeter-deep tissue phantoms with high signal-to-noise ratio (SNR) and high sensitivity (3.4 picomoles of fluorophore in a volume of 68 nanoliters can be detected). We have achieved these results in both tissue-mimic phantoms and porcine muscle tissues. We have also demonstrated multi-color USF to image and distinguish two fluorophores with different wavelengths, which might be very useful for simultaneously imaging of multiple targets and observing their interactions in the future. This work has opened the door for future studies of high-resolution centimeter-deep tissue fluorescence imaging.     

A 16-Channel Receive,Forced Current Excitation Dual-Transmit Coil for Breast Imaging at 7T

Purpose

To enable high spatial and temporal breast imaging resolution via combined use of high field MRI, array coils, and forced current excitation (FCE) multi channel transmit.

Materials and Methods

A unilateral 16-channel receive array insert was designed for use in a transmit volume coil optimized for quadrature operation with dual-transmit RF shimming at 7T. Signal-to-noise ratio (SNR) maps, g-factor maps, and high spatial and temporal resolution in vivo images were acquired to demonstrate the utility of the coil architecture.

Results

The dual-transmit FCE coil provided homogeneous excitation and the array provided an increase in average SNR of 3.3 times (max 10.8, min 1.5) compared to the volume coil in transmit/receive mode. High resolution accelerated in vivo breast imaging demonstrated the ability to achieve isotropic spatial resolution of 0.5 mm within clinically relevant 90 s scan times, as well as the ability to perform 1.0 mm isotropic resolution imaging, 7 s per dynamics, with the use of bidirectional SENSE acceleration of up to R = 9.

Conclusion

The FCE design of the transmit coil easily accommodates the addition of a sixteen channel array coil. The improved spatial and temporal resolution provided by the high-field array coil with FCE dual-channel transmit will ultimately be beneficial in lesion detection and characterization.     

Proton computed tomography applied to small biomedical samples

The computed tomography (CT) using the high-energy proton beam has the potential advantage of superior density resolution in the soft tissue analysis as compared to the conventional X-ray CT. We have developed the “first generation” type proton CT system using a 25-MeV proton beam to study the density distributions in small biomedical samples (<6 mm ϕ). The specimens were fixed in a cylindrical sample holder that was immersed in a water bath. The sample holder was scanned in front of the external semimicro beam of protons with a width of 0.06 mm at full width of half maximum (FWHM). The residual energy of the penetrated proton beam was analyzed with a solid state detector (SSD). The obtained integral density projection views were reconstructed into the density image by the filtered back projection method using the Shepp-Logan filter function. The spatial resolution of 0.1 mm and the density resolution of 0.1% were obtained by our system. The technique was applied to the eyes of small animals to study the changes in the density distributions of lenses, which develop with lens opacification in cataracts.     

Advanced radionuclide detection techniques for in vitro and in vivo animal imaging.

This review of the different methodologies used for animal imaging with radioactive compounds presents the most recent approaches developed for both in vitro and in vivo studies. The choice of a detector for analysis of the spatial distribution of radionuclides deposited in biological tissues results in a trade-off between the size and nature of the region to study (in vitro or in vivo), the required spatial resolution and the penetrating characteristics of the ionizing radiation. Real time detectors are now available for quantitative imaging of 2D or 3D radioactive samples and offer either an increased dynamic range or a lowered sensitivity in comparison with film radioautography. For high resolution imaging, two specific techniques are proposed for applications to rodents. The usefulness of self-triggering intensified charge coupled device (STIC) is illustrated for in vitro localization in radiotoxicological studies of alpha-emitters. For in vivo techniques, the performance of positron emission tomography (PET) is discussed, as a promising method of molecular imaging of biological processes.     

Imaging performance of phase-contrast breast computed tomography with synchrotron radiation and a CdTe photon-counting detector

PurposeWithin the SYRMA-CT collaboration based at the ELETTRA synchrotron radiation (SR) facility the authors investigated the imaging performance of the phase-contrast computed tomography (CT) system dedicated to monochromatic in vivo 3D imaging of the female breast, for breast cancer diagnosis.MethodsTest objects were imaged at 38 keV using monochromatic SR and a high-resolution CdTe photon-counting detector. Signal and noise performance were evaluated using modulation transfer function (MTF) and noise power spectrum. The analysis was performed on the images obtained with the application of a phase retrieval algorithm as well as on those obtained without phase retrieval. The contrast to noise ratio (CNR) and the capability of detecting test microcalcification clusters and soft masses were investigated.ResultsFor a voxel size of (60 μm)³, images without phase retrieval showed higher spatial resolution (6.7 mm⁻¹ at 10% MTF) than corresponding images with phase retrieval (2.5 mm⁻¹). Phase retrieval produced a reduction of the noise level and an increase of the CNR by more than one order of magnitude, compared to raw phase-contrast images. Microcalcifications with a diameter down to 130 μm could be detected in both types of images.ConclusionsThe investigation on test objects indicates that breast CT with a monochromatic SR source is technically feasible in terms of spatial resolution, image noise and contrast, for in vivo 3D imaging with a dose comparable to that of two-view mammography. Images obtained with the phase retrieval algorithm showed the best performance in the trade-off between spatial resolution and image noise.     

A Monte Carlo study of the influence of focal spot size,intensity distribution,breast thickness and magnification on spatial resolution of an a-Se digital mammography system using the generalized MTF

In this study the generalized Modulation Transfer Function (GMTF) and the geometric sharpness (S_geo) were used (i) to study the effects of various focal spot sizes (0.04 mm–0.3 mm), x-ray intensity distributions (Gaussian and double Gaussian), breast thicknesses (2–7 cm) and magnifications M (1.0–2.0) on the spatial resolution of an a-Se digital mammography system, (ii) to identify suitable focal spots for magnification mammography and (iii) derive optimum magnifications. For the calculation of GMTF the required components were: focal spot MTF, obtained from theory, detector MTF, scatter MTF and scatter fraction obtained from Monte Carlo simulations. The results showed that focal spots with sizes up to 0.18 mm are suitable for magnification mammography offering a GMTF which is >50% and >20% at the respective object frequencies of 6.5 mm⁻¹ and 9 mm⁻¹. Focal spots with sizes < 0.16 mm and Gaussian. intensity distribution, or sizes ≤ 0.1 mm and double Gaussian, offer a system resolution which improves or does not deteriorate with magnification for most object frequencies. For larger focal spots, i.e. 0.16–0.18 mm for a Gaussian and 0.12–0.18 mm for a double Gaussian. intensity distribution, optimum magnifications exist which depend on the object frequency and breast thickness. System resolution (in terms of S_geo) is maximized at M = 1.8–2.0 (all breast thicknesses) for Gaussian intensity distribution, and at M = 1.4–1.6 (breast thicknesses ≤ 4 cm) and M = 1.6–1.8 (thicker breasts) for double Gaussian.