Volume 35, Issue 6, June 2008
Index of content:
- Imaging Symposium: Room 342
Ultrasound for the 21st Century
35(2008); http://dx.doi.org/10.1118/1.2962369View Description Hide Description
Similar to the paradigm change of the 1980's, when ultrasound labs shifted from large, manual scanners to cart‐based, real‐time scanners, we now see a move in the industry towards small, portable ultrasound units (PUU) having sophisticated imaging capabilities. Unlike their predecessors, however, the cart‐based scan consoles are likely to continue to play an important role in medical imaging, with PUU's significantly expanding the tasks carried out by this modality. This presentation highlights these anticipated changes.
Ultrasound machines continue to use sequential, line‐by‐line echo acquisition and traditional array beam forming. This limits frame rates because of the sound propagation speed in tissue, a significant problem as 2D arrays producing 3D data sets are developed. One manufacturer has developed a “zone” transmission method with synthetic aperture processing of element data. This improves frame rate and offers new capabilities, such as adaptive mean sound speed corrections.
Operator controls, even on very basic machines, are numerous and encompass many echo acquisition and signal processing functions. Although application specific presets simplify the task of precise adjustment of these controls, there is a growing trend to develop machines that do away with traditional operator adjustments and automatically set important functions, such as overall gain, TGC, and transmit focus.
Echo amplitude information continues to serve as the primary data presented to users in non‐Doppler operating modes. This now is being supplemented by elasticityimaging, where tissue stiffness images are formed and displayed, and may soon be extended further by displays of tissueproperties that are not evident in conventional B‐mode, such as scatter sizes and attenuation.
Much development activity is taking place in PUU's, with varying capabilities and intended uses. Full featured portable machines allow sonographers to access patients not easily scanned with larger cart based machines. Inexpensive, special purpose PUU's are being directed to new users, such as emergency room physicians, and anesthesiologists.
1. Understand how current ultrasound machines operate and appreciate limitations of these devices.
2. Understand tradeoffs when choosing high end vs. portable ultrasound machines.
3. Understand sources of new information from ultrasoundscanners.
35(2008); http://dx.doi.org/10.1118/1.2962370View Description Hide Description
MO‐D‐342‐03: Breakthroughs in Elasticity, Ultrasound Computed Tomography, and Optoacoustic Tomography35(2008); http://dx.doi.org/10.1118/1.2962371View Description Hide Description
The idea of measuring the stiffness of soft tissues using ultrasound was suggested in the literature some 25 years ago. Since then, the measurement and later imaging of the local elastic properties of tissues have progressed from this idea to a new commercial reality that is based on solid fundamentals. Today, it is possible to obtain high‐resolution real‐time images (elastograms) of the axial strain components in soft tissues in vivo that are subjected to an external or internal mechanical load. These images have shown that new and potentially useful information can be obtained, far beyond that which is available from sonograms alone. The road ahead involves several additional possibilities of gleaning substantially more information relating to the mechanics of tissues. These include the calculation and imaging of the elastic modulus,imaging the Poisson's ratio and its temporal evolution for the study of fluid flow in tissues that are affected by diseases such as lymphedema, and the imaging of shear strains at tissue boundaries that characterize the bonding strength between tissue layers that may be specific for various disease states. This talk will illustrate some of the progress in the field and will demonstrate some of the diverse future possibilities.
This work was supported by the National Cancer Institute (USA) Grants R01‐CA60520, P01‐CA64597‐10, R21‐CA127291 and by the John Dunn Foundation.
35(2008); http://dx.doi.org/10.1118/1.2962372View Description Hide Description
Ultrasound(US) is an indispensable tool of medical imaging that has sustained remarkable growth over the past several decades. It is interesting to note that despite undergoing many significant technical advancements over this period, the basic imaging paradigm of US has remained unchanged, namely image formation results from transmit‐receive detection of 180 degree backscatter assuming straight‐line propagation. For more that 20 years, quantitative UltrasoundComputed Tomography (USCT), which attempts to create images from both transmitted and scattered signals, has been researched by several groups with varying degrees of success. It can be shown that conventional USimaging is actually a subset of USCT and to this end a brief historical perspective on the pioneering works of many scientists will be presented, including some present at this conference.
While pulse‐echo US provides a two‐dimensional image of relative tissue “echogenicity,” USCT attempts to compute quantitative three‐dimensional maps of tissueacoustic properties, usually sound speed, attenuation, compressibility, scatter density, etc., while accounting for refraction, reflection, multiple scattering and more. Significantly, laboratory and in vivo measurements have suggested that normal breast tissue, benign lesions and cancerous lesions may be identified by these inherent acoustic properties (particularly sound speed and attenuation). In general, due to limitations of both instrumentation and algorithms, the early methods used two‐dimensional linearization techniques to solve what is inherently a non‐linear and three dimensional problem. In order to capture a large segment of the scatter field around the object, these methods have been attempted only in the breast. From this early work it is now clear that the range of tissue properties encountered in the breast is sufficiently large that linear approximations lead to severe artifacts and inadequate spatial resolution. Recently, new methods capable of implementing advanced USCT algorithms in patients and are progressing towards clinical use. A review of these new scanner systems will be presented.
One such breast scanner, developed by Techniscan Medical Systems, Inc. (Salt Lake City, Utah), was installed at the University of California, San Diego to evaluate clinical feasibility of using USCT to analyze and detect breast masses. The system uses a multi‐frequency non‐linear 3D inverse‐scattering algorithm. Until very recently the engineering technology and mathematical methods for full‐wave inverse‐scattering 3D tomography have been so complex that practical results in humans were not realized. To solve the numerically ill‐conditioned problem of full‐wave inversion, discrete frequency domain data is used by a 3D inverse‐scattering algorithm that incorporates multiple scattering within and between the planes. The starting estimate for the 3‐D algorithm is a time‐of‐flight reconstruction followed by a series of 2‐D inverse scattering reconstructions that are concatenated together. Local minima encountered in the non‐linear optimization are avoided by discrete frequency hopping from low to high (0.3–2 MHz). Despite computational complexity of the problem, the method solves an accurate approximation to the full Helmholtz equation. Recent advancements using a GPU cluster complete the full inversion within approximately 30 minutes, a remarkable accomplishment given the very large computation cost. In its current form, 3D volumes of the entire breast are reconstructed as accurate maps of sound speed, attenuation and aberration‐corrected reflectivity.
Additional motivation for the continued interest in USCT is that conventional breast sonography is a notoriously difficult exam to perform. The quality and reproducibility of the results are highly dependent on the skill of the operator and the radiologist as well as the technical features of the scanner. In order to obtain the needed high resolution the field of view in sonography is very small, which greatly complicates interpretation and localization of a mass. USCT promises an essentially automated scanning system that does not depend on operator expertise. Furthermore, the images present a global view of the entire breast in 3D, facilitating comparison to prior exams including mammography and MRI, as well as aiding surgical intervention.
35(2008); http://dx.doi.org/10.1118/1.2962373View Description Hide Description
In 1880 Alexander G. Bell heard “a pure musical tone” in a closed gas volume that had absorbed a modulated sunlight beam. However, it would be a century before interest in the photoacoustic effect stimulated physicists to employ this discovery in novel medical instruments. In the beginning of the 21st century, opto‐acoustic tomography (OAT) emerged as a sensitive modality for visualization and quantitative characterization of malignant tumors and blood vessels. OAT combines the most compelling features of light and sound to provide maps of absorbed optical energy in optically scattering and opaque media including biological tissues. The new hybrid modality improves spatial resolution of the optical imaging and contrast of the ultrasound imaging.
The basic principles behind the optoacousticimagingsystem are that (1) laser pulses may be effectively used to produce acoustic sources in tissues with enhanced optical absorption, and (2) ultrasonic waves propagate in biological tissues as expanding spheres with minimal wavefront distortion and deliver temporarily resolved information to the surface of tissue where it may be detected. The application of transducer arrays permits reconstruction of two‐dimensional and three‐dimensional images. One of the main endogenous chromophores of tissue in the near‐infrared spectral range is the hemoglobin of blood. Therefore, blood vessels possess high optoacousticcontrast. Malignant solid tumors develop an enhanced network of microvessels to supply nutrition and oxygen to aggressively growing cancer cells. Therefore, optical contrast between normal and canceroustissues is substantially greater than the contrast utilized in ultrasound imaging and other imaging modalities. Furthermore, functional information about hemoglobin concentration and its level of oxygen saturation in tumors can serve as a basis for noninvasive diagnostic utility of OAT. The empirical rule of thumb is that optoacoustic resolution equals depth / 100, so that at the depth of 50 mm one can obtain resolution of about 0.5 mm, while typical resolution is about 50 micron at the depth of 5 mm. Experimental schemes of optoacousticimagingsystem for two‐dimensional and three‐dimensional optoacoustictomography as well as corresponding algorithms of image reconstruction will be discussed.
The niche of the optoacoustictomography in biomedical imaging is to provide high‐resolution 3D maps containing (1) functional information on blood concentration and its oxygen saturation, and (2) molecular content of endogenous or exogenous chromophores. Clinical studies performed in breast cancer patients will be presented to demonstrate that the functional imaging capability of OAT provides additional medically relevant information regarding breast tumors, which results in better sensitivity and specificity of cancer detection. The molecular imaging capability of OAT is enabled by variation of the optical wavelength for selective heating of specific chromophores administered and targeted to the site of interest. A unique opportunity for further substantial enhancement of the optoacoustic detection sensitivity comes from merging OAT with plasmonic nanotechnology. An optoacousticcontrast agent based on gold nanorods selectively delivered to cancer cells in order to substantially increase brightness of canceroustumors will be described. The same contrast agent can serve potentially as a therapeutic agent for treatment of early cancer.
Current and Future Research Trends in Nuclear Medicine
35(2008); http://dx.doi.org/10.1118/1.2962407View Description Hide Description
Nuclear Medicine instrumentation continues to progress, both hardware and software. Advances in PET/CT include increased PET sensitivity, respiratory gating, time of flightmeasurement, and improved image reconstruction algorithms that encorporate more physical effects into the acquisition model. Gamma cameras are incorporating new detector materials, and SPECT is being done with new acquisition geometries and reconstructed with new algorithms.
We will discuss new developments in nuclear medicine instrumentation, including ones that are currently available commercially and ones that are on the horizon.
1. Understand three new developments in PET that have the potential to improve image quality.
2. Understand how new detector materials allow new acquisition geometries for SPECT.
3. Understand how physic effects in the acquisition process can be modeled in the image reconstruction process.
Research sponsored by GE Healthcare.
35(2008); http://dx.doi.org/10.1118/1.2962408View Description Hide Description
The quantification of function and activity plays an important role clinically in nuclear medicine. Examples include the analysis of tracer kinetics to quantify dynamic function, the determination of the fraction of the cardiac blood pool ejected during contraction, the comparison of regional localization of uptake within the heart to that of a database to facilitate the recognition of perfusion defects in cardiacimaging, and the localization of F‐18 labeled FDG in potential lesions. Despite the present clinical utility of quantification a number of factors limit current methods. The successful addressing of these factors would expand the clinical utility of quantification and are the subject current and future investigations.
This lecture will provide an introduction to the application of quantification within nuclear medicine. It will point out some current deficiencies, and potential research directions aimed at over coming them.
1. Understand the various types of quantification employed in nuclear medicine.
2. Recognize some of the limitations to robust application of quantification clinically.
3. Appreciate some of the proposed methods to overcome these limitations.
35(2008); http://dx.doi.org/10.1118/1.2962409View Description Hide Description
Advances in our understanding of the molecular biology of cancer and other diseases have identified molecules and signaling pathways that we can now visualize, in vivo, for diagnosis, staging, and to identify optimal therapy and monitor patient response to therapy. These advances have also helped identify targets for targeted radionuclide therapy, making it possible to target radiation at the cellular and molecular level. Planning for this treatment approach is similar, in principle, to external beam radiotherapytreatment planning, but substantially different in practice. Dosimetry for systemic or locoregional administration of a therapeuticradiopharmaceutical requires an understanding of its biodistribution and pharmacokinetics, this must be coupled with a methodology for translating total number of radionuclide disintegration in a particular anatomical volume to the absorbed dose to the volume. Advances in imaging and computing technology over the past 20 to 30 years have fostered corresponding advances in the implementation of the basic radionuclide dosimetry scheme outlined above. Dosimetry in nuclear medicine is evolving from a standard anatomical model‐based calculation that provides mean absorbed dose over a target organ volume to a calculation that provides the spatial distribution of absorbed dose over the individual patient target and organ geometry and that also incorporates radiobiological modeling as a step towards assessing the biological consequences of the dose distribution. The evolution of nuclear medicinedosimetry as well as recently approved and emerging radionuclide‐based therapeutics will be reviewed.
Advances in Cardiovascular Imaging
35(2008); http://dx.doi.org/10.1118/1.2962554View Description Hide Description
Minimally invasive cardiac procedures using endovascular access have enabled treatment of patients that previously would have required open‐heart surgery. Such procedures are typically carried out in the interventional suite, with x‐ray fluoroscopy used to guide the placement of a catheter, followed by a phase of the procedure in which x‐ray fluoroscopy provides little to no information on the treatment. C‐arm CT has been under development since the early 90's, but has now entered a new phase as the ability to visualize soft tissue and low‐contrast lesions improves. By generating 3D images utilizing the x‐ray system in the interventional suite, the error prone and time‐consuming spatio‐temporal registration with prior CT or MRI exams can be avoided. However, C‐arm CTimaging in the presence of cardiac motion remains a challenge due to the slow rotation speed of the gantry, and the slow readout rates of the flat‐panel detectors. Three main data acquisition approaches to circumvent these limitations will be discussed: multi‐sweep retrospective ECG gating for myocardial visualization, single‐ or double‐sweep with volume image fusion for left atrium and pulmonary vein visualization and single‐sweep with retrospective gating for coronary artery visualization. Approaches to improve image quality, both hardware and software, such as optimized acquisition timing, motion compensation approaches and algorithms for non‐idealities such as x‐ray scatter, detector lag, limited field‐of‐view or angular coverage, data insufficiency and noise will be presented. With these developments the ultimate goal is to accomplish faster and more accurate catheter based interventions with an equivalent success rate as surgical procedures (80–90%). The C‐arm CT technology described here could eventually impact many minimally invasive procedures such as RF ablation for atrial fibrillation and coronary stenting, and also future procedures as they become available, such as endocardial injection of stem cells for treatment of myocardial infarction.
1. Learn how 3D cardiac imaging is done in the interventional suite: acquisition protocols, injection protocols, challenges and solutions.
2. Learn how image quality of 3D cardiac images can be improved using hardware and algorithm approaches.
35(2008); http://dx.doi.org/10.1118/1.2962555View Description Hide Description
Dual‐energy CT can be expected to play a new and evolving role in cardiovascular imaging. Clinical uses already reported include 1) direct CTangiography, whereby the dual energy algorithm identifies and removes bone in a 3‐D CT angiographic data set, allowing direct visualization of iodinated vessels without the need for user intervention to remove overlying bony anatomy, 2) removal of smaller hard plaques within vessels, allowing more rapid and clearer visualization of patent lumens in MIP projections, and 3) visualization of the perfused blood volume, also referred to as blood pool imaging, to demonstrate focal perfusion deficits. Dual energy bone and plaque removal techniques can be applied in retrospectively‐gated cardiacimaging, suppressing the appearance of calcified plaque and providing improved visualization of stenotic lumens. Additionally, dual‐energy CT can be used to identify and remove calcified plaque prior to image reconstruction in order to reduce the effects of calcium blooming, which creates artifactual elevation of CT numbers in voxels adjacent to calcified objects, often obscuring the dimensions of the true lumen for large or dense coronary artery calcifications. Finally, using projection‐space dual‐energy methods, mono‐energetic CTimages can be calculated, which greatly reduces beam hardening effects and increases the accuracy of CT number measurements. The use of dual‐energy CT techniques is being explored for the evaluation of myocardial perfusion deficits.
Attendees of this presentation can expect to learn about:
1) the technical approaches to dual‐energy cardiacCT currently implemented or under investigation on commercial CT systems,
2) current clinical uses of dual‐energy CT in cardiovascular imaging and
3) areas of future investigation.
35(2008); http://dx.doi.org/10.1118/1.2962556View Description Hide Description
Coronary arteriography is the standard method for determination of coronary anatomy and assessment of atherosclerosis. However, there are definite limitations to the use of visual estimation to assess the severity of coronary artery disease and luminal stenosis. These limitations include the large intraobserver and interobserver variability that result from subjective visual grading of coronary stenotic lesions. This is especially true in the case of an intermediate coronary lesion (30%–70% diameter stenosis), where coronary arteriography is very limited in distinguishing ischemia‐producing intermediate coronary lesions from non‐ischemia‐producing ones. Furthermore, pathologic findings have shown a lack of correlation between the severity of coronary stenosis as estimated from coronary arteriogram and the actual severity of stenotic lesions measured in postmortem hearts. Because of the major limitations of standard coronary arteriography, a method for functional measure of stenosis severity such as measurement of fractional flow reserve obtainable during cardiac catheterization is desirable. The fractional flow reserve measurement would provide valuable functional information in addition to the anatomical data obtained during routine coronary arteriography.
Fractional flow reserve was introduced to provide a physiological measure of coronary stenosis by quantifying the reduction in maximum coronary blood flow from a theoretical maximum normal flow in the presence of a lesion. Currently, fractional flow reserve is approximated by dividing the pressure distal to the stenosis by the aortic pressure. The distal pressure is measured using a pressure‐sensing wire that has passed across the stenosis, and the aortic pressure is measured simultaneously at the catheter tip with a pressure transducer. Pressure‐based fractional flow reserve has proven to aid the evaluation of the flow‐limiting potential of stenoses as well as the therapeutic gain of angioplasties. However, an alternative technique that can measure fractional flow reserve using only angiographic images would be a valuable tool in the cardiac catheterization laboratory because the acquired images used for visual assessment of stenosis severity can also be used to quantify physiological alterations imposed by the stenosis. In other words, fractional flow reserve could potentially be measured using only image data without the need to pass a pressure wire across a stenosis. The blood flow through the stenotic lesion is measured with a first pass distribution analysis technique and the theoretical normal blood flow is estimated by using a measurable parameter that correlates well with it. Scaling law relationships indicate that the total coronary arterial volume can be used as a surrogate measure of normal blood flow. The details of the methodology for fractional flow reserve quantification using angiographic image data will be discussed.
This lecture will provide an overview of the emerging techniques for assessment of the physiological significance of coronary lesions measured in the cardiac catheterization laboratory.
1. Understand the current techniques available for measurement of fractional flow reserve in the cardiac catheterization laboratory.
2. Understand the methodology for coronary blood flow measurement using angiographic image data.
3. Understand the methodology for fractional flow reserve measurement using angiographic image data.
35(2008); http://dx.doi.org/10.1118/1.2962557View Description Hide Description
In a recent Medical Physics “Vision 2020” paper (Medical Physics 35(1): 301–309, Jan 2008), the authors reviewed the state of endovascular image‐guided interventions (EIGI) and offered some predictions for the future. Here we review the current status of the field and of some of these advances. First, endovascular devices (such as clot busting tools, stents and their catheter delivery systems, and blood flow modifiers) are becoming finer, more complex, and are enabling the replacement of invasive surgical procedures with minimally invasive EIGI procedures. Innovative methods of actuating motion at the catheter tip, such as the use of external magnetic fields, are being introduced. Second, along with improvements in devices,imagingsystems that provide real‐time high‐resolution image guidance are being developed including a Solid State X‐ray Image Intensifier based on electron multiplying charge coupled devices (EMCCDs) that provide large on‐chip gain to overcome instrumentation noise such as that characteristic of current flat panel detectors. SSXIIs also have very high resolution capable of exceeding 10 lp/mm yet with no lag or ghosting. Third, the new high‐resolution region‐of‐interest (ROI) detectors can be used in combination with large conventional detectors for dual‐detector cone‐beam computer tomography (CB‐CT) to visualize ROIs within larger objects yet with minimal truncation artifact and with reduced integral dose. Fourth, during an interventional procedure, limited projection views can be taken to generate full 3D representations of the vasculature with accurate determination of vessel lumen morphology to enable computer fluid dynamic (CFD) calculations which in turn can be used to plan further EIGI treatment within the patient treatment time. Finally, as EIGI procedures become more complex, the consequent patient dose especially where improved image quality is implemented must be more carefully monitored. For example, we found that patient dose actually increased for certain electro‐physiology (EP) procedures performed in our EP Lab following replacement of a mobile c‐arm with a fixed unit capable of generating improved image quality. In conclusion, while progress is being made toward fulfilling the predictions made by the authors in the Vision 2020 paper published early in 2008, EIGI remains open to continuing exciting advancements.
1. Appreciate the progress being made in improved EIGI devices and imagingsystems.
2. Understand the operation of new high‐resolution micro‐angiographic systems including the SSXII and the operation of dual‐detector ROI CB‐CT systems.
3. Understand the role of limited view acquisition for providing 3D images.
4. Appreciate the patient exposure burden during EIGI procedures.
[Supported in part by NIH Grants R01 EB002873, R01 NS43924, R01 EB00842501, R01 HL52567, UB Foundation IRDF, and an equipment grant from Toshiba Medical Systems Corp.].
Advances in Breast Imaging
35(2008); http://dx.doi.org/10.1118/1.2962601View Description Hide Description
Over the last decade, there have been tremendous advances in breast imaging. As many facilities make the transition from film screen to digital mammography, it is appropriate to examine the accomplishments of the past and to acknowledge current technological limitations. In addition, we should carefully study evolving technologies in breast imaging with the goal of developing highly sensitive and highly specific techniques. This presentation will examine recent progress in digital mammography, including contrast‐enhanced imaging,tomosynthesis, and breast CT, from a breast imager's perspective.
Randomized clinical trials and meta‐analyses have shown that screening mammography results in a decrease in breast cancer mortality. Although mammography is efficacious in most women, we know that mammography is not a perfect test, epecially in women with radiographically dense breasts. Furthermore, false positive and false negative mammograms are difficult to eliminate. As digital mammography replaces film screen mammography in many practice settings, cancer detection rates may rise slightly and call back rates may decrease slightly.
Overlapping densities and areas of dense tissue often make interpretation of film screen mammograms difficult.. Overlapping parenchyma and dense tissue hinder digital mammography less than film screen mammography, but these overlying structures are not eliminated with digital mammography. Major efforts have been made in digital tomosynthesis and breast CT in order to advance beyond projection imaging.Tomosynthesis involves the acquisition of tomographic images in the digital mammography unit, while breast CT is based on a volumetric acquisition with the patient positioned prone in a machine that is similar to a unit used for stereotactic biopsies.
Conventional digital mammography, digital tomosynthesis, and breast CT may be combined with the injection of iodinated contrast material in order to visualize tumor vascularity and increase the conspicuity of breast tumors. These techniques may also be used to monitor response to neoadjuvant chemotherapy.
Over the next decade, tomosynthesis and breast CT will transition from research applications to clinical realities. Further studies are needed in order to determine how to use these emerging tools in an effective, efficacious manner.
1. To review advances in breast imaging over the last decade.
2. To understand the strengths and weaknesses of film screen and digital mammography.
3. To appreciate the potential roles of digital tomosynthesis and breast CT.
4. To understand the application of iodinated contrast material in digital mammography,tomosynthesis, and breast CT.
35(2008); http://dx.doi.org/10.1118/1.2962602View Description Hide Description
Digital tomosynthesis (or “tomo”) is revolutionizing breast imaging. Based on modified full‐field digital mammography systems, breast tomo can achieve limited‐angle cone‐beam CTimaging which produces 3D slice images of the breast. This addresses the problem of overlapping dense tissue which is the most common cause for unnecessary callbacks as well as missed cancers in mammography screening. Tomo can provide 3D images while remaining comparable to mammography in terms of speed, resolution, cost, and dose. For these reasons, tomo may be only imaging technique with the potential to completely replace the current role of mammography as the primary tool in breast cancer screening and diagnosis. FDA approval is imminent, so it is all the more important for medical physicists to understand this new modality's potential as well as limitations.
This presentation will cover both the hype and hope surrounding breast tomosynthesis. What do the initial clinical trials suggest about its performance? What are the on‐going physics issues in terms of clinical implementation, including compression, dose, and QA? What are some of the latest results from different research groups working on optimization of radiographic techniques, acquisition modes, and reconstruction algorithms? What is coming down the road in terms of advanced applications including quantitative imaging,computer aided detection, and contrast enhanced imaging? We will explore the answers to these and other questions together.
The work discussed in this presentation was supported in part by grants from NIH/NCI, US Army Breast Cancer Research Program, Susan G. Komen for the Cure, General Electric Company, Hologic, and Siemens Medical Solutions.
1. Understand the difference between breast tomosynthesis and dedicated breast CT.
2. Appreciate the many medical physics issues involved in the development and optimization of breast tomosynthesis.
3. Understand the clinical promise and concerns of using breast tomosynthesis.
35(2008); http://dx.doi.org/10.1118/1.2962603View Description Hide Description
For almost 70 years mammography has given insight into breast morphology, allowing cancer detection based on the gross impact of the disease upon body. The advent of tomographic techniques has allowed visualization of morphologic perturbations at a much smaller scale using various techniques including digital breast tomosynthesis (DBT), computed tomography(CT),ultrasound, and magnetic resonance imaging(MRI).
An alternative strategy to improve early detection is to search for functional changes. In 2007, the American Cancer Society revised their breast cancer screening recommendations. Annual screening by mammography is still recommended for women at low‐to‐medium risk for breast cancer; however, women at high risk for breast cancer are now recommended to have annual mammography and MRI. This recommendation is supported by studies which show that MRI is capable of demonstrating mammographically‐occult breast cancer both in both screening and diagnostic populations.
Contrast‐enhanced (CE) radiographic methods, including CE‐mammography, CE‐DBT and CE‐CT, have the potential to rival breast MRI as a sensitive method for diagnosis and screening of breast cancer. In a pilot study of CE‐DBT conducted at the University of Pennsylvania, suspicious enhancing lesions were demonstrated in 14 of 15 cases of breast cancer. The pre‐contrast tomosynthesisimages demonstrated lesion morphology and border characteristics in greater detail than digital mammography, while the subtracted contrast‐enhanced tomosynthesisimages demonstrated the vascular characteristics of the breast lesions in a manner consistent with breast MRI.
Further advances in CE imaging will be predicated upon shifting our attention from imaging perfusion to imaging specific molecular/cellular events. Research is ongoing in the discovery of specific biological prognostic factors, the development of appropriate imaging technologies and imaging agents, and the adaptation of these technologies to image‐guidance and monitoring of therapeutic interventions.
1. Review the development of contrast‐enhanced breast imaging.
2. Evaluate the results of the existing contrast‐enhanced clinical trials.
3. Examine the clinical roles for contrast‐enhanced imaging.
35(2008); http://dx.doi.org/10.1118/1.2962604View Description Hide Description
Dedicated breast computed tomography(CT)systems were designed and fabricated in our laboratory, and patient scanning commenced in November 2004. The breast CTscanner was designed utilizing several off‐the‐shelf components, including the x‐ray system, the flat‐panel detector, and a position encoder/bearing/motor system. These components were integrated into a custom designed scanner frame and gantry. The breast CTscanners were designed for 33 second acquisition, utilizing 500 projection images acquired over 360° around the breast. The breast CTscanner uses 80 kVp and mAs levels adjusted to the size of the woman's breast. The radiation levels are adjusted such that the mean glandular dose is equal to that of two‐view mammography for each woman. The acquisition protocols requires patient breath hold during the 17 second scan. As of May 2008, 150 patients have been scanned, including healthy volunteers (phase 1) and BIRADS 4 and 5 patients who were suspicious for having breast cancer (phase 2). Of the patients imaged, >20 were imaged with and without contrast agent injection. While our clinical evaluation of breast CT technology is still under way, initial evaluation indicates that high‐quality tomographic images of the breast can be achieved at dose levels comparable to two view mammography. The ultimate utility of breast CT may include breast cancer screening, or for diagnostic studies as part of a comprehensive diagnostic breast examination. The breast CT platform also appears ideal for robotically controlled biopsy and other interventional procedures.
Advances in X‐ray Imaging
35(2008); http://dx.doi.org/10.1118/1.2962725View Description Hide Description
In the last two decades, there have been rapid advances in projection x‐ray imaging in the areas of x‐ray detector and infrastructures for image management, processing and display. Following the development and commercialization of flat panel detectors, there have also been rapid advances in using the large area detectors to implement reconstructive 3‐D imaging techniques, including the cone beam CT and digital tomosynthesisimaging techniques. In contrast to these advances, the issues of x‐ray scatter and heavy patient attenuation remain the two biggest challenges in our effort to improve the image quality while keeping the patient dose in check. The presence of the x‐ray scatter component in the image signals biases the transmitted x‐ray intensity and results in erroneous x‐ray attenuation measurements which degrades the image quality and prevents accurate quantitative analysis in both projection and reconstructed images. Heavy patient attenuation could result in excessively low photon flux in certain anatomical regions, such as abdomen or retrocardium. This can combine with the lowered detective quantum efficiencies (DQEs) to lead to excessively low and unusable image signal‐to‐noise ratios (SNRs) Along with these two long standing issues is the long awaiting wish to develop a “weightless” x‐ray source that can be digitally controlled to shift without having to move a bulky and heavy housing. The development of a CT or digital tomosynthesisimaging system with no moving parts has become the holy grail of x‐ray imaging research. In this paper, efforts to address the scatter and exposure issues and to develop a “weightless” x‐ray source in the past two decades are reviewed with an educated guess on where we might head to in the future.
1. To review the issues of and solutions to the scatter problem in x‐ray image acquisition.
2. To review the issues of and solutions to the problem of excessive patient attenuation.
3. To review the efforts and potential use of digitally addressable “weightless” x‐ray source
WE‐C‐342‐02: The Future of Flat Panel Imagers: From Active Matrix to Active Pixel Architectures and Many Possibilities in Between35(2008); http://dx.doi.org/10.1118/1.2962726View Description Hide Description
Since the turn of the century, the phrase “Flat Panel Imager” has been increasingly used in modern x‐ray imaging venues ranging from large, institutional cardiac care, radiology and radiotherapy settings to community oral surgery offices. The phrase most commonly refers to those technologies employing a large area, monolithic array consisting of a two‐dimensional grid of imaging pixels fabricated on a thin glass substrate. Individual pixels are made addressable by means of an “active matrix” of switches — usually, in each pixel, taking the form of a single amorphous silicon (a‐Si:H) thin film transistor(TFT) coupled to some form of storage capacitor. Two variations of this relatively simple architecture (based on so‐called indirect or direct detection of the incident radiation by means of a scintillator or a photoconductor, respectively) have become almost ubiquitous for a wide variety of projection (e.g. radiography, fluoroscopy, mammography) and volumetric (e.g., CBCT, tomosynthesis) imaging applications. While offering many advantages, such Active Matrix Flat Panel Imagers (AMFPIs) are restrictive in terms of signal‐to‐noise performance, maximum frame rate, image artifacts, configurability and cost. These limitations are inspiring considerable innovation and creativity in imager development. Some approaches involve: high‐gain photoconductors such as HgI2 or avalanche‐gain with a‐Se (to improve system gain and DQE); active pixel circuits involving the inclusion of amplifiers in each pixel (to increase gain and DQE, frame rate, and to reduce artifacts); thick segmented scintillating converters (to increase x‐ray quantum detection efficiency and DQE at megavoltage energies); flexible substrates (to provide lighter, flexible and more x‐ray transparent substrates); and subtractive and additive printing of a‐Si:H or organic TFTs (to reduce costs). In this talk, a broad overview of the state of conventional AMFPI technology, its limitations, the potential for improvement, and some of the avenues being pursued to achieve these improvements will be presented. In addition, challenges for some of these approaches, along with the long‐term prospects for this general area of technology, will be reviewed, and the effect of large performance improvements on the practical implementation of advanced applications will be discussed.
1. Review the fundamental concepts behind the technology of flat panel imagers based on active matrix addressing.
2. Provide an understanding of the performance limitations on such active matrix, flat panel imagers (AMFPIs).
3. Outline the general approaches for achieving significant improvements over conventional AMFPI performance.
4. Detail specific improvement strategies involving front‐end enhancement of converter signal and pixel level circuit modifications, which preserve the major advantages of conventional AMFPIs.
5. Discuss the long‐term prospects for, and implications of flat panel imager performance improvement.
35(2008); http://dx.doi.org/10.1118/1.2962727View Description Hide Description
Two major x‐ray‐based techniques used for medical imaging are digital radiography (DR) and computed tomography(CT). Most of their detectors integrate the intensity of x‐ray flux and output gray scale images or projections. Detectors with indirect detection type heavily weigh x‐ray photons with higher energy as they generate more lights. This results in suboptimal contrast of images because the contrast of different tissues reduces in general as the energy of photons increases. In the past few years, there have been strong reviving research interests in an old concept—the use of the photon energy—to improve the quality and accuracy of diagnosis.
Recently, novel photon counting x‐ray detectors (PCXDs) with energy discrimination capabilities have been developed for x‐ray DR and CTimaging. These PCXDs counts the number of x‐ray photons within multiple energy windows. This allows us to improve the quality of the current gray scale images and the accuracy of the material decomposition. We will discuss the state‐of‐the‐art detector technologies and imaging methods unique to these PCXDs.
Seeing the Invisible: Recent Advances in MRI
35(2008); http://dx.doi.org/10.1118/1.2962763View Description Hide Description
The background underpinning the clinical use of techniques used on clinical systems to image tissues or tissue components with short T2s is reviewed. Tissue properties are discussed, and tissues are divided into those with a majority of short T2 relaxation components and those with a minority. Features of the basic physics are described including the fact that when the radiofrequency pulse duration is of the order T2, rotation of tissue magnetization into the transverse plane is incomplete. Consequences of the broad line‐width of short T2 components are also discussed including their partial saturation by off‐resonance fat suppression pulses. The need for rapid data acquisition of the order T2 is explained. Several different techniques suitable for imaging of short T2 components are available on clinical systems. These include gradient echo, ultrashort echo tissues (UTE) and swift imaging with Fourier Transformation (SWIFT). The 2D UTE pulse sequence with its half excitation pulse and radial imaging from the center of k‐space is described together with options that suppress fat and/or long T2 components. Clinical features of the imaging of cortical bone, tendons, ligaments, menisci, and periosteum as well as brain,liver and spine are illustrated. Short T2 components in all of these tissues may show high signals. Possible future developments are outlines as are technical limitations.
1. To explain the technical basis for imaging of short T2 tissue components with clinical MR systems.
2. To explain the mechanisms determining contrast and the different appearances of short T2 tissues.
Research sponsored, in part, by General Electric Healthcare.