Carl R. Crawford

Computed tomography scanning with simultaneous patient translation

This paper deals with methods of reducing the total time required to acquire the projection data for a set of contiguous computed tomography (CT) images. Normally during the acquisition of a set of slices, the patient is held stationary during data collection and translated to the next axial location during an interscan delay. We demonstrate using computer simulations and scans of volunteers on a modified scanner how acceptable image quality is achieved if the patient translation time is overlapped with data acquisition. If the concurrent patient translation is ignored, structured artifacts significantly degrade resulting reconstructions. We present a number of weighting schemes for use with the conventional convolution/backprojection algorithm to reduce the structured artifacts through the use of projection modulation using the data from individual and multiple slices. We compare the methods with respect to structured artifacts, noise, resolution and to patient motion. Review of preliminary results by a panel of radiologists indicates that the residual image degradation is tolerable for selected applications when it is critical to acquire more slices in a patient breathing cycle than is possible with conventional scanning.

Estimation of geometrical parameters and collimator evaluation for cone beam tomography.

A method is presented for estimating the geometrical parameters for a cone beam detector geometry from the coordinates of the centroid of a projected point source sampled over 360 degrees. Nonlinear expressions are derived for the coordinates of the centroids in terms of the geometrical parameters which include: the two-dimensional coordinates of the projection of the center of rotation onto the detector image plane; the focal length; the distance from the focal point to the center of rotation; and the spatial coordinates of the point source itself. Experimental data were obtained using a rotating gamma camera with a symmetrically converging collimator. The Marquardt algorithm was used to estimate the parameters for this particular cone beam geometry. The method was able to estimate the geometrical parameters and evaluate the accuracy of the collimator construction.

Guest editorial - Multirow detector and cone-beam spiral/helical CT

S PIRAL/HELICAL multirow detector computed tomog-raphy (MDCT), also referred to as multislice spiral CT (MSCT), has attracted a major interest since its recent introduction [18], [33]. Compared with single-row-detector systems, MDCT scanners allow faster data collection and thinner slices that support more demanding clinical applications and present new research opportunities. This special issue consists of 12 original papers from this rapidly expanding field. Although MDCT generally refers to commercial CT scanners used by radiologists, the work in this issue is also applicable to emission CT as well as nonmedical applications of X-ray CT, including nondestructive testing and microtomography. In this editorial, we provide a summary of the special issue, an overview of the field, and suggestions for future work. The first eight papers represent recent work on X-ray CT image reconstruction in multirow detector or cone-beam geometry. The first two papers by Schaller et al.report an adap-tive axial interpolation theory for a MDCT scanner with experimental verification [13], [29]. Adaptive axial interpolation provides increased flexibility for image reconstruction in clinical practice. By keeping slice thickness and image noise independent of pitch, the MDCT parameter selection is simplified. The paper by Proksa et al. introduces the-PI-method for spiral/helical cone-beam CT (CBCT) [27]. This method assumes a detector array shaped by a helix, permits variable pitch, and is amendable to both exact and approximate reconstruction. The paper by Hsieh addresses the degradation in image quality of spiral/helical MDCT when the gantry is tilted to produce oblique sections [17]. He formulated a model as an analytical basis and developed several compensation schemes. The paper by Bruder et al. describes two different approximate single-slice algorithms for CBCT: multirow Fourier reconstruction (MFR) and advanced single-slice rebinning (ASSR), in the framework of the generalized parallel projection using-filtering [3]. Their studies contain data valuable for the design of spiral/helical CT scanners with medium cone-beam angles. The paper by Kachel-rieß et al. proposes MDCT algorithms for cardiac imaging [19]. Based on ECG signals, the two dedicated cardiac reconstruction algorithms improve image quality compared with the standard reconstruction algorithms. The paper by Kudo et al. [21] deals with the long-object problem; that is, to reconstruct a region of interest (ROI) of a long object from data collected along a helical path that extends only marginally above and below the ROI. Their quasi-exact reconstruction algorithms require no Publisher Item Identifier S 0278-0062(00)09415-5. additional circular scans and are in the filtered …

CT filtration aliasing artifacts

Two methods for filtering computerized tomography (CT) projections for the filtered backprojection reconstruction algorithm are evaluated. The methods are based on derivations made in the spatial and Fourier domains. The Fourier method of filtration produces images with DC shifts and low-frequency shading. The spatial method does not generate similar artifacts. It is shown that the artifacts result because of aliasing artifacts that arise when a spatial waveform with infinite extent is sampled in the Fourier domain. It is also shown that it is possible to connect the artifacts generated with the Fourier method by replacing the DC and the first two frequency components with the corresponding terms from the discrete Fourier transforms of the filter used in the spatial method.

Dual Energy Volumetric X-ray Tomographic Sensor for Luggage Screening

Various imaging sensors have been used for luggage screening for explosive detection. Recent developments in dual energy X-ray volumetric computed tomography (CT) have demonstrated its advantages over other imaging sensors for luggage screening. These advantages include higher performance in automatic explosive detection, effectiveness for weapon detection, superior visualization capabilities, and automatic visual cueing for on-screen threat resolution. In this paper, we describe the performance of one such X-ray sensor using our transportation security administration (TSA) certified dual energy volumetric CT for explosive detection.

Variable Pitch Reconstruction Using John's Equation

We present an algorithm to reconstruct helical cone beam computed tomography (CT) data acquired at variable pitch. The algorithm extracts a halfscan segment of projections using an extended version of the advanced single slice rebinning (ASSR) algorithm. ASSR rebins constant pitch cone beam data to fan beam projections that approximately lie on a plane that is tilted to optimally fit the source helix. For variable pitch, the error between the tilted plane chosen by ASSR and the source helix increases, resulting in increased image artifacts. To reduce the artifacts, we choose a reconstruction plane, which is tilted and shifted relative to the source trajectory. We then correct rebinned fan beam data using Johns equation to virtually move the source into the tilted and shifted reconstruction plane. Results obtained from simulated phantom images and scanner images demonstrate the applicability of the proposed algorithm.

Moving beam helical CT scanning

Images generated with helical scanning are degraded by partial volume artifacts caused by an increased slice thickness when compared to conventional computed tomography (CT) scanning. The slice thickness for a helical scan is proportional to the sum of the thickness of the fan of radiation and the distance the patient moves during data acquisition. The authors present a method called moving beam helical scanning (MBHS) which significantly reduces the partial volume artifacts caused by helical scanning. The key element of MBHS is a rotatable collimator that is placed between the X-ray source and the patient. As the patient is translated, the collimator is used to aim the fan on a fixed position in the patient. Once sufficient data are obtained to reconstruct a slice, the collimator is quickly reset to scan a target in the next slice. The authors examined the performance of MBHS by scanning wires and phantoms on a modified scanner. The full-width-at-tenth-maximum of the slice profile at iso-center for MBHS is identical to conventional CT versus a 59% increase for conventional helical scanning. It is concluded that MBHS can be used to obtain the scan rate advantages of helical scanning with image quality comparable to conventional scanning.