摘要:

The Bloch equations have been solved using numerical techniques for a uniform fluid undergoing periodic pulsatile flow in an NMR imaging experiment. The magnetization and NMR signal have been calculated for experimental parameters appropriate for a CINE sequence (TR=40 ms, TE=14 ms) applied to the study of pulsatile aortic or other arterial flows. The flow velocity profile is obtained by Fourier superposition of different harmonics and it is shown that the steady‐state NMR signal has reduced high‐frequency components. There is also a time delay between peak signal intensity and flow because the backflow effects that can be as much as 100 ms. The apparent pulsatility depends on the NMR sequence parameters. Some limitations of the phase contrast flow‐imaging method are also discussed for nonuniform flow. Key words: magnetic resonance imaging, Bloch equations, pulsatile flow

ISSN:0269-3127    

DOI:10.1118/1.596680

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new spatial localization technique forinvivohigh‐resolution imaging is presented here. In contrast to other localization techniques that use a series of rf pulses to define a volume of interest, only one rf pulse is utilized in the proposed method for selection of a region to be imaged. Instead of rf pulses for region selection, subencoding gradient pulses are used for the localization together with a convolution process on each phase‐encoding gradient by a set of additional gradients (e.g.,ydirection). Then the 2‐D localization is completed by restricting the bandwidth in the readout direction (e.g.,xdirection). The latter is simply achieved by using a low‐pass filter in the receiver system. By applying this technique on a human body, localizedinvivohigh‐resolution images are obtained for the knee with much improved resolution. 100×100‐μm in‐plane (x,yplane) resolution images obtained from the human knee demonstrate that localizedinvivohigh‐resolution imaging for both human and animals is possible with an in‐plane resolution of below 100 μm.

ISSN:0269-3127    

DOI:10.1118/1.596681

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Positron emission tomography (PET) is potentially useful for the quantitative imaging of radiolabeled antibodies, leading in turn to improved dosimetry in radioimmunotherapy. Iodine‐124 is a positron‐emitting nuclide with appropriate chemical properties and half‐life (4.2 days) for such studies since the radiolabeling of antibodies with iodine is well understood and the half‐life permits measurements over several days. Unfortunately, I‐124 has a complex decay scheme with many high‐energy gamma rays and a positron abundance of only 25%. It has therefore been largely ignored as a PET‐imaging nuclide. However, measurements made with phantoms and animals under realistic conditions using a BGO‐based PET scanner have shown that satisfactory imaging and quantitation can be achieved. Investigations of spatial resolution, the linearity of regional observed count rate versus activity in the presence of other activity, and the visualization and quantitation of activity in spheres with different surrounding background activities were carried out with phantoms up to 22 cm in diameter. Compared with F‐18, spatial resolution was only slightly degraded (13.5 mm FWHM vs 12 mm FWHM) while linearity was the same over a 10:1 activity range (0.015 to 0.15 MBq/ml for I‐124). The visualization and quantitation of spheres was also slightly degraded when using similar imaging times. Increasing the imaging time for I‐124 reduced the difference. To verify that the technique would workinvivo, measurements were made of human neuroblastoma tumors in rats which had been injected with I‐124 labeled 3F8 antibody. Although the number of samples was small, good agreement was achieved between image‐based measurements and direct measurements of excised 4‐g tumors. Thus quantitative imaging of I‐124 labeled antibodies appears to be possible under realistic conditions.

ISSN:0269-3127    

DOI:10.1118/1.596728

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Flood field images are acquired and inspected visually during weekly quality control procedures for scintillation cameras. Nonuniformities are quantified by parameters such as integral and differential uniformity (IU and DU). This study is a first step toward understanding the relation between the standard quantitative parameters measured in regular quality control procedures and the performance of clinicians in the interpretation of studies conducted with the same scintillation camera. This study quantified the performance of observers to detect nonuniformities in an ideal case. Flood field images (64×64) were simulated on a computer. One contrasting area (2×2) was superimposed at random positions on each image. Positive and negative contrast values of 10%, 8%, 6%, 4%, 2%, and 0% were employed. The linearly scaled computer images were transferred to film. These films were evaluated by 11 observers to obtain receiver operating characteristic (ROC) curves. The areas (A) below the average ROCs for every contrast value were utilized as an indication of the detectability of the contrasting areas superimposed on the flood images. The results indicate areas with positive contrasts>6% and negative contrasts0.051 and<−0.072, respectively (assuming a gamma,G, or gradient of 2.0 for the x‐ray film characteristic curve). The contrast values and optical density differences obtained may serve as guide values, beyond which action must be taken to correct the scintillation camera nonuniformity to ensure optimum imaging. These results can be utilized by other institutions to predict the threshold contrasts of their imaging systems ifGcan be measured or estimated for the systems.

ISSN:0269-3127    

DOI:10.1118/1.596682

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The physical properties of three surgical probes containing different radiation detectors are compared: a NaI(Tl) scintillator with a flexible, fiber‐optic light guide, and two semiconductor detectors that operate at room temperature, CdTe and HgI2. Also compared are spectra, energy resolutions, and counting efficiencies measured at a variety of gamma‐ray energies between 30 and 1000 keV. The energy resolution of the NaI probe is substantially poorer than that of either semiconductor probe due in part to light losses in coupling the scintillator to the fiber optics. The semiconductor probes have complex spectral response due to charge‐carrier trapping and K x‐ray escape, and not all photoelectric interactions in these detectors contribute to the useful part of the photopeak. Above 120 keV the counting efficiency for the NaI probe is an order of magnitude higher than for the CdTe and HgI2probes. Both energy resolution and counting efficiency are slightly better for the HgI2probe than for the CdTe probe.

ISSN:0269-3127    

DOI:10.1118/1.596717

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Spatial variation in the background source distribution makes tumor detection difficult for single‐detector probes. Using a single energy window that brackets the photopeak helps discriminate against background events dominated by Compton scattering. Another approach is to use the information provided by an additional window in the Compton region. The performances of NaI(Tl), CdTe, and HgI2surgical probes have been compared under realistic simulations of a tumor‐staging procedure using optimal single‐sided energy windows and a two‐window scatter‐subtraction technique. Results showed that despite the differences in energy resolution of the detectors, the performances of the probes in a variable background were similar when optimal single energy windows were used. When the background variations were large, using information provided by a second window improved probe performance.

ISSN:0269-3127    

DOI:10.1118/1.596683

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Quantitation of planar radionuclide images is hampered by structures containing radioactivity which overlie or underlie the organ of interest. The introduction of single photon emission computerized tomography (SPECT) overcame this problem to a large extent and enhanced the contrast of the images. Attenuation of photons, however, degrades the resultant SPECT images and correction methods for photon absorption and scatter were subsequently proposed. The different correction methods have variable effects on the reconstructed images. If threshold techniques are used to quantitate organ volume, i.e., combining pixels with the same percentage of the maximum pixel count in the volume, the selected threshold values which give the most accurate volume determination, will be affected by the specific correction method used. In this study, the effect of various SPECT image correction methods on threshold was investigated. A thorax phantom containing volumes ranging from 30 to 1200 ml was used. Threshold values varying from 45.6% (210 ml without any correction) to 23.7% (1200 ml with a combination of scatter subtraction and attenuation correction) were used to produce correct quantitation when different methods were investigated. A negative correlation was found between threshold and volume. This reduction in threshold was most prominent when scatter and attenuation correction were combined. This study shows that correction methods for attenuation of photons influence the threshold value for volume quantitation and the use of a constant threshold value could lead to underestimation of larger volumes.

ISSN:0269-3127    

DOI:10.1118/1.596684

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A multinomial image model is proposed which uses intensity‐level information for reconstruction of contiguous image regions. The intensity‐level information assumes that image intensities are relatively constant within contiguous regions over the image‐pixel array and that intensity levels of these regions are determined either empirically or theoretically by information criteria. These conditions may be valid, for example, for cardiac blood‐pool imaging, where the intensity levels (or radionuclide activities) of myocardium, blood‐pool, and background regions are distinct and the activities within each region of muscle, blood, or background are relatively uniform. To test the model, a mathematical phantom over a 64×64 array was constructed. The phantom had three contiguous regions. Each region had a different intensity level. Measurements from the phantom were simulated using an emission‐tomography geometry. Fifty projections were generated over 180°, with 64 equally spaced parallel rays per projection. Projection data were randomized to contain Poisson noise. Image reconstructions were performed using an iterative maximumaposterioriprobability procedure. The contiguous regions corresponding to the three intensity levels were automatically segmented. Simultaneously, the edges of the regions were sharpened. Noise in the reconstructed images was significantly suppressed. Convergence of the iterative procedure to the phantom was observed. Compared with maximum likelihood and filtered‐backprojection approaches, the results obtained using the maximumaposterioriprobability with the intensity‐level information demonstrated qualitative and quantitative improvement in localizing the regions of varying intensities.

ISSN:0269-3127    

DOI:10.1118/1.596685

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A semiempirical model for generating molybdenum target x‐ray spectra is presented. The model is an extension of a previous model developed by the authors for tungsten and takes into account the depth of production for both bremsstrahlung and characteristic x‐ray photons. As in the previous work, the optimal model parameters were determined using nonlinear least‐squares fits to experimental data. Good agreement between the two was obtained. By varying target angle, off‐axis angle, and filtration in the model in accordance with the x‐ray tube and geometry of interest, results consistent with tabulated spectra for different conditions have been obtained.

ISSN:0269-3127    

DOI:10.1118/1.596686

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

X‐ray scatter fractions measured with two detectors are compared: a photostimulable phosphor system (PSP) and a conventional film–screen technique. For both detection methods, a beam‐stop technique was used to estimate the scatter fraction in polystyrene phantoms. These scatter fraction measurements are compared to previously reported film‐based measurements. Scatter fractions obtained with the PSP were in good agreement both with measurements using film as well as with most previously reported measurements. For the PSP measurements, repeatability was better than 1%. It was found that the PSP provides a precise x‐ray detector for quantitative scatter measurement in digital radiography.

ISSN:0269-3127    

DOI:10.1118/1.596687

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY