Volume 36, Issue 6, June 2009
Index of content:
- Imaging Symposium: Room 210A
Advances in Quantitative MRI and MR Spectroscopy
36(2009); http://dx.doi.org/10.1118/1.3182498View Description Hide Description
Cardiacmagnetic resonance imaging(MRI) is a powerful medical imaging modality that can be safely and effectively used to quantify cardiac structure and function. Imaging the beating heart poses certain challenges that are increasingly met by advances in imaging hardware, image reconstruction, and image processing. In particular, improvements in gradient hardware and coil design have facilitated faster imaging times and real‐time cardiacimaging is now routinely possible. Advances in image reconstruction have arisen from the need to robustly reconstruct sparse datasets. The increasing desire for the quantitative evaluation of image data has driven the design of more sophisticated image acquisition and quantitative image processing approaches. This talk will emphasize the latest advances in quantitative cardiac motion encoding with MRI.
The outstanding capabilities of MRI include the ability to clearly contrast different soft tissues, the ability to acquire images of arbitrary orientation, the ability to acquire true three‐dimensional image datasets, and the lack of ionizing radiation.
Most of clinical cardiacMRI focuses on the qualitative and semi‐quantitative analysis of dynamic scalar (conventional grayscale) images. Scalar images are especially well suited for appreciating gross anatomical structural relationships and global function. In particular, the measurement of right and left ventricular wall masses and volumes, cardiac output, and ejection fraction using steady‐state free precession pulse sequences are routinely used to assess cardiac function. With the administration of contrast agents, additional images can be acquired that are informative about regional myocardial perfusion and infarct location and mass.
Remarkably, MRI is not limited to acquiring scalar images. The MRI signal can be sensitized to each velocity vector component for moving structures using a technique termed phase contrast. In this way three‐dimensional images of three‐dimensional velocity‐vector fields can be acquired. In particular, cardiac phase contrast techniques can be used to quantify regional blood flow patterns. This data can be used to quantify trans‐valvular flow rates, intraventricular flow patterns and pressure gradients, turbulance, and more. Particle tracing and streamline generation is possible with this kind of data.
Other cardiac motion encoding strategies are implemented for studying regional myocardial wall motion. Such strategies include tagged MRI, harmonic phase (HARP) imaging, and displacement encoding with stimulated echoes (DENSE). Each of these techniques has unique attributes, but each ultimately provides quantitative information about regional myocardial displacement from which regional myocardial strains can be computed. Regional strains are useful engineering measures of myocardial performance. Under these circumstances, MRI can be considered a tensor field imaging method.
Finally, recent research in diffusion tensorMRI (DTMRI) provides a means for the non‐destructive evaluation of microstrutural tissue organization in non‐moving structures. The requirement for non‐moving structures generally limits the application of DTMRI in cardiacimaging to ex vivo tissue samples. The diffusion tensor data contains a wealth of information about the regional vector orientations of the tissue's myofibers and also more sophisticated measures of tissue anisotropy and the orientation of higher‐order structures. The structural organization of the myocardium is essential for understanding myocardial performance. The current goal of many research groups is to integrate the quantitative wall motion, blood flow, and microstructural information into sophisticated finite element models of healthy and diseased hearts.
1. MRI can be used to acquire scalar (conventional grayscale), vector‐field, and tensor‐field images.
2. Conventional scalar imaging is the clinical norm and is used routinely to observe cardiac structures and quantify global cardiac function.
3. CardiacMRI can be used to quantify three‐dimensional blood velocities. This information can be used to measure flowrates and pressure gradients.
4. CardiacMRI can be used to quantify regional cardiac wall strains. Regional strains are useful engineering measures of myocardial performance.
5. Ex vivo diffusion tensorMRI can be used to quantify the microstructural organization tissue. The structural organization of the myocardium is essential for understanding myocardial performance.
WE‐C‐210A‐02: Quantitative MRI of the Brain: Investigation of Cerebral Gray and White Matter Diseases36(2009); http://dx.doi.org/10.1118/1.3182499View Description Hide Description
Local and global changes in braintissue volumes can be used as diagnostic markers of various neurological diseases. Even though these volumetric measurements do not provide specific information about underlying pathology, they can be used to diagnose these diseases, monitor disease progress or treatment efficacy. For example, local and global atrophies, which are observed in diseases such as multiple sclerosis and Alzheimer's disease, can be quantified to monitor disease magnitude and extent. Quantitative MRI of cerebral gray and white matters has found widespread application in brain research. Building on these progresses, some clinical applications started to emerge in recent years. The first technique utilizes high‐resolution structural MR images to investigate local and global morphological changes in the cerebral gray matter. This technique can be used longitudinally on a single patient to monitor disease progression or it can be utilized to identify morphological differences between a patient population and healthy controls. The image comparisons can be done using a voxel by voxel analysis technique, which will search the whole brain volume for group differences. On the other hand, if the investigator has an a priori hypothesis about a specific brain region, a volume of interest (VOI) can be selected manually in each image and compared. Another popular approach is to transform each image into a stereotactic space and analyze the deformation maps used in these transformations. The second technique utilizes the diffusion of water molecules to investigate the tissue structure and organization. In a medium where the molecular diffusion is restricted by cellularmembranes, the direction and magnitude of diffusion will reveal information about the microscopic structure and organization of those tissues. It has been demonstrated that the axonal membranes in major fiber tracts lead to highly anisotropicdiffusion and using this information white matter structures can be studied in great detail. The technique is called diffusion tensor imaging (DTI) and it has been used extensively to study white matter injuries in various diseases of the central nervous system. One of the most important aspects of this technique is that it provides several quantitative measurements of the cerebral white matter. For example, mean diffusivity measurement provides information about the overall water content, thus it can be used to identify local edema. On the other hand, the magnitude of anisotropy might be used as a marker of axonal fiber density and integrity, thus an indication of the overall health of a specific fiber tract. Major axonal fiber tracts can also be constructed in 3D from DTI images and measurements of fiber density, tract volume and length can be calculated. This presentation will provide an overview of imaging and analysis techniques used in image based quantification of cerebral gray matter and white matter diseases.
1. Analysis techniques used in the quantification of volumetric changes in the gray matter and potential applications.
2. DTI acquisition and processing.
3. Quantification of cerebral white matter structures based on DTI parameters.
4. Limitations of these techniques.
36(2009); http://dx.doi.org/10.1118/1.3182500View Description Hide Description
In vivo magnetic resonancespectroscopy (MRS) based on protons (1H) but also on phosphorous (31P) or carbon (13C) nuclei aims at noninvasive determination of tissue concentrations of various metabolites and compounds in animals or humans. It therefore enables metabolism to be investigated in vivo and pathological as well as drug or exercise induced changes of the metabolism to be detected. For its use for clinical diagnostics as well as for physiological studies precise quantification of the metabolite concentration is indispensable.
Quantitative MRS relies on the fact that the intensity of the free induction decay in the time domain as well as the area underneath a resonance line in the frequency domain are proportional to the number of spins that contribute to the signal and hence also to the tissue concentration of the respective metabolite. Advanced line fitting approaches have been introduced that resolve overlapping resonance lines using prior knowledge of the contributing spin systems and included baseline models and thus enable a precise determination of the intensity of resonance lines. However, additional factors such as volume size, T1 and T2relaxation times, transmit and receive homogeneity or temperature influence the quantification results and need to be corrected for. In addition a reliable reference standard has to be used to convert signal intensities into tissue concentrations in mM. Last but not least statistical measures such as Cramer‐Rao lower bounds (CRLB) or correlation matrices along with the inspection of the spectrum for artifacts can give information about the reliability of the obtained quantification results.
After briefly reviewing the basic principle in vivo MRspectroscopy is based on, the current lecture gives an overview about state of the art line fitting approaches, discusses influence factors, possible reference standards and ways to evaluate quantification results. Practical examples are given to illustrate theory and recent developments in quantitative in vivo MRS such as electric reference to assess in vivo concentrations (ERETIC), 2‐dimensional prior knowledge fitting (ProFit) and quantitative metabolite mapping at 7T (FIDLOVS) are included.
‐ review basic principles of in vivo MRspectroscopy
‐ understand state of the art fitting methods for in vivo MRspectroscopy
‐ understand which additional factors influence the quantification results
‐ understand how to use reference standards to translate areas into tissue concentrations
‐ understand how to evaluate the quantification precision and reliability
WE‐C‐210A‐04: Quantitative Estimate of In‐Vivo Metabolites in Breast and Prostate Tissues by MR Spectroscopy36(2009); http://dx.doi.org/10.1118/1.3182501View Description Hide Description
Diagnosis of cancer is challenging despite the availability of large number of biochemical and imaging investigations. Magnetic resonance imaging(MRI) provides morphological details while in vivomagnetic resonancespectroscopy (MRS) provides metabolic information at the molecular level thus enabling to study tissuephysiology and metabolism. In vivo localization can be achieved either using single voxel spectroscopy(SVS) or multi‐voxel spectroscopy (known as MRspectroscopicimaging,MRSI or chemical shift imaging, CSI), to detect metabolites that are present in millimolar concentration or less in normal and pathological tissuesin vivo.
Breast cancer is the most common cancer in women and is a leading cause of death worldwide. MRS of normal breast tissues showed high amount of lipid with little contribution from water while malignant breast tissues contain high water content. The parameters obtained from in vivo MRS of breast tissues are water‐to‐fat (W‐F) ratio and the concentration of choline containing compounds (tCho). Recently, in vivo quantitative measurement of the concentration of tCho has been reported for differentiation of normal, benign and malignant breast tissues. Two methods are used for quantification of tCho: (a) semi‐quantitative way by calculating the signal‐to‐noise ratio (SNR), and (b) absolute quantitation of tCho concentration using water as an internal and external referencing. Further, both W‐F ratio and tCho concentration have been evaluated as markers for assessment of tumor response to therapy.
Prostate cancer is the most common cancer and is the second common cause of cancer related deaths in males. Various MR techniques have been evaluated in prostate cancerdiagnosis. MRS of prostate tissues provides relative concentrations of metabolites like citrate (Cit), creatine (Cr), choline (Cho), and polyamines. High levels of Cit are observed in normal prostate tissue and are higher in the peripheral zone (PZ) than in the central gland (CG) and transition zone (TZ). The Cit level is reduced or not detectable in prostate cancer because of a conversion from citrate‐producing to citrate‐oxidating metabolism. Cho is elevated due to a high phospholipid cell membrane turnover in the proliferating malignant tissue. These changes in Cit and Cho are quantified by using ratios of metabolites like Cit/Cho, [Cit/(Cho+Cr)] or [(Cho+Cr)/Cit]. Few studies have also reported the determination of the absolute concentration of Cit in normal and pathological prostate tissues. This talk would provide details of the various in vivo MRS methods used for diagnosis and for monitoring tumor response to therapy in breast and prostate cancer patients including details of the absolute and relative of concentration of in vivo metabolites.
1. Understand the basics of in vivoMRspectroscopy.
2. Understand the various biochemical parameters obtained from in vivo MRS on breast and prostate tissues and their significance.
3. Understand the methods used for quantitation of in vivo MRS metabolites.
Advances in Molecular Imaging
36(2009); http://dx.doi.org/10.1118/1.3182546View Description Hide Description
We are in the process of building a high performance positron emission tomography(PET)system for small animal research using a semiconductordetector material known as cadmiumzinc telluride (CZT). If successful, this system will enable substantial improvements in the ability to detect, visualize, and quantify molecular‐based signatures of disease in vivo using PET. Unlike nearly all other PETsystem designs, which use scintillation detector technology, CZT directly collects electron‐hole pairs created from the absorption of a 511 keV annihilation photon. This approach has advantages both in terms of construction as well as performance of high resolution PETsystems that we will describe in this presentation. We will also present other unusual features of the system design, results of detector spatial, energy, and temporal resolution measurements performed in the laboratory, image reconstruction strategies, as well as predicted systemphoton sensitivity, noise‐equivalent count rate, and reconstructedspatial resolution performance results obtained using Monte Carlo simulation studies.
WE‐D‐210A‐02: Photoacoustic Tomography: High‐Resolution Imaging of Optical Contrast In Vivo at New Depths36(2009); http://dx.doi.org/10.1118/1.3182547View Description Hide Description
We develop photoacoustic imaging technologies for in vivo early‐cancer detection and functional imaging by physically combining non‐ionizing electromagnetic and ultrasonic waves. Unlike ionizing x‐ray radiation, non‐ionizing electromagnetic waves, such as optical and radio waves, pose no health hazard and, at the same time, reveal new contrast mechanisms. Unfortunately, electromagnetic waves in the non‐ionizing spectral region do not penetrate biological tissue in straight paths as x‐rays do. Consequently, high‐resolution tomography based on non‐ionizing electromagnetic waves alone, as demonstrated by confocal microscopy and two‐photon microscopy as well as optical coherence tomography, is limited to superficial imaging within about one optical transport mean free path (∼1 mm in the skin) of the surface of biological tissue.Ultrasonic imaging, on the contrary, provides good image resolution but has strong speckle artifacts as well as poor contrast in early‐stage tumors. We have developed ultrasound‐mediated imaging modalities by combining electromagnetic and ultrasonic waves synergistically to overcome the above limitations. The hybrid modalities provide relatively deep penetration at high ultrasonic resolution and yield speckle‐free images with high electromagnetic contrast.
In photoacoustic computed tomography, a pulsed broad laser beam illuminates the biological tissue to generate a small but rapid temperature rise, which leads to emission of ultrasonic waves due to thermoelastic expansion. The short‐wavelength pulsed ultrasonic waves are then detected by unfocused ultrasonic transducers. High‐resolution tomographic images of optical contrast are then formed through image reconstruction. Endogenous optical contrast can be used to quantify the concentration of total hemoglobin, the oxygen saturation of hemoglobin, and the concentration of melanin. Melanoma and other tumors have been imaged in vivo in small animals. Exogenous optical contrast can be used to provide molecular imaging and reporter gene imaging.
In photoacoustic microscopy, a pulsed laser beam is focused into the biological tissue to generate ultrasonic waves. The ultrasonic waves are then detected with a focused ultrasonic transducer to form a depth resolved 1D image directly. Raster scanning yields 3D high‐resolution tomographic images. Super‐depth beyond the optical transport mean free path has been reached with high spatial resolution.
Thermoacoustictomography is similar to photoacoustictomography except that low‐energy microwave pulses, instead of laser pulses, are used. Although long‐wavelength microwaves diffract rapidly, the short‐wavelength microwave‐induced ultrasonic waves provide high spatial resolution. Microwavecontrast measures the concentrations of water and ions.
1. Understand the motivation for developing photoacoustictomography and thermoacoustictomography.
2. Understand the advantages and limitations of photoacoustictomography and thermoacoustictomography.
3. Understand the potential applications of photoacoustictomography and thermoacoustictomography.
36(2009); http://dx.doi.org/10.1118/1.3182548View Description Hide Description
Specialized ultrasound systems and targeted microbubbles optimized for vascular molecular imaging continue to advance. The development of methods for sensitive and selective imaging of adherent, targeted contrast agents requires improvements in both the physical systems and probes. We have developed transducers and signal processing techniques to maximize transducer bandwidth and integrated the components with a clinical ultrasoundscanner. The transducer integrates low and high‐frequency arrays; the center‐row high‐frequency array is surrounded on each side with confocally‐focused low‐frequency arrays. By transmitting from the low‐frequency (1.6 MHz) arrays and receiving through the high‐frequency (7 MHz) array, harmonic microbubble echoes can be detected and distinguished from the surrounding tissue. Further, by combining echoes over a pulse train, a targeted agent to free agent signal ratio of 15∼22 dB can be obtained without waiting for agent clearance. Further, we report on radiolabeling methods to fully assess microbubble biodistribution with and without molecular targeting or insonation. We find that microbubbles are rapidly accumulated within the liver and spleen, although a small (but significant) mass of the microbubble shell accumulates as a result of local insonation.
We acknowledge the support of NIH CA 103828 and CA 112356.
WE‐D‐210A‐04: Novel PET Probes to Image the Immune System and Cancer — From Discovery to Clinical Applications36(2009); http://dx.doi.org/10.1118/1.3182549View Description Hide Description
Monitoring immune function using molecular imaging could significantly impact the diagnosis and treatment evaluation of immunological disorders and therapeutic immune responses. Positron Emission Tomography(PET) is a molecular imaging modality with applications in cancer and other diseases. PET studies of immune function have been limited by a lack of specialized probes. We identified [18F]FAC (1‐(2′‐deoxy‐2′‐[18F]fluoroarabinofuranosyl) cytosine) by differential screening as a new PET probe for the deoxyribonucleotide salvage pathway. [18F]FAC PET enabled visualization of lymphoid organs and was sensitive to localized immune activation in a mouse model of anti‐tumor immunity. [18F]FAC microPET also detected early changes in lymphoid mass in systemic autoimmunity and allowed evaluation of immunosuppressive therapy. We have also developed a series of [18F]FAC analogs with improved pharmacokinetic properties. Data in animalmodels and preliminary studies in humans support the use of [18F]FAC and its analogs for immune monitoring by PET and suggest a wide range of clinical applications in immune disorders and in certain types of cancer.
Research sponsored in part by Sofie Biosciences through a UC Discovery grant.
Advances in Cardiovascular Imaging: MR, Fluoroscopy and Ultrasound
WE‐E‐210A‐01: Functional Cardiovascular MRI: Assessment, Visualization and Quantification of 3D Blood Flow Characteristics36(2009); http://dx.doi.org/10.1118/1.3182578View Description Hide Description
Magnetic Resonance Imaging(MRI) techniques provide a non‐invasive method for the highly accurate anatomic depiction of the heart and vessels. Most MR‐sequences demonstrate more or less significant sensitivity to flow and motion, which can lead to artifacts in many applications. The intrinsic motion sensitivity of MRI can, however, also be used to image vessels like in phase contrast (PC) MR‐angiography or to quantify blood flow and motion of tissue. Such techniques offer the unique possibility to acquire spatially registered functional information simultaneously with the morphological data within a single experiment. Characterizations of the dynamic components of blood flow and cardiovascular function provide insight into normal and pathological physiology and have made considerable progress in recent years
To synchronize flow or motion sensitive measurements with periodic tissue motion or pulsatile flow, data acquisition is typically gated to the cardiac cycle and time resolved (CINE) anatomical images are collected to depict the dynamics of tissue motion and blood flow during the cardiac cycle. Visualization and quantification of blood flow and tissue motion using PC MRI has been widely used in a number of applications. In addition to analyzing tissue motion such as left ventricular function, time‐resolved 2D PC MRI techniques have proven to be useful tools for the assessment of blood flow within the cardiovascular system.
Moreover, 3D spatialencoding offers the possibility of isotropic high spatial resolution and thus the ability to measure and visualize the temporal evolution of complex flow and motion patterns in a 3D‐volume. In this context, ECG synchronized and respiration controlled flow sensitive 3D MRI using 3‐directional velocity encoding (also termed ‘flow sensitive 4D MRI’) can be employed to detect and visualize global and local blood flow characteristics in targeted vascular regions (aorta, cranial arteries, carotid arteries, etc.). For the analysis and visualization of complex, three‐directional blood flow within a 3D volume, various visualization tools, including 2D vector‐fields, 3D streamlines and particle traces, have been reported. In addition more advanced data quantification strategies of directly measured (e.g. flow rates) or derived parameters (e.g. pressure difference maps, wall sheer stress, pulse wave velocity, etc.) are promising as new clinical markers for the characterization of cardiovascular disease.
This lecture will provide an overview of the MR imaging principles and advanced acquisition methods, data processing, flow visualization and quantification strategies, and clinical applications of flow sensitive MRIimaging.
1. Understand the basic and advanced methods for flowmeasurements using MRI
2. Understand techniques for 3D flow visualization and quantification of blood flow and derived parameters
3. Understand the issues related to clinical applications of flow‐sensitive MRI
WE‐E‐210A‐02: Emerging X‐Ray Fluoroscopic Guidance Technologies for Challenging Cardiovascular Interventions36(2009); http://dx.doi.org/10.1118/1.3182579View Description Hide Description
X‐ray fluoroscopy remains the primary guidance modality for cardiac interventions due to its appealing combination of real‐time imaging, high spatial resolution, simplicity of use, and compatibility with conventional catheter devices. However fluoroscopy has well‐recognized limitations. The lack of depth resolution and poor soft tissuecontrast in an x‐ray projection make it difficult to precisely position devices with respect to target cardiac structures. Ionizing radiation dose is an ongoing concern, particularly for interventions with long imaging times such as catheter‐based electrical mapping and ablation of atrial fibrillation.
Technologies which reduce dose and add three‐dimensional context to the live fluoroscopic image offer a route towards overcoming these limitations. Scanning‐beam digital x‐ray (SBDX) is an inverse geometry fluoroscopic system that provides both dose reduction and real‐time tomosynthesis, a feature that can be exploited for image‐based 3D tracking of catheter electrodes. The operating principles of SBDX will be reviewed and recent investigations of 3D catheter tracking based on multiplanar tomosynthesis will be presented. Fusion of live fluoroscopy with roadmaps derived from pre‐acquired 3D image data (e.g. CT, MRI) can allow the interventionist to visualize structures lacking x‐ray contrast, or to perform novel therapies targeted to the physiologic status of cardiactissue. A fluoroscopic fusion system for recanalization of chronically occluded arteries and for targeted stem cell therapy will be discussed.
This lecture will provide an overview of the demands and constraints of fluoroscopically‐guided cardiovascular interventions, dose reduction and 3D catheter tracking capabilities enabled by the SBDX system, and clinical application of fluoroscopy fused with 3D roadmaps.
This research was supported in part by NovaRay Medical, Inc. and NIH/NHLBI.
1. Review the demands and constraints of fluoroscopically‐guided interventions
2. Understand the principles of SBDX dose reduction and 3D catheter tracking
3. Discuss x‐ray fluoroscopy combined with 3D imaging for cardiovascular interventions
36(2009); http://dx.doi.org/10.1118/1.3182580View Description Hide Description
Atherosclerosis is characterized by the development of plaques in the arterial wall, ultimately leading to heart attacks and strokes. Quantitative measurements of the progression and regression of carotid plaque burden are important in monitoring patients and evaluating new treatment options. 3D carotid ultrasound (3D US) has the potential to be an effective method to monitor the progression of carotid artery plaques in symptomatic and asymptomatic patients. To meet this challenge, we developed a 3D carotid US system that allows detailed studies of plaques in 3D. Our software includes 3D reconstruction, viewing, and segmentation of carotid plaques, surface morphology analysis, and registration software to allow quantitative tracking of plaque changes. We used our system to examine the detailed relationship between the 3D US‐based measurement of plaque volume, the 3D scanning parameters, volume measurement variability, and plaque surface morphology. In this paper, we describe the acquisition and reconstruction of 3D carotid USimages, and extended concepts used in intima‐media thickness (IMT) measurements based on 2D images to a metric we call the 3D vessel‐wall‐plus‐plaque thickness (3D VWT), which is obtained by computing the distance between the carotid wall and lumen surfaces on a point‐by‐point basis in a 3D image of the carotid arteries. The VWT measurements can be superimposed on the arterial wall to produce the 3D VWT map. Since changes in plaque thickness is important in monitoring patients who are at risk for stroke, we also compute the change of VWT by comparing the 3D VWT maps obtained at two different times. To facilitate the visualization and interpretation of the 3D VWT and 3D VWT‐Change maps, we developed a technique to flatten these maps in an area‐preserving manner.
1. Understand the limitations of conventional carotid ultrasound imaging
2. Understand the methods that can be used to overcome the limitations of the use of conventional ultrasound to monitor progression of carotid disease
3. Understand the advantages of 3D ultrasoundimages to monitor progression of carotid disease
Brain Interventional and Perfusion Imaging
36(2009); http://dx.doi.org/10.1118/1.3182647View Description Hide Description
Perfusion computed tomography (PCT) is an imaging technique that allows rapid, noninvasive, quantitative evaluation of cerebral perfusion by generating maps of cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT). The concepts behind this imaging technique were developed in the 80s, but its widespread clinical use was allowed by the recent introduction of rapid, large‐coverage multidetector‐raw CT scanners. Key clinical applications for PCT include the diagnosis of cerebral ischemia and infarction, and evaluation of vasospasm after subarachnoid hemorrhage. PCT measurements of cerebrovascular reserve after acetazolamide challenges in patients with vascular stenoses permit evaluation of candidacy for bypass surgery and endovascular treatment. PCT has also been used to assess cerebral perfusion after head trauma and microvascular permeability in the setting of intracranial neoplasm. Some controversy exists regarding this technique, including questions regarding correct selection of an input vessel, the accuracy of quantitative results, and the reproducibility of results. This presentation will provide an overview of PCT, including details of technique, major clinical applications, and limitations.
36(2009); http://dx.doi.org/10.1118/1.3182648View Description Hide Description
CT has been used as a tool for determining blood perfusion to a diverse group of organ systems including the kidney, the lungs, the myocardium and the brain. Improvements in CT scanner design have improved and expanded capabilities for perfusion assessment, for example, faster scan times have enabled shorter sample duration and increased the number of samples that can be acquired per unit time. Greater x‐ray beam width in the Z‐axis and more rapid and controlled table movement have increased the coverage area. Gating techniques have reduced patient motion problems in some organs. However, perfusion studies remain challenging in many organs and in many patients. Artifacts, including beam hardening and streaking, and inherent limitations, including noise and partial volume effects, can limit the accuracy of measurements.
In this talk, methods for CT perfusion measurements will be reviewed. Developments in CT that have improved perfusion measurements will be discussed and sources of error will be described.
1. Understand the principles of perfusion measurement with CT
2. Review the sources for error in perfusion measurements with CT
36(2009); http://dx.doi.org/10.1118/1.3182649View Description Hide Description
In the US, 795,000 people are expected to have a new or recurrent stroke in 2009. Ischemic stroke, which accounts for more than 80% of all strokes, carries a severe toll being the 3rd cause of death and the leading cause of disability. The cost of stroke, both direct and indirect, will exceed $60 billion this year alone. Hemorrhagic stroke on the other hand originates from rupture of brain aneurysms, which cause the vast majority of subarachnoid hemorrhage, a devastating medical emergency that affects more than 30,000 Americans each year and causes substantial disability and death.
As in the coronary and peripheral vascular system, surgical treatment and medical management is progressively replaced by minimally invasive endovascular treatment. In recent years we have witnessed a rapid advancement in stent and stent‐like technology. Stents do not only serve for a combined intra‐ and extrasaccular treatment of challenging aneurysms, but assist the revascularization in acute and chronic ischemic conditions of the neurovascular system. In the Neuroangiography Suite x‐ray based imaging for guidance of devices and for monitoring brain perfusion during the intervention has experienced a quantum leap. Replacement of Image Intensifiers (II) with Flat Panel Detectors (FD) has enabled to increase the rotational speed of the system and its use as cone beam CT. Today angiography techniques do not only serve to characterize local flow alterations induced by vessel implants, but also study brain perfusion prior and during revascularization maneuvers. Detailed information of vascular structures is provided in conjunction with the surrounding brain tissue and vessel wall. Imaging of temporary or permanent implants is available with high resolution that is vital for a precise placement, treatment efficacy and patient safety.
TH‐C‐210A‐04: New High Resolution Dynamic Detectors and Flow Modifying Stents for Neuro‐Endovascular Image Guided Interventions36(2009); http://dx.doi.org/10.1118/1.3182650View Description Hide Description
As the field of minimally invasive endovascular image‐guided interventions (EIGI) advances, there has been progress in the development of new endovascular devices such as the asymmetric vascular stent for treatment of aneurysms as well as progress in the image guidance systems needed to improve both the diagnoses and interventions and the methods used to evaluate the improved imaging performance. High resolution imagingsystems with MTFs extending past 8 Lp/mm and with ability to visualize not only radioopaque markers but the detailed structure of devices such as stents have motivated the development of new asymmetric devices such as the blood flow modifying asymmetric vascular stent (AVS). The AVS has now come through a number of generations from balloon expandable stainless steel strutted structures with laser micro‐welded mesh flow diverters to new super‐elastic nitinol closedcell self‐expanding stents with organic material flow diverters. The methods of laser machining, surface finishing, and deployment will be described with progress in animal models reported. In parallel with advances in EIGI devices has come new high resolution detector development. The detectors have a unique combination of features such as far superior spatial resolution compared with conventional dynamic flat panel detectors, large dynamic range of sensitivity with negligible lag and ghosting to enable both fluoroscopy and angiography, and low noise to enable quantum limited performance over the full useful range including during lowdose fluoroscopy. The Micro‐Angiographic Fluoroscope (MAF) consists of an x‐ray converter phosphor sensed by a micro‐channel plate light image intensifier which is in turn coupled to a high performance CCD camera using a fiber optic taper. The MAF is a region of interest (ROI) imager with 4 cm field of view centered at the interventional site and may be moved in front of a larger conventional detector when improved resolution is needed. The Solid State X‐ray Image Intensifier (SSXII) while having much of the benefits of the MAF in superior imaging capability achieves its great sensitivity using only electron multiplying CCDsensors. An array design is being developed so that the imaging FOV may be expanded by adding modules each with its own EMCCD‐based detector. To more fully characterize detectors, new evaluation methods are being explored. For example, the accurate determination of MTF from measurements of noise only, without the need for a slit or edge will be reported. Also from a careful analysis of noise, the exposure range for detector quantum limited performance can be well demarcated by the instrumentation noise equivalent exposure (INEE). Finally, more realistic linear system parameters that include focal‐spot size, geometry, and scatter provide generalized MTFs and DQEs or GMTFs and GDQEs. All told, there is much happening in EIGI.
1. Appreciate the progress being made in improved EIGI devices and in particular flow modifiers such as the asymmetric vascular
2. stent (AVS) for aneurysm treatment.
3. Understand the operation of new high‐resolution micro‐angiographic systems including the MAF and SSXII.
4. Understand new objective image detector evaluations including INEE, GMTF, GDQE, and determination of MTF from noise
5. measurements alone.
(Supported in part by NIH Grants R01EB002873, R01 NS43924, R01EB008425, and the Toshiba Medical Systems Corp.)