Volume 27, Issue 9, September 2000
 RADIATION PROTECTION

 Point/Counterpoint

An occupancy factor of unity should always be used for waiting rooms and other highlyoccupied public areas
View Description Hide Description  Top
 IMAGING PHYSICS


Xray videodensitometric methods for blood flow and velocity measurement: A critical review of literature
View Description Hide DescriptionBlood flow rate and velocity are important parameters for the study of vascular systems, and for the diagnosis, monitoring and evaluation of treatment of cerebro and cardiovascular disease. For rapid imaging of cerebral and cardiac blood vessels, digital xray subtraction angiography has numerous advantages over other modalities. Roentgenvideodensitometric techniques measureblood flow and velocity from changes of contrast material density in xray angiograms. Many roentgenvideodensitometric flow measurement methods can also be applied to CT, MR and rotational angiographyimages. Hence, roentgenvideodensitometric blood flow and velocity measurement from digital xray angiograms represents an important research topic. This work contains a critical review and bibliography surveying current and old developments in the field. We present an extensive survey of Englishlanguage publications on the subject and a classification of published algorithms. We also present descriptions and critical reviews of these algorithms. The algorithms are reviewed with requirements imposed by neuro and cardiovascular clinical environments in mind.

Localization of cerebral arterovenous malformations using digital angiography
View Description Hide DescriptionSince 1989 we performed stereotactic radiotherapy treatments of cerebral arterovenous malformations (AVM), estimating threedimensional (3D) localization and shape of target volumes by the Leksell stereotactic helmet on two orthogonal radiographic projections. Due to the limitations of this method, we developed a new technique for the localization of the target volume using digital subtraction angiography(DSA) and digital image processing. To achieve this result we first developed a method to correct nonlinear distortion of DSAimages using spatial relocation of image pixels based on a calibration grid. We then developed an algorithm for localization of the target volume using two independent DSA projections. Target volume coordinates in the helmet system are calculated using two DSA acquisitions taken with a free angle (∼90°), one in the AP and the other in the LL direction. The helmet can be freely positioned between the xray source and the image plane. The projections of eight reference points inserted in the helmet at a known location, are used to calculate the transformation matrix between the two coordinate systems. We performed numerical and experimental validation of the system. A hypothetical random error (up to 2 mm) on image coordinates of the reference points allowed to determine that the error in target localization was less than 0.2 mm. Using DSAimages of target points with a known location within a phantom, the error between calculated and actual location was, on average, 0.30±0.13 mm (mean±SD), with a maximum error of 0.49 mm. The results of numerical and experimental validations show that the system we have developed allows fast and accurate localization of the center of the target volume and it is suitable for efficient guiding during stereotactic radiosurgery of AVM.

Fast reconstruction with uniform noise properties in halfscan computed tomography
View Description Hide DescriptionThe hybrid algorithms developed recently for the reconstruction of fanbeam images possess computational and noise properties superior to those of the fanbeam filtered backprojection (FFBP) algorithm. However, the hybrid algorithms cannot be applied directly to a halfscan fanbeam sinogram because they require knowledge of a fullscan fanbeam sinogram. In this work, we developed halfscanhybrid algorithms for image reconstruction in halfscan computed tomography(CT). Numerical evaluation indicates that the proposed halfscanhybrid algorithms are computationally more efficient than are the widely used halfscanFFBP algorithms. Also, the results of quantitative studies demonstrated clearly that the noise levels in imagesreconstructed by use of the halfscanhybrid algorithm are generally lower and spatially more uniform than are those in imagesreconstructed by use of the halfscanFFBP algorithm. Such reduced and uniform imagenoise levels may be translated into improvement of the accuracy and precision of lesion detection and parameter estimation in noisy CTimages without increasing the radiation dose to the patient. Therefore, the halfscanhybrid algorithms may have significant implication for image reconstruction in conventional and helical CT.

Quantitative in vivo analysis of the kinematics of carpal bones from threedimensional CT images using a deformable surface model and a threedimensional matching technique
View Description Hide DescriptionThe purpose of this study was to obtain quantitative information of the relative displacements and rotations of the carpal bones during movement of the wrist. Axial helical CT scans were made of the wrists of 11 volunteers. The wrists were imaged in the neutral position with a conventional CT technique, and in 15–20 other postures (flexion–extension, radial–ulnar deviation) with a lowdose technique. A segmentation of the carpal bones was obtained by applying a deformable surface model to the regulardose scan. Next, each carpal bone, the radius, and ulna in this scan was registered with the corresponding bone in each lowdose scan using a threedimensional matching technique. A detailed definition of the surfaces of the carpal bones was obtained from the regulardose scans. The lowdose scans provided sufficient information to obtain an accurate match of each carpal bone with its counterpart in the regulardose scan. Accurate estimates of the relative positions and orientations of the carpal bones during flexion and deviation were obtained. This quantification will be especially useful when monitoring changes in kinematics before and after operative interventions, like miniarthrodeses. This technique can also be applied in the quantification of the movement of other bones in the body (e.g., ankle and cortical spine).

Evaluation of an algorithm for the assessment of the MTF using an edge method
View Description Hide DescriptionAn algorithm to calculate the presampling modulation transfer function(MTF) of an imaging system from an angled edge image has its own inherent transfer function. Factors such as the angle of the sampling aperture to the edge, registration of edge function profiles using the determined edge angle, differentiation, smoothing, and folding all combine to produce the frequency response of the algorithm. In this work, the profile registration transfer function accounting for an error in the determined edge angle has been derived. This has been incorporated with other, previously reported, algorithm component transfer functions to fully characterize the MTF calculation algorithm. When registering profiles, small errors in the edge angle determination were found to result in large errors in the MTF, as the misalignment errors increase with the number of profiles. For example, registering 50 profiles a 0.07 degree error in a 7 degree edge angle (1% error) produces a 36% error in the MTF at the system cutoff frequency when profiles are oversampled at a frequency is defined as the maximum frequency reproducible without aliasing when sampling at the limiting system Nyquist frequency These results highlight the importance of quantifying the transfer function of the algorithm used to determine an imaging system modulation transfer function. The MTF calculation algorithm and the transfer function analysis have been incorporated into a Windowsbased software program to be made available for general use.
 Top

 MAGNETIC RESONANCE IMAGING


Analytic reconstruction of magnetic resonance imaging signal obtained from a periodic encoding field
View Description Hide DescriptionWe have proposed a twodimensional PERiodicLinear (PERL) magnetic encoding field geometry and a magnetic resonance imaging pulse sequence which incorporates two fields to image a twodimensional spin density: a standard linear gradient in the x dimension, and the PERL field. Because of its periodicity, the PERL field produces a signal where the phase of the two dimensions is functionally different. The x dimension is encoded linearly, but the y dimension appears as the argument of a sinusoidal phase term. Thus, the timedomain signal and image spin density are not related by a twodimensional Fourier transform. They are related by a onedimensional Fourier transform in the x dimension and a new Bessel function integral transform (the PERL transform) in the y dimension. The inverse of the PERL transform provides a reconstruction algorithm for the y dimension of the spin density from the signal space. To date, the inverse transform has been computed numerically by a Bessel function expansion over its basis functions. This numerical solution used a finite sum to approximate an infinite summation and thus introduced a truncation error. This work analytically determines the basis functions for the PERL transform and incorporates them into the reconstruction algorithm. The improved algorithm is demonstrated by (1) direct comparison between the numerically and analytically computed basis functions, and (2) reconstruction of a known spin density. The new solution for the basis functions also lends proof of the system function for the PERL transform under specific conditions.
 Top

 NUCLEAR MEDICINE IMAGING


Simultaneous iterative reconstruction for emission and attenuation images in positron emission tomography
View Description Hide DescriptionThe quality of the attenuation correction strongly influences the outcome of the reconstructed emission scan in positron emission tomography. Usually the attenuation correction factors are calculated from the transmission and blank scan and thereafter applied during the reconstruction on the emission data. However, this is not an optimal treatment of the available data, because the emission data themselves contain additional information about attenuation: The optimal treatment must use this information for the determination of the attenuation correction factors. Therefore, our purpose is to investigate a simultaneous emission and attenuation image reconstruction using a maximum likelihood estimator, which takes the attenuation information in the emission data into account. The total maximum likelihood function for emission and transmission is used to derive a onedimensional Newtonlike algorithm for the calculation of the emission and attenuation image. Loglikelihood convergence, mean differences, and the mean of squared differences for the emission image and the attenuation correction factors of a mathematical thorax phantom were determined and compared. As a result we obtain images improved with respect to log likelihood in all cases and with respect to our figures of merit in most cases. We conclude that the simultaneous reconstruction can improve the performance of image reconstruction.
 Top

 MEDICAL ULTRASOUND


Partial wave analysis of the ultrasound comet tail artifact
View Description Hide DescriptionAn adaptation of linear acousticscattering theory is used to describe the comet tail artifact which is observed in the diagnosticultrasound imaging of small spherical masses. The calculated backscattered pressure wave is shown to correspond well with the appearance of the artifact in a variety of clinical images.
 Top

 RADIATION TREATMENT PHYSICS


Application of coloring theory to reduce intensity modulated radiotherapy dose calculations
View Description Hide DescriptionColoring theory is applied to reduce the dose calculations under intensity modulated radiotherapy.Intensity modulated radiotherapy varies the intensity profile across the beam. The beam face is divided into a panel of small squares or “bixels.” Each square may be opened or closed for different lengths of time by moving collimator leaves in and out of the beam. It has been shown that the distribution of dose from radiation directed through any open square depends on whether the adjacent squares are opened or closed. Taking the states of neighboring bixels into account greatly increases the required dose calculations. There are possible ways to select open or closed states for the eight neighbors of a given bixel. Each combination represents one coloring of a panel, and each coloring demands a separate dose calculation. The number of calculations is reduced by considering the symmetries of a square. The 256 possible colorings can be divided into 51 distinct patterns through application of Burnside’s theorem. Each pattern consists of selections of closed bixels that are the same except for a symmetric transformation of coordinates. If the symmetry between x and y coordinates is broken by collimator leaves whose ends and sides have different effects on bordering bixels, the number of patterns increases to 84. The theoretic gain in the number of calculations through the application of Burnside’s theorem is fivefold if bixel borders are symmetric, and threefold if the borders are asymmetric. The results are applied to examples of generated intensity maps. The symmetry rules divide the bixel arrangements into proportionately fewer patterns as the intensity maps become larger, allowing computational gains to be achieved.

Computer verification of fluence map for intensity modulated radiation therapy
View Description Hide DescriptionIn a treatment planning system for intensity modulated radiation therapy(IMRT), the time sequence of multileaf collimator(MLC) settings are derived from an optimal fluence map as a postoptimization process using a software module called a “leaf sequencer.” The dosimetric accuracy of the dynamic delivery depends on the functionality of the module and it is important to verify independently the correctness of the leaf sequences for each field of a patient treatment. This verification is unique to the IMRT treatment and has been done using radiographic film, electronic portal imaging device(EPID) or electronic imaging system (BIS). The measurement tests both the leaf sequencer and the dynamic multileaf collimator(MLC)delivery system, providing a reliable assurance of clinical IMRT treatment. However, this process is labor intensive and time consuming. In this paper, we propose to separate quality assurance (QA) of the leaf sequencer from the dynamic MLCdelivery system. We describe a simple computer algorithm for the verification of the leaf sequences. The software reads in the leaf sequences and simulates the motion of the MLC leaves. The generated fluence map is then compared quantitatively with the reference map from the treatment planning system. A set of predefined QA indices is introduced to measure the “closeness” between the computed and the reference maps. The approach has been used to validate the CORVUS (NOMOS Co., Sewickley, PA) treatment plans. The results indicate that the proposed approach is robust and suitable to support the complex IMRT QA process.

An optimized forwardplanning technique for intensity modulated radiation therapy
View Description Hide DescriptionIntensity modulated radiation therapy(IMRT) has stirred considerable excitement in the radiationoncology community. Its objective is to make the dose conform to the tumor and spare other organs. Instead of resorting to the rather complex inverseplanning, the technique described here is an extension of the conventional treatment planning technique. The beam orientation and wedge angles are chosen in the conventional rulebased manner. However, within each conformal beam’s eye view (BEV) field including margin, a number of subfield openings are added. The smaller field openings are designed to irradiate the tumor, while sparing the normal tissue of the organs at risk (OARs) that intrude into the target region in the BEV. As the number of intrusions into the target BEV increases, the number of subfields for each beam increases. The Cimmino simultaneous projection method was employed to obtain the optimized weighting for each field of each beam. In cases where the dose constraints for the tumor and for the OARs are reasonable, it is possible to obtain a plan with a fairly small number of beams that satisfies the specified dose objectives. This is illustrated for the treatment of prostate cancer, where the rectum creates a concavity in the planning target volume. An advantage of this technique is that the quality assurance for the delivery of these plans does not require extensive special efforts.

Estimating the dose variation in a volume of interest with explicit consideration of patient geometric variation
View Description Hide DescriptionA method to measure the effects of internal organ motion and deformation and patient setup error on cumulative dose variation in a volume of interest is proposed. The method uses multiple CT scans and electronic portal images of a single patient to numerically simulate dosevolume effects over the entire course of the patient’s external beam treatment. The results are expressed in the form of a novel dosevolume histogram, called an expected dosevolume histogram (EDVH).

A twostep algorithm for predicting portal dose images in arbitrary detectors
View Description Hide DescriptionRecently, portal imagingsystems have been successfully demonstrated in dosimetric treatment verification applications, where measured and predicted images are quantitatively compared. To advance this approach to dosimetric verification, a twostep model which predicts dose deposition in arbitrary portal image detectors is presented. The algorithm requires patient CT data, source–detector distance, and knowledge of the incident beam fluence. The first step predicts the fluence entering a portal imaging detector located behind the patient. Primary fluence is obtained through raytracing techniques, while scatter fluence prediction requires a library of Monte Carlogenerated scatter fluence kernels. These kernels allow prediction of basic radiation transport parameters characterizing the scatteredphotons, including fluence and mean energy. The second step of the algorithm involves a superposition of Monte Carlogenerated pencil beam kernels, describing dose deposition in a specific detector, with the predicted incident fluence. This process is performed separately for primary and scatter fluence, and yields a predicted doseimage. A small but noticeable improvement in prediction is obtained by explicitly modeling the offaxis energy spectrum softening due to the flattening filter. The algorithm is tested on a slab phantom and a simple lung phantom (6 MV). Furthermore, an anthropomorphic phantom is utilized for a simulated lung treatment (6 MV), and simulated pelvis treatment (23 MV). Data were collected over a range of air gaps (10–80 cm). Detectors incorporating both low and high atomic number buildup are used to measure portal image profiles. Agreement between predicted and measured portal dose is better than 3% in areas of low dose gradient (<30%/cm) for all phantoms, air gaps, beam energies, and detector configurations tested here. It is concluded that this portal dose prediction algorithm is fast, accurate, allows separation of primary and scatterdose, and can model arbitrary detectors.

Dependence of the tray transmission factor on collimator setting and source–surface distance
View Description Hide DescriptionWhen blocks are placed on a tray in megavoltage xray beams, generally a single correction factor for the attenuation by the tray is applied for each photon beam quality. In this approach, the tray transmission factor is assumed to be independent of field size and source–surface distance (SSD). Analysis of a set of measurements performed in beams of 13 different linear accelerators demonstrates that there is, however, a slight variation of the tray transmission factor with field size and SSD. The tray factor changes about 1.5% for collimator settings varying between 4×4 cm and 40×40 cm for a 1 cm thick PMMA tray and approximately 3% for a 2 cm thick PMMA tray. The variation with field size is smaller if the source–surface distance is increased. The dependence on the collimator setting is not different, within the experimental uncertainty of about 0.5% (1 s.d.), for the nominal accelerating potentials and accelerator types applied in this study. It is shown that the variation of the tray transmission factor with field size and source–surface distance can easily be taken into account in the dose calculation by considering the volume of the irradiated tray material and the position of the tray in the beam. A relation is presented which can be used to calculate the numerical value of the tray transmission factor directly. These calculated values can be checked with only a few measurements using a cylindrical beam coaxial miniphantom.

A noninvasive dose estimation system for clinical BNCT based on PGSPECT—Conceptual study and fundamental experiments using HPGe and CdTe semiconductor detectors
View Description Hide DescriptionA noninvasive method for measuring the absorbed dose distribution during the administration of clinical boronneutron capture therapy (BNCT) using an online threedimensional (3D) imagingsystem is presented. This system is designed to provide more accurate information for treatment planning and dosimetry. The singlephoton emission computed tomography (SPECT) technique is combined with prompt gammaray analysis (PGA) to provide an ideal dose estimation system for BNCT. This system is termed PGSPECT. The fundamental feasibility of the PGSPECT system for BNCT is confirmed under the following conditions: (1) a voxel size of 1×1×1 cm^{3}, comparable to the spatial resolution of our standard dosimetric technique using gold wire activation, where data are available for every 5–10 mm of wire length; (2) a reaction rate of within the measurement volume is greater than corresponding to a thermal neutron flux of and a concentration of greater than 10 ppm for the deepest part of the tumor volume under typical BNCT clinical conditions; (3) statistical uncertainty of the count rate for prompt gamma rays is 10% or less. The desirable characteristics of a detector for the PGSPECT system were determined by basic experiments using both HPGe and CdTesemiconductor detectors. The CdTesemiconductor detector has the greatest potential for this system because of its compactness and simplicity of maintenance.

Proton loss model for therapeutic beam dose calculations
View Description Hide DescriptionA transport algorithm called the proton loss (PL) model is developed for proton pencil beams of therapeutic energies. The PL model takes into account inelastic nuclear reactions, pathlength straggling, and energyloss straggling and predicts the 3D dose distribution from a proton pencil beam. In proton beams, the multiple scattering and ionizational energy loss processes approach their diffusional limit where scattering and energy loss probability densities become Gaussian. Therefore we chose to derive the PL model from the Fermi–Eyges diffusionalmultiple scatteringtheory and the Gaussian theory of energy straggling. We first introduce a generalization of the Fermi–Eyges equation for proton pencil beams, labeled the proton loss (PL) transport equation. This new equation includes terms that model inelastic nuclear reactions as a depthdependent absorption and pathlength straggling as a quasiabsorption. Then energy straggling is taken into account by using a weighted superposition of a discrete number of elementary pencil beams. These elementary pencil beams have different initial energies and lose energy according to the CSDA, thus they have different ranges of penetration. A final solution for the proton beam transport is obtained as a linear combination of elementary pencil beam solutions with weights defined by the Gaussian evolution of the proton energy spectrum with depth. A numerical comparison of the dose distribution predictions of the PL model with measurements and PTRAN Monte Carlo simulations indicates the model is both computational fast and accurate.

GammaPlan®—Leksell Gamma Knife® radiosurgery treatment planning verification method
View Description Hide DescriptionThis work provides a method for an independent check of Gamma Knife® GammaPlan® radiosurgery calculations, named the spherical approximation method or SAM. Based on skull dimension measurements, the treated volume of the head is modeled as a sphere of radius R. With this approximation, an analytical solution for fast ray tracing of the path length, for each of the 201 beamlets, of the Gamma Knife helmet collimator was possible. The dose rate at the focus of a single shot is the sum of the contributions of all active beamlets adjusted for both the collimator factor and attenuation. For an arbitrary point, the dose rate is derived at the beamlet level from the focus values adjusted for the new path length attenuation and the appropriate collimators’ offaxis profiles. The sum over all beamlets’ contributions gives the dose rate at that particular point. At the single shot level, SAM independent check results agree with the GammaPlan® for patient calculations to better than ±6% and, as expected, in spherical phantoms the agreements improve to better than ±1.0%. For an arbitrary point, multishot procedure, the agreement is better than ±3% and ±1.5, respectively.

Red marrow dosimetry for radiolabeled antibodies that bind to marrow, bone, or blood components
View Description Hide DescriptionHematologic toxicity limits the radioactivity that may be administered for radiolabeled antibody therapy. This work examines approaches for obtaining biodistribution data and performing dosimetry when the administered antibody is known to bind to a cellular component of blood, bone, or marrow. Marrow dosimetry in this case is more difficult because the kinetics of antibody clearance from the blood cannot be related to the marrow. Several approaches for obtaining antibody kinetics in the marrow are examined and evaluated. The absorbed fractions and S factors that should be used in performing marrow dosimetry are also examined and the effect of including greater anatomical detail is considered. The radiobiology of the red marrow is briefly reviewed. Recommendations for performing marrow dosimetry when the antibody binds to the marrow are provided.

Accuracy in catheter reconstruction in computed tomography planning of high dose rate prostate brachytherapy
View Description Hide DescriptionIn high dose rate prostate brachytherapy, inadequate reconstruction of catheter geometry in treatment planning may result in erroneous dose delivery. Catheters may be digitized with: (1) Parallel reconstruction: digitized at only one point and assumed parallel and horizontal; (2) Straight reconstruction: digitized at both ends and assumed straight while at an angle; (3) Slicebyslice reconstruction: digitized on all slices to obtain exact geometry. Our results show that individual catheters are often not parallel to each other, but fairly straight. Parallel reconstruction is the least accurate for dosimetric planning, while slicebyslice reconstruction is timeconsuming. Straight (twopoint) reconstruction represents a balance between accuracy and efficiency.
