Nuclear Associates 76-908 & 76-907
Operators Manual
DC-Offset Errors
DC-offset errors typically appear as a single bright pixel (sometimes as a dark pixel if overflow or processing has occurred) at the center of the image matrix. The existence of this error is assessed visually.
Receive Quadrature Errors
Receive quadrature ghosts will be evaluated using the central slice of the multi-slice sequence acquired with the phantom at the isocenter. Receive ghosts will appear upside down and reversed from the true signal producing objects (object in the upper left-hand corner will appear as a ghost in the lower right- hand corner). Regions-of-interest values are taken from both the true image and the ghost image. The receive Quadrature Error (E) is quantified by expressing the ghost ROI value (G) as a percent of the true ROI (T).
E = ((T-G/T) x 100%
Transmit Quadrature Errors
Transmit quadrature ghosts are evaluated using images acquired in the multi-slice mode in which the phantom is placed at a location offset from the isocenter. A transmit ghost appears in the slice located in the opposite offset direction at a distance equal to the distance at which the true object is located from the isocenter (mirror image from the isocenter). The ghost and true object image will be located at the same relative positions in their respective images. For example, a true object located in the upper left comer at a distance of +5 cm from the isocenter will produce a transmit quadrature ghost in the upper left comer of the image at -5 cm ROl's taken over the true object and the ghost are used to determine the percent error
(E).
E = ((T-G)/T) x 100%
2.4.4 Action Criteria
Phase related errors should typically be less than 5% of the true signal value. DC-offset errors should not be present in images from a properly functioning system.
2.4.5 Example Images
Figure 2-9 is an image of the linearity section of the phantom demonstrating the non-linearity that exists in the magnetic field especially on the outer edge of the field of view. Three black squares are markers for phantom orientations.
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