At least one of the embodiments of the present invention generally relates to improvements in a medical x-ray imaging system, and more particularly relates to an improved C-arm of an x-ray imaging system.
X-ray imaging systems typically include an x-ray source, a detector, and a positioning arm, such as a C-arm, supporting the x-ray source and the detector. In operation, an imaging table, on which a patient is positioned, is located between the x-ray source and the detector. The x-ray source typically emits a conical beam of radiation, such as x-rays, toward the patient. The conical beam has a theoretical central beam. The radiation typically passes through the patient positioned on the imaging table and impinges on the detector. As the radiation passes through the patient, anatomical structures of different densities inside the patient cause intensity variances in the radiation received at the detector. The detector then translates the radiation variances into an image which may be employed for clinical evaluations. Typically, the x-ray source is directly mounted to a distal end of the C-arm while the x-ray detector is mounted to another distal end of the C-arm. The x-ray source is positioned such that emitted x-rays are received by the x-ray detector.
The C-arm is typically supported by a support structure. The support structure may be mobile or fixed. The C-arm is mounted to the support structure through a bearing assembly. The bearing assembly allows the C-arm to rotate relative to the support structure. Therefore, anatomical structures of a patient positioned between the x-ray source and the x-ray detector may be imaged from different angles and perspectives. That is, the x-ray source and the x-ray detector rotate around the patient thereby imaging anatomical structures of the patient from various angles and perspectives.
Typically, C-arms are circular, or semi-circular. That is, the radii of a C-arm are the same through every angle of rotation. A circular C-arm has a true center of rotation. However, in mobile C-arm systems, the x-ray source and the x-ray detector typically are offset from the center of rotation. The x-ray source and the x-ray detector are offset in order to balance the C-arm system. That is, the x-ray source and the x-ray detector are positioned to mechanically balance out the C-arm. However, because the x-ray source and the x-ray detector are offset from the center of rotation, the central beam is also offset from the center of rotation.
Typically, when the C-arm is rotated through an arc of 180 degrees, the central beam shifts due to the offset positioning of the x-ray source and the x-ray detector. That is, when the central beam is positioned such that a target anatomical structure is within an imaging area of the conical beam, the target anatomical structure typically is not within the imaging area throughout the entire 180 degrees of C-arm rotation. That is, the conical beam is not isocentric throughout the rotation of the C-arm. Rather, because of the offset positioning, the anatomical structure does not remain within the imaging area of the conical beam.
Isocentric C-arms have been developed. For example, some systems position the x-ray source and the x-ray detector in line with the center of rotation of the circular C-arm. In order to balance the C-arm, the C-arm structure is extended past the x-ray source and the x-ray detector. The extended portions of the C-arm structure are typically heavily weighted. The C-arm is extended to counterbalance the weight of the C-arm. When the extended C-arm is rotated, however, the extended portions typically encroach upon the space of a treating physician. That is, if the C-arm is rotated in one direction, one extended portion may hit the treating physician in the head. If, however, the C-arm is rotated in the opposite direction, the other extended portion may hit the treating physician in the shins or knees. Overall, the extended, counterbalancing portions typically are bulky and cumbersome. Further, the extended portions may hinder access to the patient positioned within the C-arm. Additionally, the extended portions increase the weight of the C-arm. The increased weight of a mobile C-arm system including extended portions typically makes movement of the mobile system more difficult than if the extended portions were not included.
Instead of extending portions of the C-arm, isocentric C-arm motion may also be achieved through the use of powerful motors. That is, the x-ray source and the x-ray detector may be positioned in line with the center of rotation of the circular C-arm. While many fixed systems include expensive, powerful motors, motors may also be used with mobile systems. Powerful motors counteract the mechanical imbalance of the C-arm. The inclusion of powerful motors on mobile systems, however, increases the overall weight of the system. As stated above, increasing the weight of mobile systems decreases the ease of mobility of the system. Further, the motors are expensive.
Typically isocentric circular C-arm systems exhibit an additional limitation. While typical isocentric systems maintain an anatomical structure within an isocentric area, or imaging area, throughout 180 degrees of rotation, the distance between an x-ray detector and a patient""s body in many isocentric systems fluctuates. During x-ray imaging, a patient typically lies on his back or front side. Typical systems rotate through a semi-circular arc around the patient. Therefore, because human beings typically are not round, the x-ray detector typically is closer to the patient""s skin at some positions, and further away at other positions.
For example, an x-ray detector typically is closer to a patient""s skin when the patient is being imaged from the side as opposed to the top or bottom. When the x-ray detector is further away from the patient""s skin, the resulting images typically are of lesser quality than when the x-ray detector is closer to the patient. As the beam of a circular isocentric C-arm is rotated around a patient, the circular area of the conical beam, and therefore the magnification of the target anatomy at the isocenter, typically remain relatively constant. Because the isocenter is circular, however, an air gap results that increases between the patient""s skin and the detector as the system is rotated. In some rotated positions, the air gap creates x-ray scatter. Air scatter, in turn, reduces image contrast. The further the x-ray source and x-ray detector are from the patient, the greater the air gap is between the patient""s skin and the x-ray source and detector. As the air gap increases, image contrast and magnitude of magnification decrease. That is, the greater the cross-sectional area of the conical beam, the smaller the magnification of the resulting image.
Further, the patient typically is exposed to skin intensity radiation of higher magnitude when the conical beam entering the patient is more localized. Allowing the air gap to increase results in the x-ray source being closer to the patient""s skin than is necessary. The close proximity of the x-ray source causes the x-rays to enter the patient through a smaller, more localized area, thereby increasing x-ray dose to the irradiated skin. Additionally, the presence of more air between the patient and the x-ray detector causes x-ray scatter and the resulting loss of image contrast.
Some systems alleviate the problems caused by an increasing air gap and less than optimal anatomical image magnification through the use of variable source-to-image distance (SID) x-ray detectors. That is, the face of the x-ray detector moves independently of the x-ray source toward and away from the patient depending on the position of the x-ray detector through the arc of rotation. Typically, variable SID is achieved through the use of an additional motor, thereby adding weight and complexity to the system. Further, the addition of the motor, and the independent movement of the x-ray detector through the rotation of the C-arm causes additional balance problems. That is, as the x-ray detector moves toward or away from the patient during the rotation of the C-arm, the center of gravity, or moment of inertia, of the C-arm changes with the additional independent movement of the x-ray detector. Overall, the cost of the system increases with the additional variable SID feature. For example, an additional motor, additional coordinating software and hardware, and additional support structures add to the cost of the system. That is, the addition of a variable SID feature to a C-arm complicates the system and increases the cost of the system.
Therefore a need has existed for a more efficient, simpler and substantially isocentric C-arm for use with fluoroscopic imaging equipment. Further, a need has existed for a less expensive, substantially isocentric C-arm. Additionally, a need has existed for a less cumbersome mobile, substantially isocentric C-arm.
In accordance with an embodiment of the present invention, a C-arm for an x-ray imaging system has been developed that images a target anatomical structure of a patient within an isocentric area throughout the entire range of rotation of the C-arm. The rotation of the C-arm substantially conforms to the contours of a patient""s body. The x-ray imaging system includes a support base, a bearing assembly supported by the support base, a non-circular positioning arm, or C-arm, supported by the bearing assembly, an x-ray source located at a first distal end of the positioning arm; and an x-ray detector located at a second distal end of the positioning arm. The non-circular positioning arm moves arcuately with the bearing assembly to rotate the x-ray source and the x-ray detector in a semi-elliptical path relative to the support base. The x-ray source and the x-ray detector are positioned a fixed distance from each other. The distance between the x-ray source and the x-ray detector remains constant when the non-circular C-arm rotates.
The non-circular C-arm may be shaped in a linear spiral. The linear spiral is described by the equation R=kxcex8, wherein xcex8 is an angle of rotation, wherein R is a radius of the C-arm angle of rotation xcex8 and wherein k is a constant. The C-arm also includes an imaging isocentric area located between the x-ray source and the x-ray detector. The imaging isocentric area remains substantially constant when the non-circular C-arm rotates relative to the support base.
In another embodiment of the present invention, a method for manufacturing a non-circular C-arm for use in an x-ray imaging system is provided. The method includes the steps of designating an arbitrary point as an origin of a reference linear spiral, forming the reference linear spiral from the origin of the reference linear spiral, determining a suitable size for a C-arm, constructing the C-arm through a portion of the reference linear spiral, and conforming the C-arm to the portion of the reference linear spiral. The determining step includes the step of determining an adequate space for imaging a human being. The constructing step includes constructing the C-arm through an approximately 190 degree portion of the linear spiral. The method also includes the step of aligning the x-ray source so that a central beam emitted from the x-ray source passes through the origin of the linear spiral and impinges on the center of the x-ray detector.
Another embodiment of the present invention provides a method for obtaining images from an x-ray system having a non-circular C-arm operating within a patient coordinate system. The non-circular C-arm carries an x-ray source at one end and an x-ray detector at an opposite end. First the method comprises the step of positioning the non-circular C-arm at a first imaging position at which the x-ray source and x-ray detector are oriented at a first angle with respect to the patient coordinate system and obtaining a first image at the first imaging position. Next, the method includes the step of rotating the non-circular C-arm from the first imaging position to a second imaging position at which the x-ray source and x-ray detector are oriented at a second angle with respect to the patient coordinate system. Then, the step of obtaining a second image at the second imaging position is performed. The rotating step includes moving the x-ray source and x-ray detector along a pseudo-elliptical path based on a shape of the non-circular C-arm. The x-ray detector is located at a proximal distance from a patient when in the first imaging position, and at a distal distance from a patient when in the second imaging position.