摘要:

Using a new reference/test, adaptive forced‐choice method, we investigated detectability of low contrast disks in simulated x‐ray fluoroscopy image sequences. The method minimized effects of undesirable variables, such as changes in subject attention level by alternating conventional fluoroscopy (reference) and low acquisition rate fluoroscopy (test) presentations. To compare absolute target detectability and x‐ray dose requirements of reference and test presentations, the contrast of the test presentation was set equal to that of the reference presentation, and the dose of the test presentation was adapted to give equivalent detectability of disk targets. Comparing pulsed fluoroscopy at 15 acq/s (pulsed‐15) and 7.5 acq/s (pulsed‐7.5) with conventional fluoroscopy at 30 acq/s (pulsed‐30), dose savings were found in 22 of 24 experiments. When averaged over 3 disk sizes and 4 subjects, savings were 22% and 26% for pulsed‐15 and pulsed‐7.5, respectively. In most experiments, the subject was given an unlimited time to view presentations. With unlimited display times, subjects took slightly longer to respond for pulsed‐15 or pulsed‐7.5 than for pulsed‐30. In other experiments, display times were limited. As the restricted display time increased, absolute target detectability increased while the x‐ray dose for equivalent detectability was unchanged. The number of frames,N, in a repeating loop was also varied. Absolute detectability increased asNincreased from 1 to 30 but changed little as more frames were added up to 150. The new experimental paradigm gives efficient, robust comparisons of image data and is applicable to a wide variety of medical image perception problems.

ISSN:0094-2405    

DOI:10.1118/1.597666

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An electronic portal imaging device (EPID) for use in radiotherapy with high energy photons has been under development since 1985 and has been in clinical use since 1988. The x‐ray detector consists of a metal plate/fluorescent screen combination, which is monitored by a charge‐coupled device (CCD)‐camera. This paper discusses the physical quantities governing image quality. A model which describes the signal and noise propagation through the detector is presented. The predicted contrasts and signal‐to‐noise ratios are found to be in agreement with measurements based on the EPID images. Based on this agreement the visibility of low contrast structures in clinical images has been calculated with the model. Sufficient visibility of relevant structures (4–10 mm water‐equivalent thickness) has been obtained down to a delivered dose of 4 cGy at dose maximum. It is found that the described system is not limited by quantum noise but by camera read‐out noise. In addition we predict that with a new type of CCD sensor the signal‐to‐noise ratio can be increased by a factor of 5 at small doses, enabling high quality imaging, for most relevant clinical situations, with a patient dose smaller than 4 cGy. The latter system would be quantum noise limited.

ISSN:0094-2405    

DOI:10.1118/1.597836

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Simple scaling arguments are used to determine spatial resolution achievable in time‐resolved transillumination experiments involving highly diffuse media. These arguments allow us to obtain relationships linking target resolution at different planes inside an optically turbid slab to the gating times of the imaging system. We show that this approach yields the same results as those obtained previously from an approximate and rather complicated analytical derivation. In addition, we are now able to assess the effects of scattering anisotropy on spatial resolution attainable when gating times are so short that a constant scaling of photon transport scattering length is not appropriate. These results should enable one to devise more accurate and simpler image reconstruction algorithms.

ISSN:0094-2405    

DOI:10.1118/1.597748

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1118/1.597898

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1118/1.597837

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The ratio between scatter dose and scatter kerma for points on the central axis of 15‐MV x‐ray beams has been examined by Monte Carlo calculations. This ratio, β′S, behaves differently from that between the primary dose and kerma, β′p. Both the primary and scatter components of β′ undergo an initial rapid buildup, however, β′Sbegins from a much higher surface value. In addition, the depth required for longitudinal electron equilibrium is larger for β′Sthan for β′P. The variation of β′Swith field size and depth is attributed to the spatial variation of scatter kerma in the photon beam. The approximation β′S=1 is accurate enough for clinical dose‐calculation purposes, leading to less than 0.5% error in total dose, while the assumption β′S=β′pmay cause up to 2.5% error, relative to the maximum dose, near the surface at 15 MV.

ISSN:0094-2405    

DOI:10.1118/1.597749

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The electron contamination in photon beams has been investigated by means of contaminating lepton depth doses and dose profiles in different geometries with two 20 MV beams. Different components of this contamination have been investigated separately by systematically adding contamination to a “clean” reference field. At 20 MV, the air generated electrons were found to be almost negligible compared to the electrons originating from the accelerator head when measurements were performed in standard fields at SSDs between 80 and 120 cm. The total electron part of the depth dose curve was then almost the same, i.e., independent of SSD, when the collimator opening was held fixed. However, when different accessories such as a shaping block and different attenuating plates were located in the beam path below the collimators, a large SSD dependence of the electron contamination was noticed. A comparison was also made between two machines, one equipped with a multileaf collimator, with similar beam qualities at 20 MV. These measurements indicate that the interior view of the treatment head seen by the detector (mainly the flattening filter, monitor chamber, or other electron generating material) influences the magnitude of the electron contamination. When the collimator opening is decreased the electron contamination will also decrease as parts of the electron source will be shielded by the collimator blocks.

ISSN:0094-2405    

DOI:10.1118/1.597750

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Recently, a new model of radiochromic film has been developed for medical applications to provide a higher sensitivity and better uniformity of response than existing models (i.e., MD‐55). Dosimetric characteristics including sensitivity, linearity, reproducibility, uniformity, and dependence on energy and time have been studied experimentally. The characteristics of the new films were compared with those of model MD‐55. For these investigations, the two films were exposed to ionizing radiation in the dose range from 1–72 Gy, using γ‐rays from a60Co teletherapy unit and 6‐ and 18‐MV x rays from a linear accelerator. The response of the exposed film was measured with a helium–neon laser densitometer. The results indicated that the sensitivity of the improved film was about 40% greater than that of MD‐55 film. Moreover, the response of the improved film was found to be uniform within 4% only in one direction of the film. The orthogonal direction indicated a nonuniformity of up to 15%, similar to that of model MD‐55. Less than 5% energy dependence in the megavoltage photon range was observed for the new film. Complete dosimetric characteristics of the new film are presented.

ISSN:0094-2405    

DOI:10.1118/1.597747

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The continuing development of probes for use with beta (positron and electron) emitting radionuclides may result in more complete excision of tracer‐avid tumors. Perhaps one of the most promising radiopharmaceuticals for this task is18F‐labeled‐Fluoro‐2‐Deoxy‐D‐Glucose (FDG). This positron‐emitting agent has been demonstrated to be avidly and rapidly absorbed by many human cancers. We have investigated the use of ion‐implanted‐silicon detectors in intraoperative positron‐sensitive surgical probes for use with FDG. These detectors possess very high positron detection efficiency, while the efficiency for 511 keV photon detection is low. The spatial resolution, as well as positron and annihilation photon detection sensitivity, of an ion‐implanted‐silicon detector used with18F was measured at several energy thresholds. In addition, the ability of the device to detect the presence of relatively small amounts of FDG during surgery was evaluated by simulating a surgical field in which some tumor was left intact following lesion excision. The performance of the ion‐implanted‐silicon detector was compared to the operating characteristics of a positron‐sensitive surgical probe which utilizes plastic scintillator. In all areas of performance the ion‐implanted‐silicon detector proved superior to the plastic scintillator‐based probe. At an energy threshold of 14 keV positron sensitivity measured for the ion‐implanted‐silicon detector was 101.3 cps/kBq, photon sensitivity was 7.4 cps/kBq. In addition, spatial resolution was found to be relatively unaffected by the presence of distant sources of annihilation photon flux. Finally, the detector was demonstrated to be able to localize small amounts of FDG in a simulated tumor bed; indicating that this device has promise as a probe to aid in FDG‐guided surgery.

ISSN:0094-2405    

DOI:10.1118/1.597751

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This study involves a fractionated course of external radiation therapy for a 42 year old female weighing 150 kg, diagnosed with stage IIb cancer of the cervix. The patient could not be simulated in the conventional sense due to weight restrictions on the simulator couch, and body casts or molds were impractical. Using an on‐line portal imaging device, treatment fields were established during the first session, and intratreatment verification was used before every subsequent treatment to measure and, when necessary, to correct the patient setup. Two courses of treatment were prescribed with a total dose of 60 Gy delivered by a four field box technique (A/P, P/A, and two lateral fields). Out of a total of 108 treatment fields monitored, 12 anterior fields and 1 posterior field were corrected (exclusive of the first, or simulation fraction). Without corrections, 10% of the initial setup displacements would have had displacements greater than 10 mm, 21% greater than 7 mm, and 41% greater than 5 mm. With the application of intratreatment corrections, only 2% of the displacements were greater than 10 mm, 11% were greater than 7 mm, and 32% were greater than 5 mm. It was also found that the second field treated in a parallel opposed pair (i.e., anterior/posterior or left/right lateral) had lower setup displacements and did not require verification or correction.

ISSN:0094-2405    

DOI:10.1118/1.597752

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY