Neil Bromberg

A dose reduction x-ray beam positioning system for high-speed multislice CT scanners

The introduction of multislice CT scanners and the associated dose increase compared to single and dual slice scanners has concerned many radiologists, health and medical physicists, as well as members of the regulatory community. Since multislice CT scanners are inherently post-patient collimated, they are less dose efficient than single slice CT scanners, which use prepatient collimation. The x-ray beam must be wide enough in the Z axis so that the beam remains on the detector in spite of typical movements due to thermal and mechanical flexing. We describe the x-ray beam tracking system that is employed on a GE LightSpeed QX/i scanner to substantially reduce the multislice dose. The tracking system stabilizes the beam on the detector allowing a narrower x-ray exposure profile compared to the x-ray exposure profile without tracking. The tracking system measures the position of the beam every few milliseconds and continually repositions a source aperture to hold a narrow beam fixed on the detector. Using a standard LightSpeed QX/i source collimator and segmented detector, dose reductions of up to 40% were measured when tracking was employed. We also show that tracking has the potential to provide a dose efficiency approaching single slice scanners.

Veiling Glare In The Imaging Chain

It has been observed that image intensifier contrast ratio measurements taken on clinically installed systems always produce lower values than reported by the manufacturer. One common method of measuring the contrast ratio in the field uses the film recording camera which comes with the system, while the manufacturers always measure the image intensifier isolated from other system components. Evidence is presented showing that the difference between the two measurements is attributable to contrast losses in the optics. Also, evidence is presented suggesting that there is no observable difference in contrast presentation for image tubes with contrast ratios larger than 15. Contrast ratio measurement on clinically installed systems are recommended for performance monitoring purposes only.

A z gain nonuniformity correction for multislice volumetric CT scanners.

This paper presents a calibration and correction method for detector cell gain variations. A key functionality of current CT scanners is to offer variable slice thickness to the user. To provide this capability in multislice volumetric scanners, while minimizing costs, it is necessary to combine the signals of several detector cells in z, when the desired slice thickness is larger than the minimum provided by a single cell. These combined signals are then pre-amplified, digitized, and transmitted to the system for further processing. The process of combining the output of several detector cells with nonuniform gains can introduce numerical errors when the impinging x-ray signal presents a variation along z over the range of combined cells. These numerical errors, which by nature are scan dependent, can lead to artifacts in the reconstructed images, particularly when the numerical errors vary from channel-to-channel (as the filtered-backprojection filter includes a high-pass filtering along the channel direction, within a given slice). A projection data correction algorithm has been developed to subtract the associated numerical errors. It relies on the ability of calibrating the individual cell gains. For effectiveness and data flow reasons, the algorithm works on a single slice basis, without slice-to-slice exchange of information. An initial error vector is calculated by applying a high-pass filter to the projection data. The essence of the algorithm is to correlate that initial error vector, with a calibration vector obtained by applying the same high-pass filter to various z combinations of the cell gains (each combination representing a basis function for a z expansion). The solution of the least-square problem, obtained via singular value decomposition, gives the coefficients of a polynomial expansion of the signal z slope and curvature. From this information, and given the cell gains, the final error vector is calculated and subtracted from the projection data.

Algorithm to correct for z-detector gain variations in multislice volumetric CT

This paper presents a calibration and correction method for detector cell gain variations. To provide variable slice thickness capability in multislice volumetric scanners, while minimizing costs, it is necessary to combine the signals from several detector cells. The process of combining the output of several detector cells with non-uniform gains can introduce numerical errors when the impinging x-ray signal varies over the range of the combined cells. These scan dependent numerical errors can lead to artifacts in the reconstructed images, particularly when the numerical errors vary from channel-to-channel. A projection data correction algorithm has been developed to subtract the associated numerical errors. For effectiveness and data flow reasons, the algorithm works on a slice-by-slice basis. An initial error vector is calculated by applying a high-pass filter to the projection data. The essence of the algorithm is to correlate that initial error vector, with a calibration vector obtained by applying the same high-pass filter to various z-combinations of the cell gains. The solution to the least-square problem gives the coefficients of a polynomial expansion of the signal z-slope and curvature. From this information, and given the cell gains, the final error vector is calculated and subtracted from the projection data.

Evaluation Of The Contrast Ratio Of Intensified Fluoroscopic Imaging Systems

Large area contrast loss in image intensifiers results in reduced performance at low spatial frequencies and may limit the imaging capabilities of such systems for structures of clinical interest having inherently low subject contrast. The contrast ratio of image intensifiers is a measure of large area contrast loss which may be evaluated under laboratory conditions by photometric techniques. Unfortunately such techniques are not readily extended to clinically installed systems. A technique for the measurement of contrast ratio on clinically installed systems, incorporating a film recording camera, is proposed which eliminates the requirement of having a precise knowledge of the films characteristic curve. The results of measurements obtained with this technique on clinical systems, as well as comparison of this technique to photometric measurements under laboratory conditions are presented, along with a discussion of the associate problems and pitfalls.