Radiant energy – Source with recording detector – Using a stimulable phosphor
Reexamination Certificate
2001-05-02
2004-03-02
Hannaher, Constantine (Department: 2878)
Radiant energy
Source with recording detector
Using a stimulable phosphor
C250S584000
Reexamination Certificate
active
06700131
ABSTRACT:
TECHNICAL FIELD
The present invention relates to scanning of imaging plates in general and to scanning of storage phosphor medical imaging plates in particular.
BACKGROUND OF THE INVENTION
(a) Image Plate Scanning:
Imaging plates, such as storage phosphor imaging plates, have become standard in the field of Computed Radiography (CR) as the medium onto which an image of a portion of the patient's body can stored. The image on such a phosphor imaging plate is extracted by scanning the imaging plate with a scanner. Typically, a phosphor imaging plate is scanned by passing a scanning laser beam over the surface of the imaging plate while recording light emitted from the imaging plate in response to the laser beam. By recording the phosphorescence emission corresponding to each of the pixels of the imaging plate with a detector such as a photomultiplier, the image stored therein can be re-created (such that it can be displayed on a computer terminal).
The act of scanning an imaging plate by passing a scanning laser beam thereacross is inherently destructive (i.e.: it releases the energy stored in the phosphor screen). As such, a particular image stored on an imaging plate can only be scanned (i.e. read) once.
Unfortunately, when scanning an imaging plate to re-create the image stored therein (such that it can then be displayed on a computer terminal) image artifacts tend to appear in the final image. For example, alternating bands of lighter and darker regions, which run across the image, tend to be seen. As will be explained, such bands may be generated by uneven (i.e.: varying speed) movement of the imaging plate relative to the scanner (in what is commonly called the “slow scan direction”, and referred to herein as the “Y” direction). This may be due to simple repeating mechanical irregularities in the scanner which thereby positions successive scan lines at uneven spacing along the length of the imaging plate. It may also be caused by vibrations perpendicular to the plane of the imaging plate which affect the optical focus of the scanning mechanism. In addition, various multi-head scanning systems tend to generate artifacts simply due to the fact that the different scanning heads each have their own optical paths which exhibit different optical characteristics. This is especially true in the case where each of the various scanning heads has its own dedicated laser.
Therefore, unwanted image artifacts can be divided into two broad groups. The first being those unwanted image artifacts caused by variations in the speed of movement of the scanner with respect to the imaging plate or by small vibrations either in the slow scan (i.e.: “Y”) direction or normal to the imaging plate. The second being those unwanted image artifacts caused by differences between various scanning heads (when using a scanner with more than one scanning head). These two groups are discussed separately below.
(b) “Ripple” or “Banding” Artifacts:
A variety of different systems exist to scan imaging plates, such as storage phosphor imaging plates by passing one or more scanning heads over the surface of the imaging plate.
In a first existing system, a single scanning head is moved back and forth across the surface of the imaging plate while the imaging plate is moved relative to the scanner in the Y direction. Specifically, the imaging plate is moved in a direction that is perpendicular to scanner head movement such that the scanning head passes over the imaging plate along a plurality of parallel or generally parallel paths (in an “X” direction). In one type of system, a rotating or oscillating mirror directs a laser beam across the imaging plate, and the imaging plate is then advanced an incremental distance. This process is repeated such that the scanning head traces a series of parallel paths across the imaging plate. In another type of system, the imaging plate is continuously advanced as the scanning head is passed thereover, such that the scanning head traces a series of parallel paths across the imaging plate. Alternatively, the scanning head may itself be moved back and forth in the X direction across the surface of the imaging plate.
In a second existing system, the imaging plate is wrapped around a cylinder, and the cylinder is rotated while a single scanning head moves down the length of the cylinder. An example of such a system is found U.S. Pat. No. 5,635,728.
In a third system, which is novel and was developed by the present Applicants, a plurality of (typically three) scanning heads are positioned around the perimeter of a rotary scanner, and the scanner is rotated while an imaging plate is advanced thereunder. An example of such a system is found in PCT Published Application WO 00/19477. In this system, each of the scanning heads sequentially trace a curved path across the surface of the imaging plate and the movement of the imaging plate thereunder causes these curved paths to be spaced apart from one another along the length of the imaging plate. As the imaging plate is advanced under the rotating scanner, the entire surface of the plate is scanned.
Unfortunately, in all of the above described systems, any inconsistency or periodic variation in the speed of movement between the imaging plate and the scanner will result in successive scan lines (i.e.: the paths taken by the scanning head(s) across the surface of the imaging plate) being spaced unevenly apart. This unevenness between successive scan lines causes “banding” or “ripples” to occur in the final image. This is true both in the case of a linear path scanner which is kept at a fixed location with its scanning head directing a laser beam in a straight path across an imaging plate, and in the case where a plurality of scanning heads are rotating around a common center of a scanner.
As mentioned above, the scanning of an imaging plate releases the energy trapped therein. Therefore, when successive scan lines are too close together, the edges of the laser beam spot (which passes along each successive scan line) will tend to overlap such that “oversampling” of the image occurs. In other words, part of the energy representing the brightness of the image stored in a particular pixel will have already been released by the previous scan line, thereby reducing the intensity of the image when the pixel is scanned. As such, the image energy trapped within a second pixel disposed on a second scan line will have been partially released when a first (ie: previous) adjacent scan line has passed over the imaging plate. When a region of the imaging plate has been oversampled in this manner, a dark band will tend to occur which runs across the image (in a path generally parallel to the scan lines). Conversely, should the successive scan lines be positioned too far apart, the image will tend to be undersampled, resulting in a light band passing across the image.
Even a very small degree of unevenness in the scan line spacing can give rise to detectable banding artifacts in this type of scanner because the pixel intensities are preferably digitized to a high degree of precision (typically 16 or more bits per pixel).
Such alternating light and dark bands will become especially apparent when the intensities of the individual pixels in the image are scaled and presented to an operator in a final (on screen or printed) image. Such alternating banding will typically appear as thin bands in the final (on screen or printed) image such that the image appears to have “ripples” running along its length. In the case of a linear back and forth scanning head, these ripples will appear as straight lines and in the case of a rotary scanner, these ripples will appear in curved arcs.
The unevenness in the speed at which the imaging plate moves relative to the scanner is typically introduced by very small mechanical inaccuracies in the transportation system that moves the imaging plate. For example, should movement of the imaging plate be performed by a transport mechanism which comprises a worm gear, the center worm gear may itself be at least slightly off-axis. In
Nishihara H. Keith
Wilfley Brian P.
Alara, Inc.
Gabor Otilia
Hannaher Constantine
Heller Ehrman White & McAuliffe LLP
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