Visual demonstration of aliasing in planar nuclear medicine imaging: The importance of correct collimator selection by nuclear medicine practitioners

Aliasing artefact is an imaging distortion phenomenon experienced in a wide variety of medical imaging modalities. This case report illustrates its occurrence during planar gamma camera nuclear medicine imaging under non-clinical conditions using experimental incorrect selection of collimators. In accordance with provision of an optimal service, nuclear medicine practitioners are recommended to have sufficient technical expertise along with knowledge of gamma camera operation. The purpose, construction and interaction of collimators used during planar imaging are presented herein with specific regards to the aliasing phenomenon. Furthermore, this case report recommends the careful planning of worklists to avoid frequent collimator changes to reduce the risk of human error.     

医用电子直线加速器多叶准直器的维护与故障检修

本文首先简要介绍了多叶准直器的原理与组成,然后说明了多叶准直器具体的维护方法,同时就在日常的工作中容易出现的问题一一列举说明;最后,对经常出现的故障给出了一系列解决方法。     

西门子多角度双探头SPECT工作原理及探头维修一例

SPECT是进行骨癌转移检查的重要设备,主要介绍西门子公司的Signature系列varible angle双探头SPECT的简单工作原理和探头检修。     

Cost-minimization analysis: radiation treatment with and without a multi-leaf collimator

Purpose: To compare the costs of radiation treatment on a linear accelerator with a multileaf collimator (MLC) versus treatment on a linear accelerator without an MLC. The study was designed to determine whether the increased throughput of fields and decreased block cutting made the MLC cost effective from an institutional perspective.

Methods and Materials: The number of fields, basic treatment equivalent, equivalent simple treatment visits, and blocks were prospectively collected for the four linear accelerators. Building, equipment, staffing, and service costs were all obtained in 1999 Australian dollars from the manufacturers and hospital department heads. The Joint Radiation Oncology Centre at Westmead and Nepean Hospitals, which are Australian public hospitals, runs as one unit, with the same staff, and currently operates five linear accelerators. Currently, four of the linear accelerators are used for general radiotherapy, operating for exactly the same hours; the final machine operates more limited hours and is used for specialized radiotherapy techniques and emergency cases.

Results: The two machines with MLCs, on average, treated 5,169 fields each, while the two machines without MLCs treated 4,543 fields in a 3-month period, a 12% increase in throughput. The two non-MLC machines required 155 premounted trays (PMTs) in total, while the MLC machines required 17 PMTs. Linear accelerators with MLCs were demonstrably more efficient, and while their capital costs were higher, the reduction in labor costs associated with block cutting and, particularly the increased throughput, more than offset these initial costs. The total cost of a radiation field with an MLC was found to be $A101.69 compared to $A106.98 without an MLC. A multiway sensitivity analysis showed the results to be robust. The worst-case scenario was a departmental savings of $A168,000 per year; the best-case scenario was a savings of $A680,000 per year.

Conclusion: Under the conditions pertaining to the radiation oncology department in this group of hospitals, and in similar departments, the use of an MLC can be justified.     

MLC leaf width impact on the clinical dose distribution: a Monte Carlo approach

PURPOSE: The influence of the multileaf collimator (MLC) leaf width on the dose distribution in patients treated with conformal radiotherapy and intensity-modulated radiotherapy has been analyzed. This study was based on the Monte Carlo simulation with the beams generated by a linac with the double-focused MLC. MATERIALS AND METHODS: The transmission through the leaves and the exact shape of the penumbra regions are difficult to model by treatment planning system algorithms. An accurate assessment of the dose variations due to the leaf width change can be achieved by means of Monte Carlo simulation. The BEAM/EGS4 code was used at the Hospital of the Virgen Macarena to model a Siemens PRIMUS linac, featuring an MLC with a leaf width projecting 1 cm at the isocenter. Based on this real model, a virtual head was designed while allowing for a variation of the leaf width projection. Both the real linac and the virtual linac, with leaves projecting 0.5 cm, were used to obtain the dose distributions for several treatments. A few disease sites, including the prostate, head and neck, and endometrium, were selected for the design of the conformal and intensity-modulated radiotherapy treatments with a forward planning algorithm sensitive to the different shapes of the volumes of interest. Isodose curves, differential matrix, gamma function, and the dose-volume histograms (DVHs) corresponding to both MLC models were obtained for all cases. The tumor control probability and the normal tissue complication probability were derived for those cases studied featuring the greatest differences between results for both MLCs. RESULTS: The impact on the DVHs of changing leaf width projections at the isocenter from 1.0 cm to 0.5 cm was low. Radiobiologic models showed slightly better tumor control probability/normal tissue complication probability values using the virtual MLC with a leaf width projecting 0.5 cm at isocenter in those cases presenting greater differences in the DVHs. CONCLUSIONS: The impact on the clinical dose distribution due to the MLC leaf width change is low based on the design and conditions used in this study.     

Intensity modulation with the “step and shoot” technique using a commercial mlc: a planning study

Purpose/Objective: For complex planning situations where organs at risk (OAR) surrounding the target volume place stringent constraints, intensity-modulated treatments with photons provide a promising solution to improve tumor control and/or reduce side effects. One approach for the clinical implementation of intensity-modulated treatments is the use of a multileaf collimator (MLC) in the “step and shoot” mode, in which multiple subfields are superimposed for each beam direction to generate stratified intensity distributions with a discrete number of intensity levels. In this paper, we examine the interrelation between the number of intensity levels per beam for various numbers of beams, the conformity of the resulting dose distribution, and the treatment time on a commercial accelerator (Siemens Mevatron KD2) with built-in MLC.

Methods and Materials: Two typical, clinically relevant cases of patients with head and neck tumors were selected for this study. Using the inverse planning technique, optimized treatment plans are generated for 3–25 evenly distributed coplanar beams as well as noncoplanar beams. An iterative gradient method is used to optimize a physical treatment objective that is based on the specified target dose and individual dose constraints assigned to each organ at risk (brain stem, eyes, optic nerves) by the radiation oncologist. The intensity distribution of each beam is discretized within the inverse planning program into three to infinitely many intensity levels or strata. These stratified intensity distributions are converted into MLC leaf position sequences, which can be subsequently transferred via computer link to the linac console, and can be delivered without user intervention. The quality of the plan is determined by comparing the values of the objective function, dose-volume histograms (DVHs), and isodose distributions.

Results: Highly conformal dose distributions can be achieved with five intensity levels in each of seven beams. The merit of using more intensity levels or more beams is relatively small. Acceptable results are achievable even with three levels only. On average, the number of subfields per beam is about 2–2.5 times the number of intensity levels. The average treatment time per subfield is about 20 s. The total treatment time for the three-level and seven-beam case with a total of 39 subfields is 13 min.

Conclusion: Optimizing stratified intensity distributions in the inverse planning process allows us to achieve close to optimum results with a surprisingly small number of intensity levels. This finding may help to facilitate and accelerate the delivery of intensity-modulated treatments with the “step and shoot” technique.     

The determination of phantom and collimator scatter components of the output of megavoltage photon beams: measurement of the collimator scatter part with a beam-coaxial narrow cylindrical phantom

The separation of the total scatter correction factor S_{c, p} in a collimator scatter component, S_c, and a phantom scatter component, S_p, has proven to be an useful concept in megavoltage photon beam dose calculations in situations which differ from the standard treatment geometry. A clinically applicable method to determine S_c is described. Measurements are carried out with an ionization chamber, placed at a depth beyond the range of contaminant electrons, in a narrow cylindrical polystyrene phantom with a diameter of 4 cm of which the axis coincides with the beam axis. S_{c, p} is measured in a full-scatter phantom and S_p can be derived from S_{c, p} and S_c. In order to obtain a reliable separation, i.e. excluding the influence of contaminant electrons, measurements of S_{c, p} have been carried out at depths of 5 cm for photon beams with a quality index (QI) up to and including 0.75 and a depth of 10 cm with QI larger than 0.75. These depths are in accordance with recommendations given in recent dosimetry protocols. The consistency of the method was checked by comparing calculated and measured values of S_{c, p} for a set of blocked fields for a range of photon beam energies from ⁶⁰Co up to 25 MV showing a maximum deviation of 2%. The method can easily be implemented in existing procedures for the calculation of the number of monitor units to deliver a specified dose to a target volume.     

铅铛块与多叶准直器在固定野三维适形放射治疗中应用的研究

目的在多固定野三维适形放射治疗中,为科学、准确、有效地使用铅挡块与多叶准直器提供重要依据;以便较好地处理好肿瘤组织和周围正常组织间的剂量关系,使肿瘤得到最大限度的局部控制而周围正常组织和器官的放射性损伤最小,提高疗效。方法分析比较多固定野三维适形放射治疗中铅挡块与MLC的适形度、漏射剂量率、不规则靶区边缘1cm处剂量率、物理半影、质量保证和质量控制、CT扫描方向肿瘤计划靶区大小不是MLC叶片整数倍、每次治疗时间等。结果在多固定野三维适形放射治疗中,铅挡块与多叶准直器各有优缺点。结论该研究成果可用于指导多固定野三维适形放射治疗中,铅挡块与多叶准直器的选择使用。