BS 61391-2:2010 pdf download – Ultrasonics — Pulse-echo scanners Part 2: Measurement of maximum depth of penetration and local dynamic range
Scanners for which this standard applies may be grouped according to the source of the digitized image data. The first group includes systems for which digitized image data are directly available from the scanner or over an image network. Sources of digitized image data from this group include the following: Direct DICOM [25]-images from the scanner. Image data in a DICOM format are available on most scanners. Software capable of transferring and opening DICOM formatted images is available. Digital image files available from the scanner itself.
This method is used by most scanner manufacturers for in-house quality-control testing and image-processing development. Capabilities often exist to extend the method for use by clinical personnel using, for example, file-transfer-protocol (ftp) resources. Alternatively, many scanners provide image files on removable media, such as USB-thumb drives, magneto-optical disks, zip disks, or CD-ROM, and these are appropriate sources of digital images data as well. Image-archiving systems.
Many imaging centers use commercially available Picture Archiving and Communication Systems (PACS) for viewing and storing ultrasound-image data. Manufacturers of PACS systems usually provide means to acquire images in an uncompressed format, such as a ‘tiff’ (Tagged Image File Format) or a DICOM (Digital Imaging and Communications in Medicine [25])-format, to workstations that have access rights to the image data. A second group of scanners includes those simpler devices that do not provide digitized image data but provide standard video signals, image data that can be captured into a computer and then analyzed. For these scanners, a video-frame grabber may be used to acquire digitized image data.
The video signal grabbing has to be provided under exactly specified conditions to avoid or minimize signal distortions. Specific care and attention has to be taken for the following parameters: The input dynamic range of the video-frame grabber has to be adjusted according to the maximum signal amplitude of the video output. The digitizing amplitude resolution (given by the pixel byte size) must be better than that of the gray-scale resolution of the video-output signal. A minimum of 8 bits or 256 gray levels is required. Conversion-function linearity has to be assured.
The spatial resolution (given by the pixel size) of the digital picture must be better than the original video line density of the image. The video-capture frame rate of the video-frame grabber must be high enough to allow acquisition of data to keep up with input data rates, if the imaged field is moved. Keep in mind the difference between scanning frame rate and output video frame rate. A cable matched for input/output impedance has to be used to avoid reflections in the line. Alternatively, some post-processing software on ultrasound scanners enables the user to determine the pixel values within a selected region of interest (ROI).
For some tests, such as determining local dynamic range, this tool is convenient for monitoring the image pixel value. The digitized image data must be representative of those on the display monitor of the scanner. Thus, image data derived from the scanner shall not undergo any post-processing modifications before being subjected to analysis as described in this standard.
7 Measurement methods Measurement results will depend on the system transducer, frequency, and the operating conditions and mode. These shall be specified for system characterizations performed in accordance with this standard and with IEC 61 391 -1 . Maximum depth of penetration should be measured at each frequency.
7.1 System sensitivity: maximum depth of penetration
7.1.1 Scanning system settings
To determine the maximum depth of penetration, the system-sensitivity controls shall be adjusted to provide echo signals from as deep as possible into the phantom. This adjustment generally requires the following:
a) the transmit energy (labelled, for example, “output;” “power,” etc.) should be at its highest setting;
b) the transmit focal distance is positioned at or as closely as possible to the apparent maximum depth of penetration;