摘要:

A finite element reconstruction algorithm for optical data based on a diffusion equation approximation is presented. A frequency domain approach is adopted and a unified formulation for three combinations of boundary observables and conditions is described. A multidetector, multisource measurement and excitation strategy is simulated, which includes a distributed model of the light source that illustrates the flexibility of the methodology to modeling adaptations. Simultaneous reconstruction of both absorption and scattering coefficients for a tissue‐like medium is achieved for all three boundary data types. The algorithm is found to be computationally practical, and can be implemented without major difficulties in a workstation computing environment. Results using simulated data suggest that qualitative images can be produced that readily highlight the location of absorption and scattering heterogeneities within a circular background region of close to 4 cm in diameter over a range of contrast levels. Absorption images appear to more closely identify the true size of the heterogeneity; however, both the absorption and scattering reconstructions have difficulty with sharp transitions at increasing depth. Quantitatively, the reconstructions are not accurate, suggesting that absolute optical imaging involving simultaneous recovery of both absorption and scattering profiles in multicentimeter tissues geometries may prove to be extremely difficult.

ISSN:0094-2405    

DOI:10.1118/1.597488

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A Bayesian estimator has been developed as a paradigm for human observer performance in detecting lesions of unknown size in a uniform noisy background. The Bayesian observer used knowledge of the range of possible lesion sizes as a prior; its predictions agreed well with the results of a six‐observer perceptual study. The average human response to changes in collimator resolution, as measured by the detectability index,dA, was tracked by the Bayesian detector's signal‐to‐noise ratio (SNR) somewhat better than by two other estimation models based, respectively, on lesser and greater degrees of lesion size uncertainty. As the range of possible lesion sizes increased, the Bayesian detector's SNR decreased and the optimal collimator resolution shifted towards better resolution. An analytic approximation for the variance of lesion activity estimates (which included the same prior) was shown to predict the variance of the Bayesian estimator over a wide range of collimator resolution values. Because the bias of the Bayesian estimator was small (<1%), the analytic variance estimate permitted a rapid and convenient prediction of the Bayesian detection SNR. This calculation was then used to optimize the geometric parameters of a two‐layer tungsten collimator being constructed from crossed grids for a new imaging detector. A Monte Carlo program was first run to estimate all contributions to the radial point‐spread function for collimators of differing tungsten contents and spatial resolution values, imaging 140‐keV photons emitted from the center of a 15‐cm‐diameter, water‐filled attenuator. The optimal collimator design for detecting lesions with unknown diameters in the range 2.5–7.5 mm yielded a system resolution of ∼8.5‐mm FWHM, a geometric collimator efficiency of 1.21×10−4, and a single‐septum penetration probability of 1%.

ISSN:0094-2405    

DOI:10.1118/1.597466

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A contrast‐detail (CD) experiment was performed to study the effect of lossy compression on computed radiographic (CR) images. Digital CR images of a phantom were compressed by quantizing the full‐frame discrete cosine transform and Huffman encoding the result. Since low‐contrast detectability is directly linked to an important radiological task, namely, the detection of noncalcified pulmonary nodules in adult chest radiographs, the goal of the study was to quantify any loss in low‐contrast detectability due to compression. Compression ratios varied significantly among compressed images, despite the use of fixed compression parameters; detectability could be specified by a single parameter of a CD curve; there was no significant reduction in detectability for an average compression ratio of 11:1; and, there was a statistically significant degradation in detectability for an average compression ratio of 125:1.

ISSN:0094-2405    

DOI:10.1118/1.597489

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Many problems in the analysis of medical digital images, e.g., digitized x‐ray radiograph, computed tomography (CT), magnetic resonance imaging (MRI), or positron emission tomography (PET), require a detailed and precise analysis of user chosen regions of interest (ROIs). Examples of their use include calculating integrals of area, volume, mass, structural moments, and statistical measures for either organs, tumors, or the musculoskeletal system. Among various ROI scan conversion schemes, binary approximate scan conversion is usually preferred due to its efficiency. In this paper, geometric area error is tabulated for typical scan conversion techniques, including whole pixel (WP) approximation and subpixel (SP) approximation methods, and compared to exact pixel (EP) coverage methods for medical ROIs. A new efficient and general EP method for scan conversion of these ROIs is presented. The algorithm traverses the boundary of the polygon while simultaneously scan converting the ROI, and calculates the fractional area of each pixel covered at the perimeter. The resultant geometric area is substantially more accurate than the SP or WP methods, without a significant loss of speed. The numerical results for a ROI with a large ratio of boundary to polygon area demonstrated that the geometric error for a SP method was 40% of the total polygon area, and 150% of the total polygon area for a WP method. The new algorithm could “exactly” calculate the pixel coverage area, in addition to being four times faster than the widely used EP method of Catmull. Efficient and accurate calculation of ROI integrals is essential for comparative analysis.

ISSN:0094-2405    

DOI:10.1118/1.597490

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An analytic expression has been derived to calculate the geometric modulation transfer function of a transmission imaging system that uses parallel hole collimation for both the source and the SPECT camera. This expression describes the resolution of the transmission imaging system and replaces the need to use computer intensive Monte Carlo simulations for the system design. The geometric modulation transfer function, denoted as MTFg(ρ), whereρis the spatial frequency vector, is represented as MTFg(ρ)=|[A2sc(sscρ)**Acc2(sccρ)]D(ρ)|, where ** denotes two‐dimensional convolution;Asc(ρ) andAcc(ρ) are the Fourier transforms (FT) of the aperture functions for the parallel hole source collimator (SC) and the camera collimator (CC) holes, respectively;D(ρ) is the FT of the camera response; andsscandsccare scaling constants that depend on the respective collimator dimensions, the system dimensions, the object distance above camera collimator and whether MTFg(ρ) is calculated for the object or image plane. The theoretical MTFg(ρ) was verified with Monte Carlo simulations and experimental results. The formalism shows that the system resolution is characterized by the camera resolution and a combination of the resolutions of the source and camera collimators. This expression can be used to optimize the design of transmission imaging systems to be used in nuclear medicine.

ISSN:0094-2405    

DOI:10.1118/1.597478

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The primary objective of this work was to estimate the absorbed dose distribution to the bone marrow of six multiple myeloma patients who received holmium‐166 (166Ho) DOTMP (1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetramethylene‐phosphonic acid) for the purpose of bone marrow ablation. A methodology based on gamma camera images was developed to estimate the regional absorbed dose distributions delivered to the bone marrow, and this was compared with values calculated from the MIRD technique and bone marrow biopsies. The activity concentration in various skeletal regions was calculated from the activity in the region of interest (ROI) drawn on whole body gamma camera images, and the mass of bone in each ROI was derived from a dual x‐ray absorptiometry image. The radiation absorbed dose to the bone marrow was calculated from this activity concentration using an adaptation of Bragg–Gray cavity theory. The radiation absorbed dose delivered to the bone marrow in the six patients calculated from the MIRD “S” factors ranged from 15.0 to 46.3 Gy. The gamma camera measured activity concentration in skeletal regions predominantly composed of trabecular bone was approximately five to six times higher than that in cortical regions. The skeletal activity concentration in each patient ranged from highs in such regions as the ribs to lows in such regions as the shin and foot by a factor of nearly 20, producing a heterogeneous distribution of absorbed dose within the marrow. Dose volume histograms calculated for these patients indicated that 15%–20% of the marrow received an absorbed dose significantly larger than the average value, while 5%–10% of the marrow received a substantially lower dose. Weighted mean dose estimates from the regional technique were typically 30% greater than the average dose estimates calculated with the MIRD “S” factors. Finally, absorbed dose estimates for the marrow calculated from the regional technique correlated more closely with the clinical response of blood cells and abnormal proteins measured in bone marrow aspirates and peripheral blood samples than estimates from the MIRD “S” factors.

ISSN:0094-2405    

DOI:10.1118/1.597491

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An approach to a solution of two major problems in operating Annular Phased Arrays in deep body hyperthermia is presented: anE‐field sensor capable of measuring phase and amplitude at 70 MHz and the concept of a power transmission factor to determine the effective amplitude of each applicator. In the four‐waveguide Phased Array operating at 70 MHz, which is in clinical use at the department of Radiotherapy of the Academic Medical Center (AMC), the incident fields of the waveguides were scanned in phase and amplitude over the complete aperture midplane, inside an elliptical and a square phantom filled with saline. As a check on the application of the superposition principle, superpositions of the incident fields were compared with the electric field in the measured interference set‐ups. With all four applicators radiating at equal amplitude and in phase, the maximum difference over the complete midplane of the phantom between superimposed and measured interference scans was 20% and 10° in the elliptical phantom, and 20% and 30° in the square phantom. After having determined nominal amplitude and phase patterns by a vector probe, any interference set‐up can be superimposed from measurement of the actual incident field of each applicator. Therefore, the availability of a vector sensor as described here will contribute to solve a problem of hyperthermia quality assurance: the performance evaluation of Phased Arrays.

ISSN:0094-2405    

DOI:10.1118/1.597492

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dose perturbations at tissue interfaces have been of significant concern since the beginning of this century. However, comprehensive studies related to the backscatter perturbation in kilovoltage beams are still limited. The dose perturbation depends on various parameters, including beam energy, field size, and the thickness, width, position, and atomic number,Z, of the inhomogeneity creating the interface with soft tissue. Using a thin window parallel plate ion chamber having relatively flat response at low energies, the dose perturbation was measured as backscatter dose perturbation factor, BSDF, at various interfaces in kilovoltage x‐ray beams. The BSDF is defined as the ratio of doses with and without an interface for identical setup conditions. Results indicate that the BSDF is strongly dependent on beam energy, like the backscatter factor. Contrary to its behavior in megavoltage beams, BSDF in kilovoltage beams does depend on the field size, suggesting a contribution from scattered photons and fluorescent radiation, originating in the high‐Zmaterial. The thickness of the high‐Zmedium is not critical, since a fraction of a millimeter is sufficient to provide full backscatter. The interface effect with wide inhomogeneity has two distinct regions: the high dose region (BSDF>1.0), which is very localized and disappears within a fraction of a millimeter, and the low dose region (BSDF<1.0), which is observed up to 10 cm. The dependence of BSDF is neither a quadratic function ofZnor a cube root of beam energy, indicating that the interface effect is complex and not predominantly due to photoelectron transport.

ISSN:0094-2405    

DOI:10.1118/1.597594

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Scatter dose may be calculated by summing the scatter contribution from individual volume elements. These contributions may be represented by differential scatter–air ratios (dSAR). Determination of dSAR from measured data is only approximately correct for second and higher orders of scatter and yields values often limited to one significant figure. Monte Carlo calculation, on the other hand, is time intensive, requires some knowledge of the beam's x‐ray spectrum, and mastering the complexities of a program such asegs4. Total scatter dose at a point may be determined by measuring depth dose or tissue–air ratios and partitioning the dose into its primary and scatter components. Scatter may be represented by scatter–air ratios, which can be characterized by the sum of first, second, and higher orders of scatter. The first scatter dose may be computed exactly by summing the first scatter contribution from individual elements, determined from the first principle. Separation of dSAR into primary attenuation and depth‐independent terms allows the latter to be precomputed once for a given energy and stored in tabular form. Second scatter may be treated in a similar manner. The higher orders of scatter are computed by subtracting the sum of calculated first and second scatter doses from the total scatter dose. Elements close to and approximately 1 cm above the point of calculation contribute most heavily to the first scatter dose. Compared to the first scatter dose, the second scatter dose contribution is lower, particularly for elements close to the point of calculation. First scatter dose constitutes most of the scatter dose for small field sizes and shallow depths. For larger field sizes and greater depths, second and higher orders of scatter dose become more significant. As beam energy increases, the first scatter dose comprises a greater fraction of total scatter dose.

ISSN:0094-2405    

DOI:10.1118/1.597479

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The first of a new generation of microtron accelerators has been installed and tested. It is currently in use for multisegment conformal radiotherapy at our institution. The unit produces x rays and electrons from 10 to 50 MeV in 5 MeV increments. It incorporates a 64 leaf, doubly focused multileaf collimator (MLC), which can be used to shape x‐ray and electron beams. Both x‐ray and electron beams are produced by magnetically scanning the electron beams from the accelerator. The new generation unit incorporates a purging magnet to sweep away any primary or secondary electrons that pass through the target(s). In this paper, the beam characteristics of the accelerator that were studied during acceptance testing are described. Representative examples of depth doses, beam profiles, output factors, and elementary beam distributions are presented and discussed, in comparison with the earlier generation of microtron accelerators and with other radiotherapy machines.

ISSN:0094-2405    

DOI:10.1118/1.597587

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY