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Phantom Alignment Using Modified XRETIC and Camera

IP.com Disclosure Number: IPCOM000201651D
Original Publication Date: 2010-Nov-17
Included in the Prior Art Database: 2010-Nov-17
Document File: 9 page(s) / 307K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

When using an LinAc (Linear Particle Accelerator) to support an medical imaging system, image tests have to be performed on a regular basis to ensure the quality of CT images produced by the system (CT: Computed Tomography). This testing procedure is known as acceptance QA (Quality Assurance). The tests are performed by placing an image quality phantom device at the isocenter of the machine and reconstructing the volume of the phantom device by using subsequently acquired projection images. The geometric correctness of the reconstruction and the quality of the CT images are measured and checked against a given specification. The geometry of the reconstructed volume is validated by assuming that the phantom device is placed at the isocenter. Therefore the test results depend on the correctness of the manual positioning of the phantom device itself. The manual positioning of the phantom device is a laborious and time-consuming step, especially for an inexperienced user. Therefore the test itself tends to fail quite frequently, making it difficult to distinguish a real failure due to bad quality from a failure due to a positioning error. Currently, the acceptance QA tests are run once in three months for every LinAc, but for newer versions of LinAc systems this have to be done on a daily basis since a new daily QA test is being introduced. Simplification of positioning the phantom device is all the more important in such a scenario.

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Phantom Alignment Using Modified XRETIC and Camera

Idea: Satheesh Chandran, DE-Erlangen; Anup John, IN-Bangalore

When using an LinAc (Linear Particle Accelerator) to support an medical imaging system, image tests have to be performed on a regular basis to ensure the quality of CT images produced by the system (CT: Computed Tomography). This testing procedure is known as acceptance QA (Quality Assurance). The tests are performed by placing an image quality phantom device at the isocenter of the machine and reconstructing the volume of the phantom device by using subsequently acquired projection images. The geometric correctness of the reconstruction and the quality of the CT images are measured and checked against a given specification. The geometry of the reconstructed volume is validated by assuming that the phantom device is placed at the isocenter. Therefore the test results depend on the correctness of the manual positioning of the phantom device itself. The manual positioning of the phantom device is a laborious and time-consuming step, especially for an inexperienced user. Therefore the test itself tends to fail quite frequently, making it difficult to distinguish a real failure due to bad quality from a failure due to a positioning error. Currently, the acceptance QA tests are run once in three months for every LinAc, but for newer versions of LinAc systems this have to be done on a daily basis since a new daily QA test is being introduced. Simplification of positioning the phantom device is all the more important in such a scenario.

The imaging quality is currently measured by placing the phantom device at the isocenter of the imaging system. The machine consists of a radiation source, a patient-table and a flat panel detector. The radiation source and the detector can be rotated, allowing the radiation to pass through an object at any angle. The machine set-up with the image quality phantom device applied is shown in Figure 1. The phantom device itself which is used for measuring the image quality is shown in Figure 2. This phantom device is used to estimate the measures on the image quality like uniformity, spatial frequency response, noise, etc. Additionally it is used to validate the geometry of the reconstructed volume. It is of utmost importance, to place the phantom device at the exact isocenter of the imaging system with a minimum offset including the three translation and rotation offsets. An incorrectly placed phantom device contributes to the failure of the image quality tests as these tests are done on specific slices of the reconstructed volume.

The phantom device is currently placed at the isocenter by aligning the shadow of an accessory called XRETIC (X-shaped RETICle), depicted in Figure 3, with markings on the phantom, depicted in Figure
4. Currently three gantry angles or directions, 0°, 90...