摘要:

Lanthanide‐based compounds such as Gd‐DTPA are currently used as contrast agents in MRI. Recent experiments using CT and transmission radiography show that Gd agents can increase image contrast by up to a factor of 2 relative to more commonly used iodinated agents on an equi‐molar basis. It has also been suggested that beam hardening artifacts may be reduced with Gd. This hypothesis was experimentally tested on three different CT scanners using a circular water equivalent phantom with contrast filled tube inserts. It was found that the artifacts were a factor of 1.3–1.8 more pronounced with the iodinated contrast compared with Gd‐DTPA. A theoretical model which uses an experimentally derived photon energy spectrum is proposed which relates the strength of beam hardening artifacts to the variance (over the energy spectrum) of the attenuation coefficient of the contrast agent. This allows easy assessment of the relative magnitudes of the artifact for different contrast agents.

ISSN:0269-3127    

DOI:10.1118/1.597495

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Digital radiographic systems based on photoconductive layers with the latent charge image readout by photoinduced discharge (PID) are investigated theoretically. Previously, a number of different systems have been proposed using sandwiched photoconductor and insulator layers and readout using a scanning laser beam. These systems are shown to have the general property of being very closely coupled (i.e., optimization of one imaging characteristic usually impacts negatively on others). The presence of a condensed state insulator between the photoconductor surface and the readout electrode does, however, confer a great advantage over systems using air gaps with their relatively low breakdown field. The greater breakdown field of condensed state dielectrics permits the modification of the electric field during the period between image formation and image readout. The trade‐off between readout speed and noise makes this system suitable for instant general radiography and even rapid sequence radiography, however, the system is unsuitable for the low exposure rates used in fluoroscopy.

ISSN:0269-3127    

DOI:10.1118/1.597644

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effect of motor activity on the left fronto‐central region of the human brain was analyzed spatially and temporally by using noninvasive near‐infrared light (NIR) topography. The changes in oxygenation states caused by motor activity were measured using intensity‐modulated NIR spectroscopy at ten measurement positions on the head surface. The subject randomly performed unilateral finger opposition for 30 s as motor stimulation. When the subject performed contralateral (right) finger movement, significant increases in both oxygenated hemoglobin (oxy‐Hb) and total hemoglobin (total‐Hb) and decreases in deoxygenated hemoglobin (deoxy‐Hb) were observed in a particular area. By mapping the static topograms of the changes of each Hb and comparing them with an anatomical image of MRI, it was found that the particular area was located on the motor cortex along the central sulcus. By mapping the dynamic topograms of the changes of total‐Hb, which reflect the cerebral blood volume, and analyzing the spatiotemporal hemodynamic changes associated with the brain activity, it was found that the regional change in cerebral blood volume in the primary motor area overlaps the global change around the motor cortex. These results demonstrate that NIR topography can be used to effectively observe the human brain activity.

ISSN:0269-3127    

DOI:10.1118/1.597496

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The ratio method involves forming the ratio of the elastic to inelastic x‐ray scatter signals from a localized region of a scattering medium to determine its mean atomic number. An analysis is presented of two major error sources influencing the ratio method: firstly statistical (photon) noise and secondly multiple scattering and self‐attenuation of the primary and scatter radiations in the medium. It is shown that a forward scattering geometry minimizes errors of both types for substances composed of elements with low and medium atomic number. However, owing to the small energy separation (∼100 eV) of coherent and Compton scatter for this geometry, they cannot be distinguished directly with semiconductor (e.g., Ge) detectors. A novel K edge filter technique is described which permits separation of the elastic and Compton signals in the forward‐scatter geometry. The feasibility of this method is demonstrated by experimental results obtained with Ta fluorescence radiation provided by a fluorescent x‐ray source filtered with an Er foil. The extension of this technique to the “invivo” measurement of low momentum transfer inelastic scattering from biological tissues, possibly providing useful diagnostic information, is briefly discussed.

ISSN:0269-3127    

DOI:10.1118/1.597497

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We studied the performance of linear scatter correction methods for single‐photon imaging with Tc‐99mand Tl‐201, using a numerical model of the Rollo phantom and measurements with a gamma camera modified to record position and energy information in list mode form. We compared the performance of these methods to per‐image optimized linear methods and to locally adaptive linear methods, and developed estimates of the limits on accuracy of scatter correction imposed by the presence of Poisson noise. For both Tc‐99mand Tl‐201 imaging at a fixed depth, particularly at low count rates, the performance of dual‐window methods, or of adaptive methods, is near the best possible for linear methods. Smoothing of the scatter estimate results in minor improvement for Tl‐201. Substantial gaps between the performance of any of these linear methods and the limits imposed by Poisson noise remain and are due primarily to bias, with the gap for Tl‐201 being larger than that for Tc‐99m.

ISSN:0269-3127    

DOI:10.1118/1.597645

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597646

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The purpose of this investigation is to determine if electronic portal imaging devices (EPIDs) can be used for the design and verification of compensating filters. In order to do this, we investigated the operating characteristics of a commercially available EPID and the variation in transmitted dose for various measurement situations. We performed four initial tests to determine the EPID response specific to compensator situations. The tests determined EPID response to variable patient SSDs, different gantry angles, positions of an inhomogeneity within a phantom, and the sensitivity variation of different parts of the imager. After these tests, we determined the attenuation functions relating EPID response to phantom thickness for various phantom materials. With these functions, we tested simple compensation situations to demonstrate that missing tissue compensators can both be designed and verified using EPIDs.

ISSN:0269-3127    

DOI:10.1118/1.597647

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In photon beam convolution, the distribution of energy deposition about a primary photon interaction site due to charged particles set in motion at that site is represented by the primary kernel. Energy deposited due to scattered photons, bremsstrahlung, and annihilation photons is represented by the scatter kernel. As the energy deposited in each kernel voxel is normalized to the energy imparted at the interaction site, it is known as afractionalenergydistribution. Interma‐basedconvolution, where kernels are normalized to total energy imparted at the interaction site and are convolved with the terma in the dose calculation process, the sum of fractional energies contained in the primary kernel is equal to the ratio of collision kerma (Kc) to terma (T) corresponding to the energy spectrum used to generate the kernel. Since the ratio of collision kerma to terma increases with depth as the beam hardens, theintegralfractional energy in a primary kernel formed for the spectrum at the surface is less than the ratioKc/Tat depth. This causes primary dose to be increasingly underestimated with depth and scatter dose to be increasingly overestimated.Singlepolyenergeticconvolution(using polyenergetic primary and scatter kernels formed using a polyenergetic primary photon spectrum) is thus not as rigorous as if a separate convolution is performed for each energy component. The ratio of true primary dose to single polyenergetic primary dose increases almost linearly with depth and is almost equal to theKc/Tratio. Primary and scatter dose are calculated correctly if a single polyenergetic convolution is performed in terms ofKc(for primary) andT−Kc(for scatter), where the kernels are weighted sums of monoenergetic kernels normalized toKcandT−Kc. With this method, it is ensured that total primary energy deposited due to primary photon interactions in a unit mass at a point is equal toKcat that point.

ISSN:0269-3127    

DOI:10.1118/1.597493

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An analytic representation proposed for the relative intensity distribution of the extra‐focal source in a two‐component x‐ray source model serves as the basis for calculation. From this representation, a closed‐form expression for head scatter factors defined on the central beam axis is derived by integrating over the area of the extra‐focal source plane visible from the measurement point. The resulting expression is applicable to photon beams from different Varian accelerators and different photon energies, and includes effects arising from beam shaping with cerrobend blocks or multi‐leaf collimators (MLCs). For 6‐ and 15‐MV photon beams from Varian 600C and 2300CD accelerators (Varian Oncology Systems, Palo Alto, CA), 361 measured head scatter factors for square, rectangular, asymmetric, and arbitrarily shaped fields, formed by either the X and Y jaws, the MLC and Y jaws, or by the MLC alone, were compared with model calculations. Results show that 93.4% of calculated values match corresponding experimental points to within 0.5%, the average deviation being 0.23% and the maximum deviation 0.9%. Thus, as a consequence of this work, the different influence of the X jaws, the Y jaws, and the MLC on head scatter factors is quantitatively described. In particular, it is demonstrated that in the case of radially symmetric scattering, the collimator exchange effect arises as a result of the different distances of the X and Y jaws from the focal spot.

ISSN:0269-3127    

DOI:10.1118/1.597494

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The TG21 protocol introduced theNgascalibration method for parallel plate chambers in high‐energy electron beams. This calibration method was performed for a Markus parallel plate chamber in proton and electron beams of various energies as well as a60Co beam. For an individual chamber, theNgasvalue in proton beams differs from theNgasvalue in cobalt and electron beams by the ratio of (W̄/e) for proton beams to that of a60Co beam. While the replacement correction factor is essential for Markus chambers in low‐energy electron beams, the results of ourNgasppmeasurements in proton beams showed that the Markus chamber does not need a replacement correction factor for therapeutic proton beams of energy 20–170 MeV. These results indicate that the 0.7‐mm guard ring of the Markus chamber is adequate to prevent the in‐scattering of secondary electrons produced by proton irradiation of the chamber wall or medium.

ISSN:0269-3127    

DOI:10.1118/1.597649

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY