S. A. Suddarth

A high-temperature superconducting Helmholtz probe for microscopy at 9.4 T

The design and operation of a high‐temperature superconducting (HTS) probe for magnetic resonance microscopy (MRM) at 400 MHz are presented. The design of the probe includes a Helmholtz coil configuration and a stable open‐cycle cooling mechanism. Characterization of coil operating parameters is presented to demonstrate the suitability of cryo‐cooled coils for MRM. Specifically, the performance of the probe is evaluated by comparison of signal‐to‐noise (SNR) performance with that of a copper Helmholtz pair, analysis of B1 field homogeneity, and quantification of thermal stability. Images are presented to demonstrate the SNR advantage of the probe for typical MRM applications. Magn Reson Med 41:1032–1038, 1999.

Registered 1H and 3He magnetic resonance microscopy of the lung

Using in vivo magnetic resonance microscopy, registered 1H and hyperpolarized 3He images of the rat lung were obtained with a resolution of 0.098 × 0.098 × 0.469 mm (4.5 × 10–3 mm3). The requisite stability and SNR was achieved through an integration of scan‐synchronous ventilation, dual‐frequency RF coils, anisotropic projection encoding, and variable RF excitation. The total acquisition time was 21 min for the 3He images and 64 min for the 1H image. Airways down to the 6th and 7th orders are clearly visible. Magn Reson Med 45:365–370, 2001.

Histogram-directed processing of digital chest images.

One of the potential advantages of digital chest imaging is the ability to process these images. However, such processing, when uniformly applied to the entire image, is often unsatisfactory due to the different processing requirements of lung field and mediastinum. Therefore, a method to selectively process these regions based upon the histogram of the original image has been developed. Thirteen conventional chest films were digitized with a laser film scanner. Analysis of individual lung field and mediastinum histograms showed that the chest image histogram is essentially bimodal with significant lung field-mediastinum histogram peak separation; overlap between these peaks is small (9% of the total histogram) and insensitive to minor pathologic change or radiographic technique. Using these histograms, a gray level threshold distinguishing mediastinum from lung field was selected and used to direct the regionally-selective processing of several chest images. This technique may prove especially useful for digital enhancement of the underexposed mediastinum often encountered on conventional chest radiographs.

Digital synthesis of lung nodules.

Studies evaluating observer accuracy and visual perception of pulmonary nodules usually are based upon test films obtained from clinical practice in patients with proven pulmonary nodules. Unfortunately, such nodules do not always occur in the optimal size and location to facilitate testing. Such studies would be enhanced by the ability to place nodules of desired size and location on chest radiographs. This report describes a method of placing a computer-generated (synthesized) nodule on a digitized chest radiograph. To demonstrate the similarity of these synthesized nodules to real nodules, each digitized radiograph with a computer-generated nodule was paired with a digitized chest radiograph of a patient with a clinically proven pulmonary nodule. A total of 22 pairs of chest radiographs were then shown to 13 radiologists, who were asked to distinguish the synthesized nodule from the real nodule. With this two alternative forced-choice test, the radiologists were only able to distinguish the synthesized nodule in 51% of the cases, strongly suggesting that computer generated nodules may be used to simulate real pulmonary nodules in future tests of nodule detection.

Implementation Of Adaptive Filtration For Digital Chest Imaging

Previous work has demonstrated the potential for adaptive filtration in processing digital chest images. The technique uses the histogram of the image to determine the pixels (and regions) in which edge enhancement is applied. This paper extends that work by investigating the choice of parameters used in selectively enhancing the mediastinum. The image is separated into its low and high frequency components by convolution with a square kernel. The effect of kernel size was studied with a choice of 17 x 17 mm, which was found to be sufficient to include the frequencies of interest. A serious deficiency in previous implementations of this technique is the existence of ringing artifacts at the juncture of the lung and mediastinum. These result in part from the use of a step function to specify the low frequency image intensity above which high frequencies are amplified. By replacing this step with a smoother (cosine) function, the artifact can be removed. Finally, the amplification constant was examined in light of its effect on both structure and noise in the image.

Performance of high-resolution monitors for digital chest imaging

High-resolution cathode-ray tubes (CRTs) are currently the most viable soft-copy display for digital radiography. We present here methods for measuring large-area contrast ratio and detail contrast ratio. A two-dimensional charge coupled device (ccd) array signal-averaged with a video frame buffer permits linear microradiometric measure of individual beam lines. Results from three different 1000-line monitors demonstrate the shift variance of resolution. The detail contrast ratio (or modulation depth) was found to vary from 100% to less than 10% across the face of one CRT. Dynamic focus in both the horizontal and vertical deflection circuitry proved effective in reducing this shift variance. Comparisons of three phosphors demonstrate the utility of long persistence phosphors (P164) for static display in producing brighter images with less flicker. Recommendations for CRT design and selection for high-resolution digital radiography are included.

Film-Based Digital Tomosynthesis Of The Chest

Digital tomosynthesis involves tomographic motion about a single plane during which separate radiographs are acquired at equal angular increments. The resulting series of images can be shifted and recombined to yield planes in focus at any depth in the patient. Previous work has used image intensifiers for acquisition. However, conventional image intensifiers suffer from a limited field of view and from problems arising from the curved image input. The present work, which uses digitized 14 x 17 in. films, eliminates these problems in order to explore the potential for high resolution tomosynthesis of the chest. The method utilizes linear tomographic motion about one plane, acquiring films every 2.5° over a 50° arc. These 21 films are digitized with a laser scanner, registered to ±1 pixel accuracy, shifted, and added in order to generate images in any desired plane. These tomosynthesized images are then displayed on a 23-in. 1000-line monitor. Work has been directed specifically at chest imaging. A tomography phantom, an anthropomorphic chest phantom, and a dog have been imaged. In addition to the synthesis of tomographic planes, thin slice images with differential tissue emphasis have been accomplished. These tomograms in digital format readily lend themselves to image processing.

A System for Digital Videodensitometry

The first few years of any new imaging technology are characterized by rapid developments in equipment. Digital subtraction angiography (DSA) is no exception. These improvements must be accompanied by an increase in understanding of the interaction between equipment and the patient. Several articles have discussed the impact of various components on the signal to noise ratio (SNR). While most users have recognized the importance of certain biological factors, e.g. heart rate, cardiac output, etc., only recently has the equipment evolved to the state permitting systematic study of these variables.