摘要:

We report the development of a heterogeneous resin‐tube model to study the influence of blood vessels on the apparent absorption of the system, μa(sys), using a time‐resolved technique. The experimental results show that μa(sys) depends on the absorption inside the tubes, μa(tube), tube diameters, and tube‐to‐sample volume ratios. A mathematical expression relating μa(sys) and μa(tube) is derived based on the experimental results and is verified by time‐resolved Monte Carlo simulations for heterogeneous models. This analytical formula predicts that the apparent absorption coefficient measured on a biological organ is a volume‐weighted sum of the absorption coefficients of different absorbing components. We present some apparent absorption coefficients measuredinvivoin animals and humans and discuss improved algorithms that calculate the hemoglobin saturation by including background‐tissue absorption and blood vessel distribution.

ISSN:0269-3127    

DOI:10.1118/1.597520

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We present a new technique based on the method developed by Metz and Fencil for estimation of the 3D imaging geometry and 3D object configurations from biplane angiographic acquisitions. The new method employs the 3D configuration of points calculated by the Metz–Fencil technique as an initial estimate. A 3D Procrustes algorithm is employed to translate, rotate, and scale the configuration until it aligns optimally with the set of lines that connects a focal spot with the corresponding set of image points. This alignment procedure is applied independently for each view. The rotation and translation that relate the two aligned data sets are then determined by an additional 3D Procrustes calculation. These steps are applied iteratively. Evaluations were based on Monte Carlo simulation and phantom studies. With this new technique, the mean absolute errors in magnification, in the relative position of the points, and in the angles defining the rotation and translation matrices were approximately 3.0%, 1.5 mm, and 5° and 3°, respectively, for rms input errors in the image data up to 2.0 pixels (0.7 mm). Errors in the results can be as small as 0.5%, 0.16 mm, 0.6°, and 0.3°, respectively, if input image‐data error is 0.035 mm. The improvement of the Metz–Fencil technique described here may provide a basis for precise estimation of the biplane imaging geometry and the 3D positions of vessel bifurcation points.

ISSN:0269-3127    

DOI:10.1118/1.597559

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The coherent‐to‐Compton scattering ratio (CCSR) is a technique that has been proposed for measuring trabecular bone mineral density (TBMD). This paper investigates the effect of fat on the CCSR and its correlation to the error in TBMD measurements. It is a computational study to determine the relationship between the magnitude of fat error and the momentum‐transfer variablex, which represents the incident photon energy and the scattering angle. Variation in fat content contributes significantly to the error in CCSR measurements. When employing a typical241Am source (Eγ=59.45 keV), the resulting error decreases with increasing momentum‐transfer variable or angle. For example, the error ranges from +14 mg/cc at an angle of 45° (x=18.3) to +3 mg/cc at an angle of 135° (x=44.3) for an osteoporotic trabecular region (100 mg/cc mineral) of a calcaneus that contains 6% less fat than a calibration standard. The error is about 0.3–1.2 mg/cc less for regions containing 2–3× more bone mineral and is reduced and opposite in sign for regions containing about 7% more fat than the calibration standards (e.g., −9 mg/cc at 45° and −1.5 mg/cc at 135°). Others have shown that the intrinsic sensitivity of the CCSR method for measuring TBMD at a given photon energy generally increases with increasing detector angle. Thus large angles are advantageous both for reduced sensitivity to fat variation and increased sensitivity to bone mineral variation. The primary disadvantage is reduced count rates that degrade precision unless long counting times are employed. For experimental studies, a compromise angle must be chosen to insure adequate counting statistics for reasonable precision and examination times while providing moderate mineral sensitivity and moderate fat error.

ISSN:0269-3127    

DOI:10.1118/1.597560

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

To calculate the response of an image receptor to the x rays emerging from a scattering medium, it is necessary to know the x‐ray spectrum and intensity as a function of the angle of incidence on the receptor. To permit this calculation for any x‐ray spectrum incident on a medium, these functions must be known for monoenergetic x rays. For monoenergetic x rays in the range 20–70 keV we have measured with a high‐purity germanium detector the spectrum and intensity of x rays emitted from a water phantom at angles of 0°–50° to the direction of the primary beam. The spectrum and intensity of emitted x rays have also been calculated by the Monte Carlo method. At small exit angles, most of the x rays have energies close to the incident energy. As the exit angle increases, the fraction of multiply scattered x rays increases. At very large exit angles, the dominant feature of the spectrum is the peak due to these multiply scattered x rays. For small scattering angles the Monte Carlo calculations are in good agreement with the measurements over the range of energies. For large scattering angles the scattered photon fluence predicted by Monte Carlo modeling is consistently lower than the measurement in the region just below the full energy peak. The cause of the discrepancies is not fully understood, but cannot be accounted for by Compton broadening alone. An alternate approach to model incoherent scattering is proposed.

ISSN:0269-3127    

DOI:10.1118/1.597561

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A novel multistage algorithm is proposed for the automatic segmentation of microcalcification clusters (MCCs) in digital mammography. First, a previously reported tree structured nonlinear filter is proposed for suppressing image noise, while preserving image details, to potentially reduce the false positive (FP) detection rate for MCCs. Second, a tree structured wavelet transform (TSWT) is applied to the images for microcalcification segmentation. The TSWT employs quadrature mirror filters as basic subunits for both multiresolution decomposition and reconstruction processes, where selective reconstruction of subimages is used to segment MCCs. Third, automatic linear scaling is then used to display the image of the segmented MCCs on a computer monitor for interpretation. The proposed algorithms were applied to an image database of 100 single view mammograms at a resolution of 105 μm and 12 bits deep (4096 gray levels). The database contained 50 cases of biopsy proven malignant MCCs, 8 benign cases, and 42 normal cases. The measured sensitivity (true positive detection rate) was 94% with a low FP detection rate of 1.6 MCCs/image. The image details of the segmented MCCs were reasonably well preserved, for microcalcification of less than 500 μm, with good delineation of the extent of the microcalcification clusters for each case based on visual criteria.

ISSN:0269-3127    

DOI:10.1118/1.597562

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Previously, it has been shown that Bayesian image estimation (BIE) can reduce the effects of scattered radiation and improve contrast‐to‐noise ratios (CNR) in digital radiographs of anthropomorphic chest phantoms by improving contrast while constraining noise. Here, the use of BIE as a noise reduction technique is reported. An anthropomorphic phantom was imaged with a previously calibrated photostimulable phosphor system using standard bedside chest radiography protocols. The Bayesian technique was then used to process this image. BIE incorporates a radial exponential convolution scatter model with two adjustable parameters. In previous reports, these parameters were optimized to reduce the residual fraction of scattered radiation in the processed image. Here, the parameters were adjusted to evaluate the potential of BIE to reduce image noise. While the full width at half maximum of the scatter model was held constant, the magnitude was varied. Evaluation was based on residual scatter fractions and CNR. The magnitude of the kernel in the scatter model was varied from 0.0 to 2.5 in steps of 0.5. Previously, it was found that an “ideal” scatter kernel magnitude of 2.33 provided a minimum residual scatter fraction. This magnitude corresponds to the average scatter‐to‐primary ratio in the chest radiograph. As the magnitude was increased, the residual scatter fraction decreased and the CNR increased in both the lungs and the mediastinum. However, as the magnitude was decreased, the percent noise also decreased; therefore, a lower magnitude kernel reduces noise. By varying the magnitude of the kernel used, differing amounts of noise reduction and contrast enhancement can be obtained. These results demonstrate that Bayesian image estimation can be used to both increase contrast and decrease noise in digital chest radiography.

ISSN:0269-3127    

DOI:10.1118/1.597563

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Monte Carlo techniques were used to study a generalized CT dose indexD(r) as a function of the radiusrof a cylindrical dosimetry phantom. The relationship betweenD(r) and the energy deposited in the phantom was investigated. For a specified x‐ray spectrum, the energy imparted to head or body dosimetry phantoms can be obtained from measuredD(r) values. This approach to CT dosimetry permits the energy imparted to phantoms (or patients) to be determined as CT technique parameters, or type of scanner, are changed.

ISSN:0269-3127    

DOI:10.1118/1.597564

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597512

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have derived a maximumaposteriori(MAP) approach for iterative reconstruction based on a weighted least‐squares conjugate gradient (WLS‐CG) algorithm. The WLS‐CG algorithm has been shown to have initial convergence rates up to 10× faster than the maximum‐likelihood expectation maximization (ML‐EM) algorithm, but WLS‐CG suffers from rapidly increasing image noise at higher iteration numbers. In our MAP‐CG algorithm, the increasing noise is controlled by a Gibbs smoothing prior, resulting in stable, convergent solutions. Our formulation assumes a Gaussian noise model for the likelihood function. When a linear transformation of the pixel space is performed (the “relaxation” acceleration method), the MAP‐CG algorithm obtains a low‐noise, stable solution (one that does not change with further iterations) in 10–30 iterations, compared to 100–200 iterations for MAP‐EM. Each iteration of MAP‐CG requires approximately the same amount of processing time as one iteration of ML‐EM or MAP‐EM. We show that the use of an initial image estimate obtained from a single iteration of the Chang method helps the algorithm to converge faster when acceleration is not used, but does not help when acceleration is applied. While both the WLS‐CG and MAP‐CG methods suffer from the potential for obtaining negative pixel values in the iterated image estimates, the use of the Gibbs prior substantially reduces the number of pixels with negative values and restricts them to regions of little or no activity. We use SPECT data from simulated hot‐sphere phantoms and from patient studies to demonstrate the advantages of the MAP‐CG algorithm. We conclude that the MAP‐CG algorithm requires 10%–25% of the processing time of EM techniques, and provides images of comparable or superior quality.

ISSN:0269-3127    

DOI:10.1118/1.597614

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Utilization of radiopharmaceuticals that directly target radioactivity to tumors for treatment has a great deal of promise. Ideally, lethal doses of radiation could be delivered precisely to areas of disease, while, for the most part, sparing normal tissues. This potential, however, has not yet been fully realized. Current limitations of this approach are low tumor uptake of radiopharmaceuticals and dose‐limiting radiotoxicity. In an effort to offset low uptake, radionuclides that emit high average‐energy electrons have been proposed. Unfortunately, use of these radionuclides increases myelosuppression on a per decay basis. In order to allow for the utilization of high doses of this class of high‐energy beta emitters, we propose the application of a strong static homogeneous magnetic field to constrain the beta particles. Monte Carlo computer simulations indicate that application of a 10 T magnetic field can decrease the total radiation dose from bone‐avid tracers to marrow located in shafts of human long bones by 14%. More significantly, however, the penetration depth of high‐energy electrons from the bone surface into the marrow can be reduced by up to 74.6%. Preservation of marrow in areas distal to the bone has previously been shown to facilitate relatively rapid recovery from pancytopenia produced by radiation damage to trabecular marrow (without marrow transplantation). Magnetically enhanced protection of bone marrow, therefore, may allow administered doses of high‐energy beta‐emitting radionuclides to be increased. By raising the limits on injected quantities of such highly ionizing radionuclides, amounts of the radiation dose absorbed by both soft and calcified tissue tumors will be increased, compared to conventional treatments. Hence, it is possible that this technique will, in part, aid in the realization of the promise of radionuclide therapy by permitting the delivery of therapeutic levels of radiation exposure to tumors.

ISSN:0269-3127    

DOI:10.1118/1.597513

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY