Index of content:
Volume 15, Issue 1, January 1988

Bulk flow model for multislice magnetic resonance imaging sequences with phantom validation
View Description Hide DescriptionA simple conceptual model for describing the effects of one‐dimensional bulk flow on image intensities is presented and validated using a multislice imaging sequence specific to one vendor. The model allows quantitative estimates of echo amplitudes versus velocity by using the equation of motion to follow the pulse history of fluid volumes influenced by the readout pulse for any slice of interest. Each affected volume is divided into equal elemental components and the state of each component is computed at discrete times in a pulse position‐timing diagram until readout occurs. The final amplitude for the composite volume is determined by the signed summation of each of the elemental components. Validation of the quantitative model was performed by imaging a rotating bulk flow phantom centered at each of the slices of a multislice, partial saturation, spin‐echo sequence. Effects due to rf field inhomogeneities were normalized by dividing the results of the dynamic scans by the corresponding static one. The results presented here are relatively insensitive to odd‐echo dephasing (even echo rephasing).

Total inhomogeneity correction including chemical shifts and susceptibility by view angle tilting
View Description Hide DescriptionA correction technique of the total magnetic field inhomogeneity effects including the localized object induced inhomogeneities such as chemical shift and susceptibility is developed and its usefulness is experimentally demonstrated. With this new and simple technique all the inhomogeneity induced artifacts can be corrected simultaneously. The basic idea of this method is to add a compensation gradient of the same amplitude as the selection gradient in simultaneity with the reading gradient in such a way that the view angle is tilted. Thereby all the inhomogeneity induced geometrical shifts and hence the intensity changes are corrected, since the addition of the compensation which is independent from the field inhomogeneities including both chemical shifts and susceptibility. This technique has been theoretically examined and its usefulness is demonstrated by experiments.

Solutions of two paramagnetic ions for use in nuclear magnetic resonance phantoms
View Description Hide DescriptionThe introduction of paramagnetic ions to affect relaxation times has been used in a variety of nuclear magnetic resonance(NMR) applications. All such relaxants used in NMR phantoms to date have consisted of a single paramagnetic ion. The disadvantage of this is that only one relaxation time can be adjusted as desired, either T _{1} or T _{2}. This study demonstrates that, by properly choosing two paramagnetic ions, it is possible to adjust both T _{1} and T _{2} independently over a wide range of values. Specifically, solutions of MnCl_{2} and NiCl_{2} were prepared that simultaneously matched target T _{1} and T _{2} values to within approximately 6%.

Energy deposition patterns in an amputated human lower leg heated with a miniannular phased array
View Description Hide DescriptionThe energy deposition patterns in both alcohol‐fixed and unfixed amputated human lower legs produced by a miniannular phased array (MAPA) applicator have been determined. The nontumor bearing portions of four human legs, amputated for therapeutic purposes, were heated within the MAPA. Experimental measurements of the time rate of temperature rise at many locations inside the leg (between 125 and 150) were transformed to specific absorption rate (SAR) values at each point. A simple model was developed which predicts the axial variations in SAR inside the heated limb based upon quantitative details of the leg’s geometry obtained from computerized tomography scans. The axial location of the region of maximum energy deposition was predicted by the model with a precision of approximately 1 to 2 cm. Significant time rate of temperature rise was measured inside the cortical portion of the tibia, while the temperature rise in the cancellous (marrow) portion of the tibia was negligible. The alcohol fixation process appears to have no significant effect on the energy deposition patterns within the various leg tissues.

An algorithm for three‐dimensional visualization of radiation therapy beams
View Description Hide DescriptionA computer algorithm to display radiation beams superimposed on three‐dimensional (3‐D) views of patient anatomy has been developed. It may be implemented as a postprocessing step to existing software for 3‐D presentation and display. The algorithm takes as input a shaded 3‐D view (reconstructed, for example, from computed tomography scans), together with the associated depth map, and generates as output an enhanced 3‐D view highlighting in color the visible points which lie within the projected beam outlines. The algorithm is independent of the method used to generate the 3‐D view (surface or volume rendering techniques may be used) and is independent of beam shape (beams may be modified with shielding blocks). It is not restricted to external surfaces and will correctly show radiation beams projected onto cut‐away views of internal organs. The method is illustrated by application to a tangential pair for breast malignancy, using 3‐D views generated with volume rendering software.

Slit design considerations for rotating‐aperture, scanning‐beam radiography
View Description Hide DescriptionThe rotating‐aperture wheel (RAW) scanning‐beam device is uniquely applicable for scatter elimination in short‐time, rapid‐sequence, and real‐time radiographicimaging because of the continuous rotary motion of its slit pattern. This rotary motion places special restrictions on the slit pattern design. Although simple sector‐shaped slits provide uniform primary transmission, they entail an unacceptable degree of slit widening on small‐diameter wheels. The use of multiple slit zones with slits of different angular width has reduced the extent of this widening on a prototype RAW; however, interzone boundary artifacts caused by differential primary x‐ray cutoff are apparent on some clinical images. This problem is eliminated with a unique spiral‐shaped aperture pattern which consists of slits of constant width and constant spacing. Each slit is radially continuous and provides uniform primary transmission without interzone artifacts. This spiral pattern satisfies all requirements and appears to be the pattern of choice for rotating‐aperture scanning‐beam radiography.

Characteristics of an ethylene–polyethylene high‐pressure ionization chamber and its potential for deriving radiation dose and quality information in neutron‐gamma radiation fields
View Description Hide DescriptionA recently developed method for measuringdose and quality parameters in mixed neutron‐gamma radiation fields is extended for neutron energy regions relevant to current neutron therapy. It is theoretically shown that if a polyethylene high‐pressure ionization chamber filled with a gas mixture of 95% ethylene and 5% carbon dioxide is used at suitable gas pressures, it may overcome the limitations found for a similar tissue‐equivalent chamber and measure both the gamma ray absorbed dose fraction and the mean incident neutron energy for neutrons between 0.1 and at least 30 MeV, if certain conditions exist. The simultaneous use of the polyethylene high‐pressure ionization chamber as a conventional neutrondosimeter also allows determination of both the neutron and gamma ray absorbed doses. Estimated accuracies are 15% for the mean incident neutron energy and 15%–30% for the doses determination. The present method may complement currently available techniques as it presents certain advantages in situations where the neutron spectrum is poorly known. If coupled with other dosimeters, the accuracy of the method can be improved.

The role of humidity and other correction factors in the AAPM TG‐21 dosimetry protocol
View Description Hide DescriptionA detailed derivation is presented of the formulas required to determine N _{gas} and D _{med} in the AAPM TG‐21 dosimetry protocol. This protocol specifies how to determine the absorbed dose in an electron or photonbeam when using exposure or absorbed dose calibrated ion chambers. It is shown that the expression given in TG‐21’s recent letter of clarification is incorrect. Accounting for humidity correctly increases, by 0.4%, all absorbed dose determinations using an exposure calibrated ion chamber. Taking into account other correction factors in the equation for exposure could also have varying, but significant effects (possibly over 1%). These are the stem scatter correction, the axial nonuniformity correction and the electrode correction for electrodes made of different materials from the wall. Attention is drawn to differences in the definitions of the exposure and absorbed dose calibration factors N _{ x } and N _{ D }, respectively, as supplied by the NBS and the NRCC.

The influence of beam parameters on percentage depth dose in electron arc therapy
View Description Hide DescriptionThe dependence of rotational or arc electron beam percentage depth doses on the depth of isocenter d _{ i } and nominal beam field width w is presented. A characteristic angle β, which uniquely depends on w and d _{ i }, is defined and the dependence of the radial percentage depth doses on angle β discussed. It is shown that the characteristic angle β concept can be used in clinical situations to predict the shape of the percentage depth dose curve when w and d _{ i } are known, or, more importantly, it can be used to determine the appropriate w when d _{ i } and the percentage depth dose characteristics are known.

Two‐dimensional dose distribution around a commercial ^{1} ^{2} ^{5}I seed
View Description Hide DescriptionThe Monte Carlo method was used to investigate the dose distribution around a 3M Company model 6711 ^{1} ^{2} ^{5}I seed immersed in a water phantom. Dose rate per unit activity data are presented as a matrix of 63 points surrounding the seed. Relative dose data are presented graphically for two mutually perpendicular directions and compared with the corresponding data for the only other ^{1} ^{2} ^{5}I seed currently available, the 3M Company model 6702 ^{1} ^{2} ^{5}I seed. The 6711 relative dose distribution decreases more rapidly with distance from the seed than does the 6702 relative dose distribution. Uncertainties in the 6711 seed dose distribution produced by end‐weld thickness variations were investigated and found to be substantial at certain points.

Problems with organic materials for magnetic resonance imaging phantoms
View Description Hide DescriptionOrganic liquids are frequently used as magnetic resonance(MR) phantom materials. It is often not appreciated that indirect spin–spin coupling can dramatically affect signal amplitudes in spin‐echo experiments. These effects are evidenced in three ways: (1) modulation of spin‐echo amplitudes; (2) decrease in measured T _{2}relaxation times; and (3) dependence of spin‐echo modulation and measured T _{2} values on the echo time (T _{ E }) in multiecho Carr–Purcell–Meibroom–Gill experiments. This paper illustrates these effects and warns against the inappropriate use of organic liquids as MR phantoms.

Dependence of noise on array width and depth in digital radiography
View Description Hide DescriptionMeasurements are reported for the image noise levels in digital radiographs using 256^{2}, 256^{2}‐zoom, 512^{2}, and 1024^{2} storage arrays. The noise was essentially the same with 256^{2}‐zoom or 512^{2} arrays, while it was 25% lower for a 256^{2} array, and only 15% larger for a 1024^{2} array than for a 512^{2} array. Compatibility of these results with different models is evaluated. Also the number of bits/pixel was varied to determine the effect on noise; noise before digitization ranged from 4.3 to 0.13 gray levels. The noise level was found to be increased only if the noise was less than approximately one‐half of a gray level.

Reconstruction for fan beam with an angular‐dependent displaced center‐of‐rotation
View Description Hide DescriptionA convolutional backprojection algorithm is derived for a fan beam geometry that has an angular‐dependent displacement in its center‐of‐rotation from the midline of the fan beam. In both x‐ray computed tomography and single photon emission computed tomography, misalignment can occur when the mechanical center‐of‐rotation is not colinear with midline of the fan beam. In some cases the shift in the center‐of‐rotation is constant for every angle, whereas, in other cases it varies with angular position. Standard reconstruction algorithms, which directly filter and backproject the fan beam data without rebinning into parallel beam geometry, have been derived for a geometry having its center‐of‐rotation at the midline of the fan beam. However, in the case of any misalignment of the center‐of‐rotation, if these conventional reconstruction algorithms are used to reconstruct the fan beam projections, structured artifacts and a loss of resolution will result. Simulations are performed that illustrate these artifacts and demonstrate how the new algorithm corrects for this misalignment. A method for estimating the parameters of the fan beam geometry, including the angular‐dependent shift in the center‐of‐rotation, is also described.

A simple isocenter checking procedure for radiotherapy treatment machines using the optical pointer
View Description Hide DescriptionTechniques are described for simple and convenient checks on the position and size of the isocenter of radiotherapytreatment machines by measuring the displacement of the optical pointer on a flat scale on the surface of the treatment couch.

Practical application of the differential Batho method for inhomogeneity correction on kerma in a photon beam
View Description Hide DescriptionThe Batho equation gives a satisfactory method to correct the dose for points in the electronic equilibrium region for a uniform slab of inhomogeneity in a photon beam. In spite of the many investigations, we believe no simple and adequate method has been found for routine clinical dose calculations which require dose correction of a small‐volume inhomogeneity in an arbitrary location. In the present report, we combine the values of the two calculation types of the differential Batho method, which we have developed previously, to give a new calculated value for the scatter perturbation due to an annulus of inhomogeneity. The coefficients in the combination, which we derived from a detailed analysis of the scatter perturbation, are simple geometrical ratios. The new calculated values are in good agreement with measured values. We believe this application of the differential Batho method can provide a practical and accurate method of correcting for inhomogeneities of any size and shape in clinical dose calculations.

Interface dose perturbation as a measure of megavoltage photon beam energy
View Description Hide DescriptionThe description of the quality of a photon beam has usually been characterized by a single value such as the half‐value layer, the effective attenuation coefficient, the percent depth dose, and most recently by the ionization ratio (IR). Although the IR is simple and easy to measure, it lacks sensitivity at photon energies above 10 MV. This paper describes a method based on dose perturbation at an interface and defines the forward dose perturbation factor (FDPF) as a measure of beam quality. Comparisons between the two methods are given for photon energies ranging from ^{6} ^{0}Co to 24 MV. The results show that the FDPF method is more sensitive to spectral changes at photon energies above 10 MV than the IR.

Characteristics of the high‐energy photon beam of a 25‐MeV accelerator
View Description Hide DescriptionThe CGR Saturne 25 is an isocentrically mounted standing wave medicallinear accelerator that produces dual‐energy photonbeams and a scanned electron beam with six selectable energies between 4 and 25 MeV. The highest energy photonbeam is nominally referred to as 23 MV. For this beam the mean energy of the accelerated electron beam on the 1.3 radiation length (4 mm) tungsten x‐ray target is found to be approximately 21 MeV, with the energy acceptance stated to be ±5%. The electron beam traverses a 270° bending magnet upstream of the x‐ray production target. The resulting bremsstrahlung beam passes through a combination steel and lead flattening filter, 4‐cm maximum thickness. Dosimetric data for the 23‐MV beam are presented with respect to rectangular field output factor, depth of maximum dose as a function of field size, surface and buildup dose, central axis percent depth dose, tissue‐phantom ratios, beam profile, applicability of inverse square, and block transmission. Some data are also presented on the effect of different flattening filter designs on apparent beam energy.

Output factor constituents of a high‐energy photon beam
View Description Hide DescriptionMeasurements designed to separate primary and various scatter components of central axis dose of the highest energy photon beam of the CGR Saturne 25 linear accelerator are described. This beam has an unusually large output variation with field size. The measurements are performed both in air and in a water phantom, with and without an aperture external to the collimator system. Results are presented in the form of relative output factors for different field sizes due to (i) flattening filter scatter, (ii) water phantom scatter, (iii) collimatorbackscatter into the monitor chamber, and (iv) collimator forward scatter onto the central axis. It is found that the flattening filter is the single largest scatter component, but that each of the other factors is significant in determining the output dose per monitor unit as a function of field size.

Dose computations for asymmetric fields defined by independent jaws
View Description Hide DescriptionAsymmetric fields defined by independent jaws can be used to split a beam or to match adjacent fields. We have extended a method originally developed for symmetric fields to calculate the dose for asymmetric fields. The dose to a point is computed as the product of the tissue maximum ratio (TMR), the off center ratio (OCR), and the inverse square factor. The TMR is computed from the measured central axis depth doses for symmetric fields. The OCR is obtained by multiplying the primary OCR (POCR) and the boundary factors (BF’s) for the four jaws. The POCR’s and BF’s were derived from measured beam profiles, which include the effect of off‐axis beam quality variations. Using this method, the beam profiles and isodose distributions for asymmetric fields of a 6‐MV accelerator were calculated and compared with the measured data. The agreement is within experimental errors both in the penumbra region and along the central ray of the asymmetric field.

Electron backscatter corrections for parallel‐plate chambers
View Description Hide DescriptionThe wall of an ionization chamber is commonly assumed to have a negligible effect on chamber response in electron beams. For cylindrical chambers with thin walls this assumption is valid. However, parallel‐plate chambers commonly possess large mechanical supports which may affect chamber response in a manner not accounted for in current dosimetry protocols. This is due to changes with energy in the relative backscattered electron fluence between chamber support and phantom materials. To investigate this effect, electron backscatter from low atomic number materials has been measured with electrons from 6 to 20 MeV. The effect of the diameter and thickness of the backscatteringmaterial has also been studied. Based on these data, Lucite and polystyrene chambers in water phantoms are expected to underrespond by 1% and 2% at 6 MeV. The expected underresponse decreases to 0.8% and 0.4% for polystyrene and Lucite at 12 MeV and is insignificant above 16 MeV. Two commercially available parallel‐plate chambers were compared with a cylindrical chamber in electron beams from 6 to 20 MeV. Using the 20‐MeV intercomparison, the expected chamber responses at the lower energies were calculated and compared with measurements. Both parallel‐plate chambers underresponded by approximately 1% at 6 MeV and 0.5% at 9 MeV which is qualitatively consistent with the electron backscatter data. Recommendations for minimizing electron backscatter effects through chamber design are discussed.