Chinese-German J Clin Oncol
May 2013, Vol. 12, No. 5, P237–P242
DOI 10.1007/s10330-012-1133-3
Quality assurance for computed-tomography simulator: In home Z-phantom for mechanical tests of the couch and the gantry Hassan Shafeik Abou-Elenein Department of Radiotherapy, Children Cancer Hospital, Cairo, Egypt Received: 26 December 2012 / Revised: 21 February 2013 / Accepted: 25 March 2013 © Huazhong University of Science and Technology and Springer-Verlag Berlin Heidelberg 2013 Abstract Objective: The main purpose of this work was to present a Z-phantom manufactured in home (at National Cancer Institute Cairo University) and it’s use in a simple way to check the accuracy of the computed-tomography (CT) table movement and CT gantry tilt, also the other general quality control (QC) tests of the CT simulator used at radiotherapy department. Methods: The laser phantom was used to check the external mobile laser position accuracy, for internal image indicator laser beam (light field) the coincidence between light field and radiation exposure at CT simulator was checked using X-Omat ready back film. The Z-phantom was used to check the slice thickness and the table movement and so the gantry tilts. The image quality testes were checked using the CT image quality phantom. TLDs were inserted to the Cicil phantom at the center of each scan volume to estimate the patient dose. Results: The results showed that the difference in the fixed distance between the external mobile laser and the internal image indicator laser beam was less than ± 1 mm; the orientation of the two mobile lateral lasers was coincident. The mechanical movement and the image quality of the CT simulator were within the tolerances and the results were 0.5 mm, 0.2% and 0.6% for the mechanical movement, noise and image uniformity respectively. Conclusion: A CT simulator with a good performance is important for the radiotherapy treatment planning specially with the extremely revolution in radiotherapy techniques, also a rotten quality assurance (QA) program is very important to be shore about the reproducibility of the CT performance. The use of Z-phantom to check the gantry tilt and the table movement is faster than the use of ready back films in these tests. Key words
computed-tomography; computed-tomography simulator; quality control
The demands of modern radiotherapy planning are quite different from those 20 years ago. Clinicians now require defining the target volume more precisely, not just in two dimensions (2D), but also in three dimensions (3D). It has therefore become necessary to visualize anatomy in 3D to enable planning to conform the dose around the target volume in order to irradiate the tumor to as high dose as possible, whilst saving the normal tissues [1]. Quality assurance of CT scanners is mandatory and so the AAPM Report Number 39, “Specification and acceptance testing of computed tomography scanners” [2] has described in great detail acceptance testing and QA procedures for CT scanners. Several other references have been published that address this issue [3]. Also a good source of information is the www.impactscan.org website. The QA Program should address radiation safety, CT scanner dosimetry, electromechanical performance, X-ray generator Correspondence to: Hassan Shafeik Abou-Elenein. Email:
[email protected]
operation, and imaging performance. For a successful CT-simulation process, the CT-scanner should consistently produce patient images with the highest possible quality and accurate geometrical information. Image quality directly affects the physician’s ability to define target volumes and critical structures, and the spatial integrity of the CT study establishes how accurately radiation can be delivered to target volumes. The CT-scanner evaluation process consists of an evaluation of patient dose from the CT scanner, radiation safety, electromechanical components, and image quality. The CT-simulator is a CT scanner equipped with a flat tabletop and, preferably, external patient positioning lasers. The scanner is accompanied with specialized software which allows treatment planning on volumetric patient CT scans in a manner consistent with conventional radiation therapy simulators. The CT scanner used in the CT simulation process can be located in the radiation oncology department or in the diagnostic radiology department. Depending on the CT-scanner location and primary use,
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acceptance testing, commissioning, and QA can be the responsibility of a therapy medical physicist, diagnostic physicist, or a joint responsibility of diagnostic and therapy physicists. The commissioning and periodic QA of the accompanying software and the QA of the CT-simulation process is always the responsibility of the therapy physicist [4]. The American College of Radiology Imaging Network is recommended that a qualified medical physicist supervise the image quality assurance tests on the helical CT scanner.
Materials and methods CT simulator and virtual simulation A CT-simulator of GEHC (Middle EAST Health American), modal light speed RT 16 with image acquisition of helical and axial sine was used in this study. Where this CT-scanner version consists of a curved table top we have adopted a flat table top to make it similar to the flat top of the linear accelerator. There are three machines of external mobile laser system. And there is a CT-simulation/3D treatment planning software. The CT-scanner is used to acquire a volumetric CT-scan of a patient, which represents the ‘‘virtual’’ or digital patient. The CT-simulation software provides virtual representations of the geometric capabilities of a treatment machine. A virtual simulator is a set of software which recreates the treatment machine and which allows import, manipulation, display, and storage of images from CT. CT-simulation process has been described by several authors [5]. Laser system and patient localization Laser system of GEHC, CT-simulator consists of three separate components: gantry lasers, wall mounted mobile lasers, and an overhead mobile sagittal laser. These lasers are used to position the patient in the treatment position assuring that patients are straight and properly rotated. These lasers are also used to place positioning marks on patient skin (Origin). Just as the treatment room lasers possess a well-defined and precise spatial relationship to the treatment machine isocenter, the CT-simulation patient marking lasers must possess a similar relationship to the CT-scanner image center. Thus, the accuracy of the lasers directly affects the ability to localize treatment volumes relative to patient skin marks and the reproducibly of patient positioning from the CT-scanner to the treatment machine. The external mobile laser position accuracy and orientation were checked using laser phantom Fig. 1. The laser phantom has 2 horizontal groves one is right and the other is left at the same level as seen in Fig. 1. These 2 horizontal groves are used to check that the two external horizontal lateral lasers are at the same level. There is also one vertical grove all around the phantom used to check that the 2 vertical external lasers are
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at the same level, also this vertical grove is used to test that the external lasers always have the fixed distance of 50 cm from the internal CT image indicating laser. Laser accuracy tolerances depend on the goals of radiation therapy and required accuracy of treatment procedures. The tolerance of wall and sagittal lasers orientation shifts with respect to the imaging plan is ± 2 mm [5]. CT-simulator table tests
Table parallelism Fig. 2 Showed table parallelism test. Table parallelism was checked by putting white paper on the table top and defines a point on the sajital laser as showed in Fig. 2. To check the table parallelism the table was moved in and out for long distances and the offset of the fixed point from the longitudinal laser beam position was measured.
Vertical and longitudinal movement long distance Table vertical and longitudinal digital indicators are used for patient treatment isocenter marking during CTsimulation. Therefore, the digital indicators and table motion accuracy directly affect the ability to accurately correlate internal patient anatomy with skin marks. A longitudinal motion accuracy test is inherent to the previously described laser QA wherein the separation between gantry and wall lasers was verified. To check the digital indicator of the table movement for long distances a ruler was fixed as shown in Fig. 3. The table was moved longitudinally toward the gantry and the matching of digital and ruler mechanical distances indicators were measured. The table also was moved away from the gantry and the matching of digital and mechanical distances indicators were measured. The vertical laser beam of External lateral laser was used as external fixed point to move the table longitudinally around it. Vertical digitally indicated motion accuracy and reproducibility is tested by placing a long ruler vertically on the tabletop and observing a laser position on the ruler as the table is raised and lowered. Of course, care should be taken to ensure that the ruler is perpendicular to the table top for all measurements. Both, longitudinal and vertical digital table position indicators should be accurate within ± 2 mm [5].
Longitudinal movement short distance and slice thickness accuracy This test were performed using the Z shape phantom, Fig. 4a which consists of 0.5 mm diameter lead wire of Z shape inserted between two Perspex layers of 5 mm thickness for each. Z shape phantom was fixed perpendicular to the table top as seen in Fig. 4b, and radiographic CT scan for the Z shape phantom from F to D was performed with slice thickness of 1 cm. On the CT disk tope the AB and BC distances, Fig. 5 were measured. For slice No. 1 the AB distance is correspond to AC distance and equal 20 cm and the BC distance equal zero. The AB distances will be decrease from 20 cm at slice No. 1 to zero at slice
Chinese-German J Clin Oncol, May 2013, Vol. 12, No. 5
Fig. 1 CT laser phantom on the left and it's central axial CT image on the right Fig. 2 Table parallelism check for CT simulator
Fig. 3 Fig. 4
Longitudinal movement long distance (a) Lateral view of the Z shape phantom; (b) Z shape phantom fixed on CT table top
Fig. 5 Fig. 6 Fig. 7
Schematic diagram for Z shape phantom Ready back film fixed on the CT table top to test the light radiation field alignment a, b: Schematic diagram shows changes in CT image dimensions with gantry angles
Fig. 8 Quality control CT phantom connected to the CT couch Fig. 9 Axial CT image for the homogenous water part of the CT phantom used for CT image quality check Fig. 10 The Z shape phantom
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No. 21 and vice-versa for BC distances. Light and radiation field alignment accuracy The radiation field should be alignment with the central laser of the gantry which called the light field. To test the Light and radiation field alignment ready back xOmat film was fixed on the CT table top as shown in Fig. 6 and two Pin holes were created on the image position laser indicator. The film exposed to suitable dose with image thickness of 1 cm and then developed. The matching between the Pin holes and the center of the irradiated area indicates the Light and radiation field alignment. Gantry angle tilt test To test the mechanical offset in the gantry angle with respect to the digital indicator the Z shape phantom was used: the Z-phantom was fix perpendicularly to the CT table top and an image was taken at the phantom center which indicated by AB Fig. 7a, according to the phantom dimensions |AB| =20 cm. The differences between |AB| and |A`B`|. Fig. 7b indicated the gantry tilt where cosignθ = |AB| / |A`B`| Image quality For image quality the quality control CT phantom Fig. 8 was used where one CT image at the center of the homogenous water part of the phantom was taken and five small circles (5 regions of interest) with the same diameter was drawn on this CT image. The circles were distributed in the homogenous slice radiograph as seen in Fig. 9. Using the CT tools the noise B and uniformity U were determined according to the following equations.
• NHair is the Hounsfield number for air • NHeau is the Hounsfield number for water • σ is the standard deviation
www.springerlink.com/content/1613-9089 Table 1 lator
Mechanical and digital long distance movements for CT-simu-
Mechanical long distance 10 30 50 70
Digital (Out) 10.01 30 50 70
Digital (In) 10.02 30.03 50 70
Table parallelism Table parallelism was checked with respect to sagittal laser beam and showed that the offset of the table parallelism was less than 1 mm. The parallelism in the CT table movement is important in radiotherapy treatment planning where it affects on both the volume and the location of the target volume and the organ at risk. Longitudinal movement, long distance The mechanical distances with respect to the digital indicator was tested at long distance movements. The results showed that the offset between mechanical distances indicated by the roller and the digital distance indicator were less than 0.4 mm. Table 1 showed the results for long distances 10, 30, 50 and 70 cm at in and out directions of movement. Longitudinal movement, short distance For short distances the Z shape phantom, Fig. 10 was scanned from the beginning to the end or from F to D with slice thickness of 1 cm at gantry angle 0. The distances AB to BC were measured as described in section. Fig. 11 and Table 2 showed that the precision in the slice position and slice thickness were acceptable and it were less than 0.1 mm. This test is more important specially at radiotherapy CT simulator to be sure that there are not any overexposure to normal tissues or missing parts of the tumor and so the coincidence between the dimensions on the images with that on the patient.
• NHC is the Hounsfield number for the central circle • NHP is the Hounsfield number for the peripheral circle
Results Laser system The results showed that the Accuracy and spatial orientation of lasers therefore were correct and comparable to treatment machine laser accuracy, the distance between the external mobile laser and the internal image indicator laser was 50 cm without any differences from the initial adjustment.
Fig. 11 The relation between AB and BC with respect to slice position
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Table 2 AB and BC distances measurements for the 21 CT image slices of the Z-phantom
Table 3 The standard deviation σ and noise B
Slice position (cm)
Center Right (270) Left (90) Anterior (0) Posterior (180)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
AB (cm) 20 19.1 18.1 17.1 16.1 15.1 14 13 12 11 10 9.1 8.1 7.1 6.1 5 4 3.1 2 1 0
BC (cm) 0 0.9 1.9 2.9 3.9 4.9 6 7 8 9 10 10.9 12 13 13.9 15 16 17 17.9 19 20
Gantry angle The results showed that the measured distances of the summation of AB and BC distances at the middle of the Z-phantom were as equal as the actual separation of the Z-phantom. This indicates that there in no deviation between the mechanical CT gantry angle and the digital indicator. From the same test where AB = BC = 10 cm, so there is no offset in the gantry angle. The use of Z-phantom to check the gantry tilt is more useful, accurate and faster than the use of ready back films where it can differentiate if the deviation is coming from an actual gantry tilt or it is just an offset in the gantry angle. Also it’s reducing the time of the developing of the film.
Position
Standard deviation (σ) 2 2 1.5 2 1.5
Noise % (B) 0.2 0.2 0.15 0.2 0.15
If a graphic cursor is used to display pixel CT numbers in an image of a uniform phantom (e.g., a phantom containing all water), it is seen that the CT numbers are not uniform but rather fluctuate around an average value (which should be approximately 0 for water): Some pixels are 0, some are +1, some +2, some –2, and so forth. These random fluctuations in the CT number of otherwise uniform materials appear as graininess on CT images. Table 3 showed the standard deviation σ and noise for the points of interest which were taken from the CT image data presented in Fig. 9. The results showed that the noise was about 0.2%. The noise B was calculated according to the following formula:
NH: Hounsfield number In radiography, image noise is related to the numbers of X-ray photons contributing to each small area of the image. To understand how CT technique affects noise, one should imagine how each factor in the technique affects the number of detected x-rays. Examples are as follows: (1) X-ray tube amperage: Changing the mA value changes the beam intensity and thus the number of xrays proportionally. For example, doubling the mA value will double the beam intensity and the number of x-rays detected by each measurement. (2) Scan (rotation) time: Changing the scan time chang-
Light and radiation field alignment The results showed that there are complete alignments between Light and radiation field at the CT simulator, Fig. 12 showed that the pin holes which indicate to the position of the image indicator laser (light field) are located at the center of irradiated area. Image quality Image quality in CT, as in all medical imaging, depends on 4 basic factors: image contrast, spatial resolution, image noise, and homogeneity. Depending on the diagnostic task, these factors interact to determine sensitivity (the ability to perceive low-contrast structures) and the visibility of details [6]. Image Noise B
Fig. 12 Developed film showing the pin holes at the center of the radiation field
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Table 4
Hounsfield number (NH) and uniformity (U)
Position
Hounsfield number (NH)
Uniformity% (U)
5 5 10 8 5
0 0 0.5 0.3 0
Center Right (270) Left (90) Anterior (0) Posterior (180)
es the duration of each measurement and thus the number of detected x-rays proportionally. Because amperage and scan time similarly affect noise and patient dose, they are usually considered together as mAs. (3) Slice thickness: Changing the thickness changes the beam width entering each detector and thus the number of detected X-rays approximately proportionally. (4) Peak kilovoltage: Increasing the peak kilovoltage increases the number of X-rays penetrating the patient and reaching the detectors. Thus, increasing the kilovoltage reduces image noise but can (slightly) reduce subject contrast as well.
Image Uniformity Table 4 showed both of the Hounsfield numbers (NH) for the central region of the CT image and these of the four peripheral regions and uniformity of each position relative to the central one. The results showed that the uniformity was less than 0.6%. The uniformity U was calculated according to the following formula:
Conclusion This work was performed to improve accuracy of patient treatments and efficiency of the treatment planning process. The basic principles presented in this document should be preserved whenever possible. As with other components of radiation treatment planning and deliv-
ery, CT-simulation is a constantly evolving process. CTscanners and virtual simulation software are continually being improved and new devices are being developed. The QA process performed in this work can help the other institutes for establishment of a CT-simulation QA program. This program should evolve and adapt as the device used for CT simulation process change. The modified QA program should continue to ensure accurate and efficient delivery of radiation therapy. A CT simulator with a good performance is important for the radiotherapy treatment planning special with the extremely revolution in radiotherapy techniques, also a rotten quality assurance (QA) program is very important to be shore about the reproducibility of the CT performance. The use of Z-phantom to check the gantry tilt and the table movement is more accurate and faster than the use of ready back films in these tests.
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