Photon-Counting Detector CT: Key Points Radiologists Should Know

Photon-counting detector (PCD) CT is a new CT technology utilizing a direct conversion X-ray detector, where incident X-ray photon energies are directly recorded as electronical signals. The design of the photon-counting detector itself facilitates improvements in spatial resolution (via smaller detector pixel design) and iodine signal (via count weighting) while still permitting multi-energy imaging. PCD-CT can eliminate electronic noise and reduce artifacts due to the use of energy thresholds. Improved dose efficiency is important for low dose CT and pediatric imaging. The ultra-high spatial resolution of PCD-CT design permits lower dose scanning for all body regions and is particularly helpful in identifying important imaging findings in thoracic and musculoskeletal CT. Improved iodine signal may be helpful for low contrast tasks in abdominal imaging. Virtual monoenergetic images and material classification will assist with numerous diagnostic tasks in abdominal, musculoskeletal, and cardiovascular imaging. Dual-source PCD-CT permits multi-energy CT images of the heart and coronary arteries at high temporal resolution. In this special review article, we review the clinical benefits of this technology across a wide variety of radiological subspecialties.     

Flat-detector computed tomography (FD-CT)

Flat-panel detectors or, synonymously, flat detectors (FDs) have been developed for use in radiography and fluoroscopy with the defined goal to replace standard X-ray film, film-screen combinations and image intensifiers by an advanced sensor system. FD technology in comparison to X-ray film and image intensifiers offers higher dynamic range, dose reduction, fast digital readout and the possibility for dynamic acquisitions of image series, yet keeping to a compact design. It appeared logical to employ FD designs also for computed tomography (CT) imaging. Respective efforts date back a few years only, but FD-CT has meanwhile become widely accepted for interventional and intra-operative imaging using C-arm systems. FD-CT provides a very efficient way of combining two-dimensional (2D) radiographic or fluoroscopic and 3D CT imaging. In addition, FD technology made its way into a number of dedicated CT scanner developments, such as scanners for the maxillo-facial region or for micro-CT applications. This review focuses on technical and performance issues of FD technology and its full range of applications for CT imaging. A comparison with standard clinical CT is of primary interest. It reveals that FD-CT provides higher spatial resolution, but encompasses a number of disadvantages, such as lower dose efficiency, smaller field of view and lower temporal resolution. FD-CT is not aimed at challenging standard clinical CT as regards to the typical diagnostic examinations; but it has already proven unique for a number of dedicated CT applications, offering distinct practical advantages, above all the availability of immediate CT imaging in the interventional suite or the operating room.     

Interferometer-based phase-contrast X-ray computed tomography of colon cancer specimens: comparative study with 4.74-T magnetic resonance imaging and optical microscopy

OBJECTIVE: Phase-contrast X-ray computed tomography (PCCT) with an interferometer can reveal the inner soft tissue structures of biological objects without contrast agent, and the image quality is thought to resemble that of magnetic resonance imaging (MRI). Comparative study among PCCT, MRI, and optical microscopy was undertaken. METHODS: Three formalin-fixed colon cancer specimens from nude mice were imaged both by PCCT with a reconstructed volumetric resolution of (0.018)3 mm3 and 4.74-T MRI with that of (0.075)3 mm3. RESULTS: Phase-contrast X-ray computed tomography with an interferometer clearly demonstrated the inner structures of colon cancer masses, such as cancer, necrosis, surrounding tumor vessels, and skin, in a similar way to low-magnified optical microscopic images and had approximately 4.0-fold higher signal-to-noise ratio than MRI. CONCLUSIONS: With formalin-fixed biological samples, PCCT exhibited higher image quality than MRI and was thought to be suitable for detailed imaging of soft tissue with high volumetric resolution.     

Body and cardiovascular MR imaging at 3.0 T

Potential advantages of magnetic resonance (MR) imaging at 3 T include higher signal-to-noise ratios, better image contrast, particularly in gadolinium-enhanced applications, and better spectral separation for spectroscopic applications. In terms of clinical imaging, these advantages can mean higher-spatial-resolution images, faster imaging, and improved MR spectroscopy. However, achieving superior imaging and spectroscopic quality at 3 T can be challenging. This review discusses many of the problems encountered in body and cardiovascular MR imaging at 3 T, such as increased susceptibility, B1 field inhomogeneity, and increased specific absorption rate. The article also considers solutions that are being pursued, such as parallel imaging, variable-rate selective excitation, and variable flip angle sequences. A review of the most commonly used pulse sequences provides practical tips on how these can be optimized for 3-T imaging. In the coming few years, substantial improvements in 3-T technology for clinical imaging and spectroscopy will undoubtedly be seen. An understanding of the basic principles on which these developments are based will help radiologists translate the advances into better imaging studies and, ultimately, better patient care.     

Wire chambers revisited

Detectors used for radioisotope imaging have, historically, been based on scintillating crystal/photomultiplier combinations in various forms. From the rectilinear scanner through to modern gamma cameras and positron cameras, the basic technology has remained much the same. Efforts to overcome the limitations of this form of technology have foundered on the inability to reproduce the required sensitivity, spatial resolution and sensitive area at acceptable cost. Multiwire proportional chambers (MWPCs) have long been used as position-sensitive charged particle detectors in nuclear and high-energy physics. MWPCs are large-area gas-filled ionisation chambers in which large arrays of fine wires are used to measure the position of ionisation produced in the gas by the passage of charged particles. The important properties of MWPCs are high-spatial-resolution, large-area, high-count-rate performance at low cost. For research applications, detectors several metres square have been built and small-area detectors have a charged particle resolution of 0.4 mm at a count rate of several million per second. Modification is required to MWPCs for nuclear medicine imaging. As gamma rays or X-rays cannot be detected directly, they must be converted into photo- or Compton scatter electrons. Photon-electron conversion requires the use of high atomic number materials in the body of the chamber. Pressurised xenon is the most useful form of gas only photon-electron convertor and has been used successfully in a gamma camera for the detection of gamma rays at energies below 100 keV This camera has been developed specifically for highcount-rate first-pass cardiac imaging. This high-pressure xenon gas MWPC is the key to a highly competitive system which can outperform scintillator-based systems. The count rate performance is close to a million counts per second and the intrinsic spatial resolution is better than the best scintillator-based camera. The MWPC camera produces quantitative ejection fraction information of the highest quality. The detection of higher energy gamma rays has proved more problematical, needing a solid photon-electron convertor to be incorporated into the chamber. Several groups have been working on this problem with modest success so far. The only clinical detectors have been developed for positron emission tomography, where thin lead or lead-glass can provide an acceptable convertor for 511 keV photons. Two MWPC positron cameras have been evaluated clinically and one is now in routine use in clinical oncology. The problems of detection efficiency have not been solved by these detectors although reliability and large-area PET imaging have been proven. The latest development involves a hybrid system in which crystals of barium fluoride are viewed by an MWPC filled with a photosensitive gas. The high detection efficiency of the scintillator is combined with the large sensitive area and good spatial resolution of the MWPC to overcome the limitations of previous MWPC-based positron cameras. The first large-area, clinical camera using this technology is now under development and is expected to perform at least as well as multicrystal positron cameras but at a fraction of the cost.     

Impact of technology on the utilisation of positron emission tomography in lymphoma: current and future perspectives

Positron emission tomography (PET) has now gained a place in the management of patients with cancer, including those with Hodgkin's disease and non-Hodgkin's lymphoma. Restaging studies and those addressing the monitoring of response to treatment are especially in focus. Most of the knowledge gained has been achieved with dedicated BGO-based PET technology, but there are a number of developments that will impact on the use of this metabolic imaging technique in the investigation of patients with lymphoma. The challenges ahead are determined by the need for high-quality whole-body imaging associated with increased patient throughput and the need to investigate the role of new labelled ligands. The latter are likely to yield new insights into tumour cell characterisation, tumour behaviour and tumour outcome assessment. The study of new radiolabelled ligands will impose further demands for rapid dynamic data acquisition and accurate tracer quantification. Current and future developments in PET technology range from the use of new detector materials to different detector geometries and data acquisition modes. The search for alternatives to BGO scintillation materials for PET has led to the development of PET instruments utilising new crystals such as LSO and GSO. The use of these new detectors and the increased sensitivity achieved with 3D data acquisitions represent the most significant current developments in the field. With the increasing demands imposed on the clinical utilisation of PET, issues such as study cost and patient throughput will emerge as significant future factors. As a consequence, low-cost units are being offered by the manufacturers through the utilisation of gamma camera-based SPET systems for PET coincidence imaging. Unfortunately, clinical studies in lymphoma and other cancers have already demonstrated the limitations of this technology, with 20% of lesions <15 mm in size escaping detection. On the other hand, the recent development of combined PET/CT devices attempts to address the lack of anatomical information inherent with PET images, taking advantage of further improvement in patient throughput and hence cost-effectiveness. Preliminary studies using this multimodality imaging approach have already demonstrated the potential of the technique. Although the potential exists, certain technical issues with PET/CT require refinement of the methodology. Such issues include organ movement (such as respiratory motion), which strongly influences the image fusion of a rapidly acquired CT scan with the slower acquisition of a PET dataset, and the derivation of CT-based attenuation coefficients in the presence of contrast agents or metallic implants. The application of the technology for radiotherapy planning also poses a number of associated challenges. Finally, the development of dedicated PET systems based on planar detector arrangements with new detector components has the potential to improve clinical throughput by over 100%, but clinical trials using such systems have still to be carried out in order to establish the associated whole-body image quality.

     

Physical attributes, limitations, and future potential for PET and SPECT

Advances in SPECT and PET imaging hardware, software, and radiotracers are vastly improving the non-invasive evaluation of myocardial perfusion and function. In contrast to traditional dual-headed, sodium iodide crystal and photomultiplier cameras with mechanical collimators, new SPECT camera designs utilize novel, collimators, and solid-state detectors that convert photons directly to electrical signals. These cameras simultaneously collect data from as many as 76 small detectors narrowly focused on the heart. New noise regularization and resolution recovery/noise reduction reconstruction software interprets emitted counts more efficiently and thus more effectively discriminates between useful signals and noise. As a result, shorter acquisition times and/or lower tracer doses produce higher quality SPECT images than were possible before. PET perfusion imaging has benefitted from the introduction of novel detectors that now allow true 3D imaging, new radiopharmaceuticals, and precise attenuation correction (AC). These developments have resulted in perfusion images with higher spatial and contrast resolution that may be acquired in shorter protocols and/or with less patient radiation exposure than traditional SPECT. Hybrid SPECT/CT and PET/CT cameras utilize transmission computed tomographic (CT) scans for AC, and offer the additional clinical advantages of evaluating coronary calcium, myocardial anatomy (including non-invasive CT angiography), myocardial function, and myocardial perfusion in a single imaging procedure.     

Energy-sensitive photon counting detector-based X-ray computed tomography

Energy-sensitive photon counting detectors (PCDs) have recently been developed for medical X-ray computed tomography (CT) imaging and a handful of prototype PCD-CT systems have been built and evaluated. PCDs detect X-rays by using mechanisms that are completely different from the current CT detectors (i.e., energy integrating detectors or EIDs); PCDs count photons and obtain the information of the object tissues (i.e., the effective atomic numbers and mass densities) to be imaged. Therefore, these PCDs have the potential not only for evolution—to improve the current CT images such as providing dose reduction—but also for a revolution—to enable novel applications with a new concept such as molecular CT imaging. The performance of PCDs, however, is not flawless, and thus, it requires integrated efforts to develop PCD-CT for clinical use. In this article, we review the current status and the prediction for the future of PCDs, PCD-CT systems, and potential clinical applications.     

New computed radiography technologies in digital radiography

Digital radiography (DR) has become integral to modern diagnostic radiology. One of the earliest forms of DR, computed radiography (CR) using storage phosphors, has established itself as the mainstay of DR-based diagnostic imaging over the past 20 years. More recently, flat-panel DR systems based on solid state X-ray detectors with integrated, large-area, active-matrix readout electronics are promising further improvements in clinical workflow and image quality. Despite CR's longevity, innovations continue to be made. New developments in CR screen technologies, like structured (needle) screens, and new scanner concepts based on line-at-a-time reading promise major improvements in image quality (comparable to that of flat-panel systems), system through-put and physical size, at a cost comparable to that of today's systems. Thus, despite the advent of flat-panel acquisition systems, there will still be an important role for CR in the foreseeable future. After a brief review of the current state of CR technology, this paper will explore several of these new CR developments and present some examples of their potential clinical impact.     

Impact of technology on the utilisation of positron emission tomography in lymphoma: current and future perspectives

Positron emission tomography (PET) has now gained a place in the management of patients with cancer, including those with Hodgkin's disease and non-Hodgkin's lymphoma. Restaging studies and those addressing the monitoring of response to treatment are especially in focus. Most of the knowledge gained has been achieved with dedicated BGO-based PET technology, but there are a number of developments that will impact on the use of this metabolic imaging technique in the investigation of patients with lymphoma. The challenges ahead are determined by the need for high-quality whole-body imaging associated with increased patient throughput and the need to investigate the role of new labelled ligands. The latter are likely to yield new insights into tumour cell characterisation, tumour behaviour and tumour outcome assessment. The study of new radiolabelled ligands will impose further demands for rapid dynamic data acquisition and accurate tracer quantification. Current and future developments in PET technology range from the use of new detector materials to different detector geometries and data acquisition modes. The search for alternatives to BGO scintillation materials for PET has led to the development of PET instruments utilising new crystals such as LSO and GSO. The use of these new detectors and the increased sensitivity achieved with 3D data acquisitions represent the most significant current developments in the field. With the increasing demands imposed on the clinical utilisation of PET, issues such as study cost and patient throughput will emerge as significant future factors. As a consequence, low-cost units are being offered by the manufacturers through the utilisation of gamma camera-based SPET systems for PET coincidence imaging. Unfortunately, clinical studies in lymphoma and other cancers have already demonstrated the limitations of this technology, with 20% of lesions <15 mm in size escaping detection. On the other hand, the recent development of combined PET/CT devices attempts to address the lack of anatomical information inherent with PET images, taking advantage of further improvement in patient throughput and hence cost-effectiveness. Preliminary studies using this multimodality imaging approach have already demonstrated the potential of the technique. Although the potential exists, certain technical issues with PET/CT require refinement of the methodology. Such issues include organ movement (such as respiratory motion), which strongly influences the image fusion of a rapidly acquired CT scan with the slower acquisition of a PET dataset, and the derivation of CT-based attenuation coefficients in the presence of contrast agents or metallic implants. The application of the technology for radiotherapy planning also poses a number of associated challenges. Finally, the development of dedicated PET systems based on planar detector arrangements with new detector components has the potential to improve clinical throughput by over 100%, but clinical trials using such systems have still to be carried out in order to establish the associated whole-body image quality.     

Ex vivo characterization of pathologic fluids with quantitative phase-contrast computed tomography

PurposeX-ray phase-contrast imaging (PCI) provides additional information beyond absorption characteristics by detecting the phase shift of the X-ray beam passing through material. The grating-based system works with standard polychromatic X-ray sources, promising a possible clinical implementation. PCI has been shown to provide additional information in soft-tissue samples. The aim of this study was to determine if ex vivo quantitative phase-contrast computed tomography (PCCT) may differentiate between pathologic fluid collections.Materials and methodsPCCT was performed with the grating interferometry method. A protein serial dilution, human blood samples and 17 clinical samples of pathologic fluid retentions were imaged and correlated with clinical chemistry measurements. Conventional and phase-contrast tomography images were reconstructed. Phase-contrast Hounsfield Units (HUp) were used for quantitative analysis analogously to conventional HU. The imaging was analyzed using overall means, ROI values as well as whole-volume-histograms and vertical gradients. Contrast to noise ratios were calculated between different probes and between imaging methods.ResultsHUp showed a very good linear correlation with protein concentration in vitro. In clinical samples, HUp correlated rather well with cell count and triglyceride content. PCI was better than absorption imaging at differentiating protein concentrations in the protein samples as well as at differentiating blood plasma from cellular components. PCI also allowed for differentiation of watery samples (such as lymphoceles) from pus.ConclusionPhase-contrast computed tomography is a promising tool for the differentiation of pathologic fluids that appear homogenous with conventional attenuation imaging.     

CT of the musculoskeletal system: What is left is the days of MRI?

Magnetic resonance imaging (MRI) plays a central role in the modern imaging of musculoskeletal disorders, due to its ability to produce multiplanar images and characterise soft tissues accurately. However, computed tomography (CT) still has an important role to play, not merely as an alternative to MRI, but as being the preferred imaging investigation in some situations. This article briefly reviews the history of CT technology, the technical factors involved and a number of current applications, as well as looking at future areas where CT may be employed. The advent of ever-increasing numbers of rows of detectors has opened up more possible uses for CT technology. However, diagnostic images may be obtained from CT systems with four rows of detectors or more, and their ability to produce near isotropic voxels and therefore multiplanar reformats.     

Artificial intelligence in cardiovascular CT: Current status and future implications

Artificial intelligence (AI) refers to the use of computational techniques to mimic human thought processes and learning capacity. The past decade has seen a rapid proliferation of AI developments for cardiovascular computed tomography (CT). These algorithms aim to increase efficiency, objectivity, and performance in clinical tasks such as image quality improvement, structure segmentation, quantitative measurements, and outcome prediction. By doing so, AI has the potential to streamline clinical workflow, increase interpretative speed and accuracy, and inform subsequent clinical pathways. This review covers state-of-the-art AI techniques in cardiovascular CT and the future role of AI as a clinical support tool.     

PET/MR在心血管疾病中的应用进展

PET/MR作为新型的多模态成像技术,集合了MRI高软组织对比度、多序列、多参数、可定量和PET多分子探针显像的高灵敏度的优势,实现了PET分子功能影像与具有精细解剖结构和组织特征的MRI影像同步扫描,在多种心血管疾病中的应用和研究越来越广泛。笔者总结PET/MR心血管成像的优点和局限性,并探讨其在临床中的主要应用。     

Grundlagen der Flachdetektor-CT (FD-CT)

Flat detectors (FDs) have been developed for use in radiography and fluoroscopy to replace standard X-ray film, film-screen combinations and image intensifiers (II). In comparison to X-ray film and II, FD technology offers higher dynamic range, dose reduction, fast digital readout and the possibility for dynamic acquisitions of image series, yet keeping to a compact design. It appeared logical to employ FD designs also for computed tomography (CT) imaging. FDCT has meanwhile become widely accepted for interventional and intra-operative imaging using C-arm systems. Additionally, the introduction of FD technology was a milestone for soft-tissue CT imaging in the interventional suite which was not possible with II systems in the past. This review focuses on technical and performance issues of FD technology and its wide range of applications for CT imaging. FDCT is not aimed at challenging standard clinical CT as regards to the typical diagnostic examinations, but it has already proven unique for a number of dedicated CT applications offering distinct practical advantages, above all the availability of immediate CT imaging during an intervention.     

The Changing Design of Positron Imaging Systems

This is a time of rapid change in the evolution of clinical positron imaging systems, spurred by the wider availability of (18)F-fluorodeoxyglucose, continued increase in clinical research trials showing the clinical efficacy of PET in numerous cancers, coronary artery disease, and a variety of neurological disorders, recent successes in changing the FDA approval process for PET radiopharmaceuticals, and the recent decision of HCFA to reimburse for PET procedures. To adapt to this new environment for clinical positron imaging, a number of approaches are being taken to cut the cost of positron imaging systems while still maintaining the inherent sensitivity and quantitative advantages that relate to coincidence imaging. In this article, we present some of the newer developments in positron imaging systems, discuss some of the limitations with currently available "low-cost" systems, and point the way to some future developments that will rapidly result in improved systems for clinical positron imaging. It should be noted that PET scanners have gone through many generations of technology advancements. These advancements along with new detector materials, new image reconstruction algorithms that improve signal-to-noise, lower cost/higher performance computer technology, and the present infectious enthusiasm for new ideas are all converging to produce a highly competitive marketplace of new camera solutions for clinical positron imaging.     

The role of hybrid cameras in oncology

The rapid advances in imaging technologies are a challenge for nuclear medicine physicians, radiologists, and clinicians who must integrate these technologies for optimal patient care and outcome at minimal cost. Multiple indications for functional imaging using F-18-fluorodeoxyglucose (FDG) are now well accepted in the field of oncology, including differentiation of benign from malignant lesions, staging malignant lesions, detection of malignant recurrence, and monitoring therapy. The use of FDG imaging was first shown using dedicated positron emission tomography (PET) with multiple full rings of bismuth germanate detectors. Most manufacturers now have available hybrid gamma cameras capable of imaging conventional single-photon emitters, as well as positron emitters such as FDG. This new technology was developed to make FDG imaging more widely accessible, first using single photon emission computed tomography (SPECT) with high-energy collimators, and then using dualhead coincidence (DHC) detection with multihead gamma cameras that improved spatial resolution. Most hybrid gamma cameras are now equipped with thicker NaI(TI) crystals to improve sensitivity. Technical developments are still evolving with correction for attenuation and new iterative reconstruction algorithms to improve the quality of the images. Users need to be familiar with the rapid developments of the technology as well as its limitations. Currently, one model of hybrid gamma camera is equipped with an integrated x-ray transmission system for attenuation correction, anatomic mapping, and image fusion. This powerful tool has promising clinical applications including intensity-modulated radiation therapy.     

Second annual Mario S. Verani,md, memorial lecture: nuclear cardiology,the next 10 years

The nuclear cardiology of the future will be based on new clinical and biologic targets. It will be driven by modern concepts of molecular and cell biology and molecular genetics. A major effort involves detection of atherosclerosis and vascular vulnerability. Approaches include targeting proliferating smooth muscle cells, angiogenesis, vascular injury, inflammation through a variety of mechanisms, defining cell death and protease activation, and imaging gene expression. Another new clinical target involves imaging stem cells and various progenitor cells. To meet these new objectives, advanced imaging technology is required. This involves the development of micro-single photon emission computed tomography and micro-positron emission tomography systems as well as fusion technology involving radiologic computed tomography imaging together with nuclear imaging. Vascular lesion detection imaging may require intravascular detectors. The future of nuclear cardiology, based on molecular imaging, is extraordinarily exciting. The newly defined biologic targets will allow the answering of many of the key clinical questions that will dominate cardiovascular care in cardiovascular investigation over the next decade.     

Single-photon emission computed tomography in the year 2001: instrumentation and quality control

OBJECTIVE: SPECT instrumentation is more complex than that used for whole-body and planar imaging, and requires careful quality control to ensure optimum performance. Conventional and new hybrid SPECT imaging systems (coincidence and SPECT/CT) will be discussed. New imaging detector materials such as LSO and CZT will also be discussed, along with their potential advantages. Finally, basic SPECT quality control will be reviewed. After reading this article, the nuclear medicine technologist should be able to: (a) explain the use of single and multihead gamma cameras for SPECT imaging; (b) have an understanding of the potential of new hybrid SPECT imaging systems; (c) be aware of future developments in SPECT imaging technology; (d) understand the requirements for SPECT quality control, including field uniformity and center of rotation corrections; and (e) explain the benefits of using phantoms to augment SPECT quality control.