Showing papers on "Imaging phantom published in 1981"

Computed tomography using proton energy loss

Abstract: An experiment has been performed to demonstrate the feasibility of proton computed tomography. The proton energy loss was used to measure the projections of the relative stopping power of the phantom. High quality reconstructions were obtained from scans of 19 cm and 30 cm diameter performance phantoms. Comparison with reconstructions from an EMI CT-5005 X-ray scanner showed the proton technique is more dose efficient by a large factor.

Factors affecting the measurement of size and CT number in computed tomography.

Abstract: Computed tomography scanners may be used to make anatomic size measurement; however, manipulating the viewer controls (especially the window center) can have a significant influence on the apparent size of structures in the image. A special phantom was constructed to study the effect of window center adjustments on the apparent size of lone cylindrical objects (no variation within slice thickness) and also for spherical objects (maximum variation within slice thickness). Using this phantom, the authors were able to identify several commonly encountered situations in which both the apparent size and CT numbers may be seriously in error. It was found that for cylindrical objects larger than about one transverse resolution element and aligned with the scanner axis, CT numbers may be determined accurately; however, the apparent diameter changes by several millimeters as the window center setting is changed. CT numbers for spheres are inaccurate when the diameter is comparable to or less than the slice thickness and, as a result, diameters of spheres cannot be measured accurately with fixed window center settings. Diameters of spheres can be measured accurately (+/- 1 mm) if the sphere is centered in the slice and the full width at half maximum of the CT number profile is used.

The exponential edge-gradient effect in x-ray computed tomography.

Abstract: The exponential edge-gradient effect must arise in any x-ray transmission CT scanner whenever long sharp edges of high contrast are encountered. The effect is non-linear and is due to the interaction of the exponential law of x-ray attenuation and the finite width of the scanning beam in the x-y plane. The error induced in the projection values is proved to be always negative. While the most common effect is lucent streaks emerging from single straight edges, it is demonstrated that dense streaks from pairs of edges are possible. It is shown that an exact correction of the error is possible only under very special (and rather unrealistic) circumstances in which an infinite number of samples per beam width are available and all thin rays making up the beam can be considered parallel. As a practical matter, nevertheless, increased sample density is highly desirable in making good approximate corrections; this is demonstrated with simulated scans. Two classes of approximate correction algorithms are described and their effectiveness evaluated on simulated CT phantom scans. One such algorithm is also shown to work well with a real scan of a physical phantom on a machine that provides approximately four samples per beam width.

The effect of mAs variation upon computed tomography image quality as evaluated by in vivo and in vitro studies

Abstract: The effect of different mAs settings on computed tomobraphy (CT) image quality was evaluated by a clinical study and phantom studies. In the clinical portion of this study, a number of CT images, obtained at various mAs settings, were reviewed by experienced CT interpreters. From these same images, graphs were charted of the root-mean-standard deviation (RMSD) vs. body size; there appeared to be a linear relationship between noise, as measured by RMSD, and body size. A performance phantom was used to evaluate the effect of the different mAs settings on resolution. There was some loss in low contrast resolution at 6 and 20 mAs; however, the resolution was identical at 40 and 100 mAs.

Optimal x-ray spectra for screen-film mammography.

Abstract: Theoretical and experimental techniques have been used to study optimal x-ray for screen-film mammography. A simple model of mammographic imaging predicts optimum x-ray energies which are significantly higher than the K-characteristic energies of Mo. A subjective comparison of x-ray spectra from Mo-anode and W-anode tubes indicates that spectra produced by a W-anode tube filtered with materials of atomic number just above that of Mo are more suitable for screen-film mammography than spectra produced by the Mo-anode/Mo-filter system. The imaging performance of K-edge filtered, W-anode tube spectra was compared to the performance of Mo-anode spectra using phantom measurements and mastectomy specimen radiography. It was shown that optimal W-anode spectra can produce equal contrast with an exposure reduction of a factor of two to three, a dose reduction of a factor of two, and equal or reducing tube loading, compared to Mo-anode spectra. A computer simulation was carried out to extend the initial, monoenergetic theory to the case of real, polychromatic sources. The effects of varying filter material and thickness, tube operating potential, and breast thickness were all studied. Since W-anode x-ray tubes are considered to be better for Xerox mammography than Mo-anode tubes, this study has shown that both Xerox and screen-film techniques can be performed optimally with a single, properly designed, W-anode x-ray tube.

The tissue-equivalence of the Alderson Rando anthropomorphic phantom for x-rays of diagnostic qualities.

Abstract: Comparative measurements were made of depth-dose profiles in an Alderson Rando phantom and a similarly shaped phantom of water which closely simulates the attenuation properties of human muscle. Measurements were carried out at three different kV settings of the X-ray generator, covering the lower-energy range of normal diagnostic practice over which attenuation differences are likely to be most important. The results show that whilst the Alderson phantom can undoubtedly be extremely useful in medical dosimetry, corrections to the dose distributions observed for diagnostic qualities of radiation may sometimes be necessary to allow for departures from tissue-equivalence.

Accurate CT Measurement of the Spinal Cord Using Metrizamide: Physical Factors

Abstract: Computed tomography (CT) using metrizamide is a valuable tool in the neuroradiologic evaluation of the spinal cord, but CT must be used with care when measuring object size because size may vary with different window center (level) settings. A simple method to determine appropriate window center (level) settings during CT object measurement is described. A polystyrene core with a Teflon outer cylinder designed to simulate the spinal column, spinal cord, and subarachnoid space was scanned with varying concentrations of metrizamide with an EMI 1010 unit. Computer printouts of the CT pixel matrix were analyzed and a constant relationship of the phantom cord true size to the CT numbers of the metrizamide concentration and the phantom spinal cord was found. Window center (level) selection greatly influences measurement of cord size, whereas window width does not. The appropriate window center (level) selection is the mean between the metrizamide CT attenuation number and the cord CT attenuation number.

Some Experimental Results on the Thermoacoustic Imaging of Tissue Equivalent Phantom Materials

Abstract: Thermoacoustic waves were induced in vivo and in soft tissue phantoms consisting of layers of muscle tissue phantom gel and vegetable oil by 0.4 microsecond duration electric current pulses. The acoustic signals were detected by a 0.5 MHz transducer after digital signal averaging of typically 10 to 103 waveforms. The observed signals had the amplitude and time delay characteristics expected for thermoacoustic emission. The results support the feasibility of thermoacoustic imaging of soft-tissue by a transducer-amplifier-averaging system optimized for this application which might give information complementary to conventional ultrasonic and x-ray systems.

A simplified method of quantitating organ uptake using a gamma camera

Abstract: A simplified method of absolute quantitation of activity in any region of the body is presented utilizing a series of views acquired with a scintillation camera linked to a computer. An accuracy of ±6% may be achieved. No additional programming needs to be added to the standard nuclear medicine computer software and a knowledge of the attenuation coefficients involved and the shape or position of the organs examined is not presupposed or estimated. Attenuation correction is performed using a transmission image as one of the sequence of camera views. The phantom measurements used to evaluate the technique are decsribed and some clinical results presented.

Accurate CT measurement of the spinal cord using metrizamide: physical factors

Abstract: Computed tomography (CT) using metrizamide is a valuable tool in the neuroradiologic evaluation of the spinal cord, but CT must be used with care when measuring object size because size may vary with different window center (level) settings. A simple method to determine appropriate window center (level) settings during CT object measurement is described. A polystyrene core with a Teflon outer cylinder designed to simulate the spinal column, spinal cord, and subarachnoid space was scanned with varying concentrations of metrizamide with an EMI 1010 unit. Computer printouts of the CT pixel matrix were analyzed and a constant relationship of the phantom cord true size to the CT numbers of the metrizamide concentration and the phantom spinal cord was found. Window center (level) selection greatly influences measurement of cord size, whereas window width dose not. The appropriate window center (level) selection is the mean between the metrizamide CT attenuation number and the cord CT attenuation number.

Imaging capability of an experimental digital subtraction angiography unit.

Abstract: We evaluated an experimental digital subtraction unit in terms of field uniformity, linearity of signal response to x-radiation, response of the log amplifier, and imaging capability as measured with the Rose phantom. For comparison, conventional film subtraction phantom measurements were made. Our results indicate that this unit exhibits greater sensitivity to contrast media than conventional film subtraction units.

Tomographic reconstruction of ultrasonic attenuation with correction for refractive errors

Abstract: The nonionizing and noninvasive characteristics of ultrasonics promote its increasing use in medical applications. Computerized ultrasonic attenuation tomography is one area where medically significant images may be reconstructed for diagnostic purposes. The objective of this work is to make an initial correction for refraction error and other problems present in computerized ultrasonic tomography. As a refinement of the scanning technique, our method attempts to provide high-quality projection profiles to the reconstruction algorithms. The important aspect of this new scanning mode lies in its correction for refraction in the measured attenuation profiles. In reconstructing phantom targets scanned by our experimental system, results superior to traditional line-of-sight scanning were obtained.

Limitations to iodine isolation using a dual beam non-K-edge approach.

Abstract: In dual‐beam selective iodine imaging, images of an object are made with each of two spectrally different x‐ray beams. The mean beam energies may either straddle the 33 keV iodine K‐edge or both lie above the K‐edge. Both patient exposure considerations and the availability of sufficient x‐ray flux make the latter approach favorable for tissue thicknesses exceeding 5 cm. Consider such an approach in which image contrast from tissue is suppressed in the difference image. It is proven theoretically that the residual bone–to–iodine contrast is a constant independent of the two mean beam energies used. This invariance principle is demonstrated experimentally by comparing images made from different pairs of x‐ray spectra. Observed contrast ratios match the predicted value very well. In dual‐beam imaging, contrast from only one material may be suppressed. Other substances yield residual signals which compete with the iodine. Subtleties of this incomplete cancellation are demonstrated, discussed, and quantitated. A contrast enhancement factor (CEF) is defined as the factor by which iodine contrast is enhanced in a multiple beam subtraction technique relative to monoenergetic imaging at 40 keV. CEFs are determined for tissue and bone cancellation separately and their limits are discussed. Images of a simulated artery containing iodine superimposed over a Rando head and neck phantom show that the CEF limitation for dual beam imaging is quite severe compared to a time dependent mask mode imaging approach. Finally, optimum energies for dual beam images are discussed.

Three-beam K-edge imaging of iodine using differences between fluoroscopic video images: experimental results.

Abstract: In an earlier article we discussed the rationale for using differences between video images in three‐beam selective iodine K‐edge imaging. Rather than combining three initial imagesL i linearly to yield the final imagek 1 L 1+k 2 L 2+k 3 L 3, differences between the L i were first generated and then combined either to linear or quadratic order. This approach was motivated by the desire to suppress the large multiplicative biases of fluoroscopic imaging and justified by theoretically proving that k 1+k 2+k 3≊0. In this paper we discuss the instrumentation and experimental results obtained from this difference‐based technique. A specially‐constructed apparatus is described which automatically selects the optimum combination coefficients and combines the difference images up to quadratic order at realtime video rates. Three methods for generating K‐edge subtraction images are compared: the former approach in which the L i are linearly combined and combination of differences to linear and quadratic order. In imaging phantoms in which the iodine distribution is known, the resultant subtraction images from all three methods appear similar. Inspection of signal sizes shows that the quadratic difference‐based approach provides superior bone and tissue residual suppression by about a factor of 2. In imaging phantoms in which the iodine distribution is unknown, incomplete suppression of x‐ray scatter and image intensifier veiling glare prevent a quantitative comparison of performance of the three algorithms. An experiment verification is provided of the theorem which states that k 1+k 2+k 3≊0.

On the sensitivity and the resolution of microwave imaging using ART

Abstract: The feasibility of using the presently available linear reconstruction techniques in microwave (MW) imaging was suggested and numerically demonstrated in a recent communication [1]. It was shown that although such techniques neglect reflection and refraction effects, they still can be used to obtain suitable MW images. This letter presents some calculated microwave images of phantoms simulating biological objects. These images were obtained using the algebraic reconstruction technique (ART) that was originally used in X-ray and ultrasound computerized tomographic imaging. The resolution and sensitivity of the microwave images are examined and suggestions for improvements are discussed.

Phantom Sensations (Phantom Arm) in Plexus Paralysis

Abstract: Phantom limb is the feeling that a part of the body, lost mainly through amputation, still exists. Furthermore one speaks of phantom feelings, experiences, or limbs if the limb still exists but cannot be moved or experienced due to motor or sensory paralysis, and this despite an often partially movable, sometimes painfully sensitive limb. In transverse lesions of the cord with paraplegia the phantom feelings are projected mainly to the legs. The cases reported below concern phantom experiences in a paralyzed upper extremity; namely, “phantom arm.” Phantom sensations in paralyzed plexus brachialis were first reported by Mayer-Gross [5], though Hasenjager and Potzl [4] were the first to use the term phantom arm in paralyzed plexus. It was thus several decades after the first description of phantom limbs by Weir Mitchell 1871 [14] that phantom sensations in existing, but not experienced paralyzed limbs (consequently in paraplegia) were to be reported.

Intravenous arteriography using scanned projection radiography.

Abstract: Visualization of arteries using intravenous injections of contrast material requires an imaging system capable of (a) excellent sensitivity to dilute concentrations of contrast media and (b) adequate temporal resolution to minimize the effects of motion during the exposure. A line-scanned radiography system based on a CT fan-beam detector (high-pressure xenon ionization chamber) was used for phantom and animal studies of intravenous arteriography to demonstrate the potential advantages of this method. Even though line-scanned systems require long scan times compared to existing radiographic methods, they are capable of showing rapidly moving arteries without blurring. Concentrations of 3 mg/ml of iodine could be seen in vessels 2 mm in diameter. Phantom studies using kVp switching showed that bone or soft tissue could be selectively cancelled.

Digital Imaging For Radiation Therapy Verification

Abstract: A linear diode array was used to scan a radiation field for pulsed high energy x-ray imaging. With a signal of about 50 pico-coulombs of charge from each diode for each beam pulse, the signal to noise ratio for an image of an open field was 119. An image of a porcine femur in a wax phantom was obtained by scanning the transmitted radiation. The resultant electronic image was superior in quality when compared to the standard radiotherapy port film. The simplicity of the detector system has demonstrated the feasibility of constructing an electronic portal imaging system for clinical use.

Rotating conventional gamma camera single-photon tomographic system: physical characterization.

Abstract: A whole-body single-photon emission computed tomography system has been evaluated in terms of its physical performance. It consists of a standard GE 400 T maxi camera rotating on a ring stand, coupled to an Informatek Simis 3 computer system. In its standard mode of operation, 64 or 128 successive views of 64 X 64 or 128 X 64 matrices are collected at regular angular samples. The reconstruction of up to 64 possible adjacent transverse sections, sorted into coronal, sagittal, and oblique sections, is obtained by filtered backprojection. A conventional parallel-hole collimator is used. The uniformity of the camera field of view is corrected by the GE hardware module. The resolution in the transverse plane is typically of 15.5 mm in a 20 cm Lucite phantom, and independent of radial distance. The physical slice thickness is 19 mm (full width at half maximum of the longitudinal response using a point source) with only slight variations along the diameter of the image. The sensitivity is approximately 7,000 cps mCi-1 as measured using a 99m Tc thin source, enabling images of reasonable signal-to-noise ratio to be obtained in 10 min. The effect of the energy window, the number of angular samples, and themore » linear sampling modes on the transverse resolution and contrast is measured and discussed.« less

Reduced-dose magnification mammography.

Abstract: The authors describe a new reduce-dose magnification mammography system which uses a microfocal-spot/tungsten-target x-ray tube in conjunction with a high-speed, rare-earth film/screen system. Physical measurements of imaging parametes, phantom tests, and limited clinical trials are reported. Although this system is not yet ready for general use, preliminary results show that diagnostic magnification images of breast masses and microcalcifications are feasible with low radiation doses.

Voltage, energy, and material dependence of secondary radiation

Abstract: Measurement results of the relative contribution of primary and secondary radiation are presented. Measurements were done with both monoenergetic and broad spectrum x-ray beams. A variety of phantoms were used: graphite, water, lucite, a skull phantom, and aluminum. We found that the primary radiation fraction is nearly independent of photon energy and tube potential. Calculations using a simple single Compton scattering model agree with the experiment results.

Multisegmented ion chamber for CT scanner dosimetry.

Abstract: A multisegmented, ionization chamber capable of determining dosimetric profiles from a CT scanner has been developed and tested. The chamber consists of a number of 2 mm wide electrically isolated segments from which ionization currents may be measured. Presented here are the performance characteristics of the chamber including energy response, dose linearity, and corrections for ''cross talk'' between segments. Sample dosimetric profiles are depicted for 3 and 6 mm nominal beam widths at two locations in a dosimetric phantom positioned in the x-ray beam of a fourth generation CT scanner. The results agree well with the conventional method of obtaining dosimetry measurements with TLD chips.

An Analysis of Minimally Perturbing Temperature Probe and Thermographic Measurements in Microwave Diathermy

Abstract: Temperature measurements in fat-muscle phantoms using thermography and a minimally perturbing temperature probe were investigated. Two microwave applicators (915- and 2450-MHz) were used to induce the heating in the phantom. Discrepancies between data taken with the thermographic camera versus the probe were measured. These discrepancies were shown to be primarily caused by a 40-s time delay in performing temperature measurements with the thermographic camera, which resulted in additional thermal diffusion in the phantom.

Computerized Heavy-Ion Tomography

Abstract: Several techniques for heavy-ion computerized tomography are being investigated at Lawrence Berkeley Laboratory. Using beams of carbon and neon from the Bevalac, we have demonstrated that these methods are feasible and capable of high resolution. We describe in some detail the method of heavy-ion CT imaging using nuclear track detectors, including a discussion of procedures for optical scanning and digitization of data and computerized distortion corrections. Comparisons between a heavy-ion CT image and X-ray CT image of a simple phantom are discussed. Preliminary results from two techniques using active, online detector systems for performing heavy-ion computerized tomography are presented. One method uses a multiplane, multiwire ionization chamber for detecting the heavy ions in a mode allowing true three-dimensional reconstructions. The other technique uses a system of position-sensitive silicon solid-state detectors for spacial information and high-purity germanium detectors to measure accurately the residual energy of the ions.

The contribution of internal scatter to radiation dose during CT scan of the head.

Abstract: An acrylic head phantom was irradiated during a computed tomographic scan with four commercial scanners. Measurements of the spatial distribution of the radiation dose on the surface and internal to the phantom were performed for the scan plane and the scattered beam at various distances from the scan plane. The surface scatter dose was found to be considerably smaller than that for internal scatter. A significant increase in radiation exposure within the head phantom due to internal radiation scatter, and an asymmetrical primary beam profile for dual slice scanners were also noted.

Deadtime measurements with a multihole phantom: concise communication.

Abstract: The performance characteristics of any Anger camera are defined by that camera's spatial and temporal resolutions. Procedures using first-pass techniques - such as ejection fraction or left-to-right shunt determinations - require that deadtimes be minimal. We have evaluated a simplified method for measuring a camera's deadtime; it uses a multihole phantom consisting of a lead sheet with 196 holes, 7/16 in. diameter, arranged in a 14 x 14 matrix. Each hole can be filled with a removable lead plug. Count rates are determined following incremental removal of the plugs. Deadtime curves were generated for each of three cameras by this and standard techniques. For each camera, the three curves were virtually identical. Using the phantom, deadtimes could be determined in 15 to 20 min. This phantom provides a simple, rapid, and accurate means of determining camera deadtime and minimizing personnel radiation exposure.

Phantom Tooth Phenomenon: Painless and Painful Sensations

Abstract: For centuries teeth have been frequently extracted structures in humans, but few reports on phantom phenomena actually comment on the teeth [15, 23, 28]. Phantom tooth pain is discussed in detail by Marbach [8, 9]. This study shows evidence that painless phantom phenomena exist even after tooth removal and could, therefore, contribute to further understanding of the phantom phenomenon.

Screening for brain lesions with digital radiography of the head using a CT scanner.

Abstract: Digital radiography was tested in a clinical setting as a screening device for detecting brain lesions and for following the response of known enhancing cranial masses to therapy. A concurrent phantom study was conducted to determine the low contrast sensitivity of the system. Contrast resolution was superior to that of film -- screen systems but suffered at a density level of 2 to 4% because of system noise. This noise appeared to be nonquantum in nature, related to electronic and mechanical inconsistencies of the computed tomographic instrument. System noise and patient motion combined to nullify the advantages of digital radiography for screening and monitoring intracranial masses.