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PubMedDate not supplied

Analysis of the significance of scattered radiation in reduced dose mammography, including magnification effects, scatter suppression, and focal spot and detector blurring.

A systems analysis of reduced dose mammography with scatter removal has quantitatively specified the relationships between the several parameters in mammography systems. While the results are only preliminary in the sense that they need to be proven experimentally, they do seem to provide a good feel for the nature of the important trade-offs. The major uncertainties in the analysis probably have to do with the formal equivalence, that is implicit in the aperture theory, between large area and detail contrast, as well as the output versus focal spot relationship that was adopted. Also, there are important questions about specifying motion unsharpness as a function of exposure time in mammography examinations.

Mammography↗
PubMed1982

[Model calculations of the Energy distribution of scattered radiation in a patient (author's transl)].

To obtain quantitative results on scattered radiation produced in a patient by externally applied x-rays or gamma rays, the model of a pencil beam penetrating a water layer is used. By means of the Monte-Carlo method, the energy distribution of scattered photons in a plane parallel to the absorbing layer is calculated for primary photon energies from 60 to 1250 keV. This yields information on the distribution of single-scatter and multiple-scatter photons.

Elementary Particles↗
PubMedDate not supplied

Scattered radiation from a neutron collimator.

Fast-neutron beams are being employed in radiotherapy trials and associated radiobiology studies at numerous centers in the U.S., Europe, and Japan. Since collimated beams of various sizes and shapes are employed, it is desirable to know the composition of the scattered radiation component contributed by the collimator. A simple method is shown for deducing the field composition in terms of a three-component model, from measurements made with three ionization chambers (tissue-equivalent, graphite, and magnesium). The dose contributed by the scattered radiation in the present example was found to be predominantly due to fast neutrons indistinguishable from those in the primary spectrum (from 35-MeV D+ on Be). This method may prove useful for measurements in phantoms as well.

Fast Neutrons↗
PubMed2007

Depolarization ratio of Rayleigh scattered radiation by molecules.

The depolarization ratios of Rayleigh scattered radiation by molecules has been used to measure the ratio of anisotropic polarizability to isotropic (average) polarizability in the laboratory. The authors have calculated the depolarization ratio for N2, CH3Cl, and H2O from the first principles. It is shown that the depolarization ratio derived from input polarizabilities differs by a factor of about 4 for 1Sigma diatom N2, by a factor of about 2.9 for symmetric top CH3Cl, and by a factor of about 5 for asymmetric top H2O. These large discrepancies arise because in deriving the ratio of anisotropic polarizability to isotropic (average) polarizability from the measured depolarization ratio, the constraints imposed by the conservation of angular momentum have been completely ignored.

Journal Article↗
PubMed2001

Effect of scattered radiation on image noise in cone beam CT.

Cone beam CT has a capability for the 3-dimensional imaging of large volumes with isotropic resolution, and has a potentiality for 4-dimensional imaging (dynamic volume imaging), because cone beam CT acquires data of a large volume with one rotation of an x-ray tube-detector pair. However, one of the potential drawbacks of cone beam CT is a larger amount of scattered x-rays, which may enhance the noise in reconstructed images, and thus affect the low-contrast detectablity. Our aim in this work was to estimate the scatter fractions and effects of scatter on image noise, and to seek methods of improving image quality in cone beam CT. First we derived a relationship between the noise in a reconstructed image and in an x-ray intensity measurement. Then we estimated the scatter to primary ratios in x-ray measurements using a Monte-Carlo simulation. From these we estimated the image noise under relevant clinical conditions. The results showed that the scattered radiation made a substantial contribution to the image noise. However, focused collimators could improve it by decreasing the scattered radiation drastically while keeping the primary radiation at nearly the same level. A conventional grid also improved the image noise, though the improvement was less than that of focused collimators.

Head↗
PubMedDate not supplied

Effects of scattered radiation and veiling glare in dual-energy tissue-bone imaging: a theoretical analysis.

Dual-energy subtraction imaging allows tissue and bone structures to be separated from each other and attenuating thicknesses measured. Potential applications include chest imaging, bone mineral measurement, angiography, and mammography. However, intrinsic to most x-ray detectors is the acceptance of scattered radiation as part of the image signal. Added to that is the veiling glare component when an image intensifier is used. Together, they result in erroneous transmission measurement and degrade the accuracy of energy subtraction processing. In this paper, the effects of scattered radiation and veiling glare on energy subtraction images are examined theoretically. A model is derived and used to compute the effects on the thickness signals, image contrast, and image noise as a function of the scatter glare to primary ratios. The ratios were measured on a point-by-point basis for a Rando chest phantom. For 96% of the image field studied, the thickness signals may be subject to an error ranging from 0 to -22.5 cm for tissue and 0 to 5.2 cm for bone. The image contrast in the tissue image may be reduced by a factor ranging from 1 to 59. The percentage of the uncanceled bone signals ranges from -52% to 52%. The contrast-to-noise ratio may be reduced by a factor ranging from 1 to 18.

Bone and Bones↗
PubMedDate not supplied

[Experimental determination of radiation scattering and absorption coefficients in a homogeneous layer of highly-dispersive biological medium].

A method for experimental determination of optical characteristics of a highly-dispersive medium (radiation scattering and absorption coefficients) is described. The method is based on two mathematical models of ultrashort laser pulse propagation through a highly-dispersive medium (HDM), an axial model and a diffusion model. Milk dissolved in water was used as HDM. Dependences of optical characteristics of HDM on the concentration of milk in water are obtained. The limits of applicability of the axial and diffusion models to media with different scattering and absorption characteristics are determined.

Animals↗
PubMedDate not supplied

The measurement of multidimensional myocardial dynamics using scattered radiation fields.

A new radiographic device, based upon the analysis of scattered radiation fields, has been developed to measure myocardial dynamics. The device consists of an array of detectors arranged to monitor photons scattering from the epicardial surface. Data synthesis permits real-time dynamic displays of the epicardial surface in two and three dimensions. The system has been tested on close-chest canines. Epicardial surface displacements within the closed chest cavity can be measured to 0.1 mm (S.D.). Right or left ventricular surfaces may be monitored on a given scan. Surfaces behind or in front of outer myocardial boundaries within the direct field of view of the detectors are located with equal accuracy. Except for the use of low levels of fluoroscopic x-rays, the procedure is completely noninvasive. Radiation dose levels and component costs for the prototype system are modest. The detector system attaches to a standard fluoroscopic x-ray generator.

Animals↗
PubMedDate not supplied

Dosimetry of radiation scattered to thyroid gland from prophylactic cranial irradiation for childhood leukemia.

Dosimetry of radiation scattered to the thyroid gland was performed in 17 children (9 boys, 8 girls) who were treated for acute lymphoblastic leukemia and received cranial irradiation for prophylaxis against central nervous system leukemia at a median age of 4 years and 2 months (range, 1 year and 1 month to 14 years). The absorbed dose to the thyroid gland in these children ranged from 0.7% to 7.3% of the dose delivered to the cranium. Thus the total dose to the thyroid gland ranged from 0.13 to 1.32 Gy by the end of the entire course of cranial irradiation. Doses tended to be larger in younger children, but the radiation source also had a large influence on the dose to the thyroid gland; that is, the absorbed dose to the thyroid gland with delivery by linear accelerator was smaller than that by cobalt irradiation. Long-term survivors treated with cranial irradiation for acute leukemia during childhood should be followed for the possible development of thyroid diseases, including malignant tumors, for a long period.

Adolescent↗
PubMed1996

Active minimisation of radiation scatter during breast radiotherapy: management implications for young patients with good-prognosis primary neoplasms.

BACKGROUND AND PURPOSE: Radiotherapy is used to reverse or prevent local tumour growth but is also a carcinogen in its own right. A recent audit of post-radiotherapy second malignancies in this institution revealed a striking preponderance of tumours originating near the outside edge of the treatment field. Since this finding suggests the existence of a critical subtherapeutic dose range predisposing to tumourigenesis, we attempted to define and reduce this radiation scatter dose. MATERIALS AND METHODS: We undertook a dosimetric review of 6 MV scatter from a linear accelerator in sites matching the putative tumourigenic region, and then extended this analysis to patients and tissue phantoms. RESULTS: A wide range of radiation scatter doses was confirmed-for example, doses 3 cm from the field edge varied from 1.7 to 22% of the therapeutic dose depending upon the field parameters. Scatter doses were then assessed in a sample of eight patients undergoing standard breast radiotherapy. Contralateral breast sites 4-12 cm from the midline received 4-10% of the therapeutic dose, or 200-500 cGy for a 50 Gy treatment, approximating historical estimates of the tumourigenic range. The deep component of this scatter dose from medial field breast irradiation was reduced 19% simply by replacing the 15 degrees medial tangential field wedge with a 30 degrees lateral wedge. Other manoeuvres which reduced contralateral breast dose by up to 46% included making the posterior field edges co-planar and shielding the breast during medial field irradiation. CONCLUSIONS: These results suggest that the risk of radiogenic second malignancies could be significantly decreased by careful attention to the treatment details. Greater awareness of these measures may prove particularly relevant to the conservative management of young patients with good-prognosis breast neoplasms such as ductal carcinoma in situ.

Breast↗
PubMed1998

Scattered radiation in scanning slot mammography.

Monte Carlo simulations were used to quantify the amount of scattered radiation a scanning slot detector geometry designed for use in digital mammography. Ratios of the scatter to primary (S/P) x-ray photon energy absorbed in the detector were obtained for a Lucite phantom, and were investigated as a function of photon energy, phantom thickness, and slot detector width. Over a Lucite phantom thickness range of 2-6 cm, the S/P ratios range from about 0.10 to 0.17 for a 4 mm wide slot detector at the x-ray photon energies used in mammography. These ratios increased by a factor of approximately 1.8 when the slot width was increased to 10 mm. In general, 20 keV photons gave S/P ratios similar to those of a 30 kVp x-ray spectrum (Mo target + 30 microns Mo filtration). The use of a 3 cm air gap reduced the S/P ratios by a factor of between 2.5 and 3.4, depending on the phantom thickness. For a constant primary energy fluence, coherent scatter was reduced as photon energy increased, whereas Compton scatter increased with increasing photon energy. With no air gap, the contributions of coherent and Compton scatter were found to be equal at 25 keV, whereas the introduction of a 3 cm air gap resulted in equal contributions for the two scatter processes at 36 keV. A 10 mm wide slot detector consisting of a 36.7 mg/cm2 thick Gd2O2S:Tb phosphor screen was compared to an ideal detector absorbing all incident primary/scatter photons. Average differences in the S/P ratios for these two detectors were 7% with no air gap and approximately 4% with a 3 cm air gap. The results obtained in this study will assist in the design of an optimal slot detector for use in digital mammography.

Breast↗
PubMed1979

Scattered radiation from trimmer bars in the cobalt-60 teletherapy machines.

The amount of scattered radiation resulting from four Cobalt-60 teletherapy units in four different hospitals when trimmer bars were fully extended was studied. Scatter was present for all machines checked at between 10% and 20% of the central axis dose. Clinical significance is also discussed.

Cobalt Radioisotopes↗
PubMed1979

A new technique for the calculation of scattered radiation from 60Co-teletherapy beams.

An alternative to the Clarkson technique for calculating total scattered radiation is presented. Resultant TAR values are compared with those obtained by the Clarkson technique for the same field. The new scatter technique is always more than 2.8 times faster than the Clarkson technique and more than 4 times faster on a large number of fields. The new approach appears to offer a simple and expedient alternative to the more traditional methods for iiregular field dose calculations.

Cobalt Radioisotopes↗
PubMedDate not supplied

A method for testing scattered radiation grids.

A simple method of evaluating contrast improvement factors and other related quantities has been developed. With this method a water or lucite phantom is imaged with a lead disk on a nonscreen film using a scattered radiation grid with unknown characteristics. The image is then compared with that obtained with the same arrangement but with a calibrated scattered radiation grid. The method described does not need sophisticated instruments and can be used in any department of diagnostic radiology

Technology, Radiologic↗
PubMed1976

A method to absorb scattered radiation without attenuation of the primary beam.

A method is presented for improving image quality by decreasing the scattered radiation. Two pairs of lead plates, one above and one below the patient, move in arcs with a common pivot point, the focal spot of the x-ray tube. The lower plates are separated by a 1-cm slot. The upper plates are also separated by a slot, the edges of which lie on common radii from the focal spot. The image is formed by moving the slots so that the X-ray beam emerging from the object sweeps across the film with a uniform speed during the exposure.

Humans↗
PubMed1999

Investigation of scattered radiation in 3D whole-body positron emission tomography using Monte Carlo simulations.

The correction of scattered radiation is one of the most challenging tasks in 3D positron emission tomography (PET) and knowledge about the amount of scatter and its distribution is a prerequisite for performing an accurate correction. One concern in 3D PET in contrast to 2D PET is the scatter contribution from activity outside the field-of-view (FOV) and multiple scatter. Using Monte Carlo simulations, we examined the scatter distribution for various phantoms. The simulations were performed for a whole-body PET system (ECAT EXACT HR+, Siemens/CTI) with an axial FOV of 15.5 cm and a ring diameter of 82.7 cm. With (without) interplane septa, up to one (two) out of three detected events are scattered (for a centred point source in a water-filled cylinder that nearly fills out the patient port), whereby the relative scatter fraction varies significantly with the axial position. Our results show that for an accurate scatter correction, activity as well as scattering media outside the FOV have to be taken into account. Furthermore it could be shown that there is a considerable amount of multiple scatter which has a different spatial distribution from single scatter. This means that multiple scatter cannot be corrected by simply rescaling the single scatter component.

Algorithms↗
PubMedDate not supplied

A new technique for the calculation of scattered radiation for 10 MV photon beams.

An alternative to the Clarkson technique for calculating total scattered radiation is presented for use with 10 MV photon beams. When compared to the same field size and depth parameters, the resultant TMR values are in good agreement with results utilizing the Clarkson technique. Lengthy iterative procedures are avoided in this new scatter technique in favor of a more efficient SMR algorithm, thereby reducing the amount of required computation time. The speed and simplicity of this method make it especially amenable to use with small desk top computers and even programmable pocket calculators.

Computers↗