Patent Number: 050671448
Section: description

DETAILED DESCRIPTION In a preferred but non-limiting example of the invention, an x-ray source/modulator assembly 10 slides up and down along column 12 so that it can be positioned at a selected vertical (y-direction) level prior to the x-ray procedure. When energized, assembly 10 generates a fan beam 14 which is thin in the horizontal (x) direction and tall in the vertical direction, and sweeps beam 14 horizontally across an object position 16 while selectively and individually modulating vertically spaced sectors of beam 14 to vary the intensity of the radiation delivered to object position 16 (or to an object 16 at that position) by the respective beam sectors. Also prior to the x-ray procedure, a detector/film assembly 18 slides vertically along column 20, preferably in synchronism with assembly 10, to a vertical position at which assembly 18 can receive an object-attenuated fan beam 14' exiting object 16. As fan beam 14 sweeps horizontally across object 16, assembly 18 receives the post-object radiation and uses it for two purposes: (i) to measure the radiation intensity distribution of fan beam 14' and in response to generate feedback information; and (ii) to image object 16. The feedback information, along with information related to the effect that the modulation has on individual sectors of fan 14, is used to estimate the attenuating effect of object 16 and to control the modulator to increase or decrease the local amount of radiation delivered to object 16 in order to selectively equalize the radiation delivered to assembly 18. As best seen in FIGS. 2 and 3a-3c, source/modulator assembly 10 comprises an x-ray tube 22 having a focal spot 22a. In order to sweep object 16 with fan 14 horizontally, source rotation drive 24 selectively pivots tube 22 horizontally, with focal spot 22a serving as the center of rotation. Affixed to tube 22 to pivot therewith is a modulator assembly 26. In order to shape the radiation from tube 22 into the desired fan beam, assembly 26 has a fixed collimator 28 defining the maximum vertical extent of fan 14, a patient field collimator 29 which can be adjusted to define the size of the irradiated, typically rectangular area at the image plane, and an adjustable collimator 30 defining a vertically extending collimator slit that determines the horizontal dimension of fan 14. For a given sweep of fan 14, the setting of patient field collimator 29 typically is fixed so that the fan at the film plane would irradiate only the field of the desired size and shape, e.g., for standard x-ray film sizes and orientations. The slit aperture of collimator 30 can be set depending on factors such as the overall size and expected attenuation properties of the object, etc. to deliver the desired overall intensity to the object. For example, the slit aperture could be set to a horizontal dimension in the range of 0" to 0.5" at the aperture plane, which corresponds to about 0" to 3.5" at the image plane. In order to individually and selectively modulate vertically spaced sectors of fan 14, modulator assembly 26 uses modulator pins 32 which are arranged in two generally vertical rows, inner row 34 and outer row 36 (FIG. 3b), along respective arcs which are centered at focal spot 22a of tube 22 and are in a vertical plane that includes focal spot 22a. Pins 32 slide individually and selectively into fan beam 14 in the horizontal, x-direction (along the plane of the paper in FIG. 3a and normal to the paper in FIGS. 3b and 3c) such that the pin section in a vertical plane within fan 14 is generally triangular, as illustrated in FIGS. 3b and 3c. This triangular area and the attenuation of a respective beam sector increase as a pin moves further into fan 14. As best seen in FIGS. 4a-4h, a modulator pin 32 comprises an elongated stem 32a and a wedge-shaped head 32b. Base 38 of head 32b is rectangular. Ridge 40 slopes from the back (the stem) toward the front of the pin such that the area of the triangular section gradually decreases toward the tip of the wedge-shaped head. Stated differently, the z-direction dimension of the triangular section (i.e., the dimension along the direction of propagation of the appropriate ray in fan 14) decreases gradually in moving toward the tip of the pin. Ridge 40 can be rounded or truncated, as illustrated in FIG. 4a, and the tip of the pin can be truncated as also seen in FIG. 4a. In fact, FIGS. 4a and 4b illustrate pin 32 close to scale. In an alternative embodiment, the ridge can be truncated to a significantly greater extent, to arrive at the shape illustrated in FIGS. 9a-9d. Bases 38 of the pins in row 34 are as close to each other as allowed by factors such as mounting and sliding movement constraints, and the same is true of the pins in row 36. The pins of the two rows are offset such that the center line of ridge 40 of a pin in row 36 is along a ray of fan 14 passing through the small distance between two adjacent pins 32 of row 34. Accordingly, if all pins 32 are all the way into fan 14, most rays of fan 14 would pass through both a pin of row 34 and a pin of row 36. The exception would be the relatively few rays that would pass through the small distance between two adjacent pins in row 34 or two adjacent pins of row 36. Since any one pin 32 can slide into fan 14 independently of the other pins of either row, a given ray within fan 14 can pass through no pin, through one pin, or through two pins. In order to reduce artifacts while taking into account reasonably expected relative positions of pins 32, factors such as the relative dimensions, shapes and placement of focal spot 22a and pins 32 are carefully selected to achieve a smooth intensity profile in the vertical direction within fan 14. For example, when a vertical plane of fan 14 passes through a section of a pin 32 as illustrated at FIG. 5a, the intensity profile I of that plane as affected by that section varies smoothly as illustrated. Similarly, when two approximately equal pin sections 32 are in a fan plane as illustrated in FIG. 5b, the intensity profile in that plane as affected by those two sections varies smoothly as illustrated. Still similarly, when two unequal area sections of pins 32 are in a plane of the fan as illustrated in FIG. 5c, the intensity profile still varies smoothly as illustrated. In a currently preferred embodiment, modulator pins 32 vary the intensity of respective sectors of fan 14 in ratio of about 5:1 between most and least attenuation. A ray can pass through a minimum of less than 0.125" of pin material to a maximum of about 1"; at 140 KV source voltage this corresponds to a transmitted radiation ratio of about 1:5. The geometry is such that the maximum intensity change can occur within about 0.5" at the film plane in the vertical direction. In order to achieve a smooth intensity profile by taking into account typical focal spot sizes, pins 32 are placed about 7 to 15 inches from the focal spot and the base 35 of a pin in a plane normal to the scan direction is about 2 to 5 times the size of the focal spot, e.g., if the focal spot diameter is about 1 mm (e.g., 1.2 mm), the base is about 3 mm (e.g., 3.2 mm). A typical distance between the focal spot and the image plane is about 72 inches. A typical time for a scan is 1 sec. While the pins in rows 34 and 36 are shown in a preferred configuration in which the ridge (i.e., the apex of the triangular section) of each pin of one row faces the apices of the triangular sections of the pins of the other row, other configurations are possible. Any ridge can point toward or away from the focal spot which is the origin of the fan beam. For example, the apices of all pins, of both rows, can face in the same general direction such that the apices of one row face the bases of the other row, or the apices of the two rows can point to opposite directions such that the bases of one row face the bases of the other row, and in each case both rows of modulator pins can be at one side of the collimator 30 (at the source side, as shown, or at the opposite side), or collimator 30 can be between the two rows on modulator pins. In addition, while as shown the tips of the modulator pins all point in the same direction (the x-direction), the pins can be mounted such that the tips of the two rows face each other such that the pins of one row slide into fan 14 from the left but the pins of the other row slide into the fan from the right. As seen best in FIGS. 6a and 6b, pins 32 are driven by respective modulator motors 42 to slide selectively into fan 14. Each motor 42 can be a stepper motor connected to the respective pin 32 through a flexible linkage 44 passing through one or more wire guide blocks 46. Pins 32 are slidably mounted in a pin guide block 48 in which each pin stem slides in the x-direction in a respective slot that helps maintain a precise sliding path for the pin. In the preferred but non-limiting example, the stroke of a modulator pin can be about 1", performed by about 40 contiguous steps of a motor 42, each resulting in a change of about 4% in transmitted intensity of a respective sector of fan 14. As best seen in FIG. 7, beam 14' exiting the object impinges on detector/film assembly 18 which comprises the following elements arranged in the propagation, z-direction of the x-rays: an anti-scatter grid 50, a feedback detector 52, and a film cassette 54. These elements are mounted in a supporting arm 56 slidably mounted on column 20. Grid 50 can comprise a 12:1 scatter rejection grid for reducing the amount of scattered radiation reaching detector 52. Detector 52 can comprise a flat plate Xenon detector having an active volume of, e.g., 17.times.17.times.0.25" and filled with Xenon at about 1 atmosphere pressure. The electrodes on one side can be horizontally extending strips 0.23" high and 17" long, separated vertically by insulating spaces of 0.02". Preferably it attenuates fan beam 14' as little as possible, e.g., about 12% attenuation Film cassette 54 can be a standard 14.times.17" cassette mountable in either orientation. In an alternative embodiment, modulator pins 32 can be replaced with the structure illustrated in FIGS. 8a-8c which comprises a flexible diaphragm 58 of an attenuating material such as leaded rubber, mounted on a frame 59, and pusher pins 60. Pusher pins 60 slide toward or into fan 14 similarly to modulator pins 32. However, while they may provide some selected attenuation, their main purpose is to push a selected portion of diaphragm 58 into fan 14 to a selected degree. Because of the way diaphragm 58 stretches, the intensity along the vertical direction of fan 14 varies smoothly. While as illustrated pusher pins 60 push the diaphragm from only one side, it is possible to have pusher pins on both sides of the diaphragm, pushing from both directions to ensure that the diaphragm can be made to curve as sharply as desired. In operation, the operator places a film cassette in detector/film assembly 18 and positions the patient (or an object) at position 16 between assemblies 10 and 18, and moves assemblies 10 and 18 up or down along the respective columns 12 and 20 to match the position of the patient or object. The operator uses appropriate data entry devices at console 11 (FIG. 1) to select the exposure setting, the voltage setting and the scan mode, and the system takes the exposure during which fan beam 14 scans object 16, feedback detector 52 generates a modulation feedback signal delivered to a modulator feedback circuit 62 (FIG. 10) which in turn feeds the desired control signals to modulator assembly 26 to control the position of pins 32 or 60 in order to equalize the exposure at a film in cassette 54. Power to the x-ray tube comes from auxiliary equipment 13 (FIG. 1). Other types of penetrating radiation can be used in place of x-rays such as, without limitation, gamma rays or other radiation. As earlier noted, in an alternate equalized radiography system using the invention, a fan of penetrating radiation can scan generally vertically, and the detailed description set forth above applies equally well, except for interchanging the terms horizontal and vertical as appropriate.