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Nov 1982

Volume 9, Issue 6, pp. 807-935

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The physics of proton NMR

Robert L. Dixon and Kenneth E. Ekstrand

Med. Phys. 9, 807 (1982); http://dx.doi.org/10.1118/1.595189 (12 pages) | Cited 12 times

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The physics of pulse NMR which is pertinent to an understanding of proton NMR imaging has been condensed and directed toward the medical physicist. The basic physical principles of spin manipulations using rf pulses are presented, and the relation between the quantum mechanical and the classical descriptions is covered in a rigorous fashion. The physics of relaxation is described and the relaxation times T1 and T2 are explained in some detail. Application of these spin manipulation techniques is illustrated by showing how they may be used in creating an image.
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87.57.-s Medical imaging
87.63.-d Non-ionizing radiation equipment and techniques
87.85.Pq Biomedical imaging
32.90.+a Other topics in atomic properties and interactions of atoms with photons (restricted to new topics in section 32)

Resolution and noise in xeromammography

Panos P. Fatouros

Med. Phys. 9, 819 (1982); http://dx.doi.org/10.1118/1.595190 (11 pages) | Cited 1 time

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The resolution and noise characteristics of the xeromammographic image are analyzed by means of a multistage model which considers the exposure, toner deposition, and toner transfer steps. A frequency‐dependent theory of xeromammographic noise is developed and comparison is made with experimental results. Central to this theory is the concept of a toner electrode formed in the immediate vicinity of the Se layer. Comparison is also made between the imaging capabilities of xeromammography and screen/film systems. The paper concludes by addressing the question of normalization of the transfer function in an electrostatic system.
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87.57.-s Medical imaging
87.63.-d Non-ionizing radiation equipment and techniques
87.85.Pq Biomedical imaging
87.80.-y Biophysical techniques (research methods)

Scatter dose decrement values for rectangular fields

Bengt E. Bjärngard, Lisa H. Brown, and Göran K. Svensson

Med. Phys. 9, 830 (1982); http://dx.doi.org/10.1118/1.595191 (5 pages) | Cited 1 time

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The scatter dose decrement value (SDV) has been studied by calculational methods for 60Co γ and 4‐ and 8‐MV x‐ray beams of rectangular cross section. In a principal plane and inside the field, the SDV is nearly independent of field size, energy, and depth when the position of the point considered is expressed as the fractional distance to the field edge. From this, it follows that the scatter dose profiles are quite similar in planes parallel to a principal plane. A second corollary is that the SDV for an arbitrary point approximately equals the product of the two SDV values for the projected points in the principal planes. Outside the field, the SDV loses this independence of the various beam conditions, and as a consequence thereof, the ‘‘product rule’’ leads to errors. Mathematical expressions have been derived for the SDV inside and outside small fields. The errors resulting when these small‐field formulas are used for large fields have been studied. Expressed as fractions of the total dose on the centerline, these errors are of the order of ±1% inside the field and ±2% outside.
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87.53.Bn Dosimetry/exposure assessment

Phosphorus activation neutron dosimetry and its application to an 18‐MV radiotherapy accelerator

James R. Bading, Louis Zeitz, and John S. Laughlin

Med. Phys. 9, 835 (1982); http://dx.doi.org/10.1118/1.595192 (9 pages) | Cited 5 times

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Neutron fluxes and dose rates in and near the 18‐MV x‐ray beam of a Therac‐20 accelerator were determined with measured activities from the nuclear reactions 31P(n,p)31Si (fast neutrons) and 31P(n,γ)32P (thermal neutrons), published cross sections, and neutron energy spectra from Monte Carlo calculations. Measurements were made in the patient plane in air and at a 10‐cm depth in a tissue‐similar phantom, and in a plane containing the x‐ray target. Orthophosphoric acid solution was identified as a suitable and convenient phosphorus dosimeter material. In the 31P activation method, fluxes and dose rates are determined as the product of measured saturation activity per 31P atom and a conversion factor, which depends on the shape of the assumed neutron spectrum. For fast neutrons, which deliver most of the dose, the accuracy error in the saturation activity determinations was shown to be ≲25%. An inconsistency resulting from neglect of the accelerator’s adjustable collimator in the Monte Carlo calculations was demonstrated between the measured saturation activities and the theoretical neutron spectra. The maximum neutron dose equivalent rate observed was 5.9 mSv/Gy of x‐ray absorbed dose at the accelerator calibration point. Surface dose equivalent rates of the present study are less than those of fluxmeter and remmeter studies at sites outside Therac‐20 treatment fields by as much as factors of 2.4 and 2.8, respectively. The phantom study showed that at 18 MV internally produced neutrons have a negligible effect on the neutron field within the patient.
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87.55.-x Treatment strategy
87.53.-j Effects of ionizing radiation on biological systems
87.53.Bn Dosimetry/exposure assessment

A Laplace transform pair model for spectral reconstruction

Benjamin R. Archer and Louis K. Wagner

Med. Phys. 9, 844 (1982); http://dx.doi.org/10.1118/1.595193 (4 pages) | Cited 14 times

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A four‐parameter Laplace transform pair model, which accurately reconstructs an experimental bremsstrahlung spectrum from attenuation data, is presented. Computed spectral values with both aluminum and copper attenuators generally agree with experimental 70‐kVp data to better than 2%. Reconstructed spectra at other kVp’s also show good agreement with published data.
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07.85.-m X- and γ-ray instruments
61.80.Cb X-ray effects
61.85.+p Channeling phenomena (blocking, energy loss, etc.)
87.57.-s Medical imaging
87.63.-d Non-ionizing radiation equipment and techniques
87.85.Pq Biomedical imaging

Acoustic backscattering from ultrasonically tissuelike media

Michael F. Insana, James A. Zagzebski, and Ernest L. Madsen

Med. Phys. 9, 848 (1982); http://dx.doi.org/10.1118/1.595194 (8 pages) | Cited 1 time

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The acoustic backscattering coefficients of a tissue‐mimicking (TM) material have been measured using a modified Sigelmann–Reid technique in order to assess the similarity of its acoustic properties to those of soft tissues. A simple reference scatter material was used to probe the strengths and the limitations of the measurement technique. Backscatter coefficients measured from the TM material exhibit a frequency dependence of the form fm in the range 1.0–8.0 MHz where 3.5≤m≤3.8. The backscatter coefficient is also found to be approximately proportional to the particle concentration in the TM material up to concentrations that yield an attenuation coefficient of 0.9 db/cm/MHz. Comparisons of backscatter coefficients measured from the TM material are made with published values for liver, myocardium, and blood.
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87.57.-s Medical imaging
87.63.-d Non-ionizing radiation equipment and techniques
87.85.Pq Biomedical imaging
87.50.Y- Biological effects of acoustic and ultrasonic energy

Modification of the 50% maximum dose depth for 41‐MeV ( p+,Be) neutrons by use of filtration and/or transmission targets

J. B. Smathers, R. G. Graves, L. Earls, V. A. Otte, and P. R. Almond

Med. Phys. 9, 856 (1982); http://dx.doi.org/10.1118/1.595132 (4 pages)

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Several target configurations for the 41‐MeV ( p+,Be) reaction have been evaluated for the characteristics of the radiation field produced; depth dose, dose rate per μA. From analysis, it is concluded that to achieve the desired 13.2‐cm depth for 50% of maximum dose and acceptable dose rate at a target‐to‐skin distance (TSD) of 125–150 cm, the neutron spectra must be filtered to preferentially absorb the lower‐energy neutrons. Further increases in depth of 50% of maximum dose and a significant reduction in beryllium heating problems result if a partial transmission target is used with the terminal 30% of proton energy being deposited in a copper target backing.
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87.53.Bn Dosimetry/exposure assessment
29.25.Dz Neutron sources
07.77.-n Atomic, molecular, and charged-particle sources and detectors

Compton scatter effects in CT reconstructions

G. H. Glover

Med. Phys. 9, 860 (1982); http://dx.doi.org/10.1118/1.595197 (8 pages) | Cited 52 times

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Compton scatter of x‐ray quanta is the dominant attenuation mechanism in medical diagnostic imaging. In this paper, it is shown that the scatter‐to‐primary ratio determines the nature and intensity of scatter artifacts in computed tomography (CT) reconstructions and that this ratio, while significantly lower than in radiographic and fluoroscopic examinations, can still be significant in CT. It is found that high spatial‐frequency artifacts can arise even though the detected scatter intensity has little or no high‐frequency modulation. Reconstructions from x‐ray data are presented which show ‘‘cupping’’ as well as dark streaks connecting high‐attenuation regions. Correction of the data from measurements of scatter eliminates these artifacts. It is, moreover, observed that the intensity of the artifacts is often diminished when a beam‐shaping attenuator is employed. Calculations of scatter intensity are developed from a model which includes single‐event and two‐event scatter. This analysis is in good agreement with measurements on round water phantoms. Extension to other detector geometries shows, not unexpectedly, that detectors with poorer collimation yield larger scatter artifacts.
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87.57.-s Medical imaging
87.63.-d Non-ionizing radiation equipment and techniques
87.85.Pq Biomedical imaging
42.30.Va Image forming and processing
32.80.-t Photoionization and excitation

Calculation of scattering cross sections for increased accuracy in diagnostic radiology. I. Energy broadening of Compton‐scattered photons

Gudrun Alm Carlsson, Carl A. Carlsson, Karl‐Fredrik Berggren, and Roland Ribberfors

Med. Phys. 9, 868 (1982); http://dx.doi.org/10.1118/1.595195 (12 pages) | Cited 15 times

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In this work, scattering cross sections differential with respect to both the scattering angle and the energy of the scattered photon are derived in the relativistic impulse approximation for the light elements H, Be, and Al, and photon energies between 30 and 200 keV. The energy broadening of the scattered photons reflects the momentum distribution of the target electrons. It increases with both increasing atomic number of the scatterer and with scattering angle. Even in light elements, the energy broadening is comparable with the intrinsic energy resolution of modern Ge spectrometers. In reconstructing primary photon energy spectra by means of a Ge spectrometer and Compton scattering techniques, i.e., by measuring the photons incoherently scattered at a given angle, the energy resolution is markedly impaired compared to direct measurements in the primary beam. This is usually explained as an effect of the nonzero acceptance angle of the detector. It is shown, however, that the fundamental energy broadening of the scattered photons is alone sufficient as an explanation. The Compton scattering technique is valuable in determining energy spectra in clinical situations. Aspects of its optimal performance are discussed. The commonly used scattering angle of 90° seems adequate. At small scattering angles, the incoherent‐scattering cross section is badly known due to electron–electron interactions and, for photon energies <100 keV, coherent scattering contributes appreciably to the total scattering even in media of low atomic number. In cases where coherent scattering dominates and where the energy degradation of the incoherently scattered photons is small compared to the energy resolution of the spectrometer, the reconstruction is simplified. The double‐differential cross sections derived can be used to simplify calculations of the Compton component of the mass–energy absorption coefficient.
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87.57.-s Medical imaging
87.63.-d Non-ionizing radiation equipment and techniques
87.85.Pq Biomedical imaging
32.80.-t Photoionization and excitation
87.50.S- Radiofrequency/microwave fields effects
87.50.W- Optical/infrared radiation effects

Scatter in 240‐kVp mobile chest radiography

Louis K. Wagner and Gerald Cohen

Med. Phys. 9, 880 (1982); http://dx.doi.org/10.1118/1.595196 (4 pages)

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The effects of scatter radiation on signal‐to‐noise ratio and optical density contrast are quantitatively compared for three tissue interfaces at 85, 110, and 240 kVp in mobile chest techniques. It is shown that scatter in the mediastinal portion of the radiograph is virtually independent of energy, while it substantially increases with energy in the lung fields.
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87.57.-s Medical imaging
87.63.-d Non-ionizing radiation equipment and techniques
87.85.Pq Biomedical imaging
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