Narrow Beam Attenuation Coefficients Research Articles

Technical Note: Comparison of megavoltage, dual-energy, and single-energy CT-based μ-maps for a four-channel breast coil in PET/MRI.

The purpose of this study was to describe and evaluate methods for calculating a megavoltage computed tomography (MVCT)-derived MR hardware attenuation map (μ-map) and dual-energy CT (DECT) for 511 keV photons. Phantom measurements were acquired on a whole-body hybrid PET/MRI system, using a four-channel receive-only MR radiofrequency (RF) breast coil. Two acquisitions were performed: with the phantoms positioned in the four-channel RF breast coil, and without the breast coil. PET attenuation from the breast coil was corrected using three different CT-derived hardware μ-maps: (a) Single-energy CT (SECT), (b) DECT, and (c) MVCT. Each attenuation-corrected (AC) PET volume was evaluated and compared with the acquisition performed without the breast coil. The breast coil attenuated PET photons by 10% overall. Threshold values were applied to the SECT μ-map to reduce the effects of metal artifacts, but overcorrection occurred in more highly attenuated regions. The DECT-derived virtual monochromatic image reduced beam-hardening artifacts, but other metal artifacts remained. Despite the remaining metal artifacts in the DECT image, it led to an improvement in the more attenuated regions. The MVCT images appear to be free from metal artifacts leading to an artifact-free μ-map and a further improvement AC-PET images. Our MVCT-based approach for creating μ-maps for MR RF coils greatly reduces artifacts produced by metal in a SECT approach. This eliminates the need for other artifact reduction methods, including the application of a threshold of narrow beam attenuation coefficients, or disassembling hardware to remove high-Z components before imaging with a kilovoltage source.

Measurement of in-air output ratios using different miniphantom materials

In-air output ratios, Sc, were measured using miniphantoms made of PMMA (thickness 2.4–24 g cm−2), graphite (1.8–26.5 g cm−2), copper (1.6–23.3 g cm−2) and lead (2.3–21.6 g cm−2), for collimator settings of 3 × 3 to 40 × 40 cm2, and x-ray energies of 6 MV and 15 MV, respectively. The effects of the miniphantom on Sc were quantified as correction factors as functions of collimator setting, material types and miniphantom thickness for each photon energy to correct the measured values. For miniphantoms with sufficient thickness to eliminate electron disequilibrium, the total correction factors can be expressed as multiplications of three factors: the attenuation correction factor, the mass energy absorption correction factor and the phantom scatter correction factor. This formalism implies that the collimator setting dependence of the correction factor is mainly caused by the energy spectrum shift. The narrow-beam attenuation coefficients in various phantom materials for different collimator settings were determined in narrow-beam geometries using a specially constructed collimator mounted on the tray holder of the accelerator. We have determined that the maximum total correction factor is approximately 1.01. For miniphantoms made of PMMA, graphite, copper and lead, at the miniphantom thickness of 10 g cm−2, the maximum total correction factors are 1.002, 1.003, 1.005, 1.007, and 1.002, 1.003, 1.008 and 1.009 for 6 MV and 15 MV, respectively.

Dose rate calculations around 192Ir brachytherapy sources using a Sievert integration model

The classical Sievert integral method is a valuable tool for dose rate calculations around brachytherapy sources, combining simplicity with reasonable computational times. However, its accuracy in predicting dose rate anisotropy around 192Ir brachytherapy sources has been repeatedly put into question. In this work, we used a primary and scatter separation technique to improve an existing modification of the Sievert integral (Williamson's isotropic scatter model) that determines dose rate anisotropy around commercially available 192Ir brachytherapy sources. The proposed Sievert formalism provides increased accuracy while maintaining the simplicity and computational time efficiency of the Sievert integral method. To describe transmission within the materials encountered, the formalism makes use of narrow beam attenuation coefficients which can be directly and easily calculated from the initially emitted 192Ir spectrum. The other numerical parameters required for its implementation, once calculated with the aid of our home-made Monte Carlo simulation code, can be used for any 192Ir source design. Calculations of dose rate and anisotropy functions with the proposed Sievert expression, around commonly used 192Ir high dose rate sources and other 192Ir elongated source designs, are in good agreement with corresponding accurate Monte Carlo results which have been reported by our group and other authors.

Transmission-based scatter correction of 180 degrees myocardial single-photon emission tomographic studies.

Meaningful comparison of single-photon emission tomographic (SPET) reconstructions for data acquired over 180 degrees or 360 degrees can only be performed if both attenuation and scatter correction are applied. Convolution subtraction has appeal as a practical method for scatter correction; however, it is limited to data acquired over 360 degrees. A new algorithm is proposed which can be applied equally well to data acquired over 180 degrees or 360 degrees. The method involves estimating scatter based on knowledge of reconstructed transmission data in combination with a reconstructed estimate of the activity distribution, obtained using attenuation correction with broad beam attenuation coefficients. Processing is implemented for planes of activity parallel to the projection images for which a simplified model for the scatter distribution may be applied, based on the measured attenuation. The appropriate broad beam (effective) attenuation coefficients were determined by considering the scatter buildup equation. It was demonstrated that narrow beam attenuation coefficients should be scaled by 0.75 and 0.65 to provide broad beam attenuation coefficients for technetium-99m and thallium-201 respectively. Using a thorax phantom, quantitative accuracy of the new algorithm was compared with conventional transmission-based convolution subtraction (TDCS) for 360 degrees data. Similar heart to lung contrasts were achieved and correction of 180 degrees data yielded a 10.4% error for cardiac activity compared to 5.2% for TDCS. Contrast for myocardium to ventricular cavity was similarly good for scatter-corrected 180 degrees and 360 degrees data, in contrast to attenuation-corrected data, where contrast was significantly reduced. The new algorithm provides a practical method for correction of scatter applicable to 180 degrees myocardial SPET.

Effects of scatter generated by beam‐modifying absorbers in megavoltage photon beams

Transmission through a beam-modifying absorber consists of attenuated primary beam and scattered radiation generated by the absorber. The primary component of the transmitted beam is characterized by the narrow beam attenuation coefficient which depends upon the energy of the beam and type of the absorber. In addition to beam energy and absorber material, the scatter component also depends on field size, thickness and shape of the absorber, location of the absorber with respect to the source, and the point of calculation. Based upon Compton first-scatter, a method has been developed to calculate effective broad beam transmission through any arbitrarily shaped absorber with variable thickness for any points on and off the central axis. The method requires predetermined narrow beam attenuation coefficients as a function of thickness. Transmission calculations for various absorbers such as wedges and attenuators were performed for cobalt-60 and 6-MV beams and were compared with measured data. For a cobalt-60 beam, the measured transmission fraction through a 1.33-cm-thick absorber (alloy, consisting of 55% bismuth and 45% lead) for a field size of 24 x 24 cm2 is 17% higher than the calculated value using a narrow beam attenuation coefficient. Also, for the same absorber, measured central axis transmission is as much as 3.6% higher compared to off-axis locations. The measured transmission fraction through a 1.33-cm absorber was found to differ by as much as 13% and 14% for Cobalt-60 and 6 MV, respectively, as the chamber-to-source distance was varied from 70 to 110 cm. The agreement between calculated and measured values is within 0.5% for both energies whereas conventional narrow beam calculations would have yielded errors of 18% and 19%, respectively. Similar agreement was obtained when comparing calculated and measured wedge factors as a function of field size, with the maximum deviation being 0.7%. Measured scattered doses, due to an attenuator covering part of a beam, show a maximum for a thickness of approximately one mean-free path. This is also predicted by calculations with an agreement of 0.3%.

A dosimetric investigation of scatter conditions for dual asymmetric collimators in open fields.

An investigation of the scattering effects of dual asymmetric collimators on open fields is presented. It has been determined that the output measured in air and in a phantom is radially symmetric and that the scatter as a function of field size is independent of field center offset, for a wide variety of square and rectangular field shapes. The analysis has led to a new and simplified method to represent primary off-center ratios (POCRs), which requires a minimum set of symmetric field data, including beam profiles, collimator and phantom scatter factors, and tissue maximum ratios. The field edge correction (FEC) method proposed has been shown to accurately model the POCRs to a distance of 17.5 cm from the collimator rotation axis in a plane perpendicular to the isocenter and at depths up to 30 cm without the need for extrapolation of the POCRs or special measurements setups, which may be required for POCR modeling techniques that measure narrow beam attenuation coefficients.

Implementation of a three-dimensional compensation system based on computed tomography generated surface contours and tissue inhomogeneities.

A computed tomography (CT) based system that compensates for patient surface contour and internal tissue inhomogeneity was implemented in our clinic. The compensators are fabricated with a mixture of tin granules and bee's wax. The tin/wax mixture was optimized for tin granule size and tin granule to wax ratio. The narrow beam attenuation coefficients were measured for 4-, 6-, 10-, and 24-MV photon beams. The compensator design and fabrication methodology were verified by measuring the dose distribution for a known surface contour irradiated with a compensated beam and for a known inhomogeneity that was submerged in a water phantom and irradiated with a compensated beam. For the surface contour, the uncompensated isodose levels varied by as much as 10% in the compensation plane and the compensator restored the isodose level to a variation of less than 1.3%. Measured and calculated doses for this surface contour were found to differ by less than 3.4%. For the inhomogeneity, the uncompensated isodose levels varied by 27% in the compensation plane and the compensator restored the isodose level to a variation of less than 1.5%. Measured and calculated doses for the known inhomogeneity were found to differ by less than 2%. Measurements of depth-dose curves indicate that the presence of the compensator in the beam does not significantly increase the surface dose. Twenty-six compensators have now been fabricated for clinical cases. In these patients, dose variations as great as 19% occurred in the plane of compensation prior to placing the compensator in the beam.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of gypsum attenuators for radiotherapy dose modification.

Gypsum has significant value as a compensator material for use with high energy x rays in radiation therapy. Compensator thickness may be calculated by using an effective attenuation coefficient (mu eff). Detailed measurements using narrow and broad beam geometry were collected to determine this effective attenuation coefficient as a function of energy (4-15 MV), field size, depth in tissue, and thickness of the compensator. An effective attenuation coefficient relation was defined using a least-square method. It was then determined that extrapolating the broad beam data to a 0 x 0 cm2 field size resulted in a good approximation to the measured narrow beam attenuation coefficient. The variations in surface dose produced by gypsum attenuators were compared to open beam results. For the energies studied, it was determined that the increase in surface dose was acceptable for clinical application.

Measurement of narrow beam attenuation coefficients using a broad beam geometry configuration for 145.4 keV photons

Narrow beam attenuation coefficients, μ ρ , of 145.4 keV γ-rays, emitted from the 141Ce radioisotope have been measured in various high Z materials such as Pb; (CH 3COO) 2. Pb · 3H 2O; Pb(NO 3) 2; PbO 2; PbO; Na 2 WO 4 · 2H 2O; BiPO 4 for the first time using a broad beam geometrical configuration, employing a 5.10 cm × 2.54 cm NaI (T1) detector spectrometer system coupled to a 1 K multichannel analyser. The measured μ ρ values, in cm 2/g for the above materials in the transmission range from 50% to 20% respectively are 2.12 ± 0.02 (2.2); 1.25 ± 0.03 (1.27); 1.40 ± 0.01 (1.43); 1.78 ± 0.06 (1.92); 1.96 ± 0.03 (2.05); 0.97 ± 0.02 (1.04) and 1.52 ± 0.03 (1.6). The corresponding theoretical values estimated from the mixture rule using tabulated values of Hubbell (1982), are given in parentheses. It is observed that for high Z compounds at 145.4 keV energy, the exponential law of attenuation will be valid even for a broad beam geometry configuration in the transmission range from 50% to 20%. The attenuation coefficients of elements derived from this method were compared with the tabulated narrow beam attenuation coefficients indicating the validity of the mixture-rule in the above region of transmission.

Determination of narrow beam attenuation coefficients from a broad beam geometrical configuration for 320 keV photons

Narrow beam mass attenuation coefficients, μ/ρ, of 320 keV photons, emitted from the 51Cr radioisotope have been measured in various high Z materials such as Pb; BiPO 4; (CH 3COO) 2·Pb·3H 2O; Pb(NO 3) 2; Na 2WO 4·2H 2O, using a broad beam geometrical configuration, employing a 2′' × 1′' (5.1 cm × 2.54 cm) (0.051 m × 0.025 m) NaI (Tl) detector spectrometer system coupled to a 1 k multichannel analyser. The measured μ/ρ values in cm 2/g for the above materials in the transmission range from 50% to 20% respectively are 0.34 ± 0.01 (0.35); 0.295 ± 0.002 (0.30); 0.23 ± 0.01 (0.24); 0.256 ± 0.003 (0.26); 0.255 ± 0.003 (0.256). The values given in parentheses are estimated theoretically from the mixture rule using tabulated values of Hubbell (1982). It is observed that for high Z compounds at low energies, the exponential law of attenuation will be valid even for a broad beam set up in the transmission range from 50% to 20%. The attenuation coefficients of elements derived from this method agree fairly well with the tabulated narrow beam attenuation coefficients indicating the validity of the mixture rule in this region of transmission.

Monte Carlo calculations of scatter to primary ratios for normalisation of primary and scatter dose

The separation of total absorbed dose into primary and scatter components is a commonly used technique in photon dose calculations. The primary dose component can be characterised by a measured narrow beam attenuation coefficient and a single normalisation value which establishes the relative proportion of the primary to the total dose at some reference depth and field size. The determination of this normalisation value from measured data requires an extrapolation of measured values for finite field sizes to obtain a zero field size value. The authors have used Monte Carlo simulations to score primary and scatter dose for photon beams of 4, 6, 10, 15 and 24 MV and report values of the scatter to primary ratio at the depth of dose maximum for the circular equivalent of a 10 cm*10 cm field. These values have an uncertainty of less than 1% and can be used in lieu of extrapolation of measured data to establish the relative magnitude of the primary dose for a wide range of photon beam energies.

Broad beam and narrow beam attenuation in Lipowitz's metal.

Attenuation properties of Lipowitz's metal have been studied for narrow and broad beams of cobalt-60 gamma rays and 4-10 MV x-rays. The measured transmitted fraction for geometries used in radiotherapy depends on the field size and depth of measurement. Therefore a calculation of dose for partially attenuated beams based on narrow beam attenuation coefficients can cause large errors in dosimetry. Our simple calculation of transmitted fractions based on primary attenuation and scattered radiation agrees quite well with the measured data for therapeutic geometries. Also given is a table for linear, mass attenuation, and mass energy absorption coefficients of Lipowitz's metal in the photon energy range from 10 keV to 10 MeV.

The selection of stopping power and mass energy absorption coefficient data for the HPA Code of Practice for dosimetry

All the recent protocols for high-energy X-ray dosimetry, introduced by the AAPM (1983), NACP (1980) and the HPA (1983), result in absorbed dose conversion factors significantly different from those that have been in use since 1969 when ICRU Report 14 was published. Clearly for all protocols, general or specific, it is necessary to select appropriate stopping power and mass absorption coefficient ratios for a range of X-ray beam qualities. Cunningham and Shultz (1984) have recently described a method of selecting these ratios, taking into account differences in spectral properties of radiation beams rather than the peak accelerating voltage which is not an adequate index of radiation quality. A table is given showing quality of X-ray or gamma -ray radiation versus narrow beam attenuation coefficient in water.

Attenuation of 5.9 keV Photons by Air

The narrow beam attenuation coefficient of air was measured at 5.895 keV. An 55Fe source of Mn x rays with a CrKβ filter and a collimated low-energy sodium iodide counter were used. The photon intensity as a function of the source-detector distance was determined in vacuum and in atmospheric air. The value of the measured coefficient is 24.55±0.25 cm2/g.

Transmission spectra and x-ray attenuation coefficients of alkali-halide crystals

It is shown that the spectrum of x-rays is altered by transmission through matter. The mean quantum energy is increased by removal of the soft component. It is possible to use the narrow-beam attenuation coefficient as an integral characteristic of the radiation instead of spectra. This coefficient as a function of absorber thickness decreases as the thickness increases, the measured rate of decrease being proportional to the rate of increase in the mean energy calculated from spectra recorded with a scintillation counter. The broad-beam attenuation coefficient increases with the thickness of the absorber because such a beam accumulates secondary radiation more rapidly than it loses primary radiation. Hence the mean energy may decrease as the thickness increases.