The New German Standard DIN , 2007

Transcription

1 The New German Standard DIN , 2007 Constancy testing of medical X-ray equipment for fluoroscopy The constancy testing for fluoroscopy using an X-ray image intensifier and with images from the exit screen of the X-ray image intensifier was previously regulated in the German Standard DIN from January A revision was urgently needed: The existing standard had to be adapted to include constancy testing on digital X-ray equipment for fluoroscopy with the possibility of producing images for dynamic flat detectors and to meet new image quality requirements. In October 2007, the final version of the newly revised DIN was released. This standard now also contains, due to the inclusion of the previous DIN , regulations for the constancy testing of digital subtraction angiography equipment. This new standard now also includes a differentiation between analog and digital logarithm processing. With the publication of DIN , the DIN standard was taken out of circulation. Furthermore, the new standard also contains a recommendation for testing the spatial resolution for 3D image operating mode. This article is limited to the application of the new test device for fluoroscopic and image mode. Required Testing Equipment 1. The new test device In accordance to the newly revised DIN , a new test device for the fluoroscopic and image mode is required (Fig. 1). It consists of two components: a structural body that is made up of one 17 mm thick PMMA block (Polymethylmethacrylate) and a 1.5 mm thick copper plate (Cu). It is arranged close to the image receiver and is available with an edge length of 200 mm or 300 mm an attenuator that is available in two models: - X-ray tube side, 25 mm aluminum block (alternatively, the patient-equivalent filter of a dosemeter can be used for the constancy test, e.g. the DOSIMAX plus I dosemeter) - one or more attenuators made of PMMA with an overall thickness of 30 mm and an additional 1 mm thick copper plate. Fig. 1: The structural body of the new test device in accordance to DIN with a 300 mm edge length

2 The structural body (Fig. 2) made of 17 mm PMMA and 1.5 mm copper includes: (1) a marking to identify the tube axis (2) 17 dynamic steps (stage 1: 0.00 mm Cu; stage 17: 3.48 mm Cu), circular arrangement (3) 16 detailed contrast objects for estimating the contrast resolution in steps 1 to 8 and 10 to 17 with a diameter of 4 mm and a depth of 2.5 mm in the PMMA (6) 8 detail contrast objects for defining the contrast resolution in the operating point with a diameter of 10 mm and a depth of 0.4 mm to 4 mm (8) a resolution test of 0.6 to 5 lp/mm; the Cu thickness here is only 1.1 mm (4) a kv test field of 0.78 mm thick ytterbium (Yb) Furthermore, a cross (5) is present as a center marker, the inner field (7) remains free of structures. Fig. 2: The structural body according to DIN (for explanations, see the text links on the left side)

3 Performing the constancy test 2. The Dosemeter A dosemeter has to be used that has a repetition accuracy of ± 5 % (standard deviation, 2 σ) in the utilized dose and voltage range, in accordance to DIN V ENV If the measurement is performed using pulsed radiation, the dosemeter has to be suitable for this type of radiation. It must be stated whether the dosemeter displays peak values of the dose rate, average values of the dose rate or an integrated dose as well as, if applicable, the integration time and the measured pulse rate. Test arrangement With the arrangement of the test device, make sure that the structural body is always positioned near the image detector; the attenuator must always be placed near the tube. The orientation of the test device to the tube axis is configured such that the influence of the anode position on the line pair spatial resolution to be determined is as low as possible. To adjust the operating point for the fluoroscopic and image mode, if required, additional filters of the required thickness are placed on the test device so that an X-ray tube voltage of 75 kv ± 7 kv results. As an indicator for this, a field made of ytterbium is inserted into the test device, which, at the right setting, displays the same brightness as the surroundings. For testing with various formats, it may be necessary to change the setting of the operating point (Fig. 3-4). Fig. 3: Fig. 4: Use of the structural body Use of the structural body with the patient-equivalent filter with the PMMA / copper as attenuator attenuator

4 Determining the parameters 1. Dose and dose rate The dose and the dose rate are always measured on the radiation entry port of the test device always at the same spot (as close as possible to the central beam) and in the same direction. In the image mode, the dose has to be measured per image. The reference values for the dose or the dose rate result from the average value based on three tests. The measured values may not deviate from the reference value by more than ± 30 % for automatic exposure. In addition, the X-ray tube voltage and the current strength or the current-time product should be documented. The limit values of the dose indicator must be stated by the manufacturer. 3. Line pair spatial resolution The line pair spatial resolution is determined using a lead test grid. Disrupting moiré effects can be avoided by rotating the test device. As reference values for the line pair spatial resolution, the value for the highest resolved line group has to be defined. The line pair spatial resolution recorded as part of the constancy testing may not be lower than the defined reference value. visible detail contrast objects (see Fig. 2, No.6) has to be determined. They have to correspond to the values determined in the acceptance test. 5. Limiting the useful beam field For all image receiving formats, the apertures are fully opened in all directions. The aperture edges have to be visible on at least two non-opposing sides. On the test images, during postprocessing, the limits of the useful beam field may not be covered by electronic masks. 2. Dose indicator If a dose indicator is available for the image mode, it is obtained from the assigned data of the test image. Here, too, the reference value is a result of the average value based on three tests. The dose indicator's deviations from the reference value may not be greater than changes that are the equivalent of ± 50 % of the image receiver dose. 4. Contrast resolution in the fluoroscopic and image mode The visible steps of the dynamic step wedge have to be determined. The same steps of the dynamic step wedge must be visible as was determined in the acceptance test for the reference values (at least steps 4-12). In addition, the number of 6. Checking for artifacts The visual check of the images may not indicate any disrupting artifacts such as phantom images, lines and absent pixels, or moiré structures. The limit values for the dose, dose rate and even the dose indicator have to be observed independently of one another. If the limit values are exceeded, the test must first be repeated. If these values are again exceeded in a repeat test, the cause must be immediately determined and repaired (Fig. 5).

5 Fig. 5: Test device image at 69.8 kv. The ytterbium field shows the same brightness as the surroundings Scope of the test In the image mode, the dose per image, the line pair spatial resolution, the limitation of the useful beam field and artifact checking are generally tested for all image formats using the characteristic curves and image processing parameters that were defined in the acceptance test. A test scope deviating lower than this can, depending on the X-ray equipment to be tested, be defined in the acceptance test. Dose indicator and contrast resolution are only tested for the image format that is used most frequently. In the fluoroscopic mode, the dose rate for all image formats with continual fluoroscopy or maximum pulse rate is determined using the most frequently used image format with the most frequently used pulse rate. Line pair spatial resolution; limitation of the useful beam field and artifact checking are generally tested for all image formats with the respective pulse rate and characteristic curve as well as the image processing parameters that were determined for the acceptance test. A test scope deviating lower than this can, depending on the X-ray equipment to be tested, be defined in the acceptance test. Contrast resolution is only tested for the image format that is used most frequently. Prior to determining the reference values, the successful acceptance test for the image documentation system has to have been performed in accordance to DIN or the image reproduction device in accordance to DIN V Temporary provision According to the announcement of the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety from December 19, 2007, a temporary provision concerning the introduction of DIN was added to the quality assurance directive: For all analog image intensifier-tv fluoroscopic equipment, as well as those digital image intensifier-tv fluoroscopic devices that were put into operation prior to October 1, 2007, the previous test devices (e.g. ETR1 test device) may continue to be used. For all digital image intensifier-tv fluoroscopic devices that were put into operation after October 1, 2007, the new test devices have to be put into use no later than July 1, For the fluoroscopic equipment using flat detectors, regardless of the date of initial start-up, DIN ( ) must be applied. The new test device has to be used as of July 1, When switching over to the new test device to meet DIN ( ), the reference values for the constancy test have to be newly determined. This temporary provision should ensure that there are no scheduling, organizational or financial problems caused by the switch-over. Literature 1. DIN ( ): Image quality assurance in diagnostic X-ray departments Part 4: Constancy testing of medical X- ray equipment for fluoroscopy 2. Announcement of the Federal Ministry of the Environment, Nature Conservation and Nuclear Safety from December 19, 2007 RS II Author Prof. Dr. Cornelia H. Lipfert Director Strategic Marketing / Regulatory Affairs, Radiodiagnostics IBA Dosimetry GmbH Date June 2008