PubMed Health⌕ Search

Biomedical subjects

R T Droege

Publications and source records attributed to R T Droege.

15 recordsLinked to original sources
PubMed1986

Boundary artifact in inversion-recovery images.

Modulus reconstruction of inversion-recovery images causes a pixel value to be assigned the absolute magnitude of the signal amplitude. Consequently, contrast between tissues is reduced if they have amplitudes of opposite sign. When contrast is reduced between adjacent tissues, dark line artifacts are created at the tissue boundaries. The artifacts may enable tissues to be spatially distinguished even if there is no contrast between them. However, the artifacts may lead to misinterpretation of the image since they may be difficult to distinguish from real anatomical structures.

Data Display↗
PubMed1985

Portal film technique charts.

A method to produce consistent radiographic density in portal film images is described. The exposure required to produce the desired film density can be calculated on the basis of tissue depth dose data. The required exposure is tabulated as a function of patient thickness, field size, and air gap between the patient and the cassette. The table, referred to as a "technique chart," helps to maintain consistent image quality by minimizing the frequency of over and under exposed films. Alternatively, treatment planning software may be modified to determine the film exposure required for a specific patient. The calculated exposures are accurate only if the image receptor employs a sufficiently thick front metal screen, a requirement consistent with optimum image contrast.

Cobalt Radioisotopes↗
PubMed1984

A strategy for magnetic resonance imaging of the head: results of a semi-empirical model. Part I.

This paper is an introduction to lesion detection problems of MR. A mathematical model previously developed for normal anatomy has been extended to predict the appearance of any hypothetical lesion in magnetic resonance (MR) images of the head. The model is applied to selected clinical images to demonstrate the loss of lesion visibility attributable to "crossover" and "boundary effect". The model is also used to explain the origins of these problems, and to demonstrate that appropriate gray-scale manipulations can remedy these problems. Specifically, pulse sequences that cause lesions to be the brightest tissue in the image are shown to eliminate the problems of crossover and boundary effect.

Brain Diseases↗
PubMed1984

A strategy for magnetic resonance imaging of the head: results of a semi-empirical model. Part II.

A semi-empirical model was used to identify specific pulse sequences that cause most lesions to appear distinctly brighter than normal tissues in magnetic resonance (MR) images of the head. Clinical trials confirm the utility of these sequences for patient screening. As a result, a strategy for effective and efficient MR imaging of the head is proposed. The previously described gray-scale model has been modified to account for the effect of image noise. By means of computer simulation, 13,800 different hypothetical cerebral lesions were imaged with a variety of pulse sequences. A number of conclusions resulted. First, two sequences are expected to be sufficient to visualize most intracranial lesions, a "diagonal" SE sequence (e.g., SE 2500/80) and an IR sequence with a short inversion time (e.g., IR 1800/200). These sequences are orthogonal, i.e., lesions missed by one are likely to be detected by the other. Second, signal averaging the screening sequences is expected to be more effective than optimized sequences when lesion tissue parameters differ little from brain. Finally, the effectiveness of unaveraged screening sequences suggests that improved signal-to-noise ratio (SNR) is not necessary for the detection of most large lesions. Therefore, the increased SNR achievable through signal averaging or increased field strength might best be utilized to improve spatial resolution so that smaller lesions can be detected.

Brain Diseases↗
PubMed1983

A quality assurance protocol for CT scanners.

Numerous CT performance-evaluation procedures have been described in the literature, but they are generally unsuitable for routine quality-assurance (QA) testing. This report describes the QA test protocol performed at two institutions. The protocol is comprehensive and practical to perform on a routine basis. Test data demonstrate that clinically significant changes in CT scanner performance do occur, and that this protocol is capable of detecting these changes.

Methods↗
PubMed1983

Nuclear magnetic resonance: a gray scale model for head images.

The gray scale of nuclear magnetic resonance (NMR) head images is explained in terms of tissue and machine parameters. Tissue parameters considered here include the spin-lattice relaxation time, the spin-spin relaxation time, and the proton density. Machine parameters include the pulse sequence (saturation recovery, inversion recovery, or spin echo), the repetition time, and the delay time. The ability of the operator to alter the NMR gray scale predictably is examined by computer simulation. The simulation computes NMR pixel values for a selected combination of tissue and machine parameters. The computed values are compared with those extracted from clinical NMR images. The agreement between simulation and clinical pixel values implies that the operator can use the machine parameters to alter the NMR gray scale, and thereby control contrast, appropriate to the diagnostic requirements. The extent to which the NMR gray scale can be predictably controlled is illustrated through graphs, simulated contrast displays, and representative NMR images.

Brain Chemistry↗
PubMedDate not supplied

A megavoltage MTF measurement technique for metal screen-film detectors.

A technique to measure the MTF of metal screen-film detectors at megavoltage x-ray energies has been devised. It employs an exposure through a slit 25 micron wide and a method to average numerous line spread functions from a single film image. That the slit is wide compared to those used at diagnostic x-ray energies serves two purposes: it facilitates the use of a laser alignment procedure designed to ensure correct orientation of the slit-forming tungsten blocks in the x-ray beam, and it also increases the contrast in the slit image, thereby reducing the data analysis problems caused by film noise. Further reduction in the effects of film noise is accomplished by the averaging method. A correction is made to account for the effect of the wide slit on the MTF. The validity of this correction, the laser alignment, and the averaging method have been confirmed by simulations.

Absorptiometry, Photon↗
PubMedDate not supplied

Influence of metal screens on contrast in megavoltage x-ray imaging.

The radiographic contrast of metal screen-film detectors was investigated in order to determine the contrast capabilities of these detectors applied to megavoltage x-ray imaging. The film contrast gamma was found to be independent of the metal screen composition. Measurement of the scatter-to-primary film dose ratio in contact geometry demonstrated that a thick front screen of either 1.5 g/cm2 copper of 2.5 g/cm2 lead provides optimum contrast for the photon energies studied (60Co and 4- and 8-MV x rays). The same thickness were also found to be suitable in an air gap geometry which significantly improved the contrast compared to the contact geometry. Rear lead screens were found to provide no contrast improvement.

Metals↗
PubMedDate not supplied

Metal screen-film detector MTF at megavoltage x-ray energies.

The MTF of metal screen film detectors used in radiation treatment verification has been measured at 4 and 8 MV x-ray energies. The results show that lead screens provide better resolution than copper screens, and a single-emulsion film offers considerable advantage over the traditional double-emulsion film. A rear lead screen was found to seriously degrade the resolution properties of a front lead screen single-emulsion film detector. The detector MTF was found to be energy dependent. In general, both the low and the high spatial frequency response decreased with increasing x-ray energy. This, in part, accounts for the noticeable image quality difference between 4 and 8 MV radiographs.

Metals↗
PubMedDate not supplied

A practical method to routinely monitor resolution in digital images.

A quality assurance (QA) test has been developed as a practical means to routinely monitor digital image resolution. The method is based upon a parameter termed the image modulation M. When noise is present M cannot be directly measured. Instead, a biased estimate of M, designated M', is measured. This report describes the application of the parameter M' to QA monitoring of digital image resolution. A measurement of M' is generally simpler than alternative measurements, and is shown to outperform visual evaluation in the clinical QA environment.

Models, Structural↗
PubMedDate not supplied

An MTF method immune to aliasing.

The variance derived from the images of cyclic bar patterns can be used to determine the modulation transfer function (MTF) of an imaging system. Unlike most MTF methods, it is applicable even when the imaging system undersamples the test object and generates aliasing errors. The validity of the variance method in the presence of aliasing is established theoretically and by computer simulation.

Analysis of Variance↗
PubMedDate not supplied

Measurement of pituitary gland height with MR imaging.

From sagittal magnetic resonance (MR) images with 10 mm slice thickness, the mean vertical height of the pituitary gland in 42 normal patients was found to be 5.4 +/- 0.9 mm. Computed tomographic (CT) scans in the coronal plane showed the same. Measurements of pituitary size in 13 patients with tumors using both CT and MR were also essentially equivalent. The ease, comfort, and accuracy of the MR pituitary measurement supports its use as the examination of choice for measuring pituitary height.

Adult↗