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Volume 21, Issue 6, June 1994

Analysis of the spatial‐frequency‐dependent DQE of optically coupled digital mammography detectors
View Description Hide DescriptionThe effect of optical coupling efficiency on the spatial‐frequency‐dependent propagation of signal and noise is considered for x‐ray image detectors for digital mammography in which a phosphor screen is optically coupled to a charge‐coupleddevice(CCD)image array. For experimental purposes, optical coupling between a Gd_{2}O_{2}S:Tb phosphor screen and a CCDimage array was provided by relay lenses. Neutral density filters were inserted between the lenses to vary the optical coupling efficiency without altering the inherent spatial resolution. The total coupling efficiency, defined as the number of electrons (e ^{−}) recorded in the CCD per x‐ray interaction in the phosphor, was calculated in each case. The modulation transfer function, and the contributions to the total noise power spectrum (NPS) of x‐ray quantum noise, secondary quantum noise, and inherent detectornoise were measured as a function of coupling efficiency. These data were used to calculate the spatial‐frequency‐dependent detective quantum efficiency [DQE(f)]. The NPS due to x‐ray quantum noise had a significant spatial‐frequency dependence for coupling efficiencies of more than 9 e ^{−} per x‐ray interaction, but little spatial‐frequency dependence for coupling efficiencies of less than 2 e ^{−} per x‐ray interaction. These results indicate that to preserve high spatial‐frequency values of DQE(f), and to ensure that images are x‐ray quantum‐noise limited at high spatial frequencies, a coupling efficiency on the order of 10 e ^{−} per x‐ray interaction is required. This is contrary to the common belief, based on zero spatial‐frequency analysis, that a coupling efficiency on the order of 1 e ^{−} per x‐ray interaction is sufficient to acquire x‐ray quantum‐noise‐limited images.

A laboratory CT scanner for dynamic imaging
View Description Hide DescriptionA high‐resolution laboratory CTscanner has been developed for imaging objects undergoing periodic motion. The scanner comprises an x‐ray image intensifier, optically coupled to a linear photodiode array. Gated time‐evolved projections of a single slice of the moving object are acquired, reformatted, and reconstructed. The resulting series of CTimages shows the object at different phases of its motion cycle. The scanner has an adjustable field of view (FOV) and the resolution can be as high as 3.2 mm^{−1} (for the 40‐mm FOV). The spatial resolution depends on the inherent resolution of the scanner and on the object’s velocity. For objects moving at 1 cm s^{−1}, the spatial resolution is reduced by 9% in the direction of motion. The signal intensity in the reconstructed image is linear for materials with attenuation coefficients as high as 1.5 cm^{−1} (for a 90‐kVp x‐ray beam), with an average accuracy of ±0.02 cm^{−1}. The average accuracy of circumference measurements made from the CTimages is ±0.3 mm. Lastly, an application of this dynamic CTscanner to imaging excised human arterial specimens under simulated physiological pressure conditions is presented as an example.

Evaluation of new algorithms for the interactive measurement of surface area and volume
View Description Hide DescriptionThe maximum unit normal component (MUNC) method used for surface area measurement and the divergence theorem algorithm (DTA) used for volume measurement were evaluated. The accuracy and precision of these methods were investigated at varying signal‐to‐noise ratios (SNRs), sampling, spatial averaging, and orientation. The accuracy of the MUNC measured surface area, as indicated by the mean error, was 2.0% for seven spherical samples, with SNRs ranging from 5:1 to 39:1. The precision, as indicated by the percent coefficient of variation (% CV) for these samples, was less than 3.0%. Likewise, the accuracy and precision of the DTA measured volume for these samples were both less than 1.0%. MUNC surface area measurement from 23 samples of a computed tomography(CT)image of a wooden sphere (51.44‐mm diameter) with x,y voxel size ranging from 1 to 10 mm and z voxel size ranging from 2 to 14 mm yielded an accuracy of 1.3% and a precision of 2.2%. The DTA volume measurements from 18 samples of the wooden sphere with x,yvowel size ranging from 1 to 8 mm and z size ranging from 2 to 14 mm provided an accuracy of 1.2% and a precision of 1.8%. Measurement of surface area for a cylindrical rod scanned by CT in five different orientations, ranging from along each axis to between all three axes, yielded an accuracy of 3.7% and a precision of 2.0%. The volume of the cylindrical rod measured by the DTA method for these orientations produced an accuracy of 4.0% and a precision of 3.7%. The volume measured by DTA compared well with the volume measured by a modified voxel counting method. The MUNC surface area method was superior to counting surface voxels. The accuracy and precision for five interactive surface area and volume measurements, using paired cut planes to select subsets of a computer‐generated sphere with radius 25 pixels, were both less than 1.0%.

Theoretical FWTM values in helical CT
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An objective method for evaluating electronic portal imaging devices
View Description Hide DescriptionA statistical technique is described for analyzing the image of a contrast‐detail phantom acquired by a radiotherapyelectronic portal imaging device. By using Student’s t test, the image analysis algorithm computes which holes in the contrast‐detail phantom can be objectively resolved from the background signal. The specific pattern of the resolved holes and the total number of the resolved holes characterize the performance of the electronic portal imaging device.Image processing techniques are used to align the holes automatically. Because the use of this algorithm is completely automated, the procedure removes operator subjectivity from the evaluation.

Pseudocorrelation: A fast, robust, absolute, grey‐level image alignment algorithm
View Description Hide DescriptionA new image alignment algorithm—pseudocorrelation—has been developed based on the application of Monte Carlo techniques to the calculation of a cross‐correlation integral for grey‐scale images. It has many advantages over cross‐correlation: it is at least a factor of 10 faster than fast‐Fourier‐transform‐based cross‐correlation, and requires 8 times less memory. Its high speed allows for the search space of geometric transformations between images to include magnification and rotation as well as translations without the search time becoming too long. It allows noise to be taken into account, making calculation of a robust, absolute probability of good alignment possible. It is relatively insensitive to differences in quality between images. This article describes the pseudocorrelation algorithm in detail and presents the results of tests of the effects of contrast enhancement, resolution differences, and noise on the algorithm’s performance. These tests show that the algorithm is well suited to the task of automated alignment of very low contrast images from video electronic portal imaging devices.

Attenuation correction in PET using single photon transmission measurement
View Description Hide DescriptionThe use of single photontransmission measurement with a rotating rod source has been evaluated to measure the attenuation correction factors in positron emission tomography(PET). The singles projections are resampled into the coincidence geometry using the detector positions and the rod source location. A nonparalyzable dead time correction algorithm was developed for the block detectors used in the McMaster PET scanner. This enables accurate attenuation correction factors (ACFs) to be computed using a wide range of source strengths for transmission scanning. Transaxial resolution is approximately 6 mm, which is comparable to emission scanning performance. Axial resolution is about 25 mm, with only crude source collimation. ACFs are underestimated by as much as 10% due to increased cross‐plane scatter, compared to coincidence transmission accuracy. The response of the correction factors to object density is within 15%, when comparing singles transmission measurement to current coincidence transmission measurement. The major advantage of using singles transmission measurement is a dramatically increased count rate. A factor of 7 increase in count rate over coincidence scanning is possible with a 2‐mCi transmission rod source. Uniformity of 2% in the transmission images is possible with this source strength and a 2‐min acquisition. There are no randoms counted in singles transmission scans, which makes the measured count rate vary linearly with source activity. Singles detector dead time losses are approximately 6% in the detectors opposite a 2‐mCi rod source.

A superconducting cyclotron for neutron radiation therapy
View Description Hide DescriptionThe physical and clinical specifications of a neutron therapy facility utilizing a superconducting cyclotron are presented. The cyclotron and its support system are described. Details of the operation of the cyclotron in a hospital environment are given; the requirements of the helium liquifier and cryogenic system are described together with a summary of its mode of operation. The simplicity of the cyclotron control system is discussed. The physical characteristics of the neutron beam are described. The central axis percent depth dose curve is equivalent to that of a 4 MV x‐ray beam. The depth of maximum dose occurs at approximately 9 mm depth and the surface dose is between 40% and 45%. The multirod collimator allows for the production of irregularly shaped fields of size up to 26.5×30 cm, without excessive exposure to operating personnel.

Measurement of augmentation of ^{252}Cf implant by ^{10}B and ^{157}Gd neutron capture
View Description Hide Description^{252}Cf has been used as a brachytherapysource since the early 1970s. The dominant mechanism of interactions of ^{252}Cf neutrons with tissue is elastic scattering. The scatteredneutrons lose part of their energy, which is released as kinetic energy of the recoiling nuclei. By multiple scattering,neutrons lose their energy and eventually become thermalized (in energetic equilibrium with tissue atoms with an average energy of 0.025 eV) and do not play any role in radiotherapy. These thermal neutrons may interact with hydrogen nuclei or with nitrogen, but the cell killing effects by these reaction products are negligibly small compared to the elastic scattering by fast neutrons or by photons emitted by californium. Nonetheless, these thermal neutrons are still potentially usable for neutron capture therapy and can be used to enhance californiumbrachytherapy effects. Neutron capture therapy is a two‐part therapy relying on the selective loading of tumor cells with compounds containing ^{10}B or ^{157}Gd and subsequent irradiation with thermal neutrons. To calculate neutron capture doses one has to know thermal neutron flux. This paper presents results of an experimental study of thermal neutron flux and calculations of boron neutron capture and gadolinium neutron capture doses in the vicinity of ^{252}Cf sources.

Reducing electron contamination for photon beam‐quality specification
View Description Hide DescriptionThe percentage depth dose at 10 cm in a 10×10‐cm^{2}photon beam at an SSD of 100 cm, %dd(10), is a better beam‐quality specifier for radiotherapy beams than the commonly used values of or nominal accelerating potential. The presence of electron contamination affects the measurement of %dd(10) but can be removed by the use of a 0.1‐cm lead filter, which reduces surface dose from contaminant electrons from the accelerator by more than 95% for radiotherapy beams with energies from ^{60}Co to 50 MV. The filter performs best when it is placed immediately below the head. An electron‐contamination correction factor is introduced to correct for electron contamination from the filter and air. It converts the %dd(10) which includes the electron contamination with the filter in place [hereafter %dd(10)_{m}], into %dd(10) for just the photons in the filtered beam. The correction factor is a linear function of %dd(10)_{m} for all filtered beams with %dd(10)_{m}>70%. A small correction for the photon filtering effect converts the pure photon %dd(10) for the filtered beam into that for the unfiltered beam, which can be used to determine stopping‐power ratio. Calculations show that the values of water‐to‐air stopping power ratio in the unfiltered beam are related to the values of %dd(10)_{m} in the filtered beam by a cubic function. The uncertainty of stopping‐power ratios in unfiltered beams for the same value of the %dd(10)_{m} is within 0.2% for all beams.

On‐axis and off‐axis primary dose component in high energy photon beams
View Description Hide DescriptionThe depth dose of the primary dose component, on axis and off axis of six different x‐ray beams, has been determined from transmission measurements in narrow beam geometry with and without flattening filter using a Perspex column of a cross section large enough to ensure electronic equilibrium. In order to derive the primary photon fluence, a correction for the scatter from the column has been applied according to the following method: A number of spectra taken from the literature have been used for computing a scatter coefficient S _{ c } at different depths by convolution of dose spread arrays. Using the relationship between S _{ c } and the single attenuation coefficient μ_{ i } to represent each entire spectrum, it has been possible to correct the experimental transmission curves iteratively, until the corresponding values of μ were stabilized and representative of the primary. The measured attenuation coefficients were found to have a linear increase as a function of the distance from the central axis for all the energies and types of linear accelerators. For the same nominal energy, this increase is different from one accelerator to another. The same phenomenon was observed for the attenuation coefficients obtained without the flattening filter in the same experimental conditions. The results are tentatively interpreted considering the angular variation of bremsstrahlung energy spectra with and without a flattening filter as calculated by a Monte Carlo method and they are consistent and useful to take accurately into account the softening of the beam as the off‐axis distance increases.

On three‐dimensional dose calculation of photon beam with wedge filters
View Description Hide DescriptionA wedge filter is one of the most commonly used beam modifying devices. The introduction of a wedge filter alters both the distribution of the primary photon fluence and the first and second scatter patterns. The effect of the wedge filter is normally expressed in terms of an exponential attenuation function that is used to modify the primary photon fluence. This paper describes a new three‐dimensional (3‐D) wedged field dose calculation algorithm. This algorithm automatically takes into account (1) the scatter distribution pattern correction due to the effect of wedge filters and (2) the fixed primary photon fluence correction, by utilizing a tissue maximum ratio table calculated from the measured percent depth dose of the wedge, a measured in‐plane scan and a measured cross‐plane scan of the largest rectangular wedge field size. As a result, the calculation accuracy achieved by this algorithm is better than the calculation accuracy achieved by other algorithms. The results show better than 1% accuracy of the wedge central axis percent depth dose calculation and better than 3% accuracy of the off‐axis dose calculation when compared with measurements.

Diode dosimetry of ^{103}Pd model 200 seed in water phantom
View Description Hide DescriptionThe relative dose distribution around the ^{103}Pd model 200 implant seed was measured with a computerized data acquisition system employing a p‐n junction silicon diode immersed in a water phantom. Data are acquired in polar coordinates by computer control of (1) the diode distance from the seed center and (2) the rotation angle of seed about a transverse axis. Transverse axis data are compared with thermoluminescent dosimeter(TLD) measurements and a Monte Carlo calculation by others.

Dosimetric problems at low monitor unit settings for scanned and scattering foil electron beams
View Description Hide DescriptionElectron beamdosimetry at low monitor unit (MU) settings is important for dosimetric applications. Dose linearity, beam flatness, and beam energies were studied at low MU settings with various dose rates for different types of linear accelerators. It is observed that for the scattering foil units, the dose/MU is a smooth function of MU for all beam energies. Discrepancies in dose/MU are highest at the lowest MU. Significant variation (5%–245%) in dose linearity is observed among various linear accelerators at low MU settings. Dose rate has no effect on the dose linearity for all energies for the scattering foil units tested. On the contrary, for the scanned beam, there is no predictable pattern as dose/MU is random in nature and varies with time and beam energy. The maximum dosimetric error is observed for the highest energy beam where the beam width is most narrow. Using film, the beam uniformity was noticed to be very poor at low MU and high energy for scanned beams. The beam uniformity and dose linearity are random at low MU due to the random nature of the scan cycle. Under the adverse conditions, the deviation in dosimetric parameters was observed up to 200 MU.

An analytical expression for electron beam central axis depth doses
View Description Hide DescriptionAn analytical expression based on four fitting parameters is proposed for a mathematical description of electron beam central axis depth dose distributions. The expression approximates well the measured electron beam data in the field size range from 4×4 cm^{2} to 25×25 cm^{2} and in the energy range from 6 to 20 MeV in all four regions of the electron depth dose curve: build‐up, dose maximum, dose fall‐off, and bremsstrahlung contamination.

Obituary for Donald W. Kerst
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ICRU Report 50—Prescribing, Recording and Reporting Photon Beam Therapy
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