A variable torque motor compatible with magnetic resonance imaging.

High magnetic fields used in magnetic resonance imaging (MRI) do not allow the employment of conventional motors due to various incompatibility issues. This paper reports on a new motor that can operate in or near high field magnets used for MRI. The motor was designed to be operational with the MRI equipment and could be used in a rotating imaging gantry inside the magnet designed for dual modality imaging. Furthermore, it could also be used for image guided robotic interventional procedures inside a MRI system if so desired. The prototype motor was developed using magnetic resonance (MR) compatible materials, and its functionality with MR imaging was evaluated experimentally by measuring the performance of the motor and its effect on the MR image quality. Since in our application, namely, single photon emission tomography, the motor has to perform precise stepping of the gantry in small angular steps the most important parameter is the start-up torque. The experimental results showed that the motor has a start-up torque up to 1.37 Nm and rotates at 196 rpm when a constant voltage difference of 12 V is applied at a magnetic field strength of 1 T. The MR image quality was quantified by measuring the signal-to-noise of images acquired under different conditions. The results presented here indicate that the motor is MR compatible and could be used for rotating an imaging gantry or a surgical device inside the magnet.

Analysis of contributing factors to the occurrence of off-focus radiation (OFR).

Off-focus radiation (OFR) has previously been described by many investigators [R. Thoraeus, Acta Radiologica 18, 753 (1937); L. Mallet and R. Maurin, Radiology 48, 628-632 (1947); J. F. Timmer, Medicamundi 19(2), 52-54 (1974); G. U. Rao, Appl. Radiol. 3(3), 45-49 (1974); R. Birch, Br. J. Radiol. 49, 951-955 (1976); W. W. Roeck, AAPM Symposium, 217-247 (1991)]. Off-focus radiation is frequently manifested on radiographs by the appearance of faint images of anatomical structures outside the collimated field of interest, i.e., soft tissues, ear lobes and hair, which we will refer to as "penumbral images." The observable small detail resolution within these penumbral images led to the assumption that minute sources of increased radiation intensity (secondary microfocal spots) must be present outside the area of the primary focal spot. The existence of these multiple secondary microfocal spots has been established and their locations within the area of the anode surface from which the general OFR originates was identified. The number, size, and distribution of the secondary focal spots vary over time and their magnitudes vary widely. The source of the well-focused electron beams creating the spots is attributed to the cold cathode emission principle.

Absolute cross-sectional area measurements in quantitative coronary arteriography by dual-energy DSA.

Recent studies have emphasized the limitations of conventional coronary angiography. These limitations include the lack of correlation between the severity of coronary stenosis as estimated from coronary angiograms and the actual severity of stenotic lesions measured in postmortem hearts. As a result, attempts have been made to quantitate luminal dimension more precisely. The application of quantitative digital subtraction angiography (DSA) in the assessment of coronary artery lesion dimension has been limited by cardiac and respiratory motion artifacts. We have reported previously on a motion-immune dual-energy (DE) cardiac mode in which kVp and filtration are switched at 30 Hz. To assess the potential advantages of a videodensitometric technique for quantification of absolute vessel cross-sectional area (CSA), three different quantitative coronary arteriography (QCA) algorithms were compared. The three algorithms under comparison were a videodensitometric (V) algorithm, which does not require any geometric assumption for absolute vessel CSA measurement, and videodensitometric (VC) and edge detection (ED) algorithms, which do require the assumption of circular cross-section for CSA measurements. A cylindrical vessel phantom (0.5-4.75 mm in diameter) and a crescentic vessel phantom, producing 25% to 90% area stenosis, were imaged over the chest of a humanoid phantom. The low- and high-energy images were corrected for scatter and veiling glare before energy subtraction. For CSA measurements in crescentic vessel phantoms, the V algorithm produced significantly improved results (slope = 0.87, intercept = 0.51 mm2, r = .95) when compared to the VC (slope = 1.05, intercept = 4.19 mm2, r = .75) and the ED (slope = 1.57, intercept = 5.21 mm2, r = .60) algorithms.

On the relationship of image intensifier contrast ratios measured by two techniques.

In recent years the contrast ratio (CR) has played an important role in the quantitation of imaging performance of X-ray image intensifiers. In practice, one uses either a lead disc or strip for measurement of contrast ratios. A theoretical relationship relating contrast ratios measured by both disc and strip techniques is discussed and experimental results are presented. The theoretical analysis uses the point spread function model to describe veiling glare for an image intensifier. It has been shown that one can predict strip contrast ratios after a determination of the veiling glare point spread function from a measurement of disc contrast ratios.

Characterization of the veiling glare PSF in x-ray image intensified fluoroscopy.

A theoretical derivation for the point spread function (PSF) which describes the veiling glare in x-ray image intensifiers (II) is presented. The PSF is dependent on two parameters which can be determined experimentally for a given II. An experimental investigation of the linearity of veiling glare phenomenon is undertaken. The experimental results indicate that veiling glare could be described as a linear process to a high degree of accuracy.

Development of a radiographic chest phantom with disease simulation.

An anthropomorphic chest phantom with realistic disease simulation is described with a review of previously available chest phantoms. The need for subjective image analysis is discussed and compared with the existing physics parameters as a means of evaluating image quality in chest radiology. An introduction to the use of such a radiographically realistic chest phantom with lung parenchymal detail and disease simulation is illustrated.

A highly accurate focal spot camera--laboratory and field model.

A device has been developed to determine accurately and quickly the shape, size, and intensity distribution of an x-ray tube focal spot. The pinhole device weighs only 1 kg and is portable, enabling it to be used either as a field unit or in a laboratory. The special design obviates any risk of irradiation danger to the investigator, or thermal overloading of the x-ray tube under investigation. The degree of accuracy with which the central beam can be located is better than +/-250 micron, well beyond the requirements of the latest NEMA standards regarding dimensional measurements of diagnostic x-ray tube focal spots. A star test pattern picture may be taken instead of the pinhole picture if required. The device is described and areas of application are discussed.

The production of apparent image bifurcation.

It is well known that x-ray focal spots with split-intensity distributions can produce radiographs exhibiting image bifurcations. The authors observed such bifurcations in optical analogue images which were produced with both split- and uniform-intesity focal spots. The bifurcations observed with the split focal spot were real, while those observed with the uniform focal spot were not observed in photometer scans of the object. The authors explain these bifurcations by a consideration of the response function of the eye.