CAT scanner

Details CAT scanner CAT scanner

1986-10-15

1988-09-06

Fields, Carolyn E.

X-ray or gamma ray systems or devices

Specific application

Computerized tomography

378114, A61B 602

Patent

active

047698276

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

1. Technical Field
The present invention relates to a CAT (computerized axial tomography) scanner using x-rays produced continuously and, more particularly, to a CAT scanner which is not materially affected by pulsating changes in the intensity of the used x-rays.
2. Background Art
The concept of the structure of a CAT scanner is illustrated in FIG. 1, where an x-ray tube 1 and an array 2 of x-ray detectors are rotatably disposed on opposite sides of an object 11 to be examined. The tube 1 emits x-rays in the form of a fan-shaped beam, most of which strikes x-ray detectors 2.sub.2 -2.sub.501 through the object 11. The x-rays are converted into electrical signals by the detectors. The x-rays in the vicinity of both ends of the fan-shaped beam hit reference x-ray detectors A(2.sub.1) and B(2.sub.502) at both ends of the array 2 without passing through the object 11. The x-rays falling on the reference x-ray detectors are also converted into electrical signals. Capacitors 3.sub.1 -3.sub.502 are connected with the detectors 2.sub.1 -2.sub.502, respectively, and are charged by the output currents from the detectors. The voltages developed across the capacitors 3.sub.1 -3.sub.502 are applied via first switches 4.sub.1 -4.sub.502 and an amplifier 5 to an analog-to-digital converter 7, where they are converted to digital form. The digital signals delivered from the converter 7 are supplied to a computer 8, where the data is arithmetically processed in accordance with a given procedure to reconstruct an image of the object. The capacitors 3.sub.1 -3.sub.502 are discharged by closing a second switch 6 while the first switches 4.sub.1 -4.sub.502 are closed.
X-rays are produced from the x-ray tube 1 in the form of pulses at certain intervals. Each time one x-ray pulse hits each x-ray detector, the output current is measured. The measurements of the output currents from the detectors are made by controlling the sequence in which the first switches 4.sub.1 -4.sub.502 and the second switch 6 are selectively closed by a control circuit 9 as illustrated by the timing chart of FIG. 4. More specifically, the first switches 4.sub.1`-4.sub.502 are simultaneously closed at instant t.sub.1 before each x-ray detector produces an output current I. Then, they are simultaneously opened at instant t.sub.2. During this interval the second switch 6 is maintained closed and so the capacitors 3.sub.1 -3.sub.502 are discharged. The output current I rises at instant t.sub.3 when an x-ray pulse falls on the detector, and drops at instant t.sub.4. The second switch 6 is opened at instant t.sub.5 after a certain period elapses since the output current I drops. During this interval the capacitors 3.sub.1 -3.sub.502 are charged by the output currents I from their respective x-ray detectors. Thereafter, the first switches 4.sub.1 -4.sub.502 are closed for a certain period in a given order to measure the voltages set up across the capacitors 3.sub.1 -3.sub.502. In particular, if the second switch 6 is opened at instant t.sub.5, the switch 4.sub.249, for example, is then closed to measure the voltage developed across the capacitor 3.sub.249. Then, the switch 4.sub.249 is opened at instant t.sub.6. Subsequently, the switch 4.sub.250 is closed to measure the voltage produced across the capacitor 3.sub.250. Subsequently, similar measurements are made of the voltages across the capacitors 3.sub.248, 3.sub.251, . . . , 3.sub.1, 3.sub.502. This sequence is shown in FIG. 5, where the numbers that the capacitors bear are shown against the numbers that the corresponding x-ray detectors bear.
In this measurement of the output current from each detector on which an x-ray pulse falls, the output current persists for a short time, compared with the period of the measurement. Therefore, in order to obtain a sufficient amount of output current, it is necessary to irradiate the patient with intense x-rays. This has tended to increase the absorbed dose.
In order to reduce the dose absorbed by the patient, continuous x-radiation may be contemplated. Sp

REFERENCES:
patent: 4041315 (1977-08-01), Hounsfield
patent: 4211926 (1980-07-01), Nakaya et al.
patent: 4484340 (1984-11-01), Yamaguchi et al.

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