Rigid-body motion correction in hybrid PET/MRI using spherical navigator echoes

Integrated positron emission tomography and magnetic resonance imaging (PET/MRI) is an imaging technology that provides complementary anatomical and functional information for medical diagnostics. Both PET and MRI are highly susceptible to motion artifacts due, in part, to long acquisition times. The simultaneous acquisition of the two modalities presents the opportunity to use MRI navigator techniques for motion correction of both PET and MRI data. For this task, we propose spherical navigator echoes (SNAVs)-3D k-space navigators that can accurately and rapidly measure rigid body motion in all six degrees of freedom. SNAVs were incorporated into turbo FLASH (tfl)-a product fast gradient echo sequence-to create the tfl-SNAV pulse sequence. Acquiring in vivo brain images from a healthy volunteer with both sequences first compared the tfl-SNAV and product tfl sequences. It was observed that incorporation of the SNAVs into the image sequence did not have any detrimental impact on the image quality. The SNAV motion correction technique was evaluated using an anthropomorphic brain phantom. Following a stationary reference image where the tfl-SNAV sequence was acquired along with simultaneous list-mode PET, three identical PET/MRI scans were performed where the phantom was moved several times throughout each acquisition. This motion-up to 11° and 14 mm-resulted in motion artifacts in both PET and MR images. Following SNAV motion correction of the MRI and PET list-mode data, artifact reduction was achieved for both the PET and MR images in all three motion trials. The corrected images have improved image quality and are quantitatively more similar to the ground truth reference images.

Extending the dynamic range of biomedical micro-computed tomography for application to geomaterials

BACKGROUND

X-ray computed tomography (CT) can non-destructively examine objects by producing three-dimensional images of their internal structure. Although the availability of biomedical micro-CT offers the increased access to scanners, CT images of dense objects are susceptible to artifacts particularly due to beam hardening.

OBJECTIVE

This study proposes and evaluates a simple semi-empirical correction method for beam hardening and scatter that can be applied to biomedical scanners.

METHODS

Novel calibration phantoms of varying diameters were designed and built from aluminum and poly[methyl-methacrylate]. They were imaged using two biomedical micro-CT scanners. Absorbance measurements made through different phantom sections were fit to polynomial and inversely exponential functions and used to determine linearization parameters. Corrections based on the linearization equations were applied to the projection data before reconstruction.

RESULTS

Correction for beam hardening was achieved when applying both scanners with the correction methods to all test objects. Among them, applying polynomial correction method based on the aluminum phantom provided the best improvement. Correction of sample data demonstrated a high agreement of percent-volume composition of dense metallic inclusions between using the Bassikounou meteorite from the micro-CT images (13.7%) and previously published results using the petrographic thin sections (14.6% 8% metal and 6.6% troilite).

CONCLUSIONS

Semi-empirical linearization of X-ray projection data with custom calibration phantoms allows accurate measurements to be obtained on the radiodense samples after applying the proposed correction method on biomedical micro-CT images.

Morphology-Specific Discrimination between MS White Matter Lesions and Benign White Matter Hyperintensities Using Ultra-High-Field MRI

BACKGROUND AND PURPOSE

Recently published North American Imaging in Multiple Sclerosis guidelines call for derivation of a specific radiologic definition of MS WM lesions and mimics. The purpose of this study was to use SWI and magnetization-prepared FLAIR images for sensitive differentiation of MS from benign WM lesions using the morphologic characteristics of WM lesions.

MATERIALS AND METHODS

Seventeen patients with relapsing-remitting MS and 18 healthy control subjects were enrolled retrospectively. For each subject, FLAIR and multiecho gradient-echo images were acquired using 7T MR imaging. Optimized postprocessing was used to generate single-slice SWI of cerebral veins. SWI/FLAIR images were registered, and 3 trained readers performed lesion assessment. Morphology, location of lesions, and the time required for assessment were recorded. Analyses were performed on 3 different pools: 1) lesions of >3 mm, 2) nonconfluent lesions of >3 mm, and 3) nonconfluent lesions of >3 mm with no or a single central vein.

RESULTS

The SWI/FLAIR acquisition and processing protocol enabled effective assessment of central veins and hypointense rims in WM lesions. Assessment of nonconfluent lesions with ≥1 central vein enabled the most specific and sensitive differentiation of patients with MS from controls. A threshold of 67% perivenous WM lesions separated patients with MS from controls with a sensitivity of 94% and specificity of 100%. Lesion assessment took an average of 12 minutes 10 seconds and 4 minutes 33 seconds for patients with MS and control subjects, respectively.

CONCLUSIONS

Nonconfluent lesions of >3 mm with ≥1 central vein were the most sensitive and specific differentiators between patients with MS and control subjects.

Design and construction of a multipath vessel phantom for interventional training

This short communication reports on the design and construction of a catheter manipulation skill enhancement phantom for use by residents and fellows outside the clinical environment. The phantom contains a variety of path trajectories and vessel diameter transitions, allowing trainees to manipulate catheters through vessel paths of varying difficulty. The multipath phantom, which is easy to construct and provides easily visualised paths, provides a simple, cost-effective training platform to facilitate and accelerate interventional training.

Implementation of dual- and triple-energy cone-beam micro-CT for postreconstruction material decomposition

Micro-CT has become a powerful tool for small animal research, having the ability to obtain high-resolution in vivo and ex vivo images for analyzing bone mineral content, organ vasculature, and bone microarchitecture extraction. The use of exogenous contrast agents further extends the use of micro-CT techniques, but despite advancements in contrast agents, single-energy micro-CT is still limited in cases where two different materials share similar grey-scale intensity values. This study specifically addresses the development of multiple-energy cone-beam micro-CT, for applications where bone must be separated from blood vessels filled with a Pb-based contrast material (Microfil) in ex vivo studies of rodents and tissue specimens. The authors report the implementation of dual- and triple-energy CT algorithms for material-specific imaging using postreconstruction decomposition of micro-CT data; the algorithms were implemented on a volumetric cone-beam micro-CT scanner (GE Locus Ultra). For the dual-energy approach, extrinsic filtration was applied to the x-ray beam to produce spectra with different proportions of x rays above the K edge of Pb. The optimum x-ray tube energies (140 kVp filtered with 1.45 mm Cu and 96 kVp filtered with 0.3 mm Pb) that maximize the contrast between bone and Microfil were determined through numerical simulation. For the triple-energy decomposition, an additional low-energy spectrum (70 kVp, no added filtration) was used. The accuracy of decomposition was evaluated through simulations and experimental verification of a phantom containing a cortical bone simulating material (SB3), Microfil, and acrylic. Using simulations and phantom experiments, an accuracy greater than 95% was achieved in decompositions of bone and Microfil (for noise levels lower than 11 HU), while soft tissue was separated with accuracy better than 99%. The triple-energy technique demonstrated a slightly higher, but not significantly different, decomposition accuracy than the dual-energy technique for the same achieved noise level in the micro-CT images acquired at the multiple energies. The dual-energy technique was applied to the decomposition of an ex vivo rat specimen perfused with Microfil; successful decomposition of the bone and Microfil was achieved, enabling the visualization and characterization of the vasculature both in areas where the vessels traverse soft tissue and when they are surrounded by bone. In comparison, in single energy micro-CT, vessels surrounded by bone could not be distinguished from the cortical bone, based on grey-scale intensity alone. This work represents the first postreconstruction application of material-specific decomposition that directly takes advantage of the K edge characteristics of a contrast material injected into an animal specimen; the application of the technique resulted in automatic, accurate segmentation of 3D micro-CT images into bone, vessel, and tissue components. The algorithm uses only reconstructed images, rather than projection data, and is calibrated by an operator with signal values in regions identified as being comprised entirely of either cortical bone, contrast-enhanced vessel, or soft tissue; these required calibration values are observed directly within reconstructed CT images acquired at the multiple energies. These features facilitate future implementation on existing research micro-CT systems.

In vivo small-animal imaging using micro-CT and digital subtraction angiography

Small-animal imaging has a critical role in phenotyping, drug discovery and in providing a basic understanding of mechanisms of disease. Translating imaging methods from humans to small animals is not an easy task. The purpose of this work is to review in vivo x-ray based small-animal imaging, with a focus on in vivo micro-computed tomography (micro-CT) and digital subtraction angiography (DSA). We present the principles, technologies, image quality parameters and types of applications. We show that both methods can be used not only to provide morphological, but also functional information, such as cardiac function estimation or perfusion. Compared to other modalities, x-ray based imaging is usually regarded as being able to provide higher throughput at lower cost and adequate resolution. The limitations are usually associated with the relatively poor contrast mechanisms and potential radiation damage due to ionizing radiation, although the use of contrast agents and careful design of studies can address these limitations. We hope that the information will effectively address how x-ray based imaging can be exploited for successful in vivo preclinical imaging.

In vivo characterization of lung morphology and function in anesthetized free-breathing mice using micro-computed tomography

Lung morphology and function in human subjects can be monitored with computed tomography (CT). Because many human respiratory diseases are routinely modeled in rodents, a means of monitoring the changes in the structure and function of the rodent lung is desired. High-resolution images of the rodent lung can be attained with specialized micro-CT equipment, which provides a means of monitoring rodent models of lung disease noninvasively with a clinically relevant method. Previous studies have shown respiratory-gated images of intubated and respirated mice. Although the image quality and resolution are sufficient in these studies to make quantitative measurements, these measurements of lung structure will depend on the settings of the ventilator and not on the respiratory mechanics of the individual animals. In addition, intubation and ventilation can have unnatural effects on the respiratory dynamics of the animal, because the airway pressure, tidal volume, and respiratory rate are selected by the operator. In these experiments, important information about the symptoms of the respiratory disease being studied may be missed because the respiration is forced to conform to the ventilator settings. In this study, we implement a method of respiratory-gated micro-CT for use with anesthetized free-breathing rodents. From the micro-CT images, quantitative analysis of the structure of the lungs of healthy unconscious mice was performed to obtain airway diameters, lung and airway volumes, and CT densities at end expiration and during inspiration. Because the animals were free breathing, we were able to calculate tidal volume (0.09 +/- 0.03 ml) and functional residual capacity (0.16 +/- 0.03 ml).

3-D image guidance for minimally invasive robotic coronary artery bypass

BACKGROUND

The introduction of a robot-assisted microsurgical system has made endoscopic coronary artery bypass grafting (ECABG) possible. Despite the success of this approach, surgeons still require better visualization tools for pre-surgical planning and intra-operative image guidance. Such visualization tools could, for example, assist in the placement of thoracic ports to acquire optimum access to the target vessels. In this paper we discuss the essential steps toward image-guided completely endoscopic coronary bypass surgery with robot assistance, and we present our preliminary efforts toward the development of a three-dimensional (3-D) virtual cardiac surgical planning platform (VCSP) for ECABG.

METHODS

Preoperative 3-D images of the thorax acquired with computed tomography and electrocardiogram-gated magnetic resonance imaging are imported into VCSP. Using VCSP, a user may interactively visualize and manipulate the simulated thoracic ports in 3-D within the reconstructed thoracic region. We have also implemented a virtual endoscope to simulate the endoscopic view observed by the surgeon during the operation. Once the port placements for optimal access to the target vessels are determined, the positions of the simulated tools can be recorded and marked on the patient to specify the positions for port incisions.

RESULTS

A static thorax phantom was used to verify the port placements obtained from VCSP simulations. The angles and the distances between the ports, the endoscope and the markers that were placed on the surface of the phantom were measured, and the results were compared with those obtained from simulation. The physical measured distances and angles agreed with the simulated results with average errors of 4 mm and 2 degrees, respectively.

CONCLUSIONS

The VCSP image-guided surgical system allows a surgeon to visualize a patient's thorax in a 3-D interactive environment for planning surgical procedures, and to determine the optimum port placement based on preoperative 3-D images. However, during an operation, the positions and orientation of the heart and the coronary arteries are changed from their corresponding locations in the preoperative images due to carbon-dioxide insufflation, lung deflation, and dynamic motions of the beating heart. One of our future goals of this project is the use of mathematical models that correct for these changes so that our system could be applied to intra-operative image guidance.

In vitro simulation and quantification of temporal jitter artifacts in ECG-gated dynamic three-dimensional echocardiography

The image quality of dynamic 3-D echocardiography is limited by temporal jitter artifacts that result from the asynchronous acquisition of video frames with the cardiac cycle. This paper analyzes the source and extent of these artifacts using in vitro studies. Dynamic 3-D images of a myocardial motion phantom were reconstructed and analyzed for eight cardiac motion patterns. The extent of temporal jitter artifacts was quantified, first, from the images by computing temporal jitter maps and, second, predicted from the motion waveforms. Temporal jitter appeared as a pattern of streak artifacts converging at the axis of rotation of the imaging plane, for the rotational scanning approach used in our study. The results of the experimental analysis techniques were compared with the waveform analysis using linear regression analysis. The least squares line showed good correlation between the data (r > 0.9) and its deviation from the line of identity was calculated to be <9%.

Optical detection of triggered atherosclerotic plaque disruption by fluorescence emission analysis

Fluorescence emission analysis (FEA) has proven to be very sensitive for the detection of elastin, collagen and lipids, which are recognized as the major sources of autofluorescence in vascular tissues. FEA has also been reported to detect venous thromboemboli. In this paper we have tested the hypothesis that FEA can reproducibly detect in vivo and in vitro triggered plaque disruption and thrombosis in a rabbit model. Fluorescence emission (FE) spectra, recorded in vivo, detected Russell's viper venom (RVV)-induced transformation of atherosclerotic plaque. FE intensity at 410-490 nm 4 weeks after angioplasty was significantly lower (P < 0.0033 by analysis of variance) in RVV-treated rabbits when compared to control animals with stable plaque. FE spectral profile analyses also demonstrated a significant change in curve shape as demonstrated by polynomial regression analysis (R2 from 0.980 to 0.997). We have also demonstrated an excellent correlation between changes in FE intensity and the structural characteristics detected at different stages of "unstable atherosclerotic plaque" development using multiple regression analysis (R2 = 0.989). Thus, FEA applied in vivo is a sensitive and highly informative diagnostic technique for detection of triggered atherosclerotic plaque disruption and related structural changes, associated with plaque transformation, in a rabbit model.

Evaluation of 3-D colour Doppler ultrasound for the measurement of proximal isovelocity surface area

Three-dimensional (3-D) colour Doppler ultrasound (US) enables flow rate estimation across a diseased valve without the need for a priori geometric assumptions. This study quantitatively evaluates the accuracy of 3-D colour Doppler US for measuring the flow rate (8. 3-75 mL/s) through a valve using the proximal flow convergence field. Flow rate measurements by this 3-D technique underestimate flow through finite circular orifices due to two major sources of error: 1. surface area slicing technique (18.3% +/- 3.8%) and 2. Doppler angle effect (41.0% +/- 1.5%). Combined total underestimation is 51% +/- 3.3%. To utilize 3-D US, the development of an improved proximal isovelocity surface area (PISA) measurement technique and a correction factor for the Doppler angle effect is required.

Three-dimensional echocardiography: assessment of inter- and intra-operator variability and accuracy in the measurement of left ventricular cavity volume and myocardial mass

Accurate left ventricular (LV) volume and mass estimation is a strong predictor of cardiovascular morbidity and mortality. We propose that our technique of 3D echocardiography provides an accurate quantification of LV volume and mass by the reconstruction of 2D images into 3D volumes, thus avoiding the need for geometric assumptions. We compared the accuracy and variability in LV volume and mass measurement using 3D echocardiography with 2D echocardiography, using in vitro studies. Six operators measured the LV volume and mass of seven porcine hearts, using both 3D and 2D techniques. Regression analysis was used to test the accuracy of results and an ANOVA test was used to compute variability in measurement. LV volume measurement accuracy was 9.8% (3D) and 18.4% (2D); LV mass measurement accuracy was 5% (3D) and 9.2% (2D). Variability in LV volume quantification with 3D echocardiography was %SEMinter = 13.5%, %SEMintra = 11.4%, and for 2D echocardiography was %SEMinter = 21.5%, %SEMintra = 19.1%. We derived an equation to predict uncertainty in measurement of LV volume and mass using 3D echocardiography, the results of which agreed with our experimental results to within 13%. 3D echocardiography provided twice the accuracy for LV volume and mass measurement and half the variability for LV volume measurement as compared with 2D echocardiography.

Quantitative angiographic blood-flow measurement using pulsed intra-arterial injection

A technique for quantitative blood-flow measurement using a novel pulsed injection of radiographic contrast agent is reported. A pressurized source of contrast agent is interrupted by a rotary valve at rates ranging from 1 to 30 Hz, producing well-defined boli at the end of a catheter. The position of these boli can be recorded by a digital radiographic system and analyzed by one of several previously reported techniques, to produce quantitative measurements of blood velocity and flow rate throughout the cardiac cycle. The contrast-agent flow wave form produced by the pulsed injector has been measured with an electromagnetic flow meter, for driving pressures ranging from 600 to 1500 kPa. Excellent modulation of the contrast agent is observed for injection frequencies up to 20 Hz, through catheters up to 100 cm in length. Preliminary in vitro angiographic flow measurements have been performed using an x-ray image intensifier, coupled to a linear photodiode array as the digital detector. Both constant flow and pulsatile human blood-flow wave forms were simulated within a 6.4-mm-diam straight tube and monitored with an electromagnetic flow meter. These experiments indicate that the pulsed injector can be used to provide estimates of arterial blood flow over the entire cardiac cycle (including reverse flow), to within about +/-11%, following injection of less than 10 ml of iodinated contrast agent.

Application of dynamic computed tomography for measurements of local aortic elastic modulus

A novel computed tomographic (CT) technique used for the instantaneous measurement of the dynamic elastic modulus of intact excised porcine aortic vessels subjected to physiological pressure waveforms is described. This system was comprised of a high resolution X-ray image intensifier based computed tomographic system with limiting spatial resolution of 3.2 mm-1 (for a 40 mm field of view) and a computer-controlled flow simulator. Utilising cardiac gating and computer control, a time-resolved sequence of 1 mm thick axial tomographic slices was obtained for porcine aortic specimens during one simulated cardiac cycle. With an image acquisition sampling interval of 16.5 ms, the time sequences of CT slices were able to quantify the expansion and contraction of the aortic wall during each phase of the cardiac cycle. Through superficial tagging of the adventitial surface of the specimens with wire markers, measurement of wall strain in specific circumferential sectors and subsequent calculations of localised dynamic elastic modulus were possible. The precision of circumferential measurements made from the CT images utilising a cluster-growing segmentation technique was approximately +/- 0.25 mm and allowed determination of the dynamic elastic modulus E(dyn) with a precision of +/- 8 kPa. Dynamic elastic modulus was resolved as a function of the harmonics of the physiological pressure waveform and as a function of the angular position around the vessel circumference. Application of this dynamic CT (DCT) technique to seven porcine thoracic aortic specimens produced a circumferential average (over all frequency components) E(dyn) of 373 +/- 29 kPa. This value was not statistically different (p < 0.05) from the values of 430 +/- 77 and 390 +/- 47 kPa obtained by uniaxial tensile testing and volumetric measurements respectively.

In vitro verification of myocardial motion tracking from phase-contrast velocity data

The ability to track motion from cine phase-contrast (PC) magnetic resonance (MR) velocity measurements was investigated using an in vitro model. A computer-controlled deformable phantom was used for the characterization of the accuracy and precision of the forward-backward and the compensated Fourier integration techniques. Trajectory accuracy is limited by temporal resolution when the forward-backward technique is used. With this technique the extent of the calculated trajectories is underestimated by an amount related to the motion period and the sequence repetition time, because of the band-limiting caused in the cine interpolation step. When the compensated Fourier integration technique is used, trajectory accuracy is independent of temporal resolution and is better than 1 mm for excursions of less than 15 mm, which are comparable to those observed in the myocardium. Measurement precision is dominated by the artifact level in the phase-contrast images. If no artifacts are present precision is limited by the inherent signal-to-noise ratio of the images. In the presence of artifacts, similar in magnitude to those observed in vivo, the reproducibility of tracking a 2.2 x 2.2 mm2 region of interest is better than 0.5 mm. When the Fourier integration technique is used, the improved accuracy is accompanied by a reduction in precision. We verified that tracking three-dimensional (3D) motion from velocity measurements of a single slice can lead to underestimations of the trajectory if there is a through-plane component of the motion that is not truly represented by the measured velocities. This underestimation can be overcome if volumetric cine phase-contrast velocity data are acquired and full three-dimensional analysis is performed.

Estimation of deformation gradient and strain from cine-PC velocity data

Phase contrast magnetic resonance imaging (MRI) can provide in vivo myocardial velocity field measurements. These data allow densely spaced material points to be tracked throughout the whole heart cycle using, for example, the Fourier tracking algorithm. To process the tracking results for myocardial deformation and strain quantification, we developed a method that is based on fitting the tracking results to an appropriate local deformation model. We further analyzed the accuracy and precision of the method and provided performance predictions for several local models. In order to validate the method and the theoretical performance analysis, we conducted controlled computer simulations and a phantom study. The results agreed well with expectations. Human heart data were also acquired and analyzed, and provided encouraging results. At the signal-to-noise ratio (SNR) level and spatial resolution expected in clinical settings, the study predicts strain quantification accuracy and precision that may allow the technique to become a practical and powerful noninvasive approach for the study of cardiac function, although clinically acceptable data acquisition strategies for three-dimensional (3-D) data are still a challenge.

Effect of artifacts due to flowing blood on the reproducibility of phase-contrast measurements of myocardial motion

The reproducibility of myocardial motion trajectories calculated from cine phase-contrast (PC) velocity data is reduced by artifacts due to the inconsistent motion of intracardiac blood. Spatial presaturation reduces these artifacts but requires a longer sequence TR, with a potentially negative effect on trajectory accuracy and reproducibility. We investigated the effect of spatial presaturation on trajectory reproducibility. A mid-ventricular transaxial slice was imaged in five normal volunteers. The same slice was imaged three times each with sequences using spatial presaturation or not. Because the most serious artifacts originate in the heart chambers and propagate in the phase-encoded direction, myocardial regions that were in line with the heart chambers (in the phase-encode direction) had the highest artifact level in the scans without spatial presaturation. The reproducibility of trajectories for regions placed in these areas (the anterior wall, septum and posterior wall in the transaxial scans with phase encoding in the anterior-posterior direction) improved by a factor of two when presaturation was used (P < .001). In areas that were not in line with the heart chambers (eg, the anterior aspect of the lateral wall in the transaxial scans), the effect of presaturation was not significant. These results correlate well with the measured reduction in artifact level. The reproducibility of myocardial motion trajectories over large areas of the heart is improved to approximately 1 mm when presaturation is used. Therefore, use of presaturation is recommended for myocardial motion studies using cine PC velocity data.

Physiologic motion phantom for MRI applications

To address the need for a complex physiologic motion phantom for use in MR applications, such as the verification of techniques for measuring myocardial motion dynamics and motion insensitive pulse sequences, a computer-controlled motion phantom has been designed. The phantom, which consists of a deformable silicone gel annulus mounted on a translation stage, can undergo a range of bulk motions and deformations. Available motions include bulk rotation and translation, rotational shear, axial shear, and combinations of some or all of these motions. In this paper, the capability of the phantom to produce accurate constant and time-varying waveforms is demonstrated. In the current implementation, peak linear translation and rotation rates are 175 mm s-1 and 10 rad s-1, respectively. Cycle-to-cycle reproducibility is excellent, with variations of less than .003 radians over the period of hours while undergoing rotational shear. The phantom has been designed in a flexible fashion so that various test objects can be scanned while undergoing bulk translation and can be adapted to produce different deformations.

Fourier tracking of myocardial motion using cine-PC data

A closed-form integration method is derived and analyzed for computing motion trajectories from velocity field data, particularly as measured by phase contrast (PC) cine MR imaging. By modeling periodic motion as composed of Fourier harmonics and integrating the material velocity of the tracked point in the frequency domain, this method gives an unbiased trajectory estimate in the presence of white measurement noise and eddy current effects. When applied to cine PC data, the method can incorporate compensation for the frequency response of the cine interpolation, offering a further improvement on the tracking accuracy. In simulation and phantom studies, the estimated trajectories were in excellent agreement with the true trajectories. Encouraging results have also been obtained on data from volunteers.

Decomposition of inflow and blood oxygen level-dependent (BOLD) effects with dual-echo spiral gradient-recalled echo (GRE) fMRI

Image contrast with gradient-recalled echo sequences (GRE) used for fMRI can have both blood oxygen level-dependent (BOLD) and inflow components, and the latter is often undesirable. A dual-echo technique can be used to differentiate these mechanisms, because modulation of signal from inflow is common to both echoes, whereas susceptibility and diffusion-related signal losses are larger in the second echo. An efficient dual-echo interleaved spiral sequence was developed for use with a conventional scanner. It uses a k-space trajectory that spirals out from the origin while the first echo is collected, then spirals back in while collecting the second echo. Decomposition of the data provides separate images of the inflow and T2-weighted components. Results demonstrate the decomposition with phantom experiments and with photic stimulation in normal volunteers.

Artifacts and signal loss due to flow in the presence of B(o) inhomogeneity

An in vitro study was performed to investigate the effects of B(o) inhomogeneity on magnetic resonance images of flow. Controlled inhomogeneity gradients (Gi) were applied and the magnitude of the artifacts produced was quantified for different echo delay times (TE). Both steady and pulsatile flows were examined. In the presence of an inhomogeneity gradient, signal loss is apparent if the flow is pulsatile and/or if the slice thickness is large. The signal loss increases with increasing TE and Gi. With pulsatile flow, ghosting artifacts are also generated. These increase in intensity with increasing TE and Gi. In vivo, field inhomogeneity due to susceptibility variations is large enough to produce these effects. Representative time-of-flight images obtained of a normal volunteer with two different TEs demonstrate the effect in vivo. Flow-related signal loss and artifacts, therefore, increase with increasing TE independent of the moments of the applied gradients.

Tracking of cyclic motion with phase-contrast cine MR velocity data

A method of computing trajectories of objects by using velocity data, particularly as acquired with phase-contrast magnetic resonance (MR) imaging, is presented. Starting from a specified location at one time point, the method recursively estimates the trajectory. The effects of measurement noise and eddy current-induced velocity offsets are analyzed. When the motion is periodic, trajectories can be computed by integrating in both the forward and backward temporal directions, and a linear combination of these trajectories minimizes the effect of velocity offsets and maximizes the precision of the combined trajectory. For representative acquisition parameters and signal-to-noise ratios, the limitations due to measurement noise are acceptable. In a phantom with reciprocal rotation, the measured and true trajectories agreed to within 3.3%. Sample trajectory estimates of human myocardial regions are encouraging.

A laboratory CT scanner for dynamic imaging

A high-resolution laboratory CT scanner has been developed for imaging objects undergoing periodic motion. The scanner comprises an x-ray image intensifier, optically coupled to a linear photodiode array. Gated time-evolved projections of a single slice of the moving object are acquired, reformatted, and reconstructed. The resulting series of CT images shows the object at different phases of its motion cycle. The scanner has an adjustable field of view (FOV) and the resolution can be as high as 3.2 mm-1 (for the 40-mm FOV). The spatial resolution depends on the inherent resolution of the scanner and on the object's velocity. For objects moving at 1 cm s-1, the spatial resolution is reduced by 9% in the direction of motion. The signal intensity in the reconstructed image is linear for materials with attenuation coefficients as high as 1.5 cm-1 (for a 90-kVp x-ray beam), with an average accuracy of +/- 0.02 cm-1. The average accuracy of circumference measurements made from the CT images is +/- 0.3 mm. Lastly, an application of this dynamic CT scanner to imaging excised human arterial specimens under simulated physiological pressure conditions is presented as an example.

A modified x-ray image intensifier with continuously variable field of view: resolution considerations

A conventional x-ray image intensifier (XRII) has been modified to enable the field of view (FOV) to be varied continuously, by adjusting the potentials at the focusing electrodes. The benefit, to system resolution, from decreasing the FOV has been characterized by measuring the modulation transfer function (MTF) of the XRII coupled to a high-resolution photo-diode array (PDA), at a number of different FOVs achieved either by electronic or optical zooming. Electronic zooming of the XRII from FOV = 24 cm to FOV = 10 cm led to an increase in f0.1 (the frequency at which MTF = 0.1) from 1.41 to 3.05 mm-1, while optical zooming increased f0.1 from 1.41 mm-1 only to 1.88 mm-1. It is proposed that the advantage, with respect to resolution gain, of electronic zooming over optical zooming was realized only when the XRII limits system resolution. The MTF of the XRII coupled to a video camera, with lower resolving power than the PDA, was measured at different FOVs to show that using electronic zooming is only marginally beneficial when the optical detector and the XRII contribute equally to the resolution degradation. However, when a higher-resolution optical detector is used, electronic zooming always yields a greater gain in resolution.

Elasticity and geometry measurements of vascular specimens using a high-resolution laboratory CT scanner

Vascular diseases are frequently associated with changes in the mechanical properties of the arterial wall. Existing techniques for studying arterial geometry and mechanical properties in vitro are often destructive, since they involve sectioning of the specimen into strips, or provide average measurements of the mechanical properties over the volume of intact specimens. We developed a high-resolution computed tomography (CT) scanner for in vitro studies of arterial geometry and static elastic properties. The x-ray image intensifier based system can acquire single transverse images, or a volume image, with 2 mm-1 resolution. Images were obtained through an intact abdominal aortic aneurysm at five pressures. The incremental circumferential Young's modulus E(inc) was calculated from the internal and external circumferences, and at physiological pressures E(inc) of the aneurysm was found to be 275 times greater than that of the normal aorta proximal to it. A volume image of the specimen provided landmarks that allowed histological sections to be obtained at locations coincident with those where the elasticity was measured. The histological analysis revealed a sixfold decrease in elastin content in the aneurysm, compared to the normal aorta. We have demonstrated that the static mechanical properties and geometry of vascular specimens can be quantified in vitro with the new high-resolution CT scanner and can be compared subsequently with histological analysis to provide further insight into the understanding of atherogenesis.

A geometrically accurate vascular phantom for comparative studies of x-ray, ultrasound, and magnetic resonance vascular imaging: construction and geometrical verification

A technique for producing accurate models of vascular segments for use in experiments that assess vessel geometry and flow has been developed and evaluated. The models are compatible with x-ray, ultrasound, and magnetic resonance (MR) imaging systems. In this paper, a model of the human carotid artery bifurcation, is evaluated that has been built using this technique. The phantom consists of a thin-walled polyester-resin replica of the bifurcation through which a blood-mimicking fluid may be circulated. The phantom is surrounded by an agar tissue-mimicking material and a series of fiducial markers. The blood- and tissue-mimicking materials have x-ray, ultrasound, and MR properties similar to blood and tissue; fiducial markers provide a means of aligning images acquired by different modalities. The root-mean-square difference between the inner wall geometry of the constructed model and the desired dimensions was 0.33 mm. Static images were successfully acquired using x-ray, ultrasound, and MR imaging systems, and are free of significant artifacts. Flow images acquired with ultrasound and MR agree qualitatively with each other, and with previously published flow patterns. Volume-flow measurements obtained with ultrasound and MR were within 4.4% of the actual values.

A high-resolution XRII-based quantitative volume CT scanner

A laboratory volume CT scanner has been developed, with high spatial resolution in all three dimensions, which can be used for quantitative analysis of excised tissue samples in vitro. The system incorporates an x-ray image intensifier, optically coupled to a time-delay integration (TDI) CCD to obtain low-noise and low-scatter projections of the sample volume. A water bath surrounds the sample to equalize the exposure to the image intensifier, thereby reducing the dynamic range of the input signal. The scanner operates in two modes, producing either a single, transverse image through the sample or a three-dimensional image of the sample volume. Spatial resolution is adjustable over the range of 1.2 to 2.8 mm-1. System response is linear over the range -1000 to 3500 Houndsfield units (HU), with an average precision of +/- 80 HU. The precision of geometric measurements in the transverse plane allows circumference measurements to within +/- 0.1 mm. Finally, applications of this technique of nondestructive analysis in biomedical research are discussed.

Experimental and theoretical x-ray imaging performance comparison of iodine and lanthanide contrast agents

Contrast agents based on the lanthanide elements gadolinium and holmium have recently been developed for magnetic resonance imaging (MRI). Because of the increased atomic number of these elements relative to iodine, these new compounds, used as x-ray contrast agents, may yield higher radiographic contrast, and hence improved x-ray image quality, relative to conventional iodinated compounds, for clinically useful x-ray spectra. This possibility has been investigated, in independent experimental and theoretical studies, for two x-ray imaging systems: a digital radiographic system, using an x-ray image intensifier (XRII) and charge-coupled device (CCD) detector; and a conventional screen/film system, using a Lanex Regular screen. Iodine, gadolinium, and holmium contrast agents were investigated over a wide range of concentration-thickness products (0.1-0.6 M cm) and diagnostic x-ray spectra (60-120 kVp). A simple theoretical model of x-ray detector response predicts the experimental radiographic contrast measurements with a mean absolute error of 8.0% for the XRII/CCD system and 5.9% for the screen/film system, and shows that the radiographic contrast for these two systems is representative of all XRII and screen/film systems. An index of image quality is defined, and its dependence on radiographic contrast, x-ray fluence per unit dose, and detective quantum efficiency (DQE) is shown. Theoretical values of the index, predicted by our model, are then used to compare the performance of the three contrast agents for the two systems investigated. In general, iodine performance decreases steadily with increasing kVp, gadolinium performance has a broad maximum near 85 kVp, and gadolinium outperforms holmium. Gadolinium outperforms iodine for spectra above (and vice versa below) about 72 kVp, depending slightly on spectrum filtration, object thickness, and detector type. Thus, raising the kVp to shorten exposure times or reduce x-ray tube heat loading results in a loss of image quality with iodine, but not with gadolinium. Similarly, beam-hardening artifacts in performing video densitometry with iodine would be reduced with gadolinium. Gadolinium-based contrast agents are thus shown to offer several practical advantages over conventional iodinated contrast agents.

Computer-controlled flow simulator for MR flow studies

A novel computer-controlled flow simulator for use in magnetic resonance (MR) flow experiments was evaluated. The accuracy in constant-flow mode was better than 1%. The accuracy in pulsatile-flow mode was found to be dependent on the interconnecting tubing. The short-term and long-term reproducibilities of pulsatile waveforms were less than or equal to 0.4 mL/sec (1 standard deviation). Increased response times due to the lengths of tubing required in MR flow experiments were surmounted by using a modified tubing configuration and precompensated waveforms. Piston reversal was found not to cause major difficulties in MR flow experiments.

Computer-controlled positive displacement pump for physiological flow simulation

A computer-controlled pump for use both in the study of vascular haemodynamics and in the calibration of clinical devices which measure blood flow is designed. The novel design of this pump incorporates two rack-mounted pistons, driven into opposing cylinders by a micro-stepping motor. This approach allows the production of nearly uninterrupted steady flow, as well as a variety of pulsatile waveforms, including waveforms with reverse flow. The capabilities of this pump to produce steady flow from 0.1 to 60 ml s-1, as well as sinusoidal flow and physiological flow, such as that found in the common femoral and common carotid arteries are demonstrated. Cycle-to-cycle reproducibility is very good, with an average variation of 0.1 ml s-1 over thousands of cycles.

Measurement of the spatial Wiener spectrum of nonstorage imaging devices

All previous methods for measuring image noise spectra require a noise realization, a static image, typified as a photograph which can be scanned to create the Wiener spectrum. We wished to analyze the spatial noise power spectrum at the output phosphor of a continuously irradiated imaging device, an x-ray image intensifier (XRII), which is incapable of image storage and thus the image is continually changing as a function of both time and space. Our new method utilizes a pair of slits to measure the relative Wiener spectrum of the temporally changing components of the image (i.e., x-ray quantum and XRII gain noises). By measuring the modulation transfer function and the Wiener spectrum of the same XRII on the same apparatus it was possible to demonstrate the spatial frequency dependence of the detective quantum efficiency. Adaptations of the method should permit the measurement of Wiener spectra of fluoroscopic television systems directly from the TV monitor.

Stereotactic surgical planning with magnetic resonance imaging, digital subtraction angiography and computed tomography

Over the past 2 years at the Montreal Neurological Institute and Hospital, we have evolved an integrated environment for the planning of stereotactic procedures, based on images from magnetic resonance imaging, digital subtraction angiography and computed tomography modalities. These procedures rely on fiducial marker sets which are attached to our 'OBT' stereotactic frame, and which may be recognized in the images. The software package is modular and operates in both minicomputer (PDP-11 and VAX) and IBM personal computer environments. In addition to routine tasks for stereotactic planning, the package also supports dosimetry planning for stereotactic radiosurgery.

Optical factors affecting the detective quantum efficiency of radiographic screens

Parameters related to the detective quantum efficiency (DQE) of several representative screens of different thicknesses, phosphor grain sizes, and optical properties were measured by the scintillation spectrum method, using monoenergetic x rays produced from x-ray fluorescence. The experimental results, including those for spectral shape and average light energies (EA) emitted, are compared with conventional theories of the operation of screens. It was hoped that this would vindicate the theory of the effect of optical properties and so permit the simple calculation of all parameters related to DQE from standard x-ray attenuation tables. Rather more substantial energy-dependent deviations of EA are found than was previously realized, which preliminary analysis suggests are due to both optical effects and photoelectron escape. We conclude that although DQE for a single energy can be calculated by simplified methods to within +/- 10%, the effective DQE when polyenergetic beams are used is much less accurately estimated and requires a fuller theoretical treatment.