摘要:

The concept of magnetic susceptibility is central to many current research and development activities in magnetic resonance imaging (MRI); for example, the development of MR‐guided surgery has created a need for surgical instruments and other devices with susceptibility tailored to the MR environment; susceptibility effects can lead to position errors of up to several millimeters in MR‐guided stereotactic surgery; and the variation of magnetic susceptibility on a microscopic scale within tissues contributes to MR contrast and is the basis of functional MRI. The magnetic aspects of MR compatibility are discussed in terms of two levels of acceptability: Materials with the first kind of magnetic field compatibility are such that magnetic forces and torques do not interfere significantly when the materials are used within the magnetic field of the scanner; materials with the second kind of magnetic field compatibility meet the more demanding requirement that they produce only negligible artifacts within the MR image and their effect on the positional accuracy of features within the image is negligible or can readily be corrected. Several materials exhibiting magnetic field compatibility of the second kind have been studied and a group of materials that produce essentially no image distortion, even when located directly within the imaging field of view, is identified. Because of demagnetizing effects, the shape and orientation, as well as the susceptibility, of objects within and adjacent to the imaging region is important in MRI. The quantitative use of susceptibility data is important to MRI, but the use of literature values for the susceptibility of materials is often difficult because of inconsistent traditions in the definitions and units used for magnetic parameters—particularly susceptibility. The uniform use of SI units for magnetic susceptibility and related quantities would help to achieve consistency and avoid confusion in MRI.

ISSN:0269-3127    

DOI:10.1118/1.597854

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We designed a novel force/torque moment transducer with six degrees of freedom. This new sensor is used for measuring the forces and torque moments exerted by static magnetic resonance imaging (MRI) fields on various medical implants, instruments, and ballistic shell fragments. To determine the location of strongest effect the inhomogeneity of the magnetic field between the entrance of the MRI‐tube (portal), and the imaging section was measured. Force and torque moments of a sample of 45 objects were measured at two positions, at the portal, and in the center of the imaging section. We classified the objects regarding to the magnetism‐related forces measured in the homogeneous part of the MRI tube. About one‐third of the objects was non‐ or weakly magnetic, one‐third moderately, and the last third was strongly magnetic. Forces within the inhomogeneous area of the magnetic field at the entrance part of the tube were up to half a magnitude larger than forces inside the homogeneous imaging section. Here the greatest potential risks to patients might occur.

ISSN:0269-3127    

DOI:10.1118/1.597853

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In this work we analyze the effects of inherent random noise on the detectability of low‐contrast vessel structures that possess slow flow. When flow is encoded in more than one direction, the number of independent noise contributions increases in addition to the scan time. In a fast‐flow scenario, only the noise contribution from sampling along the direction of flow is of any significance. At slow flow rates, however, it becomes necessary to account for the noise in each encoded Cartesian direction. The degree to which noise affects low‐contrast detectability also depends on the method of phase contrast image processing employed. A theoretical analysis of the statistical properties of signal and noise in processed phase contrast magnitude images is presented and verified from experimental MR image data. Results show a progressively increased bias in the processed phase contrast image magnitude at slow flow rates due to contributions from inherent random noise. The amount of this bias increases with the number of physical directions in which flow is encoded and is larger for complex difference processed images than for phase difference processing. Correspondingly, the output signal‐to‐noise ratio associated with flow is compromised.

ISSN:0269-3127    

DOI:10.1118/1.597729

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have developed a method for correcting for the scatter produced by the computed radiography plates in single‐shot dual‐energy imaging. In our technique, which uses a series of four computed radiography plates (the middle two plates act as a filter), the back plate can have scatter fractions as high as 30% due to the scatter from the first three plates. We use the deconvolution method to correct for this scatter, as the scatter medium is uniform and isotropic with a constant air gap. The ratio of the modulation transfer function (MTF) of a standard computed radiography plate to that of a dual‐energy plate gives us the correction factor we need to scale the Fourier transforms of our images, thus compensating for the reduced response of the system due to scatter. We also investigated the effect of having an air gap between the back dual‐energy plate and the first three plates. Our measurements show that this degrades the signal‐to‐noise response of the back plate slightly at low spatial frequencies. We also used independent scatter fraction measurements to verify the validity of our MTF measurements. Applying the correction to a back plate image improves the fine detail contrast by approximately 30%.

ISSN:0269-3127    

DOI:10.1118/1.597855

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A parameter accumulation method based on the Hough transformation is proposed to register three‐dimensional (3‐D) multimodality medical images. The estimation of registration parameters is decomposed into separate estimations of rotation, using directional vectors, and translation, using positional vectors. Similarly, the rotation parameters are decomposed into the rotation axis and angle, which are then estimated separately. This kind of decomposition reduces the parametric dimension and improves the computing efficiency which has been a major concern in implementing the Hough transformation. When 3‐D rotation is involved, evaluating registration error is not straightforward. This paper introduces an equivalent error angle as a criterion to evaluate the performance of 3‐D registration methods. Experimental results indicate that a least‐squares fitting is superior to the parameter accumulation with data contaminated by additive noise only. When mismatched feature points (outliers) exist in the data set, however, the parameter accumulation approach is more accurate. The application of the proposed approach to the registration of 3‐D PET and CT images is demonstrated.

ISSN:0269-3127    

DOI:10.1118/1.597730

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Many solutions have been proposed to overcome the problem of imaging the radiographically dense breast with high contrast mammographic film of limited dynamic range. In previous works, we have proposed utilizing a modulated fan‐beam in a scan‐rotate geometry RSER (Rotary Scanning Equalization Radiography), as an efficient method for producing exposure equalized mammograms. The image quality of RSER is similar to that attained with the inefficient single beam, raster scanning SER (Scanning Equalization Radiography) geometry. RSER has the potential to be a practical, efficient method for improving the detection of cancer in the dense breast. In this work, we present a theoretical analysis of the imaging properties of the RSER geometry in two regimes defined by the variation of x‐ray transmission within the object. For low contrast objects, the imaging geometry was analyzed as a linear system, whereas the high contrast regime was studied by determining the contrast limit at which RSER requires nonphysical (negative) exposure modulation for a breast‐like object. The low contrast transfer function of the RSER system is shown to be very similar to that of the SER geometry. We show that RSER enables the use of wide scanning beam of approximately 4 cm and thereby significantly reduces x‐ray tube heat loading. Analysis of the high contrast behavior shows that a wide range of object contrasts and sizes can be equalized. For example, RSER can equalize a region of 100% glandular tissue within a 4.0 cm thick compressed breast composed of 100% adipose tissue. Thus, the RSER geometry produces images very similar to the more inefficient SER geometry, and is able to produce entrance exposure distributions appropriate for equalization of the range of contrast typically encountered in mammography.

ISSN:0269-3127    

DOI:10.1118/1.597731

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An analysis is presented of the exposures received by TLDs placed on the breasts of 4400 women obtaining mammograms at 170 institutions across the United States. Mean glandular dose and exposure were examined as a function of compressed breast thickness. The exposure and mean glandular dose were found to increase linearly with breast thickness. The mean glandular dose typically delivered by the institutions was well below the limit of 3.0 mGy for the 4.5‐cm breast. However, some institutions tend to uniformly give higher doses. In such institutions approximately 25% of 4.5‐cm‐thick breasts received a mean glandular dose exceeding 3.0 mGy.

ISSN:0269-3127    

DOI:10.1118/1.597824

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The harmonic mean formalism is investigated as a nondivergent specification of the mean treatment dose in three distinct applications of interstitial brachytherapy: (a) permanent125I prostate implant; (b) double plane192Ir linear source array; and (c) multiple catheter optimized high dose rate implant. In each case the harmonic treatment dose (HTD) can be calculated in regions that contain radioactive sources or otherwise high dose gradient, to yield objective and consistent results with little computational effort. The need to identify local dose minima, geometric centers, or seed planes is largely avoided, which makes the formalism suitable for automated computation. Dose distribution uniformity is analyzed in terms of the fractional spread for the HTD and, where the periphery of the target volume is delineated, the uniformity number and the peripheral uniformity number. It is shown that the harmonic mean dose formalism is a practical alternative to the ICRU dose specification parameters in certain areas of brachytherapy.

ISSN:0269-3127    

DOI:10.1118/1.597732

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Shields made of high atomic number material are commonly used in vaginal applicators with high dose‐rate (HDR)192Ir remotely afterloaded brachytherapy sources. However little data is available for the dose distribution around such shields. Heterogeneity correction factors (HCFs) are defined as the ratio of the dose to a point with the heterogeneity (shield) in place, divided by the dose to the same point with no heterogeneity. Using thermoluminescent dosimeters (TLDs) in solid water phantom we have measured the HCFs behind 6 and 20 mm diam tungsten alloy disks, 4 and 2 mm thick and a 4 mm thick steel disk, positioned 15 mm from the source. For each measurement point, the heterogeneity correction factors were also inferred from Monte Carlo simulations, which accurately modeled the experimental geometry. The agreement between measured and calculated HCFs on the average was within 6%. Tungsten alloy disks resulted in about two times greater dose reduction in water (HCF≊0.4, for 20×4 mm disk) than for a steel disk with the same dimensions (HCF≊0.85). Reducing the disk diameter to 6 mm increased the dose transmission up to about 25%. Increasing the source‐to‐detector distance from 4 to 7 cm caused a change in HCF from 2% to more than 20%, depending on disk material and diameter. The detector artifact effects arising from the finite size and different composition of the TLD chips were determined.

ISSN:0269-3127    

DOI:10.1118/1.597733

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Conventional source localization algorithms require a one‐to‐one source correspondence between films. This requirement makes source localization cumbersome and error prone because multiple sources must be carefully digitized and some sources can be obscured or missed. A new source localization algorithm is described in this paper. The algorithm fits a ribbon or needle image on film to a linear‐quadratic equation, then analytically determines the 3‐D ribbon locus by its image on the other projection, and finally localizes the sources in the ribbon by tracing along the ribbon image. Only three points per ribbon per film are required, and corresponding points need not be identified on the other film. Phantom experiments and tests on clinical cases demonstrate that the source localization algorithm can increase the efficiency by a factor of up to 5, improve accuracy to about 1 mm, and reconstruct obscured or shifted sources without decreased accuracy and efficiency. The simplicity and minimal entry of data make this technique desirable for clinical use.

ISSN:0269-3127    

DOI:10.1118/1.597734

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY