X-ray or gamma ray systems or devices – Electronic circuit – X-ray source power supply
Reexamination Certificate
2001-08-30
2003-01-14
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
X-ray source power supply
C378S016000, C378S145000, C378S097000
Reexamination Certificate
active
06507639
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method and apparatus for modulating the radiation dose from an x-ray tube, and in particular to such a method and apparatus for modulating the radiation dose to achieve a predetermined effect or result associated with the radiation dose.
2. Description of the Prior Art and Related Applications
Computed tomography (CT) is recognized as a diagnostic procedure employing x-rays emitted from an x-ray tube with a relatively high dose. This means measures must be taken to maintain the exposure to radiation, to which the patient and attending personnel are subjected, to levels which do not represent a radiation hazard. With the introduction of spiral (helical) CT and multi-slice volume scanning techniques, new examination procedures have become available. The primary advantages of these new scanning techniques, such as section-to-section continuity, detection of small lesions, and rapid acquisition of data, have produced an increase in the number of patients which can be examined within a given time period, and thus have also produced an increase in the average dose per individual of the population. According to recent legislation (e.g. Council Directive 97/43/EURATON 1997) all doses due to medical exposure for radiological purposes, except for radiological therapeutic procedures, must be kept as low as is reasonably achievable. Dose reduction for CT purposes is strongly recommended and supported by the relevant national and international regulating authorities.
In accordance with these desires, scanning techniques are beginning to be employed in the field of CT wherein the x-ray level is adjusted dynamically during a scan. For this purpose, modulation of the radiation dose must be undertaken during the course of a scan. Dose modulation can be accomplished by adjusting the x-ray intensity during the rotation of the gantry of a CT apparatus, as a function of the gantry rotation angle and dependent on the instantaneous patient x-ray absorption at each particular tube angle position. Such techniques are described, for example, in U. S. Pat. Nos. 5,822,393 and 5,867,555 and 5,379,333. It is also known to modulate the x-ray dose dependent on the scanned body region by automatically adjusting the x-ray level during a longer spiral scan by making a compromise between the requirements for image quality and the applied dose, as described in U.S. Pat. Nos. 5,764,721 and 5,696,807 and 5,625,662.
It is also known to dynamically adjust the x-ray intensity during a scan dependent on the physiological condition of the organ under examination, such as the phase of a heart cycle in the case of cardio-dynamic scans, as described in German Application 19957083.3, corresponding to co-pending U.S. application Ser. No. 09/724,055 filed Nov. 28, 2000, and German Application P19957082.5, corresponding to co-pending U.S. application Ser. No. 09/724,057, filed Nov. 28, 2000. It is also known to dynamically adjust the x-ray intensity during gantry rotation as a function of the tube angle in order to protect the eyes of the patient in the case of a head scan, or to protect the physician's hands in a biopsy scan, as described in the aforementioned U.S. Pat. Nos. 5,764,721 and 5,696,807 and 5,625,662.
None of the above-identified known techniques, however, adequately addresses the practical problem of the dose modulation speed which is available with conventional x-ray tubes. As described in U.S. Pat. No. 5,625,662, dose modulation is achieved in an x-ray tube used in a CT apparatus by modulating the tube current. The tube current modulation is, in turn, indirectly achieved by modulating the heating current supplied to the tube filament. Because of this, conventional x-ray tubes do not react fast enough to modulate the dose dependent on the instantaneous patient absorption during gantry rotation at fast speed. The failure to reproduce radiation peaks where the patient absorption is maximum increases the quantum noise in the measured signal, and significantly degrades the image quality.
The slow reaction of x-ray tubes to rapid changes in x-ray intensity is due to physical limitations in the filament structure, as well as the thermal inertia of the filament.
It is known from U.S. Pat. No. 5,822,393 that higher modulation speeds may be achieved using specially designed x-ray tubes, which have voltage-controlled gate electrode. This is a relatively expensive approach, however, and increases the complexity and cost of the x-ray tube and the associated electronics. It is known from the aforementioned U.S. Pat. No. 5,379,333, and co-pending U.S. application Ser. No. 09/376,361, filed Aug. 6, 1999, to artificially limit the modulation curve shape to a sinusoidally shaped template and to artificially limit the modulation index to a maximum of 50%. This approach is compatible with existing CT systems, but fails to achieve the maximum possible dose reduction because of the artificial limitation of the modulation index to 50%. Moreover, the sinusoidal template does not always match the peaks in the patient absorption profile, particularly in abdominal scans. Moreover, this technique does not achieve the maximum dose reduction, or satisfy requirements of other modulation techniques such as cardio-dynamic scans.
Another known technique is described in the article “Dose Reduction in CT by Anatomically Adapted Tube Current Modulation, II. Phantom Measurements,” Kalender et al., Med. Phys., Vol. 26 (1999) pp. 2248-2253 to artificially limit the modulation index dependent only on the gantry rotation speed, for example, a maximum of 90% at 2 sec/rot to a maximum of 60% at 0.75 sec/rot. This known method, however, ignores the dependence of the modulation speed on the nominal tube current and on the focus size, and also is unable to achieve the maximum dose saving with conventional tubes.
Another known technique suggests the use of a pulsed x-ray tube with the x-ray pulse duration being adjusted to patient absorption. This technique, however, has a poor compatibility with the actual angle in angle triggered systems, and with time-triggered CT systems which use an integration period on the order of a few hundreds of microseconds. This technique also imposes significant requirements on the data measurement system. Moreover, the complete switching off of the x-rays during the pulse pauses may not be supported by all other sub-systems of a CT system, or may not be desired, such as in the case of cardio-dynamic scans.
All of the above-described known methods involve calculation of an initial x-ray profile which is desired to be achieved during a scan. An operating parameter of the x-ray tube, such as the tube current, is then varied during the scan in an effort to cause the x-ray tube to reproduce the desired x-ray profile. A conventional x-ray tube, however, as noted above cannot reproduce this profile because of its limited dynamic capabilities.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and an apparatus for modulating the radiation dose from an x-ray tube which allows a predetermined exposure effect, such as an x-ray profile, to be achieved using a conventional x-ray tube.
A further object is to provide such a method and apparatus wherein maximum dose reduction at target pixel noise is achieved by modulating the tube current in a conventional x-ray tube.
The above objects are achieved in accordance with the principles of the present invention in a method and an x-ray tube-containing apparatus, such as a computed tomography apparatus, wherein the x-ray tube has at least one variable operating parameter which, when varied, modulates the radiation dose, with a modulation speed, for x-rays produced by the x-ray tube, and wherein the x-ray tube is operated while varying the aforementioned parameter through a parameter range to generate modulation speed data, representing modulation speeds of the x-ray tube respectively for different values of the operating parameter. When an examination subject is to
Dunn Drew A.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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