David S. Lalush

A realistic spline-based dynamic heart phantom

We develop a realistic computerized heart phantom for use in medical imaging research. This phantom is a hybrid of realistic patient-based phantoms and flexible geometry-based phantoms. The surfaces of heart structures are defined using non-uniform rational B-splines (NURBS), as used in 3D computer graphics. The NURBS primitives define continuous surfaces allowing the phantom to be defined at any resolution. Also, by fitting NURBS to patient data, the phantom is more realistic than those based on solid geometry. An important innovation is the extension of NURBS to the fourth dimension, time, to model heart motion. Points on the surfaces of heart structures were selected from a gated MRI study of a normal patient. Polygon surfaces were fit to the points for each time frame, and smoothed. 3D NURBS surfaces were fit to the smooth polygon surfaces and then a 4D NURBS surface was fit through these surfaces. Each of the principal 4D surfaces (atria, ventricles, inner and outer walls) contains approximately 200 control points, We conclude that 4D NURBS are an efficient and flexible way to describe the heart and other anatomical objects for a realistic phantom.

Modeling respiratory mechanics in the MCAT and spline-based MCAT phantoms

Respiratory motion can cause artifacts in myocardial SPECT and computed tomography (CT). The authors incorporate models of respiratory mechanics into the current 4D MCAT and into the next generation spline-based MCAT phantoms. In order to simulate respiratory motion in the current MCAT phantom, the geometric solids for the diaphragm, heart, ribs, and lungs were altered through manipulation of parameters defining them. Affine transformations were applied to the control points defining the same respiratory structures in the spline-based MCAT phantom to simulate respiratory motion. The Non-Uniform Rational B-Spline (NURBS) surfaces for the lungs and body outline were constructed in such a way as to be linked to the surrounding ribs. Expansion and contraction of the thoracic cage then coincided with expansion and contraction of the lungs and body. The changes both phantoms underwent were spline-interpolated over time to create time continuous 4D respiratory models. The authors then used the geometry-based and spline-based MCAT phantoms in an initial simulation study of the effects of respiratory motion on myocardial SPECT. The simulated reconstructed images demonstrated distinct artifacts in the inferior region of the myocardium. It is concluded that both respiratory models can be effective tools for researching effects of respiratory motion.

Namic heart phantom

We develop a realistic computerized heart phantom for use in medical imaging research. This phantom is a hybrid of realistic patient-based phantoms and flexible geometry-based phantoms. The surfaces of heart structures are defined using non-uniform rational B-splines (NURBS), as used in 3D computer graphics. The NURBS primitives define continuous surfaces allowing the phantom to be defined at any resolution. Also, by fitting NURBS to patient data, the phantom is more realistic than those based on solid geometry. An important innovation is the extension of NURBS to the fourth dimension, time, to model heart motion. Points on the surfaces of heart structures were selected from a gated MRI study of a normal patient. Polygon surfaces were fit to the points for each time frame, and smoothed. 3D NURBS surfaces were fit to the smooth polygon surfaces and then a 4D NURBS surface was fit through these surfaces. Each of the principal 4D surfaces (atria, ventricles, inner and outer walls) contains approximately 200 control points, We conclude that 4D NURBS are an efficient and flexible way to describe the heart and other anatomical objects for a realistic phantom.

The importance and implementation of accurate 3D compensation methods for quantitative SPECT

The purpose of this study was to investigate the importance of 2D versus 3D compensation methods in SPECT. The compensation methods included in the study addressed two important degrading factors, namely attenuating and collimator-detector response in SPET. They can be divided into two general categories. The conventional methods are based on the filtered backprojection algorithm, the Chang algorithm for attenuation compensation and the Metz filter for detector response compensation. These methods, which were computationally efficient, could only achieve approximate compensation due to the assumptions made. The quantitative compensation methods provide accurate compensation by modelling the degrading effects at the expense of large computational requirements. Both types of compensation methods were implemented in 2D and 3D reconstructions. The 2D and 3D reconstruction/compensation methods were evaluated using data from simulation of brain and heart, and patient thallium SPECT studies. Our results demonstrate the importance of compensation methods in improving the quality and quantitative accuracy of SPECT images and the relative effectiveness of the different 2D and 3D reconstruction/compensation methods. We concluded that 3D implementation of the quantitative compensation methods provides the best SPECT image in terms of quantitative accuracy, spatial resolution, and noise at a cost of high computational requirements.

Block-iterative techniques for fast 4D reconstruction using a priori motion models in gated cardiac SPECT

We introduce a fast block-iterative maximum a posteriori (MAP) reconstruction algorithm and apply it to four-dimensional reconstruction of gated SPECT perfusion studies. The new algorithm, called RBI-MAP, is based on the rescaled block iterative EM (RBI-EM) algorithm. We develop RBI-MAP based on similarities between the RBI-EM, ML-EM and MAP-EM algorithms. RBI-MAP requires far fewer iterations than MAP-EM, and so should result in acceleration similar to that obtained from using RBI-EM or OS-EM as opposed to ML-EM. When complex four-dimensional clique structures are used in the prior, however, evaluation of the smoothing prior dominates the processing time. We show that a simple scheme for updating the prior term in the heart region only for RBI-MAP results in savings in processing time of a factor of six over MAP-EM. The RBI-MAP algorithm incorporating 3D collimator-detector response compensation is demonstrated on a simulated 99mTc gated perfusion study. Results of RBI-MAP are compared with RBI-EM followed by a 4D linear filter. For the simulated study, we find that RBI-MAP provides consistently higher defect contrast for a given degree of noise smoothing than does filtered RBI-EM. This is an indication that RBI-MAP smoothing does less to degrade resolution gained from 3D detector response compensation than does a linear filter. We conclude that RBI-MAP can provide smooth four-dimensional reconstructions with good visualization of heart structures in clinically realistic processing times.

Simulation evaluation of Gibbs prior distributions for use in maximum a posteriori SPECT reconstructions

The effects of several of Gibbs prior distributions in terms of noise characteristics, edge sharpness, and overall quantitative accuracy of the final estimates obtained from an iterative maximum a posteriori (MAP) procedure applied to data from a realistic chest phantom are demonstrated. The effects of the adjustable parameters built into the prior distribution on these properties are examined. It is found that these parameter values influence the noise and edge characteristics of the final estimate and can generate reconstructions closer to the actual solution than maximum likelihood (ML). In addition, it is found that the choice of the shape of the prior distribution affects the noise characteristics and edge sharpness in the final estimate.

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.

Design and characterization of a spatially distributed multibeam field emission x-ray source for stationary digital breast tomosynthesis

Digital breast tomosynthesis (DBT) is a limited angle computed tomography technique that can distinguish tumors from its overlying breast tissues and has potentials for detection of cancers at a smaller size and earlier stage. Current prototype DBT scanners are based on the regular full-field digital mammography systems and require partial isocentric motion of an x-ray tube over certain angular range to record the projection views. This prolongs the scanning time and, in turn, degrades the imaging quality due to motion blur. To mitigate the above limitations, the concept of a stationary DBT (s-DBT) scanner has been recently proposed based on the newly developed spatially distributed multibeam field emission x-ray (MBFEX) source technique using the carbon nanotube. The purpose of this article is to evaluate the performance of the 25-beam MBFEX source array that has been designed and fabricated for the s-DBT system. The s-DBT system records all the projection images by electronically activating the multiple x-ray beams from different viewing angles without any mechanical motion. The configuration of the MBFEX source is close to the published values from the Siemens Mammomat system. The key issues including the x-ray flux, focal spot size, spatial resolution, scanning time, beam-to-beam consistency, and reliability are evaluated using the standard procedures. In this article, the authors describe the design and performance of a distributed x-ray source array specifically designed for the s-DBT system. They evaluate the emission current, current variation, lifetime, and focal spot sizes of the source array. An emission current of up to 18 mA was obtained at 0.5 x 0.3 mm effective focal spot size. The experimentally measured focal spot sizes are comparable to that of a typical commercial mammography tube without motion blurring. Trade-off between the system spatial resolution, x-ray flux, and scanning time are also discussed. Projection images of a breast phantom were collected using the x-ray source array from 25 different viewing angles without motion. These preliminary results demonstrate the feasibility of the proposed s-DBT scanner. The technology has the potential to increase the resolution and reduce the imaging time for DBT. With the present design of 25 views, they demonstrated experimentally the feasibility of achieving 11 s scanning time at full detector resolution with 0.5 x 0.3 mm source resolution without motion blur. The flexibility in configuration of the x-ray source array will also allow system designers to consider imaging geometries that are difficult to achieve with the conventional single-source rotating approach.

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.

Effects of upward creep and respiratory motion in myocardial SPECT

In this study, the authors investigate the effects of two patient involuntary motions, namely upward creep (UC) and respiratory (RSP) motion, in myocardial SPECT images. A new realistic torso phantom was developed based on data from the Visual Human Project and using non-uniform rational B-splines (NURBS) modeling. The heart and diaphragm of the phantom move with a linear upward translation to model UC and move in a sinusoidal up and down fashion to model RSP motion. Simulated emission and transmission CT data sets were generated from the phantom using a L-shaped dual-camera SPECT system with a radioactivity distribution modeling that of a Tl-210 study with UC of 2 cm. The effects of attenuation and collimator-detector response are included in the simulation. A patient study with the same extent of UC was used for comparison. Both simulated and patient data were reconstructed with and without correction for attenuation and UC. Similar data sets were generated from the phantom with RSP motion. The simulated reconstructed images demonstrated distinct UC and RSP artifacts in the inferior region of the myocardium. The UC artifact can be greatly reduced with simple UC correction. However, the correction of RSP artifact may require respiratory gating.