The invention generally relates to radiographic imaging and, more particularly, relates to a method and apparatus for reading a computed radiography phosphor plate or sheet that has been exposed by x-rays by supplying pumping light thereto.
It is well known that, by using x-ray systems, features can be visualized within the human body or within industrial products, or the like. Current X-ray systems often use x-ray film which must be developed.
In the alternative, computed tomography installations are available but are very expensive and require large amounts of computer power.
In addition systems exist which use a technique called computed radiography. A patient or object is exposed with x-rays and a latent x-ray image is formed on a phosphor-containing computed radiography plate or sheet that is similar to a sheet of film. The phosphor-containing sheet typically may include a rare earth, such as europium, in combination with barium and fluorine. Other sheet formulations also are available. The sheet is sensitive to x-rays and can store latent x-ray image thereon. Because the sheet is also sensitive to light it is kept in the dark. A sheet containing a latent x-ray image is imaged in a scanner by exposing the sheet and its latent image to a raster-scanned laser beam. Areas of the sheet which have preferentially received x-ray energy phosphorus, making the latent X-ray image visible.
While the scanner is convenient and allows reuse of the computed radiography sheets multiple numbers of times, it does suffer from certain drawbacks. It is difficult to obtain a high-spatial resolution image because the pumping laser beam, although only covering a small spot-size at a time, tends to leave illumination energy behind, which causes bloom; thereby smearing the image and reducing its resolution. This is because the image is built up in the way that an image would be in a flying spot device wherein only a single optical detector is used. The single optical detector can capture radiation from almost any position on the sheet. The optical detector, however, is unable to determine whether the photons it is receiving are coming from unwanted bloom or coming from active phosphorescence caused by excitation by the laser beam.
The prior raster-scanned laser systems introduce spatial non-linearities in the image for which there must be compensation. The non-linearities are due to the difference in the effective beam scan rate when the beam is substantially perpendicular to the latent image containing sheet at the center portion of the sheet and when it is sweeping at an angle to the sheet near the sheet edges. As a result, since the image is constructed based upon on pumping beam timing and orientation, elaborate methods would have to be used in order to effectively relinearize the beam scan to provide an undistorted image.
U.S. Pat. No. 4,737,641 discloses an apparatus for producing x-ray images by computer tomography using a high energy excitation beam such as a red laser beam which is focused on the storage plate by suitable optics and the beam is deflected across a line of the plate by a rotating mirror. The storage plate is shifted in steps relative to the fan of the laser beam so that the entire image is read line-by-line by the x-ray beam. The photo-stimulated luminescence is successively supplied point-by-point to a common photomultiplier and then to an amplifier via a light conductor comprising a plurality of optical fibers. The emissions are connected into electrical signals are supplied to analog-to-digital converters and a computer forms an image that is visible on a display unit.
A particular problem with systems using the rotating mirror to deflect the laser beam across the line is that vibrations jiggle the mirror and cause a loss of sensitivity or tolerance. An acceptable tolerance is often only 0.004 inch which can be a problem when the mirror is being vibrated. Further, these rotating mirror and focusing lens systems require a light-sealed, large volume or space within an enclosed housing. Further, such systems may be too delicate to be used in the field such as for military x-rays of wounded soldiers or for being carried into remote rugged locations for non-military use. Thus, there is a need for a smaller and more rugged apparatus for producing x-ray images by computer tomography.
One of the particular problems of a rotating laser used in a radiographic imaging and computed radiographic phosphor plate system is that the rotating laser must be accurately positioned and kept in its exact position as it is rotated at high speed, for example, 6600 rpm. Thus the laser, which is experiencing considerable G-forces, must be locked in an exact position to keep its focus with respect to the circular or arcuate array of fiber ends to which it is supplying a beam of light. In addition to being locked in position it is important that the laser also be adjusted so that it can be positioned quite accurately and focused both radially so that the axis of rotation of the laser is substantially matched and co-axial with the axis of the circular array of fiber ends. Also, there is a need for an axial focus adjustment to shift the laser in the axial direction towards or from the fiber ends to make sure that the beam is focused properly on the fiber ends. Typically, X and Y-track systems for shifting an element such as a laser head do not have a zero backlash which is desirable to prevent slop or movement and creep of the laser beam out of focus. Additionally, typical X and Y-tracks are relatively large and expensive and they are to heavy to be rotated at high speed, for example, the 6600 rpm rotational velocity of the laser head in the illustrated embodiment. Thus there is a need for an adjustment device for locking the laser in its focused and adjusted position which is capable of being rotated with the laser head.
Another problem with the use of a rotating laser and a computer tomography, x-ray imaging system is that the power must be delivered to the laser which is also provided with a power supply and a thermal control. The power being supplied must allow the laser to be electrically insulated from the support with the laser being grounded. It will be appreciated that the electrical power system for the rotating laser head should be relatively inexpensive and simple and have a relatively long life because the laser head that is rotating at high speed, for example, at 6600 rpm.
What is needed, then, is a system and apparatus which can quickly and conveniently provide highly-accurate and high resolution computed radiography visible images without the need for expensive equipment.