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A Polacin

Publications and source records attributed to A Polacin.

12 recordsLinked to original sources
PubMed2003

Image reconstruction and image quality evaluation for a 16-slice CT scanner.

We present a theoretical overview and a performance evaluation of a novel approximate reconstruction algorithm for cone-beam spiral CT, the adaptive multiple plane reconstruction (AMPR), which has been introduced by Schaller, Flohr et al. [Proc. SPIE Int. Symp. Med. Imag. 4322, 113-127 (2001)] AMPR has been implemented in a recently introduced 16-slice CT scanner. We present a detailed algorithmic description of AMPR which allows for a free selection of the spiral pitch. We show that dose utilization is better than 90% independent of the pitch. We give an overview on the z-reformation functions chosen to allow for a variable selection of the spiral slice width at arbitrary pitch values. To investigate AMPR image quality we present images of anthropomorphic phantoms and initial patient results. We present measurements of spiral slice sensitivity profiles (SSPs) and measurements of the maximum achievable transverse resolution, both in the isocenter and off-center. We discuss the pitch dependence of image noise measured in a centered 20 cm water phantom. Using the AMPR approach, cone-beam artifacts are considerably reduced for the 16-slice scanner investigated. Image quality in MPRs is independent of the pitch and equivalent to a single-slice CT system at pitch p approximately 1.5. The full width at half-maximum (FWHM) of the spiral SSPs shows only minor variations as a function of the pitch, nominal, and measured values differ by less than 0.2 mm. With 16 x 0.75 mm collimation, the measured FWHM of the smallest reconstructed slice is about 0.9 mm. Using this slice width and overlapping image reconstruction, cylindrical holes with 0.6 mm diameter can be resolved in a z-resolution phantom. Image noise for constant effective mAs is nearly independent of the pitch. Measured and theoretically expected dose utilization are in good agreement. Meanwhile, clinical practice has demonstrated the excellent image quality and the increased diagnostic capability that is obtained with the new generation of multislice CT systems.

Algorithms↗
PubMed1996

Unwrapping Cochlear implants by spiral CT.

Multielectrode, intracochlear implants were designed for individuals with profound sensorineural hearing loss who derive little or no benefit form acoustic hearing aids. Determination of each electrode's position in a patient's 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 axis 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.

Algorithms↗
PubMed1994

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.

Biophysical Phenomena↗
PubMed1994

[Basic principles of vascular imaging with spiral CT].

Vascular investigations by CT have experienced a decisive advance and found a high acceptance since the introduction of fast volume scanning (spiral CT). We have investigated the underlying physical foundations and optimized the operational aspects of the method now introduced as CT angiography (CTA). Investigations are carried out with a table feed of 1-10 mm/s. Images are reconstructed at 1-2 mm separations by use of algorithms which optimize the layer profile. The parameters must be adapted to the region being investigated. The diagnosis is generally made with interactive cine runs; for this the original images, multiplanar reformations, 3D surface shaded displays (SSD), and maximum intensity projection (MIP) images are used. The 3D representations are discussed in the context of the principle and illustrative examples. Important applications for CTA are the evaluation of aortic aneurysms and dissections, pulmonary vessels, renal arteries, and vessel stents. CTA is characterized by short examination times, low invasiveness, and relatively low cost; in typical cases it is associated with an effective dose of 2-10 mSv. The advantages and disadvantages of the new method are discussed in terms of diagnostic value, image quality, patient dose, contrast medium techniques, and practical aspects in comparison to other angiographic methods.

Algorithms↗
PubMed1993

[Vascular imaging with spiral-CT. The path to CT-angiography].

Spiral CT is a technique that allows for high-quality two-dimensional angiographic projections and 3D imaging of vascular structures. The authors present the technical and methodological principles of the technique, including scan parameters and parameters of contrast application for various clinical imaging tasks. They present their experience with over 150 clinical cases using spiral CT angiography. Suitable applications of this technique include congenital anomalies, aneurysms, dissections, stenoses, thrombi and vascular tumor involvement. Given a problem-adapted examination technique, pathologic changes in vessels of as little as 2 mm can be visualized. In some cases with complex vascular anatomy, spiral CT angiography can be superior to arterial angiography.

Forecasting↗
PubMed1992

Evaluation of section sensitivity profiles and image noise in spiral CT.

Spiral computed tomography (CT) offers continuous volume scanning of complete organs or body sections within a single breath hold. Almost all image quality characteristics of spiral CT are identical to those of conventional section-by-section CT; however, there is a change in pixel noise values and degradation in the shape of the section sensitivity profiles (SSPs). Computer simulations, phantom measurements, and clinical studies were used in evaluating the SSP and noise characteristics of two new section-interpolation algorithms. The results were compared with standard CT and spiral CT data processed with the commonly employed linear section-interpolation algorithm. Degradation of SSP quality was insignificant for a table feed distance per 360 degrees revolution equal to the section thickness when the new algorithms were applied; noise values, however, increased. SSP width increased for table feed distances greater than the section width, the effect being less pronounced with the new algorithms. The value of these algorithms is primarily seen in the improved quality of multiplanar reformations and cine and three-dimensional displays.

Algorithms↗
PubMedDate not supplied

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.

Cerebrovascular Circulation↗
PubMedDate not supplied

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 degrees 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 analogous fashion.

Computer Simulation↗
PubMedDate not supplied

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.

Humans↗
PubMedDate not supplied

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)

Administration, Inhalation↗
PubMedDate not supplied

The wash-in/washout protocol in stable xenon CT cerebral blood flow studies.

PURPOSE: We conducted a comparative study to optimize the scanning and inhalation protocols for xenon CT cerebral blood flow (CBF) examination (Xe CT), with the aim of improving the practical performance of Xe CT as a routine clinical examination. MATERIALS AND METHODS: Four different inhalation protocols, including 3-min, 6-min, and 8-min wash-in protocols, and a 3-min wash-in/5-min washout protocol, were compared in five healthy volunteers. Each subject underwent two serial Xe CT examinations with an interval of 30 min between the first one (wash-in) and the second one (wash-in/washout). A computer simulation was also performed to support the results of the clinical study. The rate of success was calculated from our experience of 110 clinical cases examined with the wash-in/washout protocol over the last 9 months. RESULTS: The mean CBF values with 6-min and 8-min wash-in protocols were 59.0 and 59.5 mL/100-g brain per min in the thalamus, and 19.5 and 19.0 mL/100-g brain per min in the frontal white matter, respectively. The mean CBF values with 3-min wash-in/5-min washout protocol were 60.0 mL/100-g brain per min in the thalamus and 18.5 mL/100-g brain per min in the frontal white matter, respectively. Computer simulation showed improved signal-to-noise ratio by employing the 3-min wash-in/5-min washout protocol instead of 8-min wash-in protocol for the same number of data points. The rate of success improved to 99.1% due to the significant decrease in head motion with the shorter period of inhalation. CONCLUSION: A wash-in/washout protocol is a useful alternative in Xe CT CBF measurement and more useful than the wash-in method for clinical purposes.

Administration, Inhalation↗