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The purpose of this study is to review and compare simulation methods for describing the transport of charge clouds in silicon based semiconductor detectors and investigate the effects on energy spectrum for silicon based photon-counting strip detectors. Charge clouds and detailed carrier transport are simulated and compared using two different approaches including analytical and Monte Carlo schema. The results of the simulations are evaluated using pulse-height spectra (PHS) for a silicon strip detector with edge on geometry at two energies (25 and 75 keV) at various X-ray absorption locations relative to the pixel boundary and detector depth. The findings confirm carrier diffusion plays a large role in the charge sharing effect in photon counting detectors, in particular when the photon is absorbed near the pixel boundary far away from the pixel electrode. The results are further compared in terms of the double-counting probability for X-ray photons absorbed near the pixel boundary as a function of the threshold energy. Monte Carlo and analytical models show reasonable agreement (2% relative error in swank factor) for charge sharing effects for a silicon strip detector with edge-on geometry. For 25 keV mono-energetic photons absorbed at 5 μ m from the pixel boundary, the theoretical threshold energy at 10% double-counting probability based on charge sharing is 5.5, 8.5 and 9.2 keV for absorption depths of 50, 250 and 450 μ m from the electrode, respectively. The transport of charge clouds affects the spectral characteristics of photon counting detectors and the double-counting probability results show the theoretical threshold energy to avoid double-counting as a function of X-ray energy and X-ray interaction locations for silicon and can be considered for future studies of charge sharing effects.

Rationale and Objective We have installed an improved x-ray/MR (XMR) truly hybrid system with higher imaging signal-to-noise ratio (SNR) and versatility than our first prototype. In our XMR design, a fixed anode x-ray fluoroscopy system is positioned between the two donut-shaped magnetic poles of a 0.5T GE Signa-SP magnet (SP-XMR). This paper describes the methods for increased compatibility between the upgraded x-ray and MR systems that have helped improve patient management. Materials and Methods A GE OEC 9800 system (GE OEC Salt Lake City, UT) was specially reconfigured for permitting x-ray fluoroscopy inside the interventional magnet. A higher power x-ray tube, a new permanent tube mounting system, automatic exposure control (AEC), remote controlled collimators, choice of multiple frame rates, DICOM image compatibility, magnetically shimmed x-ray detector, x-ray compatible MR coil, and better RF shielding are the highlights of the new system. A total of 23 clinical procedures have been conducted with SP-XMR guidance of which five were performed using the new system. Results The 70% increased power for fluoroscopy, and a new 6 times higher power single frame imaging mode, has improved imaging capability. The choice of multiple imaging frame rates, AEC, and collimator control allow reduction in x-ray exposure to the patient. The DICOM formatting has permitted easy transfer of clinical images over the hospital PACS network. The increased MR compatibility of the detector and the x-ray transparent MR coil has enabled faster switching between x-ray and MR imaging modes. Conclusion The improvements introduced in our SP-XMR system have further streamlined x-ray/MR hybrid imaging. Additional clinical procedures could benefit from the new SP-XMR imaging.

In this issue of Academic Radiology, Hasegawa et al (1) present an extensive review of developments in integrated-modality imaging. Hasegawa’s group (2) has pioneered the development of a system integrating single photon emission computed tomography (SPECT) with computed tomography (CT)—SPECT/CT; and a group working with David Townsend in Pittsburgh, Pa, has done the same with a system integrating positron emission tomography (PET) with CT—PET/CT (3). Currently, the widespread clinical introduction of PET/CT and SPECT/CT systems is proceeding at a staggering pace. The reasons for the sudden success of such systems are multiple and due to several factors that are partly unrelated to each other. These are discussed below. Furthermore, this new trend may be only the beginning of an expanded use of hybrid systems incorporating multiple, synergistic imaging modalities.

Rationale and Objectives The purpose of this study was to provide in vivo demonstrations of the functionality of a truly hybrid interventional x-ray/magnetic resonance (MR) system. Materials and Methods A digital flat-panel x-ray system (1,024 2 array of 200 μm pixels, 30 frames per second) was integrated into an interventional 0.5-T magnet. The hybrid system is capable of MR and x-ray imaging of the same field of view without patient movement. Two intravascular procedures were performed in a 22-kg porcine model: placement of a transjugular intrahepatic portosystemic shunt (TIPS) (x-ray–guided catheterization of the hepatic vein, MR fluoroscopy–guided portal puncture, and x-ray–guided stent placement) and mock chemoembolization (x-ray–guided subselective catheterization of a renal artery branch and MR evaluation of perfused volume). Results The resolution and frame rate of the x-ray fluoroscopy images were sufficient to visualize and place devices, including nitinol guidewires (0.016–0.035-inch diameter) and stents and a 2.3-F catheter. Fifth-order branches of the renal artery could be seen. The quality of both real-time (3.5 frames per second) and standard MR images was not affected by the x-ray system. During MR-guided TIPS placement, the trocar and the portal vein could be easily visualized, allowing successful puncture from hepatic to portal vein. Conclusion Switching back and forth between x-ray and MR imaging modalities without requiring movement of the patient was demonstrated. The integrated nature of the system could be especially beneficial when x-ray and MR image guidance are used iteratively.

The sensitivity of nuclear magnetic resonance to motion can be harnessed to produce quantitative measurements of low flow velocity and volume flow rate. MR flow quantification is noninvasive, can be performed in arbitrary locations and in any image orientation, and is fairly rapid. This article discusses the basic techniques with particular attention to phase contrast quantitation of volume flow rate, especially the determinants of accuracy and reproducibility in these measurements.

Although a proposed advantage of bileaflet mechanical valves is that if one leaflet is thrombosed shut, the second leaflet may continue to function normally, clinical experience suggests that patients with one thrombosed leaflet demonstrate impaired opening of the second leaflet and therefore, extremely high levels of wall shear stress. Our goal was to quantitatively assess the flow field generated by a thrombosed valve versus a normal bileaflet valve.

A method and apparatus to reduce the noise in three-dimensional phase contrast magnetic resonance velocity measurements exploits the property that blood is incompressible and therefore the velocity field describing its flow must be divergence-free. The divergence-free condition can be incorporated by the projection operation on Hilbert space where the velocity measurements are projected onto the space of divergence-free velocity fields. The reduction of noise is achieved since the projection operation eliminates the noise component that is not divergence-free. Higher quality angiograms are produced due to the noise reduction in the velocity measurement signals.

An NMR system produces an angiogram using the complex-difference method. Artifacts caused by system phase errors are reduced by calculating a corrected phase angle using the acquired NMR data sets and using the corrected phase angle in calculating the complex difference in the corresponding elements of the image domain NMR data sets.

Motion and deformation is assessed and imaged using phase contrast magnetic resonance imaging for determining velocity of a plurality of regions at a plurality of frames during a period of interest such as the cardiac cycle. A region of interest is tracked from frame to frame and based on a known last position and the velocity at the known position. Strain and rotation are determined from the changes in velocity along selected axes.

A pulse sequence of an NMR system is disclosed which produces two spin echo signals which may be acquired. In addition, one or more gradient echos are produced and acquired. The gradient echo data is combined with the spin echo data to improve signal to noise ratio, provide T 2 ' mapping, reduce motion induced artifacts, measure and correct for RF pulse imperfections, and to provide improved T 2 measurement.

A comparison of experimental imaging results obtained with linearly polarized and circularly polarized radiofrequency excitation and reception is presented. Simulation images in good agreement with the experimental scans are described. The simulations are calculated with a model in which a homogeneous, isotropic cylinder of lossy dielectric material and infinite axial extent is immersed in a uniform rf magnetic field perpendicular to the axis. It is found that with the usual linear polarization, reconstructions of uniform objects have regions of decreased intensity. These artifacts are shown to arise from dielectric standing wave effects and eddy currents. The effects become more severe as the frequency or object size is increased, and depend upon the complex conductivity of the object. Results indicate that a significant reduction in the artifact intensity is achieved when circular polarization is employed for both transmission and reception. The expected benefits of circular polarization over linear polarization in reduction of excitation power (up to 50% reduction) and signal-to-noise advantage (√2) have been realized in practice with cylindrical objects and human subjects.

We present a method for rapid measurement of T1 relaxation times using gradient refocused images at limited flip angles and short repetition times. This "variable nutation" techniques was investigated using a T1 phantom. There was a high correlation between measurements obtained with the variable nutation and partial saturation techniques. The ability of this method to create calculated T1 images is also demonstrated. We conclude that the variable nutation method may allow measurement of T1 relaxation times with a significant reduction in acquisition time compared to partial saturation techniques.

In a system for reducing artifacts in a desired image due to substantially periodic variations in the imaging signal detected from a portion of an object under examination, a method and apparatus for generating from a signal y(t) related to the variations and whose values are not equally likely, a signal φ(t) whose values are substantially equally likely for use in distributing events substantially evenly over the variations. In order to provide the signal φ(t), an apparatus is disclosed for executing the following steps: (a) establishing a historical data base derived from values of the signal y(t) for a period of time preceding the present time, (b) measuring parameters related to the present value of the signal y(t); and (c) producing a value for φ(t) using the present measured parameters and the historical data base; wherein the values of φ(t) are more evenly distributed than the values of y(t).

A method for reducing image artifacts due to signal variations in the course of examining a subject using nuclear magnetic resonance (NMR) techniques includes the acquisition of scan data for imaging the object. The scan data is composed of a number of views. The acquisition of each view includes the implementation of a pulse sequence to generate an NMR signal and application of a magnetic gradient along at least one dimensional axis of the object. The magnetic field gradient is characterized by a parameter (e.g., amplitude or direction) adjustable from view to view to encode spatial information into the NMR signal. The parameter value is selected just prior to implementation of the pulse sequence for that view and depends on the phase of the signal variation at that point in time. The final view order depends on the measured phase during the scan. In this manner, the view order can be continuously adjusted to obtain optimum artifact reduction. The method is applicable to various NMR imaging techniques, including Fourier transform and multiple angle projection reconstruction.

A method for reducing image artifacts due to signal variations in the course of examining a subject using nuclear magnetic resonance (NMR) techniques includes the acquisition of NMR data for imaging the object. The NMR data is composed of a number of views. The acquisition of each view includes the implementation of a pulse sequence to generate an NMR signal and application of a magnetic gradient along at least one dimensional axis of the object. A gating technique is used to define a window during a portion of each respiratory cycle and NMR data is acquired only during each window. A view ordering technique is employed during the data acquisition by altering the magnitude of a field gradient in a nonmonotonic manner.