Index of content:
Volume 13, Issue 2, March 1986

Investigation of basic imaging properties in digital radiography. 7. Noise Wiener spectra of II–TV digital imaging systems
View Description Hide DescriptionWe used Wiener spectral analysis in order to investigate the different noise sources and the effects of various parameters such as pixel size, image intensifier (II) field size, and exposure level on the noise in an II–TV digital system. The digital Wiener spectra in terms of relative x‐ray intensity were determined directly from the digital noise data in terms of pixel values, by use of the characteristic curve of the imaging system. From averaged, subtracted, and/or combination images, the amount of structure mottle relative to the amount of quantum mottle was estimated. We found that a substantial amount of structure mottle was included in our II–TV digital subtraction angiography system, whereas the electronic noise of the TV system was quite small relative to the quantum and structure mottle. The effects of time jitter on the noise in single‐frame images (consisting of multiple video frames) and in subtracted and averaged images were also investigated.

Variation of the count‐dependent Metz filter with imaging system modulation transfer function
View Description Hide DescriptionA systematic investigation was conducted of how a number of parameters which alter the system modulation transfer function(MTF) influence the count‐dependent Metz filter. Since restoration filters are most effective at those frequencies where the object power spectrum dominates that of the noise, it was observed that parameters which significantly degrade the MTF at low spatial frequencies strongly influence the formation of the Metz filter. Thus the radionuclide imaged and the depth of the source in a scattering medium had the most influence. This is because they alter the relative amount of scattered radiation being imaged. For low‐energy photon emitters, the collimator employed and the distance from the collimator were found to have less of an influence but still to be significant. These cause alterations in the MTF which are more gradual, and hence are most pronounced at mid to high spatial frequencies. As long as adequate spatial sampling is employed, the Metz filter was determined to be independent of the exact size of the sampling bin width, to a first approximation. For planar and single photon emission computed tomographic (SPECT) imaging, it is shown that two‐dimensional filtering with the Metz filter optimized for the imaging conditions is able to deconvolve scatter and other causes of spatial resolution loss while diminishing noise, all in a balanced manner.

Optical factors affecting the detective quantum efficiency of radiographic screens
View Description Hide DescriptionParameters related to the detective quantum efficiency (DQE) of several representative screens of different thicknesses, phosphor grain sizes, and optical properties were measured by the scintillation spectrum method, using monoenergetic x rays produced from x‐ray fluorescence. The experimental results, including those for spectral shape and average light energies (E _{ A }) emitted, are compared with conventional theories of the operation of screens. It was hoped that this would vindicate the theory of the effect of optical properties and so permit the simple calculation of all parameters related to DQE from standard x‐ray attenuation tables. Rather more substantial energy‐dependent deviations of E _{ A } are found than was previously realized, which preliminary analysis suggests are due to both optical effects and photoelectron escape. We conclude that although DQE for a single energy can be calculated by simplified methods to within ±10%, the effective DQE when polyenergetic beams are used is much less accurately estimated and requires a fuller theoretical treatment.

Scatter rejection by electronic collimation
View Description Hide DescriptionAn electronic scanning‐slit technology is under development which involves replacing paired fore and aft slits for scatter rejection with only one fore slit. As the slit scans across the portion of the patient to be imaged, pulsed x‐ray exposures produce images of the slit in successive positions, which are digitized and stored in computer memory. Software techniques are used for tracking the slit image, and discriminating against scatter and veiling glare lying outside the slit image. Such ‘‘electronic collimation’’ does not require synchronization between the slit‐scanning and detector readout, which makes it much simpler than alternative methods and potentially adaptable to any digital fluorography system. The performance characteristics of a prototype unit are described and images of a low‐contrast phantom are presented.

Applications of time‐varying gradients in existing magnetic resonance imaging systems
View Description Hide DescriptionThis paper describes several applications of magnetic resonance imaging(MRI) with time‐varying gradients within the framework of existing imaging systems. An alternative form of slice selection is shown where a time‐varying gradient is used, during the reception interval, to isolate the slice of interest (the reconstruction of the slice itself is treated by the conventional techniques). This system allows for precise control of the slice, thinner slices (less than 1 mm) and the ability to select multiple, closely packed slices using a single imaging sequence. All this is done by postprocessing so the slice(s) of interest can be selected after the measurements have been completed. It is also shown how time‐varying gradients can be used to generate the signals required for conventional projection‐reconstruction and two‐dimensional Fourier transform techniques (yet achieving higher resolution and using a resonant gradient system). In addition to cross‐sectional imaging, this same approach provides a simple system for projection imaging of the entire volume. All of these techniques constitute a good starting point for the exploitation of time‐varying gradients for fast, high‐resolution MRI with existing imaging systems.

The precision of T _{ R } extrapolation in magnetic resonance image synthesis
View Description Hide DescriptionWe present a model of noise propagation from acquired magnetic resonance(MR)images to T _{ R } ‐extrapolated synthetic images. This model assumes that images acquired at two repetition times and are used to generate synthetic images at arbitrary repetition times . The predictions of the model are compared with experimentally acquired phantom data, and show excellent agreement. The model is utilized in an analysis of two applications of MRimage synthesis: scan time reduction and multiple‐image synthesis. Scan time is reduced by acquiring data at two short repetition times, and synthesizing at a longer repetition time, with less than T _{ R }. For T _{1}=800 ms, a reduction of 20% in scan time results in a 45% reduction in signal‐to‐noise ratio SNR, when compared to direct acquisition. Reducing scan time by much more than 20% produces large noise levels in the synthetic image, and is unlikely to be useful. In multiple‐image synthesis, images are synthesized at any repetition time in the range 0 to , for contrast optimization. If =800 ms, and =2000 ms, the optimum combination of results in synthetic images whose SNR is at worst 22% less than the SNR of directly acquired images. For many values of T _{ R }, the synthetic images have SNR superior to that obtainable by direct acquisition.

Flow‐enhanced magnetic resonance imaging
View Description Hide DescriptionA new technique for detecting blood flow in magnetic resonance imaging is proposed. This technique is tailored to enhance areas containing flow while suppressing static and nonsignal areas with the objective of optimizing the contrast of vascularized tumors. Unlike flow phase imaging, in‐plane flow directionality (parallel versus antiparallel to applied flow gradient) is removed by the proposed method to reduce phase cancellation of flow signals. This signal loss is apt to occur in instances of complicated vascularity patterns consisting of many small vessels having multiple flow directions. The new flow‐enhanced imaging method is compared to flow phase imaging by computer simulation of simple objects containing many small vessels. The results indicate that flow‐enhanced imaging yields significantly greater detectability of regions of complicated small‐vessel patterns than phase imaging.

Toward an automated analysis system for nuclear magnetic resonance imaging. I. Efficient pulse sequences for simultaneous T _{1}–T _{2} imaging
View Description Hide DescriptionSimultaneous T _{1} and T _{2}images have been obtained using multiple‐echo self‐normalizing sequences. Differential T _{1} and T _{2} discrimination for two of these sequences, double saturation recovery and inversion–saturation recovery, have been explored in detail. The inversion–saturation recovery sequence exhibits T _{1} discrimination comparable to, and in many cases better than, that of optimal T _{1} weighted conventional pulse sequences, but is poor for T _{2}imaging. The double saturation recovery sequence yields comparable tissue discrimination for T _{1} and T _{2}imaging, but is poor compared to optimal T _{1} weighted and T _{2} weighted conventional pulse sequences. Simultaneous T _{1} and T _{2}images represent quantitative maps of intrinsic tissue properties, and hence form a natural basis for an automated image analysis and tissue classification system.

Fluence perturbation in photon beams under nonequilibrium conditions
View Description Hide DescriptionThe perturbation effect in parallel‐plate ionization chambers used for buildup measurements has been investigated. The fluence perturbation due to electrons emitted through the side walls are thoroughly investigated by measurements using film and extrapolation chambers and by calculations. The electron fluence varies both with side wall material and chamber geometry. In order to obtain a small perturbation effect, the chamber should have a large guard width compared to the electrode separation and the side walls should have as large an angle as possible with the central axis. The side wall should be of the same material as the rest of the chamber. The perturbation effect is also dependent on the electron contamination of the beam and angular distribution of the electron fluence. It is thus not possible to correct the perturbation effect in one parallel‐plate chamber with fixed plate separation with correction factors obtained with extrapolation chambers of other dimensions. In order to make accurate surface dose measurements extrapolation chambers are therefore strongly recommended in favor of fixed parallel‐plate chambers.

Interface effects in the Monte Carlo simulation of electron tracks
View Description Hide DescriptionAn analog of Fano’s theorem for ionization in cavities is shown to hold for the stepwise representation of electron paths used in Monte Carlocomputermodels of electron transport. This brings to light an error in the distribution of electron paths and hence energy deposition which is induced by interrupting steps which cross the interface between media of different densities. The magnitude of the error depends on the shape of the cavity and its size relative to the electron path length in the cavity gas. In a typical calculation of a cylindrical chamber exposed to ^{6} ^{0}Co radiation, if the electron step size is taken as 10% of the remaining path, then a 3% energy deficit in the cavity results. An algorithm for crossing an interface is described which does not produce this error.

Correlation of microdosimetric measurements with relative biological effectiveness from clinical experience for two neutron therapy beams
View Description Hide DescriptionMicrodosimetric measurements were made for the neutron therapy beams at the University of Chicago and at the Cleveland Clinic with the same geometry and phantom material using the same tissue‐equivalent spherical proportional counter and standard techniques. The energy deposition spectra (dose distributions in lineal energy) are compared for these beams and for their scattered components (direct beam blocked). The model of dual radiation action (DRA) of Kellerer and Rossi is employed to interpret these data in terms of biological effectiveness over this limited range of radiation qualities. The site‐diameter parameter of the DRA theory is determined for the Cleveland beam by setting the biological effectiveness (relative to ^{6} ^{0}Co gamma radiation) equal to the relative biological effectiveness value deduced from radiobiology experiments and clinical experience. The resulting value of this site‐diameter parameter is then used to predict the biological effectiveness of the Chicago beam. The prediction agrees with the value deduced from radiobiology and clinical experience. The biological effectiveness of the scattered components of both beams is also estimated using the model.

Dosimetric considerations of d(15)+Be and p(26)+Be neutron beams from an isocentric cyclotron facility
View Description Hide DescriptionTo select the optimum therapeuticneutron beam available from a CS30 medical cyclotron (manufactured by the Cyclotron Corporation, Berkeley, California), central axis depth dose data and output dose rates were compared for the bombardment of beryllium with either the proton or deuteron beams available from the machine. The effect on these parameters of filtering the beams with either pure polyethylene, polyethylene loaded with 5% boron, or polyethylene loaded with 10% lithium was studied. A 4‐cm, 10% lithiated filter used with a 26‐MeV proton beam was selected for therapeutic use. Buildup curves, beam profiles at several transverse planes for different field sizes, and comparison of beam profiles with ^{6} ^{0}Co are given.

A prototype epithermal neutron beam for boron neutron capture therapy
View Description Hide DescriptionAn epithermal neutron beam has been designed and tested at the Georgia Insitute of Technology’s 5‐MW Research Reactor. The prototype facility consists of aluminum and sulfur disks in a tangential beam port for fast neutron filtration. A cadmium sheet at the port exit removes the thermal neutrons from the transmitted beam, leaving an intensely epithermal neutron beam spanning five energy decades, each contributing to the flux demanded by boronneutron capture therapy. The thermal neutron flux generated by the incident epithermal neutrons in a polyethylene head phantom peaks at a depth of 3 cm and remains above the incident thermal flux to a 7‐cm depth. The beam thus provides the penetration required for treating deep‐seated gliomas. Photon contamination in the prototype facility is high, and a number of basic modifications are proposed for reducing it to safer levels.

The propagation of relativistic heavy ions in multielement beam lines
View Description Hide DescriptionWe describe calculations of the energy loss, range, stopping power,multiple scattering, and other related properties of a high‐energy heavy‐ion beam at any one of a set of beam line elements. A beam line element (e.g., any beam modification, detection, or control device) is characterized by its thickness, areal density, aperture, and function. The loss of multiply scattered particles to any finite‐aperture detector is calculated in the small‐angle approximation, and the position of the Bragg peak, as given by particles stopping in the second of two ionization chambers used for Bragg curve measurements, is estimated. A general purpose computer program, propagate, has been written to allow addition, deletion, and modification of the beam line elements used in the calculation and to provide a convenient means of repeating such calculations for arbitrary beam lines. Calculations and experimental measurements are compared and found to be in satisfactory agreement.

Theoretical analysis of regional blood flow studies
View Description Hide DescriptionRegional blood flow measurements were analyzed using a two‐compartment model. The distribution of injected tracer between blood and tissue was assumed to be in equilibrium only at the beginning of the experiment. Equations for the determination of the partition coefficient of injected radionuclides from i n v i v o measurements were derived. These equations can also be used for estimation of the relative blood content of a tumor or an organ. The results were applied to experiments, where three diffusible radionuclides were injected simultaneously into patients with diagnosed tumors.

Thermodynamic considerations of acrylic cement implant at the site of giant cell tumors of the bone
View Description Hide DescriptionA discussion of the thermodynamic aspects of a relatively new treatment method for giant cell tumors of the bone is presented in this paper. The advantages of implanting methylmethacrylate acrylic bone cement into a curetted tumor site are briefly discussed and placed in perspective relative to more prevalent surgical treatments. As the bone cement self‐heats while curing, the possibility of heat necrosis in the bone exists. However, the damage due to heat may be beneficial in reducing the rate of tumor recurrence. A thermodynamic consideration of the treatment situation appears to be warranted. After a general introduction and a brief literature review, the theoretical thermodynamic equations are developed. Once the basic equations for the heat transfer from the cement or the bone are derived, there is then a discussion of the various characteristics of bone and methylmethacrylate crucial to the analysis, such as, thermal conductivity,specific heat, density, and heat generation parameters. Finally, in order to reduce the theory to a form which may be used practically, the equations derived are written in terms of finite‐difference equations, which approximate them numerically. Different equations are written for each type of heat transfer condition encountered in the cement–bone system as spacial variances in material and geometry occur. The equations derived may be used to model the system allowing one to predict the time‐dependent temperature distribution in bone during the curing of acrylic cement. Using computer techniques to reduce the equations obtained from this analysis, and knowing the temperature at which adjacent cells die, a zone of necrosis may be mapped surrounding the acrylic impact.

Generation of pseudorandom numbers
View Description Hide DescriptionComputational studies requiring the generation of pseudorandom numbers are becoming increasingly common. These include Monte Carlo methodologies and studies which require the addition of ‘‘random’’ noise to more structured data. Although well‐established random number generators exist, many of these are not suitable for implementation on micro‐ or minicomputer systems. An algorithm for an additive generator that can be used on any computer system is described. The performance of this algorithm as implemented on a microcomputer system is discussed. Extensive testing of the statistical behavior of the resulting number sequence was performed. The algorithm appears to be an appropriate one for a variety of applications.

Absorbed dose calorimetry: Bridge nulling and voltage analysis techniques
View Description Hide DescriptionA theoretical analysis is made of the conventional calorimeter approach utilizing a two‐thermistor Wheatstone bridge—an approach which requires balancing the bridge by adjusting variable resistances. However, with the current availability of computers and digital precision measuring instruments, the conventional calorimeter approach can now be readily replaced by an automated, nonmechanical, electrical systems approach called differential calorimetry. The analytical theory outlining such an approach is also presented.

Magnetic resonance fast Fourier imaging
View Description Hide DescriptionA new method for fast magnetic resonance imaging is presented. It provides a more rapid data acquisition than two‐dimensional Fourier imaging (2DFI) by a factor which may be chosen depending on the required signal‐to‐noise ratio of the image. In addition to the readout gradient of 2DFI, the present method employs an oscillating modulation gradient. In this way, a curved alternating trajectory in k space is sampled after each spin excitation. For a p‐times accelerated data acquisition, the trajectory consists of p periods, where p is of the order of 2 to 8 for low‐frequency gradient modulation but can be chosen higher if certain hardware requirements are met. Adequate sampling density in k space is obtained by scanning shifted trajectories after subsequent spin excitations. The method can be combined with volume imaging (3DFI) and multiple slice 2DFI. It was implemented on a standard Philips Gyroscan system without any hardware modifications. Results obtained for an acceleration factor p=4 are shown.

Characterization of the point spread function and modulation transfer function of scattered radiation using a digital imaging system
View Description Hide DescriptionA digital radiographic system was used to measure the distribution of scattered x radiation from uniform slabs of Lucite at various thicknesses. Using collimation and air gap techniques, [primary+scatter] images and primary images were digitally acquired, and subtracted to obtain scatter images. The scatter distributions measured using small circular apertures were computer fit to an analytical function, representing the circular aperture function convolved with a modified Gaussian point spread function (PSF). On the basis of goodness of fit criterion, the proposed Gaussian function is a very good model for the scatter PSF. The measured scatter PSF’s are reported for various Lucite thicknesses. Using the PSF’s, the modulation transfer functions are calculated, and this spatial frequency information may have value in analytical scatter removal techniques, grid design, and air gap optimization.