摘要:

In this review of tissue substitute materials, the historical development of the important systems is traced from the early 1900's. Tabulations of the constituents, elemental compositions, specific gravities, and the photon and electron interaction characteristics of 64 materials are given together with recommendations of systems having useful simulation properties. Formulation and manufacturing procedures are described and possible future developments in both materials and phantom research are outlined.

ISSN:0094-2405    

DOI:10.1118/1.594456

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In recent years the modulation transfer function (MTF) has played an important role in the quantitation of imaging performance of gamma cameras. The most common method of MTF determination requires line spread function (LSF) measurement. This paper reviews methods used for LSF measurements with special consideration given to the practical aspects of LSF measurement and MTF calculation. An analysis of errors in LSF measurements is made and means to reduce or to avoid these errors are discussed. Recommendations regarding practical considerations for LSF mesurement and MTF calculation are presented in tabular form for convenience.

ISSN:0094-2405    

DOI:10.1118/1.594457

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Computed tomography is sensitive to changes in the imaged distribution during acquisition of the projection data. Previous investigations have emphasized discrete or discontinuous changes in the imaged object. Recent advances have motivated our investigation of object time‐dependence characterized by a continuous function in time at each point. Formal mathematical and computer simulation approaches have been developed, and are presented along with simple examples of their applications. Further applications in three distinct ongoing studies are outlined.

ISSN:0094-2405    

DOI:10.1118/1.594458

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Lack of resolution (unsharpness) can reduce contrast in diagnostic radiography if the proper conditions of magnification and unsharpness are met. To describe this phenomenon, a modification of the contrast reduction factor (CRF) was introduced which used the response function of a semi‐opaque edge to predict contrast reduction for small bar‐shaped objects. To predict CRF, unsharpness is employed as a single‐term description of resolution and is obtained experimentally from the edge response function. The unsharpness term is defined as the distance over which the response goes from 16.5% to 83.5% of the maximum. Measured and predicted CRFs were compared and the CRF concept was found to be an excellent predictor of contrast reduction. The individual components of unsharpness were determined experimentally and the sum‐of‐squares rule predicted adequately their combination. Three methods to measure unsharpness were compared: (a) the ICRU prescription using pinhole radiographs of the focal spot, (b) one‐dimensional integration of the focal‐spot pinhole radiograph, and (c) the unsharpness term produced by a semi‐opaque edge. The latter two were measured using a microdensitometer.

ISSN:0094-2405    

DOI:10.1118/1.594459

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A common artifact in CT head‐section images is a cupping or broad ’'whitening” effect near the skull which is due at least in part to the polychromaticity of the x‐ray beam. In this paper, a general method is presented for removing this artifact empirically by a combination of two approaches. The gross cupping is removed by modifying the raw transmission data prior to reconstruction. The residual whitening near the bone is removed conveniently by modifying the reconstruction filter function. Examples of the modifications are shown using the AS&E CT scanner. The method convolves or deconvolves the CT image with an appropriate point spread function. Since the filter‐function modifications are done conceptually in real space rather than in frequency space, the details of the modifications are more easily understood.

ISSN:0094-2405    

DOI:10.1118/1.594486

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A system for the generation of microwave images from homogeneous and heterogeneous dielectric targets is presented. The argument is posited that microwave interrogation may address uniquely relevant features of biological targets. Dielectrically loaded antennas, electromechanical scanning, and the methods of microwave network analysis were employed.

ISSN:0094-2405    

DOI:10.1118/1.594460

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method is described which overcomes the problems of multipath propagation and range ambiguity that is suffered by the single‐frequency continuous‐wave microwave‐imaging system described in part I. This technique is essentially a variation of chirp radar techniques, which have been adapted to time delay and attenuation measurements through a target. The feasibility of discriminating between paths whose differential time delay is on the order of 100 ps is demonstrated. Further, the need for small physical aperture in the transmitting and receiving antennas is demonstrated.

ISSN:0094-2405    

DOI:10.1118/1.594461

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Nuclear magnetic resonance (NMR) pulse techniques were used to obtain values for the spin‐lattice relaxation time (T1) of whole blood, plasma, blood cells,in vitroliver samples, and live and necrotic tail samples for adultTaricha granulosa. TheT1for whole blood, is (0.80±0.01) s, for plasma (0.76±0.02) s and for blood cells (0.83±0.01) s, and did not change over several hours of measurement. The necrotic liver gave a singleT1of (0.28±0.02) s within the first 20 min of excision with a gradual increase over the next 3.5 h. Live and dead tail samples gave twoT1values: a shortT1of about 0.15 s remaining essentially constant and a longT1starting at 0.68 s and increasing to 0.9 s during the 5 h of the experiment.

ISSN:0094-2405    

DOI:10.1118/1.594462

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A semiempirical analytic description of the accelerator depth‐dose curve is described along with its physical explanation. The results of Monte Carlo calculations are presented and compared with experimental data to test this model. Calculations were made for different atomic number (Z) materials used as x‐ray targets and flatteners, with the results showing that medium‐Zmaterials are the logical choice. It is demonstrated empirically thatDmaxis a simple function of the average energy (Ē) of the x‐ray spectrum. The variation ofĒwithZof the target and flattener is demonstrated. As a practical example, Monte Carlo calculations and experimental data for old and new ClinacR35 accelerators are presented.

ISSN:0094-2405    

DOI:10.1118/1.594487

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The photoneutrons produced by high‐energy x‐ray machines can diffuse through the mazes usually employed at the treatment‐room entrance and readily penetrate the lead‐lined doors used for x‐ray shielding. The measurement of these neutrons in the presence of x rays and the determination of dose equivalent poses a problem for which there is currently no standard method of solution. In order to separate x‐ray dose from neutron dose, the author employed an ionization chamber alternately filled with argon or propane. The response characteristics of this chamber to x ray and neutrons are described. Quality factors were determined from a calculated neutron spectrum. As a result of these measurements, a 10‐in. polyethylene door was added to the entranceway of a 25‐MV linear accelerator.

ISSN:0094-2405    

DOI:10.1118/1.594443

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY