X-ray or gamma ray systems or devices – Accessory – Testing or calibration
Reexamination Certificate
1998-09-08
2002-09-24
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Accessory
Testing or calibration
C378S158000
Reexamination Certificate
active
06454460
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to a system for evaluating and tuning a radiation generator. In particular, the present invention relates to an apparatus intended for (a) performance evaluation of an x-ray generator, (b) evaluation of associated imaging system signals for servicing, (c) optimization of radiographic parameters for image contrast, patient entrance exposure and x-ray tube loading, and (d) on line feed back of performance parameters to x-ray generator control for improving the radiation output characteristics. This device, which is capable of handling several radiation induced and non-radiation based signal inputs, combined with novel data sampling and processing methods, provides a self-consistent method of measurement for every single x-ray exposure.
BACKGROUND OF THE INVENTION
X-ray imaging is a popular clinical diagnostic imaging modality. X-rays cause damage to tissues due to their ionizing power. Image quality is fundamental to diagnosis while minimal x-ray exposure reduces patient radiation risk. In order to effect these twin goals, government bodies and professional organizations have enacted performance standards for x-ray imaging systems. Performance of x-ray systems is determined by measuring radiation. If a system is found to be non-compliant with the expected standards, corrective action is taken and servicing of the equipment is performed. Service personnel use test devices to measure electrical signals and identify problems for correction. An apparatus, which can provide both types of measurements, would help quick service and recalibration. Radiation based measurements, [ref. 12,9,7,6,5,4,3,1] currently used, include kVp (kilovolts peak), mA (milliamperes), HVL (half value layer), radiation exposure [ref. 8,2,1], exposure time, and radiation waveform [ref. 12].
X-ray energy spectrum is continuous, modeled by bremsstruhlung theory, with the highest energy of the spectrum [ref. 11] determined by the peak applied potential kVp. For the same kVp, different types of generators such as single phase, three phase six pulse, three phase twelve pulse, single phase medium frequency (variable), or high frequency inverters would produce different energy distributions. The mA determines the intensity or number of x-rays of an exposure without changing the energy distribution of x-rays. Since kVp can not describe the energy of radiation due to the spectral distribution, measurement of Half Value Layer (HVL) representing the effective energy with respect to attenuation by aluminum is necessary. This is usually obtained using data in several separate exposures of radiation output placing different thickness of aluminum filters in the path of radiation. This measurement procedure requires the x-ray generator is working under reproducible conditions. This assumption is questionable, particularly in the context of quality control testing. If we have determined the distribution of applied voltage and attenuation of radiation at each voltage for several thickness of filters, for the same radiation exposure simultaneously, then we can compute HVL consistently. In order to accomplish the above measurements, several novel design concepts have to be developed and implemented.
From a servicing point of view, rise time of x-ray exposure, pulse overshoot, and any high voltage breakdown due to x-ray tube problems require sample times in the range of 10-50 microseconds. Devices with analog electronics having a bandwidth of 1 kHz and 10 KHz analog-to-digital converter system can not detect these problems.
For self-consistent x-ray measurements, radiation parameters [ref. 10] such as kVp. HVL, mA, exposure time, exposure or kerma should be evaluated at the same time for an exposure. Corresponding waveforms should be processed for the same exposure as well. If this can be accomplished, this device would reduce number of exposures for evaluation, identify system performance with minimal uncertainty and self-consistency.
Non-invasive methods of estimating kVp from radiation measurements have proven to be useful. Currently popular method of kVp evaluation is based on radiation measurements using differential methods. The kVp measurement, ideally, should be performed for the whole exposure. Several designs have set a practical limit on the inclusion of exposure data for 100 milliseconds to 300 milliseconds. Actual exposures, as per generator specifications can be as long as 10 seconds. Exposure stability problems could appear in a long exposure, as x-ray tubes become gassy. Thus, exposure time limit for kVp measurement leaves critical problem areas unattended.
Among the inputs to the performance evaluation apparatus, two classes of signals are involved. Signals from x-ray assembly are fast, following the generator frequency. Signals from ionization chamber, mA meter, photo-timer output are integrated signals with less than 1 KHz bandwidth. Signals from circuits for exposure start, exposure terminate, rotor ready, filament ready etc. are, perhaps, pulses with several seconds of delay between their occurrence.
SUMMARY
Accordingly, the present invention offers the flexibility for measuring any combination of signals, thus advancing measurement procedures to technological limits.
This invention has accomplished to overcome the design limitations by devising a suitable component architecture and implementation. Measurement limitations have been removed by designing a multiple sensor inputs assembly and an automatic scan method to sample high frequency and low frequency signals from radiation sensors and electrical interfaces in the same exposure. Suitable computational procedures have been developed to arrive at accurate performance parameters and waveforms of the x-ray generator. Thus, this invention has succeeded in achieving self-consistent performance parameters of x-ray system. In addition to performance evaluation, this apparatus is useful for optimization of patient entrance skin exposure and image contrast. This apparatus can be extended to feed performance information back to generator control for on line adjustments for improved performance levels for accurate x-ray imaging applications.
In general, a system is provided that includes a multiple sensor assembly, a filter assembly, and a processor assembly. The multiple sensor assembly has a plurality of radiation sensors arranged to receive a radiation signal from a radiation generator. Each radiation sensor has a sensor output for providing a radiation sensor signal to the processor assembly. The filter assembly has a filter panel for at least one of the radiation sensors with each filter panel having an associated radiation sensor and being operably interposed between the radiation generator and its associated radiation sensor. The processor assembly is operably connected to the multiple sensor assembly to communicate with the multiple sensor assembly in order to receive the radiation sensor signals for evaluating the performance of the radiation generator.
In one embodiment, the invention includes (1) a multi-sensor assembly, (2) personal general purpose computer controlled electronics for signal conditioning and optimization, (3) a personal computer interface card that includes a programmable gain amplifier, multiplexer, analog-to-digital converter and digital input-output interface, (4) a personal computer with storage, (5) an application software or firmware for x-ray system performance evaluation and (6) personal computer compatible input and output devices. The multi-sensor assembly includes several x-ray sensitive sensors, which are substantially more efficient [ref. 18,14,13,1] for x-rays than simple silicon photodiodes. This is accomplished either by optically coupling silicon photodiodes with x-rays-to-light converting screens or materials used in radiography and fluoroscopy or using large area photo-conductive devices. The sensor devices are operated with bias and with device output optimized by load resistors. The computer controls th
Ramanathan Naganathasastrigal
Ramanathan Vijay
Dorsey & Whitney LLP
Porta David P.
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