Yueh Z. Lee

Stationary scanning x-ray source based on carbon nanotube field emitters

We report a field emission x-ray source that can generate a scanning x-ray beam to image an object from multiple projection angles without mechanical motion. The key component of the device is a gated carbon nanotube field emission cathode with an array of electron emitting pixels that are individually addressable via a metal–oxide–semiconductor field effect transistor-based electronic circuit. The characteristics of this x-ray source are measured and its imaging capability is demonstrated. The device can potentially lead to a fast data acquisition rate for laminography and tomosynthesis with a simplified experimental setup.

Carbon nanotube based microfocus field emission x-ray source for microcomputed tomography

Microcomputed tomography is now widely used for in vivo small animal imaging for cancer studies. Achieving high imaging quality of live objects requires the x-ray source to have both high spatial and temporal resolutions. Preliminary studies have shown that carbon nanotube (CNT) based field emission x-ray source has significant intrinsic advantages over the conventional thermionic x-ray tube including better temporal resolution and programmability. Here we report the design and characterization of a CNT based field emission x-ray source that also affords a high spatial resolution. The device uses modified asymmetric Einzel lenses for electron focusing and an elliptical shaped CNT cathode patterned by photolithography. Stable and small isotropic x-ray focal spot sizes were obtained.

Quantitative measurements of cerebral blood flow in patients with unilateral carotid artery occlusion: A PET and MR study

Although it has been demonstrated that quantitative measures of cerebral blood flow (CBF) can be obtained with the singular value decomposition (SVD) algorithm, the extent to which quantitative CBF measurements can be utilized under pathophysiological conditions has not been systematically studied. A total of five healthy volunteers and five patients with unilateral carotid artery occlusion were studied. Only magnetic resonance (MR) images were acquired for the volunteer group while both MR and positron emission tomography (PET) images were acquired for the patient group. Assessments of CBF from normal volunteers compared favorably with values reported in the literature. However, while a linear relationship was observed for each patient when MR measured CBF was compared to that obtained from PET, this linear relationship diminished when all patients were analyzed as a group (r = 0.41). A correction factor (CF) was proposed that was equal to the ratio of the area of the venous output function (VOF) in each patient to the mean VOF obtained from the volunteer group. After globally scaling the CBF of each patient based on the experimentally derived CF, a substantial improvement was observed (a slope of 1.02 and r = 0.8 for the linear regression line) in the relationship between MR estimated CBF and those obtained from PET. J. Magn. Reson. Imaging 2001;14:659–667.

A dynamic micro-CT scanner based on a carbon nanotube field emission x-ray source

Current commercial micro-CT scanners have the capability of imaging objects ex vivo with high spatial resolution, but performing in vivo micro-CT on free-breathing small animals is still challenging because their physiological motions are non-periodic and much faster than those of humans. In this paper, we present a prototype physiologically gated micro-computed tomography (micro-CT) scanner based on a carbon nanotube field emission micro-focus x-ray source. The novel x-ray source allows x-ray pulses and imaging sequences to be readily synchronized and gated to non-periodic physiological signals from small animals. The system performance is evaluated using phantoms and sacrificed and anesthetized mice. Prospective respiratory-gated micro-CT images of anesthetized free-breathing mice were collected using this scanner at 50 ms temporal resolution and 6.2 lp mm(-1) at 10% system MTF. The high spatial and temporal resolutions of the micro-CT scanner make it well suited for high-resolution imaging of free-breathing small animals.

Magnetic resonance cerebral metabolic rate of oxygen utilization in hyperacute stroke patients

The purpose of this study was to explore the feasibility of obtaining magnetic resonance–measured cerebral metabolic rate of oxygen utilization (MR‐CMRO2) in acute ischemic stroke patients. Seven stroke patients were serially imaged: 4.5 ± 0.9 hours (tp1), 3 to 5 days (tp2), and 1 to 3 months (tp3) after symptom onset. Diffusion‐weighted, perfusion‐weighted, and multiecho gradient‐echo/spin‐echo images were acquired; cerebral blood flow and oxygen extraction fraction maps were obtained from which CMRO2 was calculated as the product of cerebral blood flow and oxygen extraction fraction. The final infarct lesions obtained from tp3 T2‐weighted images and the “penumbra” obtained from the tp1 perfusion‐weighted image–defined lesion were coregistered onto tp1 CMRO2 maps. CMRO2 values in the region of brain that eventually infarcted were reduced to 0.40 ± 0.24 of the respective region on the contralateral hemisphere. The “salvaged penumbra” defined by the area of mismatch between the final infarct and the tp1 perfusion‐weighted lesion demonstrated an average CMRO2 value of 0.55 ± 0.11 of the contralateral hemisphere. Although our results are preliminary and require further evaluation, the ability to obtain in vivo measurements of MR‐CMRO2 noninvasively potentially can provide information for determining brain tissue viability in acute ischemic stroke patients.

Multiplexing radiography using a carbon nanotube based x-ray source

Speed and temporal resolution are critical for tomographic imaging of objects in rapid motion. Current x-ray scanners record images sequentially in the time domain. The serial approach limits their performance and demands increasingly high x-ray peak power and gantry speed. We have developed a multipixel carbon nanotube based field emission x-ray source that produces spatially and temporally modulated radiations. Using this device we show the feasibility of multiplexing radiography that enables simultaneous collection of multiple projection images using frequency multiplexing. A drastic increase of the speed and reduction of the x-ray peak power are achieved without compromising the imaging quality.

Dynamic radiography using a carbon-nanotube-based field-emission x-ray source

We report a dynamic radiography system with a carbon nanotube based field-emission microfocus x-ray source. The system can readily generate x-ray radiation with continuous variation of temporal resolution as short as nanoseconds. Its potential applications for dynamic x-ray imaging are demonstrated. The performance characteristics of this compact and versatile system are promising for noninvasive imaging in biomedical research and industrial inspection.

A nanotube-based field emission x-ray source for microcomputed tomography

Microcomputed tomography (micro-CT) is a noninvasive imaging tool commonly used to probe the internal structures of small animals for biomedical research and for the inspection of microelectronics. Here we report the development of a micro-CT scanner with a carbon nanotube- (CNT-) based microfocus x-ray source. The performance of the CNT x-ray source and the imaging capability of the micro-CT scanner were characterized.

Matching Visual Saliency to Confidence in Plots of Uncertain Data

Conveying data uncertainty in visualizations is crucial for preventing viewers from drawing conclusions based on untrustworthy data points. This paper proposes a methodology for efficiently generating density plots of uncertain multivariate data sets that draws viewers to preattentively identify values of high certainty while not calling attention to uncertain values. We demonstrate how to augment scatter plots and parallel coordinates plots to incorporate statistically modeled uncertainty and show how to integrate them with existing multivariate analysis techniques, including outlier detection and interactive brushing. Computing high quality density plots can be expensive for large data sets, so we also describe a probabilistic plotting technique that summarizes the data without requiring explicit density plot computation. These techniques have been useful for identifying brain tumors in multivariate magnetic resonance spectroscopy data and we describe how to extend them to visualize ensemble data sets.

Quantification of Microvascular Tortuosity during Tumor Evolution Using Acoustic Angiography

The recent design of ultra-broadband, multifrequency ultrasound transducers has enabled high-sensitivity, high-resolution contrast imaging, with very efficient suppression of tissue background using a technique called acoustic angiography. Here we perform the first application of acoustic angiography to evolving tumors in mice predisposed to develop mammary carcinoma, with the intent of visualizing and quantifying angiogenesis progression associated with tumor growth. Metrics compared include vascular density and two measures of vessel tortuosity quantified from segmentations of vessels traversing and surrounding 24 tumors and abdominal vessels from control mice. Quantitative morphologic analysis of tumor vessels revealed significantly increased vascular tortuosity abnormalities associated with tumor growth, with the distance metric elevated approximately 14% and the sum of angles metric increased 60% in tumor vessels versus controls. Future applications of this imaging approach may provide clinicians with a new tool in tumor detection, differentiation or evaluation, though with limited depth of penetration using the current configuration.