Arkadiusz Polacin

Physical performance characteristics of spiral CT scanning

CT scanning in spiral geometry is achieved by continuously transporting the patient through the gantry in synchrony with continuous data acquisition over a multitude of 360-deg scans. Data for reconstruction of images in planar geometry are estimated from the spiral data by interpolation. The influence of spiral scanning on image quality is investigated. Most of the standard physical performance parameters, e.g., spatial resolution, image uniformity, and contrast, are not affected; results differ for pixel noise and slice sensitivity profiles. For linear interpolation, pixel noise is expected to be reduced by a factor of 0.82; reduction factors of 0.81 to 0.83 were measured. Slice sensitivity profiles are changed as a function of table feed d, measured in millimeters per 360-deg scan; they are smoothed as the original profile is convolved with the object motion function. The motion function is derived for linear interpolation that constitutes a triangle with a base line width of 2d and a maximal height equal to 1/d. Calculations of both the full width at half-maximum and the shape of the profiles were in good agreement with experimental results. The effect of the widened profiles, in particular of their extended tail ends, on image quality is demonstrated in phantom measurements.

A Comparison of Conventional and Spiral Ct: An Experimental Study on the Detection of Spherical Lesions

Objective It is accepted that spiral CT scanning may offer significant advantages in a number of clinical applications. There is still some concern with respect to image quality, however, since slice sensitivity profiles are slightly broadened due to the table motion. We carried out theoretical analysis, phantom measurements, and computer simulations to evaluate and to compare contrast and spatial resolution for conventional and for spiral scanning. Special emphasis was put on the task of detecting spherical lesions. Materials and Methods For standard test objects that measure only resolution in the scan plane, no significant difference between conventional and spiral scanning was observed. We therefore designed a phantom setup that allowed us to place spheres of arbitrary diameter and contrast in arbitrary positions to test three-dimensional (3D) resolution. Results For conventional CT, both lesion contrast and the degree of spatial separation of lesions observed depend on the relation of the start position of the scan series to the random location of a sphere or lesion. Spiral CT offers space-invariant resolution due to its continuous scanning. Small lesion contrast may be improved by up to a factor of 1.8 when compared with conventional CT since slices can be centered retrospectively. Measurements and simulations were in excellent agreement. Conclusion We conclude that spiral CT can offer improved 3D contrast and spatial resolution. To exploit these advantages, images should be reconstructed in spiral CT at increments of less than half the distance traveled during one 360° tube rotation. With four to five images per such interval, usually equal to the slice width, results very close to the theoretical optimum are achieved. Many of the presented considerations and results apply to other slice imaging modalities like MRI in an analogous fashion.

MEASUREMENT OF SLICE SENSITIVITY PROFILES IN SPIRAL CT

In conventional computed tomography (CT), ramps, typically thin sheets of aluminum inclined at 45 degrees, are established tools for measurements of slice sensitivity profiles (SSP). In spiral CT, however, they yield inconsistent results for different positions along the longitudinal axis. It is explained herein how ramp profiles result as a superposition of true SSPs and artifacts, the artifacts being caused by the slice interpolation process due to the difficult interpolation condition in this particular geometry. In direct consequence, ramp tests yield spatially variant results. As an alternative tool for measuring the slice sensitivity profile, the use of a thin high-contrast sheet held between two disks which approximate the ideal test of a delta impulse in the longitudinal direction is suggested. Resulting SSPs are shown for both methods; the delta method, in agreement with theoretical predictions, provided smooth symmetrical SSPs independent of table position in agreement with theoretical predictions. It is concluded that ramps are inadequate test objects to determine SSPs in spiral CT.

Unwrapping cochlear implants by spiral CT

Multielectrode, intracochlear implants were designed for individuals with profound sensorineural hearing loss who derive little or no benefit from acoustic hearing aids. Determination of each electrodes position in a patients inner ear may improve speech processor programming to maximize speech recognition. In this paper, an approach is described to use as input a volumetric spiral computed tomography (CT) image of the Nucleus electrode array (Cochlear Pty. Ltd, Lane Cove, NSW, Australia) to unwrap it, and to measure its implanted length given starting and end points. Representative curvilinear structures were digitally synthesized in image volumes of isotropic 0.1-mm voxels. The electrode array was spirally CT-scanned in vitro and in vivo, and reconstructed on an isotropic grid in 0.1-mm steps. Two algorithms were constructed to track and measure these curvilinear structures. The first algorithm is Karhunen-Loeve (K-L)-transform based, in which the K-L transform is locally applied at a current main aids position to determine the eigenvectors of the main axis voxels, the next main axis position is estimated from the current position along the principal eigendirection, adjusted to the mass center of the orthogonal cross section passing through the estimated position, and then scaled to have a prespecified step. The second algorithm is similar to the first one but avoids use of the K-L transform, in the second algorithm, the next position is directly estimated along the local direction and then processed with the same correction and scaling operations. With user-specified starting and end points as well. As a local direction at the starting point, a curvilinear structure can be automatically tracked using either of the algorithms. The first algorithm is more robust, while the second one is more efficient. In the numerical and in vitro studies, the lengths of the curvilinear structures were accurately measured. Given local directions determined in the tracking process, an electrode array image can be unwrapped into a linear array with the central electrode axis as the abscissa. The unwrapping approach allows longitudinally and cross-sectionally accurate measurement and better visualization of cochlear implant images. With preimplantation knowledge of length, width, and center electrode distance, the position of individual electrodes can be estimated after unwrapping.

Brain perfusion studies by xenon-enhanced CT using washin/washout study protocols.

Very short inhalation times and short total examination times are desirable in cerebral blood flow measurements by xenon enhanced CT to minimize the possibility of flow activation and--more importantly for practical purposes--the probability of patient motion due to the effects of xenon. We have investigated washin/washout procedures and have compared them with conventional washin scanning protocols by simulation and in clinical studies. Examination protocols with only 3 min of inhalation and up to eight scans, all taken at 1 min intervals, provide flow estimates with smaller SDs than would be obtained for washin studies taken with the same total radiation dose. Compared with a standard 8 min washin procedure, a 3 min washin/5 min washout study using the same dose yields an SD reduction by a factor of 1.3 for low flow areas and of 1.8 for high flow gray matter. A 3 min washin/3 min washout study, employing only 78% of the dose of an 8 min washin study, will still provide an SD reduction factor of 1.7 in gray matter. These results have been confirmed qualitatively by studies carried out both in volunteers and in patients.

Simulation study of cerebral blood flow measurements in xenon‐CT: Evaluation of washin/washout procedures

Simulation programs have been created that allow one to vary image pixel noise, the number and the distribution of scans with time, cerebral tissue parameters, and the type of xenon CT inhalation procedure in order to investigate CBF measurements with respect to accuracy and signal-to-noise ratio (SNR). In particular, standard washin studies were compared to washin/washout studies. Based on the results of these simulations, a new protocol is suggested; it consists of only 3 min of xenon inhalation (washin phase) and 3 min of washout, monitored by one reference and six enhancement scans taken at 1-min intervals. Compared with a standard 8-min washin study of equal total dose, flow standard deviation (s.d.) for an unconstrained least-squares algorithm is reduced by factors of 2.2 and 1.2 for gray and white matter, respectively; for flow distributed uniformly from 20 to 80 ml/min/100 g, an average s.d. reduction factor of 1.7 is achieved. This was confirmed experimentally in a volunteer study using noise power spectrum analysis. In addition, effects of tissue heterogeneity have been investigated; both the bias and s.d. of flow estimates due to varying proportions of white and gray matter in a given volume element are reduced in washin/washout protocols. When compared to a short washin-only study of 4.5 min, the 3-min washin/3-min washout study provides an improvement of flow s.d. by a factor of 1.6 and 1.9 for gray and white matter, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)