摘要:

A theoretical description of the nuclear magnetic resonance (NMR) signal from flowing nuclei has been developed for rapid imaging sequences that use small flip angles and gradient refocused echoes. Both laminar and plug flow models have been considered and formulas derived relating mean image signal intensity to flip angle, pulse sequence repetition interval (TR), and flow velocity. It is shown that the rate of approach to steady‐state conditions determines the degree of flow enhancement. Experimental measurements have been performed on a flow phantom in a whole‐body NMR imaging system operating at 0.15 T using the spoiled FLASH sequence with different radiofrequency pulse flip angles and flow rates. There is excellent agreement between the experimental results and the theoretical predictions up to the onset of turbulence.

ISSN:0269-3127    

DOI:10.1118/1.596197

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Nuclear magnetic resonance (NMR) microscopy with 4‐μm resolution, a step closer to the 1‐μm resolution with whichinvivocellular imaging would be possible is described. An analysis of the ultimate resolution and voxel size dependent signal‐to‐noise ratio (SNR) in NMR microscopy is presented and experimentally verified. For microscopic scale objects (<1‐mm diameter), the SNR based on the geometrical scale factor (s) is found to be proportional tosnwheren<2, rather thann=3 as previously supposed. This comes about because of a drastic reduction in sample noise coupled with a significant sensitivity gain realized in small diameter radiofrequency coils. A new pulse sequence which reduces both diffusion dependent resolution degradation and signal attenuation is presented. The selection of optimal bandwidth and acquisition time for maximal SNR is discussed. Experimental results obtained on both a 2.0‐T whole‐body system and a 7.0‐T small bore system adapted for microscopy indicate the potentials of 4‐μm resolution microscopy with the existing magnets.

ISSN:0269-3127    

DOI:10.1118/1.596287

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An artifact is identified in magnetic resonance images produced by the three‐dimensional FLASH technique, which features a short repetition timeTR. The artifact is caused by differential spoiling of transverse magnetization by the phase‐encoding gradients. The image intensity in different slices becomes altered, especially for shortTRand large flip angle, which are conditions for achieving strongT1‐weighted contrast. The effectiveness of spoiler gradients and rephasing gradients in suppressing the artifact is evaluated experimentally in images of a uniform phantom. Spoiler gradients that are incremented in amplitude cause even more slices to deviate in intensity, and are therefore less effective than in two‐dimensional techniques. Rephasing gradients make the slices uniformly intense, but also enhance the intensity of tissues that have longerT2. The further addition of constant spoiler gradients has reduced this intensity increase by one‐half and allowed for an intensity difference between white matter and gray matter comparable to without a rephasing gradient.

ISSN:0269-3127    

DOI:10.1118/1.596299

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper displays the results of a blind study used to determine the capability of a Laplace transform pair model to accurately reconstruct diagnostic x‐ray spectra from experimental attenuation data. Spectra reconstructed from attenuation measurements are compared to experimental spectra obtained on the same unit using an intrinsic germanium spectrometer system. The results show that when pure attenuation materials are used, good agreement is obtained between the experimental and computed spectra. If an alloy attenuator like 1100 aluminum is used, the proportion of contaminants must be included in the Laplace formulation for accurate reconstruction.

ISSN:0269-3127    

DOI:10.1118/1.596198

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have studied the effect of real‐time optical image processing (OIP) in an image intensifier/television (II–TV) radiographic imaging system by using a liquid‐crystal display (LCD) placed between the II and the TV camera. The LCD compresses the dynamic range of the transmitted image by modulating the spatial distribution of the light intensity of the image from the output phosphor of the II. The degree of dynamic‐range compression can be designed so that the dependence of the signal‐to‐noise ratio (SNR) of the LCD–TV system on x‐ray intensity matches that of the quantum noise. We measured the physical properties of an LCD and evaluated its capability for OIP. Our experimental results demonstrate that it is feasible to use an LCD to compress the dynamic range and to improve the SNR of the image. The advantages of implementing OIP with an LCD in image acquisition systems in which a TV camera is used are discussed.

ISSN:0269-3127    

DOI:10.1118/1.596199

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Multiple‐slit beam imaging technique with an image intensifier (II)–TV digital system has been developed to remove scatter and veiling glare while high x‐ray beam utilization is maintained. Although the contrast and signal‐to‐noise ratio (SNR) are improved with this technique, the overall image quality obtainable with an II–TV digital system is still limited due to low spatial resolution, which is mainly caused by the large pixel size, i.e., by the small matrix size used. In order to overcome the limitation of the pixel size, we have developed a new method of improving the resolution properties of the II–TV digital system by use of a multiple‐slit assembly (MSA) having a narrow slit width. When the slit width of the MSA is narrower than the pixel size of the II–TV digital system, two signals from a given slit due to different MSA placements may be detected by the same pixel in different image frames, and the detected signals of the slit images are mapped to a large matrix. In this way, the spatial resolution in the direction perpendicular to the slit openings can be improved along with the increased contrast and SNR as the scatter and veiling glare can be removed. Experimental results are presented, and the effect of an anisotropic resolution property on the overall image quality is discussed.

ISSN:0269-3127    

DOI:10.1118/1.596200

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The accurate quantification of theinvivodistribution of111In labeled platelets, other cells, and proteins with a scintillation camera is important in clinical and experimental medicine. Planar techniques of image quantification were therefore evaluated with the aim of improving on the accuracy, and simplifying the techniques currently in use. The attenuation of the 172‐ and 247‐keV photons of111In, singly and in combination, was determined for varying diameter flat sources (3.4 to 16.9 cm). The influence of region of interest (ROI) selection on the shape of the attenuation curves was also determined for five different ROI's. Defining the attenuation curves mathematically generated parameters of fit for three approaches toinvivoquantification, namely: a single exponential geometric mean approach that takes into account source size, depth‐dependent, and depth‐independent buildup factor approaches to account for the contribution of scatter. The accuracy of these techniques was ascertained and compared to the classical geometric mean method. This was done in a waxen phantom of a human thorax with a hollow liver and spleen. The results indicated that the depth‐independent buildup factor is the best method; the error for quantification in the spleen was 0.8%±2.2%. The classical geometric mean approach gave a corresponding error of 43.3%±3.4%. Since the attenuation of the two energies of111In differ, their ratio changes with depth. This phenomenon was investigated with the goal of determining whether the depth of an object can be estimated from one set of planar images. This was not successful. Based on these results, we recommend that quantification of organ radioactivity is best done by using both energy peaks of111In, acquiring parallel opposing images and correcting for attenuation and scatter with the depth‐independent buildup factor.

ISSN:0269-3127    

DOI:10.1118/1.596288

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An investigation of iron metabolism in a female patient volunteer by administration of stable iron isotopes as tracers was performed. The applied methodology had already been tested in rabbits in comparison with radioactive tracer technique. The subject under study was given58Fe solution intravenously and about 45 min later57Fe solution orally. Ten blood samples were drawn at different times within 522 min from injection. Single iron isotopes content in plasma samples was determined by proton nuclear activation. A Compton suppressor system was utilized to improve the detector limits. The characteristic parameters of iron plasma clearance and of iron intestinal absorption were determined.

ISSN:0269-3127    

DOI:10.1118/1.596168

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Tumor therapy using radiolabeled antibodies presents a challenging problem in absorbed dose determination. The purpose of this study is to evaluate the effect of tumor size on the absorbed dose distribution from beta‐emitters when the radiolabeled antibody is not uniformly distributed throughout the tumor. Two theoretical dosimetry models are constructed, one for nonvascularized micrometastases and the other for vascularized tumors. All calculations assume no penetration of radionuclide into the tumor. These are compared to an even distribution of radionuclide throughout the tumor. In micrometastases of 1‐mm diameter or less, emitters of low energy such as131I give higher dose rates than emitters of higher energy because less energy is lost outside the target volume. However, even with131I, a significant proportion of the energy is not absorbed in the tumor and, as a result, the concentration of radionuclide necessary for a therapeutic radiation dose becomes higher as the tumor diameter gets smaller. Because it may be impossible to achieve these concentrations in very small tumors (<0.5‐mm diameter), alpha‐emitters may be useful in combination with beta‐emitters for therapy of micrometastatic disease. In vascularized tumors, higher energy emitters such as90Y yield higher doses because of overlapping dose distributions from multiple vascular sources. This also produces a more even dose distribution across a tumor, even when there is poor penetration of the radiolabeled antibody. Thus tumor size, antibody penetration, and tumor vascularity all influence the choice of radionuclide and, depending on the circumstances, alpha‐emitters, low‐energy beta‐emitters, high‐energy beta‐emitters, or some combination of the three may be most efficacious.

ISSN:0269-3127    

DOI:10.1118/1.596169

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

When calculating cerebral blood flow by the inhalation regional cerebral blood flow technique, radionuclide activity associated with exhaled alveolar gas is used to represent the arterial input function for each brain region. In this study, tidal CO2data are used to identify respiratory gas samples that contain alveolar gas. Traditional methods identify alveolar gas samples by searching for maxima and minima in the raw air curve. The raw air curve is determined by sequentially counting radionuclide activity in respiratory gases sampled at the mouth. Traditional methods sometimes erroneously identify and use maxima or minima that do not represent alveolar gas. The use of CO2data is advantageous since the range of CO2during exhalation can identify those exhalations that approach the functional reserve capacity and hence represent alveolar gas. The arterial input function is represented by counting intervals from the raw air curve which coincide with exhalation of alveolar gas as identified by CO2data. This approach for representing the arterial input function is fully automatic, accurate, and reproducible.

ISSN:0269-3127    

DOI:10.1118/1.596290

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY