Estimating of radiation output of X-RAY tube using mathematical model: Case of high-frequency machines.

PURPOSE: During any radiological procedure, it is important to know the dose to be-administered to the patient and this can be done by estimating the output of the X-ray tube either with a dosimeter or with a mathematical equation or Monte Carlo simulations. The aim of this work is to develop a new mathematical model equation (NMME) for estimating the output of high-frequency X-ray tubes. METHODS: To achieve this, data collected from ten machines in many regions of Cameroon were used (for nine machines) to build an initial model that does not take into account the anode angle and the tenth machine was used to test the model. Using the SpekCalc software, some simulations were carried out to evaluate the influence of the anode angle. This allowed the NMME to be proposed. RESULTS: The deviations frequencies between 0.65% and 19.61% were obtained by comparing the output values obtained using initial model with the measured values. The statistical hypothesis test showed that the estimated values using initial model and NMME are in agreement with those measured unlike the Kothan and Tungjai model. For the tenth machine, the percentage difference between estimated and measured values is less than 8 %. CONCLUSION: These results show that the proposed model performed better than the previous models. In the absence of a dosimeter, the NMME could be used to estimate the output of high frequency X-ray machines and therefore the radiation doses received by patients during diagnostic X-ray examinations.

Feasibility of low-dose CT protocols for evaluating the sinonasal cavity and reducing radiation exposure in dogs.

With the increasing use of radiation therapy for treatment of canine sinonasal neoplasia, there is a need for developing low-dose CT protocols to help minimize radiation exposure. The purpose of this study was to assess the trade-off between image quality and reduced radiation exposure of a low-dose CT technique in the canine sinonasal cavity. In this prospective, experimental study, CT images of the sinonasal cavities from 10 normal Beagles were acquired using high-dose (130 kVp) or low-dose (110 kVp, 80 kVp) protocol. Radiation dose and image quality were compared. Radiation exposure measured by the volume-weighted CT dose index and dose-length product was reduced by 36% at 110 kVp and 74% at 80 kVp respectively, compared to the corresponding values at 130 kVp (P = 0.000). Low-dose protocol resulted in higher image noise and reduced signal-to-noise ratio and contrast-to-noise ratio than 130 kVp in most evaluated regions of interest (P < 0.05). CT numbers of the contrast-enhanced structures were highest at 80 kVp (P = 0.000). Conspicuity of most sinonasal structures was similar for high dose and both lower dose protocols. The results of this study indicate that 80 or 110 kVp can be used for sinonasal CT examinations to reduce radiation exposure to the patient without compromising image quality.

Accuracy of weighted CTDI in estimating average dose delivered to CTDI phantoms: An experimental study.

PURPOSE: The concept of the weighted computed tomography dose index ( CTDI w ) was proposed in 1995 to represent the average CTDI across an axial section of a cylindrical phantom. The purpose of this work was to experimentally re-examine the validity of the underlying assumptions behind CTDI w for modern MDCT systems. METHODS: To enable experimental mapping of CTDI 100 in the axial plane, in-house 16 and 32 cm cylindrical phantoms were fabricated to allow the pencil chamber to reach any arbitrary axial location within the phantoms. The phantoms were scanned on a clinical MDCT with five beam collimation widths, three bowtie filters, and four kV levels. To evaluate the linearity and rotational invariance assumptions implicitly made when the weighting factors of 1/3 and 2/3 in the CTDI w formula were originally derived, CTDI 100 was measured at different radial and angular locations within the phantom for different collimation, bowtie, and kV combinations. The average CTDI ( CTDI avg ) across the axial plane was calculated from the experimental two-dimensional (2D) dose distribution and was compared with the traditional CTDI w . RESULTS: For both phantoms under all scan conditions, the axial dose distributions were found to have significant angular dependence, potentially due to the x-ray attenuation by the patient couch or the head holder. The radial dose profiles were also found to significantly deviate from linearity in many cases due to the presence of the bowtie filter. When only the 12 o'clock peripheral CTDI 100 and the traditional weighting factors were used to calculate CTDI w , the average dose was overestimated in the 16 cm phantom by up to 8.4% at isocenter and up to 35.3% when the phantom was off-centered by 6 cm; in the 32 cm phantom at isocenter, the average dose was overestimated by up to 12.8%. Using an average of the four peripheral CTDI 100 measurements at the 12, 3, 6, and 9 o'clock locations reduced the error of CTDI w to within 1.2% in the 16 cm phantom. For the 32 cm phantom, even by using the average of the peripheral measurements, the traditional CTDI w underestimated the average dose by up to 4.3% due to aggressive drop-off of the CTDI 100 at the phantom periphery. CONCLUSIONS: The linearity and rotational-invariance assumptions behind the traditional CTDI w formalism may not be valid for modern CT systems and thus CTDI w may not accurately represent the average dose or radiation output within a CTDI phantom. Utilizing data from all four peripheral locations always improves accuracy of CTDI w in representing the true average dose. For the large (32 cm) phantom, nonlinear models and more measurement points are needed if a more precise estimation of the average axial dose is required.

Estimation of primary radiation output for wide-beam computed tomography scanner.

PURPOSE: To estimate in-air primary radiation output in a wide-beam multidetector computed tomography (CT) scanner. MATERIALS AND METHODS: A 6-cc ionization chamber was placed free-in-air at the isocenter, and two sheets of lead (1-mm thickness) were placed on the bottom of the gantry cover, forming apertures of 40-80 mm in increments of 8 mm. The air-kerma rate profiles were measured with and without the apertures ( K ˙ w - A , K ˙ w / o - A ) for 4.8 s at tube potentials of 80, 100, 120, and 135 kVp, tube current of 50 mA, and rotation time of 0.4 s. The nominal beam width was varied from 40 to 160 mm in increments of 40 mm. Upon completion of data acquisition, the K ˙ w / o - A were plotted as a function of the measured beam width, and the extrapolated dose rates ( K ˙ 0 - w / o - A ) at zero beam width were calculated by second-order least-squares estimation. Similarly, the K ˙ w - A were plotted as a function of the radiation field (measured beam width × aperture size at the isocenter), and the extrapolated dose rates ( K ˙ 0 - w - A ) were compared with the K ˙ 0 - w / o - A . RESULTS: The means and standard errors of the K ˙ w / o - A with 40-, 80-, 120-, and 160-mm nominal beam widths at 120 kVp were 10.94 ± 0.01, 11.13 ± 0.01, 11.22 ± 0.01, and 11.31 ± 0.01 mGy/s, respectively, and the K ˙ 0 - w / o - A was reduced to 10.67 ± 0.02 mGy/s. The K ˙ 0 - w - A of 40-, 80-, 120-, and 160-mm beam widths were reduced to 10.6 ± 0.1, 10.6 ± 0.2, 10.5 ± 0.1, and 10.6 ± 0.1 mGy/s and were not significantly different from the K ˙ 0 - w / o - A . CONCLUSIONS: A method for describing the in-air primary radiation output in a wide-beam CT scanner was proposed that provides a means to characterize the scatter-to-primary ratio of the CT scanner.

Teletherapy sources with imported and indigenous Co activity.

Board of Radiation and Isotope Technology, a unit of the Department of Atomic Energy, fabricates and supplies radioactive sources for medical, industrial, agriculture and research applications. High specific activity cobalt-60, required for teletherapy is normally imported. There was a proposal for manufacturing high specific activity sources indigenously. A study was carried out to observe the feasibility of mixing imported and indigenous cobalt-60 pellets to fabricate teletherapy source capsules. The specific activity of imported pellets is more than 300 Ci/g, whereas that of indigenous pellets obtained from Indian power reactors is 140 Ci/g. The radiation output from a capsule for different combinations of specific activity was evaluated. Losses due to self-absorption were accounted in the evaluations. In another study, the optimized lengths of the capsule for an output of 200 RMM and the additional activity to be added to compensate losses due to self-absorption were also estimated for different specific activity pellets. Sources fabricated on the basis of this study showed a good agreement with the estimations. Source capsules with a combination of different specific activities are yet to be fabricated.